Duration of TGF-β3 Exposure Impacts the Chondrogenic Maturation of Human MSCs in Photocrosslinked Carboxymethylcellulose Hydrogels

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.

Advanced 3D dynamic culture system with transforming growth factor-β3 enhances production of potent extracellular vesicles with modified protein cargoes via upregulation of TGF-β signaling

INTRODUCTION

Mesenchymal stromal cells (MSCs) release extracellular vesicles (MSC-EVs) containing various cargoes. Although MSC-EVs show significant therapeutic effects, the low production of EVs in MSCs hinders MSC-EV-mediated therapeutic development.

OBJECTIVES

Here, we developed an advanced three-dimensional (a3D) dynamic culture technique with exogenous transforming growth factor beta-3 (TGF-β3) treatment (T-a3D) to produce potent MSC-EVs.

METHODS

Our system enabled preparation of a highly concentrated EV-containing medium for efficient EV isolation and purification with higher yield and efficacy.

RESULTS

MSC spheroids in T-a3D system (T-a3D spheroids) showed high expression of CD9 and TGF-β3, which was dependent on TGF-β signaling. Treatment with EVs produced under T-a3D conditions (T-a3D-EVs) led to significantly improved migration of dermal fibroblasts and wound closure in an excisional wound model. The relative total efficacy (relative yield of single-batch EVs (10-11-fold) × relative regeneration effect of EVs (2-3-fold)) of T-a3D-EVs was approximately up to 33-fold higher than that of 2D-EVs. Importantly the quantitative proteomic analyses of the T-a3D spheroids and T-a3D-EVs supported the improved EV production as well as the therapeutic potency of T-a3D-EVs.

CONCLUSION

TGF-β signalling differentially regulated by fluid shear stress produced in our system and exogenous TGF-β3 addition was confirmed to play an important role in the enhanced production of EVs with modified protein cargoes. We suggest that the T-a3D system leads to the efficient production of MSC-EVs with high potential in therapies and clinical development.

Genipin-crosslinked fibrin seeded with oxidized alginate microbeads as a novel composite biomaterial strategy for intervertebral disc cell therapy

Discectomy procedures alleviate disability caused by intervertebral disc (IVD) herniation, but do not repair herniation-induced annulus fibrosus (AF) defects. Cell therapy shows promise for IVD repair, yet cell delivery biomaterials capable of sealing AF defects and restoring biomechanical function have poor biological performance. To balance the biomechanical and biological demands of IVD cell delivery biomaterials, we engineered an injectable composite biomaterial using cell-laden, degradable oxidized alginate (OxAlg) microbeads (MBs) to deliver AF cells within high-modulus genipin-crosslinked fibrin (FibGen) hydrogels (FibGen + MB composites). Conceptually, the high-modulus FibGen would immediately stabilize injured IVDs, while OxAlg MBs would protect and release cells required for long-term healing. We first showed that AF cells microencapsulated in OxAlg MBs maintained high viability and, upon release, displayed phenotypic AF cell morphology and gene expression. Next, we created cell-laden FibGen + MB composites and demonstrated that OxAlg MBs functionalized with RGD peptides (MB-RGD) minimized AF cell apoptosis and retained phenotypic gene expression. Further, we showed that cell-laden FibGen + MB composites are biomechanically stable and promote extracellular matrix (ECM) synthesis in long-term in vitro culture. Lastly, we evaluated cell-laden FibGen + MB-RGD composites in a long-term bovine caudal IVD organ culture bioreactor and found that composites had low herniation risk, provided superior biomechanical and biological repair to discectomy controls, and retained anabolic cells within the IVD injury space. This novel injectable composite hydrogel strategy shows promise as an IVD cell delivery sealant with potentially broad applications for its capacity to balance biomechanical and biological performance.

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.

Recent advances of hydrogel-based biomaterials for intervertebral disc tissue treatment: A literature review

Low back pain is an increasingly prevalent symptom mainly associated with intervertebral disc (IVD) degeneration. It is highly correlated with aging, as the nucleus pulposus (NP) dehydrates and annulus fibrosus fissure formatting, which finally results in the IVD herniation and related clinical symptoms. Hydrogels have been drawing increasing attention as the ideal candidates for IVD degeneration because of their unique properties such as biocompatibility, highly tunable mechanical properties, and especially the water absorption and retention ability resembling the normal NP tissue. Numerous innovative hydrogel polymers have been generated in the most recent years. This review article will first briefly describe the anatomy and pathophysiology of IVDs and current therapies with their limitations. Following that, the article introduces the hydrogel materials in the classification of their origins. Next, it reviews the recent hydrogel polymers explored for IVD regeneration and analyses what efforts have been made to overcome the existing limitations. Finally, the challenges and prospects of hydrogel-based treatments for IVD tissue are also discussed. We believe that these novel hydrogel-based strategies may shed light on new possibilities in IVD degeneration disease.

