Stem cell therapy for intervertebral disc regeneration: obstacles and solutions

Macrophage Changes and High-Throughput Sequencing in Aging Mouse Intervertebral Disks

Background

Intervertebral disk (IVD) degeneration is associated with lower back pain and aging; however, the mechanisms underlying age-related changes and the changes in macrophage polarization in aging intervertebral disks require further elucidation. The aim of this study was to evaluate changes in macrophages, the differential expression of senescence genes, and their relationship with hub genes in IVDs during aging in mice.

Methods

Twenty-eight male wild C57 mice aged 4 weeks were divided into two groups. Four mice per group were selected for high-throughput sequencing and 10 for tail IVD immunohistochemical analysis. Adult and aged mouse IVD specimens were stained with hematoxylin-eosin, Fast Green, and Alcian Blue to determine collagen (Col) 1, Col2, proteoglycan, P16, P21, P53, CD11b, CD86, CD206, IL-1, TGF-β, and IL-4 expression. High-throughput sequencing was performed on adult and aged mouse IVD tissues.

Results

Aged mouse IVDs showed reduced height and marked degeneration, with decreased Col2 and proteoglycan expression and increased Col1 expression. The expression of senescence markers, senescence-associated IL-1, TGF-β, and IL-4, and macrophage-related markers, CD11b, CD86, and CD206, increased markedly with age. High-throughput sequencing revealed 1975 differentially expressed genes in adult and aged mice, with 797 genes showing upregulated expression (top five: Kcna7, Mmp9, Panx3, Myl10, and Bglap) and 1178 showing downregulated expression (top five: Srd5a2, Slc38a5, Gm47283, Npy, and Pcdh8). Gene Ontology and pathway enrichment analyses highlighted aging-related cellular components, biological processes, and metabolic pathways. The identified hub genes included Cox5a, Ndufs6, and Ndufb9.

Conclusions

Disk senescence and reduced height in aged mice are linked to upregulated expression of senescence-associated phenotypes and macrophage polarization markers. These findings suggest that macrophages and differential gene expression play key roles in age-related IVD degeneration, indicating that they can be used as potential targets for therapeutic intervention.

Bioinspired hydrogel microspheres enhance nucleus pulposus regeneration through N-cadherin interaction with extracellular matrix mimicry

Intervertebral disc degeneration (IVDD) is a common cause of debilitating spinal conditions, necessitating regenerative therapies to restore tissue function. This study explores the potential of enhancing nucleus pulposus cell (NPC) viability and extracellular matrix (ECM) synthesis through surface modification of GelMA microspheres with His-Ala-Val (HAV) peptides. The HAV peptides, mimicking N-cadherin's adhesive properties, aim to promote cell-cell interactions akin to NPCs' native environment. In vitro studies demonstrated enhanced ECM secretion by NPCs cultured on HAV-functionalized GelMA microspheres, suggesting a potential for improved regenerative capacity. The microspheres promoted NP tissue regeneration when implanted in rat tail IVDs post-discectomy, indicating their therapeutic efficacy in vivo. This research provides insights into novel strategies for enhancing cell-material interactions in tissue engineering applications to mitigate IVDD.

Effects of 3D-printed exosome-functionalized brain acellular matrix hydrogel on neuroinflammation in rats following cerebral hemorrhage

BACKGROUND

Exosome-based therapeutics have garnered significant attention for intracerebral hemorrhage (ICH) treatment due to their capacity to regulate metabolic dysregulation, restore cellular homeostasis, and modulate the injury microenvironment via bioactive cargoes such as microRNAs and proteins. However, rapid systemic clearance and enzymatic degradation critically limit their therapeutic efficacy. To address this challenge, we engineered a three-dimensional (3D) bioprinted scaffold capable of encapsulating and sustaining the release of human umbilical cord mesenchymal stem cell-derived exosomes (hUCMSC-exos).

METHODS

Based on previous research [1-3], the scaffold was composed of a decellularized brain matrix (dECM), gelatin-methacryloyl (GelMA), and silk fibroin (SF) crosslinked with a photoinitiator. hUCMSC-exos were precisely incorporated via extrusion-based 3D bioprinting. Release kinetics were assessed via in vitro elution and in vivo imaging. An ICH rat model received stereotaxic implantation of the exosome-laden scaffold (dECM@exo). Neuroinflammatory markers (IL-6, TNF-α, IL-10) and apoptotic activity (JC-1, Annexin V/PI, TUNEL) were quantified. Neurological outcomes were longitudinally evaluated using the modified Longa scale, Bederson scoring, and sensorimotor tests (rotarod, forelimb placement) at 1, 4, 7 and 14 days post-ICH.

RESULTS

dECM@exo demonstrated sustained exosome release over 14 days, significantly promoting neural tissue regeneration while attenuating perihematomal edema. Mechanistically, the scaffold modulated pathological MMP activity and inflammatory cytokine expression, thereby restoring extracellular matrix homeostasis and reducing neuronal apoptosis.

CONCLUSIONS

The findings demonstrate that the 3D biological scaffold dECM@exo effectively maintains microenvironmental homeostasis in the early stages of ICH and improves outcomes associated with the condition. dECM@exo is poised to serve as a robust platform for drug delivery and biotherapy in ICH treatment, offering a viable alternative for managing this condition.

Injectable hydrogel microspheres promoting inflammation modulation and nucleus pulposus-like differentiation for intervertebral disc regeneration

Local inflammation modulation and stem cell therapy have attracted much attention in the treatment of intervertebral disc degeneration (IDD). However, severe oxidative stress and limited nucleus pulposus (NP)-like differentiation of stem cells largely impair biomaterial implantation's therapeutic efficacy. Due to their excellent performance in injectability and flowability, and minor compression to NP tissue, hydrogel microspheres have become an attractive carrier for IDD treatment. Herein, an injectable hydrogel microsphere consisting of Wnt5a-mimetic peptide Foxy5- and the antioxidative peptide-grafted gelatin methacryloyl matrix (GFA), was developed as a stem cell delivery system for IDD therapy. Being fabricated and encapsulating bone marrow-derived mesenchymal stem cells (BMSCs) using the microfluidic technology, GFA hydrogel microspheres ameliorate IDD by promoting inflammation inhibition, NP-like differentiation and extracellular matrix regeneration. They efficiently eliminated reactive oxygen species, and downregulated the inflammation level through the inhibition of interleukin-17B/nuclear factor-κB signaling pathway. Moreover, the NP-like differentiation of BMSCs was effectively stimulated by Foxy5 via the calcium/calmodulin dependent protein kinase kinase 2/protein kinase A/sex determining region Y box protein 9 signaling pathway, thereby leading to a rebalance between the generation and degradation of NP matrix. In vivo rat IDD model demonstrated that BMSC-loaded GFA hydrogel microspheres mitigated local inflammation, preserved disc height, and promoted intervertebral disc regeneration. In conclusion, this study introduces an BMSC-loaded injectable hydrogel microspheres as a promising therapy for regulating the microenvironment and alleviating the progression of IDD.

Designing hydrogel for application in spinal surgery

Spinal diseases and injuries are prevalent in clinical settings and impose a substantial burden on healthcare systems. Current treatments for spinal diseases are predominantly limited to surgical interventions, drug injections, and conservative treatments. Generally, these treatment modalities have limited or no long-term benefits. Hydrogel-based treatments have emerged as potentially powerful paradigms for improving therapeutic outcomes and the quality of life of patients. Hydrogels can be injected into target sites, including the epidural, intraspinal, and nucleus pulposus spaces, in a minimally invasive manner and fill defects to provide mechanical support. Hydrogels can be designed for the localized and controlled delivery of pharmacological agents to enhance therapeutic effects and reduce adverse reactions. Hydrogels can act as structural supports for transplanted cells to improve cell survival, proliferation, and differentiation, as well as integration into adjacent host tissues. In this review, we summarize recent advances in the design of hydrogels for the treatment of spinal diseases and injuries commonly found in clinical settings, including intervertebral disc degeneration, spinal cord injury, and dural membrane injury. We introduce the design considerations for different hydrogel systems, including precursor polymers and crosslinking mechanisms. Herein, we discuss the therapeutic outcomes of these hydrogels in terms of providing mechanical support, delivering cells/bioactive agents, regulating local inflammation, and promoting tissue regeneration and functional recovery.

3-D Sustained-Release Culture Carrier Alleviates Rat Intervertebral Disc Degeneration by Targeting STING in Transplanted Skeletal Stem Cells

The hypoxic and high-pressure microenvironment of the intervertebral discs poses a major challenge to the survival and therapeutic efficiency of exogenous stem cells. Therefore, improving the utilization efficiency and therapeutic effect of exogenous stem cells to delay intervertebral disc degeneration (IVDD) is of great importance. Here, hypoxic induction studies are conducted in vivo and in vitro using rat costal cartilage-derived skeletal stem cells (SSCs) and find that hypoxia activates the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)/stimulator of interferon genes (STING) signaling pathway and increased reactive oxygen species (ROS) accumulation, triggering ferroptosis in SSCs through hypoxia-inducible factor-1 alpha-dependent mitophagy. Progressive hypoxia preconditioning reduce STING expression and ROS accumulation, inducing SSCs differentiation into nucleus pulposus-like cells via the Wnt signaling pathway. Considering this, a 3-D sustained-release culture carrier is generated by mixing SSCs with methacrylated hyaluronic acid and polydopamine nanoparticles coated with the STING inhibitor C-176 and evaluated its inhibitory effect on IVDD. This carrier is demonstrated to inhibit the cGAS/STING pathway and prevent ROS accumulation by continuously releasing C-176-coated polydopamine nanoparticles, thereby reducing ferroptosis, promoting differentiation, and ultimately attenuating IVDD, suggesting its potential as a novel treatment strategy.

