Differentiation of epidermal keratinocytes is dependent on glucosylceramide:ceramide processing

Abstract

Skin barrier function is primarily assigned to the outer epidermal layer, the stratum corneum (SC), mainly composed of corneocytes and lipid-enriched extracellular matrix. Epidermal ceramides (Cers) are essential barrier lipids, containing ultra-long-chain (ULC) fatty acids (FAs) with a unique ω-hydroxy group, which is necessary for binding to corneocyte proteins. In the SC, Cers are believed to derive from glucosylated intermediates, namely glucosylceramides (GlcCers), as surmised from human Gaucher's disease and related mouse models. Tamoxifen (TAM)-induced deletion of the endogenous GlcCer-synthesizing enzyme UDP-glucose:ceramide glucosyltransferase (UGCG) in keratin K14-positive cells resulted in epidermal GlcCer depletion. Although free extractable Cers were elevated in total epidermis and as well in SC, protein-bound Cers decreased significantly in Ugcg(f/fK14CreERT2) mice, indicating glucosylation to be required for regular Cer processing as well as arrangement and extrusion of lipid lamellae. The almost complete loss of protein-bound Cers led to a disruption of the water permeability barrier (WPB). UGCG-deficient mice developed an ichthyosis-like skin phenotype marked by impaired keratinocyte differentiation associated with delayed wound healing. Gene expression profiling of Ugcg-mutant skin revealed a subset of differentially expressed genes involved in lipid signaling and epidermal differentiation/proliferation, correlating to human skin diseases such as psoriasis and atopic dermatitis. Peroxisome proliferator-activated receptor beta/delta (PPARβ/δ), a Cer-sensitive transcription factor was identified as potential mediator of the altered gene sets.

PNPLA1 has a crucial role in skin barrier function by directing acylceramide biosynthesis

“…3a). It is likely that the enhanced expression of inflammatory cytokines and chemokines is a secondary effect resulting from impaired barrier function, since similar changes have also been observed in several genetically distinct mouse models with barrier defects333435363738 and patients with skin diseases such as ichthyosis, atopic dermatitis and psoriasis3940. Interestingly, expression levels of genes associated with synthesis and processing of epidermal acylceramide were consistently elevated in Pnpla1 −/− mice relative to Pnpla1 +/+ and Pnpla1 +/− mice (Supplementary Fig.…”

“…In contrast, keratin 6 ( Krt6a and Krt6b ) was expressed in the lower suprabasal layer in Pnpla1 -deficient but not in control skin, reflecting the hyperproliferative state of the mutant epidermis. Abnormal differentiation of keratinocytes has also been observed in several mouse lines with targeted disruption of genes implicated in epidermal ceramide metabolism2235. Therefore, the neonatal lethality of Pnpla1 −/− mice due to skin barrier defect is likely dependent upon both altered lipid composition and impaired differentiation of keratinocytes.…”

PNPLA1 Deficiency in Mice and Humans Leads to a Defect in the Synthesis of Omega-O-Acylceramides

“…Consistent with this delay in differentiation and cornification, the expression levels of involucrin and loricrin, markers for the proper assembly of the corneocyte envelope, were reduced in mutant mice (Figure 1e). Furthermore, the expression level of FA-binding protein 5, which has been reported to be elevated in the epidermis of patients with psoriasis or ichthyosis due to abnormal keratinocyte differentiation and disturbance of the epidermal lipid metabolism (Amen et al, 2013; Siegenthaler et al, 1994), was increased in the Pnpla1 −/− skin (Figure 1e). Taken together, these pathological alterations in the skin are characteristics of ichthyoses and indicate that PNPLA1 deficiency in mice results in a condition similar to those present in humans and dogs affected by PNPLA1 mutations.…”

Simplifying complexity: genetically resculpting glycosphingolipid synthesis pathways in mice to reveal function

“…Their skin showed broadened epidermal layers, abnormal lamellar bodies, keratinocyte apoptosis, and inflammation. When epidermal deletion was induced after birth using a tamoxifen-inducible (TAM) K14Cre transgene to bypass the early lethality, TAM - K14Cre - Ugcg KO mice showed epidermal abnormalities similar to the K14Cre - Ugcg KO mice [25]. These studies supported the conclusion that Ugcg expression is essential for the formation of a functional epidermal permeability barrier through the generation of GlcCer, a precursor for the formation of ceramide in the skin.…”