A homolog of Drosophila grainy head is essential for epidermal integrity in mice

Negative regulation of Caenorhabditis elegans epidermal damage responses by death-associated protein kinase

Wounding of epidermal layers triggers multiple coordinated responses to damage. We show here that the Caenorhabditis elegans ortholog of the tumor suppressor death-associated protein kinase, dapk-1, acts as a previously undescribed negative regulator of barrier repair and innate immune responses to wounding. Loss of DAPK-1 function results in constitutive formation of scar-like structures in the cuticle, and up-regulation of innate immune responses to damage. Overexpression of DAPK-1 represses innate immune responses to needle wounding. Up-regulation of innate immune responses in dapk-1 requires the TIR-1/p38 signal transduction pathway; loss of function in this pathway synergizes with dapk-1 to drastically reduce adult lifespan. Our results reveal a previously undescribed function for the DAPK tumor suppressor family in regulation of epithelial damage responses.

ABCA12 maintains the epidermal lipid permeability barrier by facilitating formation of ceramide linoleic esters

Harlequin ichthyosis is a congenital scaling syndrome of the skin in which affected infants have epidermal hyperkeratosis and a defective permeability barrier. Mutations in the gene encoding a member of the ABCA transporter family, ABCA12, have been linked to harlequin ichthyosis, but the molecular function of the protein is unknown. To investigate the activity of ABCA12, we generated Abca12 null mice and analyzed the impact on skin function and lipid content. Abca12-/- mice are born with a thickened epidermis and die shortly after birth, as water rapidly evaporates from their skin. In vivo skin proliferation measurements suggest a lack of desquamation of the skin cells, rather than enhanced proliferation of basal layer keratinocytes, accounts for the 5-fold thickening of the Abca12-/- stratum corneum. Electron microscopy revealed a loss of the lamellar permeability barrier in Abca12-/- skin. This was associated with a profound reduction in skin linoleic esters of long-chain omega-hydroxyceramides and a corresponding increase in their glucosyl ceramide precursors. Because omega-hydroxyceramides are required for the barrier function of the skin, these results establish that ABCA12 activity is required for the generation of long-chain ceramide esters that are essential for the development of normal skin structure and function.

Gene trapping identifies a putative tumor suppressor and a new inducer of cell migration

Tumor necrosis factor alpha (TNFalpha) is a pleiotropic cytokine involved in apoptotic cell death, cellular proliferation, differentiation, inflammation, and tumorigenesis. In tumors it is secreted by tumor associated macrophages and can have both pro- and anti-tumorigenic effects. To identify genes regulated by TNFalpha, we performed a gene trap screen in the mammary carcinoma cell line MCF-7 and recovered 64 unique, TNFalpha-induced gene trap integration sites. Among these were the genes coding for the zinc finger protein ZC3H10 and for the transcription factor grainyhead-like 3 (GRHL3). In line with the dual effects of TNFalpha on tumorigenesis, we found that ZC3H10 inhibits anchorage independent growth in soft agar suggesting a tumor suppressor function, whereas GRHL3 strongly stimulated the migration of endothelial cells which is consistent with an angiogenic, pro-tumorigenic function.

Cancer as an overhealing wound: an old hypothesis revisited

What is the relationship between the wound-healing process and the development of cancer? Malignant tumours often develop at sites of chronic injury, and tissue injury has an important role in the pathogenesis of malignant disease, with chronic inflammation being the most important risk factor. The development and functional characterization of genetically modified mice that lack or overexpress genes that are involved in repair, combined with gene-expression analysis in wounds and tumours, have highlighted remarkable similarities between wound repair and cancer. However, a few crucial differences were also observed, which could account for the altered metabolism, impaired differentiation capacity and invasive growth of malignant tumours.

Genomic survey of candidate stress-response genes in the estuarine anemone Nematostella vectensis

