Reduced tissue osmolarity increases TRPV4 expression and pro-inflammatory cytokines in intervertebral disc cells

Abstract

The mechanical behaviour and cellular metabolism of intervertebral discs (IVDs) and articular cartilage are strongly influenced by their proteoglycan content and associated osmotic properties. This osmotic environment is a biophysical signal that changes with disease and may contribute to the elevated matrix breakdown and altered biologic response to loading observed in IVD degeneration and osteoarthritis. This study tested the hypothesis that changes in osmo-sensation by the transient receptor potential vallinoid-4 (TRPV4) ion channel occur with disease and contribute to the inflammatory environment found during degeneration. Immunohistochemistry on bovine IVDs from an inflammatory organ culture model were used to investigate if TRPV4 is expressed in the IVD and how expression changes with degeneration. Western blot, live-cell calcium imaging, and qRT-PCR were used to investigate whether osmolarity changes or tumour necrosis factor α (TNFα) regulate TRPV4 expression, and how altered TRPV4 expression influences calcium signalling and pro-inflammatory cytokine expression. TRPV4 expression correlated with TNFα expression, and was increased when cultured in reduced medium osmolarity and unaltered with TNFα-stimulation. Increased TRPV4 expression increased the calcium flux following TRPV4 activation and increased interleukin-1β (IL-1β) and IL-6 gene expression in IVD cells. TRPV4 expression was qualitatively elevated in regions of aggrecan depletion in degenerated human IVDs. Collectively, results suggest that reduced tissue osmolarity, likely following proteoglycan degradation, can increase TRPV4 signalling and enhance pro-inflammatory cytokine production, suggesting changes in TRPV4 mediated osmo-sensation may contribute to the progressive matrix breakdown in disease.

The role of transient receptor potential channels in joint diseases

“…Evidence suggests that another member of the TRPV family, namely TRPV4, is functionally expressed in bovine IVD cells stimulated by proinflammatory cytokines (Walter et al, 2016). TRPV4 could also be involved in mediating thermal hyperalgesia and inflammatory swelling in inflamed joints (Keeble et al, 2005).…”

“…In NP cells, reduced osmolarity activates the expression of TRPV4 and at the same time induces TRPV4-mediated Ca 2+ signalling and gene expression of pro-inflammatory cytokines. Albeit based on the analysis of only one sample, it is proposed that TRPV4 expression is elevated in regions of aggrecan depletion in degenerated human IVD, indicating that TRPV4 can possibly play a role in swelling-related IVD inflammation and ECM breakdown (Walter et al, 2016). The authors suggest that reduced tissue osmolarity, following proteoglycan degradation, can increase TRPV4 signalling, enhance proinflammatory cytokine production and contribute to progressive matrix breakdown (Walter et al, 2016).…”

“…Albeit based on the analysis of only one sample, it is proposed that TRPV4 expression is elevated in regions of aggrecan depletion in degenerated human IVD, indicating that TRPV4 can possibly play a role in swelling-related IVD inflammation and ECM breakdown (Walter et al, 2016). The authors suggest that reduced tissue osmolarity, following proteoglycan degradation, can increase TRPV4 signalling, enhance proinflammatory cytokine production and contribute to progressive matrix breakdown (Walter et al, 2016). The mechanism of osmolarity-dependent TRPV4 gene expression in chondrocytes includes phosphorylation of ERK1/2 (Hdud et al, 2014) and possibly also osmolarity-associated NFAT5 expression, but the role of NFAT5 in TRPV4-mediated effects has not yet been investigated.…”

TRPC6 in simulated microgravity of intervertebral disc cells

“…Furthermore, free calcium seems to contribute to IVD degeneration through the calcium-sensing receptor (CaSR) [22]. Recently, the expression of transient receptor potential (TRP) channels, a superfamily of ion channel with relevance in calcium regulation [23], has been demonstrated in the IVD [24][25][26]. The members of the canonical family (TRPC) are especially interesting.…”

The potential of CRISPR/Cas9 genome editing for the study and treatment of intervertebral disc pathologies

“…Similar to these cell lines, IVD cells with stably expressed Cas9 could aid in explaining the relationship between IVD genotype and phenotype in the future. Recent studies on IVD biology highlighted the importance of membrane proteins, namely, of cell surface receptors (eg, toll‐like receptor (TLR) TLR2), channels (eg, aquaporins, transient receptor potential cation channel subfamily V member 4 (TRPV4)), and adhesion molecules (eg, integrins) as these proteins allow IVD cells to sense their environment and respond to its changes. Although the expression and activity of these molecules are altered during DDD, understanding their exact function has been challenging so far as inhibitors/antagonists and activators/agonists are often nonselective or have short half‐lives .…”