Abstract
Intervertebral disk (IVD) degeneration is a common, chronic, and complex degeneration process that frequently leads to back pain and disability, resulting in a major public health issue. In this review we describe biological therapies under preclinical or clinical development with an emphasis on stem cell-based multimodal approaches that target prevention and treatment of IVD degeneration. Systematical review of the basic science and clinical literature was performed to summarize the current status of devising biological approaches to treating IVD degeneration. Since the exact mechanisms underlying IVD degeneration have not yet been fully elucidated and conservative managements appear to be mostly ineffective, current surgical treatment focuses on removal of the pathological disk tissues combined with spinal fusion. The treatment options, however, often produce insufficient efficacy and even serious complications. Therefore, there have been growing demands and endeavors for developing novel regenerative biology-guided strategies for repairing the IVD via delivery of exogenous growth factors, introduction of therapeutic genes, and transplantation of stem cells, or combinatorial therapies. Overall, the data suggest that when applied under a recovery neurobiology principle, multimodal regimens comprising ex vivo engineered stem cell-based disks hold a high potential promise for efficacious clinical translations.
Introduction
About 80% of the global population experience at least one episode of low back pain (or lumbago) at some point during their lifetime, and low back pain is a leading cause of adulthood disability. Intervertebral disk (IVD) degeneration is a leading cause of low back pain, even though the definitive etiology of IVD pathology per se still remains largely unclear (38,87). In general, conservative treatments for IVD degeneration such as medication or physical therapy are presently used as the first-line management for neck or back pain (33,55). When conservative interventions fail, a necessity for surgical treatments may be indicated. Currently, the most common surgical therapy is diskectomy/discectomy (excision of the degenerated disk) alone or in combination with a spinal fusion procedure. Though encouraging symptom improvement is often encountered following surgeries, most symptomatic relief lasts for a limited time course, and the postdiskectomy environment has been speculated as a causal factor for nerve root compression, acceleration of disk degeneration, and de novo adjacent IVD disorders, which could result in recurrent back and lower limb pain. In spinal fusion treatment, the entire IVD is surgically removed, and the disk implant is inserted between the two vertebrae for providing the immobilization and support of the adjacent vertebrae. However, drawbacks to the fusion procedure include diminished flexibility and the potential to develop recurrent severe back pain due to an acceleration of disk degeneration adjacent to the fused segments (96). The current opinion seems to be that there may be no major outcome differences between different surgical procedures when assessed 1 year after operation. Moreover, the efficacy of standard nonsurgical treatments is also questioned (96).
In response to an increasing recognition of efficacy limitations and complications, disk anthroplasty (i.e., total disk replacement) has been developed with the aim of retaining normal spinal biomechanics, which can be lost following a traditional, rigid fixation treatment; nevertheless, mechanical devices for artificial disk replacements are suitable only for a small fraction of patients with IVD degeneration, and there is evidence of many potential consequential complications including device migration, prolapsed implants, and even partial or complete failure of the artificial prostheses (72). Clearly, current surgical treatments for symptomatic IVD degeneration primarily focus on relieving back pain, rather than arresting the progression of the degeneration by specific inhibition of the underlying disease mechanisms or biologically rebuilding the disk structure and function. Therefore, there is a strong clinical demand for developing biological approaches to impeding degeneration and/or protecting and regenerating the degenerated IVD to cure back pain with optimized functional recovery. The need to develop truly efficacious treatment for IVD degeneration becomes more imperative due to the global trends in population aging. Among the effective biological approaches developed to date, stem cell-based therapies are leading the invention endeavors of regenerative strategies for repairing the degenerated disk. In this review we emphasize stem cell-based therapeutic strategies together with systematic discussions of other biological therapy initiatives for treating degenerative IVD under the principles of functional multipotency of stem cells and recovery neurobiology that may help to reach the goal of efficacious application of biological disks for patients (80).
