A possible role of the junctional face protein JP-45 in modulating Ca2+ release in skeletal muscle

摘要

We investigated the functional role of JP-45, a recently discovered protein of the junctional face membrane (JFM) of skeletal muscle. For this purpose, we expressed JP-45 C-terminally tagged with the fluorescent protein DsRed2 by nuclear microinjection in myotubes derived from the C2C12 skeletal muscle cell line and performed whole-cell voltage-clamp experiments. We recorded in parallel cell membrane currents and Ca2+ signals using fura-2 during step depolarization. It was found that properties of the voltage-activated Ca2+ current were not significantly changed in JP-45–DsRed2-expressing C2C12 myotubes whereas the amplitude of depolarization-induced Ca2+ transient was decreased compared to control myotubes expressing only DsRed2. Converting Ca2+ transients to Ca2+ input flux using a model fit approach to quantify Ca2+ removal, the change could be attributed to an alteration in voltage-activated Ca2+ permeability rather than to altered removal properties or a lower Ca2+ content of the sarcoplasmic reticulum (SR). Determining non-linear capacitive currents revealed a reduction of Ca2+ permeability per voltage-sensor charge. The results may be explained by a modulatory effect of JP-45 related to its reported in vitro interaction with the dihydropyridine receptor and the SR Ca2+ binding protein calsequestrin (CSQ).