Defects in Stratum Corneum Desquamation Are the Predominant Effect of Impaired ABCA12 Function in a Novel Mouse Model of Harlequin Ichthyosis.

Harlequin Ichthyosis is a severe skin disease caused by mutations in the human gene encoding ABCA12. Here, we characterize a novel mutation in intron 29 of the mouse Abca12 gene that leads to the loss of a 5' splice donor site and truncation of the Abca12 RNA transcript. Homozygous mutants of this smooth skin or smsk allele die perinatally with shiny translucent skin, typical of animal models of Harlequin Ichthyosis. Characterization of smsk mutant skin showed that the delivery of glucosylceramides and CORNEODESMOSIN was defective, while ultrastructural analysis revealed abnormal lamellar bodies and the absence of lipid lamellae in smsk epidermis. Unexpectedly, mutant stratum corneum remained intact when subjected to harsh chemical dissociation procedures. Moreover, both KALLIKREIN 5 and -7 were drastically decreased, with retention of desmoplakin in mutant SC. In cultured wild type keratinocytes, both KALLIKREIN 5 and -7 colocalized with ceramide metabolites following calcium-induced differentiation. Reducing the intracellular levels of glucosylceramide with a glucosylceramide synthase inhibitor resulted in decreased secretion of KALLIKREIN proteases by wild type keratinocytes, but not by smsk mutant keratinocytes. Together, these findings suggest an essential role for ABCA12 in transferring not only lipids, which are required for the formation of multilamellar structures in the stratum corneum, but also proteolytic enzymes that are required for normal desquamation. Smsk mutant mice recapitulate many of the pathological features of HI and can be used to explore novel topical therapies against a potentially lethal and debilitating neonatal disease

Immunocytochemical mapping of the biosynthetic pathways of two major proteins expressed in terminally differentiated epidermal keratinocytes

In the process of constant renewal of mammalian epidermis, cells divide and move upwards from the basal layer as they embark on a program of terminal differentiation. Cells located higher in the epidermis - the spinous, granular, and cornified layers - represent progressively later steps in this pathway. Commitment to terminal differentiation is accompanied by major changes in gene expression and morphology.Inter alia, large-scale synthesis of several proteins is initiated: these include filaggrin, a protein thought to be responsible for aggregating keratin filaments, and a recently discovered protein called loricrin, which has been implicated as a likely major component of the covalently cross-linked protein lining of the cornified cell envelope. We have used immunoelectron microscopy to investigate the biosynthetic pathways of filaggrin and loricrin.Immediately after sacrifice, skin samples were taken from newborn mice, and diced into blocks of 1-2 mm3. They were then either (i) fixed with 1% glutaraldehyde, 0.2% picric acid, in PBS and embedded in LR White, or (ii) fixed with 1% glutaraldehyde in PBS, and embedded in Lowicryl K4M. Essentially the same conclusions were reached from the experiments with both resins.</jats:p