Highly Upregulated Lhx2 in the Foxn1−/− Nude Mouse Phenotype Reflects a Dysregulated and Expanded Epidermal Stem Cell Niche

Abstract

Hair cycling is a prime example of stem cell dependent tissue regeneration and replenishment, and its regulatory mechanisms remain poorly understood. In the present study, we evaluated the effect of a blockage in terminal keratinocytic lineage differentiation in the Foxn1−/− nude phenotype on the epithelial progeny. Most notably we found a constitutive upregulation of LIM homeobox protein 2 (Lhx2), a marker gene of epithelial stem cellness indispensible for hair cycle progression. However, histological evidence along with an erratic, acyclic rise of otherwise suppressed CyclinD1 levels along with several key markers of keratinocyte lineage differentiation indicate a frustrated expansion of epithelial stem cell niches in skin. In addition, CD49f/CD34/CD200–based profiling demonstrated highly significant shifts in subpopulations of epithelial progeny. Intriguingly this appeared to include the expansion of Oct4+ stem cells in dermal fractions of skin isolates in the Foxn1 knock-out opposed to wild type. Overall our findings indicate that the Foxn1−/− phenotype has a strong impact on epithelial progeny and thus offers a promising model to study maintenance and regulation of stem cell niches within skin not feasible in other in vitro or in vivo models.

Neotenic phenomenon in gene expression in the skin of Foxn1- deficient (nude) mice - a projection for regenerative skin wound healing

“…Correspondingly, our previous data showed that large populations of nude dermal fibroblasts display stem cell biomarkers compared to wild type B6 dermal fibroblasts [45]. Therefore, we assume that the lack of Foxn1 activity prevents the final stages of differentiation [14] and keeps cells in an immature progenitor-like state [43, 46]. …”

“…In our regenerative models, we detected down-regulation of some transcription factors that promote differentiation and block proliferation of epidermal progeny cells (genes repressed by PcG; Table 1). This can lead to an increase in the number of undifferentiated, presumably epidermal stem cells (ESC) [43]. Previously, it was shown that a local population of ESC exerts a positive impact on skin regenerative capacity [44].…”

FOXN1 Transcription Factor in Epithelial Growth and Wound Healing

“…Interestingly, hair cycling was determined to be functional by the presence of cyclin D1 mRNA in nude mice (52). It is well known that these mice do not produce hair cortex (16) and form an abrogated inner root sheath (IRS) (27).…”

“…There were reported differences in the distribution of SC/progenitor fractions in HFs of nude mice versus wild-type controls. The stem cell markers SOX2 (sex-determining region Y [SRY] box 2) and OCT4 (octamer-binding transcription factor 4) are downregulated, in contrast to the upregu- lation of LHX2 (LIM homeobox protein 2), which is responsible for anagen progression, and the Wnt signaling target gene, ␤-catenin, which is uniformly expressed at high levels in the nude mice (52). Similarly, in spiny mice, which are another case of mammals able to regenerate their tissues, the readout of Wnt inductive signaling, LEF1, was present at high levels in both epidermal and dermal cells of forming hair placodes (53).…”

“…In vitro studies showed that mitogen-activated protein kinase (MAPK) is an upstream regulator of FOXN1, as inhibition of MAPK allows the expression of Foxn1 and induces the onset of differentiation (35). Interestingly, nude mouse skin has a large population of OCT4-positive dermal cells (52). In vitro studies of primary dermal fibroblasts from nude mice revealed that they produce a higher content of both collagen I and III, although at the same time, they produce active matrix metalloproteinase 9 (MMP-9) and MMP-13, which are completely absent under the wild-type conditions (15).…”