JNK SIGNALLING PATHWAY AND ITS IMPLICATION IN RTT SYNDROME: STUDY ON SYNAPTIC PLASTICITY AND MORPHOLOGICAL PROFILE IN HUMAN AND IN MOUSE MODEL.

Rett syndrome (RTT) is a rare and progressive neurodevelopmental disorder that occurs in 1:10,000-15,000 females. RTT is characterized by a normal early growth followed by, above all, motor and cognitive regression. RTT is caused by mutations of MECP2 gene, located on X-chromosome and subjected to random inactivation, that generate a very variable phenotype. Methyl-CpG-binding protein-2 (MeCP2) is a transcriptional factor involved in brain connectivity, neural circuits and importantly, in synaptic plasticity and deficits. However, the molecular mechanisms related with these defects are largely unknown. In previous works, we showed that c-Jun N-terminal protein kinase (JNK), a stress-activated kinase, was strictly involved in synaptic dysfunction related to neurodegenerative disease (Alzheimer’s disease and ischemic stroke) and that its specific inhibition, using the cell permeable D-JNKI1 peptide, led to a recovery of dendritic spines structure and to restoration of functionality supported by a rescue of cognitive deficits. We here proved for the fist time that JNK signalling is powerfully activated in RTT mice and acts as a key modulator of synaptic dysfunctions. The MeCP2 tm 1.1 Bird male mice (referred as Mecp2 y/-) were chosen for our evaluation because, despite not presenting mosaicism, they show an early onset and a most severe phenotype within a homogeneity of genetic background. D-JNKI1 treatment (22mg/kg) was administrated for the first time at 3rd week of age with an intraperitoneal injection and repeated after 3 weeks. Well-being and behavioural studies were effectuated with a weekly examination of food and water intake, weight and locomotor abilities. At 7 weeks, mice were sacrificed and tissues were processed for biochemical evaluations. In brain, we found a strong activation of JNK’s preferential target, c-Jun, a nuclear transcription factor. D-JNKI1 chronic treatment improved general mice wellbeing. Treated mice showed a rescue of motor deficits and an improvement of motor coordination, parameters evaluated with Rotarod and Open field behavioural tests. Since RTT is characterized by locomotor impairment, we checked in cerebellum the synaptic dysfunction evaluating AMPA and NMDA receptors levels. Isolating the post-synaptic region, we found that D-JNKI1 treatment rescued receptor levels. Moreover, PSD95 and Shank3 analysis revealed that the decrement of Mecp2 -/y mice was reported to control level thanks to D-JNKI1 treatment. Moreover, our studies were focused on inflammatory pathway triggered by JNK and we found an astrogliosis and a microgliosis activation, completely rescue by D-JNKI1 treatment. We then move to translational medicine to strengthen the JNK pivotal role in RTT by using Human RTT iPSCs. The mutant neuronal-IPSc presented JNK activation while isogenic control neuronal-IPSc did not; furthermore, we found that D-JNKI1 inhibited JNK activity. The results on iPSCs added value to the clinical relevance of the proposed treatment. RTT is a rare and incurable progressive postnatal female neurodegenerative disorder and the manipulation of JNK pathway may represent the development of an innovative strategy to tackle Rett Syndrome. JNK plays had shown a key role in both mice and human mutated neuronal-IPSc and consequently its relevance in clinical study. We now need to better characterize D-JNKI1 effect in female mosaicist model, closer to the human phenotype.