Epidermal growth factor (EGF) stimulation of cultured brain cells. II. Increased production of extracellular soluble proteins

Epidermal Growth Factor in the CNS: A Beguiling Journey from Integrated Cell Biology to Multiple Sclerosis. An Extensive Translational Overview

This article reviews the wealth of papers dealing with the different effects of epidermal growth factor (EGF) on oligodendrocytes, astrocytes, neurons, and neural stem cells (NSCs). EGF induces the in vitro and in vivo proliferation of NSCs, their migration, and their differentiation towards the neuroglial cell line. It interacts with extracellular matrix components. NSCs are distributed in different CNS areas, serve as a reservoir of multipotent cells, and may be increased during CNS demyelinating diseases. EGF has pleiotropic differentiative and proliferative effects on the main CNS cell types, particularly oligodendrocytes and their precursors, and astrocytes. EGF mediates the in vivo myelinotrophic effect of cobalamin on the CNS, and modulates the synthesis and levels of CNS normal prions (PrP^Cs), both of which are indispensable for myelinogenesis and myelin maintenance. EGF levels are significantly lower in the cerebrospinal fluid and spinal cord of patients with multiple sclerosis (MS), which probably explains remyelination failure, also because of the EGF marginal role in immunology. When repeatedly administered, EGF protects mouse spinal cord from demyelination in various experimental models of autoimmune encephalomyelitis. It would be worth further investigating the role of EGF in the pathogenesis of MS because of its multifarious effects.

The expressions of epidermal growth factor receptor mRNA and protein gene product 9.5 in developing rat brain

To clarify the role of epidermal growth factor receptor (EGFR) in brain development, especially with regard to neuron differentiation, EGFR mRNA expression was studied by in situ hybridization in embryonic day (E)12, E16, postnatal day (P)1, P4, P15, P29 and adult rat, using protein gene product (PGP) 9.5 as a neuron marker. The primary germinal zone (neuroepithelium) expressed neither PGP 9.5 immunoreactivity (IR) nor EGFR mRNA. In the developing brain, cells expressing EGFR mRNA but not PGP 9.5 IR were found in the secondary germinal zone such as the subventricular zone and cerebellar external germinal layer, the cortical plate and, in later stage animals, the fiber tracts. Cells expressing both EGFR mRNA and PGP 9.5 IR appeared in a differentiating field. In the adult brain, strong EGFR mRNA expression was observed in Purkinje cells, Golgi cells, some hippocampal cells and neurons of the diencephalon, pontine and medullary nuclei, and weak expression was seen in neurons of the cerebral cortex. These results suggest that EGFR is related to the process of differentiation and maturation of neurons and the maintenance of some types of adult neurons.

The neurotrophic action and signalling of epidermal growth factor

Epidermal growth factor (EGF) is a conventional mitogenic factor that stimulates the proliferation of various types of cells including epithelial cells and fibroblasts. EGF binds to and activates the EGF receptor (EGFR), which initiates intracellular signalling and subsequent effects. The EGFR is expressed in neurons of the cerebral cortex, cerebellum, and hippocampus in addition to other regions of the central nervous system (CNS). In addition, EGF is also expressed in various regions of the CNS. Therefore, EGF acts not only on mitotic cells, but also on postmitotic neurons. In fact, many studies have indicated that EGF has neurotrophic or neuromodulatory effects on various types of neurons in the CNS. For example, EGF acts directly on cultured cerebral cortical and cerebellar neurons, enhancing neurite outgrowth and survival. On the other hand, EGF also acts on other cell types, including septal cholinergic and mesencephalic dopaminergic neurons, indirectly through glial cells. Evidence of the effects of EGF on neurons in the CNS is accumulating, but the mechanisms of action remain essentially unknown. EGF-induced signalling in mitotic cells is better understood than that in postmitotic neurons. Studies of cloned pheochromocytoma PC12 cells and cultured cerebral cortical neurons have suggested that the EGF-induced neurotrophic actions are mediated by sustained activation of the EGFR and mitogen-activated protein kinase (MAPK) in response to EGF. The sustained intracellular signalling correlates with the decreased rate of EGFR down-regulation, which might determine the response of neuronal cells to EGF. It is likely that EGF is a multi-potent growth factor that acts upon various types of cells including mitotic cells and postmitotic neurons.

