TBX2 specifies and maintains inner hair and supporting cell fate in the Organ of Corti.

The auditory function of the mammalian cochlea relies on two types of mechanosensory hair cells and various non-sensory supporting cells. Recent studies identified the transcription factors INSM1 and IKZF2 as regulators of outer hair cell (OHC) fate. However, the transcriptional regulation of the differentiation of inner hair cells (IHCs) and their associated inner supporting cells (ISCs) has remained enigmatic. Here, we show that the expression of the transcription factor TBX2 is restricted to IHCs and ISCs from the onset of differentiation until adulthood and examine its function using conditional deletion and misexpression approaches in the mouse. We demonstrate that TBX2 acts in prosensory progenitors as a patterning factor by specifying the inner compartment of the sensory epithelium that subsequently gives rise to IHCs and ISCs. Hair cell-specific inactivation or misexpression causes transdifferentiation of hair cells indicating a cell-autonomous function of TBX2 in inducing and maintaining IHC fate.

GATA6 is a crucial factor for Myocd expression in the visceral smooth muscle cell differentiation program of the murine ureter.

Smooth muscle cells (SMCs) are a crucial component of the mesenchymal wall of the ureter, as they account for the efficient removal of the urine from the renal pelvis to the bladder by means of their contractile activity. Here, we show that the zinc-finger transcription factor gene Gata6 is expressed in mesenchymal precursors of ureteric SMCs under the control of BMP4 signaling. Mice with a conditional loss of Gata6 in these precursors exhibit a delayed onset and reduced level of SMC differentiation and peristaltic activity, as well as dilatation of the ureter and renal pelvis (hydroureternephrosis) at birth and at postnatal stages. Molecular profiling revealed a delayed and reduced expression of the myogenic driver gene Myocd, but the activation of signaling pathways and transcription factors previously implicated in activation of the visceral SMC program in the ureter was unchanged. Additional gain-of-function experiments suggest that GATA6 cooperates with FOXF1 in Myocd activation and SMC differentiation, possibly as pioneer and lineage-determining factors, respectively.

Notch signaling is a novel regulator of visceral smooth muscle cell differentiation in the murine ureter.

The contractile phenotype of smooth muscle cells (SMCs) is transcriptionally controlled by a complex of the DNA-binding protein SRF and the transcriptional co-activator MYOCD. The pathways that activate expression of Myocd and of SMC structural genes in mesenchymal progenitors are diverse, reflecting different intrinsic and extrinsic signaling inputs. Taking the ureter as a model, we analyzed whether Notch signaling, a pathway previously implicated in vascular SMC development, also affects visceral SMC differentiation. We show that mice with a conditional deletion of the unique Notch mediator RBPJ in the undifferentiated ureteric mesenchyme exhibit altered ureter peristalsis with a delayed onset, and decreased contraction frequency and intensity at fetal stages. They also develop hydroureter 2 weeks after birth. Notch signaling is required for precise temporal activation of Myocd expression and, independently, for expression of a group of late SMC structural genes. Based on additional expression analyses, we suggest that a mesenchymal JAG1-NOTCH2/NOTCH3 module regulates visceral SMC differentiation in the ureter in a biphasic and bimodal manner, and that its molecular function differs from that in the vascular system.

Proteomic analysis identifies ZMYM2 as endogenous binding partner of TBX18 protein in 293 and A549 cells.

The TBX18 transcription factor regulates patterning and differentiation programs in the primordia of many organs yet the molecular complexes in which TBX18 resides to exert its crucial transcriptional function in these embryonic contexts have remained elusive. Here, we used 293 and A549 cells as an accessible cell source to search for endogenous protein interaction partners of TBX18 by an unbiased proteomic approach. We tagged endogenous TBX18 by CRISPR/Cas9 targeted genome editing with a triple FLAG peptide, and identified by anti-FLAG affinity purification and subsequent LC-MS analysis the ZMYM2 protein to be statistically enriched together with TBX18 in both 293 and A549 nuclear extracts. Using a variety of assays, we confirmed the binding of TBX18 to ZMYM2, a component of the CoREST transcriptional corepressor complex. Tbx18 is coexpressed with Zmym2 in the mesenchymal compartment of the developing ureter of the mouse, and mutations in TBX18 and in ZMYM2 were recently linked to congenital anomalies in the kidney and urinary tract (CAKUT) in line with a possible in vivo relevance of TBX18-ZMYM2 protein interaction in ureter development.

