Functional and genetic crosstalks linking Wnt signalling to energy balance in the liver – WNT-METABOLIV

The liver plays a key role in metabolic homeostasis, as it can either store excess energy or provide nutriments for the other organs following the nutritional state. Liver anabolic and catabolic functions are realized in different regions of the lobule, along a porto-central gradient defining the liver metabolic zonation. Partner 1 recently showed that Wnt signalling is a master regulator of liver zonation. It induces a pericentral genetic program while its repression is required for the expression of the periportal genetic program and is thus a main determinant of the gene expression gradient that exists along the porto-central axis for most of the key metabolic regulators. We have identified the ammonia metabolism as a main target of the Wnt pathway both at the molecular and physiological level. But, although our recent data points to a role of the Wnt pathway on the glucose metabolism, its implication on the control of energy metabolism remains to be determined.
In this proposal we will use combined genetic and metabolic approaches to unravel the role of the Wnt/beta-catenin in the control of energy balance, with a methodology based on the intensive use of murine models of conditional gene invalidation of the beta-catenin gene, generating a periportalized liver and of the tumor suppressor gene Apc generating a pericentralized liver. These mice models have already allowed to undertake a global high-throughput analysis to unravel in the liver the nuclear network responsible for beta-catenin dependent gene expression, using a mass spectrometry approach to identify nuclear partners of beta-catenin, and deep-sequencing approaches to identify RNA targets and DNA-binding sites for the beta-catenin/Tcf4 complex. Preliminary analysis of the data suggests that a cross-talk between the beta-catenin signaling and nuclear receptors (Hnf4, PPAR or COUPTF-II, which are metabolic regulators) may be involved in the transcriptional control of a set of beta-catenin target genes. We also have unpublished data supporting a cross-talk between the beta-catenin cascade and insulin signaling, which is a key regulatory pathway for energy metabolism.
The objectives of this proposal are: i) to dissect the molecular mechanisms by which Wnt/ beta-catenin signaling imposes in the liver a unique metabolic genetic program; ii) to determine how the Wnt/ beta-catenin signalling controls the hepatic energy metabolism and regulates the energy balance, and Wnt/ beta-catenin interrelations with insulin and nuclear receptor pathways.
The raw data generated by high throughput analyses are currently available, and will be treated through a software pipeline developed by a bioinformatician in Partner 1 team. Reliable in silico analyses, coupled with in vitro and in vivo validations, will be performed to better define the role of Tcf4 in beta-catenin dependent and independent transcription, to discover and validate DNA regulatory modules that relay liver zonation, and to define the beta-catenin metabolic targetome. The association with partners who are expert in metabolic studies will allow, by both ex vivo and in vivo approaches, to decipher the role of the beta-catenin cascade on liver glucose and lipid homeostasis and its consequences on the energy balance of the organism. This will also be tested under challenging metabolic conditions, a high fat-high sucrose diet, known to induce hepatic steatosis, obesity and insulin resistance. In addition, the contribution of the insulin, Hnf4, PPAR and COUPTF-II pathways to this metabolic phenotype will be depicted through the use of mice models of various insulin sensitivity and mice deficient in Hnf4a, PPARa or COUPTF-II, intercrossed with mouse models of beta-catenin gain and loss of function.