Carsten Rudat

Tbx18 and the fate of epicardial progenitors.

Arising from: C.-L. Cai et al. 454, 104–108 (2008)10.1038/nature06969; Cai et al. replyUncovering the origins of myocardial cells is important for understanding and treating heart diseases. Cai et al. suggest that Tbx18-expressing epicardium provides a substantial contribution to myocytes in the ventricular septum and the atrial and ventricular walls. Here we show that the T-box transcription factor gene 18 (Tbx18) itself is expressed in the myocardium, showing that their genetic lineage tracing system does not allow conclusions of an epicardial origin of cardiomyocytes in vivo to be drawn.

Mechanisms of T-box gene function in the developing heart

The multi-chambered mammalian heart arises from a simple tube by polar elongation, myocardial differentiation and morphogenesis. Members of the large family of T-box (Tbx) transcription factors have been identified as crucial players that act in distinct subprogrammes during cardiac regionalization. Tbx1 and Tbx18 ensure elongation of the cardiac tube at the anterior and posterior pole, respectively. Tbx1 acts in the pharyngeal mesoderm to maintain proliferation of mesenchymal precursor cells for formation of a myocardialized and septated outflow tract. Tbx18 is expressed in the sinus venosus region and is required for myocardialization of the caval veins and the sinoatrial node. Tbx5 and Tbx20 function in the early heart tube and independently activate the chamber myocardial gene programme, whereas Tbx2 and Tbx3 locally repress this programme to favour valvuloseptal and conduction system development. Here, we summarize that these T-box factors act in different molecular circuits and control target gene expression using diverse molecular strategies including binding to distinct protein interaction partners.

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 &bgr; 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.

Wt1 and Epicardial Fate Mapping

Rationale: The embryonic epicardium is a crucial cell source of the cardiac fibrous skeleton as well as of the coronary system. Genetic lineage tracing systems based on Wt1 regulatory sequences provided evidence that epicardium-derived cells also adopt a myocardial fate in the mouse. Objective: To define the adequacy of Wt1-based lineage tracing systems for epicardial fate mapping. Methods and Results: Using in situ hybridization analysis and immunofluorescence on tissue sections, we detected endogenous expression of Wt1 mRNA and Wt1 protein in the proepicardium and epicardium and also in endothelial cells throughout cardiogenesis. Expression analysis of a sensitive GFP reporter showed that recombination mediated by cre recombinase in the Wt1creEGFP line occurs randomly and sporadically in all cells of the embryo. Recombination in cardiomyocytes was found in the linear heart tube before establishment of a (pro)epicardium. In contrast, the tamoxifen-inducible Wt1creERT2 mouse line mediated poor and variable recombination in the epicardium. Recombination in cardiomyocytes was not detected in this case. Conclusions: Frequently used Wt1 based cre-mediated lineage tracing systems are not suitable for epicardial fate mapping because of endogenous endothelial expression of Wt1, ectopic recombination (Wt1creEGFP), and poor recombination efficiency (Wt1creERT2) in the developing heart. We conclude that claims of a cardiomyocyte fate of epicardial cells in the mouse are not substantiated.

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.

Repression of Sox9 by Jag1 Is Continuously Required to Suppress the Default Chondrogenic Fate of Vascular Smooth Muscle Cells

Acquisition and maintenance of vascular smooth muscle fate are essential for the morphogenesis and function of the circulatory system. Loss of contractile properties or changes in the identity of vascular smooth muscle cells (vSMCs) can result in structural alterations associated with aneurysms and vascular wall calcification. Here we report that maturation of sclerotome-derived vSMCs depends on a transcriptional switch between mouse embryonic days 13 and 14.5. At this time, Notch/Jag1-mediated repression of sclerotome transcription factors Pax1, Scx, and Sox9 is necessary to fully enable vSMC maturation. Specifically, Notch signaling in vSMCs antagonizes sclerotome and cartilage transcription factors and promotes upregulation of contractile genes. In the absence of the Notch ligand Jag1, vSMCs acquire a chondrocytic transcriptional repertoire that can lead to ossification. Importantly, our findings suggest that sustained Notch signaling is essential throughout vSMC life to maintain contractile function, prevent vSMC reprogramming, and promote vascular wall integrity.

