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

Dual role of GRHL3 in bladder carcinogenesis depending on histological subtypes

The effect of grainyhead-like transcription factor 3 (GRHL3) on cancer development depends on the cancer subtypes as shown in tumor entities such as colorectal or oral squamous cell carcinomas. Here, we analyzed the subtype-specific role of GRHL3 in bladder carcinogenesis, comparing common urothelial carcinoma (UC) with squamous bladder cancer (sq-BLCA). We examined GRHL3 mRNA and protein expression in cohorts of patient samples, its prognostic role and its functional impact on tumorigeneses in different molecular and histopathological subtypes of bladder cancer. We showed for GRHL3 a reverse expression in squamous and urothelial bladder cancer subtypes. Stably GRHL3-overexpressing EJ28, J82, and SCaBER in vitro models revealed a tumor-suppressive function in squamous and an oncogenic role in the urothelial cancer cells affecting cell and colony growth, and migratory and invasive capacities. Transcriptomic profiling demonstrated highly subtype-specific GRHL3-regulated expression networks coined by the enrichment of genes involved in integrin-mediated pathways. In SCaBER, loss of ras homolog family member A (RHOA) GTPase activity was demonstrated to be associated with co-regulation of eukaryotic translation initiation factor 4E family member 3 (EIF4E3), a potential tumor suppressor gene. Thus, our data provide for the first time a detailed insight into the role of the transcription factor GRHL3 in different histopathological subtypes of bladder cancer.

Grainyhead-like 2 interacts with noggin to regulate tissue fusion in mouse

Defective tissue fusion during mammalian embryogenesis results in congenital anomalies, such as exencephaly, spina bifida and cleft lip and/or palate. The highly conserved transcription factor grainyhead-like 2 (Grhl2) is a crucial regulator of tissue fusion, with mouse models lacking GRHL2 function presenting with a fully penetrant open cranial neural tube, facial and abdominal clefting (abdominoschisis), and an open posterior neuropore. Here, we show that GRHL2 interacts with the soluble morphogen protein and bone morphogenetic protein (BMP) inhibitor noggin (NOG) to impact tissue fusion during development. The maxillary prominence epithelium in embryos lacking Grhl2 shows substantial morphological abnormalities and significant upregulation of NOG expression, together with aberrantly distributed pSMAD5-positive cells within the neural crest cell-derived maxillary prominence mesenchyme, indicative of disrupted BMP signalling. Reducing this elevated NOG expression (by generating Grhl2-/-;Nog+/- embryos) results in delayed embryonic lethality, partial tissue fusion rescue, and restoration of tissue form within the craniofacial epithelia. These data suggest that aberrant epithelial maintenance, partially regulated by noggin-mediated regulation of BMP-SMAD pathways, may underpin tissue fusion defects in Grhl2-/- mice.

Differential Occupancy and Regulatory Interactions of KDM6A in Bladder Cell Lines

Epigenetic deregulation is a critical theme which needs further investigation in bladder cancer research. One of the most highly mutated genes in bladder cancer is KDM6A, which functions as an H3K27 demethylase and is one of the MLL3/4 complexes. To decipher the role of KDM6A in normal versus tumor settings, we identified the genomic landscape of KDM6A in normal, immortalized, and cancerous bladder cells. Our results showed differential KDM6A occupancy in the genes involved in cell differentiation, chromatin organization, and Notch signaling depending on the cell type and the mutation status of KDM6A. Transcription factor motif analysis revealed HES1 to be enriched at KDM6A peaks identified in the T24 bladder cancer cell line; moreover, it has a truncating mutation in KDM6A and lacks a demethylase domain. Our co-immunoprecipitation experiments revealed TLE co-repressors and HES1 as potential truncated and wild-type KDM6A interactors. With the aid of structural modeling, we explored how truncated KDM6A could interact with TLE and HES1, as well as RUNX and HHEX transcription factors. These structures provide a solid means of studying the functions of KDM6A independently of its demethylase activity. Collectively, our work provides important contributions to the understanding of KDM6A malfunction in bladder cancer.

C. elegans molting requires rhythmic accumulation of the Grainyhead/LSF transcription factor GRH-1

C. elegans develops through four larval stages that are rhythmically terminated by molts, that is, the synthesis and shedding of a cuticular exoskeleton. Each larval cycle involves rhythmic accumulation of thousands of transcripts, which we show here relies on rhythmic transcription. To uncover the responsible gene regulatory networks (GRNs), we screened for transcription factors that promote progression through the larval stages and identified GRH-1, BLMP-1, NHR-23, NHR-25, MYRF-1, and BED-3. We further characterize GRH-1, a Grainyhead/LSF transcription factor, whose orthologues in other animals are key epithelial cell-fate regulators. We find that GRH-1 depletion extends molt durations, impairs cuticle integrity and shedding, and causes larval death. GRH-1 is required for, and accumulates prior to, each molt, and preferentially binds to the promoters of genes expressed during this time window. Binding to the promoters of additional genes identified in our screen furthermore suggests that we have identified components of a core molting-clock GRN. Since the mammalian orthologues of GRH-1, BLMP-1 and NHR-23, have been implicated in rhythmic homeostatic skin regeneration in mouse, the mechanisms underlying rhythmic C. elegans molting may apply beyond nematodes.

GRHL2-controlled gene expression networks in luminal breast cancer

Grainyhead like 2 (GRHL2) is an essential transcription factor for development and function of epithelial tissues. It has dual roles in cancer by supporting tumor growth while suppressing epithelial to mesenchymal transitions (EMT). GRHL2 cooperates with androgen and estrogen receptors (ER) to regulate gene expression. We explore genome wide GRHL2 binding sites conserved in three ER⍺/GRHL2 positive luminal breast cancer cell lines by ChIP-Seq. Interaction with the ER⍺/FOXA1/GATA3 complex is observed, however, only for a minor fraction of conserved GRHL2 peaks. We determine genome wide transcriptional dynamics in response to loss of GRHL2 by nascent RNA Bru-seq using an MCF7 conditional knockout model. Integration of ChIP- and Bru-seq pinpoints candidate direct GRHL2 target genes in luminal breast cancer. Multiple connections between GRHL2 and proliferation are uncovered, including transcriptional activation of ETS and E2F transcription factors. Among EMT-related genes, direct regulation of CLDN4 is corroborated but several targets identified in other cells (including CDH1 and ZEB1) are ruled out by both ChIP- and Bru-seq as being directly controlled by GRHL2 in luminal breast cancer cells. Gene clusters correlating positively (including known GRHL2 targets such as ErbB3, CLDN4/7) or negatively (including TGFB1 and TGFBR2) with GRHL2 in the MCF7 knockout model, display similar correlation with GRHL2 in ER positive as well as ER negative breast cancer patients. Altogether, this study uncovers gene sets regulated directly or indirectly by GRHL2 in luminal breast cancer, identifies novel GRHL2-regulated genes, and points to distinct GRHL2 regulation of EMT in luminal breast cancer cells. Video Abstract.

