Spatiotemporal expression pattern of keratins in skin of AP-2alpha-deficient mice

Evolution of an Epidermal Differentiation Complex (EDC) Gene Family in Birds

The transition of amniotes to a fully terrestrial lifestyle involved the adaptation of major molecular innovations to the epidermis, often in the form of epidermal appendages such as hair, scales and feathers. Feathers are diverse epidermal structures of birds, and their evolution has played a key role in the expansion of avian species to a wide range of lifestyles and habitats. As with other epidermal appendages, feather development is a complex process which involves many different genetic and protein elements. In mammals, many of the genetic elements involved in epidermal development are located at a specific genetic locus known as the epidermal differentiation complex (EDC). Studies have identified a homologous EDC locus in birds, which contains several genes expressed throughout epidermal and feather development. A family of avian EDC genes rich in aromatic amino acids that also contain MTF amino acid motifs (EDAAs/EDMTFs), that includes the previously reported histidine-rich or fast-protein (HRP/fp), an important marker in feather development, has expanded significantly in birds. Here, we characterize the EDAA gene family in birds and investigate the evolutionary history and possible functions of EDAA genes using phylogenetic and sequence analyses. We provide evidence that the EDAA gene family originated in an early archosaur ancestor, and has since expanded in birds, crocodiles and turtles, respectively. Furthermore, this study shows that the respective amino acid compositions of avian EDAAs are characteristic of structural functions associated with EDC genes and feather development. Finally, these results support the hypothesis that the genes of the EDC have evolved through tandem duplication and diversification, which has contributed to the evolution of the intricate avian epidermis and epidermal appendages.

Cystathionine β-synthase deficiency causes infertility by impairing decidualization and gene expression networks in uterus implantation sites

Hyperhomocysteinemia has been reported in human reproduction as a risk factor for early pregnancy loss, preeclampsia, and congenital birth defects like spina bifida. Female infertility was also observed in cystathionine beta synthase-deficient mice ( Cbs-KO) as an animal model for severe hyperhomocysteinemia. The aim for the present research was to elucidate the time-point of pregnancy loss and to pinpoint gene and cellular changes involved in the underlying pathological mechanism. By mating 90-day-old wild-type and Cbs-KO female mice with their homologous male partners, we found that pregnancy loss in Cbs-KO occurred between the 8th and 12th gestation day during placenta formation. DNA microarrays were carried out on uterus from implantation and interimplantation samples obtained on day 8. The results allowed us to select genes potentially involved in embryo death; these were individually confirmed by RT-qPCR, and their expressions were also followed throughout pregnancy. We found that changes in expression of Calb1, Ttr, Expi, Inmt, Spink3, Rpgrip1, Krt15, Mt-4, Gzmc, Gzmb, Tdo2, and Afp were important for pregnancy success, since a different regulation in Cbs-KO mice was found. Also, differences in relationships among selected genes were observed, indicating a dysregulation of these genes in Cbs-KO females. In conclusion, our data provide more information on the gene expression cascade and its timely regulated process required for a successful pregnancy. In addition, we unveil new potential avenues to explore further investigations in pregnancy loss.

Mechanistic effects of long-term ultraviolet B irradiation induce epidermal and dermal changes in human skin xenografts

UVB irradiation has been reported to induce photoaging and suppress systemic immune function that could lead to photocarcinogenesis. However, because of the paucity of an UVB-induced photodamaged skin model, precise and temporal mechanism(s) underlying the deleterious effects of long-term UVB exposure on human skin have yet to be delineated. In this study, we established a model using human skin xenografted onto severe combined immunodeficient mice, which were subsequently challenged by repeated UVB irradiation for 6 weeks. Three-dimensional optical image analysis of skin replicas and noninvasive biophysical measurements illustrated a significant increase in skin surface roughness, similar to premature photoaging, and a significant loss of skin elasticity after long-term UVB exposure. Resembling authentically aged skin, UVB-exposed samples exhibited significant increases in epithelial keratins (K6, K16, K17), elastins, and matrix metalloproteinases (MMP-1, MMP-9, MMP-12) as well as degradation of collagens (I, IV, VII). The UVB-induced deterioration of fibrous keratin intermediate filaments was also observed in the stratum corneum. Additionally, similarities in gene expression patterns between our model and chronologically aged skin substantiated the plausible relationship between photodamage and chronological age. Furthermore, severe skin photodamage was observed when neutralizing antibodies against TIMP-1, an endogenous inhibitor of MMPs, were administered during the UVB exposure regimen. Taken together, these findings suggest that our skin xenograft model recapitulates premature photoaged skin and provides a comprehensive tool with which to assess the deleterious effects of UVB irradiation.

