[Prokaryotic expression and purification of mouse recombinant myelin proteolipid protein polypeptide]

Objective To establish and optimize the prokaryotic expression method for the recombinant mouse myelin proteolipid protein (PLP, 139-208 aa) which is a critical immunogenic polypeptide of PLP. Methods The sequence coding for PLP139-208 polypeptide was cloned into pET-32a(+) vector. Afterwards, the expression vector prepared in this research was transformed into E. coli BL21, and the recombinant PLP polypeptide was induced to express by isopropyl-β-D-thiogalactoside (IPTG). Two key prokaryotic expression conditions, IPTG's induction length and temperature, were analyzed for further optimization. The recombinant PLP polypeptide was induced to express by the expression method under the optimal expression conditions, and then was purified by Ni-NTA agarose and amylose resin. Finally, the gain of PLP139-208 polypeptide was verified by Western blot analysis. Results The results in the combinatorial optimization revealed that the expression of PLP139-208 was obtained at a satisfactory level when it was incubated at 23DegreesCelsius for 20 hours with the IPTG concentration of 0.5 mmol/L. Conclusion The optimized prokaryotic expression method for the recombinant mouse PLP139-208 was successfully established and effectively performed. This will shed light on the further researches on the improved preparation for experimental autoimmune encephalitis (EAE, an animal model of multiple sclerosis) and the underlying mechanism underlying PLP-induced autoimmune demyelination.

Diverse changes in myelin protein expression in rat brain after perinatal methadone exposure

The national incidence of neonatal abstinence syndrome has dramatically increased over the last decade due to an increase in antenatal opioid exposure. Recent human and animal studies suggest that antenatal opioid exposure impacts the developing brain. The purpose of this study is to evaluate the effects of perinatal methadone exposure on myelination in multiple regions in the developing rat brain. Pregnant Sprague-Dawley rats were randomly assigned into three experimental groups and subsequently exposed to drinking water alone or drinking water containing methadone from 7 days post coitum through day 7 or through day 19 after delivery. Two male neonatal rats were randomly selected from each litter and terminated at day 19. The cerebral cortex, hippocampus, cerebellum, and brainstem were dissected and analyzed for three myelin specific proteins - CNP, PLP, and MBP - by Western blot analysis. All pups with exposure to methadone demonstrated decreased expression of CNP, PLP, and MBP in the cerebral cortex and hippocampus. In the cerebellum, PLP expression was down‑regulated without apparent alteration of CNP and MBP expression. PLP and MBP expression, but not CNP expression, were significantly inhibited in the brainstem. Compared to the pups with postnatal methadone exposure via maternal milk through day 7, partial recovery of CNP and PLP expression only occurred in the cerebral cortices of the pups exposed through day 19. The findings show that antenatal opioid exposure in rat pups is associated with regionally‑specific alterations in brain myelination that diversely affects myelin proteins.

Increased Plp1 gene expression leads to massive microglial cell activation and inflammation throughout the brain

PMD (Pelizaeus-Merzbacher disease) is a rare neurodegenerative disorder that impairs motor and cognitive functions and is associated with a shortened lifespan. The cause of PMD is mutations of the PLP1 [proteolipid protein 1 gene (human)] gene. Transgenic mice with increased Plp1 [proteolipid protein 1 gene (non-human)] copy number model most aspects of PMD patients with duplications. Hypomyelination and demyelination are believed to cause the neurological abnormalities in mammals with PLP1 duplications. We show, for the first time, intense microglial reactivity throughout the grey and white matter of a transgenic mouse line with increased copy number of the native Plp1 gene. Activated microglia in the white and grey matter of transgenic mice are found as early as postnatal day 7, before myelin commences in normal cerebra. This finding indicates that degeneration of myelin does not cause the microglial response. Microglial numbers are doubled due to in situ proliferation. Compared with the jp (jimpy) mouse, which has much more oligodendrocyte death and hardly any myelin, microglia in the overexpressors show a more dramatic microglial reactivity than jp, especially in the grey matter. Predictably, many classical markers of an inflammatory response, including TNF-α (tumour necrosis factor-α) and IL-6, are significantly up-regulated manyfold. Because inflammation is believed to contribute to axonal degeneration in multiple sclerosis and other neurodegenerative diseases, inflammation in mammals with increased Plp1 gene dosage may also contribute to axonal degeneration described in patients and rodents with PLP1 increased gene dosage.

PLP overexpression perturbs myelin protein composition and myelination in a mouse model of Pelizaeus-Merzbacher disease

Duplication of PLP1, an X-linked gene encoding the major myelin membrane protein of the human CNS, is the most frequent cause of Pelizaeus-Merzbacher disease (PMD). Transgenic mice with extra copies of the wild type Plp1 gene, a valid model of PMD, also develop a dysmyelinating phenotype dependant on gene dosage. In this study we have examined the effect of increasing Plp1 gene dosage on levels of PLP/DM20 and on other representative myelin proteins. In cultured oligodendrocytes and early myelinating oligodendrocytes in vivo, increased gene dosage leads to elevated levels of PLP/DM20 in the cell body. During myelination, small increases in Plp1 gene dosage (mice hemizygous for the transgene) elevate the level of PLP/DM20 in oligodendrocyte soma but cause only minimal and transient effects on the protein composition and structure of myelin suggesting that cells can regulate the incorporation of proteins into myelin. However, larger increases in dosage (mice homozygous for the transgene) are not well tolerated, leading to hypomyelination and alteration in the cellular distribution of PLP/DM20. A disproportionate amount of PLP/DM20 is retained in the cell soma, probably in autophagic vacuoles and lysosomes whereas the level in myelin is reduced. Increased Plp1 gene dosage affects other myelin proteins, particularly MBP, which is transitorily reduced in hemizygous mice but consistently and markedly lower in homozygotes in both myelin and naïve or early myelinating oligodendrocytes. Whether the reduced MBP is implicated in the pathogenesis of dysmyelination is yet to be established.

p38 MAP kinase regulation of oligodendrocyte differentiation with CREB as a potential target

Despite a substantial understanding of the factors regulating oligodendrocyte differentiation, the signaling mechanisms involved in this process are not well-understood. This study elaborates on the findings (Bhat NR, Zhang P (1997) FASEB J 11:A925; Baron W, Metz B, Bansal R, Hoekstra D, de Vries H (2000) Mol Cell Neurosci 15:314-329) of a role for p38 MAP kinase signaling in oligodendrocyte differentiation and myelin gene expression. When proliferating oligodendrocyte progenitors were switched to a growth factor-free differentiation medium, there was a rapid activation of p38 kinase that correlated with an increased phosphorylation of CREB, a down-stream target and a factor involved in oligodendrocyte differentiation. Addition of forskolin, a known inducer of intracellular c-AMP and of oligodendrocyte differentiation, also stimulated CREB phosphorylation in a p38 kinase dependent way. Pharmacological inhibition of p38 interfered with the morphological and antigenic changes associated with differentiating oligodendrocytes as well as with the developmental and forskolin-induced expression of myelin basic protein, thereby supporting an essential role for p38 MAPK pathway in oligodendrocyte differentiation.

Electrophysiological abnormalities precede apparent histological demyelination in the central nervous system of mice overexpressing proteolipid protein

Myelin proteolipid protein (plp), a major myelin protein in the CNS, has been proposed to function in myelin assembly. Transgenic mice overexpressing the plp gene by introduction of two extra wild-type (Wt) mouse plp genes (plp(tg/-)) exhibit normal myelination and ion channel clustering at the age of 2 months. However, at the age of 5 months, demyelination becomes observable, accompanied by a reduction in the number of K+ channel clusters at Ranvier's node and a progressive increase in motor deficit. To clarify how these age-dependent changes are related to nerve conduction in the CNS, we analyzed the conduction velocity (CV) and relative refractory period (RRP) of identified spinal ascending or descending tracts, such as the dorsal column pathway, the vestibulospinal and reticulospinal tracts, and the pyramidal tract, in plp(tg/-) mice 2, 5, and 8 months of age. We found that CVs decreased as age increased. Importantly, CVs were significantly reduced and prolonged RRPs were observed in 2-month-old (2M) plp(tg/-) mice that had no apparent demyelination. Immunohistological examination revealed that densities of Na+ and K+ channel clusters decreased as plp(tg/-) and Wt mice aged. However, a clear correlation was not observed between CVs and mean channel cluster densities or between mean channel cluster densities and progress of demyelination. Performance in the rotarod test was normal in 2M plp(tg/-) mice but deteriorated in mice older than age 5 months. These results suggest that electrophysiological analysis can detect the abnormalities of the plp(tg/-) mice earlier than histological or behavioral measures.

