Deletion of voltage-gated calcium channels in astrocytes decreases neuroinflammation and demyelination in a murine model of multiple sclerosis

The experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis was used in combination with a Cav1.2 conditional knock-out mouse (Cav1.2KO) to study the role of astrocytic voltage-gated Ca++ channels in autoimmune CNS inflammation and demyelination. Cav1.2 channels were specifically ablated in Glast-1-positive astrocytes by means of the Cre-lox system before EAE induction. After immunization, motor activity was assessed daily, and a clinical score was given based on the severity of EAE symptoms. Cav1.2 deletion in astrocytes significantly reduced the severity of the disease. While no changes were found in the day of onset and peak disease severity, EAE mean clinical score was lower in Cav1.2KO animals during the chronic phase of the disease. This corresponded to better performance on the rotarod and increased motor activity in Cav1.2KO mice. Furthermore, decreased numbers of reactive astrocytes, activated microglia, and infiltrating lymphocytes were found in the lumbar section of the spinal cord of Cav1.2KO mice 40 days after immunization. The degree of myelin protein loss and size of demyelinated lesions were also attenuated in Cav1.2KO spinal cords. Similar results were found in EAE animals treated with nimodipine, a Cav1.2 Ca++ channel inhibitor with high affinity to the CNS. Mice injected with nimodipine during the acute and chronic phases of the disease exhibited lower numbers of reactive astrocytes, activated microglial, and infiltrating immune cells, as well as fewer demyelinated lesions in the spinal cord. These changes were correlated with improved clinical scores and motor performance. In summary, these data suggest that antagonizing Cav1.2 channels in astrocytes during EAE alleviates neuroinflammation and protects the spinal cord from autoimmune demyelination.

The expression of ceruloplasmin in astrocytes is essential for postnatal myelination and myelin maintenance in the adult brain

Ceruloplasmin (Cp) is a ferroxidase enzyme that is essential for cell iron efflux. The absence of this protein in humans and rodents produces progressive neurodegeneration with brain iron accumulation. Astrocytes express high levels of Cp and iron efflux from these cells has been shown to be central for oligodendrocyte maturation and myelination. To explore the role of astrocytic Cp in brain development and aging we generated a specific conditional KO mouse for Cp in astrocytes (Cp cKO). Deletion of Cp in astrocytes during the first postnatal week induced hypomyelination and a significant delay in oligodendrocyte maturation. This abnormal myelin synthesis was exacerbated throughout the first two postnatal months and accompanied by a reduction in oligodendrocyte iron content, as well as an increase in brain oxidative stress. In contrast to young animals, deletion of astrocytic Cp at 8 months of age engendered iron accumulation in several brain areas and neurodegeneration in cortical regions. Aged Cp cKO mice also showed myelin loss and oxidative stress in oligodendrocytes and neurons, and at 18 months of age, developed abnormal behavioral profiles, including deficits in locomotion and short-term memory. In summary, our results demonstrate that iron efflux-mediated by astrocytic Cp-is essential for both early oligodendrocyte maturation and myelin integrity in the mature brain. Additionally, our data suggest that astrocytic Cp activity is central to prevent iron accumulation and iron-induced oxidative stress in the aging CNS.

Transferrin Receptor Is Necessary for Proper Oligodendrocyte Iron Homeostasis and Development

To test the hypothesis that the transferrin (Tf) cycle has unique importance for oligodendrocyte development and function, we disrupted the expression of the Tf receptor (Tfr) gene in oligodendrocyte progenitor cells (OPCs) on mice of either sex using the Cre/lox system. This ablation results in the elimination of iron incorporation via the Tf cycle but leaves other Tf functions intact. Mice lacking Tfr, specifically in NG2 or Sox10-positive OPCs, developed a hypomyelination phenotype. Both OPC differentiation and myelination were affected, and Tfr deletion resulted in impaired OPC iron absorption. Specifically, the brains of Tfr cKO animals presented a reduction in the quantity of myelinated axons, as well as fewer mature oligodendrocytes. In contrast, the ablation of Tfr in adult mice affected neither mature oligodendrocytes nor myelin synthesis. RNA-seq analysis performed in Tfr cKO OPCs revealed misregulated genes involved in OPC maturation, myelination, and mitochondrial activity. Tfr deletion in cortical OPCs also disrupted the activity of the mTORC1 signaling pathway, epigenetic mechanisms critical for gene transcription and the expression of structural mitochondrial genes. RNA-seq studies were additionally conducted in OPCs in which iron storage was disrupted by deleting the ferritin heavy chain. These OPCs display abnormal regulation of genes associated with iron transport, antioxidant activity, and mitochondrial activity. Thus, our results indicate that the Tf cycle is central for iron homeostasis in OPCs during postnatal development and suggest that both iron uptake via Tfr and iron storage in ferritin are critical for energy production, mitochondrial activity, and maturation of postnatal OPCs.SIGNIFICANCE STATEMENT By knocking-out transferrin receptor (Tfr) specifically in oligodendrocyte progenitor cells (OPCs), we have established that iron incorporation via the Tf cycle is key for OPC iron homeostasis and for the normal function of these cells during the postnatal development of the CNS. Moreover, RNA-seq analysis indicated that both Tfr iron uptake and ferritin iron storage are critical for proper OPC mitochondrial activity, energy production, and maturation.

Lanthionine Ketimine Ethyl Ester Accelerates Remyelination in a Mouse Model of Multiple Sclerosis

Although over 20 disease modifying therapies are approved to treat Multiple Sclerosis (MS), these do not increase remyelination of demyelinated axons or mitigate axon damage. Previous studies showed that lanthionine ketenamine ethyl ester (LKE) reduces clinical signs in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS and increased maturation of oligodendrocyte (OL) progenitor cells (OPCs) in vitro. In the current study, we used the cuprizone (CPZ) demyelination model of MS to test if LKE could increase remyelination. The corpus callosum (CC) and somatosensory cortex was examined by immunohistochemistry (IHC), electron microscopy and for mRNA expression changes in mice provided 5 weeks of CPZ diet followed by 2 weeks of normal diet in the presence of LKE or vehicle. A significant increase in the number of myelinated axons, and increased myelin thickness was observed in the CC of LKE-treated groups compared to vehicle-treated groups. LKE also increased myelin basic protein and proteolipid protein expression in the CC and cortex, and increased the number of mature OLs in the cortex. In contrast, LKE did not increase the percentage of proliferating OPCs suggesting effects on OPC survival and differentiation but not proliferation. The effects of LKE on OL maturation and remyelination were supported by similar changes in their relative mRNA levels. Interestingly, LKE did not have significant effects on GFAP or Iba1 immunostaining or mRNA levels. These findings suggest that remyelinating actions of LKE can potentially be formulated to induce remyelination in neurological diseases associated with demyelination including MS.

