Epidermal growth factor-induced survival and proliferation of neuronal precursor cells from embryonic rat mesencephalon

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.

Tl(I) and Tl(III) alter the expression of EGF-dependent signals and cyclins required for pheochromocytoma (PC12) cell-cycle resumption and progression

The effects of thallium [Tl(I) and Tl(III)] on the PC12 cell cycle were evaluated without (EGF(-)) or with (EGF(+)) media supplementation with epidermal growth factor (EGF). The following markers of cell-cycle phases were analyzed: cyclin D1 (G1 ); E2F-1, cyclin E and cytosolic p21 (G1 →S transition); nuclear PCNA and cyclin A (S); and cyclin B1 (G2). The amount of cells in each phase and the activation of the signaling cascade triggered by EGF were also analyzed. Tl(I) and Tl(III) (5-100 μM) caused dissimilar effects on PC12 cell proliferation. In EGF(-) cells, Tl(I) increased the expression of G1 →S transition markers and nuclear PCNA, without affecting cyclin A or cyclin B1. In addition to those, cyclin B1 was also increased in EGF(+) cells. In EGF(-) cells, Tl(III) increased the expression of cyclin D1, all the G1→S and S phase markers and cyclin B1. In EGF(+) cells, Tl(III) increased cyclin D1 expression and decreased all the markers of G1 →S transition and the S phase. Even when these cations did not induce the activation of EGF receptor (EGFR) in EGF(-) cells, they promoted the phosphorylation of ERK1/2 and Akt. In the presence of EGF, the cations anticipated EGFR phosphorylation without affecting the kinetics of EGF-dependent ERK1/2 and Akt phosphorylation. Altogether, results indicate that Tl(I) promoted cell proliferation in both EGF(-) and EGF(+) cells. In contrast, Tl(III) promoted the proliferation of EGF(-) cells but delayed it in EGF(+) cells, which may be related to the toxic effects of this cation in PC12 cells.

Implantation of bioactive growth factor-secreting rods enhances fetal dopaminergic graft survival, outgrowth density, and functional recovery in a rat model of Parkinson's disease

One of the drawbacks with fetal ventral mesencephalic (VM) grafts in Parkinson's disease is the limited outgrowth into the host striatum. In order to enhance graft outgrowth, epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) were administered by implantation of bioactive rods to the lateral part of the striatum to support grafted fetal VM implanted to the medial portion of the striatum. The polymer-based bioactive rods allow for a local secretion of neurotrophic factors over a time period of approximately 2 weeks. Moreover, glial cell line-derived neurotrophic factor (GDNF) and transforming growth factor-beta1 (TGFbeta1) were administered using the same technique. Concomitant administration of GDNF and TGFbeta1 was achieved by insertion of one GDNF and one TGFbeta1 rod. This was performed to investigate possible additive effects between GDNF and TGFbeta1. Rotational behavior, outgrowth from and nerve fiber density within the VM graft, and the number of TH-positive cells were studied. Functional compensation by reduction of rotational behavior was significantly enhanced in animals carrying bFGF and GDNF rods in comparison with animals carrying only VM graft. EGF and bFGF significantly increased the innervation density. Moreover, the nerve fiber density within the grafts was significantly enhanced by bFGF. Cell counts showed that a significantly higher number of TH-positive neurons was found in grafts treated with bFGF than that found in GDNF-treated grafts. An additive effect of TGFbeta1 and GDNF was not detectable. These results suggest that bioactive rods is a useful tool to deliver neurotrophic factors into the brain, and since bFGF was a potent factor concerning both functional, immunohistochemical and cell survival results, it might be of interest to use bFGF-secreting rods for enhancing the overall outcome of VM grafts into patients suffering from Parkinson's disease.

