Alibardi Lorenzo
Department of Evolutionary Experimental Biology, University of Bologna, Bologna, Italy.
Acta Histochem. 2003;105(3):273-83. doi: 10.1078/0065-1281-00713.
Interkeratin histidine-rich proteins (filaggrins) play a functional role in aggregation of keratin filaments into dense bundles during terminal differentiation of mammalian keratinocytes when forming the dense matrix of the stratum corneum. The origin of the stratum corneum during adaptation to land in amphibious vertebrate progenitors was probably linked with the synthesis of matrix proteins. However, whether similar proteins are present in living amphibians is unknown. The possible involvement of interkeratin matrix molecules rich in basic amino acids such as histidine during keratinization of amphibian epidermis has been evaluated in the present study by ultrastructural autoradiography after administration of tritiated histidine. At 4 and 8 h post-injection, labelling was mainly localized over electron-dense amorphous material or irregular granules in between keratin filaments in cells of the upper intermedium, replacement, and immature corneous layers. Nuclear material incorporating tritiated histidine was also present in the maturing corneous layer. Small mucous-like granules did not take up tritiated histidine and X-ray microanalysis indicated that the latter granules contained sulphur. The present study suggests that small amounts of histidine-rich molecules which were not sufficient to form microscopically-visible keratohyalin granules were present in ancestral amphibian epidermis. However, this material was sufficient to promote aggregation of keratin filaments in the cytoplasm of amphibian differentiating keratinocytes, especially near the external corneous cell envelope. Electron-dense material associated with the corneous cell envelope also contained sulphur as indicated by X-ray microanalysis. It is unknown whether sulphur is derived from either sulphated mucins, or disulphide bonds in aggregated keratins, or specific sulphur-rich proteins.
角蛋白富含组氨酸的蛋白质（聚角蛋白微丝蛋白）在哺乳动物角质形成细胞终末分化形成角质层致密基质的过程中，对角蛋白丝聚集成致密束发挥着功能性作用。两栖类脊椎动物祖先在适应陆地生活过程中角质层的起源可能与基质蛋白的合成有关。然而，尚不清楚现存两栖动物中是否存在类似的蛋白质。在本研究中，通过给予氚标记的组氨酸后进行超微结构放射自显影，评估了富含碱性氨基酸（如组氨酸）的角蛋白间基质分子在两栖类表皮角质化过程中可能的参与情况。注射后4小时和8小时，标记主要定位于上中间层、替代层和未成熟角质层细胞中角蛋白丝之间的电子致密无定形物质或不规则颗粒上。掺入氚标记组氨酸的核物质也存在于成熟角质层中。小的黏液样颗粒未摄取氚标记的组氨酸，X射线微分析表明后者颗粒含有硫。本研究表明，在两栖类祖先的表皮中存在少量富含组氨酸的分子，这些分子不足以形成显微镜下可见的透明角质颗粒。然而，这种物质足以促进两栖类分化角质形成细胞胞质中角蛋白丝的聚集，尤其是在靠近外部角质细胞包膜处。X射线微分析表明，与角质细胞包膜相关的电子致密物质也含有硫。尚不清楚硫是来自硫酸化黏蛋白、聚集角蛋白中的二硫键，还是特定的富含硫的蛋白质。
