Hypoxic/ischemic conditions induce expression of the putative pro-death gene Clca1 via activation of extrasynaptic N-methyl-D-aspartate receptors

The stimulation of extrasynaptic N-methyl-D-aspartate (NMDA) receptors triggers cell death pathways and has been suggested to play a key role in cell degeneration and neuron loss associated with glutamate-induced excitotoxicity. In contrast, synaptic NMDA receptors promote neuronal survival. One mechanism through which extrasynaptic NMDA receptors damage neurons may involve Clca1, which encodes a putative calcium-activated chloride channel. Here we show that Clca1 expression is induced in cultured rat hippocampal neurons exposed to oxygen/glucose-free media; this induction is mediated by a signaling pathway activated by extrasynaptic NMDA receptors. Clca1 mRNA levels also increased in the gerbil hippocampus following a transient forebrain ischemia caused by bilateral carotid occlusion. Microelectrode array recordings revealed that oxygen-glucose deprivation enhances hippocampal network firing rates, which induces c-fos transcription through a signaling pathway that, in contrast to Clca1, is activated by synaptic but not extrasynaptic NMDA receptors. Thus, conditions of low oxygen/glucose lead to the activation of both extrasynaptic and synaptic NMDA receptors that regulate distinct target genes. Clca1 may be part of the genomic death program triggered by extrasynaptic NMDA receptors; it could be a marker for ischemic brain damage and a possible target for therapeutic interventions.

Extrasynaptic NMDARs oppose synaptic NMDARs by triggering CREB shut-off and cell death pathways

Here we report that synaptic and extrasynaptic NMDA (N-methyl-D-aspartate) receptors have opposite effects on CREB (cAMP response element binding protein) function, gene regulation and neuron survival. Calcium entry through synaptic NMDA receptors induced CREB activity and brain-derived neurotrophic factor (BDNF) gene expression as strongly as did stimulation of L-type calcium channels. In contrast, calcium entry through extrasynaptic NMDA receptors, triggered by bath glutamate exposure or hypoxic/ischemic conditions, activated a general and dominant CREB shut-off pathway that blocked induction of BDNF expression. Synaptic NMDA receptors have anti-apoptotic activity, whereas stimulation of extrasynaptic NMDA receptors caused loss of mitochondrial membrane potential (an early marker for glutamate-induced neuronal damage) and cell death. Specific blockade of extrasynaptic NMDA receptors may effectively prevent neuron loss following stroke and other neuropathological conditions associated with glutamate toxicity.

CREB/CBP and SRE-interacting transcriptional regulators are fast on-off switches: duration of calcium transients specifies the magnitude of transcriptional responses

Transient increases in the intracellular calcium concentration, which are associated with electrical activation of neurones, control synapse-to-nucleus communication. Calcium signals differ in time and space but it is unclear exactly how this translates into stimulus-specific gene expression. Analysis of transcription induced by calcium transients with defined durations revealed that the evoked genomic responses, unlike those following neurotrophin exposure, are not all-or-none but graded events. The CRE-binding protein CREB, its coactivator CREB-binding protein (CBP), and SRE-interacting transcriptional regulators are fast on-off switches: their activities are induced by short-lasting calcium signals, remain active for the duration of the signal and are rapidly shut-off after calcium concentrations have returned to basal levels. CREB is switched on by a fast, nuclear calmodulin (CaM) kinase-dependent mechanism that mediates CREB phosphorylation on serine 133 within 30 s of calcium entry. The second calcium-activated route to CREB involves the MAP kinase/extracellular signal-regulated kinase (ERK1/2) cascade. This pathway can be triggered by brief, 30-60 s calcium transients. ERK1/2 activity peaks several minutes after calcium entry and can outlast the calcium transient. The shut-off of CREB and ERK1/2 involves rapid dephosphorylation of their activator sites. These properties of transcription factors and their regulating kinases and phosphatases provide a mechanism through which the duration of calcium signals specifies the magnitude of the transcriptional response. The decoding of temporal features of calcium transients is likely to contribute to impulse-specific gene expression.

