Yolk Formation and Degradation During Oocyte Maturation in Seabream Sparus Aurata: Involvement of Two Lysosomal Proteinases

Oocyte growth within the follicle is preponderantly due to the accumulation of hepatically derived yolk protein (vitellogenin, VTG) by receptor-mediated endocytosis; once in the oocyte, VTG is partially processed and stored in yolk globules. In some pelagic egg-laying marine teleosts, additional cleavages of yolk proteins followed by a pronounced water uptake occur concomitantly with final oocyte maturation. The aim of this study was to establish the lysosomal enzymes involved in these two proteolytic processes that characterize oocyte maturation of seabream Sparus aurata. The enzymatic activities of several cathepsins were assessed in the various classes of oocytes. Changes in cathepsin B, D, and L activity were found depending on the oocyte maturation stage; cathepsin B and D were found to be at maximum level in early-vitellogenesis oocytes, and cathepsin L in mid-vitellogenesis ones. Cathepsin D and L were purified from seabream ovary, and their roles in VTG and lipovitellin (LV) proteolysis, respectively, were analyzed. Here we demonstrate directly that one of the catalysts for the intraoocytic processing of VTG in yolk proteins is cathepsin D; however, we cannot exclude also a role of cathepsin B in the same process. On the other hand, cathepsin L is responsible for the second proteolytic cleavage of the LV components. We postulate that the acquisition of buoyancy by eggs through the hydration process may be regulated by enzymatic activation at the appropriate time of oocyte maturation, this process probably being the key event in the reproduction of this marine pelagic egg spawner.