Neonatal mouse intervertebral discs heal with restored function following herniation injury

Scleraxis-expressing progenitor cells are critical for the maturation of the annulus fibrosus and demonstrate therapeutic potential

Background

Annulus fibrosus (AF) is an important part of the intervertebral disc (IVD) and its injury leads to back pain and impaired mobility. The stem/progenitor cells are essential for the maturation and repair of the AF, however, the identity of AF stem/progenitor cells remain elusive.

Methods

In this study, we sorted cells from the murine IVDs and performed the single-cell RNA sequencing. Using single-cell transcriptomics, genetic lineage tracing, in vitro stem cell experiment, ablation models and cell transplantation, we elucidate the role of AF progenitor cells in maturation and injury.

Results

On the basis of single-cell RNA-sequencing (scRNA-seq) analysis of the intervertebral disc, we found that the transcription factor Scleraxis (Scx) can specifically label a progenitor cell population of the outer AF. By lineage tracing assay, Scx-lineage AF cells proliferate mainly prior to sexual maturity, but barely proliferate after age of 8 weeks. The Scx-expressing AF cells are enriched for stem/progenitor cell markers and show a higher proliferative capacity and differentiation potential than the Scx - cells. The ablation of Scx-expressing AF cells impairs the maturation of AF. The Scx + AF cells are enriched for TGFβ signaling. Transplantation of Scx-lineage cells to injured AF with Connective tissue growth factor (CTGF) improved the AF healing.

Conclusions

Scleraxis-expressing progenitor cells are critical for the maturation of AF and demonstrate therapeutic potential for AF regeneration.

The translational potential of this article

These findings expand the important role of stem cells in maturation and repair and provide new strategy for cellular therapy of AF repair.

Sex-specific divergences in the types and timing of infiltrating immune cells during the intervertebral disc acute injury response and their associations with degeneration

OBJECTIVE

Inadequate repair of the intervertebral disc (IVD) contributes to low back pain. Infiltrating immune cells into damaged tissues are critical mediators of repair, yet little is known about the identities, roles, and temporal regulation following IVD injury. By analyzing transcripts of immune cell markers, histopathologic analysis, immunofluorescence, and flow cytometry, we aimed to define the temporal cascade of infiltrating immune cells and their associations with IVD degeneration.

METHODS

Caudal IVDs from 12-week-old C57BL6/J mice were injured and monitored for 42 days post-injury. Transcriptional markers identifying myeloid, B, and T immune cells, and angiogenic factors were measured from the IVDs every 2-3 days. Histopathologic degeneration of the IVD was measured throughout. Flow cytometry and immunofluorescence were used to identify and localize cells including B, T, natural killer T (NKT) cells, monocytes, neutrophils, macrophages, eosinophils, and dendritic cells.

RESULTS

The injured IVD revealed distinct phases of inflammation and proliferation. Robust temporal oscillation in the myeloid and T cell transcripts was observed in females. Cd3+ T cells were more abundant in females than in males. The Cd3+Cd4-Cd8- T cells that dominate the female cascade contain rare γδ T cells. Injury-mediated degeneration was prevalent in both sexes but more severe in males.

CONCLUSIONS

This study defines the coordinated infiltration of immune cells in the IVD following injury. We report the discovery of γδ T cells in the female IVD, and this was associated with less severe degeneration. γδ T cells have potent anti-inflammatory roles and may suppress degeneration following IVD injury.

Regenerative potential of mouse neonatal intervertebral disc depends on collagen crosslink density

Intervertebral disc (IVD) defects heal poorly and can cause back pain and disability. We identified that IVD herniation injury heals regeneratively in neonatal mice until postnatal day 14 (p14) and shifts to fibrotic healing by p28. This age coincides with the shift in expansive IVD growth from cell proliferation to matrix elaboration, implicating collagen crosslinking. β-aminopropionitrile treatment reduced IVD crosslinking and caused fibrotic healing without affecting cell proliferation. Bulk sequencing on naive IVDs was depleted for matrix structural organization from p14 to p28 to validate the importance of crosslinking in regenerative healing. We conclude that matrix changes are key drivers in the shift to fibrotic healing, and a stably crosslinked matrix is needed for IVD regeneration.

Seeing the unseen: The role of bioimaging techniques for the diagnostic interventions in intervertebral disc degeneration

Intervertebral Disc Degeneration is a pathophysiological condition that primarily affects the spinal discs, causing back pain and neurological deficits. It is caused by the contribution of several factors such as genetic predisposition, age-related degeneration, and lifestyle choices like obesity and physical activity. Even though there are medications to treat pain, there is a lack of medicines for a complete cure. The main difficulty lies in poor diagnosis of the morphological and functional changes in the disc. With the ever-increasing research on bioimaging techniques, new techniques are being developed and repurposed to evaluate disc shape and composition, and their defects like thinning or deformities on the disc, leading to the proper diagnostic intervention in intervertebral disc degeneration. In this review, we aim to present a comprehensive overview of the imaging techniques used in the pre-clinical and clinical stages for the diagnosis of intervertebral disc degeneration. First, we will discuss about patho-anatomy and the pathophysiology of degenerative disc disease with the significance and a brief description of various dyes and tracers utilized for bioimaging. Then we will shed light on the latest advancements in diagnostic modalities in intervertebral disc degeneration; concluded by an analysis of the repercussions of the methodologies and experimental systems employed in identifying mechanisms and developing therapeutic strategies in intervertebral disc degeneration.

