Fibroblast growth factors, depolarization, and substratum interact in a combinatorial way to promote neuronal survival

Role of brain-derived neurotrophic factor in the protective action of N-methyl-D-aspartate in the apoptotic death of cerebellar granule neurons induced by low potassium

Several neurotrophic factors, including brain-derived neurotrophic factor (BDNF), and neurotransmitters, such as glutamate, may influence neuronal apoptotic death. Rat cerebellar granule neurons (CGN) cultured in low potassium (5 or 10 mM KCl) for more than 5 days in vitro (DIV) die apoptotically. These cells survive in the presence of high potassium (25 mM KCl, K25) or N-methyl-D-aspartate (NMDA), an agonist of glutamatergic receptors. CGN transferred from high to low potassium die apoptotically. Here, we characterized the effect of BDNF and NMDA on the apoptotic death induced by low potassium in CGN. Cell death of CGN by culturing in low potassium for 6 DIV was inhibited by BDNF and NMDA. When CGN were cultured in K25 and transferred to a low-potassium medium, 65% of neurons died after 48 hr. Under these conditions, BDNF, NMDA, or BDNF + NMDA increased CGN survival. Both BDNF and NMDA decreased caspase-9 activity and mRNA caspase-3 levels and activity induced by low potassium. CGN survival induced by BDNF is mediated by TrkB activation, whereas that induced by NMDA is mediated by NMDA receptor and TrkB activation. NMDA, but not BDNF, raised [Ca(2+)](i), which was reduced by low-potassium treatment. These results suggest that NMDA receptor stimulation induces CGN survival through the influx of extracellular Ca(2+) that may evoke the release of BDNF and the activation of TrkB. Complementary mechanisms induced by depolarization and changes in Ca(2+) levels would also contribute to the neuroprotection exerted by NMDA and potassium.

Temporal dynamics of neurite outgrowth promoted by basic fibroblast growth factor in chick ciliary ganglia

Basic fibroblast growth factor (bFGF) is a potent and multifunctional neurotrophic factor that can influence neuronal survival and differentiation. It has been shown to modulate growth and orientation of neuritic processes both in intact organs and in neuronal cultures, with a wide spectrum of effects on different preparations. Here we report that it promotes neurite growth in developing parasympathetic neurons from the chick ciliary ganglion. We have used both organotypic cultures and dissociated neurons, and we have combined assessment of global neurite growth by immunocytochemical techniques with evaluation of dynamic parameters of single neurites via time-lapse microscopy. We show that laminin, a molecule of the extracellular matrix that has been associated with stimulation of neurite extension, has only a limited and short-lived effect on neurite outgrowth. In contrast, bFGF can promote global growth of the neuritic network both in whole ganglia and in dissociated cultures for times up to 48 hr, and this effect is related to an increase in the growth rate of single neurites. Moreover, the effect can be observed even in enriched neuronal cultures, pointing to a direct action of bFGF on neurons.

Expression of bFGF and NGF and their receptors in chick’s auditory organ following overexposure to noise

Growth factors are known to activate signaling cascades for DNA replication; they participate in the regulation of cell differentiation and are required as positive signals for cell survival. Thus, many of them may be regarded as potential candidates stimulating regeneration processes in the inner ear. We analyzed the expression of basic fibroblast growth factor (bFGF) and nerve growth factor (NGF) and their receptor (bFGFR and NGFR)-like immunoreactivity in chick basilar papillae, along with bFGF and NGF mRNA expression. The evaluation was made 1 and 5 days after exposure to wide-band noise with two increasing levels of acoustic energy. For both factors, the immunoreactivity was shown predominantly in the middle part of basilar papilla, in noise-exposed, but not control birds. It was localized in the cytoplasm of hair cells, nuclei of supporting cells and cytoplasm of ganglion cells. Strong immunoreactivity of bFGFR and NGFR was found both in control and noise-exposed animals, with the cell localization similar to that of growth factors. The increase in mRNA expression for bFGF and NGF was found in noise-exposed animals only after lower exposure to noise, on day 5 after exposure (p<0.01). A lack of increased expression after higher exposure could be excused by larger damage of hair cells followed by the increase of mRNA for beta-actin to which the results were referred. The results suggest bFGF and NGF involvement in postinjury regeneration of the basilar papilla.

