SH2-B and APS are multimeric adapters that augment TrkA signaling

Neurotrophins influence growth and survival of sympathetic and sensory neurons through activation of their receptors, Trk receptor tyrosine kinases. Previously, we identified Src homology 2-B (SH2-B) and APS, which are structurally similar adapter proteins, as substrates of Trk kinases. In the present study, we demonstrate that both SH2-B and APS exist in cells as homopentamers and/or heteropentamers, independent of Trk receptor activation. Structure-function analyses revealed that the SH2-B multimerization domain resides within its amino terminus, which is necessary for SH2-B-mediated nerve growth factor (NGF) signaling. Overexpression of SH2-B enhances both the magnitude and duration of TrkA autophosphorylation following exposure of PC12 cells to NGF, and this effect requires the amino-terminal multimerization motif. Moreover, the amino terminus of SH2-B is necessary for TrkA/SH2-B-mediated morphological differentiation of PC12 cells. Together, these results indicate that the multimeric adapters SH2-B and APS influence neurotrophin signaling through direct modulation of Trk receptor autophosphorylation.

Spatial considerations for stimulus-dependent transcription in neurons

Most neurons have elaborate dendrites as well as an axon emanating from the cell body that form synaptic connections with one or many target cells, which may be located a considerable distance from the cell body. Such complex and impressive morphologies allow some types of neurons to integrate inputs from one to many thousands of pre-synaptic partners and to rapidly propagate electrical signals, often over long distances, to post-synaptic target cells. Much slower, non-electrical signals also propagate from dendrites and distal axons to neuronal nuclei that influence survival, growth, and plasticity. The distances between distal dendrites and/or distal axons and cell bodies of neurons can be hundreds of microns to more than one meter. This long-range biochemical signal propagation from distal dendrites and distal axons to neuronal nuclei is entirely unique to neurons. This review is focused on excitatory neurotransmitter signaling from dendritic synapses to neuronal nuclei as well as on retrograde growth factor signaling from distal axons to neuronal nuclei.

Spatially and functionally distinct roles of the PI3-K effector pathway during NGF signaling in sympathetic neurons

NGF is a target-derived growth factor for developing sympathetic neurons. Here, we show that application of NGF exclusively to distal axons of sympathetic neurons leads to an increase in PI3-K signaling in both distal axons and cell bodies. In addition, there is a more critical dependence on PI3-K for survival of neurons supported by NGF acting exclusively on distal axons as compared to neurons supported by NGF acting directly on cell bodies. Interestingly, PI3-K signaling within both cell bodies and distal axons contributes to survival of neurons. The requirement for PI3-K signaling in distal axons for survival may be explained by the finding that inhibition of PI3-K in the distal axons attenuates retrograde signaling. Therefore, a single TrkA effector, PI3-K, has multiple roles within spatially distinct cellular locales during retrograde NGF signaling.

Dynamic regulation of neuronal NO synthase transcription by calcium influx through a CREB family transcription factor-dependent mechanism

Neuronal nitric oxide (NO) synthase (nNOS) is dynamically regulated in response to a variety of physiologic and pathologic stimuli. Although the dynamic regulation of nNOS is well established, the molecular mechanisms by which such diverse stimuli regulate nNOS expression have not yet been identified. We describe experiments demonstrating that Ca(2+) entry through voltage-sensitive Ca(2+) channels regulates nNOS expression through alternate promoter usage in cortical neurons and that nNOS exon 2 contains the regulatory sequences that respond to Ca(2+). Deletion and mutational analysis of the nNOS exon 2 promoter reveals two critical cAMP/Ca(2+) response elements (CREs) that are immediately upstream of the transcription start site. CREB binds to the CREs within the nNOS gene. Mutation of the nNOS CREs as well as blockade of CREB function results in a dramatic loss of nNOS transcription. These findings suggest that nNOS is a Ca(2+)-regulated gene through the interactions of CREB on the CREs within the nNOS exon 2 promoter and that these interactions are likely to be centrally involved in the regulation of nNOS in response to neuronal injury and activity-dependent plasticity.

Neuropilin-2 is required in vivo for selective axon guidance responses to secreted semaphorins

Neuropilins are receptors for class 3 secreted semaphorins, most of which can function as potent repulsive axon guidance cues. We have generated mice with a targeted deletion in the neuropilin-2 (Npn-2) locus. Many Npn-2 mutant mice are viable into adulthood, allowing us to assess the role of Npn-2 in axon guidance events throughout neural development. Npn-2 is required for the organization and fasciculation of several cranial nerves and spinal nerves. In addition, several major fiber tracts in the brains of adult mutant mice are either severely disorganized or missing. Our results show that Npn-2 is a selective receptor for class 3 semaphorins in vivo and that Npn-1 and Npn-2 are required for development of an overlapping but distinct set of CNS and PNS projections.

Mediation by a CREB family transcription factor of NGF-dependent survival of sympathetic neurons

Nerve growth factor (NGF) and other neurotrophins support survival of neurons through processes that are incompletely understood. The transcription factor CREB is a critical mediator of NGF-dependent gene expression, but whether CREB family transcription factors regulate expression of genes that contribute to NGF-dependent survival of sympathetic neurons is unknown. CREB-mediated gene expression was both necessary for NGF-dependent survival and sufficient on its own to promote survival of sympathetic neurons. Moreover, expression of Bcl-2 was activated by NGF and other neurotrophins by a CREB-dependent transcriptional mechanism. Overexpression of Bcl-2 reduced the death-promoting effects of CREB inhibition. Together, these data support a model in which neurotrophins promote survival of neurons, in part through a mechanism involving CREB family transcription factor-dependent expression of genes encoding prosurvival factors.

