Postnatal development of temporal integration, spike timing and spike threshold regulation by a dendrotoxin-sensitive K+current in rat CA1 hippocampal cells

Roles of KCNA2 in Neurological Diseases: from Physiology to Pathology

Potassium voltage-gated channel subfamily a member 2 (Kv1.2, encoded by KCNA2) is highly expressed in the central and peripheral nervous systems. Based on the patch clamp studies, gain-of function (GOF), loss-of-function (LOF), and a mixed type (GOF/LOF) variants can cause different conditions/disorders. KCNA2-related neurological diseases include epilepsy, intellectual disability (ID), attention deficit/hyperactive disorder (ADHD), autism spectrum disorder (ASD), pain as well as autoimmune and movement disorders. Currently, the molecular mechanisms for the reported variants in causing diverse disorders are unknown. Consequently, this review brings up to date the related information regarding the structure and function of Kv1.2 channel, expression patterns, neuronal localizations, and tetramerization as well as important cell and animal models. In addition, it provides updates on human genetic variants, genotype-phenotype correlations especially highlighting the deep insight into clinical prognosis of KCNA2-related developmental and epileptic encephalopathy, mechanisms, and the potential treatment targets for all KCNA2-related neurological disorders.

Postnatal Maturation of Membrane Potential Dynamics during in Vivo Hippocampal Ripples

Sharp-wave ripples (SWRs) are transient high-frequency oscillations of local field potentials (LFPs) in the hippocampus and play a critical role in memory consolidation. During SWRs, CA1 pyramidal cells exhibit rapid spike sequences that often replay the sequential activity that occurred during behavior. This temporally organized firing activity gradually emerges during 2 weeks after the eye opening; however, it remains unclear how the organized spikes during SWRs mature at the intracellular membrane potential (Vm) level. Here, we recorded Vm of CA1 pyramidal cells simultaneously with hippocampal LFPs from anesthetized immature mice of either sex after the developmental emergence of SWRs. On postnatal days 16 and 17, Vm dynamics around SWRs were premature, characterized by prolonged depolarizations without either pre- or post-SWR hyperpolarizations. The biphasic hyperpolarizations, features typical of adult SWR-relevant Vm, formed by approximately postnatal day 30. This Vm maturation was associated with an increase in SWR-associated inhibitory inputs to pyramidal cells. Thus, the development of SWR-relevant inhibition restricts the temporal windows for spikes of pyramidal cells and allows CA1 pyramidal cells to organize their spike sequences during SWRs.SIGNIFICANCE STATEMENT Sharp-wave ripples (SWRs) are prominent hippocampal oscillations and play a critical role in memory consolidation. During SWRs, hippocampal neurons synchronously emit spikes with organized temporal patterns. This temporal structure of spikes during SWRs develops during the third and fourth postnatal weeks, but the underlying mechanisms are not well understood. Here, we recorded in vivo membrane potentials from hippocampal neurons in premature mice and suggest that the maturation of SWR-associated inhibition enables hippocampal neurons to produce precisely controlled spike times during SWRs.

Upregulation of A-type potassium channels suppresses neuronal excitability in hypoxic neonatal mice

Neuronal excitability is a critical feature of central nervous system development, playing a fundamental role in the functional maturation of brain regions, including the hippocampus, cerebellum, auditory and visual systems. The present study aimed to determine the mechanism by which hypoxia causes brain dysfunction through perturbation of neuronal excitability in a hypoxic neonatal mouse model. Functional brain development was assessed in humans using the Gesell Development Diagnosis Scale. In mice, gene transcription was evaluated via mRNA sequencing and quantitative PCR; furthermore, patch clamp recordings assessed potassium currents. Clinical observations revealed disrupted functional brain development in 6- and 18-month-old hypoxic neonates, and those born with normal hearing screening unexpectedly exhibited impaired central auditory function at 3 months. In model mice, CA1 pyramidal neurons exhibited reduced spontaneous activity, largely induced by excitatory synaptic input suppression, despite the elevated membrane excitability of hypoxic neurons compared to that of control neurons. In hypoxic neurons, Kcnd3 gene transcription was upregulated, confirming upregulated hippocampal Kv 4.3 expression. A-type potassium currents were enhanced, and Kv 4.3 participated in blocking excitatory presynaptic inputs. Elevated Kv 4.3 activity in pyramidal neurons under hypoxic conditions inhibited excitatory presynaptic inputs and further decreased neuronal excitability, disrupting functional brain development in hypoxic neonates.

mTORC1 function in hippocampal parvalbumin interneurons: regulation of firing and long-term potentiation of intrinsic excitability but not long-term contextual fear memory and context discrimination

