Redundant and additive functions of the four Lef/Tcf transcription factors in lung epithelial progenitors.

In multicellular organisms, paralogs from gene duplication survive purifying selection by evolving tissue-specific expression and function. Whether this genetic redundancy is also selected for within a single cell type is unclear for multimember paralogs, as exemplified by the four obligatory Lef/Tcf transcription factors of canonical Wnt signaling, mainly due to the complex genetics involved. Using the developing mouse lung as a model system, we generate two quadruple conditional knockouts, four triple mutants, and various combinations of double mutants, showing that the four Lef/Tcf genes function redundantly in the presence of at least two Lef/Tcf paralogs, but additively upon losing additional paralogs to specify and maintain lung epithelial progenitors. Prelung-specification, pan-epithelial double knockouts have no lung phenotype; triple knockouts have varying phenotypes, including defective branching and tracheoesophageal fistulas; and the quadruple knockout barely forms a lung, resembling the Ctnnb1 mutant. Postlung-specification deletion of all four Lef/Tcf genes leads to branching defects, down-regulation of progenitor genes, premature alveolar differentiation, and derepression of gastrointestinal genes, again phenocopying the corresponding Ctnnb1 mutant. Our study supports a monotonic, positive signaling relationship between CTNNB1 and Lef/Tcf in lung epithelial progenitors as opposed to reported repressor functions of Lef/Tcf, and represents a thorough in vivo analysis of cell-type-specific genetic redundancy among the four Lef/Tcf paralogs.

Transcription factor 7-like 1 is involved in hypothalamo-pituitary axis development in mice and humans.

Aberrant embryonic development of the hypothalamus and/or pituitary gland in humans results in congenital hypopituitarism (CH). Transcription factor 7-like 1 (TCF7L1), an important regulator of the WNT/ß-catenin signaling pathway, is expressed in the developing forebrain and pituitary gland, but its role during hypothalamo-pituitary (HP) axis formation or involvement in human CH remains elusive. Using a conditional genetic approach in the mouse, we first demonstrate that TCF7L1 is required in the prospective hypothalamus to maintain normal expression of the hypothalamic signals involved in the induction and subsequent expansion of Rathke's pouch progenitors. Next, we reveal that the function of TCF7L1 during HP axis development depends exclusively on the repressing activity of TCF7L1 and does not require its interaction with ß-catenin. Finally, we report the identification of two independent missense variants in human TCF7L1, p.R92P and p.R400Q, in a cohort of patients with forebrain and/or pituitary defects. We demonstrate that these variants exhibit reduced repressing activity in vitro and in vivo relative to wild-type TCF7L1. Together, our data provide support for a conserved molecular function of TCF7L1 as a transcriptional repressor during HP axis development in mammals and identify variants in this transcription factor that are likely to contribute to the etiology of CH.

Tcf7l1 proteins cell autonomously restrict cardiomyocyte and promote endothelial specification in zebrafish.

Tcf7l1 (formerly Tcf3) proteins are conserved transcription factors whose function as transcriptional repressors is relieved through interactions with ß-catenin. Although the functions of Tcf7l1 proteins have been studied in many developmental contexts, whether this conserved mediator of Wnt signaling is required for appropriate cardiomyocyte (CM) development has not been investigated. We find that Tcf7l1 proteins are necessary during two developmental periods to limit CM number in zebrafish embryos: prior to gastrulation and after the initial wave of CM differentiation. In contrast to partially redundant roles in anterior neural patterning, we find that Tcf7l1a and Tcf7l1b have non-redundant functions with respect to restricting CM specification during anterior mesodermal patterning, suggesting that between the two zebrafish Tcf7l1 paralogs there is a limit to the transcriptional repression provided during early CM specification. Using cell transplantation experiments, we determine that the Tcf7l1 paralogs are required cell autonomously to restrict CM specification and promote endothelial cell (EC) specification, which is overtly similar to the ability of Wnt signaling to direct a transformation between these progenitors in embryonic stem cells. Therefore, these results argue that during anterior-posterior patterning of the mesoderm Tcf7l1 proteins are cell autonomously required to limit Wnt signaling, which balances CM and EC progenitor specification within the anterior lateral plate mesoderm. This study expands our understanding of the in vivo developmental requirements of Tcf7l1 proteins and the mechanisms directing CM development in vertebrates.

Repressor transcription factor 7-like 1 promotes adipogenic competency in precursor cells.

The identification of factors that define adipocyte precursor potential has important implications for obesity. Preadipocytes are fibroblastoid cells committed to becoming round lipid-laden adipocytes. In vitro, this differentiation process is facilitated by confluency, followed by adipogenic stimuli. During adipogenesis, a large number of cytostructural genes are repressed before adipocyte gene induction. Here we report that the transcriptional repressor transcription factor 7-like 1 (TCF7L1) binds and directly regulates the expression of cell structure genes. Depletion of TCF7L1 inhibits differentiation, because TCF7L1 indirectly induces the adipogenic transcription factor peroxisome proliferator-activated receptor γ in a manner that can be replaced by inhibition of myosin II activity. TCF7L1 is induced by cell contact in adipogenic cell lines, and ectopic expression of TCF7L1 alleviates the confluency requirement for adipocytic differentiation of precursor cells. In contrast, TCF7L1 is not induced during confluency of non-adipogenic fibroblasts, and, remarkably, forced expression of TCF7L1 is sufficient to commit non-adipogenic fibroblasts to an adipogenic fate. These results establish TCF7L1 as a transcriptional hub coordinating cell-cell contact with the transcriptional repression required for adipogenic competency.