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Mitochondria are critical for energy production and Ca2+ homeostasis and undergo fission and fusion reactions, perturbation of which can contribute to neuronal injury and disease. Mitochondrial fission is catalyzed by Drp1 (dynamin-related protein 1), a large GTPase tightly controlled by various posttranslational modifications, including phosphorylation. Bβ2 is a neuron-specific postnatally induced protein phosphatase 2A (PP2A) regulatory subunit that mediates PP2A translocation to the outer mitochondrial membrane (OMM) to promote mitochondrial fragmentation and sensitize neurons to various injuries. Opposing PP2A/Bβ2's effect on mitochondrial morphology and cell death is protein kinase A (PKA) anchored to the OMM via A kinase anchoring protein 1 (AKAP1). This dissertation describes how reversible phosphorylation of Drp1 at a conserved Serine residue by an outer mitochondrial kinase (PKA/AKAP1) and phosphatase complex (PP2A/Bβ2) affects dendrite and synapse development in hippocampal neurons and synaptic plasticity and learning and memory in vivo.
Inducing mitochondria fragmentation decreases dendritic arbor complexity, but increases spine and synapse number. Mitochondrial elongation induces opposite effects. L-carnitine increases mitochondria membrane potential and recapitulates the dendritic and synaptic effects of mitochondrial elongation. Epistasis experiments substantiate our hypothesis that PP2A/Bβ2 dephosphorylates and PKA/AKAP1 phosphorylates Drp1 to change mitochondrial shape and regulate mitochondria localization, dendrite outgrowth, and synapse development.
Bβ2 null mice are viable and fertile, without obvious abnormalities. Bβ2 null mice demonstrate significantly larger cortical and hippocampal neuronal mitochondria than in wildtype. Bβ2 deletion decreases spine number on apical and basal cortical dendrites and hippocampal dendrites. Bβ2 null mice display significantly decreased input/output relationship in the hippocampus, consistent with a decrease in synapse number. In a combined context and cued fear-conditioning protocol, the hippocampal-dependent context recall trial revealed significant deficits in Bβ2 null and heterozygous mice. This deficit is also seen in hippocampal-dependent Barnes maze performance. These results are consistent with the reduced hippocampal long-term potentiation (LTP) found in Bβ2 null mice and demonstrate the importance of Bβ2 in hippocampal synaptic plasticity and memory. In conclusion, PP2A/Bβ2 and PKA/AKAP1 have important roles in mitochondria regulation and dendritic and synaptic development as seen in our results in vitro with rat hippocampal cultures and in vivo with Bβ2 null mice.