Proteolipid protein gene product can be secreted and exhibit biological activity during early development

PLP1-lacZ transgenic mice reveal that splice variants containing "human-specific" exons are relatively minor in comparison to the archetypal transcript and that an upstream regulatory element bolsters expression during early postnatal brain development

Much of what is known about the mechanisms that control the developmental expression of the myelin proteolipid protein gene (PLP1) has been attained through use of transgenic animal models. In this study, we analyzed expression of related transgenes which utilize PLP1 genomic DNA from either human or mouse to drive expression of a lacZ reporter. Human PLP1 (hPLP1) sequence span either the proximal 6.2 or 2.7 kb of 5'-flanking DNA to an internal site in Exon 2, while those from mouse comprise the proximal 2.3 kb of 5'-flanking DNA to an analogous site in Exon 2. Transgenes with hPLP1 sequence were named, in part, to the amount of upstream sequence they have [6.2hPLP(+)Z/FL and 2.7hPLP(+)Z]. The transgene containing mouse sequence is referred to here as mPLP(+)Z, to denote the species origin of PLP1 DNA. Mice which harbor the 6.2hPLP(+)Z/FL transgene were used as a model system to investigate the developmental expression of splice variants that incorporate supplementary exons from what is classically defined as PLP1 intron 1. While expression of the splice variants were detected in brain through RT-PCR analysis, they are present at much lower levels relative to the archetypal (classic) transcript. Additionally, we show that mice which harbor the 6.2hPLP(+)Z/FL transgene demonstrate wide-ranging expression throughout brain at P2, whereas expression of mPLP(+)Z is quite limited at this age. Therefore, we generated new transgenic mouse lines with the 2.7hPLP(+)Z transgene, which contains hPLP1 sequence orthologous to just that in mPLP(+)Z. Of the seven lines analyzed, six showed higher levels of 2.7hPLP(+)Z expression in brain at P21 compared to P2; the other line expressed the transgene, only weakly, at either age. This trend, coupled with the robust expression observed for 6.2hPLP(+)Z/FL at P2, suggests that the distal 3.5 kb of 5'-flanking PLP1 DNA specific to 6.2hPLP(+)Z/FL contains regulatory element(s) important for promoting early postnatal expression in brain.

Myelinated axons are the primary target of hemin-mediated oxidative damage in a model of the central nervous system

Iron released from oligodendrocytes during demyelination or derived from haemoglobin breakdown products is believed to amplify oxidative tissue injury in multiple sclerosis (MS). However, the pathophysiological significance of iron-containing haemoglobin breakdown products themselves is rarely considered in the context of MS and their cellular specificity and mode of action remain unclear. Using myelinating cell cultures, we now report the cytotoxic potential of hemin (ferriprotoporphyrin IX chloride), a major degradation product of haemoglobin, is 25-fold greater than equimolar concentrations of free iron in myelinating cultures; a model that reproduces the complex multicellular environment of the CNS. At low micro molar concentrations (3.3 - 10 μM) we observed hemin preferentially binds to myelin and axons to initiate a complex detrimental response that results in targeted demyelination and axonal loss but spares neuronal cell bodies, astrocytes and the majority of oligodendroglia. Demyelination and axonal loss in this context are executed by a combination of mechanisms that include iron-dependent peroxidation by reactive oxygen species (ROS) and ferroptosis. These effects are microglial-independent, do not require any initiating inflammatory insult and represent a direct effect that compromises the structural integrity of myelinated axons in the CNS. Our data identify hemin-mediated demyelination and axonal loss as a novel mechanism by which intracerebral degradation of haemoglobin may contribute to lesion development in MS.

Polarizing receptor activation dissociates fibroblast growth factor 2 mediated inhibition of myelination from its neuroprotective potential

Fibroblast growth factor (FGF) signaling contributes to failure of remyelination in multiple sclerosis, but targeting this therapeutically is complicated by its functional pleiotropy. We now identify FGF2 as a factor up-regulated by astrocytes in active inflammatory lesions that disrupts myelination via FGF receptor 2 (FGFR2) mediated activation of Wingless (Wnt) signaling; pharmacological inhibition of Wnt being sufficient to abrogate inhibition of myelination by FGF2 in tissue culture. Using a novel FGFR1-selective agonist (F2 V2) generated by deleting the N-terminal 26 amino acids of FGF2 we demonstrate polarizing signal transduction to favor FGFR1 abrogates FGF mediated inhibition of myelination but retains its ability to induce expression of pro-myelinating and immunomodulatory factors that include Cd93, Lif, Il11, Hbegf, Cxcl1 and Timp1. Our data provide new insights into the mechanistic basis of remyelination failure in MS and identify selective activation of FGFR1 as a novel strategy to induce a neuroprotective signaling environment in multiple sclerosis and other neurological diseases.

