The Polarity Protein Scribble Regulates Myelination and Remyelination in the Central Nervous System

The development and regeneration of myelin by oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), requires profound changes in cell shape that lead to myelin sheath initiation and formation. Here, we demonstrate a requirement for the basal polarity complex protein Scribble in CNS myelination and remyelination. Scribble is expressed throughout oligodendroglial development and is up-regulated in mature oligodendrocytes where it is localised to both developing and mature CNS myelin sheaths. Knockdown of Scribble expression in cultured oligodendroglia results in disrupted morphology and myelination initiation. When Scribble expression is conditionally eliminated in the myelinating glia of transgenic mice, myelin initiation in CNS is disrupted, both during development and following focal demyelination, and longitudinal extension of the myelin sheath is disrupted. At later stages of myelination, Scribble acts to negatively regulate myelin thickness whilst suppressing the extracellular signal-related kinase (ERK)/mitogen-activated protein kinase (MAP) kinase pathway, and localises to non-compact myelin flanking the node of Ranvier where it is required for paranodal axo-glial adhesion. These findings demonstrate an essential role for the evolutionarily-conserved regulators of intracellular polarity in myelination and remyelination.     

Cellular Mechanism of Myelination in the Central Nervous System

A study of myelination with electron microscopy has been carried out on the spinal cord of young rats and cats. In longitudinal and transverse sections the intimate relationship of the growing axons with the oligodendrocytes was observed. Early naked axons appear to be embedded within the cytoplasm and processes of the oligodendrocytes from which they are limited only by the intimately apposed membranes of both elements (axon-oligocytic membrane). In a transverse section several axons are observed to be in a single oligodendrocyte. The process of myelination consists in the laying down, within the cytoplasm of the oligodendrocyte and around the axon, of concentric membranous myelin layers. The first of these layers is deposited at a certain distance (200 to 600 A or more) from the axon-oligocytic membrane. This and all the other subsequently formed membranes have higher electron density and are apparently formed by the coalescence and fusion of vesicles (of 200 to 800 A) and membranes found in large amounts within the cytoplasm of the oligodendrocytes. At an early stage the myelin layers may be discontinuous and some vesicular material may even be trapped among them or between the myelin proper and the axon-oligocytic membrane. Then, when the 8th to 10th layer is deposited, the complete coalescence and alignment of the lamellae leads to the characteristic orderly multilayered organization of the myelin sheath. Myelination in the central nervous system appears to be a process of membrane synthesis within the cytoplasm of the oligodendrocyte and not a result of the wrapping of the plasma membranes as postulated in Geren's hypothesis for the peripheral nerve fibers. The possible participation of Schwann cell cytoplasm in peripheral myelination is now being investigated.     

Effects of Fumaric Acids on Cuprizone Induced Central Nervous System De- and Remyelination in the Mouse

Background

Fumaric acid esters (FAE) are a group of compounds which are currently under investigation as an oral treatment for relapsing-remitting multiple sclerosis. One of the suggested modes of action is the potential of FAE to exert a neuroprotective effect.

Methodology/Principal Findings

We have investigated the impact of monomethylfumarate (MMF) and dimethylfumaric acid (DMF) on de- and remyelination using the toxic cuprizone model where the blood-brain-barrier remains intact and only scattered T-cells and peripheral macrophages are found in the central nervous system (CNS), thus excluding the influence of immunomodulatory effects on peripheral immune cells. FAE showed marginally accelerated remyelination in the corpus callosum compared to controls. However, we found no differences for demyelination and glial reactions in vivo and no cytoprotective effect on oligodendroglial cells in vitro. In contrast, DMF had a significant inhibitory effect on lipopolysaccharide (LPS) induced nitric oxide burst in microglia and induced apoptosis in peripheral blood mononuclear cells (PBMC).

Conclusions

These results contribute to the understanding of the mechanism of action of fumaric acids. Our data suggest that fumarates have no or only little direct protective effects on oligodendrocytes in this toxic model and may act rather indirectly via the modulation of immune cells.     

