Expression and characteristics of ion channels in osteoblasts : putative roles for TRP and K+ channels

Bone turnover is regulated by a cocktail of hormones and signalling factors controlling key cell processes such as proliferation, differentiation, mineralisation and apoptosis. Disruption to the overall mineralisation-resorption balance leads to bone disorders, such as osteoporosis - a 'silent' disease affecting around 7 million people in England and Wales. Ion channels that are presumed targets for bone signalling factors include voltage-gated K channels, ATP-dependent K channels and transient receptor potential TRP channels, and several of these channel-types reportedly have roles in cell proliferation, apoptosis, and differentiation in various tissues. This Thesis shows that human osteoblasts express a number of channels in these families, including maxi-K, ATP-dependent K channels, TRPV1 and TRPM7. The maxi-K channel, displaying characteristic electrophysiological hallmarks, is abundant in patch-clamp recordings of primary human osteoblasts implying a functional role, and the Katp agonist pinacidil is shown to promote osteoblast proliferation. Electrophysiological evidence for the TRPVI channel is not found, although the mRNA signal for a TRPVI splice variant TRPVlb may provide an answer, as it renders the channel less sensitive to capsaicin and protons. However, Ca imaging indicates that osteoblastic TRPV1 channels allow Ca2 influx, and are sensitive to 1 µM capsaicin and protons. In functional studies the TRPVI ligands capsaicin and capsazepine do not influence mineralisation, but interestingly the TRPVI agonists capsaicin, resiniferatoxin and anandamide appear to prevent differentiation of osteoblastic pre-cursor cells to adipocytes, and instead encourage maturation along the osteoblast pathway, whilst TRPV1 antagonists do not affect adipocyte differentiation. In conclusion, a number of K channels and the TRPV1 channel are expressed in osteoblasts and may have important putative roles in osteoblast cell function. Further steps are required to confirm this before the channels can be considered targets for drug development to treat bone disorders.