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Communications Physics 2020Sensing neuronal action potential associated magnetic fields (APMFs) is an emerging viable alternative of functional brain mapping. Measurement of APMFs of large axons...
Sensing neuronal action potential associated magnetic fields (APMFs) is an emerging viable alternative of functional brain mapping. Measurement of APMFs of large axons of worms have been possible due to their size. In the mammalian brain, axon sizes, their numbers and routes, restricts using such functional imaging methods. With a segmented model of mammalian pyramidal neurons, we show that the APMF of intra-axonal currents in the axon hillock are two orders of magnitude larger than other neuronal locations. Expected 2D magnetic field maps of naturalistic spiking activity of a volume of neurons via widefield diamond-nitrogen-vacancy-center-magnetometry were simulated. A dictionary-based matching pursuit type algorithm applied to the data using the axon-hillock's APMF signature allowed spatiotemporal reconstruction of action potentials in the volume of brain tissue at single cell resolution. Enhancement of APMF signals coupled with magnetometry advances thus can potentially replace current functional brain mapping techniques.
PubMed: 33072889
DOI: 10.1038/s42005-020-00439-6 -
Nature Sep 2022Neurons are highly polarized cells that face the fundamental challenge of compartmentalizing a vast and diverse repertoire of proteins in order to function properly. The...
Neurons are highly polarized cells that face the fundamental challenge of compartmentalizing a vast and diverse repertoire of proteins in order to function properly. The axon initial segment (AIS) is a specialized domain that separates a neuron's morphologically, biochemically and functionally distinct axon and dendrite compartments. How the AIS maintains polarity between these compartments is not fully understood. Here we find that in Caenorhabditis elegans, mouse, rat and human neurons, dendritically and axonally polarized transmembrane proteins are recognized by endocytic machinery in the AIS, robustly endocytosed and targeted to late endosomes for degradation. Forcing receptor interaction with the AIS master organizer, ankyrinG, antagonizes receptor endocytosis in the AIS, causes receptor accumulation in the AIS, and leads to polarity deficits with subsequent morphological and behavioural defects. Therefore, endocytic removal of polarized receptors that diffuse into the AIS serves as a membrane-clearance mechanism that is likely to work in conjunction with the known AIS diffusion-barrier mechanism to maintain neuronal polarity on the plasma membrane. Our results reveal a conserved endocytic clearance mechanism in the AIS to maintain neuronal polarity by reinforcing axonal and dendritic compartment membrane boundaries.
Topics: Animals; Axon Initial Segment; Caenorhabditis elegans; Cell Membrane; Cell Polarity; Dendrites; Diffusion; Endocytosis; Endosomes; Humans; Mice; Protein Transport; Proteolysis; Rats; Receptors, Cell Surface
PubMed: 35978188
DOI: 10.1038/s41586-022-05074-5 -
Annals of the New York Academy of... May 2018The axon initial segment (AIS) is located at the proximal axon and is the site of action potential initiation. This reflects the high density of ion channels found at... (Review)
Review
The axon initial segment (AIS) is located at the proximal axon and is the site of action potential initiation. This reflects the high density of ion channels found at the AIS. Adaptive changes to the location and length of the AIS can fine-tune the excitability of neurons and modulate plasticity in response to activity. The AIS plays an important role in maintaining neuronal polarity by regulating the trafficking and distribution of proteins that function in somatodendritic or axonal compartments of the neuron. In this review, we provide an overview of the AIS cytoarchitecture, mechanism of assembly, and recent studies revealing mechanisms of differential transport at the AIS that maintain axon and dendrite identities. We further discuss how genetic mutations in AIS components (i.e., ankyrins, ion channels, and spectrins) and injuries may cause neurological disorders.
Topics: Action Potentials; Ankyrins; Axon Initial Segment; Cell Polarity; Humans; Ion Channels; Nervous System Diseases; Neurons; Protein Transport; Ranvier's Nodes
PubMed: 29749636
DOI: 10.1111/nyas.13718 -
Human Molecular Genetics Jul 2023To characterize a novel neurodevelopmental syndrome due to loss-of-function (LoF) variants in Ankyrin 2 (ANK2), and to explore the effects on neuronal network dynamics...
ANK2 loss-of-function variants are associated with epilepsy, and lead to impaired axon initial segment plasticity and hyperactive network activity in hiPSC-derived neuronal networks.
PURPOSE
To characterize a novel neurodevelopmental syndrome due to loss-of-function (LoF) variants in Ankyrin 2 (ANK2), and to explore the effects on neuronal network dynamics and homeostatic plasticity in human-induced pluripotent stem cell-derived neurons.
METHODS
We collected clinical and molecular data of 12 individuals with heterozygous de novo LoF variants in ANK2. We generated a heterozygous LoF allele of ANK2 using CRISPR/Cas9 in human-induced pluripotent stem cells (hiPSCs). HiPSCs were differentiated into excitatory neurons, and we measured their spontaneous electrophysiological responses using micro-electrode arrays (MEAs). We also characterized their somatodendritic morphology and axon initial segment (AIS) structure and plasticity.
