Astrocytes promote myelination in response to electrical impulses

Myelin, the insulating layers of membrane wrapped around axons by oligodendrocytes, is essential for normal impulse conduction. It forms during late stages of fetal development but continues into early adult life. Myelination correlates with cognitive development and can be regulated by impulse activity through unknown molecular mechanisms. Astrocytes do not form myelin, but these nonneuronal cells can promote myelination in ways that are not understood. Here, we identify a link between myelination, astrocytes, and electrical impulse activity in axons that is mediated by the cytokine leukemia inhibitory factor (LIF). These findings show that LIF is released by astrocytes in response to ATP liberated from axons firing action potentials, and LIF promotes myelination by mature oligodendrocytes. This activity-dependent mechanism promoting myelination could regulate myelination according to functional activity or environmental experience and may offer new approaches to treating demyelinating diseases.     

Volume transmission in activity-dependent regulation of myelinating glia

The importance of neural impulse activity in regulating neuronal plasticity is widely appreciated; increasingly, it is becoming apparent that activity-dependent communication between neurons and glia is critical in regulating many aspects of nervous system development and plasticity. This communication takes place not only at the synapse, but also between premyelinating axons and glia, which form myelin in the PNS and CNS. Recent work indicates that neural impulse activity releases ATP and adenosine from non-synaptic regions of neurons, which activates purinergic receptors on myelinating glia. Acting through this receptor system, neural impulse activity can regulate gene expression, mitosis, differentiation, and myelination of Schwann cells (SCs) and oligodendrocytes, helping coordinate nervous system development with functional activity in the perinatal period. ATP and adenosine have opposite effects on differentiation of Schwann cells and oligodendrocytes, providing a possible explanation for the opposite effects of impulse activity reported on myelination in the CNS and PNS.     

Individual axons regulate the myelinating potential of single oligodendrocytes in vivo

The majority of axons in the central nervous system (CNS) are eventually myelinated by oligodendrocytes, but whether the timing and extent of myelination in vivo reflect intrinsic properties of oligodendrocytes, or are regulated by axons, remains undetermined. Here, we use zebrafish to study CNS myelination at single-cell resolution in vivo. We show that the large caliber Mauthner axon is the first to be myelinated (shortly before axons of smaller caliber) and that the presence of supernumerary large caliber Mauthner axons can profoundly affect myelination by single oligodendrocytes. Oligodendrocytes that typically myelinate just one Mauthner axon in wild type can myelinate multiple supernumerary Mauthner axons. Furthermore, oligodendrocytes that exclusively myelinate numerous smaller caliber axons in wild type can readily myelinate small caliber axons in addition to the much larger caliber supernumerary Mauthner axons. These data indicate that single oligodendrocytes can myelinate diverse axons and that their myelinating potential is actively regulated by individual axons.     

PCDHα在髓鞘形成和少突胶质细胞发育中的作用

该文通过免疫组化及蛋白免疫印迹的方法分别对Pcdhα基因敲除和对照组小鼠的中枢神经系统内的髓鞘碱性蛋白表达以及少突胶质细胞的发育进行了测定。结果表明:1)Pcdhα基因缺失小鼠中枢神经系统中的髓鞘碱性蛋白较对照组小鼠明显减少;2)Pcdhα基因敲除可导致少突胶质细胞发育异常:在小脑中,处于成熟期的少突胶质细胞减少,而处于前体细胞阶段的少突胶质细胞增多。上述结果提示Pcdhα可以通过调控少突胶质细胞的成熟过程进而影响髓鞘的形成。     

Integrin-mediated axoglial interactions initiate myelination in the central nervous system

All but the smallest-diameter axons in the central nervous system are myelinated, but the signals that initiate myelination are unknown. Our prior work has shown that integrin signaling forms part of the cell–cell interactions that ensure only those oligodendrocytes contacting axons survive. Here, therefore, we have asked whether integrins regulate the interactions that lead to myelination. Using homologous recombination to insert a single-copy transgene into the hypoxanthine phosphoribosyl transferase (hprt) locus, we find that mice expressing a dominant-negative β1 integrin in myelinating oligodendrocytes require a larger axon diameter to initiate timely myelination. Mice with a conditional deletion of focal adhesion kinase (a signaling molecule activated by integrins) exhibit a similar phenotype. Conversely, transgenic mice expressing dominant-negative β3 integrin in oligodendrocytes display no myelination abnormalities. We conclude that β1 integrin plays a key role in the axoglial interactions that sense axon size and initiate myelination, such that loss of integrin signaling leads to a delay in myelination of small-diameter axons.     