Developmental morphogens direct human induced pluripotent stem cells toward an annulus fibrosus-like cell phenotype

Introduction

Therapeutic interventions for intervertebral disc herniation remain scarce due to the inability of endogenous annulus fibrosus (AF) cells to respond to injury and drive tissue regeneration. Unlike other orthopedic tissues, such as cartilage, delivery of exogenous cells to the site of annular injury remains underdeveloped, largely due to a lack of an ideal cell source and the invasive nature of cell isolation. Human induced pluripotent stem cells (iPSCs) can be differentiated to specific cell fates using biochemical factors and are, therefore, an invaluable tool for cell therapy approaches. While differentiation protocols have been developed for cartilage and fibrous connective tissues (e.g., tendon), the signals that regulate the induction and differentiation of human iPSCs toward the AF fate remain unknown.

Methods

iPSC-derived sclerotome cells were treated with various combinations of developmental signals including transforming growth factor beta 3 (TGF-β3), connective tissue growth factor (CTGF), platelet derived growth factor BB (PDGF-BB), insulin-like growth factor 1 (IGF-1), or the Hedgehog pathway activator, Purmorphamine, and gene expression changes in major AF-associated ECM genes were assessed. The top performing combination treatments were further validated by using three distinct iPSC lines and by assessing the production of upregulated ECM proteins of interest. To conduct a broader analysis of the transcriptomic shifts elicited by each factor combination, and to compare genetic profiles of treated cells to mature human AF cells, a 96.96 Fluidigm gene expression array was applied, and principal component analysis was employed to identify the transcriptional signatures of each cell population and treatment group in comparison to native AF cells.

Results

TGF-β3, in combination with PDGF-BB, CTGF, or IGF-1, induced an upregulation of key AF ECM genes in iPSC-derived sclerotome cells. In particular, treatment with a combination of TGF-β3 with PDGF-BB for 14 days significantly increased gene expression of collagen II and aggrecan and increased protein deposition of collagen I and elastin compared to other treatment groups. Assessment of genes uniquely highly expressed by AF cells or SCL cells, respectively, revealed a shift toward the genetic profile of AF cells with the addition of TGF-β3 and PDGF-BB for 14 days.

Discussion

These findings represent an initial approach to guide human induced pluripotent stem cells toward an AF-like fate for cellular delivery strategies.

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.

Neonatal Enthesis Healing Involves Noninflammatory Acellular Scar Formation through Extracellular Matrix Secretion by Resident Cells

Wound healing typically recruits the immune and vascular systems to restore tissue structure and function. However, injuries to the enthesis, a hypocellular and avascular tissue, often result in fibrotic scar formation and loss of mechanical properties, severely affecting musculoskeletal function and life quality. This raises questions about the healing capabilities of the enthesis. Herein, this study established an injury model to the Achilles entheses of neonatal mice to study the effectiveness of early-age enthesis healing. Histology and immunohistochemistry analyses revealed an atypical process that did not involve inflammation or angiogenesis. Instead, healing was mediated by secretion of collagen types I and II by resident cells, which formed a permanent hypocellular and avascular scar. Transmission electron microscopy showed that the cellular response to injury, including endoplasmic reticulum stress, autophagy, and cell death, varied between the tendon and cartilage ends of the enthesis. Single-molecule in situ hybridization, immunostaining, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling assays verified these differences. Finally, gait analysis showed that these processes effectively restored function of the injured leg. These findings reveal a novel healing mechanism in neonatal entheses, whereby local extracellular matrix secretion by resident cells forms an acellular extracellular matrix deposit without inflammation, allowing gait restoration. These insights into the healing mechanism of a complex transitional tissue may lead to new therapeutic strategies for adult enthesis injuries.

In vivo Mouse Intervertebral Disc Degeneration Models and Their Utility as Translational Models of Clinical Discogenic Back Pain: A Comparative Review

Low back pain is a leading cause of disability worldwide and studies have demonstrated intervertebral disc (IVD) degeneration as a major risk factor. While many in vitro models have been developed and used to study IVD pathophysiology and therapeutic strategies, the etiology of IVD degeneration is a complex multifactorial process involving crosstalk of nearby tissues and systemic effects. Thus, the use of appropriate in vivo models is necessary to fully understand the associated molecular, structural, and functional changes and how they relate to pain. Mouse models have been widely adopted due to accessibility and ease of genetic manipulation compared to other animal models. Despite their small size, mice lumbar discs demonstrate significant similarities to the human IVD in terms of geometry, structure, and mechanical properties. While several different mouse models of IVD degeneration exist, greater standardization of the methods for inducing degeneration and the development of a consistent set of output measurements could allow mouse models to become a stronger tool for clinical translation. This article reviews current mouse models of IVD degeneration in the context of clinical translation and highlights a critical set of output measurements for studying disease pathology or screening regenerative therapies with an emphasis on pain phenotyping. First, we summarized and categorized these models into genetic, age-related, and mechanically induced. Then, the outcome parameters assessed in these models are compared including, molecular, cellular, functional/structural, and pain assessments for both evoked and spontaneous pain. These comparisons highlight a set of potential key parameters that can be used to validate the model and inform its utility to screen potential therapies for IVD degeneration and their translation to the human condition. As treatment of symptomatic pain is important, this review provides an emphasis on critical pain-like behavior assessments in mice and explores current behavioral assessments relevant to discogenic back pain. Overall, the specific research question was determined to be essential to identify the relevant model with histological staining, imaging, extracellular matrix composition, mechanics, and pain as critical parameters for assessing degeneration and regenerative strategies.

