HOXB4 homeodomain protein is expressed in developing epidermis and skin disorders and modulates keratinocyte proliferation

HOX Protein Activity Regulation by Cellular Localization

While the functions of HOX genes have been and remain extensively studied in distinct model organisms from flies to mice, the molecular biology of HOX proteins remains poorly documented. In particular, the mechanisms involved in regulating the activity of HOX proteins have been poorly investigated. Nonetheless, based on data available from other well-characterized transcription factors, it can be assumed that HOX protein activity must be finely tuned in a cell-type-specific manner and in response to defined environmental cues. Indeed, records in protein–protein interaction databases or entries in post-translational modification registries clearly support that HOX proteins are the targets of multiple layers of regulation at the protein level. In this context, we review here what has been reported and what can be inferred about how the activities of HOX proteins are regulated by their intracellular distribution.

Dysfunctional Keratinocytes Increase Dermal Inflammation in Systemic Sclerosis: Results From Studies Using Tissue-Engineered Scleroderma Epidermis

OBJECTIVE

Evidence suggests that keratinocyte-fibroblast interactions are abnormal in systemic sclerosis (SSc). The present study was undertaken to investigate potential epidermal dysfunction in SSc and its effects on dermal homeostasis.

METHODS

Epidermal equivalents (EEs) were generated using keratinocytes from 6 healthy donors and 4 individuals with SSc. Skin and EE expression of markers of proliferation, differentiation, and activation was evaluated by immunohistochemistry. The transcriptomic profile of SSc EEs and healthy donor EEs was identified by RNA sequencing. EE conditioned medium (CM) was used to stimulate fibroblasts, and their production of interleukin-6 (IL-6), IL-8, matrix metalloproteinase 1 (MMP-1), type I collagen, and fibronectin was assessed by enzyme-linked immunosorbent assay.

RESULTS

Compared to healthy donor EEs, SSc EEs exhibited aberrant differentiation, enhanced expression of activation markers, and a lower rate of basal keratinocyte mitosis, reproducing most of the abnormalities observed in SSc epidermis. RNA sequencing analysis revealed that, compared to healthy donor EEs, SSc EEs were characterized by lower expression of homeobox gene family members and by enhanced metabolic and oxidative stress molecular pathways. EE CM enhanced fibroblast production of IL-6, IL-8, MMP-1, type I collagen, and fibronectin (P < 0.05). Except for type I collagen and fibronectin, this effect was 2-fold higher in the presence of CM generated form SSc EEs. IL-1 was responsible, at least in part, for keratinocyte-dependent fibroblast activation.

CONCLUSION

SSc EEs recapitulate the in vivo characteristics of SSc epidermis, demonstrating that SSc keratinocytes have an intrinsically altered differentiation program, possibly due to the dysregulation of genes from the homeobox family. The increased metabolic and oxidative stress associated with SSc epidermis may contribute to chronic inflammation and fibrosis of the dermis.

Accelerated human epidermal turnover driven by increased hyaluronan production

BACKGROUND

Hyaluronan (HA) is an essential component of extracellular matrix in the skin, but its functions in the epidermis remain elusive.

OBJECTIVE

We examined the interaction of increased HA production mediated by 1-ethyl-β-N-acetylglucosaminide (β-NAG2), a newly developed highly selective inducer of HA production which is intracellularly converted to UDP-N-acetylglucosamine, a substrate of HA, with epidermal proliferation and differentiation.

METHODS

The amount, molecular size and epidermal tissue distribution of HA and expression of CD44, a cell surface receptor for HA, were analyzed in β-NAG2-treated organ cultured human skin, reconstructed human skin equivalents or cultured human skin keratinocytes. The relationship between HA and epidermal proliferation or differentiation was examined.

