Abstract
Myelin is a dynamic membrane that is important for coordinating the fast propagation of action potentials along small or large caliber axons (0.1–10 μm) some of which extend the entire length of the spinal cord. Due to the heterogeneity of electrical and energy demands of the variable neuronal populations, the axo-myelinic and axo-glial interactions that regulate the biophysical properties of myelinated axons also vary in terms of molecular interactions at the membrane interfaces. An important topic of debate in neuroscience is how myelin is maintained and modified under neuronal control and how disruption of this control (due to disease or injury) can initiate and/or propagate neurodegeneration. One of the key molecular signaling cascades that have been investigated in the context of neural injury over the past two decades involves the myelin-associated inhibitory factors (MAIFs) that interact with Nogo receptor 1 (NgR1). Chief among the MAIF superfamily of molecules is a reticulon family protein, Nogo-A, that is established as a potent inhibitor of neurite sprouting and axon regeneration. However, an understated role for NgR1 is its ability to control axo-myelin interactions and Nogo-A specific ligand binding. These interactions may occur at axo-dendritic and axo-glial synapses regulating their functional and dynamic membrane domains. The current review provides a comprehensive analysis of how neuronal NgR1 can regulate myelin thickness and plasticity under normal and disease conditions. Specifically, we discuss how NgR1 plays an important role in regulating paranodal and juxtaparanodal domains through specific signal transduction cascades that are important for microdomain molecular architecture and action potential propagation. Potential therapeutics designed to target NgR1-dependent signaling during disease are being developed in animal models since interference with the involvement of the receptor may facilitate neurological recovery. Hence, the regulatory role played by NgR1 in the axo-myelinic interface is an important research field of clinical significance that requires comprehensive investigation.
Introduction
Central nervous system (CNS) myelination is a developmentally regulated process governed by key molecular events that integrate dynamic changes at axonal and oligodendroglial cell membranes. Myelination ensures efficient propagation of action potentials along axons. CNS myelin-forming oligodendrocytes initially contact axons that they subsequently may ensheath depending on their electrical activity (Foster et al., ). Myelinated axons of the adult CNS demonstrate a substantial degree of plasticity at the axo-glial and axo-myelinic membrane contacts, that are now known to be dynamically modified according to patterns of neural activity (Mitew et al., ; Hughes and Appel, ). These specific axo-myelinic contacts are coordinated by the myelin-associated glycoprotein (MAG) in cooperation with contactin 1 and Caspr paranodal adhesion molecules to structure the integral myelin domains (Djannatian et al., ). Modifications of axonal, and oligodendroglial membranes are regulated by these integral adhesion molecules, being arranged according to structural domains for appropriate morphometry, that are required for axonal propagation through saltatory conduction. A new hypothesis that may shed light on the molecular organization of the structural subdomains of the CNS axo-myelinic unit, is derived from evidence that the Reticulon 4 receptor (RTN4R), known as the Nogo-66 receptor (NgR1), can modify the integral paranodal protein, Caspr, preventing its cleavage and turnover (Lee et al., ). This molecular inhibition of Caspr cleavage may ensure the tight segregation of key voltage-gated ion channels, preventing their lateral diffusion from the node of Ranvier and the juxtaparanode through the barrier established by the septate-like junctions at the paranodal domains.
During disease and trauma, NgR1 has been reported to be upregulated in neurons that exhibit axonal transection or are undergoing degeneration, thereby governing neurite outgrowth inhibition in an extracellular milieu rich in myelin-associated inhibitory factors (MAIFs) or astroglial-derived chondroitin sulfate proteoglycans (CSPGs; for review see Petratos et al., ; Lee and Petratos, ). During neuroinflammation, NgR1 is also increased in neurons that may potentiate axonal degeneration through downstream signaling that can destabilize or disassemble the axonal cytoskeleton following Nogo-A-dependent ligation (Petratos et al., ; Lee et al., ). Of functional importance, is that during the development of the CNS visual system, NgR1 is strongly expressed in parvalbumin-positive interneurons to restrict the critical period of ocular dominance plasticity based on a disinhibitory microcircuit (Stephany et al., ). This raises the possibility that the expression of NgR1 throughout the neuronal soma, dendrites, and axons can be variable and inducible to control activity-dependent plasticity that may include axo-glial synapse-like structures. With recent evidence implicating that NgR1 can coordinate plasticity and memory formation in specific cortical regions (Karlsson et al., ) and the evidence for myelin plasticity to be center stage of human learning and cognition (Sampaio-Baptista et al., ), the investigative path to NgR1-dependent myelin plasticity (see Figure 1) is an integral open question in neurobiology that requires elucidation due to the numerous therapeutic strategies being developed for individuals with mainly acquired neurological diseases.
