Spatiotemporal expression pattern of an encephalopsin orthologue of the sea urchin Hemicentrotus pulcherrimus during early development, and its potential role in larval vertical migration

We have cloned and studied Hp-ECPN, an encephalopsin orthologue of the sea urchin Hemicentrotus pulcherrimus. Hp-ecpn cDNA was produced and found to contain a 1461-bp open reading frame that encodes 486 amino acids. Accumulation of Hp-ecpn mRNA and protein expression occurred at the 14 h postfertilization (hpf) swimming blastula stage and thereafter. The Hp-ECPN protein was N-glycosylated, and the amino acid sequence was similar to that of vertebrate encephalopsins. Whole-mount immunohistochemistry revealed the presence of Hp-ECPN in cells (ECPN cells) that appeared initially around the tip of the archenteron in 20 hpf early gastrulae. By the 54 hpf pluteus stage, ECPN cells had spread through the aboral ectoderm, and, by the eight-arm pluteus stage, were restricted to the tips of the larval arms and the posterior end of the body. The number of ECPN cells increased under conditions of continuous light, but decreased under continuous dark. Knockdown of Hp-ecpn mRNA using morpholino antisense oligonucleotides decreased the number of ECPN cells considerably, and inhibited the vertical swimming of the larvae. This suggested that Hp-ECPN plays a role in photosensitive larval swimming vertical migration. In adult tissues, the ECPN cells were detected exclusively in tube feet.