White matter injury across neurodegenerative disease

Oligodendrocytes (OLs), the myelin-generating cells of the central nervous system (CNS), are active players in shaping neuronal circuitry and function. It has become increasingly apparent that injury to cells within the OL lineage plays a central role in neurodegeneration. In this review, we focus primarily on three degenerative disorders in which white matter loss is well documented: Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). We discuss clinical data implicating white matter injury as a key feature of these disorders, as well as shared and divergent phenotypes between them. We examine the cellular and molecular mechanisms underlying the alterations to OLs, including chronic neuroinflammation, aggregation of proteins, lipid dysregulation, and organellar stress. Last, we highlight prospects for therapeutic intervention targeting the OL lineage to restore function.

Inhibition of lipid synthesis by the HIV integrase strand transfer inhibitor elvitegravir in primary rat oligodendrocyte cultures

Combined antiretroviral therapy (cART) has greatly decreased mortality and morbidity among persons with HIV; however, neurologic impairments remain prevalent, in particular HIV-associated neurocognitive disorders (HANDs). White matter damage persists in cART-treated persons with HIV and may contribute to neurocognitive dysfunction as the lipid-rich myelin membrane of oligodendrocytes is essential for efficient nerve conduction. Because of the importance of lipids to proper myelination, we examined the regulation of lipid synthesis in oligodendrocyte cultures exposed to the integrase strand transfer inhibitor elvitegravir (EVG), which is administered to persons with HIV as part of their initial regimen. We show that protein levels of genes involved in the fatty acid pathway were reduced, which correlated with greatly diminished de novo levels of fatty acid synthesis. In addition, major regulators of cellular lipid metabolism, the sterol regulatory element-binding proteins (SREBP) 1 and 2, were strikingly altered following exposure to EVG. Impaired oligodendrocyte differentiation manifested as a marked reduction in mature oligodendrocytes. Interestingly, most of these deleterious effects could be prevented by adding serum albumin, a clinically approved neuroprotectant. These new findings, together with our previous study, strengthen the possibility that antiretroviral therapy, at least partially through lipid dysregulation, may contribute to the persistence of white matter changes observed in persons with HIV and that some antiretrovirals may be preferable as life-long therapy.

Antiretroviral treatment reveals a novel role for lysosomes in oligodendrocyte maturation

White matter deficits are a common neuropathologic finding in neurologic disorders, including HIV-associated neurocognitive disorders (HAND). In HAND, the persistence of white matter alterations despite suppressive antiretroviral (ARV) therapy suggests that ARVs may be directly contributing to these impairments. Here, we report that a frontline ARV, bictegravir (BIC), significantly attenuates remyelination following cuprizone-mediated demyelination, a model that recapitulates acute demyelination, but has no impact on already formed mature myelin. Mechanistic studies utilizing primary rat oligodendrocyte precursor cells (OPCs) revealed that treatment with BIC leads to significant decrease in mature oligodendrocytes accompanied by lysosomal deacidification and impairment of lysosomal degradative capacity with no alterations in lysosomal membrane permeability or total lysosome number. Activation of the endolysosomal cation channel TRPML1 prevents both lysosomal deacidification and impairment of oligodendrocyte differentiation by BIC. Lastly, we show that deacidification of lysosomes by compounds that raise lysosomal pH is sufficient to prevent maturation of oligodendrocytes. Overall, this study has uncovered a critical role for lysosomal acidification in modulating oligodendrocyte function and has implications for neurologic diseases characterized by lysosomal dysfunction and white matter abnormalities.

Update on Central Nervous System Effects of HIV in Adolescents and Young Adults

PURPOSE OF REVIEW

: Behaviorally acquired (non-perinatal) HIV infection during adolescence and young adulthood occurs in the midst of key brain developmental processes such as frontal lobe neuronal pruning and myelination of white matter, but we know little about the effects of new infection and therapy on the developing brain.

RECENT FINDINGS

Adolescents and young adults account for a disproportionately high fraction of new HIV infections each year. Limited data exist regarding neurocognitive performance in this age group, but suggest impairment is at least as prevalent as in older adults, despite lower viremia, higher CD4 + T cell counts, and shorter durations of infection in adolescents/young adults. Neuroimaging and neuropathologic studies specific to this population are underway. The full impact of HIV on brain growth and development in youth with behaviorally acquired HIV has yet to be determined; it must be investigated further to develop future targeted treatment and mitigation strategies.

Motor Learning and Physical Exercise in Adaptive Myelination and Remyelination

The idea that myelination is driven by both intrinsic and extrinsic cues has gained much traction in recent years. Studies have demonstrated that myelination occurs in an intrinsic manner during early development and continues through adulthood in an activity-dependent manner called adaptive myelination. Motor learning, the gradual acquisition of a specific novel motor skill, promotes adaptive myelination in both the healthy and demyelinated central nervous system (CNS). On the other hand, exercise, a physical activity that involves planned, structured and repetitive bodily movements that expend energy and benefits one's fitness, promotes remyelination in pathology, but it is less clear whether it promotes adaptive myelination in healthy subjects. Studies on these topics have also investigated whether the timing of motor learning or physical exercise is important for successful addition of myelin. Here we review our current understanding of the relationship of motor skill learning and physical exercise on myelination.

Further Delineation of the Clinical and Pathologic Features of HIKESHI-Related Hypomyelinating Leukodystrophy

BACKGROUND

A recurrent homozygous missense variant, c.160G>C;p.(Val54Leu) in HIKESHI, was found to cause a hypomyelinating leukodystrophy with high frequency in the Ashkenazi Jewish population. We provide extended phenotypic classification of this disorder based on clinical history of a further seven affected individuals, assess carrier frequency in the Ashkenazi Jewish population, and provide a neuropathological study.

METHODS

Clinical information, neuroimaging, and biosamples were collected. Brain autopsy was performed for one case.

RESULTS

Individuals with HIKESHI-related disease share common clinical features: early axial hypotonia evolving to dystonia or with progressive spasticity, hyperreflexia and clonus, feeding difficulties with poor growth, and nystagmus. Severe morbidity or death during febrile illness occurred in five of the nine affected individuals. Magnetic resonance images of seven patients were analyzed and demonstrated diffuse hypomyelination and thin corpus callosum. Genotyping data of more than 125,000 Ashkenazi Jewish individuals revealed a carrier frequency of 1 in 216. Gross pathology examination in one case revealed abnormal white matter. Microscopically, there was a near-total absence of myelin with a relative preservation of axons. The cerebral white matter showed several reactive astrocytes and microglia.

CONCLUSIONS

We provide pathologic evidence for a primary disorder of the myelin in HIKESHI-related leukodystrophy. These findings are consistent with the hypomyelination seen in brain magnetic resonance imaging and with the clinical features of early-onset spastic/dystonic quadriplegia and nystagmus. The high carrier rate of the recurrent variant seen in the Ashkenazi Jewish population requires increased attention to screening and diagnosis of this condition, particularly in this population.