FSTL1 accelerates nucleus pulposus-derived mesenchymal stem cell apoptosis in intervertebral disc degeneration by activating TGF-β-mediated Smad2/3 phosphorylation

BACKGROUND

Intervertebral disc degeneration (IVDD) is the leading cause of low back pain, and repair using nucleus pulposus-derived mesenchymal stem cells (NP-MSCs) represents a promising therapeutic approach. However, both endogenous and transplanted NP-MSCs demonstrate limited proliferative capacity, increased apoptosis, and reduced resilience to the harsh microenvironment within the degenerative intervertebral disc (IVD).

METHODS

RNA sequencing (RNA-seq) was utilized to identify genes and associated mechanisms that mediate the responses of NP-MSCs to acidic conditions. Western blotting, qPCR, and immunofluorescence were used to evaluate follistatin-like 1 (FSTL1) expression in NP-MSCs. Apoptosis and extracellular matrix (ECM) anabolism were assessed via flow cytometry, TUNEL staining and Western blotting, while the TGF-β/Smad2/3 pathway was analyzed using Western blotting and immunofluorescence. FSTL1 knockdown with small interfering RNA (siRNA) was performed to determine its role in apoptosis and ECM regulation. The FSTL1 siRNA pretreatment was assessed in a puncture-induced rat IVDD model using MRI and histological staining.

RESULTS

Using RNA-seq, we identified FSTL1 as the primary acid-responsive gene in NP-MSCs. We further observed elevated FSTL1 expression in NP-MSCs isolated from degenerative IVDs in both humans and rats compared to normal IVDs. Acidic conditions upregulated FSTL1 expression in NP-MSCs in a pH-dependent manner. Notably, recombinant FSTL1 was shown to enhance cellular apoptosis and disrupt ECM metabolism. Conversely, silencing FSTL1 with siRNA reduced NP-MSC apoptosis and improved ECM anabolism. Importantly, TGF-β pathway inhibition partially reversed the pro-apoptotic and ECM catabolism effects of FSTL1. In the rat model of IVDD, pretreatment of NP-MSCs with FSTL1 siRNA significantly suppressed IVDD progression.

CONCLUSIONS

This study provides novel insights into the mechanistic role of FSTL1 in acid-induced apoptosis of NP-MSCs and its contribution to the progression of IVDD. These findings offer valuable perspectives for developing targeted therapeutic strategies to mitigate IVDD progression.

Extracellular vesicles deliver thioredoxin to rescue stem cells from senescence and intervertebral disc degeneration via a feed-forward circuit of the NRF2/AP-1 composite pathway

Intervertebral disc degeneration (IDD) is largely attributed to impaired endogenous repair. Nucleus pulposus-derived stem cells (NPSCs) senescence leads to endogenous repair failure. Small extracellular vesicles/exosomes derived from mesenchymal stem cells (mExo) have shown great therapeutic potential in IDD, while whether mExo could alleviate NPSCs senescence and its mechanisms remained unknown. We established a compression-induced NPSCs senescence model and rat IDD models to evaluate the therapeutic efficiency of mExo and investigate the mechanisms. We found that mExo significantly alleviated NPSCs senescence and promoted disc regeneration while knocking down thioredoxin (TXN) impaired the protective effects of mExo. TXN was bound to various endosomal sorting complex required for transport (ESCRT) proteins. Autocrine motility factor receptor (AMFR) mediated TXN K63 ubiquitination to promote the binding of TXN on ESCRT proteins and sorting of TXN into mExo. Knocking down exosomal TXN inhibited the transcriptional activity of nuclear factor erythroid 2-related factor 2 (NRF2) and activator protein 1 (AP-1). NRF2 and AP-1 inhibition reduced endogenous TXN production that was promoted by exosomal TXN. Inhibition of NRF2 in vivo diminished the anti-senescence and regenerative effects of mExo. Conclusively, AMFR-mediated TXN ubiquitination promoted the sorting of TXN into mExo, allowing exosomal TXN to promote endogenous TXN production in NPSCs via TXN/NRF2/AP-1 feed-forward circuit to alleviate NPSCs senescence and disc degeneration.

Application of Self-Healing Hydrogels in the Treatment of Intervertebral Disc Degeneration

Intervertebral disc degeneration (IDD) is one of the leading causes of chronic pain and disability, and traditional treatment methods often struggle to restore its complex biomechanical properties. This article explores the innovative application of self-healing hydrogels in the treatment of IDD, offering new hope for disc repair due to their exceptional self-repair capabilities and adaptability. As a key support structure in the human body, intervertebral discs are often damaged by trauma or degenerative changes. Self-healing hydrogels not only mimic the mechanical properties of natural intervertebral discs but also self-repair when damaged, thereby maintaining stable functionality. This article reviews the self-healing mechanisms and design strategies of self-healing hydrogels and, for the first time, outlines their potential in the treatment of IDD. Furthermore, the article looks forward to future developments in the field, including intelligent material design, multifunctional integration, encapsulation and release of bioactive molecules, and innovative combinations with tissue engineering and stem cell therapy, offering new perspectives and strategies for IDD treatment.

CLEC11A-Driven Molecular Mechanisms in Intervertebral Disc Degeneration: A Comprehensive Multi-Omics Study

Background

Intervertebral disc degeneration (IVDD) is a common chronic degenerative disease with a complex etiology involving genetic and environmental factors. However, the genetic pathogenesis and key driving factors of IVDD remain largely unknown.

Methods

In this study, we combined MR with transcriptomic sequencing to identify key pathogenic genes implicated in IVDD. Further exploration using single-cell transcriptomics elucidated the specific cell types and pathways through which these genes modulate IVDD. Mediational MR analysis provided insights into the intermediary roles of 91 inflammatory factors and serum metabolites in the genetic causation pathway of IVDD. Finally, we validated these findings through in vitro experiments, confirming the regulatory roles of these critical genes in the progression of IVDD.

Results

Transcriptomic and MR analyses identified six candidate pathogenic genes (AEN, CLEC11A, HMGN1, LRRC25, TAF7, and TREM1) significantly associated with IVDD. Subsequent single-cell analysis suggested that CLEC11A, TREM1, and HMGN1 may play pivotal roles in IVDD progression by modulating chondrocyte function and inflammatory responses. Mediation MR analysis further indicated that CLEC11A might significantly elevate IVDD risk by upregulating the inflammatory mediator ARTN and the uncharacterized serum metabolites X-12731 and X-18901 (ARTN: OR=1.078, 95% CI: 1.004-1.158, P=0.038; X-12731: OR=0.906, 95% CI: 0.852-0.960, P=0.043; X-18901: OR=1.090, 95% CI: 1.007-1.179, P=0.034). In vitro experiments demonstrated that overexpression of CLEC11A in nucleus pulposus cells significantly enhanced mRNA and protein expression of IVDD-related inflammatory markers; conversely, silencing CLEC11A markedly reduced these expressions. Similarly, overexpression of ARTN significantly increased, while knockdown decreased, the expression of these inflammatory markers in nucleus pulposus cells.

Conclusion

Our integrative multi-omics analysis indicates that CLEC11A exacerbates IVDD by upregulating ARTN and inducing metabolic dysregulation, thereby amplifying the inflammatory pathways that drive disease progression.

Endplate chondrocyte-derived exosomal miR-128-3p mitigates intervertebral disc degeneration by targeting TRAF6 via the miR-128-3p/TRAF6 axis to suppress pyroptosis

Intervertebral disc degeneration (IVDD) is a leading cause of chronic back pain and significantly impacts quality of life. The pathogenesis of IVDD is largely driven by inflammation, pyroptosis, and extracellular matrix (ECM) degradation, which current therapies fail to adequately address. In this study, we explore the therapeutic potential of exosomes derived from endplate chondrocytes (EPCs), with a particular focus on the microRNA miR-128-3p. Our findings reveal that exosomes isolated from third-generation EPCs, enriched with miR-128-3p, exhibit potent anti-inflammatory and anti-pyroptotic effects in lipopolysaccharide-treated nucleus pulposus cells, which are key contributors to IVDD pathology. Specifically, we demonstrate that miR-128-3p delivered via EPC-derived exosomes directly targets TRAF6, effectively suppressing activation of the NF-κB signaling pathway, which is known to play a pivotal role in inflammation and ECM breakdown, leading to a marked reduction in pro-inflammatory cytokine release and mitigation of ECM degradation. Importantly, third-generation EPC exosomes, with higher levels of miR-128-3p, showed superior efficacy compared to fifth-generation EPCs, underscoring the critical role of miR-128-3p in mediating these protective effects. Our research highlights the promise of EPC-derived exosomes, particularly those rich in miR-128-3p, as a novel, cell-free therapeutic approach for IVDD. Unlike current treatments that focus primarily on symptom management, our approach targets key molecular pathways underlying IVDD progression, including inflammation, pyroptosis, and ECM degradation. By elucidating the miR-128-3p/TRAF6 axis, this study provides a foundation for the development of targeted, biologically based interventions aimed at halting or even reversing IVDD, thereby offering hope for more effective and lasting therapeutic options.

High impact works on stem cell transplantation in intervertebral disc degeneration

BACKGROUND

Low back pain is a major disorder that causes disability and is strongly associated with intervertebral disc degeneration (IDD). Because of the limitations of contemporary interventions, stem cell transplantation (SCT) has been increasingly used to regenerate degenerative discs. Nevertheless, analyses of high-impact papers in this field are rare. This study aimed to determine and analyze the 100 highest-cited documents on SCT in IDD.

METHODS

The 100 highest-cited documents were retrieved from the Web of Science (WoS) database. Descriptive statistics were calculated and correlation analysis was conducted to determine the relationship between WoS citations, the Altmetric Attention Score (AAS), and Dimensions citations.

