Regulation of yolk degradation, or how to make sleepy lysosomes.

Yolk is the major internal food supply on which most embryos rely. Strict regulation of its utilization is essential to provide nutrients at the right time to the developing tissues, and to ensure survival of the embryo until it becomes a free-living organism able to feed. Yolk proteins are derived from a maternal serum protein, vitellogenin (Vg). Vg is internalized by the growing oocyte via receptor-mediated endocytosis, partially processed in an endosomal compartment, and then stored in specialized organelles, the yolk granules (YGs or yolk platelets) (Wallace, 1985; Opresko et al., 1980; Opresko and Karpf, 1987). YGs appear to be modified lysosomes, containing the whole set of hydrolases necessary for yolk degradation. Yet, paradoxically, YGs do not degrade their content until specific developmental stages, sometimes weeks after their formation, in striking contrast to classical lysosomes, which are voracious degradative organelles, capable of rapidly reducing almost any protein down to free amino acids, sugars and other small products. The lysosomal nature of the YGs and their latency was hypothesized more than a quarter of century ago by Pasteels (1966), who first detected acid phosphatase activity in the YGs of a molluscan egg. His observations have since been confirmed in many other species, but the regulation of yolk degradation remained a puzzle. A recent series of studies on both vertebrate and invertebrate models has brought new light on this phenomenon. Two factors appear to be responsible for the regulation of yolk degradation: pH and enzymatic latency.