TGF-β1-supplemented decellularized annulus fibrosus matrix hydrogels promote annulus fibrosus repair

Engineering Intervertebral Disc Regeneration: Biomaterials, Cell Sources and Animal Models

Intervertebral disc (IVD) degeneration is an age-related problem triggering chronic spinal issues, such as low back pain and IVD herniation. Standard surgical treatment for such spinal issues is the removal of the degenerated or herniated IVD and fusion of adjacent vertebrae to stabilise the joint and locally decompress the spinal cord and/or nerve roots to relieve pain. However, a key challenge of current surgical strategies is the increasing risk of adjacent segment degeneration due to the disruption of native biomechanics of the functional spinal unit, dominated by the loss of the IVD. In the past two decades, research has focused on developing a number of bioengineering approaches to repair and regenerate the IVD; in particular, tissue engineering of the IVD, using bioscaffolds and stem cells represents a promising area. This review highlights the current tissue engineering approaches utilising biomaterials, animal models and cell sources for IVD regeneration and discusses future opportunities.

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.

Modic changes: From potential molecular mechanisms to future research directions (Review)

Low back pain (LBP) is a leading cause of disability worldwide. Although not all patients with Modic changes (MCs) experience LBP, MC is often closely associated with LBP and disc degeneration. In clinical practice, the focus is usually on symptoms related to MC, which are hypothesized to be associated with LBP; however, the link between MC and nerve compression remains unclear. In cases of intervertebral disc herniation, nerve compression is often the definitive cause of symptoms. Recent advances have shed light on the pathophysiology of MC, partially elucidating its underlying mechanisms. The pathogenesis of MC involves complex bone marrow‑disc interactions, resulting in bone marrow inflammation and edema. Over time, hematopoietic cells are gradually replaced by adipocytes, ultimately resulting in localized bone marrow sclerosis. This process creates a barrier between the intervertebral disc and the bone marrow, thereby enhancing the stability of the vertebral body. The latest understanding of the pathophysiology of MC suggests that chronic inflammation plays a significant role in its development and hypothesizes that the complement system may contribute to its pathological progression. However, this hypothesis requires further research to be confirmed. The present review we proposed a pathological model based on current research, encompassing the transition from Modic type 1 changes (MC1) to Modic type 2 changes (MC2). It discussed key cellular functions and their alterations in the pathogenesis of MC and outlined potential future research directions to further elucidate its mechanisms. Additionally, it reviewed the current clinical staging and pathogenesis of MC, recommended the development of an updated staging system and explored the prospects of integrating emerging artificial intelligence technologies.

Robustly Injectable Tetra-PEG Hydrogel Sealants for Annulus Fibrosus Repair

Discectomy serves as the primary therapeutic approach for lumbar disc herniation, but the annular fibrosus defects after discectomy may lead to recurrence of disc herniation. Despite recent advances in bioinspired adhesives to seal the AF defect, the growing popularity of endoscopic discectomy has put forward high requirements for the tissue bioadhesives with rapid injectability, easy operation, and robust tissue adhesion in underwater environments. Herein, a rapidly in situ forming injectable tetra-PEG bioadhesive (ISG) comprising of FDA-approved tetra-armed poly (ethylene glycol) amine (tetra-PEG-NH2) and tetra-armed poly (ethylene glycol) succinimidyl glutarate (tetra-PEG-SG) for the sutureless closure of AF defects, is reported. Relying on quick ammonolysis reaction between N-hydroxysuccinimide (NHS)-ester of tetra-PEG-SG polymer and amine groups of tetra-PEG-NH2 polymer and tissue proteins, the uniform networks are formed within seconds with easy injection, efficient waterproofness, instant tissue adhesion, and durable compliance. The goat lumbar discectomy model was used to assess the effect of ISG hydrogels in vivo. The results reveal that the resultant ISG bioadhesive can effectively maintain the disc height, fuse with the host tissue, ameliorate IVD degeneration, and retain the initial biomechanics. Together, this study provides an efficient strategy of in situ injectable glue for the minimally invasive treatment of AF defects.

Harnessing hydrogen sulfide in injectable hydrogels that guide the immune response and osteoclastogenesis balance for osteoporosis treatment

Elevated levels of oxidative stress, inflammation, and a dysregulated osteoclastogenesis balance frequently characterize the microenvironment of osteoporosis, which impedes the processes of healing and repair. Existing treatment approaches are limited in scope and rely primarily on factors and drugs. An injectable hydrogel designed for the ROS-responsive release of H2S gas is presented in this study. The first network of the hydrogel comprises sodium alginate (SA-SATO) chelated with S-aroylthiooxime (SATO) and an H2S-generating group, while the second network is composed of photocrosslinkable PEGDA. Through the integration of Cys-releasing microspheres that are reactive with ROS, a composite hydrogel was developed that exhibited advantageous mechanical characteristics and biosafety. The composite hydrogel effectively promoted osteogenic differentiation of mesenchymal stem cells, modulated macrophage polarization, decreased inflammatory responses, and halted cell apoptosis, as evidenced by in vitro experiments. Additionally, it released H2S gas and mitigated excess ROS in cells. The efficacy of the composite hydrogel in promoting bone defect repair and regeneration in an osteoporotic model was further validated by in vivo findings. In summary, the composite hydrogel exhibits potential as a viable approach to address osteoporotic bone defects by harmonizing osteogenesis and osteoclast activity, modulating the microenvironment of bone injuries, and reducing inflammation. Consequently, it presents a viable strategy for the efficient repair of bone defects.

