Understanding and treating neuronal trafficking defects in Mecp2-pathies – ANTARES

Mecp2-pathies represent an emerging field among severe X-linked mental retardation syndromes. Rett Syndrome (RTT) represents the prototype of these pathologies and accounts for 10% of the cases of profound intellectual disability in females. The causative gene is the methyl CpG-binding protein 2 (MECP2) gene, encoding a multifunctional protein thought to "modulate" the expression of genes by several independent mechanisms. In addition to Rett syndrome, we now know that there are twice more patients (men and women) with a mutation in the MECP2 gene causing a neurological disease of varying severity. Globally, MECP2 mutations are affecting 30,000 new patients each year worldwide. Given that almost all of these cases are sporadic, genetic counselling and prenatal diagnosis are not possible and the prevalence of these diseases will not change. To date, no treatments are available in Mecp2-pathies. Recently, we made an important discovery demonstrating that axonal transport is altered in the absence of Mecp2. The brain derived neurotrophic factor (Bdnf) gene is one of Mecp2 targets. We found that the expression of several genes involved in Bdnf trafficking, such as huntingtin (Htt) and huntingtin-associated protein (Hap1), is reduced in the brain of adult Mecp2-deficient mouse. We showed that the velocity of Bdnf containing vesicles of Mecp2-deficient neurons is reduced and that re-expression of Mecp2 in these cells rescues this phenotype. The present project aims to further investigate the mechanisms leading to a neuronal trafficking impairment in Mecp2-deficient neurons and to identify potential pharmacological therapies to counterbalance such defects. Neuronal trafficking is not only dependent on the quantity of the molecular motors and associated proteins (dynein, dynactin, Htt, Hap1), but it also requires the proper function of several kinases. Akt pathway has been described by partner 2 to directly phosporylate Htt, leading to the stimulation of a dynamic neuronal trafficking pathway. Recently, it was shown that the phosporylation of Akt at serine 473 was decreased in the cortex of Mecp2-deficient mice. Therefore we will determine if the main partners of the Akt-pathway are affected in Mecp2 deficiencies, with a particular emphasis on Htt (a direct Akt-downstream target). We will also evaluate the direct impact of pharmacological interventions (FK506) able to maintain the level of Htt phosphorylation in Mecp2-deficient mice. The Akt pathway is also strongly dependent on the activation of insulin/IGF1 receptors. IGF1 is downregulated in the brain of Mecp2-deficient mouse. We will now evaluate wether the pharmacological stimulation of insulin/IGF1 receptors can alleviate the possible defects of the Akt signaling pathway, in vitro (cell cultures) and in vivo (mouse model). Our previous results showed that not only Htt and Hap1, but also other molecular motors, were drastically affected in neurons lacking Mecp2. Interestingly, these factors are involved in the vesicular Bdnf transport throughout the axon and also in the transport of mitochondria. Therefore, we will evaluate the possibility that part of the RTT phenotype is due to alterations of the mitochondrial transport, cell metabolism and energy supply, especially in neurons. The Mecp2 protein is located in the cell nucleus and is known as a master transcriptional regulator while molecular motors are located in the cytoplasm. In our previous results we proposed that Mecp2 dosage could alter the neuronal trafficking through transcriptional deregulation. We will further investigate this hypothesis as well as evaluate the alternative hypothesis that Mecp2 could act directly at the cytoplasmic level by a novel, still unsuspected mechanism. Altogether our aim is to improve the understanding of Mecp2-pathies, allowing us to devise and test new pharmacological therapies in pre-clinical animal models that will eventually be used to improve the clinical outcome of the patients.