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.

Application of q-Space Diffusion MRI for the Visualization of White Matter.

White matter abnormalities in the CNS have been reported recently in various neurological and psychiatric disorders. Quantitation of non-Gaussianity for water diffusion by q-space diffusional MRI (QSI) renders biological diffusion barriers such as myelin sheaths; however, the time-consuming nature of this method hinders its clinical application. In the current study, we aimed to refine QSI protocols to enable their clinical application and to visualize myelin signals in a clinical setting. For this purpose, animal studies were first performed to optimize the acquisition protocol of a non-Gaussian QSI metric. The heat map of standardized kurtosis values derived from optimal QSI (myelin map) was then created. Histological validation of the myelin map was performed in myelin-deficient mice and in a nonhuman primate by monitoring its variation during demyelination and remyelination after chemical spinal cord injury. The results demonstrated that it was sensitive enough to depict dysmyelination, demyelination, and remyelination in animal models. Finally, its utility in clinical practice was assessed by a pilot clinical study in a selected group of patients with multiple sclerosis (MS). The human myelin map could be obtained within 10 min with a 3 T MR scanner. Use of the myelin map was practical for visualizing white matter and it sensitively detected reappearance of myelin signals after demyelination, possibly reflecting remyelination in MS patients. Our results together suggest that the myelin map, a kurtosis-related heat map obtainable with time-saving QSI, may be a novel and clinically useful means of visualizing myelin in the human CNS. SIGNIFICANCE STATEMENT: Myelin abnormalities in the CNS have been gaining increasing attention in various neurological and psychiatric diseases. However, appropriate methods with which to monitor CNS myelin in daily clinical practice have been lacking. In the current study, we introduced a novel MRI modality that produces the "myelin map." The myelin map accurately depicted myelin status in mice and nonhuman primates and in a pilot clinical study of multiple sclerosis patients, suggesting that it is useful in detecting possibly remyelinated lesions. A myelin map of the human brain could be obtained in <10 min using a 3 T scanner and it therefore promises to be a powerful tool for researchers and clinicians examining myelin-related diseases.

Effects of the jimpy mutation on mouse retinal structure and function.

The Jimpy mutant mouse has a point mutation in the proteolipid protein gene (plp1). The resulting misfolding of the protein leads to oligodendrocyte death, myelin destruction, and failure to produce adequately myelinated axons in the central nervous system (CNS). It is not known how the absence of normal myelination during development influences neural function. We characterized the Jimpy mouse retina to find out whether lack of myelination in the optic nerve during development has an effect on normal functioning and morphology of the retina. Optokinetic reflex measurements showed that Jimpy mice had, in general, a functional visual system. Both PLP1 antibody staining and reverse transcriptase-polymerase chain reaction for plp1 mRNA showed that plp1 is not expressed in the wild-type retina. However, in the optic nerve, plp1 is normally expressed, and consequently, in Jimpy mutant mice, myelination of axons in the optic nerve was mostly absent. Nevertheless, neither axon count nor axon ultrastructure in the optic nerve was affected. Physiological recordings of ganglion cell activity using microelectrode arrays revealed a decrease of stimulus-evoked activity at mesopic light levels. Morphological analysis of the retina did not show any significant differences in the gross morphology, such as thickness of retinal layers or cell number in the inner and outer nuclear layer. The cell bodies in the inner nuclear layer, however, were larger in the peripheral retina of Jimpy mutant mice. Antibody labeling against cell type-specific markers showed that the number of rod bipolar and horizontal cells was increased in Jimpy mice. In conclusion, whereas the Jimpy mutation has dramatic effects on the myelination of retinal ganglion cell axons, it has moderate effects on retinal morphology and function.

Pharmacokinetic analysis of in vivo disposition of heparin-superoxide dismutase.

To improve the half-life and tissue targeting of SOD to suppress reactive oxygen species (ROS)-mediated injury, chemically modified derivative of superoxide dismutase (SOD) with heparin, anionized SOD (Hep-SOD), was designed. In this study, the pharmacokinetics of Hep-SOD had been studied. This study aimed to investigate the pharmacokinetics, tissue distribution and cell targeting. ¹²5I-radiolabeled Hep-SOD conjugate was administered to healthy mice by intravenous (i.v.) bolus injection. Compared with native SOD, the half-life of Hep-SOD conjugate, including t(1/2α) and of t(1/2ß), was lengthen and area under the plasma concentration versus time curve (AUC) of Hep-SOD was increased. The study showed that both native SOD and Hep-SOD was rapidly and widely distributed in the livers, kidneys, spleens, hearts and lungs. Furthermore, compared with Hep-SOD, radioactivity of native SOD decreased more sharply over time in most tissues. Compared with native SOD, higher amount of Hep-SOD radioactivity was found in the livers. Since livers are not the known target of ¹²5I, the most possible reason is that Hep-SOD binds to its specific targets in the livers.

