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.

STAT5 activation promotes progression and chemotherapy resistance in early T-cell precursor acute lymphoblastic leukemia

Interleukin-7 (IL-7) supports the growth and chemoresistance of T-cell acute lymphoblastic leukemia (T-ALL), particularly the early T-cell precursor subtype (ETP-ALL), which frequently has activating mutations of IL-7 signaling. Signal transducer and activator of transcription (STAT5) is an attractive therapeutic target because it is almost universally activated in ETP-ALL, even in the absence of mutations of upstream activators such as the IL-7 receptor (IL-7R), Janus kinase, and Fms-like tyrosine kinase 3 (FLT3). To examine the role of activated STAT5 in ETP-ALL, we have used a Lmo2-transgenic (Lmo2Tg) mouse model in which we can monitor chemoresistant preleukemia stem cells (pre-LSCs) and leukemia stem cells (LSCs) that drive T-ALL development and relapse following chemotherapy. Using IL-7R-deficient Lmo2Tg mice, we show that IL-7 signaling was not required for the formation of pre-LSCs but essential for their expansion and clonal evolution into LSCs to generate T-ALL. Activated STAT5B was sufficient for the development of T-ALL in IL-7R-deficient Lmo2Tg mice, indicating that inhibition of STAT5 is required to block the supportive signals provided by IL-7. To further understand the role of activated STAT5 in LSCs of ETP-ALL, we developed a new transgenic mouse that enables T-cell specific and doxycycline-inducible expression of the constitutively activated STAT5B1∗6 mutant. Expression of STAT5B1∗6 in T cells had no effect alone but promoted expansion and chemoresistance of LSCs in Lmo2Tg mice. Pharmacologic inhibition of STAT5 with pimozide-induced differentiation and loss of LSCs, while enhancing response to chemotherapy. Furthermore, pimozide significantly reduced leukemia burden in vivo and overcame chemoresistance of patient-derived ETP-ALL xenografts. Overall, our results demonstrate that STAT5 is an attractive therapeutic target for eradicating LSCs in ETP-ALL.

Dysregulation of Grainyhead-like 3 expression causes widespread developmental defects

BACKGROUND

The gene encoding the transcription factor, Grainyhead-like 3 (Grhl3), plays critical roles in mammalian development and homeostasis. Grhl3-null embryos exhibit thoraco-lumbo-sacral spina bifida and soft-tissue syndactyly. Additional studies reveal that these embryos also exhibit an epidermal proliferation/differentiation imbalance. This manifests as skin barrier defects resulting in peri-natal lethality and defective wound repair. Despite these extensive analyses of Grhl3 loss-of-function models, the consequences of gain-of-function of this gene have been difficult to achieve.

RESULTS

In this study, we generated a novel mouse model that expresses Grhl3 from a transgene integrated in the Rosa26 locus on an endogenous Grhl3-null background. Expression of the transgene rescues both the neurulation and skin barrier defects of the knockout mice, allowing survival into adulthood. Despite this, the mice are not normal, exhibiting a range of phenotypes attributable to dysregulated Grhl3 expression. In mice homozygous for the transgene, we observe a severe Shaker-Waltzer phenotype associated with hearing impairment. Micro-CT scanning of the inner ear revealed profound structural alterations underlying these phenotypes. In addition, these mice exhibit other developmental anomalies including hair loss, digit defects, and epidermal dysmorphogenesis.

CONCLUSION

Taken together, these findings indicate that diverse developmental processes display low tolerance to dysregulation of Grhl3.

YBX1 integration of oncogenic PI3K/mTOR signalling regulates the fitness of malignant epithelial cells

In heterogeneous head and neck cancer (HNC), subtype-specific treatment regimens are currently missing. An integrated analysis of patient HNC subtypes using single-cell sequencing and proteome profiles reveals an epithelial-mesenchymal transition (EMT) signature within the epithelial cancer-cell population. The EMT signature coincides with PI3K/mTOR inactivation in the mesenchymal subtype. Conversely, the signature is suppressed in epithelial cells of the basal subtype which exhibits hyperactive PI3K/mTOR signalling. We further identify YBX1 phosphorylation, downstream of the PI3K/mTOR pathway, restraining basal-like cancer cell proliferation. In contrast, YBX1 acts as a safeguard against the proliferation-to-invasion switch in mesenchymal-like epithelial cancer cells, and its loss accentuates partial-EMT and in vivo invasion. Interestingly, phospho-YBX1 that is mutually exclusive to partial-EMT, emerges as a prognostic marker for overall patient outcomes. These findings create a unique opportunity to sensitise mesenchymal cancer cells to PI3K/mTOR inhibitors by shifting them towards a basal-like subtype as a promising therapeutic approach against HNC.

Identification of a Novel GRHL3/HOPX/Wnt/β-Catenin Proto-oncogenic Axis in Squamous Cell Carcinoma of the Esophagus

BACKGROUND & AIMS

Esophageal squamous cell carcinoma (ESCC) is an aggressive malignancy with a poor long-term prognosis. The molecular mechanisms underlying the initiation and progression of this tumor are largely unknown. The transcription factor GRHL3 functions as a potent tumor suppressor in SCC of skin, head, and neck. This study aims to determine whether GRHL3 also plays a role in the homeostasis of the esophageal epithelium and in the development of ESCC.

