Recognition and clearance of apoptotic cells during the colonial blastogenetic cycle in the compound ascidian Botryllus schlosseri.

Colonies of the compound ascidian Botryllus schlosseri feature cyclical generation changes, allowing continuous renewing of the colony, during which old zooids synchronously cease filtering and are resorbed in 24-26 h, while a new generation of buds reach the functional maturity, open their siphons and start filtering. It is possible to define a colonial blastogenetic cycle corresponding to the time interval of functional activity of a zooid generation (one week at 20°C), starting with the opening of the siphons of a new zooid generation and ending with the resorption or take-over (TO) of old zooids. During the TO, diffuse apoptosis occurs in zooid tissues followed by the massive recruitment of phagocytes which leave the circulation and infiltrate zooid tissues. The interaction between phagocytes and effete cells, leading to their internalisation, is fundamental for the progression of the TO as colonies do not feed during the generation change and the products of the digestion of senescent cells represent the main source of nutrients for the colony during this period. The recognition and engulfment of apoptotic cells is greatly promoted by molecules with opsonic activity, able to act as a bridge between the surface of effete cells and that of phagocytes. B. schlosseri rhamnose-binding lectin (BsRBL) is a new member of the rhamnose-binding protein family, which exerts an opsonic role towards non-self cells. During the generation change, we observed an increase in the levels of mRNA, as revealed by sqRT-PCR, and of the protein, in immunoblot analysis, during the TO with respect to phases of the blastogenetic cycle far from the TO (mid-cycle; MC). This goes in parallel with a change in surface carbohydrates in apoptotic cells and an increased number of cells recognised by anti-BsRBL antibody at TO with respect to MC. All these data are consistent with a pivotal role of BsRBL in the interaction between phagocytes and apoptotic cells and corpses, never described before, and open new perspectives on the possible role of RBLs in vertebrates.