Foxn1 promotes keratinocyte differentiation by regulating the activity of protein kinase C

Dermal white adipose tissue development and metabolism: The role of transcription factor Foxn1

Intradermal adipocytes form dermal white adipose tissue (dWAT), a unique fat depot localized in the lower layer of the dermis. However, recognition of molecular factors regulating dWAT development, homeostasis, and bioactivity is limited. Using Foxn1-/- and Foxn1+/+ mice, we demonstrated that epidermally expressed Foxn1 regulates dWAT development and defines the adipogenic capacity of dermal fibroblasts. In intact and post-wounded skin, Foxn1 contributes to the initial stimulation of dWAT adipogenesis and participates in the modulation of lipid metabolism processes. Furthermore, Foxn1 activity strengthens adipogenic processes through Bmp2 and Igf2 signaling and regulates lipid metabolism in differentiated dermal fibroblasts. The results reveal the contribution of Foxn1 to dWAT metabolism, thus identifying possible targets for modulation and regulation of dWAT in physiological and pathological processes in the skin.

Foxn1 overexpression promotes thymic epithelial progenitor cell proliferation and mTEC maintenance, but does not prevent thymic involution

The transcription factor FOXN1 is essential for fetal thymic epithelial cell (TEC) differentiation and proliferation. Postnatally, Foxn1 levels vary widely between TEC subsets, from low/undetectable in putative TEC progenitors to highest in differentiated TEC subsets. Correct Foxn1 expression is required to maintain the postnatal microenvironment; premature downregulation of Foxn1 causes a rapid involution-like phenotype, and transgenic overexpression can cause thymic hyperplasia and/or delayed involution. We investigated a K5.Foxn1 transgene that drives overexpression in mouse TECs, but causes neither hyperplasia nor delay or prevention of aging-related involution. Similarly, this transgene cannot rescue thymus size in Foxn1lacZ/lacZ mice, which undergo premature involution as a result of reduced Foxn1 levels. However, TEC differentiation and cortico-medullary organization are maintained with aging in both K5.Foxn1 and Foxn1lacZ/lacZ mice. Analysis of candidate TEC markers showed co-expression of progenitor and differentiation markers as well as increased proliferation in Plet1+ TECs associated with Foxn1 expression. These results demonstrate that the functions of FOXN1 in promoting TEC proliferation and differentiation are separable and context dependent, and suggest that modulating Foxn1 levels can regulate the balance of proliferation and differentiation in TEC progenitors.

Hypoxia reveals a new function of Foxn1 in the keratinocyte antioxidant defense system

Skin exposed to environmental threats, including injuries and oxidative stress, develops an efficient but not fully recognized system of repair and antioxidant protection. Here, using mass spectrometry analysis (LC-MS/MS), followed by in vitro and in vivo experiments, we provided evidence that Foxn1 in keratinocytes regulates elements of the electron transport chain and participates in the thioredoxin system (Txn2, Txnrd3, and Srxn1) induction, particularly in a hypoxic environment. We first showed that Foxn1 in keratinocytes upregulates glutathione thioredoxin reductase 3 (Txnrd3) protein expression, and high levels of Txnrd3 mRNA were detected in injured skin of Foxn1^+/+ mice. We also showed that Foxn1 strongly downregulated the Ccn2 protein expression, participating in epidermal reconstruction after injury. An in vitro assay revealed that Foxn1 controls keratinocyte migration, stimulating it under normoxia and suppressing it under hypoxia. Keratinocytes overexpressing Foxn1 and exposed to hypoxia displayed a reduced ability to promote angiogenesis by downregulating Vegfa expression. In conclusion, this study showed a new mechanism in which Foxn1, along with hypoxia, participates in the activation of antioxidant defense and controls the functional properties of keratinocytes.

