Oligodendroglial maturation is dependent on intracellular protein shuttling.

Multiple sclerosis is an autoimmune disease of the CNS resulting in degeneration of myelin sheaths and loss of oligodendrocytes, which means that protection and electrical insulation of axons and rapid signal propagation are impaired, leading to axonal damage and permanent disabilities. Partial replacement of lost oligodendrocytes and remyelination can occur as a result of activation and recruitment of resident oligodendroglial precursor cells. However, the overall remyelination capacity remains inefficient because precursor cells often fail to generate new oligodendrocytes. Increasing evidence points to the existence of several molecular inhibitors that act on these cells and interfere with their cellular maturation. The p57kip2 gene encodes one such potent inhibitor of oligodendroglial differentiation and this study sheds light on the underlying mode of action. We found that subcellular distribution of the p57kip2 protein changed during differentiation of rat, mouse, and human oligodendroglial cells both in vivo and in vitro. Nuclear export of p57kip2 was correlated with promoted myelin expression, higher morphological phenotypes, and enhanced myelination in vitro. In contrast, nuclear accumulation of p57kip2 resulted in blocked oligodendroglial differentiation. Experimental evidence suggests that the inhibitory role of p57kip2 depends on specific interactions with binding proteins such as LIMK-1, CDK2, Mash1, and Hes5 either by controlling their site of action or their activity. Because functional restoration in demyelinating diseases critically depends on the successful generation of oligodendroglial cells, a therapeutic need that is currently unmet, the regulatory mechanism described here might be of particular interest for identifying suitable drug targets and devising novel therapeutic approaches.

Immunoglobulins stimulate cultured Schwann cell maturation and promote their potential to induce axonal outgrowth.

BACKGROUND: Schwann cells are the myelinating glial cells of the peripheral nervous system and exert important regenerative functions revealing them as central repair components of many peripheral nerve pathologies. Intravenous immunoglobulins (IVIG) are widely used to treat autoimmune and inflammatory diseases including immune-mediated neuropathies. Nevertheless, promotion of peripheral nerve regeneration is currently an unmet therapeutical goal. We therefore examined whether immunoglobulins affect glial cell homeostasis, differentiation, and Schwann cell dependent nerve regenerative processes. METHODS: The responses of different primary Schwann cell culture models to IVIG were investigated: immature or differentiation competent Schwann cells, myelinating neuron/glial cocultures, and dorsal root ganglion explants. Immature or differentiating Schwann cells were used to study cellular proliferation, morphology, and gene/protein expression. Myelination rates were determined using myelinating neuron/glia cocultures, whereas axonal outgrowth was assessed using non-myelinating dorsal root ganglion explants. RESULTS: We found that IVIG specifically bind to Schwann cells and detected CD64 Fc receptor expression on their surface. In response to IVIG binding, Schwann cells reduced proliferation rates and accelerated growth of cellular protrusions. Furthermore, we observed that IVIG treatment transiently boosts myelin gene expression and myelination-related signaling pathways of immature cells, whereas in differentiating Schwann cells, myelin expression is enhanced on a long-term scale. Importantly, myelin gene upregulation was not detected upon application of IgG1 control antibodies. In addition, we demonstrate for the first time that Schwann cells secrete interleukin-18 upon IVIG stimulation and that this cytokine instructs these cells to promote axonal growth. CONCLUSIONS: We conclude that IVIG can positively influence the Schwann cell differentiation process and that it enhances their regenerative potential.

Molecules involved in the crosstalk between immune- and peripheral nerve Schwann cells.

Schwann cells are the myelinating glial cells of the peripheral nervous system and establish myelin sheaths on large caliber axons in order to accelerate their electrical signal propagation. Apart from this well described function, these cells revealed to exhibit a high degree of differentiation plasticity as they were shown to re- and dedifferentiate upon injury and disease as well as to actively participate in regenerative- and inflammatory processes. This review focuses on the crosstalk between glial- and immune cells observed in many peripheral nerve pathologies and summarizes functional evidences of molecules, regulators and factors involved in this process. We summarize data on Schwann cell's role presenting antigens, on interactions with the complement system, on Schwann cell surface molecules/receptors and on secreted factors involved in immune cell interactions or para-/autocrine signaling events, thus strengthening the view for a broader (patho) physiological role of this cell lineage.