Death of oligodendrocytes and microglial phagocytosis of myelin precede immigration of Schwann cells into the spinal cord

Small, circumscribed electrolytic lesions were made in the upper cervical corticospinal tract in adult rats. In the centre of the lesion, the axons and all other tissue elements were totally destroyed. Surrounding this region of destruction is an area of tissue which is only partially damaged. In this area TUNEL positive staining of contiguous rows of tract glial cells indicates massive oligodendrocytic apoptosis at 1–3 days after operation, but axons, astrocytes and blood vessels survive. From around 4 days, the corticospinal axons in this area are demyelinated, and the microglia contain ingested myelin, identified in electron micrographs as characteristic MBP immunoreactive laminar cytoplasmic bodies. After around 3 weeks, large numbers of Schwann cells, continuous with those on the pial surface of the spinal cord, accumulate along the lesion track and selectively infiltrate the perilesional reactive area, where they mingle intimately with the phagocytic microglia. Electron micrographs show that at this time basal lamina-enclosed Schwann cell processes establish non-myelinated ensheathment of axons. From around 4 weeks after operation, prominent Schwann cell myelination is indicated by P0 immunoreactivity, and peripheral type, one-to-one myelination in electron micrographs. Thus the effect of the selective loss of oligodendrocytes is to first activate microglia, and then to induce a replacement of myelin by Schwann cells.     

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.     

A culture system to study oligodendrocyte myelination processes using engineered nanofibers

Current methods for studying central nervous system myelination necessitate permissive axonal substrates conducive to myelin wrapping by oligodendrocytes. We have developed a neuron-free culture system in which electron-spun nanofibers of varying sizes substitute for axons as a substrate for oligodendrocyte myelination, thereby allowing manipulation of the biophysical elements of axonal-oligodendroglial interactions. To investigate axonal regulation of myelination, this system effectively uncouples the role of molecular (inductive) cues from that of biophysical properties of the axon. We use this method to uncover the causation and sufficiency of fiber diameter in the initiation of concentric wrapping by rat oligodendrocytes. We also show that oligodendrocyte precursor cells display sensitivity to the biophysical properties of fiber diameter and initiate membrane ensheathment before differentiation. The use of nanofiber scaffolds will enable screening for potential therapeutic agents that promote oligodendrocyte differentiation and myelination and will also provide valuable insight into the processes involved in remyelination.     

Does oligodendrocyte survival depend on axons?

BACKGROUND: We have shown previously that oligodendrocytes and their precursors require signals from other cells in order to survive in culture. In addition, we have shown that about 50% of the oligodendrocytes produced in the developing rat optic nerve normally die, apparently in a competition for the limiting amounts of survival factors. We have hypothesized that axons may control the levels of such oligodendrocyte survival factors and that the competition-dependent death of oligodendrocytes serves to match their numbers to the number of axons that they myelinate. Here we test one prediction of this hypothesis - that the survival of developing oligodendrocytes depends on axons. RESULTS: We show that oligodendrocyte death occurs selectively in transected nerves in which the axons degenerate. This cell death is prevented by the delivery of exogenous ciliary neurotrophic factor (CNTF) or insulin-like growth factor I (IGF-1), both of which have been shown to promote oligodendrocyte survival in vitro. We also show that purified neurons promote the survival of purified oligodendrocytes in vitro. CONCLUSION: These results strongly suggest that oligodendrocyte survival depends upon the presence of axons; they also support the hypothesis that a competition for axon-dependent survival signals normally helps adjust the number of oligodendrocytes to the number of axons that require myelination. The identities of these signals remain to be determined.     