Effects of irradiation on the postnatal development of the brain in a genetic mouse model of globoid cell leukodystrophy

Irradiation is one way to condition Twitcher mice--a natural model of globoid cell leukodystrophy (GLD)--prior to receive bone marrow transplantation (BMT). BMT showed to delay but not to completely prevent GLD disease in treated mutants. The reasons why BMT is not completely preventive in Twitchers are unclear but we speculate that irradiation might contribute to worsen the neurological impairments generated by the disease by altering postnatal neurogenesis. To test this hypothesis, we examined proliferation, migration and differentiation of neural precursors in neurogenic areas of the Twitcher brain after exposure of 5 day-old mutant pups to 620 rad, a non-lethal dose that leads to 80-90% of bone-marrow engraftment in classic BMT. Twitchers showed to be sensitive to irradiation, leading to a severe retardation of body growth of irradiated mutants. Irradiated Twitchers had reduced proliferation of neural precursors and increased astrogliosis and microgliosis, with reduced numbers of migratory neuroblasts and significantly less brain myelination. These effects were accompanied by caspase-3 activation and appeared largely irreversible in the lifespan of the Twitcher. Our work confirms that exposure of the neonatal brain to irradiation conditions such as those performed prior to BMT, can lead to long-lasting alterations of postnatal neurogenesis and myelination, which might contribute to worsen the progression of disease in these myelin mutants and to reduce the success of BMT.

Expression of a myelin proteolipid protein (Plp)-lacZ transgene is reduced in both the CNS and PNS of Plp(jp) mice

Jimpy (Plp(jp)) is an X-linked recessive mutation in mice that causes CNS dysmyelination and early death in affected males. It results from a point mutation in the acceptor splice site of myelin proteolipid protein (Plp) exon 5, producing transcripts that are missing exon 5, with a concomitant shift in the downstream reading frame. Expression of the mutant PLP product in Plp(jp) males leads to hypomyelination and oligodendrocyte death. Expression of our Plp-lacZ fusion gene, PLP(+)Z, in transgenic mice is an excellent readout for endogenous Plp transcriptional activity. The current studies assess expression of the PLP(+)Z transgene in the Plp(jp) background. These studies demonstrate that expression of the transgene is decreased in both the central and peripheral nervous systems of affected Plp(jp) males. Thus, expression of mutated PLP protein downregulates Plp gene activity both in oligodendrocytes, which eventually die, and in Schwann cells, which are apparently unaffected in Plp(jp) mice.

Processing of PLP in a model of Pelizaeus-Merzbacher disease/SPG2 due to the rumpshaker mutation

The rumpshaker mutation of the X-linked myelin proteolipid protein (PLP1) gene causes spastic paraplegia type 2 or a mild form of Pelizaeus-Merzbacher disease in man. The identical mutation occurs spontaneously in mice. Both human and murine diseases are associated with dysmyelination. Using the mouse model, we show that the low steady state levels of PLP result from accelerated proteasomal degradation rather than decreased synthesis. The T(1/2) for degradation of rumpshaker PLP is 11 h compared with 23 h for wild type. A minority of newly synthesized PLP is incorporated into myelin in the correct orientation but at a reduced rate compared with wild type. However, inhibition of proteasomal degradation does not increase the level of PLP incorporated into myelin. As Plp null mice do not have a similar myelin deficiency, it is unlikely that the reduced PLP levels are the main cause of the dysmyelination. Rumpshaker oligodendrocytes also have a reduced level of other myelin proteins, such as MBP, although the mechanisms are not yet defined but are likely to operate at a translational or post-translational level.

Circadian expression of clock and screening of clock-controlled genes in peripheral lymphocytes of rat

In this paper, the circadian pattern of Clock and genes mediated by the Clock was investigated in peripheral lymphocytes of rats. Circadian rhythms of Clock are found under the regimes of constant darkness (DD) and 12-h light-12-h dark (LD12:12h), with the peak phase at CT7 and ZT21, respectively. Ten differential cDNA fragments were identified to be mediated by the Clock, including three known genes (catalase, myelin proteolipid protein, and histone acetylase), four known expressed sequence tags (ESTs), and three novel ESTs. Experiment of the RNA interference revealed that these ESTs were down-regulated by the Clock gene and three of them were identified as clock-controlled genes. Understanding of clock-mediated genes may lead to a new direction in drug design for control of circadian rhythms.

Myelinogenesis and axonal recognition by oligodendrocytes in brain are uncoupled in Olig1-null mice

Myelin-forming oligodendrocytes facilitate saltatory nerve conduction and support neuronal functions in the mammalian CNS. Although the processes of oligodendrogliogenesis and differentiation from neural progenitor cells have come to light in recent years, the molecular mechanisms underlying oligodendrocyte myelinogenesis are poorly defined. Herein, we demonstrate the pivotal role of the basic helix-loop-helix transcription factor, Olig1, in oligodendrocyte myelinogenesis in brain development. Mice lacking a functional Olig1 gene develop severe neurological deficits and die in the third postnatal week. In the brains of these mice, expression of myelin-specific genes is abolished, whereas the formation of oligodendrocyte progenitors is not affected. Furthermore, multilamellar wrapping of myelin membranes around axons does not occur, despite recognition and contact of axons by oligodendrocytes, and Olig1-null mice develop widespread progressive axonal degeneration and gliosis. In contrast, myelin sheaths are formed in the spinal cord, although the extent of myelination is severely reduced. At the molecular level, we find that Olig1 regulates transcription of the major myelin-specific genes, Mbp, Plp1, and Mag, and suppresses expression of a major astrocyte-specific gene, Gfap. Together, our data indicate that Olig1 is a central regulator of oligodendrocyte myelinogenesis in brain and that axonal recognition and myelination by oligodendrocytes are separable processes.

Microarray analysis of brain RNA in mice with methylenetetrahydrofolate reductase deficiency and hyperhomocysteinemia

Methylenetetrahydrofolate reductase (MTHFR) deficiency is the most common genetic cause of hyperhomocysteinemia, which is associated with increased risk for cardiovascular disease, stroke and possibly other neurological disorders. Microarray analysis of brain RNA from day 14 Mthfr(-/-) mice revealed several genes with altered expression. Expression changes in inositol 1,4,5-triphosphate receptor, type 1 (Itpr1), proteolipid protein (Plp), neurogenic differentiation factor 1 (Neurod1), S100 calcium binding protein A8 (S100a8), and methylenetetrahydrofolate dehydrogenase (NAD+ dependent), methenyltetrahydrofolate cyclohydrolase (Mthfd2) were confirmed by RT-PCR. We propose that neuronal damage by hyperhomocysteinemia may involve disruption of intracellular calcium.

Differential prevention of experimental autoimmune encephalomyelitis with antigen-specific DNA vaccination

We compared the potential therapeutic effect of vaccination with DNA constructs encoding two encephalitogenic proteins, PLP and MOG, on the outcome of subsequent sensitization of EAE induced in SJL/J and C57/B6 mice. Early sensitization for EAE (4 weeks after DNA vaccination) caused recipient animals to develop enhanced disease with DNA-encoding PLP but not with DNA-encoding MOG. Late sensitization (more than 10 weeks) resulted in an amelioration of EAE in animals vaccinated with both PLP and MOG DNA constructs. These results, confirming the DNA-mediated ameliorating effect on EAE, also indicate significant differences in the kinetics of development of EAE tolerance in response to vaccination with different DNA-encoding myelin antigens. Since PLP and MOG require different MHC presentation and induce different EAE models, the results point to potential differences in immune system requirements for efficient DNA-induced amelioration of the autoimmune response.