Figure 1
These questions can only be addressed when we investigate the molecular dynamics of the axo-glial and axo-myelinic interfaces during development and disease. From an ultrastructural vantage point, key subdomains are present at paranodal regions that flank the nodes of Ranvier, enriched in voltage-gated sodium channels (Caldwell et al.,
Figure 2

Unaltered ion channel distribution in the axo-myelinic junction of the CNS of ngr1−/− mice. Schematic representation of the proposed ion channel distribution in the CNS myelinated fibers of wild-type, ngr1−/− and caspr−/− mice. In caspr−/−, juxtaparanodal Kv channels are displaced and distributed throughout the paranodal region, whereas in ngr1−/−, although lengthening of paranodal Caspr was found, ion channel distribution could not be identified (Lee et al.,
Evidence gathered from auditory processing fibers within the brainstem has shown that elevated CVs occur in the large diameter myelinated axons that respond to low-frequency sound waves, with reduced internodal distances (Ford et al.,
Nogo Receptor 1 (Ngr1) Regulates Neuronal Morphology and Synaptic Plasticity
The NgR1 is a high-affinity receptor for the Nogo-66 extracellular C-terminus domain of Nogo-A, an integral oligodendroglial and myelin membrane protein. NgR1 regulates the experience-dependent turnover of dendritic spines and limits synaptic plasticity in the cortical gray matter (Akbik et al.,
Figure 3

Altered neuronal architecture within the frontal cortex and altered axonal and myelin architecture in the spinal cord of ngr1−/− mice. For detailed methods please see the Supplementary Information. (A) Illustration of the M1 frontal motor cortex region from which samples were imaged. (B–J) Semi-thin sections from the M1 regions of ngr1+/+ and ngr1−/− mice revealed a different neuronal morphology. (B,C) Light microscopic images of the M1 cortical layer from layer I to V in ngr1+/+ and ngr1−/− mice (scale bar = 50 μm). (D) Measurements of the molecular layer width; layer I, was significantly shorter in ngr1−/− compared to ngr1+/+ mice. (E–I) Representative high magnification images of pyramidal neurons from (E,F) Layer II/III (H,I) and layer V. Both (G) area and (J) length measurements obtained for the apical dendrites of neurons within the cortical layers II-V were found to be increased in ngr1−/− compared to ngr1+/+ mice. (K) Illustration of the dorsolateral white matter region of the spinal cord from which samples were imaged. Representative image showing how internodal length and axonal diameters measured is shown below. (K–Q) Toluidine blue-stained semi-thin (1 μm) dorsolateral white matter sections of lumbosacral (LSSC) and thoracic-cervical (TCSC) spinal cords (red rectangle region) of both adult ngr1+/+ and ngr1−/− mice (scale bars = 50 μm). (P) Internodes of ngr1−/− were significantly longer in both LSSC and TCSC when compared with ngr1+/+. (Q) Axonal diameters of ngr1−/− mice were significantly smaller in both LSSC and TCSC when compared with ngr1+/+ mice. (R,S) Ultra-thin (100 nm) electron micrograph longitudinal sections of LSSC from adult (R)ngr1+/+ and (S)ngr1−/− mice showing normal ultra-structure of neurofilaments in both genotypes (scale bar = 100 nm). (T) Quantification of nearest neighbor distances between neurofilaments in myelinated axons of descending fiber tracts in ngr1+/+ and ngr1−/− LSSC (****P < 0.0001, n = 8 for both genotypes).