Salt marshes are challenging habitats due to natural variability in key environmental parameters including temperature, salinity, ultraviolet light, oxygen, sulfides, and reactive oxygen species. Compounding this natural variation, salt marshes are often heavily impacted by anthropogenic insults including eutrophication, toxic contamination, and coastal development that alter tidal and freshwater inputs. Commensurate with this environmental variability, estuarine animals generally exhibit broader physiological tolerances than freshwater, marine, or terrestrial species. One factor that determines an organism's physiological tolerance is its ability to upregulate "stress-response genes" in reaction to particular stressors. Comparative studies on diverse organisms have identified a number of evolutionarily conserved genes involved in responding to abiotic and biotic stressors. We used homology-based scans to survey the sequenced genome of Nematostella vectensis, the starlet sea anemone, an estuarine specialist, to identify genes involved in the response to three kinds of insult-physiochemical insults, pathogens, and injury. Many components of the stress-response networks identified in triploblastic animals have clear orthologs in the sea anemone, meaning that they must predate the cnidarian-triploblast split (e.g., xenobiotic receptors, biotransformative genes, ATP-dependent transporters, and genes involved in responding to reactive oxygen species, toxic metals, osmotic shock, thermal stress, pathogen exposure, and wounding). However, in some instances, stress-response genes known from triploblasts appear to be absent from the Nematostella genome (e.g., many metal-complexing genes). This is the first comprehensive examination of the genomic stress-response repertoire of an estuarine animal and a member of the phylum Cnidaria. The molecular markers of stress response identified in Nematostella may prove useful in monitoring estuary health and evaluating coastal conservation efforts. These data may also inform conservation efforts on other cnidarians, such as the reef-building corals.

Grainyhead-like factor Get1/Grhl3 regulates formation of the epidermal leading edge during eyelid closure

Grainyhead transcription factors play an evolutionarily conserved role in regulating epidermal terminal differentiation. One such factor, the mammalian Grainyhead-like epithelial transactivator (Get1/Grhl3), is important for epidermal barrier formation. In addition to a role in barrier formation, Grainyhead genes play roles in closure of several structures such as the mouse neural tube and Drosophila wounds. Consistent with these observations, we found that Get1 knockout mice have an eye-open at birth phenotype. The failure of eyelid closure appears to be due to critical functions of Get1 in promoting F-actin polymerization, filopodia formation, and the cell shape changes that are required for migration of the keratinocytes at the leading edge during eyelid closure. The expression of TGFalpha, a known regulator of leading edge formation, is decreased in the eyelid tip of Get1(-/-) mice. Levels of phospho-EGFR and phospho-ERK are also decreased at the leading edge tip. Furthermore, in an organ culture model, TGFalpha can increase levels of phospho-EGFR and promote cell shape changes as well as leading edge formation in Get1(-/-) eyelids, indicating that in eyelid closure Get1 acts upstream of TGFalpha in the EGFR/ERK pathway.

Grainy head promotes expression of septate junction proteins and influences epithelial morphogenesis

Transcription factors of the Grainy head (Grh) family are required in epithelia to generate the impermeable apical layer that protects against the external environment. This function is conserved in vertebrates and invertebrates, despite the differing molecular composition of the protective barrier. Epithelial cells also have junctions that create a paracellular diffusion barrier (tight or septate junctions). To examine whether Grh has a role in regulating such characteristics, we used an epidermal layer in the Drosophila embryo that has no endogenous Grh and lacks septate junctions, the amnioserosa. Expression of Grh in the amnioserosa caused severe defects in dorsal closure, a process similar to wound closure, and induced robust expression of the septate junction proteins Coracle, Fasciclin 3 and Sinuous. Grh-binding sites are present within the genes encoding these proteins, consistent with them being direct targets. Removal of Grh from imaginal disc cells caused a reduction in Fasciclin 3 and Coracle levels, suggesting that Grh normally fine tunes their epithelial expression and hence contributes to barrier properties. The fact that ectopic Grh arrests dorsal closure also suggests that this dynamic process relies on epithelia having distinct adhesive properties conferred by differential deployment of Grh.

Perturbed desmosomal cadherin expression in grainy head-like 1-null mice

In Drosophila, the grainy head (grh) gene plays a range of key developmental roles through the regulation of members of the cadherin gene family. We now report that mice lacking the grh homologue grainy head-like 1 (Grhl1) exhibit hair and skin phenotypes consistent with a reduction in expression of the genes encoding the desmosomal cadherin, desmoglein 1 (Dsg1). Grhl1-null mice show an initial delay in coat growth, and older mice exhibit hair loss as a result of poor anchoring of the hair shaft in the follicle. The mice also develop palmoplantar keratoderma, analogous to humans with DSG1 mutations. Sequence analysis, DNA binding, and chromatin immunoprecipitation experiments demonstrate that the human and mouse Dsg1 promoters are direct targets of GRHL1. Ultrastructural analysis reveals reduced numbers of abnormal desmosomes in the interfollicular epidermis. These findings establish GRHL1 as an important regulator of the Dsg1 genes in the context of hair anchorage and epidermal differentiation, and suggest that cadherin family genes are key targets of the grainy head-like genes across 700 million years of evolution.