Hippocampal CA1 parvalbumin-expressing interneurons (PV INs) play a central role in controlling principal cell activity and orchestrating network oscillations. PV INs receive excitatory inputs from CA3 Schaffer collaterals and local CA1 pyramidal cells, and they provide perisomatic inhibition. Schaffer collateral excitatory synapses onto PV INs express Hebbian and anti-Hebbian types of long-term potentiation (LTP), as well as elicit LTP of intrinsic excitability (LTP_IE). LTP_IE requires the activation of type 5 metabotropic glutamate receptors (mGluR5) and is mediated by downregulation of potassium channels Kv1.1. It is sensitive to rapamycin and thus may involve activation of the mammalian target of rapamycin complex 1 (mTORC1). LTP_IE facilitates PV INs recruitment in CA1 and maintains an excitatory-inhibitory balance. Impaired CA1 PV INs activity or LTP affects network oscillations and memory. However, whether LTP_IE in PV INs plays a role in hippocampus-dependent memory remains unknown. Here, we used conditional deletion of the obligatory component of mTORC1, the Regulatory-Associated Protein of mTOR (Raptor), to directly manipulate mTORC1 in PV INs. We found that homozygous, but not heterozygous, conditional knock-out of Rptor resulted in a decrease in CA1 PV INs of mTORC1 signaling via its downstream effector S6 phosphorylation assessed by immunofluorescence. In whole-cell recordings from hippocampal slices, repetitive firing of CA1 PV INs was impaired in mice with either homozygous or heterozygous conditional knock-out of Rptor. High frequency stimulation of Schaffer collateral inputs that induce LTP_IE in PV INs of control mice failed to do so in mice with either heterozygous or homozygous conditional knock-out of Rptor in PV INs. At the behavioral level, mice with homozygous or heterozygous conditional knock-out of Rptor showed similar long-term contextual fear memory or contextual fear memory discrimination relative to control mice. Thus, mTORC1 activity in CA1 PV INs regulates repetitive firing and LTP_IE but not consolidation of long-term contextual fear memory and context discrimination. Our results indicate that mTORC1 plays cell-specific roles in synaptic plasticity of hippocampal inhibitory interneurons that are differentially involved in hippocampus-dependent learning and memory.

The mono-ADP-ribosyltransferase ARTD10 regulates the voltage-gated K+ channel Kv1.1 through protein kinase C delta

Background

ADP-ribosylation is a ubiquitous post-translational modification that involves both mono- and poly-ADP-ribosylation. ARTD10, also known as PARP10, mediates mono-ADP-ribosylation (MARylation) of substrate proteins. A previous screen identified protein kinase C delta (PKCδ) as a potential ARTD10 substrate, among several other kinases. The voltage-gated K⁺ channel Kv1.1 constitutes one of the dominant Kv channels in neurons of the central nervous system and the inactivation properties of Kv1.1 are modulated by PKC. In this study, we addressed the role of ARTD10-PKCδ as a regulator of Kv1.1.

Results

We found that ARTD10 inhibited PKCδ, which increased Kv1.1 current amplitude and the proportion of the inactivating current component in HeLa cells, indicating that ARTD10 regulates Kv1.1 in living cells. An inhibitor of ARTD10, OUL35, significantly decreased peak amplitude together with the proportion of the inactivating current component of Kv1.1-containing channels in primary hippocampal neurons, demonstrating that the ARTD10-PKCδ signaling cascade regulates native Kv1.1. Moreover, we show that the pharmacological blockade of ARTD10 increases excitability of hippocampal neurons.

Conclusions

Our results, for the first time, suggest that MARylation by ARTD10 controls neuronal excitability.

Development of Action Potential Waveform in Hippocampal CA1 Pyramidal Neurons

CA1 pyramidal neurons undergo intense morphological and electrophysiological changes from the second to third postnatal weeks in rats throughout a critical period associated with the emergence of exploratory behavior. Using whole cell current-clamp recordings in vitro and neurochemical methods, we studied the development of the somatic action potential (AP) waveform and some of the underlying channels in this critical period. At the third postnatal week, APs showed a more hyperpolarized threshold, higher duration and amplitude. Subthreshold depolarization broadened APs and depolarized their peak overshoots more pronouncedly in immature neurons (2 weeks old). These features were mimicked by pharmacologically blocking the fast-inactivating A-type potassium current (I_A) and matched well with the higher concentrations of K_v4.2 and K_v4.3 and the lower concentrations of BK and K_v1.2 channels detected by Western blotting. Repetitive stimulation with high frequency trains (50 Hz) reproduced AP broadening associated to inactivation of the A-type current in immature cells. Moreover, repetitive firing showed changes in AP amplitude consistent with the inactivation of both sodium and potassium subthreshold currents, which resulted in higher AP amplitudes in the more immature neurons. We propose that maturation of AP waveform and excitability in this critical developmental period could be related to the onset of exploratory behaviors.