Human endogenous retrovirus type W envelope protein inhibits oligodendroglial precursor cell differentiation.

OBJECTIVE: Differentiation of oligodendroglial precursor cells is crucial for central nervous system remyelination and is influenced by both extrinsic and intrinsic factors. Recent studies showed that human endogenous retrovirus type W (HERV-W) contributes significantly to brain damage. In particular, its envelope protein ENV can mediate injury to specific cell types of the brain and immune system. Here, we investigated whether ENV protein affects oligodendroglial differentiation. METHODS: Immunostaining and gene expression analyses were performed to establish the expression and regulation of the known ENV receptor, Toll-like receptor 4 (TLR4), on oligodendroglial precursor cells in human brain tissue and in culture. Cultured primary oligodendroglial precursor cells were stimulated with ENV protein to determine the effects of this ligand/receptor interaction. RESULTS: We demonstrated that the ENV protein is present in close proximity to TLR4-expressing oligodendroglial precursor cells adjacent to multiple sclerosis lesions. Human and rat oligodendroglial precursor cells expressed TLR4, and the ENV-mediated activation of TLR4 led to the induction of proinflammatory cytokines and inducible nitric oxide synthase as well as the formation of nitrotyrosine groups and a subsequent reduction in myelin protein expression. INTERPRETATION: Our findings suggest that ENV-mediated induction of nitrosative stress via activation of TLR4 results in an overall reduction of the oligodendroglial differentiation capacity, thereby contributing to remyelination failure. Therefore, pharmacological or antibody-mediated inhibition of ENV may prevent the blockade of myelin repair in the diseased or injured central nervous system.

Histone methyltransferase enhancer of zeste homolog 2 regulates Schwann cell differentiation.

Epigenetic control is crucial for the differentiation of a variety of cells including oligodendrocytes, the myelinating glial cells of the central nervous system. However, studies about the implication of epigenetic factors in peripheral nervous system maturation are just emerging. Here, we demonstrate for the first time the impact of a histone methyltransferase, encoded by the enhancer of zeste homolog 2 (EZH2) gene, on Schwann cell differentiation. In sciatic nerves, EZH2 expression was found in Schwann cells and to peak perinatally. Suppression of EZH2 expression in cultured primary rat Schwann cells reduced the length of cell processes. These morphological changes were accompanied by widespread alterations in the gene expression pattern, including downregulation of myelin genes and induction of p57kip2, which we have recently identified as an intrinsic inhibitory regulator of Schwann cell maturation. In addition, we show that EZH2 suppression in dorsal root ganglion cocultures interferes with in vitro myelination. Chromatin immunoprecipitation analysis revealed binding of EZH2 at the p57kip2 promoter and reduction of histone H3K27 trimethylation upon gene suppression. EZH2 suppression-dependent effects on morphology and myelin genes could be reversed by concomitant suppression of p57kip2, indicating that p57kip2 is a downstream effector of EZH2. Furthermore, we describe Hes5 as transcriptional repressor of myelin genes in Schwann cells, which was induced upon EZH2 suppression and downregulated in p57kip2-suppressed Schwann cells. Therefore, we have identified a molecular link between histone methylation and control of Schwann cell differentiation and demonstrate that this epigenetic mechanism is crucial for glial differentiation to proceed.

Fingolimod induces the transition to a nerve regeneration promoting Schwann cell phenotype.

