Mesenchymal Stem Cell Based Therapies for Disc Repair/Regeneration
Authors: Hoyland Judith A. , Clarke Louise , Hodgkinson Tom , Richardson Stephen M. 
Published: April 2016

Low back pain (LBP) is one of the most common musculoskeletal disorders, with an estimated 84% of the population experiencing LBP at some point in their lifetime. The prevalence of LBP increases with age, suggesting incidences of LBP are likely to increase in the future due to a global aging population, changes in lifestyle and occupational stresses. Although the causes of LBP are multifactorial, increasing evidence implicates intervertebral disc (IVD) degeneration as a major contributor, with loss of IVD integrity leading to destabilization of the spinal motion segment, resulting in pain and disability. The IVD is a complex structure that allows movement between adjacent vertebrae and sustains the load applied through the spine. It consists of an outer annulus fibrosus (AF), a ligamentous lamellar structure composed predominantly of type I collagen fibers, and a central gelatinous nucleus pulposus (NP) composed predominantly of the proteoglycan aggrecan, interspersed with type II collagen fibers. In degeneration there is an alteration in NP cell biology leading to diminished cell numbers and altered cell function (largely increased catabolism) resulting in an imbalance between matrix synthesis and degradation, particularly within the NP. Current medical treatments for IVD degeneration rely on conservative therapies (e.g., pain relief, exercise therapy) and, when these fail, surgery. Surgical treatments such as spinal fusion and disc replacement have shown satisfactory results in alleviating pain, but are not devoid of complications and long-term clinical outcomes still remain poor. Thus, there is an urgent need for alternative therapies focused on correcting the underlying pathogenesis and aberrant cell biology of IVD degeneration. As such many researchers, including ourselves, are focusing on the development of novel cell-based therapies. However, in order for these to be successful an appropriate cell source for implantation, together with a suitable growth factor to direct cell differentiation and formation of a functional matrix formation must be identified. Additionally, extensive in vitro studies are needed to establish and support further pre-clinical and potential commercial development. Having characterized the phenotype of human NP cells (a prerequisite for tissue engineering/regenerative strategies to ensure correct differentiation of cells to the target native cell) we have applied this knowledge to demonstrate that both bone marrow MSCs (BM-MSCs) and adipose-derived MSCs (AD-MSCs) are capable of differentiation toward an NP-like phenotype. Specifically, we have demonstrated that stimulation of both BM-MSCs and AD-MSCs with GDF6 (compared with other members of the TGF-β superfamily) results in improved differentiation to an NP-like phenotype and, importantly, synthesis and deposition of an extracellular matrix, rich in proteoglycan and having micromechanical properties akin to the native healthy NP. Significantly, these studies have highlighted that AD-MSCs (rather than BM-MSCs) are the more appropriate cell source for NP regeneration and that GDF6 (alias CDMP-2 or BMP-13) is the most suitable growth factor for directing differentiation. Additionally we have shown that factors in the IVD niche, namely, hypoxia and load can modulate AD-MSC differentiation and that when these factors are combined they act synergistically to promote matrix formation and increase proteoglycan synthesis. For regeneration strategies, MSCs will be implanted into the degenerate IVD niche which is a milieu of catabolic and pro-inflammatory cytokines, particularly IL-1, and thus their response such pro-inflammatory factors needs to be ascertained to ensure that catabolic events are not exacerbated. Interestingly, when AD-MSCs that have been differentiated to NP-like cells (i.e., aNPCs) are exposed to IL-1, there is no significant fold changes in gene expression for the matrix molecules (ACAN, COL2A1), matrix degrading enzymes (MMP3, MMP13), or proteoglycan synthesis when compared with untreated aNPCs. This suggests that these cells may be able to withstand the effects of the catabolic milieu in the degenerate IVD niche. Furthermore, our studies have shown that GDF6 has anabolic effects on degenerate human NP cells, stimulating adoption of a more normal NP phenotype and increasing appropriate matrix synthesis. This suggests that delivery of GDF6 as part of an MSC-based regenerative therapy may be beneficial both in directing appropriate, lineage-specific MSC differentiation, but also in restoring a healthier, more anabolic phenotype in native NP cells, thereby having a dual regenerative effect. Importantly, these in vitro studies demonstrating that GDF6 promotes AD-MSC differentiation to NP cells and synthesis of an NP-like matrix, as well as potential effects of GDF6 on resident NP cells, suggests that our proposed combined biologic and cellular therapy can provide a significant step-change from existing interventions, potentially bridging the gap between symptomatic care and aggressive surgical interventions. Disclosures: None