Obscurins’ Mechanistic Involvement in Signal Transduction 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. Here we focus on the molecular and functional description of two novel members of the ID, obscurin-80 and obscurin-40.Obscurins are a family of proteins expressed in striated muscles where they localize to distinct subdomains. The members of the obscurin family are multidomain proteins composed of adhesion modules and signaling domains, resulting from extensive alternative splicing of transcripts arising from the single OBSCN gene. Recent work from our laboratory has demonstrated that complex splicing at the 3’ end of the obscurin transcript gives rise to at least two novel obscurins, obscurin-80 (obsc-80) and obscurin-40 (obsc-40), named after their predicted molecular weights.Using immunofluorescence and immunoelectron microscopy, we show that obsc-80 and obsc-40 localize to the ID of developing and adult murine cardiomyocytes. Using biochemical assays we further demonstrate that both obsc-80 and obsc-40 exist in a complex with major ID proteins, including N-cadherin, connexin-43, vinculin, and ankyrin-G. The PH domain present in both obsc-80 and obsc-40 binds specifically and directly binds to phosphatidylinositol 3,4 and 4,5 bisphosphates, likely targeting both proteins to the ID membrane. Overexpression of the obscurin PH domain results in decreased phosphorylation of Akt, therefore reducing Akt activation. This suggests a potential role for obsc-80 and obsc-40 in the regulation of cell growth and proliferation via the Akt pathway. Further experiments are underway to examine the functional activities of obsc-80 and obsc-40 at the ID and their regulation in health and disease.