Oligodendrocytes, key cells in myelination and target in demyelinating diseases

Effects of antidepressant drug exposure on gene expression in the developing cerebral cortex

To clarify the basis of limited responses in children and adolescents to antidepressant treatments considered standard in the treatment of adult major depressive disorder, juvenile Sprague-Dawley rats were subjected to 21-day treatment with dissimilar antidepressant drugs fluoxetine, imipramine, or vehicle control. Total RNA was extracted from brain frontal cortices and hybridized to the Affymetrix 230.2 chip. A total of 18 microarrays were analyzed (i.e., six biological replicates in three treatment groups). Transcripts identified were validated using Taqman real-time quantitative PCR methodology, and the relative expression of each gene was also determined. In both the imipramine- and fluoxetine-treated animals, expression of six genes was down-regulated (ANOVA-filtered gene expression data using dChip [version 2005]): Gpd1; Lrrn3; Sult1A1; Angptl4; Mt1a; Unknown. Furthermore, four genes were over-expressed: P4Ha1; RDG1311476; Rgc32; and SLC25A18-like by both imipramine and fluoxetine. These data demonstrate that antidepressant drugs interfere with the expression of genes involved in cell signaling, survival, and protein metabolism. Our results show that antidepressants regulate the induction of highly specific transcriptional programs in the developing frontal cortex. These findings provide novel insights into the long-term molecular actions of antidepressant drugs in the developing brain.

A1 adenosine receptor activation induces ventriculomegaly and white matter loss

A1 adenosine receptors (A1ARs) are widely expressed in the brain during development. To examine whether A1AR activation can alter postnatal brain formation, neonatal rats from postnatal days 3 to 14 were treated with the A1AR agonist N6-cyclopentyladenosine (CPA) in the presence or absence of the peripheral A1AR antagonist 8-(p-sulfophenyl)-theophylline (8SPT). CPA or CPA + 8SPT treatment resulted in reductions in white matter volume, ventriculomegaly, and neuronal loss. Quantitative electron microscopy revealed reductions in total axon volume following A1AR agonist treatment. We also observed reduced expression of myelin basic protein in treated animals. Showing that functional A1ARs were present over the ranges of ages studies, high levels of specific [3H]CCPA binding were observed at PD 4, 7 and 14, and receptor-G protein coupling was present at each age. These observations show that activation of A1ARs with doses of CPA that mimic the effects of high adenosine levels results in damage to the developing brain.

Novel oligodendrocyte transmembrane signaling systems. Investigations utilizing antibodies as ligands

Antibodies are increasingly being used as tools to study the function of cell surface markers. Several types of responses may occur upon the selective binding of an antibody to an epitope on a receptor. Antibody binding may trigger signals that are normally transduced by endogenous ligands. Moreover, antibody binding may activate normal signals in a manner that disrupts a sequence of events that coordinates either differentiation, mitogenesis, or morphogenesis. Alternately, it is possible that binding elicits either a modified signal or no signal. This article focuses on the cascade of events that occur following specific antibody binding to myelin markers expressed by cultured murine oligodendrocytes. Binding of specific antibodies to the oligodendrocyte membrane surface markers myelin/oligodendrocyte glycoprotein (MOG), myelin/oligodendrocyte specific protein (MOSP), galactocerebroside (GalC), and sulfatide on cultured murine oligodendrocytes results in different effects with regard to phospholipid turnover, Ca2+ influxes, and antibody:marker distribution. The consequence of each antibody-elicited cascade of events appears to be the regulation of the cytoskeleton within the oligodendroglial membrane sheets. The antibody binding studies described in this article demonstrate that these myelin surface markers are capable of transducing signals. Since endogenous ligands for these myelin markers have yet to be identified, it is not known if these signals are normally transduced or are a modification of normally transduced signals.

Pathogenesis of virus-induced demyelination

Demyelination is a component of several viral diseases of humans. The best known of these are subacute sclerosing panencephalitis (SSPE) and progressive multifocal leukoencephalopathy (PML). There are a number of naturally occurring virus infections of animals that involve demyelination and many of these serve as instructive models for human demyelinating diseases. In addition to the naturally occurring diseases, many viruses have been shown to be capable of producing demyelination in experimental situations. In discussing virus-associated demyelinating disease, the chapter reviews the architecture and functional organization of the CNS and considers what is known of the interaction of viruses with CNS cells. It also discusses the immunology of the CNS that differs in several important aspects from that of the rest of the body. Experimental models of viral-induced demyelination have also been considered. Viruses capable of producing demyelinating disease have no common taxonomic features; they include both DNA and RNA viruses, enveloped and nonenveloped viruses. The chapter attempts to summarize the important factors influencing viral demyelination, their common features, and possible mechanisms.

