[
International C. elegans Meeting,
2001]
A wide range of cellular processes such as muscle contraction, neurotransmitter release, secretion of hormones and cell cycling are regulated by the second messenger calcium. Intracellular calcium stores hold a key position in calcium homeostasis. These stores accumulate and release calcium in order to control cytosolic calcium levels and thus allow rapid establishment of local gradients of calcium. In addition, calcium in the lumen of the stores affects synthesis, folding, proteolytic cleavage and sorting of proteins in the endoplasmic reticulum. Removal of cytosolic calcium and filling of the stores is regulated by the activity of the Sarcoplasmic/Endoplasmic Reticulum Calcium transport ATPase (SERCA). SERCA genes are implied in a number of diseases such as Darier-White disease, Brody myopathy, cardiac hypertrophy, heart failure and type II diabetes. In higher vertebrates, three different SERCA genes exist. Protein diversity is increased by alternative splicing. The C. elegans genome contains a single SERCA gene whose transcript undergoes alternative splicing in a manner remeniscent of vertebrate SERCA2. The C. elegans and mammalian proteins show 70% identity and 80% similarity. To characterize the C. elegans SERCA gene we combined approaches in the fields of molecular biology, genetics, pharmacology and biochemistry. C. elegans SERCA cDNAs were expressed in COS cells and microsomes representing fragmented endoplasmic reticulum membranes were prepared. Phosphorylation studied showed a calcium-dependent phosphorylated intermediate, an important step in the catalytic cycle of the enzyme. Moreover, Ca 2+ -uptake experiments showed that the two isoforms have a 2-fold different affinity for Ca 2+ . Inactivation of SERCA by RNAi or gene ablation leads to embryonic or early larval lethality respectively, indicating that embryogenesis requires maternally contributed SERCA. Arrested young larvae lacking SERCA show defects in movement, pharyngeal pumping and defecation. Rescue experiments indicate an additional role for SERCA in gonadal sheath contractility. Similar defects as observed in animals lacking SERCA could be induced pharmacologically using the SERCA-specific inhibitor thapsigargin, indicating conservation of the thapsigargin-binding site. Together, these results show that SERCA is required for the function of various contractile tissues in C. elegans . This notion is supported by strong SERCA expression in all muscle types, the intestine and the gonadal myoepithelium as shown by GFP fusion constructs of both isoforms. Thus, the data presented here open a path to study SERCA function and regulation in C. elegans .