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axon initial segment

Ryan B Griggs, Leonid M Yermakov, Keiichiro Susuki
Communication in the central nervous system (CNS) occurs through initiation and propagation of action potentials at excitable domains along axons. Action potentials generated at the axon initial segment (AIS) are regenerated at nodes of Ranvier through the process of saltatory conduction. Proper formation and maintenance of the molecular structure at the AIS and nodes are required for sustaining conduction fidelity. In myelinated CNS axons, paranodal junctions between the axolemma and myelinating oligodendrocytes delineate nodes of Ranvier and regulate the distribution and localization of specialized functional elements, such as voltage-gated sodium channels and mitochondria...
October 4, 2016: Neuroscience Research
David Albrecht, Christian M Winterflood, Mohsen Sadeghi, Thomas Tschager, Frank Noé, Helge Ewers
The axon initial segment (AIS) is enriched in specific adaptor, cytoskeletal, and transmembrane molecules. During AIS establishment, a membrane diffusion barrier is formed between the axonal and somatodendritic domains. Recently, an axonal periodic pattern of actin, spectrin, and ankyrin forming 190-nm-spaced, ring-like structures has been discovered. However, whether this structure is related to the diffusion barrier function is unclear. Here, we performed single-particle tracking time-course experiments on hippocampal neurons during AIS development...
October 10, 2016: Journal of Cell Biology
Yu-Mei Huang, Matthew N Rasband
What prevents the movement of membrane molecules between axonal and somatodendritic domains is unclear. In this issue, Albrecht et. al. (2016. J. Cell Biol. demonstrate via high-speed single-particle tracking and superresolution microscopy that lipid-anchored molecules in the axon initial segment are confined to membrane domains separated by periodically spaced actin rings.
October 10, 2016: Journal of Cell Biology
Katarzyna Dover, Christopher Marra, Sergio Solinas, Marko Popovic, Sathyaa Subramaniyam, Dejan Zecevic, Egidio D'Angelo, Mitchell Goldfarb
Neurons in vertebrate central nervous systems initiate and conduct sodium action potentials in distinct subcellular compartments that differ architecturally and electrically. Here, we report several unanticipated passive and active properties of the cerebellar granule cell's unmyelinated axon. Whereas spike initiation at the axon initial segment relies on sodium channel (Nav)-associated fibroblast growth factor homologous factor (FHF) proteins to delay Nav inactivation, distal axonal Navs show little FHF association or FHF requirement for high-frequency transmission, velocity and waveforms of conducting action potentials...
2016: Nature Communications
Winnie Wefelmeyer, Christopher J Puhl, Juan Burrone
Neurons in the brain are highly plastic, allowing an organism to learn and adapt to its environment. However, this ongoing plasticity is also inherently unstable, potentially leading to aberrant levels of circuit activity. Homeostatic forms of plasticity are thought to provide a means of controlling neuronal activity by avoiding extremes and allowing network stability. Recent work has shown that many of these homeostatic modifications change the structure of subcellular neuronal compartments, ranging from changes to synaptic inputs at both excitatory and inhibitory compartments to modulation of neuronal output through changes at the axon initial segment (AIS) and presynaptic terminals...
October 2016: Trends in Neurosciences
Rinki Saha, Stephanie Knapp, Darpan Chakraborty, Omer Horovitz, Anne Albrecht, Martin Kriebel, Hanoch Kaphzan, Ingrid Ehrlich, Hansjürgen Volkmer, Gal Richter-Levin
Inhibitory synaptic transmission in the amygdala plays a pivotal role in fear learning and its extinction. However, the local circuits formed by GABAergic inhibitory interneurons within the amygdala and their detailed function in shaping these behaviors are not well understood. Here, we employed lentiviral-mediated knockdown of the cell adhesion molecule neurofascin in the basolateral amygdala (BLA) to specifically remove inhibitory synapses at the axon initial segment (AIS) of BLA projection neurons. Quantitative analysis of GABAergic synapse markers and measurement of miniature inhibitory postsynaptic currents in BLA projection neurons after neurofascin knockdown ex vivo confirmed the loss of GABAergic input...
September 16, 2016: Neuropsychopharmacology: Official Publication of the American College of Neuropsychopharmacology
Ludovic Telley, Christelle Cadilhac, Jean-Michel Cioni, Veronique Saywell, Céline Jahannault-Talignani, Rosa E Huettl, Catherine Sarrailh-Faivre, Alexandre Dayer, Andrea B Huber, Fabrice Ango
Subcellular target recognition in the CNS is the culmination of a multiple-step program including axon guidance, target recognition, and synaptogenesis. In cerebellum, basket cells (BCs) innervate the soma and axon initial segment (AIS) of Purkinje cells (PCs) to form the pinceau synapse, but the underlying mechanisms remain incompletely understood. Here, we demonstrate that neuropilin-1 (NRP1), a Semaphorin receptor expressed in BCs, controls both axonal guidance and subcellular target recognition. We show that loss of Semaphorin 3A function or specific deletion of NRP1 in BCs alters the stereotyped organization of BC axon and impairs pinceau synapse formation...