The origin of remyelinating cells in the adult central nervous system: the role of the mature oligodendrocyte

The sequence of events by which new oligodendrocytes are generated in the adult mammalian central nervous system has not been clearly defined. Here we review old evidence that remyelinating cells can arise from the division of mature oligodendrocytes. In addition, we report the results of a tissue culture study comparing oligodendrocytes and immature progenitors with regard to their capacity for proliferation, for the generation of new oligodendrocytes and for myelination. Monoclonal antibodies 04 and 01 were used to distinguish oligodendrocytes (04+01+) from progenitors (04+01-). Dissociated cell suspensions from adult rat spinal cord were separated by flow cytometry into 01+ and 01- cell fractions, at greater than 93% purity. The 01+ fraction contained approximately 0.7% 04+01- cells while the 01- fraction contained approximately 4.4% 04+01- cells. Cells from these fractions were plated onto cultures of purified dissociated dorsal root ganglion neurons. The cultures that received 01+ cells developed numerous expanding colonies of cells expressing both 01 and 04, or 04 only, by 8 days and were essentially covered by oligodendrocytes by 16 days. In marked contrast, oligodendrocyte colonies were rare in cultures receiving 01- cells. By 24 days, myelination was extensive in cultures receiving 01+ cells; in contrast, only a few myelin segments were observed in cultures receiving the 01- fraction. Thus, oligodendrocytes (01+ cells) appear more capable than progenitors (04+01-) of generating new myelinating oligodendrocytes in this culture system.

Fetal alcohol delays the developmental expression of myelin basic protein and transferrin in rat primary oligodendrocyte cultures

This study has examined the development of immunoreactive myelin basic protein and transferrin in primary glial cell cultures. Cultures were initiated from control and experimental Sprague-Dawley rats 1-2 days postnatally. Experimental treatment involved exposure to 5% (w/v) ethanol in a liquid diet during the last two weeks of gestation. Prenatal alcohol administration delayed the expression of myelin basic protein and transferrin during the first three weeks postnatally. Other oligodendroglial and astroglial markers were little affected, if at all, by fetal alcohol exposure.

Complement mediated serum cytotoxicity against oligodendrocytes: a comparison with other cells of the oligodendrocyte-type 2 astrocyte lineage

Rat oligodendrocytes are known to be susceptible to complement attack when exposed to homologous serum as a consequence of anti-myelin antibody independent classical pathway complement activation and attack. We have now compared this susceptibility with that of other cells of the oligodendrocyte-type 2 astrocyte (O-2A) lineage, and show that while type 2 astrocytes are not sensitive, O-2A progenitors are only relatively resistant to serum cytotoxicity, higher concentrations of complement resulting in cell damage. The implications of these findings for the pathogenesis of demyelinating disease are discussed.

Rubrospinal tract of the cat: superposition of antidromic responses and changes in axonal excitability following orthodromic activity

Activity was recorded from rubrospinal neurons (RSNs) in anesthetized, paralyzed, artificially ventilated cats. Multiple-unit microelectrodes were used to simultaneously record the activity of neighboring RSNs. When antidromically activated, the RSNs responded forming 'stacks' of superimposed spikes. By using appropriate collision tests, it was found that the spikes forming a stack arose from different neurons. In addition, single extracellular and intracellular recordings were obtained from RSNs. The changes in the axonal excitability of rubrospinal axons were tested following synaptically evoked (by contralateral interpositus (IP) stimulation) and/or directly evoked (by injection of current through the intracellular electrode) action potentials at different postspike delays. Subthreshold stimuli for antidromic activation in absence of orthodromic activity were well suprathreshold for most fibers in a wide range of postspike delays. The supernormal axonal periods were longer-lasting when tested after synaptic spikes (up to an average delay of 100.4 ms; range, 10-500 ms) than after directly evoked spikes (mean delay, 78.8 ms; range, 10-296 ms). If synaptic stimulation fires more RSNs than direct stimulation, then the longer-lasting supernormal periods might be due to the activity of adjacent fibers. An additional increase in external potassium concentration in the vicinity of the axon would explain these results.

