Macroautophagy Is Required for Multicellular Development of the Social Amoeba Dictyostelium discoideum

Abstract

Macroautophagy is a mechanism employed by eukaryotic cells to recycle non-essential cellular components during starvation, differentiation, and development. Two conjugation reactions related to ubiquitination are essential for autophagy: Apg12p conjugation to Apg5p, and Apg8p conjugation to the lipid phosphatidylethanolamine. These reactions require the action of the E1-like enzyme, Apg7p, and the E2-like enzymes, Apg3p and Apg10p. In Dictyostelium, development is induced by starvation, conditions under which autophagy is required for survival in yeast and plants. We have identified Dictyostelium homologues of 10 budding yeast autophagy genes. We have generated mutations in apg5 and apg7 that produce defects typically associated with an abrogation of autophagy. Mutants are not grossly affected in growth, but survival during nitrogen starvation is severely reduced. Starved mutant cells show little turnover of cellular constituents by electron microscopy, whereas wild-type cells show significant cytoplasmic degradation and reduced organelle number. Bulk protein degradation during starvation-induced development is reduced in the autophagy mutants. Development is aberrant; the autophagy mutants do not aggregate in plaques on bacterial lawns, but they do proceed further in development on nitrocellulose filters, forming defective fruiting bodies. The autophagy mutations are cell autonomous, because wild-type cells in a chimaera do not rescue development of the autophagy mutants. We have complemented the mutant phenotypes by expression of the cognate gene fused to green fluorescent protein. A green fluorescent protein fusion of the autophagosome marker Apg8 mislocalizes in the two autophagy mutants. We show that the Apg5-Apg12 conjugation system is conserved in Dictyostelium.Protein turnover in eukaryotes is accomplished by two major mechanisms, autophagy or proteasomal degradation. Three modes of autophagy have been identified: chaperone-mediated autophagy, microautophagy, and macroautophagy. In chaperone-mediated autophagy, specific proteins containing targeting sequences are bound by chaperones that mediate direct transport across the lysosomal membrane (1). A lysosomal receptor, lysosomal-associated membrane protein type 2a, interacts with

The pleiotropic role of autophagy: from protein metabolism to bactericide

“…In S. cerevisiae, autophagy mutants are defective in spore formation, 8 while autophagy mutants of D. discoideum are defective in normal multicellular developmental processes such as aggregation formation and fruiting body formation. 9 Premature death from the third larval to pupal stages was reported in D. melanogaster mutants. 10,49 Finally, dauer formation is abnormal in Caenorhabditis elegans autophagy mutants.…”

“…The critical role played by autophagy in maintaining viability during starvation has been shown in Sacharomyces cerevisiae, 8 Dictyostelium discoideum 9 and Drosophila melanogaster. 10 In addition, we recently revealed that autophagy is also quite important to help mammals survive a unique period of starvation, the birthing process.…”

Autophagy: for better or for worse

“…Unicellular organisms, such as yeast (S. cerevisiae and S. pombe) and ameba (D. discoideum) activate adaptive processes in nutrient poor conditions that result in sporulation or the generation of fruiting bodies, respectively. During these processes, autophagy recycles amino acids needed for de novo protein synthesis and increases the organism's chance of survival [57,58]. Also C. elegans uses autophagy to adapt to stressful conditions [59].…”

Autophagy and cell death in model organisms

“…Mutations in the Dictyostelium autophagy gene orthologues ATG5, ATG6, ATG7 and ATG8 have subtle effects on growth in the presence of nutrients, but survival during nitrogen starvation is severely reduced. 16,17 Loss-of-function mutations in ATG genes in plants reduce tolerance to nitrogen or carbon depletion, resulting in enhanced chlorosis, reduced seed set, accelerated leaf senescence and limited lateral root elongation. [18][19][20] Autophagy is rapidly induced during exposure of Arabidopsis seedlings to oxidative stress.…”