The Ca2+ Bridge: From Neurons to Circuits in Rett Syndrome.

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused primarily by mutations in the gene encoding the methyl-CpG-binding protein 2 (Mecp2). Mecp2 binds to methylated cytosines, playing a crucial role in chromatin organization and transcriptional regulation. At the neurobiological level, RTT is characterized by dendritic spine dysgenesis and altered excitation-inhibition balance, drawing attention to the mechanisms that scale from mutations in a nuclear protein to altered neuronal connectivity. Although Mecp2 dysfunction disrupts multiple neuronal processes, emerging evidence highlights altered calcium (Ca2+) signaling as a central contributor to RTT pathophysiology. This review explores the link between Mecp2 and Ca2+ regulation by highlighting how Mecp2 affects Ca2+-dependent transcriptional pathways, while Ca2+ modulates Mecp2 function by inducing post-translational modifications. We discuss this crosstalk in light of evidence from RTT models, with a particular focus on the brain-derived neurotrophic factor BDNF-miR132-Mecp2 axis and the dysregulation of ryanodine receptors (RyRs). Additionally, we examine how these perturbations contribute to the reduced structural plasticity and the altered activity-driven gene expression that characterizes RTT. Understanding the intersection between Mecp2 function and Ca2+ homeostasis will provide critical insights into RTT pathogenesis and potential therapeutic targets aimed at restoring neuronal connectivity.