Primary dissociated cell culture of fetal rat central nervous tissue. I. Immunocytochemical and ultrastructural studies of cell development and synaptogenesis

We have tried to establish a method of primary dissociated cell culture of the central nervous system (CNS) for successful development of large numbers of synapses and myelinated axons. Cerebra from 18-day-old fetal rats were enzymatically dissociated into single cells and plated onto poly-D-lysine-precoated coverslips at high cell density. With the progress of cell maturation, mixed neuronal and non-neuronal cell processes grew heavily and piled up on each other, making three-dimensional structures which corresponded to 'neuropil' in vivo. Within these structures we could observe not only many mature neurons and remarkable synaptogenesis but also many myelinated axons. The synapses were mainly axo-dendritic but axo-somatic synapses were also occasionally observed. Although most of the axon terminals contained many round clear vesicles which were about 30 nm in diameter, some of them contained both round clear vesicles and 50 nm in diameter vesicles with electron-dense cores. Also a small number of large electron-dense core vesicles (about 130 nm in diameter) were found in the perikarya of mature neurons. The numerous synapse formations observed in 3-dimensional structures suggest that neurons can remain in a stable state and carry out an active metabolism through neurotransmitters. So these structures are considered to provide a favorable microenvironment for both synaptogenesis and myelinogenesis.

Differential regulation of oligodendrocyte markers by glucocorticoids: post-transcriptional regulation of both proteolipid protein and myelin basic protein and transcriptional regulation of glycerol phosphate dehydrogenase

During neonatal development glucocorticoids potentiate oligodendrocyte differentiation and myelinogenesis by regulating the expression of myelin basic protein, proteolipid protein, and glycerol phosphate dehydrogenase (sn-glycerol-3-phosphate: NAD+ 2-oxidoreductase, EC 1.1.1.8). The actual locus at which hydrocortisone exerts its developmental influence on glial physiology is, however, not well understood. Glycerol phosphate dehydrogenase is glucocorticoid-inducible in oligodendrocytes at all stages of development both in vivo and in vitro. In newborn rat cerebral cultures, between 9 and 15 days in vitro, a 2- to 3-fold increase in myelin basic protein and proteolipid protein mRNA levels occurs in oligodendrocytes within 12 hr of hydrocortisone treatment. Immunostaining demonstrates that this increase in mRNAs is followed by a 2- to 3-fold increase in the protein levels within 24 hr. In vitro transcription assays performed with oligodendrocyte nuclei show an 11-fold increase in the transcriptional activity of glycerol phosphate dehydrogenase in response to hydrocortisone but no increase in transcription of myelin basic protein or proteolipid protein. These results indicate that during early myelinogenesis, glucocorticoids influence the expression of key oligodendroglial markers by different processes: The expression of glycerol phosphate dehydrogenase is regulated at the transcriptional level, whereas the expression of myelin basic protein and proteolipid protein is modulated via a different, yet uncharacterized, mechanism involving post-transcriptional regulation.

Primary dissociated cell culture of fetal rat central nervous tissue. II. Immunocytochemical and ultrastructural studies of myelinogenesis

It is generally considered that the dissociated cell culture is a type of monolayer culture, and when this method is used with the central nervous system (CNS) cells, synapses may be formed but myelin is either not formed at all or formed in very small quantities. So we tried to establish a dissociated cell culture of the CNS which contains many myelinated axons. Cerebra from 18-day-old fetal rat were dissociated into single cells and plated onto poly-D-lysine-precoated coverslips at high cell density. With the progress of cell maturation, neuronal and non-neuronal cell processes intermingled and piled up on each other, forming 3-dimensional structures. We were able to observe not only many mature neurons and remarkable synapses but also many myelinated axons. Moreover direct connections of oligodendrocyte (ODC) somas to myelin sheaths were demonstrated either immunocytochemically or ultrastructurally. The formation of the 3-dimensional structure is considered to provide a favorable microenvironment for the intimate interaction of neurons and ODCs leading to the formation of many myelinated axons. So this culture system may provide a useful model for the investigation of the details in myelinogenesis under physiological conditions and demyelination or remyelination under pathological conditions such as multiple sclerosis and allied diseases.

