Cholesterol dysregulation in Rett syndrome: implications for synaptic function

Rett syndrome (RTT) is a progressive neurodevelopmental disorder mainly caused by mutations in the X-linked MECP2 gene. By affecting 1 in 10.000 live female births, it represents the main genetic cause of intellectual disability in girls worldwide. Since MeCP2 is highly expressed in mature neurons, the majority of studies are focused on this cell type. However, besides neurons, astrocytes have been identified as active contributors to RTT pathogenesis, as Mecp2 knock-out (KO) astrocytes fail to correctly support neuronal maturation and synaptogenesis. Indeed, culturing wild-type (WT) neurons with KO astrocytes or treating them with KO astrocyte-conditioned medium (ACM) affects their synaptic phenotype. Of note, one of the key synaptogenic factors released by astrocytes is cholesterol, which plays a crucial role in synapse formation and functioning. Several data highlight a defective cholesterol metabolism in RTT, supporting the hypothesis that abnormalities in astrocyte-produced cholesterol might contribute to synaptic dysfunctions. In this study, we report a downregulation of genes involved in cholesterol synthesis and secretion in Mecp2 KO astrocytes cultured alone and in MACS-sorted astrocytes. Moreover, we highlight an alteration in the activation of Srebp2, which represents the master regulator of cholesterol metabolism, and in ApoE lipidation, critically impairing the transport of cholesterol to neurons. Since properly lipidated ApoE is required to deliver functional cholesterol, this alteration likely compromises cholesterol activity at synapses and contributes to synaptic dysfunction. In a therapeutic perspective, we highlight that cholesterol treatment completely rescues the synaptic alterations both in WT neurons treated with KO-ACM and Mecp2 heterozygous (HET) neurons, which better recapitulate the genetic pattern of RTT patients. To corroborate in vitro evidence, we also find a deregulation of key factors involved in cholesterol metabolism in the brain of Mecp2-deficient animals. Collectively, these data demonstrate an alteration of cerebral cholesterol metabolism in RTT and suggest cerebral cholesterol supply as a possible therapeutic strategy for repairing synaptic dysfunctions in RTT.