HIV-induced neuroinflammation inhibits oligodendrocyte maturation via glutamate-dependent activation of the PERK arm of the integrated stress response

Despite combined antiretroviral therapy (cART), HIV-associated neurocognitive disorder (HAND) affects 30-50% of HIV-positive patients. Importantly, persistent white matter pathologies, specifically corpus callosum thinning and disruption of white matter microstructures observed in patients with HAND despite viral control through cART, raise the possibility that HIV infection in the setting of suboptimal cART may perturb oligodendrocyte (OL) maturation, function and/or survival, influencing HAND persistence in the cART era. To examine the effect of HIV infection on OL maturation, we used supernatants of primary human monocyte-derived macrophages infected with HIV (HIV/MDMs) to treat primary cultures of rat oligodendrocyte precursor cells (OPCs) during their differentiation to mature OLs. Using immunostaining for lineage-specific markers, we found that HIV/MDMs significantly inhibited OPC maturation. Based on our previous studies, we examined the potential role of several signaling pathways, including ionotropic glutamate receptors and the integrated stress response (ISR), and found that AMPA receptors (AMPAR)/kainic acid (KA) receptors (KARs) mediated the HIV/MDMs-induced defect in OL maturation. We also found that the treatment of OPC cultures with glutamate or AMPAR/KAR agonists phenocopied this effect. Blocking ISR activation, specifically the PERK arm of the ISR, protected OPCs from HIV/MDMs-mediated inhibition of OL maturation. Further, while glutamate, AMPA, and KA activated the ISR, inhibition of AMPAR/KAR activation prevented ISR induction in OPCs and rescued OL maturation. Collectively, these data identify glutamate signaling via ISR activation as a potential therapeutic pathway to ameliorate white matter pathologies in HAND and highlight the need for further investigation of their contribution to cognitive impairment.

Inducible knockout of Clec16a in mice results in sensory neurodegeneration

CLEC16A has been shown to play a role in autophagy/mitophagy processes. Additionally, genetic variants in CLEC16A have been implicated in multiple autoimmune diseases. We generated an inducible whole-body knockout, Clec16aΔUBC mice, to investigate the loss of function of CLEC16A. The mice exhibited a neuronal phenotype including tremors and impaired gait that rapidly progressed to dystonic postures. Nerve conduction studies and pathological analysis revealed loss of sensory axons that are associated with this phenotype. Activated microglia and astrocytes were found in regions of the CNS. Several mitochondrial-related proteins were up- or down-regulated. Upregulation of interferon stimulated gene 15 (IGS15) were observed in neuronal tissues. CLEC16A expression inversely related to IGS15 expression. ISG15 may be the link between CLEC16A and downstream autoimmune, inflammatory processes. Our results demonstrate that a whole-body, inducible knockout of Clec16a in mice results in an inflammatory neurodegenerative phenotype resembling spinocerebellar ataxia.

Differential effects of integrase strand transfer inhibitors, elvitegravir and raltegravir, on oligodendrocyte maturation: A role for the integrated stress response

Regardless of adherence to combined antiretroviral therapy, white matter and myelin pathologies persist in patients with HIV-associated neurocognitive disorders, a spectrum of cognitive, motor, and behavioral impairments. We hypothesized that antiretroviral therapy alters the maturation of oligodendrocytes which synthesize myelin. We tested whether specific frontline integrase strand transfer inhibitors would alter oligodendrocyte differentiation and myelination. To model the effect of antiretrovirals on oligodendrocytes, we stimulated primary rat oligodendrocyte precursor cells to differentiate into mature oligodendrocytes in vitro in the presence of therapeutically relevant concentrations of elvitegravir or raltegravir and then assessed differentiation with lineage specific markers. To examine the effect of antiretrovirals on myelination, we treated mice with the demyelinating compound cuprizone, for 5 weeks. This was followed by 3 weeks of recovery in absence of cuprizone, during which time some mice received a daily intrajugular injection of elvitegravir. Brains were harvested, sectioned and processed by immunohistochemistry to examine oligodendrocyte maturation and myelination. Elvitegravir inhibited oligodendrocyte differentiation in vitro in a concentration-dependent manner, while raltegravir had no effect. Following cuprizone demyelination, administration of elvitegravir to adult mice reduced remyelination compared with control animals. Elvitegravir treatment activated the integrated stress response in oligodendrocytes in vitro, an effect which was completely blocked by pretreatment with the integrated stress response inhibitor Trans-ISRIB, preventing elvitegravir-mediated inhibition of oligodendrocyte maturation. These studies demonstrate that elvitegravir impairs oligodendrocyte maturation and remyelination and that the integrated stress response mediates this effect and may be a possible therapeutic target.

Reducing Th2 inflammation through neutralizing IL-4 antibody rescues myelination in IUGR rat brain

BACKGROUND

Intrauterine growth restriction (IUGR) is a common complication of pregnancy and is associated with significant neurological deficits in infants, including white matter damage. Previous work using an animal model of IUGR has demonstrated that IUGR rats exhibit neurobehavioral deficits and developmental delays in oligodendrocyte maturation and myelination, but the mechanisms which cause this delay are unknown. Inflammation may be an important etiological factor in IUGR and has been recognized as playing a fundamental role in the pathogenesis of myelin disorders, including cerebral palsy.

METHODS

To create the model, the uterine arteries of pregnant rats were ligated at embryonic day 15. Rats delivered spontaneously. Cytokine and chemokine expression was evaluated at one prenatal and three postnatal time points, and myelin protein expression and oligodendrocyte cell numbers were evaluated by several methods at postnatal day 14. IL-4 was identified as a potential inhibitor of myelination, and rat pups were injected with IL-4 function blocking antibody from postnatal days 1-5 and myelination was assessed.

RESULTS

Here, we show a novel mechanism of white matter injury. IUGR induces an exaggerated Th2 response in the developing rat brain, including upregulation of several Th2 cytokines. Of these, IL-4 is significantly increased during the period corresponding to robust developmental myelination. We show that neutralizing IL-4 antibody therapy given in the newborn period ameliorates inflammation and restores myelin protein expression and oligodendrocyte cell number in the IUGR brain to control levels, demonstrating a novel role for Th2 responses and IL-4 in IUGR and white matter injury. In addition, IL-4 directly affects oligodendrocytes in vitro decreasing differentiation.

CONCLUSIONS

In this study, we have identified inflammation as a factor in the decrease in myelin seen in an animal model of IUGR. IL-4, an inflammatory protein often thought to be protective in the adult, is specifically increased, and treatment of these animals to prevent this increase ameliorates white matter damage. Our results suggest that the immune system plays a role in IUGR that is different in the perinatal period than in the adult and preventing this exaggerated Th2 response may be a potential therapeutic target.

Prenatal hypoxemia alters microglial morphology in fetal sheep

OBJECTIVE

Neuroimmune cells, particularly microglia and astrocytes, play a critical role in neurodevelopment. Neurocognitive delays are common in children with congenital heart disease, but their etiology is poorly understood. Our objective was to determine whether prenatal hypoxemia, at levels common in congenital heart disease, induced neuroimmune activation to better understand the origins of neurobehavioral disorders in congenital heart disease.

METHODS

Eight fetal sheep at gestational age 109 ± 3 days (term ∼145 days) were cannulated onto a pumpless extracorporeal oxygenator via the umbilical vessels and supported in a fluid environment for 22 ± 2 days under normoxic (n = 4) or hypoxic (n = 4) conditions. Control fetuses (n = 7) were harvested at gestational age 133 ± 4 days. At necropsy, brains were stained with ionized calcium-binding adaptor molecule 1 and glial fibrillary acidic protein antibodies to quantify microglia and astrocytes, respectively, in gray and white matter in frontotemporal and cerebellar sections. Microglia were classified into 4 morphologic types based on cell shape. Data were analyzed with 1-way analysis of variance or Fisher exact test, as appropriate.

RESULTS

Oxygen delivery was significantly reduced in hypoxic fetuses (15.6 ± 1.8 mL/kg/min vs 24.3 ± 2.3 mL/kg/min; P < .01). Rates of apoptosis were similar in hypoxic, normoxic, and intrauterine control animals in all examined areas. There were also no differences between groups in area occupied by glial fibrillary acidic protein-labeled astrocytes or ionized calcium-binding adaptor molecule 1-labeled microglia in all examined areas. However, round microglia were significantly increased in hypoxic animals compared with normoxic animals (33% vs 6%; P < .01) and control animals (33% vs 11%; P < .01).