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.

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Development of a novel 3D culture system for screening features of a complex implantable device for CNS repair

Tubular scaffolds which incorporate a variety of micro- and nanotopographies have a wide application potential in tissue engineering especially for the repair of spinal cord injury (SCI). We aim to produce metabolically active differentiated tissues within such tubes, as it is crucially important to evaluate the biological performance of the three-dimensional (3D) scaffold and optimize the bioprocesses for tissue culture. Because of the complex 3D configuration and the presence of various topographies, it is rarely possible to observe and analyze cells within such scaffolds in situ. Thus, we aim to develop scaled down mini-chambers as simplified in vitro simulation systems, to bridge the gap between two-dimensional (2D) cell cultures on structured substrates and three-dimensional (3D) tissue culture. The mini-chambers were manipulated to systematically simulate and evaluate the influences of gravity, topography, fluid flow, and scaffold dimension on three exemplary cell models that play a role in CNS repair (i.e., cortical astrocytes, fibroblasts, and myelinating cultures) within a tubular scaffold created by rolling up a microstructured membrane. Since we use CNS myelinating cultures, we can confirm that the scaffold does not affect neural cell differentiation. It was found that heterogeneous cell distribution within the tubular constructs was caused by a combination of gravity, fluid flow, topography, and scaffold configuration, while cell survival was influenced by scaffold length, porosity, and thickness. This research demonstrates that the mini-chambers represent a viable, novel, scale down approach for the evaluation of complex 3D scaffolds as well as providing a microbioprocessing strategy for tissue engineering and the potential repair of SCI.

Expression alterations of genes on both neuronal and glial development in rats after developmental exposure to 6-propyl-2-thiouracil

The present study was performed to determine target gene profiles associated with pathological mechanisms of developmental neurotoxicity. For this purpose, we selected a rat developmental hypothyroidism model because thyroid hormones play an essential role in both neuronal and glial development. Region-specific global gene expression analysis was performed at postnatal day (PND) 21 on four brain regions representing different structures and functions, i.e., the cerebral cortex, corpus callosum, dentate gyrus and cerebellar vermis of rats exposed to 6-propyl-2-thiouracil in the drinking water at 3 and 10ppm from gestational day 6 to PND 21. Expression changes of gene clusters of neuron differentiation and development, cell migration, synaptic function, and axonogenesis were detected in all four regions. Characteristically, gene expression profiles suggestive of affection of ephrin signaling and glutamate transmission were obtained in multiple brain regions. Gene clusters suggestive of suppression of myelination and glial development were specifically detected in the corpus callosum and cerebral cortex. Immunohistochemically, immature astrocytes immunoreactive for vimentin and glial fibrillary acidic protein were increased, and oligodendrocytes immunoreactive for oligodendrocyte lineage transcription factor 2 were decreased in the corpus callosum. Immunoreactive intensity of myelin basic protein was also decreased in the corpus callosum and cerebral cortex. The hippocampal dentate gyrus showed downregulation of Ptgs2, which is related to synaptic activity and neurogenesis, as well as a decrease of cyclooxygenase-2-immunoreactive granule cells, suggesting an impaired synaptic function related to neurogenesis. These results suggest that multifocal brain region-specific microarray analysis can determine the affection of neuronal or glial development.

