MeCP2 deficiency in astrocytes alters synaptogenesis: new insights on Rett syndrome

Rett syndrome (RTT; OMIM# 312750) is a rare devastating neurodevelopmental disorder that represents the most common genetic cause of severe intellectual disability in girls. Mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene have been reported in over 95% cases of classical forms of RTT. Although initial studies supported a role for MeCP2 exclusively in neurons, recent data indicate a function also in astrocytes, which emerged as critical players involved in RTT pathogenesis through non-cell autonomous effects. Nevertheless, many aspects of RTT astrocyte dysfunctions remain unknown. RTT is considered a synaptopathy characterized by dendritic spine dysgenesis, impaired spine plasticity, alteration of neuronal excitability and disrupted excitatory/inhibitory balance of circuits. According to the crucial role of astrocytes in promoting synapse formation and functioning, we have investigated the influence of Mecp2 null astrocytes on synaptic phenotype. By using in vitro transwell-based co-cultures or neuronal treatment with astrocyte conditioned medium (ACM), we have demonstrated that the lack of Mecp2 in cortical astrocytes dramatically influences the synaptogenesis of WT neurons. In particular, KO astrocytes release one or more thermo-labile factors, that detrimentally affect both the pre- and post-synaptic terminals. To gain insights on the involved molecular mechanisms, we used an indirect approach, by performing bulk RNA-Sequencing on WT neurons co-cultured with KO astrocytes. We have thus profiled the molecular pathways activated in WT neurons by the paracrine effects triggered by KO cortical astrocytes and by bioinformatic analyses we confirmed that KO astrocytes influence neuronal pathways mainly associated with synaptic maturation and functions and elicit inflammatory responses. qPCR and luminex assay indicated the up-regulated release of a subset of cytokines by null astrocytes, with a possible synaptotoxic effect. We are currently validating the role of two of these proteins on the occurrence of synaptic defects, being aware that the identification of the involved factors might reveal novel therapeutic approaches for the treatment of RTT.