GRHL2 Enhances Phosphorylated Estrogen Receptor (ER) Chromatin Binding and Regulates ER-Mediated Transcriptional Activation and Repression

Phosphorylation of estrogen receptor α (ER) at serine 118 (pS118-ER) is induced by estrogen and is the most abundant posttranslational mark associated with a transcriptionally active receptor. Cistromic analysis of pS118-ER from our group revealed enrichment of the GRHL2 motif near pS118-ER binding sites. In this study, we used cistromic and transcriptomic analyses to interrogate the relationship between GRHL2 and pS118-ER. We found that GRHL2 is bound to chromatin at pS118-ER/GRHL2 co-occupancy sites prior to ligand treatment, and GRHL2 binding is required for maximal pS118-ER recruitment. pS118-ER/GRHL2 co-occupancy sites were enriched at active enhancers marked by H3K27ac and H3K4me1, along with FOXA1 and p300, compared to sites where each factor binds independently. Transcriptomic analysis yielded four subsets of ER/GRHL2-coregulated genes revealing that GRHL2 can both enhance and antagonize E2-mediated ER transcriptional activity. Gene ontology analysis indicated that coregulated genes are involved in cell migration. Accordingly, knockdown of GRHL2, combined with estrogen treatment, resulted in increased cell migration but no change in proliferation. These results support a model in which GRHL2 binds to selected enhancers and facilitates pS118-ER recruitment to chromatin, which then results in differential activation and repression of genes that control estrogen-regulated ER-positive breast cancer cell migration.

Novel GRHL3 Variants in a South African Cohort With Cleft Lip and Palate

OBJECTIVE

The etiology of cleft palate (CP) is poorly understood compared with that of cleft lip with or without palate (CL ± P). Recently, variants in Grainyhead like transcription factor 3 (GRHL3) were reported to be associated with a risk for CP in European and some African populations including Nigeria, Ghana, and Ethiopia. In order to identify genetic variants that may further explain the etiology of CP, we sequenced GRHL3 in a South African population to determine if rare variants in GRHL3 are associated with the presence of syndromic or nonsyndromic CP.

DESIGN

We sequenced the exons of GRHL3 in 100 cases and where possible, we sequenced the parents of the individuals to determine the segregation pattern and presence of de novo variants.

SETTING

The cleft clinics from 2 public, tertiary hospitals in Durban, South Africa (SA), namely Inkosi Albert Luthuli Central Hospital and KwaZulu-Natal Children's Hospital.

PATIENTS, PARTICIPANTS

One hundred patients with CL ± P and their parents.

INTERVENTIONS

Saliva samples were collected.

MAIN OUTCOME MEASURES

To ascertain the genetic variants in the GRHL3 gene in patients with CL ± P in SA.

RESULTS

Five variants in GRHL3 were observed; 3 were novel and 2 were known variants. The novel variants were intronic variants (c.1062 + 77A>G and c.627 + 1G>A) and missense variant (p.Asp169Gly).

CONCLUSIONS

This study provides further evidence that variants in GRHL3 contribute to the risk of nonsyndromic CP in African populations, specifically, in the South African population.

USP39 is essential for mammalian epithelial morphogenesis through upregulation of planar cell polarity components

Previously, we have shown that the translocation of Grainyhead-like 3 (GRHL3) transcription factor from the nucleus to the cytoplasm triggers the switch from canonical Wnt signaling for epidermal differentiation to non-canonical Wnt signaling for epithelial morphogenesis. However, the molecular mechanism that underlies the cytoplasmic localization of GRHL3 protein and that activates non-canonical Wnt signaling is not known. Here, we show that ubiquitin-specific protease 39 (USP39), a deubiquitinating enzyme, is involved in the subcellular localization of GRHL3 as a potential GRHL3-interacting protein and is necessary for epithelial morphogenesis to up-regulate expression of planar cell polarity (PCP) components. Notably, mouse Usp39-deficient embryos display early embryonic lethality due to a failure in primitive streak formation and apico-basal polarity in epiblast cells, resembling those of mutant embryos of the Prickle1 gene, a crucial PCP component. Current findings provide unique insights into how differentiation and morphogenesis are coordinated to construct three-dimensional complex structures via USP39.

The ubiquitin specific protease 39 (USP39) interacts with the transcription factor and cytoplasmic regulator of planar cell polarity (PCP), Grainyheadlike 3 (Grhl3). USP39-dependent PCP gene upregulation contributes to epithelial morphogenesis.

Grainyhead-like (Grhl) Target Genes in Development and Cancer

Grainyhead-like (GRHL) factors are essential, highly conserved transcription factors (TFs) that regulate processes common to both natural cellular behaviours during embryogenesis, and de-regulation of growth and survival pathways in cancer. Serving to drive the transcription, and therefore activation of multiple co-ordinating pathways, the three GRHL family members (GRHL1-3) are a critical conduit for modulating the molecular landscape that guides cellular decision-making processes during proliferation, epithelial-mesenchymal transition (EMT) and migration. Animal models and in vitro approaches harbouring GRHL loss or gain-of-function are key research tools to understanding gene function, which gives confidence that resultant phenotypes and cellular behaviours may be translatable to humans. Critically, identifying and characterising the target genes to which these factors bind is also essential, as they allow us to discover and understand novel genetic pathways that could ultimately be used as targets for disease diagnosis, drug discovery and therapeutic strategies. GRHL1-3 and their transcriptional targets have been shown to drive comparable cellular processes in Drosophila, C. elegans, zebrafish and mice, and have recently also been implicated in the aetiology and/or progression of a number of human congenital disorders and cancers of epithelial origin. In this review, we will summarise the state of knowledge pertaining to the role of the GRHL family target genes in both development and cancer, primarily through understanding the genetic pathways transcriptionally regulated by these factors across disparate disease contexts.

Mouse models in palate development and orofacial cleft research: Understanding the crucial role and regulation of epithelial integrity in facial and palate morphogenesis

Cleft lip and cleft palate are common birth defects resulting from genetic and/or environmental perturbations of facial development in utero. Facial morphogenesis commences during early embryogenesis, with cranial neural crest cells interacting with the surface ectoderm to form initially partly separate facial primordia consisting of the medial and lateral nasal prominences, and paired maxillary and mandibular processes. As these facial primordia grow around the primitive oral cavity and merge toward the ventral midline, the surface ectoderm undergoes a critical differentiation step to form an outer layer of flattened and tightly connected periderm cells with a non-stick apical surface that prevents epithelial adhesion. Formation of the upper lip and palate requires spatiotemporally regulated inter-epithelial adhesions and subsequent dissolution of the intervening epithelial seam between the maxillary and medial/lateral nasal processes and between the palatal shelves. Proper regulation of epithelial integrity plays a paramount role during human facial development, as mutations in genes encoding epithelial adhesion molecules and their regulators have been associated with syndromic and non-syndromic orofacial clefts. In this chapter, we summarize mouse genetic studies that have been instrumental in unraveling the mechanisms regulating epithelial integrity and periderm differentiation during facial and palate development. Since proper epithelial integrity also plays crucial roles in wound healing and cancer, understanding the mechanisms regulating epithelial integrity during facial development have direct implications for improvement in clinical care of craniofacial patients.

GRHL3 Promotes Tumor Growth and Metastasis via the MEK Pathway in Colorectal Cancer

GRHL3 is a factor associated with a tumor, of which the molecular mechanism remains a further investigation. We explored the underlying mechanism of tumor-promoting effect of GRHL3 in colorectal cancer (CRC), which is involved in the MEK1/2 pathway. The expression of GRHL3 was measured in CRC and adjacent normal tissue using qPCR and immunohistochemical staining. Lentivirus-mediated knockdown expression of GRHL3 was performed in the CRC cell line HT29. Cell proliferation and metastasis were assayed in vitro, and tumorigenicity was investigated in vivo. We found higher GRHL3 expression in colorectal cancer, which was negatively correlated with patients' prognosis. Results from studies in vitro and in vivo indicated that downregulation of GRHL3 expression inhibited tumor growth and metastasis and inhibited the activation of the MEK1/2 pathway. The effect of GRHL3 downexpression was the same as that of MEK1/2 antagonists on suppression of tumor growth and metastasis. Our results suggested that GRHL3 may act as an oncogene to promote tumor growth and metastasis via the MEK pathway in colorectal cancer.

