Unbound Corneocyte Lipid Envelopes in 12R-Lipoxygenase Deficiency Support A Specific Role in Lipid-Protein Crosslinking.

ABSTRACT Loss-of-function mutations in arachidonate lipoxygenase 12B (ALOX12B) are an important cause of autosomal recessive congenital ichthyosis (ARCI). 12R-lipoxygenase (12R-LOX), the protein product of ALOX12B, has been proposed to covalently bind the corneocyte lipid envelope (CLE) to the proteinaceous corneocyte envelope (CE), thereby providing a scaffold for the assembly of barrier-providing, mature lipid lamellae. To test this hypothesis, an in-depth ultrastructural examination of CLEs was performed in ALOX12B-/- human and Alox12b-/- mouse epidermis, extracting samples with pyridine to distinguish covalently attached CLEs from unbound (i.e., non-covalently bound) CLEs. ALOX12B-/- stratum corneum contained abundant pyridine-extractable (i.e., unbound) CLEs, compared to normal stratum corneum. These unbound CLEs were associated with defective post-secretory lipid processing, and were specific to 12R-LOX deficiency, since they were not observed with deficiency of the related ARCI-associated proteins, patatin-like phospholipase 1 (Pnpla1)- or abhydrolase domain containing 5 (Abhd5). These results suggest that 12R-LOX contributes specifically to CLE-CE crosslinking, which appears to be a prerequisite for post-secretory lipid processing, and provide insights into the pathogenesis of 12R-LOX deficiency in this subtype of ARCI, as well as other conditions that display a defective CLE.