Oligodendrocytes, Schwann Cells and Myelination
8.2 Myelination
system tauopathy, and oligodendroglial degeneration is the histological hallmark of multiple system atrophy (MSA).
Other components of the cytoskeleton are also essential for process outgrowth and myelin formation. Disruption of the actin cytoskeleton prevents spiralling of the myelinating process in Schwann cells. Rho kinase (ROCK) is a key regu- lator of myelinating process extension in Schwann cells and oligodendrocytes.
ROCK phosphorylates myosin and is an effector of Rho, which regulates the actin cytoskeleton. ROCK signalling regulates the 1:1 relationship between Schwann cells and axons, and disruption of ROCK results in Schwann cells myelinating multiple internodes. In oligodendrocytes, activation of ROCK results in myelin process retraction involving S1P5 and Fyn. By contrast, activation of Cdc42 by Fyn causes process extension.
8.2 Myelination
8.2.1 Functional development of oligodendrocytes
During development, both oligodendrocytes and Schwann cells pass through a number of physiologically distinct stages that can be identified by the expression of stage-specific antigens and are characterized by marked changes in prolif- eration, migration, morphology and function. Oligodendrocyte progenitor cells (OPCs) develop in highly localized ventricular zones in the brain and spinal cord, by a process that depends on the transcription factors Olig1, Olig2, SOX10 and Nkx2.2, and the signalling molecules sonic hedgehog (Shh), bone morphogenic protein (BMP), and Notch. These transcription factors regulate key stages of OPC development and oligodendrocyte specification; Olig2 and Nkx2.2 are expressed by NG2-glia and have a continued role in the generation of oligodendrocytes throughout life. OPC migrate to their final sites in the brain and spinal cord, where they undergo local proliferation and differentiation, or apoptosis, in response to diffusible growth factors, interactions with the extracellular matrix, and diffusible and membrane-bound signals from axons and glia. OPCs can be identified by their expression of platelet-derived growth factor alpha receptors (PDGFR), the GD3 ganglioside, and the NG2 CSPG. As OPCs differentiate they gain expression of the sulphatides recognized by the O4 antibody and develop into cells variously called pro-oligodendrocytes, pre-oligodendrocytes, or promyelinating oligodendrocytes.
These ‘promyelinating’ oligodendrocytes lose PDGFR, GD3 and NG2 as they gain expression of GalC, without losing O4. As these cells form contacts with their target axons they exit the cell cycle and develop into premyelinating oligo- dendrocytes, which begin to express myelin-related gene products, prior to axon ensheathment and myelination. CNP is one of the earliest oligodendrocyte-specific proteins expressed, and the two isoforms are temporally regulated, with CNP2 being expressed by OPCs, and both CNP1 and CNP2 being expressed at the time of oligodendrocyte differentiation. This is followed by expression of MBP, PLP and
142 CH08 OLIGODENDROCYTES, SCHWANN CELLS AND MYELINATION
MAG, prior to myelination. The antibody Rip is a useful tool for studying terminal oligodendrocyte differentiation and myelination, but its epitope is unknown. The expression of PLP/DM20 is of interest, because DM20 dominates during the early stages of development and the mRNA can be expressed by OPCs. During differ- entiation, DM20 declines and PLP is the dominant form during myelination and later. MOBPs are expressed in the late stages of myelination, significantly later than expression of MBP. MOG is one of the last myelin proteins to be expressed and is often used as a marker for mature oligodendrocytes.
The various markers determine oligodendrocyte functions at different stages of differentiation. PDGFR mediates the effects of PDGF-AA, which is a potent mitogen and survival factor for OPCs and stimulates migration. The loss of PDGFR signifies a decrease in the migratory and proliferative capacity of OPCs, and a switch to the dependence on axon-derived factors for survival and differen- tiation. The precise functions of GD3 and NG2 in OPCs are unresolved, but their interactions with the ECM, integrins and the intracellular cytoskeleton indicate they are likely to mediate migration, process outgrowth, cell–cell recognition and adhesion. The developmental expression of galactolipids, sulphatides and myelin proteins are essential for myelination and axon development and integrity. The early expression of CNP, PLP and MAG are likely to be important in process outgrowth, axon recognition, and ensheathment (see Chapter 8.3.3).
