Myelin proteolipid protein gene structure and its regulation of expression in normal and jimpy mutant mice

The wmN1 Enhancer Region of the Mouse Myelin Proteolipid Protein Gene (mPlp1) is Indispensable for Expression of an mPlp1-lacZ Transgene in Both the CNS and PNS

The myelin proteolipid protein gene (PLP1) encodes the most abundant protein in CNS myelin. Expression of the gene must be strictly regulated, as evidenced by human X-linked leukodystrophies resulting from variations in PLP1 copy number, including elevated dosages as well as deletions. Recently, we showed that the wmN1 region in human PLP1 (hPLP1) intron 1 is required to promote high levels of an hPLP1-lacZ transgene in mice, using a Cre-lox approach. The current study tests whether loss of the wmN1 region from a related transgene containing mouse Plp1 (mPlp1) DNA produces similar results. In addition, we investigated the effects of loss of another region (ASE) in mPlp1 intron 1. Previous studies have shown that the ASE is required to promote high levels of mPlp1-lacZ expression by transfection analysis, but had no effect when removed from the native gene in mouse. Whether this is due to compensation by another regulatory element in mPlp1 that was not included in the mPlp1-lacZ constructs, or to differences in methodology, is unclear. Two transgenic mouse lines were generated that harbor mPLP(+)Z/FL. The parental transgene utilizes mPlp1 sequences (proximal 2.3 kb of 5'-flanking DNA to the first 37 bp of exon 2) to drive expression of a lacZ reporter cassette. Here we demonstrate that mPLP(+)Z/FL is expressed in oligodendrocytes, oligodendrocyte precursor cells, olfactory ensheathing cells and neurons in brain, and Schwann cells in sciatic nerve. Loss of the wmN1 region from the parental transgene abolished expression, whereas removal of the ASE had no effect.

Effects of Intron 1 Sequences on Human PLP1 Expression: Implications for PLP1-Related Disorders

Alterations in the myelin proteolipid protein gene ( PLP1) may result in rare X-linked disorders in humans such as Pelizaeus-Merzbacher disease and spastic paraplegia type 2. PLP1 expression must be tightly regulated since null mutations, as well as elevated PLP1 copy number, both lead to disease. Previous studies with Plp1-lacZ transgenic mice have demonstrated that mouse Plp1 ( mPlp1) intron 1 DNA (which accounts for slightly more than half of the gene) is required for the mPlp1 promoter to drive significant levels of reporter gene expression in brain. However not much is known about the mechanisms that control expression of the human PLP1 gene ( hPLP1). Therefore this review will focus on sequences in hPLP1 intron 1 DNA deemed important for hPLP1 gene activity as well as a couple of "human-specific" supplementary exons within the first intron which are utilized to generate novel splice variants, and the potential role that these sequences may play in PLP1-linked disorders.

Termination of lesion-induced plasticity in the mouse barrel cortex in the absence of oligodendrocytes

Termination of developmental plasticity occurs at specific points in development, and the mechanisms responsible for it are not well understood. One hypothesis that has been proposed is that oligodendrocytes (OLs) play an important role. Consistent with this, we found that OLs appeared in the mouse somatosensory cortex at the end of the critical period for whisker lesion-induced barrel structural plasticity. To test this hypothesis, we used two mouse lines with defective OL differentiation: Olig1-deficient and jimpy. In Olig1-deficient mice, although OLs were totally absent, the termination of lesion-induced plasticity was not delayed. The timing was normal even when the cytoarchitectonic barrel formation was temporarily blocked by pharmacological treatment in Olig1-deficient mice. Furthermore, the termination was not delayed in jimpy mice. These results demonstrate that, even though OLs appear at the end of the critical period, OLs are not intrinsically necessary for the termination of lesion-induced plasticity. Our findings underscore a mechanistic distinction between the termination of thalamocortical axonal plasticity in the barrel cortex and that in the visual cortex, in which OL-derived Nogo-A/B was recently suggested to be essential.

The pathobiology of myelin mutants reveal novel biological functions of the MBP and PLP genes

Substantial biological data indicate that the myelin basic protein (MBP) and myelin proteolipid protein (PLP/DM20) genes produce products with functions beyond that of serving as myelin structural proteins. Much of this evidence comes from studies on naturally-occurring and man-made mutations of these genes in mice and other species. This review focuses upon recent evidence showing the existence of other products of these genes that may account for some of these other functions, and recent studies providing evidence for alternative biological functions of PLP/DM20. The MBP and PLP/DM20 genes each encode the classic MBP and PLP isoforms, as well as a second family of proteins that are not involved in myelin structure. The biological roles of these other products of the genes are becoming clarified. The non-classic MBP gene products appear to be components of transcriptional complexes in the nucleus, and they also may be involved in signaling pathways in T-cells and in neural cells. The non-classic PLP/DM20 gene products appear to be components of intracellular transport vesicles in oligodendrocytes. There is evidence for other functions of the classic PLP/DM20 proteins, including a role in neural cell death mechanisms, autocrine and paracrine regulation of oligodendrocytes and neurons, intracellular transport and oligodendrocyte migration.

Evolution of myelin proteolipid proteins: Gene duplication in teleosts and expression pattern divergence

The coevolution of neurons and their supporting glia to the highly specialized axon-myelin unit included the recruitment of proteolipids as neuronal glycoproteins (DMbeta, DMgamma) or myelin proteins (DMalpha/PLP/DM20). Consistent with a genome duplication at the root of teleosts, we identified three proteolipid pairs in zebrafish, termed DMalpha1 and DMalpha2, DMbeta1 and DMbeta2, DMgamma1 and DMgamma2. The paralogous amino acid sequences diverged remarkably after gene duplication, indicating functional specialization. Each proteolipid has adopted a distinct spatio-temporal expression pattern in neural progenitors, neurons, and in glia. DMalpha2, the closest homolog to mammalian PLP/DM20, is coexpressed with P0 in oligodendrocytes and upregulated after optic nerve lesion. DMgamma2 is expressed in multipotential stem cells, and the other four proteolipids are confined to subsets of CNS neurons. Comparing protein sequences and gene structures from birds, teleosts, one urochordate species, and four invertebrates, we have reconstructed major steps in the evolution of proteolipids.