Desmosomes as Signaling Hubs in the Regulation of Cell Behavior

Desmosomes are intercellular junctions, which preserve tissue integrity during homeostatic and stress conditions. These functions rely on their unique structural properties, which enable them to respond to context-dependent signals and transmit them to change cell behavior. Desmosome composition and size vary depending on tissue specific expression and differentiation state. Their constituent proteins are highly regulated by posttranslational modifications that control their function in the desmosome itself and in addition regulate a multitude of desmosome-independent functions. This review will summarize our current knowledge how signaling pathways that control epithelial shape, polarity and function regulate desmosomes and how desmosomal proteins transduce these signals to modulate cell behavior.

Transcription factors Krüppel-like factor 4 and paired box 5 regulate the expression of the Grainyhead-like genes

Genes from the Grainyhead-like (GRHL) family code for transcription factors necessary for the development and maintenance of various epithelia. These genes are also very important in the development of many types of cancer. However, little is known about the regulation of expression of GRHL genes. Previously, there were no systematic analyses of the promoters of GRHL genes or transcription factors that bind to these promoters. Here we report that the Krüppel-like factor 4 (KLF4) and the paired box 5 factor (PAX5) bind to the regulatory regions of the GRHL genes and regulate their expression. Ectopic expression of KLF4 or PAX5 alters the expression of GRHL genes. In KLF4-overexpressing HEK293 cells, the expression of GRHL1 and GRHL3 genes was upregulated by 32% and 60%, respectively, whereas the mRNA level of GRHL2 gene was lowered by 28% when compared to the respective controls. The levels of GRHL1 and GRHL3 expression were decreased by 30% or 33% in PAX5-overexpressing HEK293 cells. The presence of minor frequency allele of single nucleotide polymorphism rs115898376 in the promoter of the GRHL1 gene affected the binding of KLF4 to this site. The evidence presented here suggests an important role of KLF4 and PAX5 in the regulation of expression of GRHL1-3 genes.

GRHL3 activates FSCN1 to relax cell-cell adhesions between migrating keratinocytes during wound reepithelialization

The migrating keratinocyte wound front is required for skin wound closure. Despite significant advances in wound healing research, we do not fully understand the molecular mechanisms that orchestrate collective keratinocyte migration. Here, we show that, in the wound front, the epidermal transcription factor Grainyhead like-3 (GRHL3) mediates decreased expression of the adherens junction protein E-cadherin; this results in relaxed adhesions between suprabasal keratinocytes, thus promoting collective cell migration and wound closure. Wound fronts from mice lacking GRHL3 in epithelial cells (Grhl3-cKO) have lower expression of Fascin-1 (FSCN1), a known negative regulator of E-cadherin. Assay for Transposase-Accessible Chromatin using sequencing (ATAC-seq) on wounded keratinocytes shows decreased wound-induced chromatin accessibility near the Fscn1 gene in Grhl3-cKO mice, a region enriched for GRHL3 motifs. These data reveal a wound-induced GRHL3/FSCN1/E-cadherin pathway that regulates keratinocyte-keratinocyte adhesion during wound-front migration; this pathway is activated in acute human wounds and is altered in diabetic wounds in mice, suggesting translational relevance.

Loss of Grhl3 is correlated with altered cellular protrusions in the non-neural ectoderm during neural tube closure

BACKGROUND

The transcription factor Grainyhead-like 3 (GRHL3) has multiple roles in a variety of tissues during development including epithelial patterning and actin cytoskeletal regulation. During neural tube closure (NTC) in the mouse embryo, GRHL3 is expressed and functions in the non-neural ectoderm (NNE). Two important functions of GRHL3 are regulating the actin cytoskeleton during NTC and regulating the boundary between the NNE and neural ectoderm. However, an open question that remains is whether these functions explain the caudally restricted neural tube defect (NTD) of spina bifida observed in Grhl3 mutants.

RESULTS

Using scanning electron microscopy and immunofluorescence based imaging on Grhl3 mutants and wildtype controls, we show that GRHL3 is dispensable for NNE identity or epithelial maintenance in the caudal NNE but is needed for regulation of cellular protrusions during NTC. Grhl3 mutants have decreased lamellipodia relative to wildtype embryos during caudal NTC, first observed at the onset of delays when lamellipodia become prominent in wildtype embryos. At the axial level of NTD, half of the mutants show increased and disorganized filopodia and half lack cellular protrusions.

CONCLUSION

These data suggest that altered cellular protrusions during NTC contribute to the etiology of NTD in Grhl3 mutants.

Grainyhead-like transcription factors: guardians of the skin barrier

There has been selective pressure to maintain a skin barrier since terrestrial animals evolved 360 million years ago. These animals acquired an unique integumentary system with a keratinized, stratified, squamous epithelium surface barrier. The barrier protects against dehydration and entry of microbes and toxins. The skin barrier centres on the stratum corneum layer of the epidermis and consists of cornified envelopes cemented by the intercorneocyte lipid matrix. Multiple components of the barrier undergo cross-linking by transglutaminase (TGM) enzymes, while keratins provide additional mechanical strength. Cellular tight junctions also are crucial for barrier integrity. The grainyhead-like (GRHL) transcription factors regulate the formation and maintenance of the integument in diverse species. GRHL3 is essential for formation of the skin barrier during embryonic development, whereas GRHL1 maintains the skin barrier postnatally. This is achieved by transactivation of Tgm1 and Tgm5, respectively. In addition to its barrier function, GRHL3 plays key roles in wound repair and as an epidermal tumour suppressor. In its former role, GRHL3 activates the planar cell polarity signalling pathway to mediate wound healing by providing directional migration cues. In squamous epithelium, GRHL3 regulates the balance between proliferation and differentiation, and its loss induces squamous cell carcinoma (SCC). In the skin, this is mediated through increased expression of MIR21, which reduces the expression levels of GRHL3 and its direct target, PTEN, leading to activation of the PI3K-AKT signalling pathway. These data position the GRHL family as master regulators of epidermal homeostasis across a vast gulf of evolutionary history.

Mesenchymal-to-Epithelial Transitions in Development and Cancer

The evolutionary emergence of the mesenchymal phenotype greatly increased the complexity of tissue architecture and composition in early Metazoan species. At the molecular level, an epithelial-to-mesenchymal transition (EMT) was permitted by the innovation of specific transcription factors whose expression is sufficient to repress the epithelial transcriptional program. The reverse process, mesenchymal-to-epithelial transition (MET), involves direct inhibition of EMT transcription factors by numerous mechanisms including tissue-specific MET-inducing transcription factors (MET-TFs), micro-RNAs, and changes to cell and tissue architecture, thus providing an elegant solution to the need for tight temporal and spatial control over EMT and MET events during development and adult tissue homeostasis.

