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

The epidermis of grhl3-null mice displays altered lipid processing and cellular hyperproliferation

The presence of an impermeable surface barrier is an essential homeostatic mechanism in almost all living organisms. We have recently described a novel gene that is critical for the developmental instruction and repair of the integument in mammals. This gene, Grainy head-like 3 (Grhl3) is a member of a large family of transcription factors that are homologs of the Drosophila developmental gene grainy head (grh). Mice lacking Grhl3 fail to form an adequate skin barrier, and die at birth due to dehydration. These animals are also unable to repair the epidermis, exhibiting failed wound healing in both fetal and adult stages of development. These defects are due, in part, to diminished expression of a Grhl3 target gene, Transglutaminase 1 (TGase 1), which encodes a key enzyme involved in cross-linking of epidermal structural proteins and lipids into the cornified envelope (CE). Remarkably, the Drosophila grh gene plays an analogous role, regulating enzymes involved in the generation of quinones, which are essential for cross-linking structural components of the fly epidermis. In an extension of our initial analyses, we focus this report on additional defects observed in the Grhl3-null epidermis, namely defective extra-cellular lipid processing, altered lamellar lipid architecture and cellular hyperproliferation. These abnormalities suggest that Grhl3 plays diverse mechanistic roles in maintaining homeostasis in the skin.

Genomic profiling of a human organotypic model of AEC syndrome reveals ZNF750 as an essential downstream target of mutant TP63

The basis for impaired differentiation in TP63 mutant ankyloblepharon-ectodermal dysplasia-clefting (AEC) syndrome is unknown. Human epidermis harboring AEC TP63 mutants recapitulated this impairment, along with downregulation of differentiation activators, including HOPX, GRHL3, KLF4, PRDM1, and ZNF750. Gene-set enrichment analysis indicated that disrupted expression of epidermal differentiation programs under the control of ZNF750 and KLF4 accounted for the majority of disrupted epidermal differentiation resulting from AEC mutant TP63. Chromatin immunoprecipitation (ChIP) analysis and ChIP-sequencing of TP63 binding in differentiated keratinocytes revealed ZNF750 as a direct target of wild-type and AEC mutant TP63. Restoring ZNF750 to AEC model tissue rescued activator expression and differentiation, indicating that AEC TP63-mediated ZNF750 inhibition contributes to differentiation defects in AEC. Incorporating disease-causing mutants into regenerated human tissue can thus dissect pathomechanisms and identify targets that reverse disease features.

Wound healing in development

Wound healing is the inherent ability of an organism to protect itself against injuries. Cumulative evidence indicates that the healing process patterns in part embryonic morphogenesis and may result in either organ regeneration or scarring, phenomena that are developmental stage- or age-dependent. Skin is the largest organ. Its morphogenesis and repair mechanisms have been studied extensively due not only to its anatomical location, which allows easy access and observation, but also to its captivating structure and vital function. Thus, this review will focus on using skin as a model organ to illustrate new insights into the mechanisms of wound healing that are developmentally regulated in mammals, with special emphasis on the role of the Wnt signaling pathway and its crosstalk with TGF-β signaling. Relevant information from studies of other organs is discussed where it applies, and the clinical impact from such knowledge and emerging concepts on regenerative medicine are discussed in perspective.

Interferon regulatory factor 6 promotes differentiation of the periderm by activating expression of Grainyhead-like 3

Interferon Regulatory Factor 6 (IRF6) is a transcription factor that, in mammals, is required for the differentiation of skin, breast epithelium, and oral epithelium. However, the transcriptional targets that mediate these effects are currently unknown. In zebrafish and frog embryos Irf6 is necessary for differentiation of the embryonic superficial epithelium, or periderm. Here we use microarrays to identify genes that are expressed in the zebrafish periderm and whose expression is inhibited by a dominant-negative variant of Irf6 (dnIrf6). These methods identify Grhl3, an ancient regulator of the epidermal permeability barrier, as acting downstream of Irf6. In human keratinocytes, IRF6 binds conserved elements near the GHRL3 promoter. We show that one of these elements has enhancer activity in human keratinocytes and zebrafish periderm, suggesting that Irf6 directly stimulates Grhl3 expression in these tissues. Simultaneous inhibition of grhl1 and grhl3 disrupts periderm differentiation in zebrafish, and, intriguingly, forced grhl3 expression restores periderm markers in both zebrafish injected with dnIrf6 and frog embryos depleted of Irf6. Finally, in Irf6 deficient mouse embryos, Grhl3 expression in the periderm and oral epithelium is virtually absent. These results indicate that Grhl3 is a key effector of Irf6 in periderm differentiation.

GRHL3/GET1 and trithorax group members collaborate to activate the epidermal progenitor differentiation program

The antagonistic actions of Polycomb and Trithorax are responsible for proper cell fate determination in mammalian tissues. In the epidermis, a self-renewing epithelium, previous work has shown that release from Polycomb repression only partially explains differentiation gene activation. We now show that Trithorax is also a key regulator of epidermal differentiation, not only through activation of genes repressed by Polycomb in progenitor cells, but also through activation of genes independent of regulation by Polycomb. The differentiation associated transcription factor GRHL3/GET1 recruits the ubiquitously expressed Trithorax complex to a subset of differentiation genes.

Human epidermal keratinocyte differentiation provides a highly suitable system to understand how progenitor cells become specialized. Previous work has implicated resolution of repressive histone modifications in the activation of the terminal differentiation gene expression program. Our work shows that this mechanism only accounts for the regulation of a subset of the differentiation gene expression program and that activating histone modifications by Trithorax chromatin modifiers, acting alone or in combination with the release from repressive chromatin changes, is essential. Furthermore, we show that the Trithorax complex is recruited to a subset of differentiation gene promoters by the transcription factor Grhl3, an evolutionarily conserved regulator of the epidermal differentiation program. Altered differentiation is characteristic for several skin diseases, including skin cancer and inflammatory diseases such as psoriasis. While genetic abnormalities play a role in these diseases, the cellular and macro-environment may also alter the course of these diseases through chromatin changes (epigenetics). Understanding the epigenetic regulation of keratinocyte differentiation may in the future lead to the development of new drugs for skin diseases.

A molecular wound response program associated with regeneration initiation in planarians

Planarians are capable of regenerating any missing body part and present an attractive system for molecular investigation of regeneration initiation. The gene activation program that occurs at planarian wounds to coordinate regenerative responses remains unknown. We identified a large set of wound-induced genes during regeneration initiation in planarians. Two waves of wound-induced gene expression occurred in differentiated tissues. The first wave includes conserved immediate early genes. Many second-wave genes encode conserved patterning factors required for proper regeneration. Genes of both classes were generally induced by wounding, indicating that a common initial gene expression program is triggered regardless of missing tissue identity. Planarian regeneration uses a population of regenerative cells (neoblasts), including pluripotent stem cells. A class of wound-induced genes was activated directly within neoblasts, including the Runx transcription factor-encoding runt-1 gene. runt-1 was required for specifying different cell types during regeneration, promoting heterogeneity in neoblasts near wounds. Wound-induced gene expression in neoblasts, including that of runt-1, required SRF (serum response factor) and sos-1. Taken together, these data connect wound sensation to the activation of specific cell type regeneration programs in neoblasts. Most planarian wound-induced genes are conserved across metazoans, and identified genes and mechanisms should be important broadly for understanding wound signaling and regeneration initiation.