Characterization of an NGF-P-TrkA retrograde-signaling complex and age-dependent regulation of TrkA phosphorylation in sympathetic neurons

Nerve growth factor (NGF) is a target-derived trophic factor for developing sympathetic and cutaneous sensory neurons. NGF promotes growth and survival of neurons via activation of the receptor tyrosine kinase TrkA. We used compartmentalized cultures of sympathetic neurons to address the mechanism of NGF signaling from distal axons and terminals to proximal axons and cell bodies. Our results demonstrate that an NGF-phospho-TrkA (NGF-P-TrkA)-signaling complex forms in distal axons and is retrogradely transported as a complex to cell bodies of sympathetic neurons. Although a minor fraction of both NGF and TrkA is retrogradely transported, a large fraction of the NGF that is retrogradely transported is found complexed with retrogradely transported TrkA. Interestingly, the metabolism of the P-TrkA complex is dramatically different in young, NGF-dependent sympathetic neurons as compared to older, NGF-independent sympathetic neurons. After withdrawal of NGF from distal axons of young neurons, P-TrkA within distal axons, as well as within proximal axons and cell bodies, dephosphorylates rapidly. In contrast, after withdrawal of NGF from distal axons of older neurons, P-TrkA within distal axons dephosphorylates completely, although more slowly than that in young neurons, whereas dephosphorylation of P-TrkA within proximal axons and cell bodies occurs markedly more slowly, with at least one-half of the level of P-TrkA remaining 2 d after NGF withdrawal. Thus, P-TrkA within the cell bodies of young, NGF-dependent sympathetic neurons is derived from distal axons. A more stable P-TrkA complex within cell bodies of mature sympathetic neurons may contribute to the acquisition of NGF independence for survival of mature sympathetic neurons.

A late phase of cerebellar long-term depression requires activation of CaMKIV and CREB

Recently, it has been shown that cerebellar LTD has a late phase that may be blocked by protein synthesis inhibitors. To understand the mechanisms underlying the late phase, we interfered with the activation of transcription factors that might couple synaptic activation to protein synthesis. Particle-mediated transfection of cultured Purkinje neurons with an expression vector encoding a dominant inhibitory form of CREB resulted in a nearly complete blockade of the late phase. Kinases that activate CREB were inhibited, and LTD was assessed. Inhibition of PKA or the MAPK/RSK cascades were without effect on the late phase, while constructs designed to interfere with CaMKIV function attenuated the late phase. These results indicate that the activation of CaMKIV and CREB are necessary to establish a late phase of cerebellar LTD.

Patterning of cortical efferent projections by semaphorin-neuropilin interactions

Cortical neurons communicate with various cortical and subcortical targets by way of stereotyped axon projections through the white matter. Slice overlay experiments indicate that the initial growth of cortical axons toward the white matter is regulated by a diffusible chemorepulsive signal localized near the marginal zone. Semaphorin III is a major component of this diffusible signal, and cortical neurons transduce this signal by way of the neuropilin-1 receptor. These observations indicate that semaphorin-neuropilin interactions play a critical role in the initial patterning of projections in the developing cortex.

Effects of PS1 deficiency on membrane protein trafficking in neurons

We have examined the trafficking and metabolism of the beta-amyloid precursor protein (APP), an APP homolog (APLP1), and TrkB in neurons that lack PS1. We report that PS1-deficient neurons fail to secrete Abeta, and that the rate of appearance of soluble APP derivatives in the conditioned medium is increased. Remarkably, carboxyl-terminal fragments (CTFs) derived from APP and APLP1 accumulate in PS1-deficient neurons. Hence, PS1 plays a role in promoting intramembrane cleavage and/or degradation of membrane-bound CTFs. Moreover, the maturation of TrkB and BDNF-inducible TrkB autophosphorylation is severely compromised in neurons lacking PS1. We conclude that PS1 plays an essential role in modulating trafficking and metabolism of a selected set of membrane and secretory proteins in neurons.

Identification and characterization of novel substrates of Trk receptors in developing neurons

Neurotrophins influence growth and survival of specific populations of neurons through activation of Trks, members of the receptor tyrosine kinase (RTK) family. In this report, we describe the identification and characterization of two substrates of Trk kinases, rAPS and SH2-B, which are closely related Src homolog 2 (SH2) domain-containing signaling molecules. rAPS and SH2-B are substrates of TrkB and TrkC in cortical neurons and SH2-B is a substrate of TrkA in sympathetic neurons. Moreover, rAPS and SH2-B bind to Grb2, and both are sufficient to mediate NGF induction of Ras, MAP kinase (MAPK), and morphological differentiation of PC12 cells. Lastly, antibody perturbation and transient transfection experiments indicate that SH2-B, or a closely related molecule, is necessary for NGF-dependent signaling in neonatal sympathetic neurons. Together, these observations indicate that rAPS and SH2-B mediate Trk signaling in developing neurons.

Neuropilin-2 is a receptor for semaphorin IV: insight into the structural basis of receptor function and specificity

Neuropilins bind secreted members of the semaphorin family of proteins. Neuropilin-1 is a receptor for Sema III. Here, we show that neuropilin-2 is a receptor for the secreted semaphorin Sema IV and acts selectively to mediate repulsive guidance events in discrete populations of neurons. neuropilin-2 and semaIV are expressed in strikingly complementary patterns during neurodevelopment. The extracellular complement-binding (CUB) and coagulation factor domains of neuropilin-2 confer specificity to the Sema IV repulsive response, and these domains of neuropilin-1 are necessary and sufficient for binding of the Sema III semaphorin (sema) domain. The coagulation factor domains alone are necessary and sufficient for binding of the Sema III immunoglobulin- (Ig-) basic domain and the unrelated ligand, vascular endothelial growth factor (VEGF). Lastly, neuropilin-1 can homomultimerize and form heteromultimers with neuropilin-2. These results provide insight into how interactions between neuropilins and secreted semaphorins function to coordinate repulsive axon guidance during neurodevelopment.

