Biology subjects

Semyanov, A.

Publications and source records attributed to Semyanov, A..

15 recordsLinked to original sources

bioRxiv2025

Four phases and temporal threshold of population calcium response during locomotion in cortical astrocytes

Calcium activity is a major form of astrocyte excitability essential to their function and plasticity. Astrocytic calcium events arise both spontaneously and in response to behaviour 1. In this study, we examined how these unitary events shape population-level calcium activity during quiescence and locomotion. Using two-photon imaging of astrocytic calcium in the primary somatosensory cortex (S1) of awake mice running on a freely rotating disk, we found that spontaneous fluctuations during quiescence were driven mainly by the co-occurrence of new calcium events rather than by their enlargement. During locomotion, population responses progressed through four phases: the emergence of new events, their merging into a superevent, fragmentation of this superevent, and an afterburst. New events appeared immediately at locomotion onset, indicating a rapid astrocytic response. The response also exhibited a temporal threshold of [~]5 seconds: when locomotion was shorter than this, the superevent phase failed to develop, and the population response was markedly reduced. In suprathreshold responses, the superevent displayed a {Delta}F/F intensity pattern that reproduced reliably across locomotion episodes. Hotspots within this pattern had shorter latencies and were also more active during quiescence, suggesting that the response is at least partly deterministic. Consistent with this, we were able to reproduce both the response phases and the superevent pattern using a dynamic mode decomposition with control (DMDc) model driven by synthetic or real locomotion inputs. Together, these findings dissect the population calcium response into quantifiable properties of unitary calcium events and reveal the formation and reproducibility of the superevent pattern, offering a potential framework for the astrocytic component of the engram 2,3.

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bioRxiv2025

Spatiotemporal differences of GABAergic polarization and shunting during dendritic integration

In the adult brain, GABA exerts either depolarizing or hyperpolarizing effects on neuronal membranes, depending on neuron type, subcellular location, and neuronal activity. Depolarizing GABA typically inhibits neurons through shunting, which is characterized by increased membrane conductance upon GABAA receptor activation; however, it can also excite neurons by recruiting voltage-dependent conductances. The net influence of these opposing actions of depolarizing GABA on glutamatergic synaptic inputs remains incompletely understood. Here, we examined the spatiotemporal characteristics of membrane polarization and shunting mediated by GABAA receptors and assessed their functional impact on the integration of GABAergic and glutamatergic inputs along dendrites. Using whole-cell current-clamp recordings in CA1 pyramidal neurons and dentate gyrus granule cells (GCs) from rat hippocampal slices, we mimicked GABAergic and glutamatergic inputs with local GABA puffs and glutamate spot-uncaging, respectively. A mathematical model further quantified the relative effects of local shunting and polarization. Depolarizing GABAergic postsynaptic responses (GPSRs) exhibited biphasic actions, exerting inhibitory effects at the synapse through shunting, and excitatory effects distally, where depolarization predominated. The excitatory component also persisted longer than the shunting inhibition. In contrast, hyperpolarizing GPSRs remained consistently inhibitory across both spatial and temporal dimensions. These findings highlight the complex spatiotemporal interplay between shunting and membrane polarization mediated by GABAergic inputs, providing new insights into dendritic computation and neuronal network dynamics.

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bioRxiv2024

Cerebral perfusion and metabolic response of astrocytes and neurons during locomotion

