Characterization of TRPV4 in inherited disorders

© 2012 Yuan YuanThe Transient Receptor Potential Vanilloid 4 channel (TRPV4) is a Ca2+ permeable, nonselective cation channel that is important for sensation of several physical and chemical stimuli and also involved in multiple physiological functions. Recently it gained immense scientific and clinical interest as several independent studies demonstrated that TRPV4 mutations cause heritable genetic disorders including several skeletal dysplasias and peripheral neuropathies. TRPV4 thus plays a critical role in skeletal and neurological development. Our recent gene linkage studies on a novel skeletal disorder called Familial Digital Arthropathy-Brachydactyly (FDAB) identified mutations in TRPV4. The main objective of the present study was to comprehensively understand the effects of FDAB mutations on channel function. To facilitate this aim, stable cell lines expressing TRPV4 wild type and mutants were generated. FDAB mutants expressed in HEK293 cells showed a defect in maturation from high mannose to complex glycosylation. Our data also show that constitutive channel activity was increased in cells expressing mutant ion channels compared to wild type TRPV4. Responses to the synthetic agonists, GSK1016790A and 4αPDD, were reduced, reflecting less cell surface expression of TRPV4. In addition, responses to hypotonicity were significantly reduced, suggesting a disruption of the normal mechanism of hypotonic activation of TRPV4. Close analysis of the data obtained from these naturally occurring genetic mutants as well as other TRPV4 mutants suggest that it is not the altered channel activity of these mutants per se, but the involvement and interaction of other factors that seem to modulate oligomerization, trafficking and degradation of TRPV4 channels. TRPV4 functions not alone but as part of a multiprotein signaling complex that is critical for mechanosensation, pain, hyperalgesia and other physiological functions. Signaling complexes are an evolutionary adaptation that enhance the sensitivity and efficiency of signaling between sensors and effectors (e.g., ion channels), and which confine signaling events to precise subcellular locations. The study was also designed to define key components of the TRPV4 signaling complex and to examine the hypothesis that disruption of components of the signaling complex will affect channel function. A receptor for activated protein kinase C (RACK1) was identified as a novel candidate signaling complex component, however, the role of RACK1 in TRPV4 function still needs to be verified experimentally. In summary, this work identifies TRPV4 mutations that lead to reduced channel activation as a new mechanism underlying an inherited osteoarthropathy and raises the possibility that modulating TRPV4 activity could provide a new therapeutic approach for osteoarthritis. The identification of RACK1 as a component of the TRPV4 protein complex validates our hypothesis that TRPV4 exists in a signaling complex and also presents insights into novel therapeutic strategies for pain syndromes