Dermal White Adipose Tissue (dWAT) Is Regulated by Foxn1 and Hif-1α during the Early Phase of Skin Wound Healing

Dermal white adipose tissue (dWAT) is involved in the maintenance of skin homeostasis. However, the studies concerning its molecular regulation are limited. In the present paper, we ask whether the introduction of two transcription factors, Foxn1 and Hif-1α, into the post-wounded skin of Foxn1-/- mice regulates dWAT during wound healing (days 3 and 6). We have chosen lentivirus vectors (LVs) as a tool to deliver Foxn1 and Hif-1α into the post-wounded skin. We documented that combinations of both transgenes reduces the number, size and diameter of dermal adipocytes at the wound bed area. The qRT-PCR analysis of pro-adipogenic genes, revealed that LV-Hif-1α alone, or combined with LV-Foxn1, increases the mRNA expression of Pparγ, Glut 4 and Fasn at post-wounding day 6. However, the most spectacular stimulatory effect of Foxn1 and/or Hif-1α was observed for Igf2, the growth factor participating in adipogenic signal transduction. Our data also shows that Foxn1/Hif-1α, at post-wounding day 3, reduces levels of CD68 and MIP-1γ mRNA expression and the percentage of CD68 positive cells in the wound site. In conclusion, the present data are the first to document that Foxn1 and Hif-1α cooperatively (1) regulate dWAT during the proliferative phase of skin wound healing through the Igf2 signaling pathway, and (2) reduce the macrophages content in the wound site.

Molekularne mechanizmy działania czynnika transkrypcyjnego FOXN1 w skórze

Artykuł jest przeglądem wyników badań dotyczących funkcji i mechanizmu działania czynnika transkrypcyjnego FOXN1. Lokalizacja FOXN1 u przedstawicieli wszystkich ssaków ogranicza się do nabłonka tylko dwóch organów: skóry i grasicy. W skórze FOXN1 stymuluje różnicowanie się keratynocytów, reguluje proces pigmentacji i bierze udział w rozwoju włosów. W skórze objętej urazem FOXN1 jest zaangażowany w bliznowy proces gojenia poprzez udział w reepitelializacji oraz w procesie przejścia epitelialno-mezenchymalnego (epithelial-mesenchymal transition; EMT). Pozbawione aktywnego czynnika transkrypcyjnego FOXN1 dorosłe myszy (Foxn1-/-) goją urazy skórne w unikalnym, charakterystycznym jedynie dla płodów ssaków, procesie bezbliznowej (scar-free) regeneracji. Analiza porównawcza transkryptomów skóry: dorosłych myszy Foxn1-/- oraz skóry płodów myszy (14. dzień rozwoju płodowego) wykazała istotne podobieństwa w ekspresji genów związanych przede wszystkim z przebudową tkanek, budową cytoszkieletu, gojeniem urazów, odpowiedzią immunologiczną oraz różnicowaniem. Wyniki te wskazują, iż FOXN1 może być głównym elementem szlaku sygnałowego na drodze tzw. punktu tranzycyjnego czyli przejścia z etapu gojenia bezbliznowego (płodowego) do bliznowego (dorosłego) w trakcie rozwoju płodowego.

Impairment of the Hif-1α regulatory pathway in Foxn1-deficient (Foxn1-/- ) mice affects the skin wound healing process

Hypoxia and hypoxia-regulated factors (eg, hypoxia-inducible factor-1α [Hif-1α], factor inhibiting Hif-1α [Fih-1], thioredoxin-1 [Trx-1], aryl hydrocarbon receptor nuclear translocator 2 [Arnt-2]) have essential roles in skin wound healing. Using Foxn1^-/- mice that can heal skin injuries in a unique scarless manner, we investigated the interaction between Foxn1 and hypoxia-regulated factors. The Foxn1^-/- mice displayed impairments in the regulation of Hif-1α, Trx-1, and Fih-1 but not Arnt-2 during the healing process. An analysis of wounded skin showed that the skin of the Foxn1^-/- mice healed in a scarless manner, displaying rapid re-epithelialization and an increase in transforming growth factor β (Tgfβ-3) and collagen III expression. An in vitro analysis revealed that Foxn1 overexpression in keratinocytes isolated from the skin of the Foxn1^-/- mice led to reduced Hif-1α expression in normoxic but not hypoxic cultures and inhibited Fih-1 expression exclusively under hypoxic conditions. These data indicate that in the skin, Foxn1 affects hypoxia-regulated factors that control the wound healing process and suggest that under normoxic conditions, Foxn1 is a limiting factor for Hif-1α.