Increased NG2+ glial cell proliferation and oligodendrocyte generation in the hypomyelinating mutant shiverer

Glial cells that express the NG2 proteoglycan (NG2(+) cells) are considered to be oligodendrocyte progenitors (OPCs) in the central nervous system (CNS), based on their ability to give rise to mature oligodendrocytes in vitro. To understand how dysmyelinated conditions influence OPC proliferation and differentiation, we studied proliferation and differentiation of NG2(+) OPCs in vivo in the shiverer mutant (shi), which do not form compact myelin due to a deletion in the myelin basic protein gene. Acute bromodeoxyuridine (BrdU) labeling studies revealed a 4- to 6-fold increase in NG2(+) cell proliferation in shi spinal cord between postnatal day18 (P18) and P60, and most BrdU(+) cells were NG2(+) after P18. The increased proliferation was accompanied by a 2-fold increase in the number of OPCs and oligodendrocytes. Survival studies following a single injection of BrdU at P18 revealed a decline in the number of BrdU(+)/NG2(+) cells with a concomitant increase in the number of BrdU(+) oligodendrocytes over time, suggesting that the proliferated NG2(+) cells had differentiated into oligodendrocytes. BrdU(+) oligodendrocytes were generated over a longer period of time in shi spinal cord and persisted longer in shi than in wild type spinal cord. These findings suggest that new oligodendrocytes continue to be generated in the dysmyelinated shi spinal cord by enhanced proliferation and differentiation of NG2(+) oligodendrocyte progenitor cells.

Pelizaeus-Merzbacher disease and spastic paraplegia type 2: two faces of myelin loss from mutations in the same gene

Pelizaeus-Merzbacher disease and X-linked spastic paraplegia type 2 are two sides of the same coin. Both arise from mutations in the gene encoding myelin proteolipid protein. The disease spectrum for Pelizaeus-Merzbacher disease and spastic paraplegia type 2 is extraordinarily broad, ranging from a spastic gait in the pure form of spastic paraplegia type 2 to a severely disabling form of Pelizaeus-Merzbacher disease featuring hypotonia, respiratory distress, stridor, nystagmus, and profound myelin loss. The diverse disease spectrum is mirrored by the underlying pathogenesis, in which a blockade at any stage of myelin proteolipid protein synthesis and assembly into myelin spawns a unique phenotype. The continuing definition of pathogenetic mechanisms operative in Pelizaeus-Merzbacher disease and spastic paraplegia type 2, together with advances in neural cell transplant therapy, augurs well for future treatment of the severe forms of Pelizaeus-Merzbacher disease.

Myelin proteolipid protein (Plp) intron 1 DNA is required to temporally regulate Plp gene expression in the brain

The myelin proteolipid protein (Plp) gene encodes the most abundant protein found in mature CNS myelin. Expression of the gene is regulated spatiotemporally, with maximal expression occurring in oligodendrocytes during the myelination period of CNS development. Plp gene expression is tightly controlled. Misregulation of the gene in humans can result in the dysmyelinating disorder Pelizaeus-Merzbacher disease, and in transgenic mice carrying a null mutation or extra copies of the gene can result in a variety of conditions, from late onset demyelination and axonopathy, to severe early onset dysmyelination. In this study we have examined the effects of Plp intron 1 DNA in mediating proper developmental expression of Plp-lacZ fusion genes in transgenic mice. Our results reveal the importance of Plp intron 1 sequences in instigating the expected surge in Plp-lacZ gene activity during (and following) the active myelination period of brain development. Transgene expression was also detected in the testis (Leydig cells), however, the presence or absence of Plp intron 1 sequences had no effect on the temporal profile in the testis. Surprisingly, expression of the transgene missing Plp intron 1 DNA was always higher in the testis, as compared to the brain, in all of the transgenic lines generated.

Multiple splice isoforms of proteolipid M6B in neurons and oligodendrocytes

Proteolipids are abundant integral membrane proteins, initially described as structural proteins of CNS myelin. More recently, two neuronal proteins related to proteolipid protein (PLP), termed M6A and M6B, were identified, suggesting a common function of proteolipids in oligodendrocytes and neurons. We have analyzed the X-linked M6B gene and discovered an unexpected complexity of protein isoforms. Two promoters and alternative exons yield at least eight M6B proteins and polypeptides, differentially expressed in neurons and oligodendrocytes. Six isoforms are tetraspan membrane proteins that differ by highly conserved amino- and carboxy-terminal domains, termed alpha, beta, psi, and omega. In MDCK cells, the beta-domain of M6B stabilizes tetraspan proteolipids at the cell surface, whereas non-beta isoforms are more abundant in intracellular compartments. Cotransfection experiments suggest a physical interaction of M6B and mutant PLP, when retained in the endoplasmic reticulum, that may also contribute to oligodendrocyte dysfunction in Pelizaeus-Merzbacher disease.

