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that phagocytes have restricted access to the dying cells. In
Drosophila
, the
adult midgut forms around the degrading larval midgut isolating the dying
cells from the rest of the tissues. Similarly,
in vitro
models of mammary lumen
formation, where the dying cells are isolated from phagocytes, implicate the
necessity of both caspases and autophagy for elimination of the dying cells
(
Debnath et al., 2002; Mills, Reginato, Debnath, Queenan, & Brugge,
2004
). Alternatively, large cells and tissues, such as the giant larval salivary
glands, may be too big to degrade by phagocytosis alone, and they require
autophagy for the bulk degradation of their cytoplasm. Finally, autophagy
may contribute to nutrient resource reallocation and survival in multicellular
organisms. In yeast and mammalian cell culture, autophagy degrades cellular
content to produce ATP and resources to protect the cell during starvation.
Interestingly, autophagic cell death of tissues in
Drosophila
occurs during a
time when the animal receives no external nutrients and must rely on its nu-
trient stores for survival and development of adult structures. Further, the
majority of
Atg
mutants are pupal lethal, suggesting that autophagy is nec-
essary to survive metamorphosis. Thus, although autophagy is killing indi-
vidual cells and tissues, this form of cell death could be promoting organism
survival. Future studies in
Drosophila
will hopefully lead to a better under-
standing of autophagy's dual roles in life and death.
ACKNOWLEDGMENTS
We thank the Baehrecke lab for constructive comments. Work on this subject is supported by
NIH grants GM079431 and CA159314, and the Ellison Medical Foundation to E. H. B
.
E.
H. B. is a member of the UMass DERC (DK32520).
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