Involucrin mRNA Is More Abundant in Human Hair Follicles Than in Normal Epidermis

Involucrin is a precursor protein of the cornified cell envelope in epidermal keratinocytes, where it has been located by immunohistochemistry in the upper spinous and granular layers of human epidermis. In the hair follicle, involucrin has been found in the inner root sheath and in the upper layers of the infundibulum and the isthmus (upper outer root sheath), whereas its presence in the lower outer root sheath and the cortex has been controversial. Therefore, we analyzed the distribution of involucrin mRNA in adult scalp by Northern blotting and in situ hybridization. Northern blots showed more abundant involucrin mRNA in the follicular fraction than in the epidermal fraction of dissected scalp. In situ hybridization matched the immunohistologic results; transcripts of involucrin were expressed not only in the infundibulum and isthmus, but also in the hair cortex and medulla, in all layers of the inner root sheath, and in the inner cells of the lower outer root sheath (all of which lack a cell envelope at the ultrastructural level). However, involucrin was absent in the hair cuticle, which is the only compartment of the follicle possessing a morphologically distinct cell envelope. Our results suggest, first, that involucrin does not serve as a precursor protein of the cornified cell envelope in adult hair follicles, and second, that it is perhaps not necessary for the formation of the cell envelope in keratinocytes of the hair cuticle, as we did not find this precursor protein with highly sensitive methodology

Quasi-Normal Cornified Cell Envelopes in Loricrin Knockout Mice Imply the Existence of a Loricrin Backup System

The cornified cell envelope, a lipoprotein layer that assembles at the surface of terminally differentiated keratinocytes, is a resilient structure on account of covalent crosslinking of its constituent proteins, principally loricrin, which accounts for up to 60%-80% of total protein. Despite the importance of the cell envelope as a protective barrier, knocking out the loricrin gene in mice results in only mild syndromes. We have investigated the epidermis and forestomach epithelium of these mice by electron microscopy. In both tissues, corneocytes have normal-looking cell envelopes, despite the absence of loricrin, which was confirmed by immunolabeling, and the absence of the distinctive loricrin-containing keratohyalin granules (L-granules). Isolated cell envelopes were normal in thickness (≈15 nm) and mass per unit area (≈7.3 kDa per nm2); however, metal shadowing revealed an altered substructure on their cytoplasmic surface. Their amino acid compositions indicate altered protein compositions. Analysis of these data implies that the epidermal cell envelopes have elevated levels of the small proline-rich proteins, and cell envelopes of both kinds contain other protein(s) that, like loricrin, are rich in glycine and serine. These observations imply that, in the absence of loricrin, the mechanisms that govern cell envelope assembly function normally but employ different building-blocks