Macrophage cell-conditioned medium promotes cholinergic differentiation of undifferentiated progenitors and synergizes with nerve growth factor action in the developing basal forebrain

Conditioned medium from stimulated microglia and from the monocyte/macrophage cell line (RAW 264.7; MC-CM) promotes the differentiation of cholinergic neurons from undifferentiated progenitors in the septal nuclei and adjacent basal forebrain (BF). We have studied the regulation of this process by measuring the activity of choline acetyltransferase (ChAT) in cultured BF taken from embryonic day 16 rat brain. Inhibition of either xanthine oxidase with allopurinol or nitric oxide synthase with N(G)-monomethyl-l-arginine produces a small but significant improvement in the efficacy of MC-CM while inclusion of pyrrolidine dithiocarbamate, a hydroxyl radical scavenger widely used as an antioxidant, lowers MC-CM-induced ChAT activity. Addition of nerve growth factor (NGF) but not brain-derived neurotrophic factor or glial-derived neurotrophic factor together with MC-CM has a synergistic effect on both ChAT activity and ChAT mRNA, raising ChAT activity as much as 29-fold and ChAT mRNA almost 15-fold. While MC-CM raised mRNA for trkA, the effect was not synergistic with NGF. mRNA for the common neurotrophin receptor (p75NTR) showed a modest synergistic increase. Blockade of the Ras/Raf/ERK [extracellular signal-regulated kinase, also known as mitogen-activated protein [(MAP) kinase] signal transduction pathway with either PD28059 (an inhibitor of MAP kinase/ERK kinase kinase or MEK) or N-acetyl-S-farnesyl-l-cysteine (an inhibitor of Ras farnesylation and, hence, activation) inhibited the action of MC-CM. Moreover, a subpopulation of cells responded rapidly to MC-CM with an increased appearance of phosphorylated ERK. Because NGF also utilizes this pathway, synergy may occur along this signal transduction pathway.

Somatic gene therapy in animal models of Parkinson's disease

Gene therapy in Parkinson's disease (PD) emerged about 10 years ago but until now, no clinical trials are under way, because most approaches have failed to show long-term therapeutic effects in PD animal models and because safety concerns precluded the use in humans so far. This review tries to give an overview on the development of different strategies in gene therapy in PD animal models and point out new and possibly more successful directions, including the transplantation of neural precursor cells and pig tissue.

Survival of purified rat photoreceptors in vitro is stimulated directly by fibroblast growth factor-2

Basic fibroblast growth factor (FGF-2) influences the differentiation and survival of retinal photoreceptors in vivo and in vitro, but it is not known whether it acts directly on photoreceptor FGF receptors or indirectly through activation of surrounding cells. To clarify the effects of FGF-2 on photoreceptor survival, we developed a purified photoreceptor culture system. The outer nuclear layers of postnatal day 5-15 rat retinas were isolated by vibratome sectioning, and the photoreceptor fractions obtained were enzymatically dissociated. Photoreceptors were maintained in monolayer culture for 1 week in a chemically defined medium. Immunocytochemical labeling showed that >99.5% of cells were photoreceptors, and glial contamination represented approximately 0. 2%. Photoreceptors from postnatal day 5-9 retinas survived for at least 24 hr in vitro, whereas cells from postnatal day 10-15 retinas died rapidly. Subsequent studies performed with postnatal day 5 photoreceptors showed that their survival was increased in a dose-dependent manner after the addition of FGF-2. In control cultures, 36% of originally seeded photoreceptors were alive after 5 d in vitro, and in the presence of 20 ng/ml FGF-2 this number was doubled to 62%. This increase was not caused by proliferation of photoreceptor precursors. Denaturing or blocking FGF-2 prevented enhancement of survival. Conversely, only 25.5% of photoreceptors survived in the presence of epidermal growth factor (EGF). FGF- and EGF-receptor mRNA and proteins were detected in purified photoreceptors in vitro, and addition of FGF-2 or EGF led to tyrosine phosphorylation of photoreceptor proteins. These data support a direct mechanism of action for FGF-2 stimulation of photoreceptor survival.