Regulation of otocyst patterning by Tbx2 and Tbx3 is required for inner ear morphogenesis in the mouse.

All epithelial components of the inner ear, including sensory hair cells and innervating afferent neurons, arise by patterning and differentiation of epithelial progenitors residing in a simple sphere, the otocyst. Here, we identify the transcriptional repressors TBX2 and TBX3 as novel regulators of these processes in the mouse. Ablation of Tbx2 from the otocyst led to cochlear hypoplasia, whereas loss of Tbx3 was associated with vestibular malformations. The loss of function of both genes (Tbx2/3cDKO) prevented inner ear morphogenesis at midgestation, resulting in indiscernible cochlear and vestibular structures at birth. Morphogenetic impairment occurred concomitantly with increased apoptosis in ventral and lateral regions of Tbx2/3cDKO otocysts around E10.5. Expression analyses revealed partly disturbed regionalisation, and a posterior-ventral expansion of the neurogenic domain in Tbx2/3cDKO otocysts at this stage. We provide evidence that repression of FGF signalling by TBX2 is important to restrict neurogenesis to the anterior-ventral otocyst and implicate another T-box factor, TBX1, as a crucial mediator in this regulatory network.

Combined genomic and proteomic approaches reveal DNA binding sites and interaction partners of TBX2 in the developing lung.

BACKGROUND: Tbx2 encodes a transcriptional repressor implicated in the development of numerous organs in mouse. During lung development TBX2 maintains the proliferation of mesenchymal progenitors, and hence, epithelial proliferation and branching morphogenesis. The pro-proliferative function was traced to direct repression of the cell-cycle inhibitor genes Cdkn1a and Cdkn1b, as well as of genes encoding WNT antagonists, Frzb and Shisa3, to increase pro-proliferative WNT signaling. Despite these important molecular insights, we still lack knowledge of the DNA occupancy of TBX2 in the genome, and of the protein interaction partners involved in transcriptional repression of target genes. METHODS: We used chromatin immunoprecipitation (ChIP)-sequencing and expression analyses to identify genomic DNA-binding sites and transcription units directly regulated by TBX2 in the developing lung. Moreover, we purified TBX2 containing protein complexes from embryonic lung tissue and identified potential interaction partners by subsequent liquid chromatography/mass spectrometry. The interaction with candidate proteins was validated by immunofluorescence, proximity ligation and individual co-immunoprecipitation analyses. RESULTS: We identified Il33 and Ccn4 as additional direct target genes of TBX2 in the pulmonary mesenchyme. Analyzing TBX2 occupancy data unveiled the enrichment of five consensus sequences, three of which match T-box binding elements. The remaining two correspond to a high mobility group (HMG)-box and a homeobox consensus sequence motif. We found and validated binding of TBX2 to the HMG-box transcription factor HMGB2 and the homeobox transcription factor PBX1, to the heterochromatin protein CBX3, and to various members of the nucleosome remodeling and deacetylase (NuRD) chromatin remodeling complex including HDAC1, HDAC2 and CHD4. CONCLUSION: Our data suggest that TBX2 interacts with homeobox and HMG-box transcription factors as well as with the NuRD chromatin remodeling complex to repress transcription of anti-proliferative genes in the pulmonary mesenchyme.

Loss of Anks6 leads to YAP deficiency and liver abnormalities.

ANKS6 is a ciliary protein that localizes to the proximal compartment of the primary cilium, where it regulates signaling. Mutations in the ANKS6 gene cause multiorgan ciliopathies in humans, which include laterality defects of the visceral organs, renal cysts as part of nephronophthisis and congenital hepatic fibrosis (CHF) in the liver. Although CHF together with liver ductal plate malformations are common features of several human ciliopathy syndromes, including nephronophthisis-related ciliopathies, the mechanism by which mutations in ciliary genes lead to bile duct developmental abnormalities is not understood. Here, we generated a knockout mouse model of Anks6 and show that ANKS6 function is required for bile duct morphogenesis and cholangiocyte differentiation. The loss of Anks6 causes ciliary abnormalities, ductal plate remodeling defects and periportal fibrosis in the liver. Our expression studies and biochemical analyses show that biliary abnormalities in Anks6-deficient livers result from the dysregulation of YAP transcriptional activity in the bile duct-lining epithelial cells. Mechanistically, our studies suggest, that ANKS6 antagonizes Hippo signaling in the liver during bile duct development by binding to Hippo pathway effector proteins YAP1, TAZ and TEAD4 and promoting their transcriptional activity. Together, this study reveals a novel function for ANKS6 in regulating Hippo signaling during organogenesis and provides mechanistic insights into the regulatory network controlling bile duct differentiation and morphogenesis during liver development.