Expression of soma-restricted proteolipid/DM20 proteins in lymphoid cells

A new family of the myelin proteolipid protein (PLP/DM20) gene products, srPLP/DM20, has been identified recently in thymus and brain. In the central nervous system, srPLP/DM20 products are not localized in the myelin membrane, unlike their classic PLP/DM20 counterparts. In the immune system, the classic PLP/DM20 products appear to be expressed predominantly in thymic cortical epithelium. In this study, we examined the cellular expression of sr-PLP/DM20 proteolipids in lymphoid tissues and cells by immunohistochemistry, FACS analysis and RT-PCR. We found that in contrast to the classic PLP/DM20 products, sr-proteins are mainly expressed in developing thymocytes in thymus and in T- and B-lymphocytes in spleen. These results are of importance in our further understanding, not only the different role of these new PLP gene products in central and peripheral tolerance, but also the function of such products in lymphocyte biology.

Stimulation of myelin proteolipid protein gene expression by eicosapentaenoic acid in C6 glioma cells

In this study, the role of exogenous fatty acids in the regulation of proteolipid protein (PLP) gene expression was investigated using the following model culture system: C6 glioma cells expressing the green-fluorescent protein (eGFP) driven by different segments of PLP promoter. Eicosapentanoic acid (EPA; 20:5 n-3), but not arachidonic acid (AA; 20:4 n-6), induced a significant increase in medium fluorescence intensity (MFI) determined by fluorescence-activated cell sorting (FACS). The induction of PLP promoter was time-dependent showing maximal activity between 24 and 48 h after EPA exposure. PLP promoter activation was dependent on fatty acid concentration, with maximum activation at 200 microM. Northern blot analysis confirmed the fluorescence data in C6 cells incubated with EPA. Furthermore, this treatment increased the adenylyl cyclase-cyclic AMP (cAMP) levels and the mitogen-activated protein kinase (MAPK) activation in C6 cells. PLP promoter activity was inhibited by pre-treatment with H89 (protein kinase A (PKA) inhibitor), but not with PD98059 (MAPK inhibitor), suggesting that EPA stimulates the expression of PLP via cAMP-mediated pathways.

Calpain inhibitor prevented apoptosis and maintained transcription of proteolipid protein and myelin basic protein genes in rat spinal cord injury

Spinal cord injury (SCI) is associated with progressive neurodegeneration and dysfunction. Multiple cellular and molecular mechanisms are involved in this pathogenesis. In particular, the activation of proteases following trauma can cause apoptosis in the spinal cord. Calpain, a calcium-dependent cysteine protease, plays a major role in apoptosis following trauma. We identified apoptosis and decrease in transcription of the genes for proteolipid protein (PLP) and myelin basic protein (MBP) in five 1-cm long spinal cord segments (S1, distant rostral; S2, near rostral; S3, lesion; S4, near caudal; and S5, distant caudal) 24 h after induction of SCI (40 g.cm force) in rats by weight-drop method. Sham rats underwent laminectomy and did not receive injury. Internucleosomal DNA fragmentation occurred prominently in the lesion (S3), moderately in near segments (S2 and S4), and slightly in distant segments (S1 and S5) of injured rats, indicating the occurrence of apoptosis in the lesion and penumbra. Levels of transcription of PLP and MBP were reduced highly in the lesion and moderately in near segments, suggesting that apoptotic loss of cells impaired biosynthesis of two important structural components of myelin. Immediate administration of the calpain inhibitor E-64-d (1 mg/kg) to injured rats prevented apoptosis and restored transcription of these genes, indicating the therapeutic efficacy of calpain inhibitor for treatment of SCI.

Evidence for a second wave of oligodendrogenesis in the postnatal cerebral cortex of the mouse

The existing view is that cortical oligodendrocytes (OLs) in rodents are born from the cortical subventricular zone (SVZ) after birth, but recent data suggest that many forebrain oligodendrocyte progenitor cells (OPCs) are specified much earlier (between E9.5 and E13.5 in the mouse) in the ventricular zone of the ventral forebrain under the control of sonic hedgehog (Shh) and migrate into the cortex afterward. We examined expression of specific early OL markers (PDGFRalpha, PLP/DM20, Olig2, and NG2) in the developing forebrain to clarify this issue. We propose that OPCs colonize the developing cortex in two temporally distinct waves. The gray matter is at least partially populated by a first wave of OPCs that arises in the medial ganglionic eminence and the entopeduncular area and spreads into the cortex via the developing cortical plate. The cerebral cortex benefits from the second wave of OPCs coming from residential SVZ. In the second wave, there might be two different types of precursor cells: PLP/DM20(+) cells populating only inner layers and PDGFRalpha(+) cells, which might eventually myelinate the outer regions as well.

Proteolipid protein gene mutation induces altered ventilatory response to hypoxia in the myelin-deficient rat

Pelizaeus Merzbacher disease is an X-linked dysmyelinating disorder of the CNS, resulting from mutations in the proteolipid protein (PLP) gene. An animal model for this disorder, the myelin-deficient (MD) rat, carries a point mutation in the PLP gene and exhibits a phenotype similar to the fatal, connatal disease, including extensive dysmyelination, tremors, ataxia, and death at approximately postnatal day 21 (P21). We postulated that early death might result from disruption of myelinated neural pathways in the caudal brainstem and altered ventilatory response to oxygen deprivation or hypercapnic stimulus. Using barometric plethysmography to measure respiratory function, we found that the MD rat develops lethal hypoxic depression of breathing at P21, but hypercapnic ventilatory response is normal. Histologic examination of the caudal brainstem in the MD rat at this age showed extensive dysmyelination and downregulation of NMDA and to a lesser extent GABA(A) receptors on neurons in the nucleus tractus solitarius, hypoglossal nucleus, and dorsal motor nucleus of the vagus. Unexpectedly, immunoreactive PLP/DM20 was detected in neurons in the caudal brainstem. Not all biosynthetic functions and structural elements were altered in these neurons, because phosphorylated and nonphosphorylated neurofilament and choline acetyltransferase expression were comparable between MD and wild-type rats. These findings suggest that PLP is expressed in neurons in the developing brainstem and that PLP gene mutation can selectively disrupt central processing of afferent neural input from peripheral chemoreceptors, leaving the central chemosensory system for hypercapnia intact.

Soma-restricted products of the myelin proteolipid gene are expressed primarily in neurons in the developing mouse nervous system

The myelin proteolipid gene encodes two sets of proteins, the classic PLP and DM20 and the sr (soma-restricted)-PLP and sr-DM20. Unlike the classic proteolipids, the sr-products are expressed in both neurons and oligodendrocytes (OLs) and are not components of the myelin sheath. In OLs, the sr-isoforms are associated with endosomes and recycling vesicles indicating a possible nonmyelin function for these proteins. In this study, a purified antibody specific for the sr-products was used to examine the expression of these proteins during the development of the mouse brain. We found that while sr-PLP and sr-DM20 are expressed in OLs, the highest levels of immunoreactivity were found in neuronal populations. During early embryonic development (E13-E15), sr-proteolipids were detected in the dorsal root ganglion and motor neurons in the spinal cord. By E17, immunostaining for sr-PLP and sr-DM20 in the brain increased dramatically. The highest levels of immunoreactivity were found during the first and second weeks postnatal after which staining intensity declined to adult levels and the pattern of expression was more restricted. Robust staining persisted in many neuronal populations including nuclei in the hindbrain, Purkinje and granule neurons in the cerebellum, pyramidal cells in the cortex and mitral cells in the olfactory bulb. The spatial and temporal pattern of sr-PLP and sr-DM20 expression is very similar to that of the endosomal protein, syntaxin 13, consistent with the finding that the sr-PLPs may play a role in vesicular transport in neurons.

