Sex- and tissue-specific expression of "similar to nothepsin" and cathepsin D in relation to egg yolk formation in Gallus gallus.

Egg yolk constitutes the main storage compartment of the avian egg and the first nutritional source that supports embryonic growth. Most egg yolk components are synthesized by the liver of laying hens at sexual maturity and are secreted into the blood to be further transferred into the ovarian oocyte (yolky follicle) by receptor-mediated endocytosis. Egg yolk proteins are secreted as precursors and must undergo proteolytic processing to be bioactive. It is assumed that chicken cathepsin D, an aspartic protease, is a key enzyme in this process. Very recently, a novel aspartic protease, namely "similar to nothepsin," has been identified in the egg yolk. Previous experiments conducted in Antarctic fish have shown that the expression of nothepsin is tissue- and sex-specific. To gain insight into the specificities of expression of both cathepsin D and "similar to nothepsin" in Gallus gallus, we compared their distribution in various tissues, in male and females. Cathepsin D is ubiquitously expressed in all tissues examined, including liver of both male and female adults, and its expression is stable during sexual maturation. In contrast, "similar to nothepsin" expression is unique to the liver of adult females and is sex steroid-dependent as it increases gradually in the liver of hens during sexual maturation. The sexual dimorphic expression of the "similar to nothepsin" gene suggests that the activity of this protein is regulated by the steroid environment of laying hens and is specifically adapted for inclusion in the yolk. Further studies are needed to assess whether "similar to nothepsin" assists cathepsin D in the proteolytic processing of egg yolk proteins during follicular growth.