TBX2-positive cells represent a multi-potent mesenchymal progenitor pool in the developing lung.

BACKGROUND: In the embryonic mammalian lung, mesenchymal cells act both as a signaling center for epithelial proliferation, differentiation and morphogenesis as well as a source for a multitude of differentiated cell types that support the structure of the developing and mature organ. Whether the embryonic pulmonary mesenchyme is a homogenous precursor pool and how it diversifies into different cell lineages is poorly understood. We have previously shown that the T-box transcription factor gene Tbx2 is expressed in the pulmonary mesenchyme of the developing murine lung and is required therein to maintain branching morphogenesis. METHODS: We determined Tbx2/TBX2 expression in the developing murine lung by in situ hybridization and immunofluorescence analyses. We used a genetic lineage tracing approach with a Cre line under the control of endogenous Tbx2 control elements (Tbx2cre), and the R26mTmG reporter line to trace TBX2-positive cells in the murine lung. We determined the fate of the TBX2 lineage by co-immunofluorescence analysis of the GFP reporter and differentiation markers in normal murine lungs and in lungs lacking or overexpressing TBX2 in the pulmonary mesenchyme. RESULTS: We show that TBX2 is strongly expressed in mesenchymal progenitors in the developing murine lung. In differentiated smooth muscle cells and in fibroblasts, expression of TBX2 is still widespread but strongly reduced. In mesothelial and endothelial cells expression is more variable and scattered. All fetal smooth muscle cells, endothelial cells and fibroblasts derive from TBX2+ progenitors, whereas half of the mesothelial cells have a different descent. The fate of TBX2-expressing cells is not changed in Tbx2-deficient and in TBX2-constitutively overexpressing mice but the distribution and abundance of endothelial and smooth muscle cells is changed in the overexpression condition. CONCLUSION: The fate of pulmonary mesenchymal progenitors is largely independent of TBX2. Nevertheless, a successive and precisely timed downregulation of TBX2 is necessary to allow proper differentiation and functionality of bronchial smooth muscle cells and to limit endothelial differentiation. Our work suggests expression of TBX2 in an early pulmonary mesenchymal progenitor and supports a role of TBX2 in maintaining the precursor state of these cells.

Delayed onset of smooth muscle cell differentiation leads to hydroureter formation in mice with conditional loss of the zinc finger transcription factor gene Gata2 in the ureteric mesenchyme.

The establishment of the peristaltic machinery of the ureter is precisely controlled to cope with the onset of urine production in the fetal kidney. Retinoic acid (RA) has been identified as a signal that maintains the mesenchymal progenitors of the contractile smooth muscle cells (SMCs), while WNTs, SHH, and BMP4 induce their differentiation. How the activity of the underlying signalling pathways is controlled in time, space, and quantity to activate coordinately the SMC programme is poorly understood. Here, we provide evidence that the Zn-finger transcription factor GATA2 is involved in this crosstalk. In mice, Gata2 is expressed in the undifferentiated ureteric mesenchyme under control of RA signalling. Conditional deletion of Gata2 by a Tbx18cre driver results in hydroureter formation at birth, associated with a loss of differentiated SMCs. Analysis at earlier stages and in explant cultures revealed that SMC differentiation is not abrogated but delayed and that dilated ureters can partially regain peristaltic activity when relieved of urine pressure. Molecular analysis identified increased RA signalling as one factor contributing to the delay in SMC differentiation, possibly caused by reduced direct transcriptional activation of Cyp26a1, which encodes an RA-degrading enzyme. Our study identified GATA2 as a feedback inhibitor of RA signalling important for precise onset of ureteric SMC differentiation, and suggests that in a subset of cases of human congenital ureter dilatations, temporary relief of urine pressure may ameliorate the differentiation status of the SMC coat. © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Mesothelial mobilization in the developing lung and heart differs in timing, quantity, and pathway dependency.

