Role and regulation of a differentiation-specific keratin in keratinocyte adaptation to mechanical stress

Abstract Skin epithelia provides a multimodal barrier that protects us against the external world, essential for water retention, mechanical protection, sensory perception, and immune surveillance. Skin on the ventral aspect of the feet and hands, known as palmoplantar or glabrous skin, bears higher mechanical stress in daily life; however, the cellular and molecular components that convey mechanical resilience in this specialized tissue are only partially understood. As an adaptation to the unique stresses it experiences, the epidermis of palmoplantar skin maintains an intricate pattern of keratin protein expression. Keratins are intermediate filament (IF) forming proteins whose expression patterns are tightly regulated in response to developmental stage, differentiation, and stress in epithelial tissues. Importantly, the type I keratin 9 (KRT9/K9) is specifically expressed, at high levels, in differentiating keratinocytes of the epidermis in human and mouse glabrous skin. The type 1 keratin 16 (KRT16/K16), associated with damage response in interfollicular epidermis, is also constitutively expressed in the glabrous skin. Mutations in both KRT9/K9 and KRT16/K16 are associated with the formation of palmoplantar keratoderma (PPK), characterized by hyperkeratosis of the glabrous skin. KRT9/K9 mutation patients develop epidermolytic PPK, with tissue fragility and cell lysis observed in the suprabasal layers of the glabrous epidermis. Krt9-/- mice, available in the Coulombe lab, recapitulate the EPPK phenotype. However, despite the clear importance of Krt9/K9 in maintaining the homeostasis and mechanical integrity of the glabrous epidermis, recent work in the Coulombe lab, including data in this proposal, suggest that Krt9/K9 is undetectable on the protein level and extremely low on the mRNA level immediately prior to birth in mice. This is temporally uncoupled from the expression of Krt9/K9’s type 2 partner, keratin 1 (Krt1/K1). Additionally, data in this proposal shows aberrant nuclear-localized Yap1 in the suprabasal layers of the lesional glabrous skin of Krt9-/- mice, suggesting a potential role for Krt9/K9 in regulating the mechanosensitive Hippo signaling pathway in the glabrous epidermis. In this study, I aim to define the role and regulation of Krt9/K9 in response to stress in the developing palmoplantar skin. I hypothesize that Krt9/K9 is responsive to mechanical stress signals, particularly the expression of Krt16/K16, and maintains the homeostatic balance of the glabrous epidermis via negative regulation of mechanosensitive signaling pathways, such as Hippo signaling. Completion of this work will illuminate critical mechanisms by which the keratinocyte differentiation program adapts to physical stresses, as well as provide insight into the pathogenesis of many keratin-linked diseases which manifest at the sites of mechanical stress.