Novel Locations; Familiar Functions: Obscurin at the Cardiac Intercalated Disc

Abstract

The intercalated disc (ID) of cardiac muscle embodies a highly ordered, multifunctional network, which is essential for the transmission of electrical stimuli and mechanical force resulting in the synchronous contraction of the heart. Recently, a plethora of proteins have been identified as novel components of the ID. The challenge now lies in their characterization as it relates to the mechanical and electrical coupling of neighbouring cardiomyocytes. Here we focus on the molecular and functional description of one of these novel members, obscurin-90.Obscurins are a family of proteins expressed in striated muscles where they localize to distinct subdomains, i.e. the sarcoplasmic reticulum, the sarcomeric cytoskeleton, and the sarcolemma, and function in their assembly and integration. Previous studies have shown that transcripts arising from the single OBSCN gene are extensively spliced, resulting in several obscurin isoforms. Complex splicing at its 3' end gives rise to obscurin-90 (obsc90), the isoform that preferentially localizes to the ID. Obsc90 contains tandem RhoGEF and PH motifs, two Ig domains, and a non-modular COOH-terminus with ankyrin-binding sites and ERK phosphorylation cassettes. Using immunofluorescence and immunoelectron microscopy, we show that obsc90 localizes to the ID, and specifically at the outer edge of the transition zone. Consistent with this, biochemical assays demonstrated that obsc90 is in a complex with major ID proteins, including N-cadherin, connexin-43, vinculin, and ankyrinG. Obsc90 is targeted to the ID through the direct binding of its PH domain to membrane lipids, and its functions are likely regulated by phosphorylation of its COOH-terminus. Preliminary evidence indicates that the phosphorylation profile of obsc90 is altered in hypertrophic cardiomyopathy.. Further experiments are underway to examine the function of obsc90 at the ID and its regulation via phosphorylation in health and disease.