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

Rett syndrome (RTT; OMIM# 312750) is a devastating and progressive neurodevelopmental disorder representing the first cause of severe intellectual disability in girls worldwide. Over 95% of individuals suffering from the classic form of RTT carry sporadic mutations in the X-linked methyl-CpG-binding protein 2 (MECP2) gene. No cure is available and the identification of therapies is hampered by the need of maintaining physiological levels of MeCP2 in brain and the limited comprehension of RTT pathogenesis. An hallmark of RTT is represented by synaptic abnormalities, leading to consider the disease as a synaptopathy. Although initial studies supported a role for MeCP2 exclusively in neurons, experiments on cell cultures and on conditional Mecp2 KO animals demonstrated that also astrocytes are integral components of RTT, but many aspects of their contribution remain still unknown. Given the fundamental role of astrocytes in promoting synapse formation and functioning, we have investigated the influence of Mecp2 KO astrocytes on synaptic phenotype. By using in vitro transwell-based co-cultures, we have demonstrated that the lack of Mecp2 in cortical astrocytes dramatically influences the synaptogenesis of WT neurons by non-cell autonomous mechanisms. To gain insights on the involved molecular mechanisms, we performed bulk RNA-Sequencing on WT neurons co-cultured with Mecp2 KO astrocytes, unveiling an increased inflammatory response and subsequent molecular investigations highlighted Interleukin-6 (IL-6) as a cytokine excessively secreted by KO astrocytes. Importantly, we demonstrated the causative role of IL-6 in synaptic impairments, since the addition of a neutralizing anti-IL-6 antibody prevented synaptic alterations. All in all, these findings unveiled a novel IL-6 mediated non-cell autonomous mechanism triggered by Mecp2 KO astrocytes leading to synaptic alterations, providing a novel therapeutic target for RTT and other Mecp2-related disorders.