METHODS

The effects of Grhl3 deletion on squamous epithelial homeostasis in embryos and adult mice were examined using immunohistochemistry, transmission electron microscopy, and real-time polymerase chain reaction. The conditionally deleted mice were subsequently used to determine susceptibility to ESCC. Whole-transcriptome sequencing (RNA-seq) was performed on ESCC in wild-type and Grhl3 deleted animals. To decipher the signaling pathways, real-time polymerase chain reaction, immunohistochemistry, analysis of chromatin immunoprecipitation sequencing, chromatin immunoprecipitation-polymerase chain reaction, and RNA seq datasets were used. Primary human samples were used to validate the findings in the mouse model.

RESULTS

Loss of Grhl3 perturbs the proliferation-differentiation balance in the esophageal epithelium, thereby increasing the susceptibility to esophageal carcinogenesis in adult mice. Grhl3 imparts its tumor suppressor function by regulating the expression of HOPX. We have identified the Wnt/β-catenin pathway as the downstream effectors of GRHL3 and HOPX through our integrated approach using patient-derived ESCC samples and mouse models.

CONCLUSIONS

GRHL3 conveys its tumor suppressor function in ESCC through regulating its target gene HOPX, which limits Wnt/β-catenin signaling. Targeted therapies to inhibit this pathway could be a potential treatment strategy for ESCC patients with reduced GRHL3 expression.

The Hsp70 chaperone system: distinct roles in erythrocyte formation and maintenance

Erythropoiesis is a tightly regulated cell differentiation process in which specialized oxygen- and carbon dioxide-carrying red blood cells are generated in vertebrates. Extensive reorganization and depletion of the erythroblast proteome leading to the deterioration of general cellular protein quality control pathways and rapid hemoglobin biogenesis rates could generate misfolded/aggregated proteins and trigger proteotoxic stresses during erythropoiesis. Such cytotoxic conditions could prevent proper cell differentiation resulting in premature apoptosis of erythroblasts (ineffective erythropoiesis). The heat shock protein 70 (Hsp70) molecular chaperone system supports a plethora of functions that help maintain cellular protein homeostasis (proteostasis) and promote red blood cell differentiation and survival. Recent findings show that abnormalities in the expression, localization and function of the members of this chaperone system are linked to ineffective erythropoiesis in multiple hematological diseases in humans. In this review, we present latest advances in our understanding of the distinct functions of this chaperone system in differentiating erythroblasts and terminally differentiated mature erythrocytes. We present new insights into the protein repair-only function(s) of the Hsp70 system, perhaps to minimize protein degradation in mature erythrocytes to warrant their optimal function and survival in the vasculature under healthy conditions. The work also discusses the modulatory roles of this chaperone system in a wide range of hematological diseases and the therapeutic gain of targeting Hsp70.

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.

Delineating the roles of Grhl2 in craniofacial development through tissue-specific conditional deletion and epistasis approaches in mouse

BACKGROUND

The highly conserved Grainyhead-like (Grhl) family of transcription factors play critical roles in the development of the neural tube and craniofacial skeleton. In particular, deletion of family member Grainyhead-like 2 (Grhl2) leads to mid-gestational embryonic lethality, maxillary clefting, abdominoschisis, and both cranial and caudal neural tube closure defects. These highly pleiotropic and systemic defects suggest that Grhl2 plays numerous critical developmental roles to ensure correct morphogenesis and patterning.

RESULTS

Here, using four separate Cre-lox conditional deletion models, as well as one genetic epistasis approach (Grhl2^+/- ;Edn1^+/- double heterozygous mice) we have investigated tissue-specific roles of Grhl2 in embryonic development, with a particular focus on the craniofacial skeleton. We find that loss of Grhl2 in the pharyngeal epithelium (using the Shh^Cre driver) leads to low-penetrance micrognathia, whereas deletion of Grhl2 within the ectoderm of the pharynx (Nestin^Cre ) leads to small, albeit significant, differences in the proximal-distal elongation of both the maxilla and mandible. Loss of Grhl2 in endoderm (Sox17-2a^iCre ) resulted in noticeable lung defects and a single instance of secondary palatal clefting, although formation of other endoderm-derived organs such as the stomach, bladder and intestines was not affected. Lastly, deletion of Grhl2 in cells of the neural crest (Wnt1^Cre ) did not lead to any discernible defects in craniofacial development, and similarly, our epistasis approach did not detect any phenotypic consequences of loss of a single allele of both Grhl2 and Edn1.

CONCLUSION

Taken together, our study identifies a pharyngeal-epithelium intrinsic, non-cell-autonomous role for Grhl2 in the patterning and formation of the craniofacial skeleton, as well as an endoderm-specific role for Grhl2 in the formation and establishment of the mammalian lung.

Small molecule inhibition of Dynamin-dependent endocytosis targets multiple niche signals and impairs leukemia stem cells

Intensive chemotherapy for acute leukemia can usually induce complete remission, but fails in many patients to eradicate the leukemia stem cells responsible for relapse. There is accumulating evidence that these relapse-inducing cells are maintained and protected by signals provided by the microenvironment. Thus, inhibition of niche signals is a proposed strategy to target leukemia stem cells but this requires knowledge of the critical signals and may be subject to compensatory mechanisms. Signals from the niche require receptor-mediated endocytosis, a generic process dependent on the Dynamin family of large GTPases. Here, we show that Dynole 34-2, a potent inhibitor of Dynamin GTPase activity, can block transduction of key signalling pathways and overcome chemoresistance of leukemia stem cells. Our results provide a significant conceptual advance in therapeutic strategies for acute leukemia that may be applicable to other malignancies in which signals from the niche are involved in disease progression and chemoresistance.

