Dorsal and ventral mossy cells differ in their axonal projections throughout the dentate gyrus of the mouse hippocampus

Dominant activities of fear engram cells in the dorsal dentate gyrus underlie fear generalization in mice

Over-generalized fear is a maladaptive response to harmless stimuli or situations characteristic of posttraumatic stress disorder (PTSD) and other anxiety disorders. The dorsal dentate gyrus (dDG) contains engram cells that play a crucial role in accurate memory retrieval. However, the coordination mechanism of neuronal subpopulations within the dDG network during fear generalization is not well understood. Here, with the Tet-off system combined with immunostaining and two-photon calcium imaging, we report that dDG fear engram cells labeled in the conditioned context constitutes a significantly higher proportion of dDG neurons activated in a similar context where mice show generalized fear. The activation of these dDG fear engram cells encoding the conditioned context is both sufficient and necessary for inducing fear generalization in the similar context. Activities of mossy cells in the ventral dentate gyrus (vMCs) are significantly suppressed in mice showing fear generalization in a similar context, and activating the vMCs-dDG pathway suppresses generalized but not conditioned fear. Finally, modifying fear memory engrams in the dDG with "safety" signals effectively rescues fear generalization. These findings reveal that the competitive advantage of dDG engram cells underlies fear generalization, which can be rescued by activating the vMCs-dDG pathway or modifying fear memory engrams, and provide novel insights into the dDG network as the neuronal basis of fear generalization.

Comparative Anatomy of the Dentate Mossy Cells in Nonhuman Primates: Their Spatial Distributions and Axonal Projections Compared With Mouse Mossy Cells

Glutamatergic mossy cells (MCs) mediate associational and commissural connectivity, exhibiting significant heterogeneity along the septotemporal axis of the mouse dentate gyrus (DG). However, it remains unclear whether the neuronal features of MCs are conserved across mammals. This study compares the neuroanatomy of MCs in the DG of mice and monkeys. The MC marker, calretinin, distinguishes two subpopulations: septal and temporal. Dual-colored fluorescence labeling is utilized to compare the axonal projection patterns of these subpopulations. In both mice and monkeys, septal and temporal MCs project axons across the longitudinal axis of the ipsilateral DG, indicating conserved associational projections. However, unlike in mice, no MC subpopulations in monkeys make commissural projections to the contralateral DG. In monkeys, temporal MCs send associational fibers exclusively to the inner molecular layer, while septal MCs give rise to wide axonal projections spanning multiple molecular layers, akin to equivalent MC subpopulations in mice. Despite conserved septotemporal heterogeneity, interspecies differences are observed in the topological organization of septal MCs, particularly in the relative axonal density in each molecular layer along the septotemporal axis of the DG. In summary, this comparative analysis sheds light on both conserved and divergent features of MCs in the DG of mice and monkeys. These findings have implications for understanding functional differentiation along the septotemporal axis of the DG and contribute to our knowledge of the anatomical evolution of the DG circuit in mammals.

Intrinsic and Synaptic Contributions to Repetitive Spiking in Dentate Granule Cells

Repetitive firing of granule cells (GCs) in the dentate gyrus (DG) facilitates synaptic transmission to the CA3 region. This facilitation can gate and amplify the flow of information through the hippocampus. High-frequency bursts in the DG are linked to behavior and plasticity, but GCs do not readily burst. Under normal conditions, a single shock to the perforant path in a hippocampal slice typically drives a GC to fire a single spike, and only occasionally more than one spike is seen. Repetitive spiking in GCs is not robust, and the mechanisms are poorly understood. Here, we used a hybrid genetically encoded voltage sensor to image voltage changes evoked by cortical inputs in many mature GCs simultaneously in hippocampal slices from male and female mice. This enabled us to study relatively infrequent double and triple spikes. We found GCs are relatively homogeneous and their double spiking behavior is cell autonomous. Blockade of GABA type A receptors increased multiple spikes and prolonged the interspike interval, indicating inhibitory interneurons limit repetitive spiking and set the time window for successive spikes. Inhibiting synaptic glutamate release showed that recurrent excitation mediated by hilar mossy cells contributes to, but is not necessary for, multiple spiking. Blockade of T-type Ca2+ channels did not reduce multiple spiking but prolonged interspike intervals. Imaging voltage changes in different GC compartments revealed that second spikes can be initiated in either dendrites or somata. Thus, pharmacological and biophysical experiments reveal roles for both synaptic circuitry and intrinsic excitability in GC repetitive spiking.