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

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

Cell-specific loss of kappa-opioid receptors in oligodendrocytes of the dysmyelinating jimpy mouse.

Jimpy is a murine mutation in myelin proteolipid protein, leading to premature death of oligodendrocytes and severe central nervous system hypomyelination. Jimpy is a bona fide model of human Pelizaeus-Merzbacher disease. This paper describes a severe reduction in expression of kappa-opioid receptors (KOP) in oligodendrocytes of jimpy mice. A cell-specific reduction of >90% is apparent by 5 days of age. Expression is not reduced in neurons, and mu-opioid receptor expression is normal. Mechanism(s) leading to deficient KOP expression in jimpy mice remain unclear. We speculate that loss of KOP may be related to increased [Ca(2+)](i) and premature death of jimpy oligodendrocytes.

Expression of distinct splice variants of the stem cell marker prominin-1 (CD133) in glial cells.

Prominin-1 (CD133) is a cholesterol-interacting pentaspan membrane glycoprotein specifically associated with plasma membrane protrusions. Prominin-1 is expressed by various stem and progenitor cells, notably neuroepithelial progenitors found in the developing embryonic brain. Here, we further investigated its expression in the murine brain. Biochemical analyses of brain membranes at early stages of development revealed the expression of two distinct splice variants of prominin-1, s1 and s3, which have different cytoplasmic C-terminal domains. The relative abundance of the s3 variant increased toward adulthood, whereas the opposite was observed for the s1 variant. Our combined in situ hybridization and immunohistochemistry revealed the expression of prominin-1 in a subpopulation of Olig-2-positive oligodendroglial cells present within white matter tracts of postnatal and adult brain. Furthermore, immunohistological and biochemical characterization suggested strongly that the s3 variant is a novel component of myelin. Consistent with this, the expression of prominin-1.s3 was significantly reduced in the brain of myelin-deficient mice. Finally, oligodendrocytes expressed selectively the s3 variant whereas GFAP-positive astrocytes expressed the s1 variant in primary glial cell cultures derived from embryonic brains. Collectively, our data demonstrate a complex expression pattern of prominin-1 molecules in developing adult brain. Given that prominin-1 is thought to act as an organizer of plasma membrane protrusions, they further suggest that a specific prominin-1 splice variant might play a role in morphogenesis and/or maintenance of the myelin sheath.

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.

Astrocytic hypertrophy in dysmyelination influences the diffusion anisotropy of white matter.

The effect of a proteolipid protein (PLP) mutation on the developing white matter anisotropy was examined by diffusion tensor magnetic resonance imaging (DT-MRI) in a noninvasive study of a mouse model of Pelizaeus-Merzbacher disease (PMD). The jimpy PLP mutation in mice produces an irreversible dysmyelination in jimpy males, whereas heterozygous females exhibit a transient hypomyelination, as assessed by a longitudinal study of the same mice during development. Modifications of the different individual DT-MRI parameters were highlighted by specific changes in tissue structures caused by the mutation that includes the hypomyelination, axonal abnormalities, and recovery. Astrocytic hypertrophy is a striking cellular event in dysmyelinated jimpy brain, where most axons or bundles of fibers are entirely wrapped by astrocyte cytoplasmic processes, so its influences on DT-MRI parameters in dysmyelination were examined for the first time. DT-MRI data of the jimpy brain were compared with those obtained from dysmyelination of (oligo-TTK) transgenic mice, induced by oligodendrocyte killing, which have a mild astrocyte hypertrophy (Jalabi et al., 2005), and from recovering jimpy females, which have reduced astrocyte hypertrophy. The unique morphological feature of astrocytes in jimpy males coupled with an increase in the water channel protein aquaporin 4 (AQP4) was found to facilitate the directional water diffusion in the white matter. In addition to the major changes of DT-MRI parameters in the two dysmyelinated mice caused by the myelin loss and axonal modifications, the amplified magnitude of radial and axial diffusions in jimpy males was attributed principally to the strongly pronounced astrocyte hypertrophy.

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

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

New insights on neuronal alterations in jimpy mutant brain.