Development and homeostasis of the skin epidermis

The skin epidermis is a stratified epithelium that forms a barrier that protects animals from dehydration, mechanical stress, and infections. The epidermis encompasses different appendages, such as the hair follicle (HF), the sebaceous gland (SG), the sweat gland, and the touch dome, that are essential for thermoregulation, sensing the environment, and influencing social behavior. The epidermis undergoes a constant turnover and distinct stem cells (SCs) are responsible for the homeostasis of the different epidermal compartments. Deregulation of the signaling pathways controlling the balance between renewal and differentiation often leads to cancer formation.

The Caenorhabditis elegans epidermis as a model skin. I: development, patterning, and growth

The skin of the nematode Caenorhabditis elegans is composed of a simple epidermal epithelium and overlying cuticle. The skin encloses the animal and plays central roles in body morphology and physiology; its simplicity and accessibility make it a tractable genetic model for several aspects of skin biology. Epidermal precursors are specified by a hierarchy of transcriptional regulators. Epidermal cells form on the dorsal surface of the embryo and differentiate to form the epidermal primordium, which then spreads out in a process of epiboly to enclose internal tissues. Subsequent elongation of the embryo into a vermiform larva is driven by cell shape changes and cell fusions in the epidermis. Most epidermal cells fuse in mid-embryogenesis to form a small number of multinucleate syncytia. During mid-embryogenesis the epidermis also becomes intimately associated with underlying muscles, performing a tendon-like role in transmitting muscle force. Post-embryonic development of the epidermis involves growth by addition of new cells to the syncytia from stem cell-like epidermal seam cells and by an increase in cell size driven by endoreplication of the chromosomes in epidermal nuclei.

The transcription factor Grainy head and the steroid hormone ecdysone cooperate during differentiation of the skin of Drosophila melanogaster

The arthropod epidermis is an epithelium that deposits the apical cuticle, which is a stratified extracellular matrix (ECM) protecting the animal against pathogens, preventing dehydration and also serving as an exoskeleton. Differentiation of the cuticle conceivably implies coordinated production, secretion and localization of its components. The underlying molecular mechanisms are poorly explored. In this work, we show that the transcription factor Grainy head and the steroid hormone ecdysone drive the production of two partially overlapping sets of cuticle factors. Nevertheless, Grainy head is needed to modulate the expression of ecdysone signalling factors; the significance of this cross-talk is yet unclear. In addition, we found that ecdysone signalling negatively regulates its own impact. In conclusion, our findings suggest that at least two independently triggered pathways have evolved in parallel to cooperatively ensure the stereotypic implementation of the cuticle. As Grainy head is also essential for epithelial differentiation in vertebrates, we speculate that it acts to decode the ancient skin programme common to all animals. Full differentiation of the skin necessitates a second, complementing taxon-specific programme that requires its own decoder, which is represented by ecdysone in arthropods, whereas the vertebrate specific one remains to be identified.

The Grainyhead transcription factor Grhl3/Get1 suppresses miR-21 expression and tumorigenesis in skin: modulation of the miR-21 target MSH2 by RNA-binding protein DND1

Epidermal differentiation and stratification, crucial for barrier formation, are regulated by a complex interplay of transcription factors, including the evolutionarily conserved Grainyhead-like 3 (Grhl3/Get1); Grhl3-deleted mice exhibit impaired epidermal differentiation and decreased expression of multiple differentiation genes. To test whether Grhl3 regulates epidermal genes indirectly by controlling the expression of specific microRNAs (miRs), we performed miR profiling and identified 11 miRs that are differentially regulated in Grhl3(-/-) skin, one of which is miR-21, previously shown to be upregulated in diseased skin, including in psoriasis and squamous cell skin cancer. We found that miR-21 is normally expressed in the post-mitotic suprabasal layers of the epidermis, overlapping with Grhl3. The miR-21 promoter is bound and repressed by Grhl3 indicating that these two factors are involved in a regulatory loop maintaining homeostasis in the epidermis. Although miR-21 overexpression in normal keratinocytes had mild effects on the expression of several known miR-21 targets, an enhanced downregulation of the miR-21 tumor-related targets, including MSH2, was observed in Ras-transformed keratinocytes. The increased sensitivity of transformed keratinocytes to miR-21's effects occurs in part through downregulation of the RNA-binding protein DND1 during the transformation process. Additionally, we observed increased tumorigenesis in mice subcutaneously injected with transformed keratinocytes lacking Grhl3. These findings indicate that decreased Grhl3 expression contributes to tumor progression and upregulation of the oncomir miR-21 in squamous cell carcinoma of the skin.

The functions of grainy head-like proteins in animals and fungi and the evolution of apical extracellular barriers

The Grainy head (GRH) family of transcription factors are crucial for the development and repair of epidermal barriers in all animals in which they have been studied. This is a high-level functional conservation, as the known structural and enzymatic genes regulated by GRH proteins differ between species depending on the type of epidermal barrier being formed. Interestingly, members of the CP2 superfamily of transcription factors, which encompasses the GRH and LSF families in animals, are also found in fungi--organisms that lack epidermal tissues. To shed light on CP2 protein function in fungi, we characterized a Neurospora crassa mutant lacking the CP2 member we refer to as grainy head-like (grhl). We show that Neurospora GRHL has a DNA-binding specificity similar to that of animal GRH proteins and dissimilar to that of animal LSF proteins. Neurospora grhl mutants are defective in conidial-spore dispersal due to an inability to remodel the cell wall, and we show that grhl mutants and the long-known conidial separation-2 (csp-2) mutants are allelic. We then characterized the transcriptomes of both Neurospora grhl mutants and Drosophila grh mutant embryos to look for similarities in the affected genes. Neurospora grhl appears to play a role in the development and remodeling of the cell wall, as well as in the activation of genes involved in defense and virulence. Drosophila GRH is required to activate the expression of many genes involved in cuticular/epidermal-barrier formation. We also present evidence that GRH plays a role in adult antimicrobial defense. These results, along with previous studies of animal GRH proteins, suggest the fascinating possibility that the apical extracellular barriers of some animals and fungi might share an evolutionary connection, and that the formation of physical barriers in the last common ancestor was under the control of a transcriptional code that included GRH-like proteins.

The human epidermal differentiation complex: cornified envelope precursors, S100 proteins and the 'fused genes' family

  The skin is essential for survival and protects our body against biological attacks, physical stress, chemical injury, water loss, ultraviolet radiation and immunological impairment. The epidermal barrier constitutes the primordial frontline of this defense established during terminal differentiation. During this complex process proliferating basal keratinocytes become suprabasally mitotically inactive and move through four epidermal layers (basal, spinous, granular and layer, stratum corneum) constantly adapting to the needs of the respective cell layer. As a result, squamous keratinocytes contain polymerized keratin intermediate filament bundles and a water-retaining matrix surrounded by the cross-linked cornified cell envelope (CE) with ceramide lipids attached on the outer surface. These cells are concomitantly insulated by intercellular lipid lamellae and hold together by corneodesmosmes. Many proteins essential for epidermal differentiation are encoded by genes clustered on chromosomal human region 1q21. These genes constitute the 'epidermal differentiation complex' (EDC), which is divided on the basis of common gene and protein structures, in three gene families: (i) CE precursors, (ii) S100A and (iii) S100 fused genes. EDC protein expression is regulated in a gene and tissue-specific manner by a pool of transcription factors. Among them, Klf4, Grhl3 and Arnt are essential, and their deletion in mice is lethal. The importance of the EDC is further reflected by human diseases: FLG mutations are the strongest risk factor for atopic dermatitis (AD) and for AD-associated asthma, and faulty CE formation caused by TG1 deficiency causes life-threatening lamellar ichthyosis. Here, we review the EDC genes and the progress in this field.

