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Immunohistochemical comparison of whisker pad cutaneous innervation in Swiss Webster and hairless mice

To establish the mouse mutant, hairless (Hr), as a useful model for future analyses of target-ending interactions, we assessed the cutaneous innervation in the whisker pad after loss of primary hair targets. Postnatal (P) development of fur in Hr begins similarly to that of "normal" Swiss Webster (SW) mice. Around P10, hairs are shed and the follicles rendered permanently incompetent. Hair loss progresses rostrocaudally until the entire skin is denuded. Substantial alterations in the distribution and density of sensory and autonomic endings in the mystacial pad vibrissal and intervibrissal fur innervation were discovered. Pilo-neural complexes innervating fur hairs were dismantled in Hr. Epidermal innervation in SW was rich; only a few endings expressed growth-associated protein-43 kdal (GAP), suggesting limited changes in axonal elongation. Innervation in Hr formed a dense layer passing upward through the thickened epidermis, with substantial increases among all types of endings. Vibrissal follicle-sinus complexes were also hyperinnervated. Endings in Hr vibrissae and fur were strongly GAP-positive, suggesting reorganization of innervation. Dermal and vascular autonomic innervation in both strains co-localized tyrosine hydroxylase and neuropeptide Y, but only in Hr did neuropeptide Y co-localize calcitonin gene-related peptide (CGRP) and express GAP immunolabeling. Stereological quantitation of trigeminal ganglia revealed no differences in neuron number between Hr and SW, although there were small increases in cell volume in Hr trigeminal ganglion cells. These results suggested that a form of collateral sprouting was active in Hr mystacial pads, not in response to local injury, but as a result of loss of primary target tissues.

The "bald Mill Hill" mutation in the mouse is associated with an abnormal, mislocalized HR bmh protein

We have previously identified a mutation in the mouse hairless locus-hairless rhino bald Mill Hill (Hr(rhbmh)). The genetic alteration in these mice consists in a large 296 bp deletion at the 3' part of the hairless gene (ID:MGI:3039558; J:89321). Here, we show that this deletion removes the stop codon and creates a new reading frame at the C terminus of the hairless protein, generating a larger mutant protein harboring an additional sequence of 117 amino acids. The mutant hairless gene mRNA is expressed during the embryonic and post-natal development of the hair follicle. The mutant protein is identified in bmh mouse skin at different stages of development by a specific antibody. We demonstrate that the HR bmh protein is able to interact with the vitamin D receptor (VDR), but is not able to repress VDR-mediated transactivation. Immunofluorescence analysis reveals that HR bmh protein displays an abnormal cellular localization in transfected cell lines, as well as in the epidermis and hair follicle of bmh mutant mice. We discuss the relevance of the hairless protein mis localization in cell signalling pathways and with respect to the specific skin phenotype of mouse hairless mutants.