In vitro and in vivo characterisation of glial cells immortalised with a temperature sensitive SV40 T antigen-containing retrovirus

O-Vanillin Attenuates the TLR2 Mediated Tumor-Promoting Phenotype of Microglia

Malignant gliomas are primary brain tumors with poor prognoses. These tumors are infiltrated by brain intrinsic microglia and peripheral monocytes which promote glioma cell invasion. In our previous studies, we discovered that the activation of Toll-like receptor 2 (TLR2) on microglia/brain macrophages converts them into a protumorigenic phenotype through the induction of matrix metalloproteinases (MMP) 9 and 14. In the present study, we used in vitro and in situ microglia-glioma interaction experimental models to test the impact of a novel inhibitor of TLR 2, ortho vanillin (O-Vanillin) to block TLR2 mediated microglia protumorigenic phenotype. We demonstrate that O-Vanillin inhibits the TLR2 mediated upregulation of MMP 9, MMP 14, IL 6 and iNOS expression. Similarly, the glioma supernatant induced MMP 9 and MMP 14 expression in murine and human microglia is abrogated by O-Vanillin treatment. O-Vanillin is not toxic for microglia, astrocytes or oligodendrocytes. Glioma growth in murine brain slice cultures is significantly reduced after treatment with O-Vanillin, and this reduced glioma growth depends on the presence of microglia. In addition, we also found that O-Vanillin inhibited the glioma induced proliferation of murine primary microglia. In summary, O-Vanillin attenuates the pro-tumorigenic phenotype of microglia/brain macrophages and thus qualifies as a candidate for glioma therapy.

Non-steroidal anti-inflammatory drug indometacin enhances endogenous remyelination

Multiple sclerosis is the most frequent demyelinating disease in the CNS that is characterized by inflammatory demyelinating lesions and axonal loss, the morphological correlate of permanent clinical disability. Remyelination does occur, but is limited especially in chronic disease stages. Despite effective immunomodulatory therapies that reduce the number of relapses the progressive disease phase cannot be prevented. Therefore, promotion of neuroprotective and repair mechanisms, such as remyelination, represents an attractive additional treatment strategy. A number of pathways have been identified that may contribute to impaired remyelination in MS lesions, among them the Wnt/β-catenin pathway. Here, we demonstrate that indometacin, a non-steroidal anti-inflammatory drug (NSAID) that has been also shown to modulate the Wnt/β-catenin pathway in colorectal cancer cells promotes differentiation of primary human and murine oligodendrocytes, myelination of cerebellar slice cultures and remyelination in cuprizone-induced demyelination. Our in vitro experiments using GSK3β inhibitors, luciferase reporter assays and oligodendrocytes expressing a mutant, dominant stable β-catenin indicate that the mechanism of action of indometacin depends on GSK3β activity and β-catenin phosphorylation. Indometacin might represent a promising treatment option to enhance endogenous remyelination in MS patients.

Glioma-derived versican promotes tumor expansion via glioma-associated microglial/macrophages Toll-like receptor 2 signaling

BACKGROUND

Accumulation and infiltration of microglia/brain macrophages around and into glioma tissue promote tumor invasion and expansion. One tumor-promoting mechanism of microglia/brain macrophages is upregulation of membrane type 1 matrix metalloprotease (MT1-MMP), which promotes the degradation of extracellular matrix. MT1-MMP upregulation is induced by soluble factors released by glioma cells activating microglial Toll-like receptor 2 (TLR2).

METHODS

Versican identified by proteomics was silenced in glioma cells by short interference RNA and short hairpin RNA approaches and studied in vitro and after injection into mouse brains or organotypic brain slices.

RESULTS

The splice variants V0/V1 of the endogenous TLR2 ligand versican are highly expressed in mouse and human glioma tissue. Versican-silenced gliomas induced less MT1-MMP expression in microglia both in vitro and in vivo, which resulted in smaller tumors and longer survival rates as compared with controls. Recombinant versican V1 induced significantly higher levels of MT1-MMP in wild-type microglia compared with untreated and treated TLR2 knockout microglial cells. Using glioma-injected organotypic brain slices, we found that the impact of versican signaling on glioma growth depended on the presence of microglia. Moreover, we found that TLR2 expression is upregulated in glioma-associated microglia but not in astrocytes. Additionally, an established TLR2 neutralizing antibody reduced glioma-induced microglial MT1-MMP expression as well as glioma growth ex vivo.