RESULTS

β-NAG2 significantly increased HA production in the epidermis of skin explants or skin equivalents without affecting molecular size of HA (>2000 kDa) or CD44 mRNA expression. Histochemical experiments revealed that β-NAG2 enhances HA signals in the basal to granular layers of the epidermis of skin equivalents, accompanying increased epidermal stratification. Immunohistochemical experiments demonstrated that signals of Ki67, transglutaminase 1 and filaggrin are increased in β-NAG2-treated skin equivalents, and these observations were confirmed by the data showing that mRNA expression of PCNA, transglutaminase 1 (TGM1) and filaggrin (FLG) is significantly up-regulated by β-NAG2 in skin equivalents. Importantly, blockade of HA production by inhibiting conversion of β-NAG2 to UDP-NAG abolished β-NAG2-mediated up-regulation of PCNA, TGM1 and FLG mRNA expression in cultured keratinocytes.

CONCLUSION

These results suggest that increased epidermal HA production plays a key role in epidermal morphogenesis and homeostasis by accelerating keratinocyte proliferation and differentiation.

Context-dependent HOX transcription factor function in health and disease

During animal development, HOX transcription factors determine the fate of developing tissues to generate diverse organs and appendages. The power of these proteins is striking: mis-expressing a HOX protein causes homeotic transformation of one body part into another. During development, HOX proteins interpret their cellular context through protein interactions, alternative splicing, and post-translational modifications to regulate cell proliferation, cell death, cell migration, cell differentiation, and angiogenesis. Although mutation and/or mis-expression of HOX proteins during development can be lethal, changes in HOX proteins that do not pattern vital organs can result in survivable malformations. In adults, mutation and/or mis-expression of HOX proteins disrupts their gene regulatory networks, deregulating cell behaviors and leading to arthritis and cancer. On the molecular level, HOX proteins are composed of DNA binding homeodomain, and large regions of unstructured, or intrinsically disordered, protein sequence. The primary roles of HOX proteins in arthritis and cancer suggest that mutations associated with these diseases in both the structured and disordered regions of HOX proteins can have substantial functional effects. These insights lead to new questions critical for understanding and manipulating HOX function in physiological and pathological conditions.

Use of homeobox gene expression patterns to determine anatomical regions of origin for body surface tissues derived from adult mice

Anatomical regions of the skin have distinct functions and anatomical characteristics, including thicker or thinner epidermis, more or fewer hair follicles, and lighter or darker skin. For a better therapeutic outcome of skin transplantation, site-specific characteristics of grafted tissues need to be taken into account in terms of their functionality and beauty. However, there is no method for evaluating positional information of epidermal cells. Homeobox genes are expressed along the anterior-posterior axis and direct the body plan in the animal development process. Although the expression of several HOX genes is known to be retained as the positional information in adult tissue, their expression patterns in the body surface tissues in adult mammals are still incompletely understood. In this study, we investigated the expression patterns of 40 homeobox genes, including 39 Hox genes and the paired box 6 (Pax6) gene, in body surface tissues of adult mice. On the basis of the results obtained, we proposed, for the first time, a method for determining anatomical regions of origin for body surface tissues derived from adult mice using Hox genes and Pax6. Evaluation of expression levels of at least 7 Hox genes and Pax6 should be sufficient to distinguish 11 anatomical body surface tissues derived from the adult mouse body. The proposed method may be useful not only for determining the origin of surface tissues from specific anatomical regions of the mammalian body but also for predicting positional information of epithelial cells generated from pluripotent stem cells.

Cellular and molecular insights into Hox protein action

Hox genes encode homeodomain transcription factors that control morphogenesis and have established functions in development and evolution. Hox proteins have remained enigmatic with regard to the molecular mechanisms that endow them with specific and diverse functions, and to the cellular functions that they control. Here, we review recent examples of Hox-controlled cellular functions that highlight their versatile and highly context-dependent activity. This provides the setting to discuss how Hox proteins control morphogenesis and organogenesis. We then summarise the molecular modalities underlying Hox protein function, in particular in light of current models of transcription factor function. Finally, we discuss how functional divergence between Hox proteins might be achieved to give rise to the many facets of their action.

A review of fetal scarless healing

Wound healing is a complex process involving a number of processes. Fetal regeneration has been shown to have a number of differences compared to scar-forming healing. This review discusses the number of differences identified in fetal regeneration. Understanding these differences may result in new therapeutic targets which may reduce or even prevent scarring in adult healing.