September 21, 2016: Neuron
J J Winters, L L Isom
Voltage-gated Na(+) channels (VGSCs) isolated from mammalian neurons are heterotrimeric complexes containing one pore-forming α subunit and two non-pore-forming β subunits. In excitable cells, VGSCs are responsible for the initiation of action potentials. VGSC β subunits are type I topology glycoproteins, containing an extracellular amino-terminal immunoglobulin (Ig) domain with homology to many neural cell adhesion molecules (CAMs), a single transmembrane segment, and an intracellular carboxyl-terminal domain...
2016: Current Topics in Membranes
Miguel A Marin, Silmara de Lima, Hui-Ya Gilbert, Roman J Giger, Larry Benowitz, Matthew N Rasband
UNLABELLED: Action potential initiation and propagation in myelinated axons require ion channel clustering at axon initial segments (AIS) and nodes of Ranvier. Disruption of these domains after injury impairs nervous system function. Traditionally, injured CNS axons are considered refractory to regeneration, but some recent approaches challenge this view by showing robust long-distance regeneration. However, whether these approaches allow remyelination and promote the reestablishment of AIS and nodes of Ranvier is unknown...
August 31, 2016: Journal of Neuroscience: the Official Journal of the Society for Neuroscience
Katalin Kerti-Szigeti, Zoltan Nusser
Hippocampal pyramidal cells (PCs) express many GABAAR subunit types and receive GABAergic inputs from distinct interneurons. Previous experiments revealed input-specific differences in α1 and α2 subunit densities in perisomatic synapses, suggesting distinct IPSC decay kinetics. However, IPSC decays evoked by axo-axonic, parvalbumin- or cholecystokinin-expressing basket cells were found to be similar. Using replica immunogold labeling, here we show that all CA1 PC somatic and AIS synapses contain the α1, α2, β1, β2, β3 and γ2 subunits...
2016: ELife
Joseph Lombardo, Melissa A Harrington
KCNQ/Kv7 channels form a slow non-inactivating K(+) current, also known as the M-current. They activate in the sub-threshold range of membrane potentials and regulate different aspects of excitability in neurons of the central nervous system. In spinal motoneurons (MNs), KCNQ/Kv7 channels have been identified in the somata, axonal initial segment (AIS) and nodes of Ranvier where they generate a slow, non-inactivating, K+ current sensitive to both muscarinic receptor-mediated inhibition and KCNQ/Kv7 channel blockers...
August 10, 2016: Journal of Neurophysiology
Milos Radivojevic, David Jäckel, Michael Altermatt, Jan Müller, Vijay Viswam, Andreas Hierlemann, Douglas J Bakkum
A detailed, high-spatiotemporal-resolution characterization of neuronal responses to local electrical fields and the capability of precise extracellular microstimulation of selected neurons are pivotal for studying and manipulating neuronal activity and circuits in networks and for developing neural prosthetics. Here, we studied cultured neocortical neurons by using high-density microelectrode arrays and optical imaging, complemented by the patch-clamp technique, and with the aim to correlate morphological and electrical features of neuronal compartments with their responsiveness to extracellular stimulation...
2016: Scientific Reports
Steven L Jones, Tatyana M Svitkina
The axon initial segment (AIS) is a specialized structure in neurons that resides in between axonal and somatodendritic domains. The localization of the AIS in neurons is ideal for its two major functions: it serves as the site of action potential firing and helps to maintain neuron polarity. It has become increasingly clear that the AIS cytoskeleton is fundamental to AIS functions. In this review, we discuss current understanding of the AIS cytoskeleton with particular interest in its unique architecture and role in maintenance of neuron polarity...
2016: Neural Plasticity
Masoumeh Nozari, Toshimitsu Suzuki, Marcello G P Rosa, Kazuhiro Yamakawa, Nafiseh Atapour
Plasticity of the axon initial segment (AIS) is a newly discovered type of structural plasticity that regulates cell excitability. AIS plasticity has been reported to happen during normal development of neocortex and also in a few pathological conditions involving disruption of the inhibition/excitation balance. Here we report on the impact of early environmental interventions on structural plasticity of AIS in the mouse neocortex. C57BL/6 mice were raised in standard or enriched environment (EE) from birth up to the time of experiments and were injected with saline or MK-801 [N-Methyl-D-Aspartate (NMDA) receptor antagonist, 1 mg/kg] on postnatal days (P) 6-10...
