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. It has been largely investigated on the structural events occurring during the organization of transverse (T) tubules, sarcoplasmic reticulum (SR), and Calcium Release Units (CRUs), and it has been proposed that during SKM differentiation, membrane systems follow a highly coordinated plan. In addition, SKM cell differentiation and remodeling requires synthesis and transport of an enormous amount of proteins addressed to build up the contractile apparatus, the SR and the sarcolemma. This process most probably requires an important involvement of endoplasmic reticulum (ER), ER exit sites (ERES), Golgi Complex (GC), or early secretory pathway components, and vesicular traffic in general. In terminally differentiated skeletal muscle cells early secretory pathway components are regularly arranged; the GC is organized in a unique fashion, fiber-type dependent and, formed by very small GC elements, which are localized around the nucleus and in all the fiber. GC elements in mature muscle fibers are very small compared to other cells, but still formed by cisternae. It has been shown that GC undergoes a dramatic reorganization during SKM cell differentiation resulting in the formation of small elements, and interestingly, in mature myotubes GC is found 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 is aimed at a 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 transformation of the secretory pathway structures as CG and ER exit sites (ERES) during SKM differentiation. The project envisaged the analysis of the localization of selected markers of secretory pathway components in C2C12 differentiating myoblasts and regenerating fibers by applying imaging techniques in order to follow the differentiation of the early secretory pathway compartments. We report preliminary data on the morphological modifications occurring to GC and ERES in differentiated C2C12 cells.