Ionic responses and growth stimulation in rat astroglial cells: differential mechanisms of gangliosides and serum

Gliogenesis and glial pathology in depression

Recent research has changed the perception of glia from being no more than silent supportive cells of neurons to being dynamic partners participating in brain metabolism and communication between neurons. This discovery of new glial functions coincides with growing evidence of the involvement of glia in the neuropathology of mood disorders. Unanticipated reductions in the density and number of glial cells are reported in fronto-limbic brain regions in major depression and bipolar illness. Moreover, age-dependent decreases in the density of glial fibrillary acidic protein (GFAP) - immunoreactive astrocytes and levels of GFAP protein are observed in the prefrontal cortex of younger depressed subjects. Since astrocytes participate in the uptake, metabolism and recycling of glutamate, we hypothesize that an astrocytic deficit may account for the alterations in glutamate/GABA neurotransmission in depression. Reductions in the density and ultrastructure of oligodendrocytes are also detected in the prefrontal cortex and amygdala in depression. Pathological changes in oligodendrocytes may be relevant to the disruption of white matter tracts in mood disorders reported by diffusion tensor imaging. Factors such as stress, excess of glucocorticoids, altered gene expression of neurotrophic factors and glial transporters, and changes in extracellular levels of neurotransmitters released by neurons may modify glial cell number and affect the neurophysiology of depression. Therefore, we will explore the role of these events in the possible alteration of glial number and activity, and the capacity of glia as a promising new target for therapeutic medications. Finally, we will consider the temporal relationship between glial and neuronal cell pathology in depression.

Homotypic cell contact-dependent inhibition of astrocyte proliferation

The normal adult vertebrate nervous system is a relative quiescent tissue in terms of cell proliferation. However, astrocytes in many regions of the central nervous system (CNS) retain the capacity to undergo cell division. To examine the mechanisms that regulate the proliferation of astrocytes in the CNS we have utilized an in vitro assay in which astrocyte density and cellular environment could be regulated. We demonstrate that type 1 astrocytes derived from the cerebral cortex of developing rats exhibit a profound density-dependent inhibition of proliferation. This inhibition of proliferation was cell type specific, but not restricted to type 1 astrocytes. NIH 3T3 cells but not smooth muscle cells inhibited astrocyte proliferation, while contact-inhibited astrocytes did not inhibit oligodendrocyte proliferation. Co-culture of type 1 astrocytes with neurons from a variety of sources resulted in induction of a process-bearing astrocyte morphology and promoted glial cell proliferation. Thus, induction of a process-bearing astrocyte morphology does not lead to a cessation of proliferation. The inhibition of astrocyte proliferation did not appear to be mediated through the release or sequestration of soluble factors but rather could be induced by membrane-associated factors.

Immunologically induced electrophysiological dysfunction: implications for inflammatory diseases of the CNS and PNS

During inflammation of the central or peripheral nervous system, a high number of immunologically active molecules, including bacterial or viral products as well as host-derived cytokines, are released. Patients suffering from inflammatory CNS or PNS diseases often develop transient symptoms with a rapid recovery, which obviously cannot be accounted for by immunologically induced tissue damage. These observations led to the hypothesis that immunologically active molecules can affect directly the electrophysiological functions of neurons and glial cells. Evidence for this hypothesis came from in vitro studies showing that cytokines, such as interleukins or tumor necrosis factors, arachidonic acid and its metabolites, interfere with electrophysiological properties of neurons or glial cells. These molecules affect ion currents, intracellular Ca2+ homeostasis, membrane potentials, and suppress or enhance the induction and maintenance of long-term potentiation. Similarly, virus proteins from human immunodeficiency virus type I were found to alter intracellular Ca2+ concentrations of neurons and astrocytes by modulating either transmitter receptors and channels or membrane transporters. Cerebrospinal fluid from MS patients contains factors which increase Na+ current inactivation and thereby reduce neuronal excitability. Immunoglobulins in sera of patients suffering from multifocal motor neuropathy and from acquired neuromyotonia interfere with nerve fibers, inducing alterations of conduction. Increased knowledge of these mechanisms will help to explain the pathogenesis of neurological symptoms and may provide a rationale for new therapeutic strategies.

Bacterial endotoxins impair electrophysiological properties of cultured astrocytes but not of cultured neurons

The endotoxins of bacteria are lipopolysaccharides which are released in the central nervous system during bacterial meningitis. Endotoxin titers in cerebrospinal fluid correspond to the appearance of severe neurological symptoms like seizures and coma. The pathogenic mechanism, however, by which endotoxins disturb neuronal function, is unclear. The functional deficit may originate either from direct alteration of neuronal excitability or from indirect effects mediated by glial cells. Therefore, we investigated the effects of lipopolysaccharides on electrophysiological properties of cortical neurons and astrocytes in separate cell cultures. Membrane potential, resistance and membrane currents of neurons were unaffected. By contrast, astrocytes depolarized markedly in a dose dependent manner (concentration range 1.0-10.0 micrograms/ml). The depolarization was Na+ dependent and amiloride sensitive (250 microM), both indicating an activation of an electrogenic sodium dependent transport system like the Na+/Ca2+ exchanger as a source of the depolarization. These results suggest that endotoxin induced neurological deficits are not caused by direct effects on neurons, but may result from an impaired glial cell function.

Identification of potassium flux pathways and their role in the cytotoxicity of estramustine in human malignant glioma, prostatic carcinoma and pulmonary carcinoma cell lines

Clinically-used drugs such as furosemide, bumetanide and cardiac glycosides, are modulators of transmembrane fluxes of cations. Recently, it has been suggested that the regulation of intracellular cation concentrations could be a primary target for anti-neoplastic drugs, and that the cytotoxic activity may be altered by inhibitors of cation fluxes at the level of the plasma membrane. Therefore, we investigated the mechanisms by which cations are translocated across the plasma membrane of malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines. The interactions between cation flux inhibitors and the cytotoxicity of estramustine were also evaluated. Ouabain, the classical inhibitor of Na+, K+ATPase, markedly reduced 86Rb (K+) influx in all three lines, indicating that this ion transport system is present in the cells. Furosemide and especially bumetanide inhibited the 86Rb influx, indicating the presence of the Na+, K+, Cl- co-transport system. The potassium channel blocker, tetraethylammonium, but not apamin reduced the influx of 86Rb showing that high conductance K+ channels are present, but that channels of low conductance probably do not exist in these cell lines. The Na+, K+, Cl- co-transport inhibitors furosemide and bumetanide significantly reduced cytotoxicity of estramustine in P31 cells, whereas no interaction between other K+ flux inhibitors and the anti-neoplastic drugs were detected in any of the cell lines investigated. Thus, the data show that Na+, K+, ATPase and NA+, K+, Cl- co-transport systems and K+ channels of high conductance are present in malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines, and that inhibition of the Na+, K+, Cl- co-transport system in P31 is associated with reduced cytotoxicity of estramustine. The results justify further studies evaluating the role of these cation flux pathways in terms of targets for anti-neoplastic therapy.

Exogenously administered gangliosides fail to increase in vivo metastatic frequency or in vitro growth of murine neoplastic cells

The influence of gangliosides on tumor growth and frequency of metastasis in vivo, as well as growth of neoplastic cells in vitro, was tested utilizing the mouse fibrosarcoma cell line MN4. In mice receiving intramuscular tumor transplants, injections of a ganglioside mixture twice daily did not influence the tumor volume or the number of spontaneous metastases per animal. Furthermore, in mice receiving the cells by tail vein injection, injections of a ganglioside mixture once or twice daily did not affect the number of metastatic foci in the lungs. Preincubation of neoplastic cells with the ganglioside mixture decreased the number of metastatic foci in the lungs of mice receiving the cells by tail i.v. injection. The addition of ganglioside mixture to the culture medium for up to a 48-h incubation period had no effect, independently of the cell density utilized, on either the rate of DNA synthesis or the relative numbers of neoplastic cells as compared to controls; at higher ganglioside concentrations, growth was actually reduced. These results are interpreted to indicate that gangliosides, under the present conditions, do not influence tumor growth and metastatic neoplastic capacity in vivo, and growth in vitro.

TIS gene expression in cultured rat astrocytes: multiple pathways of induction by mitogens