Identification of microRNA-Associated-ceRNA Networks Regulating Crop Milk Production in Pigeon (Columba livia)

Pigeon belongs to altrices. Squab cannot forage independently. Nutrition can only be obtained from crop milk secreted by male and female pigeon. miRNA could regulate many biological events. However, the roles of miRNA and ceRNA in regulating crop milk production are still unknown. In this study, we investigated the miRNAs expression profile of female pigeon crop, explored the potential key genes, and found the regulatory mechanisms of crop milk production. A total of 71 miRNAs were identified differentially expressed significantly. Meanwhile, miR-20b-5p, miR-146b-5p, miR-21-5p, and miR-26b-5p were found to be the key miRNAs regulating lactation. Target genes of these miRNAs participated mainly in cell development; protein and lipid synthesis; and ion signaling processes, such as cell-cell adhesion, epithelial cell morphogenesis, calcium signaling pathway, protein digestion, and absorption. In the ceRNA network, miR-193-5p was located in the central position, and miR-193-5p/CREBRF/LOC110355588, miR-460b-5p/GRHL2/MSTRG.132954, and miR-193-5p/PIK3CD/LOC110355588 regulatory axes were believed to affect lactation. Collectively, our findings enriched the miRNA expression profile of pigeon and provided novel insights into the microRNA-associated-ceRNA networks regulating crop milk production in pigeon.

Transcriptional and signalling regulation of skin epithelial stem cells in homeostasis, wounds and cancer

The epidermis and skin appendages are maintained by their resident epithelial stem cells, which undergo long-term self-renewal and multilineage differentiation. Upon injury, stem cells are activated to mediate re-epithelialization and restore tissue function. During this process, they often mount lineage plasticity and expand their fates in response to damage signals. Stem cell function is tightly controlled by transcription machineries and signalling transductions, many of which derail in degenerative, inflammatory and malignant dermatologic diseases. Here, by describing both well-characterized and newly emerged pathways, we discuss the transcriptional and signalling mechanisms governing skin epithelial homeostasis, wound repair and squamous cancer. Throughout, we highlight common themes underscoring epithelial stem cell plasticity and tissue-level crosstalk in the context of skin physiology and pathology.

Effect of SUV39H1 Histone Methyltransferase Knockout on Expression of Differentiation-Associated Genes in HaCaT Keratinocytes

Keratinocytes undergo a complex differentiation process, coupled with extensive changes in gene expression through which they acquire distinctive features indispensable for cells that form the external body barrier—epidermis. Disturbed epidermal differentiation gives rise to multiple skin diseases. The involvement of epigenetic factors, such as DNA methylation or histone modifications, in the regulation of epidermal gene expression and differentiation has not been fully recognized yet. In this work we performed a CRISPR/Cas9-mediated knockout of SUV39H1, a gene-encoding H3K9 histone methyltransferase, in HaCaT cells that originate from spontaneously immortalized human keratinocytes and examined changes in the expression of selected differentiation-specific genes located in the epidermal differentiation complex (EDC) and other genomic locations by RT-qPCR. The studied genes revealed a diverse differentiation state-dependent or -independent response to a lower level of H3K9 methylation. We also show, by means of chromatin immunoprecipitation, that the expression of genes in the LCE1 subcluster of EDC was regulated by the extent of trimethylation of lysine 9 in histone H3 bound to their promoters. Changes in gene expression were accompanied by changes in HaCaT cell morphology and adhesion.

Spotlight on the Granules (Grainyhead-Like Proteins) - From an Evolutionary Conserved Controller of Epithelial Trait to Pioneering the Chromatin Landscape

Among the transcription factors that are conserved across phylogeny, the grainyhead family holds vital roles in driving the epithelial cell fate. In Drosophila, the function of grainyhead (grh) gene is essential during developmental processes such as epithelial differentiation, tracheal tube formation, maintenance of wing and hair polarity, and epidermal barrier wound repair. Three main mammalian orthologs of grh: Grainyhead-like 1-3 (GRHL1, GRHL2, and GRHL3) are highly conserved in terms of their gene structures and functions. GRHL proteins are essentially associated with the development and maintenance of the epithelial phenotype across diverse physiological conditions such as epidermal differentiation and craniofacial development as well as pathological functions including hearing impairment and neural tube defects. More importantly, through direct chromatin binding and induction of epigenetic alterations, GRHL factors function as potent suppressors of oncogenic cellular dedifferentiation program - epithelial-mesenchymal transition and its associated tumor-promoting phenotypes such as tumor cell migration and invasion. On the contrary, GRHL factors also induce pro-tumorigenic effects such as increased migration and anchorage-independent growth in certain tumor types. Furthermore, investigations focusing on the epithelial-specific activation of grh and GRHL factors have revealed that these factors potentially act as a pioneer factor in establishing a cell-type/cell-state specific accessible chromatin landscape that is exclusive for epithelial gene transcription. In this review, we highlight the essential roles of grh and GRHL factors during embryogenesis and pathogenesis, with a special focus on its emerging pioneering function.

HDAC3 ensures stepwise epidermal stratification via NCoR/SMRT-reliant mechanisms independent of its histone deacetylase activity

Here, Szigety et al. investigated the function of histone deacetylases in epidermal development, and they found that HDAC3 operates in conjunction with KLF4 to repress inappropriate expression of Tgm1, Krt16, and Aqp3, and suppresses expression of inflammatory cytokines through a Rela-dependent mechanism. Their data identify HDAC3 as a hub coordinating multiple aspects of epidermal barrier acquisition.

Chromatin modifiers play critical roles in epidermal development, but the functions of histone deacetylases in this context are poorly understood. The class I HDAC, HDAC3, is of particular interest because it plays divergent roles in different tissues by partnering with tissue-specific transcription factors. We found that HDAC3 is expressed broadly in embryonic epidermis and is required for its orderly stepwise stratification. HDAC3 protein stability in vivo relies on NCoR and SMRT, which function redundantly in epidermal development. However, point mutations in the NCoR and SMRT deacetylase-activating domains, which are required for HDAC3's enzymatic function, permit normal stratification, indicating that HDAC3's roles in this context are largely independent of its histone deacetylase activity. HDAC3-bound sites are significantly enriched for predicted binding motifs for critical epidermal transcription factors including AP1, GRHL, and KLF family members. Our results suggest that among these, HDAC3 operates in conjunction with KLF4 to repress inappropriate expression of Tgm1, Krt16, and Aqp3. In parallel, HDAC3 suppresses expression of inflammatory cytokines through a Rela-dependent mechanism. These data identify HDAC3 as a hub coordinating multiple aspects of epidermal barrier acquisition.

Potassium channels as potential drug targets for limb wound repair and regeneration

Background

Ion channels are a large family of transmembrane proteins, accessible by soluble membrane-impermeable molecules, and thus are targets for development of therapeutic drugs. Ion channels are the second most common target for existing drugs, after G protein-coupled receptors, and are expected to make a big impact on precision medicine in many different diseases including wound repair and regeneration. Research has shown that endogenous bioelectric signaling mediated by ion channels is critical in non-mammalian limb regeneration. However, the role of ion channels in regeneration of limbs in mammalian systems is not yet defined.

Methods

To explore the role of potassium channels in limb wound repair and regeneration, the hindlimbs of mouse embryos were amputated at E12.5 when the wound is expected to regenerate and E15.5 when the wound is not expected to regenerate, and gene expression of potassium channels was studied.

Results

Most of the potassium channels were downregulated, except for the potassium channel kcnj8 (Kir6.1) which was upregulated in E12.5 embryos after amputation.

Conclusion

This study provides a new mouse limb regeneration model and demonstrates that potassium channels are potential drug targets for limb wound healing and regeneration.

