Expression of hornerin in stratified squamous epithelium in the mouse: a comparative analysis with profilaggrin

We have recently identified a novel protein named hornerin, the structural features of which are most similar to those of profilaggrin, an essential protein for keratinization of epidermal tissues. In this study we examined the expression of hornerin compared with that of profilaggrin in various mouse tissues. Hornerin was expressed in the upper epidermis of newborn mouse skin, as was profilaggrin. In addition, both hornerin and profilaggrin were expressed in the tongue, esophagus, and forestomach. In all four tissues, immunostaining for hornerin and profilaggrin showed a granular pattern, and most of the signals for the two proteins were co-localized on keratohyalin granules. This was confirmed by double immunoelectron microscopy. Within keratohyalin granules, hornerin was detected more frequently in the periphery, whereas profilaggrin was equally distributed. A quantitative RT-PCR revealed that both genes were expressed at highest levels in the forestomach and at the next highest levels in skin. Profilaggrin mRNA was most abundant in the forestomach. In skin, the amount of hornerin mRNA was more than fourfold greater than the amount of profilaggrin mRNA. These results form the basis for a better understanding of possible overlapping and/or differential functions of hornerin and profilaggrin.