RESULTS

The citation counts of the top 100 most cited papers ranged from 13 to 372. These studies were conducted in 17 countries and were published in 48 journals between 2003 and 2021. The top three contributing countries were the China (31), United States (22), and Japan (14). Bone marrow-derived stem cells were the most common type of stem cells (70.00%), followed by adipose-derived stem cells (13.75%), and nucleus pulposus-derived stem cells (7.50). Rabbit was the most studied species (41.25%), followed by rat (21.25%), human (13.75%), sheep (8.75%), dog (8.75%), and pig (6.25%). Tokai University School of Medicine (11) had the largest number of documents, followed by The University of Hong Kong (8), and Southeast University (4). Sakai D (10) was the most fruitful author, followed by Cheung KMC (6), Melrose J (3), Pettine K (3), Lotz JC (3), and Murphy MB (3). We observed a very high correlation between the WoS and Dimensions citations (p < 0.001, r = 0.994).

CONCLUSIONS

This study highlights the highest impact works on SCT in IDD, thereby providing a deeper understanding of the historical works related to SCT in IDD, as well as benefits for future studies in this field.

A novel spine tester TO GO

Background

Often after large animal experiments in spinal research, the question arises-histology or biomechanics? While biomechanics are essential for informed decisions on the functionality of the therapy being studied, scientists often choose histological analysis alone. For biomechanical testing, for example, flexibility, specimens must be shipped to institutions with special testing equipment, as spine testers are complex and immobile. The specimens must usually be shipped frozen, and, thus, biological and histological investigations are not possible anymore. To allow both biomechanical and biological investigations with the same specimen and, thus, to reduce the number of required animals, the aim of the study was to develop a spine tester that can be shipped worldwide to test on-site.

Methods

The "Spine Tester TO GO" was designed consisting of a frame with three motors that initiate pure moments and rotate the specimen in three motion planes. A load cell and an optical motion tracking system controlled the applied loads and measured range of motion (ROM) and neutral zone (NZ). As a proof of concept, the new machine was validated and compared under real experimental conditions with an existing testing machine already validated employing fresh bovine tail discs CY34 (n = 10).

Results

The new spine tester measured reasonable ROM and NZ from hysteresis curves, and the ROM of the two testing machines formed a high coefficient of determination R 2 = 0.986. However, higher ROM results of the new testing machine might be explained by the lower friction of the air bearings, which allowed more translational motion.

Conclusions

The spine tester TO GO now opens up new opportunities for on-site flexibility tests and contributes hereby to the 3R principle by limiting the number of experimental animals needed to obtain full characterization of spine units at the macroscopic, biomechanical, biochemical, and histological level.

A Novel Superparamagnetic-Responsive Hydrogel Facilitates Disc Regeneration by Orchestrating Cell Recruitment, Proliferation, and Differentiation within Hostile Inflammatory Niche

In situ disc regeneration is a meticulously orchestrated process, which involves cell recruitment, proliferation and differentiation within a local inflammatory niche. Thus far, it remains a challenge to establish a multi-staged regulatory framework for coordinating these cellular events, therefore leading to unsatisfactory outcome. This study constructs a super paramagnetically-responsive cellular gel, incorporating superparamagnetic iron oxide nanoparticles (SPIONs) and aptamer-modified palladium-hydrogen nanozymes (PdH-Apt) into a double-network polyacrylamide/hyaluronic acid (PAAm/HA) hydrogel. The Aptamer DB67 within magnetic hydrogel (Mag-gel) showed a high affinity for disialoganglioside (GD2), a specific membrane ligand of nucleus pulposus stem cells (NPSCs), to precisely recruit them to the injury site. The Mag-gel exhibits remarkable sensitivity to a magnetic field (MF), which exerts tunable micro/nano-scale forces on recruited NPSCs and triggers cytoskeletal remodeling, consequently boosting cell expansion in the early stage. By altering the parameters of MF, the mechanical cues within the hydrogel facilitates differentiation of NPSCs into nucleus pulposus cells to restore disc structure in the later stage. Furthermore, the PdH nanozymes within the Mag-gel mitigate the harsh inflammatory microenvironment, favoring cell survival and disc regeneration. This study presents a remote and multi-staged strategy for chronologically regulating endogenous stem cell fate, supporting disc regeneration without invasive procedures.

Current Therapeutic Strategies of Intervertebral Disc Regenerative Medicine

Intervertebral disc degeneration (IDD) is one of the most frequent causes of low back pain. No treatment is currently available to delay the progression of IDD. Conservative treatment or surgical interventions is only used to target the symptoms of IDD rather than treat the underlying cause. Currently, numerous potential therapeutic strategies are available, including molecular therapy, gene therapy, and cell therapy. However, the hostile environment of degenerated discs is a major problem that has hindered the clinical applicability of such approaches. In this regard, the design of drugs using alternative delivery systems (macro-, micro-, and nano-sized particles) may resolve this problem. These can protect and deliver biomolecules along with helping to improve the therapeutic effect of drugs via concentrating, protecting, and prolonging their presence in the degenerated disc. This review summarizes the research progress of diagnosis and the current options for treating IDD.

Allogenic bone marrow-derived mesenchymal stromal cell-based therapy for patients with chronic low back pain: a prospective, multicentre, randomised placebo controlled trial (RESPINE study)

OBJECTIVES

To assess the efficacy of a single intradiscal injection of allogeneic bone marrow mesenchymal stromal cells (BM-MSCs) versus a sham placebo in patients with chronic low back pain (LBP).

METHODS

Participants were randomised in a prospective, double-blind, controlled study to receive either sham injection or intradiscal injection of 20 million allogeneic BM-MSC, between April 2018 and December 2022. The first co-primary endpoint was the rate of responders defined by improvement of the Visual Analogue Scale (VAS) for pain of at least 20% and 20 mm, or improvement of the Oswestry Disability Index (ODI) of 20% between baseline and month 12. The secondary structural co-primary endpoint was assessed by the disc fluid content measured by quantitative MRI T2, between baseline and month 12. Secondary endpoints included pain VAS, ODI, the Short Form (SF)-36 and the minimal clinically important difference in all timepoints (1, 3, 6, 12 and 24 months). We determined the immune response associated with allogeneic cell injection between baseline and 6 months. Serious adverse events (SAEs) were recorded.

RESULTS

114 patients were randomised (n=58, BM-MSC group; n=56, sham placebo group). At 12 months, the primary outcome was not reached (74% in the BM-MSC group vs 69% in the placebo group; p=0.77). The groups did not differ in all secondary outcomes. No SAE related to the intervention occurred.

CONCLUSIONS

While our study did not conclusively demonstrate the efficacy of allogeneic BM-MSCs for LBP, the procedure was safe. Long-term outcomes of MSC therapy for LBP are still being studied.

TRIAL REGISTRATION NUMBER

EudraCT 2017-002092-25/ClinicalTrials.gov: NCT03737461.

Role of IL3RA in a Family with Lumbar Spinal Stenosis

Lumbar spinal stenosis (LSS) is a degenerative spinal condition characterized by the narrowing of the spinal canal, resulting in low back pain (LBP) and limited leg mobility. Twin and family studies have suggested that genetics contributes to disease progression. However, the genetic causes of familial LSS remain unclear. We performed whole-exome and direct sequencing on seven female patients from a Han Chinese family with LBP, among whom four developed LSS. Based on our genetic findings, we performed gene knockdown studies in human chondrocytes to study possible pathological mechanisms underlying LSS. We found a novel nonsense mutation, c.417C > G (NM_002183, p.Y139X), in IL3RA, shared by all the LBP/LSS cases. Knockdown of IL3RA led to a reduction in the total collagen content of 81.6% in female chondrocytes and 21% in male chondrocytes. The expression of MMP-1, -3, and/or -10 significantly increased, with a more pronounced effect observed in females than in males. Furthermore, EsRb expression significantly decreased following IL3RA knockdown. Moreover, the knockdown of EsRb resulted in increased MMP-1 and -10 expression in chondrocytes from females. We speculate that IL3RA deficiency could lead to a reduction in collagen content and intervertebral disk (IVD) strength, particularly in females, thereby accelerating IVD degeneration and promoting LSS occurrence. Our results illustrate, for the first time, the association between IL3RA and estrogen receptor beta, highlighting their importance and impact on MMPs and collagen in degenerative spines in women.

Exosomes Derived from Rejuvenated Stem Cells Inactivate NLRP3 Inflammasome and Pyroptosis of Nucleus Pulposus Cells via the Transfer of Antioxidants

BACKGROUND

Accumulating evidence supports the potential of exosomes as a promising therapeutic approach for intervertebral disc degeneration (IDD). Nevertheless, enhancing the efficiency of exosome treatment remains an urgent concern. This study investigated the impact of quercetin on the characteristics of mesenchymal stem cells (MSCs) and their released exosomes.

METHODS

Exosomes were obtained from quercetin pre-treated MSCs and quantified for the production based on nanoparticle tracking and western blot analysis. The molecules involved in the secretion and cargo sorting of exosomes were investigated using western blot and immunofluorescence analysis. Based on the in vitro biological analysis and in vivo histological analysis, the effects of exosomes derived from conventional or quercetin-treated MSCs on nucleus pulposus (NP) cells were compared.

RESULTS

A significant enhancement in the production and transportation efficiency of exosomes was observed in quercetin-treated MSCs. Moreover, the exosomes derived from quercetin-treated MSCs exhibited a greater abundance of antioxidant proteins, specifically superoxide dismutase 1 (SOD1), which inhibit the activation of NOD-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome in NP cells. Through in vitro and in vivo experiments, it was elucidated that exosomes derived from quercetin-treated MSCs possessed enhanced anti-inflammatory and antioxidant properties.

CONCLUSION

Collectively, our research underscores an optimized therapeutic strategy for IDD utilizing MSC-derived exosomes, thereby augmenting the efficacy of exosomes in intervertebral disc regeneration.