A null mutation in TGF-alpha leads to a reduction in midbrain dopaminergic neurons in the substantia nigra

Transforming growth factor (TGF)-alpha is neurotrophic for midbrain dopaminergic neurons in vitro. Here I investigated whether a null mutation in the TGF-alpha gene affects the normal development or survival of dopaminergic neurons in either the substantial nigra (SN) or the ventral tegmental area (VTA). The SN of TGF-alpha knockout mice contained 50% fewer dopaminergic neurons than the control SN, but VTA neuron number was unchanged. In addition, the overall volume of the dorsal striatum was reduced by 20%. Newborn mice showed a similar decrease in the number of SN dopaminergic neurons, suggesting that TGF-alpha is unlikely to regulate developmental neuron death. These studies indicate that TGF-alpha is required for the normal proliferation or differentiation of a select population of dopaminergic neurons within the SN.

Opposing actions of the EGF family and opioids: heparin binding-epidermal growth factor (HB-EGF) protects mouse cerebellar neuroblasts against the antiproliferative effect of morphine

Endogenous opioids and opiate drugs of abuse inhibit the proliferation of cerebellar external granular layer (EGL) neuroblasts by mechanisms that are incompletely understood. Opioids do not act alone, rather multiple extracellular factors regulate granule cell neurogenesis and these undoubtedly act in concert with opioids to shape developmental outcome. We examined whether, heparin binding-epidermal growth factor-like growth factor (HB-EGF), a recently described member of the epidermal growth factor (EGF) family, might compete with an inhibitory opioid signal. The results confirmed our ongoing studies that morphine inhibited neuroblast proliferation, while HB-EGF enhanced cell replication. HB-EGF not only counteracted the antiproliferative morphine signal, but invariably enhanced DNA synthesis irrespective of morphine treatment. Our findings suggest that regional and temporal differences in the availability of endogenous HB-EGF may serve to limit the response of EGL neuroblasts to opioids, and HB-EGF may be neuroprotective in opiate drug abuse. If similar responses occur in vivo, then the EGF family and the opioid system may represent distinct and contrasting components of an extracellular signaling system serving to coordinate EGL neurogenesis.

Heparin, but not other proteoglycans potentiates the mitogenic effects of FGF-2 on mesencephalic precursor cells

There is increasing evidence that the proteoglycan heparin plays a critical role in the regulation of the activity of FGF-2 by either interacting with its receptor or modifying its stability and functioning. In this study precursor cells were isolated from the rat E14 ventral mesencephalon and cultured as free floating spheres in FGF-2 alone or in combination with heparin or other related proteoglycans, including chondroitin sulfate, keratin sulfate, dermatan sulfate, or hyaluronic acid. Our results show the mitogenic effects of FGF-2 could be potentiated by heparin but not the other four proteoglycans. Sodium chlorate, which blocks the cells ability to sulfate its proteoglycans, was shown to reduce the mitogenic effects of FGF-2 alone to below that of control levels, suggesting that endogenous sulfated molecules are required for the FGF-2 effects on mesencephalic precursors. Cells expanded for 7 days with either FGF-2 or FGF-2 + heparin were plated onto a substrate and allowed to differentiate for a further 7 days in the absence of growth factors. Approximately 6% of the precursors developed into neurons whether grown with or without heparin and none were positive for TH, a marker for dopamine neurons. However, there was a significant decrease in the number of astrocytes developing from cultures grown in FGF-2 + heparin when compared to FGF-2 alone. Interestingly we could not find an EGF responsive cell in the mesencephalon at this embryonic age in the absence or presence of heparin. However, there was a synergistic effect of combining EGF + FGF-2, which could be potentiated by heparin. We conclude that heparin, but not other closely related proteoglycans, is vital for the growth of FGF-2-responsive mesencephalic neural precursors.