Transcriptional control of wound repair

Injury to the skin initiates a complex process of events involving inflammation as well as the formation and remodeling of new tissue. These processes result in at least partial reconstitution of the injured skin. However, wounds in adult mammals heal with a scar, which is accompanied by functional and aesthetic impairments. In addition to this problem, a large number of patients, in particular in the aged population, suffer from chronic, nonhealing ulcers. Therefore, there is a strong need to improve the wound healing process. This requires a thorough understanding of the underlying molecular and cellular mechanisms. During the past several years, important regulators of the wound healing process have been identified. In particular, the growth factors and matrix proteins, which orchestrate skin repair, have been characterized in detail. By contrast, much less is known about the transcription factors, which regulate gene expression at the wound site. This review summarizes recent data on the expression of transcription factors in skin wounds and their functions in the repair process.

A Cysteine-rich Protein from an Arthropod Stabilizes Clotting Mesh and Immobilizes Bacteria at Injury Sites

Hemolymph coagulation in arthropods plays key roles in host defense, including sealing wounds to staunch bleeding and immobilizing invading microorganisms. We have previously reported that horseshoe crab transglutaminase (TGase) promotes cross-linking of a clotting protein (coagulin) with hemocyte-derived proteins (proxins), resulting in the formation of stable coagulin fibrils. Here we show that TGase also cross-links proxins to another hemocyte-derived protein named stablin. Stablin is a cysteine-rich protein of 131 residues. Surface plasmon resonance analysis revealed the specific interaction of stablin with proxin-1 at K(d) = 4.0 x 10(-9) m. Stablin was predominantly localized in the large granules of hemocytes and secreted by lipopolysaccharide-induced exocytosis. Interestingly, stablin bound to chitin at K(d) = 1.5 x 10(-8) m, as determined by using a quartz-crystal microbalance. Stablin also interacted with lipopolysaccharides and lipoteichoic acids and exhibited bacterial agglutinating activity against Gram-positive and -negative bacteria. Immunostaining showed that stablin is co-localized with coagulin in the clotting fibrils that effectively trap bacteria. Moreover, an anti-stablin antibody strongly inhibited the proper formation of the clotting fibrils. These data suggest that stablin promotes the formation of the clotting mesh and the immobilization of invading microbes at injury sites. In arthropods, the TGase-mediated cross-linking may play an important role in the initial stage of host defense, wound closure, and healing, as in the case of mammals.

Hypertrophic scar formation following burns and trauma: new approaches to treatment

The authors examine the process of hypertrophic scar formation, the results of current treatments, and areas of research likely to lead to significant advances in the field.

Increased expression of Grainyhead-like-3 rescues spina bifida in a folate-resistant mouse model

Neural tube defects (NTDs), such as spina bifida, are common and severe birth defects in humans but the underlying causes are poorly understood. The pathogenesis and etiology of spina bifida in the curly tail mouse closely resemble defects in humans, providing a well-characterized model of NTDs. Grainyhead-like-3 (Grhl3), which encodes a transcription factor, was recently identified as a candidate gene for curly tail based on chromosomal location and the occurrence of spina bifida in Grhl3 null mice. However, the causative curly tail mutation has not been established, while the relationship between Grhl3 gene expression and the known cellular defect leading to NTDs in curly tail is unknown. Spina bifida in curly tail results from a cell type-specific proliferation defect in the hindgut endoderm, and we find that Grhl3 is expressed specifically in this tissue during the final stages of spinal neural tube closure in wild type embryos. Moreover, Grhl3 expression is diminished in the spinal region of neurulation-stage curly tail embryos. Curly tail mice do not carry a coding region mutation in Grhl3, however, we found a putative regulatory mutation upstream of the Grhl3 gene, which may be responsible for the expression deficit. In order to test the hypothesis that spina bifida in curly tail mice results from insufficient expression of Grhl3, we generated Grhl3-expressing curly tail mice by bacterial artificial chromosome-mediated transgenesis and demonstrated complete rescue of spina bifida. This study provides evidence for a critical role of diminished Grhl3 expression in causing spinal NTDs in the curly tail mouse model.

