Stem cell therapy in multiple sclerosis: promise and controversy

Stem cells offer the potential for regeneration of lost tissue in neurological disease, including multiple sclerosis (MS). Their development in vitro and their use in vivo in animal models of degenerative neurological disease and recent first efforts in human clinical trials were the topics of a recent international meeting sponsored by the Multiple Sclerosis International Federation and the National Multiple Sclerosis Society on “Stem Cells & MS: Prospects and Strategies” Participants reviewed the current state of knowledge about the potential use of stem and progenitor cells in MS and other degenerative neurological disorders and outlined a series of urgent fundamental and applied clinical research priorities that should allow the potential of regeneration of damaged tissue in MS to be assessed and pursued.

Normal CNS myelination in transgenic mice overexpressing MHC class I H-2L(d) in oligodendrocytes

In demyelinating diseases, such as multiple sclerosis, an upregulation of MHC class I expression is thought to contribute to oligodendrocyte/myelin damage. In order to investigate potential physiological consequences of upregulated MHC class I expression in oligodendrocytes, we generated transgenic mice that overexpress H-2L(d) under the control of the proteolipid protein (PLP) promoter (PLP-L(d) mice). We focused our studies on the MHC class I molecule H-2L(d), because of its unique intracellular transport characteristics. In the CNS of PLP-L(d) mice, H-2L(d) was expressed by oligodendrocytes. Furthermore, H-2L(d) protein was transported to and expressed on the surface of oligodendrocytes. Most importantly, this upregulation of MHC class I expression in the CNS of PLP-L(d) mice did not by itself result in a de- or dysmyelinating phenotype. These transgenic mice are likely to provide a unique and novel tool for the analysis of potential roles of MHC class I-mediated mechanisms in demyelinating pathologies.

Akt-mediated survival of oligodendrocytes induced by neuregulins

Neuregulins have been implicated in a number of events in cells in the oligodendrocyte lineage, including enhanced survival, mitosis, migration, and differentiation. At least two signaling pathways have been shown to be involved in neuregulin signaling: the phosphatidylinositol (PI)-3 kinase and the mitogen-activated protein kinase pathways. In the present studies, we examined the signaling pathway involved in the survival function of heregulin, focusing on heregulin-induced changes in Akt activity in cultured glial cells, and the consequences of Akt activation in cells in the oligodendrocyte lineage. Heregulin binds erbB receptors, and in our studies, primary cultures of both oligodendrocyte progenitor cells and differentiating oligodendrocytes expressed erbB2, erbB3, and erbB4 receptors. In C6 glioma cells and primary cultures of oligodendrocytes, heregulin induced time- and dose-dependent Akt phosphorylation at Ser(473) in a wortmannin-sensitive manner. To investigate further the signaling pathway for heregulin in glial cells, BAD was overexpressed in C6 glioma cells. In these cells, heregulin induced phosphorylation of BAD at Ser(136). Apoptosis of oligodendrocyte progenitor cells induced by growth factor deprivation was effectively blocked by heregulin in a wortmannin-sensitive manner. Overexpression of dominant negative Akt but not of wild-type Akt by adenoviral gene transfer in primary cultures of both oligodendrocytes and their progenitors induced significant apoptosis through activation of the caspase cascade. The present data suggest that the survival function of heregulin is mediated through the PI-3 kinase/Akt pathway in cells in the oligodendrocyte lineage and that the Akt pathway may be quite important for survival of cells in this lineage.

Akt-mediated survival of oligodendrocytes induced by neuregulins

Neuregulins have been implicated in a number of events in cells in the oligodendrocyte lineage, including enhanced survival, mitosis, migration, and differentiation. At least two signaling pathways have been shown to be involved in neuregulin signaling: the phosphatidylinositol (PI)-3 kinase and the mitogen-activated protein kinase pathways. In the present studies, we examined the signaling pathway involved in the survival function of heregulin, focusing on heregulin-induced changes in Akt activity in cultured glial cells, and the consequences of Akt activation in cells in the oligodendrocyte lineage. Heregulin binds erbB receptors, and in our studies, primary cultures of both oligodendrocyte progenitor cells and differentiating oligodendrocytes expressed erbB2, erbB3, and erbB4 receptors. In C6 glioma cells and primary cultures of oligodendrocytes, heregulin induced time- and dose-dependent Akt phosphorylation at Ser(473) in a wortmannin-sensitive manner. To investigate further the signaling pathway for heregulin in glial cells, BAD was overexpressed in C6 glioma cells. In these cells, heregulin induced phosphorylation of BAD at Ser(136). Apoptosis of oligodendrocyte progenitor cells induced by growth factor deprivation was effectively blocked by heregulin in a wortmannin-sensitive manner. Overexpression of dominant negative Akt but not of wild-type Akt by adenoviral gene transfer in primary cultures of both oligodendrocytes and their progenitors induced significant apoptosis through activation of the caspase cascade. The present data suggest that the survival function of heregulin is mediated through the PI-3 kinase/Akt pathway in cells in the oligodendrocyte lineage and that the Akt pathway may be quite important for survival of cells in this lineage.

Proteolipid protein mRNA stability is regulated by axonal contact in the rodent peripheral nervous system

Proteolipid protein (PLP) and its alternatively spliced isoform, DM20, are the main intrinsic membrane proteins of compact myelin in the CNS. PLP and DM20 are also expressed by Schwann cells, the myelin-forming cells in the PNS, and are necessary for normal PNS function in humans. We have investigated the expression of PLP in the PNS by examining transgenic mice expressing a LacZ transgene under the control of the PLP promoter. In these animals, myelinating Schwann cells expressed beta-galactosidase more prominently than nonmyelinating Schwann cells. PLP/DM20 mRNA levels, but not those of LacZ mRNA, increased during sciatic nerve development and decreased after axotomy, with resultant Wallerian degeneration. PLP/DM20 transcription rates, in nuclear run off experiments, however, did not increase in developing rat sciatic nerve despite robust increases in PLP/DM20 mRNA levels during the same period. In RNAse protection studies, PLP mRNA levels fell to undetectable levels following nerve transection whereas levels of DM20 were essentially unchanged despite both being transcribed from the same promoter. Finally, cotransfection studies demonstrated that PLP-GFP, but not DM20-GFP mRNA is down-regulated in Schwann cells cultured in the absence of forskolin. Taken together these data demonstrate that steady state levels of PLP mRNA are regulated at a posttranscriptional level in Schwann cells, and that this regulation is mediated by Schwann cell-axonal contact. Since the difference between these two mRNAs is a 105-bp sequence in PLP and not in DM20, this sequence is likely to play a role in the regulation of PLP mRNA.