Kv1 potassium channels control action potential firing of putative GABAergic deep cerebellar nuclear neurons

Low threshold voltage activated Kv1 potassium channels play key roles in regulating action potential (AP) threshold, neural excitability, and synaptic transmission. Kv1 channels are highly expressed in the cerebellum and mutations of human Kv1 genes are associated to episodic forms of ataxia (EAT-1). Besides the well-established role of Kv1 channels in controlling the cerebellar basket-Purkinje cells synapses, Kv1 channels are expressed by the deep cerebellar nuclear neurons (DCNs) where they regulate the activity of principal DCNs carrying the cerebellar output. DCNs include as well GABAergic neurons serving important functions, such as those forming the inhibitory nucleo-olivary pathway, the nucleo-cortical DCNs providing feed-back inhibition to the cerebellar cortex, and those targeting principal DCNs, but whether their function is regulated by Kv1 channels remains unclear. Here, using cerebellar slices from mature GAD67-GFP mice to identify putative GABAergic-DCNs (GAD + DCN) we show that specific Kv1 channel blockers (dendrotoxin-alpha/I/K, DTXs) hyperpolarized the threshold of somatic action potentials, increased the spontaneous firing rate and hampered evoked high frequency repetitive responses of GAD + DCNs. Moreover, DTXs induced somatic depolarization and tonic firing in previously silent, putative nucleo-cortical DCNs. These results reveal a novel role of Kv1 channels in regulating GABAergic-DCNs activity and thereby, cerebellar function at multiple levels.

Maternal Creatine Supplementation Positively Affects Male Rat Hippocampal Synaptic Plasticity in Adult Offspring

Creatine plays a crucial role in developing the brain, so much that its genetic deficiency results in mental dysfunction and cognitive impairments. Moreover, creatine supplementation is currently under investigation as a preventive measure to protect the fetus against oxidative stress during difficult pregnancies. Although creatine use is considered safe, posing minimal risk to clinical health, we found an alteration in morpho-functional maturation of neurons when male rats were exposed to creatine loads during brain development. In particular, increased excitability and enhanced long-term potentiation (LTP) were observed in the hippocampal pyramidal neurons of weaning pups. Since these effects were observed a long time after creatine treatment had been terminated, long-lasting modifications persisting into adulthood were hypothesized. Such modifications were investigated in the present study using morphological, electrophysiological, and calcium imaging techniques applied to hippocampal Cornu Ammonis 1 (CA1) neurons of adult rats born from dams supplemented with creatine. When compared to age-matched controls, the treated adult offspring were found to retain enhanced neuron excitability and an improved LTP, the best-documented neuronal substrate for memory formation. While translating data from rats to humans does have limitations, our findings suggest that prenatal creatine supplementation could have positive effects on adult cognitive abilities.

Functional roles of Kv1-mediated currents in genetically identified subtypes of pyramidal neurons in layer 5 of mouse somatosensory cortex

We used voltage-clamp recordings from somatic outside-out macropatches to determine the amplitude and biophysical properties of putative Kv1-mediated currents in layer 5 pyramidal neurons (PNs) from mice expressing EGFP under the control of promoters for etv1 or glt. We then used whole cell current-clamp recordings and Kv1-specific peptide blockers to test the hypothesis that Kv1 channels differentially regulate action potential (AP) voltage threshold, repolarization rate, and width as well as rheobase and repetitive firing in these two PN types. We found that Kv1-mediated currents make up a similar percentage of whole cell K⁺ current in both cell types, and only minor biophysical differences were observed between PN types or between currents sensitive to different Kv1 blockers. Putative Kv1 currents contributed to AP voltage threshold in both PN types, but AP width and rate of repolarization were only affected in etv1 PNs. Kv1 currents regulate rheobase, delay to the first AP, and firing rate similarly in both cell types, but the frequency-current slope was much more sensitive to Kv1 block in etv1 PNs. In both cell types, Kv1 block shifted the current required to elicit an onset doublet of action potentials to lower currents. Spike frequency adaptation was also affected differently by Kv1 block in the two PN types. Thus, despite similar expression levels and minimal differences in biophysical properties, Kv1 channels differentially regulate APs and repetitive firing in etv1 and glt PNs. This may reflect differences in subcellular localization of channel subtypes or differences in the other K⁺ channels expressed. NEW & NOTEWORTHY In two types of genetically identified layer 5 pyramidal neurons, α-dendrotoxin blocked approximately all of the putative Kv1 current (on average). We used outside-out macropatches and whole cell recordings at 33°C to show that despite similar expression levels and minimal differences in biophysical properties, Kv1 channels differentially regulate action potentials and repetitive firing in etv1 and glt pyramidal neurons. This may reflect differences in subcellular localization of channel subtypes or differences in the other K⁺ channels expressed.

