Sialic acid-containing glycosphingolipids, i.e., gangliosides, constitute a major component of neuronal cells and are thought to be essential for brain function. UDP-glucose:ceramide glucosyltransferase (Ugcg) catalyzes the initial step of glycosphingolipid (GSL) biosynthesis. To gain insight into the role of GSLs in brain development and function, a cell-specific disruption of Ugcg was performed as indicated by the absence of virtually all glucosylceramide-based GSLs. Shortly after birth, mice showed dysfunction of cerebellum and peripheral nerves, associated with structural defects. Axon branching of Purkinje cells was significantly reduced. In primary cultures of neurons, dendritic complexity was clearly diminished, and pruning occurred early. Myelin sheaths of peripheral nerves were broadened and focally severely disorganized. GSL deficiency also led to a down-regulation of gene expression sets involved in brain development and homeostasis. Mice died ≈3 weeks after birth. These results imply that GSLs are essential for brain maturation.

Ceramides are vital components of the water barrier in mammalian skin. Epidermis-specific, a major ceramide portion contains omega-hydroxy very long chain fatty acids (C30-C36). These omega-hydroxy ceramides (Cers) are found in the extracellular lamellae of the stratum corneum either as linoleic acyl esters or protein bound. Glucosylceramide is the major glycosphingolipid of the epidermis. Synthesized from ceramide and UDP-glucose, it is thought to be itself an intracellular precursor and carrier for extracellular omega-hydroxy ceramides. To investigate whether GlcCer is an obligatory intermediate in ceramide metabolism to maintain epidermal barrier function, a mouse with an epidermis-specific glucosylceramide synthase (Ugcg) deficiency has been generated. Four days after birth animals devoid of GlcCer synthesis in keratinocytes showed a pronounced desquamation of the stratum corneum and extreme transepidermal water loss leading to death. The stratum corneum appeared as a thick unstructured mass. Lamellar bodies of the stratum granulosum did not display the usual ordered inner structure and were often irregularly arranged. Although the total amount of epidermal protein-bound ceramides remained unchanged, epidermal-free omega-hydroxy ceramides increased 4-fold and omega-hydroxy sphingomyelins, almost not detectable in wild type epidermis, emerged in quantities comparable with lost GlcCer. We conclude that the transient formation of GlcCer is vital for a regular arrangement of lipids and proteins in lamellar bodies and for the maintenance of the epidermal barrier.

Urinary ammonium excretion by the kidney is essential for renal excretion of sufficient amounts of protons and to maintain stable blood pH. Ammonium secretion by the collecting duct epithelia accounts for the majority of urinary ammonium; it is driven by an interstitium-to-lumen NH ₃ gradient due to the accumulation of ammonium in the medullary and papillary interstitium. Here, we demonstrate that sulfatides, highly charged anionic glycosphingolipids, are important for maintaining high papillary ammonium concentration and increased urinary acid elimination during metabolic acidosis. We disrupted sulfatide synthesis by a genetic approach along the entire renal tubule. Renal sulfatide-deficient mice had lower urinary pH accompanied by lower ammonium excretion. Upon acid diet, they showed impaired ammonuria, decreased ammonium accumulation in the papilla, and chronic hyperchloremic metabolic acidosis. Expression levels of ammoniagenic enzymes and Na ⁺ -K ⁺ /NH ₄ ⁺ -2Cl ⁻ cotransporter 2 were higher, and transepithelial NH ₃ transport, examined by in vitro microperfusion of cortical and outer medullary collecting ducts, was unaffected in mutant mice. We therefore suggest that sulfatides act as counterions for interstitial ammonium facilitating its retention in the papilla. This study points to a seminal role of sulfatides in renal ammonium handling, urinary acidification, and acid–base homeostasis.

Glycosphingolipids (GSLs) constitute major components of enterocytes and were hypothesized to be potentially important for intestinal epithelial polarization. The enzyme UDP-glucose ceramide glucosyltransferase (Ugcg) catalyzes the initial step of GSL biosynthesis. Newborn and adult mice with enterocyte-specific genetic deletion of the gene Ugcg were generated. In newborn mutants lacking GSLs at day P0, intestinal epithelia were indistinguishable from those in control littermates displaying an intact polarization with regular brush border. However, those mice were not consistently able to absorb nutritional lipids from milk. Between postnatal days 5 and 7, severe defects in intestinal epithelial differentiation occurred accompanied by impaired intestinal uptake of nutrients. Villi of mutant mice became stunted, and enterocytes lacked brush border. The defects observed in mutant mice caused diarrhea, malabsorption, and early death. In this study, we show that GSLs are essential for enterocyte resorptive function but are primarily not for polarization; GSLs are required for intracellular vesicular transport in resorption-active intestine.

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.