Epithelial and Mesenchymal Cell Biology CGI-58 Is an /-Hydrolase within Lipid Transporting Lamellar Granules of Differentiated Keratinocytes

CGI-58 is the causative molecule underlying DorfmanChanarin syndrome, a neutral lipid storage disease exhibiting apparent clinical features of ichthyosis. CGI-58, associated with triacylglycerol hydrolysis, has an / -hydrolase fold and is also known as the / -hydrolase domain-containing protein 5. The purpose of this study was to elucidate the function of CGI-58 and the pathogenic mechanisms of ichthyosis in Dorfman-Chanarin syndrome. Using an antiCGI-58 antibody, we found CGI-58 to be expressed in the upper epidermis, predominantly in the granular layer cells, as well as in neurons and hepatocytes. Immunoelectron microscopy revealed that CGI-58 was also localized to the lamellar granules (LGs), which are lipid transport and secretion granules found in keratinocytes. CGI-58 expression was markedly reduced in the epidermis of patients with harlequin ichthyosis, demonstrating defective LG formation. In cultured keratinocytes, CGI-58 expression was mildly up-regulated under high Ca conditions and markedly up-regulated in three-dimensional, organotypic cultures. In the developing human epidermis, CGI-58 immunostaining was observed at an estimated gestational age of 49 days, and CGI-58 mRNA expression was up-regulated concomitantly with both epidermal stratification and keratinocyte differentiation. CGI-58 knockdown reduced expression of keratinocyte differentiation/keratinization markers in cultured human keratinocytes. Our results indicate that CGI-58 is expressed and packaged into LGs during keratinization and likely plays crucial role(s) in keratinocyte differentiation and LG lipid metabolism, contributing to skin lipid barrier formation. (Am J Pathol 2008, 173:1349–1360; DOI: 10.2353/ajpath.2008.080005) CGI-58 (ABHD5) is one of the genes that has been identified using the comparative proteomic approaches between Caenorhabditis elegans and humans. CGI was named after comparative gene identification. CGI-58 ( / -hydrolase domain-containing protein 5; ABHD5) is a member of / -hydrolase family and a putative esterase/ lipase/thioesterase. The physiological roles of CGI-58 protein were reported to be lipolytic degradation of fat in the lipid storing cells. However, its roles in various tissues including the epidermis have not yet been clarified. In 2001, CGI-58 mutations were identified in DorfmanChanarin syndrome (DCS; MIM 275630) families from the Mediterranean region and, later, in a DCS patient of Japanese origin. Additional DCS cases with CGI-58 mutations have been reported. DCS is an autosomal recessively inherited neutral lipid storage disease and is characterized by ichthyosis. This entity shows leukocyte lipid vacuoles and involvement of several internal organs, including liver dysfunction, myopathy, cataracts, and a variety of neurological symptoms. The most characteristic feature of DCS is ragged skin over the entire body, caused by thickening of the cornified layers of skin surface, termed ichthyosis. All DCS cases present with this skin manifestation, ie, moderate to severe non-bullous congenital ichthyosiform erythroderma. It was reported that truncation of CGI-58 protein results in abnormal lamellar granule (LG) formation in DCS. LGs have been shown to form a continuous network from the Golgi apparatus (trans-Golgi network; TGN). LGs are multifunctional and are involved in packaging and secretion of a variety of enzymes and structural proteins. LGs are known to be involved in lipid transport and secretion in keratinocytes and are thought to play a crucial role in epidermal lipid barrier formation. To date, the function of CGI-58 in the skin, the liver and the brain has not been clarified and the exact mechanism of pathogenesis of ichthyosis, liver dysfunction and men-