Maladaptation of dentate gyrus mossy cells mediates contextual discrimination deficit after traumatic stress

Fear overgeneralization is a maladaptive response to traumatic stress that is associated with the inability to discriminate between threat and safety contexts, a hallmark feature of post-traumatic stress disorder (PTSD). However, the neural mechanisms underlying this deficit remain unclear. Here, we show that traumatic stress exposure impairs contextual discrimination between threat and safety contexts in the learned helplessness (LH) model. Mossy cells (MCs) in the dorsal hippocampus are suppressed in response to traumatic stress. Bidirectional manipulation of MC activity in the LH model reveals that MC inhibition is causally linked to impaired contextual discrimination. Mechanistically, MC inhibition increases the number of active granule cells in a given context, significantly overlapping context-specific ensembles. Our study demonstrates that maladaptive inhibition of MCs after traumatic stress is a substantial mechanism underlying fear overgeneralization with contextual discrimination deficit, suggesting a potential therapeutic target for cognitive symptoms of PTSD.

Audiogenic kindling activates glutamatergic system in the hippocampus of rats with genetic predisposition to audiogenic seizures

Temporal lobe epilepsy (TLE) development is associated with dysregulation of glutamatergic transmission in the hippocampus; however, detailed molecular mechanisms of pathological changes are still poorly understood. In the present study, we performed the complex analysis of glutamatergic system in the hippocampus of Krushinsky-Molodkina (KM) rats genetically prone to audiogenic seizures (AGS). Daily AGS stimulations (audiogenic kindling) were used to reproduce the dynamics of TLE development. Naïve KM rats were used as a control. After 14 AGS, at the stage of developing TLE, KM rats demonstrated significant upregulation of extracellular signal-regulated kinases (ERK) 1 and 2, cAMP response element-binding protein (CREB), and c-Fos in the hippocampus indicating activation of the hippocampal cells. These changes were accompanied with an increase in glutaminase and vesicular glutamate transporter (VGLUT) 2 suggesting the activation of glutamate production and loading into the synaptic vesicles. After 21 AGS, when TLE was fully-established, alterations were similar but more pronounced, with higher activation of glutaminase, increase in glutamate production, upregulation of VGLUT1 and 2, and Fos-related antigen 1 (Fra-1) along with c-Fos. Analysis of glutamate receptors showed variable changes. Thus, after 14 AGS, simultaneous increase in metabotropic glutamate receptor mGluR1 and decrease in ionotropic N-methyl-D-aspartate (NMDA) receptors could reflect compensatory anti-epileptic mechanism, while further kindling progression induced upregulation of ionotropic receptors, probably, contributing to the hippocampal epileptization. However, we revealed practically no alterations in the expression of synaptic proteins. Altogether, obtained results suggested that overactivation of glutamate production in the hippocampus strongly contributed to TLE development in KM rats.

Antiseizure effects of Lilii Bulbus on pentylenetetrazol kindling-induced seizures in mice: Involvement of Reelin, Netrin-1, and semaphorin

Lilii Bulbus (Lilium lancifolium Thunberg) has a proneurogenic effect on the hippocampus. However, its effects on epilepsy and associated pathological features remain unknown. In this study, we investigated the antiseizure effects of a water extract of Lilii Bulbus (WELB) in mouse model of pentylenetetrazol (PTZ)-induced seizure. Mice were injected with PTZ once every 48 h until full kindling was achieved. WELB (100 and 500 mg/kg) was orally administered once daily before PTZ administration and during the kindling process. We found that WELB treatment protected against PTZ-induced low seizure thresholds and high seizure severity. Further, WELB-treated mice showed attenuated PTZ kindling-induced anxiety and memory impairment. Immunostaining and immunoblots showed that hyperactivation and ectopic migration of dentate granule cells (DGCs) were significantly reduced by WELB treatment in PTZ kindling-induced seizure mice. Staining for mossy fiber sprouting (MFS) using Timm staining and ZnT3 showed that WELB treatment significantly decreased PTZ kindling-induced MFS. Furthermore, the increased or decreased expression of proteins related to ectopic DGCs (Reelin and Dab-1), MFS (Netrin-1, Sema3A, and Sema3F), and their downstream effectors (ERK, AKT, and CREB) in the hippocampus of PTZ kindling mice was significantly restored by WELB treatment. Overall, our findings suggest that WELB is a potential antiseizure drug that acts by reducing ectopic DGCs and MFS and modulating epileptogenesis-related signaling in the hippocampus.