CONCLUSIONS

Prenatal hypoxemia altered microglial morphology without significant gliosis. Additional studies characterizing these mechanisms may provide insight into the origins of neurobehavioral disabilities in children with congenital heart disease.

White matter loss and oligodendrocyte dysfunction in HIV: A consequence of the infection, the antiretroviral therapy or both?

While the severe cognitive effects of HIV-associated dementia have been reduced by combined antiretroviral therapy (cART), nearly half of HIV-positive (HIV+) patients still suffer from some form of HIV-Associated Neurocognitive Disorders (HAND). While frank neuronal loss has been dramatically reduced in HAND patients, white matter loss, including dramatic thinning of the corpus callosum, and loss of volume and structural integrity of myelin persists despite viral control by cART. It remains unclear whether changes in white matter underlie the clinical manifestation seen in patients or whether they are the result of persistent viral reservoirs, remnant damage from the acute infection, the antiretroviral compounds used to treat HIV, secondary effects due to peripheral toxicities or other associated comorbid conditions. Both HIV infection itself and its treatment with antiretroviral drugs can induce metabolic syndrome, lipodystrophy, atherosclerosis and peripheral neuropathies by increased oxidative stress, induction of the unfolded protein response and dysregulation of lipid metabolism. These virally and/or cART-induced processes can also cause myelin loss in the CNS. This review aims to highlight existing data on the contribution of white matter damage to HAND and explore the mechanisms by which HIV infection and its treatment contribute to persistence of white matter changes in people living with HIV currently on cART.

Arx Expression Suppresses Ventralization of the Developing Dorsal Forebrain

Early brain development requires a tight orchestration between neural tube patterning and growth. How pattern formation and brain growth are coordinated is incompletely understood. Previously we showed that aristaless-related homeobox (ARX), a paired-like transcription factor, regulates cortical progenitor pool expansion by repressing an inhibitor of cell cycle progression. Here we show that ARX participates in establishing dorsoventral identity in the mouse forebrain. In Arx mutant mice, ventral genes, including Olig2, are ectopically expressed dorsally. Furthermore, Gli1 is upregulated, suggesting an ectopic activation of SHH signaling. We show that the ectopic Olig2 expression can be repressed by blocking SHH signaling, implicating a role for SHH signaling in Olig2 induction. We further demonstrate that the ectopic Olig2 accounts for the reduced Pax6 and Tbr2 expression, both dorsal specific genes essential for cortical progenitor cell proliferation. These data suggest a link between the control of dorsoventral identity of progenitor cells and the control of their proliferation. In summary, our data demonstrate that ARX functions in a gene regulatory network integrating normal forebrain patterning and growth, providing important insight into how mutations in ARX can disrupt multiple aspects of brain development and thus generate a wide spectrum of neurodevelopmental phenotypes observed in human patients.

Chronic intrauterine hypoxia alters neurodevelopment in fetal sheep

OBJECTIVE

We tested the hypothesis that chronic fetal hypoxia, at a severity present in many types of congenital heart disease, would lead to abnormal neurodevelopment.

METHODS

Eight mid-gestation fetal sheep were cannulated onto a pumpless extracorporeal oxygenator via the umbilical vessels and supported in a fluid-filled environment for 22 ± 2 days under normoxic or hypoxic conditions. Total parenteral nutrition was provided. Control fetuses (n = 7) were harvested at gestational age 133 ± 4 days. At necropsy, brains were fixed for histopathology. Neurons were quantified in white matter tracts, and the thickness of the external granular layer of the cerebellum was measured to assess neuronal migration. Capillary density and myelination were quantified in white matter. Data were analyzed with unpaired Student t tests or 1-way analysis of variance, as appropriate.

RESULTS

Oxygen delivery was reduced in hypoxic fetuses (15.6 ± 1.8 mL/kg/min vs 24.3 ± 2.3 mL/kg/min, P < .01), but umbilical blood flow and caloric delivery were not different between the 2 groups. Compared with normoxic and control animals, hypoxic fetuses had reduced neuronal density and increased external granular layer thickness. Compared with normoxic and control animals, hypoxic fetuses had increased capillary density in white matter. Cortical myelin integrity score was lower in the hypoxic group compared with normoxic and control animals. There was a significant negative correlation between myelin integrity and capillary density.

CONCLUSIONS

Chronic fetal hypoxia leads to white matter hyper-vascularity, decreased neuronal density, and impaired myelination, similar to the neuropathologic findings observed in children with congenital heart disease. These findings support the hypothesis that fetal hypoxia, even in the setting of normal caloric delivery, impairs neurodevelopment.

Reduced sterol regulatory element-binding protein (SREBP) processing through site-1 protease (S1P) inhibition alters oligodendrocyte differentiation in vitro

The formation of the myelin membrane of the oligodendrocyte in the CNS is a fundamental process requiring the coordinated synthesis of many different components. The myelin membrane is particularly rich in lipids, however, the regulation of this lipid synthesis is not understood. In other cell types, including Schwann cells, the myelin-forming cells of the PNS, lipid synthesis is tightly regulated by the sterol regulatory element-binding protein (SREBP) family of transcription factors, but this has not been previously shown in oligodendrocytes. We investigated SREBPs' role during oligodendrocyte differentiation in vitro. Both SREBP-1 and SREBP-2 were expressed in oligodendrocyte precursor cells and differentiating oligodendrocytes. Using the selective site-1 protease (S1P) inhibitor PF-429242, which inhibits the cleavage of SREBP precursor forms into mature forms, we found that preventing SREBP processing inhibited process growth and reduced the expression level of myelin basic protein, a major component of myelin. Further, process extension deficits could be rescued by the addition of exogenous cholesterol. Blocking SREBP processing reduced mRNA transcription and protein levels of SREBP target genes involved in both the fatty acid and the cholesterol synthetic pathways. Furthermore, de novo levels and total levels of cholesterol synthesis were greatly diminished when SREBP processing was inhibited. Together these results indicate that SREBPs are important regulators of oligodendrocyte maturation and that perturbation of their activity may affect myelin formation and integrity. Cover Image for this issue: doi: 10.1111/jnc.13781.

Reduced sterol regulatory element-binding protein (SREBP) processing through site-1 protease (S1P) inhibition alters oligodendrocyte differentiation in vitro

The formation of the myelin membrane of the oligodendrocyte in the CNS is a fundamental process requiring the coordinated synthesis of many different components. The myelin membrane is particularly rich in lipids, however, the regulation of this lipid synthesis is not understood. In other cell types, including Schwann cells, the myelin-forming cells of the PNS, lipid synthesis is tightly regulated by the sterol regulatory element-binding protein (SREBP) family of transcription factors, but this has not been previously shown in oligodendrocytes. We investigated SREBPs' role during oligodendrocyte differentiation in vitro. Both SREBP-1 and SREBP-2 were expressed in oligodendrocyte precursor cells and differentiating oligodendrocytes. Using the selective site-1 protease (S1P) inhibitor PF-429242, which inhibits the cleavage of SREBP precursor forms into mature forms, we found that preventing SREBP processing inhibited process growth and reduced the expression level of myelin basic protein, a major component of myelin. Further, process extension deficits could be rescued by the addition of exogenous cholesterol. Blocking SREBP processing reduced mRNA transcription and protein levels of SREBP target genes involved in both the fatty acid and the cholesterol synthetic pathways. Furthermore, de novo levels and total levels of cholesterol synthesis were greatly diminished when SREBP processing was inhibited. Together these results indicate that SREBPs are important regulators of oligodendrocyte maturation and that perturbation of their activity may affect myelin formation and integrity. Cover Image for this issue: doi: 10.1111/jnc.13781.

