Muscle contraction represents a well characterized molecular process that is, to a considerable extent, regulated by variable Ca²⁺concentrations within the cytosol.
A major player responsible for modulating this concentration is the Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase (SERCA) that transports Ca²⁺ from the cytosol into the sarcoplasmic reticulum (SR). This activity significantly reduces cytosolic Ca²⁺ levels and thus marks the beginning of the muscle relaxation phase. Accordingly, tight regulation of SERCA-activity is critical to proper functionality of muscle tissue.
As known for vertebrates and insects, activity of SERCA is regulated by peptides that bind to the enzyme and thereby modulate its activity. Vertebrate and fly loss-of-function mutants for corresponding peptides exhibit impaired Ca²⁺ transients in heart cells as well as severe heart arrhythmia. Up to now, it is largely unknown how homeostasis and turnover of such peptides are regulated.
We found that increased expression of the peptidase Neprilysin 4 (Nep4) phenocopied the depicted effects on heart physiology. Significantly, Nep4 catalytic activity was essential, thus confirming abnormal peptide hydrolysis as a causative factor. By combining in vitro and in vivo analyses, we identified Nep4 mediated hydrolysis of SERCA regulatory Sarcolamban peptides as the critical physiological event. Furthermore, Nep4 and SERCA coprecipitated and exhibited substantial colocalization with Sarcolamban in membranes of the SR, indicating that close spatial proximity is of high mechanistic relevance.
By identifying a neprilysin as a novel and essential regulator of SERCA activity, these data advance the current understanding of muscle physiology and functionality in health and disease.