Postnatal functions of Hoxa5 in the mouse brain

Hoxa5 is member of the Hox genes family coding for transcription factors which plays critical roles during the successive steps of central nervous system development. At the beginning of this work, Hoxa5 expression had been reported in the adult mouse brain in pattern and level of expression suggesting that it could be functionally required in the postnatal brain. To better understand the potential functions of Hoxa5 in the brain we first characterized its spatiotemporal expression pattern in the brain from embryonic to adult stages. Our results show that Hoxa5 expression starts in the caudal part of the hindbrain during fetal development, where it is maintained until adulthood. In the medulla oblongata and the pons Hoxa5 expression is detected mainly in the precerebellar system and in several nuclei involved in the control of autonomic functions. In these territories, HOXA5 protein is localized in glutamatergic, γ-aminobutyric acid (GABA)ergic, and few catecholaminergic neurons. In the postnatal brain, we also detected Hoxa5 transcripts, but not the protein, in the cerebellum, the thalamus and the cortex. We provide evidence that some larger variants of Hoxa5 transcripts are present in these territories. Combined with data published in parallel, these results allowed us to suggest that HOXA5 could be required for the maturation and the neuronal plasticity in the postnatal brain. To test these hypotheses, we validated a postnatal model of Hoxa5 loss-of-function using the CreERT2 system for inducible conditional mutagenesis. We then analyzed the transcriptional program downstream of HOXA5 in the brainstem at postnatal day 21. Our results show that HOXA5 regulates many genes associated to synaptic functions. Gene ontology and data mining analyses highlight that HOXA5 controls numerous transcripts involved in glutamate synapse, GABAergic synapse, calcium signaling but also cell-surface proteins and secreted proteins. Some of these data were confirmed and refined by reverse transcription quantitative polymerase chain reaction analysis. The expression of candidate target genes was shown to co-localize with Hoxa5 transcripts in precerebellar nuclei. Moreover, analysis of Allen Brain Atlas ISH data during postnatal development revealed that several HOXA5 targets are more expressed in the posterior part of pontine nuclei than in the anterior part, in a pattern similar to that described for HOXA5 and its paralogous genes. Together, our results revealed that HOXA5, through the regulation of key players of the glutamatergic/GABAergic synapses, calcium signaling, cell surface and secreted molecules, might be involved in synaptogenesis, selection of synaptic partner, synaptic transmission, and synaptic plasticity of the precerebellar circuitry in the postnatal brainstem.