In spite of abundant data on oligodendrocyte abnormalities in dysmyelinated jimpy brain, little is known about the axonal damage and the expression of neuronal genes. Recent findings indicate that Nogo-A, oligodendrocyte-myelin glycoprotein (OMgp), and myelin-associated glycoprotein (MAG) inhibit axonal growth by binding a common receptor, the Nogo-A receptor (NgR)-p75 complex. In order to evaluate neuronal modifications in the absence of myelin and in the presence of abnormal oligodendrocytes at different developmental stages, the expression of these inhibitory proteins and their receptors was investigated in jimpy mutant brain. Despite the decrease in oligodendrocyte number at P15 and P25 in jimpy, Nogo-A and OMgp mRNA levels are not significantly different compared with control, suggesting an overexpression of neuronal Nogo-A and OMgp in mutant. Double immunolabeling for Nogo-A and neurofilaments shows strong axonal staining of Nogo-A in jimpy and its down-regulation in oligodendrocytes. The current data raise questions about functions of Nogo-A other than neurite growth inhibition in the CNS. No significant changes in NgR mRNA levels were observed in jimpy, where the increase in p75 level can be correlated with the cell death of oligodendrocytes. In the paranodal region, the cell adhesion molecule neurofascin glial isoform NFN155 mRNA level is reduced by 40% whereas neuronal form NFN186 is up-regulated. These results may explain the failure of paranodal region organization, even with normal level of CASPR (paranodin) mRNA detected in jimpy brain.

Analyses of proteolipid protein mutants show levels of proteolipid protein regulate oligodendrocyte number and cell death in vitro and in vivo.

Previous tissue culture studies indicate that the level of native proteolipid protein (PLP) or mutated PLP regulates the number of oligodendrocytes (Olgs). The regulation of Olg number is most likely due to toxicity of over-expression of native PLP or mis-sense mutations of PLP. We tested, in vivo and in vitro, the hypothesis that the absence of native PLP or reduced amounts of mutated PLP leads to an increase in numbers of Olgs and a corresponding decrease in the number of apoptotic Olgs. In cultures derived from PLP deficient mice, the number of Olgs is twofold greater than in wild-type mice. In primary glial cultures or in enriched OLG cultures, in which the synthesis of native PLP is blocked using antisense technology, the number of apoptotic cells is several-fold reduced. Injection of PLP antisense oligodeoxynucleotides into jimpy (jp) mice reduces the number of dying glia in spinal cord 3x compared to controls, and increased the number of myelinated fibers. These studies demonstrate that inhibition of native or mutant PLP synthesis directly reduces apoptosis. The regulation of apoptosis by PLP gene expression occurs independently of myelination, indicating that the PLP gene has multiple primary functions.

Association of Caspr/paranodin with tumour suppressor schwannomin/merlin and beta1 integrin in the central nervous system.

Caspr/paranodin is an essential neuronal component of paranodal axoglial junctions, associated with contactin/F3. Its short intracellular domain contains a conserved motif (GNP motif) capable of binding protein 4.1 domains [FERM domains (four point one, ezrin, radixin, moesin)]. Schwannomin/merlin is a tumour suppressor expressed in many cell types, including in neurons, the function and partners of which are still poorly characterized. We show that the FERM domain of schwannomin binds to the paranodin GNP motif in glutathione S-transferase (GST)-pull down assays and in transfected COS-7 cells. The two proteins co-immunoprecipitated in brain extracts. In addition, paranodin and schwannomin were associated with integrin beta1 in transfected cells and in brain homogenates. The presence of paranodin increased the association between integrin beta1 and schwannomin or its N-terminal domain, suggesting that the interactions between these proteins are interdependent. In jimpy mutant mice, which display a severe dysmyelination with deficient paranodal junctions, the interactions between paranodin, schwannomin and integrin beta1 were profoundly altered. Our results show that schwannomin and integrin beta1 can be associated with paranodin in the central nervous system. Since integrin beta1 and schwannomin do not appear to be enriched in paranodes they may be quantitatively minor partners of paranodin in these regions and/or be associated with paranodin at other locations.

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.

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.

Ferritin binding in the developing mouse brain follows a pattern similar to myelination and is unaffected by the jimpy mutation.

We have previously provided evidence that ferritin binds selectively to white matter tracts in adult mouse and human brains. In cell culture experiments, ferritin binding is specifically localized to oligodendrocytes. The goal of the present study is to test the hypothesis that the developmental pattern for ferritin binding will coincide with the onset and progression of myelination. The first evidence of ferritin binding in the mouse brain is at 12 days of age and occurs within the brainstem. Ferritin binding persisted in the brainstem and expanded to the corpus callosum by 15-16 days of age. By 23-24 days of age ferritin binding had further extended to the striatal white matter. By adulthood, ferritin binding was strongly and selectively expressed throughout all white matter tracts. To begin to identify which factors may be involved in the induction of ferritin-binding proteins on oligodendrocytes, brains from the myelin mutant jimpy mice and unaffected littermates were examined at postnatal days 16-18. Jimpy mice were chosen because their oligodendrocytes fail to produce myelin or accumulate iron. Thus, using jimpy mice would elucidate whether these factors are necessary for ferritin-binding protein expression. Both the jimpy mutants and their controls exhibited saturable ferritin binding with similar binding densities and dissociation constants. Dissociation constants for ferritin binding in the unaffected littermates and jimpy mutant mice were 0.38 +/- 0.04 and 0.32 +/- 0.06 nM, respectively and binding densities were similar (1.1 +/- 0.09 and 0.96 +/- 0.12 fmol/mg, respectively). Our results demonstrate that expression of ferritin binding is dependent on the age of the oligodendrocytes and not dependent upon iron accumulation by oligodendrocytes or myelin production. We propose that iron delivery to oligodendrocytes is predominantly via ferritin and this method of iron uptake is unique to oligodendrocytes in the brain.