Purification and analysis of in vivo-differentiated oligodendrocytes expressing the green fluorescent protein

A complete understanding of the molecular mechanisms involved in the formation and repair of the central nervous system myelin sheath requires an unambiguous identification and isolation of in vivo-differentiated myelin-forming cells. In order to develop a novel tool for the analysis of in vivo-differentiated oligodendrocytes, we generated transgenic mice expressing a red-shifted variant of the green fluorescent protein under the control of the proteolipid protein promoter. We demonstrate here that green fluorescent protein-derived fluorescence in the central nervous system of 9-day- to 7-week-old mice is restricted to mature oligodendrocytes, as determined by its spatiotemporal appearance and by both immunocytochemical and electrophysiological criteria. Green fluorescent protein-positive oligodendrocytes could easily be visualized in live and fixed tissue. Furthermore, we show that this convenient and reliable identification now allows detailed physiological analyses of differentiated oligodendrocytes in situ. In addition, we developed a novel tissue culture system for in vivo-differentiated oligodendrocytes. Initial data using this system indicate that, for oligodendrocytes isolated after differentiation in vivo, as yet unidentified factors secreted by astrocytes are necessary for survival and/or reappearance of a mature phenotype in culture.

Disruption of the murine nuclear factor I-A gene (Nfia) results in perinatal lethality, hydrocephalus, and agenesis of the corpus callosum

The phylogenetically conserved nuclear factor I (NFI) family of transcription/replication proteins is essential both for adenoviral DNA replication and for the transcription of many cellular genes. We showed previously that the four murine NFI genes (Nfia, Nfib, Nfic, and Nfix) are expressed in unique but overlapping patterns during mouse development and in adult tissues. Here we show that disruption of the Nfia gene causes perinatal lethality, with >95% of homozygous Nfia(-/-) animals dying within 2 weeks after birth. Newborn Nfia(-/-) animals lack a corpus callosum and show ventricular dilation indicating early hydrocephalus. Rare surviving homozygous Nfia(-/-) mice lack a corpus callosum, show severe communicating hydrocephalus, a full-axial tremor indicative of neurological defects, male-sterility, low female fertility, but near normal life spans. These findings indicate that while the Nfia gene appears nonessential for cell viability and DNA replication in embryonic stem cells and fibroblasts, loss of Nfia function causes severe developmental defects. This finding of an NFI gene required for a developmental process suggests that the four NFI genes may have distinct roles in vertebrate development.

Digitized image analysis reveals diffuse abnormalities in normal-appearing white matter during acute experimental autoimmune encephalomyelitis

Demyelination of the central nervous system is a hallmark of multiple sclerosis and its widely used animal model, experimental autoimmune encephalomyelitis (EAE). Recent studies using magnetic resonance imaging and spectroscopy on multiple sclerosis patients have revealed abnormalities of central nervous system normal-appearing white matter suggesting that micro-demyelination and/or extensive membrane turnover accompanies and perhaps precedes the appearance of manifest inflammatory lesions. In the present study, we induced EAE in SWXJ mice and analyzed digitized images of immunocytochemically stained spinal cord for detection of myelin proteolipid protein (PLP). We found that digitized image analysis is a highly sensitive, objective methodology for measuring the extent of myelin loss during EAE. Our data show that two-thirds of the measured reduction of myelin PLP occurring in EAE spinal cord could be attributed to a loss of myelin in normal-appearing white matter. The marked decrease in detection of PLP was accompanied by a corresponding decrease in PLP mRNA in the central nervous system. Our results indicate that during acute EAE, diffuse myelin abnormalities extend far beyond visibly detectable inflammatory foci and are characterized by a global decrease in the expression of myelin genes and their encoded proteins.

Regulation of murine myelin proteolipid protein gene expression

To identify putative sequences that direct cell type-specific expression and/or enhance proteolipid protein (PLP) gene expression, glial or nonglial cells were transfected with various PLP-luciferase constructs that collectively span the entire mouse PLP-specific DNA present in a transgene known to direct cell type specificity in transgenic mice. These constructs were transfected into murine oligodendrocyte cell lines that transcribe the PLP gene and, hence, should contain the requisite trans-acting factors necessary for PLP gene expression. Mouse NIH/3T3 fibroblasts were used as a nonglial model. We have finely mapped the PLP promoter region for transcriptional regulatory elements and demonstrate both positive and negative elements, none of which appear to extinguish expression in nonglial cells. The 5'-flanking PLP DNA tested did not enhance the basal herpes simplex-1 virus thymidine kinase (TK) promoter, nor did PLP sequences present in the distal half of intron 1. The 5' portion of intron 1 did enhance TK promoter activity, suggesting that this region of the gene may contain enhancer elements that modulate PLP gene expression; however, the enhancement did not appear to be cell type-specific. Intriguingly, a 541 bp region of the intron that significantly enhanced TK promoter activity contains multiple JC virus repeated elements and other elements known to be important in restricting the virus to oligodendrocytes. These results suggest that intron 1 sequences may modulate expression of the PLP gene.

