Depolarization and laminin independently enable bFGF to promote neuronal survival through different second messenger pathways

PACAP support of neuronal survival requires MAPK- and activity-generated signals

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in the parasympathetic ciliary ganglion (CG) and modulates nicotinic acetylcholine receptor function. PACAP also provides trophic support, promoting partial survival of CG neurons in culture and full survival when accompanied by membrane depolarization. We probed the adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP to determine their respective roles in supporting neuronal survival and examined their interaction with signals generated by membrane activity. While PLC-dependent signaling was dispensable, AC-generated signals proved critical for PACAP to support neuronal survival. Specifically, PACAP-supported survival was mimicked by 8Br-cAMP and blocked by inhibiting either PKA or the phosphorylation of mitogen-activated protein kinase (MAPK). The ability of PACAP to promote survival was additionally dependent on spontaneous activity as blocking Na+ or Ca2+ channel currents completely abrogated trophic effects. Our results underscore the importance of coordinated MAPK- and activity-generated signals in transducing neuropeptide-mediated parasympathetic neuronal survival.

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.

Specific beta1 integrins mediate adhesion, migration, and differentiation of neural progenitors derived from the embryonic striatum

Early inductive signals within the embryonic mammalian forebrain establish two major germinal regions along the dorsal-ventral axis. The dorsal germinal zone eventually forms the cerebral cortex while the ventral ganglionic eminence primarily forms the striatum and globus pallidus. The mechanisms leading to patterning of specific forebrain structures from these distinct germinal regions are not fully understood but may involve the adhesive and migratory properties of regionally specified cells and their interactions with the extracellular environments in which they reside. In the present study, we isolated ganglionic eminence neural progenitor cells (geNPC), precursors of the adult striatum, from the ventral forebrain germinal zone and analyzed adhesion, migration, and differentiation of geNPC on various extracellular matrix (ECM) substrates in vitro. Specifically, we evaluated the role of beta1 integrins, a family of cell surface receptors important in neural development, in mediating geNPC behavior on ECM molecules expressed in embryonic brain tissue. Adhesion and migration of geNPC were significantly enhanced on laminin (LN) and fibronectin (FN) relative to other ECM substrates. Antibody perturbation experiments revealed that although geNPC express several beta1 integrins (alpha1beta1, alpha2beta1, alpha3beta1, alpha5beta1, alpha6beta1, alphavbeta1), adhesion and migration on LN and FN were primarily mediated by alpha6beta1 and alpha5beta1, respectively, and these interactions were confirmed by biochemical cross-link/extraction procedures. Finally, neuronal differentiation of geNPC was enhanced on LN, indicating a role for LN in geNPC differentiation. beta1 integrin-ECM interactions may contribute to basic mechanisms of striatal development and may explain the potent migratory capacity of geNPC transplanted into the adult brain.

Development of cranial parasympathetic ganglia requires sequential actions of GDNF and neurturin

The neurotrophic factors that influence the development and function of the parasympathetic branch of the autonomic nervous system are obscure. Recently, neurturin has been found to provide trophic support to neurons of the cranial parasympathetic ganglion. Here we show that GDNF signaling via the RET/GFR(alpha)1 complex is crucial for the development of cranial parasympathetic ganglia including the submandibular, sphenopalatine and otic ganglia. GDNF is required early for proliferation and/or migration of the neuronal precursors for the sphenopalatine and otic ganglia. Neurturin exerts its effect later and is required for further development and maintenance of these neurons. This switch in ligand dependency during development is at least partly governed by the altered expression of GFR(α) receptors, as evidenced by the predominant expression of GFR(α)2 in these neurons after ganglion formation.

Regulation of retinal neurite growth by alterations in MAPK/ERK kinase (MEK) activity

Activation of the extracellular-signal regulated kinase (ERK) cascade may be involved in the promotion of neurite outgrowth by a variety of stimuli. For example, we have previously shown that laminin (LN) and N-cadherin activate ERK2 in chick retinal neurons, and that pharmacological inhibition of MAPK/ERK kinase (MEK), the major upstream ERK2 activator, severely impairs neurite growth induced by these proteins. We have therefore hypothesized that ERK activation through MEK is required for optimal induction of neurite growth by these proteins. Here we show that expression of mutant MEK in transfected retinal neurons alters neuronal responses to LN in a manner consistent with this hypothesis. Neurons expressing a constitutively active MEK construct extended longer neurites on LN than controls, while neurons transfected with a dominant negative construct extended shorter neurites. Further, experiments in which transfected neurons were replated onto polylysine substrates suggest that activation of MEK is sufficient for neurite promotion on a non-inducing substrate, and neurons replated onto LN confirm the pharmacological data that inhibition of MEK activation inhibits LN-induced neurite growth. We conclude that ERK activation plays a direct role in the promotion of neurite outgrowth from retinal neurons by LN.

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.