The protein tyrosine phosphatase SHP-2 is expressed in glial and neuronal progenitor cells, postmitotic neurons and reactive astrocytes

In this study we examined the distribution and developmental profile of the src homology 2 (SH2) domain-containing protein tyrosine phosphatase SHP-2 in the mouse brain. We found that SHP-2 is present in both mitotically active and postmitotic cells in the forebrains of embryonic day 12 (E12) mice. In a developmental study extending from embryonic day 12 to adulthood, Western blotting analysis demonstrated equivalent levels of SHP-2 protein at all of the ages examined. Expression of SHP-2 paralleled the level of enzymatic activity at the different developmental periods. In the adult brain SHP-2 was restricted to diverse classes of neurons, while the majority of glial cells did not express detectable levels of protein. However, reactive astrocytes in response to an ischemic brain injury showed SHP-2 immunolabelling. Our data suggest that SHP-2 may play a role in pathways of neuronal and glial progenitor cells, in a broad spectrum of neuronal responses in the adult brain and in the gliotic response to the injury.

Application of ex vivo gene therapy in the treatment of Parkinson's disease

Ex vivo gene therapy approaches hold great promise for the treatment of neurodegenerative diseases where there is currently no cure or adequate treatment for affected individuals. In this review we have focused on the use of ex vivo gene transfer techniques in Parkinson's disease models; however, the issues and approaches outlined are applicable to other neurodegenerative disorders. In utilizing the ex vivo strategy two considerations are critical for delivery of therapeutic levels of transgene product to the target: (i) the vector system and (ii) the cell type for grafting. We describe herein different vector systems that are currently available and briefly review the various cell types that have been transduced and grafted into the striatum of animals with experimental Parkinson's disease. The strategies for application of gene therapy techniques to a treatment for Parkinson's disease have expanded beyond the classical dopamine replacement toward the use of neurotrophic factors in enhancing cell function or preventing cell death. In addition, we explore the utility of CNS-derived neural progenitors as alternative cell types for ex vivo gene therapy in an animal model of Parkinson's disease.

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.

Epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), and basic fibroblast growth factor (bFGF) differentially influence neural precursor cells of mouse embryonic mesencephalon

Growth factors are key elements in the process of neural cell differentiation. We examined the effects of classical mitogens on neural precursor cells, by culturing mouse cells of the embryonic (13.5 days postcoitum) mesencephalon and treating them with epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha), basic fibroblast growth factor (bFGF), nerve growth factor (NGF), and transforming growth factor-beta (TGF-beta). Our initial results show that EGF, TGF-alpha, or bFGF, but not NGF or TGF-beta, induced general proliferation of the cultured cells, followed by formation of colonies. Combinations of these three growth factors suggest that most cells with the capacity to form colonies responded to EGF, TGF-alpha, or bFGF. The number of colonies increased significantly when EGF, but not TGF-alpha, was used in combination with bFGF. Furthermore, a population responding only to EGF + bFGF was detected in the dorsal mesencephalon. The colony-forming activity of bFGF was dependent on insulin, but bFGF and insulin cooperation was indirect since we could not observe colony formation in subcultures of cells derived from colonies, even in the presence of insulin. Cells obtained from our colonies displayed neuronal and glial morphology and expressed markers of both neurons and astrocytes; nestin, a marker of neural precursor cells, was also expressed in the majority of colonies. Growth factors also influenced neuronal maturation; the best neurite outgrowth was obtained from cells derived from bFGF-induced colonies cultured in the presence of EGF + bFGF. These data indicate the existence of neural precursor cells in the embryonic mesencephalon that respond differentially to growth factors.