Tfap2 transcription factors in zebrafish neural crest development and ectodermal evolution

Transcription factor AP2 (Tfap2) genes play essential roles in development of the epidermis and migratory cells of the neural crest (NC) in vertebrate embryos. These transcriptional activators are among the earliest genes expressed in the ectoderm and specify fates within the epidermis/crest through both direct and indirect mechanisms. The Tfap2 family arose from a single ancestral gene in a chordate ancestor that underwent gene duplication to give up to five family members in living vertebrates. This coincided with the acquisition of important roles in NC development by Tfap2 genes suggesting that this gene family was important in ectodermal evolution and possibly in the origin of NC. Here, we show that a zebrafish tfap2c is expressed in the nonneural ectoderm during early development and functions redundantly with tfap2a in NC specification. In zebrafish embryos depleted of both tfap2a and tfap2c, NC cells are virtually eliminated. Cell transplantation experiments indicate that tfap2c functions cell-autonomously in NC specification. Cells of the enveloping layer, which forms a temporary skin layer surrounding the ectoderm, also fail to differentiate or to express appropriate keratins in tfap2c deficient embryos. The role of Tfap2 genes in epidermal and NC development is considered here in the broader context of ectodermal evolution. Distinct, tissue-specific functions for Tfap2 genes in different vertebrates may reflect subfunctionalisation of an ancestral gene that consequently led to the gain of novel roles for different subfamily members in patterning the epidermis and NC.

Keratin 5 knockout mice reveal plasticity of keratin expression in the corneal epithelium

We have recently demonstrated that the keratin K3 gene, which is active in the suprabasal human corneal epithelium, is missing in the genome of the mouse. We show that a normal K3 gene exists in a wide variety of mammals while in rodents the gene is converted to a pseudogene with a very strong sequence drift. The availability of K5-/- mice provides a unique opportunity to investigate type-specific keratin function during corneal differentiation in the absence of both K5 and K3. Here, we report that the deletion of K5, which in wild-type mice forms a cytoskeleton with K12, does neither cause keratin aggregation nor cytolysis in the cornea. This is due to the induction of K4 in corneal epithelial cells, normally restricted to corneal stem stem cells residing in the limbus. Using a combination of antibodies and RT-PCR, we identified additional keratins expressed in the mouse cornea including K23 which was previously thought to be specific for pancreatic carcinomas. This reflects an unexpected complexity of keratin expression in the cornea. Our data suggest that in the absence of mechanical stress, corneal differentiation does not depend on distinct keratin pairs, supporting a concept of functional redundancy, at least for certain keratins.

AP-2alpha: a regulator of EGF receptor signaling and proliferation in skin epidermis

AP-2 transcription factors have been implicated in epidermal biology, but their functional significance has remained elusive. Using conditional knockout technology, we show that AP-2α is essential for governing the balance between growth and differentiation in epidermis. In vivo, epidermis lacking AP-2α exhibits elevated expression of the epidermal growth factor receptor (EGFR) in the differentiating layers, resulting in hyperproliferation when the receptors are activated. Chromatin immunoprecipitation and promoter activity assays identify EGFR as a direct target gene for AP-2α repression, and, in the absence of AP-2α, this is manifested primarily in excessive EGF-dependent phosphoinositol-3 kinase/Akt activity. Together, our findings unveil a hitherto unrecognized repressive role for AP-2α in governing EGFR gene transcription as cells exit the basal layer and withdraw from the cell cycle. These results provide insights into why elevated AP-2α levels are often associated with terminal differentiation and why tumor cells often display reduced AP-2α and elevated EGFR proteins.