Immunocytochemical and biochemical characterization of glial phenotypes in normal and immortalized cultures derived from 3-day-old chick embryo encephalon

We examined properties of glia derived from very early neurogenesis in the chick embryo, as well as their behavior in response to extended cell passages and at various periods in culture. Primary cultures derived from the telencephalic region of 3-day-old chick embryo (stage 19) exhibited intense staining for vimentin (Vim; indicative of immature glial phenotypes) throughout the 17-day culture period, transitioning to Vim-positive/glial fibrillary acidic protein (GFAP)-positive astroblasts after a single cell passage at 13 days in vitro (DIV). With subsequent passage (P; through P6), cell continued to coexpress Vim/GFAP with the occasional appearance of fibronectin (Fib). By P11, Vim staining was faint, whereas GFAP staining gained in intensity, indicating the presence of mature astrocytes. These cultures also featured significant activity of glutamine synthetase (GS), which increased slightly with both cell passage and days in culture, correlating well with immunocytochemical findings. The activity of 2'3'-cyclic nucleotide 3'-phosphohydrolase (CNP) remained low, indicating a low percentage of mature oligodendrocytes. Thus, embryonic day (E)3H glia cultures mature into astrocytes given sufficient time in culture. To obtain a stable population of glia at various stages of astrocytic differentiation, we immortalized and subcloned homogenous colonies of E3H glia precursors at specific stages of development. A cell suspension was electroporetically transfected with the pSV40neo gene for large T antigen (E3H.SV5). After 5 passages, the parent colonies consisted of a heterogeneous population of cells, most of which were vim and GFAP positive. Following subcloning, one line (colony 7) displayed the staining pattern of mature astrocytes. However, with advancing age in culture, staining for both vim and GFAP became increasingly faint. Compared with control (normal) cultures, transfected cells exhibited a significantly lower activity of GS that did not fluctuate with days in culture but decreased with advancing cell passage. Furthermore, CNP activity in E3H.SV5 colony 7 was approximately double that observed in normal cultures at the same cell passage. This observation suggests that the phenotype of transfected glia was less stable than that observed in normal glial cultures.

Differential responsiveness of late passage C-6 glial cells and advanced passages of astrocytes derived from aged mouse cerebral hemispheres to cytokines and growth factors: glutamine synthetase activity

In this study, we were interested to compare the responsiveness to growth factors, NGF, b-FGF and EGF and cytokines, IL1 beta, and TNF-alpha, in late passages (74-79) C6 glial cells committed astrocytes and astrocytes of advanced passages (26-28) in cultures derived from aged mouse cerebral hemispheres (MACH). Cultures were grown in either DMEM or chemically defined medium (CDM/TIPS) in order to test the effects of growth factors or cytokines. The activity of glutamine synthetase (GS), a marker for astrocytes, was used as a test parameter. We found that treatment with growth factors increased GS activity in both glial cell culture systems with the exception of EGF in C-6 glial cells. Treatment with cytokines markedly decreased GS activity in the late passage C6 glial cells whereas only TNF-alpha had a similar effect on MACH astrocytes. In view of the generally opposite effects of growth factors and cytokines on GS activity, we speculate that these molecules which are also endogenously present in glial cells may play a role in the maintenance of cellular homeostasis.

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

In order to study peptide growth factor action in a three-dimensional cellular environment, aggregating cell cultures prepared from 15-day fetal rat telencephalon were grown in a chemically defined medium and treated during an early developmental stage with either bovine fibroblast growth factor (bFGF) or platelet-derived growth factor (PDGF homodimers AA and BB). A single dose (5-50 ng/ml) of either growth factor given to the cultures on day 3 greatly enhanced the developmental increase of the two glia-specific enzyme activities, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and glutamine synthetase (GS), whereas it had relatively little effect on total protein and DNA content. Distinct patterns of dose-dependency were found for CNP and GS stimulation. At low concentrations of bFGF (0.5-5 ng/ml) and at all PDGF concentrations applied, the oligodendroglial marker enzyme CNP was the most affected. A relatively small but significant mitogenic effect was observed after treatment with PDGF, particularly at higher concentrations or after repetitive stimulation. The two PDGF homodimers AA and BB were similar in their biological effects and potency. The present results show that under histotypic conditions both growth factors, bFGF and PDGF, promote the maturation rather than the proliferation of immature oligodendrocytes and astrocytes.