The tumour microenvironment provides signals to support leukaemic stem cells (LSC) maintenance and chemoresistance. Here, the authors show that disrupting niche-associated signalling by inhibiting receptor-mediated endocytosis with a dynamin GTPase inhibitor overcomes chemoresistance of LSC.

Inactivation of Zeb1 in GRHL2-deficient mouse embryos rescues mid-gestation viability and secondary palate closure

Cleft lip and palate are common birth defects resulting from failure of the facial processes to fuse during development. The mammalian grainyhead-like (Grhl1-3) genes play key roles in a number of tissue fusion processes including neurulation, epidermal wound healing and eyelid fusion. One family member, Grhl2, is expressed in the epithelial lining of the first pharyngeal arch in mice at embryonic day (E)10.5, prompting analysis of the role of this factor in palatogenesis. Grhl2-null mice die at E11.5 with neural tube defects and a cleft face phenotype, precluding analysis of palatal fusion at a later stage of development. However, in the first pharyngeal arch of Grhl2-null embryos, dysregulation of transcription factors that drive epithelial-mesenchymal transition (EMT) occurs. The aberrant expression of these genes is associated with a shift in RNA-splicing patterns that favours the generation of mesenchymal isoforms of numerous regulators. Driving the EMT perturbation is loss of expression of the EMT-suppressing transcription factors Ovol1 and Ovol2, which are direct GRHL2 targets. The expression of the miR-200 family of microRNAs, also GRHL2 targets, is similarly reduced, resulting in a 56-fold upregulation of Zeb1 expression, a major driver of mesenchymal cellular identity. The critical role of GRHL2 in mediating cleft palate in Zeb1^−/− mice is evident, with rescue of both palatal and facial fusion seen in Grhl2^−/−;Zeb1^−/− embryos. These findings highlight the delicate balance between GRHL2/ZEB1 and epithelial/mesenchymal cellular identity that is essential for normal closure of the palate and face. Perturbation of this pathway may underlie cleft palate in some patients.

Summary: Epithelial transcription factor GRHL2 is required for face closure while mesenchymal transcription factor ZEB1 is required for palate closure. Surprisingly, animals lacking both factors close their face and secondary palate.

Interrogating the Grainyhead-like 2 (Grhl2) genomic locus identifies an enhancer element that regulates palatogenesis in mouse

The highly-conserved Grainyhead-like (Grhl) transcription factors are critical regulators of embryogenesis that regulate cellular survival, proliferation, migration and epithelial integrity, especially during the formation of the craniofacial skeleton. Family member Grhl2 is expressed throughout epithelial tissues during development, and loss of Grhl2 function leads to significant defects in neurulation, abdominal wall closure, formation of the face and fusion of the maxilla/palate. Whereas numerous downstream target genes of Grhl2 have been identified, very little is known about how this crucial developmental transcription factor itself is regulated. Here, using in silico and in utero expression analyses and functional deletion in mice, we have identified a novel 2.4 ​kb enhancer element (mm1286) that drives reporter gene expression in a pattern that strongly recapitulates endogenous Grhl2 in the craniofacial primordia, modulates Grhl2 expression in these tissues, and augments Grhl2-mediated closure of the secondary palate. Deletion of this genomic element, in the context of inactivation of one allele of Grhl2 (through generation of double heterozygous Grhl2^+/-;mm1286^+/- mice), results in a significant predisposition to palatal clefting at birth. Moreover, we found that a highly conserved 325 bp region of mm1286 is both necessary and sufficient for mediating the craniofacial-specific enhancer activity of this region, and that an extremely well-conserved 12-bp sequence within this element (CTGTCAAACAGGT) substantially determines full enhancer function. Together, these data provide valuable new insights into the upstream genomic regulatory landscape responsible for transcriptional control of Grhl2 during palatal closure.

KCC1 Activation protects Mice from the Development of Experimental Cerebral Malaria

Plasmodium falciparum malaria causes half a million deaths per year, with up to 9% of this mortality caused by cerebral malaria (CM). One of the major processes contributing to the development of CM is an excess of host inflammatory cytokines. Recently K+ signaling has emerged as an important mediator of the inflammatory response to infection; we therefore investigated whether mice carrying an ENU induced activation of the electroneutral K+ channel KCC1 had an altered response to Plasmodium berghei. Here we show that Kcc1^M935K/M935K mice are protected from the development of experimental cerebral malaria, and that this protection is associated with an increased CD4+ and TNFa response. This is the first description of a K+ channel affecting the development of experimental cerebral malaria.

Arterial Thrombosis Associated with Immune Thrombocytopenia: Presence of a Platelet Aggregating IgG Synergistic with Thrombin and Adrenalin

We report the case of a 50-year-old lady who presented with arterial thrombosis in the setting of thrombocytopenia. Investigations confirmed the diagnosis of idiopathic thrombocytopenic purpura. A spontaneous platelet aggregating factor (SPAF) was isolated from the immunoglobulin fraction of the patient's plasma. The isolated IgG irreversibly aggregated platelet-rich plasma and washed platelets, an effect abolished by pretreating the platelets with aspirin. The activity of the IgG was greatly enhanced by subaggregatory concentrations of thrombin and adrenalin and was localized to the F(ab')2 of the molecule. Plasmapheresis in combination with anti-platelet therapy resulted in an increase in the patient's platelet count, reduced platelet aggregating activity of plasma and significant clinical improvement.

