A Novel Epithelial Keratin, hK6irs1, is Expressed Differentially in All Layers of the Inner Root Sheath, Including Specialized Huxley Cells (Flügelzellen) of the Human Hair Follicle

Abstract

In this study we have characterized a novel human type II keratin, hK6irs1, which is specifically expressed in the inner root sheath of the hair follicle. This keratin represents the ortholog of the recently described mouse inner root sheath keratin mK6irs. The two keratins were highly related and migrated at the same height as keratin 6 in two-dimensional gel electrophoresis. Both RNA in situ hybridization and indirect immunofluorescence studies of human hair follicles demonstrated hK6irs1 expression in the Henle and Huxley layers as well as in the cuticle of the inner root sheath. In all three layers, the expression of hK6irs1 mRNA and protein began simultaneously in adjacent cells of the lowermost bulb above the germinative cell pool. Higher up in the follicle, the detection limits for both hK6irs1 mRNA and protein precisely coincided with the asynchronous onset of abrupt terminal differentiation of the Henle layer, inner root sheath cuticle, and Huxley layer. Mainly above the level of terminal Henle cell differentiation, both indirect immunofluorescence and immunoelectron microscopy revealed the occurrence of distinct Huxley cells that developed pseudopodal hK6irs1-positive extensions passing through the fully keratinized Henle layer. These outwardly protruding foot processes abutted upon cells of the companion layer, with which they were connected by numerous desmosomes. These specialized Huxley cells have previously been termed "Flügelzellen", which means "winged cells", with reference to their characteristic foot processes. We provide evidence that, together with Henle cells, Flügelzellen ensure the maintenance of a continuous desmosomal anchorage of the companion layer along the entire inner root sheath. This tightly connected companion layer/inner root sheath unit provides an optimal molding and guidance of the growing hair shaft.

Evidence That the Satin Hair Mutant Gene Foxq1 Is among Multiple and Functionally Diverse Regulatory Targets for Hoxc13 during Hair Follicle Differentiation

“…We identified Krt2-35 and Krtap5-4 as presumptive cuticle-specific genes and validated their down-regulation in GC13 skin by Q-PCR. The Krt2-35 expression pattern in the cuticle closely resembled the follicular pattern of its human ortholog HK6IRS2 (37,38) in the "neck" region of the bulb (Fig. 3A and AЈ).…”

“…However, because of difficulty in resolving the cuticle of the IRS from that of the hair fiber (Fig. 3B), it is unclear whether Krt2-35 expression is exclusively restricted to the IRS cuticle as is the case for HK6IRS2 (37,38). By comparison, the keratin-associated protein (KAP)-encoding gene Krtap5-4, originally designated gUHS-SER-2 (39), exhibited a distinct cylindrical pattern in the cuticle restricted to a narrow region at the mid-level of the hair shaft that was shifted toward the bulb in GC13 mutant hair (Fig.…”

Involucrin, but not filaggrin and Kdap mRNA, expression is downregulated in 3-D cultures of intact rat hair bulbs after calcium stimulation

“…Because cells of Henle's layer in mid-segments are in rats nearly completely keratinized [26], the inhibitory factor would have to be produced by Huxley's layer cells and secreted into the ORS through outgrowths of the specialized cells from the Huxley's layer [38]. These outgrowths pass through the Henle's layer and contact cells of the ORS [39], or more precisely, the companion layer cells [40] where they form gap-junction like structures [39].…”

Expression of Birt–Hogg–Dubé gene mRNA in normal and neoplastic human tissues

“…A riboprobe to a keratin known to be expressed in the inner root sheath of the hair follicle, Ker6IRS1, 16 was used as a positive control and to optimize the in situ hybridization method. PCR primers for Ker6-IRS1 were designed (F 5 0 -CAT CAG CAG CAC CAG TGG CGG-3 0 and R 5 0 -CAG GAA CTA TGC TAG GTC CCA G-3 0 ) and the probe of interest was amplified by PCR at a 641C annealing temperature using standard conditions.…”

“…A riboprobe from the Ker6IRS1 gene 16 was used to optimize the in situ hybridization protocol and as a control to visualize the different regions of the hair follicle. The Ker6IRS1 riboprobe bound specifically to all three layers of the inner root sheath (Henle, Huxley and inner cuticle) and sense probe slides were void of signal (Figure 2a and b).…”