APOLIPOPHORIN-III IS DRAMATICALLY UP-REGULATED DURING THE PROGRAMMED DEATH OF INSECT SKELETAL-MUSCLE AND NEURONS

The intersegmental muscles (ISMs) of the tobacco hawkmoth Manduca sext a, participate in the emergence behavior of the adult moth and then di e during the subsequent 30 hours. In addition, several populations of interneurons and uniquely identified motor neurons also die after adul t emergence. The trigger for all of these deaths is a decline in the c irculating titer of the insect molting hormone 20-hydroxyecdysone. The ability of the muscles and neurons to die requires de novo gene expre ssion. A differential hybridization screen of a ''condemned'' ISM cDNA library permitted the isolation of clones encoding four new up-regula ted mRNAs. On sequencing, one of these recombinants was found to encod e apolipophorin III (apoLp-III), a component of lipophorin, the major hemolymph lipoprotein of insects, previously shown to be synthesized i n fat body. Although apoLp-III mRNA and protein were expressed at all stages of ISM development, levels of both molecules were dramatically elevated with the commitment of the cells to die. When ISM cell death was delayed by injection of 20-hydroxyecdysone, expression of apoLp-II I at both the RNA and protein levels was markedly reduced at the norma l time of cell death. Immunocytochemistry demonstrated that apoLp-III protein was abundantly expressed in the cytoplasm of dying muscles, in terneurons, and identified motor neurons at the time of cell death. Ap olipoproteins I and II, required components of lipophorin, were not ex pressed at detectable levels in the muscles or neurons. Furthermore, W estern blots of native gels suggest that apoLp-III was not associated with any other proteins. These data suggest that apoLp-III has activit ies independent of lipid transport that may play a role in programmed cell death. ApoLp-III joins apolipoproteins E and J (clusterin, sulfat ed glycoprotein-2) as a group of proteins that function in both lipid transfer and cell death. (C) 1995 John Wiley & Sons, Inc.