Modeling by disruption and a selected-for partner for the nude locus

A long‐standing problem in biology is how to dissect traits for which no tractable model exists. Here, we screen for genes like the nude locus (Foxn1)—genes central to mammalian hair and thymus development—using animals that never evolved hair, thymi, or Foxn1. Fruit flies are morphologically disrupted by the FOXN1 transcription factor and rescued by weak reductions in fly gene function, revealing molecules that potently synergize with FOXN1 to effect dramatic, chaotic change. Strong synergy/effectivity in flies is expected to reflect strong selection/functionality (purpose) in mammals; the more disruptive a molecular interaction is in alien contexts (flies), the more beneficial it will be in its natural, formative contexts (mammals). The approach identifies Aff4 as the first nude‐like locus, as murine AFF4 and FOXN1 cooperatively induce similar cutaneous/thymic phenotypes, similar gene expression programs, and the same step of transcription, pre‐initiation complex formation. These AFF4 functions are unexpected, as AFF4 also serves as a scaffold in common transcriptional‐elongation complexes. Most likely, the approach works because an interaction's power to disrupt is the inevitable consequence of its selected‐for power to benefit.

T-Cell Immunodeficiencies With Congenital Alterations of Thymic Development: Genes Implicated and Differential Immunological and Clinical Features

Combined Immunodeficiencies (CID) are rare congenital disorders characterized by defective T-cell development that may be associated with B- and NK-cell deficiency. They are usually due to alterations in genes expressed in hematopoietic precursors but in few cases, they are caused by impaired thymic development. Athymia was classically associated with DiGeorge Syndrome due to TBX1 gene haploinsufficiency. Other genes, implicated in thymic organogenesis include FOXN1, associated with Nude SCID syndrome, PAX1, associated with Otofaciocervical Syndrome type 2, and CHD7, one of the genes implicated in CHARGE syndrome. More recently, chromosome 2p11.2 microdeletion, causing FOXI3 haploinsufficiency, has been identified in 5 families with impaired thymus development. In this review, we will summarize the main genetic, clinical, and immunological features related to the abovementioned gene mutations. We will also focus on different therapeutic approaches to treat SCID in these patients.

β-catenin-mediated hair growth induction effect of 3,4,5-tri-O-caffeoylquinic acid

The hair follicle is a complex structure that goes through a cyclic period of growth (anagen), regression (catagen), and rest (telogen) under the regulation of several signaling pathways, including Wnt/ β-catenin, FGF, Shh, and Notch. The Wnt/β-catenin signaling is specifically involved in hair follicle morphogenesis, regeneration, and growth. β-catenin is expressed in the dermal papilla and promotes anagen induction and duration, as well as keratinocyte regulation and differentiation. In this study, we demonstrated the activation of β-catenin by a polyphenolic compound 3,4,5-tri-O-caffeoylquinic acid (TCQA) in mice model and in human dermal papilla cells to promote hair growth cycle. A complete regrowth of the shaved area of C3H mice was observed upon treatment with TCQA. Global gene expression analysis using microarray showed an upregulation in hair growth-associated genes. Moreover, the expression of β-catenin was remarkably upregulated in vivo and in vitro. These findings suggest that β-catenin activation by TCQA promoted the initiation of the anagen phase of the hair cycle.

Foxn1 in Skin Development, Homeostasis and Wound Healing

Intensive research effort has focused on cellular and molecular mechanisms that regulate skin biology, including the phenomenon of scar-free skin healing during foetal life. Transcription factors are the key molecules that tune gene expression and either promote or suppress gene transcription. The epidermis is the source of transcription factors that regulate many functions of epidermal cells such as proliferation, differentiation, apoptosis, and migration. Furthermore, the activation of epidermal transcription factors also causes changes in the dermal compartment of the skin. This review focuses on the transcription factor Foxn1 and its role in skin biology. The regulatory function of Foxn1 in the skin relates to physiological (development and homeostasis) and pathological (skin wound healing) conditions. In particular, the pivotal role of Foxn1 in skin development and the acquisition of the adult skin phenotype, which coincides with losing the ability of scar-free healing, is discussed. Thus, genetic manipulations with Foxn1 expression, specifically those introducing conditional Foxn1 silencing in a Foxn1+/+ organism or its knock-in in a Foxn1−/− model, may provide future perspectives for regenerative medicine.