Establishment of chromatin accessibility by the conserved transcription factor Grainy head is developmentally regulated

The dramatic changes in gene expression required for development necessitate the establishment of cis-regulatory modules defined by regions of accessible chromatin. Pioneer transcription factors have the unique property of binding closed chromatin and facilitating the establishment of these accessible regions. Nonetheless, much of how pioneer transcription factors coordinate changes in chromatin accessibility during development remains unknown. To determine whether pioneer-factor function is intrinsic to the protein or whether pioneering activity is developmentally modulated, we studied the highly conserved, essential transcription factor Grainy head (Grh). Prior work established that Grh is expressed throughout Drosophila development and is a pioneer factor in the larva. We demonstrated that Grh remains bound to mitotic chromosomes, a property shared with other pioneer factors. By assaying chromatin accessibility in embryos lacking maternal and/or zygotic Grh at three stages of development, we discovered that Grh is not required for chromatin accessibility in early embryogenesis, in contrast to its essential functions later in development. Our data reveal that the pioneering activity of Grh is temporally regulated and likely influenced by additional factors expressed at a given developmental stage.

Functional roles of Grainyhead-like transcription factors in renal development and disease

Proper renal function relies on the tightly regulated development of nephrons and collecting ducts. This process, known as tubulogenesis, involves dynamic cellular and molecular changes that instruct cells to form highly organized tubes of epithelial cells which compartmentalize the renal interstitium and tubular lumen via assembly of a selective barrier. The integrity and diversity of the various renal epithelia is achieved via formation of intercellular protein complexes along the apical-basal axis of the epithelial cells. In recent years, the evolutionarily conserved family of Grainyhead-like (GRHL) transcription factors which encompasses three mammalian family members (Grainyhead-like 1, 2, 3) has emerged as a group of critical regulators for organ development, epithelial differentiation, and barrier formation. Evidence from transgenic animal models supports the presence of Grainyhead-like-dependent transcriptional mechanisms that promote formation and maintenance of epithelial barriers in the kidney. In this review, we highlight different Grhl-dependent mechanisms that modulate epithelial differentiation in the kidney. Additionally, we discuss how disruptions in these mechanisms result in impaired renal function later in life.

Unraveling cancer lineage drivers in squamous cell carcinomas

Cancer hijacks embryonic development and adult wound repair mechanisms to fuel malignancy. Cancer frequently originates from de-regulated adult stem cells or progenitors, which are otherwise essential units for postnatal tissue remodeling and repair. Cancer genomics studies have revealed convergence of multiple cancers across organ sites, including squamous cell carcinomas (SCCs), a common group of cancers arising from the head and neck, esophagus, lung, cervix and skin. In this review, we summarize our current knowledge on the molecular drivers of SCCs, including these five major organ sites. We especially focus our discussion on lineage dependent driver genes and pathways, in the context of squamous development and stratification. We then use skin as a model to discuss the notion of field cancerization during SCC carcinogenesis, and cancer as a wound that never heals. Finally, we turn to the idea of context dependency widely observed in cancer driver genes, and outline literature support and possible explanations for their lineage specific functions. Through these discussions, we aim to provide an up-to-date summary of molecular mechanisms driving tumor plasticity in squamous cancers. Such basic knowledge will be helpful to inform the clinics for better stratifying cancer patients, revealing novel drug targets and providing effective treatment options.

Meta-Analysis of Grainyhead-Like Dependent Transcriptional Networks: A Roadmap for Identifying Novel Conserved Genetic Pathways

The Drosophila grainyhead (grh) and vertebrate Grainyhead-like (Grhl) transcription factors are among the most critical genes for epithelial development, maintenance and homeostasis, and are remarkably well conserved from fungi to humans. Mutations affecting grh/Grhl function lead to a myriad of developmental and adult onset epithelial disease, such as aberrant skin barrier formation, facial/palatal clefting, impaired neural tube closure, age-related hearing loss, ectodermal dysplasia, and importantly, cancers of epithelial origin. Recently, mutations in the family member GRHL3 have been shown to lead to both syndromic and non-syndromic facial and palatal clefting in humans, particularly the genetic disorder Van Der Woude Syndrome (VWS), as well as spina bifida, whereas mutations in mammalian Grhl2 lead to exencephaly and facial clefting. As transcription factors, Grhl proteins bind to and activate (or repress) a substantial number of target genes that regulate and drive a cascade of transcriptional networks. A multitude of large-scale datasets have been generated to explore the grh/Grhl-dependent transcriptome, following ablation or mis-regulation of grh/Grhl-function. Here, we have performed a meta-analysis of all 41 currently published grh and Grhl RNA-SEQ, and microarray datasets, in order to identify and characterise the transcriptional networks controlled by grh/Grhl genes across disparate biological contexts. Moreover, we have also cross-referenced our results with published ChIP and ChIP-SEQ datasets, in order to determine which of the critical effector genes are likely to be direct grh/Grhl targets, based on genomic occupancy by grh/Grhl genes. Lastly, to interrogate the predictive strength of our approach, we experimentally validated the expression of the top 10 candidate grhl target genes in epithelial development, in a zebrafish model lacking grhl3, and found that orthologues of seven of these (cldn23, ppl, prom2, ocln, slc6a19, aldh1a3, and sod3) were significantly down-regulated at 48 hours post-fertilisation. Therefore, our study provides a strong predictive resource for the identification of putative grh/grhl effector target genes.

Structural basis of gene regulation by the Grainyhead/CP2 transcription factor family

Grainyhead (Grh)/CP2 transcription factors are highly conserved in multicellular organisms as key regulators of epithelial differentiation, organ development and skin barrier formation. In addition, they have been implicated as being tumor suppressors in a variety of human cancers. Despite their physiological importance, little is known about their structure and DNA binding mode. Here, we report the first structural study of mammalian Grh/CP2 factors. Crystal structures of the DNA-binding domains of grainyhead-like (Grhl) 1 and Grhl2 reveal a closely similar conformation with immunoglobulin-like core. Both share a common fold with the tumor suppressor p53, but differ in important structural features. The Grhl1 DNA-binding domain binds duplex DNA containing the consensus recognition element in a dimeric arrangement, supporting parsimonious target-sequence selection through two conserved arginine residues. We elucidate the molecular basis of a cancer-related mutation in Grhl1 involving one of these arginines, which completely abrogates DNA binding in biochemical assays and transcriptional activation of a reporter gene in a human cell line. Thus, our studies establish the structural basis of DNA target-site recognition by Grh transcription factors and reveal how tumor-associated mutations inactivate Grhl proteins. They may serve as points of departure for the structure-based development of Grh/CP2 inhibitors for therapeutic applications.

Grainyhead-like Protein 2: The Emerging Role in Hormone-Dependent Cancers and Epigenetics

In mammals, the grainyhead-like transcription factor (GRHL) family is composed of three nuclear proteins that are responsible for driving epithelial cell fate: GRHL1, GRHL2, and GRHL3. GRHL2 is important in maintaining proper tubulogenesis during development and in suppressing the epithelial-to-mesenchymal transition. Within the last decade, evidence indicates both tumor-suppressive and oncogenic roles for GRHL2 in various types of cancers. Recent studies suggest that GRHL2 may be especially important in hormone-dependent cancers, as correlative relationships exist between GRHL2 and various steroid receptors, such as the androgen and estrogen receptors. Acting as a pioneer factor and coactivator, GRHL2 may directly affect steroid receptor transcriptional activity. This review will highlight recent discoveries of GRHL2 activity in cancer and in maintaining the epithelial state, while also exploring recent literature on the role of GRHL2 in hormone-dependent cancers and epigenetics.

