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A Framework for Understanding Power Supply and Demand in Presynaptic Nerve Terminals
Title
A Framework for Understanding Power Supply and Demand in Presynaptic Nerve Terminals.
Creator
Justs, Karlis Anthony, Macleod, Gregory T., Florida Atlantic University, Charles E. Schmidt College of Science, Department of Biological Sciences
Abstract/Description
The molecular mechanisms of synaptic function and development have been studied extensively, but little is known about the energy requirements of synapses, or the mechanisms that coordinate their energy production with their metabolic demands. These are oversights, as synapses with high energy demands are more susceptible to degeneration and degrade in the early stages of diseases such as amyotrophic lateral sclerosis, spinal muscle atrophy and Parkinson’s disease. Here, in a structure...
Show moreThe molecular mechanisms of synaptic function and development have been studied extensively, but little is known about the energy requirements of synapses, or the mechanisms that coordinate their energy production with their metabolic demands. These are oversights, as synapses with high energy demands are more susceptible to degeneration and degrade in the early stages of diseases such as amyotrophic lateral sclerosis, spinal muscle atrophy and Parkinson’s disease. Here, in a structure-function study at Drosophila motor neuron terminals, a neurophysiological model was generated to investigate how power (ATP/s) supply is integrated to satisfy the power demand of presynaptic terminals. Power demands were estimated from six nerve terminals through direct measurements of neurotransmitter release and Ca2+ entry, as well as theoretical estimation of Na+ entry and power demands at rest (cost of housekeeping). The data was leveraged with a computational model that simulated the power demands of the terminals during their physiological activity, revealing high volatility in which power demands can increase 15-fold within milliseconds as neurons transition from rest to activity. Another computational model was generated that simulated ATP production scenarios regarding feedback to the power supply machinery (Oxphos and glycolysis) through changes in nucleotide concentrations, showing that feedback from nucleotides alone fail to stimulate power supply to match the power demands of each terminal. Failure of feedback models invokes the need for feed forward mechanisms (such as Ca2+) to stimulate power supply machinery to match power demands. We also quantified mitochondrial volume, density, number and size in each nerve terminal, revealing all four features positively correlate with the terminals power demands. This suggests the terminals enhance their oxidative capacity by increasing mitochondrial content to satisfy their power demands. And lastly, we demonstrate that abolishing an ATP buffering system (the phosphagen system) does not impair neurotransmission in the nerve terminals, suggesting motor nerve terminals are capable of satisfying their power demands without the ATP buffering system.
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Date Issued
2019
PURL
http://purl.flvc.org/fau/fd/FA00013323
Subject Headings
Presynaptic Terminals, Adenosine triphosphate, Synapses--metabolism, Bioenergetics
Format
Document (PDF)
pH Dynamics within the Drosophila Synaptic Cleft During Activity
Title
pH Dynamics within the Drosophila Synaptic Cleft During Activity.
Creator
Feghhi, Touhid, Lau, Andy W.C., Macleod, Gregory T., Florida Atlantic University, Department of Physics, Charles E. Schmidt College of Science
Abstract/Description
Acute pH sensitivity of many neural mechanisms highlights the vulnerability of neurotransmission to the pH of the extracellular milieu. The dogma is that the synaptic cleft will acidify upon neurotransmission because the synaptic vesicles corelease neurotransmitters and protons to the cleft, and the direct data from sensory ribbon-type synapses support the acidification of the cleft. However, ribbon synapses have a much higher release probability than conventional synapses, and it’s not...
Show moreAcute pH sensitivity of many neural mechanisms highlights the vulnerability of neurotransmission to the pH of the extracellular milieu. The dogma is that the synaptic cleft will acidify upon neurotransmission because the synaptic vesicles corelease neurotransmitters and protons to the cleft, and the direct data from sensory ribbon-type synapses support the acidification of the cleft. However, ribbon synapses have a much higher release probability than conventional synapses, and it’s not established whether conventional synapses acidify as well. To test the acidification of the cleft in the conventional synapse, we used genetically encoded fluorescent pH reporters targeted to the synaptic cleft of Drosophila larvae. We observed alkalinization rather than acidification during activity, and this alkalinization was dependent on the exchange of protons for calcium at the postsynaptic membrane. A reaction-diffusion computational model of the pH dynamics at the Drosophila larval neuromuscular junction was developed to leverage the experimental data. The model incorporates the release of glutamate, ATP, and protons from synaptic vesicles into the cleft, PMCA activity, bicarbonate, and phosphate buffering systems. By means of numerical simulations, we reveal a highly dynamic pH landscape within the synaptic cleft, harboring deep but exceedingly rapid acid transients that give way to a prolonged period of alkalinization.
