The role of PNPLA1 in skin lipid metabolism

Autosomal recessive congenital ichthyoses (ARCI) are a heterogeneous group of keratinization disorders mainly characterized by abnormal generalized scaling of the human skin. Very recently, mutations in the PNPLA1 gene (patatin-like phospholipase domain-containing protein 1) were reported to cause the development of a new type of ARCI in humans and golden retriever dogs. However, characterization of affected individuals could neither address the enzymatic function of this protein in epidermal metabolism nor could the exact molecular mechanisms leading to disease development be identified. Considering the close phylogenetic relationship of PNPLA1 with other PNPLA family members that act as lipid hydrolases or acyltransferases in lipid metabolism, we hypothesize that PNPLA1 harbors a similar enzymatic activity. To uncover the in vivo function of PNPLA1 in epidermal and non- epidermal lipid metabolism we propose the generation and characterization of mouse models with either a global or an epidermal-specific lack of PNPLA1. The availability of PNPLA1-null mice allows us to examine the physiological and pathophysiological role of PNPLA1 in skin metabolism and in the development of ichthyosis as well as to gain new insights in the function of PNPLA1 in whole-body lipid and energy metabolism. To verify our results of PNPLA1-deficient mice we will also study PNPLA1 function in human skin using sample material from patients with PNPLA1 mutations. The results of the proposed study could reveal novel therapeutic targets or strategies for the treatment of ichthyoses and probably other lipid-associated skin disorders.

Ichthyoses are severe cornification disorders associated with an increased formation of corneocytes leading to severe scaling of the skin that usually affects the entire body. The disorder is usually caused by genetic defects, which lead to loss of function of proteins that are essential for the maintenance of a functional skin barrier. Such disease-causing mutations have been recently identified in the PNPLA1 gene (patatin-like phospholipase domain-containing protein 1) in humans and golden retriever dogs. However, characterization of affected individuals could neither address the enzymatic function of the protein in epidermal metabolism nor could the exact molecular mechanisms leading to disease development be identified. To uncover the in vivo function of PNPLA1 in epidermal lipid metabolism, we generated and characterized mice with an inactivated Pnpla1 gene. Like humans and dogs with a mutated PNPLA1 gene, these so-called Pnpla1 knock-out mice also show a defective skin barrier function. To elucidate the underlying disease-causing mechanism of ichthyosis, we analyzed the lipid composition of PNPLA1-deficient skin. These analyses demonstrated that PNPLA1-deficient skin lacks essential lipid components of the skin barrier, so-called omega-O-acylceramides (AcylCer), and accumulates their precursors, known as omega-hydroxyceramides. Consistent with the mouse model, isolated skin cells (keratinocytes) from a patient with mutated PNPLA1 showed barely detectable synthesis activity of AcylCer. In summary, our results indicate that PNPLA1 is the long-sought enzyme responsible for the final step in AcylCer biosynthesis. To provide skin barrier function, AcylCer bind to specific structural proteins of the corneocytes. This process generates a layer of protein-bound omega-hydroxyceramides that is essential for the cohesion of corneocytes and the impermeability of the skin barrier. Transmission electron microscopy revealed a complete loss of this protein-bound structure in the horny layer of PNPLA1-deficient mice. To repair the barrier defect, we isolated skin lipids from healthy newborn mice and applied them to PNPLA1-deficient skin, which led to the formation of the missing protein-bound omega-hydroxyceramide layer in the affected skin. The findings of our study have shown that treating patients with chemically synthesized AcylCer administered in skin creams would represent a promising evidence-based therapy for ichthyoses.