We suggest that the presence of this platelet aggregating IgG contributed to the development of thrombosis in our patient and postulate that a similar factor may explain the paradox of thrombosis observed in a select group of thrombocytopenic patients.

Bone marrow transplantation corrects haemolytic anaemia in a novel ENU mutagenesis mouse model of TPI deficiency

In this study, we performed a genome-wide N-ethyl-N-nitrosourea (ENU) mutagenesis screen in mice to identify novel genes or alleles that regulate erythropoiesis. Here, we describe a recessive mouse strain, called RBC19, harbouring a point mutation within the housekeeping gene, Tpi1, which encodes the glycolysis enzyme, triosephosphate isomerase (TPI). A serine in place of a phenylalanine at amino acid 57 severely diminishes enzyme activity in red blood cells and other tissues, resulting in a macrocytic haemolytic phenotype in homozygous mice, which closely resembles human TPI deficiency. A rescue study was performed using bone marrow transplantation of wild-type donor cells, which restored all haematological parameters and increased red blood cell enzyme function to wild-type levels after 7 weeks. This is the first study performed in a mammalian model of TPI deficiency, demonstrating that the haematological phenotype can be rescued.

Summary: In a novel ENU mutagenesis mouse model of TPI deficiency, bone marrow transplantation was conducted to demonstrate that haemolytic and red blood cell glycolytic defects can be effectively rescued.

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.

Stage-dependent therapeutic efficacy in PI3K/mTOR-driven squamous cell carcinoma of the skin

Cutaneous squamous cell carcinoma (SCC) is a recurrent cancer that is prevalent in predisposed subjects such as immunosuppressed patients and patients being treated for other malignancies. Model systems to trial therapies at different stages of SCC development are lacking, therefore precluding efficient therapeutic interventions. Here, we have disrupted the expression of the tumor suppressor GRHL3 to induce loss of PTEN and activation of the PI3K/mTOR signaling pathway in mice and human skin, promoting aggressive SCC development. We then examined the potential for targeting PI3K/mTOR and an oncogenic driver miR-21, alone and in combination, for the prevention and treatment of SCC during the initiation, promotion/progression and establishment stages. Treatment with PI3K/mTOR inhibitors completely prevented tumor initiation, and these inhibitors significantly delayed the course of papilloma progression to malignancy. However, established SCC did not undergo any growth regression, indicating that this therapy is ineffective in established cancers. Mechanistically, the resistant SCCs displayed increased miR-21 expression in mice and humans where antagonists of miR-21 rescued expression levels of GRHL3/PTEN, but the combination of miR-21 antagonism with PI3K/mTOR inhibition resulted in acquired SCC resistance in part via c-MYC and OCT-4 upregulation. In conclusion, our data provide molecular evidence for the efficacy of targeting oncogenic drivers of SCC during the initiation and promotion stages and indicate that combination therapy may induce an aggressive phenotype when applied in the establishment stage.

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.

A mouse model of hereditary coproporphyria identified in an ENU mutagenesis screen

A genome-wide ethyl-N-nitrosourea (ENU) mutagenesis screen in mice was performed to identify novel regulators of erythropoiesis. Here, we describe a mouse line, RBC16, which harbours a dominantly inherited mutation in the Cpox gene, responsible for production of the haem biosynthesis enzyme, coproporphyrinogen III oxidase (CPOX). A premature stop codon in place of a tryptophan at amino acid 373 results in reduced mRNA expression and diminished protein levels, yielding a microcytic red blood cell phenotype in heterozygous mice. Urinary and faecal porphyrins in female RBC16 heterozygotes were significantly elevated compared with that of wild-type littermates, particularly coproporphyrinogen III, whereas males were biochemically normal. Attempts to induce acute porphyric crises were made using fasting and phenobarbital treatment on females. While fasting had no biochemical effect on RBC16 mice, phenobarbital caused significant elevation of faecal coproporphyrinogen III in heterozygous mice. This is the first known investigation of a mutagenesis mouse model with genetic and biochemical parallels to hereditary coproporphyria.

Loss of Dynamin 2 GTPase function results in microcytic anaemia

In a dominant mouse ethylnitrosurea mutagenesis screen for genes regulating erythropoiesis, we identified a pedigree with a novel microcytic hypochromia caused by a V235G missense mutation in Dynamin 2 (Dnm2). Mutations in Dnm2, a GTPase, are highly disease-specific and have been implicated in four forms of human diseases: centronuclear myopathy, Charcot-Marie Tooth neuropathy and, more recently, T-cell leukaemia and Hereditary Spastic Paraplegia, but red cell abnormalities have not been reported to date. The V235G mutation lies within a crucial GTP nucleotide-binding pocket of Dnm2, and resulted in defective GTPase activity and incompatibility with life in the homozygous state. Dnm2 is an essential mediator of clathrin-mediated endocytosis, which is required for the uptake of transferrin (Tf) into red cells for incorporation of haem. Accordingly, we observed significantly reduced Tf uptake by Dnm2^+/V235G cells, which led to impaired endosome formation. Despite these deficiencies, surprisingly all iron studies were unchanged, suggesting an unexplained alternative mechanism underlies microcytic anaemia in Dnm2^+/V235G mice. This study provides the first in vivo evidence for the requirements of Dnm2 in normal erythropoiesis.