We know that NgR1 may be important in ocular dominance plasticity within the visual cortex of naïve mice (McGee et al.,
An alternate hypothesis that may explain the morphometric variability in the cortical architecture observed in our ngr1−/− mice may involve the level and or dynamics of intracortical myelination, regulating dendritic arborization and hence maturation. It has been demonstrated that the cognate high-affinity ligand for NgR1, Nogo-A, is not only localized to oligodendroglial plasma membranes but has been shown that its neuronal expression to be specifically observed during development and can limit dendritogenesis (Petrinovic et al.,
Ngr1 Regulates Myelinated Fiber Structure and Function
We have recently performed systematic ultrastructural analyses of the dorsolateral spinal cord white matter tracts in the thoracic and lumbosacral segments of the ngr1−/− mice that have revealed longer mean internodal lengths when compared to wildtype littermate controls. Mean axon diameters were also reduced in the ngr1−/− compared to the wild type littermate controls (Figure 3). Along with the observed reduced axonal caliber with commensurate thinner myelin exhibited by mice mutant for the ngr1 allele, we also demonstrated that these mice had increased numbers of thin fibers in spinal cord fascicles (Lee et al.,
Figure 4

Altered paw angle and rate of deceleration during the braking phase in naïve ngr1−/− mice. For detailed methods please see the Supplementary Information. (A) Absolute paw angle is the angle that the paw makes with the long axis of the direction of motion (midline). Higher absolute paw angles represent greater degrees of external rotation. (B) The forepaws of naïve NgR1-KO (ngr1−/−) mice are less externally rotated than naïve wildtype mice (ngr1+/+), while NgR1-KO hind paws are more externally rotated than WT mice. (C) The maximal rate of change of the paw area (MAX dA/dT) provides a measure of how rapidly the animal decelerates during the braking phase. An increased MAX dA/dT represents a greater change in paw area over the same amount of time; therefore, compared to its littermate (i), the animal is placing its paw down faster (ii; modified from Vincelette et al.,
Our gait analysis of naïve wildtype and ngr1−/− animals has identified altered paw angles and the rate of deceleration during the braking phase of the gait cycle. Forepaws of naïve ngr1−/− mice were more internally rotated (narrower absolute paw angles), and their hind paws more externally rotated (wider absolute paw angles) than naïve wild-type mice (Figure 4). These observations suggest ngr1−/− mice may have difficulty in balancing themselves during a stance. Another feature of abnormal gait in the ngr1−/− mouse is a significant increase in the maximal rate of change of paw area (MAX dA/dT), a measure of how rapidly the animal decelerates during the braking phase. These observations need to be further investigated to elucidate the precise neurophysiological role/s of NgR1 in gait regulation, and further dissect how changes in axon and myelin properties observed in the ngr1−/− mice can potentiate such neurobehavioral outcomes.
Neuronal Ngr1 and the Axo-Myelin Interface
So how can NgR1, a high-affinity pleiotropic receptor that inhibits neurite outgrowth, regulate the plasticity of the axo-glial unit? The dynamics of integral adhesion proteins regulating axo-myelinic membrane interactions are related to the rate of activity in axonal conduction (Mensch et al.,
We have previously demonstrated NgR1 to be a key regulator of the distribution of an integral paranodal protein, Caspr, along with the intramembranous cleavage of the paranodal protein at the junction (Lee et al.,
The presence of Caspr at the paranodal junction is a fundamental factor regulating subdomains of the nodes (in coordination with Neurofascins, the nodal and paranodal cytoskeletal scaffolds, the nodal extracellular matrix, along with myelin membrane-bound lipids and glycolipids), that the segregation of sodium and potassium channels that is necessary for the propagation of action potentials at nodes of Ranvier (Bhat et al.,
Figure 5

Chronic active MS lesions show Caspr re-distribution, increased NgR1 and axonal damage. For detailed methods please see the Supplementary Information. (A) Representative LFB-PAS stained images obtained from progressive MS patient brain tissues. From these, we selected periplaque (PP) and plaque (P) regions to further study Caspr distribution in these areas (scale bar = 500 μm). (B) In MS-PP, the co-localization of Caspr with juxtaparanodal Kv1.2 was observed. In MS-P, a significant elongation of Caspr+ segments and disrupted localization of Kv1.2 was observed (scale bar = 10 μm). (C) The ratio between the measured length of Caspr+ segments vs. their diameter was significantly increased in MS-PP and MS-P, compared with non-neurological disease control (NNDC) samples. (D) Immunostaining of SMI-32, Caspr, and DAPI in serial section showed a re-distribution of Caspr along the whole internode in SMI-32 (+) degenerative axons within the lesion border (white arrowheads indicate diffuse expression of Caspr along the internode; scale bar = 10 μm; one-way ANOVA with post hoc Tukey’s test, ****P < 0.0001, n = 4 for each patient samples). (E) Western blot for Caspr and (F) α-tubulin-loading control performed on brain white matter lysates of NNDC, Alzheimer’s disease (AD), frontotemporal dementia (FTD), Huntington’s disease (HD), and MS patients. (G) Densitometric quantification of full-length Caspr (FL-Caspr), (H) 64kDa degradation product of Caspr and (I) 45 kDa Caspr degradation product normalized by α-tubulin loading control (one-way ANOVA post hoc Tukey’s test, *P < 0.05, **P < 0.01, ***P < 0.001, n = 4 for each patient samples). (J) Immunoprecipitation of Caspr and probed with anti-PrPC. Western immunoblot for PrPC from 5% input of pre-immunoprecipitation sample shown on the bottom (K) Densitometric quantification of Caspr bound PrPC and (L) total di-PrPC. (M) Western blot for Reelin. (N) Densitometric quantification of 140 kDa Reelin.