The regenerative capacity of neonatal tissues

It is well established that humans and other mammals are minimally regenerative compared with organisms such as zebrafish, salamander or amphibians. In recent years, however, the identification of regenerative potential in neonatal mouse tissues that normally heal poorly in adults has transformed our understanding of regenerative capacity in mammals. In this Review, we survey the mammalian tissues for which regenerative or improved neonatal healing has been established, including the heart, cochlear hair cells, the brain and spinal cord, and dense connective tissues. We also highlight common and/or tissue-specific mechanisms of neonatal regeneration, which involve cells, signaling pathways, extracellular matrix, immune cells and other factors. The identification of such common features across neonatal tissues may direct therapeutic strategies that will be broadly applicable to multiple adult tissues.

Extracellular Vesicles as an Emerging Treatment Option for Intervertebral Disc Degeneration: Therapeutic Potential, Translational Pathways, and Regulatory Considerations

Emergent approaches in regenerative medicine look toward the use of extracellular vesicles (EVs) as a next-generation treatment strategy for intervertebral disc (IVD) degeneration (IVDD) because of their ability to attenuate chronic inflammation, reduce apoptosis, and stimulate proliferation in a number of tissue systems. Yet, there are no Food and Drug Administration (FDA)-approved EV therapeutics in the market with an indication for IVDD, which motivates this article to review the current state of the field and provide an IVD-specific framework to assess its efficacy. In this systematic review, 29 preclinical studies that investigate EVs in relation to the IVD are identified, and additionally, the regulatory approval process is reviewed in an effort to accelerate emerging EV-based therapeutics toward FDA submission and timeline-to-market. The majority of studies focus on nucleus pulposus responses to EV treatment, where the main findings show that stem cell-derived EVs can decelerate the progression of IVDD on the molecular, cellular, and organ level. The findings also highlight the importance of the EV parent cell's pathophysiological and differentiation state, which affects downstream treatment responses and therapeutic outcomes. This systematic review substantiates the use of EVs as a promising cell-free strategy to treat IVDD and enhance endogenous repair.

Tenomodulin and Chondromodulin-1 Are Both Required to Maintain Biomechanical Function and Prevent Intervertebral Disc Degeneration

OBJECTIVE

The underlying mechanisms and molecular factors influencing intervertebral disc (IVD) homeostasis and degeneration remain clinically relevant. Tenomodulin (Tnmd) and chondromodulin (Chm1) are antiangiogenic transmembrane glycoproteins, with cleavable C-terminus, expressed by IVD cells that are implicated in the onset of degenerative processes. We evaluate the organ-level biomechanical impact of knocking out Tnmd alone, and Tnmd and Chm1, simultaneously.

DESIGN

Caudal (c5-8) and lumbar vertebrae (L1-4) of skeletally mature male and female 9-month-old wildtype (WT), Tnmd knockout (Tnmd^-/-), and Tnmd/Chm1 double knockout (Tnmd^-/-/Chm^-/-) mice were used (n = 9-13 per group). Disc height index (DHI), histomorphological changes, and axial, torsional, creep, and failure biomechanical properties were evaluated. Differences were assessed by one-way ANOVA with post hoc Bonferroni-corrected comparisons (P < 0.05).

RESULTS

Tnmd^-/-/Chm1^-/- IVDs displayed increased DHI and histomorphological scores that indicated increased IVD degeneration compared to the WT and Tnmd^-/- groups. Double knockout IVDs required significantly less torque and energy to initiate torsional failure. Creep parameters were comparable between all groups, except for the slow time constant, which indicated faster outward fluid flow. Tnmd^-/- IVDs lost fluid faster than the WT group, and this effect was amplified in the double knockout IVDs.

CONCLUSION

Knocking out Tnmd and Chm1 affects IVD fluid flow and organ-level biomechanical function and therefore may play a role in contributing to IVD degeneration. Larger effects of the Tnmd and Chm1 double knockout mice compared to the Tnmd single mutant suggest that Chm1 may play a compensatory role in the Tnmd single mutant IVDs.

Cartilage articulation exacerbates chondrocyte damage and death after impact injury

Posttraumatic osteoarthritis (PTOA) is typically initiated by momentary supraphysiologic shear and compressive forces delivered to articular cartilage during acute joint injury and develops through subsequent degradation of cartilage matrix components and tissue remodeling. PTOA affects 12% of the population who experience osteoarthritis and is attributed to over $3 billion dollars annually in healthcare costs. It is currently unknown whether articulation of the joint post-injury helps tissue healing or exacerbates cellular dysfunction and eventual death. We hypothesize that post-injury cartilage articulation will lead to increased cartilage damage. Our objective was to test this hypothesis by mimicking the mechanical environment of the joint during and post-injury and determining if subsequent joint articulation exacerbates damage produced by initial injury. We use a model of PTOA that combines impact injury and repetitive sliding with confocal microscopy to quantify and track chondrocyte viability, apoptosis, and mitochondrial depolarization in a depth-dependent manner. Cartilage explants were harvested from neonatal bovine knee joints and subjected to either rapid impact injury (17.34 ± 0.99 MPa, 21.6 ± 2.45 GPa/s), sliding (60 min at 1 mm/s, under 15% axial compression), or rapid impact injury followed by sliding. Explants were then bisected and fluorescently stained for cell viability, caspase activity (apoptosis), and mitochondria polarization. Results show that compared to either impact or sliding alone, explants that were both impacted and slid experienced higher magnitudes of damage spanning greater tissue depths.