RESULTS
We found a broad neurodevelopmental disorder (NDD), comprising intellectual disability, autism spectrum disorders and early onset epilepsy. Using MEAs, we found that hiPSC-derived neurons with heterozygous LoF of ANK2 show a hyperactive and desynchronized neuronal network. ANK2-deficient neurons also showed increased somatodendritic structures and altered AIS structure of which its plasticity is impaired upon activity-dependent modulation.
CONCLUSIONS
Phenotypic characterization of patients with de novo ANK2 LoF variants defines a novel NDD with early onset epilepsy. Our functional in vitro data of ANK2-deficient human neurons show a specific neuronal phenotype in which reduced ANKB expression leads to hyperactive and desynchronized neuronal network activity, increased somatodendritic complexity and AIS structure and impaired activity-dependent plasticity of the AIS.
Topics: Humans; Axon Initial Segment; Induced Pluripotent Stem Cells; Ankyrins; Neurons; Epilepsy
PubMed: 37195288
DOI: 10.1093/hmg/ddad081 -
Cell Reports Dec 2023Dysregulated neuronal excitability is a hallmark of amyotrophic lateral sclerosis (ALS). We sought to investigate how functional changes to the axon initial segment...
Dysregulated neuronal excitability is a hallmark of amyotrophic lateral sclerosis (ALS). We sought to investigate how functional changes to the axon initial segment (AIS), the site of action potential generation, could impact neuronal excitability in ALS human induced pluripotent stem cell (hiPSC) motor neurons. We find that early TDP-43 and C9orf72 hiPSC motor neurons show an increase in the length of the AIS and impaired activity-dependent AIS plasticity that is linked to abnormal homeostatic regulation of neuronal activity and intrinsic hyperexcitability. In turn, these hyperactive neurons drive increased spontaneous myofiber contractions of in vitro hiPSC motor units. In contrast, late hiPSC and postmortem ALS motor neurons show AIS shortening, and hiPSC motor neurons progress to hypoexcitability. At a molecular level, aberrant expression of the AIS master scaffolding protein ankyrin-G and AIS-specific voltage-gated sodium channels mirror these dynamic changes in AIS function and excitability. Our results point toward the AIS as an important site of dysfunction in ALS motor neurons.
Topics: Humans; Axon Initial Segment; Amyotrophic Lateral Sclerosis; Induced Pluripotent Stem Cells; Motor Neurons; Action Potentials
PubMed: 38019651
DOI: 10.1016/j.celrep.2023.113509 -
Current Opinion in Neurobiology Aug 2018The axon initial segment (AIS) is a unique domain of the proximal axon serving critical electrical and structural roles including the initiation of action potentials and... (Review)
Review
The axon initial segment (AIS) is a unique domain of the proximal axon serving critical electrical and structural roles including the initiation of action potentials and maintenance of cellular polarity. Recent experimental and theoretical advances demonstrate that the anatomical site for initiation is remarkably diverse. The AIS location varies not only axially, along the axon, but axons also emerge variably from either the soma or proximal dendrites. Here, we review the evidence that the diversity of AIS and axon location has a substantial impact on the electrical properties and speculate that the anatomical heterogeneity of axon locations expands synaptic integration within cell types and improves information processing in neural circuits.
Topics: Animals; Axon Initial Segment; Axons; Cell Polarity; Membrane Potentials; Neurons; Synapses
PubMed: 29533849
DOI: 10.1016/j.conb.2018.02.016 -
The Journal of Cell Biology Jul 1968Axon hillocks and initial segments have been recognized and studied in electron micrographs of a wide variety of neurons. In all multipolar neurons the fine structure of...
Axon hillocks and initial segments have been recognized and studied in electron micrographs of a wide variety of neurons. In all multipolar neurons the fine structure of the initial segment has the same pattern, whether or not the axon is ensheathed in myelin. The internal structure of the initial segment is characterized by three special features: (a) a dense layer of finely granular material undercoating the plasma membrane, (b) scattered clusters of ribosomes, and (c) fascicles of microtubules. A similar undercoating occurs beneath the plasma membrane of myelinated axons at nodes of Ranvier. The ribosomes are not organized into Nissl bodies and are too sparsely distributed to produce basophilia. They vanish at the end of the initial segment. Fascicles of microtubules occur only in the axon hillock and initial segment and nowhere else in the neuron. Therefore, they are the principal identifying mark. Some speculations are presented on the relation between these special structural features and the special function of the initial segment.
Topics: Animals; Axons; Cell Membrane; Microscopy, Electron; Myelin Sheath; Organoids; Ranvier's Nodes; Rats; Ribosomes
PubMed: 5691973
DOI: 10.1083/jcb.38.1.193 -
The Journal of Neuroscience : the... Sep 2023Neurons are remarkably polarized structures: dendrites spread and branch to receive synaptic inputs while a single axon extends and transmits action potentials (APs) to...
