Mutations in TRPV4 have been linked to three distinct axonal neuropathies. However, the pathogenic mechanism underlying these disorders remains unclear. Both gain and loss of calcium channel activity of the mutant TRPV4 have been suggested. Here, we show that the three previously reported TRPV4 mutant channels have a physiological localization and display an increased calcium channel activity, leading to increased cytotoxicity in three different cell types. Patch clamp experiments showed that cells expressing mutant TRPV4 have much larger whole-cell currents than those expressing the wild-type TRPV4 channel. Single channel recordings showed that the mutant channels have higher open probability, due to a modification of gating, and no change in single-channel conductance. These data support the hypothesis that a "gain of function" mechanism, possibly leading to increased intracellular calcium influx, underlies the pathogenesis of the TRPV4-linked axonal neuropathies, and may have immediate implications for designing rational therapies.Scapuloperoneal spinal muscle atrophy (SPSMA), congenital distal spinal muscle atrophy (CDSMA), and Charcot-MarieTooth disease type 2C (CMT2C, also known as hereditary motor and sensory neuropathy type 2, HMSN IIC) are phenotypically heterogeneous and dominantly inherited disorders involving topographically distinct muscles and nerves (1, 2). Genetic linkage analysis mapped the disease loci to an overlapping region at 12q24 (3-5). Recently, it has been shown that these three axonal neuropathies are allelic disorders caused by mutations in the gene encoding the transient receptor potential cation channel, subfamily V (vanilloid), member 4 (TRPV4) 3 (6 -8). To date, seven mutations in TRPV4 have been identified in 15 families with axonal neuropathies (6 -10). Functional studies, however, yielded divergent hypotheses about the pathogenic mechanism of the diseases (6 -8, 10).In our original study, we tested two mutations, R316C and R269H, which were identified in the original SPSMA and CMT2C families, respectively (6). We found that in HEK293 cells, mutant TRPV4 had a physiological localization on the plasma membrane similar to wild-type TRPV4 (wtTRPV4). Calcium imaging showed that mutant TRPV4 had significantly increased calcium channel activity, at both basal and activated levels when compared with wtTRPV4. These findings were consistent with electrophysiological recordings, which showed a higher TRPV4 conductance in HEK293 cells expressing the R269H mutant compared with wtTRPV4. In another study, Landouré et al. (8) showed similar results in HEK293 cells and Xenopus laevis oocytes with R269H and R269C mutations. Furthermore, mutant TRPV4-mediated cytotoxicity was observed in transfected HEK293 cells and dorsal root ganglion neurons. This cytotoxicity was related to increased intracellular calcium concentrations (8). More recently, we have also shown that two other TRPV4 mutants (R232C and R316H) localize appropriately and cause reversible and toxic hypercalcemia in HEK293 and HeLa c...