Banked Primary Progenitor Cells for Allogeneic Intervertebral Disc (IVD) Therapy: Preclinical Qualification and Functional Optimization within a Cell Spheroid Formulation Process

Background/Objectives: Biological products are emerging as therapeutic management options for intervertebral disc (IVD) degenerative affections and lower back pain. Autologous and allogeneic cell therapy protocols have been clinically implemented for IVD repair. Therein, several manufacturing process design considerations were shown to significantly influence clinical outcomes. The primary objective of this study was to preclinically qualify (chondrogenic potential, safety, resistance to hypoxic and inflammatory stimuli) cryopreserved primary progenitor cells (clinical grade FE002-Disc cells) as a potential cell source in IVD repair/regeneration. The secondary objective of this study was to assess the cell source's delivery potential as cell spheroids (optimization of culture conditions, potential storage solutions). Methods/Results: Safety (soft agar transformation, β-galactosidase, telomerase activity) and functionality-related assays (hypoxic and inflammatory challenge) confirmed that the investigated cellular active substance was highly sustainable in defined cell banking workflows, despite possessing a finite in vitro lifespan. Functionality-related assays confirmed that the retained manufacturing process yielded strong collagen II and glycosaminoglycan (GAG) synthesis in the spheroids in 3-week chondrogenic induction. Then, the impacts of various process parameters (induction medium composition, hypoxic incubation, terminal spheroid lyophilization) were studied to gain insights on their criticality. Finally, an optimal set of technical specifications (use of 10 nM dexamethasone for chondrogenic induction, 2% O2 incubation of spheroids) was set forth, based on specific fine tuning of finished product critical functional attributes. Conclusions: Generally, this study qualified the considered FE002-Disc progenitor cell source for further preclinical investigation based on safety, quality, and functionality datasets. The novelty and significance of this study resided in the establishment of defined processes for preparing fresh, off-the-freezer, or off-the-shelf IVD spheroids using a preclinically qualified allogeneic human cell source. Overall, this study underscored the importance of using robust product components and optimal manufacturing process variants for maximization of finished cell-based formulation quality attributes.

Mapping knowledge structure and themes trends of non-surgical treatment in intervertebral disc degeneration

Background

Intervertebral disc degeneration (IDD) is a chronic disabling disease caused by degeneration of nucleus pulposus cells, decreased activity and the number of nucleus pulposus cells, decreased extracellular matrix, and infiltration of inflammatory factors, resulting in low back and leg pain. Recent studies have shown that non-surgical treatment is of great significance in reversing the progression of degenerative disc disease, and there are more relevant literature reports. However, there is no bibliometric analysis in this area. This study aimed to describe the knowledge structure and thematic trends of non-surgical treatment methods for IDD through bibliometrics.

Methods

Articles and reviews on non-surgical treatment of disc degeneration from 1998 to 2022 were collected on the Web of Science. VOSviewer 1.6.18, CiteSpace 6.1.R3, R package "bibliometrix" and two online analysis platforms were used for bibliometric and visual literature analysis.

Results

961 articles were screened for inclusion, including 821 articles and 140 reviews. The analysis of our study shows that publications in the non-surgical treatment of disc degeneration are increasing annually, with publications coming mainly from North America and Asia, with China and the United States dominating. Huazhong Univ Sci & Technol and Wang K are the most prolific institutions and authors, respectively, and Le Maitre CL is the most co-cited author. However, there is less collaboration between institutions in different countries. Spine is both the most published and the most cited journal. According to the co-citation and co-occurrence analysis results, "mesenchymal stem cells," "exosomes," "medication," and "tissue engineering" are the current research hotspots in this field.

Conclusions

This study employs bibliometric analysis to explore the knowledge structure and trends of non-surgical treatments for IDD from 2013 to 2022. Key research hotspots include mesenchymal stem cells, exosomes, medication, and tissue engineering. The number of publications, especially from China and the USA, has increased significantly, though international collaboration needs improvement. Influential contributors include Wang K and the journal Spine. These findings provide a comprehensive overview and highlight important future directions for the field.

Papain Injection Creates a Nucleotomy-like Cavity for Testing Gels in Intervertebral Discs

Biomaterials, such as hydrogels, have an increasingly important role in the development of regenerative approaches for the intervertebral disc. Since animal models usually resist biomaterial injection due to high intradiscal pressure, preclinical testing of the biomechanical performance of biomaterials after implantation remains difficult. Papain reduces the intradiscal pressure, creates cavities within the disc, and allows for biomaterial injections. But papain digestion needs time, and cadaver experiments that are limited to 24 h for measuring range of motion (ROM) cannot not be combined with papain digestion just yet. In this study, we successfully demonstrate a new organ culture approach, facilitating papain digestion to create cavities in the disc and the testing of ROM, neutral zone (NZ), and disc height. Papain treatment increased the ROM by up to 109.5%, extended NZ by up to 210.9%, and decreased disc height by 1.96 ± 0.74 mm. A median volume of 0.73 mL hydrogel could be injected after papain treatment, and histology revealed a strong loss of proteoglycans in the remaining nucleus tissue. Papain has the same biomechanical effects as known from nucleotomies or herniations and thus creates a disc model to study such pathologies in vitro. This new model can now be used to test the performance of biomaterials.

Regenerative therapies for lumbar degenerative disc diseases: a literature review

This review aimed to summarize the recent advances and challenges in the field of regenerative therapies for lumbar disc degeneration. The current first-line treatment options for symptomatic lumbar disc degeneration cannot modify the disease process or restore the normal structure, composition, and biomechanical function of the degenerated discs. Cell-based therapies tailored to facilitate intervertebral disc (IVD) regeneration have been developed to restore the IVD extracellular matrix or mitigate inflammatory conditions. Human clinical trials on Mesenchymal Stem Cells (MSCs) have reported promising outcomes exhibited by MSCs in reducing pain and improving function. Nucleus pulposus (NP) cells possess unique regenerative capacities. Biomaterials aimed at NP replacement in IVD regeneration, comprising synthetic and biological materials, aim to restore disc height and segmental stability without compromising the annulus fibrosus. Similarly, composite IVD replacements that combine various biomaterial strategies to mimic the native disc structure, including organized annulus fibrosus and NP components, have shown promise. Furthermore, preclinical studies on regenerative medicine therapies that utilize cells, biomaterials, growth factors, platelet-rich plasma (PRP), and biological agents have demonstrated their promise in repairing degenerated lumbar discs. However, these therapies are associated with significant limitations and challenges that hinder their clinical translation. Thus, further studies must be conducted to address these challenges.

Dedifferentiation-like reprogramming of degenerative nucleus pulposus cells into notochordal-like cells by defined factors

The extensive degeneration of functional somatic cells and the depletion of endogenous stem/progenitor populations present significant challenges to tissue regeneration in degenerative diseases. Currently, a cellular reprogramming approach enabling directly generating corresponding progenitor populations from degenerative somatic cells remains elusive. The present study focused on intervertebral disc degeneration (IVDD) and identified a three-factor combination (OCT4, FOXA2, TBXT [OFT]) that could induce the dedifferentiation-like reprogramming of degenerative nucleus pulposus cells (dNPCs) toward induced notochordal-like cells (iNCs). Single-cell transcriptomics dissected the transitions of cell identity during reprogramming. Further, OCT4 was found to directly interact with bromodomain PHD-finger transcription factor to remodel the chromatin during the early phases, which was crucial for initiating this dedifferentiation-like reprogramming. In rat models, intradiscal injection of adeno-associated virus carrying OFT generated iNCs from in situ dNPCs and reversed IVDD. These results collectively present a proof-of-concept for dedifferentiation-like reprogramming of degenerated somatic cells into corresponding progenitors through the development of a factor-based strategy, providing a promising approach for regeneration in degenerative disc diseases.

Exosomes derived from human urine-derived stem cells ameliorate IL-1β-induced intervertebral disk degeneration

BACKGROUND

Human intervertebral disk degeneration (IVDD) is a sophisticated degenerative pathological process. A key cause of IVDD progression is nucleus pulposus cell (NPC) degeneration, which contributes to excessive endoplasmic reticulum stress in the intervertebral disk. However, the mechanisms underlying IVDD and NPC degeneration remain unclear.

METHODS

We used interleukin (IL)-1β stimulation to establish an NPC-degenerated IVDD model and investigated whether human urine-derived stem cell (USC) exosomes could prevent IL-1β-induced NPC degeneration using western blotting, quantitative real-time polymerase chain reaction, flow cytometry, and transcriptome sequencing techniques.

RESULTS

We successfully extracted and identified USCs and exosomes from human urine. IL-1β substantially downregulated NPC viability and induced NPC degeneration while modulating the expression of SOX-9, collagen II, and aggrecan. Exosomes from USCs could rescue IL-1β-induced NPC degeneration and restore the expression levels of SOX-9, collagen II, and aggrecan.

CONCLUSIONS

USC-derived exosomes can prevent NPCs from degeneration following IL-1β stimulation. This finding can aid the development of a potential treatment strategy for IVDD.

IGF-1 Peptide Mimetic-functionalized Hydrogels Enhance MSC Survival and Immunomodulatory Activity

Human mesenchymal stem cells (MSCs) have demonstrated promise when delivered to damaged tissue or tissue defects for their cytokine secretion and inflammation modulation behaviors that can promote repair. Insulin-like growth factor 1 (IGF-1) has been shown to augment MSCs' viability and survival and promote their secretion of cytokines that signal to endogenous cells, in the treatment of myocardial infarction, wound healing, and age-related diseases. Biomaterial cell carriers can be functionalized with growth factor-mimetic peptides to enhance MSC function while promoting cell retention and minimizing off-target effects seen with direct administration of soluble growth factors. Here, we functionalized alginate hydrogels with three distinct IGF-1 peptide mimetics and the integrin-binding peptide, cyclic RGD. One IGF-1 peptide mimetic (IGM-3) was found to activate Akt signaling and support survival of serum-deprived MSCs. MSCs encapsulated in alginate hydrogels that presented both IGM-3 and cRGD showed a significant reduction in pro-inflammatory cytokine secretion when challenged with interleukin-1β. Finally, MSCs cultured within the cRGD/IGM-3 hydrogels were able to blunt pro-inflammatory gene expression of human primary cells from degenerated intervertebral discs. These studies indicate the potential to leverage cell adhesive and IGF-1 growth factor peptide mimetics together to control therapeutic secretory behavior of MSCs.

