Differential effects of cyclic AMP and cholera toxin on nerve growth factor-induced neurite outgrowth from adrenal medullary chromaffin and pheochromocytoma cells

Whole cell response to receptor stimulation involves many deep and distributed subcellular biochemical processes

Neurite outgrowth is an integrated whole cell response triggered by the cannabinoid-1 receptor. We sought to identify the many different biochemical pathways that contribute to this whole cell response. To understand underlying mechanisms, we identified subcellular processes (SCPs) composed of one or more biochemical pathways and their interactions required for this response. Differentially expressed genes and proteins were obtained from bulk transcriptomics and proteomic analysis of extracts from cells stimulated with a cannabinoid-1 receptor agonist. We used these differentially expressed genes and proteins to build networks of interacting SCPs by combining the expression data with prior pathway knowledge. From these SCP networks, we identified additional genes that when ablated, experimentally validated the SCP involvement in neurite outgrowth. Our experiments and informatics modeling allowed us to identify diverse SCPs such as those involved in pyrimidine metabolism, lipid biosynthesis, and mRNA splicing and stability, along with more predictable SCPs such as membrane vesicle transport and microtubule dynamics. We find that SCPs required for neurite outgrowth are widely distributed among many biochemical pathways required for constitutive cellular functions, several of which are termed ‘deep’, since they are distal to signaling pathways and the key SCPs directly involved in extension of the neurite. In contrast, ‘proximal’ SCPs are involved in microtubule growth and membrane vesicle transport dynamics required for neurite outgrowth. From these bioinformatics and dynamical models based on experimental data, we conclude that receptor-mediated regulation of subcellular functions for neurite outgrowth is both distributed, that is, involves many different biochemical pathways, and deep.

A defined, controlled culture system for primary bovine chromaffin progenitors reveals novel biomarkers and modulators

We present a method to efficiently culture primary chromaffin progenitors from the adult bovine adrenal medulla in a defined, serum-free monolayer system. Tissue is dissociated and plated for expansion under support by the mitogen basic fibroblast growth factor (bFGF). The cultures, although not homogenous, contain a subpopulation of cells expressing the neural stem cell marker Hes3 that also propagate. In addition, Hes3 is also expressed in the adult adrenal medulla from where the tissue is taken. Differentiation is induced by bFGF withdrawal and switching to Neurobasal medium containing B27. Following differentiation, Hes3 expression is lost, and cells acquire morphologies and biomarker expression patterns of chromaffin cells and dopaminergic neurons. We tested the effect of different treatments that we previously showed regulate Hes3 expression and cell number in cultures of fetal and adult rodent neural stem cells. Treatment of the cultures with a combination of Delta4, Angiopoietin2, and a Janus kinase inhibitor increases cell number during the expansion phase without significantly affecting catecholamine content levels. Treatment with cholera toxin does not significantly affect cell number but reduces the ratio of epinephrine to norepinephrine content and increases the dopamine content relative to total catecholamines. These data suggest that this defined culture system can be used for target identification in drug discovery programs and that the transcription factor Hes3 may serve as a new biomarker of putative adrenomedullary chromaffin progenitor cells.

Useful cell lines derived from the adrenal medulla

Five approaches for the preparation of adrenal chromaffin cell lines have been developed. Initially, continuous chromaffin lines were derived from spontaneous pheochromocytoma tumors of the medulla, either from murine or human sources, such as the rat PC12 cell line and the human KNA and KAT45 cell lines. Over the last few decades, more sophisticated molecular methods have allowed for induced tumorigenesis and targeted oncogenesis in vivo, where isolation of specific populations of mouse cell lines of endocrine origin have resulted in model cells to examine a variety of regulatory pathways in the chromaffin phenotype. As well, conditional immortalization with retroviral infection of chromaffin precursors has provided homogeneous and expandable chromaffin cells for transplant studies in animal models of pain. This same strategy of immortalization with conditionally expressed oncogenes has been expanded recently to create the first disimmortalizable chromaffin cells, with an excisable oncogenic cassette, as might be envisioned for the creation of human chromaffin cell lines. Eventually, as we increase our understanding of regulating the phenotypic fate of chromaffin cells in vitro, stem or progenitor adrenal medullary cell lines will be derived as an alternative source for expansion and clinical use.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