Epidermal growth factor influences the neurotrophic/differentiating action of nerve growth factor

Exposure of naive PC12 cells, sympathetic neurons from rat superior cervical ganglia, and brain-derived septal neurons to epidermal and nerve growth factors simultaneously resulted in some alteration of cellular events induced by nerve growth factor alone. A more pronounced decline of catecholamine content, no additional change in acetylcholinesterase activity, and additive stimulation of RNA and protein syntheses were found in PC12 cells. Earlier elevation of the enzyme activity was observed in sympathetic but not in septal neurons. Epidermal growth factor appeared to support independently the same level of acetylcholinesterase activity in septal neurons as revealed for nerve growth factor during the first week and cell survival throughout 2 weeks of observation. The data obtained indicate that epidermal growth factor augments temporarily some effects of nerve growth factor, thus supporting the idea of an important role of mitogenic growth factors in neural development as complementary and/or substitutive regulators of nerve cell differentiation and survival.

Effects of leukemia inhibitory factor on the differentiation of astrocyte progenitor cells from embryonic mouse cerebral hemispheres

The effects of leukemia inhibitory factor (LIF) on the differentiation and proliferation of astrocyte progenitor cells in an enriched culture were studied. The astrocyte progenitor cells obtained from embryonic mouse cerebral hemispheres were enriched by subculturing twice in serum-free medium containing epidermal growth factor (EGF). They exhibited a flat, non-process-bearing morphology and were negative for glial fibrillary acidic protein (GFAP), a specific marker for astrocytes. Culture of the cells with LIF resulted in the expression of GFAP and in the inhibition of growth. These results suggest that LIF promotes the differentiation of astrocyte progenitor cells.

Proliferative zones of postnatal rat brain express epidermal growth factor receptor mRNA

Two ligands for the epidermal growth factor receptor (EGF-R), EGF and transforming growth factor-alpha (TGF alpha), have recently been shown to influence the proliferation, differentiation or survival of diverse populations of fetal and neonatal neuronal and glial cells in culture. These findings suggest that EGF, TGF alpha, or another EGF-R ligand play a role in the regulation of similar cellular developmental events in vivo. In the present study, in situ hybridization with an 35S-labeled cRNA probe was used to determine if mRNA for EGF-R is expressed in two principal germinal zones of the postnatal rat brain, the forebrain ventricular/subventricular zone and the cerebellar external granule layer. Cells labeled with the EGF-R cRNA were distributed throughout the subventricular zone, particularly in the dorsolateral aspect, from birth to adulthood, although the numbers of labeled cells as well as the density of hybridization diminished during development. In the developing cerebellum, virtually all cells in the external granule layer were densely labeled with the EGF-R cRNA, as were numerous perikarya throughout the molecular layer. EGF-R mRNA was also transiently expressed at lower levels by neurons of the internal granule layer and deep cerebellar nuclei. By adulthood, cerebellar expression of EGF-R mRNA was not detected. These results demonstrate prominent expression of EGF-R mRNA within germinal zones of the developing brain and indicate a role for EGF, TGF alpha, or another member of the EGF-related family in regulating the activities of neuronal and glial progenitor cells in vivo.

Phenotypic changes induced by replating of early post-natal rat chromaffin cells

Postnatal chromaffin cells from rat adrenal medulla in culture respond to NGF by expressing neuronal traits. The replating of chromaffin cells after trypsinization produced neurite growth in a manner similar to that of NGF. The combination of replating and NGF exposure did not induce phenotypic changes over and above those observed by NGF alone. The morphological changes are independent of the preliminary culture conditions. The results of this study demonstrate for the first time that simple replating of young chromaffin cells can induce neuronal traits indistinguishable from those observed with NGF.

Neuroblast cell death in ovo and in culture: interaction of ethanol and neurotrophic factors

