JNKS AS THERAPEUTIC TARGETS TO TACKLE SYNAPTIC DYSFUNCTION IN NEURODEVELOPMENTAL AND NEURODEGENERATIVE DISEASES

Brain disorders are the first leading cause of disability and the second leading cause of death worldwide. Decoding the mechanisms of brain diseases is mandatory to improve diagnosis, neuroprotective strategies, and treatments. Although there are many different pathological and clinical manifestations, as well as causes and molecular pathways underlying neurological disorders, increasing evidence suggest that synapses, having a crucial role in neuronal communication and brain functions, are the first station that degenerates and loses functionality in neurological diseases. This concept has led to the theory of brain disease, from neurodevelopmental to neurodegenerative, as synaptopathies, in which the synaptic impairment is a shared pathogenic feature. The main aim of this thesis was to dissect this concept, focusing on a specific pathway particularly involved in brain functions and dysfunctions: the c-Jun N-terminal Kinase (JNK) signalling pathway. More in details, we studied the role of JNK in two neurodevelopmental syndromes, Rett and Angelman, and in a chronic brain illness, Alzheimer disease, focusing on the synaptopathy as the first initial mechanism of many different brain diseases. In Rett Syndrome, a rare severe developmental disease, we studied the synaptic dysfunction of two different murine models identifying JNK as an important actor downstream MECP2, the gene mutated in the pathology. We demonstrated that the specific inhibition of JNK, by D-JNKI1 treatment, strongly improved the symptoms and the molecular disorganization of the PSD region in both mice models. Then, we proved JNK activation in human neurons, differentiated from human MECP2-mutated iPSCs (MECP2mut), compared to the isogenic control expressing wild-type MECP2 allele (MECP2wt). The JNK signal was activated in the MECP2-mutated iPSCs and not in control iPSCs, and D-JNKI1 blocked the MECP2mut -induced neuronal death. In Angelman Syndrome, we analysed the synaptic dysfunction in the UBE3A+/- mouse model. JNK was strongly activated in the brain of these mice, suggesting its important role also in this disorder. The D- JNKI1 treatment improved the behavioural defects, and this correlated with the stabilization of the synaptic biomarkers. Finally, we focused on the Alzheimer’s synaptic dysfunction, the most characterize synaptopathy, to study JNK role in a chronic illness. Alzheimer mice model 5XFAD presented a powerful JNK activation together with synaptic dysfunction and cognitive impairment. We then centre our attention on the selective brain JNK isoform, JNK3, the most responsive to stress stimuli, finding that 5XFAD mice presented a powerful increase of JNK3 levels. Therefore, we decided to test the potential neuroprotective effect of the specific inhibition of JNK3. In fact, thanks to the collaboration with Prof. Falconi, we have in the laboratory the specific JNK3 inhibitor, dSIMBA2. Before testing this in-vivo, we studied the effect of JNK3 inhibition in an in-vitro model of synaptic dysfunction induced by Ab oligomers (ABO) to define the right dose and the neuroprotection against ABO-induced synaptic dysfunction, finding that dSIMBA2 prevents ABO toxicity and the induced-synaptopathy. 5 To summarize, the data obtained in this thesis participate in the reinforcement of JNK as a central actor in the degeneration mechanisms of the synapses that characterized both neurodevelopmental and neurodegenerative diseases. In addition, this work adds new elements to define JNK3, the most responsive JNK isoform to stress, as a key player in the AD, and an important target for the treatment of synaptic injury, potentially also in other brain illnesses. Developing new compounds able to inhibit one of the most responsive kinases to stress stimuli may be an intriguing field to explore to prevent synaptic dysfunction.