Foxn1 expression in keratinocytes is stimulated by hypoxia: further evidence of its role in skin wound healing

Recent studies have shown that the transcription factor Foxn1, which is expressed in keratinocytes, is involved in the skin wound healing process, yet how Foxn1 functions remains largely unknown. Our latest data indicate that Foxn1 drives skin healing via engagement in re-epithelization and the epithelial-mesenchymal transition (EMT) process. In the present study, 2D-DIGE proteomic profiling analysis of in vitro cultured keratinocytes transfected with adenoviral vector carrying Foxn1-GFP or GFP alone (control) revealed forty proteins with differential abundance between the compared groups. Among the proteins with Foxn1-dependent expression, several enable adaptation to hypoxia. Subsequent experiments revealed that hypoxic conditions (1% O₂) stimulate endogenous and exogenous (transfected Ad-Foxn1) Foxn1 expression in cultured keratinocytes. A proteomics analysis also identified proteins that can act as a factors controlling the balance between cell proliferation, differentiation and apoptosis in response to Foxn1. We also showed that in C57BL/6 keratinocytes, the stimulation of Foxn1 by hypoxia is accompanied by increases in Mmp-9 expression. These data corroborate the detected co-localization of Foxn1 and Mmp-9 expression in vivo in post-wounding skin samples of Foxn1::Egfp transgenic mice. Together, our data indicate that Foxn1 orchestrates cellular changes in keratinocytes in both physiological (self-renewal) and pathological (skin wound healing) contexts.

Wnt Signaling in Adult Epithelial Stem Cells and Cancer

Wnt/β-catenin signaling is integral to the homeostasis and regeneration of many epithelial tissues due to its critical role in adult stem cell regulation. It is also implicated in many epithelial cancers, with mutations in core pathway components frequently present in patient tumors. In this chapter, we discuss the roles of Wnt/β-catenin signaling and Wnt-regulated stem cells in homeostatic, regenerative and cancer contexts of the intestines, stomach, skin, and liver. We also examine the sources of Wnt ligands that form part of the stem cell niche. Despite the diversity in characteristics of various tissue stem cells, the role(s) of Wnt/β-catenin signaling is generally coherent in maintaining stem cell fate and/or promoting proliferation. It is also likely to play similar roles in cancer stem cells, making the pathway a salient therapeutic target for cancer. While promising progress is being made in the field, deeper understanding of the functions and signaling mechanisms of the pathway in individual epithelial tissues will expedite efforts to modulate Wnt/β-catenin signaling in cancer treatment and tissue regeneration.

FOXN1 deficient nude severe combined immunodeficiency

Nude severe combined immunodeficiency is a rare inherited disease caused by autosomal recessive loss-of-function mutations in FOXN1. This gene encodes a transcription factor essential for the development of the thymus, the primary lymphoid organ that supports T-cell development and selection. To date nine cases have been reported presenting with the clinical triad of absent thymus resulting in severe T-cell immunodeficiency, congenital alopecia universalis and nail dystrophy. Diagnosis relies on testing for FOXN1 mutations, which allows genetic counselling and guides therapeutic management. Options for treating the underlying immune deficiency include HLA-matched genoidentical haematopoietic cell transplantation containing mature donor T-cells or thymus tissue transplantation. Experience from other severe combined immune deficiency syndromes suggests that early diagnosis, supportive care and definitive management result in better patient outcomes. Without these the prognosis is poor due to early-onset life threatening infections.

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

Background

Mouse fetuses up to 16 day of embryonic development and nude (Foxn1- deficient) mice are examples of animals that undergo regenerative (scar-free) skin healing. The expression of transcription factor Foxn1 in the epidermis of mouse fetuses begins at embryonic day 16.5 which coincides with the transition point from scar-free to scar-forming skin wound healing. In the present study, we tested the hypothesis that Foxn1 expression in the skin is an essential condition to establish the adult skin phenotype and that Foxn1 inactivity in nude mice keeps skin in the immature stage resembling the phenomena of neoteny.