Phosphodiesterase I, a novel adhesion molecule and/or cytokine involved in oligodendrocyte function

One of the more complex developmental processes occurring postnatally in the CNS is the formation of the myelin sheath by oligodendrocytes. To examine the molecular events that take place during myelination, we isolated oligodendrocyte-derived cDNA clones, one of which (p421.HB) represents a putative alternatively spliced isoform of rat brain-specific phosphodiesterase I (PD-Ialpha) and a species homolog of the human cytokine autotaxin. Analysis of the structural composition of the p421.HB/PD-Ialpha protein suggests a transmembrane-bound ectoenzyme, which, in addition to the phosphodiesterase-active site contains presumed cell recognition and Ca2+-binding domains. Consequently, it may be involved in extracellular signaling events. Expression of p421.HB/PD-Ialpha is enriched in brain and spinal cord, where its mRNA can be detected in oligodendrocytes and in cells of the choroid plexus. Expression in the brain increases during development with an intermediate peak of expression around the time of active myelination and maximal expression in the adult. We have identified four presumably alternatively spliced isoforms, two of which appear to be CNS-specific. Decreased levels of p421.HB/PD-Ialpha mRNA in the dysmyelinating mouse mutant jimpy, but not shiverer, suggest a role for p421.HB/PD-Ialpha during active myelination and/or late stages of oligodendrocyte differentiation. Furthermore, p421.HB/PD-Ialpha mRNA levels were reduced in the CNS at onset of clinical symptoms in experimental autoimmune encephalomyelitis. These data together implicate the importance of p421.HB/PD-Ialpha in oligodendrocyte function, possibly through cell-cell and/or cell-extracellular matrix recognition.

Expression of molecular chaperones and vesicle transport proteins in differentiating oligodendrocytes

The major stages of oligodendrocyte differentiation are defined by the presence or absence of certain myelin-specific proteins. Events leading to the successful processing of these proteins, such as the folding, assembly, and trafficking of these proteins through the biosynthetic pathway, are largely undefined. In the present study, we have examined both cultured primary oligodendrocytes and immortalized oligodendrocyte cell lines for the presence of molecular chaperones and/or vesicle transport proteins. We find that a select set of these proteins are expressed relatively early in oligodendrocyte differentiation, whereas a characteristically different set of proteins are expressed at later stages of oligodendrocyte differentiation. In other systems, these proteins participate in the folding and assembly of protein complexes, in the prevention of protein aggregation, as well as the trafficking of proteins via vesicles to specific subcellular destinations including the plasma membrane. Some of the chaperones and/or vesicle transport proteins investigated in this study may play a pivotal role in the certain aspect of myelin biogenesis.

Use of Transgenic Systems to Investigate Oligodendrocyte Differentiation and Function

Transgenic techniques are generating new strains of animals that are of great importance for many neurological research projects. This includes new animal models of human diseases that should allow analysis of disease etiology and treatment. The insertion of new genetic material into the mouse genome enables the investigator to study the effects of overexpression of normal or mutated genes under a variety of experimental conditions. The use of cell-specific and/or developmentally regulated promoters permits studies on the expression of the specific DNA in selected cells within the nervous system at important developmental stages. This article focuses on the techniques for generating transgenic mice, noting specific advantages or problems that should be considered when designing a transgenic project. The use of reporter genes such as the LacZ gene is discussed, using the particular example of the myelin proteolipid protein promoter directing expression of the LacZ gene in differentiating oligodendrocytes.

GapIII, a new brain-enriched member of the GTPase-activating protein family

Ras GTPase-activating proteins (GAPs) are negative regulators of ras, which controls proliferation and differentiation in many cells. Ras GAPs have been found in a variety of species from yeast to mammals. We describe here a newly identified mammalian GAP, GapIII, which was obtained by differential screening of a rat oligodendrocyte cDNA library. GapIII putatively encodes a 834 amino acid protein with a predicted molecular weight of 96 kDa, which contains a consensus GAP-related domain (GRD). The protein encoded by this cDNA has high homology with Gap1m, which was recently identified as a putative mammalian homolog of Drosophila Gap1. These proteins contain three structural domains, an N-terminal calcium-dependent phospholipid binding domain, GRD, and a C-terminal PH/Btk domain. Because of the sequence homology and the structural similarities of this protein with Gap1m, we hypothesize that GapIII and Gap1m may be members of a mammalian GAP gene family, separate from p120GAP, neurofibromin (NF1), and IQGAP. To confirm the GapIII protein activity, constructs containing different GapIII-GRD domains were transformed into iral mutant yeast to determine their relative ability to replace IRA1 functionally. Constructs that contained essentially the full-length protein (all three domains), the GRD alone, or the GRD plus PH/Btk domain suppressed heat shock sensitivity of ira1, whereas constructs that contained the GRD with part of the PH/Btk domain had only a weak ability to suppress heat shock sensitivity. These results suggest that the GapIII GRD itself is sufficient to down-regulate ras proteins in yeast. Expression of GapIII mRNA (4.2 kb) was examined by Northern analysis and in situ hybridization. This mRNA was expressed at highest levels in the brain, where its expression increased with development. Lower levels of the mRNA were expressed in the spleen and lung. Among neural cells, GapIII mRNA was expressed in neurons and oligodendrocytes, but not in astrocytes. Interestingly, the expression pattern in brain is reminiscent of type 1 NF1 expression reported by Gutmann et al. (Cell Growth Differ in press, 1995). We propose that in addition to p120GAP and neurofibromin, the GapIII/Gap1m family may be important for modulating ras activity in neurons and oligodendrocytes during normal brain development and in particular in the adult brain.