Significance Statement

Insulin-like growth factor 1 (IGF-1) plays a multifaceted role in stem cell biology and may promote proliferation, survival, migration, and immunomodulation for MSCs. In this study, we functionalized alginate hydrogels with integrin-binding and IGF-1 peptide mimetics to investigate their impact on MSC function. Embedding MSCs in these hydrogels enhanced their ability to reduce inflammatory cytokine production and promote anti-inflammatory gene expression in cells from degenerative human intervertebral discs exposed to proteins secreted by the MSC. This approach suggests a new way to retain and augment MSC functionality using IGF-1 peptide mimetics, offering an alternative to co-delivery of cells and high dose soluble growth factors for tissue repair and immune- system modulation.

The role of oxidative stress in intervertebral disc degeneration: Mechanisms and therapeutic implications

Oxidative stress is one of the main driving mechanisms of intervertebral disc degeneration(IDD). Oxidative stress has been associated with inflammation in the intervertebral disc, cellular senescence, autophagy, and epigenetics of intervertebral disc cells. It and the above pathological mechanisms are closely linked through the common hub reactive oxygen species(ROS), and promote each other in the process of disc degeneration and promote the development of the disease. This reveals the important role of oxidative stress in the process of IDD, and the importance and great potential of IDD therapy targeting oxidative stress. The efficacy of traditional therapy is unstable or cannot be maintained. In recent years, due to the rise of materials science, many bioactive functional materials have been applied in the treatment of IDD, and through the combination with traditional drugs, satisfactory efficacy has been achieved. At present, the research review of antioxidant bioactive materials in the treatment of IDD is not complete. Based on the existing studies, the mechanism of oxidative stress in IDD and the common antioxidant therapy were summarized in this paper, and the strategies based on emerging bioactive materials were reviewed.

Mechanical Basis of Lumbar Intervertebral Disk Degeneration

The etiology of degenerative disk disease (DDD) is multifactorial. Among the various factors, mechanical processes contributing to endplate or discal injuries have been discussed as the initiating events in the degenerative cascade. DDD encompasses the multitudinous changes undergone by the different structures of the spinal segment, namely intervertebral disk (IVD), facet joints, vertebral end plate (VEP), adjoining marrow (Modic changes), and vertebral body. It has been etiologically linked to a complex interplay of diverse mechanisms. Mechanically, two different mechanisms have been proposed for intervertebral disk degeneration (IVDD): endplate-driven, especially in upper lumbar levels, and annulus-driven degeneration. VEP is the weakest link of the lumbar spine, and fatigue damage can be inflicted upon them under physiological loads, leading to the initiation of DDD. Disk calcification has been put forth as another initiator of inflammation, stiffening, and abnormal stresses across the IVD. The initial mechanical disruption leads to secondary IVDD through unfavorable loading of the nucleus pulposus and annulus fibrosis. The final degenerative cascade is then propagated through a combination of biological, inflammatory, autoimmune, or metabolic pathways (impaired transport of metabolites or nutrients). Abnormal spinopelvic alignment, especially pelvic incidence, also significantly impacts the degenerative process. Hence, the etiology of DDD is multifactorial. Mechanical pathways, including VEP injuries, increased disk stiffness, and abnormal spinopelvic alignment, play a significant role in the initiation of IVDD.

Progenitor-like cells contributing to cellular heterogeneity in the nucleus pulposus are lost in intervertebral disc degeneration

The nucleus pulposus (NP) in the intervertebral disc (IVD) arises from embryonic notochord. Loss of notochordal-like cells in humans correlates with onset of IVD degeneration, suggesting that they are critical for healthy NP homeostasis and function. Comparative transcriptomic analyses identified expression of progenitor-associated genes (GREM1, KRT18, and TAGLN) in the young mouse and non-degenerated human NP, with TAGLN expression reducing with aging. Lineage tracing using Tagln-CreERt2 mice identified peripherally located proliferative NP (PeriNP) cells in developing and postnatal NP that provide a continuous supply of cells to the entire NP. PeriNP cells were diminished in aged mice and absent in puncture-induced degenerated discs. Single-cell transcriptomes of postnatal Tagln-CreERt2 IVD cells indicate enrichment for TGF-β signaling in Tagln descendant NP sub-populations. Notochord-specific removal of TGF-β/BMP mediator Smad4 results in loss of Tagln+ cells and abnormal NP morphologies. We propose Tagln+ PeriNP cells are potential progenitors crucial for NP homeostasis.

Orthobiologic Management Options for Degenerative Disc Disease

Degenerative disc disease (DDD) is a pervasive condition that limits quality of life and burdens economies worldwide. Conventional pharmacological treatments primarily aimed at slowing the progression of degeneration have demonstrated limited long-term efficacy and often do not address the underlying causes of the disease. On the other hand, orthobiologics are regenerative agents derived from the patient's own tissue and represent a promising emerging therapy for degenerative disc disease. This review comprehensively outlines the pathophysiology of DDD, highlighting the inadequacies of existing pharmacological therapies and detailing the potential of orthobiologic approaches. It explores advanced tools such as platelet-rich plasma and mesenchymal stem cells, providing a historical overview of their development within regenerative medicine, from foundational in vitro studies to preclinical animal models. Moreover, the manuscript delves into clinical trials that assess the effectiveness of these therapies in managing DDD. While the current clinical evidence is promising, it remains insufficient for routine clinical adoption due to limitations in study designs. The review emphasizes the need for further research to optimize these therapies for consistent and effective clinical outcomes, potentially revolutionizing the management of DDD and offering renewed hope for patients.

Intervertebral disc degeneration and regenerative medicine

Intervertebral disc (IVD) degeneration is a common phenomenon that affects patients with increasing prevalence with increasing age. Both conservative treatments, such as the use of pain medication or physical therapy, and surgical treatments, such as fusion or disc replacement therapies, are offered to patients. Both non‐invasive and invasive treatments have been shown to improve pain and quality of life for patients. This review explores the role of regenerative medicine techniques as a promising therapeutic intervention that can be used before or in combination with conservative therapy and surgery to enhance the treatment process in patients with IVD degeneration or disc pathology. Currently, there are four major modules of regenerative medicine: genetic therapy, platelet‐rich plasma therapy, stem cell transplantation and tissue engineering. Several research studies have shown promising outcomes of stem cell transplantation and tissue engineering when combined with either surgical or conservative treatment, resulting in improved pain outcomes. The additional benefit of regenerative medicine techniques, specifically stem cell transplantation, is the potential for treating the root pathology of degeneration. Regenerative medicine techniques also have the potential to either halt or reverse degeneration as opposed to current standards of care for managing symptoms. There is a plethora of current research highlighting the benefits of regenerative medicine techniques; however, there remains clinical concerns and ethical concerns regarding the use of regenerative therapy techniques such as stem cell transplantation in the context of IVD degeneration.

Stimuli-Responsive Delivery Systems for Intervertebral Disc Degeneration

As a major cause of low back pain, intervertebral disc degeneration is an increasingly prevalent chronic disease worldwide that leads to huge annual financial losses. The intervertebral disc consists of the inner nucleus pulposus, outer annulus fibrosus, and sandwiched cartilage endplates. All these factors collectively participate in maintaining the structure and physiological functions of the disc. During the unavoidable degeneration stage, the degenerated discs are surrounded by a harsh microenvironment characterized by acidic, oxidative, inflammatory, and chaotic cytokine expression. Loss of stem cell markers, imbalance of the extracellular matrix, increase in inflammation, sensory hyperinnervation, and vascularization have been considered as the reasons for the progression of intervertebral disc degeneration. The current treatment approaches include conservative therapy and surgery, both of which have drawbacks. Novel stimuli-responsive delivery systems are more promising future therapeutic options than traditional treatments. By combining bioactive agents with specially designed hydrogels, scaffolds, microspheres, and nanoparticles, novel stimuli-responsive delivery systems can realize the targeted and sustained release of drugs, which can both reduce systematic adverse effects and maximize therapeutic efficacy. Trigger factors are categorized into internal (pH, reactive oxygen species, enzymes, etc.) and external stimuli (photo, ultrasound, magnetic, etc.) based on their intrinsic properties. This review systematically summarizes novel stimuli-responsive delivery systems for intervertebral disc degeneration, shedding new light on intervertebral disc therapy.

Anti-Stress and Anti-ROS Effects of MnOx-Functionalized Thermosensitive Nanohydrogel Protect BMSCs for Intervertebral Disc Degeneration Repair

Stem cell transplantation is proven to be a promising strategy for intervertebral disc degeneration (IDD) repair. However, replicative senescence of bone marrow-derived mesenchymal stem cells (BMSCs), shear damage during direct injection, mechanical stress, and the reactive oxygen species (ROS)-rich microenvironment in degenerative intervertebral discs (IVDs) cause significant cellular damage and limit the therapeutic efficacy. Here, an injectable manganese oxide (MnOx)-functionalized thermosensitive nanohydrogel is proposed for BMSC transplantation for IDD therapy. The MnOx-functionalized thermosensitive nanohydrogel not only successfully protects BMSCs from shear force and mechanical stress before and after injection, but also repairs the harsh high-ROS environment in degenerative IVDs, thus effectively increasing the viability of BMSCs and resident nucleus pulposus cells (NPCs). The MnOx-functionalized thermosensitive nanohydrogel provides mechanical protection for stem cells and helps to remove endogenous ROS, providing a promising stem cell delivery platform for the treatment of IDD.

