INVOLVEMENT OF THE LYSOSOMAL SYSTEM IN YOLK PROTEIN DEPOSIT AND DEGRADATION DURING VITELLOGENESIS AND EMBRYONIC-DEVELOPMENT IN TROUT

In adult female rainbow trout (Oncorhynchus mykiss), an immunocytochem ical study of the oocyte has shown that a proteolytic enzyme, cathepsi n D, is localized in multivesicular bodies (MVB) which begin to differ entiate before the phase of vitellogenesis. Estrogens cause the liver to synthesize the protein vitellogenin (VTG), which then enters the sy stemic circulation. During vitellogenesis, endocytosed VTG is co-local ized with cathepsin D in the MVB. Assays of oocyte cathepsin activitie s have shown that the only proteolytic activity of note is that of cat hepsin D. Along with the yolk proteins derived from VTG, this enzyme w ill be included in the central coalescent yolk mass. With the installa tion of the yolk syncytial layer and the surrounding yolk vascular sys tem, embryonic development is characterized by high lysosomal activity , especially in the syncytial layer. At this level, proteolytic activi ty concerns a cathepsin L, secreted as a proenzyme. We propose the hyp othesis that cathepsin D, along with the yolk proteins in the yolk glo bules that break away from the yolk mass in the vitellolysis zone, act ivates the proenzyme and leads to protein degradation, which then beco mes very rapid. (C) 1994 Wiley-Liss, Inc.