Myelin-oligodendrocyte glycoprotein (MOG) is a surface marker of oligodendrocyte maturation

The myelin-oligodendrocyte glycoprotein (MOG) is a minor component of central nervous system myelin. Using neonatal rat optic nerve oligodendrocyte cultures we have compared the development in vitro of MOG with galactocerebroside, myelin basic protein and 2' ,3'-cyclic-nucleotide 3'-phosphodiesterase. MOG appears on the surface of oligodendrocytes 1-2 days later than these other oligodendrocyte markers, suggesting that MOG may be a useful indicator of oligodendrocyte maturation. The relevance of these findings for investigating mechanisms of myelin injury in vitro and the role of oligodendrocyte damage in demyelinating disease is discussed.

Normal rat serum cytotoxicity against syngeneic oligodendrocytes. Complement activation and attack in the absence of anti-myelin antibodies

The role of complement in mediating oligodendrocyte and myelin injury has been investigated by studying the effects of normal adult rat serum on syngeneic cultured neonatal glial cells. Rat serum has cytotoxic activity directed against oligodendrocytes but not astrocytes, the potency of which increases with cell maturation. The effects of heat inactivation, decomplemented rat serum, EGTA treatment, removal of any possible anti-myelin antibody by absorption using syngeneic myelin and absence of surface staining for immunoglobulins on serum-treated oligodendrocytes, C9 depletion and reconstitution, and oligodendrocyte staining for surface C9 demonstrate that this cytotoxicity is mediated by complement via antibody independent activation of the classical pathway and is membrane attack complex dependent. These findings significantly extend the previous demonstration of complement activation by extracted myelin, and may have significance for the pathogenesis of demyelinating diseases.

Human oligodendrocytes in dissociated cell culture

Human oligodendrocytes have been successfully maintained in cell cultures for 14 weeks using a modification of a method used previously for animal brain cell cultures. Dissociated cell cultures from spinal cords of human foetuses of 10 to 20 weeks gestional age were investigated for up to 98 days. Oligodendrocytes were identified by monoclonal human antiserum specific for myelin-associated glycoprotein, by polyclonal rabbit antiserum against myelin basic protein, and by the mouse monoclonal antibody I6G1. Astrocytes were identified by polyclonal antibodies against glial fibrillary acidic protein. Immunocytochemical cell identification was corroborated by electron microscopy, by which glial cells were investigated both in situ and in culture. Immunocytochemical staining of myelin-associated glycoprotein showed specifically labelled oligodendrocytes on electron microscopy. The present study indicates that human oligodendrocytes, a putative target in demyelinating disease, can be studied in dissociated cell culture of human foetal spinal cord for several weeks in vitro under stable conditions.

Serum contains inducers and repressors of oligodendrocyte differentiation

An important stage in oligodendrocyte development is the expression of galactocerebroside (GC), the major glycolipid in myelin. Although oligodendrocyte cell lineage and differentiation in vitro have been the object of many studies, to date there is sparse information on the regulation of GC expression in oligodendrocytes already committed to be positive for GC. We report here that GC expression in these cells is controlled by three serum factors. Two of these, possibly a lipoprotein and a mucoprotein, increase GC levels, whereas the third, probably a glycoprotein, exerts an inhibitory effect. The developmental increase of GC in postnatal rat brain cerebral cultures and its induction by serum factors are reversible phenomena. The isolation of the GC-regulatory factors would allow experimental manipulation of impaired GC expression by differentiated oligodendrocytes.