Expression of cell surface markers and myelin proteins in cultured oligodendrocytes from neonatal brain of rat and mouse: a comparative study

Dissociated brain cell cultures are a useful model for investigating development and differentiation of oligodendrocytes in vitro. The current studies compare the developmental patterns of expression for oligodendrocyte lineage/myelin markers in both primary and secondary oligodendrocyte cultures derived from mouse and rat neonates. The rat and mouse dissociated brain cell cultures express the same myelin-specific antigens, but mouse oligodendrocytes produce a larger and more elaborate sheet-like membrane than rat oligodendrocytes, and some of the myelin markers (O4, GC, and MBP) show more intense membrane staining in mouse cultures. GD3 appears to be a good oligodendrocyte marker for rat cells, but it is nonspecific in mouse cells. There are fewer oligodendrocytes in mouse cultures, and they appear to require a longer differentiation time than rat oligodendrocytes. These same results are also observed in secondary oligodendrocyte cultures, although in general late myelin markers such as MBP and PLP are expressed at a much lower level in mouse cells than rat cells.

Developmental expression of protein kinase C isozymes in oligodendrocytes and their differential modulation by 4 beta-phorbol-12,13-dibutyrate

Myelin gene expression in normal oligodendrocytes (OLG) depends on developmentally regulated protein kinase C (PKC) enzyme activity (Asotra and Macklin: J Neurosci Res 34:571-588, 1993). We studied the developmental expression of the Ca(++)-dependent PKC-alpha, -beta 1, -beta II and -gamma isozymes, and the Ca(++)-independent PKC-delta, -epsilon, -zeta and -eta isozymes in enriched rat brain OLG cultures. In A2B5+ O-2A progenitors, only PKC-delta, PKC-epsilon and PKC-zeta were detected immunocytochemically. In 04+ proligondendrocytes, PKC-beta I, -delta and -zeta were expressed moderately and low levels of PKC-alpha and -epsilon were detected. GD3+ OLG, GC+ OLG and MBP+ OLG showed increased levels of PKC-alpha, -beta I, -delta and -zeta isozymes. PKC-beta II, -gamma and -eta were poorly expressed in OLG. On immunoblots, PKC-alpha was present early and increased continually up to 18 days but PKC-beta I increased until 12 days in cultured OLG. High levels of PKC-delta, PKC-epsilon and PKC-zeta, the most abundant PKC isozymes in OLG, were maintained up to 12 days and were then slightly reduced. Interestingly, relatively high levels of PKC-alpha, PKC-beta I, PKC-beta II, PKC-gamma and PKC-epsilon isozymes were detected in purified myelin membrane although greater levels of PKC-delta were found in OLG than in purified myelin. Thus, most of the PKC isozymes found in cultured OLG were also present in myelin, although at different levels. Treatment with 50 nM 4 beta-phorbol-12,13-dibutyrate (PDB) caused a delayed downregulation of PKC-delta levels after 8 hr without modulating the expression of other PKC isozymes in 1-day OLG; in the 3-day-old and 6-day-old OLG, PDB downmodulated PKC-beta I, -delta and epsilon isozymes with only a minor effect on PKC-alpha and no reduction in PKC-zeta. Induction or downmodulation of individual PKC isozymes by phorbol esters appears to depend on the differentiation state of OLG. These data suggest that PKC-beta I, -delta and -epsilon isozymes have an important function in different cellular events of OLG differentiation. We conclude that the PKC-dependent modulation of myelin gene expression in OLG results predominantly from the Ca(++)-dependent PKC-beta I isozyme activity and the CA(++)-independent PKC-delta and PKC-epsilon activitives in a cell differentiation state-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of novel mRNAs expressed in oligodendrocytes

To identify new proteins, which are expressed in oligodendrocytes and which may have a functional role in myelination, a rat oligodendrocyte cDNA library was screened using differential and subtractive screening techniques. Ten clones that have elevated levels of expression in brain were isolated. Two of these clones were characterized further and one clone, pC26.H2, was found to be closely related to mouse stearoyl-CoA desaturase 2 (SCD2), which catalyzes the synthesis of unsaturated fatty acid. From Northern blot and in situ hybridization studies, SCD2 mRNA was expressed primarily in brain with lower levels found in lung and spleen. In brain sections, SCD2 mRNA was found primarily in oligodendrocytes, although mRNA was detected at a low level in neurons, in particular in Purkinje cells in the cerebellum. Northern blot analysis of the other clone, p973.HB, indicated that it was expressed more selectively in brain. In mixed glial cultures oligodendrocytes were the only cells that expressed this mRNA, whereas in brain, neurons expressed this mRNA at a higher level than in oligodendrocytes. This clone is being actively pursued because of its unique expression exclusively in oligodendrocytes and neurons.

A myelin proteolipid protein-LacZ fusion protein is developmentally regulated and targeted to the myelin membrane in transgenic mice

Transgenic mice were generated with a fusion gene carrying a portion of the murine myelin proteolipid protein (PLP) gene, including the first intron, fused to the E. coli LacZ gene. Three transgenic lines were derived and all lines expressed the transgene in central nervous system white matter as measured by a histochemical assay for the detection of beta-galactosidase activity. PLP-LacZ transgene expression was regulated in both a spatial and temporal manner, consistent with endogenous PLP expression. Moreover, the transgene was expressed specifically in oligodendrocytes from primary mixed glial cultures prepared from transgenic mouse brains and appeared to be developmentally regulated in vitro as well. Transgene expression occurred in embryos, presumably in pre- or nonmyelinating cells, rather extensively throughout the peripheral nervous system and within very discrete regions of the central nervous system. Surprisingly, beta-galactosidase activity was localized predominantly in the myelin in these transgenic animals, suggesting that the NH2-terminal 13 amino acids of PLP, which were present in the PLP-LacZ gene product, were sufficient to target the protein to the myelin membrane. Thus, the first half of the PLP gene contains sequences sufficient to direct both spatial and temporal gene regulation and to encode amino acids important in targeting the protein to the myelin membrane.

