Biology subjects

Hemmings, H. C.

Publications and source records attributed to Hemmings, H. C..

8 recordsLinked to original sources

bioRxiv2026

Increased expression of developmental Nav1.3 promotes hippocampal CA3 hyperexcitability in early-stage 5xFAD mice

Neuronal hyperexcitability is an early and pervasive feature of Alzheimers disease (AD) that both predicts and accelerates subsequent cognitive decline. Persistent excitability depends on activation of voltage-gated sodium channels (Nav), yet most work has focused on the Nav subtypes expressed in the mature brain. Here, we show that Nav1.3, a subtype normally confined to early development, shows aberrantly increased expression in the dentate gyrus (DG)-CA3 circuit in early-stage 5xFAD mice. Using in vivo fiber photometry, three-month-old 5xFAD mice exhibited greater CA3 neuron population activity than seven-month-old 5xFAD mice or wildtype littermates. Oligomeric A{beta} expression, assessed by immunolabeling, was sparse at three months and rose significantly by seven months, indicating that CA3 hyperactivity emerges before substantial oligomeric A{beta} accumulates. This early activity increase coincided with elevated Nav1.3 expression at the DG-CA3 mossy fiber synapse, localized by immuno-electron microscopy to presynaptic mossy fiber terminals, where it exceeded levels in age-matched controls. Lentiviral shRNA-mediated knockdown of Nav1.3 expression in CA3 normalized CA3 network activity in early-stage 5xFAD mice. These findings identify Nav1.3 expression at DG-CA3 mossy fiber synapses as a driver of early hippocampal network dysfunction in AD and suggest Nav1.3 modulation as a potential target for circuit-level intervention.

neuroscience↗

bioRxiv2026

Presynaptic NMDA receptors on mouse mossy fiber terminals mediate rapid BDNF release by ketamine and hydroxynorketamine

Major depressive disorder is associated with deficits in hippocampal synaptic plasticity that depend on brain-derived neurotrophic factor (BDNF) release from both axonal and dendritic compartments. Antidepressant efficacy requires enhanced BDNF signaling, thought to be mediated by drug-induced BDNF release from postsynaptic dendritic spines. Here, we show that fast-acting antidepressants rapidly trigger BDNF secretion from presynaptic terminals in hippocampal area CA3. At antidepressant-relevant concentrations, ketamine and its metabolite (2R,6R)-hydroxynorketamine (HNK) induced BDNF release within minutes from mossy fiber terminals of dentate granule neurons in rat hippocampal cultures, with no detectable secretion from dendritic spines. This antidepressant-evoked BDNF release required presynaptic NMDA receptors (preNMDARs). Conditional genetic deletion of preNMDARs from granule neurons abolished ketamine- and HNK-induced BDNF exocytosis in acute mouse hippocampal slices, establishing a presynaptic receptor mechanism for antidepressant-induced neurotrophin release. In CA3 pyramidal neurons that receive mossy fiber input, both compounds induced rapid remodeling of dendritic spines, resulting in increased spine density. Together, these findings identify presynaptic terminals as a previously unrecognized source of antidepressant-evoked BDNF release and establish a new cellular mechanism for the rapid synaptic effects of fast-acting antidepressants.

neuroscience↗

bioRxiv2025

Sevoflurane inhibition of the developmentally expressed neuronal sodium channel Nav1.3

Neuronal voltage-gated sodium channels (Nav) are major targets for the neurophysiological actions of general anesthetics. In the adult brain, cell type-specific effects on synaptic transmission are attributed to the differential sensitivity to volatile anesthetics of specific Nav subtypes preferentially expressed in mature neurons (Nav1.1, Nav1.2, Nav1.6). Compared to mature neurons, neurons in the developing CNS are more excitable. Since the subtype-selective effects of volatile anesthetics on Nav during early development are unknown, we determined volatile anesthetic effects on Na+ currents mediated by Nav1.3, the principal Nav subtype expressed in developing neurons. Sevoflurane at clinically relevant concentrations inhibited peak Na+ current (INa) of human Nav1.3 heterologously expressed in HEK293T cells in a voltage- dependent manner, induced a -6.1 mV hyperpolarizing shift in the voltage dependence of steady- state inactivation, and slowed recovery from fast inactivation. Nav1.3-mediated Na+ currents also exhibited distinct activation properties associated with neuronal hyperexcitability, including prominent persistent currents and ramp currents, both of which were significantly reduced by sevoflurane. The major neuronal subtype Nav1.2 showed a more hyperpolarized voltage dependence of steady-state inactivation than Nav1.3. Consistent with its lower propensity for sustained repetitive firing, Nav1.2 exhibited minimal persistent and ramp currents, and these were unaffected by sevoflurane. These findings identify subtype-specific effects of the volatile anesthetic sevoflurane on neuronal Nav subtypes and suggest a mechanistic basis for increased anesthetic sensitivity in early neuronal differentiation and maturation.

