Biology subjects

Crill, S.

Publications and source records attributed to Crill, S..

3 recordsLinked to original sources

bioRxiv2025

Optimal Neuromuscular Performance Requires Motor Neuron Phosphagen Kinases

Phosphagen systems are crucial for muscle bioenergetics - rapidly regenerating ATP to support the high metabolic demands of intense musculoskeletal activity. However, their roles in motor neurons that drive muscle contraction have received little attention. Here, we knocked down expression of the primary phosphagen kinase [Arginine Kinase 1; ArgK1] in Drosophila larval motor neurons and assessed the impact on presynaptic energy metabolism and neurotransmission in situ. Fluorescent metabolic probes showed a deficit in presynaptic energy metabolism and some glycolytic compensation. Glycolytic compensation was revealed through a faster elevation in lactate at high firing frequencies, and the accumulation of pyruvate subsequent to firing. Our performance assays included two tests of endurance: enforced cycles of presynaptic calcium pumping, and, separately, enforced body-wall contractions for extended periods. Neither test of endurance revealed deficits when ArgK1 was knocked down. The only performance deficits were detected at firing frequencies that approached, or exceeded, twice the firing frequencies recorded during fictive locomotion, where both electrophysiology and SynaptopHluorin imaging showed an inability to sustain neurotransmitter release. Our computational modeling of presynaptic bioenergetics indicates that the phosphagen systems contribution to motor neuron performance is likely through the removal of ADP in microdomains close to sites of ATP hydrolysis, rather than the provision of a deeper reservoir of ATP. Taken together, these data demonstrate that, as in muscle fibers, motor neurons rely on phosphagen systems during activity that imposes intense energetic demands.

neuroscience↗

bioRxiv2023

Ultrastructural analysis reveals mitochondrial placement independent of synapse placement in fine caliber C. elegans neurons

Neurons rely on mitochondria for an efficient supply of ATP and other metabolites. However, while neurons are highly elongated, mitochondria are discrete and limited in number. Due to the slow rates of diffusion over long distances it follows that neurons would benefit from an ability to control the distribution of mitochondria to sites of high metabolic activity, such as synapses. It is assumed that neurons possess this capacity, but ultrastructural data over substantial portions of a neurons extent that would allow for tests of such hypotheses are scarce. Here, we mined the Caenorhabditis elegans electron micrographs of John White and Sydney Brenner and found systematic differences in average mitochondrial length (ranging from 1.3 to 2.4 m), volume density (3.7% to 6.5%) and diameter (0.18 to 0.24 m) between neurons of different neurotransmitter type and function, but found limited differences in mitochondrial morphometrics between axons and dendrites of the same neurons. Analyses of distance intervals found mitochondria to be distributed randomly with respect to presynaptic specializations, and an indication that mitochondria were displaced from postsynaptic specializations. Presynaptic specializations were primarily localized to varicosities, but mitochondria were no more likely to be found in synaptic varicosities than non-synaptic varicosities. Consistently, mitochondrial volume density was no greater in varicosities with synapses. Therefore, beyond the capacity to disperse mitochondria throughout their length, at least in C. elegans, fine caliber neurons manifest limited sub-cellular control of mitochondrial size and distribution. SIGNIFICANCEBrain function is unequivocally reliant on mitochondrial function for its energy needs, and the mechanisms that cells use to control these organelles is an active field of enquiry. WormImage, a decades old electron microscopy database in the public domain, contains information about the ultrastructural disposition of mitochondria within the nervous system of C elegans over previously unexamined extents. In a largely remote format, a team of students mined this database over the course of the pandemic. They found differences in mitochondrial size and density between neurons, but limited differences between different compartments of the same neurons. Also, while neurons are clearly able to disperse mitochondria throughout their extent, they found little evidence that they "install" mitochondria at synaptic varicosities.

neuroscience↗

bioRxiv2022

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

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 energy 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 10-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. SIGNIFICANCEFailure of metabolic processes to supply neurons with energy at an adequate rate can lead to synaptic dysfunction and cell death under pathological conditions. Using a quantitative approach at fruit fly motor nerve terminals we generated the first temporal profiles of presynaptic power demand during locomotor activity. This approach revealed challenges for the known mechanisms that match cellular power supply to demand. However, we discovered that motor nerve terminals in fruit flies and mice alike are supported by phosphagen systems, more commonly seen in muscles where they store energy and buffer mismatch between power supply and demand. This study highlights an understudied aspect of neuronal bioenergetics which may represent a bulwark against the progression of some neuropathologies.

neuroscience↗