ABCA12 maintains the epidermal lipid permeability barrier by facilitating formation of ceramide linoleic esters

Abstract

Harlequin ichthyosis is a congenital scaling syndrome of the skin in which affected infants have epidermal hyperkeratosis and a defective permeability barrier. Mutations in the gene encoding a member of the ABCA transporter family, ABCA12, have been linked to harlequin ichthyosis, but the molecular function of the protein is unknown. To investigate the activity of ABCA12, we generated Abca12 null mice and analyzed the impact on skin function and lipid content. Abca12-/- mice are born with a thickened epidermis and die shortly after birth, as water rapidly evaporates from their skin. In vivo skin proliferation measurements suggest a lack of desquamation of the skin cells, rather than enhanced proliferation of basal layer keratinocytes, accounts for the 5-fold thickening of the Abca12-/- stratum corneum. Electron microscopy revealed a loss of the lamellar permeability barrier in Abca12-/- skin. This was associated with a profound reduction in skin linoleic esters of long-chain omega-hydroxyceramides and a corresponding increase in their glucosyl ceramide precursors. Because omega-hydroxyceramides are required for the barrier function of the skin, these results establish that ABCA12 activity is required for the generation of long-chain ceramide esters that are essential for the development of normal skin structure and function.

FIGURE 1.

Deletion of Abca12 exon 9 in mice blocks ABCA12 expression. A, exon 9 of the Abca12 locus (wt) was disrupted by homologous recombination. The resulting targeted allele (tg) contains a NeoR cassette and truncates the 2596-amino acid ABCA12 open reading frame after amino acid 328. B, Southern and PCR analysis of DNA from day 18.5 embryos demonstrated transmission of the targeted allele and generation of the null state. Position of the DNA probe and PCR primers are indicated in the diagram. ABCA12 mRNA levels are significantly diminished in the Abca12^-/- mice as determined by quantitative RT-PCR (n = 5, ±S.D., p < 0.5) (C) and by in situ hybridization (D); scale bar, 20 μm. E, Abca12^-/- mice lack ABCA12 protein as determined by immunoblots of skin lysates using anti-ABCA12 C-terminal antibody. Serial probing of the blot for ABCA1, an ABCA12 homologue, demonstrated equal loading of the membrane, and no compensatory change in ABCA1 expression.

FIGURE 2.

Loss of ABCA12 cause neonatal lethality and epidermal hyperkeratosis. A, Abca12^-/- day 18.5 embryos (E18.5) when kept hydrated are similar in size and weight to Abca12^+/+ littermates, but after exposure to air rapidly dry out and become encased in a collodion-like shell (B). The Abca12^-/- epidermis displays a markedly thickened, hyperkeratotic epidermis as shown in hematoxylin & eosin-stained skin sections; scale bar, 20 μm (C)(sc = stratum corneum, sg = stratum granulosum, ss = stratum spinosum, and sb = stratum basale).

FIGURE 3.

Abca12^-/- mice fail to develop the epidermal permeability barrier. A, littermate E18.5 embryos stained with toluidine blue indicates loss of ABCA12 function increases inward permeability to this dye. B, loss of ABCA12 function significantly increases outward trans-epidermal water loss rates as measured in littermate E18.5 embryos using the gravimetric assay (Abca12^+/+, n = 3; Abca12^+/-, n = 7; Abca12^-/-, n = 6; ± S.E.: p = 0.11, Abca12^+/+ versus Abca12^+/-; p = 0.00005, Abca12^+/+ versus Abca12^-/-).

FIGURE 4.

Loss of ABCA12 transport function induces hyperkeratosis of the stratum corneum (SC) and disrupts interstitial lamellar lipid structure. A, electron micrograph of the Abca12^+/+ epidermis showing a normal SC (10,000×). E, Abca12^-/- epidermis showing massive hyperkeratosis of the SC (2000× magnification, which was required to capture the expanded SC). B, interstitial spaces of the Abca12^+/+ SC demonstrate the lipid lamellar structures required for permeability barrier function (arrow, 80,000×), which are absent and replaced by a disorganized multivesicular material in the interstitial spaces of the Abca12^-/- SC (F, arrow, 80,000×). C and D, lamellar storage bodies present in the Abca12^+/+ stratum granulosum (C and D, 150,000×). The Abca12^-/- stratum granulosum lacked lamellar storage bodies but did contain numerous multivesicular bodies, the presumptive precursor of the lamellar body (G and H, 150,000×).

FIGURE 5.

Loss of ABCA12 function does not alter E18.5 epidermal cell proliferation indices. A, DNA synthesis rates are similar in E18.5 Abca12^+/+ and Abca12^-/- embryos as determined by in utero incorporation of the thymidine analogue, BrdUrd. Shown are skin sections from littermate-paired embryos labeled for 1 h and stained with an anti-BrdUrd antibody (green) and counterstained for nuclei with Hoechst 33258 (blue). B, staining for the Ki67 proliferation maker (magenta) is also similar in wild-type and null samples and is confined to the basal layer. C, staining for the basal cell differentiation marker Keratin 14 (red) is expanded into the first cell layer of the stratum spinosum in the Abca12^-/- epidermis. D, staining for Keratin 10 is similar between the genotypes and is largely confined to the stratum spinosum and stratum granulosum. Staining for the cornified envelope marker loricrin (E) and filaggrin (F) show a more granular distribution in the Abca12^-/- epidermis. The basal epidermal layer is indicated by filled arrowheads, and open arrowheads denote parakeratotic nuclei in the Abca12^-/- stratum corneum. Scale bar, 20 μm.

FIGURE 6.

Loss of ABCA12 function alters filaggrin solubility but not processing. A, immunoblot analysis of Keratin 14 (K14), involucrin (Inv), profilaggrin (Profil), and filaggrin (Fil) extracted from the E18.5 skin using mild detergent buffer. B, profilaggrin and filaggrin extracted from skin using denaturing buffer.

FIGURE 7.

Loss of ABCA12 transport function blocks Cer(EOS) formation and induces a buildup of GlcCer(EOS). A, representative spectra showing the loss of the Cer(EOS) peaks in the Abca12^-/- skin sample (peak at 1020.2 m/z derived from the standard mixture is shown for comparison). B, quantitation of the Cer(EOS) and corresponding glucosyl Cer(EOS) species in the wild-type and null skin samples (n = 5, ± S.E.; ^*, p < 0.05).