Results

Uninjured skin of adult C57BL/6J (B6) mice, mouse fetuses at days 14 (E14) and 18 (E18) of embryonic development and B6.Cg-Foxn1 nu (nude) mice were characterized for their gene expression profiles by RNA sequencing that was validated through qRT-PCR, Western Blot and immunohistochemistry. Differentially regulated genes indicated that nude mice were more similar to E14 (model of regenerative healing) and B6 were more similar to E18 (model of reparative healing). The up-regulated genes in nude and E14 mice were associated with tissue remodeling, cytoskeletal rearrangement, wound healing and immune response, whereas the down-regulated genes were associated with differentiation. E14 and nude mice exhibit prominent up-regulation of keratin (Krt23, -73, -82, -16, -17), involucrin (Ivl) and filaggrin (Flg2) genes. The transcription factors associated with the Hox genes known to specify cell fate during embryonic development and promote embryonic stem cells differentiation were down-regulated in both nude and E14. Among the genes enriched in the nude skin but not shared with E14 fetuses were members of the Wnt and matrix metalloproteinases (Mmps) families whereas Bmp and Notch related genes were down-regulated.

Conclusions

In summary, Foxn1 appears to be a pivotal control element of the developmental program and skin maturation. Nude mice may be considered as a model of neoteny among mammals. The resemblance of gene expression profiles in the skin of both nude and E14 mice are direct or indirect consequences of the Foxn1 deficiency. Foxn1 appears to regulate the balance between cell proliferation and differentiation and its inactivity creates a pro-regenerative environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3401-z) contains supplementary material, which is available to authorized users.

Comparative Transcriptome Analysis of Fetal Skin Reveals Key Genes Related to Hair Follicle Morphogenesis in Cashmere Goats

Cashmere goat skin contains two types of hair follicles (HF): primary hair follicles (PHF) and secondary hair follicles (SHF). Although multiple genetic determinants associated with HF formation have been identified, the molecules that determine the independent morphogenesis of HF in cashmere goats remain elusive. The growth and development of SHF directly influence the quantity and quality of cashmere production. Here, we report the transcriptome profiling analysis of nine skin samples from cashmere goats using 60- and 120-day-old embryos (E60 and E120, respectively), as well as newborns (NB), through RNA-sequencing (RNA-seq). HF morphological changes indicated that PHF were initiated at E60, with maturation from E120, while differentiation of SHF was identified at E120 until formation of cashmere occurred after birth (NB). The RNA-sequencing analysis generated over 20.6 million clean reads from each mRNA library. The number of differentially expressed genes (DEGs) in E60 vs. E120, E120 vs. NB, and E60 vs. NB were 1,024, 0 and 1,801, respectively, indicating that no significant differences were found at transcriptomic levels between E120 and NB. Key genes including B4GALT4, TNC, a-integrin, and FGFR1, were up-regulated and expressed in HF initiation from E60 to E120, while regulatory genes such as GPRC5D, PAD3, HOXC13, PRR9, VSIG8, LRRC15, LHX2, MSX-2, and FOXN1 were up-regulated and expressed in HF keratinisation and hair shaft differentiation from E120 and NB to E60. Several genes belonging to the KRT and KRTAP gene families were detected throughout the three HF developmental stages. The transcriptional trajectory analyses of all DEGs indicated that immune privilege, glycosaminoglycan biosynthesis, extracellular matrix receptor interaction, and growth factor receptors all played dominant roles in the epithelial-mesenchymal interface and HF formation. We found that the Wnt, transforming growth factor-beta/bone morphogenetic protein, and Notch family members played vital roles in HF differentiation and maturation. The DEGs we found could be attributed to the generation and development of HF, and thus will be critically important for improving the quantity and quality of fleece production in animals for fibres.