Pathophysiology of IVD Degeneration
In contrast to articular cartilage, the IVD is a well-encapsulated and avascular tissue that has three different components [i.e., the nucleus pulposus (NP), annulus fibrosus (AF), and cartilage endplate]. The NP is located at the center of each disk and is highly hydrated and gelatinous. The ligamentous AF surrounds the NP circumferentially, and the cartilaginous endplates connect the disk to the inferior and superior vertebral bodies. Each region is populated by phenotypically distinct cells and differs in the composition of the extracelluar matrix (ECM) produced correspondingly by the unique cell types. In a healthy IVD, the cells within the NP comprise only about 1% of the disk tissue by volume. Cells of the NP are chondrocyte-like (i.e., roundish in shape and located within lacunae), whereas the AF cells are more fibroblastic with elongated morphologic appearance (31). NP cells produce a matrix that is rich in proteoglycans (PGs), predominantly aggrecan and type II collagen, whereas the AF cells produce a matrix that is rich in type I collagen with little PG or type II collagen (31,38). The spatial configuration and different hydration properties (i.e., water-retaining capacity) of NP and AF are essential for normal disk function.
Although the etiologic and pathophysiological mechanisms underlying IVD degeneration are still being actively investigated, structural changes of PG and type II collagen degradation are presently considered as a final common path for IVD degeneration (25,38). Thus, IVD degeneration could be triggered by an imbalance between the anabolic and catabolic functions of the NP cells; IVD degeneration can result from an increase in catabolic enzymes and a corresponding decrease in the ECM component that is maintained by a range of cytokines and growth factors. Well-identified anabolic growth factors include insulin-like growth factor-1 (IGF-1), transforming growth factor-β (TGF-β), and bone morphogenic proteins (BMP) (52). Matrix metalloproteases (MMPs) (97), a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) (90), and proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor-α (TNF-α) (69) are proteolytic enzymes likely responsible for ECM degradation in IVD.
Gradual degeneration of IVD could also be set in motion by chronic mechanical instability. Increased load on the NP and increased intradiskal pressure may cause the intrinsic cells or invading macrophages to produce proinflammatory cytokines or toxic amounts of MMPs, which degrade the ECM and lead to the destruction of the PGs, thereby reducing the NP's water-retaining capacity with consequent dehydration. The changes in IVD morphology are the result of a constellation of dynamic pathophysiological perturbations that are characterized by dehydration, decreased PGs and type-II collagen, and a diminished degree of nutrient diffusion, coupled with elevated levels of type I collagen, and an increased rate of cell death (34,35,44,76). Progression of these pathologic events ultimately results in structural collapse and the loss of disk function. Under such circumstances, gravity or daily life activity-related weight loading could cause disk herniation into the spinal canal, resulting in neural compression and chronic pain (35,76).
Biological strategies for IVD Regeneration
Therapeutic strategies based on manipulating biological events for IVD degeneration include delivery of molecules influencing disk cell metabolism and phenotype stability, introduction of therapeutic genes, and transplantation of cellular components (stem cells, chondrocytes, disk cells, etc.). Overall, the biological strategies for preventing, arresting, or reversing IVD degeneration are built upon the possibility of biologically improving the accumulation of healthy ECM by promoting synthesis of IVD matrix and inhibiting ECM abnormal catabolism. In the following sections, we will review each component succinctly with a detailed emphasis on stem cell-based multimodal strategies that are guided by recovery neurobiology principles.
Growth Factor and Cytokines
The homeostasis of IVD is regulated by the active maintenance of a balance between the catabolism and anabolism of disk cells. Recent understanding of the cellular and molecular events of IVD degeneration has engendered the hypothesis of arresting or preventing IVD degeneration by upregulating the production of key matrix proteins or downregulating the catabolic events induced by the proinflammatory cytokines, IL-1 and TNF-α (9,37,48,52,69,90,97). One of the most appealing biological therapeutic tactics to regenerate or repair a degenerated IVD is the direct injection of growth factors.