GRHL2-Dependent Enhancer Switching Maintains a Pluripotent Stem Cell Transcriptional Subnetwork after Exit from Naive Pluripotency

The enhancer landscape of pluripotent stem cells undergoes extensive reorganization during early mammalian development. The functions and mechanisms behind such reorganization, however, are unclear. Here, we show that the transcription factor GRHL2 is necessary and sufficient to activate an epithelial subset of enhancers as naive embryonic stem cells (ESCs) transition into formative epiblast-like cells (EpiLCs). Surprisingly, many GRHL2 target genes do not change in expression during the ESC-EpiLC transition. Instead, enhancers regulating these genes in ESCs diminish in activity in EpiLCs while GRHL2-dependent alternative enhancers become activated to maintain transcription. GRHL2 therefore assumes control over a subset of the naive network via enhancer switching to maintain expression of epithelial genes upon exit from naive pluripotency. These data evoke a model where the naive pluripotency network becomes partitioned into smaller, independent networks regulated by EpiLC-specific transcription factors, thereby priming cells for lineage specification.

Temporal Patterning in the Drosophila CNS

A small pool of neural progenitors generates the vast diversity of cell types in the CNS. Spatial patterning specifies progenitor identity, followed by temporal patterning within progenitor lineages to expand neural diversity. Recent work has shown that in Drosophila, all neural progenitors (neuroblasts) sequentially express temporal transcription factors (TTFs) that generate molecular and cellular diversity. Embryonic neuroblasts use a lineage-intrinsic cascade of five TTFs that switch nearly every neuroblast cell division; larval optic lobe neuroblasts also use a rapid cascade of five TTFs, but the factors are completely different. In contrast, larval central brain neuroblasts undergo a major molecular transition midway through larval life, and this transition is regulated by a lineage-extrinsic cue (ecdysone hormone signaling). Overall, every neuroblast lineage uses a TTF cascade to generate diversity, illustrating the widespread importance of temporal patterning.

Grainyhead-like 2 (GRHL2) knockout abolishes oral cancer development through reciprocal regulation of the MAP kinase and TGF-β signaling pathways

Grainyhead-Like 2 (GRHL2) is an epithelial-specific transcription factor that regulates epithelial morphogenesis and differentiation. Prior studies suggested inverse regulation between GRHL2 and TGF-β in epithelial plasticity and potential carcinogenesis. Here, we report the role of GRHL2 in oral carcinogenesis in vivo using a novel Grhl2 knockout (KO) mouse model and the underlying mechanism involving its functional interaction with TGF-β signaling. We developed epithelial-specific Grhl2 conditional KO mice by crossing Grhl2 floxed mice with those expressing CreER driven by the K14 promoter. After induction of Grhl2 KO, we confirmed the loss of GRHL2 and its target proteins, while Grhl2 KO strongly induced TGF-β signaling molecules. When exposed to 4-nitroquinoline 1-oxide (4-NQO), a strong chemical carcinogen, Grhl2 wild-type (WT) mice developed rampant oral tongue tumors, while Grhl2 KO mice completely abolished tumor development. In cultured oral squamous cell carcinoma (OSCC) cell lines, TGF-β signaling was notably induced by GRHL2 knockdown while being suppressed by GRHL2 overexpression. GRHL2 knockdown or KO in vitro and in vivo, respectively, led to loss of active p-Erk1/2 and p-JNK MAP kinase levels; moreover, ectopic overexpression of GRHL2 strongly induced the MAP kinase activation. Furthermore, the suppressive effect of GRHL2 on TGF-β signaling was diminished in cells exposed to Erk and JNK inhibitors. These data indicate that GRHL2 activates the Erk and JNK MAP kinases, which in turn suppresses the TGF -β signaling. This novel signaling represents an alternative pathway by which GRHL2 regulates carcinogenesis, and is distinct from the direct transcriptional regulation by GRHL2 binding at its target gene promoters, e.g., E-cadherin, hTERT, p63, and miR-200 family genes. Taken together, the current study provides the first genetic evidence to support the role of GRHL2 in carcinogenesis and the underlying novel mechanism that involves the functional interaction between GRHL2 and TGF-β signaling through the MAPK pathways.

Whole-Body Single-Cell Sequencing Reveals Transcriptional Domains in the Annelid Larval Body

Animal bodies comprise diverse arrays of cells. To characterize cellular identities across an entire body, we have compared the transcriptomes of single cells randomly picked from dissociated whole larvae of the marine annelid Platynereis dumerilii. We identify five transcriptionally distinct groups of differentiated cells, each expressing a unique set of transcription factors and effector genes that implement cellular phenotypes. Spatial mapping of cells into a cellular expression atlas, and wholemount in situ hybridization of group-specific genes reveals spatially coherent transcriptional domains in the larval body, comprising, for example, apical sensory-neurosecretory cells versus neural/epidermal surface cells. These domains represent new, basic subdivisions of the annelid body based entirely on differential gene expression, and are composed of multiple, transcriptionally similar cell types. They do not represent clonal domains, as revealed by developmental lineage analysis. We propose that the transcriptional domains that subdivide the annelid larval body represent families of related cell types that have arisen by evolutionary diversification. Their possible evolutionary conservation makes them a promising tool for evo-devo research.

Molecular characterization of gene regulatory networks in primary human tracheal and bronchial epithelial cells

BACKGROUND

Robust methods to culture primary airway epithelial cells were developed several decades ago and these cells provide the model of choice to investigate many diseases of the human lung. However, the molecular signature of cells from different regions of the airway epithelium has not been well characterized.

METHODS

We utilize DNase-seq and RNA-seq to examine the molecular signatures of primary cells derived from human tracheal and bronchial tissues, as well as healthy and diseased (cystic fibrosis (CF)) donor lung tissue.

RESULTS

Our data reveal an airway cell signature that is divergent from other epithelial cell types and from common airway epithelial cell lines. The differences between tracheal and bronchial cells are clearly evident as are common regulatory features. Only minor variation is seen between bronchial cells from healthy or CF donors.

CONCLUSIONS

These data are a valuable resource for functional genomics analysis of airway epithelial tissues in human disease.

Coordinated expression and genetic polymorphisms in Grainyhead-like genes in human non-melanoma skin cancers

BACKGROUND

The Grainyhead-like (GRHL) transcription factors have been linked to many different types of cancer. However, no previous study has attempted to investigate potential correlations in expression of different GRHL genes in this context. Furthermore, there is very little information concerning damaging mutations and/or single nucleotide polymorphisms in GRHL genes that may be linked to cancer.

METHODS

DNA and RNA were extracted from human non-melanoma skin cancers (NMSC) and adjacent normal tissues (n = 33 pairs of samples). The expression of GRHL genes was measured by quantitative real time PCR. Regulation of GRHL expression by miRNA was studied using cell transfection methods and dual-luciferase reporter system. Targeted deep sequencing of GRHL genes in tumor samples and control tissues were employed to search for mutations and single nucleotide polymorphisms. Single marker rs141193530 was genotyped with pyrosequencing in additional NMSC replication cohort (n = 176). Appropriate statistical and bioinformatic methods were used to analyze and interpret results.

RESULTS

We discovered that the expression of two genes - GRHL1 and GRHL3 - is reduced in a coordinated manner in tumor samples, in comparison to the control healthy skin samples obtained from the same individuals. It is possible that both GRHL1 and GRHL3 are regulated, at least to some extent, by different strands of the same oncogenic microRNA - miR-21, what would at least partially explain observed correlation. No de novo mutations in the GRHL genes were detected in the examined tumor samples. However, some single nucleotide polymorphisms in the GRHL genes occur at significantly altered frequencies in the examined group of NMSC patients.

CONCLUSIONS

Non-melanoma skin cancer growth is accompanied by coordinated reduced expression of epidermal differentiation genes: GRHL1 and GRHL3, which may be regulated by miR-21-3p and -5p, respectively. Some potentially damaging single nucleotide polymorphisms in GRHL genes occur with altered frequencies in NMSC patients, and they may in particular impair the expression of GRHL3 gene or functioning of encoded protein. The presence of these polymorphisms may indicate an increased risk of NMSC development in affected people.

