Acknowledgements: We thank past and present members of the Polleux and Lewis labs for feedback and discussion along the way. We thank Nelson Spruston (HHMI-Janelia) for sharing the CA1 serial EM dataset generated while EB was in his laboratory. We thank the Zuckerman Institute’s Cellular Imaging platform for instrument use and technical advice. We thank Patrycja Szybowska, Joshua Weertman, Klaudia Strucinska, Qiaolian Liu and Rhythm Sharma for excellent technical help. This research was supported by grants NIGMS R35GM137921 (TL), NIGMS P20GM103636-06 sub-project 2 (TL), Presbyterian Health Foundation (TL), NINDS R35 NS127232 (FP) and NINDS NS107483 (FP), HHMI/Janelia (EB), NIMH R01MH124047, NIMH R01MH124867, NINDS R01NS121106, NINDS U01NS115530, NINDS R01NS133381, NINDS R01NS131728, NIA, RF1AG080818 (AL), Medical Research Council (MC_UU_00028/5) (JP) and an Investigator Award from the Wellcome Trust (204766/Z/16/Z) (FMR and DGH).Funder: U.S. Department of Health & Human Services | NIH | National Institute of General Medical Sciences (NIGMS)Funder: Presbyterian Health Foundation; doi: https://doi.org/10.13039/100001298Funder: U.S. Department of Health & Human Services | NIH | National Institute of Neurological Disorders and Stroke (NINDS)Funder: U.S. Department of Health & Human Services | NIH | National Institute of Mental Health (NIMH)Neuronal mitochondria play important roles beyond ATP generation, including Ca2+ uptake, and therefore have instructive roles in synaptic function and neuronal response properties. Mitochondrial morphology differs significantly between the axon and dendrites of a given neuronal subtype, but in CA1 pyramidal neurons (PNs) of the hippocampus, mitochondria within the dendritic arbor also display a remarkable degree of subcellular, layer-specific compartmentalization. In the dendrites of these neurons, mitochondria morphology ranges from highly fused and elongated in the apical tuft, to more fragmented in the apical oblique and basal dendritic compartments, and thus occupy a smaller fraction of dendritic volume than in the apical tuft. However, the molecular mechanisms underlying this striking degree of subcellular compartmentalization of mitochondria morphology are unknown, precluding the assessment of its impact on neuronal function. Here, we demonstrate that this compartment-specific morphology of dendritic mitochondria requires activity-dependent, Ca2+ and Camkk2-dependent activation of AMPK and its ability to phosphorylate two direct effectors: the pro-fission Drp1 receptor Mff and the recently identified anti-fusion, Opa1-inhibiting protein, Mtfr1l. Our study uncovers a signaling pathway underlying the subcellular compartmentalization of mitochondrial morphology in dendrites of neurons in vivo through spatially precise and activity-dependent regulation of mitochondria fission/fusion balance