A dominant-negative inhibitor of CREB reveals that it is a general mediator of stimulus-dependent transcription of c-fos

Several studies have characterized the upstream regulatory region of c-fos, and identified cis-acting elements termed the cyclic AMP (cAMP) response elements (CREs) that are critical for c-fos transcription in response to a variety of extracellular stimuli. Although several transcription factors can bind to CREs in vitro, the identity of the transcription factor(s) that activates the c-fos promoter via the CRE in vivo remains unclear. To help identify the trans-acting factors that regulate stimulus-dependent transcription of c-fos via the CREs, dominant-negative (D-N) inhibitor proteins that function by preventing DNA binding of B-ZIP proteins in a dimerization domain-dependent fashion were developed. A D-N inhibitor of CREB, termed A-CREB, was constructed by fusing a designed acidic amphipathic extension onto the N terminus of the CREB leucine zipper domain. The acidic extension of A-CREB interacts with the basic region of CREB forming a coiled-coil extension of the leucine zipper and thus prevents the basic region of wild-type CREB from binding to DNA. Other D-N inhibitors generated in a similar manner with the dimerization domains of Fos, Jun, C/EBP, ATF-2, or VBP did not block CREB DNA binding activity, nor did they inhibit transcriptional activation of a minimal promoter containing a single CRE in PC12 cells. A-CREB inhibited activation of CRE-mediated transcription evoked by three distinct stimuli: forskolin, which increases intracellular cAMP; membrane depolarization, which promotes Ca2+ influx; and nerve growth factor (NGF). A-CREB completely inhibited cAMP-mediated, but only partially inhibited Ca2+- and NGF-mediated, transcription of a reporter gene containing 750 bp of the native c-fos promoter. Moreover, glutamate induction of c-fos expression in primary cortical neurons was dependent on CREB. In contrast, induction of c-fos transcription by UV light was not inhibited by A-CREB. Lastly, A-CREB attenuated NGF induction of morphological differentiation in PC12 cells. These results suggest that CREB or its closely related family members are general mediators of stimulus-dependent transcription of c-fos and are required for at least some of the long-term actions of NGF.

Neuropilin is a semaphorin III receptor

The semaphorin family contains a large number of phylogenetically conserved proteins and includes several members that have been shown to function in repulsive axon guidance. Semaphorin III (Sema III) is a secreted protein that in vitro causes neuronal growth cone collapse and chemorepulsion of neurites, and in vivo is required for correct sensory afferent innervation and other aspects of development. The mechanism of Sema III function, however, is unknown. Here, we report that neuropilin, a type I transmembrane protein implicated in aspects of neurodevelopment, is a Sema III receptor. We also describe the identification of neuropilin-2, a related neuropilin family member, and show that neuropilin and neuropilin-2 are expressed in overlapping, yet distinct, populations of neurons in the rat embryonic nervous system.

An NGF-TrkA-mediated retrograde signal to transcription factor CREB in sympathetic neurons

Nerve growth factor (NGF) is a neurotrophic factor secreted by cells that are the targets of innervation of sympathetic and some sensory neurons. However, the mechanism by which the NGF signal is propagated from the axon terminal to the cell body, which can be more than 1 meter away, to influence biochemical events critical for growth and survival of neurons has remained unclear. An NGF-mediated signal transmitted from the terminals and distal axons of cultured rat sympathetic neurons to their nuclei regulated phosphorylation of the transcription factor CREB (cyclic adenosine monophosphate response element-binding protein). Internalization of NGF and its receptor tyrosine kinase TrkA, and their transport to the cell body, were required for transmission of this signal. The tyrosine kinase activity of TrkA was required to maintain it in an autophosphorylated state upon its arrival in the cell body and for propagation of the signal to CREB within neuronal nuclei. Thus, an NGF-TrkA complex is a messenger that delivers the NGF signal from axon terminals to cell bodies of sympathetic neurons.

Coupling of the RAS-MAPK pathway to gene activation by RSK2, a growth factor-regulated CREB kinase

A signaling pathway has been elucidated whereby growth factors activate the transcription factor cyclic adenosine monophosphate response element-binding protein (CREB), a critical regulator of immediate early gene transcription. Growth factor-stimulated CREB phosphorylation at serine-133 is mediated by the RAS-mitogen-activated protein kinase (MAPK) pathway. MAPK activates CREB kinase, which in turn phosphorylates and activates CREB. Purification, sequencing, and biochemical characterization of CREB kinase revealed that it is identical to a member of the pp90(RSK) family, RSK2. RSK2 was shown to mediate growth factor induction of CREB serine-133 phosphorylation both in vitro and in vivo. These findings identify a cellular function for RSK2 and define a mechanism whereby growth factor signals mediated by RAS and MAPK are transmitted to the nucleus to activate gene expression.

