[Regulation of localization and function of L-type calcium channels by junctophilins]

Striated muscle L-type calcium channels (LTCC) are localized specifically to the junctional membrane (JM) where the sarcolemma is closely apposed to the sarcoplasmic reticulum. Although this allocation of LTCC is critical for efficient excitation-contraction coupling in striated muscles, its underlying molecular mechanism has not been clarified. Junctophilins (JPs) stabilize the structure of JM by bridging the sarcolemmal and SR membranes. In addition, immunoprecipitation and pull-down assay revealed that the proximal C-terminus of Ca_V1.1 subunits directly binds to both JP1 and JP2, indicating that JPs might also directly recruit and hold LTCC in JM. Indeed, expression of a JP1 mutant lacking its C-terminus including the transmembrane domain in mouse skeletal muscles exerted a dominant-negative effect on endogenous JPs by impairing LTCC-RyR coupling at triads and reducing contractile force. To investigate a role of cardiac JP2 in a similar strategy, we injected adeno-associated virus vector expressing a C-terminus lacking JP2 mutant (JP2Δ427) driven by a cardiac troponin T promoter into C57BL/6 mice. Echocardiography recorded 4 weeks after the viral injection showed that the fractional shortening in JP2Δ427 group was significantly decreased compared to that of the control group. Calcium transient of isolated ventricular myocytes was significantly decreased by JP2Δ427 expression. Immunocytochemistry showed that JP2Δ427 recruited LTCC to the surface sarcolemma from T-tubules. Taken together, expression of C-terminus lacking JP mutants down-regulated contractile force by impairing ECC of skeletal and cardiac myocytes. Thus, the physical binding between LTCC and JP is essential for contraction of striated muscles.