Injectable, redox-polymerized carboxymethylcellulose hydrogels promote nucleus pulposus-like extracellular matrix elaboration by human MSCs in a cell density-dependent manner

Reviving Intervertebral Discs: Treating Degeneration Using Advanced Delivery Systems

Intervertebral disc degeneration (IVDD) is commonly associated with many spinal problems, such as low back pain, and significantly impacts a patient's quality of life. However, current treatments for IVDD, which include conservative and surgical methods, are limited in their ability to fully address degeneration. To combat IVDD, delivery-system-based therapy has received extensive attention from researchers. These delivery systems can effectively deliver therapeutic agents for IVDD, overcoming the limitations of these agents, reducing leakage and increasing local concentration to inhibit IVDD or promote intervertebral disc (IVD) regeneration. This review first briefly introduces the structure and function of the IVD, and the related pathophysiology of IVDD. Subsequently, the roles of drug-based and bioactive-substance-based delivery systems in IVDD are highlighted. The former includes natural source drugs, nonsteroidal anti-inflammatory drugs, steroid medications, and other small molecular drugs. The latter includes chemokines, growth factors, interleukin, and platelet-rich plasma. Additionally, gene-based and cell-based delivery systems are briefly involved. Finally, the limitations and future development of the combination of therapeutic agents and delivery systems in the treatment of IVDD are discussed, providing insights for future research.

Mechanical characterization and design of biomaterials for nucleus pulposus replacement and regeneration

Biomaterials for nucleus pulposus (NP) replacement and regeneration have great potential to restore normal biomechanics in degenerated intervertebral discs following nucleotomy. Mechanical characterizations are essential for assessing the efficacy of biomaterial implants for clinical applications. While traditional compression tests are crucial to quantify various modulus values, relaxation behaviors and fatigue resistance, rheological measurements should also be conducted to investigate the viscoelastic properties, injectability, and overall stability upon deformation. To recapitulate the physiological in vivo environment, the use of spinal models is necessary to evaluate the risk of implant extrusion and the restoration of biomechanics under different loading conditions. When designing devices for NP replacement, injectable materials are ideal to fully fill the nucleus cavity and prevent implant migration. In addition to achieving biocompatibility and desirable mechanical characteristics, biomaterial implants should be optimized to avoid implant extrusion or re-herniation post-operatively. This review discusses the most commonly used testing protocols for assessing mechanical properties of biomaterial implants and serves as reference material for enabling researchers to characterize NP implants through a unified approach whereby newly developed biomaterials may be compared and contrasted to existing devices.

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

Background

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

Methods

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

Results

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

Conclusions

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

A Review: Methodologies to Promote the Differentiation of Mesenchymal Stem Cells for the Regeneration of Intervertebral Disc Cells Following Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration (IDD), a highly prevalent pathological condition worldwide, is widely associated with back pain. Treatments available compensate for the impaired function of the degenerated IVD but typically have incomplete resolutions because of their adverse complications. Therefore, fundamental regenerative treatments need exploration. Mesenchymal stem cell (MSC) therapy has been recognized as a mainstream research objective by the World Health Organization and was consequently studied by various research groups. Implanted MSCs exert anti-inflammatory, anti-apoptotic, and anti-pyroptotic effects and promote extracellular component production, as well as differentiation into IVD cells themselves. Hence, the ultimate goal of MSC therapy is to recover IVD cells and consequently regenerate the extracellular matrix of degenerated IVDs. Notably, in addition to MSC implantation, healthy nucleus pulposus (NP) cells (NPCs) have been implanted to regenerate NP, which is currently undergoing clinical trials. NPC-derived exosomes have been investigated for their ability to differentiate MSCs from NPC-like phenotypes. A stable and economical source of IVD cells may include allogeneic MSCs from the cell bank for differentiation into IVD cells. Therefore, multiple alternative therapeutic options should be considered if a refined protocol for the differentiation of MSCs into IVD cells is established. In this study, we comprehensively reviewed the molecules, scaffolds, and environmental factors that facilitate the differentiation of MSCs into IVD cells for regenerative therapies for IDD.