Midbrain-hindbrain boundary patterning and morphogenesis are regulated by diverse grainy head-like 2-dependent pathways

The isthmic organiser located at the midbrain-hindbrain boundary (MHB) is the crucial developmental signalling centre responsible for patterning mesencephalic and metencephalic regions of the vertebrate brain. Formation and maintenance of the MHB is characterised by a hierarchical program of gene expression initiated by fibroblast growth factor 8 (Fgf8), coupled with cellular morphogenesis, culminating in the formation of the tectal-isthmo-cerebellar structures. Here, we show in zebrafish that one orthologue of the transcription factor grainy head-like 2 (Grhl2), zebrafish grhl2b plays a central role in both MHB maintenance and folding by regulating two distinct, non-linear pathways. Loss of grhl2b expression induces neural apoptosis and extinction of MHB markers, which are rescued by re-expression of engrailed 2a (eng2a), an evolutionarily conserved target of the Grhl family. Co-injection of sub-phenotypic doses of grhl2b and eng2a morpholinos reproduces the apoptosis and MHB marker loss, but fails to substantially disrupt formation of the isthmic constriction. By contrast, a novel direct grhl2b target, spec1, identified by phylogenetic analysis and confirmed by ChIP, functionally cooperates with grhl2b to induce MHB morphogenesis, but plays no role in apoptosis or maintenance of MHB markers. Collectively, these data show that MHB maintenance and morphogenesis are dissociable events regulated by grhl2b through diverse transcriptional targets.

Suppression of the epithelial-mesenchymal transition by Grainyhead-like-2

Grainyhead genes are involved in wound healing and developmental neural tube closure. In light of the high degree of similarity between the epithelial-mesenchymal transitions (EMT) occurring in wound-healing processes and the cancer stem cell-like compartment of tumors, including TGF-β dependence, we investigated the role of the Grainyhead gene, Grainyhead-like-2 (GRHL2) in oncogenic EMT. GRHL2 was downregulated specifically in the claudin-low subclass breast tumors and in basal-B subclass breast cancer cell lines. GRHL2 suppressed TGF-β-induced, Twist-induced or spontaneous EMT, enhanced anoikis sensitivity, and suppressed mammosphere generation in mammary epithelial cells. These effects were mediated in part by suppression of ZEB1 expression via direct repression of the ZEB1 promoter. GRHL2 also inhibited Smad-mediated transcription and it upregulated mir-200b/c as well as the TGF-β receptor antagonist, BMP2. Finally, ectopic expression of GRHL2 in MDA-MB-231 breast cancer cells triggered an MET and restored sensitivity to anoikis. Taken together, our findings define a major role for GRHL2 in the suppression of oncogenic EMT in breast cancer cells.

Patterning skin by planar cell polarity: the multi-talented hair designer

In mammals, the skin can form complex global and local patterns to meet diverse functional requirements in different parts of the body. To date, the fundamental principles that underlie skin patterning remain poorly understood because of the involvement of multiple interacting processes. Genes involved in the planar cell polarity (PCP) signalling pathway, which is capable of polarizing cells within the planar plane of an epithelium, can control the orientation and differentiation of hair follicles, underlining their involvement in skin pattern formation. Here, we summarize recent progress that has been made to understand the PCP signalling pathway and its function in mammalian skin, including its role in hair follicle morphogenesis, ciliogenesis and wound healing. We argue that dissecting PCP signalling in the context of hair follicle formation might reveal many as-yet-undiscovered functions for PCP in the development, homeostasis and regeneration of skin.

Grainy head and its target genes in epithelial morphogenesis and wound healing

The Grainy head (Grh) family of transcription factors is characterized by a unique DNA-binding domain that binds to a conserved consensus sequence. Nematodes and flies have a single grh gene, whereas mice and humans have evolved three genes encoding Grainy head-like (Grhl) factors. We review the biological function of Grh in different animals and the mechanisms modulating its activity. grh and grhl genes play a remarkably conserved role in epithelial organ development and extracellular barrier repair after tissue damage. Recent studies in flies and vertebrates suggest that Grh factors may be primary determinants of cell adhesion and epithelial tissue formation. Grh proteins can dimerize and act as activators or repressors in different developmental contexts. In flies, tissue-specific, alternative splicing generates different Grh isoforms with different DNA-binding specificities and functions. Grh activity is also modulated by receptor tyrosine kinases: it is phosphorylated by extracellular signal regulated kinase, and this phosphorylation is selectively required for epidermal barrier repair. Two mechanisms have been proposed to explain the repressive function of Grh on target gene transcription. First, Grh can target the Polycomb silencing complex to specific response elements. Second, it can directly compete for DNA binding with transcriptional activators. Understanding the molecular mechanisms of gene regulation by Grh factors is likely to elucidate phylogenetically conserved mechanisms of epithelial cell morphogenesis and regeneration upon tissue damage.

Duox, Flotillin-2, and Src42A are required to activate or delimit the spread of the transcriptional response to epidermal wounds in Drosophila

The epidermis is the largest organ of the body for most animals, and the first line of defense against invading pathogens. A breach in the epidermal cell layer triggers a variety of localized responses that in favorable circumstances result in the repair of the wound. Many cellular and genetic responses must be limited to epidermal cells that are close to wounds, but how this is regulated is still poorly understood. The order and hierarchy of epidermal wound signaling factors are also still obscure. The Drosophila embryonic epidermis provides an excellent system to study genes that regulate wound healing processes. We have developed a variety of fluorescent reporters that provide a visible readout of wound-dependent transcriptional activation near epidermal wound sites. A large screen for mutants that alter the activity of these wound reporters has identified seven new genes required to activate or delimit wound-induced transcriptional responses to a narrow zone of cells surrounding wound sites. Among the genes required to delimit the spread of wound responses are Drosophila Flotillin-2 and Src42A, both of which are transcriptionally activated around wound sites. Flotillin-2 and constitutively active Src42A are also sufficient, when overexpressed at high levels, to inhibit wound-induced transcription in epidermal cells. One gene required to activate epidermal wound reporters encodes Dual oxidase, an enzyme that produces hydrogen peroxide. We also find that four biochemical treatments (a serine protease, a Src kinase inhibitor, methyl-ß-cyclodextrin, and hydrogen peroxide) are sufficient to globally activate epidermal wound response genes in Drosophila embryos. We explore the epistatic relationships among the factors that induce or delimit the spread of epidermal wound signals. Our results define new genetic functions that interact to instruct only a limited number of cells around puncture wounds to mount a transcriptional response, mediating local repair and regeneration.