July 30, 2016: Developmental Psychobiology
Peter J Chung, Chaeyeon Song, Joanna Deek, Herbert P Miller, Youli Li, Myung Chul Choi, Leslie Wilson, Stuart C Feinstein, Cyrus R Safinya
Tau, an intrinsically disordered protein confined to neuronal axons, binds to and regulates microtubule dynamics. Although there have been observations of string-like microtubule fascicles in the axon initial segment (AIS) and hexagonal bundles in neurite-like processes in non-neuronal cells overexpressing Tau, cell-free reconstitutions have not replicated either geometry. Here we map out the energy landscape of Tau-mediated, GTP-dependent 'active' microtubule bundles at 37 °C, as revealed by synchrotron SAXS and TEM...
2016: Nature Communications
Sungchil Yang, Roy Ben-Shalom, Misol Ahn, Alayna T Liptak, Richard M van Rijn, Jennifer L Whistler, Kevin J Bender
G-protein-coupled receptors (GPCRs) initiate a variety of signaling cascades, depending on effector coupling. β-arrestins, which were initially characterized by their ability to "arrest" GPCR signaling by uncoupling receptor and G protein, have recently emerged as important signaling effectors for GPCRs. β-arrestins engage signaling pathways that are distinct from those mediated by G protein. As such, arrestin-dependent signaling can play a unique role in regulating cell function, but whether neuromodulatory GPCRs utilize β-arrestin-dependent signaling to regulate neuronal excitability remains unclear...
August 9, 2016: Cell Reports
Bryan S Barker, Matteo Ottolini, Jacy L Wagnon, Rachel M Hollander, Miriam H Meisler, Manoj K Patel
OBJECTIVE: SCN8A encephalopathy (early infantile epileptic encephalopathy; EIEE13) is caused by gain-of-function mutations resulting in hyperactivity of the voltage-gated sodium channel Nav 1.6. The channel is concentrated at the axon initial segment (AIS) and is involved in establishing neuronal excitability. Clinical features of SCN8A encephalopathy include seizure onset between 0 and 18 months of age, intellectual disability, and developmental delay. Seizures are often refractory to treatment with standard antiepileptic drugs, and sudden unexpected death in epilepsy (SUDEP) has been reported in approximately 10% of patients...
September 2016: Epilepsia
Peter Dongmin Sohn, Tara E Tracy, Hye-In Son, Yungui Zhou, Renata E P Leite, Bruce L Miller, William W Seeley, Lea T Grinberg, Li Gan
BACKGROUND: Neurons are highly polarized cells in which asymmetric axonal-dendritic distribution of proteins is crucial for neuronal function. Loss of polarized distribution of the axonal protein tau is an early sign of Alzheimer's disease (AD) and other neurodegenerative disorders. The cytoskeletal network in the axon initial segment (AIS) forms a barrier between the axon and the somatodentritic compartment, contributing to axonal retention of tau. Although perturbation of the AIS cytoskeleton has been implicated in neurological disorders, the molecular triggers and functional consequence of AIS perturbation are incompletely understood...
2016: Molecular Neurodegeneration
Carola Städele, Wolfgang Stein
UNLABELLED: Essential to understanding the process of neuronal signal integration is the knowledge of where within a neuron action potentials (APs) are generated. Recent studies support the idea that the precise location where APs are initiated and the properties of spike initiation zones define the cell's information processing capabilities. Notably, the location of spike initiation can be modified homeostatically within neurons to adjust neuronal activity. Here we show that this potential mechanism for neuronal plasticity can also be exploited in a rapid and dynamic fashion...
June 22, 2016: Journal of Neuroscience: the Official Journal of the Society for Neuroscience
Cyril Gitiaux, Nicole Chemaly, Susana Quijano-Roy, Christine Barnerias, Isabelle Desguerre, Marie Hully, Catherine Chiron, Olivier Dulac, Rima Nabbout
OBJECTIVE: Since SCN1A is expressed in the motor neuron initial segment, we explored whether motor neuron dysfunction could contribute to gait disturbance and orthopedic misalignment in patients with Dravet syndrome due to SCN1A mutations. METHODS: We assessed 12 consecutive patients who presented to our institution between January and March 2013. All of them were older than 2 years and were positive for the SCN1A mutation. We performed nerve conduction velocity studies and needle EMG recordings...
July 19, 2016: Neurology
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