Large-scale identification of human genes implicated in epidermal barrier function

Identification of genes expressed in epidermal granular keratinocytes by ORESTES, including a number that are highly specific for these cells

Expanding the Clinical and Genetic Spectrum of KRT1, KRT2 and KRT10 Mutations in Keratinopathic Ichthyosis

Twenty-six families with keratinopathic ichthyoses (epidermolytic ichthyosis, superficial epidermolytic ichthyosis or congenital reticular ichthyosiform erythroderma) were studied. Epidermolytic ichthyosis is caused by mutations in the genes KRT1 or KRT10, mutations in the gene KRT2 lead to superficial epidermolytic ichthyosis, and congenital reticular ichthyosiform erythroderma is caused by frameshift mutations in the genes KRT10 or KRT1, which lead to the phenomenon of revertant mosaicism. In this study mutations were found in KRT1, KRT2 and KRT10, including 7 mutations that are novel pathogenic variants. Novel clinical features found in patients with congenital reticular ichthyosiform erythroderma are described, such as mental retardation, spasticity, facial dysmorphisms, symblepharon and malposition of the 4th toe

The Ubiquitous Dermokine Delta Activates Rab5 Function in the Early Endocytic Pathway

The expression of the recently identified dermokine (Dmkn) gene leads to four families of proteins with as yet unknown functions. The secreted α, β and γ isoforms share an epidermis-restricted expression pattern, whereas the δ isoform is intracellular and ubiquitous. To get an insight into Dmknδ function, we performed yeast two-hybrid screening and identified the small GTPases Rab5 as partners for Dmknδ. The Rab5 proteins are known to regulate membrane docking and fusion in the early endocytic pathway. GST pull-down assays confirmed the direct interaction between Rab5 and Dmknδ. Transient expression of Dmknδ in HeLa cells led to the formation of punctate structures colocalized with endogenous Rab5 and clathrin, indicating Dmknδ involvement in the early steps of endocytosis. Dmknδ indeed colocalized with transferrin at early stages of endocytosis, but did not modulate its endocytosis or recycling kinetics. We also showed that Dmknδ was able to bind both inactive (GDP-bound) and active (GTP-bound) forms of Rab5 in vitro but preferentially targeted GDP-bound form in HeLa cells. Interestingly, Dmknδ expression rescued the Rab5S34N-mediated inhibition of endosome fusion. Moreover, Dmknδ caused the enlargement of vesicles positive for Rab5 by promoting GTP loading onto the small GTPase. Together our data reveal that Dmknδ activates Rab5 function and thus is involved in the early endosomal trafficking

Ceramides metabolism and impaired epidermal barrier in cutaneous diseases and skin aging: focus on the role of the enzyme PNPLA1 in the synthesis of ω-O-acylceramides and its pathophysiological involvement in some forms of congenital ichthyoses

The outermost layer of the skin, the stratum corneum, is essential for the protective barrier functions of the skin. It results from the stacking of corneocytes, the dead flattened cells resulting from epidermal terminal differentiation of underlying living keratinocytes. The cornified lipid envelope, encapsulating corneocytes, and the extracellular mortar-like multilayered lipid matrix, called lamellae, are two crucial elements of the epidermal barrier. Stratum corneum extracellular lipids are mainly composed of ceramides, cholesterol and free fatty acids. Ceramides, and more specifically the epidermis specific ω-O-acylceramides, are essential for lipid-matrix organization into lamellae and formation of the corneocyte lipid envelope. Pathophysiological studies of inherited lipid metabolism disorders recently contributed to a better understanding of stratum corneum lipid metabolism. In the lab, our data from patients with Autosomal Recessive Congenital Ichthyosis and a murine knock-out model showed that the enzyme PNPLA1 is essential for the last step of synthesis of omega-O-acylceramides. Skin aging is a complex biological process caused by genetic and extrinsic factors e.g. sun exposure, smoke, and pollution. Aging skin is marked by a senescence-related decline in lipid and water content, which ultimately impairs epidermal barrier function. Thus, aged epidermis is prone to develop altered drug permeability, increased susceptibility to irritants contact dermatitis and severe xerosis. Ceramide deficiency may account, at least in part, for the dysfunction of the stratum corneum associated with ageing. Hence, treatments able to increase skin-ceramide levels could improve the epidermal barrier function in aged skin. Many animal testing and clinical trials are taken in that regard

