Calcium signaling in neurons: molecular mechanisms and cellular consequences

Neuronal activity can lead to marked increases in the concentration of cytosolic calcium, which then functions as a second messenger that mediates a wide range of cellular responses. Calcium binds to calmodulin and stimulates the activity of a variety of enzymes, including calcium-calmodulin kinases and calcium-sensitive adenylate cyclases. These enzymes transduce the calcium signal and effect short-term biological responses, such as the modification of synaptic proteins and long-lasting neuronal responses that require changes in gene expression. Recent studies of calcium signal-transduction mechanisms have revealed that, depending on the route of entry into a neuron, calcium differentially affects processes that are central to the development and plasticity of the nervous system, including activity-dependent cell survival, modulation of synaptic strength, and calcium-mediated cell death.

Requirement for BDNF in activity-dependent survival of cortical neurons

Cultured embryonic cortical neurons from rats were used to explore mechanisms of activity-dependent neuronal survival. Cell survival was increased by the activation of voltage-sensitive calcium channels (VSCCs) but not by activation of N-methyl-D-aspartate receptors. These effects correlated with the expression of brain-derived neurotrophic factor (BDNF) induced by these two classes of calcium channels. Antibodies to BDNF (which block intracellular signaling by BDNF, but not by nerve growth factor, NT3, or NT4/5) reduced the survival of cortical neurons and reversed the VSCC-mediated increase in survival. Thus, endogenous BDNF is a trophic factor for cortical neurons whose expression is VSCC-regulated and that functions in the VSCC-dependent survival of these neurons.

Identification of a signaling pathway involved in calcium regulation of BDNF expression

A signaling pathway by which calcium influx regulates the expression of the major activity-dependent transcript of BDNF in cortical neurons has been elucidated. Deletion and mutational analysis of the promoter upstream of exon III reveals that transactivation of the BDNF gene involves two elements 5' to the mRNA start site. The first element, located between 72 and 47 bp upstream of the mRNA start site, is a novel calcium response element and is required for calcium-dependent BDNF expression in both embryonic and postnatal cortical neurons. The second element, located between 40 and 30 bp upstream of the mRNA start site, matches the consensus sequence of a cAMP response element (CRE) and is required for transactivation of the promoter in postnatal but not embryonic neurons. The CRE-dependent component of the response appears to be mediated by CREB since it is part of the complex that binds to this CRE, and since dominant negative mutants of CREB attenuate transactivation of the promoter. A constitutively active mutant of CaM kinase IV, but not of CaM kinase II, leads to activation of the promoter in the absence of extracellular stimuli, and partially occludes calcium-dependent transactivation. The effects of CaM kinase IV on the promoter require an intact CRE. These mechanisms, which implicate CaM kinase IV and CREB in the control of BDNF expression, are likely to be centrally involved in activity-dependent plasticity during development.

Semaphorin 3A is a chemoattractant for cortical apical dendrites

The apical dendrites of pyramidal neurons integrate inputs from various cortical layers and are central to information processing. Here we show that the growth of apical dendrites towards the pial surface is regulated by a diffusible chemoattractant present at high levels near the marginal zone. A major component of this signal is semaphorin 3A (Sema3A), which was previously characterized as a chemorepellant for cortical axons. Soluble guanylate cyclase is asymmetrically localized to the developing apical dendrite, and is required for the chemoattractive effect of Sema3A. Thus the asymmetric localization of soluble guanylate cyclase confers distinct Sema3A responses to axons and dendrites. These observations reveal a mechanism by which a single chemotropic signal can pattern both axons and dendrites during development.

Subplate neurons pioneer the first axon pathway from the cerebral cortex

During the development of the nervous system, growing axons must traverse considerable distances to find their targets. In insects, this problem is solved in part by pioneer neurons, which lay down the first axonal pathways when distances are at a minimum. Here the existence of a similar kind of neuron in the developing mammalian telencephalon is described. These are the subplate cells, the first postmitotic neurons of the cerebral cortex. Axons from subplate neurons traverse the internal capsule and invade the thalamus early in fetal life, even before the neurons of cortical layers 5 and 6, which will form the adult subcortical projections, are generated. During postnatal life, after the adult pattern of axonal projections is firmly established, most subplate neurons disappear. These observations raise the possibility that the early axonal scaffold formed by subplate cells may prove essential for the establishment of permanent subcortical projections.

