An in vivo model of apoptosis: linking cell behaviours and caspase substrates in embryos lacking DIAP1.

The apoptotic phenotype is characterised by dynamic changes in cell behaviours such as cell rounding and blebbing, followed by chromatin condensation and cell fragmentation. Whereas the biochemical pathways leading to caspase activation have been actively studied, much less is known about how caspase activity changes cell behaviours during apoptosis. Here, we address this question using early Drosophila melanogaster embryos lacking DIAP1. Reflecting its central role in the inhibition of apoptosis, loss of DIAP1 causes massive caspase activation. We generated DIAP1-depleted embryos by either using homozygous null mutants for thread, the gene coding DIAP1, or by ectopically expressing in early embryos the RGH protein Reaper, which inhibits DIAP1. We show that (1) all cells in embryos lacking DIAP1 follow synchronously the stereotypic temporal sequence of behaviours described for apoptotic mammalian cells and (2) these cell behaviours specifically require caspase activity and are not merely a consequence of cellular stress. Next, we analyse the dynamic changes in the localisation of actomyosin, Discs large, Bazooka and DE-cadherin in the course of apoptosis. We show that early changes in Bazooka and Discs large correlate with early processing of these proteins by caspases. DE-cadherin and Myosin light chain do not appear to be cleaved, but their altered localisation can be explained by cleavage of known regulators. This illustrates how embryos lacking DIAP1 can be used to characterise apoptotic changes in the context of an embryo, thus providing an unprecedented in vivo model in which thousands of cells initiate apoptosis simultaneously.