Accumulation of TIS1 and TIS11 (Lim et al.: Oncogene 1:263-270, 1987) mRNAs in secondary cultures of rat neocortical astrocytes was much greater in response to tetradecanoyl phorbol acetate (TPA) than in response to either epidermal growth factor (EGF) or fibroblast growth factor (FGF). In contrast, EGF, FGF, and TPA were equally effective in inducing accumulation of TIS8 and TIS28/c-fos mRNAs. These data suggested that TPA and the polypeptide mitogens might induce TIS gene expression by distinct pathways. When maximally inducing concentrations of EGF and FGF were co-administered to astrocyte cultures, TIS mRNA accumulations were no greater than those observed for the individual growth factors, suggesting that EGF and FGF saturate a common, limiting step in their induction pathways. In contrast, when either EGF or FGF was presented to astrocytes in combination with maximally inducing levels of TPA, the resulting levels of accumulation of TIS mRNAs were at least as great as the sum of the levels induced by the individual mitogens. Stimulation of [3H]-thymidine incorporation demonstrated an identical pattern of interaction; EGF and FGF co-administration was no more effective than either polypeptide mitogen alone, but, when presented to astrocyte cultures along with maximally inducing concentrations of TPA, either EGF or FGF was able to increase incorporation of [3H]-thymidine. Superinduction of all the TIS genes occurred if cycloheximide (CHX) was present during TPA exposure. Once again, two distinct classes of responses of the various TIS genes occurred; superinduction of TIS1, TIS7, TIS11, and TIS28/c-fos mRNA accumulation ranged from 10- to 20-fold, while CHX superinduction of TIS8 and TIS10 was far more modest, ranging from 2- to 3-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

Ganglioside GM1 actions on cell-substratum adhesion and DNA synthesis by cultured astroglial cells

Cultures of rat astroglial (AG) cells treated with the ganglioside GM1 in serum-free medium respond with an increase in DNA labeling and cell proliferation. However, GM1 doses above 60 microM cause decreasing DNA labeling to levels even below that determined in the absence of added GM1. Quantitative determination of cell numbers in 24-hr cultures treated with various GM1 concentrations shows no loss of cells but a progressive shift in cell morphology from the usual flat to a rounded shape, suggesting a GM1-induced progressive reduction in cell-substratum adhesion. The rounded cells, which can be readily washed off the culture wells, do not carry out measurable DNA synthesis but do appear normal by several other biochemical measurements. All the GM1-treated rounded cells can regain their flat morphology and resume DNA synthesis and cell replication if fetal calf serum is added to the medium. We conclude that modulation of cell adhesion is a major component of astroglial cell responses to exogenous ganglioside treatment.

Fibroblast growth factor effects on peripheral nerve regeneration in a silicone chamber model

We have developed a silicone nerve regeneration chamber that is partitioned into two compartments by a strip of nitrocellulose paper. The modified two-compartment chamber allows the investigation of the effects on rat sciatic nerve regeneration of trophic or growth factors that are initially bound to the nitrocellulose partition. In this study we compared the effects of untreated nitrocellulose, a siliconized nitrocellulose strip, and a strip that had been soaked in a basic fibroblast growth factor (FGF) solution. FGF is a known angiogenic factor and a mitogen for endothelial cells, fibroblasts, and Schwann cells. All of these cell types are present in the peripheral nerve. In vitro analyses, using 3T3 cells as test cells, showed that some of the bound FGF remained active on the nitrocellulose paper for at least 8-10 days. In vivo experiments, examined at 16 days post-implantation, revealed that spatial migration of all cellular elements (perineurial-like cells, vasculature, and Schwann cells) across the chamber gap was slower with untreated nitrocellulose strips than with siliconized strips but was most advanced with FGF-treated ones. Most striking was the well-developed vascular arborization of the regenerate within the FGF chambers. Histologic sections from the proximal one-half of the chamber revealed that the regenerate in untreated strip chambers consisted of fibrin matrix and erythrocytes, whereas a well-developed structure with all the cellular elements of a regenerating nerve was seen in several of the FGF strip chambers. We conclude that FGF stimulates peripheral nerve regeneration in this model.

A role for gangliosides in astroglial cell differentiation in vitro

Rat cerebral astroglial cells in culture display specific morphological and biochemical behaviors in response to exogenously added gangliosides. To examine a potential function for endogenous gangliosides in the processes of astroglial cell differentiation, we have used the B subunit of cholera toxin as a ganglioside-specific probe. The B subunit, which is multivalent and binds specifically to GM1 ganglioside on the cell surface, induced a classical star-shaped (stellate) morphology in the astroglial cells and inhibited DNA synthesis in a dose-dependent manner. The morphological response was massive and complete within 2 h, with an ED50 of 0.8 nM, and appeared to depend on the direct interaction of the B subunit with GM1 on the cell surface. A B subunit-evoked inhibition of DNA synthesis and cell division (ED50 = 0.2 nM) was observed when the cells were stimulated with defined mitogens, such as epidermal growth factor and basic fibroblast growth factor. Maximal inhibition approached 80% within 24 h. The effects of the B subunit were unrelated to increases in cAMP. These observations, taken together with previous studies, demonstrate that both endogenously occurring plasma membrane gangliosides and exogenously supplied gangliosides can influence the differentiative state (as judged by morphological and growth behaviors) of astroglial cells in vitro.