To Control Site-Specific Skin Gene Expression, Autocrine Mimics Paracrine Canonical Wnt Signaling and Is Activated Ectopically in Skin Disease

Abstract

Despite similar components, the heterogeneity of skin characteristics across the human body is enormous. It is classically believed that site-specific fibroblasts in the dermis control postnatal skin identity by modulating the behavior of the surface-overlying keratinocytes in the epidermis. To begin testing this hypothesis, we characterized the gene expression differences between volar (ventral; palmoplantar) and nonvolar (dorsal) human skin. We show that KERATIN 9 (KRT9) is the most uniquely enriched transcript in volar skin, consistent with its etiology in genetic diseases of the palms and soles. In addition, ectopic KRT9 expression is selectively activated by volar fibroblasts. However, KRT9 expression occurs in the absence of all fibroblasts, although not to the maximal levels induced by fibroblasts. Through gain-of-function and loss-of-function experiments, we demonstrate that the mechanism is through overlapping paracrine or autocrine canonical WNTeb-catenin signaling in each respective context. Finally, as an in vivo example of ectopic expression of KRT9 independent of volar fibroblasts, we demonstrate that in the human skin disease lichen simplex chronicus, WNT5a and KRT9 are robustly activated outside of volar sites. These results highlight the complexities of site-specific gene expression and its disruption in skin disease. Site-specific epidermal differentiation programs define the heterogeneity of skin identity across the human body. It is generally believed that positional skin identity is regulated by epithelial (keratinocytes) and mesenchymal (fibroblasts) interactions. 1e5 Indeed, three-dimensional skin equivalent models and transplantation experiments show that epidermal stratification during the keratinocyte differentiation program is remarkably disrupted in the absence of fibroblasts, 2,4,5 demonstrating that epidermal development and homeostasis are regulated by extrinsic factors released by fibroblasts. Such interactions have been further clarified through an analysis of the differential localization of epidermal keratins in distinct epidermal layers and body sites. Volar (palmoplantar) skin is characterized by thick epidermal layers, less pigmentation, and lack of hair. In addition to these features, cytoskeleton KERATIN (KRT) 9 6 has been thought to be almost exclusively localized to volar keratinocytes, 1,6e9 indicating that KRT9 can be a unique marker for volar skin. To date, many clinical studies have reported that KRT9 mutations cause epidermolytic palmoplantar keratoderma, characterized by compensatory thickened epidermal layers in the palms and soles. 10 Conditional deletion of Krt9 in a murine model demonstrated that Krt9 is responsible for maintaining mechanical integrity and terminal differentiation of volar skin.

Antisense Long Non-Coding RNAs Are Deregulated in Skin Tissue of Patients with Systemic Sclerosis

“…KRT4 is expressed in mucosal tissue and is increased upon inflammation (Bosch et al, 1989), while KRT9 is normally expressed in soles and hand palms (Rinn et al, 2008). KRT9 is required for structural integrity of the epidermis and KRT9 was found increased in psoriasis patients (Fu et al, 2014;Kim et al, 2016). The increased expression of these keratins in skin of early SSc patients highlights the possibility of aberrant activation of these genes early in disease.…”

dsRNA Sensing Induces Loss of Cell Identity

“…Similarly, Krt9-null mice have similar symptoms of compensatory thickened volar skin, highlighting the fact that KRT9 likely provides some of the unique structural support of volar skin (Fu et al, 2014). Yamaguchi et al (1999) and Kim et al (2016) previously showed that ectopic KRT9 can be induced by volar fibroblasts. However, whether volar keratinocytes have intrinsic positional memory and expression of KRT9 is unknown.…”

Pathophysiology of pachyonychia congenita‐associated palmoplantar keratoderma: new insights into skin epithelial homeostasis and avenues for treatment

“…The latter represents a predominant element in epidermolysis bullosa simplex (EBS), a genetically determined skin blistering condition caused by mutations in either KRT14 or KRT5 . Keratinocyte fragility is also a dominant pathophysiological feature in epidermolytic PPK, which is often caused by mutations in KRT9 , the major differentiation‐specific keratin in the volar epidermis . The greater complexity of keratin gene expression in the volar epidermis likely contributes to maintain keratinocyte structural integrity in spite of mutations in individual genes such as KRT6A‐C , KRT16 and KRT17 .…”

“…At this early time point there are minimal alterations to the skin tissue histology but, already, a dramatic loss of K9 ( Krt9 /K9) expression has occurred, which then persists throughout lesion progression . Krt9 occurs exclusively in differentiating keratinocytes of volar skin and represents a predominant marker gene in this setting . In contrast to Krt9 , several differentiation markers appear to be upregulated in Krt16 null footpad skin, potentially as a compensatory mechanism .…”