Regulation of dendritic growth and remodeling by Rho, Rac, and Cdc42

The acquisition of cell type-specific morphologies is a central feature of neuronal differentiation and has important consequences for nervous system function. To begin to identify the underlying molecular mechanisms, we have explored the role of Rho-related GTPases in the dendritic development of cortical neurons. Expression of dominant negative mutants of Rac or Cdc42, the Rho-inhibitory molecule C3 transferase, or the GTPase-activating protein RhoGAP p190 causes a marked reduction in the number of primary dendrites in nonpyramidal (multipolar) neurons and in the number of basal dendrites in neurons with pyramidal morphologies. Conversely, the expression of constitutively active mutants of Rho, Rac, or Cdc42 leads to an increase in the number of primary and basal dendrites. In cortical cultures, as in vivo, dendritic remodeling leads to an apparent transformation from pyramidal to nonpyramidal morphologies over time. Strikingly, this shift in favor of nonpyramidal morphologies is also inhibited by the expression of dominant negative mutants of Cdc42 and Rac and by RhoGAP p190. These observations indicate that Rho, Rac, and Cdc42 play a central role in dendritic development and suggest that differential activation of Rho-related GTPases may contribute to the generation of morphological diversity in the developing cortex.

Distinct roles for bFGF and NT-3 in the regulation of cortical neurogenesis

To identify molecules that regulate the transition of dividing neuroblasts to terminally differentiated neurons in the CNS, conditions have been developed that allow the neuronal differentiation of cortical precursor cells to be examined in vitro. In these cultures, the proliferation of undifferentiated precursor cells is controlled by basic fibroblast growth factor (bFGF). The proliferative effects of bFGF do not preclude the action of signals that promote differentiation, since addition of neurotrophin-3 (NT-3) antagonizes the proliferative effects of bFGF and enhances neuronal differentiation. In addition, blocking NT-3 function with neutralizing antibodies leads to a marked decrease in the number of differentiated neurons, without affecting the proliferation of cortical precursors or the survival of postmitotic cortical neurons. These observations suggest that bFGF and NT-3, by their distinct effects on cell proliferation and differentiation, are key regulators of neurogenesis in the CNS.

Requirement for subplate neurons in the formation of thalamocortical connections

The neurons of layer 4 in the adult cerebral cortex receive their major ascending inputs from the thalamus. In development, however, thalamic axons arrive at the appropriate cortical area long before their target layer 4 neurons have migrated into the cortical plate. The axons accumulate and wait in the zone below the cortical plate, the subplate, for several weeks before invading the cortical plate. The subplate is a transient zone that contains the first postmitotic neurons of the telencephalon. These neurons mature well before other cortical neurons, and disappear by cell death after the thalamic axons have grown into the overlying cortical plate. The close proximity of growing thalamocortical axons and subplate neurons suggests that they might be involved in interactions important for normal thalamocortical development. Here we show that early in development the deletion of subplate neurons located beneath visual cortex prevents axons from the lateral geniculate nucleus of the thalamus from recognizing and innervating visual cortex, their normal target. In the absence of subplate neurons, lateral geniculate nucleus axons continue to grow in the white matter past visual cortex despite the presence of their target layer 4 neurons. Thus the transient subplate neurons are necessary for appropriate cortical target selection by thalamocortical axons.

Calcium activation of Ras mediated by neuronal exchange factor Ras-GRF

Tyrosine kinase receptors stimulate the Ras signalling pathway by enhancing the activity of the SOS nucleotide-exchange factor. This occurs, at least in part, by the recruitment of an SOS-GRB2 complex to Ras in the plasma membrane. Here we describe a different signalling pathway to Ras that involves activation of the Ras-GRF exchange factor in response to Ca2+ influx. In particular, we show that the ability of Ras-GRF to activate Ras in vivo is markedly enhanced by raised Ca2+ concentrations. Activation is mediated by calmodulin binding to an IQ motif in Ras-GRF, because substitutions in conserved amino acids in this motif prevent both calmodulin binding to Ras-GRF and Ras-GRF activation in vivo. So far, full-length Ras-GRF has been detected only in brain neurons. Our findings implicate Ras-GRF in the regulation of neuronal functions that are influenced by Ca2+ signals.

