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

Abstract

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).