There are numerous reports that describe cell proliferation and matrix synthesis by growth factors such as IGF-1, epidermal growth factor (EGF), basic fibroblast growth factor (bFGF or FGF2), TGF-β, BMP-2, osteogenic protein-1 (OP-1), and growth and differentiation factor-5 (GDF-5) in experimental models (9,14,15,32,52,83). Though the exact mechanisms of these commonly tested growth factors for affecting degenerated disks remain not completely understood, application of exogenous growth factors has been shown to stimulate cell proliferation and matrix synthesis in the degenerated disk induced experimentally. However, the response to different growth factors varies according to variability in cellularity and matrix architecture of the degenerating disk. For example, inner annular chondrocytes in degenerated disks are responsive to exogenous GDF-5 and TGF-β by boosting their matrix synthesis within the NP and inner annulus, but their response to IGF-1 or bFGF treatment is less potent. The outcomes suggest that limited and transient exposure to growth factors alone is unlikely to create a sustainable benefit of tissue remodeling for human IVD degeneration, especially given the fact that the quantity and distribution of viable cells clinically vary in degenerated disks. In addition, the half-lives and interstitial solubility of the factors, the proper carrier, the environmental presence of inhibiting factors, etc., are issues that need to be taken into consideration (9,14,15,32,52,83). Despite the known obstacles to successful translation due to short half-lives of biological factors and low numbers of viable cells to be stimulated in already degenerated IVD disks (38), there is strong hope to obtain valuable information from an ongoing clinical trial (ClinicalTrials.gov identifier: NCT01182337) in which intradiskal recombinant human GDF-5 (rhGDF-5) administration is tested for therapeutic efficacy in treating an early stage of lumbar disk degeneration. Furthermore, for advanced stages of IVD degeneration the use of multimodal treatment incorporating administration of growth factors, donor cells, and tissue or tissue engineering technologies is likely required, since in these cases the number of viable host cells to be stimulated would be extremely low.
The proteolytic enzymes of MMPs and ADAMTS, and proinflammatory cytokines play crucial roles in the degradation of the ECM of the NP. Therefore, administration of specific antagonists or inhibitors of these proteins directly into the disk has been reasoned as a strategy to prevent the target proteolytic enzymes from causing further ECM degradation (86). However, the effects of these agents are temporary and nonrestrictive to the disk only and could even produce unwanted side effects. Trying to avoid such unfavorable outcomes, another strategy to slow down IVD degeneration has been developed to biologically block the transcription factors that activate the genes encoding enzymatic proteins contributing to IVD degeneration. For instance, receptor activator of nuclear factor-κ B ligand (RANKL) is targeted since this transcription factor plays a critical role in the regulation of many genes active in IVD degeneration (50).
Gene Therapy
In addition, to overcome the short half-life limit of biological factors administered via bolus injection, the search for a method to provide a sustained growth factor supply within the IVD has led to the development of gene therapy strategies (20,30,45,57,59,60,61,64,75,88,89,93,94,102,103). Gene therapy involves the transfer of the therapeutic gene or genes into target cells using a vector (commonly viral and nonviral) so the functional status of recipient cells can be modified (via expressing products of donor genes). Consequently, gene therapy, if done properly, may offer many benefits including a more sustained target gene expression and a longer term biological effect, etc. Numerous methods for transfecting therapeutic nucleic acids into the target cell genes can be used, applying either viral or nonviral methods. Although viral vectors are the preferred vehicles because they provide efficient gene delivery, safety concerns related to uncontrolled gene expression, insertional mutagenesis, and immunogenicity must also be determined and handled seriously.
Many studies have provided methods for transferring a gene of interest to IVD cells. Examples of exogenous gene products that can stimulate disk-related ECM production are TGF-β, BMP-2, LIM mineralization protein-1 (LMP-1), chondroitinase ABC, and TIMP (tissue inhibitor of metalloproteinases) (43,46). LMP-1 has been reported to have a chondrogenic effect on IVD cells and induce a significant increase in the expression of aggrecan mRNA, as well as an increase in PG synthesis (43,75). Chondroitinase ABC catalyzes the removal of chondroitin sulfate and dermatan sulfate side chains of proteoglycans (i.e., to depolarize chondroitin sulfate) and has been reported to be a potential reagent for chemonucleolysis, which is an established treatment for IVD displacement (26). Patient safety should be of paramount importance as misdirected or supratherapeutic injections may have potentially toxic effects on surrounding structures (47,89). To improve the safety profile of gene therapy, nonviral vectors have received significant attention even though nonviral gene transfer is currently less efficient than viral gene delivery. Moreover, despite an overall excellent safety record, we reported that some frequently used nonviral vectors can inhibit chondrogenic differentiation of mesenchymal stem cells (MSCs), likely imposing an unfavorable effect on disk repair (82). Hence, weighing full spectrum impacts of genetic modifications is very important in designing gene manipulation therapies for IVD degeneration (82).