Grainyhead-like 3 (Grhl3) deficiency in brain leads to altered locomotor activity and decreased anxiety-like behaviors in aged mice

The highly conserved Grainyhead-like (Grhl) family of transcription factors, comprising three members in vertebrates (Grhl1-3), play critical regulatory roles during embryonic development, cellular proliferation, and apoptosis. Although loss of Grhl function leads to multiple neural abnormalities in numerous animal models, a comprehensive analysis of Grhl expression and function in the mammalian brain has not been reported. Here they show that only Grhl3 expression is detectable in the embryonic mouse brain; particularly within the habenula, an organ known to modulate repressive behaviors. Using both Grhl3-knockout mice (Grhl3^-/- ), and brain-specific conditional deletion of Grhl3 in adult mice (Nestin-Cre/Grhl3^flox/flox ), they performed histological expression analyses and behavioral tests to assess long-term effects of Grhl3 loss on motor co-ordination, spatial memory, anxiety, and stress. They found that complete deletion of Grhl3 did not lead to noticeable structural or cell-intrinsic defects in the embryonic brain; however, aged Grhl3 conditional knockout (cKO) mice showed enlarged lateral ventricles and displayed marked changes in motor function and behaviors suggestive of decreased fear and anxiety. They conclude that loss of Grhl3 in the brain leads to significant alterations in locomotor activity and decreased self-inhibition, and as such, these mice may serve as a novel model of human conditions of impulsive behavior or hyperactivity. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 775-788, 2017.

Effects of high and low sodium diet on blood pressure and heart rate in mice lacking the functional grainyhead-like 1 gene

Hypertension is a major health problem throughout the world because of its high prevalence and its association with increased risk of cardiovascular disease. Two independent studies discovered a locus conferring susceptibility to essential hypertension on chromosome 2, in the 2p25 region, but the causative gene remains unknown. Grainyhead-like 1 (GRHL1) is one of the genes located in this region. Our experiments determined that the Grhl1 -null mice, when fed standard diet, have the same blood pressure as their wild type littermate controls. However, we discovered that blood pressure of these mice increases following high sodium diet and decreases when they are fed low sodium diet, and similar effects were not observed in the control wild type littermates. This suggests that the Grhl1 -null mice are sensitive to the development of salt-sensitive hypertension. Thus it is possible that the GRHL1 gene is involved in the regulation of blood pressure, and it may be the causative gene for the locus of susceptibility to essential hypertension in the 2p25 region.

Mice lacking the conserved transcription factor Grainyhead-like 3 (Grhl3) display increased apposition of the frontal and parietal bones during embryonic development

BACKGROUND

Increased apposition of the frontal and parietal bones of the skull during embryogenesis may be a risk factor for the subsequent development of premature skull fusion, or craniosynostosis. Human craniosynostosis is a prevalent, and often serious embryological and neonatal pathology. Other than known mutations in a small number of contributing genes, the aetiology of craniosynostosis is largely unknown. Therefore, the identification of novel genes which contribute to normal skull patterning, morphology and premature suture apposition is imperative, in order to fully understand the genetic regulation of cranial development.

RESULTS

Using advanced imaging techniques and quantitative measurement, we show that genetic deletion of the highly-conserved transcription factor Grainyhead-like 3 (Grhl3) in mice (Grhl3 ^-/- ) leads to decreased skull size, aberrant skull morphology and premature apposition of the coronal sutures during embryogenesis. Furthermore, Grhl3 ^-/- mice also present with premature collagen deposition and osteoblast alignment at the sutures, and the physical interaction between the developing skull, and outermost covering of the brain (the dura mater), as well as the overlying dermis and subcutaneous tissue, appears compromised in embryos lacking Grhl3. Although Grhl3 ^-/- mice die at birth, we investigated skull morphology and size in adult animals lacking one Grhl3 allele (heterozygous; Grhl3 ^+/- ), which are viable and fertile. We found that these adult mice also present with a smaller cranial cavity, suggestive of post-natal haploinsufficiency in the context of cranial development.

CONCLUSIONS

Our findings show that our Grhl3 mice present with increased apposition of the frontal and parietal bones, suggesting that Grhl3 may be involved in the developmental pathogenesis of craniosynostosis.

Activation of the erythroid K-Cl cotransporter Kcc1 enhances sickle cell disease pathology in a humanized mouse model

We used an N-ethyl-N-nitrosurea-based forward genetic screen in mice to identify new genes and alleles that regulate erythropoiesis. Here, we describe a mouse line expressing an activated form of the K-Cl cotransporter Slc12a4 (Kcc1), which results in a semi-dominant microcytosis of red cells. A missense mutation from methionine to lysine in the cytoplasmic tail of Kcc1 impairs phosphorylation of adjacent threonines required for inhibiting cotransporter activity. We bred Kcc1(M935K) mutant mice with a humanized mouse model of sickle cell disease to directly explore the relevance of the reported increase in KCC activity in disease pathogenesis. We show that a single mutant allele of Kcc1 induces widespread sickling and tissue damage, leading to premature death. This mouse model reveals important new insights into the regulation of K-Cl cotransporters and provides in vivo evidence that increased KCC activity worsened end-organ damage and diminished survival in sickle cell disease.