Fate of constitutive endocytic vesicles formed in the growth cone: transport of vesicles from one growth cone to another in the same neuron

A developing neuron must have multiple paths of communication coordinating events among all its parts. One of these is the transport to the cell body of endocytic vesicles formed in growth cones. In order to observe this at single cell resolution, we developed a technique in which the fluorescent dye FM1-43 was applied to a single growth cone and newly formed constitutive endocytic vesicles were labeled. Using low light, time-lapse microscopy we were able to follow the movement of these vesicles throughout the neuron. The vast majority of the transported vesicles went to the cell body. However, many were observed to enter secondary neurites and to be transported to other growth cones. These new, more direct paths of transport that link the multiple growth cones of a neuron may play a role in several important developmental events involving interactions between the multiple neurites of a single neuron.

Homeostatic effects of depolarization on Ca2+ influx, synaptic signaling, and survival

Depolarization promotes neuronal survival through moderate increases in Ca(2+) influx, but the effects of survival-promoting depolarization (vs conventional trophic support) on neuronal signaling are poorly characterized. We found that chronic, survival-promoting depolarization, but not conventional trophic support, selectively decreased the somatic Ca(2+) current density in hippocampal and cerebellar granule neurons. Depolarization rearing depressed multiple classes of high-voltage activated Ca(2+) current. Consistent with the idea that these changes also affected synaptic Ca(2+) channels, chronic depolarization presynaptically depressed hippocampal neurotransmission. Six days of depolarization rearing completely abolished glutamate transmission but altered GABA transmission in a manner consistent with the alterations of Ca(2+) current. The continued survival of depolarization-reared neurons was extremely sensitive to the re-establishment of basal culture conditions and was correlated with the effects on intracellular Ca(2+) concentration. Thus, compared with cells reared on conventional trophic factors, depolarization evokes homeostatic changes in Ca(2+) influx and signaling that render neurons vulnerable to cell death on activity reduction.

Survival and differentiation of neurons cultured from the mouse cochlear nucleus on extracellular matrix components

This study was designed to determine the effects of several extracellular matrix (ECM) proteins on the survival and differentiation of cochlear nucleus (CN) neurons grown in primary cell culture. CN neurons were successfully grown on laminin, collagen and fibronectin, but neuronal survival was significantly decreased on all three substrates relative to poly-D-lysine controls. In addition, analysis of dendrite organization patterns indicated that CN neurons branched more extensively when grown on poly-D-lysine than on any of the ECM components tested. Although dendrite growth has been shown to depend strongly on the presence of ECM components in other neuronal systems, the data from these experiments do not support the hypothesis that laminin, collagen or fibronectin is necessary for the survival and differentiation of CN neurons.

A sensory neuron subpopulation with unique sequential survival dependence on nerve growth factor and basic fibroblast growth factor during development

We characterized a subpopulation of dorsal root ganglion (DRG) sensory neurons that were previously identified as preferential targets of enkephalins. This group, termed P-neurons after their "pear" shape, sequentially required nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) for survival in vitro during different developmental stages. Embryonic P-neurons required NGF, but not bFGF. NGF continued to promote their survival, although less potently, up to postnatal day 2 (P2). Conversely, at P5, they needed bFGF but not NGF, with either factor having similar effects at P2. This trophic switch was unique to that DRG neuronal group. In addition, neither neurotrophin-3 (NT-3) nor brain-derived neurotrophic factor influenced their survival during embryonic and postnatal stages, respectively. The expression of NGF (Trk-A) and bFGF (flg) receptors paralleled the switch in trophic requirement. No single P-neuron appeared to coexpress both Trk-A and flg. In contrast, all of them coexpressed flg and substance P, providing a specific marker of these cells. Immunosuppression of bFGF in newborn animals greatly reduced their number, suggesting that the factor was required in vivo. bFGF was present in the DRG and spinal cord, as well as in skeletal muscle, the peripheral projection site of P-neurons, as revealed by tracer DiIC(18)3. The lack of requirement of NT-3 for survival and immunoreactivity for the neurofilament of 200 kDa distinguished them from muscle proprioceptors, suggesting that they are likely to be unmyelinated muscle fibers. Collectively, their properties indicate that P-neurons constitute a distinct subpopulation of sensory neurons for which the function may be modulated by enkephalins.