NGF controls axonal receptivity to myelination by Schwann cells or oligodendrocytes

Axons dictate whether or not they will become myelinated in both the central and peripheral nervous systems by providing signals that direct the development of myelinating glia. Here we identify the neurotrophin nerve growth factor (NGF) as a potent regulator of the axonal signals that control myelination of TrkA-expressing dorsal root ganglion neurons (DRGs). Unexpectedly, these NGF-regulated axonal signals have opposite effects on peripheral and central myelination, promoting myelination by Schwann cells but reducing myelination by oligodendrocytes. These findings indicate a novel role for growth factors in regulating the receptivity of axons to myelination and reveal that different axonal signals control central and peripheral myelination.     

Axonal cell-adhesion molecule L1 in CNS myelination

Of the axonal signals influencing myelination, adhesion molecules expressed at the axonal surface are strong candidates to mediate interactions between myelinating cells and axons. The recognition cell-adhesion molecule L1, a member of the immunoglobulin superfamily has been shown to play important roles in neuronal migration and survival, and in PNS myelination. We have investigated the role of axonally expressed L1 in CNS myelination. In co-cultures of myelinating oligodendrocytes and neurons derived from murine brain, we demonstrate that, before myelination, L1 immunoreactivity is confined to neurites. After myelination commences, L1 expression is downregulated on myelinated axons and adjacent, but not yet myelinated, internodes.Interfering with L1 before the onset of myelination, by adding either anti-L1 antibody or L1-Fc fusion proteins to the culture medium, inhibits myelination. In addition, in purified cultures of oligodendrocytes, L1-Fc fusion protein prevents lysophosphatidic acid-induced activation of the mitogen-activated kinase (MAP)-kinase pathway. Together, our data indicate that L1 is involved in the initiation of CNS myelination, and that this effect might involve the dephosphorylation of oligodendroglial phosphoproteins.     

Evidence for Cooperative Selection of Axons for Myelination by Adjacent Oligodendrocytes in the Optic Nerve

The cellular mechanisms that regulate the topographic arrangement of myelin internodes along axons remain largely uncharacterized. Recent clonal analysis of oligodendrocyte morphologies in the mouse optic nerve revealed that adjacent oligodendrocytes frequently formed adjacent internodes on one or more axons in common, whereas oligodendrocytes in the optic nerve were never observed to myelinate the same axon more than once. By modelling the process of axonal selection at the single cell level, we demonstrate that internode length and primary process length constrain the capacity of oligodendrocytes to myelinate the same axon more than once. On the other hand, probabilistic analysis reveals that the observed juxtaposition of myelin internodes among common sets of axons by adjacent oligodendrocytes is highly unlikely to occur by chance. Our analysis may reveal a hitherto unknown level of communication between adjacent oligodendrocytes in the selection of axons for myelination. Together, our analyses provide novel insights into the mechanisms that define the spatial organization of myelin internodes within white matter at the single cell level.     

Death receptor 6 negatively regulates oligodendrocyte survival, maturation and myelination

Survival and differentiation of oligodendrocytes are important for the myelination of central nervous system (CNS) axons during development and crucial for myelin repair in CNS demyelinating diseases such as multiple sclerosis. Here we show that death receptor 6 (DR6) is a negative regulator of oligodendrocyte maturation. DR6 is expressed strongly in immature oligodendrocytes and weakly in mature myelin basic protein (MBP)-positive oligodendrocytes. Overexpression of DR6 in oligodendrocytes leads to caspase 3 (casp3) activation and cell death. Attenuation of DR6 function leads to enhanced oligodendrocyte maturation, myelination and downregulation of casp3. Treatment with a DR6 antagonist antibody promotes remyelination in both lysolecithin-induced demyelination and experimental autoimmune encephalomyelitis (EAE) models. Consistent with the DR6 antagoinst antibody studies, DR6-null mice show enhanced remyelination in both demyelination models. These studies reveal a pivotal role for DR6 signaling in immature oligodendrocyte maturation and myelination that may provide new therapeutic avenues for the treatment of demyelination disorders such as multiple sclerosis.     