Enhanced degradation of messenger RNA encoding myelin proteins by terminal complement complexes in oligodendrocytes

Sublytic terminal C complexes (TCC) are capable of stimulating cells and affect the target cell activity. Activation of TCC that generates leukotriene B4 in oligodendrocytes, the myelin-forming cells of the central nervous system, is also a required process in antibody-mediated demyelination of rodent cerebellar explants. In the present study, the effect of TCC on myelin protein gene expression was studied in primary rat oligodendrocytes in culture. Sublytic activation of serum C reduced accumulation of mRNA encoding proteolipid protein (PLP) and myelin basic protein (MBP) within 1 h, but not beta-actin mRNA. C activation, on the other hand, induced sustained expression of c-jun mRNA. Experiments using C7-deficient human serum to determine the role of TCC showed that selective MBP and PLP mRNA down-regulation was achieved only when C7 was reconstituted to form TCC. The C7 requirement was also observed in the presence of alpha-amanitin. Post-transcriptional regulation was explored by determining mRNA decay, which demonstrated that the MBP and PLP mRNA were selectively destabilized when C7 was reconstituted. Limited exploration of the signals responsible for the TCC effect revealed that down-regulation of mRNA by TCC was significantly influenced by Ca2+ on PLP, whereas MBP did not show the same Ca2+ sensitivity as PLP. The TCC-mediated MBP mRNA decay was completely abrogated by HA1004, an inhibitor for the cAMP- and cGMP-dependent protein kinases, but not by H7, a protein kinase C inhibitor.

Enhanced degradation of messenger RNA encoding myelin proteins by terminal complement complexes in oligodendrocytes

Sublytic terminal C complexes (TCC) are capable of stimulating cells and affect the target cell activity. Activation of TCC that generates leukotriene B4 in oligodendrocytes, the myelin-forming cells of the central nervous system, is also a required process in antibody-mediated demyelination of rodent cerebellar explants. In the present study, the effect of TCC on myelin protein gene expression was studied in primary rat oligodendrocytes in culture. Sublytic activation of serum C reduced accumulation of mRNA encoding proteolipid protein (PLP) and myelin basic protein (MBP) within 1 h, but not beta-actin mRNA. C activation, on the other hand, induced sustained expression of c-jun mRNA. Experiments using C7-deficient human serum to determine the role of TCC showed that selective MBP and PLP mRNA down-regulation was achieved only when C7 was reconstituted to form TCC. The C7 requirement was also observed in the presence of alpha-amanitin. Post-transcriptional regulation was explored by determining mRNA decay, which demonstrated that the MBP and PLP mRNA were selectively destabilized when C7 was reconstituted. Limited exploration of the signals responsible for the TCC effect revealed that down-regulation of mRNA by TCC was significantly influenced by Ca2+ on PLP, whereas MBP did not show the same Ca2+ sensitivity as PLP. The TCC-mediated MBP mRNA decay was completely abrogated by HA1004, an inhibitor for the cAMP- and cGMP-dependent protein kinases, but not by H7, a protein kinase C inhibitor.

Protein kinase C activity modulates myelin gene expression in enriched oligodendrocytes

Protein kinase C (PKC) and its potential role in myelin gene expression were investigated in primary cultured rat oligodendrocytes. The major myelin genes were expressed in a developmentally coordinated manner in cultured oligodendrocytes. PKC activity in these cells was similarly regulated with differential expression of PKC isozyme mRNAs. PKC-gamma mRNA was expressed transiently and was most abundant in 9-day cells in vitro. PKC-alpha and PKC-beta mRNAs were present at low levels throughout development in these cells, and their expression increased in 18-25 day cells. Immunocytochemical colocalization of PKC with oligodendrocyte-specific markers--O4, galactosyl cerebroside, MBP, and PLP--in enriched oligodendrocyte cultures suggested that the PKC enzyme activities assayed in these cultures were predominantly contributed by oligodendrocytes. PKC inhibition resulting from long-term exposure to 4 beta-phorbol-12,13-dibutyrate (4 beta-PDB) reduced steady-state levels of MBP, PLP, MAG, CNP, and PKC-alpha mRNAs, as detected by slot blots or in situ hybridization, and downregulated the oligodendrocyte-specific markers O4, galactosyl cerebroside, and the major constituent proteins MBP and PLP, as detected by immunocytochemistry. PKC-mediated downmodulation of myelin gene expression was most profound in normally differentiating oligodendrocytes at or before the onset of myelin protein synthesis. Six-day oligodendrocytes were most susceptible to such modulation. To elucidate the mechanism of reduction in various myelin gene messages upon modulation of PKC, we analyzed mRNA levels in oligodendrocytes, which were pretreated with either the transcriptional inhibitor actinomycin D or the protein synthesis blocker cycloheximide before exposure to 4 beta-PDB. Our results demonstrate that the PKC inhibition-mediated loss in myelin mRNA levels did not require the transcription of any genes, but appeared to be at least partially dependent on continuous protein synthesis.

Mutations in the myelin proteolipid protein gene alter oligodendrocyte gene expression in jimpy and jimpymsd mice

The mouse myelin proteolipid protein (PLP) gene has been studied in normal and jimpymsd mice. Potential upstream regulatory regions of the normal gene have been cloned and mapped, but when these regions were studied in jimpymsd mice by Southern blots, no alterations were observed, relative to the normal gene. To assess whether the low ratio of PLP to DM20 proteins in this mutant reflected an altered PLP/DM20 ratio mRNAs, S1 nuclease analyses were undertaken, which demonstrated that at all ages studied in both jimpy and jimpymsd mice, PLP mRNA was elevated above DM20 mRNA. When exon 3 (the site of the alternative splice signal for DM20 mRNA) of the jimpymsd PLP gene was sequenced, no mutation was identified. The transcription of the PLP gene in normal and mutant animals was studied. The transcription rate increases in normal animals with development, and in very young jimpymsd or jimpy mice, the transcription rate of the PLP gene was close to that of age-matched normal animals. However, by 10 days of age, the transcription rate of this gene in both mutants was significantly below that of age-matched controls. The transcription rate of the myelin basic protein (MBP) gene was also reduced, indicating that expression of both genes is affected by this mutation. In contrast, the transcription rate of the glycerol phosphate dehydrogenase (GPDH) gene, an early marker of oligodendrocytes, is equal to or greater than normal in both mutants. We have confirmed an earlier report of a point mutation in exon 6 of the jimpymsd PLP gene, which converts an alanine to a valine.(ABSTRACT TRUNCATED AT 250 WORDS)

Iron-enriched oligodendrocytes: a reexamination of their spatial distribution

Previous histochemical studies of iron in the brain demonstrated iron enrichment in oligodendrocytes, but the iron-enriched oligodendrocytes had a spatially restricted distribution that excluded major white matter tracts. In this study we used techniques designed to permeabilize tissue, combined with iron histochemistry, and we observed iron-enriched oligodendrocytes in regions previously thought to lack such cells. Our data suggest that most, if not all, oligodendrocytes are enriched in iron. We suggest that iron functions in the formation and/or maintenance of the myelin sheet and may play a role in the pathology of myelin diseases.

