The basal keratin network of stratified squamous epithelia: defining K15 function in the absence of K14.

Abstract

Abstract. Keratin 5 and keratin 14 have been touted as the hallmarks of the basal keratin networks of all stratified squamous epithelia. Absence of K14 gives rise to epidermolysis bullosa simplex, a human blistering skin disorder involving cytolysis in the basal layer of epidermis. To address the puzzling question of why this disease is primarily manifested in skin rather than other stratified squamous epithelia, we ablated the K14 gene in mice and examined various tissues expressing this gene. We show that a key factor is the presence of another keratin, K15, which was hitherto unappreciated as a basal cell component. We show that the levels of K15 relative to K14 vary dramatically among stratified squamous epithelial tissues, and with neonatal development. In the absence of K14, K15 makes a bona fide, but ultrastructurally distinct, keratin filament network with K5. In the epidermis of neonatal mutant mice, K15 levels are low and do not compensate for the loss of K14. In contrast, the esophagus is unaffected in the neonatal mutant mice, but does appear to be fragile in the adult. Parallel to this phenomenon is that esophageal K14 is expressed at extremely low levels in the neonate, but rises in postnatal development. Finally, despite previous conclusions that the formation of suprabasal keratin filaments might depend upon K5/K14, we find that a wide variety of suprabasal networks composed of different keratins can form in the absence of K14 in the basal layer.

Complete Cytolysis and Neonatal Lethality in Keratin 5 Knockout Mice Reveal Its Fundamental Role in Skin Integrity and in Epidermolysis Bullosa Simplex

“…On E18.5, the expression of K6 mRNA was weakly induced compared with wild-type mice, indicating that the induction of K6 mRNA was already initiated, possibly close to cytolyzing basal keratinocytes. Subsequent Western blotting demonstrated that the strong increase in K6 mRNA was also translated into the corresponding protein in neonatal mice, whereas we were unable to visualize K6 on E18.5 by this technique.The induction of K6 in K5 Ϫ/Ϫ mice seems to be in contrast to the K14 Ϫ/Ϫ mice (Lloyd et al, 1995). Whether this difference is due to the extent of tissue damage in both mice or whether there are other reasons remains to be clarified.…”

“…Besides epidermis, we studied the keratin expression in tongue, esophagus, palate, fore stomach, foot, and tail (our unpublished results). In general, the intraepidermal cleft could be observed in every stratifying epithelium that we examined, emphasizing the extreme fragility of the basal cells as a consequence of the loss of K5.In contrast to K14 Ϫ/Ϫ and K6 Ϫ/Ϫ mice (Lloyd et al, 1995;Wong et al, 2000), the basal cells on the dorsal side of the tongue were largely unaffected ( Figure 5). Here, suprabasal expression of K6 was recently shown to be essential for the maintenance of epidermal integrity (Wong et al, 2000).…”

“…On the basis of these observations, it was suggested that the overall function of epidermal keratins was to provide the reinforcement of the epidermis and the maintenance of cellular integrity under mechanical and thermal stress. Several mice carrying null or dominant keratin mutations that affect epidermal integrity have added further support to this notion (Fuchs et al, 1992;Lloyd et al, 1995;Porter et al, 1996;Ness et al, 1998; Wojcik et al, 2000;Wong et al, 2000).So far, however, none of these mouse models have addressed the question of whether the deletion of the corresponding partner keratin in the same tissue leads to the same phenotype. This is an important issue, because it was suggested that types I and II keratins may associate with distinct sets of proteins.…”

“…At least one member of each family is necessary to form heterodimeric IFs. In epidermis, keratins display a complex expression pattern that is widely assumed to reflect the structural requirements of distinct epidermal compartments (Fuchs and Cleveland, 1998).The major keratins in the basal layer of stratified epithelia are K5, K14, and K15 (Fuchs and Green, 1978, 1980;Moll et al, 1982;Leube et al, 1988;Lloyd et al, 1995). In wound healing or in hyperproliferative disorders, the keratin pair K6 and K16 is transiently expressed in the suprabasal layers instead of K1, K2e, and K10 (for review, see McGowan and Coulombe, 1998).…”

Desmosomes and Intermediate Filaments: Their Consequences for Tissue Mechanics

“…EBS is characterized by cytoplasmic keratin aggregates, cytolysis of basal keratinocytes, and bullous lesions following mild trauma to the skin. Although it is recognized that the pathomechanisms contributing to EBS and additional keratinopathies are more complex than originally considered Roth et al 2012;Bohnekamp et al 2015;Hobbs et al 2016;Kumar et al 2016), it is evident that loss of an intact keratin cytoskeleton renders keratinocytes fragile on mild physical stress, shown by KRT5 and KRT14 KO mice (Lloyd et al 1995;Peters et al 2001). Of note, even mutations causing severe disease do not prevent formation of long keratin intermediate filaments (KIFs) in vitro (Herrmann et al 2002), suggesting that mutations and physical stress act at the level of keratin bundling, network organization, dynamics, or by affecting association with other proteins.…”