Schwann cell expression of PLP1 but not DM20 is necessary to prevent neuropathy

Proteolipid protein (PLP1) and its alternatively spliced isoform, DM20, are the major myelin proteins in the CNS, but are also expressed in the PNS. The proteins have an identical sequence except for 35 amino acids in PLP1 (the PLP1-specific domain) not present in DM20. Mutations of PLP1/DM20 cause Pelizaeus-Merzbacher Disease (PMD), a leukodystrophy, and in some instances, a peripheral neuropathy. To identify which mutations cause neuropathy, we have evaluated a cohort of patients with PMD and PLP1 mutations for the presence of neuropathy. As shown previously, all patients with PLP1 null mutations had peripheral neuropathy. We also identified 4 new PLP1 point mutations that cause both PMD and peripheral neuropathy, three of which truncate PLP1 expression within the PLP1-specific domain, but do not alter DM20. The fourth, a splicing mutation, alters both PLP1 and DM20, and is probably a null mutation. Six PLP1 point mutations predicted to produce proteins with an intact PLP1-specific domain do not cause peripheral neuropathy. Sixty-one individuals with PLP1 duplications also had normal peripheral nerve function. These data demonstrate that expression of PLP1 but not DMSO is necessary to prevent neuropathy, and suggest that the 35 amino acid PLP1-specific domain plays an important role in normal peripheral nerve function.

Expression of FGF receptors 1, 2, 3 in the embryonic and postnatal mouse brain compared with Pdgfralpha, Olig2 and Plp/dm20: implications for oligodendrocyte development

Fibroblast growth factors (FGF) receptors FgfR1, FgfR2 and FgfR3 are differentially regulated during oligodendrocyte (OL) maturation in vitro: FgfR3 is expressed by OL progenitors whereas FgfR2 is expressed by differentiated OLs [Mol Cell Neurosci 1996;7:263-275], and we have recently shown that FgfR3 is required for the timely differentiation of OLs in vivo [J Neurosci 2003;23:883-894]. Here we have used in situ hybridization to investigate the expression patterns of FgfR1-3 and compare them to the putative OL progenitor markers Olig2, Pdgfralpha and Plp/dm20 as a function of development in vivo, in particular at sites of OL specification, migration or differentiation in the mouse forebrain and cerebellum. We show that at early stages FgfR1-3 expression overlaps with that of Olig2 in the embryonic ventricular zone of the lateral and medial ganglionic eminences. Further, a scattered population of cells expressing FgfR3 (but not FgfR1 or FgfR2) in the ventral telencephalon appear to arise from the ventricular zone, and at later stages are found more dorsally in the cortex, in an overall pattern similar to Olig2 and/or Pdgfralpha. Postnatal expression of FgfR2 increases with age, more prominently in specific regions, including the cortical and cerebellar white matter and optic nerve. Thus, the differential expression pattern of FgfR2 and FgfR3 observed in vivo suggests that their expression is developmentally regulated in a manner consistent with the pattern of their expression in culture. These data provide further insights into role of FgfRs in OL development, and they emphasize that these receptors are positioned both spatially and temporally to impact OL generation in vivo.

Effects of intrauterine inflammation on developing rat brain

Damage to the white matter in the brain during development can lead to cerebral palsy (CP), a heterogeneous group of clinical syndromes that results in life-long disorders of movement and posture. Periventricular leukomalacia (PVL) is a pathological process within the white matter characterized by oligodendrocyte loss and is associated with the development of CP. Clinically, CP and PVL are associated with intrauterine infection and inflammation, but mechanisms involved are not well understood. We developed a model of intrauterine inflammation in Lewis and Fischer 344 rats to study the effects of intrauterine inflammation on developing glia. Pregnant rats were intracervically injected with lipopolysaccharide (LPS) at 15 days of gestation (E15) and a dose of LPS that caused low fetal mortality was determined. At E20, treated fetuses had increased TUNEL(+) nuclei and tumor necrosis factor (TNF)-alpha-immunoreactive areas within the brains. In a second series of animals allowed to survive until postnatal day 21 (PND 21), immunostaining was performed against several glial markers. Staining for the oligodendrocyte-specific proteins 2', 3'-cyclic nucleotide phosphodiesterase (CNP) and myelin proteolipid protein (PLP) was decreased in treated pups compared to shams within the corpus callosum, a white matter structure used as a representative area of developing white matter. Treated pups had activated astrocytes lining cerebral blood vessels, as observed by glial fibrillary acidic protein (GFAP) staining, while sham pups did not. Activated microglia were not detected using OX42 as a cell marker. Our model of intrauterine inflammation causes increased TUNEL and TNF-alpha staining early after injury, suggesting increased apoptotic cell death, possibly by cytokine-related mechanisms.

Nuclear organization in differentiating oligodendrocytes

Many studies have suggested that the 3D organization of chromatin and proteins within the nucleus contributes to the regulation of gene expression. We tested multiple aspects of this nuclear organization model within a primary cell culture system. Oligodendrocyte lineage cells were examined to facilitate analysis of nuclear organization relative to a highly expressed tissue-specific gene, proteolipid protein (PLP), which exhibits transcriptional upregulation during differentiation from the immature progenitor stage to the mature oligodendrocyte stage. Oligodendrocyte lineage cells were isolated from brains of neonatal male rodents, and differentiation from oligodendrocyte progenitors to mature oligodendrocytes was controlled with culture conditions. Genomic in situ hybridization was used to detect the single copy of the X-linked PLP gene within each interphase nucleus. The PLP gene was not randomly distributed within the nucleus, but was consistently associated with the nuclear periphery in both progenitors and differentiated oligodendrocytes. PLP and a second simultaneously upregulated gene, the myelin basic protein (MBP) gene, were spatially separated in both progenitors and differentiated oligodendrocytes. Increased transcriptional activity of the PLP gene in differentiated oligodendrocytes corresponded with local accumulation of SC35 splicing factors. Differentiation did not alter the frequency of association of the PLP gene with domains of myelin transcription factor 1 (Myt1), which binds the PLP promoter. In addition to our specific findings related to the PLP gene, these data obtained from primary oligodendrocyte lineage cells support a nuclear organization model in which (1). nuclear proteins and genes can exhibit specific patterns of distribution within nuclei, and (2). activation of tissue-specific genes is associated with changes in local protein distribution rather than spatial clustering of coordinately regulated genes. This nuclear organization may be critical for complex nucleic-acid-protein interactions controlling normal cell development, and may be an important factor in aberrant regulation of cell differentiation and gene expression in transformed cells.

Selective effect of hypothyroidism on expression of myelin markers during development

Thyroid hormones are critical for maturation of the central nervous system. In a previous study, we showed a change in the pattern of mature myelinated nerve fibers by 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in developing hypothyroid animals, which suggests a possible role for thyroid hormones in myelin compaction. The classical myelin markers myelin basic protein (MBP) and proteolipidic protein (PLP) are expressed later in oligodendroglial development, when myelin sheath formation is in progress. A myelin constituent designated myelin-associated/oligodendrocytic basic protein (MOBP) has been identified and related to myelin compaction. We assessed the developmental sequence of appearance of CNPase, MBP, MOPB, and PLP proteins in cerebellum (Cb) and corpus callosum (cc) in an experimental hypothyroidism model. The appearance of both MOBP isoforms occurred at postnatal day (P)25 and P30 in cc and Cb, respectively, followed by an increase with age in the control group. However, all the MOBP isoforms were weakly detectable in both regions at P30 from the hypothyroid (H) group, and the higher molecular weight isoform remains decreased in cc, even at P90. The developmental pattern of expression of CNPase, MBP, and PLP proteins was also delayed in the H group. CNPase and MBP expression was recovered in cc and Cb, whereas PLP remained below control levels at P90 in cc. Our data show that the experimental hypothyroidism affects the developmental pattern of the oligodendrocytic/myelin markers. Furthermore, thyroid hormone may modulate specific genes, as demonstrated by permanent down-regulation of MOBP and PLP expression in adulthood.