Epidermal growth factor affects both glia and cholinergic neurons in septal cell cultures

The effects of epidermal growth factor on high density primary cultures of fetal (embryonic day 17) rat septal cells were examined. Under serum-free conditions, the continuous exposure of these cultures to epidermal growth factor for seven days significantly decreased choline acetyltransferase (EC 2.3.1.6) activity in a dose-dependent manner. Maximal decreases were observed from 1 to 10 ng/ml epidermal growth factor. This effect was completely abolished by the addition of anti-epidermal growth factor antibodies. The epidermal growth factor-mediated decrease in choline acetyltransferase activity was culture-time dependent, being first detectable after five days of factor application and may likely represent an inhibition of the spontaneous increase in enzyme activity that occurs with time in culture. Concomitant with changes in enzyme activity, epidermal growth factor produced a significant and proportional decrease in the number of acetylcholinesterase-positive neurons. This decrease in acetylcholinesterase-positive cells did not reflect a decrease in cholinergic cell survival as nerve growth factor could restore the number of acetylcholinesterase-positive neurons in epidermal growth factor-treated cultures to control levels. Furthermore, in these high-density cultures, epidermal growth factor did not affect general neuronal survival, while it did produce an increase in the number and intensity of glial fibrillary acidic protein-immunoreactive astroglia as well as in the number of macrophage-like cells. The proliferative response of these non-neuronal cells to epidermal growth factor, as assessed by [3H]thymidine incorporation, was evident after three days of epidermal growth factor application, persisted thereafter, and could be antagonized by the inclusion of the antimitotic 5-fluorodeoxyuridine. Furthermore, 5-fluorodeoxyuridine completely blocked the epidermal growth factor-mediated decrease in choline acetyltransferase activity. However, when epidermal growth factor was tested in pure glial cultures, it only directly induced proliferation of astrocytes. These results suggest that the proliferative response of either one or both of these glial cell types in the mixed cultures may be indirectly affecting cholinergic cell expression.

Expression of the proto-oncogene c-fos in three-dimensional fetal brain cell cultures and the lack of correlation with maturation-inducing stimuli

Previous work has shown that aggregating fetal brain cell cultures are able to attain a highly differentiated state, and that their development is greatly enhanced by growth and/or differentiation factors such as epidermal growth factor (EGF), basic fibroblast growth factor (bFGF), and the protein kinase C-activating tumor promoter mezerein. The present study shows that in these 3-dimensional cultures the peptide growth factors EGF and bFGF as well as mezerein are able to induce the expression of the proto-oncogene c-fos. This induction was rapid and transient, in good agreement with observations reported from a wide variety of cell types in vitro. The maximal levels of c-fos mRNA found after stimulation were low in immature cultures and increased greatly as maturation progressed. Of the three factors tested, mezerein was the most potent inducer of c-fos. In contrast to the peptide growth factors EGF and bFGF which were found to induce c-fos only in glial cells, mezerein was stimulatory in glial cells as well as in neurons. A similar cell type specificity has been observed previously for the maturation-enhancing response in immature aggregate cultures. However, in the present study no correlation was found between the degree of c-fos induction and the extent of the maturation-enhancing stimulation. Immature cultures known to be most sensitive and responsive to these maturation-enhancing agents required relatively high doses of peptide growth factors for the induction of c-fos, and the maximal levels of c-fos mRNA elicited were much lower than those in differentiated cultures which did not show any long-term response to these stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Epidermal growth factor and the nervous system

Various growth factors and their receptors are present in the nervous system. This review focuses on the presence of epidermal growth factor (EGF) and its receptors in the central nervous system (CNS). Evidence indicates that EGF in the CNS is the result of local synthesis, by intrinsic and blood-derived macrophages, glial cells and neurons, and uptake from the peripheral blood through the circumventricular organs and probably also through the blood-brain barrier. Evidence is accumulating suggesting that EGF regulates a variety of CNS functions in a specific manner. EGF influences CNS growth, differentiation and maintenance (actions proposed to promote neural regeneration and cell survival following a variety of insults). EGF also induces neuromodulatory actions, affects the neuroendocrine system, and suppresses food intake and gastric acid secretion. Acute and chronic pathological processes, e.g., various cancers, stimulate the production and release of EGF in various cell systems. Monitoring of EGF by the CNS may participate in several neurological manifestations (e.g., appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic disease. EGF and transforming growth factor-alpha (TGF-alpha, a factor that binds to the EGF receptor with high affinity and induces the same biological signals as EGF) also may be involved in the promotion of malignancy in the CNS and in the neuropathogenesis of degenerative disorders. Thus evidence is accumulating concerning the neurophysiological or neuropathophysiological significance of EGF in the nervous system.