An epidermal wound provides signals that initiate a variety of localized responses, some of which act to regenerate and repair the breach in the epidermal barrier. The Drosophila melanogaster embryonic epidermis provides an excellent system to discover new genes that regulate wound-healing processes. Using fluorescent epidermal “wound” reporters that are locally activated around wound sites, we have screened almost 5,000 Drosophila mutants for functions required to activate or delimit wound-induced transcriptional responses to a local zone of epidermal cells. Among the seven new genes required to delimit the spread of wound responses are Flotillin-2 and Src42A. These two genes are also sufficient, when overexpressed at high levels, to inhibit wound-induced transcription in epidermal cells. One new gene required to activate epidermal wound reporters encodes Dual oxidase, an enzyme that produces hydrogen peroxide. We also find that four biochemical treatments (a serine protease, a Src kinase inhibitor, methyl-ß-cyclodextrin, and hydrogen peroxide) are sufficient to globally activate epidermal wound response genes in Drosophila embryos. Our results define new genetic functions, and the interactions among them, which regulate the local transcriptional response to puncture wounds.

Hole-in-one mutant phenotypes link EGFR/ERK signaling to epithelial tissue repair in Drosophila

Background

Epithelia act as physical barriers protecting living organisms and their organs from the surrounding environment. Simple epithelial tissues have the capacity to efficiently repair wounds through a resealing mechanism. The known molecular mechanisms underlying this process appear to be conserved in both vertebrates and invertebrates, namely the involvement of the transcription factors Grainy head (Grh) and Fos. In Drosophila, Grh and Fos lead to the activation of wound response genes required for epithelial repair. ERK is upstream of this pathway and known to be one of the first kinases to be activated upon wounding. However, it is still unclear how ERK activation contributes to a proper wound response and which molecular mechanisms regulate its activation.

Methodology/Principal Findings

In a previous screen, we isolated mutants with defects in wound healing. Here, we describe the role of one of these genes, hole-in-one (holn1), in the wound healing process. Holn1 is a GYF domain containing protein that we found to be required for the activation of several Grh and Fos regulated wound response genes at the wound site. We also provide evidence suggesting that Holn1 may be involved in the Ras/ERK signaling pathway, by acting downstream of ERK. Finally, we show that wound healing requires the function of EGFR and ERK signaling.

Conclusions/Significance

Based on these data, we conclude that holn1 is a novel gene required for a proper wound healing response. We further propose and discuss a model whereby Holn1 acts downstream of EGFR and ERK signaling in the Grh/Fos mediated wound closure pathway.

A Gαq-Ca²⁺ signaling pathway promotes actin-mediated epidermal wound closure in C. elegans

BACKGROUND

Repair of skin wounds is essential for animals to survive in a harsh environment, yet the signaling pathways initiating wound repair in vivo remain little understood. In Caenorhabditis elegans, a p38 mitogen-activated protein kinase (MAPK) cascade promotes innate immune responses to wounding but is not required for other aspects of wound healing. We therefore set out to identify additional wound response pathways in C. elegans epidermis.

RESULTS

We show here that wounding the adult C. elegans skin triggers a rapid and sustained rise in epidermal Ca(2+) that is critical for survival after wounding. The wound-triggered rise in Ca(2+) requires the epidermal transient receptor potential channel, melastatin family (TRPM) channel GTL-2 and IP(3)R-stimulated release from internal stores. We identify an epidermal signal transduction pathway that includes the Gα(q) EGL-30 and its effector PLCβ EGL-8. Loss of function in this pathway impairs survival after wounding. The Gα(q)-Ca(2+) pathway is not required for known innate immune responses to wounding but instead promotes actin-dependent wound closure. Wound closure requires the Cdc42 small GTPase and Arp2/3-dependent actin polymerization and is negatively regulated by Rho and nonmuscle myosin. Finally, we show that the death-associated protein kinase DAPK-1 acts as a negative regulator of wound closure.

CONCLUSIONS

Skin wounding in C. elegans triggers a Ca(2+)-dependent signaling cascade that promotes wound closure, in parallel to the innate immune response to damage. Wound closure requires actin polymerization and is negatively regulated by nonmuscle myosin.

Targeting of the tumor suppressor GRHL3 by a miR-21-dependent proto-oncogenic network results in PTEN loss and tumorigenesis

Despite its prevalence, the molecular basis of squamous cell carcinoma (SCC) remains poorly understood. Here, we identify the developmental transcription factor Grhl3 as a potent tumor suppressor of SCC in mice, and demonstrate that targeting of Grhl3 by a miR-21-dependent proto-oncogenic network underpins SCC in humans. Deletion of Grhl3 in adult epidermis evokes loss of expression of PTEN, a direct GRHL3 target, resulting in aggressive SCC induced by activation of PI3K/AKT/mTOR signaling. Restoration of Pten expression completely abrogates SCC formation. Reduced levels of GRHL3 and PTEN are evident in human skin, and head and neck SCC, associated with increased expression of miR-21, which targets both tumor suppressors. Our data define the GRHL3-PTEN axis as a critical tumor suppressor pathway in SCC.

Deconstructing host-pathogen interactions in Drosophila

Many of the cellular mechanisms underlying host responses to pathogens have been well conserved during evolution. As a result, Drosophila can be used to deconstruct many of the key events in host-pathogen interactions by using a wealth of well-developed molecular and genetic tools. In this review, we aim to emphasize the great leverage provided by the suite of genomic and classical genetic approaches available in flies for decoding details of host-pathogen interactions; these findings can then be applied to studies in higher organisms. We first briefly summarize the general strategies by which Drosophila resists and responds to pathogens. We then focus on how recently developed genome-wide RNA interference (RNAi) screens conducted in cells and flies, combined with classical genetic methods, have provided molecular insight into host-pathogen interactions, covering examples of bacteria, fungi and viruses. Finally, we discuss novel strategies for how flies can be used as a tool to examine how specific isolated virulence factors act on an intact host.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

Swatting flies: modelling wound healing and inflammation in Drosophila

Aberrant wound healing can lead to a variety of human pathologies, from non-healing chronic wounds that can become dangerously infected, to exuberant fibrotic healing in which repair is accompanied by excessive inflammation. To guide therapeutic intervention, we need a better understanding of the fundamental mechanisms driving tissue repair; this will require complementary wound-healing studies in several model organisms. Drosophila has been used to model genetic aspects of numerous human pathologies, and is being used increasingly to gain insight into the molecular and genetic aspects of tissue repair and inflammation, which have classically been modelled in mice or cultured cells. This review discusses the advantages and disadvantages of Drosophila as a wound-healing model, as well as some exciting new research opportunities that will be enabled by its use.

The transcription factor Grainyhead like 3 (GRHL3) affects endothelial cell apoptosis and migration in a NO-dependent manner

Migratory capacity and resistance to apoptosis are crucial for proper endothelial function. In a screen for anti-apoptotic genes in a breast cancer cell line, we identified Grainyhead like 3 (GRHL3). Therefore, the aim of our study was to investigate whether GRHL3 is expressed in endothelial cells and moreover, to determine its role in migration, apoptosis and senescence. GRHL3 is expressed in human endothelial cells. GRHL3 is required for endothelial cell migration. The underlying mechanism is independent of vascular endothelial growth factor. GRHL3 induces Akt and endothelial nitric oxide synthase phosphorylation and its expression is increased by physiological concentrations of nitric oxide. Nitric oxide dependent migration is completely dependent on GRHL3 expression. Moreover, GRHL3 inhibits apoptosis of endothelial cells in an eNOS-dependent manner. Thus, loss of GRHL3 may result in endothelial dysfunction in vivo. One may consider new therapeutic strategies with the aim to conserve GRHL3 expression in the vasculature.