The role of putative intragenic control elements in c-fos regulation by calcium and growth factor signalling pathways

Sequences in the transcribed region of the c-fos gene have been suggested to control c-fos induction following exposure of cells to mitogens or stimuli that increase intracellular calcium concentrations. Using a mutational analysis we show that putative regulatory elements present in the first intron of the human c-fos gene and the fos-intragenic-regulatory-element (FIRE) are not required for c-fos regulation by growth factor and calcium signalling pathways in AtT20 and PC12 cells. Removal of the c-fos first intron and the FIRE did not increase the basal level of c-fos mRNA and only moderately reduced the magnitude of calcium-induced transcription mediated by either the entire c-fos promoter or the cAMP response element (CRE). Intragenic mutations did not affect serum response element (SRE)-dependent gene expression induced by calcium signals but caused a superinduction of c-fos expression in nerve growth factor-stimulated PC12 cells. These results indicate that c-fos promoter elements, rather than intragenic sequences, are the principal targets of transcription-regulating signalling pathways. This suggests that CRE- and SRE-bound activators of transcription initiation may also enhance, in a signal-dependent manner, c-fos transcript elongation beyond promoter-proximal pause sites.

Nuclear calcium signaling controls CREB-mediated gene expression triggered by synaptic activity

Information storage in the nervous system requires transcription triggered by synaptically evoked calcium signals. It has been suggested that translocation of calmodulin into the nucleus, initiated by submembranous calcium transients, relays synaptic signals to CREB. Here we show that in hippocampal neurons, signaling to CREB can be activated by nuclear calcium alone and does not require import of cytoplasmic proteins into the nucleus. The nucleus is particularly suited to integrate neuronal firing patterns, and specifies the transcriptional outputs through a burst frequency-to-nuclear calcium amplitude conversion. Calcium release from intracellular stores promotes calcium wave propagation into the nucleus, which is critical for CREB-mediated transcription by synaptic NMDA receptors. Pharmacological or genetic modulation of nuclear calcium may directly affect transcription-dependent memory and cognitive functions.

Transcription-dependent neuronal plasticity: The nuclear calcium hypothesis

In neurons, calcium ions control gene transcription induced by synaptic activity. The states and histories of neuronal activity are represented by a calcium code that comprises the site of calcium entry, and the amplitude, duration and spatial properties of signal-evoked calcium transients. The calcium code is used to transform specific firing patterns into qualitatively and quantitatively distinct transcriptional responses. The following hypothesis is proposed: electrical activity causes long-lasting, transcription-dependent changes in neuronal functions when synaptically evoked calcium transients associated with the stimulation propagate to the nucleus; gene transcription activated by dendritic calcium signals only is insufficient to consolidate functional alterations long-term. Similar to enduring increases in synaptic efficacy, nuclear calcium transients are induced by high-frequency firing patterns or by weak synaptic inputs coinciding with backpropagating dendritic action potentials. Nuclear calcium stimulates CREB-mediated transcription and, through inducing the activity of the transcriptional coactivator CREB-binding protein (CBP), may modulate the expression of numerous genes including neurotransmitter receptors and scaffolding proteins. Increases in the transcription rate of target genes are predicted to be transient and in many cases small, however, they collectively contribute to the maintenance of changes in synaptic efficacy. Nuclear calcium may be the common regulator of diverse transcription-dependent forms of neuronal plasticity.

Calcium-regulated protein kinase cascades and their transcription factor targets

In the nervous system, calcium signals associated with electrical activation of neurons induce gene transcription that may be important for long-lasting adaptation. The type of transcriptional response is determined by the properties of the calcium signal that include subcellular localisation, amplitude, duration and the physical site of entry. Here we review calcium-regulated protein kinase cascades and discuss potential mechanisms through which they propagate calcium signals to and within the nucleus and control the activity of transcription factors and transcriptional co-activators.