Consequences of the loss of the Grainyhead-like 1 gene for renal gene expression, regulation of blood pressure and heart rate in a mouse model

AIM

The Grainyhead-like 1 (GRHL1) transcription factor is tissue-specific and is very highly expressed in the kidney. In humans the GRHL1 gene is located at the chromosomal position 2p25. A locus conferring increased susceptibility to essential hypertension has been mapped to 2p25 in two independent studies, but the causative gene has never been identified. Furthermore, a statistically significant association has been found between a polymorphism in the GRHL1 gene and heart rate regulation. The aim of our study was to investigate the physiological consequences of Grhl1 loss in a mouse model and ascertain whether Grhl1 may be involved in the regulation of blood pressure and heart rate.

EXPERIMENTAL APPROACH

In our research we employed the Grhl1 "knock-out" mouse strain. We analyzed renal gene expression, blood pressure and heart rate in the Grhl1-null mice in comparison with their "wild-type" littermate controls. Most important results: The expression of many genes is altered in the Grhl1(-/-) kidneys. Some of these genes have previously been linked to blood pressure regulation. Despite this, the Grhl1-null mice have normal blood pressure and interestingly, increased heart rate.

CONCLUSIONS

Our work did not discover any new evidence to suggest any involvement of Grhl1 in blood pressure regulation. However, we determined that the loss of Grhl1 influences the regulation of heart rate in a mouse model.

Identification of a Novel Proto-oncogenic Network in Head and Neck Squamous Cell Carcinoma

BACKGROUND

The developmental transcription factor Grainyhead-like 3 (GRHL3) plays a critical tumor suppressor role in the mammalian epidermis through direct regulation of PTEN and the PI3K/AKT/mTOR signaling pathway. GRHL3 is highly expressed in all tissues derived from the surface ectoderm, including the oral cavity, raising a question about its potential role in suppression of head and neck squamous cell carcinoma (HNSCC).

METHODS

We explored the tumor suppressor role of Grhl3 in HNSCC using a conditional knockout (Grhl3 (∆/-) /K14Cre (+) ) mouse line (n = 26) exposed to an oral chemical carcinogen. We defined the proto-oncogenic pathway activated in the HNSCC derived from these mice and assessed it in primary human HNSCC samples, normal oral epithelial cell lines carrying shRNA to GRHL3, and human HNSCC cell lines. Data were analyzed with two-sided chi square and Student's t tests.

RESULTS

Deletion of Grhl3 in oral epithelium in mice did not perturb PTEN/PI3K/AKT/mTOR signaling, but instead evoked loss of GSK3B expression, resulting in stabilization and accumulation of c-MYC and aggressive HNSCC. This molecular signature was also evident in a subset of primary human HNSCC and HNSCC cell lines. Loss of Gsk3b in mice, independent of Grhl3, predisposed to chemical-induced HNSCC. Restoration of GSK3B expression blocked proliferation of normal oral epithelial cell lines carrying shRNA to GRHL3 (cell no., Day 8: Scramble ctl, 616±21.8 x 10(3) vs GRHL3-kd, 1194±44 X 10(3), P < .001; GRHL3-kd vs GRHL3-kd + GSK3B, 800±98.84 X 10(3), P = .003) and human HNSCC cells.

CONCLUSIONS

We defined a novel molecular signature in mammalian HNSCC, suggesting new treatment strategies targeting the GRHL3/GSK3B/c-MYC proto-oncogenic network.

Grhl3 and GEF19 in the front rho

Directional migration is a critical component of cell motility is observed in many diverse processes including embryogenesis, immune surveillance and wound repair. A central aspect of directional migration is cellular polarity, which is established through several signaling pathways that converge on the small GTPases. These factors orchestrate precise spatial and temporal organization of the actin cytoskeleton at the leading edge of the cell, and induce polarized capture and stabilization of microtubules and their associated microtubule organizing center (MTOC). Studies of the regulation of the GTPases have predominantly focused on post-translational mechanisms involving guanine nucleotide exchange factors (GEFs), GTPase activating proteins (GAPs), and guanine nucleotide dissociation inhibitors (GDIs). In this commentary, we examine the transcriptional regulation of these factors, focusing on the recently described regulation of RhoGEF19, an activator of RhoA, by the epidermal-specific transcription factor GRHL3, and the importance of this regulatory mechanism in wound repair. Our findings establish novel links between epidermal cell migration in wound healing and the planar cell polarity (PCP) signaling pathway, and establish a paradigm for tissue-specific regulation of Rho GTPase activity.

Human fetal globin gene expression is regulated by LYAR

Human globin gene expression during development is modulated by transcription factors in a stage-dependent manner. However, the mechanisms controlling the process are still largely unknown. In this study, we found that a nuclear protein, LYAR (human homologue of mouse Ly-1 antibody reactive clone) directly interacted with the methyltransferase PRMT5 which triggers the histone H4 Arg3 symmetric dimethylation (H4R3me2s) mark. We found that PRMT5 binding on the proximal γ-promoter was LYAR-dependent. The LYAR DNA-binding motif (GGTTAT) was identified by performing CASTing (cyclic amplification and selection of targets) experiments. Results of EMSA and ChIP assays confirmed that LYAR bound to a DNA region corresponding to the 5′-untranslated region of the γ-globin gene. We also found that LYAR repressed human fetal globin gene expression in both K562 cells and primary human adult erythroid progenitor cells. Thus, these data indicate that LYAR acts as a novel transcription factor that binds the γ-globin gene, and is essential for silencing the γ-globin gene.