Proliferation and cornification during development of the mammalian epidermis

The skin is the largest organ of the body and consists of the underlying dermis and outer epidermis. Proper embryonic development and continual renewal of the adult epidermis are essential to provide an impenetrable barrier against fluid loss and serve as our most important defense against insults from the external environment. During mammalian embryogenesis the epidermis develops from the surface ectoderm, which initially consists of a multipotent single-layer epithelium. Once these epithelial cells receive the appropriate developmental cues, they become committed to stratify, initiate a massive expansion program, and finally embark on a journey of terminal differentiation to produce the morphologically distinct layers of the epidermis. The culmination of this journey is the formation of an impervious cornified envelope via a highly specialized form of programmed cell death, termed "cornification" (reviewed in Candi et al.), which is distinct in many ways from the classic apoptotic pathways. The epidermal developmental program that is first seen in the fetus is recapitulated for the entire life of the organism. The basal layer of adult skin harbors stem cells, which can divide to produce daughter stem cells and transit amplifying (TA) cells that go on to differentiate and cornify (reviewed in Fuchs and Raghavan). In this review we summarize current knowledge about the molecular regulation of proliferation and cornification in the developing mammalian epidermis. We focus on events in the interfollicular epidermis, with special emphasis on transcriptional regulation by p63, Notch, NF-kappaB/IKK, Hox, AP-1, AP-2, and C/EBP factors. We end with a discussion about perturbations in epidermal proliferation and cornification as they pertain to human skin pathologies.

Type II keratins precede type I keratins during early embryonic development

We and others have recently demonstrated that the keratin (K) gene family in mammals is even more complex than previously thought [Eur. J. Cell Biol. 83, 19-26]. To address the function of keratins during early development, precise information on their spatio-temporal expression is required. Here, we examined the expression of selected mouse keratins from pre-implantation to mid-gestational embryonic stages using RT-PCR and immunofluorescence. At E0.5, transcripts encoding K5, K6, K7, K8, K14, K15, K18, and K19 are apparently absent. We report on a post-transcriptional regulation of type I keratins, preventing filament formation in 8- to 16-cell stage embryos. In these embryos, mRNAs coding for K7, K8, K18, and K19 are present, but only K7 and K8 are translated into protein which is deposited in aggregates. Following the accumulation of K18 protein at E3.5, keratin filaments are formed. Delayed onset of type I keratin protein expression was additionally observed in later embryonic stages for K5 and K14. K5 protein expression starts in the forelimb surface ectoderm as early as E9.25, while the expression of its partner, K14, begins at E9.75. From E9.25 to E9.75, K5 forms atypical filaments with K18. Remarkably, in embryonic K5-/- mice, K14 formed normal filaments until E12.5 despite the absence of its partner K5, due to the presence of K8. Following periderm formation, K14-containing filaments disappeared and K14 became localized in aggregates in basal keratinocytes. Despite the absence of a keratin cytoskeleton, there was no cytolysis. We suggest that the formation of the first embryonic cytoskeleton from soluble keratins is regulated by unknown mechanisms. Whether the premature expression of type II keratins relates to their proposed role in TNF- and Fas-mediated signalling is presently unknown.

AP-2alpha selectively regulates fragile X mental retardation-1 gene transcription during embryonic development

