Regulation and molecular function of the T-box transcription factors Tbx2 and Tbx3 in the development of the murine lung
Final Report Abstract
The development of vertebrate organs relies on the precise spatiotemporal orchestration of proliferation rates and differentiation patterns in adjacent tissue compartments. During branching morphogenesis of the mammalian embryonic lung, a number of signals secreted from the epithelial endodermal branch tips maintain the underlying mesenchyme in an undifferentiated proliferative state. This, in turn, is required to maintain precursor cells in the distal lung bud epithelium, and allow continued outgrowth of the epithelial tree. Although individual signals that maintain proliferation of precursor tissues have been identified in the development of many organs, including the lung, it has remained unclear how different epithelial and mesenchymal signals are integrated and translated into cell-cycle control. Our previous work had shown that the closely related T-box transcriptional repressor genes Tbx2 and Tbx3 are co-expressed in the mesenchymal compartment of the developing mouse lung, and that lungs of Tbx2-deficient mice are markedly hypoplastic and exhibit reduced branching morphogenesis. We found that TBX2 maintains proliferation and inhibits differentiation of the lung mesenchyme by directly repressing the transcription of genes (Cdkn1a, Cdkn1b) inhibiting the progression of the cell cycle. In our project, we wished to further characterize the phenotypic changes associated with the individual and combined loss of Tbx2 and Tbx3 in the mouse lung, and define the molecular function of these transcription factors as a regulatory hub that translates epithelial signals into the maintenance of proliferation and inhibition of differentiation in the lung mesenchyme. Our work characterized SHH, BMPs and TGFb as signals that induce and/or maintain expression of Tbx2 and Tbx3 in the embryonic lung, and defined TBX2/TBX3 as the crucial downstream mediators of epithelial SHH signals in the proliferative expansion of the lung mesenchyme. Defects in branching morphogenesis and organ size of Tbx2 mutant lungs were strongly enhanced by additional loss of the Tbx3 gene in the lung mesenchyme indicating that TBX2 and TBX3 act combinatorially in maintaining the mesenchymal progenitor pool. Although Tbx2 (and Tbx3) are widely expressed in the lung mesenchyme, 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. We identified Il33, Ccn4, and the WNT antagonists Frzb1 and Shisa3 as direct targets of repressive activity of TBX2. Repression of the latter two genes is crucial to maintain WNT signaling and its pro-proliferative activity in the lung mesenchyme. We provided evidence that TBX2 interacts with homeobox proteins such as PBX1 and the HMG transcription factor HMGB2 to achieve target gene specificity. Identification of components of the NuRD chromatin remodeling and repression complex and of additional chromatin proteins as interaction partners of TBX2 suggests that target gene repression in the lung mesenchyme is mediated by a combination of histone deacetylation, histone methylation, DNA methylation and chromatin compaction. We conclude that Tbx2 and Tbx3 act downstream of SHH signaling to maintain proliferation of the lung mesenchyme by interacting with other tissue-specific transcription factors and the NuRD complex to repress cell-cycle inhibitors and WNT antagonists. Our finding resolved a TBX2-centered signaling axis for organ growth. It will be interesting to see whether this axis is reemployed in injury conditions and contributes to disease states of the lung.
Publications
- (2016). Tbx2 and Tbx3 Act Downstream of Shh to Maintain Canonical Wnt Signaling during Branching Morphogenesis of the Murine Lung. Developmental Cell 39:239-253
Lüdtke, T.H., Rudat, C., Wojahn, I., Weiss, A.-C., Kleppa, M.-J., Kurz, J., Farin, H.F., Moon, A., Christoffels, V.M. and Kispert, A.
(See online at https://doi.org/10.1016/j.devcel.2016.08.007) - (2017). A SHH-FOXF1-BMP4 signaling axis regulating growth and differentiation of epithelial and mesenchymal tissues in ureter development. PLoS Genet 13, e1006951
Bohnenpoll, T., Wittern, A.B., Mamo, T.M., Weiss, A.C., Rudat, C., Kleppa, M.J., Schuster-Gossler, K., Wojahn, I., Lüdtke, T.H., Trowe, M.O., Kispert, A.
(See online at https://doi.org/10.1371/journal.pgen.1006951) - (2019). Mesothelial mobilization in the developing lung and heart differs in timing, quantity, and pathway dependency. Am J Physiol Lung Cell Mol Physiol. 316(5):L767- L783
Lüdtke, T.H., Rudat, C., Kurz, J., Häfner, R., Greulich, F., Wojahn, I., Aydoğdu, N., Mamo, T.M., Kleppa, M.J., Trowe, M.O., Bohnenpoll, T., Taketo, M.M., Kispert, A.
(See online at https://doi.org/10.1152/ajplung.00212.2018) - (2019). TBX2- positive cells represent a multi-potent mesenchymal progenitor pool in the developing lung. Respiratory Research 20:292
Wojahn, I., Lüdtke, T.H., Christoffels, V.M., Trowe, M.O. and Kispert A.
(See online at https://doi.org/10.1186/s12931-019-1264-y) - (2021). Combined genomic and proteomic approaches reveal DNA binding sites and interaction partners of TBX2 in the developing lung. Respiratory Research 22:85
Lüdtke, T.H., Wojahn, I., Kleppa, M.-J., Schierstaedt, J., Christoffels, V.M., Künzler, P. and Kispert, A.
(See online at https://doi.org/10.1186/s12931-021-01679-y)
