Ectopic overexpression of Drosophila lamin C is stage-specific lethal

To gain insight into the function of the developmentally regulated A-type lamins we transformed Drosophila melanogaster with a construct containing the hsp70 promoter followed by the Drosophila lamin C (an analog of vertebrate A-type lamins) cDNA. Lamin C was expressed ectopically after heat shock of embryos and localized to the nucleus. No phenotypic change was observed after lamin C expression in embryos that normally do not contain lamin C. However, ectopic expression of lamin C during most larval (but not pupal) stages stalled growth, inhibited ecdysteroid signaling (in particular during the larval-prepupal transition), resulted in development of melanotic tumors, and finally caused death. During pupation in control animals, when massive apoptosis of larval tissues takes place, lamin C is proteolyzed into a fragment with a size similar to that predicted by caspase cleavage. The ectopically expressed lamin C is identically cleaved, resulting in a large increase of the steady-state level of the lamin C fragment. A null mutation of the dcp-1 gene, one of the two known Drosophila caspase genes, also results in development of melanotic tumors and larval death, suggesting that the ectopically expressed lamin C inhibits apoptosis through competitive inhibition of caspase activity.