Knockdown of GRHL3 inhibits activities and induces cell cycle arrest and apoptosis of human colorectal cancer cells

The Grainyhead-like 3 (GRHL3) is involved in epidermal barrier formation, neural tube closure and wound repair. Previous studies have suggested that GRHL3 has been linked to many different types of cancers. However, to date, its effects on human colorectal cancer (CRC) has not been clarified yet. Our microarray analysis has indicated predominant GRHL3 expression in CRC. The purpose of this study was to investigate the expression and significance of GRHL3 in CRC tumorigenesis using CRC tissues and paired paracancerous tissues, as well as using distinct CRC cell lines (HT29 and DLD1). We observed increased GRHL3 expression at both mRNA and protein levels in CRC tissues and CRC cell lines using quantitative real-time polymerase chain reaction (qRT-PCR) and Western blotting. Moreover, silencing GRHL3 with siRNA could suppress CRC cell proliferation, viability and migration in vitro. We also found that knockdown of GRHL3 could promote cell cycle arrest at G₀/G₁ phase in HT29 cells and DLD1 cells, and induce cell apoptosis in HT29 cells. Together, our study revealed the down-regulation of GRHL3 in vitro could inhibit CRC cell activity and trigger cell cycle arrest at G₀/G₁ phase and apoptosis.

Mis-expression of grainyhead-like transcription factors in zebrafish leads to defects in enveloping layer (EVL) integrity, cellular morphogenesis and axial extension

The grainyhead-like (grhl) transcription factors play crucial roles in craniofacial development, epithelial morphogenesis, neural tube closure, and dorso-ventral patterning. By utilising the zebrafish to differentially regulate expression of family members grhl2b and grhl3, we show that both genes regulate epithelial migration, particularly convergence-extension (CE) type movements, during embryogenesis. Genetic deletion of grhl3 via CRISPR/Cas9 results in failure to complete epiboly and pre-gastrulation embryonic rupture, whereas morpholino (MO)-mediated knockdown of grhl3 signalling leads to aberrant neural tube morphogenesis at the midbrain-hindbrain boundary (MHB), a phenotype likely due to a compromised overlying enveloping layer (EVL). Further disruptions of grhl3-dependent pathways (through co-knockdown of grhl3 with target genes spec1 and arhgef19) confirm significant MHB morphogenesis and neural tube closure defects. Concomitant MO-mediated disruption of both grhl2b and grhl3 results in further extensive CE-like defects in body patterning, notochord and somite morphogenesis. Interestingly, over-expression of either grhl2b or grhl3 also leads to numerous phenotypes consistent with disrupted cellular migration during gastrulation, including embryo dorsalisation, axial duplication and impaired neural tube migration leading to cyclopia. Taken together, our study ascribes novel roles to the Grhl family in the context of embryonic development and morphogenesis.

Grainyhead-like 2 (GRHL2) regulates epithelial plasticity in pancreatic cancer progression

The epithelial‐mesenchymal transition (EMT) and mesenchymal‐epithelial transition (MET) contribute to cancer metastasis of pancreatic ductal adenocarcinoma (PDAC). We explored the role of grainyhead‐like 2 (GRHL2), a suppressor of EMT, in the progression of PDAC. Expressions of GRHL2 were assessed using surgically resected PDAC tissues by immunohistochemistry analysis, and in vitro using human and mouse PDAC cells. Effects on epithelial plasticity and stemness of GRHL2 were examined in vitro using liver metastatic PDAC cells (CFPAC‐1) with GRHL2 knockdown by specific siRNAs. GRHL2 has a significantly positive correlation with E‐cadherin and CD133 in 155 resected human primary PDAC tissues. GRHL2 is highly expressed in liver metastatic cells than in primary invasive cells of both human and mouse PDAC, accompanied by a positive correlation with E‐cadherin expression. GRHL2 knockdown CFPAC‐1 cells demonstrated morphological changes into mesenchymal appearances and reduced proliferation through EMT. Notably, knockdown studies followed by flow cytometry analysis for a subpopulation of CD133+ showed that GRHL2 facilitates CFPAC‐1 cells to maintain stem‐like characters including self‐renewal capacity and anoikis resistance. GRHL2 regulates epithelial plasticity along with stemness in PDAC, both of which are crucial for metastasis, implicating the possibility of GRHL2 as a therapeutic target for PDAC liver metastasis.

Zebrafish models of orofacial clefts

Zebrafish is a model organism that affords experimental advantages toward investigating the normal function of genes associated with congenital birth defects. Here we summarize zebrafish studies of genes implicated in orofacial cleft (OFC). The most common use of zebrafish in this context has been to explore the normal function an OFC-associated gene product in craniofacial morphogenesis by inhibiting expression of its zebrafish ortholog. The most frequently deployed method has been to inject embryos with antisense morpholino oligonucleotides targeting the desired transcript. However, improvements in targeted mutagenesis strategies have led to widespread adoption of CRISPR/Cas9 technology. A second application of zebrafish has been for functional assays of gene variants found in OFC patients; such in vivo assays are valuable because the success of in silico methods for testing allele severity has been mixed. Finally, zebrafish have been used to test the tissue specificity of enhancers that harbor single nucleotide polymorphisms associated with risk for OFC. We review examples of each of these approaches in the context of genes that are implicated in syndromic and non-syndromic OFC. Developmental Dynamics 246:897-914, 2017. © 2017 Wiley Periodicals, Inc.

Neural tube closure: cellular, molecular and biomechanical mechanisms

Neural tube closure has been studied for many decades, across a range of vertebrates, as a paradigm of embryonic morphogenesis. Neurulation is of particular interest in view of the severe congenital malformations - 'neural tube defects' - that result when closure fails. The process of neural tube closure is complex and involves cellular events such as convergent extension, apical constriction and interkinetic nuclear migration, as well as precise molecular control via the non-canonical Wnt/planar cell polarity pathway, Shh/BMP signalling, and the transcription factors Grhl2/3, Pax3, Cdx2 and Zic2. More recently, biomechanical inputs into neural tube morphogenesis have also been identified. Here, we review these cellular, molecular and biomechanical mechanisms involved in neural tube closure, based on studies of various vertebrate species, focusing on the most recent advances in the field.

Transcriptional mechanisms coordinating tight junction assembly during epithelial differentiation

Epithelial tissues form a selective barrier via direct cell-cell interactions to separate and establish concentration gradients between the different compartments of the body. Proper function and formation of this barrier rely on the establishment of distinct intercellular junction complexes. These complexes include tight junctions, adherens junctions, desmosomes, and gap junctions. The tight junction is by far the most diverse junctional complex in the epithelial barrier. Its composition varies greatly across different epithelial tissues to confer various barrier properties. Thus, epithelial cells rely on tightly regulated transcriptional mechanisms to ensure proper formation of the epithelial barrier and to achieve tight junction diversity. Here, we review different transcriptional mechanisms utilized during embryogenesis and disease development to promote tight junction assembly and maintenance of intercellular barrier integrity. We focus particularly on the Grainyhead-like transcription factors and ligand-activated nuclear hormone receptors, two central families of proteins in epithelialization.