Stem Cells and Extrusion 3D Printing for Hyaline Cartilage Engineering

Hyaline cartilage is deficient in self-healing properties. The early treatment of focal cartilage lesions is a public health challenge to prevent long-term degradation and the occurrence of osteoarthritis. Cartilage tissue engineering represents a promising alternative to the current insufficient surgical solutions. 3D printing is a thriving technology and offers new possibilities for personalized regenerative medicine. Extrusion-based processes permit the deposition of cell-seeded bioinks, in a layer-by-layer manner, allowing mimicry of the native zonal organization of hyaline cartilage. Mesenchymal stem cells (MSCs) are a promising cell source for cartilage tissue engineering. Originally isolated from bone marrow, they can now be derived from many different cell sources (e.g., synovium, dental pulp, Wharton’s jelly). Their proliferation and differentiation potential are well characterized, and they possess good chondrogenic potential, making them appropriate candidates for cartilage reconstruction. This review summarizes the different sources, origins, and densities of MSCs used in extrusion-based bioprinting (EBB) processes, as alternatives to chondrocytes. The different bioink constituents and their advantages for producing substitutes mimicking healthy hyaline cartilage is also discussed.

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.

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

Low back pain is a major cause for disability and is closely linked to intervertebral disc degeneration. Mechanical and biological dysfunction of the nucleus pulposus in the disc has been found to initiate intradiscal degenerative processes. Replacing or enriching the diseased nucleus pulposus with an injectable, stem cell-laden biomaterial that mimics its material properties can provide a minimally invasive strategy for biological and structural repair of the tissue. In this study, injectable, in situ-gelling carboxymethylcellulose hydrogels were developed for nucleus pulposus tissue engineering using encapsulated human marrow-derived mesenchymal stromal cells (hMSCs). With the goal of obtaining robust extracellular matrix deposition and faster construct maturation, two cell-seeding densities, 20 × 10⁶ cells/ml and 40 × 10⁶ cells/ml, were examined. The constructs were fabricated using a redox initiation system to yield covalently crosslinked, cell-seeded hydrogels via radical polymerization. Chondrogenic culture of the hydrogels over 35 days exhibited high cell viability along with deposition of proteoglycan and collagen-rich extracellular matrix, and mechanical and swelling properties similar to native human nucleus pulposus. Further, the matrix production and distribution in the carboxymethylcellulose hydrogels was found to be strongly influenced by hMSC-seeding density, with the lower cell-seeding density yielding a more favorable nucleus pulposus-specific matrix phenotype, while the rate of construct maturation was less dependent on the cell-seeding density. These findings are the first to demonstrate the utility of redox-polymerized carboxymethylcellulose hydrogels as hMSC carriers for potential minimally invasive treatment strategies for nucleus pulposus replacement.

Lessons learned from intervertebral disc pathophysiology to guide rational design of sequential delivery systems for therapeutic biological factors

Intervertebral disc (IVD) degeneration has been associated with low back pain, which is a major musculoskeletal disorder and socio-economic problem that affects as many as 600 million patients worldwide. Here, we first review the current knowledge of IVD physiology and physiopathological processes in terms of homeostasis regulation and consecutive events that lead to tissue degeneration. Recent progress with IVD restoration by anti-catabolic or pro-anabolic approaches are then analyzed, as are the design of macro-, micro-, and nano-platforms to control the delivery of such therapeutic agents. Finally, we hypothesize that a sequential delivery strategy that i) firstly targets the inflammatory, pro-catabolic microenvironment with release of anti-inflammatory or anti-catabolic cytokines; ii) secondly increases cell density in the less hostile microenvironment by endogenous cell recruitment or exogenous cell injection, and finally iii) enhances cellular synthesis of extracellular matrix with release of pro-anabolic factors, would constitute an innovative yet challenging approach to IVD regeneration.

Thermoresponsive, redox-polymerized cellulosic hydrogels undergo in situ gelation and restore intervertebral disc biomechanics post discectomy

Back and neck pain are commonly associated with intervertebral disc (IVD) degeneration. Structural augmentation of diseased nucleus pulposus (NP) tissue with biomaterials could restore degeneration-related IVD height loss and degraded biomechanical behaviors; however, effective NP replacement biomaterials are not commercially available. This study developed a novel, crosslinked, dual-polymer network (DPN) hydrogel comprised of methacrylated carboxymethylcellulose (CMC) and methylcellulose (MC), and used in vitro, in situ and in vivo testing to assess its efficacy as an injectable, in situ gelling, biocompatible material that matches native NP properties and restores IVD biomechanical behaviors. Thermogelling MC was required to enable consistent and timely gelation of CMC in situ within whole IVDs. The CMC-MC hydrogel was tuned to match compressive and swelling NP tissue properties. When injected into whole IVDs after discectomy injury, CMC-MC restored IVD height and compressive biomechanical behaviors, including range of motion and neutral zone stiffness, to intact levels. Subcutaneous implantation of the hydrogels in rats further demonstrated good biocompatibility of CMC-MC with a relatively thin fibrous capsule, similar to comparable biomaterials. In conclusion, CMC-MC is an injectable, tunable and biocompatible hydrogel with strong potential to be used as an NP replacement biomaterial since it can gel in situ, match NP properties, and restore IVD height and biomechanical function. Future investigations will evaluate herniation risk under severe loading conditions and assess long-term in vivo performance.