Transcriptional regulation studies of myelin-associated genes in myelin-deficient mutant rats

To identify and assess the consequences of the mutation in myelin-deficient (md) rats, the myelin proteolipid protein (PLP) gene and its expression were studied in md rats. Southern blots of the PLP gene demonstrated that no major deletions or insertions have occurred in this gene. In addition, the mutation in this gene does not result in a splicing defect in the RNAs, since all exons are represented in md PLP RNAs. These data are consistent with results in another laboratory indicating that a point mutation in the PLP gene in md rats results in a single amino acid alteration in the protein. To elucidate the molecular mechanisms producing reduced levels of PLP, myelin basic protein (MBP) and glycerol phosphate dehydrogenase (GPDH) mRNAs, and their corresponding proteins in md rats, in vitro transcription assays were performed. Transcription of the PLP gene in nuclei isolated from 23-day-old md rat brains was dramatically reduced relative to normal tissue. Thus, the single amino acid alteration in this protein alters the regulation of transcription of this gene. In contrast, the transcriptional activities of the MBP and GPDH genes in md rats were indistinguishable from normal animals. Thus, the lower level of MBP and GPDH mRNA and protein in md rats relative to normal results from a posttranscriptional event.

Developmental expression of proteolipid protein and DM20 mRNAs and proteins in the rat brain

Proteolipid protein (PLP) and DM20, two abundant proteins of myelin, are produced from alternatively spliced mRNAs from the primary PLP gene transcript. Recent studies on the mouse, bovine and human central nervous systems have found that DM20 protein is expressed prior to PLP during development. As development proceeds, however, PLP becomes the predominant protein. This implies that there must be some form of regulation controlling the ratio of these proteins during development. If this regulation occurs during transcription or splicing, the PLP/DM20 mRNA ratio should mimic the PLP/DM20 protein ratio during development. In order to study these closely related mRNAs, we developed a method that used oligonucleotide probes to specifically identify PLP or DM20 mRNA. In this study, we established that (1) as with other species studied, the DM20 protein is present at higher or equivalent levels to PLP during early rat brain development, and (2) PLP and DM20 mRNAs have essentially identical developmental profiles in the rat, although the DM20 mRNA is expressed at lower levels than PLP mRNA. Since the PLP/DM20 protein ratio in young rats does not reflect the PLP/DM20 mRNA ratio, the mechanism of regulation responsible for altering the PLP/DM20 protein ratio during development must occur after transcription and splicing. Possible posttranscriptional mechanisms controlling the ratio of these proteins during development are discussed.

Myelin proteolipid protein gene expression in jimpy and jimpy(msd) mice

Proteolipid protein (PLP) gene expression was studied in the dysmyelinating mouse mutant jimpy(msd) (jpmsd; myelin synthesis deficient) and compared with that in wild-type mice and the allelic mutant, jimpy (jp). Southern analyses of genomic DNA from jpmsd mice revealed no major rearrangements of the PLP gene relative to the wild-type mouse PLP gene. PLP-specific mRNA levels were significantly reduced in these mutant mice, although both the 3.2- and 2.4-kilobase PLP-specific mRNAs were seen. Also, no size differences in either PLP or DM20 mRNAs were found by S1 nuclease assays of brain RNA from either jpmsd or wild-type mice. Both PLP and DM20 protein were detectable at low levels in jpmsd brain homogenates, and these proteins comigrated with PLP and DM20 protein from normal mice. Western analyses showed an altered PLP:DM20 ratio in jpmsd mice relative to wild-type mice; DM20 levels exceeded PLP levels. It is surprising that a similar pattern of expression was seen in normal mice at less than 10 days of age: DM20 protein expression preceding PLP expression. Thus, jpmsd mice are capable of synthesizing normal PLP and DM20 protein; however, the PLP gene defect has affected the normal developmental pattern of expression for these two proteins.

Biotin enrichment in oligodendrocytes in the rat brain

Cytochemical localization of endogenous biotin in the rat brain was detected by two different staining methods, avidin-biotin-HRP and goat-anti-biotin with HRP-rabbit-anti-goat. In both staining methods, oligodendrocytes were labeled to a far greater degree than other brain cells. This finding may be important for identification of the role of carbonic anhydrase, which is elevated in the oligodendrocyte, and it may help to explain how the oligodendrocyte generates large quantities of lipids for myelin production.

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.

An AG----GG transition at a splice site in the myelin proteolipid protein gene in jimpy mice results in the removal of an exon

The myelin proteolipid protein gene was characterized in jimpy mice to identify the specific mutation that produces dysmyelination, oligodendrocyte cell death, and death of the animal by 30 days of age. Exon 5 and flanking intron segments were isolated from jimpy proteolipid protein genomic clones and sequenced. A single nucleotide difference was noted between the normal and jimpy proteolipid protein genes: the conversion of an AG/GT to a GG/GT in the splice acceptor signal preceding exon 5, which apparently destroys the splice signal. Thus, exon 5 of the mouse myelin proteolipid protein gene is skipped during the processing of mRNA, producing a shortened proteolipid protein mRNA.