An anti-senescence hydrogel with pH-responsive drug release for mitigating intervertebral disc degeneration and low back pain

Oxidative stress and aging lead to progressive senescence of nucleus pulposus (NP) cells, resulting in intervertebral disc (IVD) degeneration (IVDD). In some cases, degenerative IVD can further cause low back pain (LBP). Several studies have confirmed that delaying and rejuvenating the senescence of NP cells can attenuate IVDD. However, the relatively closed tissue structure of IVDs presents challenges for the local application of anti-senescence drugs. Here, we prepared an anti-senescence hydrogel by conjugating phenylboronic acid-modified gelatin methacryloyl (GP) with quercetin to alleviate IVDD by removing senescent NP cells. The hydrogel exhibited injectability, biodegradability, prominent biocompatibility and responsive release of quercetin under pathological conditions. In vitro experiments demonstrated that the hydrogel could reduce the expression of senescence markers and restore the metabolic balance in senescent NP cells. In vivo studies validated that a single injection of the hydrogel in situ could maintain IVD tissue structure and alleviate sensitivity to noxious mechanical force in the rat models, indicating a potential therapeutic approach for ameliorating IVDD and LBP. This approach helps prevent potential systemic toxicity associated with systemic administration and reduces the morbidity resulting from repeated injections of free drugs into the IVD, providing a new strategy for IVDD treatment.

Preserving the Immune-Privileged Niche of the Nucleus Pulposus: Safeguarding Intervertebral Discs from Degeneration after Discectomy with Synthetic Mucin Hydrogel Injection

Intervertebral disc (IVD) herniation is a prevalent spinal disorder, often necessitating surgical intervention such as microdiscectomy for symptomatic relief and nerve decompression. IVDs comprise a gel-like nucleus pulposus (NP) encased by an annulus fibrosus (AF), and their avascular nature renders them immune-privileged. Microdiscectomy exposes the residual NP to the immune system, precipitating an immune cell infiltration and attack that exacerbates IVD degeneration. While many efforts in the tissue engineering field are directed toward IVD regeneration, the inherently limited regenerative capacity due to the avascular and low-cellularity nature of the disc and the challenging mechanical environment of the spine often impedes success. This study, aiming to prevent IVD degeneration post-microdiscectomy, utilizes mucin-derived gels (Muc-gels) that form a gel at the surgical site, inspired by the natural mucin coating on living organisms to evade immune reorganization. It is shown that type I macrophages are present in severely degenerated human discs. Encapsulating IVDs within Muc-gels prevents fibrous encapsulation and macrophage infiltration in a mouse subcutaneous model. The injection of Muc-gels prevents IVD degeneration in a rat tail IVD degeneration model up to 24 weeks post-operation. Mechanistic investigations indicate that Muc-gels attenuate immune cell infiltration into NPs, offering durable protection against immune attack post-microdiscectomy.

Polyphenol-Mediated Adhesive and Anti-Inflammatory Double-Network Hydrogels for Repairing Postoperative Intervertebral Disc Defects

Hydrogels have garnered tremendous attention for their applications in the repair of intervertebral disk (IVD) degeneration and postoperative IVD defects. However, it is still challenging to develop a hydrogel fulfilling the requirements for high mechanical properties, adhesive capability, biocompatibility, antibacterial properties, and anti-inflammatory performance. Herein, we report a multifunctional double-network (DN) hydrogel composed of physically cross-linked carboxymethyl chitosan (CMCS) and tannic acid (TA) networks as well as chemically cross-linked acrylamide (AM) networks, which integrates the properties of high strength, adhesion, biocompatibility, antimicrobial activity, and anti-inflammation for the repair of postoperative IVD defects. The treatment with CMCS/TA/PAM DN hydrogels can significantly decrease the levels of inflammatory cytokines and degeneration-related factors and upregulated collagen type II alpha 1. In addition, the hydrogels can effectively seal the annulus fibrosus defect, prevent nucleus pulposus degeneration, retain IVD height, and restore the biomechanical properties of the disc to some extent. This polyphenol-mediated DN hydrogel is promising for sealing IVD defects and preventing herniation after lumbar discectomy.

Injectable Inflammation-Responsive Hydrogels for Microenvironmental Regulation of Intervertebral Disc Degeneration

Chronic local inflammation and excessive cell apoptosis in nucleus pulposus (NP) tissue are the main causes of intervertebral disc degeneration (IDD). Stimuli-responsive hydrogels have great potential in the treatment of IDD by facilitating localized and controlled drug delivery. Herein, an injectable drug-loaded dual stimuli-responsive adhesive hydrogel for microenvironmental regulation of IDD, is developed. The gelatin methacryloyl is functionalized with phenylboronic acid groups to enhance drug loading capacity and enable dual stimuli-responsive behavior, while the incorporation of oxidized hyaluronic acid further improves the adhesive properties. The prepared hydrogel exhibits an enhanced drug loading capacity for diol-containing drugs, pH- and reactive oxygen species (ROS)-responsive behaviors, excellent radical scavenging efficiency, potent antibacterial activity, and favorable biocompatibility. Furthermore, the hydrogel shows a beneficial protective efficacy on NP cells within an in vitro oxidative stress microenvironment. The in vivo results demonstrate the hydrogel's excellent therapeutic effect on treating IDD by maintaining water retention, restoring disc height, and promoting NP regeneration, indicating that this hydrogel holds great potential as a promising therapeutic approach for regulating the microenvironment and alleviating the progression of IDD.

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.