Elevated levels of the chemokine GRO-1 correlate with elevated oligodendrocyte progenitor proliferation in the jimpy mutant

The dysmyelinating mutant jimpy (jp) arises from a point mutation in the mouse gene encoding proteolipid protein and is characterized by severe dysmyelination attributable to oligodendrocyte death. This mutant was used to investigate the regulation of oligodendrocyte progenitor proliferation in the postnatal spinal cord. At postnatal day 18, jp spinal cord contained a three- to eightfold greater number of proliferating oligodendrocyte progenitor cells than did wild-type (wt) spinal cord. Increased proliferation in jp spinal cord was accompanied by a twofold increase in the number of progenitor cells. Semiquantitative reverse transcriptase-PCR revealed no change in the level of mRNA encoding the platelet-derived growth factor A, transforming growth factor-beta, or insulin-like growth factor-I, all of which have been implicated as regulators of proliferation and differentiation of oligodendrocyte progenitor cells. There was, however, a 17-fold increase in the level of mRNA encoding the chemokine GRO-1 and a 5- to 6-fold increase in GRO-1 protein in the jp spinal cord. Double immunofluorescence labeling revealed elevated levels of GRO-1 in reactive astrocytes in jp spinal cord white matter. In vitro studies indicated that extracts from jp spinal cord stimulated oligodendrocyte progenitor proliferation. Furthermore, removal of GRO-1 from jp extracts by immunoprecipitation reduced the proliferation of progenitor cells to a level similar to that achieved by wt extracts. These findings suggest a novel mechanism by which proliferation of oligodendrocyte progenitor cells is regulated in the postnatal spinal cord in response to insult.

Understanding glial abnormalities associated with myelin deficiency in the jimpy mutant mouse

Jimpy is a shortened life-span murine mutant showing recessive sex-linked inheritance. The genetic defect consists of a point mutation in the PLP gene and produces a severe CNS myelin deficiency that is associated with a variety of complex abnormalities affecting all glial populations. The myelin deficiency is primarily due to a failure to produce the normal amount of myelin during development. However, myelin destruction and oligodendrocyte death also account for the drastic myelin deficit observed in jimpy. The oligodendroglial cell line shows complex abnormalities in its differentiation pattern, including the degeneration of oligodendrocytes through an apoptotic mechanism. Oligodendrocytes seem to be the most likely candidate to be primarily altered in a disorder affecting myelination, but disturbances affecting astrocytes and microglia are also remarkable and may have a crucial significance in the development of the jimpy disorder. In fact, the jimpy phenotype may not be attributed to a defect in a single cell but rather to a deficiency in the normal relations between glial cells. Evidences from a variety of sources indicate that the jimpy mutant could be a model for disturbed glial development in the CNS. The accurate knowledge of the significance of PLP and its regulation during development must be of vital importance in order to understand glial abnormalities in jimpy.

The first intron of the myelin proteolipid protein gene confers cell type-specific expression by a transcriptional repression mechanism in non-expressing cell types

Chimeric genes containing portions of the mouse myelin proteolipid protein (PLP) gene fused to the lacZ reporter gene were used to detect the effect of PLP intron 1 sequences on cell type-specific expression. A transfected fusion gene containing PLP intron 1 sequences was expressed in an oligodendrocyte cell line but not in a liver cell line, consistent with endogenous PLP gene expression. However, an analogous fusion gene missing the first intron was expressed in either oligodendrocyte or liver transfected cells. These studies suggest that transcriptional repressor element(s) located in PLP intron 1 are important in extinguishing expression in non-glial cell types and that the promoter alone functions in an indiscriminate manner. This moderately large intron (>8 kb) was sequenced to aid in future fine mapping of these cell-specific regulatory element(s).

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.

Myelin proteolipid protein (PLP), but not DM-20, is an inositol hexakisphosphate-binding protein

Myelin proteolipid protein (PLP) and its alternatively spliced isoform, DM-20, are the major integral membrane proteins of central nervous system myelin. It is known that PLP and DM-20 are delivered to myelin by a finely regulated vesicular transport system in oligodendrocytes. Evolutionarily, it is believed that ancestral DM-20 acquired a PLP-specific exon to create PLP, after which PLP/DM-20 became a major component of central nervous system myelin. We purified PLP as an inositol 1,3,4,5-tetrakisphosphate-binding protein after solubilization in a non-organic solvent. However, under the isotonic condition, PLP binds inositol hexakisphosphate (InsP6) significantly, not inositol 1,3,4,5-tetrakisphosphate. Most of the InsP6-binding proteins are involved in vesicular transport, suggesting the involvement of PLP in vesicular transport. We separated DM-20 from PLP by CM-52 chromatography and showed that DM-20 has no InsP6 binding activity. These findings indicate that the PLP-specific domain confers the InsP6 binding activity and this interaction may be important for directing PLP transport to central nervous system myelin.

Tissue lipoproteins revisited: new proteolipid protein gene family members in elasmobranchs

The proteolipids (PLPs) are abundant components of mammalian CNS myelin. Recombinant DNA methodologies have enabled us to search for evolutionary antecedents of PLP/DM20. Polymerase chain reactions of Torpedo and Squalus brain cDNA were performed with degenerate primers designed according to the mammalian PLP/DM20 sequence. Three DM20-related products (DM alpha, DM beta, and DM gamma) were amplified; no cDNAs containing the PLP-specific segment were found. Regions of the DM alpha and DM gamma are similar to the pore-forming segments of certain ligand-gated channels. In embryonic Squalus CNS, DM alpha and DM gamma appear to be co-expressed with P0. Antiserum raised against Torpedo DM alpha recognizes a protein in mouse CNS myelin, demonstrating that at least one of the newly recognized fish DMs is also in mammals. Our data, as well as that of other laboratories, supports the existence of a ubiquitously expressed proteolipid gene family.

