Morphological Modifications of the Early Secretory Pathway in Differentiating Skeletal Muscle Cells

Skeletal muscle (SKM) cells present a regular and striking arrangement of organelles and membrane systems that is essential for their function. During SKM differentiation, membrane systems as transverse (T) tubules, sarcoplasmic reticulum (SR), and Calcium Release Units (CRUs) follow a highly coordinated plan. In addition to remodeling processes, SKM cell differentiation involves synthesis and transport of proteins addressed to build up the contractile apparatus, the SR and the sarcolemma. This process most probably requires an important involvement of early secretory pathway components as endoplasmic reticulum (ER), ER exit sites (ERES), Golgi Complex (GC), and vesicular traffic in general.In terminally differentiated SKM cells, early secretory pathway components are regularly arranged; the GC is organized fiber-type dependent and, formed by small elements localized around the nucleus and in throughout all the fiber. It has been shown that early secretory pathway components undergo a dramatic reorganization during SKM cell differentiation resulting in the formation of smaller GC elements that are strongly associated to ERES. Whether GC reorganization consists in fragmentation or ex-novo biogenesis is not known, and whether its co-localization with ERES is the result of structural hindrance by the myofibrils and membrane systems, or depends on interaction mechanisms has not been explained.The present work aims at better understanding how GC and ERES achieve their mature structure, what is their role during SKM differentiation, and whether morphological modifications are followed by alterations of membrane traffic efficiency. Therefore we investigated the processes underlying the morphological modifications of the secretory pathway structures as CG and ERES during SKM differentiation by applying live imaging techniques.We report preliminary data on the morphological modifications occurring to GC and ERES in differentiated C2C12 cells.