The hemodynamic response links neural oxidative metabolism changes to increased cerebral blood flow during brain activity1. Although Roy and Sherrington introduced this concept over a century ago (1890), the exact cellular mechanisms remain unclear. This study demonstrates how local blood supply increases correlate with the metabolic response of individual brain cells during locomotion. Using Raman microspectroscopy, we observed an elevation in oxygen saturation levels (sO2) in cortical venules but not in arterioles, which were already near saturation. The increased sO2 in the venules was accompanied by vasodilation, indicating blood oversupply in the local brain area, a phenomenon known as functional hyperemia. We then analyzed the metabolic response of individual neurons and astrocytes to locomotion. In neurons, the levels of reduced cytochromes of c and b types [cyt c,b (Fe2+)] rapidly decreased at the onset of locomotion. This suggests an increase in the activity of the mitochondrial respiratory chain (electron transport chain, ETC) in response to heightened energy demands in these cells2. In contrast, because astrocytes rely less on oxidative phosphorylation for their energy metabolism than neurons3, we did not observe an initial decrease in reduced cytochromes in these cells. However, as locomotion continued, the cyt c,b (Fe2+) levels steadily increased in both cell types. In neurons, this led to a slow recovery from the initial drop, while in astrocytes, the increase exceeded baseline levels. Consequently, we observed an overload of electrons in the astrocytic ETC. The distinct responses of astrocytic and neuronal mitochondria to locomotion may reflect differences in the organization of the ETC in the two cell types4. Furthermore, astrocytes may shift to glycolysis under increased neuronal activity5,6. This difference also resulted in hydrogen peroxide (H202) production in astrocytic mitochondria but not neuronal mitochondria. Since H202 is a signaling molecule critical for cognitive function in the brain7, we speculate that astrocytic mitochondria act as signaling hubs during exercise.

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bioRxiv2024

Astrocyte ezrin defines resilience to stress-induced depressive behaviours in mice

Astrocyte atrophy is the main histopathological hallmark of major depressive disorder (MDD) in humans and in animal models of depression. Here we demonstrated that manipulating with ezrin expression specifically in astrocytes significantly increases the resilience of mice to chronic unpredictable mild stress (CUMS). Overexpression of ezrin in astrocytes from prefrontal cortex (PFC) rescued depressive-like behaviours induced by CUMS, whereas down-regulation of ezrin in astrocytes from PFC increased mice susceptibility to CUMS and promoted depressive-like behaviours. These behavioural changes correlated with astrocytic morphology. Astrocytes from PFC of mice sensitive to CUMS demonstrated significant atrophy; similar atrophy was found in astrocytes from animals with down-regulated ezrin expression. To the contrary morphology remains unchanged astrocytes in animals resistant to CUMS and in animals with astrocytic overexpression of ezrin. Morphological changes also correlated with ezrin immunoreactivity which was low in mice with depressive-like behaviours and high in mice resistant to stress. We conclude that Ezrin-dependent morphological remodelling of astrocytes defines the sensitivity of mice to stress: high ezrin expression renders them stress resilient, whereas low ezrin expression promotes depressive-like behaviour in response to chronic stress.

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bioRxiv2023

Electroacupuncture prevents astrocyte atrophy to alleviate depression

Astrocyte atrophy is the main histopathological hallmark of the major depressive disorder (MDD) in humans and in animal models of depression. Here we show that electroacupuncture prevents astrocyte atrophy in the prefrontal cortex and alleviates depressive-like behaviour in mice subjected to the chronic unpredictable mild stress (CUMS). Treatment of mice with CUMS induced depressive-like phenotypes as confirmed by sucrose preference test, tail suspension test, and forced swim test. These behavioural changes were paralleled with morphological atrophy of astrocytes in the prefrontal cortex, revealed by analysis of 3D reconstructions of confocal Z-stack images of mCherry expressing astrocytes. This morphological atrophy was accompanied with a decrease in expression of cytoskeletal linker Ezrin, associated with formation of astrocytic leaflets, which form astroglial synaptic cradle. Electroacupuncture at the acupoint ST36 as well as treatment with anti-depressant fluoxetine prevented depressive-like behaviours, astrocytic atrophy and down-regulation of astrocytic ezrin. In conclusion, our data further strengthen the notion of a primary role of astrocytic atrophy in depression and reveal astrocytes as cellular target for electroacupuncture in treatment of depressive disorders.

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bioRxiv2022

Aging impairs astrocytes in the human cerebral cortex

How aging affects cellular components of the human brain active milieu remains largely unknown. We analyzed astrocytes and neurons in the neocortical access tissue of younger (22 - 50 years) and older (51 - 72 years) adult patients who underwent glioma resection. Aging decreased the amount of reduced mitochondrial cytochromes in astrocytes but not neurons. The total amount of protein was decreased in astrocytes and increased in neurons. Aged astrocytes showed morphological dystrophy quantified by the decreased length of branches, decreased volume fraction of leaflets, and shrinkage of the anatomical domain. Dystrophy correlated with the loss of gap junction coupling between astrocytes and increased input resistance. Aging was accompanied by the upregulation of glial fibrillary acidic protein (GFAP) and downregulation of membrane-cytoskeleton linker Ezrin associated with leaflets. No significant changes in neuronal excitability or spontaneous inhibitory postsynaptic signaling were observed. Thus, brain aging is associated with the impaired morphological presence and mitochondrial malfunction of astrocytes, but not neurons.