Human mesenchymal stem cells isolated from olfactory biopsies but not bone enhance CNS myelination in vitro

Spinal cord injury (SCI) is a devastating condition with limited capacity for repair. Cell transplantation is a potential strategy to promote SCI repair with cells from the olfactory system being promising candidates. Although transplants of human olfactory mucosa (OM) are already ongoing in clinical trials, the repair potential of this tissue remains unclear. Previously, we identified mesenchymal-like stem cells that reside in the lamina propria (LP-MSCs) of rat and human OM. Little is known about these cells or their interactions with glia such as olfactory ensheathing cells (OECs), which would be co-transplanted with MSCs from the OM, or endogenous CNS glia such as oligodendrocytes. We have characterized, purified, and assessed the repair potential of human LP-MSCs by investigating their effect on glial cell biology with specific emphasis on CNS myelination in vitro. Purified LP-MSCs expressed typical bone marrow MSC (BM-MSC) markers, formed spheres, were clonogenic and differentiated into bone and fat. LP-MSC conditioned medium (CM) promoted oligodendrocyte precursor cell (OPC) and OEC proliferation and induced a highly branched morphology. LP-MSC-CM treatment caused OEC process extension. Both LP and BM-MSCs promoted OPC proliferation and differentiation, but only myelinating cultures treated with CM from LP and not BM-MSCs had a significant increase in myelination. Comparison with fibroblasts and contaminating OM fibroblast like-cells showed the promyelination effect was LP-MSC specific. Thus LP-MSCs harvested from human OM biopsies may be an important candidate for cell transplantation by contributing to the repair of SCI.

Proteolipid protein dimerization at cysteine 108: Implications for protein structure

Proteolipid protein (PLP) and its alternatively spliced isoform DM20 comprise ∼50% of central nervous system (CNS) myelin protein. The two proteins are identical in sequence except for the presence of a 35 amino sequence within the intracellular loop of PLP that is absent in DM20. In this work, we compared the expression of PLP/DM20 in transfected cells, oligodendrocytes and brain. In all 3 tissues, PLP exists as both a monomer and a disulfide-linked dimer; in contrast, DM20 is found mainly as a monomer. PLP dimers were increased by both chemical crosslinking and incubation with hydrogen peroxide, and were mediated by a cysteine at amino acid 108, located within the proximal intracellular loop of both PLP and DM20. The PLP-specific sequence thus influences the accessibility of this cysteine to chemical modification, perhaps as a result of altering protein structure. Consistent with these findings, several mutant PLPs known to cause Pelizaeus-Merzbacher disease form predominantly disulfide-linked, high molecular weight aggregates in transfected COS7 cells that are arrested in the ER and are associated with increased expression of CHOP, a part of the cellular response to unfolded proteins. In contrast, the same mutations in DM20 accumulate fewer high molecular weight disulfide-linked species that are expressed at the cell surface, and are not associated with increased CHOP. Taken together, these data suggest that mutant PLP multimerization, mediated in part by way of cysteine 108, may be part of the pathogenesis of Pelizaeus-Merzbacher disease.

Time-lapse imaging of the dynamics of CNS glial-axonal interactions in vitro and ex vivo

Background

Myelination is an exquisite and dynamic example of heterologous cell-cell interaction, which consists of the concentric wrapping of multiple layers of oligodendrocyte membrane around neuronal axons. Understanding the mechanism by which oligodendrocytes ensheath axons may bring us closer to designing strategies to promote remyelination in demyelinating diseases. The main aim of this study was to follow glial-axonal interactions over time both in vitro and ex vivo to visualize the various stages of myelination.

Methodology/Principal Findings

We took two approaches to follow myelination over time: i) time-lapse imaging of mixed CNS myelinating cultures generated from mouse spinal cord to which exogenous GFP-labelled murine cells were added, and ii) ex vivo imaging of the spinal cord of shiverer (Mbp mutant) mice, transplanted with GFP-labelled murine neurospheres. We demonstrate that oligodendrocyte-axonal interactions are dynamic events with continuous retraction and extension of oligodendroglial processes. Using cytoplasmic and membrane-GFP labelled cells to examine different components of the myelin-like sheath, we provide evidence from time-lapse fluorescence microscopy and confocal microscopy that the oligodendrocytes' cytoplasm-filled processes initially spiral around the axon in a corkscrew-like manner. This is followed subsequently by focal expansion of the corkscrew process to form short cuffs, which then extend longitudinally along the axons. We predict from this model that these spiral cuffs must extend over each other first before extending to form internodes of myelin.

Conclusion

These experiments show the feasibility of visualizing the dynamics of glial-axonal interaction during myelination over time. Moreover, these approaches complement each other with the in vitro approach allowing visualization of an entire internodal length of myelin and the ex vivo approach validating the in vitro data.

