JNK as an innovative target to tackle rett syndrome

Rett syndrome (RTT) is a rare and progressive neurodevelopmental disorder that occurs in 1:10,000-15,000 females. RTT is characterized by normal early growth followed by motor and intellectual regression. RTT is caused by mutations of MECP2 gene, located on X-chromosome and subjected to random inactivation. Methyl-CpG-binding protein-2 (MeCP2) is a transcriptional factor involved in brain connectivity, neural circuits and importantly, in synaptic deficits. However, the molecular mechanisms related with these defects are largely unknown. In our 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 inhibition, using the cell permeable D-JNKI1 peptide, led to a recovery of dendritic spines structure and functionality and a rescue of cognitive deficits. We discovered that JNK signalling is a key modulator also of neurodegenerative RTT pathways. The MeCP2 tm 1.1 Bird male mice (referred as Mecp2 y/-) were chosen for our evaluation because of their most severe phenotype but present a genetic background homogeneity. 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. Wellbeing 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. To make more consistent our results, we used human iPSCs differentiated in neurons, that provides the control and mutant experimental neurons. Here we found a strong activation of JNK’s preferential target, c-Jun, a nuclear transcription factor in all brain areas considered. 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. 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. 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 human phenotype.