In vitro inhibition of antirecoverin immunoglobulin-mediated death of mammalian photoreceptor cells

Cancer-associated retinopathy (CAR) is a blinding disease, which can be mediated by autoimmune reactions with a specific calcium-binding retinal protein, recoverin. A number of recent studies demonstrate that agents that mobilize intracellular calcium can protect neurons from apoptotic death induced by a variety of insults. In this study, we investigated the effect of one such agent, potassium, on the survival of mammalian rod photoreceptors exposed to antirecoverin IgG. Primary cell cultures of rat retinal neurons were grown in a chemically defined medium, and cells were exposed to antirecoverin IgG for 72 hr in various concentrations of potassium and the surviving cells counted. Rod photoreceptors were quantitated using antirhodopsin immunofluorescence microscopy, and total cell numbers were determined by 4',6-diamidino-2-phenylindole (DAPI) staining of nuclei. Apoptosis was evaluated by TdT-mediated biotin-dUTP nick-end labeling (TUNEL), cell death-detection ELISA, and DNA laddering. The present study shows that elevated extracellular K+ ([K+](o)) protects retinal neurons from antirecoverin antibody-mediated cell death. The protective effects of ([K+](o)) were shown to be time- and dose-dependent. The inhibition of antirecoverin IgG-mediated death of photoreceptors by elevated ([K+](o)) suggests that the mobilization of internal calcium stores rescues the cells by interfering with apoptotic signal transduction pathways. These data also suggest that the death of photoreceptor cells occurring in CAR possibly can be prevented by reagents and/or environmental changes that mobilize intracellular calcium.

FGF9: a motoneuron survival factor expressed by medial thoracic and sacral motoneurons

In the nervous system, fibroblast growth factor-9 (FGF9) is produced mainly by neurons. By whole-mount in situ hybridization, on embryonic rat spinal cord, we observed Fgf9 expression in a subpopulation of motoneurons located in the thoracic and sacral regions of the median motor column that innervate the axial muscles. Furthermore, FGF9 prevented death of purified rat and chicken motoneurons in culture in the same concentration range as FGF2. The targets of FGF9 are more restricted than that of the other FGFs, however, because conversely to FGF1 or FGF2, FGF9 had only weak or inexistent survival effects on chicken ciliary neurons or rat DRG. FGF9 may therefore play a role as an autocrine/paracrine survival factor for motoneurons.

Nicotinic acetylcholine receptor agonists promote survival and reduce apoptosis of chick ciliary ganglion neurons

The abundance, diversity, and ubiquitous expression of neuronal nicotinic acetylcholine receptors (AChRs) suggest that many are involved in functions other than synaptic transmission. We now report that a major AChR class promotes neuronal survival. The 10-day survival of ciliary ganglion neurons in basal culture medium (MEM) was approximately 35%, but increased to approximately 75% in MEM containing nicotine (MEM/Nic) or carbachol, an effect similar to that achieved by chronic depolarization with KCl. Pharmacological experiments revealed that agonist-enhanced survival requires activation of AChRs sensitive to alpha-bungarotoxin (alphaBgt). alphaBgt-AChRs partly support neuronal survival by limiting apoptosis since fewer apoptotic neurons were observed in MEM/Nic compared to MEM. Moreover, nicotinic survival support was not further enhanced by fibroblast growth factor, as seen for KCl, but increased to 100% by adding PACAP, a trophic neuropeptide present in the ganglion. These results indicate that alphaBgt-AChR activation regulates neuronal survival and suggest a mechanism involving reduced apoptosis and interaction with an endogenous neuropeptide growth factor.

Membrane recycling in the neuronal growth cone revealed by FM1-43 labeling

Membrane dynamics within the chick ciliary neuronal growth cone were investigated by using the membrane-impermeant dye FM1-43. A depolarization-evoked endocytosis was observed that shared many properties with the synaptic vesicle recycling previously described at the presynaptic terminal. In addition, in the absence of depolarization a basal level of constitutive endocytotic activity was observed at approximately 30% of the rate of evoked endocytosis. This constitutive endocytosis accounted for large amounts of membrane retrieval: the equivalent of the entire growth cone surface area could be internalized within a 30 min period. Endosomes generated via constitutive and evoked processes were highly mobile and could move considerable distances both within the growth cone and out to the neurite. In addition to their different requirements for formation, evoked and constitutive endosomes displayed a significant difference in release properties. After a subsequent depolarization of labeled growth cones, evoked endosomes were released although constitutive endosomes were not released. Furthermore, treatment with latrotoxin released evoked endosomes, but not constitutive endosomes. Although the properties of evoked endosomes are highly reminiscent of synaptic vesicles, constitutive endosomes appear to be a separate pool resulting from a distinct and highly active process within the neuronal growth cone.