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bioRxiv2022

Astrocyte mitochondria produce nitric oxide from nitrite to modulate cerebral blood flow during brain hypoxia

In low oxygen conditions, increases in cerebral blood flow maintain brain oxygen delivery but the mechanisms underlying hypoxia-induced dilations of cerebral vasculature are incompletely understood. Here we show that astrocytes accumulate nitrite and in response to hypoxia produce nitric oxide via mitochondrial reduction of nitrite by a molybdenum-containing enzyme sulfite oxidase. These data suggest that astrocytes can modulate cerebral blood flow in accord with brain tissue oxygenation via mitochondrial production of nitric oxide.

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bioRxiv2021

A high-fat diet changes astrocytic metabolism to enhance synaptic plasticity and promote exploratory behavior

A high-fat diet (HFD) is generally considered to negatively influence the body, the brain, and cognitive abilities. On the other hand, fat and fatty acids are essential for nourishing and constructing brain tissue. Astrocytes are central for lipolysis and fatty acids metabolism. Here we show that exposure of young mice to one month of HFD elevates lipid content and increases the relative amount of reduced cytochromes in astrocytes but not in neurons. Metabolic changes were paralleled with an enlargement of astrocytic territorial domains due to an increased outgrowth of branches and leaflets. Astrocyte remodeling was associated with an increase in expression of ezrin and with no changes in glial fibrillary acidic protein (GFAP), glutamate transporter-1 (GLT-1), and glutamine synthetase (GS). Such physiological (non-reactive) enlargement of astrocytes in the brain active milieu promoted glutamate clearance and long-term potentiation. These changes translated into improved exploratory behavior. Thus, dietary fat intake is not invariably harmful and might exert beneficial effects depending on the biological context. In BriefA high-fat diet stimulates the metabolism and growth of astrocytes, which improves glutamate clearance, synaptic plasticity, and exploratory behavior in young mice. Thus, dietary fat arguably is an essential component of the diet for children and young adults, supporting the optimal development of the brain. HighlightsO_LIExposure of young mice to a high-fat diet elevated lipid content and increased amount of reduced cytochromes in astrocytes but not in neurons. C_LIO_LIMetabolic changes were paralleled with an enlargement of astrocytic territorial domains due to an increased outgrowth of branches and leaflets. C_LIO_LIAstrocytic enlargement was associated with increased expression of ezrin but not GFAP, hence was not reactive but physiological C_LIO_LIExpansion of astrocytes in the brain active milieu improved glutamate clearance and long-term potentiation. C_LIO_LIThe high-fat diet improved exploratory behavior in young mice. C_LI

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bioRxiv2020

K+ efflux through postsynaptic NMDA receptors suppresses local astrocytic glutamate uptake

Glutamatergic transmission prompts K+ efflux through postsynaptic NMDA receptors. K+ depolarizes local presynaptic terminals, boosting glutamate release, but whether it also affecting glutamate uptake remains unknown. Here, we find that the pharmacological blockade, or conditional knockout, of NMDA receptors suppresses the progressive use- dependent increase in the amplitude and decay time of the astrocytic glutamate transporter current (IGluT), whereas blocking the astrocytic inward-rectifying K+ channels prevents the decay time increase only. Glutamate spot-uncaging reveals that local astrocyte depolarization, rather than extracellular K+ rises on their own, reduces the amplitude and prolong the decay of IGluT. Biophysical simulations confirm that local transient elevations of extracellular K+ can inhibit local glutamate uptake in fine astrocytic processes. Optical glutamate sensor imaging and a two-pathway test relate postsynaptic K+ efflux to enhanced extrasynaptic glutamate signaling. Thus, postsynaptic K+ efflux facilitates presynaptic release while reducing local glutamate uptake.