Oligodendrocyte differentiation is regulated by a variety of growth factors.
PDGF-AA is the best characterized mitogen and survival factor for OPCs. Fibrob- last growth factor 2 (FGF2) is a mitogen for OPCs and inhibits their differentiation into oligodendrocytes, at least partly by maintaining expression of PDGFR.
Expression of FGF receptors is developmentally regulated in oligodendrocytes:
FGF-R1 increases as cells mature; FGF-R2 is expressed throughout the lineage;
FGF-R3 is expressed transiently by premyelinating oligodendrocytes and is impor- tant in the initiation of myelination. Insulin-like growth factor I (IGF-I) is a survival factor for oligodendrocytes and with thyroid hormone promotes differ- entiation. The precise interactions of these trophic factorsin vivo are unresolved.
When OPCs reach their final destinations, contact with axons may be sufficient for survival and differentiation, under the influence of neuregulin-1 (NRG-1) and Notch/Jagged signalling.
8.2.2 Functional development of Schwann cells
Schwann cell precursors originate from neural crest cells that migrate from the dorsal part of the neural tube as it closes. These cells are multipotent and give rise to both neurones and Schwann cells in the PNS. Under the influence of Delta/Notch signalling, NRG-1 and the transcription factor SOX10, crest cells develop first into Schwann cell precursors and then immature Schwann cells. These then diverge to form either nonmyelinating Schwann cells which surround small diameter axons, or myelinating Schwann cells which surround and subsequently myelinate large
8.2 MYELINATION 143 diameter axons, under the influence of the transcription factor Krox-20 and axonal signals, a key one being NRG-1. NRG-1 is involved in all stages of Schwann cell development, and is the major axon-derived mitogen, survival factor and regulator of myelination for Schwann cells.
8.2.3 Axon–glial interactions and the control of myelination
The ensheathment and myelination of axons during development proceeds in a series of steps that requires complex and reciprocal interactions between axons and the myelinating cells (Figure 8.8). In the first phase, oligodendrocytes and Schwann cells have to recognize axons that require myelination. In a second phase, the processes of the premyelinating cell must adhere to the axons and begin to ensheath them. In a third phase, axons and the myelinating cells pass through a series of interdependent maturation stages. How do the myelinating cells recognize axons to be myelinated? What determines the longitudinal and radial growth of the myelin sheath? How do the myelinating cells regulate the radial growth of axons and what are the signals that determine the differential and highly specialized organization of the axolemma and myelin sheaths at nodes of Ranvier, paranodes, and juxtaparanodes? We do not know the answers to these questions, but there is clear evidence of crucial roles for a number of candidate molecular cues and signals. The answers to these questions will also be relevant to the regulation of remyelination in demyelinating diseases and following regeneration.
