Presynaptic Mitochondrial Volume and Packing Density Scale with Presynaptic Power Demand

Abstract

Stable neural function requires an energy supply that can meet the intense episodic power demands of neuronal activity. Neurons have presumably optimized the volume of their bioenergetic machinery to ensure these power demands are met, but the relationship between presynaptic power demands and the volume available to the bioenergetic machinery has never been quantified. Here, we estimated the power demands of six motor nerve terminals in female Drosophila larvae through direct measurements of neurotransmitter release and Ca 21 entry, and via theoretical estimates of Na 1 entry and power demands at rest. Electron microscopy revealed that terminals with the highest power demands contained the greatest volume of mitochondria, indicating that mitochondria are allocated according to presynaptic power demands. In addition, terminals with the greatest power demand-to-volume ratio (;66 nmol•min 21 •ml 21 ) harbor the largest mitochondria packed at the greatest density. If we assume sequential and complete oxidation of glucose by glycolysis and oxidative phosphorylation, then these mitochondria are required to produce ATP at a rate of 52 nmol•min 21 •ml 21 at rest, rising to 963 during activity. Glycolysis would contribute ATP at 0.24 nmol•min 21 •ml 21 of cytosol at rest, rising to 4.36 during activity. These data provide a quantitative framework for presynaptic bioenergetics in situ, and reveal that, beyond an immediate capacity to accelerate ATP output from glycolysis and oxidative phosphorylation, over longer time periods presynaptic terminals optimize mitochondrial volume and density to meet power demand.

Presynaptic Mitochondria Communicate With Release Sites for Spatio-Temporal Regulation of Exocytosis at the Motor Nerve Terminal

“…Therefore, our present results also indicate that the upper limit of the RRP size in this synapse is reached at 50 Hz stimulation and that the reduction in mCa 2+ uptake cannot increase it. On the other side, the progressive decrease in the RRP with CCCP could be due, among others, to the increase in asynchronous fusions ( Figure 5E , Supplementary Figures 1C,D ), reduction in the mitochondrial ATP production (Justs et al, 2022 ), partial inactivation of P/Q-type voltage-dependent Ca 2+ channels by high Ca 2+ (Forsythe et al, 1998 ; Demaria et al, 2001 ), moderate desensitization of postsynaptic receptors, or a combination of all. In all cases, mCa 2+ uptake could minimize these effects during physiological stimuli.…”

“…In synapses, the high sensitivity of mitochondria to Ca 2+ may be relevant not only to preventing Ca 2+ accumulation in microdomains and their surroundings but also to modulate the range of responses to physiological stimuli during development and maturity. For example, in the mouse calyx of Held, mitochondrial volumes are increased to support high firing and secretion rates upon maturity (Kim et al, 2013 ; Thomas et al, 2019 ) and in Drosophila motor nerve terminals, presynaptic mitochondrial volume and packing density scale with presynaptic demands (Justs et al, 2022 ).…”

Mitochondrial phosphagen kinases support the volatile power demands of motor nerve terminals

“…In our previous study, we quantified the signaling and non-signaling (housekeeping) power demands (as ATP/s) of the different MN terminals (Justs et al, 2022), but power demand volatility was neglected. To summarize, there we estimated signaling power demands using the "bottom-up" approach of (Lu et al, 2016) through direct measurements of neurotransmitter release, calcium ion (Ca 2+ ) entry, and theoretical estimates of sodium ion (Na + ) entry, and the energy required to reverse these activities (Fig.…”

“…3D-E confirm ATP production rates do not accelerate). These terminals are thought to rely on mitochondria to produce most of their ATP (Justs et al, 2022), and so in implementing a variable rate of ATP production we adopted a model of adenine nucleotide homeostasis that centered on Ox-Phos. In this model, homeostasis is achieved by driving ATP production as a function of the reciprocal of the energy state, where energy state is defined as [ATP]/([ADP].…”

“…Following (Justs et al, 2022), we modeled the presynaptic ATP consumption rate (power) as the sum of a time-dependent signaling rate capturing the energetic cost of triggering neurotransmitter release in response to an action potential (AP), and a constant rest rate capturing all other cell activities (protein/lipid synthesis, cytoskeleton turnover, ongoing Na + /K + ATPase activity at the plasma membrane, etc). The consumption rate associated with a single AP is the sum of five distinct signaling processes: Ca 2+ pumping, Na + pumping, and three types of neurotransmitter recycling processes (NT1, NT2, NT3).…”