MISS-SA · Molecular mechanisms involved in the IGF-1R signalling setting of the somatotropic axis

Molecular mechanisms involved in the IGF-1R signalling setting of the somatotropic axis

Project context and objectives Heterozygous invalidation and that experienced as a transient nutritional restriction during the early postnatal period (suckling). In both cases, the miss-programming of the somatotropic axis observed modifies life trajectory, modulating global growth of the organism, as well as its susceptibility to develop cardio-metabolic pathologies as adult-like diabetes and arterial hypertension. In particular, we studied two major hypothalamic neurons secreting neuropeptides implicated in the control of the somatotropic axis: growth hormone releasing hormone (GHRH) and somatostatin (SRIH) neurons. Indeed, these two neuronal populations, located in the brain, demonstrate a persistent alteration in both our models. With the support of this Marie Curie fellowship, Laurent Kappeler start to determine molecular mechanisms involved in this programming of the somatotropic axis. The outgoing phase of this Marie Curie fellowship was performed in the lab of Prof. Michael Meaney (McGill University, Montreal, Canada), who is internationally renowned in studies of the neuroendocrine stress axis by maternal behaviour. There, Laurent Kappeler first counted SRIH and GHRH neurons in the transgenic mouse model. The similar number of both SRIH and GHRH neurons suggested a transcriptional alteration. These effects strongly suggest epigenetic influences in programming neuronal expression. He thus studied histone post-translational modifications (PTM) as well as DNA methylation in proximal promoter of these two genes. Accordingly, ChIP was studied against two positive marks: histone H3 acetylated on the lysine 9 (H3K9ac) or histone H4 monomethylated on the lysine 20 (H4K20me1) indicates their increased enrichment on srih promoter of adult mice previously restricted during early postnatal period when compared to the control. These changes are associated with a decreased frequency of cytosine methylation in the CpG island present in the srih promoter, and correlate well with the persistent increased transcription. In contrast, no modifications were observed in the promoter of GHRH. Similar results have been obtained with the transgenic mouse model carrying heterozygous invalidation of the IGF-1R in brain specifically. These results highlight the association between epigenetic alterations at the level of the srih promoter with the miss-programming of the somatotropic axis. Importantly, he infused epigenetic modifiers intracerebroventrically (icv) in adults for 14 days, in order to determine if epigenetic mechanisms effectively control SRIH and GHRH expression. Infusion of Trichostatin A (TSA), a deacetylase inhibitor, in adult control mice was performed to mimic the restricted phenotype regarding epigenetic status. In agreement with our hypothesis, control mice infused with TSA show increased association of H3K9ac and H4K20me1 with the srih promoter. In contrast, restricted mice icv-infused with the methyl donor L-methionine have a strongly decreased association of H4K20me1 with srih promoter. Gene expression levels are consistent with the epigenetic status: normal mice infused with TSA present increased SRIH mRNA levels and a trend for decreased GHRH ones, as compared to saline-infused litter mates. On the other hand, restricted mice infused with methionine show decreased levels of SRIH associated with a strong increase of GHRH mRNA levels, as compared to saline-infused litter mates. During the outgoing phase of this Marie Curie fellowship, Laurent Kappeler highlighted a proof of concept that somatotropic axis, and thus life trajectory, could be programmed by early postnatal food supply, largely through epigenetic mechanisms. The return phase of this fellowship was performed in the Inserm lab headed by Prof. Yves Le Bouc (St Antoine research centre, Paris, France), who is internationally renowned for the study of epigenetic implications in growth disorders, and notably those involving DNA methylation alterations. Laurent Kappeler, as permanent Inserm researcher, studied DNA methylation of icv-infused mice and implemented techniques learned in Canada (µChIP) for the study of histones PTMs. In parallel, he succeeded in encouraging a postdoc fellow to start studying IGF-1 effects on GHRH axon growth. Indeed, during the outgoing phase, he observed that programming the somatotropic axis did not involve epigenetic or neuronal loss for the GHRH population. These results indicate that formerly observed alterations of GHRH, which are responsible of the permanent pituitary hypoplasia in somatotrophs (GH+) cells, a key element in the somatotropic axis programming, may be due to a growth axon delay. Such effects have recently been reported for IGF-I and are in agreement with previous results obtained by Laurent Kappeler. Preliminary results obtained by the fellow and the postdoc Erik Mire on GHRH-eGFP mice suggest a stimulating effect of IGF-I on GHRH axon growth. This concept still requires experimental testing, but is very innovative and will open up important avenues of new research.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз