Developmental effects of basic fibroblast growth factor and platelet-derived growth factor on glial cells in a three-dimensional cell culture system

Preparation, maintenance, and use of serum-free aggregating brain cell cultures

Serum-free aggregating brain cell cultures are free-floating three-dimensional primary cell cultures able to reconstitute spontaneously a histotypic brain architecture to reproduce critical steps of brain development and to reach a high level of structural and functional maturity. This culture system offers, therefore, a unique model for neurotoxicity testing both during the development and at advanced cellular differentiation, and the high number of aggregates available combined with the excellent reproducibility of the cultures facilitates routine test procedures. This chapter presents a detailed description of the preparation, maintenance, and use of these cultures for neurotoxicity studies and a comparison of the developmental characteristics between cultures derived from the telencephalon and cultures derived from the whole brain. For culture preparation, mechanically dissociated embryonic brain tissue is used. The initial cell suspension, composed of neural stem cells, neural progenitor cells, immature postmitotic neurons, glioblasts, and microglial cells, is kept in a serum-free, chemically defined medium under continuous gyratory agitation. Spherical aggregates form spontaneously and are maintained in suspension culture for several weeks. Within the aggregates, the cells rearrange and mature, reproducing critical morphogenic events, such as migration, proliferation, differentiation, synaptogenesis, and myelination. For experimentation, replicate cultures are prepared by the randomization of aggregates from several original flasks. The high yield and reproducibility of the cultures enable multiparametric endpoint analyses, including "omics" approaches.

Temporal analysis of growth factor mRNA expression in myelinating rat brain aggregate cultures: increments in CNTF, FGF-2, IGF-I, and PDGF-AA mRNA are induced by antibody-mediated demyelination

Myelinogenesis in rat brain aggregate cultures is associated with a pattern of growth factor mRNA expression comparable to that of the developing brain. The rate of increase in platelet-derived growth factor-AA (PDGF-AA) expression was greatest just before the detection of myelin basic protein (MBP) mRNA in the cultures and remained high thereafter, consistent with in vivo observations. Levels of fibroblast growth factor-2 (FGF-2) and of ciliary neurotrophic factor (CNTF) mRNA increased continuously over the period of MBP accumulation. High rates of transforming growth factor beta1 (TGF-beta1), insulin-like growth factor-I (IGF-I), and neurotrophin-3 (NT-3) expression at early time points during the culture gradually decreased over time, indicative of a key regulatory role during oligodendrocyte development. The addition of demyelinative anti-myelin oligodendrocyte glycoprotein (anti-MOG) antibody resulted in a significant increase in MBP peptide fragments with a C-terminus at phenylalanine 89 indicating proteolytic breakdown of MBP after myelin phagocytosis. Immediately after antibody treatment the expression of CNTF mRNA was significantly increased, compared with controls, while that of FGF-2 and IGF-I, and of PDGF-AA peaked during the early and later stages of recovery respectively. Thus, specific growth factors combine to regulate myelination and remyelination in the aggregates; these data have implications for demyelinating disease in which protective growth factor secretion may be central to regeneration.

Aggregating Brain Cell Cultures as a Model to Study Ischaemia-induced Neurodegeneration

Rotation-mediated aggregating brain cell cultures at two different maturational stages (DIV 11 and DIV 20) were subjected for 1 or 2 hours to ischaemic conditions by transient immobilization (arrest of media circulation). During recovery, cell damage was evaluated by measuring changes in cell type-specific enzyme activities and total protein content. It was found that in immature cultures (DIV 11), immobilization for 1 or 2 hours did not affect the parameters measured. By contrast, at DIV 20, ischaemic conditions for 1 hour caused a pronounced decrease in the activities of glutamic acid decarboxylase and choline acetyltransferase. A significant decrease in these neuron-specific enzyme activities was found at post-ischaemic days 1-14, indicating immediate and irreversible neuronal damage. The activity of the astrocyte-specific enzyme, glutamine synthetase, was significantly increased at 4 days post-treatment; equal to control values at 6 days; and significantly decreased at 14 days after the ischaemic insult. Immobilization of DIV 20 cultures for 2 hours caused a drastic reduction in all the parameters measured at post-ischaemic day 6. Generally, the ischaemic conditions appeared to be more detrimental to neurons than to astrocytes, and GABAergic neurons were more affected than cholinergic neurons.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Macrophages in CNS remyelination: friend or foe?

Hematogenous macrophages and resident brain microglia are agents of demyelination in multiple sclerosis (MS) and paradoxically may also participate in remyelination. In vitro studies have shown that macrophage enrichment of aggregate brain cultures promotes myelination per se and enhances the capacity to remyelinate following a demyelinating episode. It has been hypothesized that remyelination in MS is implemented by surviving dedifferentiated oligodendrocytes or by newly recruited progenitors that migrate, proliferate and synthesize myelin in response to signalling molecules in the local environment. We postulate that macrophage-derived cytokines or growth factors may directly or indirectly promote oligodendroglial proliferation and differentiation, contributing to myelin repair in inflammatory demyelinating disease.

Growth factors and myelin regeneration in multiple sclerosis

Insulin-like growth factor-I (IGF-I), platelet-derived growth factor (PDGF), fibroblast growth factor (FGF) and ciliary neurotrophic factor (CNTF) are multifunctional growth factors which are found in the CNS. Oligodendroglia are the cells that form and maintain myelin sheaths and many in vitro experiments have shown that these growth factors promote the proliferation, differentiation and survival of cells in the oligodendroglial lineage. Since myelin breakdown is often severe in multiple sclerosis (MS), the possibility of growth factor use in the treatment of MS has been considered and recently, IGF-I treatment has been shown to reduce lesion severity and promote myelin regeneration in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. This review briefly summarizes the structural characteristics of these growth factors and the actions which might help reduce oligodendrocyte-myelin sheath injury in MS and promote myelin regeneration.

Long-term bFGF neuronal culture: reintroduction into serum medium yields neurons and non-neuronal cells with neuronal characteristics

The potential use of bFGF immortalized cells as hosts for delivering foreign genes into nervous tissue led us to examine the effect of maintaining, E-18 hippocampal neurons for extended periods in bFGF culture prior to transfer into a standard, serum-containing, medium. We found: (1) many, if not most, precursors seen in bFGF, mature into glia and not into primary neurons after medium exchange; (2) the electrophysiology of the neurons which do mature after medium transfer and replating, is similar to that of neurons in standard cultures; (3) extended culture in bFGF prior to cell harvesting and replating into standard medium generates neurons from the precursors that possess proper neuronal polarization, morphology, and electrophysiology; and (4) extended bFGF also induces the expression, on transfer into standard medium, of an additional cell type with a distinct non-neuronal morphology that stains with the neuronal marker MAP-2. These results illustrate the need for additional characterization of long-term growth factor effects on maintained progenitor cells prior to their use in gene therapy and transplantation.

Expression and developmental regulation of Ehk-1, a neuronal Elk-like receptor tyrosine kinase in brain

Protein tyrosine kinases are pivotal in central nervous tissue development and maintenance. Here we focus on the expression of Ehk-1, a novel Elk-related receptor tyrosine kinase. Ehk-1 gene expression is observed in the developing and adult central nervous system and is highly regulated throughout development at both the messenger RNA and protein levels. Three messenger RNA transcripts of 8.5, 5.9 and 5.1 kb are detectable in the rat brain and a variety of splice possibilities have been identified. However, a major protein species of around M(r) 120,000 predominates throughout development. Ehk-1 messenger RNA and protein levels are highest in the first postnatal week. By in situ messenger RNA hybridization the gene is expressed by all neurons of the adult brain, but mostly in the hippocampus, cerebral cortex and large neurons of the deep cerebellar nuclei, as well as the Purkinje and granular cells of the cerebellum. At earlier stages of development, transcripts are most prominent in the periventricular germinal layers of the brain. Immunohistochemistry reveals a pronounced membrane associated protein expression in immature neurons. In the adult animal, peak reactivity was found in the neuropil with sparing of most perikarya. The spatial and temporal pattern of ehk-1 gene expression suggests a role in both the development and maintenance of differentiated neurons of the central nervous system.

Myelin basic protein content of aggregating rat brain cell cultures treated with cytokines and/or demyelinating antibody: effects of macrophage enrichment

The demyelinative potential of the cytokines interleukin-1 alpha (IL-1 alpha), interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha) has been investigated in myelinating aggregate brain cell cultures. Treatment of myelinated cultures with these cytokines resulted in a reduction in myelin basic protein (MBP) content. This effect was additively increased by anti-myelin/oligodendrocyte glycoprotein (alpha-MOG) in the presence of complement. Qualitative immunocytochemistry demonstrated that peritoneal macrophages, added to the fetal telencephalon cell suspensions at the start of the culture period, successfully integrated into aggregate cultures. Supplementing the macrophage component of the cultures in this fashion resulted in increased accumulation of MBP. The effect of IFN-gamma on MBP content of cultures was not affected by the presence of macrophages in increased numbers.