Glial-restricted precursors stimulate endogenous cytogenesis and effectively recover emotional deficits in a model of cytogenesis ablation

Adult cytogenesis, the continuous generation of newly-born neurons (neurogenesis) and glial cells (gliogenesis) throughout life, is highly impaired in several neuropsychiatric disorders, such as Major Depressive Disorder (MDD), impacting negatively on cognitive and emotional domains. Despite playing a critical role in brain homeostasis, the importance of gliogenesis has been overlooked, both in healthy and diseased states. To examine the role of newly formed glia, we transplanted Glial Restricted Precursors (GRPs) into the adult hippocampal dentate gyrus (DG), or injected their secreted factors (secretome), into a previously validated transgenic GFAP-tk rat line, in which cytogenesis is transiently compromised. We explored the long-term effects of both treatments on physiological and behavioral outcomes. Grafted GRPs reversed anxiety-like deficits and demonstrated an antidepressant-like effect, while the secretome promoted recovery of only anxiety-like behavior. Furthermore, GRPs elicited a recovery of neurogenic and gliogenic levels in the ventral DG, highlighting the unique involvement of these cells in the regulation of brain cytogenesis. Both GRPs and their secretome induced significant alterations in the DG proteome, directly influencing proteins and pathways related to cytogenesis, regulation of neural plasticity and neuronal development. With this work, we demonstrate a valuable and specific contribution of glial progenitors to normalizing gliogenic levels, rescuing neurogenesis and, importantly, promoting recovery of emotional deficits characteristic of disorders such as MDD.

Increased excitability of dentate gyrus mossy cells occurs early in life in the Tg2576 model of Alzheimer's disease

INTRODUCTION

Hyperexcitability in Alzheimer's disease (AD) emerge early and contribute to disease progression. The dentate gyrus (DG) is implicated in hyperexcitability in AD. We hypothesized that mossy cells (MCs), regulators of DG excitability, contribute to early hyperexcitability in AD. Indeed, MCs generate hyperexcitability in epilepsy.

METHODS

Using the Tg2576 model and WT mice (∼1month-old), we compared MCs electrophysiologically, assessed c-Fos activity marker, Aβ expression and mice performance in a hippocampal-dependent memory task.

RESULTS

Tg2576 MCs exhibit increased spontaneous excitatory events and decreased inhibitory currents, increasing the charge transfer excitation/inhibition ratio. Tg2576 MC intrinsic excitability was enhanced, and showed higher c-Fos, intracellular Aβ expression, and axon sprouting. Granule cells only showed changes in synaptic properties, without intrinsic changes. The effects occurred before a memory task is affected.

DISCUSSION

Early electrophysiological and morphological alterations in Tg2576 MCs are consistent with enhanced excitability, suggesting an early role in DG hyperexcitability and AD pathophysiology.

HIGHLIGHTS

∘ MCs from 1 month-old Tg2576 mice had increased spontaneous excitatory synaptic input. ∘ Tg2576 MCs had reduced spontaneous inhibitory synaptic input. ∘ Several intrinsic properties were abnormal in Tg2576 MCs. ∘ Tg2576 GCs had enhanced synaptic excitation but no changes in intrinsic properties. ∘ Tg2576 MCs exhibited high c-Fos expression, soluble Aβ and axonal sprouting.

Circuit-based intervention corrects excessive dentate gyrus output in the fragile X mouse model

Abnormal cellular and circuit excitability is believed to drive many core phenotypes in fragile X syndrome (FXS). The dentate gyrus is a brain area performing critical computations essential for learning and memory. However, little is known about dentate circuit defects and their mechanisms in FXS. Understanding dentate circuit dysfunction in FXS has been complicated by the presence of two types of excitatory neurons, the granule cells and mossy cells. Here we report that loss of FMRP markedly decreased excitability of dentate mossy cells, a change opposite to all other known excitability defects in excitatory neurons in FXS. This mossy cell hypo-excitability is caused by increased Kv7 function in Fmr1 knockout (KO) mice. By reducing the excitatory drive onto local hilar interneurons, hypo-excitability of mossy cells results in increased excitation/inhibition ratio in granule cells and thus paradoxically leads to excessive dentate output. Circuit-wide inhibition of Kv7 channels in Fmr1 KO mice increases inhibitory drive onto granule cells and normalizes the dentate output in response to physiologically relevant theta-gamma coupling stimulation. Our study suggests that circuit-based interventions may provide a promising strategy in this disorder to bypass irreconcilable excitability defects in different cell types and restore their pathophysiological consequences at the circuit level.

