Dendritic Kv4.2 potassium channels selectively mediate spatial pattern separation in the dentate gyrus

Maturational Stage-Dependent Contributions of the Cav3.2 T-Type Calcium Channel to Dentate Gyrus Granule Cell Excitability

T-Type calcium channels shape neuronal excitability driving burst firing, plasticity, and neuronal oscillations that influence circuit activity. The three biophysically distinct T-type channel subtypes (Cav3.1, Cav3.2, Cav3.3) are differentially expressed in the brain, contributing to divergent physiological processes. Cav3.2 channels are highly expressed in the dentate gyrus (DG) of the hippocampus, and mice lacking Cav3.2 [knock-out (KO)] exhibit impairments in hippocampal dependent learning and memory tasks, as well as attenuated development of pilocarpine induced epilepsy. Owing to neurogenesis, granule cells (GCs) are continuously added to the DG, generating a heterogeneous population of maturational stages with distinct excitability. While initial studies identified the role of Cav3.2 in mature GC burst firing, its functional relevance in the intrinsic excitability of different GC subpopulations has not yet been examined. In this study, we used juvenile Cav3.2 KO mice to examine the contributions of Cav3.2 channels to GC excitability at three different stages of maturation. We recorded from cells throughout the GC layer using their electrophysiological and morphological features to allocate GCs into immature, intermediate, and mature groups. In immature GCs, loss of Cav3.2 channels reduced the proportion of cells that fired low-threshold calcium spikes. Conversely, Cav3.2 KO increased excitability in regular spiking intermediate and mature GCs, enabling higher-frequency firing, with little impact on the frequency-dependent response. Overall, this study shows that Cav3.2 channels differentially regulate GC excitability throughout maturation and suggest that calcium influx via Cav3.2 may have maturation-dependent contributions to DG processes such as GC survival, integration, and memory encoding.

Kv4.2 Regulates Basal Synaptic Strength by Inhibiting R-Type Calcium Channels in the Hippocampus

Kv4.2 subunits, which mediate transient A-type K+ current, are crucial in regulating neuronal excitability and synaptic responses within the hippocampus. While their contribution to activity-dependent regulation of synaptic response is well-established, the impact of Kv4.2 on basal synaptic strength remains elusive. To address this gap, we introduced a Kv4.2-specific antibody (anti-Kv4.2) into hippocampal neurons of mice of both sexes to selectively inhibit postsynaptic Kv4.2, enabling direct examination of its impact on excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) during basal synaptic activity. Our results demonstrated that blocking Kv4.2 significantly enhanced the amplitude of EPSPs. This amplification was proportional to the increase in the amplitude of EPSCs, which, in turn, correlated with the expression level of Kv4.2 in the dendritic regions of the hippocampus. Furthermore, the anti-Kv4.2-induced increase in EPSC amplitude was associated with a decrease in the failure rate of EPSCs evoked by minimal stimulation, suggesting that blocking Kv4.2 facilitates the recruitment of AMPA receptors to both silent and functional synapses to enhance synaptic efficacy. The anti-Kv4.2-induced synaptic potentiation was effectively abolished by intracellular 10 mM BAPTA or by blocking R-type calcium channels (RTCCs) and downstream signaling molecules, including protein kinases A and C. Importantly, Kv4.2 inhibition did not occlude further synaptic potentiation induced by high-frequency stimulation, suggesting that anti-Kv4.2-induced synaptic strengthening involves unique mechanisms that are distinct from long-term potentiation pathways. Taken together, these findings underscore the essential role of Kv4.2 in the regulation of basal synaptic strength, which is mediated by the inhibition of RTCCs.

The Voltage-Gated Potassium Channel Shal (Kv4) Contributes to Active Hearing in Drosophila

The full complement of ion channels which influence insect auditory mechanotransduction and the mechanisms by which their influence is exerted remain unclear. Shal (Kv4), a Shaker family member encoding voltage-gated potassium channels in Drosophila melanogaster, has been shown to localize to dendrites in some neuron types, suggesting the potential role of Shal in Drosophila hearing, including mechanotransduction. A GFP trap was used to visualize the localization of the Shal channel in Johnston's organ neurons responsible for hearing in the antenna. Shal protein was localized strongly to the cell body and inner dendritic segment of sensory neurons. It was also detectable in the sensory cilium, suggesting its involvement not only in general auditory function but specifically in mechanotransduction. Electrophysiological recordings to assess neural responses to auditory stimuli in mutant Shal flies revealed significant decreases in auditory responses. Laser Doppler vibrometer recordings indicated abnormal antennal free fluctuation frequencies in mutant lines, indicating an effect on active antennal tuning, and thus active transduction mechanisms. This suggests that Shal participates in coordinating energy-dependent antennal movements in Drosophila that are essential for tuning the antenna to courtship song frequencies.