Kruppel-like factor 5 is required for formation and differentiation of the bladder urothelium

Kruppel-like transcription factor 5 (Klf5) was detected in the developing and mature murine bladder urothelium. Herein we report a critical role of KLF5 in the formation and terminal differentiation of the urothelium. The Shh(GfpCre) transgene was used to delete the Klf5(floxed) alleles from bladder epithelial cells causing prenatal hydronephrosis, hydroureter, and vesicoureteric reflux. The bladder urothelium failed to stratify and did not express terminal differentiation markers characteristic of basal, intermediate, and umbrella cells including keratins 20, 14, and 5, and the uroplakins. The effects of Klf5 deletion were unique to the developing bladder epithelium since maturation of the epithelium comprising the bladder neck and urethra was unaffected by the lack of KLF5. mRNA analysis identified reductions in Pparγ, Grhl3, Elf3, and Ovol1expression in Klf5 deficient fetal bladders supporting their participation in a transcriptional network regulating bladder urothelial differentiation. KLF5 regulated expression of the mGrhl3 promoter in transient transfection assays. The absence of urothelial Klf5 altered epithelial-mesenchymal signaling leading to the formation of an ectopic alpha smooth muscle actin positive layer of cells subjacent to the epithelium and a thinner detrusor muscle that was not attributable to disruption of SHH signaling, a known mediator of detrusor morphogenesis. Deletion of Klf5 from the developing bladder urothelium blocked epithelial cell differentiation, impaired bladder morphogenesis and function causing hydroureter and hydronephrosis at birth.

Wound repair: toward understanding and integration of single-cell and multicellular wound responses

The importance of wound healing to medicine and biology has long been evident, and consequently, wound healing has been the subject of intense investigation for many years. However, several relatively recent developments have added new impetus to wound repair research: the increasing application of model systems; the growing recognition that single cells have a robust, complex, and medically relevant wound healing response; and the emerging recognition that different modes of wound repair bear an uncanny resemblance to other basic biological processes such as morphogenesis and cytokinesis. In this review, each of these developments is described, and their significance for wound healing research is considered. In addition, overlapping mechanisms of single-cell and multicellular wound healing are highlighted, and it is argued that they are more similar than is often recognized. Based on this and other information, a simple model to explain the evolutionary relationships of cytokinesis, single-cell wound repair, multicellular wound repair, and developmental morphogenesis is proposed. Finally, a series of important, but as yet unanswered, questions is posed.

Evolution of gene regulatory networks controlling body plan development

Evolutionary change in animal morphology results from alteration of the functional organization of the gene regulatory networks (GRNs) that control development of the body plan. A major mechanism of evolutionary change in GRN structure is alteration of cis-regulatory modules that determine regulatory gene expression. Here we consider the causes and consequences of GRN evolution. Although some GRN subcircuits are of great antiquity, other aspects are highly flexible and thus in any given genome more recent. This mosaic view of the evolution of GRN structure explains major aspects of evolutionary process, such as hierarchical phylogeny and discontinuities of paleontological change.

Grhl2 deficiency impairs otic development and hearing ability in a zebrafish model of the progressive dominant hearing loss DFNA28

Congenital and progressive hearing impairment is a common distressing disease. The progressive dominant hearing loss DFNA28 in human is associated with a frameshift mutation of Grainyhead-like 2 (GRHL2) but its etiology and mechanism remain unknown. Here we report a zebrafish grhl2b(T086) mutant line in which grhl2b expression is interrupted by an insertion of a Tol2 transposon element. The mutants exhibit enlarged otocysts, smaller or eliminated otoliths, malformed semicircular canals, insensitiveness to sound stimulation and imbalanced swimming motion. Since grainyhead-like family members can regulate epithelial adhesion, we examined the expression of some genes encoding junction proteins in mutants. We show that the expression of claudin b (cldnb) and epcam is abolished or dramatically reduced and apical junctional complexes are abnormal in otic epithelial cells of mutant embryos. Co-injection of cldnb and epcam mRNA could largely rescue the mutant phenotype. Injection of human wild-type GRHL2 mRNA but not the mutant GRHL2 mRNA derived from DFNA28 patients into grhl2b(T086) mutant embryos could rescue the inner-ear defects. Furthermore, we demonstrate that Grhl2b directly binds to the enhancers and promotes the expression of cldnb and epcam. Thus, this work reveals an evolutionarily conserved function of Grhl2 in otic development and provides a fish model for further studying mechanisms of Grhl2-related hearing loss.

Pointing in the right direction: new developments in the field of planar cell polarity

Planar cell polarity (PCP) is observed in an array of developmental processes that involve collective cell movement and tissue organization, and its disruption can lead to severe developmental defects. Recent studies in flies and vertebrates have identified new functions for PCP as well as new signalling components, and have proposed new mechanistic models. However, despite this progress, the search to simplify principles of understanding continues and important mechanistic uncertainties still pose formidable challenges.

Over-expression of Grhl2 causes spina bifida in the Axial defects mutant mouse

Cranial neural tube defects (NTDs) occur in mice carrying mutant alleles of many different genes, whereas isolated spinal NTDs (spina bifida) occur in fewer models, despite being common human birth defects. Spina bifida occurs at high frequency in the Axial defects (Axd) mouse mutant but the causative gene is not known. In the current study, the Axd mutation was mapped by linkage analysis. Within the critical genomic region, sequencing did not reveal a coding mutation whereas expression analysis demonstrated significant up-regulation of grainyhead-like 2 (Grhl2) in Axd mutant embryos. Expression of other candidate genes did not differ between genotypes. In order to test the hypothesis that over-expression of Grhl2 causes Axd NTDs, we performed a genetic cross to reduce Grhl2 function in Axd heterozygotes. Grhl2 loss of function mutant mice were generated and displayed both cranial and spinal NTDs. Compound heterozygotes carrying both loss (Grhl2 null) and putative gain of function (Axd) alleles exhibited normalization of spinal neural tube closure compared with Axd/+ littermates, which exhibit delayed closure. Grhl2 is expressed in the surface ectoderm and hindgut endoderm in the spinal region, overlapping with grainyhead-like 3 (Grhl3). Axd mutants display delayed eyelid closure, as reported in Grhl3 null embryos. Moreover, Axd mutant embryos exhibited increased ventral curvature of the spinal region and reduced proliferation in the hindgut, reminiscent of curly tail embryos, which carry a hypomorphic allele of Grhl3. Overall, our data suggest that defects in Axd mutant embryos result from over-expression of Grhl2.