H-ferritin expression in astrocytes is necessary for proper oligodendrocyte development and myelination

How iron is delivered to the CNS for myelination is poorly understood. Astrocytes are the most abundant glial cells in the brain and are the only cells in close contact with blood vessels. Therefore, they are strategically located to obtain nutrients, such as iron, from circulating blood. To determine the importance of astrocyte iron uptake and storage in myelination and remyelination, we conditionally knocked-out the expression of the divalent metal transporter 1 (DMT1), the transferrin receptor 1 (Tfr1), and the ferritin heavy subunit (Fth) in Glast-1-positive astrocytes. DMT1 or Tfr1 ablation in astrocytes throughout early brain development did not significantly affects oligodendrocyte maturation or iron homeostasis. However, blocking Fth production in astrocytes during the first postnatal week drastically delayed oligodendrocyte development and myelin synthesis. Fth knockout animals presented an important decrease in the number of myelinating oligodendrocytes and a substantial reduction in the percentage of myelinated axons. This postnatal hypomyelination was accompanied by a decline in oligodendrocyte iron uptake and with an increase in brain oxidative stress. We also tested the relevance of astrocytic Fth expression in the cuprizone model of myelin damage and repair. Fth deletion in Glast1-positive astrocytes significantly reduced myelin production and the density of mature myelinating oligodendrocytes throughout the complete remyelination process. These results indicate that Fth iron storage in astrocytes is vital for early oligodendrocyte development as well as for the remyelination of the CNS.

Ceruloplasmin deletion in myelinating glial cells induces myelin disruption and oxidative stress in the central and peripheral nervous systems

Ceruloplasmin (Cp) is a ferroxidase enzyme that is essential for cell iron efflux and has been postulated to have a neuroprotective role. During the myelination process, oligodendrocytes (OLs) and Schwann cells (SCs) express high levels of Cp, but the role of this enzyme in glial cell development and function is completely unknown. To define the function of Cp in the myelination of the central and peripheral nervous systems, we have conditionally knocked-out Cp specifically in OLs and SCs during early postnatal development as well as in aged mice. Cp ablation in early OLs (postnatal day 2, P2) significantly affects the differentiation of these cells and the synthesis of myelin through the first four postnatal weeks. The total number of mature myelinating OLs was reduced, and the density of apoptotic OLs was increased. These changes were accompanied with reductions in the percentage of myelinated axons and increases in the g-ratio of myelinated fibers. Cp ablation in young myelinating OLs (P30 or P60) did not affect myelin synthesis and/or OL numbers, however, Cp loss in aged OLs (8 months) induced cell iron overload, apoptotic cell death, brain oxidative stress, neurodegeneration and myelin disruption. Furthermore, Cp deletion in SCs affected postnatal SC development and myelination and produced motor coordination deficits as well as oxidative stress in young and aged peripheral nerves. Together, our data indicate that Cp ferroxidase activity is essential for OLs and SCs maturation during early postnatal development and iron homeostasis in matured myelinating cells. Additionally, our results suggest that Cp expression in myelinating glial cells is crucial to prevent oxidative stress and neurodegeneration in the central and peripheral nervous systems.

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Cp activity is essential for the development and function of myelinating glial cell.

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Cp ablation delays oligodendrocyte and Schwann cell maturation.

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Cp deletion interrupts the myelination of the central and peripheral nervous systems.

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Cp deletion in aged oligodendrocytes induces cell dead and brain oxidative stress.

Iron Metabolism in Oligodendrocytes and Astrocytes, Implications for Myelination and Remyelination

Iron is a key nutrient for normal central nervous system (CNS) development and function; thus, iron deficiency as well as iron excess may result in harmful effects in the CNS. Oligodendrocytes and astrocytes are crucial players in brain iron equilibrium. However, the mechanisms of iron uptake, storage, and efflux in oligodendrocytes and astrocytes during CNS development or under pathological situations such as demyelination are not completely understood. In the CNS, iron is directly required for myelin production as a cofactor for enzymes involved in ATP, cholesterol and lipid synthesis, and oligodendrocytes are the cells with the highest iron levels in the brain which is linked to their elevated metabolic needs associated with the process of myelination. Unlike oligodendrocytes, astrocytes do not have a high metabolic requirement for iron. However, these cells are in close contact with blood vessel and have a strong iron transport capacity. In several pathological situations, changes in iron homoeostasis result in altered cellular iron distribution and accumulation and oxidative stress. In inflammatory demyelinating diseases such as multiple sclerosis, reactive astrocytes accumulate iron and upregulate iron efflux and influx molecules, which suggest that they are outfitted to take up and safely recycle iron. In this review, we will discuss the participation of oligodendrocytes and astrocytes in CNS iron homeostasis. Understanding the molecular mechanisms of iron uptake, storage, and efflux in oligodendrocytes and astrocytes is necessary for planning effective strategies for iron management during CNS development as well as for the treatment of demyelinating diseases.

Calcium Signaling in the Oligodendrocyte Lineage: Regulators and Consequences

Cells of the oligodendrocyte lineage express a wide range of Ca²⁺ channels and receptors that regulate oligodendrocyte progenitor cell (OPC) and oligodendrocyte formation and function. Here we define those key channels and receptors that regulate Ca²⁺ signaling and OPC development and myelination. We then discuss how the regulation of intracellular Ca²⁺ in turn affects OPC and oligodendrocyte biology in the healthy nervous system and under pathological conditions. Activation of Ca²⁺ channels and receptors in OPCs and oligodendrocytes by neurotransmitters converges on regulating intracellular Ca²⁺, making Ca²⁺ signaling a central candidate mediator of activity-driven myelination. Indeed, recent evidence indicates that localized changes in Ca²⁺ in oligodendrocytes can regulate the formation and remodeling of myelin sheaths and perhaps additional functions of oligodendrocytes and OPCs. Thus, decoding how OPCs and myelinating oligodendrocytes integrate and process Ca²⁺ signals will be important to fully understand central nervous system formation, health, and function.

