Localization of ABCA12 from Golgi apparatus to lamellar granules in human upper epidermal keratinocytes.

ABCA12 is an ATP-binding cassette transporter and is thought to act as a transmembrane lipid transporter. We reported that deleterious ABCA12 mutations cause a disturbance in lamellar granule (LG) lipid transport in the epidermal granular layer keratinocytes, resulting in harlequin ichthyosis, a severe genodermatosis. Detailed localization of ABCA12 in comparison with glucosylceramide and Golgi apparatus markers were studied in order to obtain clues to clarify the function(s) of ABCA12 in human skin. We performed double-labelling immunofluorescent staining using antibodies against ABCA12, glucosylceramide and two Golgi apparatus markers (TGN46 and GM130) in normal human skin and cultured keratinocytes. Immunogold electron microscopy for ABCA12 and glucosylceramide was studied on postembedding and cryoultrathin sections of normal human skin. Confocal laser scanning microscopy demonstrated that ABCA12 and glucosylceramide co-localized in the granular layer keratinocytes as well as in keratinocytes cultured in high Ca2+ conditions through the Golgi apparatus to the cell periphery. Postembedding immunogold electron microscopy revealed that both ABCA12 and glucosylceramide labellings were associated with the LG of the uppermost granular layer keratinocytes. Using cryoultramicrotomy, lamellar structures in the LG were more clearly observed, and ultrastructural localization of ABCA12 and glucosylceramide was better demonstrated to LG in the uppermost granular layer cells. These results indicate that ABCA12 plays an important role in lipid transport from the Golgi apparatus to LG in human granular layer keratinocytes.