Interaction Between HCN and Slack Channels Regulates mPFC Pyramidal Cell Excitability in Working Memory Circuits

Carvedilol inhibits neuronal hyperexcitability caused by epilepsy-associated KCNT1 mutations

BACKGROUND AND PURPOSE

KCNT1 encodes a sodium-activated potassium channel (Slack channel), and its mutation can cause several forms of epilepsy. Traditional antiepileptic medications have limited efficacy in treating patients with KCNT1 mutations. Here, we describe one heterozygous KCNT1 mutation, M267T, in a patient with EIMFS. The pathological channel properties of this mutation and its effect on neuronal excitability were investigated. Additionally, this study aimed to develop a medication for effective prevention of KCNT1 mutation-induced seizures.

EXPERIMENTAL APPROACH

Wild-type or mutant KCNT1 plasmids were expressed heterologously in Xenopus laevis oocytes, and channel property assessment and drug screening were performed based on two-electrode voltage-clamp recordings. The single-channel properties were investigated using the excised inside-out patches from HEK293T cells. Through in utero electroporation, WT and M267T Slack channels were expressed in the hippocampal CA1 pyramidal neurons in male mice, followed by the examination of the electrical properties using the whole-cell current-clamp technique. The kainic acid-induced epilepsy model in male mice was used to evalute the antiseizure effects of carvedilol.

KEY RESULTS

The KCNT1 M267T mutation enhanced Slack channel function by increasing single-channel open probability. Through screening 16 FDA-approved ion channel blockers, we found that carvedilol effectively reversed the mutation-induced gain-of-function channel properties. Notably, the KCNT1 M267T mutation in the mouse hippocampal CA1 pyramidal neurons affected afterhyperpolarization properties and induced neuronal hyperexcitability, which was inhibited by carvedilol. Additionally, carvedilol exhibited antiseizure effects in the kainic acid-induced epilepsy model.

CONCLUSION AND IMPLICATION

Our findings suggest carvedilol as a new potential candidate for treatment of epilepsies.

Dynamic Network Connectivity: from monkeys to humans

Human brain imaging research using functional MRI (fMRI) has uncovered flexible variations in the functional connectivity between brain regions. While some of this variability likely arises from the pattern of information flow through circuits, it may also be influenced by rapid changes in effective synaptic strength at the molecular level, a phenomenon called Dynamic Network Connectivity (DNC) discovered in non-human primate circuits. These neuromodulatory molecular mechanisms are found in layer III of the macaque dorsolateral prefrontal cortex (dlPFC), the site of the microcircuits shown by Goldman-Rakic to be critical for working memory. This research has shown that the neuromodulators acetylcholine, norepinephrine, and dopamine can rapidly change the strength of synaptic connections in layer III dlPFC by (1) modifying the depolarization state of the post-synaptic density needed for NMDA receptor neurotransmission and (2) altering the open state of nearby potassium channels to rapidly weaken or strengthen synaptic efficacy and the strength of persistent neuronal firing. Many of these actions involve increased cAMP-calcium signaling in dendritic spines, where varying levels can coordinate the arousal state with the cognitive state. The current review examines the hypothesis that some of the dynamic changes in correlative strength between cortical regions observed in human fMRI studies may arise from these molecular underpinnings, as has been seen when pharmacological agents or genetic alterations alter the functional connectivity of the dlPFC consistent with the macaque physiology. These DNC mechanisms provide essential flexibility but may also confer vulnerability to malfunction when dysregulated in cognitive disorders.

Localization of PDE4D, HCN1 channels, and mGluR3 in rhesus macaque entorhinal cortex may confer vulnerability in Alzheimer's disease

Alzheimer's disease cortical tau pathology initiates in the layer II cell clusters of entorhinal cortex, but it is not known why these specific neurons are so vulnerable. Aging macaques exhibit the same qualitative pattern of tau pathology as humans, including initial pathology in layer II entorhinal cortex clusters, and thus can inform etiological factors driving selective vulnerability. Macaque data have already shown that susceptible neurons in dorsolateral prefrontal cortex express a "signature of flexibility" near glutamate synapses on spines, where cAMP-PKA magnification of calcium signaling opens nearby potassium and hyperpolarization-activated cyclic nucleotide-gated channels to dynamically alter synapse strength. This process is regulated by PDE4A/D, mGluR3, and calbindin, to prevent toxic calcium actions; regulatory actions that are lost with age/inflammation, leading to tau phosphorylation. The current study examined whether a similar "signature of flexibility" expresses in layer II entorhinal cortex, investigating the localization of PDE4D, mGluR3, and HCN1 channels. Results showed a similar pattern to dorsolateral prefrontal cortex, with PDE4D and mGluR3 positioned to regulate internal calcium release near glutamate synapses, and HCN1 channels concentrated on spines. As layer II entorhinal cortex stellate cells do not express calbindin, even when young, they may be particularly vulnerable to magnified calcium actions and ensuing tau pathology.