pharmacology and toxicology↗

bioRxiv2024

Functionally important binding site for a volatile anesthetic in a voltage-gated sodium channel identified by X-ray crystallography

Voltage-gated sodium channels (VGSCs), key mediators of excitability and synaptic transmission, are established and functionally relevant targets for volatile anaesthetic (VA) action. Using the structurally homologous prokaryotic VGSCs NavMs and NaChBac as models, we present a structure-function analysis of VGSC-VA interactions. We report that multiple VAs compete for binding sites on NavMs, and that these direct interactions mediate functional effects of sevoflurane on NavMs that mirror those attributed to VA effects in eukaryotic VGSCs, including human isoforms. Using X-ray crystallography, we determined the first atomic-resolution structure of a VA bound to a VGSC, showing sevoflurane displacing lipids to bind in an intramembranous hydrophobic pocket of NavMs. A conserved tyrosine residue within this binding site is critical for channel gating, and its substitution with alanine abolishes sevoflurane binding and selectively eliminates the characteristic anaesthetic-induced hyperpolarising shift of steady-state inactivation that reduces neuronal excitability at physiological membrane potentials. Finally, we provide evidence supporting VA action at the conserved sites in human VGSC isoforms. These findings define the first VA binding site in a VGSC. A membrane-mediated access pathway to the binding site leads to negative modulation of channel function that reduces neuronal activity and excitatory synaptic transmission in general anaesthesia.

biophysics↗

bioRxiv2023

Nonspecific membrane bilayer perturbations by ivermectin underlie SARS-CoV-2 in vitro activity

Since it was proposed as a potential host-directed antiviral agent for SARS-CoV-2, the antiparasitic drug ivermectin has been investigated thoroughly in clinical trials, which have provided insufficient support for its clinical efficacy. To examine the potential for ivermectin to be repurposed as an antiviral agent, we therefore undertook a series of preclinical studies. Consistent with early reports, ivermectin decreased SARS-CoV-2 viral burden in in vitro models at low micromolar concentrations, five-to ten-fold higher than the reported toxic clinical concentration. At similar concentrations, ivermectin also decreased cell viability and increased biomarkers of cytotoxicity and apoptosis. Further mechanistic and profiling studies revealed that ivermectin nonspecifically perturbs membrane bilayers at the same concentrations where it decreases the SARS-CoV-2 viral burden, resulting in nonspecific modulation of membrane-based targets such as G-protein coupled receptors and ion channels. These results suggest that a primary molecular mechanism for the in vitro antiviral activity of ivermectin may be nonspecific membrane perturbation, indicating that ivermectin is unlikely to be translatable into a safe and effective antiviral agent. These results and experimental workflow provide a useful paradigm for performing preclinical studies on (pandemic-related) drug repurposing candidates. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=61 SRC="FIGDIR/small/563088v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@a21f94org.highwire.dtl.DTLVardef@1c76751org.highwire.dtl.DTLVardef@500930org.highwire.dtl.DTLVardef@8b6c05_HPS_FORMAT_FIGEXP M_FIG Graphical abstract C_FIG

pharmacology and toxicology↗

bioRxiv2022

Volatile anesthetics inhibit presynaptic cGMP signaling to depress presynaptic excitability in rat hippocampal neurons

Volatile anesthetics alter presynaptic function including effects on Ca2+ influx and neurotransmitter release. These actions are proposed to play important roles in their pleiotropic neurophysiological effects including unconsciousness and amnesia. The nitric oxide and cyclic guanosine monophosphate (NO/cGMP) signaling pathway has been implicated in presynaptic mechanisms, and disruption of NO/cGMP signaling has been shown to alter sensitivity to volatile anesthetics in vivo. We investigated NO/cGMP signaling in relation to volatile anesthetic actions in cultured rat hippocampal neurons using pharmacological tools and genetically encoded biosensors of cGMP levels. Using the fluorescent biosensor cGull we found that electrical stmulation-evoked NMDA-type glutamate receptor-independent presynaptic cGMP transients were inhibited -33.2% by isoflurane (0.51 mM) and -23.8% by sevoflurane (0.57 mM) (p<0.0001) compared to a stimulation without anesthetic. Isoflurane and sevoflurane inhibition of stimulation-evoked increases in presynaptic Ca2+ concentration, measured with synaptophysin-GCaMP6f, and synaptic vesicle exocytosis, measured with synaptophysin-pHlourin, were reduced by in neurons expressing the cGMP scavenger sponGee. This reduction in anesthetic effect was recapitulated by inhibiting HCN channels, a cGMP-modulated effector that can facilitate glutamate release. We propose that volatile anesthetics depress presynaptic cGMP signaling and downstream effectors like HCN channels that are essential to presynaptic function and excitability. These findings identify a novel mechanism by which volatile anesthetics depress synaptic transmission via second messenger signaling involving the NO/cGMP pathway.