PAR2 absence completely rescues inflammation and ichthyosis caused by altered CAP1/Prss8 expression in mouse skin

Altered serine protease activity is associated with skin disorders in humans and in mice. The serine protease channel-activating protease-1 (CAP1; also termed protease serine S1 family member 8 (Prss8)) is important for epidermal homeostasis and is thus indispensable for postnatal survival in mice, but its roles and effectors in skin pathology are poorly defined. In this paper, we report that transgenic expression in mouse skin of either CAP1/Prss8 (K14-CAP1/Prss8) or protease-activated receptor-2 (PAR2; Grhl3^PAR2/+), one candidate downstream target, causes epidermal hyperplasia, ichthyosis and itching. K14-CAP1/Prss8 ectopic expression impairs epidermal barrier function and causes skin inflammation characterized by an increase in thymic stromal lymphopoietin levels and immune cell infiltrations. Strikingly, both gross and functional K14-CAP1/Prss8-induced phenotypes are completely negated when superimposed on a PAR2-null background, establishing PAR2 as a pivotal mediator of pathogenesis. Our data provide genetic evidence for PAR2 as a downstream effector of CAP1/Prss8 in a signalling cascade that may provide novel therapeutic targets for ichthyoses, pruritus and inflammatory skin diseases.

The activity of serine proteases, including CAP1/Prss8, is altered in some human skin disorders; however, the downstream effectors of these proteins are relatively unknown. Here, using animal models, the authors show that protease-activated receptor-2 is a critical component of the CAP1/Prss8 signalling cascade.

Phosphorylation of Grainy head by ERK is essential for wound-dependent regeneration but not for development of an epidermal barrier

Grainy head (GRH) is a key transcription factor responsible for epidermal barrier formation and repair, whose function is highly conserved across diverse animal species. However, it is not known how GRH function is reactivated to repair differentiated epidermal barriers after wounding. Here, we show that GRH is directly regulated by extracellular signal-regulated kinase (ERK) phosphorylation, which is required for wound-dependent expression of GRH target genes in epidermal cells. Serine 91 is the principal residue in GRH that is phosphorylated by ERK. Although mutations of the ERK phosphorylation sites in GRH do not impair its DNA binding function, the ERK sites in GRH are required to activate Dopa decarboxylase (Ddc) and misshapen (msn) epidermal wound enhancers as well as functional regeneration of an epidermal barrier upon wounding. This result indicates that the phosphorylation sites are essential for damaged epidermal barrier repair. However, GRH with mutant ERK phosphorylation sites can still promote barrier formation during embryonic epidermal development, suggesting that ERK sites are dispensable for the GRH function in establishing epidermal barrier integrity. These results provide mechanistic insight into how tissue repair can be initiated by posttranslational modification of a key transcription factor that normally mediates the developmental generation of that tissue.

The unique and cooperative roles of the Grainy head-like transcription factors in epidermal development reflect unexpected target gene specificity

The Grainy head-like 3 (Grhl3) gene encodes a transcription factor that plays essential roles in epidermal morphogenesis during embryonic development, with deficient mice exhibiting failed skin barrier formation, defective wound repair, and loss of eyelid fusion. Despite sharing significant sequence homology, overlapping expression patterns, and an identical core consensus DNA binding site, the other members of the Grhl family (Grhl1 and -2) fail to compensate for the loss of Grhl3 in these processes. Here, we have employed diverse genetic models, coupled with biochemical studies, to define the inter-relationships of the Grhl factors in epidermal development. We show that Grhl1 and Grhl3 have evolved complete functional independence, as evidenced by a lack of genetic interactions in embryos carrying combinations of targeted alleles of these genes. In contrast, compound heterozygous Grhl2/Grhl3 embryos displayed failed wound repair, and loss of a single Grhl2 allele in Grhl3-null embryos results in fully penetrant eyes open at birth. Expression of Grhl2 from the Grhl3 locus in homozygous knock-in mice corrects the wound repair defect, but these embryos still display a complete failure of skin barrier formation. This functional dissociation is due to unexpected differences in target gene specificity, as both GRHL2 and GRHL3 bind to and regulate expression of the wound repair gene Rho GEF 19, but regulation of the barrier forming gene, Transglutaminase 1 (TGase1), is unique to GRHL3. Our findings define the mechanisms underpinning the unique and cooperative roles of the Grhl genes in epidermal development.

The transcription factor grainyhead-like 2 regulates the molecular composition of the epithelial apical junctional complex

Differentiation of epithelial cells and morphogenesis of epithelial tubes or layers is closely linked with the establishment and remodeling of the apical junctional complex, which includes adherens junctions and tight junctions. Little is known about the transcriptional control of apical junctional complex components. Here, we show that the transcription factor grainyhead-like 2 (Grhl2), an epithelium-specific mammalian homolog of Drosophila Grainyhead, is essential for adequate expression of the adherens junction gene E-cadherin and the tight junction gene claudin 4 (Cldn4) in several types of epithelia, including gut endoderm, surface ectoderm and otic epithelium. We have generated Grhl2 mutant mice to demonstrate defective molecular composition of the apical junctional complex in these compartments that coincides with the occurrence of anterior and posterior neural tube defects. Mechanistically, we show that Grhl2 specifically associates with cis-regulatory elements localized at the Cldn4 core promoter and within intron 2 of the E-cadherin gene. Cldn4 promoter activity in epithelial cells is crucially dependent on the availability of Grhl2 and on the integrity of the Grhl2-associated cis-regulatory element. At the E-cadherin locus, the intronic Grhl2-associated cis-regulatory region contacts the promoter via chromatin looping, while loss of Grhl2 leads to a specific decrease of activating histone marks at the E-cadherin promoter. Together, our data provide evidence that Grhl2 acts as a target gene-associated transcriptional activator of apical junctional complex components and, thereby, crucially participates in epithelial differentiation.

Protein crosslinking by transglutaminase controls cuticle morphogenesis in Drosophila

Transglutaminase (TG) plays important and diverse roles in mammals, such as blood coagulation and formation of the skin barrier, by catalyzing protein crosslinking. In invertebrates, TG is known to be involved in immobilization of invading pathogens at sites of injury. Here we demonstrate that Drosophila TG is an important enzyme for cuticle morphogenesis. Although TG activity was undetectable before the second instar larval stage, it dramatically increased in the third instar larval stage. RNA interference (RNAi) of the TG gene caused a pupal semi-lethal phenotype and abnormal morphology. Furthermore, TG-RNAi flies showed a significantly shorter life span than their counterparts, and approximately 90% of flies died within 30 days after eclosion. Stage-specific TG-RNAi before the third instar larval stage resulted in cuticle abnormality, but the TG-RNAi after the late pupal stage did not, indicating that TG plays a key role at or before the early pupal stage. Immediately following eclosion, acid-extractable protein from wild-type wings was nearly all converted to non-extractable protein due to wing maturation, whereas several proteins remained acid-extractable in the mature wings of TG-RNAi flies. We identified four proteins—two cuticular chitin-binding proteins, larval serum protein 2, and a putative C-type lectin—as TG substrates. RNAi of their corresponding genes caused a lethal phenotype or cuticle abnormality. Our results indicate that TG-dependent protein crosslinking in Drosophila plays a key role in cuticle morphogenesis and sclerotization.