Calcium as a versatile second messenger in the control of gene expression

The elevation of intracellular calcium is a major effector of stimulus-induced physiological change in a variety of cell types. Such change is invariably complex and frequently involves the activation of gene expression. Calcium signals are often able to activate different subsets of genes within the same cell, the basis for which has been unclear. Recent studies have revealed that a number of differing properties of the calcium signal are responsible for distinct cellular responses.

Nuclear calcium-activated gene expression: possible roles in neuronal plasticity and epileptogenesis

Nuclear calcium signals associated with electrical activation of neurons are critical regulators of gene expression and may cause changes in neuronal structure and function. Recent studies have identified a key component of the transcriptional machinery, the coactivator CREB binding protein (CBP), as a target for a nuclear calcium signalling pathway. Because the regulation of many genes involves transcription factors that function through their interaction with CBP, this mechanism, termed 'the coactivator control model', may modulate the expression of a large number of genes. During normal working of the brain, nuclear calcium increases may be transient and initiate transcriptional responses that are important for learning and memory. However, more intense or sustained stimulations of neurons (for example those used in the kindling model) may overactivate nuclear calcium-regulated processes. This may initiate inappropriate gene expression responses and could lead to the formation of epileptic neuronal circuits and disorders of neuronal excitability.

Control of recruitment and transcription-activating function of CBP determines gene regulation by NMDA receptors and L-type calcium channels

Recruitment of the coactivator CBP by signal-regulated transcription factors and stimulation of CBP activity are key regulatory events in the induction of gene transcription following Ca2+ flux through ligand- and/or voltage-gated ion channels in hippocampal neurons. The mode of Ca2+ entry (L-type Ca2+ channels versus NMDA receptors) differentially controls the CBP recruitment step to CREB, providing a molecular basis for the observed Ca2+ channel type-dependent differences in gene expression. In contrast, activation of CBP is triggered irrespective of the route of Ca2+ entry, as is activation of c-Jun, that recruits CBP independently of phosphorylation at major regulatory c-Jun phosphorylation sites, serines 63 and 73. This control of CBP recruitment and activation is likely relevant to other CBP-interacting transcription factors and represents a general mechanism through which Ca2+ signals associated with electrical activity may regulate the expression of many genes.

c-Jun functions as a calcium-regulated transcriptional activator in the absence of JNK/SAPK1 activation

Calcium is the principal second messenger in the control of gene expression by electrical activity in neurons. Recruitment of the coactivator CREB-binding protein, CBP, by the prototypical calcium-responsive transcription factor, CREB and stimulation of CBP activity by nuclear calcium signals is one mechanism through which calcium influx into excitable cells activates gene expression. Here we show that another CBP-interacting transcription factor, c-Jun, can mediate transcriptional activation upon activation of L-type voltage-gated calcium channels. Calcium-activated transcription mediated by c-Jun functions in the absence of stimulation of the c-Jun N-terminal protein kinase (JNK/SAPK1) signalling pathway and does not require c-Jun amino acid residues Ser63 and Ser73, the two major phosphorylation sites that regulate c-Jun activity in response to stress signals. Similar to CREB-mediated transcription, activation of c-Jun-mediated transcription by calcium signals requires calcium/ calmodulin-dependent protein kinases and is dependent on CBP function. These results identify c-Jun as a calcium-regulated transcriptional activator and suggest that control of coactivator function (i.e. recruitment of CBP and stimulation of CBP activity) is a general mechanism for gene regulation by calcium signals.