neuroscience↗

bioRxiv2022

A common human brain-derived neurotrophic factor polymorphism leads to sustained depression of excitatory synaptic transmission by isoflurane in hippocampus

Multiple presynaptic and postsynaptic targets have been identified for the reversible neurophysiological effects of general anesthetics on synaptic transmission and neuronal excitability. However, the synaptic mechanisms involved in persistent depression of synaptic transmission resulting in more prolonged neurological dysfunction following anesthesia are less clear. Here, we show that brain-derived neurotrophic factor (BDNF), a growth factor implicated in synaptic plasticity and dysfunction, enhances glutamate synaptic vesicle exocytosis, and that attenuation of vesicular BDNF release by isoflurane contributes to transient depression of excitatory synaptic transmission in mice. This reduction in synaptic vesicle exocytosis was irreversible in neurons that release less endogenous BDNF due to a polymorphism (BDNF Val66Met) compared to wild-type mouse hippocampal neurons following isoflurane exposure. These effects were prevented by exogenous application of BDNF. Our findings identify a role for a common human BDNF single nucleotide polymorphism (Val66Met; rs6265) in persistent changes of synaptic function following isoflurane exposure. These persistent alterations in excitatory synaptic transmission have important implications for the role of genotype in anesthetic effects on synaptic plasticity and neurocognitive function.

neuroscience↗

bioRxiv2020

Differential inhibition of GABA release from mouse hippocampal interneuron subtypesby the volatile anesthetic isoflurane

General anesthesia is critical to modern medicine and animal research, but the cellular and molecular actions of general anesthetics on the central nervous system remain poorly understood. Volatile anesthetics such as isoflurane disrupt synaptic transmission and inhibit synaptic vesicle release in a neurotransmitter-selective manner. For example, GABA release from interneurons is less sensitive to isoflurane inhibition than are glutamate or dopamine release. Hippocampal and cortical interneuron subpopulations have diverse neurophysiological and synaptic properties, and their individual subtype-specific responses to isoflurane are unknown. We used live-cell optical imaging of exocytosis using fluorescent biosensors expressed in transgenic mouse hippocampal neuron cultures to delineate interneuron subtype-specific effects of isoflurane on synaptic vesicle exocytosis. We found that a clinically relevant concentration of isoflurane (0.5 mM) differentially modulated action potential-mediated exocytosis from GABAergic interneurons: parvalbumin-expressing interneurons were inhibited to 83.1{+/-}11.7% of control, whereas somatostatin-expressing and interneurons glutamatergic neurons were inhibited to 58.6{+/-}13.3% and 64.5{+/-}8.5% of control, respectively. The role of presynaptic voltage-gated sodium channel (Nav) subtype expression in determining isoflurane sensitivity was probed by overexpression or knockdown of specific Nav subtypes, which have distinct sensitivities to isoflurane and are differentially expressed between glutamatergic and GABAergic neurons. We found that the sensitivity of exocytosis to isoflurane was determined by the relative expression of Nav1.1 (associated with lower sensitivity) and Nav1.6 (associated with higher sensitivity). Thus the selective effects of isoflurane on synaptic vesicle exocytosis from hippocampal interneuron subtypes is determined by synaptic diversity in the relative expression of Nav1.1 and Nav1.6. Significance statementThe volatile anesthetic isoflurane inhibits hippocampal GABAergic interneuron synaptic vesicle exocytosis with differences in potency between interneuron subtypes. This neuron subtype-specific pharmacology derives in part from synaptic diversity in the expression of presynaptic voltage-gated sodium channels that have different sensitivities to anesthetic modulation of channel function. GABAergic interneurons are generally more resistant to the presynaptic effects of isoflurane owing to predominant Nav1.1 expression, whereas glutamatergic neurons are more sensitive owing to predominant Nav1.6 expression, which supports heterogenous pharmacologic effects on specific neural circuits.

neuroscience↗