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bioRxiv2020

Astrocytes dystrophy in ageing brain parallels impaired synaptic plasticity

Little is known about age-dependent changes in structure and function of astrocytes and of the impact of these into the cognitive decline in the senescent brain. The prevalent view on age-dependent increase in reactive astrogliosis and astrocytic hypertrophy requires scrutiny and detailed analysis. Using two-photon microscopy in conjunction with 3D reconstruction, Sholl and volume fraction analysis we demonstrate a significant reduction in the number and the length of astrocytic processes, in astrocytic territorial domains and in astrocyte-to-astrocyte coupling in the aged brain. Probing physiology of astrocytes with patch-clamp and Ca2+ imaging revealed deficits in K+ and glutamate clearance, and spatiotemporal reorganization of Ca2+ events in old astrocytes. These changes paralleled impaired synaptic long-term potentiation (LTP) in hippocampal CA1 in old mice. Our findings may explain astroglial mechanisms of age-dependent decline in learning and memory.

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bioRxiv2020

Attenuation of the extracellular matrix increases the number of synapses but suppresses synaptic plasticity

The brain extracellular matrix (ECM) is a proteoglycan complex that occupies the extracellular space between brain cells and regulates brain development, brain wiring, and synaptic plasticity. However, the action of the ECM on synaptic plasticity remains controversial. Here, we employed serial section electron microscopy to show that enzymatic attenuation of ECM with chondroitinase ABC (ChABC) triggers the appearance of new glutamatergic synapses onto thin dendritic spines of CA1 pyramidal neurons. The appearance of new synapses increased the ratio of the field excitatory postsynaptic potential (fEPSP) to presynaptic fiber volley (PrV), suggesting that these new synapses are formed on existing axonal fibers. However, both the mean miniature excitatory postsynaptic current (mEPSC) amplitude and AMPA/NMDA ratio were decreased, suggesting that ECM attenuation increased the proportion of unpotentiated synapses. A higher proportion of unpotentiated synapses would be expected to promote long-term potentiation (LTP). Surprisingly, theta-burst induced LTP was suppressed by ChABC treatment. The suppression of LTP was accompanied by decreased excitability of CA1 pyramidal neurons due to the upregulation of small conductance Ca2+-activated K+ (SK) channels. A pharmacological blockade of SK channels restored cell excitability and, expectedly, enhanced LTP above the level of control. This enhancement of LTP was abolished by a blockade of Rho-associated protein kinase (ROCK), which is involved in the maturation of dendritic spines. Thus, ECM attenuation enables the appearance of new synapses in the hippocampus, which is compensated for by a reduction in the excitability of postsynaptic neurons, thereby preventing network overexcitation at the expense of synaptic plasticity.

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bioRxiv2019

Tonic GABAA conductance favors temporal over rate coding in the rat hippocampus

Synaptic plasticity is triggered by different patterns of neuronal network activity. Network activity leads to an increase in ambient GABA concentration and tonic activation of GABAA receptors. How tonic GABAA conductance affects synaptic plasticity during temporal and rate-based coding is poorly understood. Here, we show that tonic GABAA conductance differently affects long-term potentiation (LTP) induced by different stimulation patterns. The LTP based on a temporal spike - EPSP order (spike-timing-dependent [st] LTP) was not affected by exogenous GABA application. Backpropagating action potential, which enables Ca2+ entry through N-methyl-D-aspartate receptors (NMDARs) during stLTP induction, was only slightly reduced by the tonic conductance. In contrast, GABA application impeded LTP dependent on spiking rate (theta-burst-induced [tb] LTP) by reducing the EPSP bust response and, hence, NMDAR-mediated Ca2+ entry during tbLTP induction. Our results may explain the changes in different forms of memory under physiological and pathological conditions that affect tonic GABAA conductance.