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Date Issued
2023
PURL
http://purl.flvc.org/fau/fd/FA00014221
Subject Headings
Synapses, pH (Chemistry), Hydrogen-ion concentration., Synaptic Transmission, Drosophila
Format
Document (PDF)
The Dynamic pH Landscape At The Drosophila NMJ Synaptic Cleft And Its Implication In Neurotransmission
Title
The Dynamic pH Landscape At The Drosophila NMJ Synaptic Cleft And Its Implication In Neurotransmission.
Creator
Hernandez, Roberto X., Macleod, Gregory T., Florida Atlantic University, Department of Biological Sciences, Charles E. Schmidt College of Science
Abstract/Description
The intricate processes governing cellular pH and its impact on protein and cellular function have been extensively explored. However, our understanding of the pH fluctuations that occur during routine cellular activities and their potential to modulate cell function remains, particularly within the highly dynamic pH landscape of a synapse. Investigating the scale, directionality, and temporal characteristics of these activity-dependent pH fluctuations at synapses is of paramount interest, as...
Show moreThe intricate processes governing cellular pH and its impact on protein and cellular function have been extensively explored. However, our understanding of the pH fluctuations that occur during routine cellular activities and their potential to modulate cell function remains, particularly within the highly dynamic pH landscape of a synapse. Investigating the scale, directionality, and temporal characteristics of these activity-dependent pH fluctuations at synapses is of paramount interest, as it carries profound implications for neurotransmitter release and signal transduction. Employing both empirical and computational modeling methods, our research explores the dynamic pH environment within the synaptic cleft of Drosophila glutamatergic motor neuron Ib terminals during synaptic activity and reveals its significance in modulating neurotransmission. Contrary to popular belief, we discovered that these terminals undergo activity-dependent extracellular alkalinization in response to both single action potentials and burst stimulation. This surprising phenomenon was also observed at the mouse calyx of Held. We found activity-dependent alkalinization to be predominantly driven by Ca2+ movement across the postsynaptic membrane, and by targeting pH indicators to subcellular domains, we identified alkalinization to primarily occur within the cleft.
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Date Issued
2023
PURL
http://purl.flvc.org/fau/fd/FA00014346
Subject Headings
Neurotransmission, Drosophila, Hydrogen-Ion Concentration, Motor Neurons, Optogenetics
Format
Document (PDF)
Neuronal Energetics: Mitochondrial Distribution and The Phosphagen System
Title
Neuronal Energetics: Mitochondrial Distribution and The Phosphagen System.
Creator
Riboul, Danielle V., Macleod, Gregory T., Florida Atlantic University, Department of Biological Sciences, Charles E. Schmidt College of Science
Abstract/Description
The relationship between neuronal function and energy metabolism is a field of intense inquiry and while bioenergetic per se are well understood, we lack a good understanding of the ways in which these mechanisms overcome the challenges presented by the unique morphology of neurons and their volatile energy demands. Here we examined the extent to which these challenges can be met through strategic mitochondrial placement and the support of a phosphagen system. We examined fluctuations in...
Show moreThe relationship between neuronal function and energy metabolism is a field of intense inquiry and while bioenergetic per se are well understood, we lack a good understanding of the ways in which these mechanisms overcome the challenges presented by the unique morphology of neurons and their volatile energy demands. Here we examined the extent to which these challenges can be met through strategic mitochondrial placement and the support of a phosphagen system. We examined fluctuations in energy demand of Drosophila larval motor neurons utilizing a combination of computational modeling and empirical analysis, and uncovered a neglected aspect of cellular energy metabolism that appears to accommodate the stress of highly volatile energy demands. Our findings highlight a reliance on the phosphagen system to buffer against rapid changes in the rate of ATP consumption induced by burst firing. The knockdown of a key element in the phosphagen system of invertebrates, arginine kinase, revealed a suppression of the mitochondrial proton motive force, and a more rapid decline in the presynaptic ATP/ADP ratio during burst firing. The knock down of arginine kinase also revealed metabolic shifts that indicated a compensatory increase in glycolysis, but, surprisingly, few consequences for either presynaptic Ca2+ handling or neurotransmission. In a final effort to ensure that we were imposing a metabolic load adequate to challenge these motor neurons, we developed an ex vivo calcium clearance assay and in vivo locomotor performance assay – currently in their final stages of validation.
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Date Issued
2024
PURL
http://purl.flvc.org/fau/fd/FA00014419
Subject Headings
Mitochondria, Neurons, Energy metabolism
Format
Document (PDF)