Ca(2+)-dependent routes to Ras: mechanisms for neuronal survival, differentiation, and plasticity?

Purinergic Ca2+ signaling as a novel mechanism of drug tolerance in BRAF mutant melanoma

Drug tolerance is a major cause of relapse after cancer treatment. In spite of intensive efforts1-9, its molecular basis remains poorly understood, hampering actionable intervention. We report a previously unrecognized signaling mechanism supporting drug tolerance in BRAF-mutant melanoma treated with BRAF inhibitors that could be of general relevance to other cancers. Its key features are cell-intrinsic intracellular Ca2+ signaling initiated by P2X7 receptors (purinergic ligand-gated cation channels), and an enhanced ability for these Ca2+ signals to reactivate ERK1/2 in the drug-tolerant state. Extracellular ATP, virtually ubiquitous in living systems, is the ligand that can initiate Ca2+ spikes via P2X7 channels. ATP is abundant in the tumor microenvironment and is released by dying cells, ironically implicating treatment-initiated cancer cell death as a source of trophic stimuli that leads to ERK reactivation and drug tolerance. Such a mechanism immediately offers an explanation of the inevitable relapse after BRAFi treatment in BRAF-mutant melanoma, and points to actionable strategies to overcome it.

Multi-parametric analysis of 57 SYNGAP1 variants reveal impacts on GTPase signaling, localization, and protein stability

SYNGAP1 is a neuronal Ras and Rap GTPase-activating protein with important roles in regulating excitatory synaptic plasticity. While many SYNGAP1 missense and nonsense mutations have been associated with intellectual disability, epilepsy, schizophrenia, and autism spectrum disorder (ASD), whether and how they contribute to individual disease phenotypes is often unknown. Here, we characterize 57 variants in seven assays that examine multiple aspects of SYNGAP1 function. Specifically, we used multiplex phospho-flow cytometry to measure variant impact on protein stability, pERK, pGSK3β, pp38, pCREB, and high-content imaging to examine subcellular localization. We find variants ranging from complete loss-of-function (LoF) to wild-type (WT)-like in their regulation of pERK and pGSK3β, while all variants retain at least partial ability to dephosphorylate pCREB. Interestingly, our assays reveal that a larger proportion of variants located within the disordered domain of unknown function (DUF) comprising the C-terminal half of SYNGAP1 exhibited higher LoF, compared to variants within the better studied catalytic domain. Moreover, we find protein instability to be a major contributor to dysfunction for only two missense variants, both located within the catalytic domain. Using high-content imaging, we find variants located within the C2 domain known to mediate membrane lipid interactions exhibit significantly larger cytoplasmic speckles than WT SYNGAP1. Moreover, this subcellular phenotype shows both correlation with altered catalytic activity and unique deviation from signaling assay results, highlighting multiple independent molecular mechanisms underlying variant dysfunction. Our multidimensional dataset allows clustering of variants based on functional phenotypes and provides high-confidence, multi-functional measures for making pathogenicity predictions.

Effects of pharmacological inhibition of the centrally-projecting Edinger-Westphal nucleus on ethanol-induced conditioned place preference and body temperature

Alcohol use disorder is a chronic disease characterized in part by repeated relapsing events. Exposure to environmental stimuli or cues that have previously been associated with the effects of alcohol can promote relapse through the triggering of craving for alcohol. Therefore, identifying and characterizing neuronal populations that may regulate these associations is of the upmost importance. Previous studies have implicated the centrally-projecting Edinger Westphal nucleus (EWcp) in this process, as the EWcp is both sensitive to, and can regulate alcohol intake. To date however, it is unclear if the EWcp is involved in the formation or expression of these alcohol-cue associations. As such, the present studies examined the involvement of the EWcp in male DBA/2J mice in the acquisition and expression of place preference for an alcohol-paired cue using the conditioned place preference (CPP) procedure. Pharmacological inhibition of the EWcp via the GABAA and GABAB receptor agonists muscimol and baclofen did not affect either the acquisition or the expression of CPP. Follow up studies did find however, that pharmacological inhibition of the EWcp increased body temperature and prevented alcohol-induced increases in c-Fos expression in the EWcp. When considered in light of previous studies, the present results indicate that the EWcp may be involved in the regulation of alcohol self-administration, and not conditioned alcohol-seeking. Additionally, the present studies provide further evidence for the involvement of the EWcp in thermoregulation and help elucidate the molecular mechanisms by which alcohol increases c-Fos in the EWcp.