Enhanced oligodendrocyte maturation and myelination in a mouse model of Timothy syndrome

To study the role of L-type voltage-gated Ca++ channels in oligodendrocyte development, we used a mouse model of Timothy syndrome (TS) in which a gain-of-function mutation in the α1 subunit of the L-type Ca++ channel Cav1.2 gives rise to an autism spectrum disorder (ASD). Oligodendrocyte progenitor cells (OPCs) isolated from the cortex of TS mice showed greater L-type Ca++ influx and displayed characteristics suggestive of advanced maturation compared to control OPCs, including a more complex morphology and higher levels of myelin protein expression. Consistent with this, expression of Cav1.2 channels bearing the TS mutation in wild-type OPCs triggered process formation and promoted oligodendrocyte-neuron interaction via the activation of Ca++ /calmodulin-dependent protein kinase II. To ascertain whether accelerated OPC maturation correlated with functional enhancements, we examined myelination in the TS brain at different postnatal time points. The expression of myelin proteins was significantly higher in the corpus callosum, cortex and striatum of TS animals, and immunohistochemical analysis for oligodendrocyte stage-specific markers revealed an increase in the density of myelinating oligodendrocytes in several areas of the TS brain. Along the same line, electron microscopy studies in the corpus callosum of TS animals showed significant increases both in the percentage of myelinated axons and in the thickness of myelin sheaths. In summary, these data indicate that OPC development and oligodendrocyte myelination is enhanced in the brain of TS mice, and suggest that this mouse model of a syndromic ASD is a useful tool to explore the role of L-type Ca++ channels in myelination.

The imidazoline I2 receptor agonist 2-BFI attenuates hypersensitivity and spinal neuroinflammation in a rat model of neuropathic pain

Chronic pain is a large, unmet public health problem. Recent studies have demonstrated the importance of neuroinflammation in the establishment and maintenance of chronic pain. However, pharmacotherapies that reduce neuroinflammation have not been successfully developed to treat chronic pain thus far. Several preclinical studies have established imidazoline I₂ receptor (I₂R) agonists as novel candidates for chronic pain therapies, and while some I₂R ligands appear to modulate neuroinflammation in certain scenarios, whether they exert anti-neuroinflammatory effects in models of chronic pain is unknown. This study examined the effects of the prototypical I₂R agonist 2-(2-benzofuranyl)-2-imidazoline hydrochloride (2-BFI) on hypersensitivity and neuroinflammation induced by chronic constriction injury (CCI), a neuropathic pain model in rats. In CCI rats, twice-daily treatment with 10 mg/kg 2-BFI for seven days consistently increased mechanical and thermal nociception thresholds, reduced GFAP and Iba-1 levels in the dorsal horn of the spinal cord, and reduced levels of TNF-α relative to saline treatment. These results were recapitulated in primary mouse cortical astrocyte cultures. Incubation with 2-BFI attenuated GFAP expression and supernatant TNF-α levels in LPS-stimulated cultures. These results suggest that I₂R agonists such as 2-BFI may reduce neuroinflammation which may partially account for their antinociceptive effects.

L-type voltage-operated calcium channels contribute to astrocyte activation In vitro

We have found a significant upregulation of L-type voltage-operated Ca(++) channels (VOCCs) in reactive astrocytes. To test if VOCCs are centrally involved in triggering astrocyte reactivity, we used in vitro models of astrocyte activation in combination with pharmacological inhibitors, siRNAs and the Cre/lox system to reduce the activity of L-type VOCCs in primary cortical astrocytes. The endotoxin lipopolysaccharide (LPS) as well as high extracellular K(+) , glutamate, and ATP promote astrogliosis in vitro. L-type VOCC inhibitors drastically reduce the number of reactive cells, astrocyte hypertrophy, and cell proliferation after these treatments. Astrocytes transfected with siRNAs for the Cav1.2 subunit that conducts L-type Ca(++) currents as well as Cav1.2 knockout astrocytes showed reduce Ca(++) influx by ∼80% after plasma membrane depolarization. Importantly, Cav1.2 knock-down/out prevents astrocyte activation and proliferation induced by LPS. Similar results were found using the scratch wound assay. After injuring the astrocyte monolayer, cells extend processes toward the cell-free scratch region and subsequently migrate and populate the scratch. We found a significant increase in the activity of L-type VOCCs in reactive astrocytes located in the growing line in comparison to quiescent astrocytes situated away from the scratch. Moreover, the migration of astrocytes from the scratching line as well as the number of proliferating astrocytes was reduced in Cav1.2 knock-down/out cultures. In summary, our results suggest that Cav1.2 L-type VOCCs play a fundamental role in the induction and/or proliferation of reactive astrocytes, and indicate that the inhibition of these Ca(++) channels may be an effective way to prevent astrocyte activation. GLIA 2016. GLIA 2016;64:1396-1415.

Golli Myelin Basic Proteins Modulate Voltage-Operated Ca(++) Influx and Development in Cortical and Hippocampal Neurons

The golli proteins, products of the myelin basic protein gene, are widely expressed in oligodendrocyte progenitor cells and neurons during the postnatal development of the brain. While golli appears to be important for oligodendrocyte migration and differentiation, its function in neuronal development is completely unknown. We have found that golli proteins function as new and novel modulators of voltage-operated Ca(++) channels (VOCCs) in neurons. In vitro, golli knock-out (KO) neurons exhibit decreased Ca(++) influx after plasma membrane depolarization and a substantial maturational delay. Increased expression of golli proteins enhances L-type Ca(++) entry and processes outgrowth in cortical neurons, and pharmacological activation of L-type Ca(++) channels stimulates maturation and prevents cell death in golli-KO neurons. In situ, Ca(++) influx mediated by L-type VOCCs was significantly decreased in cortical and hippocampal neurons of the golli-KO brain. These Ca(++) alterations affect cortical and hippocampal development and the proliferation and survival of neural progenitor cells during the postnatal development of the golli-KO brain. The CA1/3 sections and the dentate gyrus of the hippocampus were reduced in the golli-KO mice as well as the density of dendrites in the somatosensory cortex. Furthermore, the golli-KO mice display abnormal behavior including deficits in episodic memory and reduced anxiety. Because of the expression of the golli proteins within neurons in learning and memory centers of the brain, this work has profound implication in neurodegenerative diseases and neurological disorders.