A preliminary study of differentially expressed genes in expanded skin and normal skin: implications for adult skin regeneration

In adults, severely damaged skin heals by scar formation and cannot regenerate to the original skin structure. However, tissue expansion is an exception, as normal skin regenerates under the mechanical stretch resulting from tissue expansion. This technique has been used clinically for defect repair and organ reconstruction for decades. However, the phenomenon of adult skin regeneration during tissue expansion has caused little attention, and the mechanism of skin regeneration during tissue expansion has not been fully understood. In this study, microarray analysis was performed on expanded human skin and normal human skin. Significant difference was observed in 77 genes, which suggest a network of several integrated cascades, including cytokines, extracellular, cytoskeletal, transmembrane molecular systems, ion or ion channels, protein kinases and transcriptional systems, is involved in the skin regeneration during expansion. Among these, the significant expression of some regeneration related genes, such as HOXA5, HOXB2 and AP1, was the first report in tissue expansion. Data in this study suggest a list of candidate genes, which may help to elucidate the fundamental mechanism of skin regeneration during tissue expansion and which may have implications for postnatal skin regeneration and therapeutic interventions in wound healing.

Homeodomain-only protein HOP is a novel modulator of late differentiation in keratinocytes

The homeodomain-only protein (HOP) contains an atypical homeodomain which is unable to bind to DNA due to mutations in residues important for DNA binding. Recently, HOP was reported to regulate proliferation/differentiation homeostasis in different cell types. In the present study, we performed transcriptional profiling of cultured primary human keratinocytes and noted a robust induction of HOP upon calcium-induced cell differentiation. Immunohistochemistry of human skin localized HOP to the granular layer in the epidermis. Overexpression of HOP using a lentiviral vector up-regulated FLG and LOR expression during keratinocyte differentiation. Conversely, decreasing HOP expression using small interfering RNA markedly reduced the calcium-induced expression of late markers of differentiation in vitro, with the most prominent effect on profilaggrin (FLG) mRNA. Moreover, mRNA levels of profilaggrin and loricrin were downregulated in the epidermis of HOP knockout mice. Analysis of skin disorders revealed altered HOP expression in lichen planus, psoriasis and squamous cell carcinoma (SCC). Our data indicate that HOP is a novel modulator of late terminal differentiation in keratinocytes.

C/EBPδ gene targets in human keratinocytes

C/EBPs are a family of B-Zip transcription factors -TFs- involved in the regulation of differentiation in several tissues. The two most studied members -C/EBPα and C/EBPβ- play important roles in skin homeostasis and their ablation reveals cells with stem cells signatures. Much less is known about C/EBPδ which is highly expressed in the granular layer of interfollicular epidermis and is a direct target of p63, the master regular of multilayered epithelia. We identified C/EBPδ target genes in human primary keratinocytes by ChIP on chip and profiling of cells functionally inactivated with siRNA. Categorization suggests a role in differentiation and control of cell-cycle, particularly of G2/M genes. Among positively controlled targets are numerous genes involved in barrier function. Functional inactivation of C/EBPδ as well as overexpressions of two TF targets -MafB and SOX2- affect expression of markers of keratinocyte differentiation. We performed IHC on skin tumor tissue arrays: expression of C/EBPδ is lost in Basal Cell Carcinomas, but a majority of Squamous Cell Carcinomas showed elevated levels of the protein. Our data indicate that C/EBPδ plays a role in late stages of keratinocyte differentiation.

Accelerated closure of skin wounds in mice deficient in the homeobox gene Msx2

Differences in cellular competence offer an explanation for the differences in the healing capacity of tissues of various ages and conditions. The homeobox family of genes plays key roles in governing cellular competence. Of these, we hypothesize that Msx2 is a strong candidate regulator of competence in skin wound healing because it is expressed in the skin during fetal development in the stage of scarless healing, affects postnatal digit regeneration, and is reexpressed transiently during postnatal skin wound repair. To address whether Msx2 affects cellular competence in injury repair, 3 mm full-thickness excisional wounds were created on the back of C.Cg-Msx2(tm1Rilm)/Mmcd (Msx2 null) mice and the healing pattern was compared with that of the wild type mice. The results show that Msx2 null mice exhibited faster wound closure with accelerated reepithelialization plus earlier appearance of keratin markers for differentiation and an increased level of smooth muscle actin and tenascin in the granulation tissue. In vitro, keratinocytes of Msx2 null mice exhibit increased cell migration and the fibroblasts show stronger collagen gel contraction. Thus, our results suggest that Msx2 regulates the cellular competence of keratinocytes and fibroblasts in skin injury repair.

