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IScience Mar 2024The axon initial segment (AIS) is located at the proximal axon demarcating the boundary between axonal and somatodendritic compartments. The AIS facilitates the...
The axon initial segment (AIS) is located at the proximal axon demarcating the boundary between axonal and somatodendritic compartments. The AIS facilitates the generation of action potentials and maintenance of neuronal polarity. In this study, we show that the location of AIS assembly, as marked by Ankyrin G, corresponds to the nodal plane of the lowest-order harmonic of the Laplace-Beltrami operator solved over the neuronal shape. This correlation establishes a coupling between location of AIS assembly and neuronal cell morphology. We validate this correlation for neurons with atypical morphology and neurons containing multiple AnkG clusters on distinct neurites, where the nodal plane selects the appropriate axon showing enriched Tau. Based on our findings, we propose that Turing patterning systems are candidates for dynamically governing AIS location. Overall, this study highlights the importance of neuronal cell morphology in determining the precise localization of the AIS within the proximal axon.
PubMed: 38450155
DOI: 10.1016/j.isci.2024.109264 -
The Journal of Neuroscience : the... Apr 2024Dravet syndrome (DS) is a neurodevelopmental disorder characterized by epilepsy, developmental delay/intellectual disability, and features of autism spectrum disorder,...
Dravet syndrome (DS) is a neurodevelopmental disorder characterized by epilepsy, developmental delay/intellectual disability, and features of autism spectrum disorder, caused by heterozygous loss-of-function variants in encoding the voltage-gated sodium channel α subunit Nav1.1. The dominant model of DS pathogenesis is the "interneuron hypothesis," whereby GABAergic interneurons (INs) express and preferentially rely on Nav1.1-containing sodium channels for action potential (AP) generation. This has been shown for three of the major subclasses of cerebral cortex GABAergic INs: those expressing parvalbumin (PV), somatostatin, and vasoactive intestinal peptide. Here, we define the function of a fourth major subclass of INs expressing neuron-derived neurotrophic factor (Ndnf) in male and female DS (+/-) mice. Patch-clamp electrophysiological recordings of Ndnf-INs in brain slices from +/â mice and WT controls reveal normal intrinsic membrane properties, properties of AP generation and repetitive firing, and synaptic transmission across development. Immunohistochemistry shows that Nav1.1 is strongly expressed at the axon initial segment (AIS) of PV-expressing INs but is absent at the Ndnf-IN AIS. In vivo two-photon calcium imaging demonstrates that Ndnf-INs in +/â mice are recruited similarly to WT controls during arousal. These results suggest that Ndnf-INs are the only major IN subclass that does not prominently rely on Nav1.1 for AP generation and thus retain their excitability in DS. The discovery of a major IN subclass with preserved function in the +/â mouse model adds further complexity to the "interneuron hypothesis" and highlights the importance of considering cell-type heterogeneity when investigating mechanisms underlying neurodevelopmental disorders.
Topics: Animals; Interneurons; Epilepsies, Myoclonic; Mice; NAV1.1 Voltage-Gated Sodium Channel; Female; Male; Disease Models, Animal; Action Potentials; Mice, Inbred C57BL; Mice, Transgenic
PubMed: 38443186
DOI: 10.1523/JNEUROSCI.1977-23.2024 -
Scientific Reports Feb 2024The 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...
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 Na1.2 and Na1.6, 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 Ca imaging, we observed that 7 nM AaH-II produces a spurious slow Ca influx via Ca-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.
Topics: Mice; Animals; Action Potentials; Axon Initial Segment; Scorpions; Peptides; Scorpion Venoms; Animals, Poisonous
PubMed: 38424206
DOI: 10.1038/s41598-024-55315-y -
The Korean Journal of Physiology &... Mar 2024The slow and regular pacemaking activity of midbrain dopamine (DA) neurons requires proper spatial organization of the excitable elements between the soma and dendritic...
