Deconstructing the lipoxygenase-hepoxilin pathway in skin barrier formation

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Deconstructing the Lipoxygenase-Hepoxilin Pathway in Skin Barrier Formation SUMMARY A deficiency in any one of the genes involved in forming the mammalian skin permeability barrier has devastating consequences, being neonatal lethal in mice and in humans leading to congenital ichthyosis (scaly skin), a socially challenging condition for afflicted families. Skin barrier malfunction is also implicated in the common skin diseases of atopic dermatitis and psoriasis. Two genes critical to barrier formation are the lipoxygenases 12R-LOX and eLOX3, which act in series to oxygenate the essential fatty acid linoleate esterified to the omega-hydroxyl of the unique epidermal acylceramide Cer-EOS [E = esterified, O = omega- hydroxy]. The oxidized product is a linoleate-Hepoxilin (“hep” indicating a hydroxy-epoxy structure). For reasons heretofore unresolved, inactivation of the LOX genes (or other ichthyosis genes earlier in the ceramide metabolism pathway) disrupts the covalent attachment of ceramide to the proteinaceous corneocyte envelope, normally forming a key structural feature of the barrier, the “corneocyte lipid envelope”, CLE. We propose to study a new hypothesis that identifies the link between the LOX pathway oxidations of Cer-EOS and the covalent coupling of ceramides, which is the culmination of multiple steps in barrier formation. Of special importance is the activity of a recently identified orphan ichthyosis gene SDR9C7, that our preliminary data identifies as a NAD-dependent dehydrogenase that oxidizes the Cer-EOS-Hepoxilin to a Cer-EOS-keto- Hepoxilin. This keto-Hepoxilin sub-structure (9,10-epoxy-11E-13-keto) is known from chemical precedent and biochemical studies to spontaneously and specifically bind covalently to amino acid residues of protein, and as a consequence also achieve covalent coupling of the EOS-ceramide. This hypothesis thus rationalizes the need for LOX-catalyzed oxidations with the ultimate goal of binding ceramide to protein and forming the CLE. In Specific Aim 1 we will (i) define the effects of sdr9c7 gene knockout on the lipoxygenase products and ceramides in mouse skin, (ii) extend the analyses to human and pig skin for the equivalent SDR9C7-catalyzed transformations, (iii) determine the reactions of recombinant SDR9C7 with LOX pathway products. In Specific Aim 2 we will (i) prepare authentic standards of amino acid adducts of keto-Hepoxilin with amino acids and model peptides, (ii) examine epidermal proteins qualitatively and quantitatively for covalently bound ceramides and their mode of binding to amino acid residues in mouse epidermis and also (iii) in human and pig skin, ultimately with identification of the adducted proteins by LC-MS analysis of recovered peptides. In Specific Aim 3 we will use differentiated keratinocytes in culture to manipulate and dissect these pathways to help characterize the chemical mechanisms of ceramide binding to protein and the role of the LOX/SDR9C7 pathway. The results of this study will unravel the mechanisms underlying an important facet of epidermal water barrier construction. Understanding the physiology allows for the rational design of therapeutics, and it is to rationalize the role of multiple key enzymes of the epidermal water barrier that this project's ultimate goal.