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

Abstract

Neural function relies on cellular energy supplies meeting the episodic demands of synaptic activity, but little is known about the extent to which power demands (energy demands per unit time) fluctuate, or the mechanisms that match supply with demand. Here, in individually-identified glutamatergic motor neuron terminals of Drosophila larvae, we leveraged prior macroscopic estimates of power demand to generate profiles of power demand from one action potential to the next. These profiles show that signaling demands can exceed non-signaling demands by 17-fold within milliseconds, and terminals with the greatest fluctuation (volatility) in power demand have the greatest mitochondrial volume and packing density. We elaborated on this quantitative approach to simulate adenosine triphosphate (ATP) levels during activity and drove ATP production as a function of the reciprocal of the energy state, but this canonical feedback mechanism appeared to be unable to prevent ATP depletion during locomotion. Muscle cells possess a phosphagen system to buffer ATP levels but phosphagen systems have not been described for motor nerve terminals. We examined these terminals for evidence of a phosphagen system and found the mitochondria to be heavily decorated with an arginine kinase, the key element of invertebrate phosphagen systems. Similarly, an examination of mouse cholinergic motor nerve terminals found mitochondrial creatine kinases, the vertebrate analogues of arginine kinases. Knock down of arginine kinase in Drosophila resulted in rapid depletion of presynaptic ATP during activity, indicating that, in motor nerve terminals, as in muscle, phosphagen systems play a critical role in matching power supply with demand.

Brain energy metabolism: A roadmap for future research

“…It is also known that energy metabolism operates in coordination with enzymes like adenylate kinase and creatine kinase ( Erecinska et al, 2004 ). Although there is increasing evidence of its relevance for synaptic integrity and activity ( Chen et al, 2021 ; Justs et al, 2022 ), the interplay between the ATP buffering/regenerating systems and glycolysis/oxidative phosphorylation is not fully understood in the context of neuronal activity and neurotransmission.…”

Intracellular Lactate Dynamics Reveal the Metabolic Diversity ofDrosophilaGlutamatergic Neurons

“…Therefore, changes in cytosolic [lactate] are inversely related to mTFP-Venus FRET efficiency in cells expressing Laconic, which we refer to as the Laconic ratio 22,23 . Here, we expressed UAS-Laconic 24 under the control of the d42-GAL4 glutamatergic/motor neuron driver 25,26 (d42>Laconic).…”

“…We thank the Bloomington Drosophila Stock Center for fly stocks and Dr. Gregory Macleod for the UAS-Laconic fly line 24…”

“…Confocal imaging of dissociated neurons was performed in accordance with previouslydescribed methods 7,22,24,61 . Briefly, we used a Nikon A1R laser-scanning confocal for live-cell imaging on a Nikon TI-E inverted microscope equipped with Nikon 40x/1.3 NA Plan-Fluor oil immersion objective and the Perfect Focus System for maintenance of focus over time to excite Laconic with the 445 nm laser line and emission signals recorded at 488 nm (mTFP) and 535 nm (Venus).…”