Requirement of NF-κB/Rel for the development of hair follicles and other epidermal appendices

TRAF6-deficient mice display hypohidrotic ectodermal dysplasia

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is an adapter protein that links signals from members of the TNFR superfamily and Toll/IL-1 receptor family to activation of transcription factors NFkappaB and AP-1. Analysis of TRAF6-deficient mice revealed that TRAF6 is essential for normal bone formation and establishment of immune and inflammatory systems. Here we report that TRAF6 deficiency results in defective development of epidermal appendixes, including guard hair follicles, sweat glands, sebaceous glands of back skin, and modified sebaceous glands such as meibomian glands, anal glands, and preputial glands. Except the sebaceous gland impairment, these abnormal phenotypes are identical to those observed in Tabby (Ta), downless (dl), and crinkled (cr) mice, which are models of hypohidrotic (anhidrotic) ectodermal dysplasia in human. beta-catenin and mucosal addressin cell adhesion molecule-1, an early marker of developing guard-hair follicles is absent in the skin of TRAF6-deficient embryos. Thus, TRAF6 is essential for development of epidermal appendixes. TRAF6 does not associate with the cytoplasmic tail of the dl protein (DL)/ectodysplasin receptor (EDAR) receptor, which, when mutated, results in hypohidrotic (anhidrotic) ectodermal dysplasia. However, TRAF6 associates with X-linked ectodysplasin-A2 receptor (XEDAR) and TNFR super family expressed on the mouse embryo (TROY/toxicity and JNK inducer (TAJ), which are EDAR-related members of the TNFR superfamily that are expressed at high level in epidermal appendixes. Furthermore, TRAF6 is essential for the XEDAR-mediated NFkappaB activation. Our results suggest that TRAF6 may transduce signals emanating from XEDAR or TROY/TAJ that are associated with development of epidermal appendixes.

Death receptor signaling giving life to ectodermal organs

A new tumor necrosis factor (TNF) pathway has been identified that has an important function in the regulation of embryonic development. Three key components of this pathway are previously unknown proteins: the TNF ligand ectodysplasin (also known as EDA), its death domain-containing receptor EDAR, and the death domain adapter molecule EDARADD. This pathway was discovered and delineated through the cloning of genes that cause human hypohidrotic ectodermal dysplasia (HED) syndromes and by analysis of the corresponding mouse mutants (Tabby, downless, and crinkled) showing defects in hair, teeth, and several exocrine glands. EDAR signaling is mediated by the activation of nuclear factor kappa B, but other downstream targets are not known. Ectodysplasin-EDAR signaling mediates cell interactions within the ectoderm and regulates the initiation and morphogenesis of hair and teeth. It is also necessary for the development of fish scales, indicating that this pathway and its function have been conserved during the evolution of ectodermal organs.

Healing a natural knockout of epithelial organogenesis

Ectodermal dysplasias are a large group of rare genetic disorders with developmental abnormalities in skin, teeth, hair and nails. Many of them are clinically serious and impair the life of patients. The cloning of the gene for the most common of them, X-linked anhidrotic ectodermal dysplasia, in 1996 opened the door to dissect novel developmental pathways at the molecular level. Since then, several new genes and proteins with novel functions have been identified.

Designer skin: lineage commitment in postnatal epidermis

The epidermis is populated by stem cells that produce daughters that differentiate to form the interfollicular epidermis, hair follicles and sebaceous glands. Diffusible factors, cell-cell contact and extracellular matrix proteins are all important components of the microenvironment of individual stem cells and profoundly affect the differentiation pathways selected by their progeny. Here, we summarize what is known about stem-cell populations and lineage relationships within the epidermis. We also present evidence that postnatal epidermis can be reprogrammed, altering the number and location of cells that differentiate along specific epidermal lineages.

Identification of a novel death domain-containing adaptor molecule for ectodysplasin-A receptor that is mutated in crinkled mice

Hypohydrotic Ectodermal Dysplasia (HED) is a genetic disease seen in humans and mice. It is characterized by loss of hair, sweat glands, and teeth. The predominant X-linked form results from mutations in ectodysplasin-A (EDA), a TNF-like ligand. A phenotypically indistinguishable autosomal form of the disease results from mutations in the receptor for EDA (EDAR). EDAR is a NF-kappaB-activating, death domain-containing member of the TNF receptor family. crinkled, a distinct autosomal form of HED, was discovered in a mouse strain in which both the ligand (EDA) and receptor (EDAR) were wild-type, suggestive of a disruption further downstream in the signaling pathway. Employing a forward genetic approach, we have cloned crinkled (CR) and find it to encode a novel death domain-containing adaptor. crinkled binds EDAR through a homotypic death domain interaction and mediates engagement of the NF-kappaB pathway, possibly by recruiting TRAF2 to the receptor-signaling complex. This is an unprecedented example of naturally occurring mutations in ligand, receptor, or adaptor giving rise to the same phenotypic disease characterized by a defect in the proper development of epidermal appendages.

Alopecia areata in families: association with the HLA locus

Alopecia areata (AA) is a T cell mediated disease directed against hair follicles that results in bald patches. It can range in severity from patchy (AA), to total scalp hair loss (alopecia totalis; AT) or body hair loss (alopecia universalis; AU). We have previously shown that HLA-DR4 and DR11 as well as HLA-DQ*03 alleles are increased in unrelated AA patients compared with controls. To study whether class II HLA alleles are linked to AA, we investigated 81 extended families that included 192 AA patients, including 89 with AT or AU. We also performed the transmission disequilibrium test (TDT) in 143 nuclear families. Results showed an association between alleles of HLA-DQB (p = 0.014) and HLA-DR (p = 0.010). We also performed linkage analysis in 75 families whose members' genomic DNA were available for HLA typing. Results from this analysis support linkage between AA and class II loci with a maximal LOD score of 2.42 to HLA-DQB at 5% recombination, and with a maximal LOD score of 2.34 to HLA-DR at 0% recombination. There was an increased incidence of atopic dermatitis and autoimmune thyroiditis in families. AA appears to be a class II HLA restricted organ specific immune response to the hair follicle.