Biology subjects

Ashrafi, G.

Publications and source records attributed to Ashrafi, G..

7 recordsLinked to original sources

bioRxiv2025

Ketogenic diet dampens excitatory neurotransmission by shrinking synaptic vesicle pools

HIGHLIGHTSO_LIKD rewires gene expression in the hippocampus, particularly impacting synaptic genes. C_LIO_LIKD profoundly alters epigenetic modifications in the hippocampus. C_LIO_LIKD reduces excitatory synaptic gain and dampens integration of synaptic inputs. C_LIO_LIKD reduces the readily releasable vesicle pool in excitatory synapses. C_LI The ketogenic diet (KD) is a common dietary intervention for treating seizures in intractable childhood epilepsies and has been proposed to improve disease outcome in neurodegenerative disorders. Despite its clinical applications, we know little about how this diet impacts brain circuitry and neuronal function to elicit its protective effects. Here, we examined the impact of the KD on hippocampal function through integrative analysis of gene expression, epigenetics and neurotransmission. We found that KD induces profound transcriptional reprogramming of the hippocampus, including dampened expression of numerous synaptic genes. Through proteomic analysis of histone variants and post-translational modifications, we uncovered significant changes in activating and repressive histone marks in the hippocampus of KD mice. To determine how transcriptional rewiring of the hippocampus under KD impacts neurotransmission, we performed electrophysiological recordings of neurotransmission and synaptic dynamics at excitatory CA3-CA1 synapses. We found that KD diminishes synaptic gain and dampens short-term plasticity at excitatory synapses, resulting in reduced integration of synaptic inputs at the circuit level. Combining electrophysiology and electron microscopy, we determined that effects of KD in excitatory synapses are caused by a reduction in size of the readily releasable pool of synaptic vesicles, as well as the total vesicle pool. Our findings show that the ketogenic diet triggers synaptic remodeling in the hippocampus, driven by broad transcriptional and epigenetic changes that reduce synaptic vesicle pools and short-term plasticity at excitatory synapses ultimately dampening excitatory synaptic gain and integration at the circuit level. These synaptic adaptations may represent a major mechanism underlying the anti-epileptic effects of this diet.

neuroscience↗

bioRxiv2024

Mitochondrial pyruvate transport regulates presynaptic metabolism and neurotransmission

Glucose has long been considered the primary fuel source for the brain. However, glucose levels fluctuate in the brain during sleep, intense circuit activity, or dietary restrictions, posing significant metabolic stress. Here, we demonstrate that the mammalian brain utilizes pyruvate as a fuel source, and pyruvate can support neuronal viability in the absence of glucose. Nerve terminals are sites of metabolic vulnerability within a neuron and we show that mitochondrial pyruvate uptake is a critical step in oxidative ATP production in hippocampal terminals. We find that the mitochondrial pyruvate carrier is post-translationally modified by lysine acetylation which in turn modulates mitochondrial pyruvate uptake. Importantly, our data reveal that the mitochondrial pyruvate carrier regulates distinct steps in synaptic transmission, namely, the spatiotemporal pattern of synaptic vesicle release and the efficiency of vesicle retrieval, functions that have profound implications for synaptic plasticity. In summary, we identify pyruvate as a potent neuronal fuel and mitochondrial pyruvate uptake as a critical node for the metabolic control of synaptic transmission in hippocampal terminals. HIGHLIGHTSO_LISerum pyruvate is taken up by the brain and efficiently oxidized in the TCA cycle. C_LIO_LIThe mitochondrial pyruvate carrier (MPC) is essential for presynaptic energy metabolism. C_LIO_LIAcetylation of the MPC complex modulates mitochondrial pyruvate uptake. C_LIO_LIMPC activity regulates the release and retrieval of synaptic vesicles in nerve terminals. C_LI

neuroscience↗

bioRxiv2023

Metabolic Regulation of Single Synaptic Vesicle Exo- and Endocytosis in Hippocampal Synapses

Glucose has long been considered a primary source of energy for synaptic function. However, it remains unclear under what conditions alternative fuels, such as lactate/pyruvate, contribute to powering synaptic transmission. By detecting individual release events in cultured hippocampal synapses, we found that mitochondrial ATP production from oxidation of lactate/pyruvate regulates basal vesicle release probability and release location within the active zone (AZ) evoked by single action potentials (APs). Mitochondrial inhibition shifted vesicle release closer to the AZ center, suggesting that the energetic barrier for vesicle release is lower in the AZ center that the periphery. Mitochondrial inhibition also altered the efficiency of single AP evoked vesicle retrieval by increasing occurrence of ultrafast endocytosis, while inhibition of glycolysis had no effect. Mitochondria are sparsely distributed along hippocampal axons and we found that nerve terminals containing mitochondria displayed enhanced vesicle release and reuptake during high-frequency trains, irrespective of whether neurons were supplied with glucose or lactate. Thus, synaptic terminals can entirely bypass glycolysis to robustly maintain the vesicle cycle using oxidative fuels in the absence of glucose. These observations further suggest that mitochondrial metabolic function not only regulates several fundamental features of synaptic transmission but may also contribute to modulation of short-term synaptic plasticity. HighlightsO_LISynapses can sustain neurotransmission across various activity levels by bypassing glycolysis and utilizing oxidative fuels. C_LIO_LIMitochondria, but not glycolysis, regulate release probability and nanoscale organization of vesicle release within the active zone. C_LIO_LIMitochondrial inhibition increases the occurrence of vesicle retrieval via ultra-fast endocytosis. C_LIO_LIMitochondrial localization in nerve terminals enhances vesicle release and retrieval in the absence of glucose, representing a form of synaptic plasticity. C_LI

neuroscience↗

bioRxiv2023

Sirtuin3 ensures the metabolic plasticity of neurotransmission during glucose deprivation