Gas6 Promotes Oligodendrogenesis and Myelination in the Adult Central Nervous System and After Lysolecithin-Induced Demyelination

A key aim of therapy for multiple sclerosis (MS) is to promote the regeneration of oligodendrocytes and remyelination in the central nervous system (CNS). The present study provides evidence that the vitamin K-dependent protein growth arrest specific 6 (Gas6) promotes such repair in in vitro cultures of mouse optic nerve and cerebellum. We first determined expression of Gas6 and TAM (Tyro3, Axl, Mer) receptors in the mouse CNS, with all three TAM receptors increasing in expression through postnatal development, reaching maximal levels in the adult. Treatment of cultured mouse optic nerves with Gas6 resulted in significant increases in oligodendrocyte numbers as well as expression of myelin basic protein (MBP). Gas6 stimulation also resulted in activation of STAT3 in optic nerves as well as downregulation of multiple genes involved in MS development, including matrix metalloproteinase-9 (MMP9), which may decrease the integrity of the blood–brain barrier and is found upregulated in MS lesions. The cytoprotective effects of Gas6 were examined in in vitro mouse cerebellar slice cultures, where lysolecithin was used to induce demyelination. Cotreatment of cerebellar slices with Gas6 significantly attenuated demyelination as determined by MBP immunostaining, and Gas6 activated Tyro3 receptor through its phosphorylation. In conclusion, these results demonstrate that Gas6/TAM signaling stimulates the generation of oligodendrocytes and increased myelin production via Tyro3 receptor in the adult CNS, including repair after demyelinating injury. Furthermore, the effects of Gas6 on STAT3 signaling and matrix MMP9 downregulation indicate potential glial cell repair and immunoregulatory roles for Gas6, indicating that Gas6-TAM signaling could be a potential therapeutic target in MS and other neuropathologies.     

TWEAK and the Central Nervous System

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a member of the tumor necrosis factor superfamily that acts on responsive cells via binding to a cell surface receptor named fibroblast growth factor-inducible 14 (Fn14). TWEAK can regulate numerous cellular responses in vitro and in vivo. Recent studies have indicated that TWEAK and Fn14 are expressed in the central nervous system (CNS), and that in response to a variety of stimuli, including cerebral ischemia, there is an increase in TWEAK and Fn14 expression in perivascular astrocytes, microglia, endothelial cells, and neurons with subsequent increase in the permeability of the blood–brain barrier (BBB) and cell death. Furthermore, there is a growing body of evidence indicating that TWEAK induces the activation of the NF-κB in the CNS with release of proinflammatory cytokines and matrix metalloproteinases. In addition, inhibition of TWEAK activity by either treatment with a Fn14-Fc fusion protein or neutralizing anti-TWEAK antibodies has shown therapeutic efficacy in animal models of ischemic stroke, cerebral edema, and multiple sclerosis.     

Vimentin in the Central Nervous System

Intermediate filament proteins were identified by two-dimensional gel electrophoresis in urea extracts of rat optic nerves undergoing Wallerian degeneration and in cytoskeletal preparations of rat brain and spinal cord during postnatal development. The glial fibrillary acidic (GFA) protein and vimentin were the major optic nerve proteins following Wallerian degeneration. Vimentin was a major cytoskeletal component of newborn central nervous system (CNS) and then progressively decreased until it became barely identifiable in mature brain and spinal cord. The decrease of vimentin occurred concomitantly with an increase in GFA protein. A protein with the apparent molecular weight of 61,000 and isoelectric point of 5.6 was identified in both cytoskeletal preparations of brain and spinal cord, and in urea extracts of normal optic nerves. The protein disappeared together with the polypeptides forming the neurofilament triplet in degenerated optic nerves.     

