Biology subjects

Westbrook, G. L.

Publications and source records attributed to Westbrook, G. L..

8 recordsLinked to original sources

bioRxiv2026

Hilar mossy cells control structural and functional organization in the dentate gyrus

Hilar mossy cells in the dentate gyrus project widely throughout the hippocampus, broadly contributing to circuit function. Their loss in disease is associated with local functional and structural rearrangements, including retrograde granule cell axon sprouting, aberrant neurogenesis, and disinhibition. To examine how mossy cell loss contributes to these circuit rearrangements, we ablated or silenced hilar mossy cells using viral approaches in transgenic (Crlr-Cre) mice. Both mossy cell ablation and silencing dramatically altered dentate gyrus structure and function, as assessed using immunohistochemical, viral labeling, electrophysiology, and anatomical methods. Both manipulations accelerated the maturation of adult-born neurons, but did not alter neuroblast proliferation or cause granule cell axon sprouting. However, mossy cell ablation, but not silencing, caused collapse of the inner molecular layer accompanied by proximal translocation of middle molecular layer inputs. In both cases, granule cell activity measured by cFos labeling and seizure susceptibility were unchanged after mossy cell loss, indicating functional compensation for the altered network organization. Our results highlight how mossy cells influence dentate gyrus organization and adult neurogenesis but also demonstrate the resilience of the hippocampal circuit to structural or functional perturbations.

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bioRxiv2021

Adaptive mossy cell circuit plasticity after status epilepticus

Hilar mossy cells control network function in the hippocampus through both direct excitation and di-synaptic inhibition of dentate granule cells (DGCs). Substantial mossy cell loss occurs after epileptic seizures; however the contribution of surviving mossy cells to network activity in the reorganized dentate gyrus is unknown. To examine functional circuit changes after pilocarpine-induced status epilepticus, we optogenetically stimulated mossy cells in acute hippocampal slices. In control mice, activation of mossy cells produced monosynaptic excitatory and di-synaptic GABAergic currents in DGCs. In pilocarpine-treated mice, mossy cell density and excitation of DGCs were reduced in parallel, with only a minimal reduction in feedforward inhibition, enhancing the inhibition:excitation ratio. Surprisingly, mossy cell-driven excitation of parvalbumin-positive basket cells, the primary mediators of feed-forward inhibition, was maintained, indicating increased connectivity between surviving mossy cells and these targets. Our results suggest that mossy cell outputs reorganize following seizures, increasing their net inhibitory effect in the hippocampus.

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bioRxiv2019

α2δ-2 protein controls structure and function at the cerebellar climbing fiber synapse

2{delta} proteins (Cacna2d1-4) are auxiliary subunits of voltage-dependent calcium channels that also drive synapse formation and maturation. Because cerebellar Purkinje cells (PCs) only express one isoform of this family, 2{delta}-2 (Cacna2d2), we used PCs as a model system to examine roles of 2{delta} in excitatory synaptic function in a Cacna2d2 knockout mouse. Whole-cell recordings of PCs from acute cerebellar slices revealed altered climbing fiber (CF)-evoked complex spike generation, as well as increased amplitude and faster decay of CF-evoked excitatory postsynaptic currents (EPSCs). CF terminals in the KO were localized more proximally on PC dendrites, as indicated by VGLUT2+ immunoreactive puncta, and computational modeling demonstrated that the increased EPSC amplitude can be partly attributed to the more proximal location of CF terminals. In addition, CFs in KO mice exhibited increased multivesicular transmission, corresponding to greater sustained responses during repetitive stimulation, despite a reduction in the measured probability of release. Electron microscopy demonstrated that mutant CF terminals had twice as many vesicle release sites, providing a morphologic explanation for the enhanced glutamate release. Though KO CFs evoked larger amplitude EPSCs, the charge transfer was the same as wildtype as a result of increased glutamate re-uptake, producing faster decay kinetics. Together, the larger, faster EPSCs in the KO explain the altered complex spike responses, which degrade information transfer from PCs and likely contribute to ataxia in Cacna2d2 KO mice. Our results also illustrate the multidimensional synaptic roles of 2{delta} proteins. Significance Statement2{delta} proteins (Cacna2d1-4) regulate synaptic transmission and synaptogenesis, but co-expression of multiple 2{delta} isoforms has obscured a clear understanding of how various 2{delta} proteins control synaptic function. We focused on roles of the 2{delta}-2 protein (Cacna2d2), whose deletion causes cerebellar ataxia and epilepsy in mice and humans. Because cerebellar Purkinje cells only expresses this single isoform, we studied excitatory climbing fiber synaptic function onto Purkinje cells in Cacna2d2 knockout mice. Using optical and electrophysiological analysis, we provide a detailed description of the changes in Purkinje cells lacking 2{delta}-2, and provide a comprehensive mechanistic explanation for how functional synaptic phenotypes contribute to the altered cerebellar output.