Genetically-defined lineage tracing of Nkx2.2-expressing cells in chick spinal cord

In the spinal cord, generation of oligodendrocytes (OLs) is totally dependent on the presence of Olig2, a basic helix-loop-helix transcription factor. However, it also requires Nkx2.2 for its generation, whose expression follows the expression of Olig2. Although it is believed that oligodendrocytes originate from the pMN domain, Nkx2.2 is present in the p3 domain located ventral to the pMN domain. According to recent reports, it is possible that oligodendrocytes are directly derived from the p3 domain in addition to the pMN domain in the chick spinal cord. We examined this hypothesis in this paper. To analyze OL development in the spinal cord, chick embryos are widely used for genetic modification by electroporation or for transplantation experiments, because it is relatively easy to manipulate them compared with mouse embryos. However, genetic modification by electroporation is not appropriate for glial development analyses because glia proliferate vigorously before maturation. In order to overcome these problems, we established a novel method to permanently introduce exogenous gene into a specific cell type. We introduced the CAT1 gene, a murine retroviral receptor, by electroporation followed by injection of murine retrovirus. By using this method, we successfully transduced murine retrovirus into the chick neural tube. We analyzed cell lineage from the p3 domain by restricting CAT1 expression by Nkx2.2-enhancer and found that most of the labeled cells became OLs when the cells were labeled at cE4. Moreover, the labeled OLs were found throughout the white matter in the spinal cord including the most dorsal spinal cord. Thus p3 domain directly generates spinal cord OLs in the chick spinal cord.

Novel neuronal proteolipid protein isoforms encoded by the human myelin proteolipid protein 1 gene

The human myelin proteolipid protein 1 gene (hPLP1), which encodes the major structural myelin proteins of the central nervous system (CNS), is classically described as expressed in the oligodendrocytes, the CNS myelinating cells. We identified two new exons in the intron 1 of the hPLP1 gene that lead to the expression of additional mRNA and protein isoforms mainly expressed in neurons instead of oligodendrocytes. Those novel neuronal PLP isoforms are detected as soon as human fetal development and their concomitant expression is specific of the human species. As classical PLP proteins, the novel protein isoforms seem to be addressed to the plasma membrane. These results suggest for the first time that PLP may have functions in humans not only in oligodendrocytes but also in neurons and could be implicated in axono-glial communication. Moreover, this neuronal expression of the hPLP1 gene might explain the neuronal dysfunctions in patients carrying hPLP1 gene mutations.

BMP signaling through BMPRIA in astrocytes is essential for proper cerebral angiogenesis and formation of the blood-brain-barrier

Bone morphogenetic protein (BMP) signaling is involved in differentiation of neural precursor cells into astrocytes, but its contribution to angiogenesis is not well characterized. This study examines the role of BMP signaling through BMP type IA receptor (BMPRIA) in early neural development using a conditional knockout mouse model, in which Bmpr1a is selectively disrupted in telencephalic neural stem cells. The conditional mutant mice show a significant increase in the number of cerebral blood vessels and the level of vascular endothelial growth factor (VEGF) is significantly upregulated in the mutant astrocytes. The mutant mice also show leakage of immunoglobulin around cerebral microvessels in neonatal mice, suggesting a defect in formation of the blood-brain-barrier. In addition, astrocytic endfeet fail to encircle cortical blood vessels in the mutant mice. These results suggest that BMPRIA signaling in astrocytes regulates the expression of VEGF for proper cerebrovascular angiogenesis and has a role on in the formation of the blood-brain-barrier.

Oligodendrocytes secrete exosomes containing major myelin and stress-protective proteins: Trophic support for axons?