Dysmyelination, demyelination and reactive astrogliosis in the optic nerve of the taiep rat

Taiep is an autosomal recessive mutant rat that shows a highly hypomyelinated central nervous system (CNS). Oligodendrocytes accumulate microtubules (MTs) in association with endoplasmic reticulum (ER) membranes forming MT-ER complexes. The microtubular defect in oligodendrocytes, the abnormal formation of CNS myelin and the astrocytic reaction were characterized by immunocytochemical and ultrastructural methods during the first year of life. Optic nerves of both control and taiep rats were processed by the immunoperoxidase method using antibodies against tubulin, myelin basic protein (MBP) and glial fibrillary acidic protein (GFAP). Taiep oligodendrocytes are strongly immunoreactive against tubulin, indicative of a significant accumulation of microtubules. Early differentiated oligodendrocytes observed with electron microscopy show that MT-ER complexes are mainly present in the cell body. This defect increases during the first year of life; oligodendrocytes show large MT-ER complexes projected within oligodendrocyte processes. Using anti-MBP, there was a progressive reduction of immunolabeling in the myelin sheaths as taiep rats grew older. Ultrastructural analysis revealed severely dysmyelinated axons with a frequently collapsed periaxonal collar. However, through age the myelin sheath became gradually infiltrated by MTs, suggesting their contribution to premature loss of myelin in the taiep rat. Axons of one-year-old taiep rats were severely demyelinated. Modifications in astrocytes revealed by the GFAP antibody showed a strong hypertrophy with increased immunostaining in their processes. As demyelination of axons progressed, taiep rats developed a strong astrogliosis. The present findings suggest that in taiep rats the early abnormal myelination of axons affects the adequate maintenance of myelin, leading to a progressive loss of myelin components and severe astrogliosis, features that should be considered in the pathogenesis of dysmyelinating diseases.     

Gas6 enhances axonal ensheathment by MBP+ membranous processes in human DRG/OL promyelinating co-cultures

The molecular requirements for human myelination are incompletely defined, and further study is needed to fully understand the cellular mechanisms involved during development and in demyelinating diseases. We have established a human co-culture model to study myelination. Our earlier observations showed that addition of human γ-carboxylated growth-arrest-specific protein 6 (Gas6) to human oligodendrocyte progenitor cell (OPC) cultures enhanced their survival and maturation. Therefore, we explored the effect of Gas6 in co-cultures of enriched OPCs plated on axons of human fetal dorsal root ganglia explant. Gas6 significantly enhanced the number of myelin basic protein-positive (MBP⁺) oligodendrocytes with membranous processes parallel with and ensheathing axons relative to co-cultures maintained in defined medium only for 14 days. Gas6 did not increase the overall number of MBP⁺ oligodendrocytes/culture; however, it significantly increased the length of MBP⁺ oligodendrocyte processes in contact with and wrapping axons. Multiple oligodendrocytes were in contact with a single axon, and several processes from one oligodendrocyte made contact with one or multiple axons. Electron microscopy supported confocal Z-series microscopy demonstrating axonal ensheathment by MBP⁺ oligodendrocyte membranous processes in Gas6-treated co-cultures. Contacts between the axonal and oligodendrocyte membranes were evident and multiple wraps of oligodendrocyte membrane around the axon were visible supporting a model system in which to study events in human myelination and aspects of non-compact myelin formation.     

Abnormal myelination in the spinal cord of PTPα-knockout mice

PTPα interacts with F3/contactin to form a membrane-spanning co-receptor complex to transduce extracellular signals to Fyn tyrosine kinase. As both F3 and Fyn regulate myelination, we investigated a role for PTPα in this process. Here, we report that both oligodendrocytes and neurons express PTPα that evenly distributes along myelinated axons of the spinal cord. The ablation of PTPα in vivo leads to early formation of transverse bands that are mainly constituted by F3 and Caspr along the axoglial interface. Notably, PTPα deficiency facilitates abnormal myelination and pronouncedly increases the number of non-landed oligodendrocyte loops at shortened paranodes in the spinal cord. Small axons, which are normally less myelinated, have thick myelin sheaths in the spinal cord of PTPα-null animals. Thus, PTPα may be involved in the formation of axoglial junctions and ensheathment in small axons during myelination of the spinal cord.     