Viscoelastic hydrogels regulate adipose-derived mesenchymal stem cells for nucleus pulposus regeneration

Low back pain is a leading cause of disability worldwide, often attributed to intervertebral disc (IVD) degeneration with loss of the functional nucleus pulposus (NP). Regenerative strategies utilizing biomaterials and stem cells are promising for NP repair. Human NP tissue is highly viscoelastic, relaxing stress rapidly under deformation. However, the impact of tissue-specific viscoelasticity on the activities of adipose-derived stem cells (ASC) remains largely unexplored. Here, we investigated the role of matrix viscoelasticity in regulating ASC differentiation for IVD regeneration. Viscoelastic alginate hydrogels with stress relaxation time scales ranging from 100 s to 1000s were developed and used to culture human ASCs for 21 days. Our results demonstrated that the fast-relaxing hydrogel significantly enhanced ASCs long-term cell survival and NP-like extracellular matrix secretion of aggrecan and type-II collagen. Moreover, gene expression analysis revealed a substantial upregulation of the mechanosensitive ion channel marker TRPV4 and NP-specific markers such as SOX9, HIF-1α, KRT18, CDH2 and CD24 in ASCs cultured within the fast-relaxing hydrogel, compared to slower-relaxing hydrogels. These findings highlight the critical role of matrix viscoelasticity in regulating ASC behavior and suggest that viscoelasticity is a key parameter for novel biomaterials design to improve the efficacy of stem cell therapy for IVD regeneration. STATEMENT OF SIGNIFICANCE: Systematically characterized the influence of tissue-mimetic viscoelasticity on ASC. NP-mimetic hydrogels with tunable viscoelasticity and tissue-matched stiffness. Long-term survival and metabolic activity of ASCs are substantially improved in the fast-relaxing hydrogel. The fast-relaxing hydrogel allows higher rate of cell protrusions formation and matrix remodeling. ASC differentiation towards an NP-like cell phenotype is promoted in the fast-relaxing hydrogel, with more CD24 positive expression indicating NP committed cell fate. The expression of TRPV4, a molecular sensor of matrix viscoelasticity, is significantly enhanced in the fast-relaxing hydrogel, indicating ASC sensing matrix viscoelasticity during cell development. The NP-specific ECM secretion of ASC is considerably influenced by matrix viscoelasticity, where the deposition of aggrecan and type-II collagen are significantly enhanced in the fast-relaxing hydrogel.

Role of Galectin-3 in intervertebral disc degeneration: an experimental study

BACKGROUND

This study investigated the role of Galectin-3 in the degeneration of intervertebral disc cartilage.

METHODS

The patients who underwent lumbar spine surgery due to degenerative disc disease were recruited and divided into Modic I, Modic II, and Modic III; groups. HE staining was used to detect the pathological changes in endplates. The changes of Galectin-3, MMP3, Aggrecan, CCL3, and Col II were detected by immunohistochemistry, RT-PCR, and Western blot. MTT and flow cytometry were used to detect cartilage endplate cell proliferation, cell cycle, and apoptosis.

RESULTS

With the progression of degeneration (from Modic I to III), the chondrocytes and density of the cartilage endplate of the intervertebral disc decreased, and the collagen arrangement of the cartilage endplate of the intervertebral disc was broken and calcified. Meanwhile, the expressions of Aggrecan, Col II, Galectin-3, Aggrecan, and CCL3 gradually decreased. After treatment with Galectin-3 inhibitor GB1107, the proliferation of rat cartilage end plate cells was significantly reduced (P < 0.05). GB1107 (25 µmol/L) also significantly promoted the apoptosis of cartilage endplate cells (P < 0.05). Moreover, the percentage of cartilage endplate cells in the G1 phase was significantly higher, while that in the G2 and S phases was significantly lower (P < 0.05). Additionally, the mRNA and protein expression levels of MMP3, CCL3, and Aggrecan in rat cartilage end plate cells were lower than those in the control group.

CONCLUSIONS

Galectin-3 decreases with the progression of the cartilage endplate degeneration of the intervertebral disc. Galectin-3 may affect intervertebral disc degeneration by regulating the degradation of the extracellular matrix.

Hotspot Analysis and Frontier Exploration of Stem Cell Research in Intervertebral Disc Regeneration and Repair: A Bibliometric and Visualization Study

BACKGROUND

Stem cells have shown tremendous potential and vast prospects in the research of intervertebral disc (IVD) regeneration and repair, attracting considerable attention in recent years. In this study, a bibliometric analysis and visualization techniques were employed to probe and analyze the hotspots and frontiers of stem cell research in IVD regeneration and repair, aiming to provide valuable references and insights for further investigations.

METHODS

This study utilized the Science Citation Index Expanded from the Web of Science Core Collection database to retrieve and extract relevant literature records as research samples. Visual analysis tools such as VOSviewer 1.6.19, CiteSpace 6.2.R4, and bibliometric online analysis platforms were employed to construct scientific knowledge maps, providing a comprehensive and systematic exposition from various perspectives including collaboration networks, cocitation networks, and co-occurrence networks.

RESULTS

A total of 1075 relevant studies have been published in 303 journals by 4181 authors from 1198 institutions across 54 countries/regions. Over the past 20 years, the field of research has witnessed a significant growth in annual publications and citations. China and the United States have emerged as the primary participants and contributors, with the AO Research Institute Davos, Zhejiang University, and Tokai University being the top 3 leading research institutions. The most productive and highly cited author is Sakai D, who is regarded as a key leader in this research field. The journals with the highest number of publications and citations are Spine and Biomaterials, which are considered to be high-quality and authoritative core journals in this field. The current research focuses primarily on the sources and selection of stem cells, optimization of transplantation strategies, mechanisms of IVD regeneration, and the combined application of stem cells and biomaterials. However, there are still some challenges that need to be addressed, including posttransplantation stability, assessment of regenerative effects, and translation into clinical applications. Future research will concentrate on the diversity of stem cell sources, the application of novel biomaterials, personalized treatments, and the development of gene editing technologies, among other cutting-edge directions.

CONCLUSIONS

This study utilized bibliometric analysis and visualization techniques to unveil the hotspots and frontiers in the research on stem cells for IVD regeneration and repair. These research findings provide essential guidance and references for further experimental design and clinical applications. However, additional experiments and clinical studies are still needed to address the challenges and difficulties faced in the field of IVD regeneration and repair, thus offering novel strategies and approaches for the treatment of IVD diseases.

Perspective on Intradiscal Therapies for Lumbar Discogenic Pain: State of the Science, Knowledge Gaps, and Imperatives for Clinical Adoption

Specific clinical diagnostic criteria have established a consensus for defining patients with lumbar discogenic pain. However, if conservative medical management fails, these patients have few treatment options short of surgery involving discectomy often coupled with fusion or arthroplasty. There is a rapidly-emerging research effort to fill this treatment gap with intradiscal therapies that can be delivered minimally-invasively via fluoroscopically guided injection without altering the normal anatomy of the affected vertebral motion segment. Viable candidate products to date have included mesenchymal stromal cells, platelet-rich plasma, nucleus pulposus structural allograft, and other cell-based compositions. The objective of these products is to repair, supplement, and restore the damaged intervertebral disc as well as retard further degeneration. In doing so, the intervention is meant to eliminate the source of discogenic pain and avoid surgery. Methodologically rigorous studies are rare, however, and based on the best clinical evidence, the safety as well as the magnitude and duration of clinical efficacy remain difficult to estimate. Further, we summarize the US Food and Drug Administration's (FDA) guidance regarding the interpretation of the minimal manipulation and homologous use criteria, which is central to designating these products as a tissue or as a drug/device/biologic. We also provide perspectives on the core evidence and knowledge gaps associated with intradiscal therapies, propose imperatives for evaluating effectiveness of these treatments and highlight several new technologies on the horizon.

Single-Cell Microgel Encapsulation Improves the Therapeutic Efficacy of Mesenchymal Stem Cells in Treating Intervertebral Disc Degeneration via Inhibiting Pyroptosis

While mesenchymal stem cell (MSC) shows great potentials in treating intervertebral disc degeneration, most MSC die soon after intradiscal transplantation, resulting in inferior therapeutic efficacy. Currently, bulk hydrogels are the common solution to improve MSC survival in tissues, although hydrogel encapsulation impairs MSC migration and disrupts extracellular microenvironment. Cell hydrogel encapsulation has been proposed to overcome the limitation of traditional bulk hydrogels, yet this technique has not been used in treating disc degeneration. Using a layer-by-layer self-assembly technique, we fabricated alginate and gelatin microgel to encapsulate individual MSC for treating disc degeneration. The small size of microgel allowed intradiscal injection of coated MSC. We demonstrated that pyroptosis was involved in MSC death under oxidative stress stimulation, and microgel coating suppressed pyroptosis activation by maintaining mitochondria homeostasis. Microgel coating protected MSC in the harsh disc microenvironment, while retaining vital cellular functions such as migration, proliferation, and differentiation. In a rat model of disc degeneration, coated MSC exhibits prolonged retention in the disc and better efficacy of attenuating disc degeneration, as compared with bare MSC treatment alone. Further, microgel-coated MSC exhibited improved therapeutic effects in treating disc degeneration via suppressing the activation of pyroptosis in the disc. For the first time, microgel-encapsulated MSC was used to treat disc degeneration and obtain encouraging outcomes. The developed biocompatible single-cell hydrogel is an effective strategy to protect MSC and maintain cellular functions and may be an efficacious approach to improving the efficacy of MSC therapy in treating disc degeneration. The objective of this study is to improve the efficacy of cell therapy for treating disc degeneration using single-cell hydrogel encapsulation and further to understand related cytoprotective mechanisms.

The most influential articles on stem cells in intervertebral disc degeneration

BACKGROUND

Stem cell-related studies have been increasingly conducted to facilitate the regeneration of degenerative discs. However, analyses of high-impact articles focused on this topic are rare. This study aimed to determine and summarize the most-cited studies examining stem cells in the context of intervertebral disc degeneration (IDD).

METHODS

We searched the Web of Science (WoS) database for stem cell-related articles in IDD, and the 50 highest-cited papers were summarized. A correlation analysis was conducted to determine the relationship among WoS citations, Altmetric Attention Score (AAS), and Dimensions.

