The roles of ABCA12 in epidermal lipid barrier formation and keratinocyte differentiation.

ATP-binding cassette (ABC) transporters form a large superfamily of transporters that bind and hydrolyze ATP to transport various molecules across limiting membranes or into vesicles. The ABCA subfamily members are thought to transport lipid materials. ABCA12 is a keratinocyte transmembrane lipid transporter protein associated with the transport of lipids via lamellar granules. ABCA12 is considered to transport lipids including ceramides to form extracellular lipid layers in the stratum corneum of the epidermis, which is essential for skin barrier function. ABCA12 mutations are known to underlie the three major types of autosomal recessive congenital ichthyoses: harlequin ichthyosis, lamellar ichthyosis and congenital ichthyosiform erythroderma. ABCA12 mutations result in defective lipid transport via lamellar granules in the keratinocytes, leading to ichthyosis phenotypes from malformation of the stratum corneum lipid barrier. Studies on ABCA12-deficient bioengineered models have revealed that lipid transport by ABCA12 is required for keratinocyte differentiation and epidermal morphogenesis. Defective lipid transport due to loss of ABCA12 function leads to the accumulation of intracellular lipids, including glucosylceramides and gangliosides, in the epidermal keratinocytes. The accumulation of gangliosides seems to result in the apoptosis of Abca12(-/-) keratinocytes. It was reported that AKT activation occurs in Abca12(-/-) granular-layer keratinocytes, which suggests that AKT activation serves to prevent the cell death of Abca12(-/-) keratinocytes. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.