CONCLUSIONS

Our results show that versican released from glioma promotes tumor expansion through glioma-associated microglial/macrophage TLR2 signaling and subsequent expression of MT1-MMP. This signaling cascade might be a novel target for glioma therapies.

A critical role for the cholesterol-associated proteolipids PLP and M6B in myelination of the central nervous system

The formation of central nervous system myelin by oligodendrocytes requires sterol synthesis and is associated with a significant enrichment of cholesterol in the myelin membrane. However, it is unknown how oligodendrocytes concentrate cholesterol above the level found in nonmyelin membranes. Here, we demonstrate a critical role for proteolipids in cholesterol accumulation. Mice lacking the most abundant myelin protein, proteolipid protein (PLP), are fully myelinated, but PLP-deficient myelin exhibits a reduced cholesterol content. We therefore hypothesized that "high cholesterol" is not essential in the myelin sheath itself but is required for an earlier step of myelin biogenesis that is fully compensated for in the absence of PLP. We also found that a PLP-homolog, glycoprotein M6B, is a myelin component of low abundance. By targeting the Gpm6b-gene and crossbreeding, we found that single-mutant mice lacking either PLP or M6B are fully myelinated, while double mutants remain severely hypomyelinated, with enhanced neurodegeneration and premature death. As both PLP and M6B bind membrane cholesterol and associate with the same cholesterol-rich oligodendroglial membrane microdomains, we suggest a model in which proteolipids facilitate myelination by sequestering cholesterol. While either proteolipid can maintain a threshold level of cholesterol in the secretory pathway that allows myelin biogenesis, lack of both proteolipids results in a severe molecular imbalance of prospective myelin membrane. However, M6B is not efficiently sorted into mature myelin, in which it is 200-fold less abundant than PLP. Thus, only PLP contributes to the high cholesterol content of myelin by association and co-transport.

Self-segregation of myelin membrane lipids in model membranes

Rapid conduction of nerve impulses requires coating of axons by myelin sheaths, which are multilamellar, lipid-rich membranes produced by oligodendrocytes in the central nervous system. To act as an insulator, myelin has to form a stable and firm membrane structure. In this study, we have analyzed the biophysical properties of myelin membranes prepared from wild-type mice and from mouse mutants that are unable to form stable myelin. Using C-Laurdan and fluorescence correlation spectroscopy, we find that lipids are tightly organized and highly ordered in myelin isolated from wild-type mice, but not from shiverer and ceramide synthase 2 null mice. Furthermore, only myelin lipids from wild-type mice laterally segregate into physically distinct lipid phases in giant unilamellar vesicles in a process that requires very long chain glycosphingolipids. Taken together, our findings suggest that oligodendrocytes exploit the potential of lipids to self-segregate to generate a highly ordered membrane for electrical insulation of axons.

Highly malignant behavior of a murine oligodendrocyte precursor cell line following transplantation into the demyelinated and nondemyelinated central nervous system

Understanding the basic mechanisms that control CNS remyelination is of direct clinical relevance. Suitable model systems include the analysis of naturally occurring and genetically generated mouse mutants and the transplantation of oligodendrocyte precursor cells (OPCs) following experimental demyelination. However, aforementioned studies were exclusively carried out in rats and little is known about the in vivo behavior of transplanted murine OPCs. Therefore in the present study, we (i) established a model of ethidium bromide-induced demyelination of the caudal cerebellar peduncle (CCP) in the adult mouse and (ii) studied the distribution and marker expression of the murine OPC line BO-1 expressing the enhanced green fluorescent protein (eGFP) 10 and 17 days after stereotaxic implantation. Injection of ethidium bromide (0.025%) in the CCP resulted in a severe loss of myelin, marked astrogliosis, and mild to moderate axonal alterations. Transplanted cells formed an invasive and liquorogenic metastasizing tumor, classified as murine giant cell glioblastoma. Transplanted BO-1 cells displayed substantially reduced CNPase expression as compared to their in vitro phenotype, low levels of MBP and GFAP, prominent upregulation of NG2, PDGFRα, nuclear p53, and an unaltered expression of signal transducer and activator of transcription (STAT)-3. Summarized environmental signaling in the brain stem was not sufficient to trigger oligodendrocytic differentiation of BO-1 cells and seemed to block CNPase expression. Moreover, the lack of the remyelinating capacity was associated with tumor formation indicating that BO-1 cells may serve as a versatile experimental model to study tumorigenesis of glial tumors.

