PHARMACOLOGICAL MODULATION OF NEURONAL ACTIVITY FOR THE TREATMENT OF RETT SYNDROME

Rett syndrome (RTT) is a severe neurodevelopmental disorder with an incidence of 1 out of 10000 females born alive. Although considered a rare syndrome, RTT is the most common genetic cause of severe intellectual disability in females worldwide. A typical feature of RTT is the apparently normal early development, followed by a dramatic regression phase that usually occurs 6-18 months after birth. From this moment on, patients begin to present the various classic symptoms of the pathology. However, some recent reports revealed that a series of symptoms, such as hypotonia, jerkiness in limb movements and limited social interactions, are present since early infancy, pointing out that molecular impairment occurs before the onset of clinics symptoms. Nonetheless, their contribution to the pathogenesis is still not understood. More than 95% of classical RTT cases are caused by mutations in the X-linked MECP2 gene and in line with the role of MeCP2 as a master regulator of gene expression, transcriptional maturation is affected in null neurons both in vivo and in vitro, as well as the ability of null neurons to respond to external stimuli. Neuronal activity plays a key role to maintain proper functions of mature neurons, and to ensure their proper development. Accordingly, slight variations from physiological ranges lead to severe consequences, like neurodevelopmental disorders. We thus tested the possible causative link between immaturity and reduced neuronal activity by pharmacologically stimulating Mecp2 null neurons within different time windows of brain maturation. Although, most of the pre-clinical studies so far focused on juvenile or adult mouse models of the disease, one of the main purposes of this study was to identify the optimal time window of intervention for the selected therapeutic approach, also testing the value of anticipating the treatment in a pre-symptomatic period to delay the progression of the disease. To trigger neuronal activity, we selected Ampakine drugs that positively modulate AMPA receptors after glutamate binding. Their binding prolongs the flow of current through the channel enhancing synaptic response and LTP. Two different drugs of the class of Ampakines were tested in vivo using the full knock out mouse model of the disease. The efficacy of the treatments on life span and motor and cognitive defects was established. CX546 ameliorated behavioral defects and prolonged lifespan of Mecp2 null mice in translational time window of intervention. However, the drug induced some discomfort in the treated knock out animals. This evidence together with its poor value for clinical trial led us to test a new generation Ampakine (CX1632) that was already proved safe in clinical trials. This new drug, tested in pre-symptomatic and early symptomatic time windows of intervention, confirmed its total safety in vivo and, more importantly, demonstrated beneficial effects at both the behavioral and molecular level in the mouse model of RTT. However, early treatment revealed more powerful in inducing persistent therapeutic effects. Molecular analyses confirmed the activation of AMPAr associated pathways and the recovery of some RTT defects associated to neuronal activity. All in all, our results support the value of an early therapeutic approach acting on neuronal activity as a strategy for the cure of Rett syndrome.