Putting the Pieces in Place: Mobilizing Cellular Players to Improve Annulus Fibrosus Repair

The intervertebral disc (IVD) is an integral load-bearing tissue that derives its function from its composite structure and extracellular matrix composition. IVD herniations involve the failure of the annulus fibrosus (AF) and the extrusion of the nucleus pulposus beyond the disc boundary. Disc herniations can impinge the neural elements and cause debilitating pain and loss of function, posing a significant burden on individual patients and society as a whole. Patients with persistent symptoms may require surgery; however, surgical intervention fails to repair the ruptured AF and is associated with the risk for reherniation and further disc degeneration. Given the limitations of AF endogenous repair, many attempts have been made toward the development of effective repair approaches that reestablish IVD function. These methods, however, fail to recapitulate the composition and organization of the native AF, ultimately resulting in inferior tissue mechanics and function over time and high rates of reherniation. Harnessing the cellular function of cells (endogenous or exogenous) at the repair site through the provision of cell-instructive cues could enhance AF tissue regeneration and, ultimately, improve healing outcomes. In this study, we review the diverse approaches that have been developed for AF repair and emphasize the potential for mobilizing the appropriate cellular players at the site of injury to improve AF healing. Impact statement Conventional treatments for intervertebral disc herniation fail to repair the annulus fibrosus (AF), increasing the risk for recurrent herniation. The lack of repair devices in the market has spurred the development of regenerative approaches, yet most of these rely on a scarce endogenous cell population to repair large injuries, resulting in inadequate regeneration. This review identifies current and developing strategies for AF repair and highlights the potential for harnessing cellular function to improve AF regeneration. Ideal cell sources, differentiation strategies, and delivery methods are discussed to guide the design of repair systems that leverage specialized cells to achieve superior outcomes.

Transcriptional profiling of mESC-derived tendon and fibrocartilage cell fate switch

The transcriptional regulators underlying induction and differentiation of dense connective tissues such as tendon and related fibrocartilaginous tissues (meniscus and annulus fibrosus) remain largely unknown. Using an iterative approach informed by developmental cues and single cell RNA sequencing (scRNA-seq), we establish directed differentiation models to generate tendon and fibrocartilage cells from mouse embryonic stem cells (mESCs) by activation of TGFβ and hedgehog pathways, achieving 90% induction efficiency. Transcriptional signatures of the mESC-derived cells recapitulate embryonic tendon and fibrocartilage signatures from the mouse tail. scRNA-seq further identify retinoic acid signaling as a critical regulator of cell fate switch between TGFβ-induced tendon and fibrocartilage lineages. Trajectory analysis by RNA sequencing define transcriptional modules underlying tendon and fibrocartilage fate induction and identify molecules associated with lineage-specific differentiation. Finally, we successfully generate 3-dimensional engineered tissues using these differentiation protocols and show activation of mechanotransduction markers with dynamic tensile loading. These findings provide a serum-free approach to generate tendon and fibrocartilage cells and tissues at high efficiency for modeling development and disease.

Type II collagen-positive embryonic progenitors are the major contributors to spine and intervertebral disc development and repair

Type II collagen-positive embryonic progenitors are the major contributors to spine and intervertebral disc development and repair has been removed because it was published by mistake. The article will be published on October 1, 2021.

A challenging playing field: Identifying the endogenous impediments to annulus fibrosus repair

Intervertebral disc (IVD) herniations, caused by annulus fibrosus (AF) tears that enable disc tissue extrusion beyond the disc space, are very prevalent, especially among adults in the third to fifth decade of life. Symptomatic herniations, in which the extruded tissue compresses surrounding nerves, are characterized by back pain, numbness, and tingling and can cause extreme physical disability. Patients whose symptoms persist after nonoperative intervention may undergo surgical removal of the herniated tissue via microdiscectomy surgery. The AF, however, which has a poor endogenous healing ability, is left unrepaired increasing the risk for re-herniation and pre-disposing the IVD to degenerative disc disease. The lack of understanding of the mechanisms involved in native AF repair limits the design of repair systems that overcome the impediments to successful AF restoration. Moreover, the complexity of the AF structure and the challenging anatomy of the repair environment represents a significant challenge for the design of new repair devices. While progress has been made towards the development of an effective AF repair technique, these methods have yet to demonstrate long-term repair and recovery of IVD biomechanics. In this review, the limitations of endogenous AF healing are discussed and key cellular events and factors involved are highlighted to identify potential therapeutic targets that can be integrated into AF repair methods. Clinical repair strategies and their limitations are described to further guide the design of repair approaches that effectively restore native tissue structure and function.