Grainyhead-like 3 regulation of endothelin-1 in the pharyngeal endoderm is critical for growth and development of the craniofacial skeleton

Craniofacial development is a highly conserved process that requires complex interactions between neural crest cells (NCCs) and pharyngeal tissues derived from all three germ layers. Signals emanating from the pharyngeal endoderm drive differentiation of NCCs into craniofacial cartilage, and disruption of this process underpins several human craniofacial defects (CFD). Here, we demonstrate that morpholino (MO)-mediated knockdown in zebrafish of the highly conserved transcription factor grainyhead-like 3 (grhl3), which is selectively expressed in the pharyngeal endoderm, leads to severe hypoplasia of the lower jaw cartilages. Phylogenetic analysis of conserved grhl-binding sites in gene regulatory regions identified endothelin-1 (edn1) as a putative direct grhl3 target gene, and this was confirmed by chromatin precipitation (ChIP) assays in zebrafish embryos. Injection of sub-phenotypic concentrations of MOs targeting both grhl3 and edn1 induced jaw abnormalities, and injection of edn1 mRNA into grhl3-morphants rescued both pharyngeal expression of the downstream effectors of edn1, and jaw cartilage formation. This study sheds new light on the role of endodermal endothelin-1 in vertebrate jaw development, and highlights potential new genetic defects that could underpin human CFD.

Loss of Grainy head-like 1 is associated with disruption of the epidermal barrier and squamous cell carcinoma of the skin

The Grainyhead-like 1 (GRHL1) transcription factor regulates the expression of desmosomal cadherin desmoglein 1 (Dsg1) in suprabasal layers of the epidermis. As a consequence, the epidermis of Grhl1-null mice displays fewer desmosomes that are abnormal in structure. These mice also exhibit mild chronic skin barrier defects as evidenced by altered keratinocyte terminal differentiation, increased expression of inflammatory markers and infiltration of the skin by immune cells. Exposure of Grhl1^−/− mice to a standard chemical skin carcinogenesis protocol results in development of fewer papillomas than in wild type control animals, but with a rate of conversion to squamous cell carcinoma (SCC) that is strikingly higher than in normal littermates. The underlying molecular mechanism differs from mice with conditional ablation of a closely related Grhl family member, Grhl3, in the skin, which develop SCC due to the loss of expression of phosphatase and tensin homolog (PTEN) and activation of the phosphatidylinositol 3-kinase (PI3K)/AKT/mechanistic target of rapamycin (mTOR) signaling pathway.

Golgi Feels Its Own Wound

SIGNIFICANCE

The Golgi apparatus is essential for protein processing, sorting, and transport. Processing includes carbohydrate modifications and proteolytic cleavage, and transport can involve secretion from the cell or relocation to a specific cellular compartment. Rapid and synchronized reorientation of the Golgi in migrating cells is thought to facilitate polarized secretion, providing membrane and secreted products to the proximal plasma membrane. This function is a fundamental process in cell motility. Whether the Golgi structure and positioning is functionally required for directed secretion and polarity in cell migration responses, such as wound healing, is yet to be elucidated.

RECENT ADVANCES

: Exciting recent analysis examined the effects of perturbed Golgi positioning without disruption of microtubular or actin cytoskeleton assembly or protein secretion, in the context of cellular polarity and directional migration in wound repair. This was achieved by Yadav et al. (2009) through depletion of Golgin-160 or GMAP210 (Golgi microtubule associated protein of 210 kDa), which resulted in fragmentation and dispersal of Golgi without altering secretion kinetics. As a consequence, the direction of secretion, cell polarization, and cell migration in response to wounding were severely impaired. Thus, in response to a scratch wound, cell polarity requires peri-centrosomal positioning of the Golgi apparatus, implying that after initiation by a polarity cue there is a dependence on the Golgi's directed secretion to maintain the polarized state that facilitates cell migration.

CRITICAL ISSUES

Golgi peri-centrosomal positioning can now be included among the growing list of cellular processes and signaling pathways that are critical for establishment of cellular polarity in response to external stimuli-a key feature of wound repair.

FUTURE DIRECTIONS

A complete understanding of the function of Golgi components in motility merits attractive avenues for future investigations that will ultimately bring regulators of Golgi into the clinic whereby treatment of skin-related disorders will greatly benefit.

Novel mechanisms that pattern and shape the midbrain-hindbrain boundary

The midbrain-hindbrain boundary (MHB) is a highly conserved vertebrate signalling centre, acting to pattern and establish neural identities within the brain. While the core signalling pathways regulating MHB formation have been well defined, novel genetic and mechanistic processes that interact with these core components are being uncovered, helping to further elucidate the complicated networks governing MHB specification, patterning and shaping. Although formation of the MHB organiser is traditionally thought of as comprising three stages, namely positioning, induction and maintenance, we propose that a fourth stage, morphogenesis, should be considered as an additional stage in MHB formation. This review will examine evidence for novel factors regulating the first three stages of MHB development and will explore the evidence for regulation of MHB morphogenesis by non-classical MHB-patterning genes.

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.

