Mitochondria remodel near synapses to boost metabolic capacity and fuel plasticity

Brain synapses are hotspots of energy consumption, and the synaptic plasticity underlying learning sharply increases this demand. Because most synapses lie far from their cell body, their energy demands must be met locally. A dendritic mitochondrion can extend ~30 m and span many spines, yet ramps up ATP synthesis only near spines undergoing plasticity, providing immediate and sustained energy. Whether structural changes in these juxtaspinal mitochondrial regions (2 m segments at spine bases) support this response is unknown; resolving their energy-producing architecture poses substantial challenges. Compared with the lamellar cristae of axonal mitochondria, dendritic mitochondrial cristae form complex, tubular, and interconnected networks that require multiple high-resolution tomograms to characterize. We therefore developed a correlative light and electron microscopy (CLEM) pipeline with a custom deep-learning model that simultaneously segments five distinct 3D suborganellar structures from limited training data, enabling quantitative analysis across 42 tomograms at 2 nm pixel resolution. We find that juxtaspinal mitochondrial regions are larger than non-juxtaspinal regions within the same mitochondrion and enlarge further during synaptic plasticity. Juxtaspinal mitochondrial ATP synthesis and spine ATP levels rise in parallel; during single-spine plasticity, ATP rises within minutes, before detectable enlargement, indicating that juxtaspinal mitochondrial enlargement sustains rather than initiates the local energy response. The enlarged regions exhibit increased cristae membrane surface area, crista junction number, and membrane area with high curvature, together with increased MIC60 and ATP synthase copies, endoplasmic reticulum contacts, and more proximal ribosomes. Matrix-targeted PKA inhibition reduces MIC60 phosphorylation and impairs juxtaspinal mitochondrial enlargement, plasticity-associated ATP increase, and single-spine plasticity. Together, our findings support a model in which spine-specific remodeling within an individual mitochondrion boosts local metabolic capacity to sustain ATP synthesis and fuel synaptic plasticity.