An enhancer-AAV approach selectively targeting dentate granule cells of the mouse hippocampus

The mammalian brain contains a diverse array of cell types, including dozens of neuronal subtypes with distinct anatomical and functional characteristics. The brain leverages these neuron-type specializations to perform diverse circuit operations and thus execute different behaviors properly. Through the use of Cre lines, access to specific neuron types has improved over past decades. Despite their extraordinary utility, development and cross-breeding of Cre lines is time consuming and expensive, presenting a significant barrier to entry for investigators. Furthermore, cell-based therapeutics developed in Cre mice are not clinically translatable. Recently, several adeno-associated virus (AAV) vectors utilizing neuron-type-specific regulatory transcriptional sequences (enhancer-AAVs) were developed that overcome these limitations. Using a publicly available RNA sequencing (RNA-seq) dataset, we evaluated the potential of several candidate enhancers for neuron-type-specific targeting in the hippocampus. Here, we demonstrate that a previously identified enhancer-AAV selectively targets dentate granule cells over other excitatory neuron types in the hippocampus of wild-type adult mice.

Dopamine D2 receptors in hilar mossy cells regulate excitatory transmission and hippocampal function

Hilar mossy cells (MCs) are principal excitatory neurons of the dentate gyrus (DG) that play critical roles in hippocampal function and have been implicated in brain disorders such as anxiety and epilepsy. However, the mechanisms by which MCs contribute to DG function and disease are poorly understood. A defining feature of MCs is the promoter activity of the dopamine D2 receptor (D2R) gene (Drd2), and previous work indicates a key role for dopaminergic signaling in the DG. Additionally, the involvement of D2R signaling in cognition and neuropsychiatric conditions is well known. Surprisingly, though, the function of MC D2Rs remains largely unexplored. In this study, we show that selective and conditional removal of Drd2 from MCs of adult mice impaired spatial memory, promoted anxiety-like behavior, and was proconvulsant. To determine the subcellular expression of D2Rs in MCs, we used a D2R knockin mouse which revealed that D2Rs are enriched in the inner molecular layer of the DG, where MCs establish synaptic contacts with granule cells (GCs). D2R activation by exogenous and endogenous dopamine reduced MC to dentate GC synaptic transmission, most likely by a presynaptic mechanism. In contrast, exogenous dopamine had no significant impact on MC excitatory inputs and passive and active properties. Our findings support that MC D2Rs are essential for proper DG function by reducing MC excitatory drive onto GCs. Lastly, impairment of MC D2R signaling could promote anxiety and epilepsy, therefore highlighting a potential therapeutic target.

Shh from mossy cells contributes to preventing NSC pool depletion after seizure-induced neurogenesis and in aging

Epileptic seizures induce aberrant neurogenesis from resident neural stem cells (NSCs) in the dentate gyrus of the adult mouse hippocampus, which has been implicated in depletion of the NSC pool and impairment of hippocampal function. However, the mechanisms regulating neurogenesis after seizures remain unknown. Here, we demonstrate that Sonic hedgehog (Shh) from mossy cells is a major source of Shh signaling activity after seizures, by which mossy cells contribute to seizure-induced neurogenesis and maintenance of the NSC pool. Deletion of Shh from mossy cells attenuates seizure-induced neurogenesis. Moreover, in the absence of Shh from mossy cells, NSCs pool are prematurely depleted after seizure-induced proliferation, and NSCs have impaired self-renewal. Likewise, lack of Shh from mossy cells accelerates age-related decline of the NSC pool with accompanying reduction of self-renewal of NSCs outside the context of pathology such as seizures. Together, our findings indicate that Shh from mossy cells is critical to maintain NSCs and to prevent exhaustion from excessive consumption in aging and after seizures.

Circuit-based intervention corrects excessive dentate gyrus output in the Fragile X mouse model

Abnormal cellular and circuit excitability is believed to drive many core phenotypes in fragile X syndrome (FXS). The dentate gyrus is a brain area performing critical computations essential for learning and memory. However, little is known about dentate circuit defects and their mechanisms in FXS. Understanding dentate circuit dysfunction in FXS has been complicated by the presence of two types of excitatory neurons, the granule cells and mossy cells. Here we report that loss of FMRP markedly decreased excitability of dentate mossy cells, a change opposite to all other known excitability defects in excitatory neurons in FXS. This mossy cell hypo-excitability is caused by increased Kv7 function in Fmr1 KO mice. By reducing the excitatory drive onto local hilar interneurons, hypo-excitability of mossy cells results in increased excitation/inhibition ratio in granule cells and thus paradoxically leads to excessive dentate output. Circuit-wide inhibition of Kv7 channels in Fmr1 KO mice increases inhibitory drive onto granule cells and normalizes the dentate output in response to physiologically relevant theta-gamma coupling stimulation. Our study suggests that circuit-based interventions may provide a promising strategy in this disorder to bypass irreconcilable excitability defects in different cell types and restore their pathophysiological consequences at the circuit level.