Membrane topology of PLP in CNS myelin: evaluation of models

Three new models for proteolipid protein (PLP) topology in the myelin membrane have been proposed--the 4-helix N(in) and N(out) models of Popot (J. Membr. Biol. 120:233-246), and the model of Weimbs and Stoffel (Biochemistry 31:12289-12296). Unlike the earlier models proposed by Laursen (Proc. Natl. Acad. Sci. USA 81:2912-2916), Stoffel (Proc. Natl. Acad. Sci. USA. 81:5012-5016) and Hudson (J. Cell Biol. 109:717-727), the four hydrophobic clusters are all assigned as membrane-spanning domains. The Popot-N(in) and Weimbs models, which are similar to the Laursen model, both assign the positively-charged domain, which is deleted from the DM20 transcript of PLP, to the cytoplasmic surface, while the Popot-N(out) model, similar to the Stoffel and Hudson models, assigns this sequence to the extracellular surface. Our calculations of membrane surface charge shows that the disposition of this basic domain greatly influences membrane interactions, by shifting the equilibrium myelin period to alkaline pH due to the electrostatic repulsion force at the extracellular apposition. In the Laursen, Popot-N(in) and Weimbs models, the onset of swelling was calculated to be at lower pH than in the Stoffel, Hudson and Popot-N(out) models, and lower than that observed experimentally with mouse optic nerve myelin. The absolute electron density profile of the myelin membrane that is derived from the x-ray diffraction patterns shows similar density levels at its cytoplasmic and extracellular surfaces. By contrast, the electron density profile calculated from a chemical model that includes lipids plus myelin basic protein (but not PLP) shows a higher density at the cytoplasmic than at the extracellular side.(ABSTRACT TRUNCATED AT 250 WORDS)

Fate of jimpy-type oligodendrocytes in jimpy heterozygote

In the jimpy mutant mouse, as well as in many other animals with mutations in the myelin proteolipid protein (PLP) gene, oligodendrocytes degenerate before their maturation. To analyze whether this degeneration is caused by the loss of function of PLP gene products related to oligodendrocyte maturation/survival acting extrinsically, expression of the PLP gene was investigated in the jimpy heterozygote, in which one-half of the cells are jimpy type and the other half are wild type due to random X-chromosome inactivation. We first showed that jimpy PLP gene expression is normally regulated at the early stages of development in brains of jimpy hemizygotes and heterozygotes, at least to day 2 after birth. However, the great increase in the level of PLP gene transcripts observed in wild-type mouse brain is suppressed in jimpy mouse brain. This increase was also suppressed in the jimpy heterozygote, and by 2 months after birth, very few jimpy-type PLP gene transcripts were detected in heterozygotes. These results indicate that jimpy-type oligodendrocytes cannot survive or are still in the immature stage in the brain of jimpy heterozygotes. Thus, degeneration of jimpy oligodendrocytes is not caused merely by the lack of trophic factors.

Premature arrest of myelin formation in transgenic mice with increased proteolipid protein gene dosage

Proteolipid protein (PLP) is an integral membrane protein of CNS myelin. Mutations of the X chromosome-linked PLP gene cause glial cell death and myelin deficiency in jimpy mice and other neurological mutants. As part of an attempt to rescue these mutants by transgenic complementation, we generated normal mouse lines expressing autosomal copies of the entire wild-type PLP gene. Surprisingly, increase of the PLP gene dosage in nonmutant mice with only 2-fold transcriptional overexpression results in a novel phenotype characterized by severe hypomyelination and astrocytosis, seizures, and premature death. This demonstrates that precise control of the PLP gene is a critical determinant of terminal oligodendrocyte differentiation. Dysmyelination of PLP transgenic mice provides experimental evidence that Pelizaeus-Merzbacher disease, previously associated with a partial duplication of the human X chromosome, can be caused by doubling of the PLP gene dosage.

Selective expression of foreign genes in glioma cells: use of the mouse myelin basic protein gene promoter to direct toxic gene expression

We have previously demonstrated that retrovirus-mediated genes were transferred to mouse glioma cells in a meningeal gliomatosis model (Yamada et al.: Japanese Journal of Cancer Research 83:1244-1247, 1992). This retrovirus vector contains the Escherichia coli. beta-galactosidase (beta-gal) gene as a marker for integration of the lacZ gene, which is controlled by the SV40 early promoter. We investigated whether lacZ genes could be specifically controlled in mouse glioma cells by glial-specific promoters, including the 2.5 kb 5' flanking region of the mouse glial fibrillary acidic protein (GFAP) gene, the 1.3 kb 5' flanking region of the myelin basic protein (MBP) gene, and the 1.5 kb 5' flanking region of the myelin proteolipid protein (PLP) gene. Psi-2 packaging cells were transfected with each retrovirus vector (GFAP promoter-, MBP promoter-, and PLP promoter-lacZ) and the infectious virus particles were recovered from the supernatants. Blue staining for beta-gal was detected in various fibroblast, myeloma, and glioma cell lines transduced with the retrovirus BAG vector. On the other hand, blue staining was only detected in glioma cells after transduction with the lacZ gene-bearing retrovirus controlled by glial-specific promoters. The strongest promoter activity was detected after transduction with the retrovirus in which the MBP promoter controlled the lacZ gene. Mouse glioma cells transduced with retrovirus containing the MBP promoter directing the herpes simplex virus type 1 thymidine kinase (HTK) gene were extremely sensitive to ganciclovir, while the parental cells and cells transduced with retrovirus containing the lacZ gene were not sensitive to ganciclovir.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Structure, expression and chromosomal localization of the gene encoding human 2',3'-cyclic-nucleotide 3'-phosphodiesterase

Four human genomic DNA clones for 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNP) were isolated by screening a human genomic library with human CNP cDNA clones as probes. Restriction mapping and sequence analysis revealed that the human CNP gene is about 8.5 kb long and composed of four exons interrupted by three introns. There are two transcription start points and in human brain, two forms of CNP mRNA are produced from a single gene by alternative splicing, similar to mouse. A homology search of the 5'-flanking regions of exon 0 and exon 1 in the human CNP gene indicated the presence of oligodendroglia-specific elements and myelin basic protein transcription element (MBTE) motif, in addition to TATA-box-like sequences. Spot blot hybridization of flow-sorted human chromosomes with the 3'-noncoding region of the human CNP cDNA showed the localization of CNP to chromosome 17.