Nuclear Notch1 signaling and the regulation of dendritic development

To understand the function of Notch in the mammalian brain, we examined Notch1 signaling and its cellular consequences in developing cortical neurons. We found that the cytoplasmic domain of endogenous Notch1 translocated to the nucleus during neuronal differentiation. Notch1 cytoplasmic-domain constructs transfected into cortical neurons were present in multiple phosphorylated forms, localized to the nucleus and could induce CBF1-mediated transactivation. Molecular perturbation experiments suggested that Notch1 signaling in cortical neurons promoted dendritic branching and inhibited dendritic growth. These observations show that Notch1 signaling to the nucleus exerts an important regulatory influence on the specification of dendritic morphology in neurons.

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.

Canagliflozin, a novel inhibitor of sodium glucose co-transporter 2, dose dependently reduces calculated renal threshold for glucose excretion and increases urinary glucose excretion in healthy subjects

Canagliflozin, a potent, selective sodium glucose co-transporter 2 inhibitor in development for treatment of type 2 diabetes, lowers plasma glucose (PG) by lowering the renal threshold for glucose (RT(G) ) and increasing urinary glucose excretion (UGE). An ascending single oral-dose phase 1 study investigated safety, tolerability and pharmacodynamics of canagliflozin in healthy men (N = 63) randomized to receive canagliflozin (n = 48) or placebo (n = 15). Canagliflozin (10, 30, 100, 200, 400, 600 or 800 mg q.d. or 400 mg b.i.d.) was administered to eight cohorts (six subjects/cohort: canagliflozin; two subjects/cohort: placebo). Dose dependently, canagliflozin decreased calculated 24-h mean RT(G) with maximal reduction to approximately 60 mg/dl, and increased mean 24-h UGE. At doses >200 mg administered before breakfast, canagliflozin reduced postprandial PG and serum insulin excursions at that meal. Canagliflozin was generally well tolerated; most adverse events were mild and no hypoglycaemia was reported. These results support further study of canagliflozin.

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.

Regulation of CBP-mediated transcription by neuronal calcium signaling

The transcription factor CREB is involved in mediating many of the long-term effects of activity-dependent plasticity at glutamatergic synapses. Here, we show that activation of NMDA receptors and voltage-sensitive calcium channels leads to CREB-mediated transcription in cortical neurons via a mechanism regulated by CREB-binding protein (CBP). Recruitment of CBP to the promoter is not sufficient for transactivation, but calcium influx can induce CBP-mediated transcription via two distinct transactivation domains. CBP-mediated transcription is stimulus strength-dependent and can be induced by activation of CaM kinase II, CaM kinase IV, and protein kinase A, but not by activation of the Ras-MAP kinase pathway. These observations indicate that CBP can function as a calcium-sensitive transcriptional coactivator that may act as a regulatory switch for glutamate-induced CREB-mediated transcription.

Atmospheric imaging results from the Mars exploration rovers: Spirit and Opportunity

A visible atmospheric optical depth of 0.9 was measured by the Spirit rover at Gusev crater and by the Opportunity rover at Meridiani Planum. Optical depth decreased by about 0.6 to 0.7% per sol through both 90-sol primary missions. The vertical distribution of atmospheric dust at Gusev crater was consistent with uniform mixing, with a measured scale height of 11.56 +/- 0.62 kilometers. The dust's cross section weighted mean radius was 1.47 +/- 0.21 micrometers (mm) at Gusev and 1.52 +/- 0.18 mm at Meridiani. Comparison of visible optical depths with 9-mm optical depths shows a visible-to-infrared optical depth ratio of 2.0 +/- 0.2 for comparison with previous monitoring of infrared optical depths.