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.

Research progress in seed cells for cartilage tissue engineering

Cartilage defects trouble millions of patients worldwide and their repair via conventional treatment is difficult. Excitingly, tissue engineering technology provides a promising strategy for efficient cartilage regeneration with structural regeneration and functional reconstruction. Seed cells, as biological prerequisites for cartilage regeneration, determine the quality of regenerated cartilage. The proliferation, differentiation and chondrogenesis of seed cells are greatly affected by their type, origin and generation. Thus, a systematic description of the characteristics of seed cells is necessary. This article reviews in detail the cellular characteristics, research progress, clinical translation challenges and future research directions of seed cells while providing guidelines for selecting appropriate seed cells for cartilage regeneration.

New hope for intervertebral disc degeneration: bone marrow mesenchymal stem cells and exosomes derived from bone marrow mesenchymal stem cell transplantation

Bone marrow mesenchymal stem cells (BMSCs), multidirectional cells with self-renewal capacity, can differentiate into many cell types and play essential roles in tissue healing and regenerative medicine. Cell experiments and in vivo research in animal models have shown that BMSCs can repair degenerative discs by promoting cell proliferation and expressing extracellular matrix (ECM) components, such as type II collagen and protein-polysaccharides. Delaying or reversing the intervertebral disc (IVD) degeneration (IDD) process at an etiological level may be an effective strategy. However, despite increasingly in-depth research, some deficiencies in cell transplantation timing and strategy remain, preventing the clinical application of cell transplantation. Exosomes exhibit the characteristics of the mother cells from which they were secreted and can inhibit nucleus pulposus (NP) cell (NPC) apoptosis and delay IDD through intercellular communication. Furthermore, the use of exosomes effectively avoids problems associated with cell transplantation, such as immune rejection. This manuscript introduces almost all of the BMSCs and exosomes derived from BMSCs (BMSCs-Exos) described in the IDD literature. Many challenges regarding the use of cell transplantation and therapeutic exosome intervention for IDD remain to be overcome.

Effect of Transforming Growth Factor Beta (TGF-β) on the Degeneration of Intervertebral Discs by Regulating Nuclear Factor Kappa-Light-Chain-Enhancer of Activated B Cells/Mammalian Target of Rapamycin (NF-fcB/mTOR) Signaling Pathway

Intervertebral disc degenerative disease (IDDD) is common in orthopedics. TGF-β involves in inflammation and tissue repair. But its role in IDDD remains unclear. IDDD patients and normal intervertebral disc nucleus pulposus tissues were collected. IDDD was divided into prominent group and prolapse group. IDDD nucleus pulposus cells were isolated and divided into control group, TGF-β agonist group and TGF-β inhibitor group followed by analysis of cell proliferation by MTT, cell apoptosis by flow cytometry BALP and OC expression by Real time PCR, NF-/<B/mTOR signaling protein expression by Western blot as well as IL-1 and IL-6 secretion by ELISA. Compared with normal group, TGF-β mRNA and serum level in patients with IDDD was significantly decreased (P < 0.05), with more significant changes in prolapse group (P < 0.01). Pfirrmann grading scores were negatively correlated with TGF-β serum level (P < 0.001). TGF-β agonists can significantly promote cell proliferation, inhibit apoptosis, upregulate BALP and OC expression, inhibit NF-κB expression, increased p-mTOR level and decrease IL-1 and IL-6 secretion (P < 0.05). All these changes were significantly reversed by TGF-β inhibitors (P < 0.05). TGF-β expression in IDDD is reduced and associated with disease severity. Promoting TGF-β expression can inhibit inflammatory factors secretion, promote BALP and OC expression and cell proliferation, and inhibit the degeneration of intervertebral discs by regulating NF-/<B/mTOR signaling.