Gene therapy for IVD regeneration is by definition an exciting technology that aims to reprogram target cell genes for prolonged synthesis of a transfected growth factor of therapeutic interest (20,30,45,57,59,60,61,64,75,88,89,94,102,103). Conversely, its drawbacks stem from inefficiency of the gene delivery systems and the influence on regulatory pathways that might cause uncontrolled expression of growth factors. In addition, it is unclear whether degenerated IVDs contain sufficient numbers of viable cells to receive transferred genes and produce proteins for staging a clinically efficacious therapy for IVD degeneration. Most importantly, clinical translation of gene therapy for disk degeneration requires stringent assurances of safety.
Stem Cell Approaches for the Treatment of IVD Degeneration
IVD degeneration is characterized by a decrease in the number of viable and functioning cells in the NP and progressive loss of the ECM molecules (31,38,71,91). Accordingly, it is desirable to develop a method of stimulating PG and/or type II collagen syntheses by introducing exogenous cells to supplement and replenish the innate disk cell population. Autologous articular chondrocytes and disk cells can be harvested and isolated from a patient's own articular cartilage and IVD, respectively (17,23,53). However, the approach, while promising, has so far only shown partial effectiveness mainly because of the following reasons. 1) Articular chondrocytes produce aggrecan and collagen type II, but the ratio of proteoglycan to collagen is lower compared with that produced by NP cells. Hence, articular chondrocytes may not be used as a primary source for disk cell replacement therapy (58,85). 2) Clinically, direct extraction of disk cells in a sufficient amount for transplantation is technically challenging due to the limited efficiency of harvesting NP cells. The herniated and degenerated disk usually has a severely diminished number of cells. In addition, the regenerative and repair capacity of autologous disk cells harvested from herniated and degenerated disks remains questionable since these cells showed status of senescence, which seriously mitigates the potential for expanding them in vitro (7,22,70,85). 3) There is a discernible risk of genetic alteration in the cells propagated in vitro that may lose their phenotypic characteristics particularly following monolayer cultures (12). 4) Invasive procedures are required to obtain autologous cells, which may expose patients to additional morbidity.
Therefore, perhaps the most promising strategy to date for obtaining transplantable disk cells is the use of stem cells, both early developmental and adult stem cells and induced pluripotent stem cells (iPSCs) that have self-renewal, multilineage differentiation capacity, and functional multipotency (e.g., anti-inflammation, proregeneration, etc.) properties (35,80). Stem cell-based therapies for preventing, stabilizing, or reversing disk degeneration have gathered considerable attention over the past decade (3,19,40). This modality allows the injection of stem cells into the IVD to replace lost cells and to replenish ECM as well, to increase IVD proteoglycan and interstitial fluid contents. Because of potential ethical barriers and immune rejection issues related to embryonic stem cells (ESCs) and other types of nonautologous cells, MSCs with various sources (e.g., bone marrow, adipose tissues, synovial membrane, umbilical cord blood, and dental pulp) have been studied, assessing MSCs as one of optimal cell types for transplantation purposes aiming to derive autologous disk cells.
A recent review summarized different sources of MSCs, treatment strategies, cell doses, and efficacies investigated in various experimental settings of IVD degeneration, including mouse, rat, rabbit, canine, porcine, and ovine models (98). The analysis of 24 studies suggests that use of MSCs derived from bone marrow (BM-MSCs), adipose tissue (AD-MSCs), and synovial fluid (SF-MSCs) is largely safe and effective for impeding IVD degeneration; the rate of complication, particularly osteophyte formation, was ~2.7% (73,98). Osteophyte formation has been thought to be the result of leakage of implanted MSCs. Thus, application of scaffolding materials (e.g., fibrin, hyaluronan, or atelocollagen) has been strongly recommended to prevent cell leakage and mitigate the risk of ectopic osteoblast differentiation of MSCs (17,84,98). Since the degenerated IVD provides a very limited nutrient supply, the minimal effective dose of MSCs must be determined to maximize postimplantation survival. Data from larger model (canine, porcine, and ovine) studies showed that a dose of 105 or 107 BM-MSCs injected per disk protected and maintained the NP and inner AF structures well. However, administration of 106 BM-MSCs/per disk had the least number of MSC apoptosis with comparable beneficial effects. The survival duration of implanted BM-MSCs ranged from 8 weeks to 6 months (98). Among all the sources of MSCs that have so far been studied, there is no clear recommendation concerning definitive efficacy superiority of a particular type of MSCs tested in vivo. In some studies, it was suggested that AD-MSCs might be a more appropriate cell type than BM-MSCs for IVD regeneration because AD-MSCs could differentiate into cells with a more NP-like phenotype (56,85).