The role of axonal Kv1 channels in CA3 pyramidal cell excitability

Axonal ion channels control spike initiation and propagation along the axon and determine action potential waveform. We show here that functional suppression of axonal Kv1 channels with local puff of dendrotoxin (DTx), laser or mechanical axotomy significantly increased excitability measured in the cell body. Importantly, the functional effect of DTx puffing or axotomy was not limited to the axon initial segment but was also seen on axon collaterals. In contrast, no effects were observed when DTx was puffed on single apical dendrites or after single dendrotomy. A simple model with Kv1 located in the axon reproduced the experimental observations and showed that the distance at which the effects of axon collateral cuts are seen depends on the axon space constant. In conclusion, Kv1 channels located in the axon proper greatly participate in intrinsic excitability of CA3 pyramidal neurons. This finding stresses the importance of the axonal compartment in the regulation of intrinsic neuronal excitability.

Roles of specific Kv channel types in repolarization of the action potential in genetically identified subclasses of pyramidal neurons in mouse neocortex

The action potential (AP) is a fundamental feature of excitable cells that serves as the basis for long-distance signaling in the nervous system. There is considerable diversity in the appearance of APs and the underlying repolarization mechanisms in different neuronal types (reviewed in Bean BP. Nat Rev Neurosci 8: 451-465, 2007), including among pyramidal cell subtypes. In the present work, we used specific pharmacological blockers to test for contributions of Kv1, Kv2, or Kv4 channels to repolarization of single APs in two genetically defined subpopulations of pyramidal cells in layer 5 of mouse somatosensory cortex (etv1 and glt) as well as pyramidal cells from layer 2/3. These three subtypes differ in AP properties (Groh A, Meyer HS, Schmidt EF, Heintz N, Sakmann B, Krieger P. Cereb Cortex 20: 826-836, 2010; Guan D, Armstrong WE, Foehring RC. J Neurophysiol 113: 2014-2032, 2015) as well as laminar position, morphology, and projection targets. We asked what the roles of Kv1, Kv2, and Kv4 channels are in AP repolarization and whether the underlying mechanisms are pyramidal cell subtype dependent. We found that Kv4 channels are critically involved in repolarizing neocortical pyramidal cells. There are also pyramidal cell subtype-specific differences in the role for Kv1 channels. Only Kv4 channels were involved in repolarizing the narrow APs of glt cells. In contrast, in etv1 cells and layer 2/3 cells, the broader APs are partially repolarized by Kv1 channels in addition to Kv4 channels. Consistent with their activation in the subthreshold range, Kv1 channels also regulate AP voltage threshold in all pyramidal cell subtypes.

Maternal creatine supplementation affects the morpho-functional development of hippocampal neurons in rat offspring

Creatine supplementation has been shown to protect neurons from oxidative damage due to its antioxidant and ergogenic functions. These features have led to the hypothesis of creatine supplementation use during pregnancy as prophylactic treatment to prevent CNS damage, such as hypoxic-ischemic encephalopathy. Unfortunately, very little is known on the effects of creatine supplementation during neuron differentiation, while in vitro studies revealed an influence on neuron excitability, leaving the possibility of creatine supplementation during the CNS development an open question. Using a multiple approach, we studied the hippocampal neuron morphological and functional development in neonatal rats born by dams supplemented with 1% creatine in drinking water during pregnancy. CA1 pyramidal neurons of supplemented newborn rats showed enhanced dendritic tree development, increased LTP maintenance, larger evoked-synaptic responses, and higher intrinsic excitability in comparison to controls. Moreover, a faster repolarizing phase of action potential with the appearance of a hyperpolarization were recorded in neurons of the creatine-treated group. Consistently, CA1 neurons of creatine exposed pups exhibited a higher maximum firing frequency than controls. In summary, we found that creatine supplementation during pregnancy positively affects morphological and electrophysiological development of CA1 neurons in offspring rats, increasing neuronal excitability. Altogether, these findings emphasize the need to evaluate the benefits and the safety of maternal intake of creatine in humans.