Alterations in Gene Expression Associated with Primary Demyelination and Remyelination in the Peripheral Nervous System

Primary demyelination is an important component of a number of human diseases and toxic neuropathies. Animal models of primary demyelination are useful for isolating processes involved in myelin breakdown and remyelination because the complicating events associated with axonal degeneration and regeneration are not present. The tellurium neuropathy model has proven especially useful in this respect. Tellurium specifically blocks synthesis of cholesterol, a major component of PNS myelin. The resulting cholesterol deficit in myelin-producing Schwann cells rapidly leads to synchronous primary demyelination of the sciatic nerve, which is followed by rapid synchronous remyelination when tellurium exposure is discontinued. Known alterations in gene expression for myelin proteins and for other proteins involved in the sequence of events associated with demyelination and subsequent remyelination in the PNS are reviewed, and new data regarding gene expression changes during tellurium neuropathy are presented and discussed.     

Riboflavin Homeostasis in the Central Nervous System

Abstract: The mechanisms by which riboflavin, which is not synthesized in mammals, enters and leaves brain, CSF, and choroid plexus were investigated by injecting [¹⁴C]riboflavin intravenously or intraventricularly. Tracer amounts of [¹⁴C]riboflavin with or without FMN were infused intravenously at a constant rate into normal, starved, or probenecid-pretreated rabbits. At 3 h, [¹⁴C]riboflavin readily entered choroid plexus and brain, and, to a much lesser extent, CSF. Over 85% of the [¹⁴C]riboflavin in brain and choroid plexus was present as [¹⁴C]FMN and [¹⁴C]FAD. The addition of 0.2 mmol/kg FMN to the infusate markedly depressed the relative entry of [¹⁴C]riboflavin into brain, choroid plexus, and, less so, CSF, whereas starvation increased the relative entry of [¹⁴C]riboflavin into brain and choroid plexus. After intraventricular injection (2 h), most of the [¹⁴C]riboflavin was extremely rapidly cleared from CSF into blood. Some of the [¹⁴C]riboflavin entered brain, where over 85% of the ¹⁴C was present as [¹⁴C]FMN plus [¹⁴C]FAD. The addition of 1.2³μmol FAD (which was rapidly hydrolyzed to riboflavin) to the injectate decreased the clearance of [¹⁴C]riboflavin from CSF and the phosphorylation of [¹⁴C]riboflavin in brain. Probenecid in the injectate also decreased the clearance of [¹⁴C]riboflavin from CSF. These results show that the control of entry and exit of riboflavin is the mechanism, at least in part, by which total riboflavin levels in brain cells and CSF are regulated. Penetration of riboflavin through the blood-brain barrier, saturable efflux of riboflavin from CSF, and saturable entry of riboflavin into brain cells are three distinct parts of the homeostatic system for total riboflavin in the central nervous system.     

Cytokine Actions in the Central Nervous System

Cytokines and chemokines have been implicated in contributing to the initiation, propagation and regulation of immune and inflammatory responses. Also, these soluble mediators have important roles in contributing to a wide array of neurological diseases such as multiple sclerosis, AIDS Dementia Complex, stroke and Alzheimers disease. Cytokines and chemokines are synthesized within the central nervous system by glial cells and neurons, and have modulatory functions on these same cells via interactions with specific cell-surface receptors. In this article, I will discuss the ability of glial cells and neurons to both respond to, and synthesize, a variety of cytokines. The emphasize will be on three select cytokines; interferon-gamma (IFN-γ), a cytokine with predominantly proinflammatory effects; interleukin-6 (IL-6), a cytokine with both pro- and anti-inflammatory properties; and transforming growth factor-beta (TGF-β), a cytokine with predominantly immunosuppressive actions. The significance of these cytokines to neurological diseases with an immunological component will be discussed.     

Caspase与神经系统疾病

近年来,细胞凋亡发生机制的研究已取得众多进展。研究表明,许多神经系统疾病与caspase家族有着密切联系。现将细胞凋亡的最新研究结果及其与神经系统疾病的关系,尤其是caspase家族在神经系统疾病中的主导地位作简单综述,希望由此了解神经元细胞凋亡的内在机制并达到治疗目的。     