Gliopathy of Demyelinating and Non-Demyelinating Strains of Mouse Hepatitis Virus

Demyelination in the central nervous system induced by neurovirulent strains of Mouse Hepatitis Virus (MHV) is mediated by the viral spike glycoprotein, but it is not clear whether the mechanism of this disease pathology involves direct viral infection of oligodendrocytes. Detailed studies of glial cell tropism of MHV are presented, demonstrating that direct MHV infection of oligodendrocytes differs between demyelinating (RSA59) and non-demyelinating (RSMHV2) viral strains both in vitro and in vivo. Our results indicate that direct injury of mature oligodendrocytes is an important mechanism of virus-induced demyelination. In vivo, RSA59 infection was identified in spinal cord gray and white matter, but infected oligodendrocytes were restricted to white matter. In contrast, RSMHV2 infection was restricted to gray matter neurons and was not localized to oligodendrocytes. In vitro, RSA59 can infect both oligodendrocyte precursors and differentiated oligodendrocytes, whereas RSMHV2 can infect oligodendrocyte precursors but not differentiated oligodendrocytes. Viral spreading through axonal means to white matter and release of the demyelinating strain MHV at the nerve end is critical for oligodendrocytes infection and subsequent demyelination. Understanding the mechanisms by which known viruses effect demyelination in this animal model has important therapeutic implications in the treatment of human demyelinating disease.

Altered Oligodendrocyte Maturation and Myelin Maintenance: The Role of Antiretrovirals in HIV-Associated Neurocognitive Disorders

Despite effective viral suppression through combined antiretroviral therapy (cART), approximately half of HIV-positive individuals have HIV-associated neurocognitive disorders (HAND). Studies of antiretroviral-treated patients have revealed persistent white matter abnormalities including diffuse myelin pallor, diminished white matter tracts, and decreased myelin protein mRNAs. Loss of myelin can contribute to neurocognitive dysfunction because the myelin membrane generated by oligodendrocytes is essential for rapid signal transduction and axonal maintenance. We hypothesized that myelin changes in HAND are partly due to effects of antiretroviral drugs on oligodendrocyte survival and/or maturation. We showed that primary mouse oligodendrocyte precursor cell cultures treated with therapeutic concentrations of HIV protease inhibitors ritonavir or lopinavir displayed dose-dependent decreases in oligodendrocyte maturation; however, this effect was rapidly reversed after drug removal. Conversely, nucleoside reverse transcriptase inhibitor zidovudine had no effect. Furthermore, in vivo ritonavir administration to adult mice reduced frontal cortex myelin protein levels. Finally, prefrontal cortex tissue from HIV-positive individuals with HAND on cART showed a significant decrease in myelin basic protein compared with untreated HIV-positive individuals with HAND or HIV-negative controls. These findings demonstrate that antiretrovirals can impact myelin integrity and have implications for myelination in juvenile HIV patients and myelin maintenance in adults on lifelong therapy.

Long-term intermittent hypoxia elevates cobalt levels in the brain and injures white matter in adult mice

STUDY OBJECTIVES

Exposure to the variable oxygenation patterns in obstructive sleep apnea (OSA) causes oxidative stress within the brain. We hypothesized that this stress is associated with increased levels of redox-active metals and white matter injury.

DESIGN

Participants were randomly allocated to a control or experimental group (single independent variable).

SETTING

University animal house.

PARTICIPANTS

Adult male C57BL/6J mice.

INTERVENTIONS

To model OSA, mice were exposed to long-term intermittent hypoxia (LTIH) for 10 hours/day for 8 weeks or sham intermittent hypoxia (SIH).

MEASUREMENTS AND RESULTS

Laser ablation-inductively coupled plasma-mass spectrometry was used to quantitatively map the distribution of the trace elements cobalt, copper, iron, and zinc in forebrain sections. Control mice contained 62 ± 7 ng cobalt/g wet weight, whereas LTIH mice contained 5600 ± 600 ng cobalt/g wet weight (P < 0.0001). Other elements were unchanged between conditions. Cobalt was concentrated within white matter regions of the brain, including the corpus callosum. Compared to that of control mice, the corpus callosum of LTIH mice had significantly more endoplasmic reticulum stress, fewer myelin-associated proteins, disorganized myelin sheaths, and more degenerated axon profiles. Because cobalt is an essential component of vitamin B12, serum methylmalonic acid (MMA) levels were measured. LTIH mice had low MMA levels (P < 0.0001), indicative of increased B12 activity.

CONCLUSIONS

Long-term intermittent hypoxia increases brain cobalt, predominantly in the white matter. The increased cobalt is associated with endoplasmic reticulum stress, myelin loss, and axonal injury. Low plasma methylmalonic acid levels are associated with white matter injury in long-term intermittent hypoxia and possibly in obstructive sleep apnea.

Oxidative stress disrupts oligodendrocyte maturation

Periventricular white matter injury (PWMI) is the leading cause of chronic neurologic injury among survivors of preterm birth. The hallmark of PWMI is hypomyelination and a lack of mature, myelinating oligodendrocytes. Oligodendrocytes undergo a well-characterized lineage progression from neural stem cell to mature oligodendrocyte. Oligodendrocyte precursors have increased susceptibility to oxidative and free radical-mediated injury compared with mature oligodendrocytes as a result of lower levels of antioxidant enzymes and free radical scavengers. In this study, we show that oxidative stress disrupts oligodendrocyte differentiation by two mechanisms. First, oxidizing agents decrease the expression of key genes that promote oligodendrocyte differentiation from neural stem cells and increase the expression of genes known to inhibit differentiation. Second, global histone acetylation persists under conditions of oxidative stress, further contributing to the prevention of oligodendrocyte differentiation. Both of these mechanisms result in the arrest of oligodendrocyte differentiation without an increase in cell death.

Wnt signaling is sufficient to perturb oligodendrocyte maturation

The development of oligodendrocytes, the myelinating cells of the central nervous system, is temporally and spatially controlled by local signaling factors acting as inducers or inhibitors. Dorsal spinal cord tissue has been shown to contain inhibitors of oligodendrogliogenesis, although their identity is not completely known. We have studied the actions of one family of dorsal signaling molecules, the Wnts, on oligodendrocyte development. Using tissue culture models, we have shown that canonical Wnt activity through beta-catenin activation inhibits oligodendrocyte maturation, independently of precursor proliferation, cell death, or diversion to an alternate cell fate. Mice in which Wnt/beta-catenin signaling was constitutively activated in cells of the oligodendrocyte lineage had equal numbers of oligodendrocyte precursors relative to control littermates, but delayed appearance of mature oligodendrocytes, myelin protein, and myelinated axons during development, although these differences largely disappeared by adulthood. These results indicate that activating the Wnt/beta-catenin pathway delays the development of myelinating oligodendrocytes.

Bone morphogenetic proteins 4, 6, and 7 are up-regulated in mouse spinal cord during experimental autoimmune encephalomyelitis

Although spontaneous remyelination occurs in multiple sclerosis (MS), the extent of myelin repair is often inadequate to restore normal function. Oligodendrocyte precursors remaining in nonremyelinating MS plaques may be restricted by an inhibitory signal. Bone morphogenetic proteins (BMPs) have been implicated as repressors of oligodendrocyte development and inducers of astrogliogenesis. We hypothesized that BMPs are up-regulated in MS lesions and play a role in demyelination and astrogliosis. We examined expression of BMPs in an animal model of MS, chronic experimental autoimmune encephalomyelitis (EAE) induced by the myelin oligodendrocyte glycoprotein (MOG) peptide in C57BL/6 mice. By 14 days postimmunization, compared to those of control mice, the lumbar spinal cords of MOG-peptide EAE mice demonstrated prominent astrogliosis, infiltration of inflammatory cells, and disrupted expression of myelin proteins. Quantitative RT-PCR showed that expression of BMP4, BMP6, and BMP7 mRNA increased 2- to 4-fold in the lumbar spinal cords of animals with symptomatic EAE versus in vehicle-treated and untreated controls on days 14, 21, and 42 postimmunization. BMP2 mRNA expression was not altered. BMP4 mRNA was much more abundant in the spinal cords of all animals than was mRNA encoding BMP2, BMP6, and BMP7. Immunoblot analysis confirmed the increased expression of BMP4 in the EAE animals. Immunohistochemistry revealed increased BMP4 immunoreactivity in areas of inflammation in MOG-peptide EAE animals. BMP4 labeling was mostly limited to macrophages but was sometimes associated with astrocytes and oligodendrocytes. These results indicate that members of the BMP family are differentially expressed in adult spinal cord and are up-regulated during EAE. (c) 2007 Wiley-Liss, Inc.

