The role of CGl-58 in epidermal barrier formation

The formation of a functional epidermal permeability barrier depends on the correct assembly of the corneocyte lipid envelope (CLE) and requires extensive lipid remodeling. The first step in cellular triglyceride (TG) catabolism requires the TG hydrolytic activity of adipose triglyceride lipase (ATGL) and its co-activator comparative gene identification-58 (CGI-58), which stimulates ATGL TG hydrolytic activity severalfold. Mutant mouse models lacking either ATGL or CGI-58 accumulate TG in multiple organs. Interestingly, the lack of CGI-58 also causes a severe skin barrier defect associated with marked TG accumulation and an absence of acylceramides in the epidermis, whereas ATGL deficiency is not associated with ichthyosis. CGI-58-deficient mice die within hours after birth and, besides the skin barrier defect, exhibit markedly decreased blood lipids and hepatic fat accumulation. The observed normal skin development of ATGL-deficient mice implicates an ATGL-independent role for CGI-58 in epidermal lipid metabolism and as such the existence of a currently unknown epidermal TG lipase(s) or lipogenic enzyme(s) activated by CGI-58. In this project we will study the role of CGI-58 in the formation of the epidermal permeability barrier and will functionally characterize novel candidate enzymes involved in epidermal lipid metabolism. To achieve these goals we will: generate mice lacking or overexpressing CGI-58 exclusively in the epidermis to address the epidermal and non-epidermal role of CGI-58 in the formation of a functional skin permeability barrier. investigate whether CGI-58 deficiency and defective TG catabolism affect keratinocyte (KC) differentiation via the reduced availability of fatty acids (FA) as energy fuel or as ligands for nuclear receptors. functionally characterize novel candidate enzymes involved in epidermal lipid remodeling and identify the postulated, unknown CGI-58-activated TG-lipase(s) in the epidermis. The examination of the tissue-specific role of CGI-58 in epidermal barrier formation and the biochemical characterization of novel candidate enzymes involved in epidermal lipid metabolism and barrier formation might provide novel therapeutic strategies for the treatment of lipid-associated skin disorders.

The catabolism of intracellular triglyceride (TG) deposits critically depends on the TG-hydrolytic activity of Adipose triglyceride lipase (ATGL) and its coactivator Comparative gene identification-58 (CGI-58). Albeit loss of functional ATGL or CGI-58 provokes TG accumulation within multiple organs in humans, solely loss of CGI-58 function causes ichthyosis (impaired keratinocyte differentiation linked to an epidermal water barrier defect) and hepatic steatosis. In contrast, massive TG accumulation in skeletal and cardiac muscle together with severe cardiomyopathy are a hallmark of mutated ATGL alleles. In a previous study we could show that global CGI-58 deficiency leads to severe ichthyosis and hepatic steatosis in new born mice, which die several hours after birth due to rapid water loss. In contrast, mice lacking ATGL show no signs of ichthyosis but develop lethal cardiomyopathy around 12 weeks of age. These findings suggest an ATGL-independent role of CGI-58 at least in keratinocytes and skin barrier formation. The aim of this project was to examine the epidermis-specific function of CGI-58 in keratinocyte differentiation and barrier formation. Moreover, we examined the potential role of CGI-58 as a coactivator of a currently unknown epidermal TG lipase, required for the adequate supply of substrates for the synthesis of complex epidermal ceramide species. To do so, we generated mice lacking CGI-58 exclusively in the epidermis. These mice showed a virtually identical skin barrier defect compared to mice with global CGI-58 deletion, including a marked reduction of long-chain acyl-omega-hydroxy-ceramides (Acyl-?-OH-cer), an essential precursor for the establishment of the corneocyte lipid envelope (CLE). During CLE formation, the acyl group (typically linoleate) of Acyl-?-OH-cer has to be cleaved prior the covalent linkage to the corneocyte envelope. Defective CLE formation (and postnatal lethality) of CGI-58-deficient mice could be bypassed by the exclusive expression of a CGI-58 transgene in differentiated keratinocytes. In contrast, CGI-58 expression solely in basal keratinocytes failed to rescue the lethal skin barrier defect of newborn mice. These data strongly suggest that CGI-58 expression is essentially required during late-stage keratinocyte differentiation. Furthermore, the availability of an ATGL-specific inhibitor (Atglistatin) allowed us to investigate the potential ATGL-independent role of CGI-58 in epidermal lipolysis. Notably, Atglistatin reduced epidermal TG-hydrolytic activities of CGI-58 overexpressing mice to levels of Atglisatin-treated wildtype mice implicating that TG accumulation in CGI-58-deficient mice derives exclusively from impaired stimulation of ATGL activity. Considering that ATGL deficiency is compatible with normal barrier formation, these findings suggest that CGI-58 may act as an essential cofactor for an enzyme that participates in the complex pathway of Acyl-?-OH-cer synthesis. Interestingly, we could show that CGI-58 interacts with an ATGL-related protein, which has been also implicated in the development of ichthyosis in humans, dogs and mice.