A virus-induced molecular mimicry model of multiple sclerosis

Molecular mimicry is the process by which virus infection activates T cells that are cross-reactive with self antigens. Infection of SJL/J mice with the neurotropic picornavirus Theiler's murine encephalomyelitis virus (TMEV) leads to a progressive CD4(+) T cell-mediated demyelinating disease similar to multiple sclerosis. To study the potential of virus-induced molecular mimicry to initiate autoimmune demyelination, a nonpathogenic TMEV variant was engineered to encode a 30-mer peptide encompassing the immunodominant encephalitogenic myelin proteolipid protein (PLP139-151) epitope. Infection with the PLP139-151-encoding TMEV led within 10-14 days to a rapid-onset paralytic demyelinating disease characterized by PLP139-151-specific CD4(+) Th1 responses; insertion of a non-self ovalbumin sequence led to restoration of the normal late-onset disease. Early-onset disease was also observed in mice infected with a TMEV encoding PLP139-151 with an amino acid substitution at the secondary T cell receptor (TCR) contact residue (H147A), but not in mice infected with TMEV encoding a PLP139-151 substitution at the primary TCR contact (W144A). Most significantly, mice infected with TMEV encoding a Haemophilus influenzae mimic peptide, sharing only 6 of 13 amino acids with PLP139-151, displayed rapid-onset disease and developed cross-reactive PLP139-151-specific CD4(+) Th1 responses. To our knowledge, this is the first study showing that a naturally infectious virus encoding a myelin epitope mimic can directly initiate organ-specific T cell-mediated autoimmunity.

Single or multiple oligodendroglial lineages: a controversy

The text books all say oligodendrocytes are the last cell to arise during development. The analysis of the spatio-temporal pattern of expression of plp/dm-20 during embryonic development in both the chick and the mouse provides evidence that the induction of oligodendrocyte occurs much earlier than we thought. In fact, it seems as though these cells must arise nearly simultaneously with neurons and it is just that they do not mature until later. Furthermore, we review the experimental arguments in favor of the existence of at least two, if not more, oligodendrocyte precursor cells: one is defined by the expression of PDGFRalpha, another characterized by expression of plp/dm-20 is independent from PDGF-AA for its proliferation and survival. We then postulate the existence of a third family of yet unknown origin.

Embryonic stem cell-derived glial precursors: a source of myelinating transplants

Self-renewing, totipotent embryonic stem (ES) cells may provide a virtually unlimited donor source for transplantation. A protocol that permits the in vitro generation of precursors for oligodendrocytes and astrocytes from ES cells was devised. Transplantation in a rat model of a human myelin disease shows that these ES cell-derived precursors interact with host neurons and efficiently myelinate axons in brain and spinal cord. Thus, ES cells can serve as a valuable source of cell type-specific somatic precursors for neural transplantation.

The expression of myelin protein mRNAs during remyelination of lysolecithin-induced demyelination

To gain insights into the mechanisms of myelin repair in the CNS and to establish the extent to which this process resembles myelination in development we have examined the patterns of expression of transcripts of the major myelin proteins, myelin basic protein (MBP) and proteolipid protein (PLP) during remyelination of lysolecithin-induced demyelination in the adult rat spinal cord. Injection of 1 microliter 1% lysolecithin into the dorsal funiculus caused a dramatic decrease in levels of MBP exon 1 and MBP exon 2-containing transcripts and PLP/DM20 transcripts. Between 10 and 21 days post-lesion induction there was a gradual increase in levels of expression of all transcripts, which had returned to levels associated with normally myelinated spinal cord white matter at 21 days. These increases in levels of expression corresponded to the appearance of remyelinated axons, detected on toluidine blue-stained resin sections. Foci of high levels of expression occurred in regions of the lesion in which new myelin sheath formation was occurring, although the level of expression throughout the lesion never exceeded levels associated with myelin sheath maintenance in normal white matter due to the asynchronous pattern of remyelination. The changes in levels of expression of MBP exon 2 closely followed those of MBP exon 1. Our results indicate that (i) myelin protein gene expression associated with myelinogenesis during remyelination follows a similar pattern to that of myelinogenesis during development and that (ii) in rat models of demyelination changes of expression of MBP exon 1 and exon 2-containing transcripts are of equal value, an observation relevant to quantifying the effects of putative remyelination-enhancing strategies using the lysolecithin model.

Antisilencing: myelin proteolipid protein gene expression in oligodendrocytes is regulated via derepression

Antisilencer or antirepressor elements have been described, thus far, for only a few eukaryotic genes and were identified by their ability not to augment gene expression per se but to override repression mediated via negative transcription regulatory elements. Here we report the first case of antisilencing for a neural-specific gene, the myelin proteolipid protein (PLP) gene (Plp). PLP is the most abundant protein found in CNS myelin. The protein is synthesized in oligodendrocytes, and its expression is regulated developmentally. Previously we have shown that a PLP-lacZ transgene (which includes the entire sequence for Plp intron 1) is regulated in mice, in a manner consistent with the spatial and temporal expression of the endogenous Plp gene. In the present report, we demonstrate by transfection analyses, using various PLP-lacZ deletion constructs, that Plp intron 1 DNA contains multiple elements that collectively regulate Plp gene expression in oligodendrocytes. One of these regulatory elements functions as an antisilencer element, which acts to override repression mediated by at least two negative regulatory elements located elsewhere within Plp intron 1 DNA. The mechanism for antisilencing appears to be complex as the intragenic region that mediates this function binds multiple nuclear factors specifically.

Cotranslational integration of myelin proteolipid protein (PLP) into the membrane of endoplasmic reticulum: analysis of topology by glycosylation scanning and protease domain protection assay

The four transmembrane domain topology of the proteolipid protein (PLP) in the myelin membrane of the central nervous system (CNS) has been further substantiated by biochemical studies. We have analyzed the cotranslational polytopic integration of nascent PLP during protein synthesis into the membrane of the endoplasmic reticulum (ER) on two routes. Consensus sequences for N-glycosylation were introduced by site directed mutagenesis into the PLP sequence as reporter sites, which upon glycosylation monitor the intraluminal location of the respective domains corresponding to the extracellular side of the plasma membrane. Single, double, and triple mutant cDNAs were constructed for transcription/translation in vitro in the presence of ER-membranes. The glycosylation pattern of the translation products revealed that hydrophilic extramembrane regions 2 and 4 (EMR2/EMR4) and EMR3 of PLP are exposed on opposite sides of the ER membrane. Their localization either at the cytosolic or luminal side of the ER membrane leads to two different topologies. The two modes of membrane integration during in vitro cotranslational translocation were confirmed by protease protection assays with wild-type and truncated PLP polypeptides with either one, two, or three putative transmembrane domains integrated into the ER-membrane. The fragment pattern of the [35S]methionine- or [3H]leucine-labeled polypeptides revealed that EMR3 and EMR4 were exposed with opposite orientation either on the cytosolic or luminal side of the ER membrane supporting the 4-transmembrane helix (TMH) N(in) model with the N and C termini on the cytoplasmic side, as established for the myelin membrane (plasma membrane); the other inversely integrated PLP constructs indicate the 4-TMH-Nout profile. These results are discussed with regard to the PLP biogenesis and the plasma membrane topology in PLP-expressing cells.