Astrocytes are crucial for survival and maturation of embryonic hippocampal neurons in a neuron-glia cell-insert coculture assay

Synapses represent specialized cell-cell contact sites between nerve cells. These structures mediate the rapid and efficient transmission of signals between neurons and are surrounded by glial cells. Previous investigations have shown that astrocytes are important for the formation, maintenance, and function of CNS synapses. To study effects of glial-derived molecules on synaptogenesis, we have established an in vitro cell-insert coculture system for E18 rat hippocampal neurons and various glial cell types. Neurons were cultured without direct contact with glial cells for distinct time periods. First, it was confirmed that astrocytes are essential to promote survival of E18 hippocampal neurons. Beginning with 10 days in culture, the concurrent expression of pre- and postsynaptic proteins was observed. Moreover, the colocalization of the presynaptic marker Bassoon and the postsynaptic protein ProSAP1/Shank2 indicated the formation of synapses. A technique was developed that permits the semiautomated quantitative determination of the number of synaptic puncta per neuron. The culture system was used to assess effects of pharmacological treatments on synapse formation by applying blockers and activators of small GTPases. In particular, treatment with lysophosphatidic acid enhanced synaptogenesis in the coculture system.

Influence of glial-derived matrix molecules, especially chondroitin sulfates, on neurite growth and survival of cultured mouse embryonic motoneurons

Mechanisms controlling neuronal survival and regeneration play an important role during development, after birth, and under lesion conditions. Isolated embryonic mouse motoneurons have been a useful tool for studying such basic mechanisms. These cultured motoneurons depend on extracellular matrix (ECM) molecules, which are potent mediators of survival and axonal growth and guidance in the CNS and in vitro, exhibiting either attractive or repellent guidance cues. Additionally, ECM proteoglycans and glycoproteins are components of the glial scar acting as a growth barrier for regenerating axons. Compared with CNS axon outgrowth, less is known about the cues that guide motoneurons toward their peripheral targets. Because we are interested in the effects of glial-derived chondroitin sulfate proteoglycans (CSPGs), we have worked out a model system for investigating the influences of glial-derived matrix molecules on motoneuron outgrowth and survival. We used cultured embryonic mouse motoneurons to investigate axon growth effects of matrix molecules produced by the glial-derived cell lines A7, Neu7, and Oli-neu primary astrocytes as well as the immortalized Schwann cell line IMS32. The results indicate that molecules of the ECM, especially chondroitin sulfates, play an important role as axon growth-promoting cues. We could demonstrate a modifying effect of the matrix components on motoneuron survival and caspase3-induced apoptosis.

Invasive drug delivery

The central nervous system is a very attractive target for new therapeutic strategies since many genes involved in neurological diseases are known and often only local low level gene expression is required. However, as the blood brain barrier on one hand prevents some therapeutic agents given systematically from exerting their activity in the CNS, it also provides an immune privileged environment. Neurosurgical technology meanwhile allows the access of nearly every single centre of the CNS and provides the surgical tool for direct gene delivery via minimal invasive surgical approaches to the brain. Successful therapy of the central nervous system requires new tools for delivery of therapeutics in vitro and in vivo (Fig. 1). The application of therapeutic proteins via pumps into the CSF was shown to be only of limited value since the protein mostly is not sufficiently transported within the tissue and the half life of proteins limits the therapeutic success. Direct gene delivery into the host cell has been a main strategy for years, and in the beginning the direct DNA delivery or encapsulation in liposomes or other artificial encapsulation have been applied with different success. For several years the most promising tools have been vectors based on viruses. Viruses are able to use the host cell machinery for protein synthesis, and some of them are able to stably insert into the host cell genome and provide long term transgene expression as long as the cell is alive. The increasing knowledge of viruses and their live cycle promoted the development of viral vectors that function like a shuttle to the cell, with a single round of infection either integrating or transiently expressing the transgene. Viral vectors have proven to be one of the most efficient and stable transgene shuttle into the cell and have gained increasing importance. The limitations of some viral vectors like the adenoviral vector and adeno-associated viral vector have been improved by new constructs like HIV-1 based lentiviral vectors. The immune response caused by expression of viral proteins, or the inability of some viral vectors like the retroviral vector to infect only dividing cells have been overcome by these new constructs. Lentiviral vectors allow an efficient and stable transgene expression over years in vivo without effecting transgene expression or immune response. In this Chapter we will describe synthetic vectors, give an overview of the most common viral vectors and focus our attention on lentiviral vectors, since we consider them to be the most efficient tool for gene delivery in the CNS.

