Cell Changes During Autophagic Cell Death of Larval Salivary Glands During Drosophila melanogaster Metamorphosis

Abstract

Autophagic cell death has been implicated in human diseases such as neurodegeneration and cancer. In order to obtain a clearer picture of the mechanisms that regulate autophagic cell death, I have been studying the cell changes that occur during this process using Drosophila melanogaster larval salivary glands as a model. During Drosophila metamorphosis, larval salivary gland histolysis is triggered by a pulse of the steroid hormone 20-hydroxyecdysone (ecdysone) that occurs twelve hours after puparium formation. Ecdysone directly causes the activation of the early genes E93, BR-C and E74A, with ßFTZ-F1 serving as a competence factor for their induction. In turn, these transcription regulators activate a set of late genes rpr, hid, dronc, drice and ark that have a more direct role in the cellular changes that occur during salivary gland cell death. While dying salivary glands use typical apoptotic machinery, the morphology of their cells resembles autophagic cell death. The morphological changes seen during salivary gland autophagic cell death are a result of active caspase cleavage of structural proteins such as nuclear Lamin, alpha-Tubulin and alpha-Spectrin. However, some changes that occur during cell death are caspase-independent, indicating that other proteins are important for histolysis of these glands.

Proteome analyses identified 5,313 proteins that are present before and during salivary gland cell death. These proteins are involved in numerous processes such as autophagy, the ubiquitin- mediated proteolysis, cell organization, cell growth regulation and cell cycle regulation. Further analyses of these proteins may illustrate their importance during salivary gland autophagic cell death. Forward genetic screening was also used to identify mutations in genes that affect salivary gland cell death. One such gene ctp encodes a light chain of a dynein motor, and animals with mutations in ctp have salivary glands that fail to die.