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

We have previously reported a synthetic Laponite® crosslinked pNIPAM‐co‐DMAc (NPgel) hydrogel, which induces nucleus pulposus (NP) cell differentiation of human MSCs (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 IVD. Here, we investigated the viability and differentiation of hMSCs within NP gel within a catabolic microenvironment. Human MSCs were encapsulated in NPgel and cultured for 4 weeks under hypoxia (5% O2) with ± calcium, IL‐1β and TNFα either individually or in combination to mimic the degenerate environment. Cell viability, and cellular phenotype was 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 MMP's, ADAMTS 4 and Col I expression by hMSCs cultured in NPgel was upregulated by calcium but not by pro‐inflammatory 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 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.