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bioRxiv2019

Exercise-induced enhancement of synaptic functiontriggered by the inverse BAR protein, Mtss1L.

Exercise is a potent enhancer of learning and memory, yet we know little of the underlying mechanisms that likely include alterations in synaptic efficacy in the hippocampus. To address this issue, we exposed mice to a single episode of voluntary exercise, and permanently marked mature hippocampal dentate granule cells that were specifically activated during exercise using conditional Fos-TRAP mice. Only a few dentate granule cells were active at baseline, but two hours of voluntary exercise markedly increased the number of activated neurons. Activated neurons (Fos-TRAPed) showed an input-selective increase in dendritic spines and excitatory postsynaptic currents at 3 days post-exercise, indicative of exercise-induced structural plasticity. Laser-capture microdissection and RNASeq of activated neurons revealed that the most highly induced transcript was Mtss1L, a little-studied gene in the adult brain. Overexpression of Mtss1L in neurons increased spine density, leading us to hypothesize that its I-BAR domain initiated membrane curvature and dendritic spine formation. shRNA-mediated Mtss1L knockdown in vivo prevented the exercise-induced increases in spines and excitatory postsynaptic currents. Our results link short-term effects of exercise to activity-dependent expression of Mtss1L, which we propose as a novel effector of activity-dependent rearrangement of synapses. One Sentence SummarySingle episodes of voluntary exercise induced a functional increase in hippocampal synapses mediated by activity-dependent expression of the BAR protein Mtss1L, acting as a novel early effector of synapse formation.

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bioRxiv2018

Anti-NMDA receptor encephalitis in mice induced by active immunization with conformationally-stabilized holoreceptors

Autoimmunity to membrane proteins in the central nervous system has been increasingly recognized as a cause of neuropsychiatric disease. A key recent development was the discovery of antibodies to NMDA receptors in limbic encephalitis, characterized by cognitive changes, memory loss, seizures and sometimes long-term morbidity or mortality. Treatment approaches and experimental studies have largely focused on the pathogenic role of these autoantibodies. Passive antibody transfer to mice has provided useful insights, but does not produce the full spectrum of the human disease. Here we describe a de novo autoimmune mouse model of anti-NMDA receptor encephalitis. Active immunization of immune competent mice with conformationally-stabilized, native-like NMDA receptors induced a fulminant encephalitis that was strikingly similar to the behavioral and pathologic characteristics of human cases. Our results provide evidence of neuroinflammation and immune cell infiltration as early and robust features of the autoimmune response. Use of transgenic mice indicated that mature T cells as well as antibody-producing cells were required for disease induction. Our results provide new insights into disease pathogenesis as well as a platform for testing mechanisms of disease initiation and therapeutic approaches.\n\nOne Sentence SummaryWe report an active immunization model of anti-NMDA receptor encephalitis in mice that recapitulates the features of the clinical disease, provides new insights into the pathophysiology, and offers a platform for investigation of new therapeutic interventions.

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bioRxiv2018

Early detonation by sprouted mossy fibers enables aberrant dentate network activity

In temporal lobe epilepsy, sprouting of hippocampal mossy fiber axons onto dentate granule cell dendrites creates a recurrent excitatory network. However, unlike mossy fibers projecting to CA3, sprouted mossy fiber synapses depress upon repetitive activation. Thus, despite their proximal location, large presynaptic terminals, and ability to excite target neurons, the impact of sprouted mossy fiber synapses on hippocampal hyperexcitability is unclear. We find that despite their short-term depression, single episodes of sprouted mossy fiber activation in hippocampal slices initiated bursts of recurrent polysynaptic excitation. Consistent with a contribution to network hyperexcitability, optogenetic activation of sprouted mossy fibers reliably triggered action potential firing in postsynaptic dentate granule cells after single light pulses. This pattern resulted in a shift in network recruitment dynamics to an \"early detonation\" mode and an increased probability of release compared to mossy fiber synapses in CA3. A lack of tonic adenosine-mediated inhibition contributed to the higher probability of glutamate release thus facilitating reverberant circuit activity.\n\nSignificance StatementSprouted mossy fibers are one of the hallmark histopathological findings in temporal lobe epilepsy. These fibers form recurrent excitatory synapses onto other dentate granule cells that display profound short-term depression. Here, however, we show that although these sprouted mossy fibers weaken substantially during repetitive activation, their initial high probability of glutamate release can activate reverberant network activity. Furthermore, we find that a lack of tonic adenosine inhibition enables this high probability of release and, consequently, recurrent network activity.