Fragile X syndrome (FXS) is almost always caused by silencing of the FMR1 gene. The defects observed in FXS indicate that the normal FMR1 gene has a range of functions and plays a particularly prominent role during development. However, the mechanisms regulating FMR1 expression in vivo are not known. Here, we have tested the role of the transcription factor AP-2alpha in regulating Fmr1 expression. Chromatin immunoprecipitation showed that AP-2alpha associates with the Fmr1 promoter in vivo. Furthermore, Fmr1 transcript levels are reduced >4-fold in homozygous null AP-2alpha mutant mice at embryonic day 18.5 when compared with normal littermates. Notably, AP-2alpha exhibits a strong gene dosage effect, with heterozygous mice showing approximately 2-fold reduction in Fmr1 levels. Examination of conditional AP-2alpha mutant mice indicates that this transcription factor plays a major role in regulating Fmr1 expression in embryos, but not in adults. We further investigated the role of AP-2alpha in the developmental regulation of Fmr1 expression using the Xenopus animal cap assay. Over-expression of a dominant-negative AP-2alpha in Xenopus embryos led to reduced Fmr1 levels. Moreover, exogenous wild-type AP-2alpha rescued Fmr1 expression in embryos where endogenous AP-2alpha had been suppressed. We conclude that AP-2alpha associates with the Fmr1 promoter in vivo and selectively regulates Fmr1 transcription during embryonic development.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.

Involvement of changes in stratum corneum keratin in wrinkle formation by chronic ultraviolet irradiation in hairless mice

Chronic exposure to ultraviolet (UV) radiation alters components of the skin. We previously reported that chronic low-dose UV irradiation induced wrinkle formation but did not significantly alter dermal components of hairless mice. In the present study, we examined whether the epidermal changes by UV irradiation could contribute to wrinkle formation. Hairless mice were irradiated with one-third the minimum erythema dose (MED) of UVB (20 mJ/cm2) and UVA (14 J/cm2) for 10 weeks, and followed up for another 24weeks without irradiation. Fine wrinkles were detected following irradiation, which existed even 24 weeks after cessation of irradiation. An increase in the viscoelastic proportion of the total distension (Uv/Ue) was recognized in irradiated mice, which may be related to wrinkle formation. In the epidermis, an increase in the amount of transepidermal water loss and a decrease in the water content of the stratum corneum were seen after 10 weeks of UV irradiation, neither of which recovered during the 24-week observation period. In addition, there was a significant increase in the amount of stratum corneum keratin after 10 weeks of irradiation, which persisted during the 24-week follow-up. The results of our study suggest that chronic low-dose UV irradiation primarily alters the epidermal rather than the dermal components of the skin. In addition, our results indicate that the increased keratin content of the stratum corneum may be involved in the alteration of the physical properties of the skin. This process could be one of the early events of wrinkle formation.

Transcriptional regulation of Na+/H+ exchanger expression in the intact mouse

We examined regulation of the Na+/H+ exchanger (NHE1 isoform) in the developing mouse. We generated transgenic mice with the Na+/H+ exchanger promoter directing expression of the beta-Galactosidase reporter. We found that expression of the Na+/H+ exchanger was maximum in the heart and liver of 12-day-old embryonic mice. Similar results were found in mice using the green fluorescent protein reporter driven by the Na+/H+ exchanger promoter. Detailed examination of the myocardium revealed that the GFP reporter protein was expressed in the cytoplasm of cardiomyocyte cells. We examined NHE1 protein expression in transgenic mice lacking the transcription factors AP-2alpha or the transcription factor COUP-TF1. Eighteen-day-old AP-2alpha heterozygote mice show no large changes in NHE1 expression in heart, lung, liver, kidney and brain. In contrast, 18-day-old embryos from AP-2alpha null mice showed a large increase in Na+/H+ exchanger protein expression in the brain. NHE1 protein levels in COUP-TF1 knockout embryos did not differ from wild type embryos. The results suggest that AP-2alpha and COUP-TF1 are not critical to NHE1 expression in the late stage embryo and that other related transcription factors may function in regulation of the Na+/H+ exchanger.

Transcription factor AP-2 is an essential and direct regulator of epidermal development in Xenopus

Expression of the Xenopus homolog of the mammalian transcription factor AP-2alpha (XAP-2) is activated throughout the animal hemisphere shortly after the midblastula transition, and becomes restricted to prospective epidermis by the end of gastrulation, under the control of BMP signal modulation. Elevated expression in the future neural crest region begins at this time. Ectopic expression of XAP-2 can restore transcription of epidermal genes in neuralized ectoderm, both in ectodermal explants and in the intact embryo. Likewise, loss of XAP-2 function, accomplished by injection of antisense oligonucleotides or by overexpression of antimorphic XAP-2 derivatives, leads to loss of epidermal and gain of neural gene expression. These treatments also result in gastrulation failure. Thus, AP-2 is a critical regulator of ectodermal determination that is required for normal epidermal development and morphogenesis in the frog embryo.