Characterization of a brain-specific Sp1-like activity interacting with an unusual binding site within the myelin proteolipid protein promoter

The proteolipid protein (PLP) gene encodes the main integral protein of the myelin membrane of the central nervous system. The expression of the gene is regulated in a cell- and development-specific manner. Comparison of approximately 1.5 kb of the upstream noncoding region from man, mouse, and rat gene revealed an extensive sequence identity of about 95% between -250 and +100 (the most upstream transcription start site is defined as +1) but only about 50% identity further upstream. To define potential cis-acting elements in the promoter of the mouse PLP gene the upstream region was studied by transfection of C6 glioblastoma cells and CHO fibroblasts with various 5' deletion constructs fused to the reporter gene luciferase. We localized a promoter at position -184 to +90, which is active in both cell lines. Analysis of this region by DNase I foot-printing experiments and band shift analysis with nuclear extracts from myelinating brain, liver, C6, and CHO cells shows the binding of several different proteins to the promoter region. One brain-specific and two ubiquitous factors bound to the sequence AAGGGGAGGAG (DR1/2 box). This motif is also present in the upstream region of other myelin-specific genes and in some variants of the glia cell-specific virus JC. The factors bound with similar affinity to a Sp1-binding site. Therefore one of the ubiquitous factors seems to be Sp1 suggesting that Sp1 may play a role in the transcriptional regulation of the PLP gene. It has been shown that the DR1/2 box-binding factors are Zn(2+)-dependent. By Southwestern blotting it has been demonstrated that the DR1/2 box binds a protein of about 66 kDa that is enriched in brain.

Axons modulate the expression of proteolipid protein in the CNS

We examined the expression of mRNA encoding proteolipid protein (PLP), the major myelin protein in the CNS, in developing rat cerebrum, and in normal and degenerating optic nerves. PLP transcripts were initiated at two clusters of start sites that were separated by about 30 base pairs. During the peak of PLP mRNA expression in developing cerebrum, a higher proportion of PLP transcripts were initiated from the distal start site, furthest from the open reading frame, than in mature cerebrum. We enucleated one eye of immature rats to cause Wallerian degeneration in the optic nerve. In these degenerating optic nerves, the steady state levels of PLP mRNA fell markedly, and the proportion of distally initiated PLP transcripts declined to the same proportion found in normal adult nerves. Changes in myelin gene expression were not limited to PLP mRNA, as the steady-state levels of myelin basic protein (MBP) mRNA paralleled those of PLP mRNA in the developing cerebrum and in degenerating optic nerves. Thus, oligodendrocytes require axons to maintain their normal levels of PLP and MBP transcripts and the high proportion of distally initiated PLP transcripts that characterize early myelination.

Selective expression of DM-20, an alternatively spliced myelin proteolipid protein gene product, in developing nervous system and in nonglial cells

Mutations within the gene for myelin proteolipid protein (PLP), a major myelin structural protein, result in abnormal glial differentiation, suggesting that the PLP gene products play some other functional roles. Transcripts from the PLP gene were analyzed in the developing mouse brain by a sensitive method using polymerase chain reaction. The mRNA for DM-20, an alternatively spliced transcript from the PLP gene, was detected in the embryonic mouse brain as early as embryonic day 11, long before the appearance of oligodendrocytes, which were considered to be responsible for PLP production. PLP gene expression was analyzed in various cell lines to determine whether synthesis of the DM-20 mRNA is restricted to those of glial cell lineage. All of the nervous system cell lines examined, including nonglial cell lines, produced DM-20 mRNA but no or very little PLP mRNA. Peripheral sciatic nerve from adult Wistar rats also produced mainly DM-20 mRNA. These results indicate that DM-20 is not only a myelin structural protein, but it also plays other roles in the nervous system that seem to relate, at least in part, to glial differentiation.

The DM20 protein of myelin: intracellular and surface expression patterns in transfectants

DM20 is an abundant CNS myelin-specific protein whose role in myelinogenesis is unknown. We have cloned the DM20 cDNA from adult mouse brain total RNA using the polymerase chain reaction and expressed it in HeLa cells. DM20, detected by immunofluorescence in stable transfectants, is present in some cells in large, intensely fluorescent intracellular clumps that probably represent elements of the rough endoplasmic reticulum and Golgi apparatus. Frequently, intense DM20 fluorescence could be detected at the plasma membrane. These findings are consistent with previous studies demonstrating that an intracellular "pool" of DM20 and its larger isoform, proteolipid protein, exists and that a substantial lag occurs between synthesis and insertion of these proteins into the expanding myelin membrane. Permanent DM20 expressors in contact with one another do not display any ultrastructural rearrangements at regions of cell-cell contact, in contrast to what we have previously reported for P0, a PNS-specific protein shown to mediate adhesion of the extracellular faces of the Schwann cell during PNS myelinogenesis. We believe that these results indicate that if DM20 is indeed an adhesion molecule, this property is likely to be significantly more subtle than P0-mediated adhesion.

Regulation of rodent myelin proteolipid protein gene expression

The regulation of rodent proteolipid protein (PLP) gene expression was studied during rat development and in cultured cells. Nuclear run-on assays demonstrate a strong transcriptional component associated with the developmental regulation of the PLP mRNA. Transcription rates of the PLP and MBP genes parallel their respective steady-state mRNA levels throughout rat brain development. In addition, a moderate 25-h half-life was measured for PLP mRNA in 37-day-old cultured oligodendrocytes, suggesting that regulation of PLP expression occurs predominantly at the transcriptional level. Finally, 5400 and 1400 bp of mouse PLP 5'-flanking sequence demonstrate transcriptional activity 13-fold and 5-fold above background, respectively, in hamster glial cells. Far upstream elements are clearly involved in transcription of the PLP gene. The 5400 bp sequence demonstrates no more activity than the 1400 bp in a mouse hepatoma cell line suggesting that elements involved in the glial cell-specific expression of PLP lie between 1400 and 5400 bp upstream of the gene.