Induction of a nerve growth factor-sensitive kinase that phosphorylates the DNA-binding domain of the orphan nuclear receptor NGFI-B

Nerve growth factor (NGF) induces the synthesis and the phosphorylation of the orphan nuclear receptor NGFI-B in PC12 cells. Previous work has shown that phosphorylation, by protein kinase A, of a specific serine in the DNA-binding domain inhibits its binding to the NGFI-B response element. Also, cytoplasmic extracts from PC12 cells phosphorylate this serine, and phosphorylation is greater in extracts from cells treated with NGF. The present work describes the induction, identification, and partial purification of a kinase (termed NGFI-B kinase I) from PC12 cell extracts that catalyzes this phosphorylation. Phosphorylation of the DNA-binding domain with this purified preparation inhibits its binding to the NGFI-B response element. The kinase is rapidly activated by treatment of the cells with NGF, and the activation lasts for at least several hours. It also is activated by fibroblast growth factor and epidermal growth factor (EGF), but the activation by EGF is quite transient. The kinase requires Mg2+ but will use Mn2+. The molecular mass of the kinase is 95-100 kDa, and it is different from protein kinase A, Fos kinase, or pp90rsk. Comparison with a partially purified preparation of cyclic AMP response element-binding protein kinase, however, indicates that the two are either very similar or identical.

Calcium activates serum response factor-dependent transcription by a Ras- and Elk-1-independent mechanism that involves a Ca2+/calmodulin-dependent kinase

Enhanced levels of cytoplasmic Ca2+ due to membrane depolarization with elevated levels of KCl or exposure to the Ca2+ ionophore ionomycin stimulate serum response element (SRE)-dependent transcription in the pheochromocytoma cell line PC12. By using altered binding specificity mutants of transcription factors that bind to the SRE, it was demonstrated that in contrast to treatment with purified growth factors, such as nerve growth factor, the serum response factor (SRF), but not Elk-1, mediates Ca(2+)-regulated SRE-dependent transcription. Enhanced levels of cytoplasmic Ca2+ were found to trigger SRE-dependent transcription via a Ras-independent signaling pathway that appears to involve a Ca2+/calmodulin-dependent kinase (CaMK). Overexpression of a constitutively active form of CaMKIV stimulated SRF-dependent transcription. Taken together, these findings indicate that SRF is a versatile transcription factor that, when bound to the SRE, can function by distinct mechanisms and can mediate transcriptional responses to both CaMK- and Ras-dependent signaling pathways.

Serine 133-phosphorylated CREB induces transcription via a cooperative mechanism that may confer specificity to neurotrophin signals

A mechanism has been characterized by which the transcription factor CREB regulates neurotrophin-induced gene expression. Whereas CREB can mediate calcium- or cyclic AMP-induced c-fos transcription independently of other promoter-bound transcription factors, CREB mediates NGF induction of c-fos transcription via a novel mechanism that appears to require a cooperative interaction with another transcription factor, the serum response factor. A similar transcriptional mechanism may explain how neurotrophins and growth factors induce distinct subsets of delayed response genes. Neurotrophins induce the phosphorylation of CREB at a key regulatory site, Serine 133, with prolonged kinetics that are distinct from the transient kinetics of CREB phosphorylation elicited by growth factors. These results indicate that CREB is a versatile transcription factor that activates transcription via distinct mechanisms in a stimulus-specific manner. In addition, by selectively activating delayed response genes, CREB may confer specificity to neurotrophin signals that promote the survival and differentiation of neurons.

L-type voltage-sensitive Ca2+ channel activation regulates c-fos transcription at multiple levels

A mechanism by which voltage-sensitive Ca2+ channel (VSCC) activation triggers c-fos transcription has been characterized. Ca2+ influx through VSCCs stimulates phosphorylation of the transcription factor cAMP response element-binding protein (CREB) on serine 133 leading to an increase in the formation of transcription complexes that can elongate through a transcription pause site within the c-fos gene. Ca(2+)-stimulated CREB serine 133 phosphorylation is mediated by a Ca(2+)-activated kinase and is not dependent on the cAMP-dependent protein kinase (PKA). While necessary for c-fos transcriptional induction following VSCC opening, CREB serine 133 phosphorylation is not sufficient for transcriptional activation. A second, PKA-dependent event is required. Following induction, c-fos transcription is rapidly down-regulated. Dephosphorylation of CREB serine 133 parallels and likely mediates the transcriptional shut-off event. These results suggest that the phosphorylation and dephosphorylation of CREB controls its ability to regulate transcription in membrane-depolarized cells and that multiple pathways contribute to Ca(2+)-activated gene expression.

Nerve growth factor activates a Ras-dependent protein kinase that stimulates c-fos transcription via phosphorylation of CREB

A mechanism by which the nerve growth factor (NGF) signal is transduced to the nucleus to induce gene expression has been characterized. An NGF-inducible, Ras-dependent protein kinase has been identified that catalyzes the phosphorylation of the cyclic AMP response element-binding protein (CREB) at Ser-133. Phosphorylation of Ser-133 stimulates the ability of CREB to activate transcription in NGF-treated cells. These findings suggest that CREB has a more widespread function than previously believed and functions in the nucleus as a general mediator of growth factor responses.

Membrane depolarization and calcium influx stimulate MEK and MAP kinase via activation of Ras

A pathway by which calcium influx through voltage-sensitive calcium channels leads to mitogen-activated protein kinase (MAPK) activation has been characterized. In PC12 cells, membrane depolarization leading to calcium influx through L-type calcium channels activates the dual specificity MAPK kinase MEK1, which phosphorylates and activates MAPK. Calcium influx leads within 30 s to activation of the small guanine nucleotide-binding protein Ras. Moreover, activation of MAPK in response to calcium influx is inhibited by the dominant negative mutant RasAsn17, indicating that Ras activity is required for calcium signaling to MAPK. Ras is also activated by release of calcium from intracellular stores and by membrane depolarization of primary cortical neurons. The pleiotropic regulatory potential of both Ras and the MAPK pathway suggests that they may be central mediators of calcium signaling in the nervous system.