Neurotransmission is an energetically expensive process that underlies cognition. During intense electrical activity or dietary restrictions, glucose levels in the brain plummet, forcing neurons to utilize alternative fuels. However, the molecular mechanisms of neuronal metabolic plasticity remain poorly understood. Here, we demonstrate that glucose-deprived neurons activate the CREB and PGC1 transcriptional program that induces the expression of the mitochondrial deacetylase Sirtuin 3 (Sirt3) both in vitro and in vivo. We show that Sirt3 localizes to axonal mitochondria and stimulates mitochondrial oxidative capacity in hippocampal nerve terminals. Sirt3 plays an essential role in sustaining synaptic transmission in the absence of glucose by powering the retrieval of synaptic vesicles after release. These results demonstrate that the transcriptional induction of Sirt3 ensures the metabolic plasticity of synaptic transmission. HighlightsO_LIGlucose deprivation drives transcriptional reprogramming of neuronal metabolism via CREB and PGC1. C_LIO_LIGlucose or food deprivation trigger the neuronal expression of mitochondrial deacetylase sirtuin 3 (Sirt3) both in vitro and in vivo. C_LIO_LISirt3 stimulates oxidative ATP synthesis in nerve terminals. C_LIO_LISirt3 sustains the synaptic vesicle cycle in the absence of glucose. C_LI

neuroscience↗

bioRxiv2021

Semi-automated analysis of an optical ATP indicator in neural synapses

SignificanceThe firefly enzyme luciferase has been used in a wide range of biological assays, including bioluminescence imaging of ATP. The biosensor Syn-ATP utilizes subcellular targeting of luciferase to nerve terminals for optical measurement of ATP in this compartment. Manual analysis of Syn-ATP signals is challenging due to signal heterogeneity and cellular motion in long imaging sessions. Here, we have leveraged machine learning tools to develop a method for analysis of bioluminescence images. AimOur goal was to create a semi-automated pipeline for analysis of bioluminescence imaging to improve measurements of ATP content in nerve terminals. ApproachWe developed an image analysis pipeline that applies machine learning toolkits to distinguish neurons from background signals, and excludes neural cell bodies, while also incorporating user input. ResultsSide-by-side comparison of manual and semi-automated image analysis demonstrated that the latter improves precision and accuracy of ATP measurements. ConclusionsOur method streamlines data analysis and reduces user-introduced bias, thus enhancing the reproducibility and reliability of quantitative ATP imaging in nerve terminals.

neuroscience↗

bioRxiv2021

Neuronal mitochondria transport Pink1 mRNA via Synaptojanin 2 to support local mitophagy

PTEN-induced kinase 1 (PINK1) is a very short-lived protein that is required for the removal of damaged mitochondria through Parkin translocation and mitophagy. Because the short half-life of PINK1 limits its ability to be trafficked into neurites, local translation is required for this mitophagy pathway to be active far from the soma. The Pink1 transcript is associated with and cotransported with neuronal mitochondria. In concert with translation, the mitochondrial outer membrane protein Synaptojanin 2 binding protein (SYNJ2BP) and Synaptojanin 2 (SYNJ2) are required for tethering Pink1 mRNA to mitochondria via an RNA-binding domain in SYNJ2. This neuron-specific adaptation for local translation of PINK1 provides distal mitochondria with a continuous supply of PINK1 for activation of mitophagy.

neuroscience↗

bioRxiv2019

Molecular tuning of the axonal mitochondrial Ca2+ uniporter ensures metabolic flexability of neurotransmission

The brain is a vulnerable metabolic organ and must adapt to different fuel conditions to sustain function. Nerve terminals are a locus of this vulnerability but how they regulate ATP synthesis as fuel conditions vary is unknown. We show that synapses can switch from glycolytic to oxidative metabolism, but to do so, they rely on activity-driven presynaptic mitochondrial Ca2+ uptake to accelerate ATP production. We demonstrate that while in non-neuronal cells mitochondrial Ca2+ uptake requires elevated extramitochondrial Ca2+, axonal mitochondria readily take up Ca2+ in response to small changes in Ca2+. We identified the brain-specific protein MICU3 as a critical driver of this tuning of Ca2+ sensitivity. Ablation of MICU3 renders axonal mitochondria similar to non-neuronal mitochondria, prevents acceleration of local ATP synthesis, and impairs presynaptic function under oxidative conditions. Thus, presynaptic mitochondria rely on MICU3 to facilitate mitochondrial Ca2+ uptake during activity and achieve metabolic flexibility.\n\nO_LISynapses rely on activity-driven mitochondrial ATP synthesis with oxidative fuels\nC_LIO_LIMitochondrial Ca2+ uptake is required to stimulate ATP synthesis in axons\nC_LIO_LIThe mitochondria Ca2+ uptake threshold is lower in axons than in non-neuronal cells\nC_LIO_LIMICU3 controls the Ca2+ sensitivity of MCU in axonal mitochondria\nC_LI

neuroscience↗