The slow and regular pacemaking activity of midbrain dopamine (DA) neurons requires proper spatial organization of the excitable elements between the soma and dendritic compartments, but the somatodendritic organization is not clear. Here, we show that the dynamic interaction between the soma and multiple proximal dendritic compartments (PDCs) generates the slow pacemaking activity in DA neurons. In multipolar DA neurons, spontaneous action potentials (sAPs) consistently originate from the axon-bearing dendrite. However, when the axon initial segment was disabled, sAPs emerge randomly from various primary PDCs, indicating that multiple PDCs drive pacemaking. Ca measurements and local stimulation/perturbation experiments suggest that the soma serves as a stably-oscillating inertial compartment, while multiple PDCs exhibit stochastic fluctuations and high excitability. Despite the stochastic and excitable nature of PDCs, their activities are balanced by the large centrally-connected inertial soma, resulting in the slow synchronized pacemaking rhythm. Furthermore, our electrophysiological experiments indicate that the soma and PDCs, with distinct characteristics, play different roles in glutamate- induced burst-pause firing patterns. Excitable PDCs mediate excitatory burst responses to glutamate, while the large inertial soma determines inhibitory pause responses to glutamate. Therefore, we could conclude that this somatodendritic organization serves as a common foundation for both pacemaker activity and evoked firing patterns in midbrain DA neurons.
PubMed: 38414399
DOI: 10.4196/kjpp.2024.28.2.165 -
The Journal of Comparative Neurology Feb 2024Due to its proximity to the axon initial segment (AIS), the paranode of the first myelin segment can influence the threshold for action potentials and how a neuron...
Due to its proximity to the axon initial segment (AIS), the paranode of the first myelin segment can influence the threshold for action potentials and how a neuron participates in a neuronal circuit. Using serial section electron microscopy, we examined its three-dimensional (3D) organization in the ventral horn of the mouse spinal cord. The myelin loops of postnatal day 18 mice resemble those at the node of Ranvier. However, in 3-month-old mice, 13 of 22 para-AIS showed 4 types of alteration: (A) A cytoplasmic foot process, with ultrastructural characteristics of an astrocyte, was interposed between the axolemma and the myelin loops. (B) A thin extension of the inner tongue was present between the foot process and axolemma. (C) The foot process was absent. The inner tongue extension was a broad lamella from which a thin extension reached beyond the loops and spiraled around axon. (D) One set of loops was adjacent to the axon, and another was further back and underlain by compact myelin. We suggest that (A)-(C) are steps in a progression toward (D). In this progression, a glial process displaces the original loops, the inner tongue reactivates and extends beneath the foot process, then wraps around the axon to form a new set of loops. This is the first study of the 3D organization of myelin at the AIS and provides evidence for glia-mediated age-dependent remodeling at this critical region.
Topics: Mice; Animals; Myelin Sheath; Axon Initial Segment; Axons; Neurons; Microscopy, Electron
PubMed: 38411251
DOI: 10.1002/cne.25574 -
BioRxiv : the Preprint Server For... Feb 2024Changes in mitochondrial distribution are a feature of numerous age-related neurodegenerative diseases. In , reducing the activity of Cdk5 causes a neurodegenerative...
Changes in mitochondrial distribution are a feature of numerous age-related neurodegenerative diseases. In , reducing the activity of Cdk5 causes a neurodegenerative phenotype and is known to affect several mitochondrial properties. Therefore, we investigated whether alterations of mitochondrial distribution are involved in Cdk5-associated neurodegeneration. We find that reducing Cdk5 activity does not alter the balance of mitochondrial localization to the somatodendritic vs. axonal neuronal compartments of the mushroom body, the learning and memory center of the brain. We do, however, observe changes in mitochondrial distribution at the axon initial segment (AIS), a neuronal compartment located in the proximal axon involved in neuronal polarization and action potential initiation. Specifically, we observe that mitochondria are partially excluded from the AIS in wild-type neurons, but that this exclusion is lost upon reduction of Cdk5 activity, concomitant with the shrinkage of the AIS domain that is known to occur in this condition. This mitochondrial redistribution into the AIS is not likely due to the shortening of the AIS domain itself but rather due to altered Cdk5 activity. Furthermore, mitochondrial redistribution into the AIS is unlikely to be an early driver of neurodegeneration in the context of reduced Cdk5 activity.
