Analysis of the effect of the scorpion toxin AaH-II on action potential generation in the axon initial segment

The toxin AaH-II, from the scorpion Androctonus australis Hector venom, is a 64 amino acid peptide that targets voltage-gated Na+ channels (VGNCs) and slows their inactivation. While at macroscopic cellular level AaH-II prolongs the action potential (AP), a functional analysis of the effect of the toxin in the axon initial segment (AIS), where VGNCs are highly expressed, was never performed so far. Here, we report an original analysis of the effect of AaH-II on the AP generation in the AIS of neocortical layer-5 pyramidal neurons from mouse brain slices. After determining that AaH-II does not discriminate between Nav1.2 and Nav1.6, i.e. between the two VGNC isoforms expressed in this neuron, we established that 7 nM was the smallest toxin concentration producing a minimal detectable deformation of the somatic AP after local delivery of the toxin. Using membrane potential imaging, we found that, at this minimal concentration, AaH-II substantially widened the AP in the AIS. Using ultrafast Na+ imaging, we found that local application of 7 nM AaH-II caused a large increase in the slower component of the Na+ influx in the AIS. Finally, using ultrafast Ca2+ imaging, we observed that 7 nM AaH-II produces a spurious slow Ca2+ influx via Ca2+-permeable VGNCs. Molecules targeting VGNCs, including peptides, are proposed as potential therapeutic tools. Thus, the present analysis in the AIS can be considered a general proof-of-principle on how high-resolution imaging techniques can disclose drug effects that cannot be observed when tested at the macroscopic level.

Kinetics and functional consequences of BK channels activation by N-type Ca2+ channels in the dendrite of mouse neocortical layer-5 pyramidal neurons

The back-propagation of an action potential (AP) from the axon/soma to the dendrites plays a central role in dendritic integration. This process involves an intricate orchestration of various ion channels, but a comprehensive understanding of the contribution of each channel type remains elusive. In this study, we leverage ultrafast membrane potential recordings (Vm) and Ca2+ imaging techniques to shed light on the involvement of N-type voltage-gated Ca2+ channels (VGCCs) in layer-5 neocortical pyramidal neurons' apical dendrites. We found a selective interaction between N-type VGCCs and large-conductance Ca2+-activated K+ channels (BK CAKCs). Remarkably, we observe that BK CAKCs are activated within a mere 500 μs after the AP peak, preceding the peak of the Ca2+ current triggered by the AP. Consequently, when N-type VGCCs are inhibited, the early broadening of the AP shape amplifies the activity of other VGCCs, leading to an augmented total Ca2+ influx. A NEURON model, constructed to replicate and support these experimental results, reveals the critical coupling between N-type and BK channels. This study not only redefines the conventional role of N-type VGCCs as primarily involved in presynaptic neurotransmitter release but also establishes their distinct and essential function as activators of BK CAKCs in neuronal dendrites. Furthermore, our results provide original functional validation of a physical interaction between Ca2+ and K+ channels, elucidated through ultrafast kinetic reconstruction. This insight enhances our understanding of the intricate mechanisms governing neuronal signaling and may have far-reaching implications in the field.

Nav1.2 and BK channels interaction shapes the action potential in the axon initial segment

In neocortical layer-5 pyramidal neurons, the action potential (AP) is generated in the axon initial segment (AIS) when the membrane potential (V_m ) reaches the threshold for activation of the voltage-gated Na⁺ channels (VGNCs) Na_v 1.2 and Na_v 1.6. Yet, whereas it is known that these VGNCs differ in spatial distribution along the AIS and in biophysical properties, the understanding of the functional differences between the two channels remains elusive. Here, using ultrafast Na⁺ , V_m and Ca²⁺ imaging in combination with the partial block of Na_v 1.2 by the peptide G¹ G⁴ -huwentoxin-IV, we demonstrate an exclusive role of Na_v 1.2 in shaping the generating AP. Precisely, we show that a selective block of ∼30% of Na_v 1.2 widens the AP in the distal part of the AIS and we demonstrate that this effect is due to a loss of activation of BK Ca²⁺ -activated K⁺ channels (CAKCs). Indeed, Ca²⁺ influx via Na_v 1.2 activates BK CAKCs determining the amplitude and the early phase of repolarisation of the AP in the AIS. By mean of control experiments using 4,9-Anhydrotetrodotoxin, which is a moderately selective inhibitor of Na_v 1.6, we concluded that the Ca²⁺ influx shaping the early phase of the AP is exclusive of Na_v 1.2. Hence, we mimicked this result with a NEURON model where the role of the different ion channels tested reproduced the experimental evidence. The exclusive role of Na_v 1.2 reported here is important for understanding the physiology and pathology of neuronal excitability. KEY POINTS: We optically analysed the action potential that generates in the axon initial segment of mouse layer-5 neocortical pyramidal neurons and its associated Na+ and Ca2+ currents using ultrafast imaging techniques. We found that the partial selective block of the voltage-gated Na+ channel Nav1.2, produced by a recently developed peptide, widens the shape of the action potential in the distal part of the axon initial segment. We demonstrate that this effect is due to the reduction of the Ca2+ influx through Nav1.2 that activates BK Ca2+-activated K+ channels. To validate our conclusions, we generated a NEURON model that reproduces the ensemble of our experimental results. The present results indicate a specific role of Nav1.2 in the axon initial segment for the shaping of the action potential during its generation. Abstract figure legend In the axon initial segment of neocortical pyramidal neurons the voltage-gated Na⁺ channel Na_v 1.2 contributes to the generation of the action potential by providing a Na⁺ current and a Ca²⁺ current that activates BK Ca²⁺ -activated K⁺ channels shaping the kinetics of the action potential. This article is protected by copyright. All rights reserved.

