Osteoporosis and other diseases of skeletal fragility affect more than 200 million people worldwide and contributes to ~9 million factures annually. Preventing bone loss and/or restoring lost bone mass in patients is of vital importance to limiting the personal and economic impact of diseases of skeletal fragility. A key target in the stimulation of new bone formation is the protein sclerostin, an antagonist of the Wnt/beta-catenin signaling cascade, which is produced by bone embedded osteocytes. Numerous osteoanabolic cues, including mechanical load, reduce expression of the sclerostin leading to ?de-repression? of osteoblastogenesis and stimulation of de novo bone formation. However, key mechanistic details of how osteocytes sense mechanical load, transduce these load signals to biologic effectors, the identity of these biological effectors and how sclerostin bioavailability is regulated are unclear. Our preliminary data have uncovered a number of novel mediators of how osteocytes sense and respond to mechanical cues. Specifically, we show that microtubule- dependent cytoskeletal stiffness regulates mechano-activated Ca2+ influx. Furthermore, we implicate TRPV4 as a major mechano-dependent Ca2+ influx pathway that drives Ca2+ dependent activation of calcium/calmodulin-dependent kinase II (CamKII) to reduce sclerostin bioavailability in the osteocyte. In the present grant, we will use in vitro, ex vivo and in vivo models to determine the contribution of MT density and cytoskeletal crosslinking to osteocyte mechanosensing, define the contribution and mechanisms of osteocyte TRPV4 channel opening in response to mechanical stress and elucidate the mechanisms by which FFSS- dependent CamKII activation regulates sclerostin degradation and Sost gene transcription. This work will more fully explain the biological regulation of sclerostin, will mechanistically link several gaps in the knowledge of how osteocytes sense and respond to mechanical load, and will reveal novel targets to improve or preserve bone mass in aging and disease.
There is a consensus that bone embedded osteocytes sense mechanical load, leading to the regulation of sclerostin, a key modulator of bone remodeling. However, fundamental aspects of how these cells sense and respond to mechanical load and how calcium influx leads to changes in sclerostin bioavailability are unknown. In this grant, we explore the role of the microtubules in modulating mechano-responsiveness by tuning cytoskeletal stiffness, impacting calcium channel activation, signal transduction and the transcriptional and translational control of sclerostin ultimately affecting bone quality.
|Lyons, James S; Joca, Humberto C; Law, Robert A et al. (2017) Microtubules tune mechanotransduction through NOX2 and TRPV4 to decrease sclerostin abundance in osteocytes. Sci Signal 10:|
|Lyons, James S; Iyer, Shama R; Lovering, Richard M et al. (2016) Novel multi-functional fluid flow device for studying cellular mechanotransduction. J Biomech 49:4173-4179|