Calcium regulation of gene expression in neuronal cells

Long-term adaptive changes in neurons following brief periods of neuronal activity are likely to involve changes in gene expression. The mechanisms of activity-dependent gene expression have been explored in central neurons and the neuronal cell line PC12. Calcium influx through either NMDA receptors or voltage-sensitive calcium channels leads to the rapid induction of a number of immediate-early genes including c-fos. Promoter analysis indicates that Ca2+ influx through different calcium channels activates distinct signaling pathways that either target the serum response element (SRE) or the calcium response element (CaRE) within the c-fos promoter. Transcription through the CaRE requires the induced phosphorylation of the cAMP response element binding protein (CREB) at Ser133. This site on CREB is also phosphorylated in the suprachiasmatic nucleus in vivo upon light stimulation. These observations suggest Ca2+ can regulate gene expression by multiple signaling pathways including one that involves the Ca(2+)-dependent phosphorylation of the transcription factor CREB.

A growth factor-induced kinase phosphorylates the serum response factor at a site that regulates its DNA-binding activity

A signaling pathway by which growth factors may induce transcription of the c-fos proto-oncogene has been characterized. Growth factor stimulation of quiescent fibroblasts activates a protein kinase cascade that leads to the rapid and transient phosphorylation of the serum response factor (SRF), a regulator of c-fos transcription. The in vivo kinetics of SRF phosphorylation and dephosphorylation parallel the activation and subsequent repression of c-fos transcription, suggesting that this phosphorylation event plays a critical role in the control of c-fos expression. The ribosomal S6 kinase pp90rsk, a growth factor-inducible kinase, phosphorylates SRF in vitro at serine 103, the site that becomes newly phosphorylated upon growth factor stimulation in vivo. Phosphorylation of serine 103 significantly enhances the affinity and rate with which SRF associates with its binding site, the serum response element, within the c-fos promoter. These results suggest a model in which the growth factor-induced phosphorylation of SRF at serine 103 contributes to the activation of c-fos transcription by facilitating the formation of an active transcription complex at the serum response element.

Regulation of CREB phosphorylation in the suprachiasmatic nucleus by light and a circadian clock

Mammalian circadian rhythms are regulated by a pacemaker within the suprachiasmatic nuclei (SCN) of the hypothalamus. The molecular mechanisms controlling the synchronization of the circadian pacemaker are unknown; however, immediate early gene (IEG) expression in the SCN is tightly correlated with entrainment of SCN-regulated rhythms. Antibodies were isolated that recognize the activated, phosphorylated form of the transcription factor cyclic adenosine monophosphate response element binding protein (CREB). Within minutes after exposure of hamsters to light, CREB in the SCN became phosphorylated on the transcriptional regulatory site, Ser133. CREB phosphorylation was dependent on circadian time: CREB became phosphorylated only at times during the circadian cycle when light induced IEG expression and caused phase shifts of circadian rhythms. These results implicate CREB in neuronal signaling in the hypothalamus and suggest that circadian clock gating of light-regulated molecular responses in the SCN occurs upstream of phosphorylation of CREB.

Regulation of gene expression in hippocampal neurons by distinct calcium signaling pathways

Calcium ions (Ca2+) act as an intracellular second messenger and can enter neurons through various ion channels. Influx of Ca2+ through distinct types of Ca2+ channels may differentially activate biochemical processes. N-Methyl-D-aspartate (NMDA) receptors and L-type Ca2+ channels, two major sites of Ca2+ entry into hippocampal neurons, were found to transmit signals to the nucleus and regulated gene transcription through two distinct Ca2+ signaling pathways. Activation of the multifunctional Ca(2+)-calmodulin-dependent protein kinase (CaM kinase) was evoked by stimulation of either NMDA receptors or L-type Ca2+ channels; however, activation of CaM kinase appeared to be critical only for propagating the L-type Ca2+ channel signal to the nucleus. Also, the NMDA receptor and L-type Ca2+ channel pathways activated transcription by means of different cis-acting regulatory elements in the c-fos promoter. These results indicate that Ca2+, depending on its mode of entry into neurons, can activate two distinct signaling pathways. Differential signal processing may provide a mechanism by which Ca2+ controls diverse cellular functions.

Trans-synaptic regulation of gene expression

Neurotransmitters regulate gene expression through second messenger cascades that transmit the signal from the plasma membrane to the nucleus of the postsynaptic cell. Ca2+ and cAMP are two of the second messengers that regulate gene expression in response to neurotransmitters. The Ca2+ and cAMP signals induce expression of a class of genes, termed immediate early genes, within minutes of neurotransmitter receptor activation. Many of these genes encode transcription factors that regulate the expression of late response genes. The results of recent experiments have elucidated mechanisms by which neurotransmitter-induced Ca2+ and cAMP signals regulate immediate early gene expression.

The activity of cAMP-dependent protein kinase is required at a posttranslational level for induction of voltage-dependent sodium channels by peptide growth factors in PC12 cells

The synthesis and expression of voltage-dependent sodium (Na) channels is a crucial aspect of neuronal differentiation because of the central role these ion channels play in the generation of action potentials and the transfer of information in the nervous system. We have used rat pheochromocytoma (PC12) cell lines deficient in cAMP-dependent protein kinase (PKA) activity to examine the role of PKA in the induction of Na channel expression by nerve growth factor (NGF) and basic FGF (bFGF). In the parental PC12 cell line both NGF and bFGF elicit an increase in the density of functional Na channels, as determined from whole-cell patch clamp recordings. This increase does not occur in two PC12 cell lines deficient in both isozymes of PKA (PKAI and PKAII), and is strongly reduced in a third line deficient in PKAII, but not PKAI. Despite the inability of the neurotrophic factors to induce functional Na channel expression in the PKA-deficient cells, Northern blot hybridization studies and saxitoxin binding assays of intact cells indicate that NGF and bFGF are still capable of eliciting increases in both Na channel mRNA and Na channel protein in the membrane. Thus, PKA activity appears to be necessary at a posttranslational step in the synthesis and expression of functional Na channels, and thereby plays an important role in determining neuronal excitability.