Phase 1 – Axon contact: Axon diameter is a critical determinant of myelination in the PNS, and axons are not myelinated until they attain acritical diameterof approximately 0.7m. Axon diameter plays a less definitive role in the CNS, but nonetheless CNS axons are not myelinated below 0.2 m and the initiation of myelination only occurs when axons have reached this ‘critical’ diameter. How the myelinating cells recognize axons of a critical diameter is not yet resolved, but the axonal surface proteins NCAM and L1 are key candidates for negative and positive recognition signals. Disappearance of NCAM from the axonal surface is coincident with the onset of myelination, and suppression of NCAM stimulates myelination. Thus, myelinating cells may distinguish axons that are, and are not, ready for myelination by their differential expression of NCAM. L1 is another adhesion molecule expressed at the axon surface and has a demonstrated function in axon recognition in the PNS and most likely functions in the same way in the CNS. L1 is expressed by premyelinated axons and is down-regulated during myelination. Disruption of L1 strongly inhibits myelination, suggesting an induc- tive role for L1 in the initiation of myelination (although the L1 knockout mouse shows no abnormalities in myelination). The binding partners for NCAM and L1 in oligodendrocytes have not been identified. In Schwann cells, integrins and periaxin are important in axo–glial interactions. Oligodendrocytes do not express periaxin,
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Figure 8.8 Myelination. During development, myelination proceeds in a series of steps that require complex and reciprocal interactions between axons and the myelinating cells (an oligo- dendrocyte is illustrated). In essence, these interactions are the same in the PNS and CNS, although specific signals regulating myelination may differ; a major difference is that Schwann cells identify a single axon which they myelinate, whereas oligodendrocytes identify multiple axons that require myelination and the definitive number depends on oligodendrocyte phenotype and axon diameter.Phase 1– the premyelinating Schwann cell or oligodendrocyte contacts many premyelinated axons, but only those that have attained a critical diameter are myelinated; the recognition and adhesion signals are unresolved, but are likely to involve interactions between membrane surface molecules on the myelinating cell and axon.Phase 2– the myelinating cell ensheaths the axon, which requires process adhesion and longitudinal growth; the adhesion molecules have not been identified, although cell surface molecules are again likely candidates, and include MAG and NCAM. At this stage, oligodendrocytes undergo remodelling, when nonensheathing processes are lost and the definitive number of sheaths per unit is established;
this is strictly dependent on the diameter of axons in the unit, but the signals controlling oligo- dendrocyte phenotype (I–IV) divergence are unresolved.Phases 3 and 4– axons and the myelin sheath undergo interdependent growth and maturation. Myelin sheaths become compacted and the number of lamellae increases as the axons grow in diameter. Nodes of Ranvier are established (see Figure 8.11) and internodal myelin sheaths grow in direct relationship to the thickening of axons. The longitudinal and radial growth of the myelin sheath and axons, and the organization of the axolemma and myelin sheaths at nodes of Ranvier, paranodes, and juxtaparanodes must involve highly complex molecular cues and signals (see the text for more details)
8.2 MYELINATION 145 but specific integrins expressed on oligodendrocytes promote either differentia- tion and/or proliferation, dependent on the Src family kinases (SFKs) Fyn and Lyn. Early in the lineage, Lyn drives integrin-dependent progenitor proliferation, whereas at later stages Fyn regulates integrin-driven myelin formation and switches the response to neuregulin signalling from proliferation to differentiation. There is experimental evidence that N-cadherin may be important for the initial contact between myelinating oligodendrocytes and axons. Oligodendrocytes also express neurofascin, which is a ligand for the axonal L1 CAM family, but these interac- tions are likely to play a later role in myelination rather than axon contact and recognition.
Contact with axons stimulates the differentiation of OPCs into premyelinating oligodendrocytes, which begin to sequentially express myelin-related gene prod- ucts GalC, CNP, PLP/DM20, S-MAG and MBP, prior to axonal ensheathment.
Neuregulin-1 and Jagged/Notch signalling are key negative and positive regulators of oligodendrocyte differentiation, respectively. (However, in vitro oligodendro- cytes will differentiate and form myelin in appropriate culture conditions in the absence of axons.) Down-regulation of axonal Jagged is required for OPC differ- entiation, and Jagged/Notch signalling may play a particular role in the onset of myelination. Axonal neuregulin-1 induces differentiation of oligodendrocytesin vitro. Disruption of neuregulin-1 signallingin vitroand the absence of the neureg- ulin receptor ErbB2in vivoblocks OPC differentiation and myelin formation. In addition, interactions between axonal contactin and Notch receptors on OPCs play an instructive role by promoting OPC differentiation and up-regulation of myelin proteins. In the PNS, NRG-1 type III on axons determines their ensheathment fate, independent of axon diameter, and provides a key instructive signal for Schwann cell myelination.
Phase 2 – Axon ensheathment and establishment of incipient internodal myelin