Voltage-gated Ca2+ entry promotes oligodendrocyte progenitor cell maturation and myelination in vitro

We have previously shown that the expression of voltage-operated Ca(++) channels (VOCCs) is highly regulated in the oligodendroglial lineage and is essential for proper oligodendrocyte progenitor cell (OPC) migration. Here we assessed the role of VOCCs, in particular the L-type, in oligodendrocyte maturation. We used pharmacological treatments to activate or block voltage-gated Ca(++) uptake and siRNAs to specifically knock down the L-type VOCC in primary cultures of mouse OPCs. Activation of VOCCs by plasma membrane depolarization increased OPC morphological differentiation as well as the expression of mature oligodendrocyte markers. On the contrary, inhibition of L-type Ca(++) channels significantly delayed OPC development. OPCs transfected with siRNAs for the Cav1.2 subunit that conducts L-type Ca(++) currents showed reduce Ca(++) influx by ~75% after plasma membrane depolarization, indicating that Cav1.2 is heavily involved in mediating voltage-operated Ca(++) entry in OPCs. Cav1.2 knockdown induced a decrease in the proportion of oligodendrocytes that expressed myelin proteins, and an increase in cells that retained immature oligodendrocyte markers. Moreover, OPC proliferation, but not cell viability, was negatively affected after L-type Ca(++) channel knockdown. Additionally, we have tested the ability of L-type VOCCs to facilitate axon-glial interaction during the first steps of myelin formation using an in vitro co-culture system of OPCs with cortical neurons. Unlike control OPCs, Cav1.2 deficient oligodendrocytes displayed a simple morphology, low levels of myelin proteins expression and appeared to be less capable of establishing contacts with neurites and axons. Together, this set of in vitro experiments characterizes the involvement of L-type VOCCs on OPC maturation as well as the role played by these Ca(++) channels during the early phases of myelination.

Impact of simulated microgravity on oligodendrocyte development: implications for central nervous system repair

We have recently established a culture system to study the impact of simulated microgravity on oligodendrocyte progenitor cells (OPCs) development. We subjected mouse and human OPCs to a short exposure of simulated microgravity produced by a 3D-Clinostat robot. Our results demonstrate that rodent and human OPCs display enhanced and sustained proliferation when exposed to simulated microgravity as assessed by several parameters, including a decrease in the cell cycle time. Additionally, OPC migration was examined in vitro using time-lapse imaging of cultured OPCs. Our results indicated that OPCs migrate to a greater extent after stimulated microgravity than in normal conditions, and this enhanced motility was associated with OPC morphological changes. The lack of normal gravity resulted in a significant increase in the migration speed of mouse and human OPCs and we found that the average leading process in migrating bipolar OPCs was significantly longer in microgravity treated cells than in controls, demonstrating that during OPC migration the lack of gravity promotes leading process extension, an essential step in the process of OPC migration. Finally, we tested the effect of simulated microgravity on OPC differentiation. Our data showed that the expression of mature oligodendrocyte markers was significantly delayed in microgravity treated OPCs. Under conditions where OPCs were allowed to progress in the lineage, simulated microgravity decreased the proportion of cells that expressed mature markers, such as CC1 and MBP, with a concomitant increased number of cells that retained immature oligodendrocyte markers such as Sox2 and NG2. Development of methodologies aimed at enhancing the number of OPCs and their ability to progress on the oligodendrocyte lineage is of great value for treatment of demyelinating disorders. To our knowledge, this is the first report on the gravitational modulation of oligodendrocyte intrinsic plasticity to increase their progenies.

STAT3-mediated astrogliosis protects myelin development in neonatal brain injury

OBJECTIVE

Pathological findings in neonatal brain injury associated with preterm birth include focal and/or diffuse white matter injury (WMI). Despite the heterogeneous nature of this condition, reactive astrogliosis and microgliosis are frequently observed. Thus, molecular mechanisms by which glia activation contribute to WMI were investigated.

METHODS

Postmortem brains of neonatal brain injury were investigated to identify molecular features of reactive astrocytes. The contribution of astrogliosis to WMI was further tested in a mouse model in genetically engineered mice.

RESULTS

Activated STAT3 signaling in reactive astrocytes was found to be a common feature in postmortem brains of neonatal brain injury. In a mouse model of neonatal WMI, conditional deletion of STAT3 in astrocytes resulted in exacerbated WMI, which was associated with delayed maturation of oligodendrocytes. Mechanistically, the delay occurred in association with overexpression of transforming growth factor (TGF)β-1 in microglia, which in healthy controls decreased with myelin maturation in an age-dependent manner. TGFβ-1 directly and dose-dependently inhibited the maturation of purified oligodendrocyte progenitors, and pharmacological inhibition of TGFβ-1 signaling in vivo reversed the delay in myelin development. Factors secreted from STAT3-deficient astrocytes promoted elevated TGFβ-1 production in cultured microglia compared to wild-type astrocytes.

INTERPRETATION

These results suggest that myelin development is regulated by a mechanism involving crosstalk between microglia and oligodendrocyte progenitors. Reactive astrocytes may modify this signaling in a STAT3-dependent manner, preventing the pathological expression of TGFβ-1 in microglia and the impairment of oligodendrocyte maturation.

