Subcellular control of membrane excitability in the axon

Abstract

Ion channels are microscopic pore proteins in the membrane that open and close in response to chemical and electrical stimuli. This simple concept underlies rapid electrical signaling in the brain as well as several important aspects of neural plasticity. Although the soma accounts for less than 1% of many neurons by membrane area, it has been the major site of measuring ion channel function. However, the axon is one of the longest processes found in cellular biology and hosts a multitude of critical signaling functions in the brain. Not only does the axon initiate and rapidly propagate action potentials (APs) across the brain but it also forms the presynaptic terminals that convert these electrical inputs into chemical outputs. Here, we review recent advances in the physiological role of ion channels within the diverse landscape of the axon and presynaptic terminals.

Figure 1.. Heterogeneity of axon morphology and ion channel distribution

(a) Left; AiryScan imaging of rat hippocampal axon expressing QuasAr to illuminate the plasmalemma. AiryScan imaging reveals the variability in the axon diameter and extreme morphology that occurs at brach points (blue arrow) and en passant synaptic terminals (black arrow). Quantification of axon diameters (red; middle panel) and terminal diameters (yellow; right panel) from several hippocampal neurons using a fluorescent tag of synaptic vesicles to identify presynaptic terminals (not shown). Images adapted from [2**]. (b) (i) Diagram of neuron illustrating both myelinated and unmyelinated axons with branch points and herogeneous daughter branch diameters throughout the arborization, summarizing heterogeneities reported in effective sodium currents found at the AIS, nodes of Ranvier, branch points and distal branches of the axon (red). (ii) Illustration of axonal branch points that contain daughter branches with larger diameters that impose impendance mismatches (blue) [83] that can slow and or halt propagation that assume equal sodium channel activity (top) or heterogeneous sodium channel activity (bottom). (c) Possible mechanisms to regulate heterogeneous sodium currents in umyelinated branches of an axon as illustrated by green box in Figure 1b. This heterogeneity can be caused by directly trafficking more sodium channels to specific regions of the plasmalemma (i). Enrichment of sodium channels can also be caused by extracellular cues from surrounding tissues through interactions with Nav subunits or membrane binding partners that recognize extracellular molecular signals such as tenascin or brevican (ii). A third possibility is that changes in local membrane pontential or binding partners alter the local percentage of inactivated sodium channels throughout the axon through interactions with binding partners or acute signals that alter local membrane potential (iii).

Figure 2.. Regulation of excitability at the axon initial segment

(a) Diagram exploring the factors influencing AIS excitability from the somatodendritic compartments. (i) The resting membrane potential of the AIS is set by a variety of voltage-gated ion channels localized within the soma. Potassium leak channels (K⁺ leak, gray) will act to inhibit the conductance of sodium (Nav, cyan) and calcium (Cav, purple) channels, while increasing the conductance of HCN channels (HCN, red). The depolarizing HCN current from somatic channels is critical for setting the resting membrane potential at the AIS (~−3mV lower than in the soma) and for the activation of voltage-dependent conductances within the AIS. Adapted from [33] (ii) The surface area of the somatodendritic region also influences AIS excitability, where a larger somatodendritic area creates a larger capacitive load, effectively hyperpolarizing the proximal axonal membrane. (b) Diagram exploring local neurotransmitter regulation of AIS excitability. (i) NMDA receptors (NMDAR, dark purple) decrease the conductance of sodium (Nav, cyan) and potassium (Kv, green) channels at the AIS by decreasing their surface expression. The calcium entering the AIS from NMDARs and potentially from calcium channels (Cav, purple) may also activate small conductance calcium-activated K⁺ channels (SK, orange) depending on their coupling to calcium sources, leading to a hyperpolarization of the AIS, though this has yet to be experimentally confirmed (dashed lines). (ii) Dopamine D3 receptors (D3R, light green) decrease excitability by hyperpolarizing the activation range of Cav at the AIS. (iii) Serotonin receptors inhibit Nav conductance at the AIS by producing a depolarizing shift in their activation curve and facilitating slow inactivation of Na⁺ currents, decreasing excitability. Additionally, serotonin hyperpolarizes the activation range of HCN channels (HCN, red) within the AIS, increasing their conductance and thereby reducing excitability. (iv) Muscarinic acetylcholine receptors increase Cav activity as rest, resulting in an elevated intracellular calcium concentration within the AIS. This elevated calcium leads to a subsequent suppression of the M current through Kv, increasing excitability at the AIS.

Figure 3.. Presynaptic AP modulation of synaptic transmission and its local regulation by Kv isoforms

(a) Diagram exploring activation of Cav channels (closed grey) and vesicle fusion in a top down view of the presynaptic active zone from baseline (i) and by three different AP waveforms: (ii) normal, (iii) higher overshoot, and (iv) higher overshoot and broadened width. Impacts of calcium within microdomains around open channels (red) and diffuse calcium are shown in blue. An arbitrary number of calcium sensors promoting vesicle fusion when fully occupied are shown in yellow. Small changes in amplitude will alter the probability of Cav opening such as compared in (ii) and (iii), moving from 30% to 50% of Cavs activated. Changes in width of the AP can have more complex changes in calcium microdomains but generally result in more diffuse build-up within the cytoplasm near the active zone (brighter blue seen in subpanel iv). (b) Top; Illustration of a single AP evoking calcium entry and vesicle fusion where the falling or hyperpolarizing phase of the waveform is controlled by Kv1 channels. Bottom; If a subsequent AP arrives in 20 ms (50 Hz) vs 250 ms (4Hz) a larger percentage of Kv1 channels would still be inactivated at 50Hz stimulating frequency, thus broadening the AP and causing a facilitation of vesicle fusion. (c) Top; Illustration of a single AP evoking calcium entry and vesicle fusion where the falling or hyperpolarizing phase of the waveform is controlled by Kv3 and/or BK channels. Bottom; If a subsequent AP arrives in 20 ms (50 Hz) vs 250 ms (4Hz) a larger percentage of BK channels would activate at 50Hz stimulating frequency, thus narrowing the AP and restricting calcium build-up and reducing vesicle facilitation. Factors that localize the BK channel to the calcium source will strongly influence AP narrowing.