Evolution of Excitation-Contraction Coupling

In mammalian cardiomyocytes, Ca²⁺ influx through L-type voltage-gated Ca²⁺ channels (VGCCs) is amplified by release of Ca²⁺ via type 2 ryanodine receptors (RyR2) in the sarcoplasmic reticulum (SR): a process termed Ca²⁺-induced Ca²⁺-release (CICR). In mammalian skeletal muscles, VGCCs play a distinct role as voltage-sensors, physically interacting with RyR1 channels to initiate Ca²⁺ release in a mechanism termed depolarisation-induced Ca²⁺-release (DICR). In the current study, we surveyed the genomes of animals and their close relatives, to explore the evolutionary history of genes encoding three proteins pivotal for ECC: L-type VGCCs; RyRs; and a protein family that anchors intracellular organelles to plasma membranes, namely junctophilins (JPHs). In agreement with earlier studies, we find that non-vertebrate eukaryotes either lack VGCCs, RyRs and JPHs; or contain a single homologue of each protein. Furthermore, the molecular features of these proteins thought to be essential for DICR are only detectable within vertebrates and not in any other taxonomic group. Consistent with earlier physiological and ultrastructural observations, this suggests that CICR is the most basal form of ECC and that DICR is a vertebrate innovation. This development was accompanied by the appearance of multiple homologues of RyRs, VGCCs and junctophilins in vertebrates, thought to have arisen by 'whole genome replication' mechanisms. Subsequent gene duplications and losses have resulted in distinct assemblies of ECC components in different vertebrate clades, with striking examples being the apparent absence of RyR2 from amphibians, and additional duplication events for all three ECC proteins in teleost fish. This is consistent with teleosts possessing the most derived mode of DICR, with their Ca_v1.1 VGCCs completely lacking in Ca²⁺ channel activity.