Extracellular Vesicles Functional “Brick‐Cement” Bio‐Integrated System for Annulus Fibrosus Repair

Due to the deficiency of mechanical supporting after discectomy and weak proliferative capacity of annulus fibrosus (AF) cells, the AF defect repair remains a clinical challenge. Herein, a myofibroblasts derived extracellular vesicles (M‐EVs) functional “brick‐cement” bio‐integrated system (M‐EVs@PGBgel) is developed to repair AF defect. The modified Poly(glycerol‐sebacate) (PGBS), “bio‐brick” layer, exhibited excellent support features on account of its elastomeric mechanical properties. The loaded M‐EVs in the “bio‐cement” layer activated ITGA6/PI3K/AKT pathway, regulated M2 macrophage polarization, thus synergistically promoting AF cell proliferation and migration. The “bio‐cement” layer integrated PGBS and remnant tissue at the defect through the Schiff base reaction and aided M‐EVs’ sustained release. This study demonstrated that M‐EVs@PGBgel significantly improved the disc's biological and mechanical properties in the AF defect microenvironments and promoted AF regeneration in vivo. The M‐EVs@PGBgel shows promise as an effective strategy to simultaneously address the mechanical imbalance and biological disruptions resulting from AF defect.

Thread-structural microneedles loaded with engineered exosomes for annulus fibrosus repair by regulating mitophagy recovery and extracellular matrix homeostasis

Low back pain is among the most grave public health concerns worldwide and the major clinical manifestation of intervertebral disc degeneration (IVDD). The destruction of annulus fibrosus (AF) is the primary cause of IVDD. A sustainable and stable treatment system for IVDD is lacking because of the special organizational structure and low nutrient supply of AF. We here found that IVDD results in the impaired mitochondrial function of AF tissue, and mitochondrial autophagy (mitophagy) plays a protective role in this process. We therefore reported a thread-structural microneedle (T-MN) matching the ring structure of AF. Based on the adsorption effect of laminin, our T-MN could load with bone marrow mesenchymal stem cell-derived exosomes to envelope the regulating mitophagy microRNA (miRNA 378), named as T-MN@EXO@miR-378. In general, we offered in situ locking in the defect site of AF to prevent nucleus pulposus leakage and promoted AF repair. The design of the thread structure was aimed at bionically matching the layered AF structure, thereby providing stronger adhesion. The T-MN@EXO@miR-378 effectively attached to AF and slowly released therapeutic engineered exosomes, and prevented IVDD progression by restoring mitophagy, promoting AF cell proliferation and migration, and inhibiting the pathological remodeling of the extracellular matrix. This functional system can be used as an excellent tool for sustained drug release and has a certain prospect in substituting the conventional treatment of IVDD.

Microgels for Cell Delivery in Tissue Engineering and Regenerative Medicine

Microgels prepared from natural or synthetic hydrogel materials have aroused extensive attention as multifunctional cells or drug carriers, that are promising for tissue engineering and regenerative medicine. Microgels can also be aggregated into microporous scaffolds, promoting cell infiltration and proliferation for tissue repair. This review gives an overview of recent developments in the fabrication techniques and applications of microgels. A series of conventional and novel strategies including emulsification, microfluidic, lithography, electrospray, centrifugation, gas-shearing, three-dimensional bioprinting, etc. are discussed in depth. The characteristics and applications of microgels and microgel-based scaffolds for cell culture and delivery are elaborated with an emphasis on the advantages of these carriers in cell therapy. Additionally, we expound on the ongoing and foreseeable applications and current limitations of microgels and their aggregate in the field of biomedical engineering. Through stimulating innovative ideas, the present review paves new avenues for expanding the application of microgels in cell delivery techniques.

Temporal and spatial expression analysis of periostin in mice periodontitis model

OBJECTIVES

This study aimed to investigate the temporal and spatial changes in the expression of periostin during periodontal inflammation in mice.

METHODS

A periodontitis model was constructed using silk thread ligation. Mice were randomly divided into five groups including control group, 4-day ligation group, 7-day ligation group, 14-day ligation group, and self-healing group (thread removal for 14 days after 14-day ligation). Micro-CT and histological staining were performed to characterize the dynamic changes in the mouse periodontal tissue in each group. RNAscope and immunohistochemical staining were used to analyze the pattern of changes in periostin at various stages of periodontitis. The cell experiment was divided into three groups: control group, lipopolysaccharide (LPS) stimulation group (treated with LPS for 12 h), and LPS stimulation removal group (treated with LPS for 3 h followed by incubation with medium for 9 h). Quantitative real-time polymerase chain reaction (qRT-PCR) was used to detect the expression of periostin, transforming growth factor-β1 (TGF-β1), and matrix metalloproteinase 2 (MMP2).

RESULTS

Significant alveolar bone resorption was observed 7 days after ligation. With increasing duration of ligation, the damage to the mouse periodontal tissue was aggravated, which manifested as increased osteoclasts, widening of the periodontal membrane space, and decreased alveolar bone height. Some degree of periodontal tissue repair was observed in the self-healing group. Periostin expression decreased at 4 and 7 days compared with the control group and increased at 14 days compared with 4 and 7 days. A significant recovery was found in the self-healing group. The qRT-PCR results showed that the expression of periostin and TGF-β1 in the LPS stimulation group decreased compared with that in the control group but significantly recovered in the LPS removal group.

CONCLUSIONS

Periostin expression in the PDL of mice showed a downward and upward trend with inflammation progression. The significant recovery of periostin expression after removing inflammatory stimuli may be related to TGF-β1, which is crucial to maintain the integrity of the PDL.

