Discovery and Characterization of Genes Involved in Muscle Calcium Handling

Muscle tissue requires continuous cycling of calcium release and clearance to generate and sustain contraction. When the plasma membrane of a muscle fiber becomes electrically excited, a voltage sensor in the membrane, called the dihydropyridine receptor (DHPR), becomes activated and signals to the calcium release channel located in the membrane of the sarcoplasmic reticulum (SR), called the ryanodine receptor (RyR). When the RyR channel is activated, calcium is released into the cytoplasm from the SR, the primary calcium storage compartment of muscle fibers. Calcium then binds to the sarcomere, activating the motor activity of the myosin filaments, and causes the fiber to contract. Following contraction, most of the cytosolic and sarcomeric calcium is recycled back to the SR by the sarco/endoplasmic reticulum ATPase (SERCA). Although most components of the calcium handling pathway are thought to be already known, the large number muscle-specific genes with unknown functions would suggest that additional components may yet be undiscovered. The first goal of this study was to carry out initial functional characterization of the Stac3 knockout mouse, in which muscle contraction is severely defective. We found that mice lacking Stac3, a gene with skeletal muscle-specific expression, lack muscle contraction because of a defect in excitation-contraction coupling, that is the link between membrane excitation and SR calcium release. The second goal was to identify small peptides that may play a role in muscle function. We examined codon conservation in transcripts annotated as long non-coding RNAs and discovered a transcript that encodes a 34-amino-acid transmembrane peptide with cardiac and slow-twitch muscle expression that we have named Dwarf Open Reading Frame or DWORF. Overexpression of this peptide in cardiac myocytes increases the peak calcium release during contraction while also increasing the rate of calcium clearance. We conclude that DWORF likely serves to increase the apparent enzymatic activity of SERCA. Together the discoveries of Stac3 and Dworf suggest that many important genes in muscle function may be awaiting a closer look or have not yet been discovered at all.