Synergistic effects of nerve growth factor and phorbol 12-myristate 13-acetate on rapid motility and process formation in PC12 cells: the role of laminin

A126-1B2 cells, a PKA (cAMP-dependent protein kinase)-deficient variant of PC12 cells, but not parental PC12 cells, form processes within 15-30 min of exposure to both nerve growth factor (NGF) and activators of protein kinase C when grown on tissue culture plastic (Glowacka and Wagner, J Neurosci Res 25: 453-462, 1990). Time-lapse microscopy has demonstrated that these processes are formed by a novel mechanism, i.e., rapid movement of the cell body away from a point of attachment, which morphologically resembles a growth cone. These "fast" neurites are attached to the substratum at a number of points, which display membrane activity in the form of active ruffling and the extension of filopodia and membrane pleats. Thus, these processes are formed by a mechanism distinct from that used by PC12 and other neuronal cells to form processes in culture. Wild-type PC12 cells also migrate and form fast neurites in response to a combination of NGF and phorbol 12-myristate 13-acetate (PMA), when they are grown in conditioned media or plates, suggesting that a secreted factor that can bind to the substratum is essential for the rapid formation of these neurites. Similarly, wild-type PC12 cells grown on a laminin-coated substratum also migrate and form "fast neurites" in response to a combination of NGF and PMA. This rapid migration is attenuated by an anti-alpha 1, beta 1-integrin antisera, implicating a laminin-integrin interaction; and it is inhibited by alpha-lactalbumin, suggesting an involvement of a beta 1,4 galactosyltransferase in the response. The formation of fast neurites is not dependent on concurrent protein synthesis, but it is inhibited by lithium, cytochalasin D, and methylthioadenosine or pretreatment of cells with NGF. Thus PC12 cells grown on the appropriate substrate have the ability to migrate rapidly and thereby form neuron-like processes within minutes of exposure to NGF and PMA. This morphological response to a combination of agents may provide an alternative means by which nerve cells form connections. Alternatively, it may reflect a mechanism that facilitates cellular migration during developmental processes.

Induction of immediate early genes by Ca2+ influx requires cAMP-dependent protein kinase in PC12 cells

Agents that activate cAMP-dependent protein kinase (PKA) as well as agents that increase intracellular calcium induce the expression of certain immediate early genes (IEGs). Recently, it has been demonstrated that the same cis-acting element in the 5' region of the c-fos gene has the ability to mediate both cAMP- and calcium-induced c-fos expression in PC12 cells (Sheng, M., McFadden, G., and Greenberg, M. (1990) Neuron 4, 571-582). Here we demonstrate that both cAMP- and calcium-mediated induction of c-fos and egr1 are dependent on PKA activity. Addition of either depolarizing concentrations of KCl or the calcium ionophore, ionomycin, to PC12 cells increased the expression of both c-fos and egr1, but these inductions were dramatically reduced in three PKA-deficient cell lines, 123.7, AB.11, and A126-1B2. Furthermore, pretreatment of PC12 cells with 20 microM H89, a specific inhibitor of PKA, inhibited forskolin, dibutyryl cAMP, and KCl-induced c-fos and egr1 induction, while having no effect on NGF induction. Likewise, in the PKA-deficient cells, NGF or an activator of protein kinase C induced c-fos and egr1 normally. To determine if PKA deficiency modifies the ability of Ca2+ to activate calcium-dependent kinases, autophosphorylation of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) in response to Ca2+ influx was determined. In parental PC12 cells, PC12 cells pretreated with H89, and PKA-deficient cell lines, CaM kinase was activated equivalently in response to KCl depolarization. These results suggest that PKA is not required for Ca(2+)-induced increase in CaM kinase activity and that the induction of IEGs in response to Ca2+ influx is PKA-dependent. Thus, the requirement for PKA resides at a point distal to the activation of calmodulin-dependent processes.

Nerve growth factor-induced neuronal differentiation after dominant repression of both type I and type II cAMP-dependent protein kinase activities

Clonal PC12 lines deficient in cAMP-dependent protein kinase (PKA) were made by stably expressing mutant regulatory subunits (RI) of PKA that are deficient in cAMP binding (Correll, L. A., Woodford, T. A., Corbin, J. D., Mellon, P. L., and McKnight, G. S. (1989) J. Biol. Chem. 264, 16672-16678). Expression of the mutant RIs repressed cAMP-dependent activation of both PKAI and PKAII while having no effects on the cAMP binding to either free RI or RII or the level of catalytic subunit protein. These data suggest that RI and RII compete for the same pool of catalytic subunit and that the level of PKAI and PKAII are interdependent. We have used these cell lines to examine the requirement for PKA in mediating the effects of nerve growth factor (NGF) and agents that are thought to act exclusively via cAMP-dependent pathways. While several responses to cAMP were strongly compromised in these lines, NGF-dependent responses were comparable in parental and PKA-deficient cells, including: 1) protein phosphorylation, 2) transcriptional induction of the immediate early gene egr1, 3) expression of the gene for GAP-43, 4) induction of ornithine decarboxylase activity, and 5) formation of neurites. Furthermore, transient expression of the cAMP-dependent protein kinase inhibitor (RSVPKI; Day, R. N., Walder, J. A., and Maurer, R. A. (1989) J. Biol. Chem. 264, 431-436) blocked cAMP, but not NGF, induction of regulatory elements derived from the gene for egr1. These experiments support the idea that NGF can regulate neuronal differentiation by pathways that are independent of cAMP-activatable PKA.