A composite enhancer regulates p63 gene expression in epidermal morphogenesis and in keratinocyte differentiation by multiple mechanisms

p63 is a crucial regulator of epidermal development, but its transcriptional control has remained elusive. Here, we report the identification of a long-range enhancer (p63LRE) that is composed of two evolutionary conserved modules (C38 and C40), acting in concert to control tissue- and layer-specific expression of the p63 gene. Both modules are in an open and active chromatin state in human and mouse keratinocytes and in embryonic epidermis, and are strongly bound by p63. p63LRE activity is dependent on p63 expression in embryonic skin, and also in the commitment of human induced pluripotent stem cells toward an epithelial cell fate. A search for other transcription factors involved in p63LRE regulation revealed that the CAAT enhancer binding proteins Cebpa and Cebpb and the POU domain-containing protein Pou3f1 repress p63 expression during keratinocyte differentiation by binding the p63LRE enhancer. Collectively, our data indicate that p63LRE is composed of additive and partly redundant enhancer modules that act to direct robust p63 expression selectively in the basal layer of the epidermis.

miR-18b and miR-518b Target FOXN1 during epithelial lineage differentiation in pluripotent cells

MicroRNAs (miRNAs) regulate myriad biological processes; however, their role in cell fate choice is relatively unexplored. Pluripotent NT2/D1 embryonal carcinoma cells differentiate into an epithelial/smooth muscle phenotype when treated with bone morphogenetic protein-2 (BMP-2). To identify miRNAs involved in epithelial cell development, we performed miRNA profiling of NT2/D1 cells treated with BMP-2 at 6, 12, and 24 h, and on days 6 and 10. Integration of the miRNA profiling data with previously obtained gene expression profiling (GEP) data of NT2/D1 cells treated with BMP-2 at the same time points identified miR-18b and miR-518b as the top two miRNAs with the highest number of up-regulated predicted targets with known functions in epithelial lineage development. Silencing of miR-18b and miR-518b in NT2/D1 cells revealed several up-regulated TFs with functions in epithelial lineage development; among these, target prediction programs identified FOXN1 as the only direct target of both miRNAs. FOXN1 has previously been shown to play an important role in keratinocyte differentiation and epithelial cell proliferation. NT2/D1 and H9 human embryonic stem cells with silenced miR-18b and miR-518b showed up-regulation of FOXN1 and the epithelial markers CDH1, EPCAM, KRT19, and KRT7. A 3'UTR luciferase assay confirmed FOXN1 to be a target of the two miRNAs, and up-regulation of FOXN1 in NT2/D1 cells led to the expression of epithelial markers. Overexpression of the two miRNAs in BMP-2-treated NT2/D1 cells led to down-regulation of FOXN1 and epithelial lineage markers. These results show that miR-18b and miR-518b are upstream controllers of FOXN1-directed epithelial lineage development.

A positive FGFR3/FOXN1 feedback loop underlies benign skin keratosis versus squamous cell carcinoma formation in humans

Seborrheic keratoses (SKs) are common, benign epithelial tumors of the skin that do not, or very rarely, progress into malignancy, for reasons that are not understood. We investigated this by gene expression profiling of human SKs and cutaneous squamous cell carcinomas (SCCs) and found that several genes previously connected with keratinocyte tumor development were similarly modulated in SKs and SCCs, whereas the expression of others differed by only a few fold. In contrast, the tyrosine kinase receptor FGF receptor-3 (FGFR3) and the transcription factor forkhead box N1 (FOXN1) were highly expressed in SKs, and close to undetectable in SCCs. We also showed that increased FGFR3 activity was sufficient to induce FOXN1 expression, counteract the inhibitory effect of EGFR signaling on FOXN1 expression and differentiation, and induce differentiation in a FOXN1-dependent manner. Knockdown of FOXN1 expression in primary human keratinocytes cooperated with oncogenic RAS in the induction of SCC-like tumors, whereas increased FOXN1 expression triggered the SCC cells to shift to a benign SK-like tumor phenotype, which included increased FGFR3 expression. Thus,we have uncovered a positive regulatory loop between FGFR3 and FOXN1 that underlies a benign versus malignant skin tumor phenotype.