BMP signaling mutant mice exhibit glial cell maturation defects

Bone morphogenetic proteins have been implicated in the development of oligodendrocytes and astrocytes, however, a role for endogenous BMP signaling in glial development has not been demonstrated in a genetic model. Using mice in which signaling via type I BMP receptors Bmpr1a and Bmpr1b have been inactivated in the neural tube, we demonstrate that BMP signaling contributes to the maturation of glial cells in vivo. At P0, mutant mice exhibited a 25-40% decrease in GFAP+ or S100beta+ astrocytes in the cervical spinal cord. The number of oligodendrocyte precursors and the timing of their emergence was unchanged in the mutant mice compared to the normals, however myelin protein expression and mature oligodendrocyte numbers were significantly reduced. These data indicate that BMP signaling promotes the generation of astrocytes and mature, myelinating oligodendrocytes in vivo but does not affect oligodendrocyte precursor development, thus suggesting tight regulation of BMP signaling to ensure proper gliogenesis.

Pannexin1 is expressed by neurons and glia but does not form functional gap junctions

Pannexins are a newly described family of proteins that may form gap junctions. We made antisera against mouse pannexin1 (Panx1). HeLa cells expressing Panx1 have cell surface labeling, but not gap junction plaques, and do not transfer small fluorescent dyes or neurobiotin in a scrape-loading assay. Neuro2a cells expressing Panx1 are not electrophysiologically coupled. Intracellular Panx1-immunoreactivity, but not gap junction plaques, is seen in cultured oligodendrocytes, astrocytes, and hippocampal neurons. Thus, at least in these mammalian cells lines, Panx1 does not form morphological or functional gap junctions, and it remains to be demonstrated that Panx1 forms gap junction-forming protein in the CNS.

Injuring neurons induces neuronal differentiation in a population of hippocampal precursor cells in culture

A novel population of hippocampal precursor cells (HPCs) that can be induced to differentiate into astrocytes and oligodendrocytes can be derived from hippocampal cultures grown in serum-free media. The HPCs are PDGF-responsive, do not proliferate with bFGF, and grow as sheets of cells rather than gathering into neurospheres. The HPCs share many markers (A2B5, GD3, poly-sialylated neuronal common adhesion molecule (PSA-NCAM), and NG2) with oligodendrocyte precursor cells (OPCs). The HPCs do not express markers for mature neurons, astrocytes, or oligodendrocytes. Like OPCs, the HPCs differentiate into glial fibrillary acidic protein (GFAP)+ astrocytes and GalC+ oligodendrocytes with the addition of bone morphogenetic protein-4 (BMP-4) and triiodothyronine (T3), respectively. They do not differentiate into neurons with the addition or withdrawal of basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), or retinoic acid (RA). These HPCs can be stimulated to differentiate into neuron-like cells by the induction of neuronal injury or cell death in nearby cultured neurons or by conditioned medium from injured neuronal cultures. Under these conditions, HPCs grow larger, develop more extensive dendritic processes, become microtubule-associated protein-2-immunoreactive, express large voltage-dependent sodium currents, and form synaptic connections. The conversion of endogenous pluripotent precursor cells into neurons in response to local brain injury may be an important component of central nervous system homeostasis.

Oligodendrocyte maturation is inhibited by bone morphogenetic protein

Mature oligodendrocytes myelinate axons in the CNS. The development of the myelin sheath is dependent on the proper maturation of oligodendrocytes from precursors cells, a spatially restricted process that is regulated by inductive and repressive cues. Several members of the bone morphogenetic protein family (BMP2 and 4) have been implicated as repressors of oligodendrocyte development in vitro by shifting oligodendrocyte precursors into the astrocyte lineage. We now report on a second role of BMPs in oligodendrocyte development, regulation of myelin protein expression in immature oligodendrocytes. Purified immature rodent oligodendrocytes treated with BMP4 maintained galactocerebroside (GalC) expression, whereas the expression of three key myelin proteins, proteolipid protein (PLP), myelin basic protein (MBP), and 2'-3'-cyclic nucleotide 3'-phosphodiesterase (CNP), was severely decreased. Paradoxically, BMP-treated oligodendrocytes show increased process extension and complexity, normally a feature of oligodendrocyte maturation. We also investigated whether BMP4 could inhibit myelin protein expression in an E 12.5 mouse explant culture of cervical spinal cord and hindbrain that maintains the in vivo cellular relationships and architecture. Beads soaked in BMP protein implanted into these explants inhibited the expression of myelin proteins, proteolipid protein, and myelin-associated glycoprotein (MAG), in the local area surrounding the bead. Since these explants also contained precursors cells, expression of galactocerebroside and O4, an oligodendrocyte marker, were also decreased by BMP treatment but to a much lesser degree than the myelin markers. Together, these data indicate that BMPs have multiple roles in oligodendrocyte development. At earlier stages, they affect cell lineage decisions and at later stages, they inhibit cell specialization.

AMPA glutamate receptor-mediated calcium signaling is transiently enhanced during development of oligodendrocytes

Cells of the oligodendroglial lineage express Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-preferring glutamate receptors (AMPA-GluR) during development. Prolonged activation of their AMPA-GluR causes Ca2+ overload, resulting in excitotoxic death. Prior studies have shown that oligodendroglial progenitors and immature oligodendrocytes are susceptible to excitotoxicity, whereas mature oligodendrocytes are resistant. An unresolved issue has been why Ca2+-permeability of AMPA-GluR varies so markedly with oligodendroglial development, although the level of expression of edited GluR2, an AMPA-GluR subunit which blocks Ca2+ entry, is relatively constant. To address this question, we performed Ca2+ imaging, molecular and electrophysiological analyses using purified cultures of the rat oligodendroglial lineage. We demonstrate that transient up-regulation of expression of GluR3 and GluR4 subunits in oligodendroglial progenitors and immature oligodendrocytes results in the assembly by these cells, but not by oligodendroglial pre-progenitors or mature oligodendrocytes, of a population of AMPA-GluR which lack GluR2. This stage-specific up-regulation of edited GluR2-free, and hence Ca2+-permeable, AMPA-GluR explains the selective susceptibility to excitotoxicity of cells at these stages of oligodendroglial differentiation, and is likely to be important to these cells in the trans-synaptic Ca2+-signaling from glutamatergic neurons, which occurs in hippocampus

ST8Sia IV mRNA corresponds with the biosynthesis of alpha2,8sialyl polymers but not oligomers in rat oligodendrocytes

