microRNA-218-5p Coordinates Scaling of Excitatory and Inhibitory Synapses during Homeostatic Synaptic Plasticity
Homeostatic synaptic plasticity (HSP) is a fundamental neuronal mechanism that allows networks to compensate for prolonged changes in activity by adjusting synaptic strength. This process is crucial for maintaining stable brain function and has been implicated in memory consolidation during sleep. While scaling of both excitatory and inhibitory synapses plays an important role during homeostatic synaptic plasticity, molecules coordinating both of these processes are unknown.  In this study, we investigate the role of miR-218-5p as a regulator of inhibitory and excitatory synapses in the context of picrotoxin (PTX)-induced homeostatic synaptic downscaling (HSD) in rat hippocampal neurons. Using enrichment analysis of miRNA-binding sites in differentially expressed genes changing upon PTX-induced HSD, we bioinformatically predicted and experimentally validated increased miR-218-5p activity upon PTX-treatment in the process compartment. By monitoring synapse structure in vitro with confocal microscopy, we demonstrate that miR-218-5p exerts a dual effect in HSD: it prevents the downscaling of excitatory synapses and dendritic spines, while at the same time blocking inhibitory synapse upscaling. Furthermore, we identify the Neuroligin2 interacting molecule Mdga1 as a crucial target of miR-218-5p in the context of homeostatic upscaling of inhibitory synapses. By performing long-term electroencephalographic (EEG) recordings, we further revealed that local inhibition of miR-218-5p in the somatosensory cortex reduced local slow-wave activity (SWA) during non-rapid-eye-movement (NREM) sleep. In summary, this study uncovers miR-218-5p as a key player in coordinating inhibitory and excitatory synapses during homeostatic plasticity and sleep. Our findings contribute to a deeper understanding of how neural circuits maintain stability in the face of activity-induced perturbations, with potential implications for both physiological and pathological conditions. Overall design: microRNA-inhibitors called LNAs against miR-218-5p were injected into one hemisphere of the somatosensory cortex I of mice, while the other hemisphere was injected with a scrambled negative control. 4 independent biological replicates (number of animals) were used in the study and denoted as Mxx were xx is a animal identifier. Per mouse (or animal identifier) we have two conditions: LNA218 (or hemisphere injected with miR-218-5p inhibitor) and NC (negative control, injected in the contralateral hemisphere). RH or LH denote whether the right or left hemisphere was injected. RNA was then extracted from the injected site 10 days after stereotactic injections, polyA-enriched and sequenced using an Illumina Novaseq 6000).