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.