Neurons are remarkably polarized structures: dendrites spread and branch to receive synaptic inputs while a single axon extends and transmits action potentials (APs) to downstream targets. Neuronal polarity is maintained by the axon initial segment (AIS), a region between the soma and axon proper that is also the site of action potential (AP) generation. This polarization between dendrites and axons extends to inhibitory neurotransmission. In adulthood, the neurotransmitter GABA hyperpolarizes dendrites but instead depolarizes axons. These differences in function collide at the AIS. Multiple studies have shown that GABAergic signaling in this region can share properties of either the mature axon or mature dendrite, and that these properties evolve over a protracted period encompassing periadolescent development. Here, we explored how developmental changes in GABAergic signaling affect AP initiation. We show that GABA at the axon initial segment inhibits action potential initiation in layer (L)2/3 pyramidal neurons in prefrontal cortex from mice of either sex across GABA reversal potentials observed in periadolescence. These actions occur largely through current shunts generated by GABA receptors and changes in voltage-gated channel properties that affected the number of channels that could be recruited for AP electrogenesis. These results suggest that GABAergic neurons targeting the axon initial segment provide an inhibitory "veto" across the range of GABA polarity observed in normal adolescent development, regardless of GABAergic synapse reversal potential. GABA receptors are a major class of neurotransmitter receptors in the brain. Typically, GABA receptors inhibit neurons by allowing influx of negatively charged chloride ions into the cell. However, there are cases where local chloride concentrations promote chloride efflux through GABA receptors. Such conditions exist early in development in neocortical pyramidal cell axon initial segments (AISs), where action potentials (APs) initiate. Here, we examined how chloride efflux in early development interacts with mechanisms that support action potential initiation. We find that this efflux, despite moving membrane potential closer to action potential threshold, is nevertheless inhibitory. Thus, GABA at the axon initial segment is likely to be inhibitory for action potential initiation independent of whether chloride flows out or into neurons via these receptors.
Topics: Animals; Mice; Axon Initial Segment; Action Potentials; Chlorides; GABAergic Neurons; Receptors, GABA-A; gamma-Aminobutyric Acid
PubMed: 37596053
DOI: 10.1523/JNEUROSCI.0605-23.2023 -
Cells Apr 2023Brain channelopathies are a group of neurological disorders that result from genetic mutations affecting ion channels in the brain. Ion channels are specialized proteins... (Review)
Review
Brain channelopathies are a group of neurological disorders that result from genetic mutations affecting ion channels in the brain. Ion channels are specialized proteins that play a crucial role in the electrical activity of nerve cells by controlling the flow of ions such as sodium, potassium, and calcium. When these channels are not functioning properly, they can cause a wide range of neurological symptoms such as seizures, movement disorders, and cognitive impairment. In this context, the axon initial segment (AIS) is the site of action potential initiation in most neurons. This region is characterized by a high density of voltage-gated sodium channels (VGSCs), which are responsible for the rapid depolarization that occurs when the neuron is stimulated. The AIS is also enriched in other ion channels, such as potassium channels, that play a role in shaping the action potential waveform and determining the firing frequency of the neuron. In addition to ion channels, the AIS contains a complex cytoskeletal structure that helps to anchor the channels in place and regulate their function. Therefore, alterations in this complex structure of ion channels, scaffold proteins, and specialized cytoskeleton may also cause brain channelopathies not necessarily associated with ion channel mutations. This review will focus on how the AISs structure, plasticity, and composition alterations may generate changes in action potentials and neuronal dysfunction leading to brain diseases. AIS function alterations may be the consequence of voltage-gated ion channel mutations, but also may be due to ligand-activated channels and receptors and AIS structural and membrane proteins that support the function of voltage-gated ion channels.
Topics: Humans; Axon Initial Segment; Axons; Channelopathies; Ion Channels; Brain; Seizures
PubMed: 37190119
DOI: 10.3390/cells12081210 -
The Journal of Neuroscience : the... Feb 2018At the base of axons sits a unique compartment called the axon initial segment (AIS). The AIS generates and shapes the action potential before it is propagated along the... (Review)
Review
At the base of axons sits a unique compartment called the axon initial segment (AIS). The AIS generates and shapes the action potential before it is propagated along the axon. Neuronal excitability thus depends crucially on the AIS composition and position, and these adapt to developmental and physiological conditions. The AIS also demarcates the boundary between the somatodendritic and axonal compartments. Recent studies have brought insights into the molecular architecture of the AIS and how it regulates protein trafficking. This Viewpoints article summarizes current knowledge about the AIS and highlights future challenges in understanding this key actor of neuronal physiology.
Topics: Animals; Axon Initial Segment; Humans
PubMed: 29378864
DOI: 10.1523/JNEUROSCI.1922-17.2018