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bioRxiv2019

Beyond the suprachiasmatic nucleus: circadian rhythmicity shapes astrocyte morphology and neuronal function in hippocampal area CA1

Most animal species operate according to a 24-hour period set by the suprachiasmatic nucleus (SCN) of the hypothalamus. The rhythmic activity of the SCN is known to modulate hippocampal-dependent memory processes, but the molecular and cellular mechanisms that account for this effect remain largely unknown. Here, we show that there are cell-type specific structural and functional changes that occur with circadian rhythmicity in neurons and astrocytes in hippocampal area CA1. Pyramidal neurons change the surface expression of NMDA receptors, whereas astrocytes change their proximity to synapses. Together, these phenomena alter glutamate clearance, receptor activation and integration of temporally clustered excitatory synaptic inputs, ultimately shaping hippocampal-dependent learning in vivo. We identify corticosterone as a key contributor to changes in synaptic strength. These findings identify important mechanisms through which neurons and astrocytes modify the molecular composition and structure of the synaptic environment, contribute to the local storage of information in the hippocampus and alter the temporal dynamics of cognitive processing.

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bioRxiv2019

An astrocytic basis of caloric restriction action on the brain plasticity

One month of calorically restricted diet (CR) induced morphological plasticity of astrocytes in the stratum (str.) radiatum of hippocampal CA1 in three-months old mice: the volume fraction of distal perisynaptic astrocytic processes increased whereas the number of gap-junction coupled astrocytes decreased. The uncoupling was not associated with a decrease in the expression of connexin 43. Uncoupling and morphological remodeling affected spontaneous Ca2+ activity in the astrocytic network: Ca2+ events became longer, whereas their spread was reduced. The change in the pattern of astrocytic Ca2+ activity may increase the spatial resolution of the information encoding in the astroglial network. Consistent with expanded synaptic enwrapping by the astroglial processes, the spillover of synaptically released K+ and glutamate was diminished after CR. However, no significant changes in the expression of astrocytic glutamate transporter (GLT-1/EAAT2) were observed, although the level of glutamine synthetase was decreased. Glutamate uptake is known to regulate the synaptic plasticity. Indeed, the magnitude of long-term potentiation (LTP) in the glutamatergic CA3-CA1 synapses was significantly enhanced after CR. Our findings highlight an astroglial basis for improved learning and memory reported in various species subjected to CR.

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bioRxiv2018

Morphological profile determines the frequency of spontaneous calcium events in astrocytic processes

Astrocytes express a complex repertoire of intracellular Ca2+ transients (events) that represent a major form of signaling within individual cells and in the astrocytic syncytium. These events have different spatiotemporal profiles, which are modulated by neuronal activity. Spontaneous Ca2+ events appear more frequently in distal astrocytic processes and independently from each other. However, little is known about the mechanisms underlying such subcellular distribution of the Ca2+ events. Here we identify the initiation points of the Ca2+ events within the territory of single astrocytes expressing genetically encoded Ca2+ indicator GCaMP2 in culture or in hippocampal slices. We found that most of the Ca2+ events start in thin distal processes. Our mathematical model demonstrated that a high surface-to-volume (SVR) of the thin processes leads to increased amplitude of baseline Ca2+ fluctuations caused by a stochastic opening of Ca2+ channels in the plasma membrane. Suprathreshold fluctuations trigger Ca2+-induced Ca2+ release (CICR) from the Ca2+ stores by activating inositol 1,4,5-trisphosphate (IP3) receptors. In agreement with the model prediction, the spontaneous Ca2+ events frequency depended on the extracellular Ca2+ concentration. Astrocytic depolarization by high extracellular K+ increased the frequency of the Ca2+ events through activation of voltage-gated Ca2+ channels (VGCC) in cultured astrocytes. Our results suggest that the morphological profile of the astrocytic processes is responsible for tuning of the Ca2+ event frequency. Therefore, the structural plasticity of astrocytic processes can be directly translated into changes in astrocytic Ca2+ signaling. This may be important for both physiological and pathological astrocyte remodeling.\n\nMain pointsO_LIMajority of spontaneous Ca2+ events start in thin astrocytic processes\nC_LIO_LIHigher surface-to-volume ratio of the process is responsible for larger intracellular Ca2+ fluctuations\nC_LIO_LILarger intracellular Ca2+ fluctuations trigger Ca2+-dependent Ca2+ release\nC_LI

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