CAMs and FGF cause a local submembrane calcium signal promoting axon outgrowth without a rise in bulk calcium concentration

Binding of basic fibroblast growth factor (bFGF) and cell adhesion molecules to the nerve cell membrane promotes axon outgrowth. This response can be blocked by antagonists of voltage-gated calcium channels, yet no change of cytosolic calcium concentration in the growth cone can be detected upon binding of the growth factor bFGF or the cell adhesion molecule L1. Using barium as a charge carrier, we show that bFGF and L1 open a calcium influx pathway in growth cones of rat sensory neurons without changing the membrane voltage. L1 does not activate influx in cells expressing a dominant negative mutant of the fibroblast growth factor receptor (FGFR) tyrosine kinase. FGFR-activated influx is blocked by specific antagonists of L- and N-type voltage-gated calcium channels and by an inhibitor of diacylglycerol lipase. We propose that both L1 and bFGF act via the FGFR to generate polyunsaturated fatty acids which in turn cause calcium channels to flicker open and shut. Short-lived domains of raised calcium at the cytosolic mouth of open channels activate axon outgrowth without raising bulk cytosolic calcium concentration. In confirmation of this model, the rapidly-acting calcium buffer BAPTA is significantly more effective at blocking FGF-induced axon outgrowth when compared with the slower buffer EGTA. Generation of short-lived calcium domains may provide a crucial mechanism for axon guidance during development and for promoting regeneration of damaged axons.

TGFbeta1 regulates the gating properties of intermediate-conductance KCa channels in developing parasympathetic neurons

The developmental expression of Ca2+-activated K+ channels (KCa) in chick ciliary ganglion (CG) neurons is regulated by a target-derived avian isoform of TGFbeta1, which evokes a robust increase in the number of functional large-conductance (BK) KCa channels but which produces no change in their kinetics. However, CG neurons express multiple KCa channel subtypes. Here we show that TGFbeta1 regulates the gating properties of intermediate-conductance (IK) KCa channels in developing CG neurons. IK channels in inside-out patches excised from control E9 CG neurons became active on exposure to 1-5 microM free Ca2+ but then remained active on return to Ca2+-free salines. In contrast, IK channels in TGFbeta1-treated cells became active on exposure to 1-5 microM Ca2+, but became quiescent immediately on return to Ca2+-free salines. In contrast to its effects on BK channels, TGFbeta1 had no effect on the mean number of IK channels detected in excised patches. IK channels were not activated in cell-attached patches on E9 neurons depolarized by bath application of 145 mM KCl in the presence of 5 mM external Ca2+. However, BK channels were activated immediately by this procedure and were detected at a higher density in TGFbeta1-treated cells. In addition, analyses of macroscopic KCa fluctuations, and the voltage-dependence of KCa tail currents, suggest that IK channels do not contribute to voltage-evoked whole cell KCa. IK channels therefore may have some other function. These results indicate that the effects of TGFbeta1 on CG neurons entail distinct actions on multiple KCa channel subtypes.

Basic Fibroblast Growth Factor Is Expressed by CD19/CD11c-Positive Cells in Hairy Cell Leukemia

Several features are characteristic for hairy cell leukemia (HCL). Among those are pancytopenia, bone marrow fibrosis, and the appearance of a defined tumor cell phenotype in peripheral blood (PB), bone marrow (BM), and spleen. Hairy cells (HC) coexpress antigens specific for B lymphocytes and monocytes/macrophages and thus the malignant cell does not seem to be restricted to a defined lineage. When serum or bone marrow aspirate was screened by enzyme-linked immunosorbent assay (ELISA) for basic fibroblast growth factor (bFGF), specimen derived from HCL (serum: mean value, 29 pg/mL; BM aspirate: mean value, 641 pg/mL) contained significantly higher levels than those from healthy subjects. To study whether peripheral blood mononuclear cells (PBMC) derived from patients suffering from HCL and healthy donors (HD) were capable of producing bFGF, culture supernatant (conditioned medium, [CM]) was tested for the presence of this cytokine. While bFGF was not detectable in cell cultures from HD, HCL-derived CM contained relatively high levels of bFGF. CM was successfully used for stimulation of mesenchymal cell proliferation, which could be inhibited by a neutralizing anti-bFGF antibody. Cellular activation by pokeweed mitogen (PWM) or the combination of 12-o-tetradecanoyl-phorbol-13-acetate (TPA) plus calcium ionophore (Ca-Ip) led to an enhanced mRNA expression. Results of Western blot experiments showed that HC synthesize at least three isoforms (approximately 18, 23, and 25 kD), but only the 23-kD isoform is exported. To assess the nature of the producer cell, double immunofluorescence analysis using a bFGF-specific and an anti-CD11c monoclonal antibody (MoAb) was undertaken. The majority of cells scoring positive for CD11c were also reactive with the anti-bFGF MoAb. Furthermore, enrichment of CD19/CD11c-positive cells correlated with enhanced bFGF levels, thereby supporting the argument for HC being the producer cells of bFGF. A biological function of bFGF in HCL might be mediation of chemoresistance, as 2-chlorodeoxyadenosine (2-CdA)–induced inhibition of cell proliferation can be reversed by bFGF. Endogenous bFGF production by HC is not affected by this purine analogue and 2-CdA–induced apoptosis is diminished in bFGF-producing HC as compared with normal PBMC. Therefore, bFGF expression by HC might be important for resistance to chemotherapy and survival of the malignant cells.