Retinoic acid stimulates the differentiation of PC12 cells that are deficient in cAMP-dependent protein kinase

Retinoic acid (RA) induced neuronal differentiation in A126-1B2 cells and 123.7 cells, two mutant lines of PC12 that are deficient in cAMP-dependent protein kinase, but not in the parental PC12 cell line. A single exposure to RA was sufficient to cause neurite formation and inhibit cell division for a period of greater than 3 wk, suggesting that RA may cause a long-term, stable change in the state of these cells. In A126-1B2 cells, RA also induced the expression of other markers of differentiation including acetylcholinesterase and the mRNAs for neurofilament (NF-M) and GAP-43 as effectively as nerve growth factor (NGF). Neither NGF nor RA stimulated an increase in the expression of smg-25A in A126-1B2 cells, suggesting that the cAMP-dependent protein kinases may be required for an increase in the expression of this marker. RA also caused a rapid increase in the expression of the early response gene, c-fos, but did not effect the expression of egr-1. RA equivalently inhibited the division of A126-1B2 cells, 123.7 cells and parental PC12 cells, so RA induced differentiation is not an indirect response to growth arrest. In contrast, the levels of retinoic acid receptors (RAR alpha and RAR beta), and retinoic acid binding protein (CRABP) mRNA were strikingly higher in both A126-1B2 cells and 123.7 cells than in the parental PC12 cells. The deficiencies in cAMP-dependent protein kinase may increase the expression of CRABP and the RARs; and, thus, cAMP may indirectly regulate the ability of RA to control neurite formation and neural differentiation. Thus, RA appears to regulate division and differentiation of PC12 cells by a biochemical mechanism that is quite distinct from those used by peptide growth factors.

Multiple pathways for the regulation of ornithine decarboxylase in intestinal epithelial cells

The regulation of ornithine decarboxylase (ODC) was examined in an intestinal epithelial crypt cell line (IEC-6). Addition of fetal bovine serum or growth factors to quiescent preconfluent cells resulted in a 20- to 30-fold increase in the specific activity of ODC, which was maximal at approximately 4 h. In contrast, ODC mRNA levels either did not change or increased only twofold over the time period examined. The increased enzymatic activity was blocked by cycloheximide, putrescine, and the calmodulin antagonist N-(6-aminohexyl)-5-chloro-1-napthalinesulfonamide (W-7). Cycloheximide alone increased mRNA levels and potentiated the induction in response to serum, suggesting that protein synthesis is not required for the increase in mRNA accumulation. In contrast to its effect on ODC activity, W-7 was without effect on the serum-stimulated increase in ODC or c-fos mRNA levels. Putrescine decreased ODC activity, but not mRNA content, in a dose-dependent manner with an IC50 between 0.1 and 1.0 microM. Also, serum stimulation resulted in a threefold increase in the stability of the enzyme in the presence of cycloheximide; this effect was blocked by pretreatment with W-7. Enzymatic activity was paralleled by ODC protein content as determined by [3H] difluoromethylornithine binding. Finally, the induction of enzyme activity was due entirely to an increase in Vmax as no detectable change in Km for ornithine was detected. These results suggest that ODC is regulated at multiple levels by independent signaling pathways in cultured intestinal epithelial cells. Increased levels of active ODC protein and enzymatic activity are sensitive to W-7 and putrescine.(ABSTRACT TRUNCATED AT 250 WORDS)

Apparent post-transcriptional modification of ornithine decarboxylase accounts for its induction in IEC-6 cells in culture

Putrescine, the product of the ornithine decarboxylase (ODC)-catalyzed reaction, stimulates macromolecular synthesis in a duodenal crypt cell line, IEC-6 cells, grown in culture. In addition, supplementation of medium with putrescine alone reverses the inhibition of proliferation produced by inhibition of ODC with difluoromethylornithine (DFMO). A series of experiments was initiated in the IEC-6 cell line to study the regulation of induction of ODC, as this enzyme is rate-limiting in putrescine synthesis. Five percent fetal bovine serum (FBS) and 10 nM IGF-II stimulated a 21-fold and 6-fold induction of ODC activity, respectively. Kinetic analysis indicated that the effect was on the Vmax of the reaction and not on the Km, suggesting an increase in total ODC protein. This was verified by measuring [3H]DFMO binding; serum-stimulated induction of activity was accompanied by a corresponding 20-fold increase in the specific binding of DFMO to ODC. In contrast, Northern analysis demonstrated only a two-fold change in ODC mRNA level during induction. Measurement of enzyme stability showed that the half-life of the ODC protein was increased three-fold above basal level in the induced state. Inhibition of induction was produced by pretreatment with either the calmodulin antagonist, W-7, or the product of the ODC-catalyzed reaction, putrescine. Further analysis illustrated that the inhibition produced by these agents was partly the result of destabilization of the enzyme and not a decrease in message level. These results demonstrate that the induction of ODC by trophic agents is the result of post-transcriptional events rather than at the level of RNA synthesis.