Cellular and molecular aspects of myelin protein gene expression

The cellular and molecular aspects of myelin protein metabolism have recently been among the most intensively studied in neurobiology. Myelination is a developmentally regulated process involving the coordination of expression of genes encoding both myelin proteins and the enzymes involved in myelin lipid metabolism. In the central nervous system, the oligodendrocyte plasma membrane elaborates prodigious amounts of myelin over a relatively short developmental period. During development, myelin undergoes characteristic biochemical changes, presumably correlated with the morphological changes during its maturation from loosely-whorled bilayers to the thick multilamellar structure typical of the adult membrane. Genes encoding four myelin proteins have been isolated, and each of these specifies families of polypeptide isoforms synthesized from mRNAs derived through alternative splicing of the primary gene transcripts. In most cases, the production of the alternatively spliced transcripts is developmentally regulated, leading to the observed protein compositional changes in myelin. The chromosomal localizations of several of the myelin protein genes have been mapped in mice and humans, and abnormalities in two separate genes appear to be the genetic defects in the murine dysmyelinating mutants, shiverer and jimpy. Insertion of a normal myelin basic protein gene into the shiverer genome appears to correct many of the clinical and cell biological abnormalities associated with the defect. Most of the dysmyelinating mutants, including those in which the genetic defect is established, appear to exhibit pleiotropy with respect to the expression of other myelin genes. Post-translational events also appear to be important in myelin assembly and metabolism. The major myelin proteins are synthesized at different subcellular locations and follow different routes of assembly into the membrane. Prevention of certain post-translational modifications of some myelin proteins can result in the disruption of myelin structure, reminiscent of naturally occurring myelin disorders. Studies on the expression of myelin genes in tissue culture have shown the importance of epigenetic factors (e.g., hormones, growth factors, and cell-cell interactions) in modulating myelin protein gene expression. Thus, myelinogenesis has proven to be very useful system in which to examine cellular and molecular mechanisms regulating the activity of a nervous system-specific process.

A chemically defined medium for the culture of mature oligodendrocytes

A new chemically defined medium consisting of equal parts of Dulbecco modified Eagle's and Ham's F-12 media supplemented with insulin, sodium selenite, putrescine, and D+ galactose, which allows the long-term survival of mature oligodendrocyte pure cultures, is described. Immunohistochemical staining has shown that over 90% of the cells become positive for myelin proteins shortly following subculture. Contaminating astrocytes (2%) do not survive in this medium. Biochemical data have indicated that these purified oligodendrocytes express 2'3'-cyclic nucleotide 3'-phosphohydrolase and UDP-galactose ceramide galactosyltransferase activities. Electron microscopical examination revealed that the oligodendrocytes were mostly of medium-dark type and appeared to be identical to cells cultured in serum-containing medium. The ability to maintain pure oligodendrocyte cultures in such a defined medium will allow investigations concerning exogenous and endogenous factors involved in oligodendrocyte metabolism.

Transferrin: an early marker of oligodendrocytes in culture

In this study, the developmental pattern of transferrin expression, the iron transporting glycoprotein, was investigated morphologically and immunocytochemically in mixed glial cultures as well as pure cultures of mature oligodendrocytes, both derived from newborn rat brain. Double immunofluorescent labeling of pure oligodendrocyte cultures revealed that transferrin co-localizes with the oligodendroglial marker, myelin basic protein. During early development in mixed glial cultures, the presence of transferrin was detected at 3 days in vitro in small round process-bearing cells lying on top of astrocytes. These cells were galactocerebroside negative. However, at 7 days these process-bearing cells began to express galactocerebrosides and transferrin co-localized with the oligodendroglial marker. Transferrin did not co-localize with any neuronal or astroglial markers at any time. These results indicate that transferrin is an oligodendrocyte-specific marker which is expressed earlier than galactocerebroside.

Serum antibodies to central nervous system antigens: an analysis of their relation with different human neurologic disorders

In a previous paper we have presented a double ligand enzyme linked immunosorbent assay (ELISA) technique suitable for the detection of human antibodies to different brain antigens. In the present study, we have applied this technique to the analysis of 100 neurologically affected patients with regard to both a list of clinical parameters and the presence in their sera of nervous tissue specific antibodies, in an attempt to highlight the meaning of such antibodies in different neurologic disorder. We show that the presence of these antibodies cannot be used for elucidation of pathogenesis or for diagnostic purposes, but can be used as a prognostic index.

Reactive glial cells in CNS demyelination contain both GC and GFAP

The glial cell response to anti-galactocerebroside (GC) induced demyelination of the cat optic nerve was studied using electron microscopy and immunocytochemistry. Oligodendrocytes, which are a primary target for anti-GC, were depleted in the early lesions but astrocytes survived and showed reactive changes. Astrocytic processes exhibited dual staining for both GC and glialfibrillary acidic protein, a feature not seen in astrocytes outside the lesion or in normal optic nerve. These reactive astrocytes did not stain for anti-myelin basic protein, nor did they contain myelin debris, making it unlikely that the GC immunoreactivity was due to phagocytosis of myelin. Rather, it is postulated that the presence of GC in these cells represents a process of dedifferentiation to a more primitive state in which both astrocytic and oligodendrocytic determinants are synthesised, and that these reactive glial cells may be precursors of a new population of remyelinating oligodendrocytes.