Localization and regulation of basic fibroblast growth factor (FGF-2) and FGF receptor-1 in rat superior cervical ganglion after axotomy

In response to peripheral nerve lesion, synthesis of basic fibroblast growth factor (FGF-2) increases in sensory ganglia and motoneurons. Here, we investigated the axotomy-induced regulation of FGF-2 and FGF receptor-1 (FGFR-1) expression in the autonomic nervous system using the sympathetic superior cervical ganglion of the adult rat as a model. Transcripts for both proteins were detected by ribonuclease protection assay. Western blotting indicated the presence of all three FGF-2 isoforms (18, 21, and 23 kD) in the superior cervical ganglion. Immunohistochemical analysis revealed FGF-2 localization in nuclei of satellite cells surrounding postganglionic perikarya. After transection of the carotid nerves, the number of FGF-2-immunoreactive glial cells increased. FGF-2 mRNA was up-regulated within 6 h and remained elevated for 3 weeks. The 18-, 21-, and 23-kD isoforms were all increased 7 days after axotomy. FGFR-1 immunoreactivity was observed in neuronal and nonneuronal nuclei in the normal rat superior cervical ganglion. In contrast to FGF-2, expression of FGFR-1 was unchanged in ganglia after axotomy. Taken together, the present results suggest that FGF-2 participates in neuron-glial interactions of sympathetic ganglia and may be involved in sympathetic neuron survival or nerve regeneration after nerve lesion.

IGF-I enhances survival of embryonic chick ciliary ganglion neurons in a calcium-dependent way

We have shown that neurons from embryonic chick ciliary ganglia in primary culture possess receptors for insulin-like growth factor I (IGF-I). When added to serum- and insulin-free culture medium, the factor potently enhanced neuronal survival as observed after 24 and 48 h of culture. The effect saturated at 5 ng/ml. Laminin was not necessary for the trophic effects of IGF-I; in the absence of the factor, it had no effect on neuronal survival. Insulin exerted a trophic effect similar to that observed with IGF-I, but at higher doses. The trophic effect of IGF-I was sharply and specifically reduced when either a membrane-permeable calcium chelating agent or blockers of voltage-dependent calcium channels were added to the medium.

Calcium transients evoked by action potentials in the somata of chick ciliary neurons

The effect of action potentials on the calcium concentration in the somata of chick ciliary neurons ([Ca2+]s) was determined by loading these with the calcium indicator calcium green-1. Following trains of 1-10 impulses (30 Hz) to the postganglionic nerve, the [Ca2+]s increased rapidly and then declined along a single exponential with a time constant of 0.70 +/- 0.04 s (fast phase). After trains of 20 or 50 impulses, the elevated [Ca2+]s declined as the sum of two exponentials, with time constants of 0.78 +/- 0.12 s (fast phase) and 4.0 +/- 0.4 s (moderate phase). After a 600-impulse postganglionic train of impulses, the elevated [Ca2+]s declined quickly over about 1 s, and then as the sum of two exponentials: that of the moderate phase and a slower component with a time constant of 109 +/- 16 s (slow phase). Similar time courses were observed following stimuli to the preganglionic nerve. Caffeine (3 mM) and ryanodine (20 microM) both sped the fast phase and slowed the moderate phase of [Ca2+]s decline. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 2 microM) slowed the slow phase, without affecting the other phases of decline. These results are discussed in relation to identifying the mechanisms responsible for these different phases of Ca2+ removal.

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.

Inhibitory neuronal activity can compensate for adverse effects of beta-amyloid in hippocampal neurons

One of the most prominent effects of Alzheimer disease is the disruption of finely tuned neuronal circuitry of discrete brain regions associated with learning and memory. Results from the present study support a role for the intrinsic inhibitory component of neuronal circuitry in determining the magnitude of beta-amyloid peptide induced cell death in the highly vulnerable pyramidal neurons of the hippocampus. Previous efforts have mostly focused on direct effects on excitatory neurons. By contrast, less emphasis has been placed on addressing a role for the intrinsic inhibitory component of cell-cell interactions of neuronal networks in response to Abeta. The present study provides evidence demonstrating that blockage of the intrinsic inhibitory component between Abeta exposed neurons leads to destabilization of calcium homeostasis and exacerbated neuronal death compared to Abeta treated cultures. Neuronal electrical activity was first silenced by exposing cultures to tetrodotoxin (TTX; 100 nM) plus Abeta, followed by survival counts. Cell death, unexpectedly, did not significantly differ from Abeta-exposed neurons. The intrinsic inhibition in Abeta-exposed cultures was then pharmacologically removed with picrotoxin (40 microM) or bicuculline (25 microM) resulting in significantly greater death than Abeta-exposed neurons alone. From these observations, it is proposed that intrinsic functional inhibition in hippocampal circuits can reduce adverse effects of Abeta on the excitatory component. By considering not just the excitatory component of electrical activity, but the intrinsic balance between excitation and inhibition, new strategies for the treatment of Alzheimer disease may emerge.

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.

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.