Biomimetic Scaffolds-A Novel Approach to Three Dimensional Cell Culture Techniques for Potential Implementation in Tissue Engineering

Biomimetic scaffolds imitate native tissue and can take a multidimensional form. They are biocompatible and can influence cellular metabolism, making them attractive bioengineering platforms. The use of biomimetic scaffolds adds complexity to traditional cell cultivation methods. The most commonly used technique involves cultivating cells on a flat surface in a two-dimensional format due to its simplicity. A three-dimensional (3D) format can provide a microenvironment for surrounding cells. There are two main techniques for obtaining 3D structures based on the presence of scaffolding. Scaffold-free techniques consist of spheroid technologies. Meanwhile, scaffold techniques contain organoids and all constructs that use various types of scaffolds, ranging from decellularized extracellular matrix (dECM) through hydrogels that are one of the most extensively studied forms of potential scaffolds for 3D culture up to 4D bioprinted biomaterials. 3D bioprinting is one of the most important techniques used to create biomimetic scaffolds. The versatility of this technique allows the use of many different types of inks, mainly hydrogels, as well as cells and inorganic substances. Increasing amounts of data provide evidence of vast potential of biomimetic scaffolds usage in tissue engineering and personalized medicine, with the main area of potential application being the regeneration of skin and musculoskeletal systems. Recent papers also indicate increasing amounts of in vivo tests of products based on biomimetic scaffolds, which further strengthen the importance of this branch of tissue engineering and emphasize the need for extensive research to provide safe for humansbiomimetic tissues and organs. In this review article, we provide a review of the recent advancements in the field of biomimetic scaffolds preceded by an overview of cell culture technologies that led to the development of biomimetic scaffold techniques as the most complex type of cell culture.

Decellularized extracellular matrix biomaterials for regenerative therapies: Advances, challenges and clinical prospects

Tissue engineering and regenerative medicine have shown potential in the repair and regeneration of tissues and organs via the use of engineered biomaterials and scaffolds. However, current constructs face limitations in replicating the intricate native microenvironment and achieving optimal regenerative capacity and functional recovery. To address these challenges, the utilization of decellularized tissues and cell-derived extracellular matrix (ECM) has emerged as a promising approach. These biocompatible and bioactive biomaterials can be engineered into porous scaffolds and grafts that mimic the structural and compositional aspects of the native tissue or organ microenvironment, both in vitro and in vivo. Bioactive dECM materials provide a unique tissue-specific microenvironment that can regulate and guide cellular processes, thereby enhancing regenerative therapies. In this review, we explore the emerging frontiers of decellularized tissue-derived and cell-derived biomaterials and bio-inks in the field of tissue engineering and regenerative medicine. We discuss the need for further improvements in decellularization methods and techniques to retain structural, biological, and physicochemical characteristics of the dECM products in a way to mimic native tissues and organs. This article underscores the potential of dECM biomaterials to stimulate in situ tissue repair through chemotactic effects for the development of growth factor and cell-free tissue engineering strategies. The article also identifies the challenges and opportunities in developing sterilization and preservation methods applicable for decellularized biomaterials and grafts and their translation into clinical products.

A novel rat model of annulus fibrosus injury for intervertebral disc degeneration

BACKGROUND CONTEXT

In clinical practice, acute trauma and chronic degeneration of the annulus fibrosus (AF) can promote further degeneration of the intervertebral disc (IVD). Therefore, it is critical to understand the AF repair process and its consequences on IVD. However, the lack of cost-effective and reproducible in vivo animal models of AF injury has limited research development in this field.

PURPOSES

The purpose of this study was to establish and evaluate the utility of a novel animal model for full-thickness AF injury. Three foci were proposed: (1) whether this new modeling method can cause full-layer AF damage; (2) the repair processes and pathological changes in the damaged area after AF injury, and (3) the morphological and histological changes in the IVD are after AF injury.

STUDY DESIGN/SETTING

In vivo rat AF injury model with characterization of AF damage repair, IVD degeneration.

METHODS

A total of 72,300 g male rats were randomly assigned to one of the two groups: experimental or sham. Annulus fibrosus was separated layer by layer under the microscope with a #11 blade up to the AF- nucleus pulpous (NP) junction. The repair process of the horizontal AF and morphological changes in the sagittal IVD were evaluated with HE staining. Sirius red staining under polarized light. Immunofluorescence was conducted to analyze changes in the expression of COL1 and COL3 in the AF injury area and 8-OHdg, IL-6, MMP13, FSP1, and ACAN in the IVD. The disc height and structural changes after AF injury were measured using X-ray and contrast-enhanced micro-CT. Additionally, the resistance of the AF to stretching was analyzed using three-point bending.

RESULTS

Annulus fibrosus-nucleus pulpous border was identified to stably induce the full-thickness AF injury without causing immediate NP injury. The AF repair process after injury was slow and expressed inflammation factors continuously, with abundant amounts of type III collagen appearing in the inner part of the AF. The scar at the AF lesion had decreased resistance to small molecule penetration and weakened tensile strength. Full-thickness AF injury induced disc degeneration with loss of disc height, progressive unilateral vertebral collapse, and ossification of the subchondral bone. Inflammatory-induced degeneration and extracellular matrix catabolism gradually appeared in the NP and cartilage endplate (CEP).

CONCLUSIONS

We established a low-cost and reproducible small animal model of AF injury which accurately replicated the pathological state of the limited AF self-repair ability and demonstrated that injury to the AF alone could cause further degeneration of the IVD.

CLINICAL RELEVANCE

This in vivo rat model can be used to study the repair process of the AF defect and pathological changes in the gradual degeneration of IVD after AF damage. In addition, the model provides an experimental platform for in vivo experimental research of potential clinical therapeutics.