The Functional Role of Interface Tissue Engineering in Annulus Fibrosus Repair: Bridging Mechanisms of Hydrogel Integration with Regenerative Outcomes

Hydrogels are extraordinarily versatile by design and can enhance repair in diseased and injured musculoskeletal tissues. Biological fixation of these constructs is a significant determinant factor that is critical to the clinical success and functionality of regenerative technologies for musculoskeletal repair. In the context of an intervertebral disc (IVD) herniation, nucleus pulposus tissue protrudes through the ruptured annulus fibrosus (AF), consequentially impinging on spinal nerve roots and causing debilitating pain. Discectomy is the surgical standard of care to treat symptomatic herniation; however these procedures do not repair AF defects, and these lesions are a significant risk factor for recurrent herniation. Advances in tissue engineering utilize adhesive hydrogels as AF sealants; however these repair strategies have yet to progress beyond preclinical animal models because these biomaterials are often plagued by poor integration with AF tissue and lead to large variability in repair outcomes. These critical barriers to translation motivate this article to review the material composition of hydrogels that have been evaluated in situ for AF repair, proposed mechanisms of how these biomaterials interface with AF tissue, and their functional outcomes after treatment in order to inform the development of new hydrogels for AF repair. In this systematic review, we identify 18 hydrogel formulations evaluated for AF repair, all of which demonstrate large heterogeneity in their interfacing mechanisms and reported outcome measures to assess the effectiveness of repair. Hydrogels that covalently bond to AF tissue were found to be the most successful in improving IVD biomechanical properties from the injured state, but none were able to restore properties to the intact state suggesting that new repair strategies with innovative surface chemistries are an important future direction. We additionally review biomechanical evaluation methods and recommend standardization in the field of AF tissue engineering to establish mechanical benchmarks for translation and ensure clinical feasibility.

Advanced glycation end products cause RAGE-dependent annulus fibrosus collagen disruption and loss identified using in situ second harmonic generation imaging in mice intervertebral disk in vivo and in organ culture models

Aging and diabetes are associated with increased low-back pain and intervertebral disk (IVD) degeneration yet causal mechanisms remain uncertain. Advanced glycation end products (AGEs), which accumulate in IVDs from aging and are implicated in diabetes-related disorders, alter collagen and induce proinflammatory conditions. A need exists for methods that assess IVD collagen quality and degradation in order to better characterize specific structural changes in IVDs due to AGE accumulation and to identify roles for the receptor for AGEs (RAGE). We used multiphoton microscopy with second harmonic generation (SHG), collagen-hybridizing peptide (CHP), and image analysis methods to characterize effects of AGEs and RAGE on collagen quality and quantity in IVD annulus fibrosus (AF). First, we used SHG imaging on thin sections with an in vivo dietary mouse model and determined that high-AGE (H-AGE) diets increased AF fibril disruption and collagen degradation resulting in decreased total collagen content, suggesting an early degenerative cascade. Next, we used in situ SHG imaging with an ex vivo IVD organ culture model of AGE challenge on wild type and RAGE-knockout (RAGE-KO) mice and determined that early degenerative changes to collagen quality and degradation were RAGE dependent. We conclude that AGE accumulation leads to RAGE-dependent collagen disruption in the AF and can initiate molecular and tissue level collagen disruption. Furthermore, SHG and CHP analyzes were sensitive to collagenous alterations at multiple hierarchical levels due to AGE and may be useful in identifying additional contributors to collagen damage in IVD degeneration processes.

Understanding embryonic development for cell-based therapies of intervertebral disc degeneration: Toward an effort to treat disc degeneration subphenotypes

Chronic low back and neck pain are associated with intervertebral disc degeneration and are major contributors to the global burden of disability. New evidence now suggests that disc degeneration comprises a spectrum of subphenotypes influenced by genetic background, age, and environmental factors, which may be contributing to the mixed outcomes seen in clinical trials of cell-based therapies that aim to treat disc degeneration. This problem is further compounded by the fact that disc degeneration and aging coincide with an exhaustion of endogenous progenitor cells, imposing limitations on the regenerative capacity of the disc. At the bench-side, current work is focused on applying our knowledge of embryonic disc development to direct and refine differentiation of adult and human-induced pluripotent stem cells into notochord-like and nucleus pulposus-like cells for use in novel cell-based therapies. Accordingly, this review presents the salient features of intervertebral disc development, post-natal maintenance, and regeneration, with emphasis on recent advancements. We also discuss how a stratified approach can be undertaken for the development of future cell-based therapies to bring emerging subphenotypes into consideration.

Leptin signaling and the intervertebral disc: Sex dependent effects of leptin receptor deficiency and Western diet on the spine in a type 2 diabetes mouse model

Type 2 diabetes and obesity are associated with back pain in juveniles and adults and are implicated in intervertebral disc (IVD) degeneration. Hypercaloric Western diets are associated with both obesity and type 2 diabetes. The objective of this study was to determine if obesity and type 2 diabetes result in spinal pathology in a sex-specific manner using in vivo diabetic and dietary mouse models. Leptin is an appetite-regulating hormone, and its deficiency leads to polyphagia, resulting in obesity and diabetes. Leptin is also associated with IVD degeneration, and increased expression of its receptor was identified in degenerated IVDs. We used young, leptin receptor deficient (Db/Db) mice to mimic the effect of diet and diabetes on adolescents. Db/Db and Control mice were fed either Western or Control diets, and were sacrificed at 3 months of age. Db/Db mice were obese, while only female mice developed diabetes. Female Db/Db mice displayed altered IVD morphology, with increased intradiscal notochordal band area, suggesting delayed IVD cell proliferation and differentiation, rather than IVD degeneration. Motion segments from Db/Db mice exhibited increased failure risk with decreased torsional failure strength. Db/Db mice also had inferior bone quality, which was most prominent in females. We conclude that obesity and diabetes due to impaired leptin signaling contribute to pathological changes in vertebrae, as well as an immature IVD phenotype, particularly of females, suggesting a sex-dependent role of leptin in the spine.