Successful regeneration of injured peripheral nerves is mainly attributed to the plastic behavior of Schwann cells. Upon loss of axons, these cells trans-differentiate into regeneration promoting repair cells which provide trophic support to regrowing axons. Among others, activation of cJun was revealed to be involved in this process, initiating the stereotypic pattern of Schwann cell phenotype alterations during Wallerian degeneration. Nevertheless, the ability of Schwann cells to adapt and therefore the nerve's potential to regenerate can be limited in particular after long term denervation or in neuropathies leading to incomplete regeneration only and thus emphasizing the need for novel therapeutic approaches. Here we stimulated primary neonatal and adult rat Schwann cells with Fingolimod/FTY720P and investigated its impact on the regeneration promoting phenotype. FTY720P activated a number of de-differentiation markers including cJun and interfered with maturation marker and myelin expression. Functionally, FTY720P treated Schwann cells upregulated growth factor expression and these cells enhanced dorsal root ganglion neurite outgrowth on inhibitory substrates. Our results therefore provide strong evidence that FTY720P application supports the generation of a repair promoting cellular phenotype and suggest that Fingolimod could be used as treatment for peripheral nerve injuries and diseases.

Mesenchymal stem cell conditioning promotes rat oligodendroglial cell maturation.

Oligodendroglial progenitor/precursor cells (OPCs) represent the main cellular source for the generation of new myelinating oligodendrocytes in the adult central nervous system (CNS). In demyelinating diseases such as multiple sclerosis (MS) myelin repair activities based on recruitment, activation and differentiation of resident OPCs can be observed. However, the overall degree of successful remyelination is limited and the existence of an MS-derived anti-oligodendrogenic milieu prevents OPCs from contributing to myelin repair. It is therefore of considerable interest to understand oligodendroglial homeostasis and maturation processes in order to enable the development of remyelination therapies. Mesenchymal stem cells (MSC) have been shown to exert positive immunomodulatory effects, reduce demyelination, increase neuroprotection and to promote adult neural stem cell differentiation towards the oligodendroglial lineage. We here addressed whether MSC secreted factors can boost the OPC's oligodendrogenic capacity in a myelin non-permissive environment. To this end, we analyzed cellular morphologies, expression and regulation of key factors involved in oligodendroglial fate and maturation of primary rat cells upon incubation with MSC-conditioned medium. This demonstrated that MSC-derived soluble factors promote and accelerate oligodendroglial differentiation, even under astrocytic endorsing conditions. Accelerated maturation resulted in elevated levels of myelin expression, reduced glial fibrillary acidic protein expression and was accompanied by downregulation of prominent inhibitory differentiation factors such as Id2 and Id4. We thus conclude that apart from their suggested application as potential anti-inflammatory and immunomodulatory MS treatment, these cells might also be exploited to support endogenous myelin repair activities.

Interaction of the periplasmic PratA factor and the PsbA (D1) protein during biogenesis of photosystem II in Synechocystis sp. PCC 6803.

The biogenesis of photosynthetic complexes is assisted by a growing number of trans-acting factors in both chloroplasts and cyanobacteria. We have previously shown that the periplasmic PratA factor from Synechocystis sp. PCC 6803 (Synechocystis 6803) is required for adequate C-terminal processing of the PsbA (D1) subunit of photosystem II (PSII) supporting the idea that the early steps of PSII assembly occur at the plasma membrane. Here we report on the molecular analysis of the interaction between PratA and the D1 protein. Both yeast two-hybrid and glutathione S-transferase pulldown assays revealed that PratA binds to the soluble forms of both mature and precursor D1 C-terminal regions. In agreement with that finding, the binding region was mapped to amino acid positions 314-328 of D1 by applying a peptide-scanning approach. Approximately 10-20% of the soluble PratA factor was found to be associated with membranes in a D1-dependent manner. Sucrose density gradient centrifugations allowed the identification of a specific membrane subfraction that contains both PratA and D1 and which might represent a transfer and/or connecting region between plasma and thylakoid membrane. Imaging data obtained with enhanced cyan fluorescent protein-labeled D1 protein in wild-type and pratA mutant backgrounds further supported this notion.