Whereas the research data on MSC implantation appears encouraging, these cells are actually being transplanted into a harsh environment consisting of low cellularity, low glucose, low oxygen, low pH due to high lactic acid buildup, and low nutrients under an inflammatory milieu (24,29,42,69,95). All of these factors could detrimentally influence differentiation potential, viability, and metabolism of the implanted cells (42). Therefore, two important considerations need to be taken into account before effective therapies could be devised: (1) how implanted MSCs may properly differentiate into an NP-like phenotype and (2) how MSCs may modulate the inflammatory microenvironment and vice versa. It has been reported that human MSCs (hMSCs), when injected into a degenerated disk, improved matrix production by host-degenerated NP cells, boosted NP-like gene expression, and modulated NP cell immunological responses to inflammatory cytokines, resulting from effects of trophic factors and anti-inflammatory cytokines secreted by MSCs (i.e., functional multipotency of stem cells) (4,6,80). Obviously, the immunomodualtory effects of MSCs on NP cells within the degenerated disk may potentially inhibit the inflammatory milieu, which mediates ingrowth of pain-inducing vasculature and nerve fibers (2,5,41,42). On the other hand, the field presently lacks tangible data to verify whether the injected MSCs only differentiate into NP cells inside the degenerated disk. This is partly caused by difficulties encountered in defining specific markers of NP versus articular chondrocytes for formulating molecular assays. As mentioned above, inflammatory mediators are a key component to progressive IVD degeneration, and MSCs must be able to function optimally within the inflammatory, low oxygen, and low nutrient environment of the degenerated disk to ensure effective repair (42). Although reports that hMSCs exposed to hypoxia had enhanced tissue protective effects, exposure of MSCs to inflammatory factors (IL-1β and TNF-α) or hypoxic environment might negatively modulate the MSC differentiation potential because both IL-1β and TNF-α could inhibit the chondrogenic differentiation of MSCs and promote osteogenic-like mineral deposition, which is not desirable for disk repair (16,42,51,54,78,92). Thus, the field should invest in studies that define preimplantation conditioning procedures to enable MSCs to differentiate properly under the harsh conditions that occur following disk degeneration (8,27,39,42,63,68,76,77,84,85). For example, preconditioning MSCs with the application of growth factors, such as TGF-β and BMPs in vitro was shown to stimulate their differentiation into NP cells (68,77,84). Other tactics include coculture of MSCs with NP, notochordal cell-conditioned media, or disk matrix components (11,13,27,63,66,76,77,101), as well as culturing MSCs under hypoxia exposure immediately before implantation (51,54,104).
Notochordal cells (NCs) and terminal NP cells are both phenotypically correct and desirable for transplantation purposes, but NCs are scarcely present in adult human NP tissue (49). NCs are more crucial since they can generate NP cells and may potentially survive better in the unfavorable microenvironment posttransplantation (1,21,49). So, it is very pivotal to establish protocols that guide generation of high-quantity and functional NCs from pluripotent iPSCs or ESCs (18). Sheikh et al. demonstrated that ESC-derived chondroprogenitors could potentially differentiate into NCs in a rabbit model of IVD degeneration (74). Compared with disk regeneration by applying ESCs, use of iPSCs for disk repair may be more attractive due to patient specificity and fewer concerns about ethical issues and immune rejection. Liu et al. showed certain effectiveness of using natural NP tissue matrix to direct NC differentiation of iPSCs (49).