Cyclic AMP-induced upregulation of proteolipid protein and myelin associated glycoprotein gene expression in C6 cells

A model culture system of C6 rat glioma cells was used to test the involvement of cAMP in the regulation of the myelin PLP and MAG genes. The treatment of cells with isoproterenol (10(-5) to 10(-8) M) upregulated the expression of the PLP and MAG genes in a concentration-dependent manner. The mRNA for PLP reached a maximum (sevenfold higher than in control cells) after about 12-24 hr, then declined to approximately fourfold over the control level. The response of MAG gene was delayed by at least 36 hr, and the level of MAG mRNA reached a maximum of approximately 48-fold over the control level on the fourth day in culture. The co-administration of propranolol blocked the effect of isoproterenol, whereas 10(-5) M forskolin simulated the effect of isoproterenol, indicating a role of cAMP in the signal transduction cascades leading to upregulation of the myelin genes. However, the dissimilarity in the timing and the extent of upregulation of the PLP and MAG genes by cAMP-stimulating agents indicate the existence of different intracellular mechanisms for the activation of these two genes. Cycloheximide blocked the stimulatory effect of isoproterenol on both the PLP and MAG genes, indicating that the effect of cAMP on the myelin genes is mediated by protein product(s) of other cAMP-response gene(s).

Myelin deficiency in female rats due to a mutation in the PLP gene

Myelin deficiency (md) in female rats due to a mutation in the X-linked proteolipid protein (PLP) gene is caused by X-chromosome monosomy. Cytogenetic analysis revealed a single X karyotype [41,X(md/0)]. An immunocytochemical, electron microscopic, and biochemical study was performed on male and female md rats. The central nervous system (CNS) of the female md rat [41,X(md/0)] revealed the same total lack of PLP as the CNS of the affected male littermate [42,XY(md/Y)]. Immunocytochemistry for myelin basic protein (MBP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNP) revealed "islands" of myelin sheath-like reaction product in both. Electron microscopy showed great paucity of compact myelin sheaths in 41,X(md/0) and 42,XY(md/Y). Reduced levels of MPB, MAG, and CNP were confirmed for both sexes but MAG and CNP were substantially higher in 41,X(md/0). Sexual differentiation of the brain may account for the observed differences since normal female reproductive organs are present in the md female rat.

Isolation and characterization of a library of cDNA clones that are preferentially expressed in the embryonic telencephalon

In order to isolate genes involved in development of the mammalian telencephalon we employed an efficient cDNA library procedure. By subtracting an adult mouse telencephalic cDNA library from an embryonic day 15 (E15) mouse telencephalic cDNA library we generated two subtracted libraries (ES1 and ES2). We estimate that ES1 contains between 200 and 600 different cDNA clones, which approximates the number of genes that are preferentially expressed in the E15 telencephalon, compared to the adult telencephalon. Northern analysis of 20 different cDNA clones shows that 14 of these are expressed at least 5-fold more in the E15 telencephalon than the adult telencephalon. Limited sequencing of the 14 differentially expressed clones reveals that 10 have no significant identity to sequences in GenBank and EMBL databases, whereas the other 4 have significant sequence identity to vimentin, histone 3.3, topoisomerase I and the B2 repeat element. In situ hybridization using one of the differentially expressed cDNAs, TES-1, demonstrates that it is transiently expressed in the anlage of the basal ganglia. In situ hybridization with another differentially expressed cDNA clone, TES-4, shows that it is specifically expressed in differentiating cells of the neural axis with a distinctive rostral-caudal temporal pattern. These findings, and the methods that we have developed, provide a framework for future investigations of the genetic control of telencephalon development.

Structure of mouse myelin-associated glycoprotein gene

The mouse myelin-associated glycoprotein gene was isolated from a mouse gene library. This gene was split into 13 exons distributed about 15 kb in length. Each extracellular immunoglobulin-related domain was encoded by a single exon, and RNA splicing between those exons occurred between the first and second nucleotides of the junctional codon, the features of which are conserved in most of the genes of the immunoglobulin superfamily. The sequence of the 5'-flanking region appeared to have some regions homologous to other myelin proteins, which suggested that they were possible cis-elements for specific expression of oligodendrocytes.

Levels of proteolipid protein mRNAs in peripheral nerve are not under stringent axonal control

The proteolipid protein (PLP) is the major protein in the myelin sheath of the CNS. It was recently reported that PLP coding transcripts are also found in the PNS, although the protein was not detectable in peripheral nerve myelin. In the present investigation, levels of mRNA for PLP in sciatic nerve were studied during development and following transection and crush injury. Results were compared to those for P0, the major PNS myelin protein, and the myelin-associated glycoprotein (MAG). PLP transcript levels were very low at 21 days in sciatic nerve and remained unchanged in the adult sciatic nerve. This contrasts markedly with P0 and MAG mRNAs, which are expressed at high levels during development and decrease in content significantly by adulthood. The level of PLP messages was reduced approximately 40% in the quiescent Schwann cells in the distal segment of the sciatic nerve at 21 days after permanent transection, yet P0 mRNA levels were very low, and MAG mRNAs were undetectable in this tissue. The distal segment of the crush-injured sciatic nerve is characterized by transient demyelination followed by rapid myelination. PLP mRNA levels remained comparatively unaffected in the 3-week period following crush injury. RNase protection experiments using two antisense riboprobes confirmed that levels of PLP-derived protected fragments, corresponding to PLP and DM-20 messages, remained unchanged in the developing and adult sciatic nerve. These results indicate that myelin-specific P0 and MAG genes are tightly controlled at the level of transcription through Schwann cell-axonal interactions, whereas PLP transcription in the peripheral nerve remains nearly dissociated from axonal influences.