The mesothelial lining of the lung, the visceral pleura, and of the heart, the epicardium, derive from a common multipotent precursor tissue, the mesothelium of the embryonic thoracic cavity that also contributes to organ-specific mesenchymal cell types. Insight into mesothelial mobilization and differentiation has prevailedin the developing heart while the mesenchymal transition and fate of the visceral pleura are poorly understood. Here, we use the fact that the early mesothelium of both the lung and the heart expresses the transcription factor gene Wt1, to comparatively analyze mesothelial mobilization in the two organs by a genetic cre-loxP-based conditional approach. We show that epicardial cells are mobilized in a large number between E12.5 and E14.5, whereas pleural mobilization occurs only sporadically and variably in few regions of the lung in a temporally highly confined manner shortly after E12.5. Mesothelium-specific inactivation of unique pathway components using a Wt1creERT2 line excluded a requirement for canonical WNT, NOTCH, HH, TGFB, PDGFRA, and FGFR1/FGFR2 signaling in the mesenchymal transition of the visceral pleura but indicated a deleterious effect of activated WNT, NOTCH, and HH signaling on lung development. Epicardial mobilization was negatively impacted on by loss of HH, PDGFRA, FGFR1/2 signaling. Epicardial overactivation of WNT, NOTCH, and HH disturbed epicardial and myocardial integrity. We conclude that mesothelial mobilization in the developing lung and heart differs in timing, quantity and pathway dependency, indicating the organ specificity of the program.

TBX2 and TBX3 act downstream of canonical WNT signaling in patterning and differentiation of the mouse ureteric mesenchyme.

The organized array of smooth muscle cells (SMCs) and fibroblasts in the walls of visceral tubular organs arises by patterning and differentiation of mesenchymal progenitors surrounding the epithelial lumen. Here, we show that the TBX2 and TBX3 transcription factors have novel and required roles in regulating these processes in the murine ureter. Co-expression of TBX2 and TBX3 in the inner mesenchymal region of the developing ureter requires canonical WNT signaling. Loss of TBX2/TBX3 in this region disrupts activity of two crucial drivers of the SMC program, Foxf1 and BMP4 signaling, resulting in decreased SMC differentiation and increased extracellular matrix. Transcriptional profiling and chromatin immunoprecipitation experiments revealed that TBX2/TBX3 directly repress expression of the WNT antagonists Dkk2 and Shisa2, the BMP antagonist Bmper and the chemokine Cxcl12 These findings suggest that TBX2/TBX3 are effectors of canonical WNT signaling in the ureteric mesenchyme that promote SMC differentiation by maintaining BMP4 and WNT signaling in the inner region, while restricting CXCL12 signaling to the outer layer of fibroblast-fated mesenchyme.

Retinoic acid signaling maintains epithelial and mesenchymal progenitors in the developing mouse ureter.

The differentiated cell types of the mature ureter arise from the distal ureteric bud epithelium and its surrounding mesenchyme. Uncommitted epithelial cells first become intermediate cells from which both basal and superficial cells develop. Mesenchymal progenitors give rise to separated layers of adventitial fibrocytes, smooth muscle cells and lamina propria fibrocytes. How progenitor expansion and differentiation are balanced is poorly understood. Here, we addressed the role of retinoic acid (RA) signaling in these programs. Using expression analysis of components and target genes, we show that pathway activity is restricted to the mesenchymal and epithelial progenitor pools. Inhibition of RA signaling in ureter explant cultures resulted in tissue hypoplasia with a relative expansion of smooth muscle cells at the expense of lamina propria fibroblasts in the mesenchyme, and of superficial cells at the expense of intermediate cells in the ureteric epithelium. Administration of RA led to a slight reduction of smooth muscle cells, and almost completely prevented differentiation of intermediate cells into basal and superficial cells. We identified cellular programs and transcriptional targets of RA signaling that may account for this activity. We conclude that RA signaling is required and sufficient to maintain mesenchymal and epithelial progenitors in early ureter development.