Based on reported experimental studies, the MSC-based treatment of IVD degeneration has been considered to be effective, but a reliable method of direct injection of the cell suspension into a target disk appears to not have been well established; current procedures often show backflow of cells through the injection site and a low survival rate of the implanted cells (36,62,84,98). In addition, rabbits after receiving direct MSC suspension administration showed osteophyte growth in the anterolateral disk space due to leakage of the MSCs (84). To overcome the disadvantages of direct cell suspension injection and more critically to formulate cell-built structures suitable for treating degenerated disk, development of regimens that combine cells with cell-carrying matrix molecules such as fibrin and hyaluronan has launched a prototype three-dimensional (3D) approach for IVD regeneration, buoyed by the success of scaffolding delivery of neural stem cells (10,65,67,79). Therefore, the aforementioned results emphasize the importance of devising a scaffolding technology of MSC delivery with needle tract sealing capability following injection to reduce the risk of ectopic differentiation of MSCs into osteoclasts (e.g., osteophyte formation). Moreover, recovery neurobiology principle-based multimodal strategies need to be devised to comprehensively tackle the complex pathology of IVD-triggered chronic back pain, in which central neuronal and glial modulation as well as peripheral tissue nociceptive afferent signals should be targeted combinatorially by biological and novel pharmacological interventions (100).
To date, there are only two reported clinical trials on cell-mediated IVD therapies (62,99). In a case series study (62), injection of autologous BM-MSCs into the NP region without a cell carrier or annulus sealant improved low back pain and disability at 12 months postinjection in a total of 10 patients with IVD degeneration. Additionally, T2-weighted MRI showed an increase in water content, but disk height was not restored. Similar outcomes were found in a different study where two female patients with IVD degeneration experienced pain relief and increase in water content of the disk at 2 years after injection of about 20 collagen porous sponges seeded with autologous BM-MSCs. In this study, the surgical hole was sealed with an acellular collagen sponge (99).
Bioengineering Approaches to Repairing IVD Degeneration
Whereas direct cytokine and trophic factor injections may beneficially affect mild IVD pathology, implantation of cells alone appears to be more effective for both mild and moderate degeneration of IVD where numbers of live NP cells are very limited and the ECM microenvironment exceedingly poor (28). For severe IVD degeneration with loss of cell capacity (e.g., NP cell, MSC recruitment, and proliferation, etc.) and physiologic disk structure, restoration of proper disk height is necessary to ensure functionality of the IVD. Thus, a multifunctional therapeutic modality has been derived, which is to implant an ex vivo engineered tissue block directly into the degenerated IVD space. For this tactic, tissue engineering cells should ideally interact with an appropriate type of scaffolding material that closely mimics the structural, biological, and mechanical functions of the native IVD ECM environment; together, they should create a biological disk that has the right degree of stiffness. For the latter case, the field needs to develop biomaterial scaffolds fulfilling the following criteria: 1) possessing mechanical stability for load-bearing applications; 2) reliable injectable feasibility without causing cell depletion from the implant; 3) ability to carry cells and allowance of sufficient cell attachment, proliferation, and migration into the surrounding cell matrix in vivo; 4) mechanisms to recruit host MSCs and to enhance ECM synthesis to engender tissue formation and remodeling responses, and 5) adequate immune compatibility.
Summary and Future Prospects
Promising biological regenerative and recovery approaches are emerging to ultimately provide tangible alternative treatments for IVD degeneration and overcome inherent complications and deficiency of conventional treatments. In this review, we summarized the field's current opinions about the limits of the following therapy developments: 1) direct administration of growth factors per se has the disadvantage of short half-lives and/or a possibility of lacking IVD intrinsic cells as therapeutic targets in severe disk degeneration cases; 2) present gene therapy formulas, while circumventing deficits of bolus application of biological factors, have common shortcomings of inefficient gene delivery, unstable long-term expression, and safety concerns (see Table 1 for representative molecules); and 3) by contrast, cell-based transplantation regimens have shown more promising therapeutic potential. Earlier studies investigated the proliferative and regenerative capability of mature NP cells, AF cells, nondisk chondrocytes, and fibroblasts. However, these cell types have the disadvantage of being scarce (e.g., the amount of harvestable NP cells is severely diminished in degenerative IVDs) and difficult to harvest and grow in vitro. Encouragingly, natural or induced stem or progenitor cells demonstrate biological features that can overcome the aforementioned inadequacies. Among stem cell types studied to date (see Table 2 for representative reports), the MSC-based tissue engineering technology platform is considered one of the most promising candidates for treating IVD degeneration. We also discussed ideal criteria for developing stem cell scaffolds to repair and regenerate deteriorated IVDs that require a multimodal strategy comprising an autologous or nonimmunogenic cell source, biofunctional and mechanically stiff scaffolds, controllable proper biological signaling capability, and histocompatibility. Though recent studies for IVD regeneration primarily focused on regenerating a specific component of IVD, truly efficacious treatments demand cell-mediated multimodal actions that address multifaceted issues of IVD pathology following recovery-oriented neurobiological principles to maximally impede chronic back pain and optimize functional improvement (100). Therefore, for developing more satisfactory IVD degeneration therapies, we think that designing case-individualized treatment plans is a key requirement. This could be done via setting determination of specific biomarkers that help select an appropriate intervention with highest probability to be effective as one of the phases 0-2 aims in future clinical trials. Preferably, such treatments should have minimally invasive features. For example, for early degeneration stages, intradiskal injection of biological factors, and implantation of extracellular matrix molecules or/and synthetic scaffolds may provide good efficacy if particular biomarkers indicate that a proregenerative niche is still present. Direct cell injection with or without cell carrier matrix may be best used to treat both mild and moderate IVD deterioration cases. Conversely, the engineered MSC construct with characteristics of mechanical and structural support, biological signaling to enable implanted cell development, anti-inflammation, and host cell recruitment, as well as discrete tissue architecture that emulates healthy IVDs, can better serve the medical demand in treating the most advanced stage of IVD degeneration.
