Triadin Function in Sarcoplasmic Reticulum Structure

Abstract

Muscle contraction is achieved when an efficient excitation signal at the plasma membrane triggers intracellular calcium release. This process called “excitation-contraction (E-C) coupling” relies on a multimolecular protein complex, spanning the plasma membrane and the sarcoplasmic reticulum (SR), containing the calcium channel of the SR, the ryanodine receptor (RyR). Triadin is a member of this complex, present in the SR membrane and interacting with RyR in skeletal muscle. We have shown that overexpression of triadin in cultured myotubes abolishes E-C coupling, although RyR is still functional. Moreover in knock-out mice, deletion of the triadin gene leads to disorganisation of SR membranes in skeletal muscles. In the present work, we have expressed triadin in COS-7 cells to dissect its intrinsic properties on membrane organisation. We show that triadin expression leads to important modification of the endoplasmic reticulum (ER) morphology, already observed with the expression of proteins regulating ER morphology, and known as “rope-like structures”. These modifications of ER morphology are correlated to alteration of the microtubule network. Indeed, in cells expressing triadin, microtubules are bundled, often running parallel to the plasma membrane, and more stable than in untransfected cells. Surprisingly, suppression of the cytosolic N-terminal part of triadin did not reverse this phenotype. Using splice versions of the triadin protein and C-terminal deletion mutants, we show that ER/microtubules modifications depend on an intra-lumenal sequence. Altogether, our work lead to the hypothesis that modifications of ER morphology and microtubule dynamics observed in cells expressing triadin are mediated by an intermediate protein currently under investigation. Expression of triadin in COS-7 cells can modify endoplasmic reticulum morphology. It thus suggests that in skeletal muscle, triadin could play a role in the structure of sarcoplasmic reticulum to allow efficient E-C coupling.