Mf. Sire et al., INVOLVEMENT OF THE LYSOSOMAL SYSTEM IN YOLK PROTEIN DEPOSIT AND DEGRADATION DURING VITELLOGENESIS AND EMBRYONIC-DEVELOPMENT IN TROUT, The Journal of experimental zoology, 269(1), 1994, pp. 69-83
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.