DIFFERENTIAL EXPRESSION AND CELL-ENVELOPE INCORPORATION OF SMALL PROLINE-RICH PROTEIN-1 IN DIFFERENT CORNIFIED EPITHELIA

In the final stages of terminal differentiation in the epidermis and o ther squamous epithelia, a similar to 15 nm thick protein layer called the cornified cell envelope (CE) assembles on the keratinocytes' inne r surface. Its constituent proteins are covalently crosslinked by the action of transglutaminases. Recent studies have indicated that the ex pression of CE precursor proteins may vary in different tissues. To in vestigate such variations further, we have studied the CEs of two diff erent keratinizing epithelia of mouse: epidermis and forestomach, with particular focus on their contents of loricrin and the small proline- rich proteins (SPRs). To this end, we have applied electron microscopi c immunocytochemistry and estimated the CE protein compositions by mat hematical modeling of their amino acid compositions. Ultrastructurally , forestomach resembles the epidermis in having well defined cornified and granular layers. Minor but significant differences are: in forest omach, striated material resembling lamellar granules is intercalated between the cornified squames; and in forestomach granular layer cells , loricrin-containing L-granules are more abundant, and filaggrin-cont aining F-granules less abundant than in epidermis. In forestomach, den se labeling with anti-SPR1 antibody was observed at the margin of corn ified layer cells; and in the granular layer, diffuse but positive lab eling of both cytoplasm and nucleus. In contrast, epidermis was unifor mly negative. Isolated forestomach CEs (but not epidermal CEs), labele d positively on the cytoplasmic side, consistent with the presence of covalently crosslinked SPR1. Our compositional analysis predicts the c ontent of loricrin in forestomach CEs to be very high (similar to 65%) , as in the epidermis, and accompanied by similar to 18% content of to tal SPRs. Of these, a substantial proportion should be SPR1, according to our immunolabeling data. In contrast, epidermal CEs are calculated to have a much lower amount of SPRs or SPR-like proteins (similar to 8%), with a negligible content of SPR1. Thus both kinds of CEs have lo ricrin as their major constituent but differ in their respective compl ements of SPRs, which are thought to inter-connect loricrin molecules in the final phase of CE assembly. Applying a basic concept of materia ls science, it may be that the observed differences in their SPR conte nts reflect differences in the mechanical and chemical properties requ ired for the function of the respective CEs.