Regulation of Yolk Catabolism in Early Embryogenesis

Abstract

Yolk provides an important source of nutrients during the early development of oviparous (non-platental) organisms. In addition to phosphate and lipids, it is com- posed mainly of vitellogenin proteins packed into membrane-bound compartments called yolk platelets. Catabolism of yolk is initiated by acidification of the yolk platelet, leading to the activation of cathepsin-like proteases that degrade yolk contents, but it is unknown how this process is triggered. Using maternal shRNA technology in Drosophila melanogaster embryos we find that yolk catabolism depends on components of the Tor pathway, a well-characterized regulator of cellular metabolism. Knockdown of Tor also leads to severe nuclear fragmentation, abnormal gastrulation, and an increased ratio of AMP/ATP. This phenotype is more severe than inhibition of Tor in later development, or in cell culture models, suggesting that Tor may have additional functions during early development. Additionally, we identify a downstream target of Tor, Atg1, as necessary for yolk catabolism. Atg1 is responsible for initiation of autophagy, a process that de- grades both protein and organelles within the cell. While Atg1 is required for a burst of spatially-regulated autophagy during late cellularization, autophagy is not required for yolk catabolism. We find that knockdown of Atg1, but not downstream autophagy proteins, can rescue shRNA-Tor embryos, suggesting that Atg1’s role in yolk cataboilism may be through regulation of Tor. Last, we find that Rheb, a GTPase responsible for activation of Tor on the lysosome membrane, is present on Xenopus laevis yolk platelets. Therefore, regulation of yolk catabolism by the Tor pathway may function in a similar manner to Tor’s activity on the lysosome. Together, this work connects the conserved Tor and Atg1 metabolic sensing pathways to yolk catabolism, and may provide insight into the metabolic regulation of lysosomes more generally.