THE EXPRESSION OF MDP-1, A COMPONENT OF DROSOPHILA EMBRYONIC BASEMENT-MEMBRANES, IS MODULATED BY APOPTOTIC CELL-DEATH

Using a novel monoclonas antibody we have studied the expression of a large proteoglycan-type molecule in Drosophila embryos. This molecule is secreted exclusively by migratory, embryonic hemocytes/macrophagus and was therefore named MDP-1 for Macrophage-Derived Proteoglycan-l. E xpression of MDP-1 begins late during hemocyte differentiation, after these cells have left their birthplace in the head mesoderm. At this t ime, macrophages are engaged in extracellular matrix deposition and th e phagocytosis of cell debris generated by apoptotic events in various parts of the embryo, in particular from the developing central nervou s system. Embryos deficient for programed cell death display a greatly reduced amount of MDP-1 deposition in tissues that normally undergo m orphogenetic cell death. This suggests a regulatory role for apoptosis in the terminal differentiation of Drosophila hemocytes. MDP-1 is ini tially deposited around the developing central nervous system and is l ater found in basement membrane structures surrounding various other o rgans, such as the gut, Malpighian tubules and part of the tracheal sy stem. The temporal and localized deposition of MDP-1 suggests that it may play a role in delineating the central nervous system structure du ring axonogenesis and may participate in the formation of a functional 'blood-brain barrier' in Drosophila.