Developmental expression of myelin protein genes in dysmyelinating mutant mice: analysis by nuclear run-off transcription assay, in situ hybridization, and immunohistochemistry

Gene expression for myelin proteolipid protein (PLP) and myelin basic protein (MBP) in the dysmyelinating mutant mice shiverer and jimpy was analyzed by nuclear run-off transcription assay, in situ hybridization, and immunohistochemistry. The level of PLP transcription in shiverer brains was lower than that in controls at postnatal day 18 but relatively higher at later stages. In spite of the considerable amount of hybridization with PLP cDNA, immunoreaction for PLP was greatly reduced in shiverer mice throughout their lives, probably owing to a defect in the assembly of PLP into myelin. Abnormal deposition of PLP in oligodendroglial cell bodies suggested that transport of PLP to myelin is delayed in shiverer brains. The number of oligodendrocytes expressing PLP mRNA was drastically reduced in jimpy mice. MBP mRNA in jimpy mice is localized preferentially in oligodendroglial cell bodies, a result suggesting that oligodendrocytes in jimpy are mostly the immature type. Although transcriptional activity of the MBP gene in jimpy was greatly reduced, a finding reflecting the decrease in the number of mature oligodendrocytes, that of the PLP gene remained high at early stages. The discrepancy of the two gene expressions is discussed relative to the role of PLP transcripts at early stages of myelination.

Major Myelin proteolipid: the 4-alpha-helix topology

Several conflicting models have been proposed for the membrane arrangement of the major myelin proteolipid (PLP). We have compared features of the sequence of PLP with those of other eukaryotic integral membrane proteins, with the view of identifying the most likely transmembrane topology. A new, simple model is suggested, which features four hydrophobic alpha-helices spanning the whole thickness of the lipid bilayer. Its orientation may be such that both the N- and C-termini face the cytosol. None of the biochemical, biophysical or immunological experiments hitherto reported provides incontrovertible evidence against the model. The effect or absence thereof of various PLP mutations is discussed in the frame of the proposed 4-helix topology.

Folding and function of the myelin proteins from primary sequence data

To explain how the myelin proteins are involved in the organization and function of the myelin sheath requires knowing their molecular structures. Except for P2 basic protein of PNS myelin, however, their structures are not yet known. As an aid to predicting their molecular folding and possible functions, we have developed a FORTRAN program to analyze the primary sequence data for proteins, and have applied this to the myelin proteins in particular. In this program, propensities for the secondary structure conformations as well as physical-chemical parameters are assigned to the amino acids and the pattern of these parameters is examined by calculating their average values, autocorrelation functions and Fourier transforms. To compare two proteins, their sequences are aligned using a unitary scoring matrix, and homologies are searched by plotting a two-dimensional map of the correlation coefficients. Comparison of the corresponding myelin basic proteins (MBP) and P0 glycoproteins (P0) for rodent and shark showed that the conserved residues included most of the amino acids which were predicted to form the alpha or beta conformations, while the altered residues were mainly in the hydrophilic and turn or coil regions. In both rodent and shark the putative extracellular domain of P0 glycoprotein displayed consecutive peaks of beta propensity similar to that for the immunoglobulins, while the cytoplasmic domain showed alpha-beta-alpha folding. To trace the immunoglobulin fold along the P0 sequence, we compared the beta propensity curve of P0 with that of the immunoglobulin M603, whose three-dimensional structure has been determined. We propose that the flat beta-sheets of P0 are orientated parallel to the membrane surface to facilitate their homotypic interaction in the extracellular space. An extra beta-fold in the extracellular domain of shark P0 compared with rodent P0 was found, and this may result in a greater attraction between the apposed extracellular surfaces and may account for a smaller extracellular space as measured by x-ray diffraction. A computer search of the myelin protein sequences for functional motifs revealed sites for N-glycosylation, phosphorylation, nucleotide binding, and certain enzyme activities. We note especially that there are potential nucleotide binding sites in proteolipid protein (PLP), MBP and 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP). This is consistent with the experimental observations that PLP acts like an ionophore or proton channel when reconstituted into planar lipid bilayers, MBP binds GTP, and CNP catalyzes in vitro the hydrolysis of 2',3'-nucleotides into corresponding 2'-nucleotides.

Reliable transient promoter assay using fluorescein-di-beta-D-galactopyranoside substrate

The promoter region of the mouse myelin proteolipid protein (PLP) gene was cloned into a promoter testing vector, pIP111. The pIP111 vector is a promoterless derivative of pCH110 (SV40 early region promoter-lacZ) and contains the Escherichia coli lpp transcription terminator sequence at the 5' end of the cloning site. The newly constructed PLP-lacZ fusion plasmid (pWP) was transfected into PLP-nonproducing NIH-3T3 fibroblasts or PLP-producing C6 cells. When the measured beta-galactosidase activity in the pWP-transfected cells was normalized to the pCH110-transfected cells (an appropriate control if the SV40 early region promoter functions constitutively in various cell lines), the results suggested that the promoter region of the PLP gene contains the information necessary for initiation of transcription in a C6 cell-specific manner. However, the beta-galactosidase produced in viable cells was also detected by fluorescein-di-beta-D-galactopyranoside (FDG) treatment followed by image analysis using inverted fluorescent microscopy, which allowed the transfection efficiency to be calculated, and the beta-galactosidase activity obtained by the regular ONPG method was normalized with the value obtained. This procedure indicated that the promoter region of the PLP gene did not show C6-specific expression, because the SV40 early-region promoter was 10 times more active in NIH-3T3 cells than in C6 cells. Thus, the standard experiment gave misleading results. As our detection method is simple and can be used to analyze the promoter activity in a single cell, many applications should be possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Prenatal diagnosis in Pelizaeus-Merzbacher disease using RFLP analysis