Grainyhead-like 2 enhances the human telomerase reverse transcriptase gene expression by inhibiting DNA methylation at the 5'-CpG island in normal human keratinocytes

We recently identified Grainyhead-like 2 (GRHL2) as a novel transcription factor that binds to and regulates the activity of the human telomerase reverse transcriptase (hTERT) gene promoter. In this study, we investigated the biological functions of GRHL2 and the molecular mechanism underlying hTERT gene regulation by GRHL2. Retroviral transduction of GRHL2 in normal human keratinocytes (NHK) led to a significant extension of replicative life span, whereas GRHL2 knockdown notably repressed telomerase activity and cell proliferation. Using promoter magnetic precipitation coupled with Western blotting, we confirmed the binding of GRHL2 to the hTERT promoter and mapped the minimal binding region at -53 to -13 of the promoter. Furthermore, mutation analysis revealed the three nucleotides from -21 to -19 to be critical for GRHL2 binding. Because hTERT expression is regulated in part by DNA methylation, we determined the effects of GRHL2 on the methylation status of the hTERT promoter. Senescent NHK exhibited hypermethylation of the CpG island, which occurred with the loss of hTERT expression. On the contrary, the promoter remained hypomethylated in GRHL2-transduced NHK, irrespective of cell proliferation status. Also, knockdown of endogenous GRHL2 led to hypermethylation of the promoter. These results indicate that GRHL2 regulates the hTERT expression through an epigenetic mechanism and controls the cellular life span.

Epidermal wound repair is regulated by the planar cell polarity signaling pathway

The mammalian PCP pathway regulates diverse developmental processes requiring coordinated cellular movement, including neural tube closure and cochlear stereociliary orientation. Here, we show that epidermal wound repair is regulated by PCP signaling. Mice carrying mutant alleles of PCP genes Vangl2, Celsr1, PTK7, and Scrb1, and the transcription factor Grhl3, interact genetically, exhibiting failed wound healing, neural tube defects, and disordered cochlear polarity. Using phylogenetic analysis, ChIP, and gene expression in Grhl3(-)(/-) mice, we identified RhoGEF19, a homolog of a RhoA activator involved in PCP signaling in Xenopus, as a direct target of GRHL3. Knockdown of Grhl3 or RhoGEF19 in keratinocytes induced defects in actin polymerization, cellular polarity, and wound healing, and re-expression of RhoGEF19 rescued these defects in Grhl3-kd cells. These results define a role for Grhl3 in PCP signaling and broadly implicate this pathway in epidermal repair.

Zebrafish grainyhead-like1 is a common marker of different non-keratinocyte epidermal cell lineages, which segregate from each other in a Foxi3-dependent manner

Grainyhead/CP2 transcription factor family members are widely conserved among the animal kingdom and have been implicated in different developmental processes. Thus far, nothing has been known about their roles in zebrafish. Here we identify seven zebrafish grainyhead-like (grhl) / cp2 genes, with focus on grhl1, which is expressed in the periderm and in epidermal ionocyte progenitors, but downregulated when ionocytes differentiate. In addition, expression was detected in other "non-keratinocyte" cell types of the epidermis, such as pvalb8-expressing cells, which according to our lineage tracing experiments are derived from the same pool of progenitor cells like keratinocytes and ionocytes. Antisense morpholino oligonucleotide-based loss-of-function analysis revealed that grhl1 is dispensable for the development and function of all investigated epidermal cell types, but required as a negative regulator of its own transcription during ionocyte differentiation. Knockdown of the transcription factor Foxi3a, which is expressed in a subset of the grhl1 population, caused a loss of ionocytes and a corresponding increase in the number of pvalb8-expressing cells, while leaving the number of grhl1-positive cells unaltered. We propose that grhl1 is a novel common marker of all or most "non-keratinocyte" epidermal progenitors, and that the sub-functionalisation of these cells is regulated by differential positive and negative effects of Foxi3 factors.

Nonsteroidal anti-inflammatory drugs for wounds: pain relief or excessive scar formation?

The inflammatory process has direct effects on normal and abnormal wound healing. Hypertrophic scar formation is an aberrant form of wound healing and is an indication of an exaggerated function of fibroblasts and excess accumulation of extracellular matrix during wound healing. Two cytokines--transforming growth factor-beta (TGF-beta) and prostaglandin E2 (PGE2)--are lipid mediators of inflammation involving wound healing. Overproduction of TGF-beta and suppression of PGE2 are found in excessive wound scarring compared with normal wound healing. Nonsteroidal anti-inflammatory drugs (NSAIDs) or their selective cyclooxygenase-2 (COX-2) inhibitors are frequently used as a pain-killer. However, both NSAIDs and COX-2 inhibitors inhibit PGE2 production, which might exacerbate excessive scar formation, especially when used during the later proliferative phase. Therefore, a balance between cytokines and medication in the pathogenesis of wound healing is needed. This report is a literature review pertaining to wound healing and is focused on TGF-beta and PGE2.

Grainyhead and Zelda compete for binding to the promoters of the earliest-expressed Drosophila genes

Maternally contributed mRNAs and proteins control the initial stages of development following fertilization. During this time, most of the zygotic genome remains transcriptionally silent. The initiation of widespread zygotic transcription is coordinated with the degradation of maternally provided mRNAs at the maternal-to-zygotic transition (MZT). While most of the genome is silenced prior to the MZT, a small subset of zygotic genes essential for the future development of the organism is transcribed. Previous work in our laboratory and others identified the TAGteam element, a set of related heptameric DNA-sequences in the promoters of many early-expressed Drosophila genes required to drive their unusually early transcription. To understand how this unique subset of genes is regulated, we identified a TAGteam-binding factor Grainyhead (Grh). We demonstrated that Grh and the previously characterized transcriptional activator Zelda (Zld) bind to different TAGteam sequences with varying affinities, and that Grh competes with Zld for TAGteam occupancy. Moreover, overexpression of Grh in the early embryo causes defects in cell division, phenocopying Zld depletion. Our findings indicate that during early embryonic development the precise timing of gene expression is regulated by both the sequence of the TAGteam elements in the promoter and the relative levels of the transcription factors Grh and Zld.

The evolutionary diversification of LSF and Grainyhead transcription factors preceded the radiation of basal animal lineages

Background

The transcription factors of the LSF/Grainyhead (GRH) family are characterized by the possession of a distinctive DNA-binding domain that bears no clear relationship to other known DNA-binding domains, with the possible exception of the p53 core domain. In triploblastic animals, the LSF and GRH subfamilies have diverged extensively with respect to their biological roles, general expression patterns, and mechanism of DNA binding. For example, Grainyhead (GRH) homologs are expressed primarily in the epidermis, and they appear to play an ancient role in maintaining the epidermal barrier. By contrast, LSF homologs are more widely expressed, and they regulate general cellular functions such as cell cycle progression and survival in addition to cell-lineage specific gene expression.