Shh from mossy cells contributes to preventing NSC pool depletion after seizure-induced neurogenesis and in aging

Epileptic seizures induce aberrant neurogenesis from resident neural stem cells (NSCs) in the dentate gyrus of the adult mouse hippocampus, which has been implicated in depletion of the NSC pool and impairment of hippocampal function. However, the mechanisms regulating neurogenesis after seizures remain unknown. Here we demonstrate that Shh from mossy cells is a major source of Shh signaling activity after seizures, by which mossy cells contribute to seizure-induced neurogenesis and maintenance of the NSC pool. Deletion of Shh from mossy cells attenuates seizure-induced neurogenesis. Moreover, in the absence of Shh from mossy cells, NSCs pool are prematurely depleted after seizure-induced proliferation, and NSCs have impaired self-renewal. Likewise, lack of Shh from mossy cells accelerates age-related decline of the NSC pool with accompanying reduction of self-renewal of NSCs outside the context of pathology such as seizures. Together, our findings indicate that Shh from mossy cells is critical to maintain NSCs and to prevent exhaustion from excessive consumption in aging and after seizures.

A novel enhancer-AAV approach selectively targeting dentate granule cells

The mammalian brain contains the most diverse array of cell types of any organ, including dozens of neuronal subtypes with distinct anatomical and functional characteristics. The brain leverages these neuron-type-specializations to perform diverse circuit operations and thus execute different behaviors properly. Through the use of Cre lines, access to specific neuron types has steadily improved over past decades. Despite their extraordinary utility, development and cross-breeding of Cre lines is time-consuming and expensive, presenting a significant barrier to entry for many investigators. Furthermore, cell-based therapeutics developed in Cre mice are not clinically translatable. Recently, several AAV vectors utilizing neuron-type-specific regulatory transcriptional sequences (enhancer-AAVs) were developed which overcome these limitations. Using a publicly available RNAseq dataset, we evaluated the potential of several candidate enhancers for neuron-type-specific targeting in the hippocampus. Here we identified a promising enhancer-AAV for targeting dentate granule cells and validated its selectivity in wild-type adult mice.

Dopamine D2 receptors in mossy cells reduce excitatory transmission and are essential for hippocampal function

Hilar mossy cells (MCs) are principal excitatory neurons of the dentate gyrus (DG) that play critical roles in hippocampal function and have been implicated in brain disorders such as anxiety and epilepsy. However, the mechanisms by which MCs contribute to DG function and disease are poorly understood. Expression from the dopamine D2 receptor (D2R) gene (Drd2) promoter is a defining feature of MCs, and previous work indicates a key role for dopaminergic signaling in the DG. Additionally, the involvement of D2R signaling in cognition and neuropsychiatric conditions is well-known. Surprisingly, though, the function of MC D2Rs remain largely unexplored. In this study, we show that selective and conditional removal of Drd2 from MCs of adult mice impaired spatial memory, promoted anxiety-like behavior and was proconvulsant. To determine the subcellular expression of D2Rs in MCs, we used a D2R knockin mouse which revealed that D2Rs are enriched in the inner molecular layer of the DG, where MCs establish synaptic contacts with granule cells. D2R activation by exogenous and endogenous dopamine reduced MC to dentate granule cells (GC) synaptic transmission, most likely by a presynaptic mechanism. In contrast, removing Drd2 from MCs had no significant impact on MC excitatory inputs and passive and active properties. Our findings support that MC D2Rs are essential for proper DG function by reducing MC excitatory drive onto GCs. Lastly, impairment of MC D2R signaling could promote anxiety and epilepsy, therefore highlighting a potential therapeutic target.

Asymmetric dysregulation of glutamate dynamics across the synaptic cleft in a mouse model of Alzheimer's disease

Most research on glutamate spillover focuses on the deleterious consequences of postsynaptic glutamate receptor overactivation. However, two decades ago, it was noted that the glial coverage of hippocampal synapses is asymmetric: astrocytic coverage of postsynaptic sites exceeds coverage of presynaptic sites by a factor of four. The fundamental relevance of this glial asymmetry remains poorly understood. Here, we used the glutamate biosensor iGluSnFR, and restricted its expression to either CA3 or CA1 neurons to visualize glutamate dynamics at pre- and postsynaptic microenvironments, respectively. We demonstrate that inhibition of the primarily astrocytic glutamate transporter-1 (GLT-1) slows glutamate clearance to a greater extent at presynaptic compared to postsynaptic membranes. GLT-1 expression was reduced early in a mouse model of AD, resulting in slower glutamate clearance rates at presynaptic but not postsynaptic membranes that opposed presynaptic short-term plasticity. Overall, our data demonstrate that the presynapse is particularly vulnerable to GLT-1 dysfunction and may have implications for presynaptic impairments in a variety of brain diseases.