Nuclear calcium: a key regulator of gene expression

Through the evolution of multicellular organisms, calcium has emerged as the preferred ion for intracellular signalling. It now occupies a pivotal role in many cell types and nowhere is it more important than in neurons, where it mediates both the relaying and long-term storage of information. The latter is a process that enables learning and memory to be formed and requires the activation of gene expression by calcium signals. Evidence from a number of diverse organisms shows that transcription mediated by the transcription factor CREB is critical for learning and memory. Here we review the features of CREB activation by calcium signals in mammalian cells. In contrast to other transcription factors, its regulation is dependent on an elevation of nuclear calcium concentration, potentially placing this spatially distinct pool of calcium as an important mediator of information storage.

CBP: a signal-regulated transcriptional coactivator controlled by nuclear calcium and CaM kinase IV

Recruitment of the coactivator, CREB binding protein (CBP), by signal-regulated transcription factors, such as CREB [adenosine 3', 5'-monophosphate (cAMP) response element binding protein], is critical for stimulation of gene expression. The mouse pituitary cell line AtT20 was used to show that the CBP recruitment step (CREB phosphorylation on serine-133) can be uncoupled from CREB/CBP-activated transcription. CBP was found to contain a signal-regulated transcriptional activation domain that is controlled by nuclear calcium and calcium/calmodulin-dependent (CaM) protein kinase IV and by cAMP. Cytoplasmic calcium signals that stimulate the Ras mitogen-activated protein kinase signaling cascade or expression of the activated form of Ras provided the CBP recruitment signal but did not increase CBP activity and failed to activate CREB- and CBP-mediated transcription. These results identify CBP as a signal-regulated transcriptional coactivator and define a regulatory role for nuclear calcium and cAMP in CBP-dependent gene expression.

Mechanisms controlling gene expression by nuclear calcium signals

Nuclear calcium is an important regulator of gene expression following membrane depolarisation of electrically excitable cells. Here we describe nuclear calcium transients in hippocampal neurons following activation of calcium influx through L-type voltage-sensitive calcium channels and N-methyl-D-aspartate (NMDA) receptors, as well as following calcium release from intracellular caffeine-sensitive stores. Increases in nuclear calcium activate gene transcription by a mechanism that is distinct from gene regulation by cytoplasmic calcium signals and involves the cAMP response element (CRE) and the CRE binding protein, CREB. The nuclear calcium/calmodulin dependent (CaM) protein kinase IV, which is expressed in cultured hippocampal neurons and in the mouse pituitary cell line AtT20, may function as a mediator of nuclear calcium-induced transcription.

Calcium controls gene expression via three distinct pathways that can function independently of the Ras/mitogen-activated protein kinases (ERKs) signaling cascade

Calcium ions are the principal second messenger in the control of gene expression by electrical activation of neurons. However, the full complexity of calcium-signaling pathways leading to transcriptional activation and the cellular machinery involved are not known. Using the c-fos gene as a model system, we show here that the activity of its complex promoter is controlled by three independently operating signaling mechanisms and that their functional significance is cell type-dependent. The serum response element (SRE), which is composed of a ternary complex factor (TCF) and a serum response factor (SRF) binding site, integrates two calcium-signaling pathways. In PC12 cells, calcium-regulated transcription mediated by the SRE requires the TCF site and is not inhibited by expression of the dominant-negative Ras mutant, RasN17, nor by the MAP kinase kinase 1 inhibitor PD 98059. In contrast, TCF-dependent transcriptional regulation by nerve growth factor or epidermal growth factor is mediated by a Ras/MAP kinases (ERKs) pathway targeting the TCF Elk-1. In AtT20 cells and hippocampal neurons, calcium signals can stimulate transcription via a TCF-independent mechanism that requires the SRF binding site. The cyclic AMP response element (CRE), which cooperates with the TCF site in growth factor-regulated transcription, is a target of a third calcium-regulated pathway that is little affected by the expression of RasN17 or by PD 98059. Thus, calcium can stimulate gene expression via a TCF-, SRF-, and CRE-linked pathway that can operate independently of the Ras/MAP kinases (ERKs) signaling cascade in a cell type-dependent manner.