Decellularized Disc Hydrogels for hBMSCs tissue-specific differentiation and tissue regeneration

Tissue specificity, a key factor in the decellularized tissue matrix (DTM), has shown bioactive functionalities in tuning cell fate-e.g., the differentiation of mesenchymal stem cells. Notably, cell fate is also determined by the living microenvironment, including material composition and spatial characteristics. Herein, two neighboring tissues within intervertebral discs, the nucleus pulposus (NP) and annulus fibrosus (AF), were carefully processed into DTM hydrogels (abbreviated DNP-G and DAF-G, respectively) to determine the tissue-specific effects on stem cell fate, such as specific components and different culturing methods, as well as in vivo regeneration. Distinct differences in their protein compositions were identified by proteomic analysis. Interestingly, the fate of human bone marrow mesenchymal stem cells (hBMSCs) also responds to both culturing methods and composition. Generally, hBMSCs cultured with DNP-G (3D) differentiated into NP-like cells, while hBMSCs cultured with DAF-G (2D) underwent AF-like differentiation, indicating a close correlation with the native microenvironments of NP and AF cells, respectively. Furthermore, we found that the integrin-mediated RhoA/LATS/YAP1 signaling pathway was activated in DAF-G (2D)-induced AF-specific differentiation. Additionally, the activation of YAP1 determined the tendency of NP- or AF-specific differentiation and played opposite regulatory effects. Finally, DNP-G and DAF-G specifically promoted tissue regeneration in NP degeneration and AF defect rat models, respectively. In conclusion, DNP-G and DAF-G can specifically determine the fate of stem cells through the integrin-mediated RhoA/LATS/YAP1 signaling pathway, and this tissue specificity is both compositional and spatial, supporting the utilization of tissue-specific DTM in advanced treatments of intervertebral disc degeneration.

Balancing biological and biomechanical performance in intervertebral disc repair: a systematic review of injectable cell delivery biomaterials

Discogenic back pain is a common condition without approved intervertebral disc (IVD) repair therapies. Cell delivery using injectable biomaterial carriers offers promise to restore disc height and biomechanical function, while providing a functional niche for delivered cells to repair degenerated tissues. This systematic review advances the injectable IVD cell delivery biomaterials field by characterising its current state and identifying themes of promising strategies. Preferred Reporting Items for Systematic Reviews and Meta- Analyses (PRISMA) guidelines were used to screen the literature and 183 manuscripts met the inclusion criteria. Cellular and biomaterial inputs, and biological and biomechanical outcomes were extracted from each study. Most identified studies targeted nucleus pulposus (NP) repair. No consensus exists on cell type or biomaterial carrier, yet most common strategies used mesenchymal stem cell (MSC) delivery with interpenetrating network/co-polymeric (IPN/CoP) biomaterials composed of natural biomaterials. All studies reported biological outcomes with about half the studies reporting biomechanical outcomes. Since the IVD is a load-bearing tissue, studies reporting compressive and shear moduli were analysed and two major themes were found. First, a competitive balance, or 'seesaw' effect, between biomechanical and biological performance was observed. Formulations with higher moduli had inferior cellular performance, and vice versa. Second, several low-modulus biomaterials had favourable biological performance and matured throughout culture duration with enhanced extracellular matrix synthesis and biomechanical moduli. Findings identify an opportunity to develop next-generation biomaterials that provide high initial biomechanical competence to stabilise and repair damaged IVDs with a capacity to promote cell function for long-term healing.