Growth factors attenuate the cholinotoxic effects of ethanol during early neuroembryogenesis in the chick embryo

The interaction between growth factors and ethanol on cholinergic neuronal expression was studied in the chick embryo during early neuroembryogenesis using choline acetyltransferase activity as a cholinergic marker. As we have previously reported (Brodie & Vernadakis, Dev. Brain Res. 56: 223-228, 1990; Kentroti and Vernadakis, Dev. Brain Res. 56: 205-210, 1990), ethanol administration in ovo at embryonic days 1-3 produced a 30% decrease in choline acetyltransferase activity. Nerve growth factor and epidermal growth factor administration alone, at embryonic days 1-3, produced a slight increase in choline acetyltransferase activity of both brain and spinal cord when examined at embryonic day 8. Concomitant administration of either nerve growth factor or epidermal growth factor with ethanol eliminated the decrease in choline acetyltransferase activity produced by ethanol. Moreover, administration of either nerve growth factor or epidermal growth factor at embryonic days 4-7 to embryos pretreated with ethanol at days 1-3 raised choline acetyltransferase activity to a level similar to that observed in controls. Thus the growth factors reversed the ethanol-induced cholinergic insult and restored the cholinergic population to normal. These findings provide evidence of a possible role of NGF and EGF in interfering with the neurotoxic effects of ethanol during embryogenesis.

Epidermal growth factor enhances the expression of an endogenous lectin in aggregating fetal brain cell cultures

Aggregating cell cultures prepared from fetal rat telencephalon express the two subunits [cerebellar soluble lectins (CSL) 1 and 2] of a soluble, mannose-specific endogenous lectin (CSL) in a development-dependent manner. Increased CSL synthesis was found at an early postmitotic stage as well as during the period of maximal myelination. Repetitive treatment of early cultures with epidermal growth factor (EGF, 3nM) caused a great stimulation of CSL biosynthesis. Immunocytochemical studies revealed particularly intense CSL-specific staining in small, EGF-responsive cells, presumably glial cells. Large quantities of CSL-immunoreactive material were found also in the extracellular space and on the external side of the plasma membrane, indicating abundant release of CSL. The present findings suggest that EGF or EGF-related factors in the brain are able to regulate the expression of an endogenous lectin, affecting brain ontogeny.

Use of aggregating cell cultures for toxicological studies

Relatively simple techniques are now available which allow the preparation of large quantities of highly reproducible aggregate cultures from fetal rat brain or liver cells, and to grow them in a chemically defined medium. Since these cultures exhibit extensive histotypic cellular reorganization and maturation, they offer unique possibilities for developmental studies. Therefore, the purpose of the present study was to investigate the usefulness of these cultures in developmental toxicology. Aggregating brain cell cultures were exposed at different developmental stages to model drugs (i.e., antimitotic, neurotoxic, and teratogenic agents) and assayed for their responsiveness by measuring a set of biochemical parameters (i.e., total protein and DNA content, cell type-specific enzyme activities) which permit a monitoring of cellular growth and maturation. It was found that each test compound elicited a distinct, dose-dependent response pattern, which may ultimately serve to screen and classify toxic drugs by using mechanistic criteria. In addition, it could be shown that aggregating liver cell cultures are capable of toxic drug activation, and that they can be used in co-culture with brain cell aggregates, providing a potential model for complementary toxicological and metabolic studies.

Secretion of newly synthesized proteins into the extracellular fluid of the rabbit hippocampus

Extracellular proteins were collected continuously from the hippocampus of the rabbit by slow perfusion of a protein-permeable thin tubing. After pulse labelling with radioactive amino acids via the tubing a rapid appearance of labelled proteins was seen in the extracellular fluid. The half-life of the proteins in the extracellular space varied between 15 min and 2 h. Protein fractions were separated with gel electrophoresis or HPLC. The extracellular fraction differed distinctly from intracellular proteins with respect to the labelling pattern.

Visualization of growth factor receptor sites in rat forebrain

It is now known that various growth factors may also act in the central nervous system. Among them, it has recently been shown that epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) may possess trophic effects in the mammalian brain. We report here on the respective autoradiographic distribution of [125I]EGF and [125I]IGF-I receptor binding sites in the rat brain, both during ontogeny and in adulthood. It appears that [125I]EGF sites are mostly found in the rat forebrain during brain development. On the other hand, [125I]IGF-I sites are more widely distributed both during ontogeny and in adulthood. These results reveal the plasticity of the expression of EGF and IGF-I receptor sites in the mammalian brain. This could be relevant for the respective role of these two growth factors in the development and maintenance of neuronal function.