Overproduction and immuno-affinity purification of myelin proteolipid protein (PLP), an inositol hexakisphosphate-binding protein, in a baculovirus expression system

Myelin proteolipid protein (PLP) is a major integral membrane protein of central nervous system myelin and is considered to play a significant role in myelination. PLP has a four-transmembrane structure, judging from the hydropathy profile. In addition, it has InsP6 binding activity. Here, we have succeeded in producing PLP in large quantities of 3.9 pg/cell (6 mg/L) by using a baculovirus expression system and developing an efficient purification method, maintaining InsP6 binding activity. The recombinant PLP (rPLP) was purified by ion-exchange and immunoaffinity chromatography in a nonorganic solvent. The final yield of purified rPLP was 36%. The Kd and Bmax values for the InsP6-PLP binding were 55 nM and 33 pmol/microgram protein, respectively. The Kd value of purified rPLP is equal to that of mouse brain PLP. These results indicate that purified rPLP keeps its native conformation and binds InsP6 in an almost one-to-one ratio.

Sp1 as a target site for metal-induced perturbations of transcriptional regulation of developmental brain gene expression

Differential gene expression is partially regulated by zinc finger proteins (ZFP) such as Sp1, which may be potential targets for perturbations by environmental metals. In this paper, we discuss the selective effects of lead (Pb) and other heavy metals on the in vitro and in vivo DNA-binding of Sp1, and the developmental expression of its target genes. We have found that the presence of Pb, Zn and Cd in a DNA-binding assay differentially modulated the binding of Sp1 to its specific DNA sequence, while Ca, Mg and Ba, did not. In PC12 cells, cultured in the presence of low concentrations of Pb, a premature enhancement of Sp1 DNA-binding was observed. Similarly, Sp1 DNA-binding in the cerebellum of Pb-exposed animals was shifted to the first week after birth, while the developmental profile of a non-ZFP, NFkB, was not. Furthermore, selective premature peaks of myelin basic protein and proteolipid protein mRNA expression were observed to occur in a manner relative to the changes in Sp1 DNA-binding. Since these genes are high targets for Sp1, these data suggest that exposure to heavy metals may alter developmental gene expression and brain development through selective modulation of the transcriptional activity of Sp1.

HLA-DP: a class II restriction molecule involved in epitope spreading during the development of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system. It is widely believed that complex polygenic inheritance patterns involving HLA-DR and -DQ class II genes contribute to MS susceptibility, and current evidence indicates that disease risk vs disease outcome may be associated with distinctly different HLA class II alleles. We have recently shown that the early development of MS is accompanied by an extensive plasticity of myelin self-recognition with the acquisition of neo-autoreactivity, or epitope spreading, as a prominent feature. Although we did not observe a common determinant recognized by patients sharing identical HLA-DR or -DQ class II alleles, we did observe epitope spreading to the p50-63 determinant of myelin proteolipid protein (PLP) in two study subjects showing complete disparity at HLA-DR and -DQ but identity at the HLA-DP allele DPB1*0301. In the present study we show that self-recognition during the early stages in the development of MS involves HLA-DP class II restricted responses to the PLP 50-63 spreading determinant. Our results suggest that self-presentation by HLA-DP may play an important role in epitope spreading and in the propagation of self-recognition during the clinical progression of MS.

Intracellular transport of the DM-20 bearing shaking pup (shp) mutation and its possible phenotypic consequences

Paralytic tremor (pt) in rabbits and shaking pup (shp) in dogs are allelic dysmyelinated mutants of the proteolipid protein (Plp) gene. Both mutations affect the same amino acid, histidine36, which is replaced by glutamine in pt and by proline in shp. Phenotypic expression of these two mutations is very different. Paralytic tremor presents a much milder form of dysmyelination than shaking pup. The number of oligodendrocytes in the mutant rabbit is normal, while in the dog, the oligodendrocyte number is reduced due to early death or incomplete maturation. We have previously reported an abnormal intracellular transport of the PLPpt, whereas DM-20pt was normally transported to the cell membrane. In the present study, we show that the transport of the two isoforms containing the shp mutation is impaired in transfected Cos-7 cells. Cotransfecting cells with different ratios and combinations of mutated PLP and DM-20 cDNAs, we demonstrated that DM-20pt, but not DM-20shp, facilitates intracellular trafficking and integration into the plasma membrane of either of the two mutated PLPs. The phenotypic difference between these two allelic mutations can result from differences in DM-20 protein trafficking and sorting. These results show that the loss of function of PLP is not position-dependent but depends on the nature of the mutation.

Delayed and incomplete myelination in a transgenic mouse mutant with abnormal oligodendrocytes

In search of animal models suitable for investigating myelin repair, we have analysed myelinogenesis in a transgenic mouse mutant with delayed myelination, but with a normal life-span. The 2-50 mutant which carries a c-myc gene under the regulation of the myelin basic protein promoter has been described previously (Orian et al.: J Neurosci Res 39:604-612, 1994). Here we show that appropriate mRNA transcripts and their corresponding protein products are generated, but that the accumulation of these products is delayed in transgenic mice with respect to nontransgenic littermates. This phenomenon is associated with aberrant myelin and paucity of normal oligodendrocytes. Myelination appears to be carried out by abnormal, oligodendrocyte-like cells. We propose that the primary defect in the 2-50 mutant is an inability to generate the normal number of mature oligodendrocytes. This mutant represents a novel class of mutant in which oligodendrocyte development and myelination can be studied in the absence of interference with a gene for a structural protein of myelin, in an animal with normal survival. It may also represent a new tool to investigate in vivo gliogenesis and regulatory events bringing about the coordinated regulation of myelin protein synthesis.

Reduced size of retinal ganglion cell axons and hypomyelination in mice lacking brain-derived neurotrophic factor

While brain-derived neurotrophic factor (BDNF) delays the death of axotomized retinal ganglion cells in rodents, it is unclear if it affects any aspect of the normal development of these cells. Here we examined the optic nerve of bdnf-/- mice. Axonal numbers were normal, but their diameter, as well as the proportion of myelinated axons, was reduced at postnatal day 20 (P20). In contrast, the facial nerve was not hypomyelinated. Expression levels of mRNAs coding for the myelin proteins PLP and MBP were substantially reduced in the hippocampus and cortex at P20, but not in the sciatic nerve. Intraventricular injections of BDNF into the ventricles of wild-type mice at P10 and P12 up-regulated expression of PLP in the hippocampus at P14. These results indicate a role of BDNF, discussed as indirect, in the control of myelination in the central nervous system.

Induction of myelin-associated glycoprotein expression through neuron-oligodendrocyte contact

The role of neurons on expression of myelin-associated glycoprotein (MAG) in oligodendrocytes and oligodendroglial differentiation was examined. Primary cultures of oligodendrocytes prepared from neonatal mouse brains were co-cultured with neuronal cells derived from embryonal carcinoma P19 cells. The levels of MAG mRNAs following this co-culture were determined by reverse transcription (RT)-PCR. In oligodendrocytes co-cultured in direct contact with P19-derived neurons, the levels of MAG mRNAs, particularly that of the L-type isoform, were markedly higher than those in cultures without any neuronal cells. On the other hand, when the P19-derived neurons were present, but not in direct contact, no significant induction of MAG expression was found, though oligodendrocytes appeared to mature morphologically. The L-MAG expression was also stimulated when just the neuronal cell membrane fraction was added, which implies that there might be some effecter(s) in the cell membrane which are possibly exerting a signal transduction for myelin formation. These results suggest that morphological differentiation and functional maturation of oligodendrocytes are due to independent factors. The former is caused by some humoral factor(s) liberated from neuronal cells, while the latter resulted from cellular contact with neuronal cells.