As oligodendrocytes mature they progress through a series of distinct differentiation steps characterized by the expression of specific markers. One such marker, polysialic acid found on the neural cell adhesion molecule (NCAM), is detected by antibodies and is present on progenitor oligodendrocytes, but is not detected to the same extent on mature oligodendrocytes. Two closely related polysialyltransferases, ST8Sia II (STX) and ST8Sia IV (PST) have been cloned previously and shown to synthesize polysialic acid on NCAM and other glycoproteins. To determine whether or not polyalpha2,8sialyltransferases are downregulated during the differentiation of oligodendrocytes, the enzyme activity and expression of ST8Sia II and ST8Sia IV mRNA at two stages of maturation in JS12/1 and JS3/16 oligodendrocytes were examined. Differentiation in both oligodendroglial cell lines was accompanied by more than a 50% reduction in the biosynthesis of polymers of alpha2,8sialic acid when fetuin was used as substrate. Most interestingly, extracts of JS12/1 mature cells synthesized 60% more short oligomers of alpha2,8sialic acid than the progenitor cells, whereas JS3/16 mature cells synthesized barely detectable amounts of the short oligomers. Transcripts for ST8Sia IV mRNA were present in both JS12/1 and JS3/16 and were reduced when the biosynthesis was markedly reduced. In contrast ST8Sia II mRNA was barely detectable in JS3/16 cells and although detectable in JS12/1 cells, there was no clear modulation with maturation. These results were supported by the examination of the brains of rats from embryonic to Day 21 ages. The enzyme activity and mRNA experiments show that polyalpha2,8sialyltransferase itself is down regulated to cause the reduction in sialyl polymers on mature oligodendrocytes. Moreover, ST8Sia IV is responsible for the polysialylation of NCAM in oligodendrocytes.

Caspase-3 activation in oligodendrocytes from the myelin-deficient rat

The myelin-deficient (MD) rat has a point mutation in its proteolipid protein (PLP) gene that causes severe dysmyelination and oligodendrocyte cell death. Using an in vitro model, we have shown that MD oligodendrocytes initially differentiate similarly to wild-type cells, expressing galactocerebroside, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and myelin basic protein. However, at the time when PLP expression would normally begin, the MD oligodendrocytes die via an apoptotic pathway involving caspase activation. The active form of caspase-3 was detected, along with the cleavage products of poly-(ADP-ribose) polymerase (PARP) and spectrin, major targets of caspase-mediated proteolysis. A specific inhibitor of casapse-3, Ac-DEVD-CMK, reduced apoptosis in MD oligodendrocytes, but the rescued cells did not mature fully or express myelin-oligodendrocyte glycoprotein. These results suggest that mutant PLP affects not only cell death but also oligodendrocyte differentiation.

Epidermal growth factor receptor agonists increase expression of glutamate transporter GLT-1 in astrocytes through pathways dependent on phosphatidylinositol 3-kinase and transcription factor NF-kappaB

The glial glutamate transporter GLT-1 may be the predominant Na(+)-dependent glutamate transporter in forebrain. Expression of GLT-1 correlates with astrocyte maturation in vivo and increases during synaptogenesis. In astrocyte cultures, GLT-1 expression parallels differentiation induced by cAMP analogs or by coculturing with neurons. Molecule(s) secreted by neuronal cultures contribute to this induction of GLT-1, but little is known about the signaling pathways mediating this regulation. In the present study, we determined whether growth factors previously implicated in astrocyte differentiation regulate GLT-1 expression. Of the six growth factors tested, two [epidermal growth factor (EGF) and transforming growth factor-alpha] induced expression of GLT-1 protein in cultured astrocytes. Induction of GLT-1 protein was accompanied by an increase in mRNA and in the V(max) for Na(+)-dependent glutamate transport activity. The effects of dibutyryl-cAMP and EGF were additive but were independently blocked by inhibitors of protein kinase A or protein tyrosine kinases, respectively. The induction of GLT-1 in both EGF- and dibutyryl-cAMP-treated astrocytes was blocked by inhibitors targeting phosphatidylinositol 3-kinase (PI3K) or the nuclear transcription factor-kappaB. Furthermore, transient transfection of astrocyte cultures with a constitutively active PI3K construct was sufficient to induce expression of GLT-1. These data suggest that independent but converging pathways mediate expression of GLT-1. Although an EGF receptor-specific antagonist did not block the effects of neuron-conditioned medium, the induction of GLT-1 by neuron-conditioned medium was completely abolished by inhibition of PI3K or nuclear factor-kappaB. EGF also increased expression of GLT-1 in spinal cord organotypic cultures. Together, these data suggest that activation of specific signaling pathways with EGF-like molecules may provide a novel approach for limiting excitotoxic brain injury.

Stage-specific effects of bone morphogenetic proteins on the oligodendrocyte lineage

Oligodendrocyte maturation is regulated by multiple secreted factors present in the brain during critical stages of development. Whereas most of these factors promote oligodendrocyte proliferation and survival, members of the bone morphogenetic protein family (BMPs) recently have been shown to inhibit oligodendrocyte differentiation in vitro. Oligodendrocyte precursors treated with BMPs differentiate to the astrocyte lineage. Given that cells at various stages of the oligodendrocyte lineage have distinct responses to growth factors, we hypothesized that the response to BMP would be stage-specific. Using highly purified, stage-specific cultures, we found that BMP has distinct effects on cultured oligodendrocyte preprogenitors, precursors, and mature oligodendrocytes. Oligodendrocyte preprogenitors (PSA-NCAM+, A2B5-) treated with BMP2 or BMP4 developed a novel astrocyte phenotype characterized by a morphological change and expression of glial fibrillary acidic protein (GFAP) but little glutamine synthetase expression and no labeling with A2B5 antibody. In contrast, treating oligodendrocyte precursors with BMPs resulted in the accumulation of cells with the traditional type 2 astrocyte phenotype (GFAP+, A2B5+). However, many of the cells with an astrocytic morphology did not express GFAP or glutamine synthetase unless thyroid hormone was present in the medium. The addition of fibroblast growth factor along with BMP to either oligodendrocyte preprogenitor or the oligodendrocyte precursor cells inhibited the switch to the astrocyte lineage, whereas platelet-derived growth factor addition had no effect. Treatment of mature oligodendrocytes with BMP elicited no change in morphology or expression of GFAP. These data suggest that as cells progress through the oligodendrocyte lineage, they show developmentally restricted responses to the BMPs.

Cell cycle arrest induced by ectopic expression of p27 is not sufficient to promote oligodendrocyte differentiation

Oligodendrocyte differentiation is accompanied by dramatic changes in gene expression as well as cell cycle arrest. To determine whether cell cycle arrest is sufficient to induce the changes in cell phenotype associated with differentiation, we inhibited oligodendrocyte precursor proliferation in vitro by overexpressing p27, a cyclin kinase inhibitor, using a recombinant adenovirus. Ectopic expression of p27 efficiently inhibited oligodendrocyte precursor cell division, even in the presence of exogenous mitogens, by blocking the activity of the cyclin-dependent kinase, cdk2. Although the cells had stopped dividing, they did not express galactocerebroside (GalC) or myelin basic protein (MBP), changes associated with oligodendrocyte differentiation, suggesting that they had not differentiated. After removal of exogenous mitogens, however, adenovirus-expressing oligodendrocyte precursors differentiated with a temporal profile similar to that of control, uninfected oligodendrocytes, as indicated by expression of GalC and MBP. We conclude that cell cycle arrest is not sufficient to induce differentiation of dividing oligodendrocyte precursors, and that modulation of additional, as yet unknown, signaling pathways is required for this to occur.