Effects of opioid peptides and naloxone on nervous tissue in culture

It was shown that opioid peptides stimulate nervous tissue growth in culture in the rat, which manifests itself in augmented outgrowth of neurites from explants and in an increase in the number of glial and fibroblast-like cells in the growth zone. The effects of opioid peptides ([Leu]- and [Met]-enkephalins, beta- and gamma-endorphins and some synthetic analogues of [Leu]-enkephalin) on the growth of organotypic cultures of rat sympathetic and dorsal root ganglia and spinal cord were investigated. Neurite outgrowth, cell composition, and size of the growth zone as well as the dynamics of its formation were estimated. Changes in the survival of neurons in dorsal root ganglion cultures were determined. The experiments were performed with living cultures as well as with fixed preparations. In experiments with sympathetic ganglia, it was demonstrated that a significant growth-promoting effect is exerted by peptides taken at concentrations of 10(-8) M to 10(-14) M. Naloxone does not eliminate the effects of peptides, but stimulates the growth at 10(-5) M to 10(-7) M. Studies with spinal cord revealed that naloxone (10(-6) M) enhances the response to [Leu]-enkephalin (10(-9) M). The survival of dorsal root ganglion neurons under the influence of a [leu]-enkephalin analog (10(-9) M) exceeds control values by approximately two to four times. Thus, opioid peptides were shown to exert a strong growth-promoting effect on nervous tissue in culture. This effect is dual: in neurons the peptides stimulate the outgrowth of neurites and their survival, while in glial cells they change the rate of their migration and, probably, their proliferation. It is suggested that opioid peptides, besides their already established functions, may play a role in the development and regeneration of nervous tissue.

Protein kinase C-activating tumor promoters enhance the differentiation of astrocytes in aggregating fetal brain cell cultures

Serum-free aggregating cell cultures of fetal rat telencephalon treated with the potent tumor promoter phorbol 12-myristate 13-acetate (PMA) showed a marked, rapid, and sustained increase in the activity of the astrocyte-specific enzyme glutamine synthetase (GS). This effect was accompanied by a small increase in RNA synthesis and a progressive reduction in DNA synthesis. Only mitotically active cultures were responsive to PMA treatments. Since in aggregate cultures astrocytes are the preponderant cell type, both in number and mitotic activity, it can be concluded that PMA induces and/or enhances the terminal differentiation of astrocytes. The developmental expression of GS was also greatly stimulated by mezerein, a potent nonphorbol tumor promoter, but not by 4 alpha-phorbol 12,13-didecanoate, a nonpromoting phorbol ester. Since both tumor promoters, PMA and mezerein, are potent and specific activators of C-kinase, it is suggested that C-kinase plays a regulatory role in the growth and differentiation of normal astrocytes.

Epidermal growth factor in synaptosomal fractions of mouse cerebral cortex

Using a specific and sensitive epidermal growth factor radioimmunoassay (EGF-RIA) we measured EGF concentrations in whole brain, cerebral cortex, and cerebral cortical synaptosomal (pinched-off presynaptic nerve terminals) fractions of 26-day-old mouse brain. The relative EGF concentration in synaptosomal fractions was significantly greater than the growth factor concentrations in whole brain or cerebral cortex. Intracerebral injection, in an amount of EGF, several-fold greater than whole brain EGF content, did not appreciably increase synaptosomal EGF concentration, suggesting that no artifact was involved. The high synaptosomal EGF content suggests a neurotransmitter or a neuromodulator role for EGF in the CNS.

Epidermal growth factor and bovine growth hormone stimulate differentiation and myelination of brain cell aggregates in culture

Bovine growth hormone (bGH) and epidermal growth factor (EGF) increased the activity of ornithine decarboxylase (ODC) in brain cell aggregates cultured in a serum-free chemically defined medium. ODC is considered as a marker of cell growth and differentiation. The effect of bGH and EGF on myelination was investigated by measuring two myelin markers, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and myelin basic protein (MBP). EGF treatment at days 2 and 5 caused a dose-dependent increase of both myelin markers at culture day 12. This increase could still be observed at culture day 19, indicating a prolonged action of EGF. The continual presence of bGH in the culture medium produced a large accumulation of MBP at day 19. This effect was dose-dependent and required the presence of triiodothyronine (T3). In contrast, the effect of bGH on CNP activity did not require the presence of T3. This is the first report showing a direct effect of bGH on CNS myelination in vitro and of EGF on both MBP accumulation and ODC activity.