Transgenic Kallikrein 14 Mice Display Major Hair Shaft Defects Associated with Desmoglein 3 and 4 Degradation, Abnormal Epidermal Differentiation, and IL-36 Signature

Abstract

Netherton syndrome is a rare autosomal recessive skin disease caused by loss-of-function mutations in SPINK5 encoding LEKTI protein that results in unopposed activity of epidermal kallikrein-related peptidases (KLKs), mainly KLK5, KLK7, and KLK14. Although the function of KLK5 and KLK7 has been previously studied, the role of KLK14 in skin homeostasis and its contribution to Netherton syndrome pathogenesis remains unknown. We generated a transgenic murine model overexpressing human KLK14 (TghKLK14) in stratum granulosum. TghKLK14 mice revealed increased proteolytic activity in the granular layers and in hair follicles. Their hair did not grow and displayed major defects with hyperplastic hair follicles when hKLK14 was overexpressed. TghKLK14 mice displayed abnormal epidermal hyperproliferation and differentiation. Ultrastructural analysis revealed cell separation in the hair cortex and increased thickness of Huxley's layer. Desmoglein (Dsg) 2 staining was increased, whereas Dsg3 and Dsg4 were markedly reduced. In vitro studies showed that hKLK14 directly cleaves recombinant human DSG3 and recombinant human DSG4, suggesting that their degradation contributes to hair abnormalities. Their skin showed an inflammatory signature, with enhanced expression of IL-36 family members and their downstream targets involved in innate immunity. This in vivo study identifies KLK14 as an important contributor to hair abnormalities and skin inflammation seen in Netherton syndrome.

Advances in understanding of Netherton syndrome and therapeutic implications

“…Because KLKs participate in various processes such as desquamation, antimicrobial defense and lipid permeability [46], their unrestrained activity in the context of LEKTI deficiency can have several impacts on the skin that can be classified into two main categories: (1) structural and (2) related to innate immunity and skin inflammation. Transgenic mouse models engineered to express human or mouse KLKs in the epidermis have been instrumental in delineating the contribution of each of these proteases to the skin barrier defects and skin inflammation in NS [29,43,[47][48][49][50][51][52].…”

“…At the structural level, KLKs hyperactivity causes premature desquamation by cleavage of corneodesmosin and the corneodesmosomal cadherins Desmoglein 1 (DSG1) and Desmocollin 1 (DSC1) [37,[53][54][55], proteolytic processing of pro-Filaggrin [56] and degradation of Filaggrin [27,28]. Furthermore, cleavage of DSG3 and DSG4 by KLK14 contributes to hair abnormalities and alopecia [48]. KLKs regulate stratum corneum lipid permeability barrier, hence transepidermal water loss, by proteolytic degradation of the lipidprocessing enzymes β-Glucocerebrosidase and Acid sphingomyelinase [57,58].…”

“…Infection and keratinocyte damage/stress signals activate pattern recognition receptor signaling and induce release of alarmins, thereby triggering the expression of pro-inflammatory cytokines such as IL-6, IL-17C or IL-36 cytokines in keratinocytes. Downstream chemokine release leads to neutrophil recruitment and activation of dendritic cells, triggering the IL-23/Th17 immune signaling axis [18,48,67]. Immune cell infiltrates in NS skin consist mainly of neutrophils, mast cells, eosinophils, Th2 and Th17 cells.…”

“…Transgenic mice overexpressing KLK5 [52], Klk6 [47,190], KLK7 [49-51], or KLK14 [48,191] are viable and have helped to study the role of each of these proteases in the development of skin barrier defect, skin inflammation and hair defects resembling NS. Over-expression of hKLK5 in the epidermis of mice leads to the development of a skin phenotype resembling NS with features of cutaneous and systemic inflammation [52].…”

The Proteolytic Network in Palmoplantar Keratoderma: SERPINA12 Joins the Family

“…With KLK7 overexpression, there is increased acanthosis, hyperkeratosis, and inflammation, resulting in chronic and itchy dermatitis. Acanthosis and hyperkeratosis are also observed in KLK14 overexpression models, similar to the skin histology of patients with SERPINA12 mutations; however, clinical skin inflammation is not apparent (Gouin et al, 2020).…”