A SHH-FOXF1-BMP4 signaling axis regulating growth and differentiation of epithelial and mesenchymal tissues in ureter development.

The differentiated cell types of the epithelial and mesenchymal tissue compartments of the mature ureter of the mouse arise in a precise temporal and spatial sequence from uncommitted precursor cells of the distal ureteric bud epithelium and its surrounding mesenchyme. Previous genetic efforts identified a member of the Hedgehog (HH) family of secreted proteins, Sonic hedgehog (SHH) as a crucial epithelial signal for growth and differentiation of the ureteric mesenchyme. Here, we used conditional loss- and gain-of-function experiments of the unique HH signal transducer Smoothened (SMO) to further characterize the cellular functions and unravel the effector genes of HH signaling in ureter development. We showed that HH signaling is not only required for proliferation and SMC differentiation of cells of the inner mesenchymal region but also for survival of cells of the outer mesenchymal region, and for epithelial proliferation and differentiation. We identified the Forkhead transcription factor gene Foxf1 as a target of HH signaling in the ureteric mesenchyme. Expression of a repressor version of FOXF1 in this tissue completely recapitulated the mesenchymal and epithelial proliferation and differentiation defects associated with loss of HH signaling while re-expression of a wildtype version of FOXF1 in the inner mesenchymal layer restored these cellular programs when HH signaling was inhibited. We further showed that expression of Bmp4 in the ureteric mesenchyme depends on HH signaling and Foxf1, and that exogenous BMP4 rescued cell proliferation and epithelial differentiation in ureters with abrogated HH signaling or FOXF1 function. We conclude that SHH uses a FOXF1-BMP4 module to coordinate the cellular programs for ureter elongation and differentiation, and suggest that deregulation of this signaling axis occurs in human congenital anomalies of the kidney and urinary tract (CAKUT).

Tbx2 and Tbx3 Act Downstream of Shh to Maintain Canonical Wnt Signaling during Branching Morphogenesis of the Murine Lung.

Numerous signals drive the proliferative expansion of the distal endoderm and the underlying mesenchyme during lung branching morphogenesis, but little is known about how these signals are integrated. Here, we show by analysis of conditional double mutants that the two T-box transcription factor genes Tbx2 and Tbx3 act together in the lung mesenchyme to maintain branching morphogenesis. Expression of both genes depends on epithelially derived Shh signaling, with additional modulation by Bmp, Wnt, and Tgfß signaling. Genetic rescue experiments reveal that Tbx2 and Tbx3 function downstream of Shh to maintain pro-proliferative mesenchymal Wnt signaling, in part by direct repression of the Wnt antagonists Frzb and Shisa3. In combination with our previous finding that Tbx2 and Tbx3 repress the cell-cycle inhibitors Cdkn1a and Cdkn1b, we conclude that Tbx2 and Tbx3 maintain proliferation of the lung mesenchyme by way of at least two molecular mechanisms: regulating cell-cycle regulation and integrating the activity of multiple signaling pathways.

MicroRNA-199a-5p inhibition enhances the liver repopulation ability of human embryonic stem cell-derived hepatic cells.

BACKGROUND & AIMS: Current hepatic differentiation protocols for human embryonic stem cells (ESCs) require substantial improvements. MicroRNAs (miRNAs) have been reported to regulate hepatocyte cell fate during liver development, but their utility to improve hepatocyte differentiation from ESCs remains to be investigated. Therefore, our aim was to identify and to analyse hepatogenic miRNAs for their potential to improve hepatocyte differentiation from ESCs. METHODS: By miRNA profiling and in vitro screening, we identified miR-199a-5p among several potential hepatogenic miRNAs. Transplantation studies of miR-199a-5p-inhibited hepatocyte-like cells (HLCs) in the liver of immunodeficient fumarylacetoacetate hydrolase knockout mice (Fah(-/-)/Rag2(-/-)/Il2rg(-/-)) were performed to assess their in vivo liver repopulation potential. For target determination, western blot and luciferase reporter assay were carried out. RESULTS: miRNA profiling revealed 20 conserved candidate hepatogenic miRNAs. By miRNA screening, only miR-199a-5p inhibition in HLCs was found to be able to enhance the in vitro hepatic differentiation of mouse as well as human ESCs. miR-199a-5p inhibition in human ESCs-derived HLCs enhanced their engraftment and repopulation capacity in the liver of Fah(-/-)/Rag2(-/-)/Il2rg(-/-) mice. Furthermore, we identified SMARCA4 and MST1 as novel targets of miR-199a-5p that may contribute to the improved hepatocyte generation and in vivo liver repopulation. CONCLUSIONS: Our findings demonstrate that miR-199a-5p inhibition in ES-derived HLCs leads to improved hepatocyte differentiation. Upon transplantation, HLCs were able to engraft and repopulate the liver of Fah(-/-)/Rag2(-/-)/Il2rg(-/-) mice. Thus, our findings suggest that miRNA modulation may serve as a promising approach to generate more mature HLCs from stem cell sources for the treatment of liver diseases.

