Alterations in somatodendritic bioenergetics in Drosophila models of tauopathy

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Ionic homeostasis in the somatodendritic compartment of neurons is maintained by pumps that utilize the energy of ATP hydrolysis to set the resting membrane potential and prevent toxic elevations in cytosolic [Ca2+]. The relative contributions of glycolysis and mitochondrial respiration in meeting the ATP burden associated with the activity of these pumps is poorly understood. Also unclear is how these two axes of ATP production are perturbed in neurodegenerative disease such as Alzheimer’s and related dementias (ADRDs), which exhibit bioenergetic deficits and ionic dyshomeostasis. In this proposal, we detail our plans to elucidate the relative contributions of glycolysis and mitochondrial ATP synthesis to the somatodendritic ATP burden stemming with depolarization and the release of Ca2+ from the endoplasmic reticulum (ER). By imaging of cytosolic/mitochondrial [Ca2+] and [ATP]/[ADP] ratio in live dissociated Drosophila neurons we have formulated the model that ATP burden of depolarization is so tightly coupled to ATP synthesis such that somatodendritic [ATP]/[ADP] ratio remained stable in depolarized neurons. Our preliminary data also suggest that depolarization elicits ATP production in the somatodendritic compartment without a necessity for concomitant changes in mitochondrial [Ca2+]. Given that in the absence of matrix [Ca2+] elevations mitochondrial ATP production is not potentiated, we hypothesize that glycolysis, rather than OXPHOS, is the favored bioenergetic response to depolarization. We will test this hypothesis in aim 1, and also determine whether the Na+/K+ ATPase and plasma membrane Ca2+ ATPase (PMCA) are the recipients of glycolysis-derived ATP in depolarized neurons. Aim 2 is driven by our preliminary finding that ER Ca2+ release via inositol trisphosphate receptors (IP3Rs) in depolarized neurons desynchronized ATP production from consumption. We will also probe the significance of Ca2+ transfer between the ER and mitochondria, and attendant changes in neuronal bioenergetics in a fly model of tauopathies, which stem from our preliminary findings show that expression of a toxic human Tau variant in fly glutamatergic neurons disrupted Ca2+ transfer between ER and mitochondria, and evoked toxicity that was consistent with Ca2+ dyshomeostasis. In summary, we will use a range of imaging tools and direct measures of bioenergetics to address fundamental questions of metabolic regulation in neurons, and interrogate the mechanism by which these parameters are perturbed in a model of ADRD.