We used two experimental paradigms to examine the influence of the neurotrophins, NGF, EGF, and bFGF on normal neuroblast survival and also after ethanol insult. In the first paradigm, chick embryos received in ovo at embryonic day 1 and 2 (E1 and E2) saline (control) ethanol (10mg/50 microliters/day), NGF (50 ng/50 microliters/day), or EGF (25 ng/50 microliters/day), or ethanol+NGF or EGF. At E3, cultures were prepared from whole embryos separately from each group. At C2, all cultures were labeled with [3H]thymidine and assessed for effects or neuronal survival. In the second paradigm, cultures were prepared from 3-day-old whole embryos and at C0, cultures were treated with either ethanol (50 mM) alone, NGF (50 ng/ml) alone, EGF (25 ng/ml) alone, bFGF (50 ng/ml) alone, or were treated concomitantly with ethanol plus one of the neurotrophins; control had only the culture medium, DMEM + 5% FBS. We obtained the following findings. 1) Cultures derived from embryos treated with either of the three neurotrophins exhibited a higher neuronal survival as compared to controls (1st paradigm). 2) The survival-promoting effect was also observed when the neurotrophins were added directly to the cultures (2nd paradigm). 3) As reported previously, cultures derived from ethanol-treated embryos exhibited a marked decline in neuronal survival as compared to controls. 4) All three neurotrophins attenuated the decline in neuronal survival produced by ethanol. The 'rescuing' effects of the neurotrophins support our early hypothesis that ethanol administration during early neurogenesis interferes with microenvironmental trophic signals essential for neuroblast survival and differentiation.

Localization and characterization of epidermal growth-factor receptors in the developing rat medial septal area in culture

The presence and binding properties of epidermal growth-factor receptors (EGF-Rs) in different cell types purified from the rat medial septal area in culture were investigated. We report that astrocytes, oligodendrocytes and neurons from this area possess EGF-Rs while microglia do not. EGF-binding sites are detectable on astrocytes derived from the medial septum of both embryonic and neonatal rats. Scatchard analysis of the data for astrocytes from the fetal rats show that EGF specifically binds to both high- (Kd = 7.21 x 10(-10) M, Bmax = 3602 receptors/cell) and low-affinity (Kd = 3.99 x 10(-8) M, Bmax = 86,265 receptors/cell) receptors on these cells. On the other hand, astrocytes purified from neonatal tissue possess a greater number of high-affinity receptors (Bmax = 10,938 receptors/cell) when compared with the embryonic astroglia. With time in culture, the number of both types of receptors on neonatal astrocytes decreases. Oligodendrocytes also possess high- and low-affinity EGF-Rs with dissociation constants of 3.25 x 10(-10) M and 3.85 x 10(-8) M, respectively. The number of receptors on oligodendrocytes is significantly lower than those of neonatal astrocytes (Bmax = 1185 and 25,081 receptors/cell for high- and low-affinity binding sites, respectively). Finally, neurons from this area also exhibit two different EGF-R types with dissociation constants similar to those described for astrocytes. As the number of receptors/neuron (Bmax = 136 and 1159 receptors/cell for high- and low-affinity binding sites, respectively) appears to be extremely low, it is possible that EGF specifically binds only to a subpopulation of neurons from this area. These studies demonstrate which cell types in the developing medial septal area possess EGF-Rs and provide a detailed characterization of these binding sites. These EGF-R-bearing cells may be potential targets for this growth factor or for transforming growth factor alpha in this brain area.

Growth factor modulation of injury-reactive ependymal cell proliferation and migration

Injury-reactive ependymal cells from regenerating axolotl spinal cord can be maintained in their mesenchymal outgrowth phase in culture (O'Hara et al., 1992). To address the ability of specific growth factors in stimulating or maintaining migration and proliferation, mesenchymal ependymal cell cultures derived from injured axolotl spinal cord at 2 weeks post-lesioning were used to determine the potential effects of epidermal growth factor, platelet-derived growth factor and transforming growth factor-beta 1. In our cultures, medium containing epidermal growth factor alone or in combination with the other growth factors permitted significant migration and proliferation from ependymal explants. Platelet-derived growth factor alone was shown to have a small positive effect on ependymal cell migration and no effect on proliferation. Transforming growth factor-beta 1 alone did not support cell migration and was found to be inhibitory towards cellular proliferation. Lastly, medium containing platelet-derived growth factor and transforming growth factor-beta 1, but not epidermal growth factor, caused ependymal cell explants to break apart and migrate on the dish as cords. Migration and proliferation of injury-reactive ependymal cells was shown to be dependent on epidermal growth factor in vitro. These results suggest that epidermal growth factor may be a critical component in vivo during the initiation of ependymal migration and proliferation following transection of the axolotl spinal cord. The reorganization of cultured ependymal cells in response to the combination of platelet-derived growth factor and transforming growth factor-beta shows that ependymal organization can be modulated by growth factors. This suggests that the progressive changes observed during regeneration may be under the control of growth factors.