Cell-based delivery of cytokines allows for the differentiation of a doxycycline inducible oligodendrocyte precursor cell line in vitro

BACKGROUND

Stem cells, having the property of self renewal, offer the promise of lifelong repair of damaged tissue. However, somatic tissue-committed primary stem cells are rare and difficult to expand in vitro. Genetically modified stem-like cells with the ability to expand conditionally provide a valuable tool with which to study stem cell biology, especially the cellular events of proliferation and differentiation. In addition, stem cells may be appropriate candidates for therapeutic applications.

METHODS

Double transgenic mice possesing SV40 T antigen (Tag) under the control of the reverse tetracycline-transactivator (rtTA) were used to establish cell lines. One brain cell line was partially characterized by DNA sequencing, morphology, antigen expression using flow cytometry, confocal microscopy, and electrophysiology using the patch clamp technique. Cell cycle analysis was performed using propidium iodide staining; cell viability and H3-thymidine incorporation assays. The ability of this cell line to differentiate was assessed by confocal microscopy following co-culture with stem cells secreting cytokines.

RESULTS

We report here the establishment and partial characterization of a cell line derived from the brain tissue of rtTA-SV40 Tag transgenic mice. Analysis of the morphology and antigen markers has shown that this cell line mimics some aspects of primary glial precursors. The results of electrophysiology are consistent with this and suggest that the cell line is derived from O2A glial precursor cells. Cell cycle progression of this cell line is doxycycline-dependent. In the absence of doxycycline, cells become apoptotic. Differentiation into mature type 2 astrocytes and (precursor) oligodendrocytes can be induced upon withdrawal of doxycycline and addition of epithelial stem cells secreting cytokine, such as hIL3 (human Interleukine 3) or hIL6 to the culture. In contrast, co-culturing with hCNTF (human Ciliary NeuroTrophic Factor)-secreting epithelial stem cells did not induce them to mature into progeny cell types.

CONCLUSION

The differentiation of this O2A glial precursor line does not occur automatically in culture. Additional external help is required from the cell-based delivery of appropriate transgenic cytokines. Withdrawal of doxycycline from the culture medium removes the proliferation signals and induces a fatal outcome.

The AN2 protein is a novel marker for the Schwann cell lineage expressed by immature and nonmyelinating Schwann cells

The expression of the 330 kDa AN2 glycoprotein was studied in the rodent peripheral nervous system. AN2 is expressed by immature Schwann cells in vitro and in vivo and downregulated as the cells upregulate myelin genes. A subpopulation of nonmyelinating Schwann cells in the adult sciatic nerve retains expression of AN2. In rat sciatic nerve crushes, where Schwann cell numbers increase after initial axonal loss and markers of immature Schwann cells show an upregulation, no increased expression of AN2 was observed. In contrast, AN2 expression was upregulated in nerves from peripheral myelin protein-22-transgenic rats, where immature Schwann cells expand without axonal loss. Furthermore, coculture with neurons upregulated AN2 expression on Schwann cells in vitro. Polyclonal antibodies against AN2 inhibited the migration of an immortalized Schwann cell clone in an in vitro migration assay, and the purified AN2 protein was shown to be neither inhibitory nor permissive for outgrowing dorsal root ganglion neurites. AN2 is thus a novel marker for the Schwann cell lineage. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of purified AN2 from early postnatal mouse brain demonstrated that AN2 is the murine homolog of the rat NG2 proteoglycan.