Radicicol potentiates neurotrophin-mediated neurite outgrowth and survival of cultured sensory neurons from chick embryo

Radicicol, an antifungal antibiotic with markedly low toxicity, is a potent inhibitor of the Src family of protein tyrosine kinases and causes morphological reversion of v-src-transformed fibroblasts. Recently, this antibiotic was also found to inhibit Raf kinase. In the present study, we found that nanomolar concentrations of radicicol (10 ng/ml) enhanced the survival and neurite outgrowth of neurons from embryonic chick dorsal root ganglia (DRGs) and sympathetic ganglia. It potentiated the trophic effects of nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 on the cultured DRG neurons. This concentration of radicicol did not alter the tyrosine phosphorylation of Trk receptors or the activity of mitogen-activated protein (MAP) kinases. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), did not inhibit radicicol, excluding the involvement of PI3-kinase in the radicicol-dependent trophic actions. These results suggest that radicicol mediates neuronal growth presumably via a mechanism not involving the activation of Trk receptors, MAP kinase, or PI3-kinase.

Neuronal survival and calcium influx induced by basic fibroblast growth factor in chick ciliary ganglion neurons

Basic fibroblast growth factor (bFGF/FGF2) exhibits widespread biological activities in the nervous system. However, little is known about the cascade of intracellular events that links the activation of its tyrosine kinase receptors to these effects. Here we report that, in ciliary ganglion neurons from chick embryo, this trophic factor significantly enhanced neuronal survival. The percentage of surviving neurons was reduced when intracellular calcium was chelated by adding a membrane-permeable BAPTA ester to the culture medium, while antagonists of L- and N-type voltage-dependent calcium channels were ineffective. The ionic signals in response to bFGF stimulation have been studied using cytofluorimetric and patch-clamp techniques. In single-cell Fura-2 measurements, bFGF elicited a long lasting rise of the cytosolic calcium concentration that was dependent on [Ca2+]o. In whole-cell experiments, we observed a reversible depolarization of the membrane resting potential and an inward cationic current. Single channel experiments, performed in the cell-attached configuration, provide evidence for the activation of two families of Ca2+-permeable cationic channels. Moreover, inositol 1,4,5-trisphosphate opens channels with similar properties, suggesting that this cytosolic messenger can be responsible for the calcium influx induced by bFGF.

Responses of mature and aged sympathetic neurons to laminin and NGF: an in vitro study

Whilst the potent effects of NGF and laminin on developing neurons are well documented, relatively little is known about the effects of, or altered availability of or altered responsiveness to, these substances on the growth of adult neurons. We have therefore examined this question using explant cultures of sympathetic neurons from the superior cervical ganglion (SCG) of mature and aged rats. Explants were grown on substrata containing different doses of laminin, either with or without added NGF in culture medium containing FCS. Individually, laminin and NGF had relatively small effects on neurite outgrowth and length, which tended to be reduced in old neurons. In contrast, laminin in the presence of exogenous NGF exerted a powerful effect on nerve growth which was substantially greater than the sum of the effects of the individual factors. This synergy was evident in all experimental groups and was greatest in old explants at high doses of laminin, where growth was comparable to that of mature neurons. The dose-response curve of old neurons to laminin in the presence of added NGF indicated reduced responsiveness. These results suggest that variations in the availability of laminin and/or exogenous NGF, together with altered patterns of neuronal responsiveness, may contribute to impaired neuronal plasticity in old age.