Bioactive decellularized adipose matrix prepared using a rapid, nonchemical/enzymatic method for adipogenesis

Recent developments in the field of regenerative surgeries and medical applications have led to a renewed interest in adipose tissue-enriched mesenchymal stem cell scaffolds. Various advantages declared for the decellularized adipose matrix (DAM) have caused its extensive use in the transfer of stem cells or growth factors for soft tissue regeneration induction. Meanwhile, the long-term application of detergents toward DAM regeneration has been assumed as a risky obstacle in this era. Herein, a rapid, mechanical protocol was developed to prepare DAM (M-DAM) without chemicals/enzymes and was comprehensively compared with the ordinary DAM (traditional chemical method). Accordingly, this method could effectively hinder oils and cells, sustain the structural and biological elements, and contain a superior level of collagen content. In addition, more protein numbers, as well as higher basement membrane elements, glycoproteins, and extracellular matrix-related proteins were detected in the regenerated M-DAM. Also, superior adipogenesis and angiogenesis proteins were distinguished. The noncytotoxicity of the M-DAM was also approved, and a natural ecological niche was observed for the proliferation and differentiation of stem cells, confirming its great potential for vascularization and adipogenesis in vivo. The suggested technique could effectively prepare the modified DAM in variant constructions of tablets, powders, emulsions, hydrogels, and different three-dimensional-printed structures. Hence, this rapid, mechanical process can produce bioactive DAM, which has the potential to be widely used in various research fields of regenerative medicine.

Design principles in mechanically adaptable biomaterials for repairing annulus fibrosus rupture: A review

Annulus fibrosus (AF) plays a crucial role in the biomechanical loading of intervertebral disc (IVD). AF is difficult to self-heal when the annulus tears develop, because AF has a unique intricate structure and biologic milieu in vivo. Tissue engineering is promising for repairing AF rupture, but construction of suitable mechanical matching devices or scaffolds is still a grand challenge. To deeply know the varied forces involved in the movement of the native annulus is highly beneficial for designing biomimetic scaffolds to recreate the AF function. In this review, we overview six freedom degrees of forces and adhesion strength on AF tissue. Then, we summarize the mechanical modalities to simulate related forces on AF and to assess the characteristics of biomaterials. We finally outline some current advanced techniques to develop mechanically adaptable biomaterials for AF rupture repair.

Self-healing hydrogels based on biological macromolecules in wound healing: A review

The complete healing of skin wounds has been a challenge in clinical treatment. Self-healing hydrogels are special hydrogels formed by distinctive physicochemically reversible bonds, and they are considered promising biomaterials in the biomedical field owing to their inherently good drug-carrying capacity as well as self-healing and repair abilities. Moreover, natural polymeric materials have received considerable attention in skin tissue engineering owing to their low cytotoxicity, low immunogenicity, and excellent biodegradation rates. In this paper, we review recent advances in the design of self-healing hydrogels based on natural polymers for skin-wound healing applications. First, we outline a variety of natural polymers that can be used to construct self-healing hydrogel systems and highlight the advantages and disadvantages of different natural polymers. We then describe the principle of self-healing hydrogels in terms of two different crosslinking mechanisms-physical and chemical-and dissect their performance characteristics based on the practical needs of skin-trauma applications. Next, we outline the biological mechanisms involved in the healing of skin wounds and describe the current application strategies for self-healing hydrogels based on these mechanisms. Finally, we analyze and summarize the challenges and prospects of natural-material-based self-healing hydrogels for skin applications.

Recent advances in the repair of degenerative intervertebral disc for preclinical applications

The intervertebral disc (IVD) is a load-bearing, avascular tissue that cushions pressure and increases flexibility in the spine. Under the influence of obesity, injury, and reduced nutrient supply, it develops pathological changes such as fibular annulus (AF) injury, disc herniation, and inflammation, eventually leading to intervertebral disc degeneration (IDD). Lower back pain (LBP) caused by IDD is a severe chronic disorder that severely affects patients' quality of life and has a substantial socioeconomic impact. Patients may consider surgical treatment after conservative treatment has failed. However, the broken AF cannot be repaired after surgery, and the incidence of re-protrusion and reoccurring pain is high, possibly leading to a degeneration of the adjacent vertebrae. Therefore, effective treatment strategies must be explored to repair and prevent IDD. This paper systematically reviews recent advances in repairing IVD, describes its advantages and shortcomings, and explores the future direction of repair technology.

Decellularization of Dense Regular Connective Tissue-Cellular and Molecular Modification with Applications in Regenerative Medicine

Healing of dense regular connective tissue, due to a high fiber-to-cell ratio and low metabolic activity and regeneration potential, frequently requires surgical implantation or reconstruction with high risk of reinjury. An alternative to synthetic implants is using bioscaffolds obtained through decellularization, a process where the aim is to extract cells from the tissue while preserving the tissue-specific native molecular structure of the ECM. Proteins, lipids, nucleic acids and other various extracellular molecules are largely involved in differentiation, proliferation, vascularization and collagen fibers deposit, making them the crucial processes in tissue regeneration. Because of the multiple possible forms of cell extraction, there is no standardized protocol in dense regular connective tissue (DRCT). Many modifications of the structure, shape and composition of the bioscaffold have also been described to improve the therapeutic result following the implantation of decellularized connective tissue. The available data provide a valuable source of crucial information. However, the wide spectrum of decellularization makes it important to understand the key aspects of bioscaffolds relative to their potential use in tissue regeneration.

Kartogenin-enhanced dynamic hydrogel ameliorates intervertebral disc degeneration via restoration of local redox homeostasis

INTRODUCTION

Over-activation of oxidative stress due to impaired antioxidant functions in nucleus pulpous (NP) has been identified as a key factor contributing to intervertebral disc degeneration (IVDD). While Kartogenin (KGN) has previously demonstrated antioxidant properties on articular cartilage against osteoarthritis, its effects on NP degeneration have yet to be fully understood.

OBJECTIVES

This study aimed to investigate the protective effects of KGN on nucleus pulpous cells (NPCs) against an inflammatory environment induced by interleukin (IL)-1β, as well as to explore the therapeutic potential of KGN-enhanced dynamic hydrogel in preventing IVDD.