Cell motility and spreading are suppressed by HOXA4 in ovarian cancer cells: possible involvement of beta1 integrin

HOX genes are transcription factors that control morphogenesis, organogenesis and differentiation. Increasing evidence suggests that HOX genes play a role in ovarian cancer progression; however few studies have defined functional roles and mechanisms of action. We showed previously that HOXA4 expression is increased in invasive, compared to noninvasive, epithelial ovarian tumors. However, HOXA4 suppressed cell migration suggesting that elevated HOXA4 expression in invasive tumors constitutes a homeostatic response. In the present study, we used siRNA and forced-expression in multiple cell lines to define the role of HOXA4 in the regulation of transwell migration/invasion and cellular/colony morphology. Knockdown of endogenous HOXA4 increased migration, but not Matrigel invasion, of OVCAR-8 and OVCAR-3 cells. HOXA4 knockdown also increased cell spreading on plastic or fibronectin, reduced cell-cell adhesion, and increased filopodia in two- and three-dimensional cultures. These changes were not associated with significant changes in alphaV or beta3 integrin and E- or N-cadherin. However, down-regulation of HOXA4 significantly reduced beta1 integrin protein levels within cell colonies and cell aggregates, but not of single, nonadherent cells. It had no effect on beta1 integrin, alpha5 integrin, or fibronectin mRNA levels. Conversely, overexpression of HOXA4 in CaOV-3 cells suppressed transwell migration and increased beta1 integrin protein levels. Our results confirm that HOXA4 inhibits cell motility, show that it suppresses cell spreading and filopodia formation while enhancing cell-cell adhesion, and suggest a role for beta1 integrin in mediating these changes. These observations support the hypothesis that overexpression of HOXA4 in invasive ovarian tumors is a homeostatic, invasion-suppressive response.

MicroRNA-126 regulates HOXA9 by binding to the homeobox

The PicTar program predicted that microRNA-126 (miR-126), miR-145, and let-7s target highly conserved sites within the Hoxa9 homeobox. There are increased nucleotide constraints in the three microRNA seed sites among Hoxa9 genes beyond that required to maintain protein identity, suggesting additional functional conservation. In preliminary experiments, forced expression of these microRNAs in Hoxa9-immortalized bone marrow cells downregulated the HOXA9 protein and caused loss of biological activity. The microRNAs were shown to target their predicted sites within the homeobox. miR-126 and Hoxa9 mRNA are coexpressed in hematopoietic stem cells and downregulated in parallel during progenitor cell differentiation; however, miR-145 is barely detectable in hematopoietic cells, and let-7s are highly expressed in bone marrow progenitors, suggesting that miR-126 may function in normal hematopoietic cells to modulate HOXA9 protein. In support of this hypothesis, expression of miR-126 alone in MLL-ENL-immortalized bone marrow cells decreased endogenous HOXA9 protein, while inhibition of endogenous miR-126 increased expression of HOXA9 in F9 cells.

Tissue engineering of replacement skin: the crossroads of biomaterials, wound healing, embryonic development, stem cells and regeneration