NGF-mediated transcriptional targets of p53 in PC12 neuronal differentiation

BACKGROUND

p53 is recognized as a critical regulator of the cell cycle and apoptosis. Mounting evidence also suggests a role for p53 in differentiation of cells including neuronal precursors. We studied the transcriptional role of p53 during nerve growth factor-induced differentiation of the PC12 line into neuron-like cells. We hypothesized that p53 contributed to PC12 differentiation through the regulation of gene targets distinct from its known transcriptional targets for apoptosis or DNA repair.

RESULTS

Using a genome-wide chromatin immunoprecipitation cloning technique, we identified and validated 14 novel p53-regulated genes following NGF treatment. The data show p53 protein was transcriptionally activated and contributed to NGF-mediated neurite outgrowth during differentiation of PC12 cells. Furthermore, we describe stimulus-specific regulation of a subset of these target genes by p53. The most salient differentiation-relevant target genes included wnt7b involved in dendritic extension and the tfcp2l4/grhl3 grainyhead homolog implicated in ectodermal development. Additional targets included brk, sdk2, sesn3, txnl2, dusp5, pon3, lect1, pkcbpb15 and other genes.

CONCLUSION

Within the PC12 neuronal context, putative p53-occupied genomic loci spanned the entire Rattus norvegicus genome upon NGF treatment. We conclude that receptor-mediated p53 transcriptional activity is involved in PC12 differentiation and may suggest a contributory role for p53 in neuronal development.

Mechanisms regulating epithelial stratification

The epidermis is a stratified epithelium that functions as a barrier protecting the organism from dehydration, mechanical trauma, and microbial insults. This barrier function is established during embryogenesis through a complex and tightly controlled stratification program. Whereas the morphological changes that occur during epidermal development have been extensively studied, the molecular mechanisms that govern this process remain poorly understood. In this review we summarize the current advances that have been made in understanding the molecular mechanisms that regulate epidermal morphogenesis.

Grainyhead-related transcription factor is required for duct maturation in the salivary gland and the kidney of the mouse

Duct epithelial structure is an essential feature of many internal organs, including exocrine glands and the kidney. The ducts not only mediate fluid transfer but also help to maintain homeostasis. For instance, fluids and solutes are resorbed from or secreted into the primary fluid flowing through the lumen of the ducts in the exocrine glands and kidneys. The molecular mechanism underlying the functional maturation of these ducts remains largely unknown. Here, we show that a grainyhead-related transcription factor, CP2-like 1 (CP2L1), is required for the maturation of the ducts of the salivary gland and kidney. In the mouse, Cp2l1 is specifically expressed in the developing ducts of a number of exocrine glands, including the salivary gland, as well as in those of the kidney. In Cp2l1-deficient mice, the expression of genes directly involved in functional maturation of the ducts was specifically reduced in both the salivary gland and kidney, indicating that Cp2l1 is required for the differentiation of duct cells. Furthermore, the composition of saliva and urine was abnormal in these mice. These results indicate that Cp2l1 expression is required for normal duct development in both the salivary gland and kidney.

The Grainyhead-like epithelial transactivator Get-1/Grhl3 regulates epidermal terminal differentiation and interacts functionally with LMO4

Defective permeability barrier is an important feature of many skin diseases and causes mortality in premature infants. To investigate the control of barrier formation, we characterized the epidermally expressed Grainyhead-like epithelial transactivator (Get-1)/Grhl3, a conserved mammalian homologue of Grainyhead, which plays important roles in cuticle development in Drosophila. Get-1 interacts with the LIM-only protein LMO4, which is co-expressed in the developing mammalian epidermis. The epidermis of Get-1(-/-) mice showed a severe barrier function defect associated with impaired differentiation of the epidermis, including defects of the stratum corneum, extracellular lipid composition and cell adhesion in the granular layer. The Get-1 mutation affects multiple genes linked to terminal differentiation and barrier function, including most genes of the epidermal differentiation complex. Get-1 therefore directly or indirectly regulates a broad array of epidermal differentiation genes encoding structural proteins, lipid metabolizing enzymes and cell adhesion molecules. Although deletion of the LMO4 gene had no overt consequences for epidermal development, the epidermal terminal differentiation defect in mice deleted for both Get-1 and LMO4 is much more severe than in Get-1(-/-) mice with striking impairment of stratum corneum formation. These findings indicate that the Get-1 and LMO4 genes interact functionally to regulate epidermal terminal differentiation.