Neuronal localization of pituitary adenylate cyclase-activating polypeptide 38 in the adrenal medulla and growth-inhibitory effect on chromaffin cells

The chromaffin cells of the adult rat adrenal medulla are essentially growth arrested in situ, but can proliferate in vitro, suggesting the existence of growth inhibitory factors in the adrenal gland. We have investigated whether pituitary adenylate cyclase-activating polypeptide 38 (PACAP38) could be involved in the growth arrest of adrenal chromaffin cells. In adult rat adrenal gland, PACAP38 was detected by radioimmunoassay and high-performance liquid chromatography and its concentration in the medulla was estimated as 24 nmol/kg wet tissue. Immunohistochemistry of the neonatal and adult rat adrenal medulla showed PACAP38 immunoreactivity in a widely distributed network of delicate nerve fibers surrounding the chromaffin cells. In a primary culture system, PACAP38 inhibited growth factor-stimulated DNA synthesis by 90% in neonatal and adult rat chromaffin cells with half-maximal inhibition at 4 and 0.5 nM, respectively, as demonstrated by bromodeoxyuridine pulse-labeling and immunocytochemical staining of cell nuclei. In comparison, corticosterone inhibited neonatal and adult chromaffin cell proliferation by 70% and 95%, respectively, with half-maximal effect at 100 nM. In neonatal chromaffin cells, 100 nM PACAP38 and 1 microM corticosterone added together abolished proliferation completely (99.8% inhibition). Finally, PACAP38 increased cell survival but showed little neurite-promoting activity in the chromaffin cells. Our data suggest that neurally derived PACAP38, in conjunction with glucocorticoids, may override growth factor mitogenic signals, leading to the postmitotic state of chromaffin cells in the adult adrenal medulla.

Chronic exposure to an activator of protein kinase C mimics early effects of NGF in chromaffin cells

Adrenal chromaffin cells respond to nerve growth factor (NGF) in vitro by expressing neuronal characteristics and, over a period of 2 to 4 weeks, transdifferentiating into postmitotic sympathetic neurons. Phorbol myristate acetate (PMA) is a potent activator of protein kinase C (PKC); chronic exposure to PMA mimics the initial actions of NGF by promoting the outgrowth of neurites and increasing the incorporation of [3H] thymidine in primary cultures of adrenal chromaffin cells from young rats. PMA and NGF affect the same populations of cells and even individual neurites. These effects are specific for active phorbol ester and do not result from the release of NGF or FGF in the cultures. As in the case of NGF, the effects are inhibited by glucocorticoids. The PKC inhibitor staurosporine inhibits the effects of PMA, as well as those of NGF, in a dose-dependent manner. These results suggest that a modulation in activity of PKC is important in the neuritogenic and proliferative effects of NGF, at least for an initial period of approximately 1 week.

Regulation of neurotensin content in adrenal medullary cells: comparison of PC12 cells to normal rat chromaffin cells in vitro

Radioimmunoassay studies of cultures of PC12 pheochromocytoma cells have shown progressive increments in content and release of neurotensin in response to combinations of dexamethasone, nerve growth factor, activators of adenylate cyclase and lithium. We have studied the distribution of immunoreactive neurotensin by immunocytochemistry in cultures of PC12 cells and normal rat chromaffin cells, with two objectives: (i) to determine how changes measured by radioimmunoassay in extracts of PC12 cell populations are manifested at the level of individual cells and (ii) to determine whether normal chromaffin cells respond to combinations of agents similarly to PC12 cells. Staining for immunoreactive neurotensin is not identifiable in PC12 cells maintained in control medium or with any of the medium supplements alone. Approximately 3% of cells are stained after maintenance with dexamethasone plus nerve growth factor, verus 17% with dexamethasone plus nerve growth factor plus forskolin, and 33% with all four agents. This heterogeneity does not appear to result from clonal diversity, or to be cell cycle-dependent. Individual PC12 cells recruited to produce neurotensin in response to particular signals may, however, have passed a critical stage of differentiation toward a chromaffin cell, rather than neuronal phenotype before exposure to those signals. Staining for immunoreactive neurotensin is observed in up to 18% of normal chromaffin cells maintained with dexamethasone plus nerve growth factor, up to 45% with dexamethasone plus nerve growth factor plus forskolin, and up to 54% with all four agents. Proportions of cells stained under the various culture conditions are established before birth and in fetal cultures staining is confined for the most part to cells which do not undergo neuronal differentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Mimicry and inhibition of nerve growth factor effects: interactions of staurosporine, forskolin, and K252a in PC12 cells and normal rat chromaffin cells in vitro