Synchronization of neuronal activity promotes survival of individual rat neocortical neurons in early development

Neural activity is thought to play a significant role during the development of the cerebral cortex. In this study, we examined the effects of global activity block or enhancement and the effects of patterned firing on the ability of cultured rat neocortical neurons to survive during the second week in vitro, beyond the beginning of synaptogenesis. Blockade of neuronal activity by adding tetrodotoxin (TTX) and increasing magnesium concentration in the medium strongly reduced the survival of cortical cells. Increasing neuronal activity by raising the external potassium concentration significantly improved the survival of cortical neurons. We postulated that in a developing neuronal network the survival of nerve cells is regulated by synaptically mediated events that involve changes in the intracellular calcium concentration. To examine this question further, we monitored the activity of the developing network by optically recording the intracellular calcium signals of many neurons simultaneously. These recordings show that in low magnesium neocortical neurons express synchronized oscillation of their intracellular calcium concentration. The ability of a network to synchronize the changes in intracellular calcium of multiple cells appeared gradually during the second week in culture, paralleled by both an increase in the synaptic density and a decline in the number of surviving neurons. By examining the fate of identified cells several days after a recording session, we found that those nerve cells that were co-activated with other neurons had a significantly higher chance to survive than cells that did not participate in synchronized events. These experiments demonstrate that during early cortical network development cortical neurons show synchronized firing activity and that the survival of neurons is at least partially dependent on this pattern of neuronal activity.

Trophic support of cultured spiral ganglion neurons by depolarization exceeds and is additive with that by neurotrophins or cAMP and requires elevation of [Ca2+]i within a set range

Spiral ganglion neurons (SGNs) require both pre- and postsynaptic contacts to maintain viability. BDNF, NT-3, chlorphenylthio-cAMP, and depolarization (veratridine or elevated [K+]o) all promote survival of SGNs in vitro, depolarization being the most effective. Combining different trophic stimuli increases survival in an additive manner. Neurotrophins and depolarization maintain comparable soma size and neurite extension, but SGNs are shrunken in cAMP. Elevated [K+]o has a biphasic effect on SGN survival; survival improves as [K+]o is raised to 30 mM (30K) and falls as [K+]o is further increased; SGN survival in 80 mM [K+]o (80K) is poor relative to survival in 30K. These responses to elevated [K+]o are potentiated by an L-type channel agonist, whereas L-type Ca2+ channel blockers antagonize the trophic effect of depolarization. Four hours after depolarization, steady-state [Ca2+]i is elevated in SGNs in 30K and further elevated in SGNs in 80K. At 22 hr after depolarization, by which time death of neurons in 80K has begun, elevated [Ca2+]i levels in surviving neurons in 80K are not higher than those in neurons in 30K ( approximately 150-450 nM), suggesting that neurons with high [Ca2+]i are preferentially lost. Veratridine causes oscillatory increases in [Ca2+]i to 250-350 nM. Thus, [Ca2+]i is predictive of cell survival; [Ca2+]i elevated to 100-500 nM in a sustained or oscillatory manner permits SGN survival independent of exogenous neurotrophic factors. Higher [Ca2+]i is associated with cell death.

Target and afferents interact to control developmental cell death in the mesencephalic parabigeminal nucleus of the rat

During the period of natural cell death in the developing mammalian brain, both target cells and afferents have been shown to be important for neuronal survival. Here we demonstrate that afferents and targets have interactive roles in the maintenance of cells during development of the mesencephalic parabigeminal nucleus (PB) in rats. Pyknotic nuclei were counted in the PB of developing rats that received a bilateral lesion of the superior colliculus on the day of birth (P0). We observed that simultaneous deafferentation and deeferentation leads to a large peak of cell death at P1-2 in all three divisions of PB. Later the rate of pyknosis decreases and a second period of elevated cell death is observed just before the complete disappearance of the nucleus at P7-8. Counts of healthy neurones indicates two separate periods of increased neuronal loss. The first period occurs at P1-2, and the last and dramatic episode of cell loss at P8 leads to the disappearance of the PB. The combined effects of simultaneous target removal and deafferentation were different from the sum of the individual effects, indicating that the axonal targets and the afferents interact to control cell survival in the PB.

Adrenocorticotropic hormone activation of adenylate cyclase in raphe neurons: multiple regulatory pathways control serotonergic neuronal differentiation