Gene regulation by nuclear and cytoplasmic calcium signals

Calcium entry into neuronal cells through N-methyl-D-aspartate (NMDA)-type glutamate receptors or L-type voltage-gated calcium channels is a key event in the control of gene expression following electrical activation. Calcium acts both in the cytoplasm and the nucleus to activate signalling pathways that stimulate gene expression through different DNA regulatory elements. Differential control of transcription by spatially distinct calcium signals provides a mechanism by which a single second messenger can generate diverse transcriptional responses. This may allow for stimulation-specific modulation of gene expression critical for adaptive changes in the nervous system.

Distinct functions of nuclear and cytoplasmic calcium in the control of gene expression

Calcium entry into neuronal cells through voltage or ligand-gated ion channels triggers neuronal activity-dependent gene expression critical for adaptive changes in the nervous system. Cytoplasmic calcium transients are often accompanied by an increase in the concentration of nuclear calcium, but the functional significance of such spatially distinct calcium signals is unknown. Here we show that gene expression is differentially controlled by nuclear and cytoplasmic calcium signals which enable a single second messenger to generate diverse transcriptional responses. We used nuclear microinjection of a non-diffusible calcium chelator to block increases in nuclear, but not cytoplasmic, calcium concentrations following activation of L-type voltage-gated calcium channels. We showed that increases in nuclear calcium concentration control calcium-activated gene expression mediated by the cyclic-AMP-response element (CRE), and demonstrated that the CRE-binding protein CREB can function as a nuclear calcium-responsive transcription factor. A second signalling pathway, activating transcription through the serum-response element (SRE), is triggered by a rise in cytoplasmic calcium and does not require an increase in nuclear calcium.

N-methyl-D-aspartate receptors are critical for mediating the effects of glutamate on intracellular calcium concentration and immediate early gene expression in cultured hippocampal neurons

The mechanisms by which activation of excitatory amino acid receptors is coupled to the regulation of gene transcription were studied using cultured hippocampal neurons from neonatal rats. Voltage recording, calcium imaging, specific RNA analysis and immunocytochemistry were carried out on sister cultures. This allowed analysis of the expression of functional glutamate receptor subtypes, examination of their role in controlling intracellular free calcium ([Ca2+]i), and determination of their relative contributions to the transcriptional regulation of six immediate early genes c-fos, fosB, c-jun, junB, zif/268 (also termed Egr-1; NGFI-A; Krox-24) and nur/77 (also termed NGFI-B). Expression of all six immediate early genes was induced in hippocampal neurons by glutamate treatment. Nuclear run-on assays demonstrated that this induction occurred at the transcriptional level. Activation of the N-methyl-D-aspartate subtype of glutamate receptor was necessary and sufficient for the transcriptional response. Non-N-methyl-D-aspartate receptors, while present in cultured hippocampal neurons, contributed relatively little to the regulation of transcription. Calcium imaging showed that glutamate-induced changes in [Ca2+]i were almost entirely mediated by N-methyl-D-aspartate receptors, rather than by L-type voltage-sensitive calcium channels. Previous studies have shown that stimulation with selective agonists of either N-methyl-D-aspartate receptors, non-N-methyl-D-aspartate receptors, or L-type calcium channels can lead to an increase in [Ca2+]i and c-fos expression. Here we demonstrate that in our hippocampal culture system glutamate controls [Ca2+]i and induces immediate early gene transcription primarily by activating N-methyl-D-aspartate receptors.

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.

Regionally selective stimulation of mitogen activated protein (MAP) kinase tyrosine phosphorylation after generalized seizures in the rat brain

Immunoblot analysis using a phosphotyrosine-specific antibody was performed to investigate tyrosine phosphorylation of the mitogen activated protein (MAP) kinase in the rat brain. Epileptic seizures induced by systemic injection of bicuculline caused a rapid and transient stimulation of MAP kinase tyrosine phosphorylation in hippocampus and somatosensory cortex. This increase in tyrosine phosphorylation was markedly attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801. In contrast, in the cerebellum, tyrosine phosphorylation of MAP kinase remained undetectable after bicuculline-induced seizures. These results demonstrate that generalized seizures stimulate tyrosine phosphorylation of MAP kinase in a regionally selective manner.