Putrescine stimulates DNA synthesis in intestinal epithelial cells

Experiments were designed to examine the effects of exogenously supplied putrescine on the synthesis of DNA, RNA, and protein in cultured epithelial cells (IEC-6). Putrescine increased aphidicolin-sensitive DNA synthesis at concentrations as low as 0.3 microM putrescine with maximal stimulation (267% control) at 10 microM. This response appeared to be an effect of increases in the intracellular concentration of putrescine as the intracellular levels of spermidine and spermine did not change over the time period examined. Furthermore, pulse-chase experiments revealed that putrescine that entered the cell was not metabolized to another polyamine or degraded. In addition, 10 microM putrescine enhanced both cycloheximide-sensitive lysine incorporation and actinomycin D-sensitive uridine incorporation, indexes of protein and RNA synthesis, respectively. Incorporation of both lysine and uridine was maximal 12 h after the addition of putrescine, whereas thymidine incorporation was still increasing at 24 h, the longest time point examined. These data suggest that putrescine synthesis and/or transport during mucosal proliferation is directly involved in the stimulation of epithelial DNA, RNA, and protein synthesis.

Polyamine-dependent growth and calmodulin-regulated induction of ornithine decarboxylase

The proliferation of cultured gastrointestinal crypt epithelial cells (IEC-6) and the role of calcium in polyamine biosynthesis were examined after serum stimulation of quiescent cells. Ornithine decarboxylase (ODC) activity was high when cells grew in 5% fetal calf serum (FCS) and dropped to nearly nondetectable levels when cells reached contact inhibition of growth. Polyamines appeared to be necessary for the proliferation of these cells, as growth was completely inhibited by the addition of 5 mM difluoromethylornithine, a specific inhibitor of ODC, to the media. This effect was reversed by 10 microM putrescine. Serum deprivation of preconfluent cells resulted in a fall in ODC activity. Readdition of serum led to an increase in ODC activity, which peaked at 4 h after addition and preceded both putrescine accumulation and [3H]thymidine incorporation into acid-precipitable material. Furthermore, readdition of serum to serum-deprived cells resulted in an approximately twofold increase in the level of free, ionized, intracellular Ca2+ as measured spectrophotometrically by monitoring fura-2 fluorescence. Inhibition of calmodulin-mediated processes with N-(6-aminohexyl)-5-chloro-1-naphthalelesulfonamide (W-7), a calmodulin antagonist, inhibited the serum-stimulated induction of ODC in a dose-dependent manner, with an IC50 of approximately 10 microM. Similar results were obtained with trifluoperazine. Lastly, 30 microM W-7 completely inhibited serum-stimulated [3H]thymidine incorporation into acid-precipitable material. These data demonstrate that polyamine biosynthesis and subsequent DNA synthesis after serum refeeding of cells is regulated by a Ca2+ activated, calmodulin-dependent process. Furthermore, the production of polyamines is essential for normal proliferation of these epithelial cells in culture.

Neurogenic pulmonary edema in a pulmonary normotensive model

In the present study our aim was to determine whether or not neurogenic pulmonary edema would develop from a brief pulse of intracranial pressure (ICP) in the absence of any obvious pulmonary hypertension. There were three groups of cats: sham-operated controls, ICP only, and ICP plus variable occlusion of the pulmonary artery. Partial occlusion of the pulmonary artery was carried out by placing a ligature around the pulmonary trunk and mechanically constricting the artery to maintain pulmonary arterial pressure (PAP) and left atrial pressure (LAP) at pre-ICP levels. In sham-operated animals the extravascular lung water/blood free dry weight ratio (EVLW/BFDW) was 3.26 +/- 0.07 and broncho-alveolar lavage (BAL) protein, 6.49 +/- 0.62 mg/g lung. ICP-only caused a rise in PAP, left atrial pressure, and EVLW/BFDW to 3.67 +/- 0.08 (P less than 0.05). ICP with partial occlusion of the pulmonary artery prevented any rise in PAP or LAP while EVLW/BFDW rose to 3.67 +/- 0.10 (P less than 0.05) and BAL protein was 8.37 +/- 1.27 mg/g lung. Our results show that EVLW/BFDW can increase with neurogenic pulmonary edema in cats in the absence of an obvious increase in pulmonary arterial or left atrial pressure.

A pyrene fluorescence technique and microchamber for measurement of oxygen consumption of single isolated axons

Pyrene fluorescence is quenched by oxygen in an inverse and linear manner related to the partial pressure of O2 in solution. We have developed a microchamber for measuring QO2 of a single isolated axon, monitoring the change in fluorescence of a pyrene probe. The probe consists of a Spectra/Por dialysis hollow fiber filled with 2.5 mM pyrene in paraffin oil. The probe is inserted into a 1-mm-i.d. 2-cm-long quartz capillary tube with a freshly isolated crayfish medial giant axon. The capillary is mounted in an apparatus that forms an air- and water-tight seal except for a 0.2-mm-i.d. stainless steel tube at both ends permitting the exchange of solutions. An Olympus inverted microscope, equipped with epifluorescence optics and a 150-W xenon lamp, is used to view the preparation, generate the excitation light, and monitor the emitted fluorescence with a photomultiplier tube placed in the microscope TV port. A dichroic filter unit is utilized to select an excitation wavelength of 350 nm and collect emitted light above 420 nm. The signal is amplified with a Keithley 480 picoammeter and recorded on a strip chart. QO2 of isolated axons was 552 +/- 70 X 10(-6) mol O2/liter tissue X min. Following sequential treatment with 2 mM ouabain and 2 mM NaCN, QO2 decreased by 22 and 82%, respectively. These data are consistent with QO2 measurements of whole nerve cord made with a Clark electrode O2 monitor.(ABSTRACT TRUNCATED AT 250 WORDS)