RESULTS

The number of citations of the top 50 manuscripts ranged from 92 to 370. The top three countries were the United States (14), China (10), and Japan (9). Spine (12) was the most prevalent journal, and this was followed by Biomaterials (6). Bone marrow-derived stem cells were the most common subject (38), and they were followed by nucleus pulposus-derived stem cells (4) and annulus fibrosus-derived stem cells (4). Humans were the most studied species (31), and the next most studied were rabbits (9) and rats (7). There was a very high correlation between WoS and Dimension citations (p < 0.001, r = 0.937).

CONCLUSIONS

For the first time, the highest impact articles examining stem cells in the context of IDD were assessed together. The current study provides a deepened understanding of historical studies focused on stem cells in IDD and is beneficial for future studies in this field.

Conditioned Medium of Intervertebral Disc Cells Inhibits Osteo-Genesis on Autologous Bone-Marrow-Derived Mesenchymal Stromal Cells and Osteoblasts

Low back pain (LBP) is associated with the degeneration of human intervertebral discs (IVDs). Despite progress in the treatment of LBP through spinal fusion, some cases still end in non-fusion after the removal of the affected IVD tissue. In this study, we investigated the hypothesis that the remaining IVD cells secrete BMP inhibitors that are sufficient to inhibit osteogenesis in autologous osteoblasts (OBs) and bone marrow mesenchymal stem cells (MSCs). A conditioned medium (CM) from primary human IVD cells in 3D alginate culture was co-cultured with seven donor-matched OB and MSCs. After ten days, osteogenesis was quantified at the transcript level using qPCR to measure the expression of bone-related genes and BMP antagonists, and at the protein level by alkaline phosphatase (ALP) activity. Additionally, cells were evaluated histologically using alizarin red (ALZR) staining on Day 21. For judging ALP activity and osteogenesis, the Noggin expression in samples was investigated to uncover the potential causes. The results after culture with the CM showed significantly decreased ALP activity and the inhibition of the calcium deposit formation in alizarin red staining. Interestingly, no significant changes were found among most bone-related genes and BMP antagonists in OBs and MSCs. Noteworthy, Noggin was relatively expressed higher in human IVD cells than in autologous OBs or MSCs (relative to autologous OB, the average fold change was in 6.9, 10.0, and 6.3 in AFC, CEPC, and NPC, respectively; and relative to autologous MSC, the average fold change was 2.3, 3.4, and 3.2, in AFC, CEPC, and NPC, respectively). The upregulation of Noggin in residual human IVDs could potentially inhibit the osteogenesis of autologous OB and MSC, thus inhibiting the postoperative spinal fusion after discectomy surgery.

Preclinical Animal Study and Pilot Clinical Trial of Using Enriched Peripheral Blood-Derived Mononuclear Cells for Intervertebral Disc Degeneration

Low back pain (LBP) is a leading cause of long-term disability globally. Intervertebral disk degeneration (IVDD) is mainly responsible for discogenic pain in LBP-affected young patients. There is no effective therapy to reverse disease severity and IVDD progression. This study investigates the effect of human peripheral blood-derived mononuclear cells (PBMCs) on pain relief and life quality improvement in IVDD patients. The enriched monocytes of the PBMCs could differentiate into CD14 and CD206 double-positive M2 macrophages in vitro. Preclinical evidence in rats showed that the transplanted PBMCs exhibited anti-inflammatory and moderate tissue-repair effects on controlling IVDD progress in the rat model. The PBMCs significantly steered the aggrecan and type II collagen expressions and attenuated the pro-inflammatory cytokines in the affected disk. Based on the animal results, 36 patients with chronic low back pain (CLBP) were included in clinical trials. The control group was conservative care only, and the experimental group was platelet-rich plasma (PRP) and PBMCs intradiscal injections. We first confirmed the single lumbar disk causing the discogenic pain by provocative discography or magnetic resonance imaging (MRI). Discogenic LBP participants received one intradiscal injection of autologous PBMCs and followed for 6 months. Our clinical trial showed that patients' LBP and disability were significantly ameliorated after the PBMCs transplantation rather than PRP. These preclinical and pilot clinical studies indicate that intradiscal injection of the enriched PBMCs might be a feasible and potential cell therapy to control pain and disability in IVDD patients.

Exoskeleton Partial-Coated Stem Cells for Infarcted Myocardium Restoring

The integration of abiotic materials with live cells has emerged as an exciting strategy for the control of cellular functions. Exoskeletons consisting ofmetal-organic frameworks are generated to produce partial-coated bone marrow stem cells (BMSCs) to overcome low cell survival leading to disappointing effects for cell-based cardiac therapy. Partially coated exoskeletons can promote the survival of suspended BMSCs by integrating the support of exoskeletons and unimpaired cellular properties. In addition, partial exoskeletons exhibit protective effects against detrimental environmental conditions, including reactive oxygen species, pH changes, and osmotic pressure. The partial-coated cells exhibit increased intercellular adhesion forces to aggregate and adhere, promoting cell survival and preventing cell escape during cell therapy. The exoskeletons interact with cell surface receptors integrin α5β1, leading to augmented biological functions with profitable gene expression alteration, such as Vegfa, Cxcl12, and Adm. The partial-coated BMSCs display enhanced cell retention in infarcted myocardium through non-invasive intravenous injections. The repair of myocardial infarction has been achieved with improved cardiac function, myocardial angiogenesis, proliferation, and inhibition of cell apoptosis. This discovery advances the elucidation of potential molecular and cellular mechanisms for cell-exoskeleton interactions and benefits the rational design and manufacture of next-generation nanobiohybrids.

Selective cargo sorting in stem cell-derived small extracellular vesicles: impact on therapeutic efficacy for intervertebral disc degeneration

BACKGROUND

Growing evidence has suggested the role of stem cell-derived small extracellular vesicles (sEVs) in intervertebral disc degeneration (IVDD). The cargo sorting of sEVs, particularly miRNAs, may be influenced when the donor cell is subjected to oxidative stress. Here, we discovered that miRNAs containing specific motifs are selectively sorted into intraluminal vesicles within mesenchymal stem cells (MSCs) in response to oxidative stress.

METHODS

Analysis of miRNA cargoes in sEVs derived from normal MSCs (C-sEVs) or stressed MSCs (T-sEVs) was conducted using miRNA sequencing. Differential expressed miRNAs in sEVs and the identification of motifs were evaluated through bioinformatics analysis. Protein binding was assessed using immunofluorescent staining and immunoprecipitation analysis. Additionally, RNA pull down and RNA immunoprecipitation (RIP) immunoprecipitation were employed to determine the binding between miRNAs and proteins. The effects of C-sEVs and T-sEVs on IVDD were compared by detecting the expression levels of phenotypic genes in vitro or histological evaluation in vivo.

RESULTS

The sorting process of miRNAs is mediated by the nucleocytoplasmic transport of heterogeneous nuclear ribonucleoproteins, which in turn facilitates the phosphorylation of SNAP25 and promotes the transport and secretion of sEVs. Additionally, CHMP1B plays a role in membrane repair and protects against cell ferroptosis upon oxidative stress, concurrently affecting the release of sEVs. Notably, stem cell-derived sEVs associated with ferroptosis impair the therapeutic efficacy for IVDD. However, the application of engineered sEVs containing a specific miRNA inhibitor exhibits the potential to reinstate the therapeutic efficacy for IVDD both in vitro and in vivo.

CONCLUSIONS

Taken together, our findings shed light on the mechanism of miRNAs sorting into sEVs and offer new insights for the optimization of sEV-based treatments during intervertebral disc regeneration. regeneration.

Hypoxic Preconditional Engineering Small Extracellular Vesicles Promoted Intervertebral Disc Regeneration by Activating Mir-7-5p/NF-Κb/Cxcl2 Axis

Chronic low back pain (LBP) caused by intervertebral disc (IVD) degradation is a serious socioeconomic burden that can cause severe disabilities. Addressing the underlying pathogenic mechanisms of IVD degeneration may inspire novel therapeutic strategy for LBP. Herein, hypoxic preconditioning improves both the biological function of MSCs in hostile microenvironments and enhances the production of small extracellular vesicles (sEVs) with desirable therapeutic functions. In vitro results reveal that hypoxic preconditional engineering sEVs (HP-sEVs) alleviate the inflammatory microenvironments of IVD degradation, enhance the proliferation of nucleus pulposus (NP) cells, and promote proteoglycan synthesis and collagen formation. Transcriptomic sequencing reveales the excellent therapeutic effects of HP-sEVs in promoting extracellular matrix regeneration through the delivery of microRNA(miR)-7-5p, which further suppresses p65 production and thus the inhibition of Cxcl2 production. Moreover, in vivo results further confirm the robust therapeutic role of HP-sEVs in promoting IVD regeneration through the same mechanism mediated by miR-7-5p delivery. In conclusion, this study provides a novel therapeutic strategy for treating IVD degradation and is thus valuable for understanding the mechanism-of-action of HP-sEVs in IVD regeneration associated with chronic lower back pain.

Combining adhesive and nonadhesive injectable hydrogels for intervertebral disc repair in an ovine discectomy model

Background

Intervertebral disc (IVD) disorders (e.g., herniation) directly contribute to back pain, which is a leading cause of global disability. Next-generation treatments for IVD herniation need advanced preclinical testing to evaluate their ability to repair large defects, prevent reherniation, and limit progressive degeneration. This study tested whether experimental, injectable, and nonbioactive biomaterials could slow IVD degeneration in an ovine discectomy model.

Methods

Ten skeletally mature sheep (4-5.5 years) experienced partial discectomy injury with cruciate-style annulus fibrosus (AF) defects and 0.1 g nucleus pulposus (NP) removal in the L1-L2, L2-L3, and L3-L4 lumbar IVDs. L4-L5 IVDs were Intact controls. IVD injury levels received: (1) no treatment (Injury), (2) poly (ethylene glycol) diacrylate (PEGDA), (3) genipin-crosslinked fibrin (FibGen), (4) carboxymethylcellulose-methylcellulose (C-MC), or (5) C-MC and FibGen (FibGen + C-MC). Animals healed for 12 weeks, then IVDs were assessed using computed tomography (CT), magnetic resonance (MR) imaging, and histopathology.