Ciliary IFT80 is essential for intervertebral disc development and maintenance

The intervertebral disc degeneration (IVDD)-related diseases occur in more than 90% of the population older than 50 years. Owing to the lack of understanding of the cellular mechanisms involved in IVDD formation effective treatment options are still unavailable. Primary cilia are microtubule-based organelles that play important roles in the organ development. Intraflagellar transport (IFT) proteins are essential for the assembly and bidirectional transport within the cilium. Role of cilia and IFT80 protein in intervertebral disc (IVD) development, maintenance, and degeneration are largely unknown. Using cilia-GFP mice, we found presence of cilia on growth plate (GP), cartilage endplate (EP) annulus fibrosus (AF), and nucleus pulposus (NP) with varying ciliary length. Cilia length in NP and AF during IVDD were significantly decreased. However, cilia numbers increased by 63% in AF during repair. Deletion of IFT80 in type II collagen-positive cells resulted in cilia loss in GP and EP, and disrupted IVD structure with disorganized and decreased GP, EP, and internal AF (IAF), and less compact and markedly decreased gel-like matrix in the NP. Deletion of IFT80 in type I collagen-positive cells led to a disorganized outer AF (OAF) with thinner, loosened, and disconnected fiber alignment. Mechanistic analyses showed that loss of IFT80 caused a significant increase in cell apoptosis in the IVD, and a marked decrease in expression of chondrogenic markers - type II collagen, sox9, aggrecan, and hedgehog (Hh) signaling components, including Gli1 and Patch1 in the IVD of IFT80^fl/fl ; Col2-creERT mice, and Gli1 and Patch1 expression in the OAF of IFT80^fl/fl ; Col1-creERT mice. Interestingly, Smoothened agonist-SAG rescued OAF cell proliferation and osteogenic differentiation. Our findings demonstrate that ciliary IFT80 is important for the maintenance of IVD cell organization and function through regulating the cell survival and Hh signaling.

Functional cell phenotype induction with TGF-β1 and collagen-polyurethane scaffold for annulus fibrosus rupture repair

Appropriate cell sources, bioactive factors and biomaterials for generation of functional and integrated annulus fibrosus (AF) tissue analogues are still an unmet need. In the present study, the AF cell markers, collagen type I, cluster of differentiation 146 (CD146), mohawk (MKX) and smooth muscle protein 22α (SM22α) were found to be suitable indicators of functional AF cell induction. In vitro 2D culture of human AF cells showed that transforming growth factor β1 (TGF-β1) upregulated the expression of the functional AF markers and increased cell contractility, indicating that TGF-β1-pre-treated AF cells were an appropriate cell source for AF tissue regeneration. Furthermore, a tissue engineered construct, composed of polyurethane (PU) scaffold with a TGF-β1-supplemented collagen type I hydrogel and human AF cells, was evaluated with in vitro 3D culture and ex vivo preclinical bioreactor-loaded organ culture models. The collagen type I hydrogel helped maintaining the AF functional phenotype. TGF-β1 supplement within the collagen I hydrogel further promoted cell proliferation and matrix production of AF cells within in vitro 3D culture. In the ex vivo IVD organ culture model with physiologically relevant mechanical loading, TGF-β1 supplement in the transplanted constructs induced the functional AF cell phenotype and enhanced collagen matrix synthesis. In conclusion, TGF-β1-containing collagen-PU constructs can induce the functional cell phenotype of human AF cells in vitro and in situ. This combined cellular, biomaterial and bioactive agent therapy has a great potential for AF tissue regeneration and rupture repair.

Aging aggravates intervertebral disc degeneration by regulating transcription factors toward chondrogenesis

Osterix is a critical transcription factor of mesenchymal stem cell fate, where its loss or loss of Wnt signaling diverts differentiation to a chondrocytic lineage. Intervertebral disc (IVD) degeneration activates the differentiation of prehypertrophic chondrocyte-like cells and inactivates Wnt signaling, but its interactive role with osterix is unclear. First, compared to young-adult (5 mo), mechanical compression of old (18 mo) IVD induced greater IVD degeneration. Aging (5 vs 12 mo) and/or compression reduced the transcription of osterix and notochordal marker T by 40-75%. Compression elevated the transcription of hypertrophic chondrocyte marker MMP13 and pre-osterix transcription factor RUNX2, but less so in 12 mo IVD. Next, using an Ai9/td reporter and immunohistochemical staining, annulus fibrosus and nucleus pulposus cells of young-adult IVD expressed osterix, but aging and compression reduced its expression. Lastly, in vivo LRP5-deficiency in osterix-expressing cells inactivated Wnt signaling in the nucleus pulposus by 95%, degenerated the IVD to levels similar to aging and compression, reduced the biomechanical properties by 45-70%, and reduced the transcription of osterix, notochordal markers and chondrocytic markers by 60-80%. Overall, these data indicate that age-related inactivation of Wnt signaling in osterix-expressing cells may limit regeneration by depleting the progenitors and attenuating the expansion of chondrocyte-like cells.