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bioRxiv2017

Preferential targeting of lateral entorhinal inputs onto newly integrated granule cells

Mature dentate granule cells in the hippocampus receive input from the entorhinal cortex via the perforant path in precisely arranged lamina, with medial entorhinal axons innervating the middle molecular layer and lateral entorhinal cortex axons innervating the outer molecular layer. Although vastly outnumbered by mature granule cells, adult-generated newborn granule cells play a unique role in hippocampal function, which has largely been attributed to their enhanced excitability and plasticity (Schmidt-Hieber et al., 2004; Ge et al., 2007). Inputs from the medial and lateral entorhinal cortex carry different informational content, thus the distribution of inputs onto newly integrated granules will affect their function in the circuit. Therefore we examined the functional innervation and synaptic maturation of newly-generated dentate granule cells using retroviral labeling in combination with selective optogenetic activation of medial or lateral entorhinal inputs. Our results indicate that lateral entorhinal inputs provide nearly all the functional innervation of newly integrated granule cells. Despite preferential functional targeting, the dendritic spine density of immature granule cells was not increased in the outer molecular layer compared to the middle molecular layer. However, chronic blockade of neurotransmitter release in medial entorhinal axons with tetanus toxin disrupted normal synapse development from both medial and lateral entorhinal inputs. Our results support a role for preferential lateral perforant path input onto newly generated neurons in mediating pattern separation, but also indicates that medial perforant path input is necessary for normal synaptic development.\n\nSignificance StatementThe formation of episodic memories involves the integration of contextual and spatial information. Newly integrated neurons in the dentate gyrus of the hippocampus play a critical role in this process, despite constituting only a minor fraction of total granule cells. Here we demonstrate that these neurons preferentially receive information thought to convey the context of an experience - a unique role that each newly integrated granule cell serves for about a month before reaching maturity.

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bioRxiv2017

Distinct Temporal Filters In Mitral Cells And External Tufted Cells Of The Olfactory Bulb

Short-term synaptic plasticity is a critical regulator of neural circuits, and largely determines how information is temporally processed. In the olfactory bulb, afferent olfactory receptor neurons respond to increasing concentrations of odorants with barrages of action potentials, and their terminals have an extraordinarily high release probability (Sicard, 1986; Murphy et al., 2004). These features suggest that during naturalistic stimuli, afferent input to the olfactory bulb is subject to strong synaptic depression, presumably truncating the postsynaptic response to afferent stimuli. To examine this issue, we used single glomerular stimulation in mouse olfactory bulb slices to measure the synaptic dynamics of afferent-evoked input at physiological stimulus frequencies. In cell-attached recordings, mitral cells responded to high frequency stimulation with sustained responses, whereas external tufted cells responded transiently. Consistent with previous reports (Murphy et al., 2004), olfactory nerve terminals onto both cell types had a high release probability (0.7), from a single pool of slowly recycling vesicles, indicating that the distinct responses of mitral and external tufted cells to high frequency stimulation did not originate presyaptically. Rather, distinct temporal response profiles in mitral cells and external tufted cells could be attributed to slow dendrodendritic responses in mitral cells, as blocking this slow current in mitral cells converted mitral cell responses to a transient response profile, typical of external tufted cells. Our results suggest that despite strong axodendritic synaptic depression, the balance of axodendritic and dendrodendritic circuitry in external tufted cells and mitral cells, respectively, tunes the postsynaptic responses to high frequency, naturalistic stimulation.\n\nKey PointsO_LIThe release probability of the ORN is reportedly one of the highest in the brain (Murphy et al., 2004), which is predicted to impose a transient temporal filter on postsynaptic cells.\nC_LIO_LIMitral cells responded to high frequency ORN stimulation with sustained transmission, whereas external tufted cells responded transiently.\nC_LIO_LIThe release probability of ORNs (0.7) was equivalent across mitral and external tufted cells and could be explained by a single pool of slowly recycling vesicles.\nC_LIO_LIThe sustained response in mitral cells resulted from dendrodendritic amplification in mitral cells, which was blocked by NMDA and mGluR1 receptor antagonists, converting mitral cell responses to transient response profiles.\nC_LIO_LIOur results suggest that although the afferent ORN synapse shows strong synaptic depression, dendrodendritic circuitry in mitral cells produces robust amplification of brief afferent input, thus the relative strength of axodendritic and dendrodendritic input determines the postsynaptic response profile.\nC_LI

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