Wnt11 is required for oriented migration of dermogenic progenitor cells from the dorsomedial lip of the avian dermomyotome.

The embryonic origin of the dermis in vertebrates can be traced back to the dermomyotome of the somites, the lateral plate mesoderm and the neural crest. The dermal precursors directly overlying the neural tube display a unique dense arrangement and are the first to induce skin appendage formation in vertebrate embryos. These dermal precursor cells have been shown to derive from the dorsomedial lip of the dermomyotome (DML). Based on its expression pattern in the DML, Wnt11 is a candidate regulator of dorsal dermis formation. Using EGFP-based cell labelling and time-lapse imaging, we show that the Wnt11 expressing DML is the source of the dense dorsal dermis. Loss-of-function studies in chicken embryos show that Wnt11 is indeed essential for the formation of dense dermis competent to support cutaneous appendage formation. Our findings show that dermogenic progenitors cannot leave the DML to form dense dorsal dermis following Wnt11 silencing. No alterations were noticeable in the patterning or in the epithelial state of the dermomyotome including the DML. Furthermore, we show that Wnt11 expression is regulated in a manner similar to the previously described early dermal marker cDermo-1. The analysis of Wnt11 mutant mice exhibits an underdeveloped dorsal dermis and strongly supports our gene silencing data in chicken embryos. We conclude that Wnt11 is required for dense dermis and subsequent cutaneous appendage formation, by influencing the cell fate decision of the cells in the DML.

Tbx2 terminates shh/fgf signaling in the developing mouse limb bud by direct repression of gremlin1.

Vertebrate limb outgrowth is driven by a positive feedback loop that involves Sonic hedgehog (Shh) and Gremlin1 (Grem1) in the posterior limb bud mesenchyme and Fibroblast growth factors (Fgfs) in the overlying epithelium. Proper spatio-temporal control of these signaling activities is required to avoid limb malformations such as polydactyly. Here we show that, in Tbx2-deficient hindlimbs, Shh/Fgf4 signaling is prolonged, resulting in increased limb bud size and duplication of digit 4. In turn, limb-specific Tbx2 overexpression leads to premature termination of this signaling loop with smaller limbs and reduced digit number as phenotypic manifestation. We show that Tbx2 directly represses Grem1 in distal regions of the posterior limb mesenchyme allowing Bone morphogenetic protein (Bmp) signaling to abrogate Fgf4/9/17 expression in the overlying epithelium. Since Tbx2 itself is a target of Bmp signaling, our data identify a growth-inhibiting positive feedback loop (Bmp/Tbx2/Grem1). We propose that proliferative expansion of Tbx2-expressing cells mediates self-termination of limb bud outgrowth due to their refractoriness to Grem1 induction.

Phenotypical analysis of atypical PKCs in vivo function display a compensatory system at mouse embryonic day 7.5.

