Novel Roles for Obscurin Proteins in Cardiac Muscle

Obscurin is a giant myofibrillar protein important for sarcomere structure, signaling, and maintenance of sarcoplasmic reticulum (SR) organization. Surprisingly, obscurin knockout mice develop normally and only show signs of a mild skeletal muscle myopathy. However, little is known about cardiac functions of obscurin. We characterized cardiac roles of obscurin and found no changes to cardiac physiology. One explanation for the lack of a cardiac phenotype in obscurin knockout mice is that other obscurin protein family members possess a functionally redundant role. We hypothesized that obscurin-like 1 (Obsl-1) is responsible for this redundancy, in part due to its shared ability to bind the sarcomeric proteins titin and myomesin. We generated double knockout (dKO) mice by crossing the obscurin-KO mice with a cardiac specific Obsl1-KO line to investigate the roles for these proteins in cardiac development and function. Although a majority of the dKO-mice died after 12 months of age, microscopic and ultrastructural analysis failed to reveal overt changes to sarcomere organization. Dramatic changes were observed, however, in SR structure and function. Specifically we observed: 1.) loss of SR structural integrity and apparent loss of SR content, and 2.) alterations to Ca2+ transients and in the levels of Ca2+ handling proteins in KO animals. The majority of these changes were more severe in dKO-mice. Furthermore, we employed serial blockface TEM technology to precisely measure the volume of the SR and mitochondria and noticed significant differences between KO and control animals. Physiologically, these mice develop a late onset cardiomyopathy characterized by increased systolic left ventricular internal dimension and alterations in hemodynamic properties at ∼12 mo. of age. Our data suggests that obscurin and Obsl1 are not critical for sarcomerogenisis, however are important for jointly organizing SR organization, cellular Ca2+ handling, and long-term survival in mice.