Glucagon-like peptide-1 receptor differentially controls mossy cell activity across the dentate gyrus longitudinal axis

Understanding the role of dentate gyrus (DG) mossy cells (MCs) in learning and memory has rapidly evolved due to increasingly precise methods for targeting MCs and for in vivo recording and activity manipulation in rodents. These studies have shown MCs are highly active in vivo, strongly remap to contextual manipulation, and that their inhibition or hyperactivation impairs pattern separation and location or context discrimination. Less well understood is how MC activity is modulated by neurohormonal mechanisms, which might differentially control the participation of MCs in cognitive functions during discrete states, such as hunger or satiety. In this study, we demonstrate that glucagon-like peptide-1 (GLP-1), a neuropeptide produced in the gut and the brain that regulates food consumption and hippocampal-dependent mnemonic function, might regulate MC function through expression of its receptor, GLP-1R. RNA-seq demonstrated that most, though not all, Glp1r in hippocampal principal neurons is expressed in MCs, and in situ hybridization revealed strong expression of Glp1r in hilar neurons. Glp1r-ires-Cre mice crossed with Ai14D reporter mice followed by co-labeling for the MC marker GluR2/3 revealed that almost all MCs in the ventral DG expressed Glp1r and that almost all Glp1r-expressing hilar neurons were MCs. However, only ~60% of dorsal DG MCs expressed Glp1r, and Glp1r was also expressed in small hilar neurons that were not MCs. Consistent with this expression pattern, peripheral administration of the GLP-1R agonist exendin-4 (5 μg/kg) increased cFos expression in ventral but not dorsal DG hilar neurons. Finally, whole-cell patch-clamp recordings from ventral MCs showed that bath application of exendin-4 (200 nM) depolarized MCs and increased action potential firing. Taken together, this study adds to known MC activity modulators a neurohormonal mechanism that may preferentially affect ventral DG physiology and may potentially be targetable by several GLP-1R pharmacotherapies already in clinical use.

Mossy cells of the dentate gyrus: Drivers or inhibitors of epileptic seizures?

Mossy cells (MCs) are glutamatergic cells of the dentate gyrus with an important role in temporal lobe epilepsy. Under physiological conditions MCs can control both network excitations via direct synapses to granule cells and inhibition via connections to GABAergic interneurons innervating granule cells. In temporal lobe epilepsy mossy cell loss is one of the major hallmarks, but whether the surviving MCs drive or inhibit seizure initiation and generalization is still a debate. The aim of the present review is to summarize the latest findings on the role of mossy cells in healthy and overexcited hippocampus.

Connectivity and synaptic features of hilar mossy cells and their effects on granule cell activity along the hippocampal longitudinal axis

The hippocampus is an elongated brain structure which runs along a ventral-to-dorsal axis in rodents, corresponding to the anterior-to-posterior axis in humans. A glutamatergic cell type in the dentate gyrus (DG), the mossy cells (MCs), establishes extensive excitatory collateral connections with the DG principal cells, the granule cells (GCs), and inhibitory interneurons in both hippocampal hemispheres along the longitudinal axis. Although coupling of two physically separated GC populations via long-axis projecting MCs is instrumental for information processing, the connectivity and synaptic features of MCs along the longitudinal axis are poorly defined. Here, using channelrhodopsin-2 assisted circuit mapping, we showed that MC excitation results in a low synaptic excitation-inhibition (E/I) balance in the intralamellar (local) GCs, but a high synaptic E/I balance in the translamellar (distant) ones. In agreement with the differential E/I balance along the ventrodorsal axis, activation of MCs either enhances or suppresses the local GC response to the cortical input, but primarily promotes the distant GC activation. Moreover, activation of MCs enhances the spike timing precision of the local GCs, but not that of the distant ones. Collectively, these findings suggest that MCs differentially regulate the local and distant GC activity through distinct synaptic mechanisms. KEY POINTS: Hippocampal mossy cell (MC) pathways differentially regulate granule cell (GC) activity along the longitudinal axis. MCs mediate a low excitation-inhibition balance in intralamellar (local) GCs, but a high excitation-inhibition balance in translamellar (distant) GCs. MCs enhance the spiking precision of local GCs, but not distant GCs. MCs either promote or suppress local GC activity, but primarily promote distant GC activation.