Ectopic bone morphogenetic proteins 5 and 4 in the chicken forebrain lead to cyclopia and holoprosencephaly

Proper dorsal-ventral patterning in the developing central nervous system requires signals from both the dorsal and ventral portions of the neural tube. Data from multiple studies have demonstrated that bone morphogenetic proteins (BMPs) and Sonic hedgehog protein are secreted factors that regulate dorsal and ventral specification, respectively, within the caudal neural tube. In the developing rostral central nervous system Sonic hedgehog protein also participates in ventral regionalization; however, the roles of BMPs in the developing brain are less clear. We hypothesized that BMPs also play a role in dorsal specification of the vertebrate forebrain. To test our hypothesis we implanted beads soaked in recombinant BMP5 or BMP4 into the neural tube of the chicken forebrain. Experimental embryos showed a loss of the basal telencephalon that resulted in holoprosencephaly (a single cerebral hemisphere), cyclopia (a single midline eye), and loss of ventral midline structures. In situ hybridization using a panel of probes to genes expressed in the dorsal and ventral forebrain revealed the loss of ventral markers with the maintenance of dorsal markers. Furthermore, we found that the loss of the basal telencephalon was the result of excessive cell death and not a change in cell fates. These data provide evidence that BMP signaling participates in dorsal-ventral patterning of the developing brain in vivo, and disturbances in dorsal-ventral signaling result in specific malformations of the forebrain.

Maturation-dependent apoptotic cell death of oligodendrocytes in myelin-deficient rats

Mutations in the proteolipid protein gene (PLP/plp), which encodes the major intrinsic membrane protein in central nervous system (CNS) myelin, cause inherited dysmyelination in mammals. One of these mutants, the myelin-deficient (md) rat, has severe dysmyelination that is associated with oligodendrocyte cell death. Using the terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) assay, which labels apoptotic cells, we find that cell death is increased in multiple white matter tracts of md rats. The tracts that myelinate the earliest show the earliest increase in cell death, and cell death persists for at least 22 days, the lifespan of these mutant animals. In all tracts, and at all developmental ages examined, apoptotic cells expressed the markers of mature oligodendrocytes, such as myelin basic protein, myelin-associated glycoprotein, and the Rip antigen, but not chondroitin sulfate proteoglycan, a marker of oligodendrocyte precursors. Mature oligodendrocytes fail to accumulate in md brain because they die before they fully mature.

MEBA derepresses the proximal myelin basic protein promoter in oligodendrocytes

The central nervous system expression of myelin basic protein (MBP) is restricted to oligodendrocytes and is developmentally regulated; these regulatory features are transcriptionally mediated. We have previously shown that the proximal 149 nucleotides of the MBP promoter were both necessary and sufficient to activate the transcription of MBP in cultured oligodendrocytes, but not in other cell types. Sequences within the distal portion of this promoter, which contains a nuclear factor 1 (NF1) binding site, repressed activation of the MBP promoter in Cos-7 cells, but not in oligodendrocytes. We now describe a sequence upstream of and partially overlapping the NF1 site that activates the MBP promoter in oligodendrocytes, but not in Cos-7 cells. A protein complex binds to this site, designated MEBA (myelinating glia-enriched DNA binding activity), and is enriched in nuclear extracts prepared from the brain, oligodendrocytes, and Schwann cells. The amount of MEBA parallels MBP expression and myelinogenesis in the developing brain and parallels new MBP expression as purified oligodendrocytes differentiate. Mutational analyses of binding and function distinguish MEBA, an activator, from NF1, a repressor of MBP transcription, and suggest that MEBA consists of at least two proteins. Because the binding sites of MEBA and NF1 overlap, we suggest that MEBA may either compete with or modify NF1 binding, thereby activating the MBP promoter in oligodendrocytes.

Re-entry into the cell cycle is required for bFGF-induced oligodendroglial dedifferentiation and survival

Remyelination in the CNS following demyelinating disease may be accomplished by surviving mature oligodendrocytes that dedifferentiate, proliferate, migrate, and finally regenerate myelin. We previously reported that basic fibroblast growth factor (bFGF) induces oligodendrocytes in primary mixed glial cultures to dedifferentiate and synthesize DNA (Grinspan et al.: J Neurosci Res 36:672-680, 1993). We now show that this effect is direct and not mediated through the effects of bFGF on other cell types, because we were able to demonstrate similar changes in oligodendrocyte phenotype in enriched oligodendrocyte cultures prepared by immunopanning. The bFGF-induced reversion to the precursor stage of the oligodendroglial lineage can be blocked by agents that inhibit entry to the cell cycle; thus oligodendroglial dedifferentiation is dependent on proliferation. We also report that 2 days of bFGF treatment inhibits oligodendroglial apoptosis. However, when oligodendroglia are prevented from entering the cell cycle in the presence of bFGF, apoptotic cell death is increased. Thus, bFGF induces oligodendroglial dedifferentiation if oligodendroglial DNA synthesis can occur but causes oligodendroglial apoptosis when oligodendroglial DNA synthesis is prevented.

Axonal interactions regulate Schwann cell apoptosis in developing peripheral nerve: neuregulin receptors and the role of neuregulins

Programmed cell death during development resulting from the lack of appropriate survival factors has been demonstrated in both neurons and oligodendrocytes and occurs mostly in the form of apoptosis. We now demonstrate that Schwann cells in the rat sciatic nerve undergo apoptosis during early postnatal development and that the amount of apoptosis is markedly increased by axotomy. The apoptotic Schwann cells express the low-affinity nerve growth factor receptor but not myelin-related proteins, indicating that they are in the premyelinating state. Apoptosis resulting from normal development or from axotomy can be inhibited markedly by exogenous neuregulin. Consistent with this, the neuregulin receptor components erbB2 and erbB3 are expressed and phosphorylated in developing sciatic nerve. These data suggest that Schwann cell number in developing peripheral nerve is regulated by apoptosis through competition for axonally derived neuregulin.

Connexin32 is a myelin-related protein in the PNS and CNS

We have examined the expression of a gap junction protein, connexin32 (Cx32), in Schwann cells and oligodendrocytes. In peripheral nerve, Cx32 is found in the paranodal myelin loops and Schmidt-Lanterman incisures of myelinating Schwann cells, and the levels of Cx32 protein and mRNA change in parallel with those of other myelin-related genes during development, Wallerian degeneration, and axonal regeneration. In the central nervous system, Cx32 is found in oligodendrocytes and their processes, but not in compact myelin, and the levels of Cx32 protein and mRNA increase during development in parallel with those of the other myelin genes. Thus, Cx32 is expressed as part of the myelinating phenotype of both Schwann cells and oligodendrocytes, indicating that this gap junction protein plays in important role in the biology of myelin-forming cells.

Myelin gene expression in glia treated with oligodendroglial trophic factor

Oligodendroglia synthesize myelin in the CNS. In vitro, oligodendroglia may be identified by the binding of monoclonal antibodies against galactocerebroside, a myelin-specific galactolipid. Oligodendroglial trophic factor is a protein mitogen for cells of the oligodendroglial lineage. When oligodendroglia in cerebral white matter cultures are treated with oligodendroglial trophic factor, galactocerebroside-positive cells undergo mitosis but fail to express the myelin structural proteins, myelin basic protein and proteolipid protein. Oligodendroglia treated with oligodendroglial trophic factor, however, do express 2',3'-cyclic nucleotide 3'-phosphodiesterase and myelin-associated glycoprotein in a manner similar to oligodendroglia treated with platelet-derived growth factor. Oligodendroglial trophic factor, therefore, generates a population of somewhat 'immature' oligodendroglia, which are galactocerebroside, myelin-associated glycoprotein and 2', 3'-cyclic nucleotide 3' phosphodiesterase positive but myelin basic protein and proteolipid protein negative.

Platelet-derived growth factor is a survival factor for PSA-NCAM+ oligodendrocyte pre-progenitor cells

Mature oligodendroglia, which synthesize and express lipids and proteins characteristic of myelin, are generated from precursor cells which are formed in germinal matrix, then migrate widely through the neuraxis. We now demonstrate that these precursor cells can be recognized at a very early stage by their surface expression of polysialylated neural cell adhesion molecules (PSA-NCAM), and only later bind anti-ganglioside antibodies that had previously been used to recognize "O-2A" oligodendroglial precursor cells. PSA-NCAM expression by these cells is likely to be of functional significance, since a recent study demonstrated that O-2A cells become immobile when stripped of PSA-NCAM. Platelet-derived growth factor (PDGF) proved to be a survival factor for these PSA-NCAM+cells, and in a defined medium, PDGF was sufficient to ensure maturation of immunopurified PSA-NCAM+cells to oligodendroglia.

