Autophagy-Mediated Yolk Absorption during Fish Embryogenesis

* Abstract Proteolytic degradation of yolk proteins is essential process for embryogenesis, homeostasis in fish. Since two different major pathways, i. e. ubiquitin-proteasome system and autophagy, have been well known in the animal cells, their roles for the yolk protein proteolysis during embryogenesis were characterized in the embryos of zebrafish and other fish species. Ubiquitin-proteasome system regulates ATP-dependent protein degradation specifi c to ubiquitinated proteins by proteasome, while autophagy regulates a process of bulky protein degradation via lysosomal/vacuolar system. The proteolytic processing of yolk protein in fish embryogenesis has been identified to be undertaken by lysosomal proteases. In order to characterize the molecular mechanisms on the proteolytic degradation of yolk protein, we examined roles of autophagy during embryogenesis. We established transgenic zebrafish lines, which are expressed a maker protein of autophagosome, microtubule-associated protein 1 light chain 3 (MAP1LC3) fused to green fluorescent protein (GFP), to visualize autophagosome formed by autophagy. The transgenic embryos were very useful to observe autophagic cells in embryos, and we detected the induction of autophagosome during early embryogenesis. The treatment of embryos with protease inhibitors, such as E-64 (inhibitor of cathepsins B and L) and proteasome inhibitor, effectively repressed yolk protein degradation and embryogenesis, suggesting that both autophagy and proteasome pathways are essential for degradation of yolk proteins.