Autism-associated mutations in KV7 channels induce gating pore current

Autism spectrum disorder (ASD) adversely impacts >1% of children in the United States, causing social interaction deficits, repetitive behaviors, and communication disorders. Genetic analysis of ASD has advanced dramatically through genome sequencing, which has identified >500 genes with mutations in ASD. Mutations that alter arginine gating charges in the voltage sensor of the voltage-gated potassium (KV) channel KV7 (KCNQ) are among those frequently associated with ASD. We hypothesized that these gating charge mutations would induce gating pore current (also termed ω-current) by causing an ionic leak through the mutant voltage sensor. Unexpectedly, we found that wild-type KV7 conducts outward gating pore current through its native voltage sensor at positive membrane potentials, owing to a glutamine in the third gating charge position. In bacterial and human KV7 channels, gating charge mutations at the R1 and R2 positions cause inward gating pore current through the resting voltage sensor at negative membrane potentials, whereas mutation at R4 causes outward gating pore current through the activated voltage sensor at positive potentials. Remarkably, expression of the KV7.3/R2C ASD-associated mutation in vivo in midbrain dopamine neurons of mice disrupts action potential generation and repetitive firing. Overall, our results reveal native and mutant gating pore current in KV7 channels and implicate altered control of action potential generation by gating pore current through mutant KV7 channels as a potential pathogenic mechanism in autism.

The murburn precepts for cellular electrophysiology

Based on murburn model of bioenergetics, we had recently proposed the rationale for cation distribution in cells (Manoj & Tamagawa, 2020) and also demonstrated a potential generation based on murzyme activity (Tamagawa et al., 2021). Herein, the precepts for murburn concept based electrophysiological phenomena- differentiation of ions at the interface, and action potential generation/conduction along a neuron are briefly charted. Contrary to the ionic theories that prevail, the murburn model is electronic; although, the contributions of ions are not discounted.

Activity-mediated accumulation of potassium induces a switch in firing pattern and neuronal excitability type

During normal neuronal activity, ionic concentration gradients across a neuron’s membrane are often assumed to be stable. Prolonged spiking activity, however, can reduce transmembrane gradients and affect voltage dynamics. Based on mathematical modeling, we investigated the impact of neuronal activity on ionic concentrations and, consequently, the dynamics of action potential generation. We find that intense spiking activity on the order of a second suffices to induce changes in ionic reversal potentials and to consistently induce a switch from a regular to an intermittent firing mode. This transition is caused by a qualitative alteration in the system’s voltage dynamics, mathematically corresponding to a co-dimension-two bifurcation from a saddle-node on invariant cycle (SNIC) to a homoclinic orbit bifurcation (HOM). Our electrophysiological recordings in mouse cortical pyramidal neurons confirm the changes in action potential dynamics predicted by the models: (i) activity-dependent increases in intracellular sodium concentration directly reduce action potential amplitudes, an effect typically attributed solely to sodium channel inactivation; (ii) extracellular potassium accumulation switches action potential generation from tonic firing to intermittently interrupted output. Thus, individual neurons may respond very differently to the same input stimuli, depending on their recent patterns of activity and/or the current brain-state.

Curbing action potential generation or ATP-synthase leads to a decrease in in-cell pyruvate dehydrogenase activity in rat cerebrum slices

Direct and real-time monitoring of cerebral metabolism exploiting the drastic increase in sensitivity of hyperpolarized 13C-labeled metabolites holds the potential to report on neural activity via in-cell metabolic indicators. Here, we followed the metabolic consequences of curbing action potential generation and ATP-synthase in rat cerebrum slices, induced by tetrodotoxin and oligomycin, respectively. The results suggest that pyruvate dehydrogenase (PDH) activity in the cerebrum is 4.4-fold higher when neuronal firing is unperturbed. The PDH activity was 7.4-fold reduced in the presence of oligomycin, and served as a pharmacological control for testing the ability to determine changes to PDH activity in viable cerebrum slices. These findings may open a path towards utilization of PDH activity, observed by magnetic resonance of hyperpolarized 13C-labeled pyruvate, as a reporter of neural activity.