Epidermal growth factor and basic fibroblast growth factor have independent actions on mesencephalic dopamine neurons in culture

Epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) are both trophic for dopamine neurons in cultures of dissociated embryonic rat mesencephalon, but the significance of this apparent overlap in neurotrophic activity is not yet known. In this study, we investigated the mechanisms of action of these two growth factors and the potential relationship between them. Using a nuclease protection assay, we determined that bFGF mRNA was expressed in the cultures. Double-label immunocytochemistry revealed that bFGF immunoreactive material could be detected in tyrosine hydroxylase immunoreactive neurons and glial fibrillary acidic protein immunoreactive astrocytes. EGF treatment increased bFGF mRNA content per culture dish. As we have previously demonstrated that EGF exerts its dopaminergic neurotrophic activity via an intermediate cell type, studies were designed to address whether the pathway by which EGF acts on dopaminergic neurons is mediated by the release of bFGF. However, the trophic action of EGF on dopamine neurons, represented by high-affinity neuronal dopamine uptake, could not be blocked by immunoneutralization of bFGF, suggesting that the actions of EGF were not mediated by bFGF release. The time course of the effects of EGF and bFGF on dopamine uptake were similar, with significant increases detectable only after 5 days in culture. Both growth factors were active in the picomolar-to-nanomolar range with maximal trophic activity between 0.4 and 2.5 nM. EGF, however, was the more potent mitogen under these conditions. When cultures were simultaneously incubated with maximal concentrations of EGF and bFGF, the effect on dopamine uptake was significantly greater than with either growth factor alone and, in fact, approximated the sum of the individual effects. On the basis of these results we conclude that these growth factors have independent effects on dopamine neurons of the mesencephalon.

Oligodendrocyte-type-2 astrocyte (O-2A) progenitors increase the survival of rat mesencephalic, dopaminergic neurons from death induced by serum deprivation

When a primary culture of E16 rat striatal cells was grown in a serum-free medium, treatment with basic fibroblast growth factor (bFGF, 10 ng/ml) caused the generation of the progenitor cell for oligodendrocytes and type-2 astrocytes (O-2A). Immunostaining tests confirmed that > 90% of the cells were positive for A2B5, and < 5% positive for glial fibrillary acidic protein (GFAP). When E14, mesencephalic, dopaminergic neurons were co-cultured with established O-2A progenitor cells in a serum-free growth medium, the survival of tyrosine hydroxylase-positive (TH+) neurons increased 23-fold and 668-fold at the 5th and 10th days, respectively, compared with control cultures plated on poly-D-lysine. Conditioned medium from the O-2A progenitor cultures also decreased the death of TH+ neurons. The mitotic inhibitor, cytosine arabinoside (1.0 microM), did not block the protective effect of the O-2A progenitor cells. O-2A progenitor cells produce a potent, soluble factor, that mediates the increased survival of dopaminergic neurons in vitro.