The structurally similar compounds staurosporine and K252a are potent inhibitors of protein kinases. K252a has previously been reported to inhibit most or all of the effects of nerve growth factor (NGF) on PC12 pheochromocytoma cells, and staurosporine has been reported both to inhibit and to mimic NGF-induced neurite outgrowth from a PC12 cell subclone in a dose-dependent manner. We have studied the interactions of these agents with each other, with NGF, and with forskolin, an activator of adenylate cyclase, on the parent PC12 cell line and on normal neonatal and adult rat chromaffin cells. Staurosporine alone or in conjunction with forskolin induces outgrowth of short neurites from PC12 cells but does not substitute for NGF in promoting cell survival. It does not abolish NGF-induced neurite outgrowth but does reverse the effects of NGF on catecholamine synthesis. K252a abolishes NGF-induced neurite outgrowth but only partially decreases outgrowth induced by NGF plus forskolin. It does not inhibit neurite outgrowth produced by staurosporine or staurosporine plus forskolin. These findings with PC12 cells suggest that staurosporine might act downstream from K252a and NGF on components of one or more signal transduction pathways by which NGF selectively affects the expression of certain traits. Both neonatal and adult rat chromaffin cells show dramatic flattening and extension of filopodia in response to staurosporine, an observation suggesting that some of the same pathways might remain active in cells that do not exhibit a typical NGF response. Only a small amount of neurite outgrowth is observed, however, and only in neonatal cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of cyclic AMP on neuritic outgrowth in explant cultures of developing chick olfactory epithelium

The effect of cyclic AMP (cAMP) analogs and phosphodiesterase (PDE) inhibitors on neurite outgrowth was studied in explant cultures of olfactory neurons. Nasal pits from 5- or 6-day-old chick embryos were minced, explanted into culture dishes, and grown in a serum-free medium. One of the cyclic AMP analogs, dibutyryl cyclic AMP (dbcAMP) or 8-bromo-cyclic AMP (8-Br-cAMP), or one of the PDE inhibitors, theophylline or isobutylmethylxanthine (IBMX), was added to the culture medium. The explants were examined for neurite outgrowth after 2 days in vitro. Db-cAMP increased the number of explants expressing neurites by 25-35% over control cultures, whereas 8-Br-cAMP had essentially no effect at the same concentrations. Addition of dibutyryl cyclic GMP (dbcGMP) gave no increase in neurite outgrowth, thus indicating that the effect of enhancing neuritic growth is specific to cAMP and not cyclic nucleotides in general. The resulting increase in neurite outgrowth is due to the cyclic nucleotide component of dbcAMP, since both IBMX and theophylline, which elevate intracellular cAMP, also increased neurite outgrowth significantly. When forskolin was added to the culture medium, there was a trend to increased neurite outgrowth; this was significantly enhanced when a subthreshold concentration of theophylline was added in addition to the forskolin.

Nerve growth factor increases the cyclic GMP level and activates the cyclic GMP phosphodiesterase in PC12 cells

Nerve growth factor (NGF) rapidly increases the cyclic GMP (cGMP) level about 2-3-fold and enhances the cGMP phosphodiesterase (PDE) activity about 2-fold in rat pheochromocytoma PC12 cells. No changes in the level of cyclic AMP (cAMP) and in the activity of cAMP PDE were found. GTP and a nonhydrolysable analog of GTP, GMP-PCP, at 100 microM, were able to mimic the effect of NGF on the cGMP PDE activity. These results suggest that the cGMP system may be one of the second messengers of NGF action in PC12 cells.