Pelizaeus-Merzbacher disease (PMD) is a rare X-linked recessive disorder with severe psychomotor retardation and neurological symptoms due to an inborn abnormality of proteolipid protein (PLP), the major protein component of myelin. A tight linkage between the gene of PLP and PMD locus has been suggested. We have carried out a series of RFLP studies using a cDNA probe for PLP and an anonymous DNA-fragment DXYS12 in a large Finnish family with at least three affected individuals. DNA analysis on chorionic villus specimens allowed us to exclude the disease in a male fetus of a possible carrier mother and, likewise, to demonstrate carrier status in a female fetus in another at-risk pregnancy.

Tissue-specific in vitro transcription from the mouse myelin basic protein promoter

The mouse myelin basic protein promoter was transcribed in brain nuclear extracts. The distal promoter region from -253 to -54 directed preferential transcription in brain extracts, whereas the same region repressed transcription activity in liver extracts. Stimulation of transcription was observed when the distal region was located only in a native orientation. The proximal region downstream from -53 alone still directed preferential transcription. It is suggested that cooperative function by the two promoter regions may be required for higher specificity.

Mutation of the proteolipid protein gene PLP in a human X chromosome-linked myelin disorder

Myelin is a highly specialized membrane unique to the nervous system that ensheaths axons to permit the rapid saltatory conduction of impulses. The elaboration of a compact myelin sheath is disrupted in a diverse spectrum of human disorders, many of which are of unknown etiology. The X chromosome-linked human disorder Pelizaeus-Merzbacher disease is a clinically and pathologically heterogeneous group of disorders that demonstrate a striking failure of oligodendrocyte differentiation. This disease appears pathologically and genetically to be similar to the disorder seen in the dysmyelinating mouse mutant jimpy, which has a point mutation in the gene encoding an abundant myelin protein, proteolipid protein (PLP). We report that the molecular defect in one Pelizaeus-Merzbacher family is likewise a point mutation in the PLP gene. A single T----C transition results in the substitution of a charged amino acid residue, arginine, for tryptophan in one of the four extremely hydrophobic domains of the PLP protein. The identification of a mutation in this Pelizaeus-Merzbacher family should facilitate the molecular classification and diagnosis of these X chromosome-linked human dysmyelinating disorders.

The initial events in myelin synthesis: orientation of proteolipid protein in the plasma membrane of cultured oligodendrocytes

Proteolipid protein (PLP) is the most abundant transmembrane protein in myelin of the central nervous system. Conflicting models of PLP topology have been generated by computer predictions based on its primary sequence and experiments with purified myelin. We have examined the initial events in myelin synthesis, including the insertion and orientation of PLP in the plasma membrane, in rat oligodendrocytes which express PLP and the other myelin-specific proteins when cultured without neurons (Dubois-Dalcq, M., T. Behar, L. Hudson, and R. A. Lazzarini. 1986. J. Cell Biol. 102:384-392). These cells, identified by the presence of surface galactocerebroside, the major myelin glycolipid, were stained with six anti-peptide antibodies directed against hydrophilic or short hydrophobic sequences of PLP. Five of these anti-peptide antibodies specifically stained living oligodendrocytes. Staining was only seen approximately 10 d after PLP was first detected in the cytoplasm of fixed and permeabilized cells, suggesting that PLP is slowly transported from the RER to the cell surface. The presence of PLP domains on the extracellular surface was also confirmed by cleavage of such domains with proteases and by antibody-dependent complement-mediated lysis of living oligodendrocytes. Our results indicate that PLP has only two transmembrane domains and that the great majority of the protein, including its amino and carboxy termini, is located on the extracellular face of the oligodendrocyte plasma membrane. This disposition of the PLP molecule suggests that homophilic interactions between PLP molecules of apposed extracellular faces may mediate compaction of adjacent bilayers in the myelin sheath.

Tissue-specific in vitro transcription from the mouse myelin basic protein promoter

The mouse myelin basic protein promoter was transcribed in brain nuclear extracts. The distal promoter region from -253 to -54 directed preferential transcription in brain extracts, whereas the same region repressed transcription activity in liver extracts. Stimulation of transcription was observed when the distal region was located only in a native orientation. The proximal region downstream from -53 alone still directed preferential transcription. It is suggested that cooperative function by the two promoter regions may be required for higher specificity.

The promoter elements of the mouse myelin basic protein gene function efficiently in NG108-15 neuronal/glial cells

We measured transiently-expressed beta-galactosidase activity by introducing the mouse myelin basic protein (MBP)-lacZ chimeric gene (MBP-lacZ) into the NG108-15 neuronal/glial hybrid cell line. Deletion studies of the promoter region of the MBP gene showed that the promoter region between -1318 bp and -254 bp might contain sequences that repress MBP promoter activity. Fine deletion analysis using BAL 31 exonuclease revealed sequences between bp -208 and -140, -139 and -118, and -89 and -75 which were critical for promoter activity in NG 108-15 cells. DNaseI footprinting analysis revealed a cellular factor(s) that bind to the promoter region between bp -127 and -106 with NG108-15 whole cell extracts. The SV40 promoter was activated by insertion of the sequences around the region protected in footprinting experiments, in a manner independent of its orientation in NG108-15 cells. This protected region is thought to be one of the critical cis-acting DNA elements for efficient transcription.