Expression of endogenous NMDAR1 transcripts without receptor protein suggests post-transcriptional control in PC12 cells

Expression of RNA for the NMDAR1 subunit of the N-methyl-D-aspartate receptor was detected by Northern hybridization in both nerve growth factor-differentiated and undifferentiated rat pheochromocytoma (PC12) cells. The NMDA receptor type 1 (NMDAR1) message in PC12 cells was similar in size to that expressed in hippocampal neurons. PC12 cell cDNAs that were amplified by polymerase chain reaction with primers flanking the coding region of NMDAR1 corresponded to the NMDAR1 splice variant NMDA receptor type 1 isoform C (NMDAR1C). Using calcium imaging or patch-clamp recording, no functional NMDA-gated ion channels were found in PC12 cells. A monoclonal antibody against NMDAR1 was developed in order to investigate whether or not NMDAR1 protein was present in PC12 cells. Only trace amounts of NMDAR1 protein were found in native PC12 cells. However, expression of NMDAR1 protein was detected in PC12 cells that were transfected with an expression vector containing an NMDAR1C clone under control of a cytomegalovirus promoter. These findings suggest that the expression of NMDAR1 protein in PC12 cells may be controlled by post-transcriptional mechanisms. The PC12 cell line may serve as a model system for the study of the transcriptional, post-transcriptional, and translational regulation of NMDAR1. Furthermore, the presence of NMDAR1 RNA in a particular cell type may not necessarily indicate expression of NMDAR1 protein.

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.

Heterogeneous expression of c-myb protein in human leukemia detected by simultaneous two color flow cytometric analysis

The expression of c-myb mRNA and protein was analyzed in fresh leukemic cells by Northern-blot analyses and by immunofluorescent staining using monoclonal c-myb specific antibodies. Staining of the cells was evaluated by flow cytometry. The results demonstrate c-myb mRNA expression predominantly in acute lymphocytic leukemia (ALL, 4/4 cases), acute myeloic leukemia (AML, 17/17) and chronic myeloic leukemia (CML, 12/12) but rarely in chronic lymphocytic leukemia (CLL, 1/17). Immunofluorescent analyses revealed expression of c-myb protein in the nucleus of ALL (5/7) and AML (9/9) with a good correlation of c-myb-positive cells and with the number of proliferating (Ki67-positive) blast cells.

Stimulation of protein tyrosine phosphorylation by NMDA receptor activation

The N-methyl-D-aspartate (NMDA) receptor, a subtype of glutamate receptors, plays a key role in synaptic plasticity in the nervous system. After NMDA receptor activation, calcium entry into the postsynaptic neuron is a critical initial event. However, the subsequent mechanisms by which the NMDA receptor signal is processed are incompletely understood. Stimulation of cultured rat hippocampal cells with glutamate resulted in the rapid and transient tyrosine phosphorylation of a 39-kilodalton protein (p39). Tyrosine phosphorylation of p39 was triggered by the NMDA receptor and required an influx of Ca2+ from the extracellular medium. Because p39 was found to be highly related or identical to the microtubule-associated protein 2 kinase, the NMDA receptor signal may be processed by a sequential activation of protein kinases.

