Mesenchymal stem cell therapies for intervertebral disc degeneration: Consideration of the degenerate niche

We have previously reported a synthetic Laponite crosslinked poly N-isopropylacrylamide-co-N, N'-dimethylacrylamide (NPgel) hydrogel, which induces nucleus pulposus (NP) cell differentiation of human mesenchymal stem cells (hMSCs) without the need for additional growth factors. Furthermore NP gel supports integration following injection into the disc and restores mechanical function to the disc. However, translation of this treatment strategy into clinical application is dependent on the survival and differentiation of hMSC to the correct cell phenotype within the degenerate intervertebral disc (IVD). Here, we investigated the viability and differentiation of hMSCs within NP gel within a catabolic microenvironment. hMSCs were encapsulated in NPgel and cultured for 4 weeks under hypoxia (5% O₂) with ± calcium, interleukin-1β (IL-1β), and tumor necrosis factor alpha (TNFα) either individually or in combination to mimic the degenerate environment. Cell viability and cellular phenotype were investigated. Stem cell viability was maintained within hydrogel systems for the 4 weeks investigated under all degenerate conditions. NP matrix markers: Agg and Col II and NP phenotypic markers: HIF-1α, FOXF1, and PAX1 were expressed within the NPgel cultures and expression was not affected by culture within degenerate conditions. Alizarin red staining demonstrated increased calcium deposition under cultures containing CaCl₂ indicating calcification of the matrix. Interestingly matrix metalloproteinases (MMPs), ADAMTS 4, and Col I expression by hMSCs cultured in NPgel was upregulated by calcium but not by proinflammatory cytokines IL-1β and TNFα. Importantly IL-1β and TNFα, regarded as key contributors to disc degeneration, were not shown to affect the NP cell differentiation of mesenchymal stem cells (MSCs) in the NPgel. In agreement with our previous findings, NPgel alone was sufficient to induce NP cell differentiation of MSCs, with expression of both aggrecan and collagen type II, under both standard and degenerate culture conditions; thus could provide a therapeutic option for the repair of the NP during IVD degeneration.