Intranasal administration of aTf protects and repairs the neonatal white matter after a cerebral hypoxic-ischemic event

Our previous studies showed that the intracerebral injection of apotransferrin (aTf) attenuates white matter damage and accelerates the remyelination process in a neonatal rat model of cerebral hypoxia-ischemia (HI) injury. However, the intracerebral injection of aTf might not be practical for clinical treatments. Therefore, the development of less invasive techniques capable of delivering aTf to the central nervous system would clearly aid in its effective clinical use. In this work, we have determined whether intranasal (iN) administration of human aTf provides neuroprotection to the neonatal mouse brain following a cerebral hypoxic-ischemic event. Apotransferrin was infused into the naris of neonatal mice and the HI insult was induced by right common carotid artery ligation followed by exposure to low oxygen concentration. Our results showed that aTf was successfully delivered into the neonatal HI brain and detected in the olfactory bulb, forebrain and posterior brain 30 min after inhalation. This treatment successfully reduced white matter damage, neuronal loss and astrogliosis in different brain regions and enhanced the proliferation and survival of oligodendroglial progenitor cells (OPCs) in the subventricular zone and corpus callosum (CC). Additionally, using an in vitro hypoxic model, we demonstrated that aTf prevents oligodendrocyte progenitor cell death by promoting their differentiation. In summary, these data suggest that iN administration of aTf has the potential to be used for clinical treatment to protect myelin and to induce remyelination in demyelinating hypoxic-ischemic events in the neonatal brain.

Golli myelin basic proteins stimulate oligodendrocyte progenitor cell proliferation and differentiation in remyelinating adult mouse brain

Golli myelin basic proteins are necessary for normal myelination, acting via voltage and store-dependent Ca(2+) entry at multiple steps during oligodendrocyte progenitor cell (OPC) development. To date nothing is known regarding the role of golli proteins in demyelination or remyelination events. Here the effects of golli ablation and overexpression in myelin loss and recovery were examined using the cuprizone (CPZ) model of demyelination/remyelination. We found severe demyelination in the corpus callosum (CC) of golli-overexpressing mice (JOE) during the CPZ treatment, which was accompanied by an increased number of reactive astrocytes and activation of microglia/macrophages. During demyelination of JOE brains, a significant increase in the number of proliferating OPCs was found in the CC as well as in the subventricular zone, and our data indicate that these progenitors matured and fully remyelinated the CC of JOE animals after CPZ withdrawal. In contrast, in the absence of golli (golli-KO mice) delayed myelin loss associated with a smaller immune response, and a lower number of OPCs was found in these mice during the CPZ treatment. Furthermore, incomplete remyelination was observed after CPZ removal in large areas of the CC of golli-KO mice, reflecting irregular recovery of the oligodendrocyte population and subsequent myelin sheath formation. Our findings demonstrate that golli proteins sensitize mature oligodendrocytes to CPZ-induced demyelination, while at the same time stimulate the proliferation/recruitment of OPCs during demyelination, resulting in accelerated remyelination.

Proline substitutions and threonine pseudophosphorylation of the SH3 ligand of 18.5-kDa myelin basic protein decrease its affinity for the Fyn-SH3 domain and alter process development and protein localization in oligodendrocytes

The developmentally regulated myelin basic proteins (MBPs), which arise from the golli (gene of oligodendrocyte lineage) complex, are highly positively charged, intrinsically disordered, multifunctional proteins having several alternatively spliced isoforms and posttranslational modifications, and they play key roles in myelin compaction. The classic 18.5-kDa MBP isoform has a proline-rich region comprising amino acids 92-99 (murine sequence -T(92)PRTPPPS(99)-) that contains a minimal SH3 ligand domain. We have previously shown that 18.5-kDa MBP binds to several SH3 domains, including that of Fyn, a member of the Src family of tyrosine kinases involved in a number of signaling pathways during CNS development. To determine the physiological role of this binding as well as the role of phosphorylation of Thr92 and Thr95, in the current study we have produced several MBP variants specifically targeting phosphorylation sites and key structural regions of MBP's SH3 ligand domain. Using isothermal titration calorimetry, we have demonstrated that, compared with the wild-type protein, these variants have lower affinity for the SH3 domain of Fyn. Moreover, overexpression of N-terminal-tagged GFP versions in immortalized oligodendroglial N19 and N20.1 cell cultures results in aberrant elongation of membrane processes and increased branching complexity and inhibits the ability of MBP to decrease Ca(2+) influx. Phosphorylation of Thr92 can also cause MBP to traffic to the nucleus, where it may participate in additional protein-protein interactions. Coexpression of MBP with a constitutively active form of Fyn kinase resulted in membrane process elaboration, a phenomenon that was abolished by point amino acid substitutions in MBP's SH3 ligand domain. These results suggest that MBP's SH3 ligand domain plays a key role in intracellular protein interactions in vivo and may be required for proper membrane elaboration of developing oligodendrocytes and, further, that phosphorylation of Thr92 and Thr95 can regulate this function.

Modulation of canonical transient receptor potential channel 1 in the proliferation of oligodendrocyte precursor cells by the golli products of the myelin basic protein gene

Golli proteins, products of the myelin basic protein gene, function as a new type of modulator of intracellular Ca(2+) levels in oligodendrocyte progenitor cells (OPCs). Because of this, they affect a number of Ca(2+)-dependent functions, such as OPC migration and process extension. To examine further the Ca(2+) channels regulated by golli, we studied the store-operated Ca(2+) channels (SOCCs) in OPCs and acute brain slice preparations from golli knock-out and golli-overexpressing mice. Our results showed that pharmacologically induced Ca(2+) release from intracellular stores evoked a significant extracellular Ca(2+) entry after store depletion in OPCs. They also indicated that, under these pharmacological conditions, golli promoted activation of Ca(2+) influx by SOCCs in cultured OPCs as well as in tissue slices. The canonical transient receptor potential family of Ca(2+) channels (TRPCs) has been postulated to be SOCC subunits in oligodendrocytes. Using a small interfering RNA knockdown approach, we provided direct evidence that TRPC1 is involved in store-operated Ca(2+) influx in OPCs and that it is modulated by golli. Furthermore, our data indicated that golli is probably associated with TRPC1 at OPC processes. Additionally, we found that TRPC1 expression is essential for the effects of golli on OPC proliferation. In summary, our data indicate a key role for golli proteins in the regulation of TRPC-mediated Ca(2+) influx, a finding that has profound consequences for the regulation of multiple biological processes in OPCs. More important, we have shown that extracellular Ca(2+) uptake through TRPC1 is an essential component in the mechanism of OPC proliferation.