Nerve growth factor rapidly stimulates arachidonate metabolism in PC12 cells: potential involvement in nerve fiber growth

Homogenates prepared from pheochromocytoma (PC12) cells that are extending nerve fibers in response to nerve growth factor (NGF) have an increased capacity to metabolize exogenous arachidonate compared with homogenates prepared from cells untreated with NGF. These changes are not a consequence of cell attachment, since they are also seen in NGF-treated PC12 cells grown in suspension and are not found in attached cells grown in the absence of NGF. This NGF-stimulated increase in arachidonate metabolic capacity occurs rapidly and before the extension of nerve fibers. In contrast to NGF, epidermal growth factor does not alter the metabolism of exogenous arachidonate by PC12 cells. Radioimmunoassay of medium from PC12 cultures indicates that intact cells produce and release increased amounts of prostaglandin (PGE) in response to NGF. Drugs that inhibit arachidonate liberation from membrane phospholipids (mepacrine or 4-bromphenacyl bromide) block NGF-stimulated nerve fiber growth by PC12 cells. Selective inhibitors of cyclooxygenase metabolism of arachidonate (indomethacin and aspirin) fail to block growth, but inhibitors of lipoxygenase metabolism (baicalein, BW755, and eicosatetraynoic acid) are potent blockers. In cultures of dorsal root ganglion neurons, inhibitors of arachidonate release (mepacrine, 4-bromphenacyl bromide) or its subsequent metabolism by lipoxygenases (nordihydroquaiaretic acid, eicosatetraynoic acid) also prevent the early morphological events of nerve fiber growth. Our data suggest that NGF rapidly and specifically increases the capacity of PC12 cells to synthesize arachidonate metabolites, and that arachidonate metabolism may be important in nerve fiber growth by both PC12 cells and dorsal root ganglion neurons.

Cholera toxin and dibutyryl cyclic AMP inhibit the expression of neurofilament protein induced by nerve growth factor in cultures of naive and primed PC12 cells

The effects of nerve growth factor (NGF), dibutyryl cyclic AMP (db cAMP), and cholera toxin on neurofilament protein expression in cultures of PC12 rat pheochromocytoma cells were examined using an enzyme-linked immunoadsorbent assay (ELISA). Morphological differentiation induced by NGF was associated with up to 30-fold increases in the level of neurofilament protein recognised by monoclonal antibody RT97. A more rapid response was apparent from primed as compared to naive PC12 cells. Cholera toxin and db cAMP both induced morphological differentiation of naive PC12 cells, but failed to promote neurite regeneration from primed cells. Neither response was associated with a significant induction of neurofilament protein. Both cholera toxin and db cAMP, but not B-cholera toxin nor antibodies to the toxin receptor, were found to inhibit the neurofilament protein response induced by NGF. Primed cells were more susceptible to this inhibition, and both cholera toxin and db cAMP inhibited neurite regeneration from these cells. These data suggest that increased intracellular cyclic AMP can suppress the expression of neuronal differentiation antigens induced by NGF, and are consistent with a role for neurofilament protein in promoting or facilitating the formation of a stable neuritic network.

Hypertrophy of pheochromocytoma cells treated with nerve growth factor and activators of adenylate cyclase

PC12 pheochromocytoma cells treated with nerve growth factor (NGF) in combination with high concentrations of the activators of adenylate cyclase, forskolin or cholera toxin, become more neuron-like in size than cells treated with NGF or with activators of adenylate cyclase alone. Cells treated simultaneously with NGF plus forskolin or cholera toxin paradoxically show less process outgrowth than cells treated with NGF alone. Addition of forskolin or cholera toxin to cells pretreated with NGF, however, produces enlarged cells with intact processes that are indistinguishable from cultured neurons. One possible implication of these findings is that NGF might act in concert with agents that increase intracellular cyclic AMP to cause neuronal maturation during embryogenesis, and that the proper sequence of exposure to these signals is necessary for normal development. Specific activity of acetylcholinesterase is increased by NGF but is unaffected or slightly decreased by forskolin, suggesting that individual aspects of the developing neuronal phenotype are subject to different types of control.

