EFFECTS OF ATM KINASE INHIBITION ON BRAIN DEVELOPMENT

The maintenance of a proper balance between excitatory and inhibitory neurotransmission (E/I ratio) is crucial for correct brain development, function and plasticity. Through its inhibitory action, GABA (gamma-aminobutyric acid) neurotransmitter is the principal regulator of E/I ratio, exerting a precise modulation of excitatory transmission. Conversely, at the early stages of neuronal maturation, GABA operates as an excitatory neurotransmitter directly evoking action potentials. Then, during development, it acquires its typical role of brake for neuronal activity through the fundamental process called “excitatory-to-inhibitory switch of GABA”, the postnatal transition of GABA transmission from excitatory to inhibitory, directly related to the action of the potassium-chloride co-transporter KCC2. Defects in GABA switch have been largely described in neurodevelopmental disorders such as epilepsy, autism and schizophrenia. In this context, we have recently unveiled a new role of ATM (Ataxia Telangiectasia Mutated), a protein kinase involved in DNA double strand breaks (DSB) repair, in orchestrating the maturation of GABAergic inhibition. Here we demonstrate that the exposure of wild-type neurons in a “critical window” during development to an inhibitor of ATM kinase activity (KU), a drug already exploited as therapeutic tool in oncology, accelerates the excitatory-to-inhibitory switch of GABA. We show that the molecular mechanism underlying KU effect involves the transcription factor Egr4 and the epigenetic regulator MeCP2, which independently and in parallel boost KCC2 expression both in vitro and in vivo. The resultant neuronal network exhibits a potentiated inhibitory synaptic transmission and appears resistant to a hyper-excitability paradigm. Surprisingly, we found an increased expression of ATM associated to low levels of KCC2 in Mecp2y/- mice, the genetic model of Rett syndrome (RTT), a neurodevelopmental disorder associated to mental retardation. Coherently, KU treatment in Mecp2y/- neurons, potentiating Egr4 activity on Kcc2b promoter and restoring proper KCC2 expression, rescues the delayed GABA switch and counteracts the pharmacologically-induced hyper-excitability, suggesting that increased ATM levels contribute to the generation of the altered neuronal phenotype in RTT. The results collected in this thesis provide new evidence and molecular mechanisms of ATM crucial role in the physiological development of central neurons and highlight ATM inhibition as a prospective therapeutic tool in neurodevelopmental disorders.