Transcriptional down-regulation of c-myc expression by protein synthesis-dependent and -independent pathways in a human T lymphoblastic tumor cell line

We show that in the human T lymphoblastic tumor cell line Molt4 c-myc mRNA and protein expression is down-regulated after exposure to dimethyl sulfoxide, to phorbol myristate acetate, or to the calcium ionophore A23187, which raises the intracellular calcium concentration. A block to RNA elongation is largely responsible for decreased c-myc transcription. Although negative regulation by dimethyl sulfoxide takes place even when protein synthesis is inhibited by cycloheximide, the phorbol myristate acetate effect is blocked to some extent only by cycloheximide. The calcium ionophore-induced c-myc suppression, however, strictly requires de novo protein synthesis. Therefore, two different negative regulatory pathways are involved in c-myc regulation: one which is independent and one which depends on de novo protein synthesis. The latter one appears to be mediated by a rapidly calcium-dependent induced gene product.

Distribution of c-myc, c-myb, and Ki-67 antigens in interphase and mitotic human cells evidenced by immunofluorescence staining technique

Using specific antibodies and the immunofluorescence staining technique we found a similar subcellular distribution pattern of the cellular proto-oncogene proteins c-myc and c-myb in interphase and mitotic HL60 and Molt4 cells. Antibodies against c-myc as well as those against c-myb protein gave rise to a nuclear staining excluding the nucleoli. In mitotic cells both proteins are apparently not associated with the chromatin of the condensed chromosomes, but appear diffusely distributed throughout the cytoplasm. In contrast, immunostaining using the proliferation marker antibody Ki-67 yielded in both cell lines several prominent specks in the nucleus and a weak finely dispersed staining throughout the nucleoplasm. No fluorescence was detectable in the cytoplasm. In dividing cells Ki-67 immunofluorescence was found to be associated with the surface of the chromosomes. The functional significance of the different localizations of the proteins is discussed in light of what is currently known about nuclear antigens.

Expression of p64c-myc and neuroendocrine properties define three subclasses of small cell lung cancer

Twelve human small cell lung cancer (SCLC) cell lines and 6 non-SCLC cell lines were analysed with respect to expression of the c-myc, c-myb, and c-raf1 protooncogenes at the protein level. Analysis of p64c-myc protein expression in 12 SCLC cell lines resulted in the observation that it is present at high levels not only in cells with low, but also in those with moderate neuroendocrine differentiation. Neuroendocrine differentiation was based on parameters such as growth rate, colony formation, L-Dopa decarboxylase (DDC) activity, bombesin, and neurotensin described before. Surprisingly, in two cell lines with low neuroendocrine differentiation but without c-myc protein expression (SCLC-86M1 and NCI-H526) p75c-myb expression was observed which may therefore be able to substitute for the p64c-myc protein. Analysis of p74c-raf1 expression did not result in correlation with any growth or differentiation parameter since it was expressed at low levels in 11 out of 12 cases. We conclude that SCLC in vitro can be classified in three rather than two previously defined subclasses. In addition to the classic subclass with slow growth, high neuroendocrine differentiation, and absent or very low p64c-myc expression and the variant subclass with fast growth, absent to very low neuroendocrine differentiation, and high p64c-myc expression, we suggest a third subclass designated as transitional with moderate growth, moderate neuroendocrine differentiation, and high p64c-myc expression. Data on a small number of non-SCLC cell lines tested showed that high levels of p64c-myc correlate with high in vitro growth rates. This indicates that high p64c-myc levels may be associated with high proliferative activity, and lack of differentiation in lung cancer in general. The p74c-raf1 protein was found in all non-SCLC cell lines. Whether this classification of SCLC cell lines is applicable to SCLC in vivo remains to be determined.

Mapping of a small phosphopeptide at the carboxyterminus of the viral myb protein by monoclonal antibodies

Several myb-specific monoclonal antibodies were produced and their antigen recognition sites characterized using a series of bacterially expressed truncated myb proteins. The monoclonal antibodies were used for analysing the in vivo phosphorylation site of the oncogene protein from avian myeloblastosis virus (AMV), p48v-myb. The p48v-myb protein labeled metabolically with [32P]orthophosphate was isolated from the AMV-transformed chicken myeloblast cell line BM-2 by immunoaffinity chromatography. Phosphoamino acid analysis indicated that it was phosphorylated mainly on serine and to a lesser extent (less than 5%) on threonine residues. Indirect immunoprecipitation of phosphopeptides from trypsin-digested [32P]-labeled purified p48v-myb protein by use of the myb-specific monoclonal antibodies allowed the mapping of a small phosphopeptide at the carboxyterminus of p48v-myb.

