Programming gene expression in developing epidermis

Abstract

As the major proteins of adult keratinocytes, keratins provide biochemical markers for exploring mouse epidermal embryogenesis. Here, we used a modified method of whole-mount in situ hybridization to track skin-specific expression of endogenous keratin mRNAs throughout embryogenesis. To monitor transcriptional regulation, we coupled this with beta-galactosidase expression of a human epidermal keratin promoter-driven transgene. These studies have radically changed our perception of how the program of gene expression becomes established during epidermal development. Specifically, we have discovered that (1) basal keratin (K5 and K14) genes are first detected at E9.5 in a highly regional fashion, and surprisingly as early as the single layered ectodermal stage; (2) the early patterns do not correlate with morphogenesis per se, but rather with regional variations in the embryonic origin of underlying mesenchyme, supporting morphogenetic criteria that early inductive cues are mesenchymal; (3) epidermal keratin genes are expressed in periderm, supporting the notion that this layer arises from ectodermal stratification, even though it is simple epithelial-like in morphology and is subsequently sloughed during development; (4) later embryonic patterns of K5 and K14 gene expression parallel proliferative capacity and not stratification; and (5) K1 and K10 mRNAs are first detected as early as E13.5, and their patterns correlate with differentiation and not stratification. These patterns of epidermal gene expression led us to explore whether potential transcriptional regulators of these genes are expressed similarly. We show that AP2 (but not Sp1) cRNAs hybridize in a pattern similar to, but preceding that of basal keratin cRNAs. Finally, using gene expression in cultured cells, we demonstrate that AP2 has a strong inductive effect on basal keratin expression in a cellular environment that does not normally possess AP2 activity.

Identification of a Functional Determinant of Differentiation-dependent Expression in the Involucrin Gene

“…A great deal of evidence has accumulated to support a role for AP2 in tissue-specific expression of keratinocyte genes. Although AP2 is present in a number of cell types, it is highly expressed in the epidermis during development as well as in the adult tissue (20). A functional AP2 response element is necessary for tissue-specific expression of K14 (19,37) and AP2 binding sequences have been implicated in the expression of K1 and K5 (18,19).…”

“…An AP-2 like element and associated DNAbinding protein, KTF-1 which is related to or identical to AP2, appears to be involved in the epidermal specific expression of an embryonic keratin gene in Xenopus (38). Importantly, AP2 expression precedes that the responsive keratin genes in the developing mouse embryo (20). However, it appears that AP2 may not be strictly involved in dictating appropriate differentiation-dependent expression of keratinocyte genes.…”

Transcriptomic and Functional Evidence Show Similarities between Human Amniotic Epithelial Stem Cells and Keratinocytes

“…According to Figure 2 a, when compared to KCs, AESCs showed high expressions of a distinct group of keratins, including KRT24 , KRT72 , KRT27 , KRT19 , and KRT7 . Although KRT7 and KRT19 were hardly detected in KCs (isolated from adult skin), their expression has been identified in fetal epidermis in previous studies [ 20 , 21 ]. To verify this, KRT7 and KRT19 were assessed by immunostaining, which confirmed their expression in both fetal skin and amnion, but not in adult skin ( Figure 2 d and Figure S2b ).…”

“…The keratin family is a characteristic of distinct epithelial cells and show different expression patterns during cellular differentiation from embryonic to adult stages [ 19 ]. During embryonic development, keratins KRT8/KRT18 and KRT7/KRT19 appear earlier in single-layer multipotent epithelial cells [ 20 , 21 , 37 ] and are involved in placental barrier function [ 19 ]. Later, prior to stratification, these keratins are replaced with KRT5/KRT14 [ 38 ], marking the beginning of epidermal commitment [ 20 ].…”

“…During embryonic development, keratins KRT8/KRT18 and KRT7/KRT19 appear earlier in single-layer multipotent epithelial cells [ 20 , 21 , 37 ] and are involved in placental barrier function [ 19 ]. Later, prior to stratification, these keratins are replaced with KRT5/KRT14 [ 38 ], marking the beginning of epidermal commitment [ 20 ]. KRT5/KRT14 are markers of the undifferentiated basal stem cell layer and parallel the proliferative capacity.…”