Ischemic damage and subsequent proliferation of oligodendrocytes in focal cerebral ischemia

In order to achieve a better understanding of the pathophysiology of ischemic white matter lesions, oligodendrocytic degeneration and subsequent proliferation were examined in the mouse model of middle cerebral artery occlusion. In situ hybridization histochemistry for proteolipid protein messenger RNA was employed as a sensitive and specific marker of oligodendrocytes, and immunohistochemistry for myelin basic protein was used as a compact myelin marker. Immunohistochemistry for microtubule-associated protein 2 and albumin was employed to monitor neuronal degeneration and the breakdown of the blood brain barrier, respectively. In the ischemic core of the caudoputamen, the immunoreactivity for microtubule-associated protein 2 disappeared and massive albumin extravasation occurred several hours after vessel occlusion, while proteolipid protein messenger RNA signals remained relatively strong at this time. The messenger RNA signals began to attenuate 12 h after ischemia and were hardly detectable 24 h after ischemia in the whole ischemic lesion. In situ end-labeling of fragmented DNA showed some cells with proteolipid protein messenger RNAs to have DNA fragmentation at this period. In contrast to proteolipid protein messenger RNA signals, the immunoreactivity for myelin basic protein was detected as long as five days after ischemia. An apparent increase in the cells possessing strong proteolipid protein messenger RNA signals was found five days after ischemia, mainly in the corpus callosum and the cortex bordering the infarcted areas. A double simultaneous procedure with in situ hybridization for proteolipid protein messenger RNA and immunohistochemistry for glial fibrillary acid protein or lectin histochemistry for macrophages/microglia showed proliferating oligodendrocytes to be co-localized with reactive astrocytes and macrophages/microglia. These findings show that oligodendrocytic damage occurred following ischemic neuronal damage and the breakdown of the blood brain barrier, but preceded the breakdown of myelin proteins in the ischemic lesion, that an apoptosis-like process was involved in ischemic oligodendrocytic death, and that surviving oligodendrocytes responded and proliferated in the outer border of the infarcted area.

Molecular bases of myelin formation as revealed by investigations on mice deficient in glial cell surface molecules

Several glia-associated cell surface molecules have been implicated in myelin formation in the central (CNS) and peripheral nervous system (PNS). Recent studies in mice deficient for such molecules have been instrumental in understanding the role of these molecules during the formation of the spiraling loops around the axon, compaction of the spiraling loops, determination of the thickness of the myelin sheath, and myelin maintenance. In the PNS, the major peripheral myelin protein PO and the peripheral myelin protein (PMP) 22 are involved in spiral formation as reflected by retarded myelin formation in mice deficient for the respective molecules. An involvement of the myelin-associated glycoprotein (MAG) in this process is detectable only in mice deficient in both PO and MAG, suggesting that PO can replace MAG during the formation of the spiraling loops. Myelin compaction is mediated by both PO and the intracellular myelin component myelin basic protein (MBP). The determination of the correct myelin thickness is mediated by PO, MBP, and PMP22, with PO and MBP fostering and PMP22 attenuating myelin growth. For the maintenance of the association of the Schwann cell and myelin with its ensheathed axon, the myelin components PO, PMP22, MAG, and Connexin 32 are crucial. In the CNS, recognition of oligodendrocytes and axons and the formation of the spiraling loops is mediated by MAG. MAG is additionally responsible for the maintenance of myelin. Myelin compaction is mediated by MBP and by PLP, which fulfills some analogous functions in the CNS as PO in the PNS. These studies reveal that myelin-related cell surface molecules can play distinct but also partially overlapping roles during the formation and maintenance of myelin.

Single intracranial injection of apotransferrin in young rats increases the expression of specific myelin protein mRNA

Transferrin (Tf) is a possible regulator of oligodendrocyte development in vitro (Espinosa de los Monteros et al., 1989). At least two different mechanisms may account for the effects of Tf on myelin synthesis. It may act as a trophic factor and enhance the formation of new myelin sheaths. Tf may also induce the synthesis of myelin proteins in the central nervous system. We recently demonstrated that a single intracranial injection of apotransferrin (aTf) in young rats induces an increased myelination (Escobar Cabrera et al., 1994). In the present study, we investigated the in vivo effect of aTf on the expression of mRNAs of specific myelin genes. Three-day-old rats were injected intracranially with aTf and killed at different ages after injection. Total brain RNA was isolated, and the expression of different mRNAs was analyzed by Northern blot. The amount of mRNAs of myelin basic protein and of 2'-3' cyclic nucleotide 3'-phosphohydrolase were markedly increased in the experimental animals, whereas myelin proteolipid protein mRNA did not show differences relative to controls. These results indicate that in the animals treated with aTf, there is a differential effect on the expression of certain specific myelin protein genes. They also suggest that aTf might exert its action at the posttranscriptional level and/or by direct transcriptional regulation of the genes.

Selenium is required for normal upregulation of myelin genes in differentiating oligodendrocytes

The purpose of this study was to characterize the selenium requirement for the normal differentiation of oligodendrocyte lineage cells. In primary mixed glial cultures prepared from newborn rat brains, the overall growth of cultures, as seen from the total RNA yield, was not significantly affected by selenium. However, 30 nM selenium was required for the normal upregulation the proteolipid protein, basic protein, and myelin-associated glycoprotein gene expression assessed by Northern blot analysis. Selenium deprivation during initial, rapid phase of the gene upregulation irreversibly suppressed the genes, indicating the existence of a critical period in oligodendrocyte differentiation. In purified oligodendrocyte cultures prepared by mechanical dislodging of progenitor (O-2A) cells from mixed glial cultures, total cell number and total RNA yield were virtually unaffected by selenium deprivation; however, the developmental upregulation of the myelin genes was profoundly attenuated. Immunocytochemical analysis confirmed the suppressive effect of selenium deficiency on the differentiation of oligodendrocyte lineage cells, as seen from a significant decrease in the population of GalC+ and O4+ cells. Because the number of GC+ cells was more reduced than the number of O4+ cells, the results indicate that selenium deficiency may specifically inhibit the progression from immature to mature oligodendrocytes.

Jimpy-4J mouse has a missense mutation in exon 2 of the Plp gene

We previously showed that the jimpy-4J mouse mutation is located on the X chromosome, in or closely linked to the proteolipid protein (Plp) gene. The phenotype is characterized by the most severe hypomyelination of any of the naturally occurring myelin mutant mice, sharp reduction in oligodendrocyte number, and virtual absence of PLP protein. Affected animals show tremor, seizures, and die at about 24 postnatal days. We now report that sequencing of Plp genomic and cDNAs identifies a single nucleotide substitution in exon 2 that predicts an Ala38Ser substitutions in a hydrophilic region of PLP/DM20 protein close to a transmembrane domain. This mutation occurs in a very different region of the mouse Plp gene than that jimpy-msd mutations, yet all three produce qualitatively similar phenotypes.

Assembly of CNS myelin in the absence of proteolipid protein

Two proteolipid proteins, PLP and DM20, are the major membrane components of central nervous system (CNS) myelin. Mutations of the X-linked PLP/DM20 gene cause dysmyelination in mouse and man and result in significant mortality. Here we show that mutant mice that lack expression of a targeted PLP gene fail to exhibit the known dysmyelinated phenotype. Unable to encode PLP/DM20 or PLP-related polypeptides, oligodendrocytes are still competent to myelinate CNS axons of all calibers and to assemble compacted myelin sheaths. Ultrastructurally, however, the electron-dense 'intraperiod' lines in myelin remain condensed, correlating with its reduced physical stability. This suggests that after myelin compaction, PLP forms a stabilizing membrane junction, similar to a "zipper." Dysmyelination and oligodendrocyte death emerge as an epiphenomenon of other PLP mutations and have been uncoupled in the PLP null allele from the risk of premature myelin breakdown.

Accelerated myelinogenesis by dietary lipids in rat brain

Our previous work showed an early development of behavioral reflexes in rats whose mothers had been fed, during pregnancy and lactation, a lipid fraction extracted from yeast grown on n-alkanes (which contain 50% odd-chain fatty acids) in comparison with controls fed a margarine diet. To clarify whether the observed changes might be linked to an early myelination, we have investigated mRNAs involved in myelin synthesis in the brains of offspring at 5 days of age by northern blot and in situ hybridization. Northern blot analysis showed that proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG) mRNAs were higher in animals on the lipid diet compared with controls. In situ hybridization with probes specific for PLP, myelin basic protein, and MOG mRNA showed significantly higher numbers of positive cells in test animals compared with controls in all brain regions. This study shows an acceleration of myelinogenesis induced by dietary lipids. These data can give a new insight in the therapeutical approaches involved to promote repair in demyelinating diseases.

