Hippocampal CA1 and CA2 dendritic compartment-specific differences in mitochondrial form and function.

Mitochondrial morphology varies by neuronal cell type and subcellular compartment; however, the functional significance of these differences is unclear. Hippocampal CA2 neurons are enriched for genes encoding mitochondrial proteins compared to CA1, suggesting a difference in metabolic demand across hippocampal circuits. However, whether CA2 neuron mitochondria are structurally or functionally distinct to support circuit-specific energy demands is unknown. Here we compared mitochondrial morphology, protein expression, and calcium levels across CA1 and CA2 circuits. We found that CA2 dendritic mitochondria were larger than in CA1. However, both subregions harbored larger mitochondria in the entorhinal cortex (EC)-contacting distal dendrites compared to CA3-contacting proximal dendrites. Together, these data demonstrate cell type- and input-specific regulation of mitochondrial morphology that likely influences the function of these distinct circuits. To determine whether differences in mitochondrial fission or fusion account for cell and/or layer specific differences in morphology, we immunostained for MFF and OPA1, which showed a general enrichment in distal relative to proximal dendrites, and an unexpected increase in CA1 distal dendrites compared to CA2 distal dendrites. To show whether these morphological differences result in functionally distinct mitochondria, we measured mitochondrial calcium levels in live slices. We found a striking enrichment of mitochondrial calcium levels in CA2 distal dendrites relative to proximal dendrites, and this layer-specific effect was significantly different from that in CA1 at baseline and after activity. Collectively, these findings reveal discrete morphological and functional differences in mitochondria across hippocampal subregions and dendritic layers, which likely confer unique circuit properties and/or vulnerability to disease.