METHODS

NPCs were isolated from rat caudal IVDs and subjected to treatment with KGN at varying concentrations (ranging from 0.01 to 1 ​μM) in the presence of IL-1β. The expression of extracellular matrix (ECM) anabolism markers was quantitatively assessed at both the mRNA and protein levels. Additionally, intracellular reactive oxygen species and antioxidant enzyme expression were evaluated, along with the role of nuclear factor erythroid 2-related factor 2 (NRF2). Based on these findings, a dynamic self-healing hydrogel loaded with KGN was developed through interconnecting networks. Subsequently, KGN-enhanced dynamic hydrogel was administered into rat caudal IVDs that had undergone puncture injury, followed by radiographic analysis and immunohistochemical staining to evaluate the therapeutic efficacy.

RESULTS

In vitro treatments utilizing KGN were observed to maintain ECM synthesis and inhibit catabolic activities in IL-1β-stimulated NPCs. The mechanism behind this protective effect of KGN on NPCs was found to involve the asctivation of NRF2 and downstream antioxidant enzymes, including glutathione peroxidase 1 and heme oxygenase 1. This was further supported by the loss of both antioxidant and anabolic effects upon pharmacological inhibition of NRF2. Furthermore, a self-healing hydrogel was developed and loaded with KGN to achieve localized and sustained release of the compound. The injection of KGN-enhanced hydrogel effectively ameliorated the degradation of NP ECM and mitigated inflammation in a rat model of puncture-induced IVDD.

CONCLUSIONS

Our results indicate that KGN exhibits potential as a therapeutic agent for NP degeneration, and that KGN-enhanced dynamic hydrogel represents a novel approach for treating IVDD by restoring redox homeostasis in NP.The translational potential of this article: The dysregulation of oxidant and antioxidant balance has been shown to impede the repair and regeneration of NP, thereby hastening the progression of IVDD following injury. The present investigation has demonstrated that the sustained release of KGN promotes the synthesis of ECM in vitro and mitigates the progression of IVDD in vivo by restoring redox equilibrium, thereby presenting a novel therapeutic candidate based on the antioxidant properties of KGN for the treatment of IVDD.

Adiponectin inhibits TGF-β1-induced skin fibroblast proliferation and phenotype transformation via the p38 MAPK signaling pathway

The aim of this study was to investigate the effects of adiponectin (APN) on the proliferation and phenotypic transformation of human skin fibroblasts (HSFs) induced by TGF-β1. Primary fibroblast cultures were collected from prepuce surgery, and the cell viability and proliferative activity of HSFs were detected by Cell Counting Kit-8 and EdU assays. In addition, cell migration was detected by Transwell assay. The protein levels of related genes in HSF were detected by Western blotting. The results showed that the proliferation and migration abilities of HSF in the TGF-β1 group were significantly improved, and the relative protein expression levels of PCNA, α-SMA, and Collagen I in the TGF-β1 group were greatly increased. Furthermore, TGF-β1 stimulated the phosphorylation of p38 in HSF, while APN pretreatment significantly inhibited the TGF-β1-induced phosphorylation of p38. Additionally, blocking the p38 MAPK signaling pathway relieved the injury in the HSF induced by TGF-β1 and enhanced the therapeutic effect of APN in the TGF-β1-treated HSF. In conclusion, APN inhibits TGF-β1-induced HSF proliferation and myofibroblast phenotypic transformation by activating the p38 MAPK signaling pathway. APN is expected to become a potential target for preventing and treating skin fibrosis and pathological scars.

Engineered high-strength biohydrogel as a multifunctional platform to deliver nucleic acid for ameliorating intervertebral disc degeneration

Intervertebral disc degeneration (IVDD) is a leading cause of low back pain. The strategy of using functional materials to deliver nucleic acids provides a powerful tool for ameliorating IVDD. However, the immunogenicity of nucleic acid vectors and the poor mechanical properties of functional materials greatly limit their effects. Herein, antagomir-204-3p (AM) shows low immunogenicity and effectively inhibits the apoptosis of nucleus pulposus cells. Moreover, a high-strength biohydrogel based on zinc-oxidized sodium alginate-gelatin (ZOG) is designed as a multifunctional nucleic acid delivery platform. ZOG loaded with AM (ZOGA) exhibits great hygroscopicity, antibacterial activity, biocompatibility, and biodegradability. Moreover, ZOGA can be cross-linked with nucleus pulposus tissue to form a high-strength collagen network that improves the mechanical properties of the intervertebral disc (IVD). In addition, ZOGA provides an advantageous microenvironment for genetic expression in which AM can play an efficient role in maintaining the metabolic balance of the extracellular matrix. The results of the radiological and histological analyses demonstrate that ZOGA restores the height of the IVD, retains moisture in the IVD, and maintains the tissue structure. The ZOGA platform shows the sustained release of nucleic acids and has the potential for application to ameliorate IVDD, opening a path for future studies related to IVD.

Polymer-Based Hydrogels Applied in Drug Delivery: An Overview

Polymer-based hydrogels are hydrophilic polymer networks with crosslinks widely applied for drug delivery applications because of their ability to hold large amounts of water and biological fluids and control drug release based on their unique physicochemical properties and biocompatibility. Current trends in the development of hydrogel drug delivery systems involve the release of drugs in response to specific triggers such as pH, temperature, or enzymes for targeted drug delivery and to reduce the potential for systemic toxicity. In addition, developing injectable hydrogel formulations that are easily used and sustain drug release during this extended time is a growing interest. Another emerging trend in hydrogel drug delivery is the synthesis of nano hydrogels and other functional substances for improving targeted drug loading and release efficacy. Following these development trends, advanced hydrogels possessing mechanically improved properties, controlled release rates, and biocompatibility is developing as a focus of the field. More complex drug delivery systems such as multi-drug delivery and combination therapies will be developed based on these advancements. In addition, polymer-based hydrogels are gaining increasing attention in personalized medicine because of their ability to be tailored to a specific patient, for example, drug release rates, drug combinations, target-specific drug delivery, improvement of disease treatment effectiveness, and healthcare cost reduction. Overall, hydrogel application is advancing rapidly, towards more efficient and effective drug delivery systems in the future.