Regulation of L-type Ca++ currents and process morphology in white matter oligodendrocyte precursor cells by golli-myelin proteins

The golli myelin basic proteins are expressed in oligodendroglial precursor cells (OPCs) where they play a role in regulating Ca(2+) homeostasis. During depolarization, they influence process outgrowth and migration through their action on voltage-operated Ca(2+) channels (VOCCs). To identify ion channels that are modulated by golli, we examined the electrophysiological properties of VOCCs in OPCs in the white matter of golli knock-out and control mice. OPCs exhibited two distinct Ca(2+) channels, which were distinguished by their voltage dependence and pharmacological profiles and which exhibited many of the hallmarks of LVA/T-type and HVA/L-type Ca(2+) channels. The density of high-voltage-activated (HVA) currents was reduced in OPCs recorded in golli-KO tissue, while low-voltage-activated (LVA) currents remained unaltered in these cells. These data indicate that golli exerts an exclusive influence on L-type Ca(2+) channels in OPCs. Oligodendrocytes (OLs) also displayed LVA and HVA currents, although the density of these currents was much reduced at this developmental stage. These currents were not altered in golli-KO OLs showing the influence of golli on L-type Ca(2+) channels is restricted to a specific time-window during the course of oligodendroglial development. The actions of golli on OPC L-type Ca(2+) channels were accompanied by changes in process morphology, including a reduction in process complexity and the appearance of enlarged varicosities that decorated these cellular processes. These data on L-type Ca(2+) channels and process development provide in situ evidence for the influence of golli on VOCCs, and offer an explanation for the hypomyelination observed in the brains of golli-KO mice.

Classical 18.5-and 21.5-kDa isoforms of myelin basic protein inhibit calcium influx into oligodendroglial cells, in contrast to golli isoforms

The myelin basic protein (MBP) family arises from different transcription start sites of the golli (gene of oligodendrocyte lineage) complex, with further variety generated by differential splicing. The "classical" MBP isoforms are peripheral membrane proteins that facilitate compaction of the mature myelin sheath but also have multiple protein interactions. The early developmental golli isoforms have previously been shown to promote process extension and enhance Ca(2+) influx into primary and immortalized oligodendrocyte cell lines. Here, we have performed similar studies with the classical 18.5- and 21.5-kDa isoforms of MBP. In contrast to golli proteins, overexpression of classical MBP isoforms significantly reduces Ca(2+) influx in the oligodendrocyte cell line N19 as well as in primary cultures of oligodendroglial progenitor cells. Pharmacological experiments demonstrate that this effect is mediated by voltage-operated Ca(2+) channels (VOCCs) and not by ligand-gated Ca(2+) channels or Ca(2+) release from intracellular stores. The pseudo-deiminated 18.5-kDa and the full-length 21.5-kDa isoforms do not reduce Ca(2+) influx as much as the unmodified 18.5-kDa isoform. However, more efficient membrane localization (of overexpressed, pseudo-deiminated 18.5-kDa and 21.5-kDa isoforms of classical MBP containing the 21-nt 3'-untranslated region transit signal) further reduces the Ca(2+) response after plasma membrane depolarization, suggesting that binding of classical MBP isoforms to the plasma membrane is important for modulation of Ca(2+) homeostasis. Furthermore, we have found that the mature 18.5-kDa isoform expressed in oligodendrocytes colocalizes with VOCCs, particularly at the leading edge of extending membrane processes. In summary, our findings suggest a key role for classical MBP proteins in regulating voltage-gated Ca(2+) channels at the plasma membrane of oligodendroglial cells and thus also in regulation of multiple developmental stages in this cell lineage.

Partial inhibition of the proteasome enhances the activity of the myelin basic protein promoter

We have previously shown that low concentrations of a specific proteasome inhibitor accelerate exit from the cell cycle and enhance oligodendroglial cell (OLGc) differentiation. To elucidate the mechanisms involved in this process, OLGcs of the N20.1 cell line, transfected with a reporter gene driven by the MBP promoter, were treated with proteasome inhibitors and/or inhibitors of different signaling pathways. Partial proteasome inhibition resulted in enhanced activation of the MBP promoter which involved the tyrosine kinase, PI3-Akt and PKC pathways, accompanied by an increase in the levels of p21(Cip1), p27(Kip1) and Sp1 and by a decrease in Nkx2.2. Binding of Sp1 to DNA was also increased. These results were not observed when the Sp1 binding site was mutated. We conclude that the enhanced activation of the MBP promoter induced by partial inhibition of the proteasome could be due, at least in part, to the stabilization of p27(Kip1) and Sp1.

Increased expression of golli myelin basic proteins enhances calcium influx into oligodendroglial cells

The myelin basic protein (MBP) gene encodes two families of proteins: the classic MBP constituents of myelin and the golli-MBPs, the function of which is less well understood. Previous work suggests that golli proteins may play a role in Ca2+ homeostasis in oligodendrocytes (OLs) and in T-cells. Overexpression of golli in OL cell lines induces elaboration of sheets and processes. Live imaging of these cells revealed a rapid retraction of the processes and sheets after depolarization with high K+. This phenomenon was associated with a significant increase in [Ca2+]int without changes in cell viability. The results indicated that golli produced its effect through Ca2+ influx, rather than Ca2+ release from intracellular stores. Furthermore, a specific [Ca2+]int chelator (BAPTA) or Cd2+, a specific blocker of voltage-operated Ca2+ channels, abolished the ability of golli to promote process extension in a dose-dependent manner. Analysis of the golli protein identified a myristoylation site at the C terminus of the golli domain, which was essential for the action of golli on Ca2+ influx, suggesting that binding of golli to the plasma membrane is important for modulating Ca2+ homeostasis. High-resolution spatiotemporal analysis along N19 processes revealed higher-amplitude local Ca2+ influx in regions with elevated levels of golli. These findings suggest a key role for golli proteins in regulating voltage-gated Ca2+ channels in OLs during process remodeling. Our observations are consistent with the hypothesis that golli proteins, as a part of a protein complex, modulate Ca2+ influx at the plasma membrane and along OL processes.