Postnatal hypofunction of N-methyl-D-aspartate receptors alters perforant path synaptic plasticity and filtering and impairs dentate gyrus-mediated spatial discrimination

BACKGROUND AND PURPOSE

Transient hypofunction of the NMDA receptor represents a convergence point for the onset and further development of psychiatric disorders, including schizophrenia. Although the cumulative evidence indicates dysregulation of the hippocampal formation in schizophrenia, the integrity of the synaptic transmission and plasticity conveyed by the somatosensorial inputs to the dentate gyrus, the perforant pathway synapses, have barely been explored in this pathological condition.

EXPERIMENTAL APPROACH

We identified a series of synaptic alterations of the lateral and medial perforant paths in animals postnatally treated with the NMDA antagonist MK-801. This dysregulation suggests decreased cognitive performance, for which the dentate gyrus is critical.

KEY RESULTS

We identified alterations in the synaptic properties of the lateral and medial perforant paths to the dentate gyrus synapses in slices from MK-801-treated animals. Altered glutamate release and decreased synaptic strength precede an impairment in the induction and expression of long-term potentiation (LTP) and CB1 receptor-mediated long-term depression (LTD). Remarkably, by inhibiting the degradation of 2-arachidonoylglycerol (2-AG), an endogenous ligand of the CB1 receptor, we restored the LTD in animals treated with MK-801. Additionally, we showed for the first time, that spatial discrimination, a cognitive task that requires dentate gyrus integrity, is impaired in animals exposed to transient hypofunction of NMDA receptors.

CONCLUSION AND IMPLICATIONS

Dysregulation of glutamatergic transmission and synaptic plasticity from the entorhinal cortex to the dentate gyrus has been demonstrated, which may explain the cellular dysregulations underlying the altered cognitive processing in the dentate gyrus associated with schizophrenia.

Environmental Enrichment Improves the Recognition Memory in Adult Mice Following Social Isolation via Downregulation of Kv4.2 Potassium Channels

Recognition memory is a cognitive process that enables us to distinguish familiar objects and situations from new items, which is essential for mammalian survival and adaptation to a changing environment. Social isolation (SI) has been implicated as a detrimental factor for recognition memory. The medial prefrontal cortex (mPFC) has been shown to carry information concerning the relative familiarity of individual stimuli, and modulating neuronal function in this region may contribute to recognition memory. The present study aimed to investigate the neuronal mechanisms in the mPFC of environmental enrichment (EE) on recognition memory in adult mice following SI. Mice were assigned into three groups: control, SI, and SI + EE groups. Novel location recognition (NLR) and novel object recognition (NOR) tests were performed to evaluate the recognition memory. The levels of Kv4 channels were assessed by qRT-PCR and western blotting. The effects of SI and SI + EE on the excitability of pyramidal neurons in the mPFC were measured using whole-cell recording. We found that SI led to a reduction in the excitability of pyramidal neurons. Specifically, we have identified that the reduction in the firing activity of pyramidal neurons resulted from alterations in the function and expression of Kv4.2 channels. Furthermore, EE regulated Kv4.2 channels, normalized the activity of pyramidal neurons, and restored the behavioral deficits following SI. Thus, the roles of Kv4.2 channels in excitability of pyramidal neurons suggest that the Kv4.2 channels present a promising therapeutic target for recognition memory impairment.

Design of Ultrapotent Genetically Encoded Inhibitors of Kv4.2 for Gating Neural Plasticity

The Kv4.2 potassium channel plays established roles in neuronal excitability, while also being implicated in plasticity. Current means to study the roles of Kv4.2 are limited, motivating us to design a genetically encoded membrane tethered Heteropodatoxin-2 (MetaPoda). We find that MetaPoda is an ultrapotent and selective gating-modifier of Kv4.2. We narrow its site of contact with the channel to two adjacent residues within the voltage sensitive domain (VSD) and, with docking simulations, suggest that the toxin binds the VSD from within the membrane. We also show that MetaPoda does not require an external linker of the channel for its activity. In neurons (obtained from female and male rat neonates), MetaPoda specifically, and potently, inhibits all Kv4 currents, leaving all other A-type currents unaffected. Inhibition of Kv4 in hippocampal neurons does not promote excessive excitability, as is expected from a simple potassium channel blocker. We do find that MetaPoda's prolonged expression (1 week) increases expression levels of the immediate early gene cFos and prevents potentiation. These findings argue for a major role of Kv4.2 in facilitating plasticity of hippocampal neurons. Lastly, we show that our engineering strategy is suitable for the swift engineering of another potent Kv4.2-selective membrane-tethered toxin, Phrixotoxin-1, denoted MetaPhix. Together, we provide two uniquely potent genetic tools to study Kv4.2 in neuronal excitability and plasticity.