Alzheimer-like tau accumulation in dentate gyrus mossy cells induces spatial cognitive deficits by disrupting multiple memory-related signaling and inhibiting local neural circuit

Abnormal tau accumulation and spatial memory loss constitute characteristic pathology and symptoms of Alzheimer disease (AD). Yet, the intrinsic connections and the mechanism between them are not fully understood. In the current study, we observed a prominent accumulation of the AD‐like hyperphosphorylated and truncated tau (hTau N368) proteins in hippocampal dentate gyrus (DG) mossy cells of 3xTg‐AD mice. Further investigation demonstrated that the ventral DG (vDG) mossy cell‐specific overexpressing hTau for 3 months induced spatial cognitive deficits, while expressing hTau N368 for only 1 month caused remarkable spatial cognitive impairment with more prominent tau pathologies. By in vivo electrophysiological and optic fiber recording, we observed that the vDG mossy cell‐specific overexpression of hTau N368 disrupted theta oscillations with local neural network inactivation in the dorsal DG subset, suggesting impairment of the ventral to dorsal neural circuit. The mossy cell‐specific transcriptomic data revealed that multiple AD‐associated signaling pathways were disrupted by hTau N368, including reduction of synapse‐associated proteins, inhibition of AKT and activation of glycogen synthase kinase‐3β. Importantly, chemogenetic activating mossy cells efficiently attenuated the hTau N368‐induced spatial cognitive deficits. Together, our findings indicate that the mossy cell pathological tau accumulation could induce the AD‐like spatial memory deficit by inhibiting the local neural network activity, which not only reveals new pathogenesis underlying the mossy cell‐related spatial memory loss but also provides a mouse model of Mossy cell‐specific hTau accumulation for drug development in AD and the related tauopathies.

Activin A regulates the excitability of hippocampal mossy cells

Mossy cells (MCs) in the hilus of the dentate gyrus (DG) receive increasing attention as a major player controlling information processing in the DG network. Furthermore, disturbed MC activity has been implicated in widespread neuropsychiatric disorders such as epilepsy and major depression. Using whole-cell patch-clamp recordings from MCs in acute hippocampal slices from wild type and transgenic mice, we demonstrate that activin, a member of the transforming growth factor-β (TGF-β) family, has a strong neuromodulatory effect on MC activity. Disruption of activin receptor signaling reduced MC firing, dampened their excitatory input and augmented their inhibitory input. By contrast, acute application of recombinant activin A strongly increased MC activity and promoted excitatory synaptic drive. Notably, similar changes of MC activity have been observed in a rodent model of depression and after antidepressant drug therapy, respectively. Given that a rise in activin signaling particularly in the DG has been proposed as a mechanism of antidepressant action, our data suggest that the effect of activin on MC excitability might make a considerable contribution in this regard.

Flexible encoding of objects and space in single cells of the dentate gyrus

The hippocampus is involved in the formation of memories that require associations among stimuli to construct representations of space and the items and events within that space. Neurons in the dentate gyrus (DG), an initial input region of the hippocampus, have robust spatial tuning, but it is unclear how nonspatial information may be integrated with spatial activity in this region. We recorded from the DG of 21 adult mice as they foraged for food in an environment that contained discrete objects. We found DG cells with multiple firing fields at a fixed distance and direction from objects (landmark vector cells) and cells that exhibited localized changes in spatial firing when objects in the environment were manipulated. By classifying recorded DG cells into putative dentate granule cells and mossy cells, we examined how the addition or displacement of objects affected the spatial firing of these DG cell types. Object-related activity was detected in a significant proportion of mossy cells. Although few granule cells with responses to object manipulations were recorded, likely because of the sparse nature of granule cell firing, there was generally no significant difference in the proportion of granule cells and mossy cells with object responses. When mice explored a second environment with the same objects, DG spatial maps completely reorganized, and a different subset of cells responded to object manipulations. Together, these data reveal the capacity of DG cells to detect small changes in the environment while preserving a stable spatial representation of the overall context.

Modeling intrahippocampal effects of anterior hippocampal hyperactivity relevant to schizophrenia using chemogenetic excitation of long axis-projecting mossy cells in the mouse dentate gyrus

Background

The anterior hippocampus of individuals with early psychosis or schizophrenia is hyperactive, as is the ventral hippocampus in many rodent models for schizophrenia risk. Mossy cells (MCs) of the ventral dentate gyrus (DG) densely project in the hippocampal long axis, targeting both dorsal DG granule cells and inhibitory interneurons. MCs are responsive to stimulation throughout hippocampal subfields and thus may be suited to detect hyperactivity in areas where it originates such as CA1. Here, we tested the hypothesis that hyperactivation of ventral MCs activates dorsal DG granule cells to influence dorsal hippocampal function.

