Abstract
TRPC6 is a nonselective cation channel, and mutations of this gene are associated with FSGS. These mutations are associated with TRPC6 current amplitude amplification and/or delay of the channel inactivation (gain-of-function phenotype). However, the mechanism of the gain-of-function in TRPC6 activity has not yet been clearly solved. We performed electrophysiologic, biochemical, and biophysical experiments to elucidate the molecular mechanism underlying calmodulin (CaM)-mediated Ca2+-dependent inactivation (CDI) of TRPC6. To address the pathophysiologic contribution of CDI, we assessed the actin filament organization in cultured mouse podocytes. Both lobes of CaM helped induce CDI. Moreover, CaM binding to the TRPC6 CaM-binding domain (CBD) was Ca2+-dependent and exhibited a 1:2 (CaM/CBD) stoichiometry. The TRPC6 coiled-coil assembly, which brought two CBDs into adequate proximity, was essential for CDI. Deletion of the coiled-coil slowed CDI of TRPC6, indicating that the coiled-coil assembly configures both lobes of CaM binding on two CBDs to induce normal CDI. The FSGS-associated TRPC6 mutations within the coiled-coil severely delayed CDI and often increased TRPC6 current amplitudes. In cultured mouse podocytes, FSGS-associated channels and CaM mutations led to sustained Ca2+ elevations and a disorganized cytoskeleton. The gain-of-function mechanism found in FSGS-causing mutations in TRPC6 can be explained by impairments of the CDI, caused by disruptions of TRPC's coiled-coil assembly which is essential for CaM binding. The resulting excess Ca2+ may contribute to structural damage in the podocytes.
Highlights
Transient Receptor Potential Canonical 6 (TRPC6) is a nonselective cation channel, and mutations of this gene are associated with FSGS
CaM binding to the TRPC6 CaM-binding domain (CBD) was Ca2+-dependent and exhibited a 1:2 (CaM/CBD) stoichiometry
Deletion of the coiled-coil slowed Ca2+-dependent inactivation (CDI) of TRPC6, indicating that the coiled-coil assembly configures both lobes of CaM binding on two CBDs to induce normal CDI
Summary
Biochemical, and biophysical experiments to elucidate the molecular mechanism underlying calmodulin (CaM)-mediated Ca2+-dependent inactivation (CDI) of TRPC6. Mutations in the CBD or coiled-coil of TRPC6 (including the deletion of coiled-coil mutation which was terminated at 875 a.a.) were generated by overlap extension PCR utilizing mutagenic primers in which segments were replaced into the SacI and XbaI sites of the TRPC6 expression vector. Coding regions at 1275 a.a. and 762145 a.a. of CaMWT obtained using PCR were respectively cloned to generate the CaM N-lobe (CaMDC) and C-lobe (CaMDN) in pCI-neo. The cDNAs encoding the CBD segment of a human TRPC6 CBD segment spanning residues 8532874 a.a. and the CBD with the coiled-coil (CBD-CC) segment spanning residues 8532927 a.a. were generated by PCR and digested at the NotI and XbaI sites, after which YFP or CFP was fused to the N-terminal side of the CBD or CBD-CC at the NheI and NotI sites in the pIRESn expression vector.
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