Prognostic significance of p53 overexpression in gastric and colorectal carcinoma

p53 expression was examined in 55 gastric and 107 colorectal carcinomas with an immunoperoxidase technique, using the polyclonal antibody CM1 on routinely fixed, paraffin embedded tissue. p53 protein was detected in 47% gastric and in 46% colorectal carcinomas and found to correlate with stage of disease and unfavourable clinical outcome (P less than 0.001). Thus, the proportion of positively reacting neoplasms increased as the stage progressed, tumours which had invaded regional lymph-nodes overexpressed p53 more frequently than localised carcinomas and an elevated level of p53 was associated with early relapse and death. In colorectal carcinoma p53 positivity was also linked with site and macroscopic configuration of the primary tumour and was most frequently expressed in carcinomas from the rectum and in ulcerative tumours. p53 overexpression was irrespective of tumour grade. Uniform negative reactivity with anti-p53 antibody was seen in normal epithelium adjacent to carcinoma, intestinal metaplasia, atrophic gastritis and in colonic adenomas. There was a good correlation between immunohistochemical staining on paraffin and frozen sections. These studies suggest that in gastric and colorectal carcinoma, immunohistochemical detection of p53 protein in routinely fixed tissue can be used along with other established parameters to assess prognostic outcome, especially to identify patients with poor short-term prognosis.

Involvement of subplate neurons in the formation of ocular dominance columns

During development of the mammalian visual system, axon terminals of lateral geniculate nucleus (LGN) neurons, initially intermixed within layer 4 of the visual cortex, gradually segregate according to eye preference to form ocular dominance columns. In addition to LGN axons and layer 4 neurons, subplate neurons may also participate in interactions leading to column formation. Deletion of subplate neurons before the formation of ocular dominance columns prevents the segregation of LGN axons within layer 4. Thus, interactions between LGN axons and layer 4 neurons are not sufficient; subplate neurons are also required for formation of ocular dominance columns in the visual cortex.

A role for subplate neurons in the patterning of connections from thalamus to neocortex

During cerebral cortical development, ingrowing axons from different thalamic nuclei select and invade their cortical targets. The selection of an appropriate target is first evident even before thalamic axons grow into the cortical plate: initially axons accumulate and wait below their cortical target area in a zone called the subplate. This zone also contains the first postmitotic neurons of the cerebral cortex, the subplate neurons. Here we have investigated whether subplate neurons are involved in the process of target selection by thalamic axons by ablating them from specific cortical regions at the onset of the waiting period and examining the subsequent thalamocortical axon projection patterns. Subplate neurons were ablated at the onset of the waiting period by intracortical injections of kainic acid. The effect of the ablation on the thalamocortical projection from visual thalamus was examined by DiI-labeling of the LGN days to weeks following the lesion. At two to four weeks post-lesion, times when LGN axons would have normally invaded the cortical plate, the axons remained below the cortical plate and grew past their appropriate cortical target in an anomalous pathway. Moreover, examination of LGN axons at one week post-lesion, a time when they would normally be waiting and branching within the visual subplate, indicated that the axons had already grown past their correct destination. These observations suggest that visual subplate neurons are involved in the process by which LGN axons select and subsequently grow into visual cortex. In contrast, subplate neurons do not appear to play a major role in the initial morphological development of the LGN itself. Subplate ablations did not alter dendritic growth or shapes of LGN projection neurons during the period under study, nor did it prevent the segregation of retinal ganglion cell axons into eye-specific layers. However, the overall size of the LGN was reduced, suggesting that there may be increased cell death of LGN neurons in the absence of subplate neurons. To examine whether subplate neurons beneath other neocortical areas play a similar role in the formation of thalamocortical connections, subplate neurons were deleted beneath auditory cortex at the onset of the waiting period for auditory thalamic axons. Subsequent DiI labeling revealed that in these animals the majority of MGN axons had grown past auditory cortex instead of innervating it. Taken together these observations underscore a general requirement for subplate neurons throughout neocortex in the process of cortical target selection and ingrowth by thalamic axons.(ABSTRACT TRUNCATED AT 400 WORDS)