The tumour-suppressor Scribble dictates cell polarity during directed epithelial migration: regulation of Rho GTPase recruitment to the leading edge

Altered expression of human Scribble is associated with invasive epithelial cancers, however, its role in tumour development remains unclear. Mutations in Drosophila Scribble result in loss of polarity, overproliferation and 3D-tumourous overgrowth of epithelial cells. Using complementation studies in Drosophila we recently demonstrated that expression of human Scribble can also regulate polarity and restrict tissue overgrowth. Here, we have undertaken a detailed study of human Scribble function in the polarized mammary cell line, MCF10A. We show that although Scribble does not seem to be required for apical-basal polarity or proliferation control in MCF10A cells, Scribble is essential for the control of polarity associated with directed epithelial cell migration. Scribble-depleted MCF10A cells show defective in vitro wound closure and chemotactic movement. The cells at the wound edge fail to polarize, show reduced lamellipodia formation and impaired recruitment of Cdc42 and Rac1 to the leading edge. Furthermore, we show that this function is relevant in vivo as Scribble mutant mice show defective epidermal wound healing. This data identifies an essential role for mammalian Scribble in the regulation of the polarity specifically involved in directed epithelial migration.

Spatial and temporal expression of the Grainyhead-like transcription factor family during murine development

The Drosophila transcription factor Grainyhead (grh) is expressed in ectoderm-derived tissues where it regulates several key developmental events including cuticle formation, tracheal elongation and dorsal closure. Our laboratory has recently identified three novel mammalian homologues of the grh gene, Grainyhead-like 1, -2 and -3 (Grhl1-3) that rewrite the phylogeny of this family. Using gene targeting in mice, we have shown that Grhl3 is essential for neural tube closure, skin barrier formation and wound healing. Despite their extensive sequence homology, Grhl1 and Grhl2 are unable to compensate for loss of Grhl3 in these developmental processes. To explore this lack of redundancy, and to gain further insights into the functions of this gene family in mammalian development we have performed an extensive in situ hybridisation analysis. We demonstrate that, although all three Grhl genes are highly expressed in the developing epidermis, they display subtle differences in the timing and level of expression. Surprisingly, we also demonstrate differential expression patterns in non-ectoderm-derived tissues, including the heart, the lung, and the metanephric kidney. These findings expand our understanding of the unique role of Grhl3 in neurulation and epidermal morphogenesis, and provide a focus for further functional analysis of the Grhl genes during mouse embryogenesis.

Current perspectives on the genetic causes of neural tube defects

Neural tube defects (NTDs) are a group of severe congenital abnormalities resulting from the failure of neurulation. The pattern of inheritance of these complex defects is multifactorial, making it difficult to identify the underlying causes. Scientific research has rapidly progressed in experimental embryology and molecular genetics to elucidate the basis of neurulation. Crucial mechanisms of neurulation include the planar cell polarity pathway, which is essential for the initiation of neural tube closure, and the sonic hedgehog signaling pathway, which regulates neural plate bending. Genes influencing neurulation have been investigated for their contribution to human neural tube defects, but only genes with well-established role in convergent extension provide an exciting new set of candidate genes. Biochemical factors such as folic acid appear to be the greatest modifiers of NTDs risk in the human population. Consequently, much research has focused on genes of folate-related metabolic pathways. Variants of several such genes have been found to be significantly associated with the risk of neural tube defects in more studies. In this manuscript, we reviewed the current perspectives on the causes of neural tube defects and highlighted that we are still a long way from understanding the etiology of these complex defects.

Lhx2 maintains stem cell character in hair follicles

During embryogenesis, stem cells are set aside to fuel the postnatal hair cycle and repair the epidermis after injury. To define how hair follicle stem cells are specified and maintained in an undifferentiated state, we developed a strategy to isolate and transcriptionally profile embryonic hair progenitors in mice. We identified Lhx2 as a transcription factor positioned downstream of signals necessary to specify hair follicle stem cells, but upstream from signals required to drive activated stem cells to terminally differentiate. Using gain- and loss-of-function studies, we uncovered a role for Lhx2 in maintaining the growth and undifferentiated properties of hair follicle progenitors.

