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Mechanisms of Node of Ranvier Assembly

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Mechanisms of Node of Ranvier Assembly REVIEWS Mechanisms of node of Ranvier assembly Matthew N. Rasband 1 ✉ and Elior Peles 2 ✉ Abstract | The nodes of Ranvier have clustered Na+ and K+ channels necessary for rapid and efficient axonal action potential conduction. However, detailed mechanisms of channel clustering have only recently been identified: they include two independent axon–glia interactions that converge on distinct axonal cytoskeletons. Here, we discuss how glial cell adhesion molecules and the extracellular matrix molecules that bind them assemble combinations of ankyrins, spectrins and other cytoskeletal scaffolding proteins, which cluster ion channels. We present a detailed molecular model, incorporating these overlapping mechanisms, to explain how the nodes of Ranvier are assembled in both the peripheral and central nervous systems. Axolemma In the early 1800s, engineers dreamed of communication mechanisms for their assembly have been revealed. In The plasma membrane of between North America and Europe using a transatlan- this Review, we emphasize and discuss nodal proteins the axon. tic telegraph cable. Their vision was realized in 1858 in the context of the multiple overlapping axon–glia and introduced a new ‘Age of Information'1. Although interactions that combine and converge on the axonal Extracellular matrix (ECM). A complex mixture of this earliest cable only transmitted a few words per cytoskeleton to efficiently cluster and maintain high + + extracellular macromolecules, hour, it still reduced the time required for the transmis- densities of Na and K channels at the nodes of Ranvier. including glycoproteins, that sion of a message by more than tenfold. Today, mod- surround cells. ern fibre- optic cable systems have further reduced this Myelinating glia control node assembly time to just milliseconds! However, these technological Myelination is a late developmental process mediated wonders lag ~400 million years behind the revolution by Schwann cells in the peripheral nervous system in speed of axonal action potential conduction that (PNS) and oligodendrocytes in the CNS. For example, occurred with the evolution of myelin and clustered ion in mice, PNS myelination begins at birth and is mostly channels at the nodes of Ranvier (Fig. 1). In addition to complete within 2 weeks, although sheath length con- speed, myelin and clustered ion channels also dramati- tinues to increase as the animal grows. CNS myelination cally reduce both the space requirements and metabolic is more protracted, with significant regional variation. costs of rapid action potential conduction. These evolu- Myelination begins at birth in the spinal cord and at tionary advances permitted the subsequent development 1 week of age in the optic nerve, being mostly complete of complex nervous systems2,3. by about 1 month of age. Ion channel clustering at the The organization of our nervous system is similar to nodes of Ranvier follows and requires myelination, sug- that of our modern transatlantic cable networks. Because gesting a causal connection. Together, myelination and signals degrade as they propagate along a cable, repeat- the associated clustering of ion channels results in a pro- ers are used to regenerate the signals and increase the found transition in the mechanism of action potential transmission range. In myelinated axons, the nodes of propagation from continuous to saltatory conduction. Ranvier function as repeaters to regenerate the action Because of this dramatic physiological change and its potential as it propagates in a saltatory manner along importance for the developing, healthy and diseased the axon to the nerve terminal, thereby dramatically brain, it is important to understand how myelinating 4,5 1Department of Neuroscience, increasing the speed of action potential propagation . glia regulate nodal ion channel clustering. Baylor College of Medicine, A prerequisite for the function of the nodes in saltatory The concept that Schwann cells and oligodendrocytes Houston, TX, USA. conduction is the high density clustering of Na+ chan- cluster nodal ion channels was originally proposed by 2Department of Molecular nels6,7 and K+ channels8–12 at the nodal axolemma (Fig. 1b). Rosenbluth13,14 and relied on his detailed descriptions of Cell Biology, The Weizmann In addition, the nodes of Ranvier, as well as their flank- myelinated axons analysed using freeze-fracture electron Institute of Science, Rehovot, Israel. ing paranodal junctions, are comprised of unique cell microscopy. Later developmental studies using antibod- + ✉e- mail: [email protected]; adhesion molecules (CAMs), cytoskeletal scaffolds and ies to detect the earliest clusters of Na channels showed [email protected] extracellular matrix (ECM) molecules (Table 1). As these that they accumulate adjacent to the edges of the form- 15–17 6,18 https://doi.org/10.1038/ various molecular components of the nodal environs ing myelin sheath in both the PNS and the CNS . s41583-020-00406-8 have come into focus (Fig. 1b,c), the specific molecular Further support that oligodendrocyte and myelinating NATURE REVIEWS | NEUROSCIENCE REVIEWS a Nerve Node of Ranvier terminal AIS Neuron cell body Myelin Axon Internode b Sciatic nerve Optic nerve Perinodal Myelin PNL glial cell JXP PNJ NOR NaCh Caspr Kv1.2 BL Microvilli c Node of Ranvier Paranodal junction PNS Schwann cell microvilli CNS PNL • β2–α2 Spectrin • ERM Perinodal process • AnkB (PNS) • EBP50 • AnkG (CNS) Syn3/Syn4 • DP116 • 4.1G (PNS) Neurocan Hyaluronan Gliomedin βDG TenR M6B αDG Versican MAG NrCAM Bral1 NF155 Perlecan NF186 Brevican NrCAM Caspr NF186 • TRAAK NrCAM Contactin • TREK1 • Kv7.2/Kv7.3 Contactin • Kv3.1b NaCh Axolemma β α β Axon MBD S C AnkyrinG/AnkyrinR 4.1B • 4– 2 Spectrin • 4– 2 Spectrin β α β α β2–α2 Spectrin • β1–α2 Spectrin • β1–α2 Spectrin Schwann cell contact control the assembly of the nodes they eventually become nodes of Ranvier remain of Ranvier was obtained from studies demonstrating unclear27. Contact- mediated mechanisms depend on that channel clustering was largely impaired upon the specific axoglial interactions that occur at the forming depletion of oligodendrocytes and myelinating Schwann nodes of Ranvier (that is, ‘nodal mechanism’) and at the cells19,20 or after demyelination21–23. While these stud- paranodal axoglial junction that border the nodes (that ies strongly support the notion that axoglial contact is, ‘paranodal mechanism’). The latter is present in both clusters Na+ channels, a second model proposed that the CNS and the PNS and is formed between the axon oligodendrocyte- secreted factors may initiate chan- and the terminal cytoplasmic loops of oligodendrocytes Microvilli nel clustering along axons24. Recently, two studies25,26 or myelinating Schwann cells28. In contrast to the para- Small membrane protrusions reported that soluble, oligodendrocyte- derived factors nodal junction, axoglial contact at the nodes differs that increase the surface area + of a cell to facilitate adhesion, promote the formation of axonal Na channel clusters between the PNS and the CNS. In the PNS, the nodal absorption, or signal prior to myelination but only in GABAergic neurons; axolemma is contacted by microvilli processes that are transduction. however, the prevalence of these clusters and whether formed by the outer aspect of the adjacent myelinating www.nature.com/nrn REVIEWS ◀ Fig. 1 | Organization of the nodal environ. a | Myelin formed by Schwann cells or that mice with a disrupted Nfasc gene fail to cluster oligodendrocytes (not shown) covers the axon in segments (internodes) separated by AnkG or Na+ channels40. However, this conclusion the nodes of Ranvier. The nodes of Ranvier, which are flanked by the paranodal junction was complicated by the recognition that the two major (green) as well as by the a xo n initial segment (AIS) contain a high density of Na+ channels Nfasc gene products are both found at or near the nodes (red). b | S c h e m atic o r g a n i z ation o f a m y e li n a ted a x o n a t t h e n od e o f Ranvier (NOR) in the peripheral nervous system (PNS; sciatic nerve) and the CNS (optic nerve). The NOR, of Ranvier: NF186 in axons and NF155 at paranodes of paranodal junction (PNJ; green) and the juxtaparanodal region (JXP; blue) are labelled. myelinating Schwann cells and oligodendrocytes. The node is contacted by Schwann cell microvilli in the PNS or by processes from Paranodal NF155 is an obligate component of the par- perinodal glia in the CNS. The PNJ is formed between the paranodal loops (PNL) of the anodal axoglial junction41,42. Thus, using a constitutive myelin and the axon. This junction separates the NOR from the JXP, which is enriched in Nfasc- deficient mouse it was not possible to distinguish Kv1 channels. In the PNS, myelinated fibres are covered by a basal lamina (BL). Immuno- the role of axonal NF186 in node formation from that labelling of sciatic and optic nerves using antibodies to nodal (Na+ channels; NaCh), of glial paranodal NF155. Recently, pathogenic human paranodal (Caspr, a paranodal marker protein) and juxtaparanodal (K+ channels; Kv1.2) variants of Nfasc and its binding partners have been components are shown on the right (scale bars 10 μm). c | Molecular composition of reported, with profound consequences for nervous nodes and the paranodal junction. The nodal axolemma is enriched in both Na+ (REF.7) system function (BOx 1). and K+ channels (TRAAK, TREK-1, Kv7.2, Kv7.3 and Kv3.1b)8,9,11,12. These channels are splice associated with neurofascin 186 (NF186) and a secreted form of NrCAM as well as with To determine the contributions of each Nfasc variant 43 ankyrin G (AnkG), ankyrin R (AnkR), β4, β1 and α2 spectrin (AnkR and β1 spectrin are , Zonta et al. used a transgenic rescue strategy located at the nodes during node assembly). In the PNS, the nodal gap contains Schwann and expressed NF186 or NF155 in neurons or myeli- cell microvilli that are enriched in several transmembrane proteins, including a complex nating glia of Nfasc knockout mice, respectively.
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