Neurochemical characteristics of a novel dorsal root ganglion X neuroblastoma hybrid cell line, F-11

We investigated the properties of the novel dorsal root ganglion (DRG) hybrid cell line F-11 to see how closely these cells resembled normal DRG cells. Under normal growth conditions, F-11 cells appeared to contain several short neurite-like processes. However, these cells could also be grown under conditions in which they showed a much more extensive neuronal morphology, exhibiting many long neurites. Several differentiated features of DRG cells were present on F-11 cells. These included the presence of delta-opioid receptors, receptors for prostaglandins and bradykinin, and dihydropyridine-sensitive calcium channels. F-11 cells also synthesized and released a substance P-like compound, as determined by immunoreactivity. Both the number of bradykinin receptors and the voltage-sensitive calcium influx increased on cell differentiation. Opioid agonists (delta-specificity) were found to decrease cyclic AMP levels in F-11 cells in a naloxone- and pertussis toxin-reversible fashion. Bradykinin stimulated the synthesis of inositol-1,4-bisphosphate and inositol-1,4,5-trisphosphate. Ca2+ channel agonists stimulated voltage-sensitive Ca2+ influx in a dose-dependent, stereospecific manner, whereas Ca2+ channel antagonists inhibited Ca2+ influx. F-11 cells should, therefore, prove useful as models for authentic DRG neurons.

Selective inhibition of responses to nerve growth factor and of microtubule-associated protein phosphorylation by activators of adenylate cyclase

To study the influence of cAMP on cellular responses to nerve growth factor (NGF) and to use elevation of intracellular cAMP to probe the NGF mechanism, cultured PC12 pheochromocytoma cells were exposed to forskolin and cholera toxin. As in other cell types, the latter agents greatly increased PC12 cell cAMP levels. Such treatment also brought about a reversible, dose-dependent suppression of NGF-promoted regeneration of neurites. In support of the role of cAMP in this effect, regeneration blockage by forskolin was potentiated by phosphodiesterase inhibitors. When tested on NGF-stimulated initiation of process outgrowth, cholera toxin and forskolin exerted a dual effect. As in previous studies, these drugs, when applied along with NGF, significantly enhanced the initial formation of short cytoplasmic extensions. However, after approximately 3 d of NGF exposure, at which time such extensions begin to acquire the morphological and ultrastructural features of neurites, these agents suppressed process outgrowth. That is, the neurites were fewer in number, significantly less branched, and much shorter than in control cultures. Such changes also occurred when these drugs were added to cultures that had been pretreated with NGF alone. Whereas forskolin and cholera toxin affect the formation and regeneration of neurites, these drugs did not interfere with the short-latency, transient changes in surface morphology that are triggered by NGF, nor did they inhibit transcription-dependent priming. In contrast, the rapidly occurring NGF-induced phosphorylation of tyrosine hydroxylase was suppressed. Moreover, forskolin and cholera toxin rapidly and selectively blocked the NGF-promoted phosphorylation of a set of microtubule-associated proteins known as chartins. Previous observations have suggested a causal relationship between NGF-induced chartin microtubule-associated protein phosphorylation and the formation and outgrowth of neurites. This is supported by the present data and provides a possible mechanism whereby elevated cAMP may interfere with neurite growth and regeneration.

Fate of cyclic nucleotides in PC12 cell cultures: uptake, metabolism, and effects of metabolites on nerve growth factor-induced neurite outgrowth

The fate of cyclic AMP (cAMP), dibutyryl-cAMP (Bt2-cAMP), and the (Sp)-isomer of adenosine 3',5'-monophosphorothioate [(Sp)-cAMPS] was studied in the PC12 culture medium by means of HPLC. In the absence of PC12 cells, cAMP and Bt2-cAMP were rapidly degraded by nonspecific esterases and cyclic nucleotide phosphodiesterase both originating from the serum commonly used as a culture medium ingredient, whereas (Sp)-cAMPS was completely stable. Since 5'-AMP, adenosine, inosine, and hypoxanthine appeared in the culture medium after incubation with cAMP or Bt2-cAMP, we have determined their effect on nerve growth factor (NGF)-induced neurite outgrowth. 5'-AMP, adenosine, and inosine were indeed potent agents in producing a potentiating effect on NGF-induced early neurite outgrowth at a concentration of 1 mM. Thus, cAMP metabolites had the capacity to induce an effect that has been described as cAMP-specific. In serum-free culture medium and in the presence of cells, all cyclic nucleotides were taken up by PC12 cells. Uptake was highly correlated with the hydrophobic nature of the compounds, and was accompanied by a simultaneous excretion of metabolites. On incubation with cAMP, NGF had a pronounced effect on the metabolic pattern found in the culture medium. In particular, dephosphorylation of 5'-AMP was specifically enhanced. This effect of NGF on the degradation of cAMP was also apparent when cAMP metabolites were incubated with PC12 cells. Whereas 5'-AMP degradation was greatly increased, NGF had no effect on the metabolism of the other purine compounds.