Deoxycytidine Transport and Metabolism in the Central Nervous System

Abstract: The mechanisms by which deoxycytidine enters and leaves brain, choroid plexus, and CSF were investigated by injecting [³H]deoxycytidine intraarterially, intravenously, and intraventricularly. After intracarotid injection of deoxycytidine (1.0 μM) into rats, deoxycytidine did not pass through the blood-brain barrier at a faster rate than sucrose. [³H]Deoxycytidine, either alone or together with unlabeled deoxycytidine, was infused at a constant rate into conscious adult rabbits. At 130 min, [³H]deoxycytidine readily entered CSF, choroid plexus, and brain. In brain, approx. 60% of the nonvolatile radioactivity was attributable to [³H]deoxycytidine phosphates. The addition of 0.22 mmol/kg unlabeled deoxycytidine to the infusion syringe decreased the phosphorylation of [³H]deoxycytidine in brain by approx. 50%; the addition of 2.2 mmol/kg of unlabeled deoxycytidine to the infusion syringe decreased the relative entry of [³H]deoxycytidine into CSF and brain by approx. 50 and 75%, respectively. Two hours after the intraventricular injection of [³H]deoxycytidine, [³H]deoxycytidine was rapidly cleared from CSF, in part, to brain, where approx. 65% of the [³H]deoxycytidine was converted to [³H]deoxycytidine phosphates. The intraventricular injection of unlabeled deoxycytidine with the [³H]deoxycytidine decreased the phosphorylation of [³H]deoxycytidine in the brain significantly and also decreased the clearance of [³H]deoxycytidine from the CSF. These results were interpreted as showing that the entry of deoxycytidine from blood into CSF occurs by a saturable transport system within the choroid plexus. Once within the CSF, the deoxycytidine can enter brain, undergo phosphorylation to deoxycytidine phosphates, and subsequently be incorporated into DNA.     

Uridine Transport and Metabolism in the Central Nervous System

Myelin and myelin-containing (P3) fractions were prepared from human white matter by discontinuous sucrose gradient centrifugation. The myelin isolated from each of the fractions of different densities was morphologically and biochemically distinct. Light myelin fractions consisted of compact, multilamellar myelin, whereas the denser fractions consisted predominantly of loose myelin with fewer lamellae. The amounts of both basic protein and lipophilin (proteolipid protein) were reduced in the denser fractions. In contrast, the high-molecular-weight components were elevated in the dense fractions. The lipid composition was similar in all the fractions studied. Analysis of basic protein by gel electrophoresis at pH 10.6 revealed differences in basic protein microheterogeneity among the fractions. The light myelin fraction was enriched in the more positively charged basic protein components (components 1, 2, and 3), whereas these components were reduced in the denser fractions. Myelin in the dense fractions was enriched in the more modified forms of basic protein (components 6, 7, and 8). The pattern of microheterogeneity was different for basic protein isolated from myelins of a 2-year-old and an adult brain; the former showed fewer components and mainly the most cationic species. On the other hand, the pattern of microheterogeneity of basic protein isolated from the different density gradient fractions was similar for both ages.     

Hypoxanthine Transport and Metabolism in the Central Nervous System

The mechanisms by which hypoxanthine, the principal purine in plasma and CSF, enters and leaves rabbit brain, choroid plexus, and CSF were investigated in the isolated choroid plexus in vitro and by injecting [14C]hypoxanthine intraventricularly and [3H]hypoxanthine intravenously. The isolated choroid plexus accumulated and extensively metabolized [14C]hypoxanthine; however, 14C was readily released from choroid plexus principally as [14C]-hypoxanthine. After infusion of [3H]hypoxanthine intravenously, [3H]hypoxanthine entered CSF and brain slowly and was converted in brain to nucleotides. Fewer than 5% of the acid-soluble purine nucleotides in brain entered rabbit brain from plasma hypoxanthine (and inosine) per 24 h. After intraventricular injection of [14C]hypoxanthine, the [14C]hypoxanthine was cleared from the CSF into the blood or accumulated by brain and largely converted into 14C-nucleotides. Little [14C]xanthine and no [14C]uric acid or allantoin were formed. These studies show that brain, unlike most other tissues, rapidly recycles hypoxanthine and converts it into purine nucleotides, and not unsalvageable purines.