Nephric duct insertion requires EphA4/EphA7 signaling from the pericloacal mesenchyme

The vesico-ureteric junction (VUJ) forms through a complex developmental program that connects the primordium of the upper urinary tract [the nephric duct (ND)] with that of the lower urinary tract (the cloaca). The signals that orchestrate the various tissue interactions in this program are poorly understood. Here, we show that two members of the EphA subfamily of receptor tyrosine kinases, EphA4 and EphA7, are specifically expressed in the mesenchyme surrounding the caudal ND and the cloaca, and that Epha4−/−;Epha7+/− and Epha4−/−;Epha7−/− (DKO) mice display distal ureter malformations including ureterocele, blind and ectopically ending ureters with associated hydroureter, megaureter and hydronephrosis. We trace these defects to a late or absent fusion of the ND with the cloaca. In DKO embryos, the ND extends normally and approaches the cloaca but the tip subsequently looses its integrity. Expression of Gata3 and Lhx1 and their downstream target Ret is severely reduced in the caudal ND. Conditional deletion of ephrin B2 from the ND largely phenocopies these changes, suggesting that EphA4/EphA7 from the pericloacal mesenchyme signal via ephrin B2 to mediate ND insertion. Disturbed activity of this signaling module may entail defects of the VUJ, which are frequent in the spectrum of congenital anomalies of the kidney and the urinary tract (CAKUT) in human newborns.

Upk3b is dispensable for development and integrity of urothelium and mesothelium

The mesothelium, the lining of the coelomic cavities, and the urothelium, the inner lining of the urinary drainage system, are highly specialized epithelia that protect the underlying tissues from mechanical stress and seal them from the overlying fluid space. The development of these epithelia from simple precursors and the molecular characteristics of the mature tissues are poorly analyzed. Here, we show that uroplakin 3B (Upk3b), which encodes an integral membrane protein of the tetraspanin superfamily, is specifically expressed both in development as well as under homeostatic conditions in adult mice in the mesothelia of the body cavities, i.e., the epicardium and pericardium, the pleura and the peritoneum, and in the urothelium of the urinary tract. To analyze Upk3b function, we generated a creERT2 knock-in allele by homologous recombination in embryonic stem cells. We show that Upk3bcreERT2 represents a null allele despite the lack of creERT2 expression from the mutated locus. Morphological, histological and molecular analyses of Upk3b-deficient mice did not detect changes in differentiation or integrity of the urothelium and the mesothelia that cover internal organs. Upk3b is coexpressed with the closely related Upk3a gene in the urothelium but not in the mesothelium, leaving the possibility of a functional redundancy between the two genes in the urothelium only.

The transcription factor GATA4 promotes myocardial regeneration in neonatal mice

Heart failure is often the consequence of insufficient cardiac regeneration. Neonatal mice retain a certain capability of myocardial regeneration until postnatal day (P)7, although the underlying transcriptional mechanisms remain largely unknown. We demonstrate here that cardiac abundance of the transcription factor GATA4 was high at P1, but became strongly reduced at P7 in parallel with loss of regenerative capacity. Reconstitution of cardiac GATA4 levels by adenoviral gene transfer markedly improved cardiac regeneration after cryoinjury at P7. In contrast, the myocardial scar was larger in cardiomyocyte‐specific Gata4 knockout (CM‐G4‐KO) mice after cryoinjury at P0, indicative of impaired regeneration, which was accompanied by reduced cardiomyocyte proliferation and reduced myocardial angiogenesis in CM‐G4‐KO mice. Cardiomyocyte proliferation was also diminished in cardiac explants from CM‐G4‐KO mice and in isolated cardiomyocytes with reduced GATA4 expression. Mechanistically, decreased GATA4 levels caused the downregulation of several pro‐regenerative genes (among them interleukin‐13, Il13) in the myocardium. Interestingly, systemic administration of IL‐13 rescued defective heart regeneration in CM‐G4‐KO mice and could be evaluated as therapeutic strategy in the future.

Diversification of Cell Lineages in Ureter Development

The mammalian ureter consists of a mesenchymal wall composed of smooth muscle cells and surrounding fibrocytes of the tunica adventitia and the lamina propria and an inner epithelial lining composed of layers of basal, intermediate, and superficial cells. How these cell types arise from multipotent progenitors is poorly understood. Here, we performed marker analysis, cell proliferation assays, and genetic lineage tracing to define the lineage relations and restrictions of the mesenchymal and epithelial cell types in the developing and mature mouse ureter. At embryonic day (E) 12.5, the mesenchymal precursor pool began to subdivide into an inner and outer compartment that began to express markers of smooth muscle precursors and adventitial fibrocytes, respectively, by E13.5. Smooth muscle precursors further diversified into lamina propria cells directly adjacent to the ureteric epithelium and differentiated smooth muscle cells from E16.5 onwards. Uncommitted epithelial progenitors of the ureter differentiated into intermediate cells at E14.5. After stratification into two layers at E15.5 and three cell layers at E18.5, intermediate cells differentiated into basal cells and superficial cells. In homeostasis, proliferation of all epithelial and mesenchymal cell types remained low but intermediate cells still gave rise to basal cells, whereas basal cells divided only into basal cells. These studies provide a framework to further determine the molecular mechanisms of cell differentiation in the tissues of the developing ureter.