Emerging tissue engineering strategies for annulus fibrosus therapy

Low back pain is a major public health concern experienced by 80% of the world's population during their lifetime, which is closely associated with intervertebral disc (IVD) herniation. IVD herniation manifests as the nucleus pulposus (NP) protruding beyond the boundaries of the intervertebral disc due to disruption of the annulus fibrosus (AF). With a deepening understanding of the importance of the AF structure in the pathogenesis of intervertebral disc degeneration, numerous advanced therapeutic strategies for AF based on tissue engineering, cellular regeneration, and gene therapy have emerged. However, there is still no consensus concerning the optimal approach for AF regeneration. In this review, we summarized strategies in the field of AF repair and highlighted ideal cell types and pro-differentiation targeting approaches for AF repair, and discussed the prospects and difficulties of implant systems combining cells and biomaterials to guide future research directions. STATEMENT OF SIGNIFICANCE: Low back pain is a major public health concern experienced by 80% of the world's population during their lifetime, which is closely associated with intervertebral disc (IVD) herniation. However, there is still no consensus concerning the optimal approach for annulus fibrosus (AF) regeneration. In this review, we summarized strategies in the field of AF repair and highlighted ideal cell types and pro-differentiation targeting approaches for AF repair, and discussed the prospects and difficulties of implant systems combining cells and biomaterials to guide future research directions.

Decellularized-disc based allograft and xenograft prosthesis for the long-term precise reconstruction of temporomandibular joint disc

Currently, no effective disc reconstruction treatment strategy is clinically available for temporomandibular joint (TMJ) disc-related diseases. To address this, we developed a prosthesis construct with laser-drilled decellularized natural disc reinforced by polycaprolactone, which mimics the natural morphology, and structural, biomechanical and biological property of the TMJ disc. The construct demonstrated good biocompatibility, safety and immunological tolerance both in vitro, and in a rat subcutaneous model. During 6 months implantation in an allogeneic rabbit TMJ disc reconstruction model, the disc prosthesis maintained its integrity, collagen fiber-orientation, mechanical property, joint structural stability and prevented articular cartilage and bone from damage. Furthermore, the "upgraded" disc prosthesis obtained from decellularized porcine disc was implanted into a goat TMJ disc reconstruction model. The xenograft prosthesis, with strength and viscoelasticity similar to a natural TMJ disc, was able to restore the structure and function of TMJ up to 20 weeks. These results demonstrate the translational feasibility of an allogeneic or xenogeneic decellularized disc prosthesis for treatment of advanced TMJ disc-related diseases. STATEMENT OF SIGNIFICANCE: This study makes a significant contribution to TMJ disc disease treatment both in theory and in clinics, because: (1) it provided an innovative approach to prepare an artificial TMJ disc with decent mechanical properties and long-term condyle-protecting effect; (2) it specified an advanced decellularized method for fibrocartilage decellularization and xenograft application; (3) it developed a facile and reproducible TMJ disc reconstruction model not only for middle size animal but also for large animal study; (4) the comprehensive and unreported biomechanical tests on the natural TMJ discs would act as a valuable reference for further research in the field of artificial TMJ disc materials or TMJ disc tissue engineering; (5) it suggested a potential treatment for patients with severe TMJ diseases that were commonly met but difficult to treat in clinics.

Core-shell oxygen-releasing fibers for annulus fibrosus repair in the intervertebral disc of rats

The repair of annulus fibrosus (AF) defect after discectomy in the intervertebral disc (IVD) has presented a challenge over the past decade. Hostile microenvironments in the IVD, including, compression and hypoxia, are critical issues that require special attention. Till date, little information is available on potential strategies to cope with the hypoxia dilemma in AF defect sites. In this study, perfluorotributylamine (PFTBA) core-shell fibers were fabricated by coaxial electrospinning to construct oxygen-releasing scaffold for promoting endogenous repair in the AF after discectomy. We demonstrated that PFTBA fibers (10% chitosan, chitosan: PCL, 1:6) could release oxygen for up to 144 ​h. The oxygen released from PFTBA fibers was found to protect annulus fibrosus stem cells (AFSCs) from hypoxia-induced apoptosis. In addition, the PFTBA fibers were able to promote proliferation, migration and extracellular matrix (ECM) production in AFSCs under hypoxia, highlighting their therapeutic potential in AF defect repair. Subsequent in vivo studies demonstrated that oxygen-supplying fibers were capable of ameliorating disc degeneration after discectomy, which was evidenced by improved disc height and morphological integrity in rats with the oxygen-releasing scaffolds. Further transcriptome analysis indicated that differential expression genes (DEGs) were enriched in "oxygen transport" and "angiogenesis", which likely contributed to their beneficial effect on endogenous AF regeneration. In summary, the oxygen-releasing scaffold provides novel insights into the oxygen regulation by bioactive materials and raises the therapeutic possibility of oxygen supply strategies for defect repair in AF, as well as other aerobic tissues.