PubMed: 38405730
DOI: 10.1101/2024.02.14.580288 -
Neuron Apr 2024Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder...
Dysfunction in sodium channels and their ankyrin scaffolding partners have both been implicated in neurodevelopmental disorders, including autism spectrum disorder (ASD). In particular, the genes SCN2A, which encodes the sodium channel Na1.2, and ANK2, which encodes ankyrin-B, have strong ASD association. Recent studies indicate that ASD-associated haploinsufficiency in Scn2a impairs dendritic excitability and synaptic function in neocortical pyramidal cells, but how Na1.2 is anchored within dendritic regions is unknown. Here, we show that ankyrin-B is essential for scaffolding Na1.2 to the dendritic membrane of mouse neocortical neurons and that haploinsufficiency of Ank2 phenocopies intrinsic dendritic excitability and synaptic deficits observed in Scn2a conditions. These results establish a direct, convergent link between two major ASD risk genes and reinforce an emerging framework suggesting that neocortical pyramidal cell dendritic dysfunction can contribute to neurodevelopmental disorder pathophysiology.
Topics: Animals; Mice; Ankyrins; Autism Spectrum Disorder; Autistic Disorder; Dendrites; NAV1.2 Voltage-Gated Sodium Channel; Neocortex; Pyramidal Cells
PubMed: 38290518
DOI: 10.1016/j.neuron.2024.01.003 -
The Journal of Cell Biology Apr 2024The compartmentalization of the plasma membrane (PM) is a fundamental feature of cells. The diffusivity of membrane proteins is significantly lower in biological than in...
The compartmentalization of the plasma membrane (PM) is a fundamental feature of cells. The diffusivity of membrane proteins is significantly lower in biological than in artificial membranes. This is likely due to actin filaments, but assays to prove a direct dependence remain elusive. We recently showed that periodic actin rings in the neuronal axon initial segment (AIS) confine membrane protein motion between them. Still, the local enrichment of ion channels offers an alternative explanation. Here we show, using computational modeling, that in contrast to actin rings, ion channels in the AIS cannot mediate confinement. Furthermore, we show, employing a combinatorial approach of single particle tracking and super-resolution microscopy, that actin rings are close to the PM and that they confine membrane proteins in several neuronal cell types. Finally, we show that actin disruption leads to loss of compartmentalization. Taken together, we here develop a system for the investigation of membrane compartmentalization and show that actin rings compartmentalize the PM.
Topics: Actins; Cell Membrane; Ion Channels; Animals; Rats; Neurons; Models, Chemical
PubMed: 38252080
DOI: 10.1083/jcb.202310138 -
Scientific Reports Jan 2024Static magnetic stimulation (SMS) is a form of non-invasive brain stimulation that alters neural activity and induces neural plasticity that outlasts the period of...
Static magnetic stimulation (SMS) is a form of non-invasive brain stimulation that alters neural activity and induces neural plasticity that outlasts the period of stimulation. This can modify corticospinal excitability or motor behaviours, suggesting that SMS may alter the intrinsic excitability of neurons. In mammalian neurons, the axon initial segment (AIS) is the site of action potential initiation and undergoes structural plasticity (changes in length and position from the soma) as a homeostatic mechanism to counteract chronic changes in neuronal activity. We investigated whether the chronic application of SMS (6 and 48 h, 0.5 T) induces structural AIS plasticity in postnatally derived primary cortical neurons. Following 6 h of SMS, we observed a shortening in mean AIS length compared to control, that persisted 24 h post stimulation. In contrast, 48 h of SMS induced an immediate distal shift that persisted 24 h post-stimulation. Pharmacological blockade of voltage gated L/T-type calcium channels during stimulation did not prevent SMS-induced AIS structural plasticity. Our findings provide the foundation to expand the use of chronic SMS as a non-invasive method to promote AIS plasticity.
Topics: Animals; Axon Initial Segment; Axons; Neurons; Action Potentials; Neuronal Plasticity; Calcium Channels; Magnetic Phenomena; Mammals
PubMed: 38233493
DOI: 10.1038/s41598-024-51845-7