Oligodendrocytes synthesize the CNS myelin sheath by enwrapping axonal segments with elongations of their plasma membrane. Spatial and temporal control of membrane traffic is a prerequisite for proper myelin formation. The major myelin proteolipid protein (PLP) accumulates in late endosomal storage compartments and multivesicular bodies (MVBs). Fusion of MVBs with the plasma membrane results in the release of the intralumenal vesicles, termed exosomes, into the extracellular space. Here, we show that cultured oligodendrocytes secrete exosomes carrying major amounts of PLP and 2'3'-cyclic-nucleotide-phosphodiesterase (CNP). These exosomes migrated at the characteristic density of 1.10-1.14 g/mL in sucrose density gradients. Treatment of primary oligodendrocytes with the calcium-ionophore ionomycin markedly increased the release of PLP-containing exosomes, indicating that oligodendroglial exosome secretion is regulated by cytosolic calcium levels. A proteomic analysis of the exosomal fraction isolated by sucrose density centrifugation revealed in addition to PLP and CNP, myelin basic protein (MBP) and myelin oligodendrocyte glycoprotein (MOG) as constituents of oligodendroglial exosomes, together with a striking group of proteins with proposed functions in the relief of cell stress. Oligodendroglial exosome secretion may contribute to balanced production of myelin proteins and lipids, but in addition exosomes may embody a signaling moiety involved in glia-mediated trophic support to axons.

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.

Human embryonic stem cell-derived oligodendrocyte progenitors for the treatment of spinal cord injury

Stem cells are self-renewing, pluripotent cells that can be manipulated in vitro to differentiate into virtually any cell type. Stem cells are highly proliferative and have the potential to expand into very large numbers of a desired cell lineage. As such, they represent an excellent source of cells for cellular replacement strategies in disease states that are typified by a loss of a particular cell population. Recent studies have indicated that spinal cord injury is accompanied by chronic progressive demyelination, and have thus identified oligodendrocytes as a desirable transplant population for remyelination strategies. To address this need, we developed a method to differentiate hESCs into high purity human oligodendrocyte progenitor cells (OPCs). Transplantation into spinal cord injury sites in adult rats resulted in remyelination and functional repair. Here, we summarize these findings and present new data concerning the effects of hESC-derived OPC transplantation on the host environment.

Selective and Antigen-Dependent Effects of Myelin Degeneration on Central Nervous System Inflammation

Damage to myelin sheath or oligodendrocytes may precede or even provoke inflammation of the central nervous system (CNS), but the extent to which these degenerative changes affect inflammation remains largely undefined. To study these processes in more detail, we used CNS antigen-specific T cells in the presence or absence of anti-myelin antibodies to induce experimental autoimmune encephalomyelitis (EAE) in transgenic Lewis rats with low-grade subclinical myelin degeneration and associated microglia cell activation, and in wild-type Lewis rats with an intact CNS. We found that myelin degeneration affects the localization of inflammatory lesions, the numbers of T cells recruited to these lesions, and the severity of the resulting clinical disease. In addition, myelin degeneration and associated microglia cell activation jointly enhance the susceptibility of the CNS to the action of anti-myelin antibodies. Our data show that even subtle alterations of myelin and oligodendrocytes may massively amplify the extent of demyelination and tissue damage, involving different immune effector mechanisms. A similar causal relationship might also operate in human patients with multiple sclerosis, where T cell-mediated inflammation and antibody-mediated demyelination have been documented, and where genetic factors might determine the susceptibility of the target tissue for immune-mediated injury.

Oligodendrogenesis is differentially regulated in gray and white matter of jimpy mice

The factors that regulate oligodendrogenesis have been studied extensively in optic nerve, where oligodendrocyte production and myelination quickly follow colonization of the nerve by progenitor cells. In contrast, oligodendrocyte production in the cerebral cortex begins approximately 1 week after progenitor cell colonization and continues for 3-4 weeks. This and other observations raise the possibility that oligodendrogenesis is regulated by different mechanisms in white and gray matter. The present study examined oligodendrocyte production in the developing cerebral cortex of jimpy (jp) and jimpy(msd) (msd) mice, which exhibit hypomyelination and oligodendrocyte death due to mutations in and toxic accumulations of proteolipid protein, the major structural protein of CNS myelin. Proliferation of oligodendrocyte progenitors and production of myelinating oligodendrocytes was reduced in jp cerebral cortex when compared to wild-type (wt) and msd mice. The incidence of oligodendrocyte cell death was similar in jp and msd cortex, but total dying oligodendrocytes were greater in msd. We confirm previous reports of increased oligodendrocyte production in white matter of both jp and msd mice. The jp mutation, therefore, reduces oligodendrocyte production in cerebral cortex but not in white matter. These data provide additional evidence that oligodendrogenesis is differentially regulated in white matter and gray matter and implicate PLP/DM20 as a modulator of these differences.