Factors That Have Regenerative Potential in Degenerative Intervertebral Disk
TGF-β1, transforming growth factor-β1; NP, nucleus pulposus; AF, annulus fibrosus; IVD, intervertebral disk; ADAMTS, a disintegrin and metalloproteinase with thrombospondin motifs; TIMP, tissue inhibitor of metalloproteinases; TNF-α, tumor necrosis factor-α; MMP, matrix metalloproteinase; IL-1Ra, IL-1 receptor antagonist; IGF-1, insulin-like growth factor-1; OP-1, osteogenic protein-1; GDF-5, growth and differentiation factor-5; BMP, bone morphogenetic protein; LMP-1, LIM mineralization protein-1.
Cell Therapies for Regeneration of the Intervertebral Disk
MSCs, mesenchymal stromal cells; AD-MSCs, adipose tissue-derived MSCs; BM-MSCs, bone marrow-derived MSCs; NP, nucleus pulposus; AF, annulus fibrosus; IVD, intervertebral disk; TGF-β, transforming growth factor-β; MRI, magnetic resonance imaging.
Future biological therapeutic development for IVD degeneration may benefit from directly addressing the need to devise effective differentiation regimens for MSCs to become NP cells and to establish specific markers of NP cells to better compare them with regular nondisk chondrocytes. The phenotypes of the NP cells are thought to be different from those of the articular chondrocytes; implantation of chondrocyte-like cells, which do not have the correct NP cell phenotype, can result in the formation of a tissue that does not restore functional IVD (73). Thus, it is imperative to implant the correctly differentiated NP cells or cells possessing biological capacities most similar to those of NP cells. In addition, establishment of technologies that deliver the optimal adjacent/local niche support (e.g., proper innervations, nutrient supply, etc.) and developmental state of stem cells with proper cell–matrix/scaffold interactions is critical for the generation of the correct cell phenotype and effective repair mechanism to ensure survival and ECM production of implanted cell in the harsh environment of degenerated IVDs (81). Regarding ex vivo engineered disk constructs, it is pivotal for investigators to demonstrate whether the engineered biological disk replacement can indeed provide adequate biomechanical support and retain levels of endurance that are close to those of a healthy host disk. Finally, transplantation biology emphasizes the importance of validating the ability of the biological implant to integrate appropriately into the surrounding tissue of the host (i.e., engraftability) in order to recover function (80).
Footnotes
Acknowledgments
I.B.H. is supported by the Korea Healthcare Technology Research and Development Project, Ministry for Health and Welfare Affairs (H114C3245). Y.D.T. is supported by VAR&D, DoD, and HMS and SRH's Gordon Project to Cure Clinical Paralysis. Y.D.T. and R.Z. thank the Cele H. & William B. Rubin Family Fund, Inc. for the support to their collaborative research work. R.Z. is also supported in part by NIH/EKS-NICHD R24 HD065688. Except for X.Z., R.Z., and Y.D.T., all authors are practicing academic neurosurgeons and previous or current Teng Laboratory Team members at HMS/BWH/SRH and VABHS. The authors declare no conflicts of interest.