Deciphering new Hox gene functions in the postnatal and adult brain

Our research activities are centred on a family of highly conserved homeodomain (HD) transcription factors, the HOX proteins, which play critical regulatory roles in both vertebrates and invertebrates. As a first step towards a functional analysis of Hox genes in the adult brain, we recently generated a neuroanatomical atlas of expression of the 39 Hox genes in the adult mouse brain. Our study provided several lines of evidence supporting the idea that Hox genes are recruited to serve new functions in mature neurons.
The main objective our group is to understand the biological pertinence of HOX presence in the brainstem after birth, a period characterized by important processes for maturation of neuronal circuitry. We also aim to decipher the underlying mechanisms of how HOX-dependent transcriptional regulatory networks control these processes. To begin addressing these issues, we focused on the HOXA5 protein and the pre-cerebellar system, which is essential for coordinated motor activity, and provide the main relay between cortex and cerebellum.

Mouse models of conditional inactivation
To investigate late functions of Hox genes in the CNS, viable conventional knock-out mice are not appropriate as their phenotype may reflect both early and late effect of the mutation. Using the CreERT2 system, we thus generated a novel loss-of-function mouse model, in which we inactivate Hoxa5 after its main phases of embryonic and fetal expression, at early postnatal stages (Hoxa5 cKO mice- Figure 1).

Identification of HOX functions in the postnatal brain : molecular and behavioural phenotyping of Hox cKO mutant mice
Using the conditional mouse model, we investigate the functional consequences of Hox gene inactivation in the postnatal brain, using several technologies: histological analysis, behavioural phenotyping and gene expression profiling of selected brainstem regions. Following the identification of the transcriptional regulatory network downstream of HOX in the brainstem, in vitro assays will be developed to evaluate the impact on neuronal behaviour (Figure 2).

Understanding Hox proteins mode of action : identification of rarget genes and interacting proteins
Very little is known about the downstream target loci HOX proteins regulate and how they are recruited to these loci as HOX protein complexes. An efficient strategy for identifying transcription factor direct targets exploits chromatin-immunoprecipitation (ChIP) methods to screen the tissue of interest. The coupling of ChIP with massively parallel DNA sequencing should allow identifying the binding sites of HOXA5 protein with high resolution. To further our understanding of the interacting network in which HOX protein are inserted, we will characterize the HOXA5 interactome.