Trafficking of PLP/DM20 and cAMP signaling in immortalized jimpy oligodendrocytes

The synthesis, transport, and insertion of jimpy proteolipid protein and DM20 were studied in normal (158N) and jimpy (158JP) immortalized oligodendrocyte lines. Four different expression vectors encoding fusion proteins composed of native PLP and DM20 or jimpy PLP or DM20 were linked to enhanced green fluorescent protein (EGFP). All four transfected fusion proteins had similar distributions in the cell bodies and processes of the two cell types. Both normal and jimpy PLP-EGFP and DM20-EGFP were detected in both cell lines as far as 200 microM from the cell body, indicating synthesis and transport of mutated PLP and DM20 toward the plasma membrane. Immunocytochemistry of fixed normal and jimpy cells with the O10 antibody, which recognizes a conformationally sensitive PLP/DM20 epitope, confirmed that normal and jimpy PLP and DM20 were transported to the plasma membrane. Live staining of normal and jimpy cells transiently transfected with the native PLP showed positive staining, indicating PLP was correctly inserted into the membrane of both normal and jimpy oligodendrocytes. However, live staining of normal and jimpy cells transiently transfected with jimpy PLP showed no positive staining, indicating the mutated protein is abnormally inserted into the plasma membrane. Electrophysiological recordings of the resting membrane potential measured in the whole cell mode of the patch-clamp technique showed the absence of a developmentally regulated negative shift in the membrane potential in jimpy cells compared to normal native or immortalized oligodendrocytes. Treatment of 158N cells and native oligodendrocytes with dibutyryl cAMP (dbcAMP) caused morphological and biochemical differentiation, but failed to do so in 158JP cells, suggesting an abnormal signaling pathway in jimpy. The defect in cAMP signaling in jimpy oligodendrocytes was associated with the suppression of increase in mRNA level of the inducible cAMP early repressor (ICER). When the jimpy oligodendrocyte line was transfected with normal PLP or DM20 and exposed to dbcAMP, the cells failed to differentiate. This finding suggests that improper insertion of jimpy protein into the plasma membrane alters the membrane in such a way that certain signaling pathways are permanently altered. The abnormal insertion of jimpy PLP/DM20 into the plasma membrane may be the basis for the lack of cell signaling and abnormal resting potential in jimpy oligodendrocytes.

Proteolipid protein gene modulates viability and phenotype of neurons

Overexpression or lack of expression of proteolipid protein (PLP) gene by oligodendrocytes causes axonal pathology. It is unclear whether dysfunction of the PLP gene mediates its effects directly on neurons or indirectly by abnormal formation of myelin sheaths. We performed experiments using cocultures and conditioned media (CM) to test the direct effect of PLP gene expression on neurons. Non-glial cell lines were stably transfected with PLP or DM20 (an alternate splice variant of PLP) cDNAs. Immunocytochemistry and enhanced green fluorescent protein expression showed that translated products were synthesized and inserted into the plasma membrane in proper conformation. The number of surviving dorsal root ganglion (DRG) neurons was significantly less than controls when cocultured for 5 d with PLP-expressing cells. The number of degenerating neurons increased in a dose-dependent manner corresponding to increasing numbers of PLP-expressing cells. However, the number of surviving DRG neurons cocultured with DM20-expressing cells was comparable to that of controls, indicating that PLP-specific products contributed to decreased neuron survival. When DRG neurons were cultured with CM from PLP- or DM20-expressing cells, significantly fewer neurons survived with CM of PLP- but not DM20-expressing cells. This suggests that secreted factors from PLP-expressing cells contribute to neuronal death. Increased neuronal death found with PLP-expressing cells cannot be attributed to density-dependent artifacts, because in each experiment the density of different cell lines was similar. This effect of CM may be mediated by a negative pH shift elicited from PLP but not DM20 expression. These results indicate that PLP gene products directly modulate neuron viability.

Myelin proteolipid protein--the first 50 years

Myelin proteolipid protein (PLP), the most abundant protein of central nervous system (CNS) myelin, is a hydrophobic integral membrane protein. Because of its physical properties, which make it difficult to work with, progress towards determining the exact function(s) and disease associations of myelin PLP has been slow. However, recent molecular biology advances have given new life to investigations of PLP, and suggest that it has multiple functions within myelin and is of importance in several neurological disorders.

