Characterization of synaptopodin in striated and smooth muscles: isoform spectrum, expression patterns, localization and protein interactions

Synaptopodin (SYNPO), founding member of the family of podin proteins, was mainly studied in the kidney and the brain. Here, SYNPO was initially described to be expressed in renal podocytes and the endoplasmic reticulum (ER-)related spine apparatus in dendritic spines, respectively. More recently, SYNPO expression was also reported in epithelial cells, and database analyses even indicate ubiquitous expression, including high expression in striated and smooth muscle cells. Hrt (hard return) our analyses at RNA and protein levels confirm expression of distinct SYNPO variants in all striated and smooth muscle cell-containing organs analyzed and identify isoform-specific expression of SYNPO in human and mouse tissues. Immunostaining of tissue sections revealed diffuse localization at myofibrillar Z-discs in striated muscle fibers and at intercalated discs of cardiomyocytes. In skeletal muscle fibers, but also in cardiomyocytes, SYNPO partially colocalizes with longitudinal sarcoplasmic reticulum (SR-)associated STIM1. In smooth muscle cells, SYNPO shows a reticular distribution, compatible with potential co-distribution with the SR in these cells. In addition to previously reported interactions with actin and α-actinin, our protein interaction studies reveal a small region of SYNPO, homologous to the filamin C (FLNc)-binding region of SYNPO2, to bind FLNc. Transfection of SYNPO isoforms indicate close association with myofibrillar Z-discs in skeletal myotubes as well as stress fiber dense bodies and focal adhesions in smooth muscle cells. Our data identify strong expression of SYNPO variants in all muscle cell types, and indicate a role for SYNPO in the interaction between the actin cytoskeleton and the sarcoplasmic reticulum.