Mecp2 knock-out astrocytes affect synaptogenesis by IL-6 dependent mechanisms

Rett syndrome (RTT) is a rare devastating neurodevelopmental disorder that represents the most common genetic cause of severe intellectual disability in girls, and mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene have been reported in over 95% cases of classical forms of RTT. Initial studies supported a role for MeCP2 exclusively in neurons, which are profoundly defective in RTT. Indeed, RTT is considered a synaptopathy, characterized by dendritic spine dysgenesis, impaired spine plasticity, and disrupted excitatory/inhibitory balance. However, many data support the involvement also of astrocytes, which can affect neuronal maturation through non-cell autonomous mechanisms. Nevertheless, many aspects of RTT astrocyte dysfunctions remain still unknown. 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 in vitro experiments. By using transwell-based co-cultures, we demonstrate that the lack of Mecp2 in cortical astrocytes dramatically affect the density of synaptic puncta and impair synaptic functionality in wild-type (WT) neurons. To gain insights into the involved molecular mechanisms, by transcriptomic analyses in WT neurons cultured with KO astrocytes we confirmed that KO astrocytes influence neuronal pathways mainly associated with synaptic maturation and functions, and we highlighted an increased inflammatory response in neurons as a putative target mechanism. qRT-PCR and flow cytometry assay indicated the up-regulated secretion of Interleukin-6 (IL-6) by Mecp2 null astrocytes. The synaptotoxic role of this cytokine was proved by observing that the recombinant IL-6 caused synaptic defects in WT neurons and, coherently, that a neutralizing IL-6 antibody rescued KO astrocytes-mediated defects. These in vitro data constitute the rationale to validate the detrimental role of IL-6 in Mecp2 heterozygous mice and explore pharmacological strategies targeting IL-6 to improve RTT symptoms.