Advanced therapies combating acute and chronic skin wounds are likely to be brought about using our knowledge of regenerative medicine coupled with appropriately tissue-engineered skin substitutes. At the present time, there are no models of an artificial skin that completely replicate normal uninjured skin. Natural biopolymers such as collagen and fibronectin have been investigated as potential sources of biomaterial to which cells can attach. The first generation of degradable polymers used in tissue engineering were adapted from other surgical uses and have drawbacks in terms of mechanical and degradation properties. This has led to the development of synthetic degradable gels primarily as a way to deliver cells and/or molecules in situ, the so-called smart matrix technology. Tissue or organ repair is usually accompanied by fibrotic reactions that result in the production of a scar. Certain mammalian tissues, however, have a capacity for complete regeneration without scarring; good examples include embryonic or foetal skin and the ear of the MRL/MpJ mouse. Investigations of these model systems reveal that in order to achieve such complete regeneration, the inflammatory response is altered such that the extent of fibrosis and scarring is diminished. From studies on the limited examples of mammalian regeneration, it may also be possible to exploit such models to further clarify the regenerative process. The challenge is to identify the factors and cytokines expressed during regeneration and incorporate them to create a smart matrix for use in a skin equivalent. Recent advances in the use of DNA microarray and proteomic technology are likely to aid the identification of such molecules. This, coupled with recent advances in non-viral gene delivery and stem cell technologies, may also contribute to novel approaches that would generate a skin replacement whose materials technology was based not only upon intelligent design, but also upon the molecules involved in the process of regeneration.

Le gènehairlessde la souris

The hairless gene in mammals encodes a nuclear factor that is highly expressed in skin and appears to control hair follicle integrity and cycling. In the absence of a normal and functional Hairless (Hr) protein, the hair bulb undergoes premature apoptosis during the first catagen stage of the hair cycle. The most striking effects of the mutation are loss of hair follicles and formation of epidermal utricles and dermal cysts. The hairless gene expression appears to be widespread and temporally regulated. The gene is strongly expressed in different compartments of the brain. Hairless mRNAs were detected in cartilage, gonads, thymus and colon. In addition to alopecia, hairless mice strains show subtle defects in the development and differentiation of various tissues and organs. The Hr protein is localised in cell nuclei and functions as a transcriptional regulator. Although its role has not been resolved in molecular terms, it was demonstrated that Hr is able to interact with multiple nuclear hormone receptors. Hr seems to be a part of a large multiprotein complex capable to repress transcription by its association to chromatin remodelling factors such as histone deacetylases. Recent experimental data suggest that Hr might be involved in Hox gene regulation, cell adhesion modulation and progenitor cells identity. At least in the skin, but probably in other organs, the Hr repressor seems to be responsible for the timing of epithelial cells differentiation.

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.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

Hoxb13 up-regulates transglutaminase activity and drives terminal differentiation in an epidermal organotypic model

Hox genes act to differentiate and pattern embryonic structures by promoting the proliferation of specific cell types. An exception is Hoxb13, which functions as a proapoptotic and antiproliferative protein during development of the caudal spinal cord and tail vertebrae and has also been implicated in adult cutaneous wound repair. The adult epidermis, which expresses several Hox genes including Hoxb13, is continually renewed in a program of growth arrest, differentiation, and a specialized form of apoptosis (cornification). Yet little is known about the function(s) of these genes in skin. Based on its role during embryogenesis, Hoxb13 is an attractive candidate to be involved in the regulation of epidermal differentiation. Here, we demonstrate that Hoxb13 overexpression in an adult organotypic epidermal model recapitulates actions of Hoxb13 reported in embryonic development. Epidermal cell proliferation is decreased, apoptosis increased, and excessive terminal differentiation observed, as characterized by enhanced transglutaminase activity and excessive cornified envelope formation. Overexpression of Hoxb13 also produces abnormal phenotypes in the epidermal tissue that resemble certain pathological features of dysplastic skin diseases. Our results suggest that Hoxb13 functions to promote epidermal differentiation, a critical process for skin regeneration and for the maintenance of normal barrier function.