Isolation of enteric glia and establishment of transformed enteroglial cell lines from the myenteric plexus of adult rat

Although enteroglial cells (EGCs) may play a key role in the inflammatory response of the enteric nervous system, little is known about their immunophysiological properties. To facilitate further characterization of enteric glia, we have developed a novel method to isolate and purify EGCs from the myenteric plexus. Myenteric plexus preparations were enzymatically dissociated and EGCs purified by complement-mediated cytolysis of contaminating cells and transformed by retroviral gene transfer. Primary and transformed cells were characterized immunohistochemically and by dot-blot analysis. Functionally, c-fos mRNA expression was assessed in primary and transformed enteroglial cells. All cells displayed robust glial fibrillary acidic protein, S-100 and vimentin immunoreactivities, but no Thy-1.1, desmin, smooth muscle alpha-actin or C3 complement receptor immunoreactivity. This confirmed their enteroglial lineage and excluded contamination with other cell types. Both primary and transformed EGCs displayed little constitutive c-fos mRNA expression. This, however, could be upregulated by various stimuli, including proinflammatory cytokines. In summary, we present a novel method to purify EGCs from rat myenteric plexus for tissue culture and to establish transformed EGC lines that retain their glial nature and functional properties. Such cell lines are now available for physiological studies of the functional properties of enteric glia in vitro.

Cyclic AMP regulates PDGF-stimulated signal transduction and differentiation of an immortalized optic-nerve-derived cell line

To facilitate the study of the molecular events underlying the development of optic-nerve-derived oligodendrocytes and their growth-factor-related signal transduction events, we immortalized perinatal rat optic nerve cells with a temperature-sensitive simian virus 40 large T-antigen, carrying the tsA58 and U19 mutations, via a retrovirus vector. The line, tsU19-9, was selected on the basis of the expression of the neural precursor marker nestin. At the permissive temperature, 33 degreesC, tsU19-9 cells had a flat epithelial morphology. In contrast, following exposure to platelet-derived growth factor (PDGF), a factor important in the lineage progression of oligodendrocytes, or in the presence of dibutyryl cyclic AMP at 39 degreesC (the non-permissive temperature), the cells underwent morphological and antigenic differentiation to cells characteristic of the oligodendrocyte lineage. We used this cell line to investigate the binding characteristics of PDGF and related signalling cascades. Competition binding, phosphoinositide hydrolysis and intracellular Ca2+ mobilization assays all demonstrated that the three different isoforms of PDGF (AA, AB and BB) bound to and acted on the cell line. Overnight exposure to forskolin, a treatment that initiated morphological and phenotypic progression into an oligodendrocyte lineage, decreased PDGF-BB-induced intracellular Ca2+ mobilization and inhibited basal and PDGF-stimulated [3H]thymidine incorporation. Our results demonstrate that tsU19-9 may serve as a resource to study early optic-nerve oligodendrocyte development.

Novel pluripotential neural progenitor lines exhibiting rapid controlled differentiation to neurotransmitter receptor-expressing neurons and glia

The immortalization of progenitor cells from embryonic murine hippocampus using oncogene-carrying retroviral vectors is described. Use of a vector encoding the oncogene v-myc results in lines of nestin-positive progenitor cells. Limited differentiation ensues if the cells are cultured in the presence of dibutyryl cyclic adenosine monophosphate. In contrast, use of a vector in which the extracellular portion of the epidermal growth factor (EGF) receptor is fused to the neu tyrosine kinase generates lines of pluripotential nestin-positive progenitor cells, which differentiate upon withdrawal of EGF into neurons and glia. Differentiated neurons expressing action potentials and neurotransmitter receptors make up a high proportion of the cells. These cell lines are useful tools to investigate the characteristics of differentiating neurons and glia, as well as to screen neuroactive drugs. This work has been reported in preliminary form as an abstract (1996 Society for Neuroscience Abstract, #606.20, p. 1537).