Conclusion
How NgR1 regulates the distribution of Caspr in a tightly orchestrated paranodal interaction with its glial membrane proteins that are integral to the synchrony of axonal myelin physiology is an important question. Future research should involve investigations into CNS remyelination in adult ngr1−/− mice where specific demyelinating lesions are observed during the repair. The cuprizone-mediated experimental demyelination model would be ideal to assess the initiation of paranode and internode formation in the adult CNS, without the influence of neuronal NgR utilizing Cre-deleted NgR1-floxed transgenic mice. Moreover, this model has the added advantage of having no invading autoreactive adaptive peripheral immune cells impacting CNS demyelination/remyelination (as can occur in models such as EAE) and axonopathy that can differentiate the role of NgR1 in myelin turnover during remyelination compared to the axonal degeneration occurring in MOG-induced models. Elucidating the precise coupling of NgR1-dependent neuronal activity with the molecular restructuring of the node of Ranvier and paranodal myelin is a critical line of investigation in neuroscience that will drive the development of future regenerative therapeutic interventions that target the Nogo-A/NgR1 cell signaling mechanism during neurodegenerative diseases governed by the sequelae of inflammation.
Statements
Ethics statement
The AMREP Animal Ethics Committee (AEC nos. E/1532/2015/M and E/1602/2015/M) reviewed and approved the use of these animals for experimentation in this study, in accordance with the guidelines and regulations set out by the National Health and Medical Research Council of Australia. All animal experiments are governed by the Australian Code for the care and use of animals for scientific purposes (2013) and comply with the Victorian Cruelty to Animals Act 1986. All frozen human deep-cortical white matter tissues used for this study were acquired from the Victorian Brain Bank Network (VBBN) under the National Health and Medical Research Council guidelines and the Monash University Human Research Ethics Committee approval number CF13/1646-2013000831.
Author contributions
SP wrote the manuscript. PT, MK, MA and JL edited the manuscript and generated data.
Funding
JL was supported by Multiple Sclerosis Research Australia and Trish Multiple Sclerosis Research Foundation Postgraduate Scholarship. SP was supported by National Multiple Sclerosis Society Project Grant #RG4398A1/1, International Progressive Multiple Sclerosis Alliance Challenge Award #PA0065, Multiple Sclerosis Research Australia and Trish Multiple Sclerosis Research Foundation #15-022 and Bethlehem Griffiths Research Foundation #BGRF1706.
Conflict of interest
JL is currently employed by the company ToolGen Inc. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fncel.2020.00227/full#supplementary-material.
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Summary
Keywords
nogo receptor, Nogo-A, Caspr, paranode, myelin, PrPc, reelin
Citation
Petratos S, Theotokis P, Kim MJ, Azari MF and Lee JY (2020) That’s a Wrap! Molecular Drivers Governing Neuronal Nogo Receptor-Dependent Myelin Plasticity and Integrity. Front. Cell. Neurosci. 14:227. doi: 10.3389/fncel.2020.00227
Received
07 March 2020
Accepted
29 June 2020
Published
04 August 2020
Volume
14 - 2020
Edited by
Domna Karagogeos, University of Crete, Greece
Reviewed by
Catherine Faivre-Sarrailh, Aix Marseille Université-INSERM UMR1249, France; Jeffrey Dupree, Virginia Commonwealth University, United States
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Copyright
© 2020 Petratos, Theotokis, Kim, Azari and Lee.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Steven Petratos steven.petratos@monash.edu
Specialty section: This article was submitted to Non-Neuronal Cells, a section of the journal Frontiers in Cellular Neuroscience
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