The nude mouse skin phenotype: the role of Foxn1 in hair follicle development and cycling

Foxn1 is required for tissue assembly and desmosomal cadherin expression in the hair shaft

The mouse nude mutation inactivates the gene encoding the Foxn1 transcription factor, causing defective hair morphogenesis. Here, we show for the first time that Foxn1 is required for proper assembly of the hair medulla, and we identify Foxn1-regulated genes by transcript profiling. One such gene encodes the desmosomal cadherin, Dsc2. Significantly, Foxn1-dependent Dsc2 expression is restricted to the hair medulla, and within these cells, Dsc2 protein is predominantly localized to specialized adhesion junctions between the cortex and the medulla. Our results reveal Foxn1 as an essential regulator of tissue assembly in the growing hair shaft and implicate Dsc2 as a downstream effector of this activity.

Premature termination of hair follicle morphogenesis and accelerated hair follicle cycling in Iasi congenital atrichia (fzica) mice points to fuzzy as a key element of hair cycle control

Inbred laboratory mice have proven to be useful model systems for studying hair biology and pathomechanisms of hair loss. Fuzzy (fz) is an autosomal recessive mutation that results in hair coat abnormalities. Though this mutant has long been known, its cutaneous abnormalities still await systematic analysis. Here, we provide a systematic skin phenotype analysis of mice that are homozygous for Iasi congenital atrichia (fzica/fzica), which is allelic to fz. Homozygous mice exhibit a sparse hair coat after birth and completely loose their hair at around postnatal day 120. Although early and mid stages of hair follicle morphogenesis are normal, late hair follicle morphogenesis reveals multifocal cell degeneration within the Huxley layer of the inner root sheath (IRS) and a complete lack of the hair shaft medulla. In addition, hair follicle development is prematurely terminated by induction of the first postnatal hair cycle with premature entry into catagen. Subsequently, a dramatically shortened telogen is immediately followed by premature anagen development, resulting in a marked, generalized acceleration of hair follicle cycling. This suggests that fuzzy is not only involved in structural hair shaft integrity and differentiation of the IRS and medulla, but also plays an important role in the control of hair follicle cycling. Our data show that fuzzy is involved in controlling both catagen and anagen initiation, designating fuzzy an exciting target for characterizing the intracutaneous oscillator system that drives hair follicle cycling.

In search of the "hair cycle clock": a guided tour

The hair follicle, a unique characteristic of mammals, represents a stem cell-rich, prototypic neuroectodermal-mesodermal interaction system. This factory for pigmented epithelial fibers is unique in that it is the only organ in the mammalian body which, for its entire lifetime, undergoes cyclic transformations from stages of rapid growth (anagen) to apoptosis-driven regression (catagen) and back to anagen, via an interspersed period of relative quiescence (telogen). While it is undisputed that the biological "clock" that drives hair follicle cycling resides in the hair follicle itself, the molecular nature of the underlying oscillator system remains to be clarified. To meet this challenge is of profound general interest, since numerous key problems of modern biology can be studied exemplarily in this versatile model system. It is also clinically important, since the vast majority of patients with hair growth disorders suffers from an undesired alteration of hair follicle cycling. Here, we sketch basic background information and key concepts that one needs to keep in mind when exploring the enigmatic "hair cycle clock"(HCC), and summarize competing models of the HCC. We invite the reader on a very subjective guided tour, which focuses on our own trials, errors, and findings toward the distant goal of unravelling one of the most fascinating mysteries of biology: Why does the hair follicle cycle at all? How does it do it? What are the key players in the underlying molecular controls? Attempting to offer at least some meaningful answers, we share our prejudices and perspectives, and define crucial open questions.

FOXN1 is critical for onycholemmal terminal differentiation in nude (Foxn1) mice

Nude mice have a mutation in the transcription factor Foxn1(nu), resulting in downregulation of hair keratins. Although hair follicles develop normally, the hair fibers become structurally weak, curl, and break off at the surface. Nails in nude mice are deformed, based on alterations of the onychocyte differentiation process. Elemental microanalysis of the nail plate reveals marked decreases in sulfur concentrations in the nude mouse nail plates. Immunohistochemistry shows a lack of keratin 1 expression in terminally differentiating keratinocytes of the nail matrix. Instead, the typical differentiation process of the matrix is altered toward an epidermis-like differentiation pattern, comprising the production of filaggrin-containing keratohyalin granules in cells resembling those of the stratum granulosum, which are never observed in normally haired mice. The nail plate has diffuse basophilic stippling. It is thinner than normal, weak, and in most Foxn1(nu)/Foxn1(nu) mice breaks where it separates from the hyponychium. These studies indicate that the Foxn1(nu) mutated gene has effects beyond downregulating keratin expression, including changes in filaggrin expression, and is critical for normal onycholemmal differentiation. The nails of nude mice provide new insights into the molecular controls of onychocyte differentiation, and they offer a useful model to investigate the pathogenesis of nail hypergranulosis, a common feature in human nail diseases.

Nude thymic rudiment lacking functional foxn1 resembles respiratory epithelium

The epithelial compartment of the thymus arises from endoderm of the 3rd pharyngeal pouch. As it moves from a cervical to a mediastinal position during development, this epithelium becomes populated by lymphoid progenitor cells from the blood and begins to support their differentiation along the T cell lineage. Productive differentiation of thymic epithelium is strictly dependent on the foxn1 transcription factor, as evidenced by the lack of functional thymic tissue in nude mice that carry a spontaneous loss-of-function mutation of foxn1. Evaluation of the thymic rudiment epithelium from nude mice revealed phenotypic properties and tissue organization that was strongly reminiscent of respiratory epithelium. These data suggest that foxn1 may be involved in directing lineage choices of multi-potential progenitor epithelial cells rather than simply affecting the terminal differentiation program of epithelial cells specified to a thymic fate.