Methods

In CD-1 mice, we targeted dorsal DG-projecting ventral MCs using an adeno-associated virus intersectional strategy. In vivo fiber photometry recording of ventral MCs was performed during exploratory behaviors. We used excitatory chemogenetic constructs to test the effects of ventral MC hyperactivation on long-term spatial memory during an object location memory task.

Results

Photometry revealed that ventral MCs were activated during exploratory rearing. Ventral MCs made functional monosynaptic inputs to dorsal DG granule cells, and chemogenetic activation of ventral MCs modestly increased activity of dorsal DG granule cells measured by c-Fos. Finally, chemogenetic activation of ventral MCs during the training phase of an object location memory task impaired test performance 24 hours later, without effects on locomotion or object exploration.

Conclusions

These data suggest that ventral MC activation can directly excite dorsal granule cells and interfere with dorsal DG function, supporting future study of their in vivo activity in animal models for schizophrenia featuring ventral hyperactivity.

Dorsal and ventral mossy cells differ in their axonal projections throughout the dentate gyrus of the mouse hippocampus

Glutamatergic hilar mossy cells (MCs) have axons that terminate both near and far from their cell body but stay within the DG, making synapses primarily in the molecular layer. The long-range axons are considered the primary projection, and extend throughout the DG ipsilateral to the soma, and project to the contralateral DG. The specificity of MC axons for the inner molecular layer (IML) has been considered to be a key characteristic of the DG. In the present study, we made the surprising finding that dorsal MC axons are an exception to this rule. We used two mouse lines that allow for Cre-dependent viral labeling of MCs and their axons: dopamine receptor D2 (Drd2-Cre) and calcitonin receptor-like receptor (Crlr-Cre). A single viral injection into the dorsal DG to label dorsal MCs resulted in labeling of MC axons in both the IML and middle molecular layer (MML). Interestingly, this broad termination of dorsal MC axons occurred throughout the septotemporal DG. In contrast, long-range axons of ventral MCs terminated in the IML, consistent with the literature. Taken together, these results suggest that dorsal and ventral MCs differ significantly in their axonal projections. Since MC projections in the ML are thought to terminate primarily on GCs, the results suggest a dorsal-ventral difference in MC activation of GCs. The surprising difference in dorsal and ventral MC projections should therefore be considered when evaluating dorsal-ventral differences in DG function.

Bidirectional Regulation of Cognitive and Anxiety-like Behaviors by Dentate Gyrus Mossy Cells in Male and Female Mice

The dentate gyrus (DG) of the hippocampus is important for cognition and behavior. However, the circuits underlying these functions are unclear. DG mossy cells (MCs) are potentially important because of their excitatory synapses on the primary cell type, granule cells (GCs). However, MCs also activate GABAergic neurons, which inhibit GCs. We used viral delivery of designer receptors exclusively activated by designer drugs (DREADDs) in mice to implement a gain- and loss-of-function study of MCs in diverse behaviors. Using this approach, manipulations of MCs could bidirectionally regulate behavior. The results suggest that inhibiting MCs can reduce anxiety-like behavior and improve cognitive performance. However, not all cognitive or anxiety-related behaviors were influenced, suggesting specific roles of MCs in some, but not all, types of cognition and anxiety. Notably, several behaviors showed sex-specific effects, with females often showing more pronounced effects than the males. We also used the immediate early gene c-Fos to address whether DREADDs bidirectionally regulated MC or GC activity. We confirmed excitatory DREADDs increased MC c-Fos. However, there was no change in GC c-Fos, consistent with MC activation leading to GABAergic inhibition of GCs. In contrast, inhibitory DREADDs led to a large increase in GC c-Fos, consistent with a reduction in MC excitation of GABAergic neurons, and reduced inhibition of GCs. Together, these results suggest that MCs regulate anxiety and cognition in specific ways. We also raise the possibility that cognitive performance may be improved by reducing anxiety.SIGNIFICANCE STATEMENT The dentate gyrus (DG) has many important cognitive roles as well as being associated with affective behavior. This study addressed how a glutamatergic DG cell type called mossy cells (MCs) contributes to diverse behaviors, which is timely because it is known that MCs regulate the activity of the primary DG cell type, granule cells (GCs), but how MC activity influences behavior is unclear. We show, surprisingly, that activating MCs can lead to adverse behavioral outcomes, and inhibiting MCs have an opposite effect. Importantly, the results appeared to be task-dependent and showed that testing both sexes was important. Additional experiments indicated what MC and GC circuitry was involved. Together, the results suggest how MCs influence behaviors that involve the DG.