The RN46A cell line was derived from embryonic day 13 rat medullary raphe cells by infection with a retrovirus encoding the temperature-sensitive mutant of SV40 large T antigen (tsT-ag). The RN46A cell line is neuronally restricted and constitutively differentiates following a shift to nonpermissive temperature. Differentiated RN46A cells express low levels of tryptophan hydroxylase (TPH) but no detectable levels of serotonin (5-HT). Treatment of cultures with the adrenocorticotropic hormone peptide ACTH4-10 up-regulates the expression of TPH immunoreactivity in differentiated RN46A cells, but 5-HT synthesis requires initial treatment with ACTH4-10, followed by partial membrane depolarizing conditions. Up-regulation of TPH by ACTH4-10 is apparently due to activation of adenylate cyclase, whereas the increased 5-HT synthesis with membrane depolarization can be blocked with the voltage-sensitive Ca(2+)-channel blockers nifedipine and omega-conotoxin. ACTH4-10 treatment also markedly up-regulates the expression of the 5-HT reuptake transporter, as do dibutyryl cyclic AMP and forskolin; chronic membrane depolarization has no effect on 5-HT reuptake. The expression of the high-affinity 5-HT1A receptor is increased threefold by ACTH4-10 treatment during differentiation and fivefold by differentiation under partial membrane depolarizing conditions. Combining ACTH4-10 treatment and membrane depolarization does not increase expression of the 5-HT1A receptor further. 5-HT release is constitutive in ACTH-treated RN46A cells and linked to spontaneous synaptic vesicle fusion in RN46A cells. Considered with previous results, these data indicate that multiple effectors, ACTH, brain-derived neurotrophic factor, and membrane depolarization, have both distinct and overlapping effects that regulate specific elements of the serotonergic neuronal phenotype during differentiation and maturation.

Investigation of the factors necessary for growth of hippocampal neurons in a defined system

We have developed an in vitro system that combines the use of a defined medium with a chemically defined surface for the differentiation of embryonic rat hippocampal neurons. Cells were grown on silica substrates modified with two chemically distinct molecules: poly-D-lysine and an amine-containing organosilane. Cells were dissociated by mechanical or enzymatic methods and grown in serum-containing versus serum-free medium on these surfaces. Our results demonstrate that optimal survival and growth in serum-free medium occurs on the artificial surfaces. X-ray photoelectron spectroscopy (XPS) was used to analyze the surfaces both before and after cell cultures. In addition, surface properties such as elemental composition, the initial thickness of the substrate material, and the thickness of material deposited during the course of cell culture were quantified after cell removal. Taken together, the results from the cell culture and surface analysis demonstrate that the media, proteins deposited from the media onto the surface, surface composition, and properties intrinsic to neuronal membranes all interact in a complex fashion to determine whether or not the cells will adhere and survive in culture. In particular, the role of material deposited from the medium onto the culture substratum may be more important than have been previously appreciated. This system allow for the study of neuronal differentiation in a well-defined environment.

FGF inhibits neurite outgrowth over monolayers of astrocytes and fibroblasts expressing transfected cell adhesion molecules

We have cultured cerebellar neurons on monolayers of cortical astrocytes in control medium or medium containing recombinant basic fibroblast growth factor (FGF). FGF was found to inhibit neurite outgrowth, with a significant effect seen at 0.5 ng/ml and a maximal effect at 10 ng/ml. FGF increased the production of arachidonic acid (AA) in cerebellar neurons, and when added directly to cultures or generated endogenously via activation of phospholipase A2 using melittin, this second messenger could mimic the inhibitory effect of FGF. FGF and AA could also specifically inhibit neurite outgrowth stimulated by three cell adhesion molecules (NCAM, N-cadherin and L1) expressed in transfected fibroblasts, or in the case of L1 bound to a tissue culture substratum. These data demonstrate that, in certain cellular contexts, FGF can act as an inhibitory cue for axonal growth and that arachidonic acid is the second messenger responsible for this activity. We discuss the possibility that arachidonic acid inhibits neurite outgrowth by desensitising the second messenger pathway underlying neuronal responsiveness to cell adhesion molecules.

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.

Brain-derived proteins that rescue spinal motoneurons from cell death in the chick embryo: comparisons with target-derived and recombinant factors