Proteolipid promoter activity distinguishes two populations of NG2-positive cells throughout neonatal cortical development

Transgenic mice expressing enhanced green fluorescent protein (EGFP) driven by the mouse myelin proteolipid protein (PLP) gene promoter have been developed to investigate cells in the oligodendrocyte lineage. Transgene expression is consistent with the developmental expression of PLP, with cells at all stages of oligodendrocyte differentiation clearly visualized. These animals were analyzed to establish the time course of oligodendrocyte progenitor migration, proliferation, and differentiation in neonatal cortex. In these animals, two populations of NG2 proteoglycan-positive oligodendrocyte progenitor cells were identified that exist in postnatal subventricular zone and cortex. These two populations are distinguished by the presence or absence of PLP gene expression. Thus, PLP gene expression defines a subpopulation of NG2-positive cells from very early postnatal ages, which migrates toward the pial surface and proliferates in situ. EGFP(+)/NG2(+) cells are present in the cortex from postnatal day 1, and they remain in the cortex as undifferentiated oligodendrocyte progenitors for up to 3 weeks before myelination begins. These data could be explained by the presence of an important inhibitor of oligodendrocyte differentiation in the cortex during this period, which is downregulated in a region-specific manner to allow myelination. On the other hand, it is possible that oligodendrocyte progenitor cells remain undifferentiated in cortex until an essential signal is produced in situ to induce differentiation.

An immortalized jimpy oligodendrocyte cell line: defects in cell cycle and cAMP pathway

Normal and jimpy oligodendrocytes in secondary cultures were transfected with plasmids containing the SV40 T-antigen gene expressed under the control of the mouse metallothionein-I promoter. Two immortalized stable cell lines, a normal (158N) and jimpy (158JP) cell line, expressed transcripts and proteins of oligodendrocyte markers, including proteolipid protein (PLP), myelin basic protein (MBP), and carbonic anhydrase II (CAII). Galactocerebroside and sulfatide were also detected with immunocytochemistry. Immunoelectron microscopy using gold particles showed that the truncated endogenous jimpy PLP was distributed throughout the cytoplasm and in association with the plasma membrane of cell bodies and processes. The length of the cell cycle in the jimpy oligodendrocytes in the absence of zinc was 31 h, about a 4-h longer cell cycle than the normal line. In the presence of 100 microM zinc, the cell cycle became 3 h shorter for both cell lines, with the jimpy cell cycle duration remaining 4 h longer than the normal line. Interestingly, the jimpy cell line showed a significant deficiency in stimulation via the cAMP pathway. While the level of oligodendrocyte markers (PLP, MBP, and CAII) were significantly increased by dibutyryl cAMP (dbcAMP) treatment in the normal cell line, no changes were observed in the jimpy cell lines. This observation, together with previous results showing jimpy oligodendrocyte's failure to respond to basic fibroblast growth factor (bFGF), suggests a role for PLP in a signal transduction pathway. Jimpy and normal oligodendrocytes transfected with the SV40T antigen gene, driven by the wild-type promoter of mouse metallothionein-I, continue to express properties of oligodendrocytes and therefore provide a powerful model to explore the function of myelin proteins and to dissect the complexity of the jimpy phenotype.

Aberrant dendrite growth in central nervous system white matter induced by a mutant proteolipid protein transgene

Transgenic mice were produced that carry a construct encoding a mutant form of the DM20 isoform of myelin proteolipid protein. The transgene is under the direction of the human Plp gene promoter, which has previously been shown to direct tissue-specific expression of transgenes. Two lines of mice were generated with this construct, both of which express the transgene at extremely low levels. Central nervous system myelination proceeds normally in the transgenic mice. However, in aged transgenic mice, areas of dendrite processes synapsed with axonal termini were observed within the white matter of the spinal cord. This phenotype was accompanied by focal areas of astrocytic hypertrophy and an increase in apoptotic cell death in white matter but not gray matter. One interpretation of these findings is that expression of the mutant DM20 alters signaling between oligodendrocytes and neurons, producing abnormal neurite outgrowth.