Immune exposure: how macrophages interact with the nucleus pulposus

Intervertebral disc degeneration (IDD) is a primary contributor to low back pain. Immune cells play an extremely important role in modulating the progression of IDD by interacting with disc nucleus pulposus (NP) cells and extracellular matrix (ECM). Encased within the annulus fibrosus, healthy NP is an avascular and immune-privileged tissue that does not normally interact with macrophages. However, under pathological conditions in which neovascularization is established in the damaged disc, NP establishes extensive crosstalk with macrophages, leading to different outcomes depending on the different microenvironmental stimuli. M1 macrophages are a class of immune cells that are predominantly pro-inflammatory and promote inflammation and ECM degradation in the NP, creating a vicious cycle of matrix catabolism that drives IDD. In contrast, NP cells interacting with M2 macrophages promote disc tissue ECM remodeling and repair as M2 macrophages are primarily involved in anti-inflammatory cellular responses. Hence, depending on the crosstalk between NP and the type of immune cells (M1 vs. M2), the overall effects on IDD could be detrimental or regenerative. Drug or surgical treatment of IDD can modulate this crosstalk and hence the different treatment outcomes. This review comprehensively summarizes the interaction between macrophages and NP, aiming to highlight the important role of immunology in disc degeneration.

Decellularized matrix for repairing intervertebral disc degeneration: Fabrication methods, applications and animal models

Intervertebral disc degeneration (IDD)-induced low back pain significantly influences the quality of life, placing a burden on public health systems worldwide. Currently available therapeutic strategies, such as conservative or operative treatment, cannot effectively restore intervertebral disc (IVD) function. Decellularized matrix (DCM) is a tissue-engineered biomaterial fabricated using physical, chemical, and enzymatic technologies to eliminate cells and antigens. By contrast, the extracellular matrix (ECM), including collagen and glycosaminoglycans, which are well retained, have been extensively studied in IVD regeneration. DCM inherits the native architecture and specific-differentiation induction ability of IVD and has demonstrated effectiveness in IVD regeneration in vitro and in vivo. Moreover, significant improvements have been achieved in the preparation process, mechanistic insights, and application of DCM for IDD repair. Herein, we comprehensively summarize and provide an overview of the roles and applications of DCM for IDD repair based on the existing evidence to shed a novel light on the clinical treatment of IDD.

Targeting Ferroptosis Holds Potential for Intervertebral Disc Degeneration Therapy

Intervertebral disc degeneration (IVDD) is a common pathological condition responsible for lower back pain, which can significantly increase economic and social burdens. Although considerable efforts have been made to identify potential mechanisms of disc degeneration, the treatment of IVDD is not satisfactory. Ferroptosis, a recently reported form of regulated cell death (RCD), is characterized by iron-dependent lipid peroxidation and has been demonstrated to be responsible for a variety of degenerative diseases. Accumulating evidence suggests that ferroptosis is implicated in IVDD by decreasing viability and increasing extracellular matrix degradation of nucleus pulposus cells, annulus fibrosus cells, or endplate chondrocytes. In this review, we summarize the literature regarding ferroptosis of intervertebral disc cells and discuss its molecular pathways and biomarkers for treating IVDD. Importantly, ferroptosis is verified as a promising therapeutic target for IVDD.

Aggressive strategies for regenerating intervertebral discs: stimulus-responsive composite hydrogels from single to multiscale delivery systems

As our research on the physiopathology of intervertebral disc degeneration (IVD degeneration, IVDD) has advanced and tissue engineering has rapidly evolved, cell-, biomolecule- and nucleic acid-based hydrogel grafting strategies have been widely investigated for their ability to overcome the harsh microenvironment of IVDD. However, such single delivery systems suffer from excessive external dimensions, difficult performance control, the need for surgical implantation, and difficulty in eliminating degradation products. Stimulus-responsive composite hydrogels have good biocompatibility and controllable mechanical properties and can undergo solution-gel phase transition under certain conditions. Their combination with ready-to-use particles to form a multiscale delivery system may be a breakthrough for regenerative IVD strategies. In this paper, we focus on summarizing the progress of research on the stimulus response mechanisms of regenerative IVD-related biomaterials and their design as macro-, micro- and nanoparticles. Finally, we discuss multi-scale delivery systems as bioinks for bio-3D printing technology for customizing personalized artificial IVDs, which promises to take IVD regenerative strategies to new heights.

Smad7 Is Highly Expressed in Human Degenerative Discs and Participates in IL-1β-Induced Apoptosis of Rat AF Cells via the Mitochondria Pathway

Background. Abnormal Smad7 expression can lead to apoptosis in different cell types. Previously, we found high expression of Smad7 in rat degenerative discs. However, the exact role of Smad7 in the apoptosis of disc cells and the possible underlying mechanism remain unclear. Methods. Degenerative and nondegenerative human lumbar intervertebral discs were collected from patients during operation. The expressions of SMAD7 mRNA and protein in the different components of these discs were measured with real-time PCR and Western blotting, respectively. Annulus fibrosus (AF) cells were isolated and cultivated from the discs of young healthy rats. Smad7 in the AF cells was overexpressed with adenovirus and knocked down with siRNA. IL-1β was used to induce apoptosis in the AF cells. Loss-and-gain cell function experiments were performed to show the effect of Smad7 on the apoptosis of AF cells. The function recovery experiments were performed to verify whether Smad7 regulates the apoptosis of AF cells through the mitochondria-mediated pathway. Results. Both the mRNA and protein expressions of Smad7 were significantly higher in the different components of human degenerative discs than in those of the nondegenerative discs. IL-1β stimulated apoptosis while upregulating the Smad7 expression in the AF cells in vitro. Overexpression of Smad7 in AF cells exaggerated the IL-1β-induced apoptosis in the cells while knockdown of Smad7 expression suppressed this apoptosis. With the exaggerated apoptosis in the AF cells with Smad7 overexpression, both active cleaved caspase-3 and cleaved caspase-9, the ratio of Bax/Bcl-2, and Cyt-c increased significantly. However, the inhibitor of caspase-9, Z-LEHD-FMK, significantly diminished the apoptosis in these cells. Conclusion. Smad7 is highly expressed in human degenerative discs and participates in IL-1β-induced apoptosis of rat AF cells via the mitochondria pathway. Smad7 may be a potential target for the prevention and treatment of degenerative disc disease.