The role of cAMP in nerve growth factor-promoted neurite outgrowth in PC12 cells

Nerve growth factor (NGF)-mediated neurite outgrowth in rat pheochromocytoma PC12 cells has been described to be synergistically potentiated by the simultaneous addition of dibutyryl cAMP. To elucidate further the role of cAMP in NGF-induced neurite outgrowth we have used the adenylate cyclase activator forskolin, cAMP, and a set of chemically modified cAMP analogues, including the adenosine cyclic 3',5'-phosphorothioates (cAMPS) (Rp)-cAMPS and (Sp)-cAMPS. These diastereomers have differential effects on the activation of cAMP-dependent protein kinases, i.e., (Sp)-cAMPS behaves as a cAMP agonist and (Rp)-cAMPS behaves as a cAMP antagonist. Our data show that the establishment of a neuritic network, as observed from PC12 cells treated with NGF alone, could not be induced by either forskolin, cAMP, or cAMP analogues alone. The presence of NGF in combination with forskolin or cAMP or its agonistic analogues potentiated the initiation of neurite outgrowth from PC12 cells. The (Sp)-cAMPS-induced stimulation of NGF-mediated process formation was successfully blocked by the (Rp)-cAMPS diastereomer. On the other hand, NGF-stimulated neurite outgrowth was not inhibited by the presence of the cAMP antagonist (Rp)-cAMPS. We conclude that the morphological differentiation of PC12 cells stimulated by NGF does not require cAMP as a second messenger. The constant increase of intracellular cAMP, caused by either forskolin or cAMP and the analogues, in combination with NGF, not only rapidly stimulated early neurite outgrowth but also exerted a maintaining effect on the neuronal network established by NGF.

Phenotypic plasticity of cultured bovine chromaffin cells. II. Fiber outgrowth induced by elevated potassium: morphology and ionic requirements

Chromaffin cells isolated from the adult bovine adrenal medulla extend neurite-like processes in culture in response to a variety of agents. In the present study we investigated the effect of chronic depolarization by high potassium on fiber outgrowth and its ionic requirements. Elevated K+ in the culture medium induced process formation of isolated bovine chromaffin cells in a dose-dependent fashion, and so did veratridine. Short-term depolarization by acetylcholine or carbachol caused increased flattening out of the cells, but no outgrowth of neurite-like processes. Formation of processes was accompanied by a significant reduction of endogenous catecholamines in cultured cells after 18 h and 8 days and a relative shift towards storage of primary amines after 8 days. K+-induced fiber outgrowth was dependent on the presence of Ca2+ in the medium and Ca2+-influx into the cells: the effect was inhibited by EDTA, EGTA, CoCl2 and verapamil and mimicked by the Ca2+ ionophore A 23187. Tetrodotoxin, tetraethylammonium, amiloride and ouabain, which interfere with Na+- and K+-fluxes, did not inhibit K+-induced process formation nor did any of them by itself evoke fiber outgrowth. Fiber outgrowth required de novo protein synthesis as shown by the inhibitory effect of cycloheximide. K+-induced formation of processes was not affected by dexamethasone and dbcAMP, both of which inhibit NGF-induced neurite formation of early postnatal rat chromaffin cells in vitro. These results document that chronic depolarization may induce de novo formation of neurite-like processes of bovine chromaffin cells in vitro by a Ca2+- and protein synthesis-dependent mechanism.