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors mediate excitotoxicity in the oligodendroglial lineage

We demonstrate by reverse transcriptase-polymerase chain reaction and Southern blotting that an immortalized rat oligodendroglial cell line (CG-4) expresses the non-N-methyl-D-aspartate (non-NMDA) glutamate receptor (GluR) genes GluR2-7, KA-1, and KA-2 and that nonimmortalized cells of the rat oligodendroglial lineage express the GluR1-3, GluR5-7, KA-1, and KA-2 genes. Lactic dehydrogenase release assays show that both immortalized and nonimmortalized cells of the oligodendroglial lineage are damaged by a 24-h exposure to 500 microM kainate or 5 mM L-glutamate, but not by a 24-h exposure to up to 10 mM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA). Damage is prevented by the non-NMDA GluR channel inhibitor 6-cyano-7-nitroquinoxaline-2,3-dione and is also averted if Ca2+ is removed from the culture medium. Cyclothiazide, which blocks desensitization of AMPA-preferring GluRs, increases cytotoxicity of kainate as well as inducing toxicity of AMPA. We conclude that cells of the oligodendroglial lineage express a population of AMPA-preferring and possibly also kainate-preferring GluR channels that are capable of mediating Ca(2+)-dependent excitotoxicity and that AMPA-induced cytotoxicity is blocked by desensitization of AMPA-preferring GluRs.

Pharmacology of sodium-dependent high-affinity L-[3H]glutamate transport in glial cultures

Pharmacological and molecular biological studies provide evidence for subtypes of sodium-dependent high-affinity glutamate (Glu) transport in the mammalian CNS. At least some of these transporters appear to be selectively expressed in different brain regions or by different cell types. In the present study, the properties of L-[3H]Glu transport were characterized using astrocyte-enriched cultures prepared from cerebellum and cortex. In both brain regions, the kinetic data for sodium-dependent transport were consistent with a single site with Km values of 91 +/- 17 microM in cortical glial cells and 66 +/- 23 microM in cerebellar glial cells. The capacities were 6.1 +/- 1.6 nmol/mg of protein/min in cortical glial cells and 8.4 +/- 0.9 nmol/mg of protein/min in cerebellar glial cells. The potencies of approximately 40 excitatory amino acid analogues for inhibition of sodium-dependent transport into glial cells prepared from cortex and cerebellum were examined, including compounds that are selective inhibitors of transport in synaptosomes prepared from either cerebellum or cortex. Of the analogues tested, 14 inhibited transport activity by > 50% at 1 mM concentrations. Unlike L-[3H]Glu transport in synaptosomes prepared from cerebellum or cortex, there were no large differences between the potencies of compounds for inhibition of transport measured in glial cells prepared from these two brain regions. With the exception of (2S,1'R,2'R)-2-(carboxycyclopropyl)glycine and L-alpha-aminoadipate, all of the compounds examined were approximately 10-200-fold less potent as inhibitors of L-[3H]Glu transport measured in glial cells than as inhibitors of transport measured in synaptosomes prepared from their respective brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial purification of a novel mitogen for oligodendroglia

A protein with a MWapp of 50-70 kDa isolated from the salt extract of crude membranes from neonatal rat brain increases the numbers of oligodendroglia in mixed glial cultures prepared from neonatal rat cerebral white matter. After partial purification by ion exchange and gel exclusion chromatography, and elution from an SDS-polyacrylamide gel, this protein ("oligodendroglial trophic factor," OTF) elicited half-maximal oligodendroglial recruitment at a concentration of 5 ng/mL. OTF is a mitogen for oligodendroglia, and to a lesser extent, for oligodendroglial progenitor (O2A) cells, but does not stimulate proliferation of astroglia, Schwann cells, or endoneurial fibroblasts. OTF, unlike platelet-derived growth factor (PDGF), is not an oligodendroglial survival factor. Antibodies against PDGF and basic fibroblast growth factor (bFGF) do not interfere with the accumulation of oligodendroglia induced by OTF. When OTF is given simultaneously with either PDGF or bFGF, there is an additive increase in the numbers of cells of the oligodendroglial lineage.

Protein growth factors as potential therapies for central nervous system demyelinative disorders

Demyelinative diseases are frequently accompanied by loss of oligodendroglia; in such instances, oligodendroglial regeneration must precede remyelination. Recent studies indicate that extracellular proteins such as platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF) profoundly influence the oligodendroglial lineage. PDGF stimulates the formation of oligodendroglia from partially differentiated progenitor cells, whereas bFGF induces mature oligodendroglia to proliferate and dedifferentiate. Manipulations of the central nervous system concentrations of these and other protein growth factors may prove of therapeutic value in multiple sclerosis.

Trophic effects of basic fibroblast growth factor (bFGF) on differentiated oligodendroglia: a mechanism for regeneration of the oligodendroglial lineage

We have investigated the effect of basic fibroblast growth factor (bFGF) on the proliferation and phenotype of differentiated oligodendroglia. Using primary cell cultures enriched in oligodendrocytes but containing few O2A-oligodendrocyte progenitor cells, we demonstrate that bFGF treatment greatly increases the proportion of O2A cells while decreasing the proportion of galactocerebroside +(GalC+), myelin basic protein +(MBP+) oligodendrocytes, and the steady state levels of MPB mRNA. Complement mediated cell lysis experiments using the A2B5 antibody to deplete existing O2A cells or the R-Mab antibody to deplete existing oligodendroglia show that bFGF elicits a rapid increase in the number of O2A cells in cultures previously depleted of O2A cells, but does not cause an early increase in O2A cells in cultures from which oligodendroglia had been removed, indicating that the oligodendrocytes are the source of the newly recruited O2A cells. This bFGF-mediated transition from oligodendrocyte to O2A cells occurs with a time course similar to the bFGF-induced increase of the proliferation rate of the GalC+ oligodendrocytes. Studies with purified, passaged cells of the oligodendroglial lineage show that bFGF augments oligodendroglial dedifferentiation and proliferation in chronologically adult oligodendrocytes and in the virtual absence of other cell types. We have thus demonstrated that mature oligodendrocytes are induced by bFGF to dedifferentiate and proliferate, suggesting a mechanism for regeneration of the oligodendroglial lineage following demyelinating disease.

Cerebral white matter contains PDGF-responsive precursors to O2A cells

Cells dissociated from the cerebral white matter of immature rats were maintained in monolayer culture. Treatment with platelet-derived growth factor (PDGF) caused a large increase in the numbers of "O2A" oligodendroglial precursor cells (which bind the monoclonal antibody A2B5) and subsequently in the numbers of galactocerebroside (galC)-positive oligodendroglia. A2B5-negative "pre-O2A cells" in cerebral white matter cultures in which O2A cells and oligodendroglia had been killed by antibody-dependent complement-mediated cytolysis were induced by PDGF to proliferate and to differentiate into O2A cells and subsequently into oligodendroglia and type 2 astroglia. The most mature pre-O2A phenotype in these cultures was a small, round, process-bearing cell which expressed vimentin but not glial fibrillary acidic protein or galC. Cells of this phenotype were not observed upon PDGF treatment of immature rat optic nerve monolayer cultures from which O2A cells and oligodendrocytes had been depleted, and PDGF also failed to elicit the accumulation of O2A cells and oligodendroglia in such cultures.