Developing nodes of Ranvier are defined by ankyrin-G clustering and are independent of paranodal axoglial adhesion.

Nodes of Ranvier are excitable regions of axonal membranes highly enriched in voltage-gated sodium channels that propagate action potentials. The mechanism of protein clustering at nodes has been a source of controversy. In this study, developmental analysis of nodes of Ranvier in optic nerve axons reveals that early node intermediates are defined by ankyrin-G. Other node components, including beta IV spectrin, voltage-gated sodium channels, and the L1 cell adhesion molecule neurofascin, are subsequently recruited to sites of ankyrin-G clustering. The role of intact paranodes in protein clustering was examined in the dysmyelinating mouse mutant jimpy. Jimpy mice do not have intact paranodal axoglial contacts, which is indicated by a complete lack of neurexin/contactin-associated protein/paranodin clustering in paranodes. In the absence of intact paranodes, ankyrin-G was still able to cluster, although fewer ankyrin clusters were seen in jimpy optic nerves than in wild-type optic nerves. Recruitment of Na(v)1.2, Na(v)1.6, beta IV spectrin, and neurofascin to sites of ankyrin-G clustering is unimpaired in jimpy mice, indicating that node formation occurs independent of intact paranodal axoglial contacts.

Essential role of oligodendrocytes in the formation and maintenance of central nervous system nodal regions.

The membrane of myelinated axons is divided into functionally distinct domains characterized by the enrichment of specific proteins. The mechanisms responsible for this organization have not been fully identified. To further address the role of oligodendrocytes in the functional segmentation of the axolemma in vivo, the distribution of nodal (Na(+) channels, ankyrin G), paranodal (paranodin/contactin-associated-protein) and juxtaparanodal (Kv1.1 K(+) channels) axonal markers, was studied in the brain of MBP-TK and jimpy mice. In MBP-TK transgenic mice, oligodendrocyte ablation was selectively induced by FIAU treatment before and during the onset of myelination. In jimpy mice, oligodendrocytes degenerate spontaneously within the first postnatal weeks after the onset of myelination. Interestingly, in MBP-TK mice treated for 1-20 days with FIAU, despite the ablation of more than 95% of oligodendrocytes, the protein levels of all tested nodal markers was unaltered. Nevertheless, these proteins failed to cluster in the nodal regions. By contrast, in jimpy mice, despite a diffused localization of paranodin, the formation of nodal clusters of Na(+) channels and ankyrin G was observed. Furthermore, K(+) channels clusters were transiently visible, but were in direct contact with nodal markers. These results demonstrate that the organization of functional domains in myelinated axons is oligodendrocyte dependent. They also show that the presence of these cells is a requirement for the maintenance of nodal and paranodal regions.

Differential sensitivity in the survival of oligodendrocyte cell lines to overexpression of myelin proteolipid protein gene products.

The proteolipid (PLP) gene encodes at least four proteins, including the classic PLP and DM20, which are important components of the myelin sheath, and the recently identified soma-restricted (sr) isoforms, srPLP and srDM20. The classic PLP and DM20 gene products have been implicated in oligodendrocyte survival by overexpression studies in vitro and in vivo. The classic and sr proteolipids are targeted to different cellular compartments in the oligodendrocyte, suggesting different cellular functions. Accordingly, we examined the effects of in vitro overexpression of the sr-PLP/DM20 isoforms on the survival of stably transfected, conditionally immortalized, oligodendroglial cell lines and compared this to overexpression of the classic and the jimpy-mutated proteolipids. The results indicate that overexpression of either normal or jimpy classic PLP/DM20 resulted in a dramatic reduction in the survival of the oligodendrocyte cell lines at the nonpermissive temperature, but not the COS-7 cell line, a cell line expressing the same oncogene constitutively. Survival of the oligodendrocyte cell lines was significantly less affected when either the sr-PLP/DM20 or the dopamine D-2 receptor, another cell membrane protein, was overexpressed in the cell lines. These results suggest that overexpression of the "classic" PLP or DM20 can compromise the survival of oligodendrocytes whether or not they are mutated. Furthermore, they suggest that the internal mechanisms for normal targeting of the PLP/DM20 isoforms of either the "classic" or the "sr" types influence the oligodendrocyte's ability to survive when these proteolipids are overexpressed.

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.