Small Obscurins at the Intercalated Disc Mediate Structure, Cell Size, and Adhesion

Abstract

The intercalated disc (ID) of cardiac muscle embodies a highly ordered, multifunctional network, essential for the transmission of electrical stimuli and mechanical force resulting in the synchronous beating 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 coupling of neighbouring cardiomyocytes and whole heart function. Here we focus on the functional characterization of two novel ID proteins, obscurin-40 and obscurin-80.Obscurins are a family of proteins expressed in striated muscle cells where they localize to distinct substructures, contributing to their assembly and integration with other sarcoplasmic elements. Our previous studies have shown that the obscurin transcript, arising from the single OBSCN gene is extensively spliced, resulting in several obscurin isoforms. Complex splicing at the 3’ end of OBSCN results in at least two novel products, obsc-40 and obsc-80, which preferentially concentrate at the ID. Consistent with their ID subcellular localization, biochemical assays demonstrated that obsc-40 and obsc-80 are in a complex with major ID proteins, including N-cadherin, connexin-43, vinculin, and ankyrin-G. In addition, the obscurin pleckstrin homology (PH) domain present in both obsc-40 and obsc-80 binds specifically and directly to phosphatidylinositol 3,4 and 4,5 bisphosphates, likely targeting both proteins to the ID membrane. Overexpression of the obscurin-PH domain in a cardiac derived cell line results in decreased activation of the PI3K/Akt and mTOR pathways. These molecular alterations lead to decreased cellular aggregation and overall cardiomyocyte size. Furthermore, both obsc-40 and obsc-80, but not giant obscurins, exhibit reduced expression in myocardial infarction and transaortic constriction cardiac-stress models. Taken together these important findings implicate these novel small obscurins in the maintenance of heart size and the prevention of hypertrophy.