Prostaglandin E1 induces c-Fos and Myc proteins and protects rat hippocampal cells against hypoxic injury

We investigated the effects of prostaglandin (PG) E1 on the hypoxic injury of fetal rat hippocampal cells. Primary hippocampal cell cultures (embryonic day 18) were established and maintained. After 72 h in culture, PGE1 was added to the serum-free medium at a final concentration of 10(-5)-10(-9) M. Cultures were divided into two groups: The normoxia group was in culture for another 48 h, and the hypoxia group was exposed to 24 h of hypoxia followed by continuation of culture for another 24 h. As a quantitative measure of cell death, lactate dehydrogenase (LDH) activity was estimated in the culture medium. The LDH activity, released by the hypoxic insult, was significantly smaller with PGE1 treatment at 10(-6), 10(-7), and 10(-8) M (p < 0.01) and 10(-9) M (p < 0.05) compared with the control. No differences in the LDH activities were observed in the normoxia group. Glial culture was not affected by the hypoxia. Western blot analysis showed an increased induction of 62-kDa c-Fos and 58, 60, and 66 kDa Myc proteins in rat hippocampal cells with 10(-7) M PGE1 treatment. We conclude that PGE1 at concentrations of 10(-6)-10(-9) M protects rat hippocampal neurons against hypoxic insult.

Proliferation of neuronal precursor cells from the central nervous system in culture

Studies over the past ten years have revealed that neuronal precursors from the central nervous system of chick, rat and mouse embryos are able to divide in culture and that their proliferation is enhanced by several nervous tissue extracts as well as by growth factors, hormones and various other molecules. In this article we present an overview of this subject. It has been found that neuronal precursors from chick embryo cerebral hemispheres proliferate in culture during the first week and that those from 6 day-old chick embryos possess the highest proliferative activity. Neuronal precursors from fetal rat cerebral cortex and spinal cord can also proliferate in vitro. The highest proliferative activity was observed between 24 and 48 h. Brain and meningeal extracts have been shown to stimulate the proliferation of chick neuroblasts. Moreover, RNAs, purine nucleotides, purine bases and transferrin present in these extracts are able to reinduce the proliferation of these cells. Other investigations have indicated that several hormones and growth factors stimulate the proliferation of rat and mouse neuronal precursors. Acidic and basic fibroblast growth factors are potent mitogens for these cells. Nerve growth factor, epidermal growth factor and insulin-like growth factor also affect the growth of the neuroblasts. The reported in vitro observations are discussed in relation to the physiological role of these molecules during neuronal proliferation in brain development.

Trophic actions of transforming growth factor alpha on mesencephalic dopaminergic neurons developing in culture

Transforming growth factor alpha messenger RNA and protein levels are highest in the striatum, the target area of mesencephalic dopaminergic neurons of the substantia nigra, suggesting a role as a target-derived neurotrophic factor for these cells. To test this hypothesis, we characterized the actions of transforming growth factor alpha on fetal rat dopaminergic neurons in culture. Transforming growth factor alpha promoted dopamine uptake in a dose- and time-dependent manner. Administration of transforming growth factor alpha at the time of plating for 2 h produced a significant increase in dopamine uptake after five days of growth in vitro. As cultures aged they became less responsive to transforming growth factor alpha, such that longer times of exposure were required to elicit a similar, but weaker, response. Dopaminergic cell survival was selectively promoted by transforming growth factor alpha, since there was an increase in the number of tyrosine hydroxylase-immunostained cells without a parallel increase in the total number of neuron-specific enolase-immunopositive cells. Neurite length, branch number and soma area of tyrosine hydroxylase-immunopositive cells also were enhanced by transforming growth factor alpha treatment. Increases in each of the dopaminergic parameters due to transforming growth factor alpha were accompanied by a rise in glial cell number, making it possible that these effects were mediated by this cell population. The neurotrophin antagonist, K252b, failed to inhibit the transforming growth factor alpha-induced increase in dopamine uptake, indicating that transforming growth factor alpha's effects were not mediated by neurotrophin mechanisms. The actions of transforming growth factor alpha on the differentiation of dopaminergic neurons only partially overlapped with those of epidermal growth factor. Thus, while transforming growth factor alpha and epidermal growth factor are believed to share the same receptor they differentially affect dopaminergic cell development in vitro. These results indicate that transforming growth factor alpha is a trophic factor for mesencephalic cells in culture and suggests that transforming growth factor alpha plays a physiological role in the development of these cells in vivo.