Differential gene expression during development in two oligodendroglial cell lines overexpressing transferrin: a cDNA array analysis

In the central nervous system, transferrin (Tf) is produced by oligodendroglial cells (OLGcs) and is essential for their development. Recently, using the complete cDNA of the human Tf gene, we obtained clones overexpressing Tf in two OLGc lines, N19 and N20.1, which represent different stages of differentiation. We showed that the overexpression of this glycoprotein promotes the maturation and myelinogenic capacity of both cell lines. In this work, using cDNA array technology, we examined changes induced by Tf in 1,176 genes. We found 41 genes differentially expressed in both cell lines, all of them involved in OLGc development. In the less mature cells (N19) overexpressing Tf, there was a significant increase in key enzymes of neurosteroid metabolism, such as cholesterol side chain cleavage cytochrome P450, 3beta-hydroxysteroid dehydrogenase and 5alpha-reductase type 1. In the more mature cell line (N20.1), Tf overexpression produced an induction of several mRNAs of the GABA(A) receptor subunits, of thyroid hormone receptors and of proteins involved in axon-glia interactions such as F3/contactin. In addition, in both cell lines, Tf overexpression induced an increase in the expression of different isoforms of transforming growth factor beta receptors and in several genes related to mitochondrial function and to complex lipid metabolism, crucial steps in myelin synthesis. Differentiation produced by Tf in both cell lines seems to occur by modulation of different genes depending on the maturational stage of the cells. Our findings provide new insights into the molecular basis of OLGc differentiation and on the role played by Tf in this process.

Apotransferrin decreases the response of oligodendrocyte progenitors to PDGF and inhibits the progression of the cell cycle

In the CNS, transferrin (Tf) is expressed by the oligodendroglial cells (OLGcs) and is essential for their development. We have previously shown that apotransferrin (aTf) accelerates maturation of OLGcs in vivo as well as in vitro. The mechanisms involved in this action appear to be complex and have not been completely elucidated. The aim of this study was to investigate if Tf participates in the regulation of the cell cycle of oligodendroglial progenitor cells (OPcs). Primary cultures of OPcs were treated with aTf and/or with different combinations of mitogenic factors. Cell cycle progression was studied by BrdU incorporation, flow cytometry and by the expression of cell cycle regulatory proteins. Apotransferrin decreased the number of BrdU+ cells, increasing the cell cycle time and decreasing the number of cells in S phase. The cell cycle inhibitors p27kip1, p21cip1 and p53 were increased, and in agreement with these results, the activity of the complexes involved in G1-S progression (cyclin D/CDK4, cyclin E/CDK2), was dramatically decreased. Apotransferrin also inhibited the mitogenic effects of PDGF and PDGF/IGF on OPcs, but did not affect their proliferation rate in the presence of bFGF, bFGF/PDGF or bFGF/IGF. Our results indicate that inhibition of the progression of the cell cycle of OPcs by aTf, even in the presence of PDGF, leads to an early beginning of the differentiation program, evaluated by different maturation markers (O4, GC and MBP) and by morphological criteria. The modulation by aTf of the response of OPcs to PDGF supports the idea that this glycoprotein might act as a key regulator of the OLGc lineage progression.

Overexpression of human transferrin in two oligodendroglial cell lines enhances their differentiation

We have previously demonstrated that the addition of apotransferrin (aTf) to oligodendroglial cell (OLGc) primary cultures accelerates their maturation. Cells treated with aTf developed a multipolar morphology and displayed increased expression of mature OLGc markers. In this work, we studied the effect of Tf overexpression in two OLGc lines, N19 and N20.1. The former cells exhibit characteristics of OLGc precursors (O2A), while N20.1 cells express markers of more mature OLGcs. Using the complete cDNA of the human Tf gene, we obtained clones overexpressing Tf in both cell lines. These clones were evaluated for the expression of OLGc differentiation markers. In agreement with our previous results, we found that in the cells overexpressing Tf, there was an increased O(4), GC, and MBP immunoreactivity. To study the myelinogenic potential of these cells, we co-cultured N19 and N20.1 Tf-transfected cells together with cortical neurons. There was a dramatic increase in the morphological differentiation of the OLGcs accompanied by enhanced GC and MBP expression. The OLGcs appeared to establish contact with neurites and extend their processes along them. Only two MBP isoforms were detected in Tf-overexpressing clones, while all the isoforms were present in the co-cultures, suggesting that there was a modulation of MBP expression by neurons. Concomitantly, we found an increase in several proteins involved in axon-glia interaction, such as MAG, N-CAM, and F3/Contactin. This co-culture system represents a potentially powerful tool to study neuron-glia interactions that occur during myelinogenesis and the role of Tf in this process.

Expression of myelin basic protein in two oligodendroglial cell lines is modulated by apotransferrin through different transcription factors

We have shown that apotransferrin (aTf) promotes the differentiation of two oligodendroglial cell (OLGc) lines, N19 and N20.1, representing different stages of OLGc maturation. Although in both cell lines aTf promoted myelin basic protein (MBP) expression, an increase in cAMP levels and CREB phosphorylation was observed only in the less mature cells (N19), suggesting that the maturation induced by aTf is achieved probably through different signaling pathways. We transfected both cell lines with the proximal region of the human MBP promoter fused to the lacZ reporter gene. In both transfected cell lines, addition of aTf produced an activation of the promoter. To elucidate the mechanisms involved in this action, Western blot analysis, EMSAs, and RT-PCR were performed for different transcription factors involved in mbp regulation. In the N20.1 line, treatment with aTf increased the expression and the DNA-binding capacity of thyroid hormone (TH) receptors, Sp1, and nuclear factor-kappaB (NFkappaB). For these cells we found that an inductor of NFkappaB (tumor necrosis factor-alpha) promoted MBP messenger synthesis, whereas mithramycin, a specific inibitor of Sp1, and a cAMP analog (db-cAMP) inhibited its transcription. In the N19 cell line, aTf stimulated NF-I and NFkappaB activation, but, aside from aTf, only db-cAMP induced mbp transcription. These data suggest that, depending on the OLGc maturational stage, aTf modulates MBP expression and OLGc differentiation through different signaling pathways and different transcription factors.