HOXB6 protein is bound to CREB-binding protein and represses globin expression in a DNA binding-dependent, PBX interaction-independent process

Although HOXB6 and other HOX genes have previously been associated with hematopoiesis and leukemias, the precise mechanism of action of their protein products remains unclear. Here we use a biological model in which HOXB6 represses alpha- and gamma-globin mRNA levels to perform a structure/function analysis for this homeodomain protein. HOXB6 protein represses globin transcript levels in stably transfected K562 cells in a DNA-binding dependent fashion. However, the capacity to form cooperative DNA-binding complexes with the PBX co-factor protein is not required for HOXB6 biological activity. Neither the conserved extreme N-terminal region, a polyglutamic acid region at the protein C terminus, nor the Ser(214) CKII phosphorylation site was required for DNA binding or activity in this model. We have previously reported that HOX proteins can inhibit CREB-binding protein (CBP)-histone acetyltransferase-mediated potentiation of reporter gene transcription. We now show that endogenous CBP is co-precipitated with exogenous HOXB6 from nuclear and cytoplasmic compartments of transfected K562 cells. Furthermore, endogenous CBP co-precipitates with endogenous HOXB6 in day 14.5 murine fetal liver cells during active globin gene expression in this tissue. The CBP interaction motif was localized to the homeodomain but does not require the highly conserved helix 3. Our data suggest that the homeodomain contains most or all of the important structures required for HOXB6 activity in blood cells.

A new allele of the mouse hairless gene interferes with Hox/LacZ transgene regulation in hair follicle primordia

A new autosomal recessive mouse mutation, causing loss of hair in homozygous mice 2-3 weeks after birth, arose spontaneously in a colony at the National Institute for Medical Research (NIMR), Mill Hill, London in early 1998. Complementation analysis confirmed that this mutation was an allele of the hairless gene (hr). The gene symbol hr(rhbm) (hairless-rhino-bald Mill Hill) was assigned to reflect the source of the colony. Here we show the molecular defect in these mutants, which is a substantial deletion at the 3'-end of the hairless gene. Morphological and immunological analysis of the new hairless mutation was performed at early postnatal stages. In an effort to address the molecular and cellular mechanisms of the hairless phenotype, we analysed developmental stages before the establishment of alopecia. Using a HoxLacZ reporter line of transgenic mice, epidermal placode formation was followed in embryos. Homozygous mutant embryos (hr(rhbmh)/hr(rhbmh)), containing the LacZ reporter under the control of a Hoxb4 gene enhancer, display sharp loss of LacZ staining in epidermal cells invaginating to form the embryonic hair follicle placode. In the light of targeted mutagenesis data involving a Hox gene in the hair development, we discuss the potential implication of the hr(rhbmh) locus in cascades of Hox gene regulation during embryogenesis.

Expression profiles of 39 HOX genes in normal human adult organs and anaplastic thyroid cancer cell lines by quantitative real-time RT-PCR system

HOX genes are well known as master control genes in embryonic morphogenesis. We hypothesized that HOX genes give cells spatial information to maintain tissue- or organ-specificity in adult body and that the deregulated expression of HOX genes results in tumor development. We established a comprehensive analysis system to quantify expression of 39 human HOX genes based on the real-time reverse transcription PCR (RT-PCR) method. Analysis of 39 HOX genes of 20 normal adult organs by this system revealed that 5' HOX genes were expressed in organs in the caudal parts of the body, and that the more caudal regions the more numbers of HOX genes were expressed. It was also found that the expression patterns of HOX genes were more similar in the adjacent genes on the same cluster rather than in those belonging to the same paralogs. Compared with normal thyroid tissues, thyroid cancer cell lines showed the altered expression of some HOX genes, especially Abd-B homeobox family genes. Our results showed that HOX genes were organ-specifically expressed in adult body and that the deregulated expressions of Abd-B family genes were implicated in thyroid tumor development.

FLASH, a component of the FAS-CAPSASE8 apoptotic pathway, is directly regulated by Hoxb4 in the notochord

The Hox genes are a family of homeodomain-containing transcription factors that confer positional identity during development. Although their regulation and function have been extensively studied, very little is known of their downstream target genes. Here we show that Hoxb4 directly induces the expression of FLASH in the notochord of embryos after neurulation. FLASH is a component of the FAS-CAPSASE8 apoptotic pathway, and blocking its activity, or that of Hoxb4, prevents apoptosis in the notochord.