Cloning and expression of the proteolipid protein DM20 cDNA from the brain of the rainbow trout, Oncorhynchus mykiss

The myelin sheath in higher vertebrates consists predominantly of proteolipid protein (PLP) and its smaller isoform DM20. Mutations in the PLP gene produces several neurological disorders such as Pelizaeus-Merzbacher disease and the rumpshaker phenotype in mice. This paper describes the cloning and expression of DM20 from the brain of Rainbow trout. We have isolated a nearly full-length cDNA clone containing 1835 bp that codes for a protein of 258 amino acids. Trout DM20 shows extensive homology with DM20 from higher vertebrates and includes the four hydrophobic regions that are believed to span the myelin membrane. The DM20 transcript is expressed throughout the central nervous system of the trout but appears at its highest levels in the spinal cord and medulla oblongata. The transcript is expressed at very low levels on hatching day but increases 179-fold by the 5th week. Contrary to higher vertebrates, there is no switch to the PLP transcript in maturing trout. Moreover, the rsh mutation (186 Thr to Ile) that produces the rumpshaking neurological disorder in mice has no effect in trout.

OLN-93: a new permanent oligodendroglia cell line derived from primary rat brain glial cultures

We have established a new permanent cell line (OLN-93), derived from spontaneously transformed cells in primary rat brain glial cultures. In growth medium supplemented with 10% fetal calf serum a doubling time of 16-18 hr was determined. OLN-93 cells in their antigenic properties resemble primary oligodendrocytes in culture. As analyzed by indirect immunofluorescence, the A2B5 surface marker is absent, they express galactocerebroside and myelin-specific proteins, such as myelin basic protein (MBP), myelin-associated glycoprotein (MAG), proteolipidprotein (PLP), and Wolfgram protein (WP), but do not exhibit astrocytic properties, such as the expression of vimentin or the glial fibrillary acidic protein (GFAP). In their morphological features they resemble bipolar O-2A-progenitor cells and, when grown at low density or on poly-L-lysine-coated culture dishes under low serum conditions, immature oligodendrocytes with a more arborized cell morphology. The cellular processes contain microfilaments, while N-CAM/D2 immunoreactivity is localized on the cell surface of the somata and processes. Immunoblot analysis further confirmed the presence of MAG, WP and MBP immunoreactivity, and the absence of vimentin and GFAP. Only a single MBP isoform (approximately 14 kDa) was detectable in the cellular extracts. PLP mRNA expression was studied by RT-PCR. The two proteolipid-specific mRNAs, DM20 and PLP, were present in OLN-93 cell extracts. Comparisons with embryonic rat cerebral cells in culture and primary oligodendrocytes suggest that OLN-93 cells in their morphological features and their antigenic properties resemble 5- to 10-day-old (postnatal time) cultured rat brain oligodendrocytes. Thus, the new cell line described in this study should provide a useful model system to investigate the specific mechanisms regulating the proliferation and differentiation of oligodendrocytes in vitro, and the molecular interactions with other cells of the nervous system.

Temporal and spatial expression of major myelin proteins in the human fetal spinal cord during the second trimester

Immunohistochemical identification of myelin basic protein (MBP) is a sensitive method for assessing myelination in the human fetal central nervous system (CNS). However, the temporospatial relationship of expression of two other major myelin proteins, proteolipid protein (PLP) and myelin-associated glycoprotein (MAG) to that of MBP during fetal development has not been assessed in human tissues. Vibratome sections of cervical, thoracic and lumbosacral levels from 37 normal spinal cords of < or = 10 to 24 gestational week (GW) fetuses were analyzed using immunohistochemical methods. Using light microscopy, MBP was the first oligodendrocyte marker detected, present by 10 GW at more rostral levels. PLP and MAG were detected rostrally between 12 to 14 GW. All myelin proteins were expressed in anterior to posterior and rostral to caudal gradients. By the late second trimester, expression of MBP, PLP and MAG was noted in all locations in the spinal white matter except for the corticospinal tract. Expression of MAG was particularly marked in the posterior root entry zone and propriospinal tracts. The results suggest that PLP and MAG are expressed later than MBP but follow similar spatial gradients.

Oligodendrocyte progenitors in the embryonic spinal cord express DM-20

Oligodendrocyte progenitors, originating in the ventral ventricular zone of the embryonic rodent spinal cord, migrate and differentiate into the oligodendrocytes myelinating the future white matter. Transcripts for the dm-20 isoform of the proteolipid protein (plp) gene are detectable initially in cells of the ventral ventricular region of the embryonic central canal and subsequently throughout the white matter. The dm-20+ cells are present several days before oligodendrocytes or myelin sheaths are detectable. The purpose of the present study was to determine if DM-20 protein is present and whether DM-20+ cells can be linked to the oligodendrocyte lineage in the mouse spinal cord. Expression of plp and dm-20 transcripts and product was monitored using reverse transcription polymerase chain reaction (RT-PCR), and in situ hybridization and immunostaining of cryosections and associated cultures. Cell identification was performed using antigenic markers characterizing different stages of oligodendrocyte differentiation. We show a temporal and spatial progression of cells expressing dm-20 transcripts and product from the ventral ventricular zone at embryonic day 13 (E13.0), via the lateral borders of the floor plate to the ventral pia and white matter. The cells, initially devoid of myelin basic protein (MBP) and PLP, co-express these myelin proteins at approximately E16.5/17.0. Some DM-20+ cells co-label with definitive markers of the early oligodendrocyte lineage, are capable of mitosis and subsequently differentiate into oligodendrocytes. Other DM-20+ cells may represent earlier precursor cells. The expression of DM-20 in oligodendrocyte progenitors is consistent with a postulated role in glial cell development.

Myelin basic protein in experimental allergic encephalomyelitis is not affected at the posttranslational level: implications for demyelinating disease

The microheterogeneity of myelin basic protein, expressed as the ratio between the least cationic (C-8) charge isomer and the most cationic (C-1), was examined in experimental allergic encephalomyelitis (EAE) cases. These included acute EAE of 2 months' duration induced with bovine proteolipid protein in complete Freund's adjuvant (CFA), chronic EAE induced with mouse spinal cord homogenate in varying doses from 0.5 to 2.0 mg in CFA, and chronic relapsing EAE of 12 months' duration induced with synthetic peptide 139-151 of the proteolipid protein sequence. The C-8/C-1 ratio was within the normal range for all groups of animals. However, the C-8/C-1 ratio was six- to sevenfold increased in a spontaneously demyelinating transgenic model, ND4, which contains 70 copies of the cDNA for DM20 (Mastronardi et al.: 1996). Since an increase in the C-8/C-1 ratio was also observed in victims of multiple sclerosis but not other neurological diseases, the ND4 model may address primary changes prior to demyelination, while the EAE model addresses the autoimmune aspects of the disease.

Loss of myelin basic protein cationicity in DM20 transgenic mice is dosage dependent

Demyelination in the transgenic mice depended on the dosage of the cDNA for DM20, in which low copy numbers (two to four and 17 copies of the minigene) showed no signs of demyelination. However when transgenic mice with 17 copies were made homozygous with 34 copies of the DM20 minigene (ND3A hm.) demyelination was observed at around 12 to 16 months compared with ND4 mice having 70 copies of the transgene which had an earlier onset of demyelinating symptoms at 3 months, demonstrating a transgene dosage effect. The process by which demyelination was initiated was associated with changes in myelin basic protein. An increased abundance of less cationic MBP (C-8) isomers occurred prior to demyelination. This increase was also associated with increased activity of peptidylarginine deiminase, the enzyme which converts arginine to citrulline in proteins, thereby providing a mechanism for generating less cationic forms of MBP. These data support a dosage effect of the DM20 transgene.