Different Sensitivity of action potential generation to the rate of depolarization in vagal afferent A-fiber versus C-fiber neurons

The vagal afferent nerves innervate the visceral organs and convey sensory information from the internal environment to the central nervous system. A better understanding of the mechanisms controlling the activation of vagal afferent neurons bears physiological and pathological significance. Although it is generally believed that the magnitude and the rising rate of membrane depolarization are both critical for the action potential generation, no direct or quantitative evidence has been documented so far for the sensitivity of vagal afferent neuron activation to the rate of depolarization and for its underlying ionic mechanisms. Here, by measuring the response of mouse nodose neurons to the suprathreshold current stimuli of varying rising rates, the slowest depolarization capable of evoking action potentials, the rate-of-depolarization threshold (dV/dtthreshold), was determined and found to be ~20 fold higher in the A-fiber neurons compared to the C-fiber neurons classified based on the capsaicin responsiveness and characteristics of action potential waveforms. Moreover, although the dV/dtthreshold varied substantially among individual neurons it was not different in any one neuron in response to different intensities of current stimuli. Finally, inhibition of low-threshold activated D-type potassium current (IK.D) by α-dendrotoxin or low concentration of 4-aminopyrydine nearly abrogated the sensitivity of action potential generation to the depolarization rate. Thus, the depolarization rate is an important independent factor contributing to the control of action potential discharge, which is particularly effective in the vagal afferent A-fiber neurons. The IK.D channel may regulate the excitability of vagal sensory neurons by setting the dV/dtthreshold for action potential discharge.

Activity-mediated accumulation of potassium induces a switch in firing pattern and neuronal excitability type

During normal neuronal activity, ionic concentration gradients across a neuron’s membrane are often assumed to be stable. Prolonged spiking activity, however, can reduce transmembrane gradients and affect voltage dynamics. Based on mathematical modeling, we here investigate the impact of neuronal activity on ionic concentrations and, consequently, the dynamics of action potential generation. We find that intense spiking activity on the order of a second suffices to induce changes in ionic reversal potentials and to consistently induce a switch from a regular to an intermittent firing mode. This transition is caused by a qualitative alteration in the system’s voltage dynamics, mathematically corresponding to a co-dimension-two bifurcation from a saddle-node on invariant cycle (SNIC) to a homoclinic orbit bifurcation (HOM). Our electrophysiological recordings in mouse cortical pyramidal neurons confirm the changes in action potential dynamics predicted by the models: (i) activity-dependent increases in intracellular sodium concentration directly reduce action potential amplitudes, an effect that previously had been attributed soley to sodium channel inactivation; (ii) extracellular potassium accumulation switches action potential generation from tonic firing to intermittently interrupted output. Individual neurons thus may respond very differently to the same input stimuli, depending on their recent patterns of activity or the current brain-state.Author summaryIonic concentrations in the brain are not constant. We show that during intense neuronal activity, they can change on the order of seconds and even switch neuronal spiking patterns under identical stimulation from a regular firing mode to an intermittently interrupted one. Triggered by an accumulation of extracellular potassium, such a transition is caused by a specific, qualitative change in of the neuronal voltage dynamics – a so-called bifurcation – which affects crucial features of action-potential generation and bears consequences for how information is encoded and how neurons behave together in the network. Also changes in intracellular sodium can induce measurable effects, like a shrinkage of spike amplitude that occurs independently of the fast amplitude-effects attributed to sodium channel inactivation. Taken together, our results demonstrate that a neuron can respond very differently to the same stimulus, depending on its previous activity or the current brain state. The finding is particularly relevant when other regulatory mechanisms of ionic homeostasis are challenged, for example, during pathological states of glial impairment or oxygen deprivation. Categorization of cortical neurons as intrinsically bursting or regular spiking may be biased by the ionic concentrations at the time of the observation, highlighting the non-static nature of neuronal dynamics.

The cell membrane

The cell membrane forms an integral part of cellular homeostasis. This article will review the structure and function of physiological membranes and how the lipid bilayer plays an important role in maintaining cellular integrity, ion permeability, and membrane potential as well as being selectively permeable to certain biological substances. This review looks at the physico-chemical and structural properties of the plasma membrane which affords it the unique property of selective permeability and action potential generation. Furthermore, the mechanisms by which permeability occurs across the membrane are also discussed.