Epidermal barrier formation and recovery in skin disorders

Skin is at the interface between the complex physiology of the body and the external, often hostile, environment, and the semipermeable epidermal barrier prevents both the escape of moisture and the entry of infectious or toxic substances. Newborns with rare congenital barrier defects underscore the skin's essential role in a terrestrial environment and demonstrate the compensatory responses evoked ex utero to reestablish a barrier. Common inflammatory skin disorders such as atopic dermatitis and psoriasis exhibit decreased barrier function, and recent studies suggest that the complex response of epidermal cells to barrier disruption may aggravate, maintain, or even initiate such conditions. Either aiding barrier reestablishment or dampening the epidermal stress response may improve the treatment of these disorders. This Review discusses the molecular regulation of the epidermal barrier as well as causes and potential treatments for defects of barrier formation and proposes that medical management of barrier disruption may positively affect the course of common skin disorders.

Grainyhead-like 3, a transcription factor identified in a microarray screen, promotes the specification of the superficial layer of the embryonic epidermis

The Xenopus ectoderm consists of two populations of cells, superficial polarised epithelial cells and deep, non-epithelial cells. These two cell types differ in their developmental fate. In the neural ectoderm, primary neurons are derived only from the deep cells. In the epidermal ectoderm, superficial cells express high levels of differentiation markers, while most of the deep cells do not differentiate until later when they produce the stratified adult epidermis. However, few molecular differences are known between the deep and superficial cells. Here, we have undertaken a systematic approach to identify genes that show layer-restricted expression by microarray analysis of deep and superficial cells at the gastrula stage, followed by wholemount in situ hybridisation. We have identified 32 differentially expressed genes, of which 26 show higher expression in the superficial layer and 6 in the deep layer and describe their expression at the gastrula and neurula stage. One of the identified genes is the transcription factor Grhl3, which we found to be expressed in the superficial layer of the gastrula ectoderm and the neurula epidermis. By using markers identified in this work, we show that Grlh3 promotes superficial gene expression in the deep layer of the epidermis. Concomitantly, deep layer specific genes are switched off, showing that Grlh3 can promote deep cells to take on a superficial cell identity in the embryonic epidermis.

Inhibition of JNK promotes differentiation of epidermal keratinocytes

In inflamed tissue, normal signal transduction pathways are altered by extracellular signals. For example, the JNK pathway is activated in psoriatic skin, which makes it an attractive target for treatment. To define comprehensively the JNK-regulated genes in human epidermal keratinocytes, we compared the transcriptional profiles of control and JNK inhibitor-treated keratinocytes, using DNA microarrays. We identified the differentially expressed genes 1, 4, 24, and 48 h after the treatment with SP600125. Surprisingly, the inhibition of JNK in keratinocyte cultures in vitro induces virtually all aspects of epidermal differentiation in vivo: transcription of cornification markers, inhibition of motility, withdrawal from the cell cycle, stratification, and even production of cornified envelopes. The inhibition of JNK also induces the production of enzymes of lipid and steroid metabolism, proteins of the diacylglycerol and inositol phosphate pathways, mitochondrial proteins, histones, and DNA repair enzymes, which have not been associated with differentiation previously. Simultaneously, basal cell markers, including integrins, hemidesmosome and extracellular matrix components, are suppressed. Promoter analysis of regulated genes finds that the binding sites for the forkhead family of transcription factors are over-represented in the SP600125-induced genes and c-Fos sites in the suppressed genes. The JNK-induced proliferation appears to be secondary to inhibition of differentiation. The results indicate that the inhibition of JNK in epidermal keratinocytes is sufficient to initiate their differentiation program and suggest that augmenting JNK activity could be used to delay cornification and enhance wound healing, whereas attenuating it could be a differentiation therapy-based approach for treating psoriasis.

Regulation of post-embryonic neuroblasts by Drosophila Grainyhead

The Drosophila post-embryonic neuroblasts (pNBs) are neural stem cells that persist in the larval nervous system where they proliferate to produce neurons for the adult CNS. These pNBs provide a good model to investigate mechanisms regulating the maintenance and proliferation of stem cells. The transcription factor Grainyhead (Grh), which is required for morphogenesis of epidermal and tracheal cells, is also expressed in all pNBs. Here, we show that grh is essential for pNBs to adopt the stem cell programme appropriate to their position within the CNS. In grh mutants the abdominal pNBs produced more progeny while the thoracic pNBs, in contrast, divided less and produced fewer progeny than wild type. We investigated three candidates; the Neuroblast identify gene Castor, the signalling molecule Notch and the adhesion protein E-Cadherin, to determine whether they could mediate these effects. Neither Castor nor Notch fulfilled the criteria for intermediaries, and in particular Notch activity was found to be dispensable for the normal proliferation and survival of the pNBs. In contrast E-Cadherin, which has been shown to regulate pNB proliferation, was present at greatly reduced levels in the grh mutant pNBs. Furthermore, ectopic expression of Grh was sufficient to promote ectopic E-Cadherin and two conserved Grh-binding sites were identified in the E-Cadherin/shotgun flanking sequences, arguing that this gene is a downstream target. Thus one way Grh could regulate pNBs is through expression of E-cadherin, a protein that is thought to mediate interactions with the glial niche.