Neonatal annulus fibrosus regeneration occurs via recruitment and proliferation of Scleraxis-lineage cells

Intervertebral disc (IVD) injuries are a cause of degenerative changes in adults which can lead to back pain, a leading cause of disability. We developed a model of neonatal IVD regeneration with full functional restoration and investigate the cellular dynamics underlying this unique healing response. We employed genetic lineage tracing in mice using Scleraxis (Scx) and Sonic hedgehog (Shh) to fate-map annulus fibrosus (AF) and nucleus pulposus (NP) cells, respectively. Results indicate functional AF regeneration after severe herniation injury occurs in neonates and not adults. AF regeneration is mediated by Scx-lineage cells that lose ScxGFP expression and adopt a stem/progenitor phenotype (Sca-1, days 3–14), proliferate, and then redifferentiate towards type I collagen producing, ScxGFP+ annulocytes at day 56. Non Scx-lineage cells were also transiently observed during neonatal repair, including Shh-lineage cells, macrophages, and myofibroblasts; however, these populations were no longer detected by day 56 when annulocytes redifferentiate. Overall, repair did not occur in adults. These results identify an exciting cellular mechanism of neonatal AF regeneration that is predominantly driven by Scx-lineage annulocytes.

Multiscale and multimodal structure-function analysis of intervertebral disc degeneration in a rabbit model

OBJECTIVES

The objective of this study was to perform a quantitative analysis of the structural and functional alterations in the intervertebral disc during in vivo degeneration, using emerging tools that enable rigorous assessment from the microscale to the macroscale, as well as to correlate these outcomes with noninvasive, clinically relevant imaging parameters.

DESIGN

Degeneration was induced in a rabbit model by puncturing the annulus fibrosus (AF) with a 16-gauge needle. 2, 4, 8, and 12 weeks following puncture, degenerative changes in the discs were evaluated via magnetic resonance imaging (MRI), whole motion segment biomechanics, atomic force microscopy, histology and polarized light microscopy, immunohistochemistry, biochemical content, and second harmonic generation imaging.

RESULTS

Following puncture, degeneration was evident through marked changes in whole disc structure and mechanics. Puncture acutely compromised disc macro and microscale mechanics, followed by progressive stiffening and remodeling. Histological analysis showed substantial anterior fibrotic remodeling and osteophyte formation, as well as an overall reduction in disc height, and disorganization and infolding of the AF lamellae into the NP space. Increases in NP collagen content and aggrecan breakdown products were also noted within 4 weeks. On MRI, NP T2 was reduced at all post-puncture time points and correlated significantly with microscale indentation modulus.

CONCLUSION

This study defined the time dependent changes in disc structure-function relationships during IVD degeneration in a rabbit annular injury model and correlated degeneration severity with clinical imaging parameters. Our findings identified AF infolding and occupancy of the space as a principle mechanism of disc degeneration in response to needle puncture, and provide new insights to direct the development of novel therapeutics.

Phlpp1 is associated with human intervertebral disc degeneration and its deficiency promotes healing after needle puncture injury in mice

Back pain is a leading cause of global disability and is strongly associated with intervertebral disc (IVD) degeneration (IDD). Hallmarks of IDD include progressive cell loss and matrix degradation. The Akt signaling pathway regulates cellularity and matrix production in IVDs and its inactivation is known to contribute to a catabolic shift and increased cell loss via apoptosis. The PH domain leucine-rich repeat protein phosphatase (Phlpp1) directly regulates Akt signaling and therefore may play a role in regulating IDD, yet this has not been investigated. The aim of this study was to investigate if Phlpp1 has a role in Akt dysregulation during IDD. In human IVDs, Phlpp1 expression was positively correlated with IDD and the apoptosis marker cleaved Caspase-3, suggesting a key role of Phlpp1 in the progression of IDD. In mice, 3 days after IVD needle puncture injury, Phlpp1 knockout (KO) promoted Akt phosphorylation and cell proliferation, with less apoptosis. At 2 and 8 months after injury, Phlpp1 deficiency also had protective effects on IVD cellularity, matrix production, and collagen structure as measured with histological and immunohistochemical analyses. Specifically, Phlpp1-deletion resulted in enhanced nucleus pulposus matrix production and more chondrocytic cells at 2 months, and increased IVD height, nucleus pulposus cellularity, and extracellular matrix deposition 8 months after injury. In conclusion, Phlpp1 has a role in limiting cell survival and matrix degradation in IDD and research targeting its suppression could identify a potential therapeutic target for IDD.

Multi-laminate annulus fibrosus repair scaffold with an interlamellar matrix enhances impact resistance, prevents herniation and assists in restoring spinal kinematics

Focal defects in the annulus fibrosus (AF) of the intervertebral disc (IVD) arising from herniation have detrimental impacts on the IVD's mechanical function. Thus, biomimetic-based repair strategies must restore the mechanical integrity of the AF to help support and restore native spinal loading and motion. Accordingly, an annulus fibrosus repair patch (AFRP); a collagen-based multi-laminate scaffold with an angle-ply architecture has been previously developed, which demonstrates similar mechanical properties to native outer AF (oAF). To further enhance the mimetic nature of the AFRP, interlamellar (ILM) glycosaminoglycan (GAG) was incorporated into the scaffolds. The ability of the scaffolds to withstand simulated impact loading and resist herniation of native IVD tissue while contributing to the restoration of spinal kinematics were assessed separately. The results demonstrate that incorporation of a GAG-based ILM significantly increased (p < 0.001) the impact strength of the AFRP (2.57 ± 0.04 MPa) compared to scaffolds without (1.51 ± 0.13 MPa). Additionally, repair of injured functional spinal units (FSUs) with an AFRP in combination with sequestering native NP tissue and a full-thickness AF tissue plug enabled the restoration of creep displacement (p = 0.134), short-term viscous damping coefficient (p = 0.538), the long-term viscous (p = 0.058) and elastic (p = 0.751) damping coefficients, axial neutral zone (p = 0.908), and axial range of motion (p = 0.476) to an intact state. Lastly, the AFRP scaffolds were able to prevent native IVD tissue herniation upon application of supraphysiologic loads (5.28 ± 1.24 MPa). Together, these results suggest that the AFRP has the strength to sequester native NP and AF tissue and/or implants, and thus, can be used in a composite repair strategy for IVDs with focal annular defects thereby assisting in the restoration of spinal kinematics.