Mice deficient for the epidermal dermokine and isoforms display transient cornification defects

International audienceExpression of the human dermokine gene (DMKN) leads to the production of four dermokine isoform families. The secreted α, β and γ isoforms have an epidermis-restricted expression pattern, with Dmkn β and γ being specifically expressed by the granular keratinocytes. The δ isoforms are intracellular and ubiquitous. Here, we performed an in-depth characterization of Dmkn expression in mouse skin and found an expression pattern that was less complex than in humans. In particular, mRNA coding for the δ family were absent. Homozygous mice null for the Dmkn β and γ isoforms had no obvious phenotype but only a temporary scaly skin during the first week of life. The pups null for the Dmkn β and γ isoforms had smaller keratohyalin granules and their cornified envelopes were more sensitive to mechanical stress. At the molecular level, amounts of profilaggrin and filaggrin monomers were reduced whereas amino acid components of the natural moisturizing factor were increased. In addition, the electrophoretic mobility of involucrin was modified, suggesting post-translational modifications. Finally, the mice null for the Dmkn β and γ isoforms strongly overexpressed Dmkn α. These data are evocative of compensatory mechanisms relevant to the temporary phenotype. Overall, we improved the knowledge of Dmkn expression in mouse and highlighted a role for Dmkn β and γ in cornification

Novel mutation in NIPAL4 in a Romanian family with autosomal recessive congenital ichthyosis

International audienceAutosomal recessive congenital ichthyosis (ARCI), a severe and highly clinically heterogeneous group of mendelian disorders of cornification, is the result of mutations in at least nine genes regulating the epidermal barrier functionality. NIPAL4 is the second most frequently mutated ARCI gene. We report two adult patients from a nonconsanguineous family of Romanian origin, who had lamellar ichthyosis. A positive in situ transglutaminase 1 activity assay excluded a putative TGM1 mutation. NIPAL4 sequencing revealed in both patients a new homozygous missense mutation, c.403A>C, affecting a highly conserved amino acid (p. Ser135Arg) and predicted to be deleterious according to in silico analysis. In addition to the ARCI features, the patients had caries and partial edentation. Although delay in dental treatment led to caries progression and extraction of secondary teeth, this finding raises the possibility of a deficiency in enamel mineralization due to NIPAL4 dysfunction as an Mg(2+) transporter. Evaluating new patients with ARCI provides fruitful clinical and molecular finding

Activography reveals aberrant proteolysis in desquamating diseases of differing backgrounds

International audienceThe role of epidermal proteolysis in overdesquamation was revealed in Netherton syndrome, a rare ichthyosis due to genetic deficiency of the LEKTI inhibitor of serine proteases. Recently, we developed activography, a new histochemical method, to spatially localize and semiquantitatively assess proteolytic activities using activity-based probes. Activography provides specificity and versatility compared to in situ zymography, the only available method to determine enzymatic activities in tissue biopsies. Here, activography was validated in skin biopsies obtained from an array of distinct disorders and compared with in situ zymography. Activography provides a methodological advancement due to its simplicity and specificity and can be readily adapted as a routine diagnostic assay. Interestingly, the levels of epidermal proteolysis correlated with the degree of desquamation independent of skin pathology. Thus, deregulated epidermal proteolysis likely represents a universal mechanism underlying aberrant desquamation

Lethal Form of Keratitis–Ichthyosis–Deafness Syndrome Caused by the GJB2 Mutation p.Ser17Phe

International audienceKeratosis–Ichthyosis–Deafness (KID) syndrome is a rare form of ichthyosis caused by mutations in the gene GJB2 encoding the gap junction protein connexin 26 (Cx26). Connexins are a family of integral membrane proteins forming gap junction channels that control and coordinate a variety of cellular activities through the exchange of small ions, metabolites and signalling molecules