Results

To illuminate the early evolution of this family and reconstruct the functional divergence of LSF and GRH, we compared homologs from 18 phylogenetically diverse taxa, including four basal animals (Nematostella vectensis, Vallicula multiformis, Trichoplax adhaerens, and Amphimedon queenslandica), a choanoflagellate (Monosiga brevicollis) and several fungi. Phylogenetic and bioinformatic analyses of these sequences indicate that (1) the LSF/GRH gene family originated prior to the animal-fungal divergence, and (2) the functional diversification of the LSF and GRH subfamilies occurred prior to the divergence between sponges and eumetazoans. Aspects of the domain architecture of LSF/GRH proteins are well conserved between fungi, choanoflagellates, and metazoans, though within the Metazoa, the LSF and GRH families are clearly distinct. We failed to identify a convincing LSF/GRH homolog in the sequenced genomes of the algae Volvox carteri and Chlamydomonas reinhardtii or the amoebozoan Dictyostelium purpureum. Interestingly, the ancestral GRH locus has become split into two separate loci in the sea anemone Nematostella, with one locus encoding a DNA binding domain and the other locus encoding the dimerization domain.

Conclusions

In metazoans, LSF and GRH proteins play a number of roles that are essential to achieving and maintaining multicellularity. It is now clear that this protein family already existed in the unicellular ancestor of animals, choanoflagellates, and fungi. However, the diversification of distinct LSF and GRH subfamilies appears to be a metazoan invention. Given the conserved role of GRH in maintaining epithelial integrity in vertebrates, insects, and nematodes, it is noteworthy that the evolutionary origin of Grh appears roughly coincident with the evolutionary origin of the epithelium.

Genetic screen in Drosophila melanogaster uncovers a novel set of genes required for embryonic epithelial repair

The wound healing response is an essential mechanism to maintain the integrity of epithelia and protect all organisms from the surrounding milieu. In the "purse-string" mechanism of wound closure, an injured epithelial sheet cinches its hole closed via an intercellular contractile actomyosin cable. This process is conserved across species and utilized by both embryonic as well as adult tissues, but remains poorly understood at the cellular level. In an effort to identify new players involved in purse-string wound closure we developed a wounding strategy suitable for screening large numbers of Drosophila embryos. Using this methodology, we observe wound healing defects in Jun-related antigen (encoding DJUN) and scab (encoding Drosophila alphaPS3 integrin) mutants and performed a forward genetics screen on the basis of insertional mutagenesis by transposons that led to the identification of 30 lethal insertional mutants with defects in embryonic epithelia repair. One of the mutants identified is an insertion in the karst locus, which encodes Drosophila beta(Heavy)-spectrin. We show beta(Heavy)-spectrin (beta(H)) localization to the wound edges where it presumably exerts an essential function to bring the wound to normal closure.

The epidermal differentiation-associated Grainyhead gene Get1/Grhl3 also regulates urothelial differentiation

Skin and bladder epithelia form effective permeability barriers through the activation of distinct differentiation gene programs. Using a genome-wide gene-expression study, we identified transcriptional regulators whose expression correlates highly with that of differentiation markers in both the bladder and skin, including the Grainyhead factor Get1/Grhl3, which is already known to be important for epidermal barrier formation. In the bladder, Get1 is most highly expressed in the differentiated umbrella cells and its mutation in mice leads to a defective bladder epithelial barrier formation due to the failure of apical membrane specialization. Genes encoding components of the specialized urothelial membrane, the uroplakins, were downregulated in Get1(-/-) mice. At least one of these genes, uroplakin II, is a direct target of Get1. The urothelial-specific activation of the uroplakin II gene is due to selective binding of Get1 to the uroplakin II promoter in urothelial cells, which is most likely regulated by histone modifications. These results show a crucial role for Get1 in urothelial differentiation and barrier formation.

The tyrosine kinase Stitcher activates Grainy head and epidermal wound healing in Drosophila

Epidermal injury initiates a cascade of inflammation, epithelial remodelling and integument repair at wound sites. The regeneration of the extracellular barrier and damaged tissue repair rely on the precise orchestration of epithelial responses triggered by the injury. Grainy head (Grh) transcription factors induce gene expression to crosslink the extracellular barrier in wounded flies and mice. However, the activation mechanisms and functions of Grh factors in re-epithelialization remain unknown. Here we identify stitcher (stit), a new Grh target in Drosophila melanogaster. stit encodes a Ret-family receptor tyrosine kinase required for efficient epidermal wound healing. Live imaging analysis reveals that Stit promotes actin cable assembly during wound re-epithelialization. Stit activation also induces extracellular signal-regulated kinase (ERK) phosphorylation along with the Grh-dependent expression of stit and barrier repair genes at the wound sites. The transcriptional stimulation of stit on injury triggers a positive feedback loop increasing the magnitude of epithelial responses. Thus, Stit activation upon wounding coordinates cytoskeletal rearrangements and the level of Grh-mediated transcriptional wound responses.

Modeling neural tube defects in the mouse

Neural tube defects (NTDs) are among the most common structural birth defects observed in humans. Mouse models provide an excellent experimental system to study the underlying causes of NTDs. These models not only allow for identification of the genes required for neurulation, they provide tractable systems for uncovering the developmental, pathological and molecular mechanisms underlying NTDs. In addition, mouse models are essential for elucidating the mechanisms of gene-environment and gene-gene interactions that contribute to the multifactorial inheritance of NTDs. In some cases these studies have led to development of approaches to prevent NTDs and provide an understanding of the underlying molecular mechanism of these therapies prevent NTDs.

Scab formation and wound healing of plant tissue by soldier aphid

In the social aphid Nipponaphis monzeni, a unique gall-repairing behaviour has been known: when a hole is made on the gall, many soldier nymphs discharge body fluid on the breach, which promptly solidifies and plugs the hole. Here, we experimentally investigated the subsequent fate of repaired galls and their inhabitants. Irrespective of natural repair by soldier nymphs or artificial repair with adhesive, repaired galls survived significantly better than non-repaired galls. Within a month after repair, the plant tissue around the hole proliferated and sealed up the hole. Many soldier nymphs were localized at the hole area and extermination of inhabiting aphids by insecticides aborted the gall regeneration, indicating that the gall regeneration requires inhabiting aphids, wherein soldier nymphs are likely to play a major role. This study provides an unprecedented case of scab formation and wound healing, which occurs at an animal-plant interface: scab derived from insect body fluid promptly plugs damaged plant tissue and subsequently the insects actively stimulate regeneration of the plant tissue, whereby the compromised plant tissue recovers. We suggest that the novel system may have evolved in the aphid lineage through enhancement and recruitment of the pre-existing capabilities of haemolymph coagulation and gall formation.

Multiple transcription factor codes activate epidermal wound-response genes in Drosophila

Wounds in Drosophila and mouse embryos induce similar genetic pathways to repair epidermal barriers. However, the transcription factors that transduce wound signals to repair epidermal barriers are largely unknown. We characterize the transcriptional regulatory enhancers of 4 genes-Ddc, ple, msn, and kkv-that are rapidly activated in epidermal cells surrounding wounds in late Drosophila embryos and early larvae. These epidermal wound enhancers all contain evolutionarily conserved sequences matching binding sites for JUN/FOS and GRH transcription factors, but vary widely in trans- and cis-requirements for these inputs and their binding sites. We propose that the combination of GRH and FOS is part of an ancient wound-response pathway still used in vertebrates and invertebrates, but that other mechanisms have evolved that result in similar transcriptional output. A common, but largely untested assumption of bioinformatic analyses of gene regulatory networks is that transcription units activated in the same spatial and temporal patterns will require the same cis-regulatory codes. Our results indicate that this is an overly simplistic view.