Eukaryotic cells are unique in their ability to form complex multicellular organisms giving rise to distinct physiological systems. However, the ability for such complexity to evolve likely stems from an early event in which endosymbiosis of an aerobic prokaryote by a eukaryotic precursor gave rise to the eukaryotic organelle we now know as mitochondria. Mitochondria are colloquially known as the “power house” of the cell due to their ability to produce ATP through oxidative phosphorylation, but perform numerous other vital functions within the cell including sequestration of cytosolic Ca2+, production and sequestration of reactive oxygen species (ROS), and initiation of various forms of cell death. Mitochondria are especially important in neurons given their high demand for ATP and the importance of Ca2+ signaling in neuron excitability and development. Neurons are highly compartmentalized and plastic cells requiring the ability to control energy supply and Ca2+ signaling locally within given specialized structures such as dendritic spines or synaptic boutons. Therefore, mitochondria must be able to localize to particular sub-cellular locales and respond functionally to signaling occurring in that environment.
Mitochondrial transport and function are heavily dependent upon the ability of mitochondria to undergo opposing and reversible fission and fusion events. Mitochondrial fission and fusion are themselves regulated by GTPase enzymes which physically catalyze constriction and fusion of the mitochondrial membranes. Mutations in mitochondrial fission and fusion enzymes specifically cause neurological disease in humans and recent work has illustrated the necessity of a proper balance of mitochondrial fission in neuron development, survival, and plasticity. Despite recognizing the importance of mitochondrial fission and fusion in neuron survival, development, and function we lack a concrete understanding of how changes in the equilibrium of fission and fusion impact these processes in vivo. In this thesis we investigate how promoting or inhibiting mitochondrial fission, through phosphoregulation of the mitochondrial fission enzyme Dynamin related protein 1 (Drp1) at mitochondria, impacts neuron survival and memory in vivo.
We find that inhibiting phosphorylation of Drp1 at Serine 656 (S656) at the mitochondria, through deletion of a mitochondrial targeted A kinase anchoring protein (AKAP) known as AKAP1 in mice, increases cerebral infarct volume following transient occlusion of the mid-cerebral artery. Oppositely, promoting phosphorylation of Drp1-S656 at the mitochondria, through deletion of the PP2A regulatory subunit Bβ2 which localizes the PP2A heterotrimer to mitochondria, decreases cerebral infarct volume following occlusion of the mid-cerebral artery. Mechanistic in vitro studies in primary neurons reveal these effects are dependent upon the phosphorylation state of Drp1-S656 and likely due to altered mitochondrial respiratory capacity, ROS production, and Ca2+ homeostasis. Interestingly, we also observe improved hippocampal dependent memory in mice in which AKAP1 has been deleted which also appears dependent upon the phosphorylation state of Drp1-S656 and Ca2+ homeostasis. Ultimately, these findings provide insight into how phosphoregulation of Drp1 at the mitochondria alters neuron survival and function through shifting the mitochondrial fission/fusion equilibrium and consequently mitochondrial function.