Immune response to murine cell lines of glial origin transplanted into the central nervous system of adult mice

Temperature-sensitive simian virus 40 (SV40) T antigen-transformed central nervous system (CNS)-derived murine cell lines were used to analyse the host response to transplantation in the mouse adult brain. The cell lines were shown to be susceptible to immune recognition in vitro by cytotoxic effector cells indicating that tissue-specific privilege was not in operation. Histological examination at time points post-implantation showed characteristic responses similar to those seen during graft rejection. Astrocytosis and up-regulation of major histocompatibility complex (MHC) class I and MHC class II activation of resident microglia and recruitment of macrophages were observed in both allogeneic and syngeneic hosts 10 days post-implantation suggesting a trauma-induced response. However, the response in allogeneic hosts was more widespread and evident when the syngeneic responses had returned to normal levels. Evidence of T-cell infiltration was also more pronounced in the allogeneic hosts. Despite quite extensive host reactions to these cellular grafts at early time-points the implants appeared to survive in the host CNS long after the responses had abated and could be detected at the maximum time-point studied of 40 days.

Immortalization and characterization of rat microglial cells

Microglial cell lines from rat brain were established by transfer of a temperature sensitive simian virus 40 large tumour antigen by means of a retrovirus. Four weeks after infection, colonies were generated in the presence of neomycin and granulocyte-macrophage colony stimulating factor (GM-CSF), and subsequently subcloned. Both bulk cell lines and clones proliferate actively at 33 degrees C, whereas the rate of division was significantly decreased at 39 degrees C when the large T antigen is non-functional. At 39 degrees C, these cells take on the microglial phenotype as demonstrated by immunoreactivity to ED-1 (an intracellular antigen), OX-42 (complement type 3 receptor), W3/25 (CD4 homologue), OX-6 (MHC class II antigen) and OX-18 (MHC class I antigen). These cells are capable of active phagocytosis and retain these properties for 10-15 passages. Long-term culture of these lines and clones, greater than 15 passages, displayed a gradual down-regulation of all cell surface specific antigens that were not rescued by lipopolysaccharide (LPS), interferon-gamma (gamma-IFN), GM-CSF or colony-stimulating factor-1 (CSF-1). The expression of the SV-40 large T antigen was unaffected. These results demonstrate the feasibility of immortalizing short-term cell lines with the SV-40 large T antigen for their use in the characterization of microglial properties.

Myelinogenic potential of an immortalized oligodendrocyte cell line

The myelinogenic potential of an oligodendrocyte cell line (N20.1) immortalized by transformation with a temperature-sensitive retrovirus (Verity et al., J Neurochem 60:577-587, 1993) has been evaluated in a co-culture system utilizing dorsal root ganglion neurons. When N20.1 cells were placed in co-culture with dorsal root ganglion neurons at 39 degrees C, the temperature at which TAg expression is decreased relative to that in cells maintained at 34 degrees C, there was a dramatic decrease in the N20.1 proliferation rate compared to cells maintained in the absence of neurons at either temperature. This decrease in proliferation was observed within 3 days of co-culture and appeared to precede a further decrease in TAg expression that occurred with time in response to the neurons. In co-cultures the immunoreactivity of N20.1 cells for galactocerebroside increased with time, and the cells appeared to establish contact with neurites and initiate formation of membranous sheets. When the duration of co-culture was extended to 52 days, myelin-like figures were noted by electron microscopy. Thus, the extent of N20.1 differentiation is dependent on the presence of neurons and the duration of co-culture. This culture system represents a potentially powerful tool for the study of neuronal-glial interactions influencing myelinogenesis and remyelination.

Nervous system modification by transplants and gene transfer

New possibilities to modify function and direct repair in the central nervous system (CNS) have been established by the merger of gene transfer technology with neural transplantation. Rapid advances in viral-mediated DNA-delivery procedures permit the study of novel gene expression in neurons and glial cells. Foreign genes, transferred by a virus vector, can be used to generate new cell lines, identify transplanted cells, and express growth factors or enzymes for neurotransmitter synthesis. In addition to CNS cell types, non-neural cells are also being studied with transgene technology in the nervous system. Functional effects have been obtained with grafts of genetically modified cells in animal models of several nervous system disorders, and the recent results set the stage for potential application of these techniques to human CNS gene therapy.