Procedure for establishing oligodendroglial cells in primary cultures based on developmental parameters

We have established cultures using dissociated cells obtained from the neopallium of mouse pups at several different stages of development. The cellular composition of these cultures changed from more than 95% astrocytes when cultures were established from the neopallium of neonates to more than 90% oligodendrocytes when cultures were established from the neopallium of 2-week-old pups. The oligodendrocytic nature of the cells was established on morphological bases as well as the presence of an oligodendrocytic marker, galactocerebroside.

Expression of myelin proteolipid and basic protein mRNAs in cultured cells

Studies were undertaken to investigate the regulation of myelin-specific mRNA expression in cultured cells. Three experimental systems were investigated: primary oligodendrocytes grown as enriched cell populations, primary oligodendrocytes grown in the presence of chick spinal cord neurons, and C6 cells. cDNA probes specific for the myelin proteolipid mRNA and the myelin basic protein mRNA were used to quantitate proteolipid and myelin basic protein mRNA levels in cells under different experimental conditions. C6 cells expressed less than 0.2% of the proteolipid mRNA that was expressed in primary oligodendrocytes. Primary oligodendrocytes expressed the myelin-specific mRNAs for at least 104 days in culture, and the level of these mRNAs in cultures was elevated fourfold by coculturing rat oligodendrocytes with chick spinal cord neurons.

Glial differentiation in dissociated cell cultures of neonatal rat brain: noncoordinate and density-dependent regulation of oligodendroglial enzymes

The mixed glial system of primary cultures of cells dissociated from neonatal rat brain was utilized to study glial differentiation. The investigation was addressed specifically to the possibility of noncoordinate regulation of two manifestations of oligodendroglial differentiation, i.e., activities of glycerol-3-phosphate dehydrogenase (GPDH) and of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), as well as the effects of initial cell density on the time of onset and the intensity of expression of these aspects of oligodendroglial differentiation. Simultaneously, glutamine synthetase activity was studied to determine effects on astrocytic differentiation. GPDH exhibited a major developmental increase in specific activity between 20 and 32 days in culture. However, CNP activity exhibited a major developmental increase that commenced approximately 2 weeks earlier. The onset of these expressions of oligodendroglial differentiation was not affected by such environmental factors as initial cell density. However, the intensity of expression of the temporally separate increases in GPDH and CNP activities was markedly density-dependent. The highest activities were attained in cultures plated at the lowest cell densities. The astrocytic enzyme, glutamine synthetase, also exhibited a striking developmental increase (approximately tenfold between 13 and 30 days in culture), but initial cell density affected neither the time of onset nor the intensity of expression of this aspect of astrocytic differentiation. The data demonstrate a striking developmental increase in GPDH activity that is not coordinate with that in CNP. The noncoordinate manifestations of oligodendroglial differentiation commence as a function of time in culture, whereas the intensity of expression of this differentiation can be influenced by such environmental factors as initial cell density.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of myelin basic protein in oligodendrocytes by a soluble neuronal factor

Myelin basic protein (MBP) is one of the most important myelin components. Based on our previous studies, we hypothesized that neurons might have regulatory effects on the production of MBP by oligodendrocytes, and we conducted studies designed to verify this hypothesis. Oligodendroglia-rich cultures from total brain of neonatal rats or mice and pure cultures of embryonic rats or chicks were prepared. Cultures of mouse fibroblasts and astrocytes were prepared as well. We show here that MBP production by oligodendrocytes was greatly enhanced by treatment with either pure neurons, rat neuronal conditioned medium, or chick neuronal conditioned medium, while chemically defined, hormonally supplemented medium or medium conditioned by astrocytes and fibroblasts had no effect on MBP expression. We conclude that the production of MBP by oligodendrocytes is regulated by a nonspecies specific soluble neuronal factor. The conservation of this phenomenon from avian to rodent species implies its critical role in myelination and suggests its potential application as a treatment in demyelination.