The temporal progression of the myelination defect in the taiep rat

The Sprague Dawley myelin mutant, the taiep rat, demonstrates a defect in CNS myelination which worsens with age and which is associated with abnormal accumulations of microtubules in oligodendrocytes. Quantitative and qualitative electron microscopic studies of myelin development and oligodendrocyte morphology were used to describe the temporal development of the defect in this mutant, in three regions of the CNS. The results indicate that the time of onset of myelination is similar in mutant and control rats, however the amount of myelin formed is reduced in the mutant, compared to controls, and there is a loss of myelin from the taiep CNS as the animals age. Thus the myelination defect in taiep has features of both hypomyelination and demyelination. Oligodendrocyte microtubule abnormalities were noted in each region of the taiep CNS at the time of onset of myelination. The earliest changes seen were close associations of oligodendrocyte microtubules with endoplasmic reticulum, with marked accumulations of microtubules filling the cytoplasm of oligodendrocytes from older taiep rats. These findings suggest that the microtubule abnormality in the taiep mutant inhibits both the initial formation and the long-term maintenance of myelin by the oligodendrocyte. In addition, there is also evidence to suggest that although the microtubule abnormality is present in oligodendrocytes throughout the taiep CNS, it results in a more marked defect in the myelination of axons of small diameter.     

Inhibition of Epidermal Growth Factor Receptor Improves Myelination and Attenuates Tissue Damage of Spinal Cord Injury

Preventing demyelination and promoting remyelination of denuded axons are promising therapeutic strategies for spinal cord injury (SCI). Epidermal growth factor receptor (EGFR) inhibition was reported to benefit the neural functional recovery and the axon regeneration after SCI. However, its role in de- and remyelination of axons in injured spinal cord is unclear. In the present study, we evaluated the effects of EGFR inhibitor, PD168393 (PD), on the myelination in mouse contusive SCI model. We found that expression of myelin basic protein (MBP) in the injured spinal cords of PD treated mice was remarkably elevated. The density of glial precursor cells and oligodendrocytes (OLs) was increased and the cell apoptosis in lesions was attenuated after PD168393 treatment. Moreover, PD168393 treatment reduced both the numbers of OX42 + microglial cells and glial fibrillary acidic protein + astrocytes in damaged area of spinal cords. We thus conclude that the therapeutic effects of EGFR inhibition after SCI involves facilitating remyelination of the injured spinal cord, increasing of oligodendrocyte precursor cells and OLs, as well as suppressing the activation of astrocytes and microglia/macrophages.     

Cellular mechanism of myelination in the central nervous system

A study of myelination with electron microscopy has been carried out on the spinal cord of young rats and cats. In longitudinal and transverse sections the intimate relationship of the growing axons with the oligodendrocytes was observed. Early naked axons appear to be embedded within the cytoplasm and processes of the oligodendrocytes from which they are limited only by the intimately apposed membranes of both elements (axon-oligocytic membrane). In a transverse section several axons are observed to be in a single oligodendrocyte. The process of myelination consists in the laying down, within the cytoplasm of the oligodendrocyte and around the axon, of concentric membranous myelin layers. The first of these layers is deposited at a certain distance (200 to 600 A or more) from the axon-oligocytic membrane. This and all the other subsequently formed membranes have higher electron density and are apparently formed by the coalescence and fusion of vesicles (of 200 to 800 A) and membranes found in large amounts within the cytoplasm of the oligodendrocytes. At an early stage the myelin layers may be discontinuous and some vesicular material may even be trapped among them or between the myelin proper and the axon-oligocytic membrane. Then, when the 8th to 10th layer is deposited, the complete coalescence and alignment of the lamellae leads to the characteristic orderly multilayered organization of the myelin sheath. Myelination in the central nervous system appears to be a process of membrane synthesis within the cytoplasm of the oligodendrocyte and not a result of the wrapping of the plasma membranes as postulated in Geren's hypothesis for the peripheral nerve fibers. The possible participation of Schwann cell cytoplasm in peripheral myelination is now being investigated.