In vivo spatiotemporal control of voltage-gated ion channels by using photoactivatable peptidic toxins

Photoactivatable drugs targeting ligand-gated ion channels open up new opportunities for light-guided therapeutic interventions. Photoactivable toxins targeting ion channels have the potential to control excitable cell activities with low invasiveness and high spatiotemporal precision. As proof-of-concept, we develop HwTxIV-Nvoc, a UV light-cleavable and photoactivatable peptide that targets voltage-gated sodium (NaV) channels and validate its activity in vitro in HEK293 cells, ex vivo in brain slices and in vivo on mice neuromuscular junctions. We find that HwTxIV-Nvoc enables precise spatiotemporal control of neuronal NaV channel function under all conditions tested. By creating multiple photoactivatable toxins, we demonstrate the broad applicability of this toxin-photoactivation technology.

Ultrafast Sodium Imaging of the Axon Initial Segment of Neurons in Mouse Brain Slices

Monitoring Na⁺ influx in the axon initial segment (AIS) at high spatial and temporal resolution is fundamental to understanding the generation of an action potential (AP). Here, we present protocols to obtain this measurement, focusing on the AIS of layer 5 (L5) somatosensory cortex pyramidal neurons in mouse brain slices. We first outline how to prepare slices for this application, how to select and patch neurons, and how to optimize the image acquisition. Specifically, we describe the preparation of optimal slices, patching and loading of L5 pyramidal neurons with the Na⁺ indicator ING-2, and Na⁺ imaging at 100 µs temporal resolution with a pixel resolution of half a micron. Then, we present a data analysis strategy in order to extract information on the kinetics of activated voltage-gated Na⁺ channels by determining the change in Na⁺ by compensating for bleaching and calculating the time derivative of the resulting fit. In sum, this approach can be widely applied when investigating the function of Na⁺ channels during initiation of an AP and propagation under physiological or pathological conditions in neuronal subtypes. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Preparation of cortical slices Basic Protocol 2: Selection, patching, and Na⁺ fluorescence recording of a neuron Support Protocol: Calibrating Na⁺ fluorescence Basic Protocol 3: Data analysis.

Cal-520FF is the Present Optimal Ca2+ Indicator for Ultrafast Ca2+ Imaging and Optical Measurement of Ca2+ Currents

Ultrafast Ca²⁺ imaging using low-affinity fluorescent indicators allows the precise measurement of the kinetics of fast Ca²⁺ currents mediated by voltage-gated Ca²⁺ channels. Thus far, only a few indicators provided fluorescence transients with sufficient signal-to-noise ratio necessary to achieve this measurement, with Oregon Green BAPTA-5N exhibiting the best performance. Here we evaluated the performance of the low-affinity Ca²⁺ indicator Cal-520FF to record fast Ca²⁺ signals and to measure the kinetics of Ca²⁺ currents. Compared to Oregon Green BAPTA-5N and to Fluo4FF, Cal-520FF offers a superior signal-to-noise-ratio providing the optimal characteristics for this important type of biophysical measurement. This ability is the result of a relatively high fluorescence at zero Ca²⁺, necessary to detect enough photons at short exposure windows, and a high dynamic range leading to large fluorescence transients associated with short Ca²⁺ influx periods. We conclude that Cal-520FF is at present the optimal commercial low-affinity Ca²⁺ indicator for ultrafast Ca²⁺ imaging applications.