Phenotypic plasticity of cultured bovine chromaffin cells. I. Morphological changes induced by non-chromaffin cells and organ extracts, but not by mouse and bovine nerve growth factor

Adult bovine chromaffin cells are known to show process outgrowth in culture, similar to that seen with rat chromaffin and pheochromocytoma cells after exposure to nerve growth factor (NGF). To determine whether bovine chromaffin cells respond to NGF or NGF-like factors, dissociated adrenal medullary cells from adult cattle were cultured for up to 4 weeks in the presence or absence of NGF (from mouse submaxillary glands and bovine seminal vesicles, respectively), adrenal non-chromaffin cells and various organ extracts. Chromaffin cells that had been freed from non-chromaffin cells by differential plating and/or gamma-irradiation showed virtually no fiber outgrowth. In the presence of adrenal non-chromaffin cells (NCC), cell-free extracts obtained from these cells or medium conditioned by them, chromaffin cells formed processes. Extracts from bovine seminal vesicles (SVE), but not from other organs including bovine brain, heart, liver, kidney, adrenal glands and male mouse submaxillary glands, also elicited fiber outgrowth. Extension of processes induced by NCC and SVE could not be blocked by administration of monospecific anti-NGF antibodies directed against NGF from mouse submaxillary glands. Purified NGF from mouse submaxillary glands and SVE did not elicited a response. We conclude that adult bovine chromaffin cells in culture show structural plasticity similar to that shown by cultured chromaffin cells from other species, but do not respond to NGF. Neurite outgrowth promoting activities appear to reside with macromolecular constituents of bovine adrenal non-chromaffin cells and SVE.

Use of isolated chromaffin cells to study basic release mechanisms

An account is given of the authors' work with isolated adrenal chromaffin cells to study the synthesis, storage and release of catecholamines and of a number of neuropeptides endogenous to the adrenal medulla. A review of other studies in the literature with the isolated chromaffin cell system is included. It is seen that the isolated chromaffin cells are a convenient in vitro system well-suited to studies of basic release mechanisms. The isolated adrenal chromaffin cells maintain high levels of catecholamines and opiates and release them by exocytosis. The cells have both nicotinic and muscarinic receptors but only the nicotinic are involved in the agonist-evoked release of catecholamines (EC50 nicotine 5 X 10(-6) M: ACh 5 X 10(-5) M). The cells can synthesize AChE and selectively release the 10S molecular form by a mechanism different from exocytosis. Substance P (SP) modulates the secretion of catecholamines and ATP evoked by ACh or nicotine but not that evoked by K+ or veratridine. SP appears to interact with the nicotinic receptor-ionophore complex to regulate Na+ entry. SP receptors on the chromaffin cells show similar structural requirements to SP receptors in other SP responsive tissues. Binding studies on isolated chromaffin cell membranes with [4-3H-Phe]SP have shown specific binding in the nM range. In addition, at high concentrations of ACh, SP protects against nicotinic receptor desensitization. Since SP is contained in the splanchnic nerve terminals that innervate the medulla, the demonstration of SP action and SP receptors on the chromaffin cells suggests a physiological role for SP in the regulation of secretion from the adrenal medulla. Somatostatin (SS) and a number of SS analogues also inhibit release, but are approximately 15-fold less potent than SP. Leu- and Met-enkephalin, which are co-stored with adrenaline in the bovine adrenal medullary cells produce a non-specific inhibition of the nicotine-evoked release of CA, but enhance the basal release of endogenous catecholamines by a mechanism that is Ca2+-dependent, stereospecific and reversible by naloxone and naltrexone. The implication of these peptide-amine interactions for physiological processes regulating homeostasis in the adrenal are discussed.

Presence of adrenal medullary chromaffin cells in primary adrenal dispersions and their persistence in long-term culture

Cell suspensions prepared by enzymatic dispersion of whole rat adrenal glands for the purpose of studying adrenocortical cells were found to contain chromaffin cells even though they are commonly thought to not survive in such preparations. These cells fluoresced when treated with methods specific for catecholamines. The fluorescent cells persisted in the cultures of "cortical" cells, took on the morphology of neurons in the cultures, maintained their specific catecholamine fluorescence in long-term cultures, and ultrastructurally were identical to chromaffin cells.