Remyelination after cuprizone-induced demyelination in the rat is stimulated by apotransferrin

Twenty-one-day-old Wistar rats were fed a diet containing 0.6% cuprizone for 2 weeks. Studies carried out after withdrawal of cuprizone showed histological evidences of marked demyelination in the corpus callosum. Biochemical studies of isolated myelin showed a marked decrease in myelin proteins, phospholipids, and galactocerebrosides as well as a marked decrease in myelin yield. Treatment of these animals with a single intracranial injection of 350 ng of apotransferrin at the time of withdrawal of cuprizone induced a marked increase in myelin deposition resulting in a significantly improved remyelination, evaluated by histological, immunocytochemical, and biochemical parameters, in comparison to what was observed in spontaneous recovery. Immunocytochemical studies of cryotome sections to analyze developmental parameters of the oligodendroglial cell population at the time of termination of cuprizone and at different times thereafter showed that in the untreated animals, there was a marked increase in the number of NG2-BrdU-positive precursor cells together with a marked decrease in MBP expression at the peak of cuprizone-induced demyelination. As expected, the amount of precursor cells decreased markedly during spontaneous remyelination and was accompanied by an increase in MBP reactivity. In the apotransferrin-treated animals, these phenomena occurred much faster, and remyelination was much more efficient than in the untreated controls. The results of this study suggest that apotransferrin is a very active promyelinating agent which could be important for the treatment of certain demyelinating conditions.

Apotransferrin promotes the differentiation of two oligodendroglial cell lines

We have previously shown that addition of apotransferrin (aTf) accelerates maturation of oligodendroglial cells (OLGcs) in primary cultures. In this work, we examined the effect of aTf on two conditionally immortalized cell lines: N19 and N20.1. These cells proliferate at 34 degrees C and differentiate into mature OLGcs at 39 degrees C. In vitro addition of aTf to both cell lines at the differentiation temperature for 7 days showed increased expression of galactocerebroside, O4, and myelin basic protein (MBP) and a drop in the percentage of BrdU+ cells. The effect on MBP expression was particularly interesting in the less mature N19 cells. These cells do not express either MBP mRNAs or proteins, so aTf induced, rather than modulated, MBP expression in this cell line. In addition, even though MBP mRNAs for all four isoforms were induced, only the 17 and 21.5 kDa appeared to be translated. OLGc differentiation has been shown to be stimulated by the cAMP-CREB pathway. In N19 cells, following a pulse of aTf, there was a 10-fold increase in cAMP levels accompanied by elevated levels of pCREB. In the more mature N20.1 cells, there were no changes in cAMP levels. We conclude that addition of aTf to immature OLGc lines can enhance their expression of differentiated markers, such as MBP. The action of aTf on MBP gene expression in the least mature line is likely to be mediated by the cAMP pathway. In the N20.1 cells, it appears that different signals and/or mechanisms are involved in modulating myelin lipid and MBP expression. The results suggest that aTf can influence OLGc gene expression and differentiation through multiple mechanisms depending on the maturation of the cell.

Inhibition of the proteasome by lactacystin enhances oligodendroglial cell differentiation

We have used lactacystin, a specific inhibitor of the 26S proteasome, in oligodendroglial cell (OLGc) primary cultures to explore the possible participation of the proteasome-ubiquitin-dependent pathway in the decision of the OLGcs to arrest their proliferation and start differentiation. Addition of lactacystin at various concentrations to cultures containing a majority of OLGc was found to produce their withdrawal from the cell cycle and to induce their biochemical and morphological differentiation, with the appearance of extensive myelin-like sheets. The three classic proteolytic activities of the proteasome were significantly decreased in the lactacystin-treated cultures, and the immunocytochemical analysis showed an increase in the number of O4-, O1-, myelin basic protein-, and myelin proteolipid protein-positive cells and a decrease in A2B5-reacting cells. Quantitative immunochemical evaluation of the expression of certain proteins controlling the cell cycle showed an increase in p27kip1-, cyclin D-, and cdk4-positive cells, with a decrease in cyclin E- and cdk2-positive cells. In the lactacystin-treated OLGcs, there was a dose-dependent decrease in the number of cells incorporating bromodeoxyuridine and in the activity of the complexes cyclin D-cdk4 and cyclin E-cdk2. Furthermore, increased levels of expression of several STAT factors were found, suggesting that proteasome inhibition in OLGcs could stabilize signals of survival and differentiation that might be processed through the JAK/STAT signaling cascade.

Apotransferrin decreases migration and enhances differentiation of oligodendroglial progenitor cells in an in vitro system

We have previously shown that a single intracranial injection of apotransferrin (aTf) in neonatal rats produces an accelerated mylinogenesis and increases the expression of certain myelin proteins such as myelin basic protein (MBP). In the present work, we studied the effects of aTf upon oligodendrocyte progenitor cell (Opc) cultures. In the presence of aTf, cells developed a multipolar morphology and showed an increased expression of O(4), MBP, O(1) and myelin-associated glycoprotein compared to controls. Migration studies using the agarose drop assay showed that aTf strongly inhibited OPc migration. This effect was not observed when an antibody against the transferrin receptor was added. The expression of two cell adhesion molecules, neural cell adhesion molecule (NCAM), N-cadherin and of polysialylated NCAM (PSA-NCAM) was evaluated by immunocytochemistry and by Western blot. Although NCAM expression did not change, there was a significant increase in N-cadherin expression and a decrease in PSA-NCAM in the aTf-treated cells. Time lapse studies of the expression of PSA-NCAM as an indicator of migration and of MBP as a marker of differentiation showed that in the cultures treated with aTf there is first a decrease in the percentage of cells expressing the former molecule which is followed by an increase in the percentage of cells expressing MBP. These results suggest that aTf added in vitro to cultured OPcs inhibits first their migration and then enhances their differentiation.