Apolipophorin III is dramatically up-regulated during the programmed death of insect skeletal muscle and neurons.

Abstract

The intersegmental muscles (ISMs) of the tobacco hawkmoth Manduca sexta, participate in the emergence behavior of the adult moth and then die during the subsequent 30 hours. In addition, several populations of interneurons and uniquely identified motor neurons also die after adult emergence. The trigger for all of these deaths is a decline in the circulating titer of the insect molting hormone 20-hydroxyecdysone. The ability of the muscles and neurons to die requires de novo gene expression. A differential hybridization screen of a "condemned" ISM cDNA library permitted the isolation of clones encoding four new up-regulated mRNAs. On sequencing, one of these recombinants was found to encode apolipophorin III (apoLp-III), a component of lipophorin, the major hemolymph lipoprotein of insects, previously shown to be synthesized in 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-III at both the RNA and protein levels was markedly reduced at the normal time of cell death. Immunocytochemistry demonstrated that apoLp-III protein was abundantly expressed in the cytoplasm of dying muscles, interneurons, and identified motor neurons at the time of cell death. Apolipoproteins I and II, required components of lipophorin, were not expressed at detectable levels in the muscles or neurons. Furthermore, Western blots of native gels suggest that apoLp-III was not associated with any other proteins. These data suggest that apoLp-III has activities independent of lipid transport that may play a role in programmed cell death. ApoLp-III joins apolipoproteins E and J (clusterin, sulfated glycoprotein-2) as a group of proteins that function in both lipid transfer and cell death.