Hoxb13 knockout adult skin exhibits high levels of hyaluronan and enhanced wound healing

In contrast to adult cutaneous wound repair, early gestational fetal cutaneous wounds heal by a process of regeneration, resulting in little or no scarring. Previous studies indicate that down-regulation of HoxB13, a member of the highly conserved family of Hox transcription factors, occurs during fetal scarless wound healing. No down-regulation was noted in adult wounds. Here, we evaluate healing of adult cutaneous wounds in Hoxb13 knockout (KO) mice, hypothesizing that loss of Hoxb13 in adult skin should result in enhanced wound healing. Tensiometry was used to measure the tensile strength of incisional wounds over a 60-day time course; overall, Hoxb13 KO wounds are significantly stronger than wild-type (WT). Histological evaluation of incisional wounds shows that 7-day-old Hoxb13 KO wounds are significantly smaller and that 60-day-old Hoxb13 KO wounds exhibit a more normal collagen architecture compared with WT wounds. We also find that excisional wounds close at a faster rate in Hoxb13 KO mice. Biochemical and histochemical analyses show that Hoxb13 KO skin contains significantly elevated levels of hyaluronan. Because higher levels of hyaluronan and enhanced wound healing are characteristics of fetal skin, we conclude that loss of Hoxb13 produces a more "fetal-like" state in adult skin.

HOXB13 homeodomain protein is cytoplasmic throughout fetal skin development

Substantial evidence suggests that HOX homeobox genes regulate aspects of body development, including hair formation. We initially isolated the HOXB13 gene from human fetal skin in experiments designed to identify candidate genes that regulate scarless fetal wound healing. Although the HOX homeodomain proteins have been proposed to function as transcription factors, we have demonstrated previously that substantial fractions of the HOXB6 and HOXB4 proteins are localized to the cytoplasm throughout epidermal development. The purpose of the current study was to identify HOXB13 protein expression patterns in developing skin to elucidate potential mechanisms by which this protein might regulate aspects of tissue development and healing. HOXB13 protein expression was detected throughout the developing epidermis, with weaker signal observed in the early developing dermis. Epidermal HOXB13 signal was detected over the entire body surface, but surprisingly, essentially all of the signal was cytoplasmic in developing skin. Low-level HOXB13 protein expression was detected in adult skin and within the telogen hair follicle, and a portion of the residual signal in adult epidermis was nuclear. Expression in hyperproliferative skin conditions remained cytoplasmic with the exception of epidermis associated with Kaposi's sarcoma, which showed strong HOXB13 expression that was partially localized to the nucleus.

Hox in hair growth and development

The evolutionarily conserved Hox gene family of transcriptional regulators has originally been known for specifying positional identities along the longitudinal body axis of bilateral metazoans, including mouse and man. It is believed that subsequent to this archaic role, subsets of Hox genes have been co-opted for patterning functions in phylogenetically more recent structures, such as limbs and epithelial appendages. Among these, the hair follicle is of particular interest, as it is the only organ undergoing cyclical phases of regression and regeneration during the entire life span of an organism. Furthermore, the hair follicle is increasingly capturing the attention of developmental geneticists, as this abundantly available miniature organ mimics key aspects of embryonic patterning and, in addition, presents a model for studying organ renewal. The first Hox gene shown to play a universal role in hair follicle development is Hoxc13, as both Hoxc13-deficient and overexpressing mice exhibit severe hair growth and patterning defects. Differential gene expression analyses in the skin of these mutants, as well as in vitro DNA binding studies performed with potential targets for HOXC13 transcriptional regulation in human hair, identified genes encoding hair-specific keratins and keratin-associated proteins (KAPs) as major groups of presumptive Hoxc13 downstream effectors in the control of hair growth. The Hoxc13 mutant might thus serve as a paradigm for studying hair-specific roles of Hoxc13 and other members of this gene family, whose distinct spatio-temporally restricted expression patterns during hair development and cycling suggest discrete functions in follicular patterning and hair cycle control. The main conclusion from a discussion of these potential roles vis-à-vis current expression data in mouse and man, and from the perspective of the results obtained with the Hoxc13 transgenic models, is that members of the Hox family are likely to fulfill essential roles of great functional diversity in hair that require complex transcriptional control mechanisms to ensure proper spatio-temporal patterns of Hox gene expression at homeostatic levels.