Current strategies for treatment of intervertebral disc degeneration: substitution and regeneration possibilities

Background

Intervertebral disc degeneration has an annual worldwide socioeconomic impact masked as low back pain of over 70 billion euros. This disease has a high prevalence over the working age class, which raises the socioeconomic impact over the years. Acute physical trauma or prolonged intervertebral disc mistreatment triggers a biochemical negative tendency of catabolic-anabolic balance that progress to a chronic degeneration disease. Current biomedical treatments are not only ineffective in the long-run, but can also cause degeneration to spread to adjacent intervertebral discs. Regenerative strategies are desperately needed in the clinics, such as: minimal invasive nucleus pulposus or annulus fibrosus treatments, total disc replacement, and cartilaginous endplates decalcification.

Main body

Herein, it is reviewed the state-of-the-art of intervertebral disc regeneration strategies from the perspective of cells, scaffolds, or constructs, including both popular and unique tissue engineering approaches. The premises for cell type and origin selection or even absence of cells is being explored. Choice of several raw materials and scaffold fabrication methods are evaluated. Extensive studies have been developed for fully regeneration of the annulus fibrosus and nucleus pulposus, together or separately, with a long set of different rationales already reported. Recent works show promising biomaterials and processing methods applied to intervertebral disc substitutive or regenerative strategies. Facing the abundance of studies presented in the literature aiming intervertebral disc regeneration it is interesting to observe how cartilaginous endplates have been extensively neglected, being this a major source of nutrients and water supply for the whole disc.

Conclusion

Several innovative avenues for tackling intervertebral disc degeneration are being reported – from acellular to cellular approaches, but the cartilaginous endplates regeneration strategies remain unaddressed. Interestingly, patient-specific approaches show great promise in respecting patient anatomy and thus allow quicker translation to the clinics in the near future.

* Optimization of Preculture Conditions to Maximize the In Vivo Performance of Cell-Seeded Engineered Intervertebral Discs

The development of engineered tissues has progressed over the past 20 years from in vitro characterization to in vivo implementation. For musculoskeletal tissue engineering in particular, the emphasis of many of these studies was to select conditions that maximized functional and compositional gains in vitro. However, the transition from the favorable in vitro culture environment to a less favorable in vivo environment has proven difficult, and, in many cases, engineered tissues do not retain their preimplantation phenotype after even short periods in vivo. Our laboratory recently developed disc-like angle-ply structures (DAPS), an engineered intervertebral disc for total disc replacement. In this study, we tested six different preculture media formulations (three serum-containing and three chemically defined, with varying doses of transforming growth factor β3 [TGF-β3] and varying strategies to introduce serum) for their ability to preserve DAPS composition and metabolic activity during the transition from in vitro culture to in vivo implantation in a subcutaneous athymic rat model. We assayed implants before and after implantation to determine collagen content, glycosaminoglycan (GAG) content, metabolic activity, and magnetic resonance imaging (MRI) characteristics. A chemically defined media condition that incorporated TGF-β3 promoted the deposition of GAG and collagen in DAPS in vitro, the maintenance of accumulated matrix in vivo, and minimal changes in the metabolic activity of cells within the construct. Preculture in serum-containing media (with or without TGF-β3) was not compatible with DAPS maturation, particularly in the nucleus pulposus (NP) region. All groups showed increased collagen production after implantation. These findings define a favorable preculture strategy for the translation of engineered discs seeded with disc cells.

Strategies in regenerative medicine for intervertebral disc repair using mesenchymal stem cells and bioscaffolds

The intervertebral disc (IVD) is a major weight bearing structure that undergoes degenerative changes with aging limiting its ability to dissipate axial spinal loading in an efficient manner resulting in the generation of low back pain. Low back pain is a number one global musculoskeletal disorder with massive socioeconomic impact. The WHO has nominated development of mesenchymal stem cells and bioscaffolds to promote IVD repair as primary research objectives. There is a clear imperative for the development of strategies to effectively treat IVD defects. Early preclinical studies with mesenchymal stem cells in canine and ovine models have yielded impressive results in IVD repair. Combinatorial therapeutic approaches encompassing biomaterial and cell-based therapies promise significant breakthroughs in IVD repair in the near future.