Nuclear and cytoplasmic distribution of cellular myb protein in human haematopoietic cells evidenced by monoclonal antibody

A monoclonal antibody was used for analysing the expression of the cellular myb (c-myb) protein in a variety of established human tumor cell lines and its decrease after induction of differentiation. Differentiated resting human T-cells and B-cells do not express detectable amounts of c-myb protein. However, upon mitogenic stimulation in vitro T-cells exhibit strong expression of the c-myb protein as demonstrated by immunocytochemical staining and indirect immunoprecipitation. In contrast to the transformed T-lymphoblastic cell line Molt-4, where c-myb protein is a nuclear antigen, it was found in proliferating normal T-cells almost exclusively distributed in the cytoplasm. Screening of a total of 70 fresh human malignant lymphomas by immunohistochemical staining indicates the presence of the c-myb protein primarily in non-Hodgkin's lymphomas with a large growth fraction, i.e. precursor cell-derived lymphoblastic lymphomas of B-cell type and T-cell type (9/10, 3/4, respectively) and anaplastic large cell Ki-1 lymphomas (5/9), which originate from activated lymphoid cells. The c-myb protein was located predominantly in the nucleus and in some cases additionally in the cytoplasm. The different subcellular locations suggest a dual functional role. While nuclear localisation is exhibited by transformed haematopoietic cells, cytoplasmic localisation appears to be characteristic for proliferating normal T-cells and points to a second property of the c-myb protein other than interaction with DNA.

Determination of the molecular weight of DNA-bound protein(s) responsible for gel electrophoretic mobility shift of linear DNA fragments examplified with purified viral myb protein

A protein-DNA complex has less gel electrophoretic mobility than the free DNA fragment. One parameter for the degree of retardation of a linear DNA fragment in a protein-DNA complex is the molecular weight of the bound protein(s). The quotient of the migration distances of free DNA (m) and protein-DNA complex (m') is a function of the molecular weight (MW) of the bound protein(s). Based on the evaluation of the lac repressor induced mobility shift of a 203 bp DNA fragment containing the lac operator in a 5% non-denaturating polyacrylamide gel a direct proportionality could be shown between (m/m'-1) and MW with the proportionality factor K = 215 kDa. The factor K depends on the acrylamide concentration in the gel, getting lower values with increasing acrylamide concentrations. A calculation is given to determine the molecular weight of DNA-binding factors responsible for the decreased electrophoretic mobility of a linear DNA fragment. As an example this calculation was used in order to analyse DNA-binding of the isolated viral myb protein. It could be demonstrated that the viral myb protein binds to DNA as a monomer and as a dimer.

Interaction of the oncogene protein myc with specific DNA fragments

The p110gag-myc protein coded for by the retrovirus MC29 was purified 3,000-fold from MC29-Q8 transformed cells by immuno-affinity chromatography using IgG specific for the N-terminal region of the gag protein. Interaction of the protein with DNA fragments was studied by filter binding assay. DNA fragments were obtained from a MC29 DNA clone by restriction endonuclease treatment. Besides the complete DNA provirus the clone contained flanking cellular sequences into which the provirus had integrated. The DNA fragments which were retained by the p110gag-myc protein were eluted from the filter and analyzed by agarose gel electrophoresis. Preferential binding of a DNA fragment originating from the flanking cellular sequences was detected. The protein did not preferentially bind to the viral LTR promoter/enhancer region as suggested by an autoregulatory model, which can therefore no longer be substantiated.