Structure and expression of the mouse myelin proteolipid protein gene

The gene for the mouse myelin proteolipid protein has been isolated and the seven exons have been sequenced. Since the sequence of a rat proteolipid protein cDNA and partial sequence of the human proteolipid protein gene have been determined, it was possible to demonstrate a very high degree of conservation for the proteolipid protein gene exons among species. While there are some nucleotide changes, the protein coding region of the mouse gene encodes protein that is totally conserved relative to both rat and human proteolipid proteins. The regulatory and noncoding regions of the proteolipid protein gene are also highly conserved. The upstream regulatory and 5'-noncoding region of the gene is 92% homologous to the comparable region of the human proteolipid protein gene, and the 3'-noncoding region of the mouse gene is approximately 90% homologous to a rat proteolipid protein cDNA through 2,200 nucleotides of 3'-noncoding DNA. S1 nuclease protection experiments indicated that the major 5'-end for proteolipid protein mRNAs from mouse, rat, human, or baboon is approximately 147-160 nucleotides upstream from the initial methionine codon of the protein coding region. Other S1 nuclease protection experiments indicated the possible existence of an alternative splice site within exon 3, which may produce mRNA for DM20. This mRNA is approximately 100 nucleotides shorter than that for the proteolipid protein, and it is missing the latter half of exon 3, that is, amino acids 116-150 of the proteolipid protein sequence.

Expression of myelin proteins in the developing human spinal cord: cloning and sequencing of human proteolipid protein cDNA

A full-length clone for the human proteolipid protein (PLP) was isolated from a cDNA library constructed from poly(A)+ RNA isolated from fetal spinal cords obtained at 15-24 weeks of conceptional age. The sequence of the human PLP cDNA was determined, and the deduced amino acid sequence was found to be identical with that of rat PLP. Comparison of human and rat PLP cDNA clones indicated that the coding regions retained 97% homology and that there were also other areas of conserved sequence. The human 5'-untranslated region was 93% homologous to that of the rat. The 3'-untranslated region was, overall, 73% homologous to that of the rat with areas containing greater than 84% homology in the first 400 and last 200 nucleotides. The most variability within the 3'-untranslated region occurred between nucleotides 2,000-2,500, where homology with the rat cDNA dropped to 55%. Expression of PLP in the human spinal cord between 11 and 23 weeks after conception was examined and compared with the expression of the myelin basic protein (MBP). RNA was isolated from pooled human spinal cords obtained at three periods of development: 11-14 weeks, 17-19 weeks, and 21-23 weeks. Northern blot analysis revealed a 3.2-kilobase (kb) PLP mRNA that was present at higher abundance in the 21-23-week spinal cord RNA than in the 17-19-week or the 11-14-week samples. The 17-19-week RNA sample also contained a PLP-hybridizing band at 2.2 kb which may possibly have arisen by utilization of alternative polyadenylation signals. Messenger RNA for MBP was detectable at 11-14 weeks but was readily evident in both the 17-19- and 21-23-week age groups. Immunoblot analysis of whole spinal cord homogenates indicated that polypeptides for MBP preceded the appearance polypeptides for PLP by 3-4 weeks.

Characterization of myelin proteolipid mRNAs in normal and jimpy mice

A clone specific for the rat myelin proteolipid protein (PLP) was isolated from a cDNA library made in pUC18 from 17-day-old rat brain stem mRNA. This clone corresponded to the carboxyl-terminal third of the PLP-coding region. The clone was used to identify PLP-specific mRNAs in mouse brain and to establish the time course of PLP mRNA expression during mouse brain development. Three PLP-specific mRNAs were seen, approximately 1,500, 2,400, and 3,200 bases in length, of which the largest was the most abundant. During brain development, the maximal period of PLP mRNA expression was from 14 to 25 days of age, and this was a similar time course to that for myelin basic protein mRNA expression. When the jimpy mouse, an X-linked dysmyelination mutant, was studied for PLP mRNA expression, low levels of PLP mRNA were seen which were approximately 5% of wild-type levels at 20 days of age. When jimpy brain RNA was analyzed by Northern blotting, the PLP-specific mRNA was shown to be 100 to 200 bases shorter than the wild-type PLP-specific mRNA. This size difference was seen in the two major PLP mRNAs, and it did not result from a loss of polyadenylation of these mRNAs.

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.

Appearance of myelin proteins during development in the chick central nervous system

The time course of the appearance of myelin-specific markers was studied in the developing chick central nervous system (CNS). Chick CNS tissue was studied for the presence of both proteolipid and myelin basic protein by electroblotting and for 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNPase) by enzyme assay. Four regions of chick spinal cord (cervical, brachial, thoracic and lumbar), brain stem, cerebellum, optic nerve and cortex were studied. In general, myelin basic protein appeared approximately 1 day earlier than proteolipid. In spinal cord and brain stem, myelin basic protein appeared at 13 days incubation. In cerebellum and optic nerve, it appeared at 17 days incubation and in cortex at hatching. CNPase activity increased in most CNS regions between 16 days incubation and hatching. These results suggest that myelination occurs earlier in the chick than in the rat and that it occurs over a shorter time period.

Chronic experimental allergic encephalomyelitis produced by bovine proteolipid apoprotein: immunological studies in rabbits

A chronic experimental allergic encephalomyelitis (EAE) has been produced in rabbits sensitized with bovine white matter proteolipid apoprotein. Eleven of 12 animals developed clinical disease one to six months after immunization with a single dose of the apoprotein. The clinical course was characterized by posterior ataxia, flaccid paralysis progressing to spastic paralysis, and incontinence. Spontaneous relapses and remissions were observed in 3 rabbits. Histologically, acute and chronic encephalomyelitis accompanied by primary demyelination were observed. Serum antibody production, assayed by both an enzyme-linked immunosorbent assay and an electroblot procedure, did not correlate with either the clinical course or the histopathological findings. Delayed hypersensitivity to proteolipid apoprotein was observed in all rabbits prior to the onset of clinical signs. The data suggest that lymphocytes specifically sensitized to the proteolipid may be involved in the pathogenesis of the demyelination in chronic EAE.

