MicroRNA-138-5p suppresses excitatory synaptic strength at the cerebellar input layer.

MicroRNAs are small, highly conserved non-coding RNAs that negatively regulate mRNA translation and stability. In the brain, miRNAs contribute to neuronal development, synaptogenesis, and synaptic plasticity. MicroRNA 138-5p (miR-138-5p) controls inhibitory synaptic transmission in the hippocampus and is highly expressed in cerebellar excitatory neurons. However, its specific role in cerebellar synaptic transmission remains unknown. Here, we investigated excitatory transmission in the cerebellum of mice expressing a sponge construct that sequesters endogenous miR-138-5p. Mossy fibre stimulation-evoked EPSCs in granule cells were ∼40% larger in miR-138-5p sponge mice compared to controls. Furthermore, we observed larger miniature EPSC amplitudes, suggesting an increased number of functional postsynaptic AMPA receptors. High-frequency train stimulation revealed enhanced short-term depression following miR-138-5p downregulation. Together with computational modelling, this suggests a negative regulation of presynaptic release probability. Overall, our results demonstrate that miR-138-5p suppresses synaptic strength through pre- and postsynaptic mechanisms, providing a potentially powerful mechanism for tuning excitatory synaptic input into the cerebellum. KEY POINTS: MicroRNAs are powerful regulators of mRNA translation and control key cell biological processes including synaptic transmission, but their role in regulating synaptic function in the cerebellum has remained elusive. In this study, we investigated how microRNA-138-5p (miR-138-5p) modulates excitatory transmission at adult murine cerebellar mossy fibre to granule cell synapses. Downregulation of miR-138-5p enhances excitatory synaptic strength at the cerebellar input layer and increases short-term depression. miR-138-5p exerts its regulatory function through both pre- and postsynaptic mechanisms by negatively regulating release probability at mossy fibre boutons, as well as functional AMPA receptor numbers in granule cells. These findings provide insights into the role of miR-138-5p in the cerebellum and expand our understanding of microRNA-dependent control of excitatory synaptic transmission and short-term plasticity.