Combined Hydrogel and Mesenchymal Stem Cell Therapy for Moderate-Severity Disc Degeneration in Goats

Intervertebral disc degeneration is a cascade of cellular, structural, and biomechanical changes that is strongly implicated as a cause of low-back pain. Current treatment strategies have poor long-term efficacy as they seek only to alleviate symptoms without preserving or restoring native tissue structure and function. The objective of this study was to evaluate the efficacy of a combined triple interpenetrating network hydrogel (comprising dextran, chitosan, and teleostean) and mesenchymal stem cell (MSC) therapy targeting moderate-severity disc degeneration in a clinically relevant goat model. Degeneration was induced in lumbar discs of 10 large frame goats by injection of chondroitinase ABC. After 12 weeks, degenerate discs were treated by injection of either hydrogel alone or hydrogel seeded with allogeneic, bone marrow-derived MSCs. Untreated healthy and degenerate discs served as controls, and animals were euthanized 2 weeks after treatment. Discs exhibited a significant loss of disc height 12 weeks after degeneration was induced. Two weeks after treatment, discs that received the combined hydrogel and MSC injection exhibited a significant, 10% improvement in disc height index, as well as improvements in histological condition. Discs that were treated with hydrogel alone exhibited reduced tumor necrosis factor-α expression in the nucleus pulposus (NP). Microcomputed tomography imaging revealed that the hydrogel remained localized to the central NP region of all treated discs after 2 weeks of unrestricted activity. These encouraging findings motivate further, longer term studies of therapeutic efficacy of hydrogel and MSC injections in this large animal model. Impact statement Low-back pain is the leading cause of disability worldwide, and degeneration of the intervertebral discs is considered to be one of the most common reasons for low-back pain. Current treatment strategies focus solely on alleviation of symptoms, and there is a critical need for new treatments that also restore disc structure and function. In this study, using a clinically relevant goat model of moderate-severity disc degeneration, we demonstrate that a combined interpenetrating network hydrogel and mesenchymal stem cell therapy provides acute improvements in disc height, histological condition, and local inflammation.

Mesenchymal Stem Cells for Regenerative Medicine

In recent decades, the biomedical applications of mesenchymal stem cells (MSCs) have attracted increasing attention. MSCs are easily extracted from the bone marrow, fat, and synovium, and differentiate into various cell lineages according to the requirements of specific biomedical applications. As MSCs do not express significant histocompatibility complexes and immune stimulating molecules, they are not detected by immune surveillance and do not lead to graft rejection after transplantation. These properties make them competent biomedical candidates, especially in tissue engineering. We present a brief overview of MSC extraction methods and subsequent potential for differentiation, and a comprehensive overview of their preclinical and clinical applications in regenerative medicine, and discuss future challenges.

Injectable cellulose-based hydrogels as nucleus pulposus replacements: Assessment of in vitro structural stability, ex vivo herniation risk, and in vivo biocompatibility

Current treatments for intervertebral disc degeneration and herniation are palliative only and cannot restore disc structure and function. Nucleus pulposus (NP) replacements are a promising strategy for restoring disc biomechanics and height loss. Cellulose-based hydrogel systems offer potential for NP replacement since they are stable, non-toxic, may be tuned to match NP material properties, and are conducive to cell or drug delivery. A crosslinked, carboxymethylcellulose-methylcellulose dual-polymer hydrogel was recently formulated as an injectable NP replacement that gelled in situ and restored disc height and compressive biomechanical properties. The objective of this study was to investigate the translational potential of this hydrogel system by examining the long-term structural stability in vitro, the herniation risk and fatigue bending endurance in a bovine motion segment model, and the in vivo biocompatibility in a rat subcutaneous pouch model. Results showed that the hydrogels maintained their structural integrity over a 12-week period. AF injury significantly increased herniation risk and reduced fatigue bending endurance in bovine motion segments. Samples repaired with cellulosic hydrogels demonstrated restored height and exhibited herniation risk and fatigue endurance comparable to samples that underwent the current standard treatment of nucleotomy. Lastly, injected hydrogels elicited a minimal foreign body response as determined by analysis of fibrous capsule development and macrophage presence over 12 weeks. Overall, this injectable cellulosic hydrogel system is a promising candidate as an NP substitute. Further assessment and optimization of this cellulosic hydrogel system in an in vivo intradiscal injury model may lead to an improved clinical solution for disc degeneration and herniation.