Defects in intervertebral disc and spine during development, degeneration, and pain: New research directions for disc regeneration and therapy

Intervertebral discs are cartilaginous joints present between vertebrae. The centers of the intervertebral discs consist of a gelatinous nucleus pulposus derived from the embryonic notochord. With age or injury, intervertebral discs may degenerate, causing neurological symptoms including back pain, which affects millions of people worldwide. Back pain is a multifactorial disorder, and disc degeneration is one of the primary contributing factors. Recent studies in mice have identified the key molecules involved in the formation of intervertebral discs. Several of these key molecules including sonic hedgehog and Brachyury are not only expressed by notochord during development, but are also expressed by neonatal mouse nucleus pulposus cells, and are crucial for postnatal disc maintenance. These findings suggest that intrinsic signals in each disc may maintain the nucleus pulposus microenvironment. However, since expression of these developmental signals declines with age and degeneration, disc degeneration may be related to the loss of these intrinsic signals. In addition, findings from mouse and other mammalian models have identified similarities between the patterning capabilities of the embryonic notochord and young nucleus pulposus cells, suggesting that mouse is a suitable model system to understand disc development and aging. Future research aimed at understanding the upstream regulators of these developmental signals and the modes by which they regulate disc growth and maintenance will likely provide mechanistic insights into disc growth and aging. Further, such findings will likely provide insights relevant to the development of effective therapies for treatment of back pain and reversing the disc degenerative process. This article is categorized under: Birth Defects > Organ Anomalies Vertebrate Organogenesis > Musculoskeletal and Vascular Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Adult Stem Cells, Tissue Renewal, and Regeneration > Stem Cells and Aging.

Biomechanical test protocols to detect minor injury effects in intervertebral discs

Intervertebral discs (IVDs) maintain flexibility of the spine and bear mechanical load. Annulus fibrosus (AF) defects are associated with IVD degeneration and herniation which disrupt biomechanical function and can cause pain. AF puncture injuries can induce IVD degeneration but are needed to inject therapies. Identifying small AF defects with biomechanical testing can be difficult because IVDs have a complex, composite structure and nonlinear biomechanical properties that are dependent on AF fiber tension. It remains unclear how choice of biomechanical testing protocols affect the sensitivity of biomechanical properties to AF injuries. This study determined whether axial preload or magnitude of cyclic axial or torsional testing affected the ability to detect minor AF defects in rat caudal motion segments using ex vivo biomechanical testing. Intact and injured motion segments were subjected to a repeated measures study design with multiple biomechanical testing protocols that varied axial tension-compression force amplitude (±1.6 N, ±8.0 N, ±16.0 N), axial preload (-1.6 N, -8.0 N, -16.0 N, corresponding to -0.1 MPa, -0.5 MPa, and -1.0 MPa, respectively), and torsional rotation angle (±10°, ±15°, and ±20°). Biomechanical properties obtained from the lowest force testing conditions for axial tension-compression (±1.6 N), axial preload (-1.6 N), and angular rotation (±10°) exhibited the largest differences in biomechanical properties between intact and injured conditions. Biomechanical properties determined under low axial force or torsion amplitudes involve less AF fiber tension and were most sensitive to injury. Low force testing protocols are recommended for detecting minor structural AF defects and may enable more precise assessments of IVD injuries, healing or repair.

Annulus fibrosus cell phenotypes in homeostasis and injury: implications for regenerative strategies

Despite considerable efforts to develop cellular, molecular, and structural repair strategies and restore intervertebral disk function after injury, the basic biology underlying intervertebral disk healing remains poorly understood. Remarkably, little is known about the origins of cell populations residing within the annulus fibrosus, or their phenotypes, heterogeneity, and roles during healing. This review focuses on recent literature highlighting the intrinsic and extrinsic cell types of the annulus fibrosus in the context of the injury and healing environment. Spatial, morphological, functional, and transcriptional signatures of annulus fibrosus cells are reviewed, including inner and outer annulus fibrosus cells, which we propose to be referred to as annulocytes. The annulus also contains peripheral cells, interlamellar cells, and potential resident stem/progenitor cells, as well as macrophages, T lymphocytes, and mast cells following injury. Phases of annulus fibrosus healing include inflammation and recruitment of immune cells, cell proliferation, granulation tissue formation, and matrix remodeling. However, annulus fibrosus healing commonly involves limited remodeling, with granulation tissues remaining, and the development of chronic inflammatory states. Identifying annulus fibrosus cell phenotypes during health, injury, and degeneration will inform reparative regeneration strategies aimed at improving annulus fibrosus healing.