Institut National de la Recherche Agronomique, UR83 Recherches Avicoles, F-37380 Nouzilly, France.
Poult Sci. 2012 Sep;91(9):2288-93. doi: 10.3382/ps.2011-01910.
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.
蛋黄是禽类蛋的主要储存部位，也是支持胚胎生长的最初营养来源。蛋黄中的大多数成分都是在产蛋母鸡性成熟时由肝脏合成的，并分泌到血液中，然后通过受体介导的内吞作用进一步转移到卵巢卵母细胞（蛋黄滤泡）中。蛋黄蛋白以前体的形式分泌出来，必须经过蛋白水解加工才能具有生物活性。人们认为，鸡组织蛋白酶 D（一种天冬氨酸蛋白酶）是这个过程中的关键酶。最近，一种新的天冬氨酸蛋白酶，即“类组织蛋白酶 N”，在蛋黄中被鉴定出来。之前在南极鱼类中进行的实验表明，组织蛋白酶 N 的表达具有组织和性别特异性。为了深入了解鸡组织蛋白酶 D 和“类组织蛋白酶 N”在鸡中的表达特异性，我们比较了它们在各种组织中的分布情况，包括雄性和雌性。组织蛋白酶 D 在所有检查的组织中都广泛表达，包括成年雄性和雌性的肝脏，其表达在性成熟过程中是稳定的。相比之下，“类组织蛋白酶 N”的表达是成年雌性肝脏所特有的，并且依赖于性类固醇，因为它在性成熟过程中逐渐增加。“类组织蛋白酶 N”基因的性别二态性表达表明，该蛋白的活性受到产蛋母鸡类固醇环境的调节，并且专门适应于包含在蛋黄中。需要进一步研究来评估“类组织蛋白酶 N”是否有助于组织蛋白酶 D 在卵泡生长过程中对蛋黄蛋白进行蛋白水解加工。