An ancient control of epithelial barrier formation and wound healing

Animal epithelia are lined with apical surface matrices, which protect against pathogens, dehydration and physical damage of the underlying cells. The proteins and polysaccharides that comprise these protective barriers vary greatly within the animal kingdom and have evolved in response to the biological needs of various organisms. Yet the genetic control of barrier formation and its regeneration upon wounding appears conserved between vertebrates and insects that are evolutionary more than several hundred millions of years apart. A key role is carried out by Grainy head, a phylogenetically conserved transcription factor expressed in epidermal cells in nematodes, flies, frogs, mice and humans.

Comprehensive sequence analysis of horseshoe crab cuticular proteins and their involvement in transglutaminase-dependent cross-linking

Arthropod cuticles play an important role as the first barrier against invading pathogens. We extensively determined the sequences of horseshoe crab cuticular proteins. Proteins extracted from a part of the ventral side of the cuticle were purified by chitin-affinity chromatography, and separated by two-dimensional SDS/PAGE. Proteins appearing on the gel were designated high molecular mass chitin-binding proteins, and these proteins were then grouped into classes based on their approximate isoelectric points and predominant amino acid compositions. Members of groups designated basic G, basic Y, and acidic S groups contained a so-called Rebers and Riddiford consensus found in arthropod cuticular proteins. Proteins designated acidic DE25 and DE29 each contained a Cys-rich domain with sequences similar to those of insect peritrophic matrix proteins and chitinases. In contrast, basic QH4 and QH10 contained no consensus sequences found in known chitin-binding proteins. Alternatively, a low molecular mass chitin-binding fraction was prepared by size exclusion chromatography, and 15 low molecular mass chitin-binding proteins, named P1 through P15, were isolated. With the exception of P9 and P15, all were found to be identical to known antimicrobial peptides. P9 consisted of a Kunitz-type chymotrypsin inhibitor sequence, and P15 contained a Cys-rich motif found in insulin-like growth factor-binding proteins. Interestingly, we observed transglutaminase-dependent polymerization of nearly all high molecular mass chitin-binding proteins, a finding suggests that transglutaminase-dependent cross-linking plays an important role in host defense in the arthropod cuticle, analogous to that observed in the epidermal cornified cell envelope in mammals.

Epidermal impermeable barriers in mouse and fly

Despite significant structural differences, the surface epithelia of flies and mice exhibit remarkable functional parallels. Genetic studies in both organisms have identified highly conserved pathways regulating cell movement and polarity, wound healing, innate immunity and appendage formation. More recently, it has emerged that the establishment and repair of the barrier function of the integument are also achieved by common mechanisms involving genes responsible both for cross-linking surface proteins and for assembly of cellular tight junctions. These studies support the model that the formation and maintenance of the epidermal impermeable barrier in a wide range of species relies on two independent and complementary pathways.

Cell Biology: Master Regulators of Sealing and Healing

The protective layer of the epidermis in Drosophila (cuticle) and mice (stratum corneum) are structurally unrelated. Yet new evidence suggests a conserved transcription factor, Grainyhead, controls both their development and the means by which both structures repair themselves.

An Epidermal Barrier Wound Repair Pathway in Drosophila Is Mediated by grainy head

We used wounded Drosophila embryos to define an evolutionarily conserved pathway for repairing the epidermal surface barrier. This pathway includes a wound response enhancer from the Ddc gene that requires grainy head (grh) function and binding sites for the Grh transcription factor. At the signaling level, tyrosine kinase and extracellular signal-regulated kinase (ERK) activities are induced in epidermal cells near wounds, and activated ERK is required for a robust wound response. The conservation of this Grh-dependent pathway suggests that the repair of insect cuticle and mammal skin is controlled by an ancient, shared control system for constructing and healing the animal body surface barrier.