Results

All repaired IVDs retained ~90% of their preoperative disc height and showed minor degenerative changes by Pfirrmann grading. All repairs had similar disc height loss and Pfirrmann grade as Injury IVDs. Adhesive AF sealants (i.e., PEGDA and FibGen) did not herniate, although repair caused local endplate (EP) changes and inflammation. NP repair biomaterials (i.e., C-MC) and combination repair (i.e., FibGen + C-MC) exhibited lower levels of degeneration, less EP damage, and less severe inflammation; however, C-MC showed signs of herniation via biomaterial expulsion.

Conclusions

All repair IVDs were noninferior to Injury IVDs by IVD height loss and Pfirrmann grade. C-MC and FibGen + C-MC IVDs had the best outcomes, and may be appropriate for enhancement with bioactive factors (e.g., cells, growth factors, and miRNAs). Such bioactive factors appear to be necessary to prevent injury-induced IVD degeneration. Application of AF sealants alone (i.e., PEGDA and FibGen) resulted in EP damage and inflammation, particularly for PEGDA IVDs, suggesting further material refinements are needed.

Can extracellular vesicles be considered as a potential frontier in the treatment of intervertebral disc disease?

As a global public health problem, low back pain (LBP) caused by intervertebral disc degeneration (IDD) seriously affects patients' quality of life. In addition, the prevalence of IDD tends to be younger, which brings a huge burden to individuals and society economically. Current treatments do not delay or reverse the progression of IDD. The emergence of biologic therapies has brought new hope for the treatment of IDD. Among them, extracellular vesicles (EVs), as nanoscale bioactive substances that mediate cellular communication, have now produced many surprising results in the research of the treatment of IDD. This article reviews the mechanisms and roles of EVs in delaying IDD and describes the prospects and challenges of EVs.

Transplantation of active nucleus pulposus cells with a keep-charging hydrogel microsphere system to rescue intervertebral disc degeneration

BACKGROUND

Cell transplantation has been demonstrated as a promising approach in tissue regeneration. However, the reactive oxygen species (ROS) accumulation and inflammation condition establish a harsh microenvironment in degenerated tissue, which makes the transplanted cells difficult to survive.

METHODS

In this study, we constructed a keep-charging hydrogel microsphere system to enable cells actively proliferate and function in the degenerated intervertebral disc. Specifically, we combined Mg2+ to histidine-functionalized hyaluronic acid (HA-His-Mg2+) through coordination reaction, which was further intercrossed with GelMA to construct a double-network hydrogel microsphere (GelMA/HA-His-Mg2+, GHHM) with microfluidic methods. In vitro, the GHHM loaded with nucleus pulposus cells (GHHM@NPCs) was further tested for its ability to promote NPCs proliferation and anti-inflammatory properties. In vivo, the ability of GHHM@NPCs to promote regeneration of NP tissue and rescue intervertebral disc degeneration (IVDD) was evaluated by the rat intervertebral disc acupuncture model.

RESULTS

The GHHM significantly enhanced NPCs adhesion and proliferation, providing an ideal platform for the NPCs to grow on. The loaded NPCs were kept active in the degenerative intervertebral disc microenvironment as charged by the Mg2+ in GHHM microspheres to effectively support the loaded NPCs to reply against the ROS-induced inflammation and senescence. Moreover, we observed that GHHM@NPCs effectively alleviated nucleus pulposus degeneration and promoted its regeneration in the rat IVDD model.

CONCLUSION

In conclusion, we constructed a keep charging system with a double-network hydrogel microsphere as a framework and Mg2+ as a cell activity enhancer, which effectively maintains NPCs active to fight against the harsh microenvironment in the degenerative intervertebral disc. The GHHM@NPCs system provides a promising approach for IVDD management.

A Biomimetic Peptide Functions as Specific Extracellular Matrix for Quiescence of Stem Cells against Intervertebral Disc Degeneration

Maintaining quiescence of stem cells is a potential way to decrease cell nutrition demand for restoring the organization. Herein, a biomimetic peptide to maintain quiescence of stem cells through C-X-C motif chemokine ligand 8 (CXCL8)-C-X-C motif chemokine receptor 1 (CXCR1) pathway against intervertebral disc degeneration (IVDD) is developed. First, it is confirmed that quiescence can be induced via inhibiting phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway in nucleus pulposus stem cells (NPSCs). Meanwhile, it is well known that CXCR1, a chemokine receptor, can be targeted by CXCL8, resulting in cell proliferation via activating PI3K/Akt/mTOR pathway. Second, a biomimetic peptide (OAFF) that can bind to CXCR1 and form fibrous networks on NPSCs, mimicking extracellular matrix formation is developed. The multivalent effect and long-term binding to CXCR1 on NPSCs of OAFF fibers offer forcefully competitive inhibition with natural CXCL8, which induces NPSCs quiescence and ultimately overcomes obstacle in intradiscal injection therapy. In rat caudal disc puncture model, OAFF nanofibers still maintain at 5 weeks after operation and inhibit degeneration process of intervertebral disc in terms of histopathology and imageology. In situ fibrillogenesis of biomimetic peptide on NPSCs provides promising stem cells for intradiscal injection therapy against IVDD.

Multifunctional injectable hydrogels with controlled delivery of bioactive factors for efficient repair of intervertebral disc degeneration

Millions of people worldwide suffer from intervertebral disc degeneration (IVDD), which imposes a significant socioeconomic burden on society. There is an urgent clinical demand for more effective treatments for IVDD because conventional treatments can only alleviate the symptoms rather than preventing the progression of IVDD. Hydrogels, a class of elastic biomaterials with good biocompatibility, are promising candidates for intervertebral disc repair and regeneration. In recent years, various hydrogels have been investigated in vitro and in vivo for the repair of intervertebral discs, some of which are ready for clinical testing. This review summarizes the latest findings and developments in using bioactive factors-released bioactive injectable hydrogels for the repair and regeneration of intervertebral discs. It focuses on the analysis and summary of the use of multifunctional injectable hydrogels to delivery bioactive factors (cells, exosomes, growth factors, genes, drugs) for disc regeneration, providing guidance for future study. Finally, we discussed and analyzed the optimal timing for the application of controlled-release hydrogels in the treatment of IVDD to meet the high standards required for intervertebral disc regeneration and precision medicine.

A hydrogel reservoir as a self-contained nucleus pulposus cell delivery vehicle for immunoregulation and repair of degenerated intervertebral disc

The strategies for modulating the local inflammatory microenvironment to inhibit intervertebral disc degeneration (IVDD) have garnered significant interest in recent years. In this study, we developed a "self-contained" injectable hydrogel capable of storing Mg2+ while carrying nucleus pulposus (NP) cells, with the aim of inhibiting IVDD through immunoregulation. The hydrogel consists of sodium alginate (SA), poly(N-isopropylacrylamide) (PNIPAAm), silicate ceramics (SC), and NP cells. When injected into the NP site, PNIPAAm gelates instantly under body temperature, forming an interpenetrating network (IPN) hydrogel with SA. Ca2+ released from the SC can crosslink the SA in situ, forming a SA/PNIPAAm hydrogel with an interpenetrating network (IPN) encapsulating the NP cells. Moreover, inside the hydrogel, Mg2+ released from SC are effectively encapsulated and maintained at a desirable concentration. These Mg2+ facilitates the local cell matrix synthesis and promotes immunomodulation (upregulating M2 / downregulating M1 macrophage polarization), thus inhibiting the IVDD progression. The proposed hydrogel has biocompatibility and is shown to enhance the expression of collagen II (COL II) and aggrecan. The potential of the injectable hydrogel in IVD repair has also been successfully demonstrated by in vivo studies. STATEMENT OF SIGNIFICANCE.

In situ forming macroporous biohybrid hydrogel for nucleus pulposus cell delivery

Degenerative intervertebral disc disease is a common source of chronic pain and reduced quality of life in people over the age of 40. While degeneration occurs throughout the disc, it most often initiates in the nucleus pulposus (NP). Minimally invasive delivery of NP cells within hydrogels that can restore and maintain the disc height while regenerating the damaged NP tissue is a promising treatment strategy for this condition. Towards this goal, a biohybrid ABA dimethacrylate triblock copolymer was synthesized, possessing a lower critical solution temperature below 37 °C and which contained as its central block an MMP-degradable peptide flanked by poly(trimethylene carbonate) blocks bearing pendant oligoethylene glycol groups. This triblock prepolymer was used to form macroporous NP cell-laden hydrogels via redox initiated (ammonium persulfate/sodium bisulfite) crosslinking, with or without the inclusion of thiolated chondroitin sulfate. The resulting macroporous hydrogels had water and mechanical properties similar to those of human NP tissue and were mechanically resilient. The hydrogels supported NP cell attachment and growth over 28 days in hypoxic culture. In hydrogels prepared with the triblock copolymer but without the chondroitin sulfate the NP cells were distributed homogeneously throughout in clusters and deposited collagen type II and sulfated glycosaminoglycans but not collagen type I. This hydrogel formulation warrants further investigation as a cell delivery vehicle to regenerate degenerated NP tissue. STATEMENT OF SIGNIFICANCE: The intervertebral disc between the vertebral bones of the spine consists of three regions: a gel-like central nucleus pulposus (NP) within the annulus fibrosis, and bony endplates. Degeneration of the intervertebral disc is a source of chronic pain in the elderly and most commonly initiates in the NP. Replacement of degenerated NP tissue with a NP cell-laden hydrogel is a promising treatment strategy. Herein we demonstrate that a crosslinkable polymer with a lower critical solution temperature below 37 °C can be used to form macroporous hydrogels for this purpose. The hydrogels are capable of supporting NP cells, which deposit collagen II and sulfated glycosaminoglycans, while also possessing mechanical properties matching those of human NP tissue.