BACKGROUND: The atypical protein kinases C (PKC) isoforms ι/λ and ζ play crucial roles in many cellular processes including development, cell proliferation, differentiation and cell survival. Possible redundancy between the two isoforms has always been an issue since most biochemical tools do not differentiate between the two proteins. Thus, much effort has been made during the last decades to characterize the functions of aPKCs using gene targeting approaches and depletion studies. However, little is known about the specific roles of each isoform in mouse development. METHODOLOGY/PRINCIPAL FINDINGS: To evaluate the importance of PKCι in mouse development we designed PKCι deletion mutants using the gene targeting approach. We show that the deletion of PKCι, results in a reduced size of the amniotic cavity at E7.5 and impaired growth of the embryo at E8.5 with subsequent absorption of the embryo. Our data also indicate an impaired localization of ZO-1 and disorganized structure of the epithelial tissue in the embryo. Importantly, using electron microscopy, embryoid body formation and immunofluorescence analysis, we found, that in the absence of PKCι, tight junctions and apico-basal polarity were still established. Finally, our study points to a non-redundant PKCι function at E9.5, since expression of PKCζ is able to rescue the E7.5 phenotype, but could not prevent embryonic lethality at a later time-point (E9.5). CONCLUSION: Our data show that PKCι is crucial for mouse embryogenesis but is dispensable for the establishment of polarity and tight junction formation. We present a compensatory function of PKCζ at E7.5, rescuing the phenotype. Furthermore, this study indicates at least one specific, yet unknown, PKCι function that cannot be compensated by the overexpression of PKCζ at E9.5.

Tbx2 controls lung growth by direct repression of the cell cycle inhibitor genes Cdkn1a and Cdkn1b.

Vertebrate organ development relies on the precise spatiotemporal orchestration of proliferation rates and differentiation patterns in adjacent tissue compartments. The underlying integration of patterning and cell cycle control during organogenesis is insufficiently understood. Here, we have investigated the function of the patterning T-box transcription factor gene Tbx2 in lung development. We show that lungs of Tbx2-deficient mice are markedly hypoplastic and exhibit reduced branching morphogenesis. Mesenchymal proliferation was severely decreased, while mesenchymal differentiation into fibrocytes was prematurely induced. In the epithelial compartment, proliferation was reduced and differentiation of alveolar epithelial cells type 1 was compromised. Prior to the observed cellular changes, canonical Wnt signaling was downregulated, and Cdkn1a (p21) and Cdkn1b (p27) (two members of the Cip/Kip family of cell cycle inhibitors) were strongly induced in the Tbx2-deficient lung mesenchyme. Deletion of both Cdkn1a and Cdkn1b rescued, to a large degree, the growth deficits of Tbx2-deficient lungs. Prolongation of Tbx2 expression into adulthood led to hyperproliferation and maintenance of mesenchymal progenitor cells, with branching morphogenesis remaining unaffected. Expression of Cdkn1a and Cdkn1b was ablated from the lung mesenchyme in this gain-of-function setting. We further show by ChIP experiments that Tbx2 directly binds to Cdkn1a and Cdkn1b loci in vivo, defining these two genes as direct targets of Tbx2 repressive activity in the lung mesenchyme. We conclude that Tbx2-mediated regulation of Cdkn1a and Cdkn1b represents a crucial node in the network integrating patterning information and cell cycle regulation that underlies growth, differentiation, and branching morphogenesis of this organ.

Notch signaling regulates smooth muscle differentiation of epicardium-derived cells.

RATIONALE: The embryonic epicardium plays a crucial role in the formation of the coronary vasculature and in myocardial development, yet the exact contribution of epicardium-derived cells (EPDCs) to the vascular and connective tissue of the heart, and the factors that regulate epicardial differentiation, are insufficiently understood. OBJECTIVE: To define the role of Notch signaling in murine epicardial development. METHODS AND RESULTS: Using in situ hybridization and RT-PCR analyses, we detected expression of a number of Notch receptor and ligand genes in early epicardial development, as well as during formation of coronary arteries. Mice with epicardial deletion of Rbpj, the unique intracellular mediator of Notch signaling, survived to adulthood and exhibited enlarged coronary venous and arterial beds. Using a Tbx18-based genetic lineage tracing system, we show that EPDCs give rise to fibroblasts and coronary smooth muscle cells (SMCs) but not to endothelial cells in the wild type, whereas in Rbpj-deficient embryos EPDCs form and surround the developing arteries but fail to differentiate into SMCs. Conditional activation of Notch signaling results in premature SMC differentiation of epicardial cells and prevents coronary angiogenesis. We further show that Notch signaling regulates, and cooperates with transforming growth factor ß signaling in SM differentiation of EPDCs. CONCLUSIONS: Notch signaling is a crucial regulator of SM differentiation of EPDCs, and thus, of formation of a functional coronary system.