Spinal motoneurons that normally die during early development can be rescued by a variety of purified growth or neurotrophic factors and target tissue extracts. There is also indirect evidence that brain or supraspinal afferent input may influence lumbar motoneuron survival during development and that this effect may be mediated by central nervous system-derived trophic agents. This report examines the biological and biochemical properties of motoneuron survival activity obtained from extracts of the embryonic chick brain. Treatment with an ammonium sulfate (25% to 75%) fraction of embryonic day 16 (E16) or E10 brain extracts rescued many spinal motoneurons that otherwise die during the normal period of cell death in vivo (E6 to E10). The same fractions also enhanced lumbar motoneuron survival following deafferentation. There were both similarities and differences between the active fractions derived from brain extracts (BEX) when compared with extracts derived from target muscles (MEX) or with purified neurotrophic factors. Survival activity from E10 BEX was as effective in promoting motoneuron survival as E10 MEX and more effective than astrocyte-conditioned media. Unlike MEX, the active fractions from BEX also rescued placode-derived nodose ganglion cells. In addition, unlike nerve growth factor and brain-derived neurotrophic factor, active BEX fractions did not rescue neural crest-derived dorsal root ganglion cells or sympathetic ganglion neurons. Interestingly, among many cranial motor and other brainstem nuclei examined, only the survival of motoneurons from the abducens nucleus was enhanced by BEX. Active proteins obtained from BEX were further separated by gel filtration chromatography and by preparative isoelectric focusing techniques. Activity was recovered in a basic (pI 8) and an acidic (pI 5) small molecular weight protein fraction (20 kD or less). The specific activity of the basic fraction was increased x66 when compared with the specific activity of crude BEX, and the basic fraction had a slightly higher specific activity than the acidic fraction. The biological and biochemical properties of these fractions are discussed in the context of known neurotrophic factors and their effects on normal and lesion-induced motoneuron death during development.

Block of neuronal apoptosis by a sustained increase of steady-state free Ca2+ concentration

Programmed death is a ubiquitous feature of the development of the vertebrate nervous system. This death is prevented in vivo by trophic factors and by afferent input. Death of neurons can also be prevented in culture models of programmed death by trophic factors and by chronic depolarization with elevated concentrations of K+ in the culture medium. The latter effect is mediated by Ca2+ influx through voltage-gated channels and may prevent death by mimicking survival-promoting effects of naturally occurring electrical activity. Little is currently known about the mechanism by which either trophic factors or increased cytoplasmic Ca2+ promote survival.

Enhanced in vitro survival and growth of foetal human mesencephalic dopaminergic neurones on laminin and collagen: implications for cell banking

Culture of second trimester mesencephalic cells on laminin and collagen substrata has been investigated in an attempt to ascertain the effects of these extracellular matrix components on survival and growth of central dopaminergic (DA) neurones. There were 156.8-186.4% more cells attached to laminin and collagen than poly-D-lysine 6 h post-plating. By 24 h there was statistically no significant difference in the total number of cells attached to the three substrate but in terms of cell type-specific survival the proportion of mesencephalic DA neurones surviving on laminin and collagen substrata after 7 days in culture increased significantly compared with poly-D-lysine (1.4-1.6% versus 0.4% of the total cellular population), an effect augmented by bFGF treatment, which led to levels of 2% or more, with a concomitant decrease in the proportion of attritic DA neurones. These results indicate a critical requirement for ECM proteins in the survival and growth of in vitro-propagated central DA neurones at the time of plating and throughout the culture period. They also imply survival-enhancing interactions of ECM proteins and neurotrophic factors in developmental neuronal regulation and provide paradigms for obtaining high yields of these cells for neural transplantation cell banks.

Expression of trk in MAH cells lacking the p75 low-affinity nerve growth factor receptor is sufficient to permit nerve growth factor-induced differentiation to postmitotic neurons

We have transfected MAH cells, an immortalized sympathoadrenal progenitor cell line, with a plasmid encoding the 140-kDa Trk protein, a nerve growth factor (NGF) receptor with protein-tyrosine kinase activity. NGF promotes neurite outgrowth and proliferation from such cells, indicating that Trk is sufficient to mediate such responses in the absence of significant levels of the endogenous 75-kDa low-affinity NGF receptor (p75). These initial NGF responses are indistinguishable from those evoked by basic fibroblast growth factor (bFGF). However, NGF is sufficient to promote terminal differentiation of a approximately 8% of trk-transfected MAH cells to postmitotic, NGF-dependent neurons, whereas all cells eventually die in medium with bFGF. Other environmental signals (such as depolarization or ciliary neurotrophic factor) can cooperate with NGF to enhance production of postmitotic NGF-dependent neurons in trk-transfected MAH cells. The terminal differentiation of sympathetic neurons thus involves sequential and cooperative actions of different growth and neurotrophic factors, as well as cell-intrinsic changes in the response to these factors.