Developmental regulation of caspases during death and regeneration events in an invertebrate chordate

Programmed cell death (PCD) is critical to the development and maintenance of all multicellular organisms. We study the colonial marine urochordate Botryllus schlosseri, which undergoes synchronized PCD throughout the entire generation of adult zooids during the takeover or death phase of its budding cycle (cyclic blastogenesis). The adult generation is then resorbed by circulating phagocytes and recycled, as a new asexual generation reaches functional maturity. Due to the cyclical and predictable nature of PCD in this clonally-modular animal, Botryllus is a great model organism to study the regulation of PCD via cysteinyl aspartases called “caspases”. It is well documented that caspases -8 and -9 play a role in the initiation phase of PCD, while caspases -3 and -7 are used in its execution. In this study, we investigated whether caspase expression was developmentally-regulated in Botryllus. An expressed sequence tag (EST) project was carried out over the past two years, and oligonucleotide primers were designed to amplify caspase orthologs from Botryllus. In the current study, caspase expression patterns were investigated using developmentally-staged, clonal replicates from two genotypically- identical colonies. Our findings indicate that expression of executioner caspases is developmentally-regulated during cyclic blastogenesis, with loss of steady-state expression levels during the resorption phase of takeover. However, substantial variation was found in caspase expression patterns between genotypically distinct colonies. Antibody localization and siRNA studies will be required to determine the functional involvement of specific caspases during takeover and resolve the apparent discrepancies uncovered in our study