ENU mutagenesis identifies the first mouse mutants reproducing human β-thalassemia at the genomic level

Forward genetic screens have been performed in many species to identify phenotypes in specific organ systems. We have undertaken a large-scale N-ethyl-N-nitrosourea (ENU) mutagenesis screen to identify dominant mutations that perturb erythropoiesis in mice. Mutant mice that displayed an erythrocyte mean cell volume (MCV) greater than three standard deviations from the population mean were identified. Two of these lines, RBC13 and RBC14, displayed a hypochromic, microcytic anemia, accompanied by a marked reticulocytosis, splenomegaly and diminished red cell survival. Timed pregnancies from heterozygous intercrosses revealed that a quarter of the embryos displayed severe anemia and did not survive beyond embryonic day (E) 18.5, consistent with homozygous β-thalassemia. Genetic complementation studies with a β-thalassemia mouse line reproduced the embryonic lethality in compound heterozygotes and a genomic custom capture array and massively parallel sequencing of the β-globin locus identified the causative mutations. The RBC13 line displayed a nonsense mutation at codon 40 in exon 2 of the β-major gene, invoking parallels with the common β(0)39 thalassemia mutation seen in humans. The RBC14 line exhibited a mutation at the polyadenylation signal of the β-major gene, exactly replicating a human β-thalassemia mutation. The RBC13 and RBC14 lines are the first β-thalassemia mouse models that reproduce human β-thalassemia at the genomic level, and as such highlight the power of ENU mutagenesis screens in generating mouse models of human disease.

The role of WDR5 in silencing human fetal globin gene expression

BACKGROUND

Histone H3 lysine 4 (K4) methylation has been linked with transcriptional activity in mammalian cells. The WD40-repeat protein, WDR5, is an essential component of the MLL complex that induces histone H3 K4 methylation, but the role of WDR5 in human globin gene regulation has not yet been established.

DESIGN AND METHODS

To study the role of WDR5 in human globin gene regulation, we performed knockdown experiments in both K562 cells and primary human bone marrow erythroid progenitor cells (BMC). The effects of WDR5 knockdown on γ-globin gene expression were determined. Biochemical approaches were also employed to investigate WDR5 interaction molecules. Chromosomal marks in the globin locus were analyzed by ChIP.

RESULTS

We found that WDR5 interacted with protein arginine methyltransferase 5 (PRMT5), a known repressor of γ-globin gene expression, and was essential for generating tri-methylated H3K4 (H3K4me3) at the γ-globin promoter in K562 cells. Enforced expression of WDR5 in K562 cells reduced γ-globin gene expression, whereas knockdown of WDR5 increased γ-globin gene expression in both K562 cells and primary human bone marrow erythroid progenitor cells. Consistent with this, both histone H3 and H4 acetylation at the γ-globin promoter were increased, while histone H4R3 and H3K9 methylation were decreased, in WDR5 knockdown cells compared to controls. We found that WDR5 interacted with HDAC1 and a PHD domaincontaining protein, ING2 (inhibitor of growth), an H3K4me3 mark reader, to enhance γ-globin gene transcriptional repression. In human BMC, levels of WDR5 were highly enriched on the γ-promoter relative to levels on other globin promoters and compared to the γ-promoter in cord blood erythroid progenitors, suggesting that WDR5 is important in the developmental globin gene expression program.

CONCLUSIONS

Our data are consistent with a model in which WDR5 binds the γ-globin promoter in a PRMT5-dependent manner; H3K4me3 induced at the γ-globin promoter by WDR5 may result in the recruitment of the ING2-associated HDAC1 component and consequent silencing of γ-globin gene expression.

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.

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.

The planar cell polarity pathway in vertebrate epidermal development, homeostasis and repair

The planar cell polarity (PCP) pathway plays a critical role in diverse developmental processes that require coordinated cellular movement, including neural tube closure and renal tubulogenesis. Recent studies have demonstrated that this pathway also has emerging relevance to the epidermis, as PCP signaling underpins many aspects of skin biology and pathology, including epidermal development, hair orientation, stem cell division and cancer. Coordinated cellular movement required for epidermal repair in mammals is also regulated by PCP signaling, and in this context, a new PCP gene encoding the developmental transcription factor Grainyhead-like 3 (Grhl3) is critical. This review focuses on the role that PCP signaling plays in the skin across a variety of epidermal functions and highlights perturbations that induce epidermal pathologies.

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.

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.

The Lmo2 oncogene initiates leukemia in mice by inducing thymocyte self-renewal

The LMO2 oncogene causes a subset of human T cell acute lymphoblastic leukemias (T-ALL), including four cases that arose as adverse events in gene therapy trials. To investigate the cellular origin of LMO2-induced leukemia, we used cell fate mapping to study mice in which the Lmo2 gene was constitutively expressed in the thymus. Lmo2 induced self-renewal of committed T cells in the mice more than 8 months before the development of overt T-ALL. These self-renewing cells retained the capacity for T cell differentiation but expressed several genes typical of hematopoietic stem cells (HSCs), suggesting that Lmo2 might reactivate an HSC-specific transcriptional program. Forced expression of one such gene, Hhex, was sufficient to initiate self-renewal of thymocytes in vivo. Thus, Lmo2 promotes the self-renewal of preleukemic thymocytes, providing a mechanism by which committed T cells can then accumulate additional genetic mutations required for leukemic transformation.