A subtractive gene expression screen suggests a role of transcription factor AP-2 alpha in control of proliferation and differentiation

The transcription factor AP-2 alpha has been implicated as a cell type-specific regulator of gene expression during vertebrate embryogenesis based on its expression pattern in neural crest cells, ectoderm, and the nervous system in mouse and frog embryos. AP-2 alpha is prominently expressed in cranial neural crest cells, a population of cells that migrate from the lateral margins of the brain plate during closure of the neural tube at day 8-9 of embryonic development. Homozygous AP-2 alpha mutant mice die perinatally with cranio-abdominoschisis, full facial clefting, and defects in cranial ganglia and sensory organs, indicating the importance of this gene for proper development. By using a subtractive cloning approach, we identified a set of genes repressed by AP-2 alpha that are described to retard cellular proliferation and induce differentiation and apoptosis. We show that these target genes are prematurely expressed in AP-2 alpha mutant mice. One of the genes isolated, the Krüppel-box transcription factor KLF-4 implicated in induction of terminal differentiation and growth regulation, is found expressed in mutant embryonic fibroblasts. We show that fibroblasts lacking AP-2 alpha display retarded growth but no enhanced apoptosis. Based on these data we suggest that AP-2 alpha might be required for cell proliferation by suppression of genes inducing terminal differentiation, apoptosis, and growth retardation.

Redistribution of transcription factor AP-2alpha in differentiating cultured human epidermal cells

Expression of the transcription factor AP-2alpha was examined in cultured human epidermal cells. Levels of AP-2alpha mRNA increased substantially after the cultures reached confluence, similar to the expression pattern of the differentiation markers involucrin and keratinocyte transglutaminase. The level of AP-2alpha protein in nuclear extracts declined markedly after confluence, however, along with its ability to form complexes with oligonucleotides containing the AP-2 response element. In contrast, the levels of AP-2alpha protein in cytoplasmic extracts increased dramatically after confluence, but these extracts had low DNA binding activity. Supershift experiments with specific antisera detected only AP-2alpha and not the beta or gamma isoforms. Examination of its localization by confocal microscopy revealed that AP-2alpha was primarily in the nucleus of basal cells and largely cytoplasmic in the most superficial cells. Localization was a dynamic phenomenon in that changing the medium resulted in accumulation of this transcription factor in the nucleus after several hours. Overall, the data indicate that AP-2alpha transcriptional activity is regulated in a differentiation-dependent manner in cultured keratinocytes and that this occurs by relocalization of the protein. Nuclear localization of the AP-2alpha protein in basal cells permits its accessibility to response elements in gene promoters, whereas sequestration in the cytoplasm as the differentiation program progresses curtails its transcriptional activity. This regulatory scheme may provide keratinocytes with the ability to restore AP-2 transcriptional activity rapidly by redistribution to the nucleus after receiving an appropriate growth signal, such as a medium change.

Regulatory roles of AP-2 transcription factors in vertebrate development, apoptosis and cell-cycle control

AP-2 transcription factors represent a family of three closely related and evolutionarily conserved sequence-specific DNA-binding proteins, AP-2alpha, -beta and -gamma. Subsequent studies have identified spatially and temporally regulated embryonic expression patterns in a number of different tissues including neural crest derivatives, neural, epidermal and urogenital tissues. Here, we review the current understanding of developmental defects in AP-2-deficient mice and consider regulatory functions of AP-2 in control of apoptosis, cell cycle, and gene expression. Recently, the first inherited human disorder, Char syndrome, was identified to be caused by AP-2beta missense mutations. In light of the manifold and essential functions of AP-2 proteins in cell growth, differentiation and programmed death, mutations or changes in precisely programmed expression patterns are likely to contribute to other congenital malformations or neoplastic diseases.