Electroblot analysis of rat myelin proteolipid protein and basic protein during development

The accumulation of the myelin proteolipid protein during rat brain development was studied in the spinal cord, lower brain stem (pons and medulla), cerebellum, and upper brain stem. Proteolipid accumulation was assessed on electroblots of total membrane samples from each region between 2 and 18 days of age. Proteolipid could be detected in rat spinal cord as early as 2 days of age, in lower brain stem and cerebellum by 4 days of age and in upper brain stem by 10 days of age. The time course of myelin basic protein accumulation was essentially the same as that of the proteolipid in each of the brain regions.

Chronic experimental allergic encephalomyelitis induced in rabbits with bovine white matter proteolipid apoprotein

A chronic, progressive form of experimental allergic encephalomyelitis was produced by immunization of rabbits with bovine brain white matter proteolipid apoprotein. Clinical signs appeared 4 to 13 months after sensitization, and were characterized by ataxia and limb paresis which progressed to flaccid paralysis and incontinence. Light and electron microscopic observations showed both acute and chronic nonsuppurative myelitis or encephalomyelitis accompanied by primary demyelination. Myelin damage was most evident in the spinal cord but was also present in the optic nerve and brain. The neuropathology was consistent with lesions of chronic experimental allergic encephalomyelitis produced by central nervous system tissue, and resembled lesions of multiple sclerosis as well. These observations suggest that protein may be involved in the pathophysiology of demyelinating diseases. A mechanism for the chronic course of the disease is discussed.

Solid-phase immunoassays for quantitation of antibody to bovine white matter proteolipid apoprotein

Two solid-phase immunoassays have been developed for quantitation of antibodies to bovine white matter proteolipid apoprotein. Conditions were established for optimal specific antibody binding. Water-soluble proteolipid apoprotein was bound to microtiter plates and plates were incubated with test serum. Goat anti-rabbit IgG conjugated with horseradish peroxidase was used as the second antibody for an enzyme-linked immunospecific assay and 125I-labeled protein A for a radioimmunoassay. Both procedures have been used to follow the time course of anti-proteolipid antibody production in rabbits and to compare different immunization protocols.

Electroblot analysis of the myelin proteolipid protein

The myelin proteolipid has been studied by the electroblot method of Towbin et al [1979]. Samples were separated by SDS-polyacrylamide gel electrophoresis, transferred to nitrocellulose, and incubated with proteolipid antibody. The proteolipid band could be identified immunologically in CNS myelin and in whole brain homogenates. No proteolipid was detected in PNS myelin. Proteolipid from human, bovine, rat, and mouse myelin all cross-react when analyzed by this method. No cross-reactivity was indicated between proteolipid and myelin basic protein.

A study of elastase peptides from bovine white matter proteolipid

Bovine white matter proteolipid has been digested with elastase in the presence of deoxycholate. After acidification, the digest was separated into an acid-soluble and an acid-insoluble fraction. The acid-insoluble fraction was enriched in nonpolar amino acids and, by a combination of solvent fractionation and chromatography, a fraction was obtained which consisted of a mixture of two peptides with a molecular weight of approximately 4000 daltons. The acid-soluble peptides were separated by molecular sieve, ion exchange and high performance liquid chromatography (HPLC) in the reverse phase mode. The purified peptides were smaller than expected on the basis of their elution position from a molecular sieve column, suggesting they were in an aggregated state during the initial chromatography. Reverse phase HPLC was shown to be useful for fingerprinting these peptide mixtures. The data demonstrate the difficulties associated with the study of this proteolipid and emphasize the tendency of both the protein and the peptides derived from it to aggregate.

Hydrophobic compounds interfere in radioimmunoassay for basic protein in myelin

Hydrophobic compounds influenced the accuracy of the radioimmunoassay for myelin basic protein when lipids (stearic acid, phosphatidylcholine, cholesterol, cerebroside, sulfatide, or GM1 ganglioside) or proteolipids (white-matter proteolipid apoprotein, kidney proteolipid apoproteins, or heart proteolipid apoproteins) were added to a known amount of basic protein and the samples assayed. All of these interfere with the assay, but the direction of the error depends on the quantity added: low concentrations of lipid decrease apparent basic protein, high concentrations enhance it. Obviously, results of basic-protein assays must be interpreted carefully.

Hydrophobic compounds interfere in radioimmunoassay for basic protein in myelin

Hydrophobic compounds influenced the accuracy of the radioimmunoassay for myelin basic protein when lipids (stearic acid, phosphatidylcholine, cholesterol, cerebroside, sulfatide, or GM1 ganglioside) or proteolipids (white-matter proteolipid apoprotein, kidney proteolipid apoproteins, or heart proteolipid apoproteins) were added to a known amount of basic protein and the samples assayed. All of these interfere with the assay, but the direction of the error depends on the quantity added: low concentrations of lipid decrease apparent basic protein, high concentrations enhance it. Obviously, results of basic-protein assays must be interpreted carefully.

Production and purification of antibody to bovine white matter proteolipid apoprotein

Circulating antibody to the bovine white matter proteolipid apoprotein was detected in rabbits 1 month ater a single injection of the water-soluble form of the apoprotein. By double immunodiffusion, the antiserum reacted specifically with the delipidated proteolipid apoprotein and the crude proteolipid fraction containing complex lipids; after exposure of the proteolipid apoprotein to sodium dodecyl sulfate (SDS), no reactivity was observed. The antiserum did not react with other myelin components, i.e., basic protein, cerebroside or GM1 ganglioside, nor was there reactivity with non-neural proteolipids. The anti-apoprotein antibody was purified by affinity chromatography. The antibody-antigen interaction is apparently very hydrophobic, since elution of the antibody from the affinity column requires buffer containing 0.5% Triton X-100-4 M-urea.