Lineage-specific dynamic and pre-established enhancer-promoter contacts cooperate in terminal differentiation

Chromosome conformation is an important feature of metazoan gene regulation; however, enhancer-promoter contact remodeling during cellular differentiation remains poorly understood. To address this, genome-wide promoter capture Hi-C (CHi-C) was performed during epidermal differentiation. Two classes of enhancer-promoter contacts associated with differentiation-induced genes were identified. The first class ('gained') increased in contact strength during differentiation in concert with enhancer acquisition of the H3K27ac activation mark. The second class ('stable') were pre-established in undifferentiated cells, with enhancers constitutively marked by H3K27ac. The stable class was associated with the canonical conformation regulator cohesin, whereas the gained class was not, implying distinct mechanisms of contact formation and regulation. Analysis of stable enhancers identified a new, essential role for a constitutively expressed, lineage-restricted ETS-family transcription factor, EHF, in epidermal differentiation. Furthermore, neither class of contacts was observed in pluripotent cells, suggesting that lineage-specific chromatin structure is established in tissue progenitor cells and is further remodeled in terminal differentiation.

Roles of Grainyhead-like transcription factors in cancer

The mammalian homologs of the D. melanogaster Grainyhead gene, Grainyhead-like 1-3 (GRHL1, GRHL2 and GRHL3), are transcription factors implicated in wound healing, tubulogenesis and cancer. Their induced target genes encode diverse epithelial cell adhesion molecules, while mesenchymal genes involved in cell migration and invasion are repressed. Moreover, GRHL2 suppresses the oncogenic epithelial-mesencyhmal transition, thereby acting as a tumor suppressor. Mechanisms, some involving established cancer-related signaling/transcription factor pathways (for example, Wnt, TGF-β, mir200, ZEB1, OVOL2, p63 and p300) and translational implications of the Grainyhead proteins in cancer are discussed in this review article.

Grainyhead-like Transcription Factors in Craniofacial Development

Craniofacial development in vertebrates involves the coordinated growth, migration, and fusion of several facial prominences during embryogenesis, processes governed by strict genetic and molecular controls. A failure in any of the precise spatiotemporal sequences of events leading to prominence fusion often leads to anomalous facial, skull, and jaw formation-conditions termed craniofacial defects (CFDs). Affecting approximately 0.1% to 0.3% of live births, CFDs are a highly heterogeneous class of developmental anomalies, which are often underpinned by genetic mutations. Therefore, identifying novel disease-causing mutations in genes that regulate craniofacial development is a critical prerequisite to develop new preventive or therapeutic measures. The Grainyhead-like ( GRHL) transcription factors are one such gene family, performing evolutionarily conserved roles in craniofacial patterning. The antecedent member of this family, Drosophila grainyhead ( grh), is required for head skeleton development in fruit flies, loss or mutation of Grhl family members in mouse and zebrafish models leads to defects of both maxilla and mandible, and recently, mutations in human GRHL3 have been shown to cause or contribute to both syndromic (Van Der Woude syndrome) and nonsyndromic palatal clefts. In this review, we summarize the current knowledge regarding the craniofacial-specific function of the Grainyhead-like family in multiple model species, identify some of the major target genes regulated by the Grhl transcription factors in craniofacial patterning, and, by examining animal models, draw inferences as to how these data will inform the likely roles of GRHL factors in human CFDs comprising palatal clefting. By understanding the molecular networks regulated by Grhl2 and Grhl3 target genes in other systems, we can propose likely pathways that mediate the effects of these transcription factors in human palatogenesis.

Alternative splicing and start sites: Lessons from the Grainyhead-like family

The two main mechanisms that expand the proteomic output of eukaryotic genes are alternative splicing and alternative translation initiation signals. Despite being essential to generate isoforms of gene products that create functional diversity during development, the impact of these mechanisms on fine-tuning regulatory gene networks is still underappreciated. In this review, we use the Grainyhead-like (Grhl) family as a case study to illustrate the importance of isoforms when investigating transcription factor family function during development and disease, and highlight the potential for differential modulation of downstream target genes. We provide insights into the importance of considering alternative gene products when designing, undertaking, and analysing primary research, and the effect that isoforms may have on development. This review also covers known mutations in Grhl family members, and postulates how genetic changes may dictate transcriptional specificity between the Grhl family members. It also contrasts and compares the available literature on the function and importance of the Grhl isoforms, and highlights current gaps in our understanding of their regulatory gene networks in development and disease.

Regulation of Transcription Factors by Reactive Oxygen Species and Nitric Oxide in Vascular Physiology and Pathology

SIGNIFICANCE

Cardiovascular diseases are the main cause of death worldwide and pose an immense economical burden. In most cases, the underlying problem is vascular occlusion by atherosclerotic plaques. Importantly, different cell types of the vascular wall and the immune system play crucial roles in atherosclerosis at different stages of the disease. Furthermore, atherosclerosis and conditions recognized as risk factors are characterized by a reduced availability of the vasoprotective molecule nitric oxide and an increase in reactive oxygen species, so-called oxidative stress. Transcription factors function as intracellular signal integrators and relays and thus, play a central role in cellular responses to changing conditions. Recent Advances: Work on specific transcriptional regulators has uncovered many of their functions and the upstream pathways modulating their activity in response to reactive oxygen and nitrogen species. Here, we have reviewed for a few selected examples how this can contribute not only to protection against atherosclerosis development but also to disease progression and the occurrence of clinical manifestations, such as plaque rupture.

CRITICAL ISSUES

Transcription factors have pleiotropic outputs and often also divergent functions in different cell types and tissues. Thus, in light of potential severe adverse side effects, a global activation or inhibition of particular transcriptions factors does not seem a feasible therapeutic option.

FUTURE DIRECTIONS

A further in-depth characterization of the cell- and stage-specific actions and regulation of transcription factors in atherosclerosis with respect to protein-protein interactions and target genes could open up new avenues for prevention or therapeutic interventions in this vascular disease. Antioxid. Redox Signal. 26, 679-699.

EMT: 2016

The significant parallels between cell plasticity during embryonic development and carcinoma progression have helped us understand the importance of the epithelial-mesenchymal transition (EMT) in human disease. Our expanding knowledge of EMT has led to a clarification of the EMT program as a set of multiple and dynamic transitional states between the epithelial and mesenchymal phenotypes, as opposed to a process involving a single binary decision. EMT and its intermediate states have recently been identified as crucial drivers of organ fibrosis and tumor progression, although there is some need for caution when interpreting its contribution to metastatic colonization. Here, we discuss the current state-of-the-art and latest findings regarding the concept of cellular plasticity and heterogeneity in EMT. We raise some of the questions pending and identify the challenges faced in this fast-moving field.

Specification of osteoblast cell fate by canonical Wnt signaling requires Bmp2

Enhanced BMP or canonical Wnt (cWnt) signaling are therapeutic strategies employed to enhance bone formation and fracture repair, but the mechanisms each pathway utilizes to specify cell fate of bone-forming osteoblasts remain poorly understood. Among all BMPs expressed in bone, we find that singular deficiency of Bmp2 blocks the ability of cWnt signaling to specify osteoblasts from limb bud or bone marrow progenitors. When exposed to cWnts, Bmp2-deficient cells fail to progress through the Runx2/Osx1 checkpoint and thus do not upregulate multiple genes controlling mineral metabolism in osteoblasts. Cells lacking Bmp2 after induction of Osx1 differentiate normally in response to cWnts, suggesting that pre-Osx1⁺ osteoprogenitors are an essential source and a target of BMP2. Our analysis furthermore reveals Grainyhead-like 3 (Grhl3) as a transcription factor in the osteoblast gene regulatory network induced during bone development and bone repair, which acts upstream of Osx1 in a BMP2-dependent manner. The Runx2/Osx1 transition therefore receives crucial regulatory inputs from BMP2 that are not compensated for by cWnt signaling, and this is mediated at least in part by induction and activation of Grhl3.