A role for calcium in resistin transcriptional activation in diabetic hearts

The adipokine resistin has been proposed to link obesity, insulin resistance and diabetes. We have previously reported that diabetic hearts express high levels of resistin while overexpression of resistin in adult rat hearts gives rise to a phenotype resembling diabetic cardiomyopathy. The transcriptional regulation of resistin in diabetic cardiac tissue is currently unknown. This study investigated the mechanism of resistin upregulation and the role of Serca2a in its transcriptional suppression. We demonstrate that restoration of Ca²⁺ homeostasis in diabetic hearts, through normalization of Serca2a function genetically and pharmacologically, suppressed resistin expression via inhibition of NFATc. H9c2 myocytes stimulated with high-glucose concentration or Ca²⁺ time-dependently increased NFATc and resistin expression while addition of the Ca²⁺ chelator BAPTA-AM attenuated this effect. NFATc expression was enhanced in hearts from ob/ob diabetic and from cardiac-specific Serca2a^−/− mice. Similarly, NFATc increased resistin expression in myocytes cultured in low glucose while the NFATc inhibitor VIVIT blocked glucose-induced resistin expression, suggesting that hyperglycemia/diabetes induces resistin expression possibly through NFATc activation. Interestingly, overexpression of Serca2a or VIVIT mitigated glucose-stimulated resistin and NFATc expression and enhanced AMPK activity, a downstream target of resistin signaling. NFATc direct activation of resistin was verified by resistin promoter luciferase activity and chromatin-immunoprecipitation analysis. Interestingly, activation of Serca2a by a novel agonist, CDN1163, mirrored the effects of AAV9-Serca2a gene transfer on resistin expression and its promoter activity and AMPK signaling in diabetic mice. These findings parse a role for Ca²⁺ in resistin transactivation and provide support that manipulation of Serca2a-NFATc-Resistin axis might be useful in hyper-resistinemic conditions.

A Link Between Alzheimer's and Type II Diabetes Mellitus? Ca+2 -Mediated Signal Control and Protein Localization

We propose protein localization dependent signal activation (PLDSA) as a model to describe pre-existing protein partitioning between the cytosol, and membrane surface, as a means to modulate signal activation, specificity, and robustness. We apply PLDSA to explain possible molecular links between type II diabetes mellitus (T2DM) and Alzheimer's disease (AD) by describing Ca+2 -mediated interactions between the Src non-receptor tyrosine kinase and p52Shc adaptor protein. We suggest that these interactions may serve as a contributing factor to disease development and progression. In particular, we propose that signaling response is regulated, in part, by Ca+2 -mediated partitioning of lipid-bound and soluble forms of Src and p52shc. Thus, protein-protein interactions that drive signaling in response to extracellular ligand binding are also mediated by partitioning of signaling proteins between membrane-bound and soluble populations. We propose that PLDSA effects may explain, in part, the evolutionary basis of promiscuous protein interaction domains and their importance in cellular function.

Age-dependent alterations in expression and co-localization of Pax6 and Ras-GAP in brain of aging mice

As the brain ages, the survival and plasticity of neurons and glia are compromised. The data-mining and in silico studies suggest interactions of Pax6 with Ras and binding sites in Ras-GAP promoter. The Pax6 also shows age-dependent alterations. Therefore, it is presumed that Pax6 may be associated with the Ras-GAP, a synaptic protein, either directly or indirectly in brain. The expression, co-localization and interaction of Pax6 and Ras-GAP in different regions of brain of mice during aging were investigated through immunofluorescence assay, co-immunoprecipitation and western blotting, respectively. The co-localization of Pax6 and Ras-GAP were observed in dentate gyrus (DG) and sub-granular zone (SGZ) of hippocampus, in glomerular (GlLa) and mitral cells (MiCe) of olfactory lobe, granular cells (GrCe), Purkinje cell (PuCe) and molecular cell layer (MoLa) of cerebellum, internal plexiform layer (InPl), molecular layer (MoLa) of cerebral cortex and in intercalated cells of amygdala (ITC), caudate nucleus regions in brain of aging mice. The expression of Pax6 and Ras-GAP was altered in hippocampus, amygdala, caudate nucleus, olfactory lobe, cerebral cortex and cerebellum from young to old mice. The Pax6 interacts with Ras-GAP in brain of mice. Results indicate impact of Pax6 on Ras-GAP-mediated activities of synapses, learning and memory, emotions and fear as well as motor functions. Alterations in expression and co-localization of Pax6 and Ras-GAP during aging may be responsible for age-associated compromised survival and plasticity of neurons and glia.

Calreticulin promotes EGF-induced EMT in pancreatic cancer cells via Integrin/EGFR-ERK/MAPK signaling pathway

Our previous study showed that Calreticulin (CRT) promoted the development of pancreatic cancer (PC) through ERK/MAPK pathway. We next investigate whether CRT promotes EGF-induced epithelial-mesenchymal transition (EMT) in PC via Integrin/EGFR-ERK/MAPK signaling, which has not been reported yet to our knowledge. EGF simultaneously induced EMT and activated Integrin/EGFR-ERK/MAPK signaling pathway in 3 PC cells. However, CRT silencing significantly inhibited EGF function, including inhibiting EGF-induced EMT-like cell morphology, EGF-enhanced cell invasion and migration, and EGF induced the decrease of E-cadherin, ZO-1, and β-catenin and the increase of the key proteins in Integrin/EGFR-ERK/MAPK signaling (pEGFR-tyr1173, Fibronectin, Integrinβ1, c-Myc and pERK). Conversely, CRT overexpression rescued the change of EMT-related proteins induced by EGF in CRT silencing PC cells. Additionally, CRT was co-stained with pEGFR1173 (with EGF), Fibronectin and Integrinβ1 by IF under confocal microscopy and was co-immunoprecipitated with Fibronectin, Integrinβ1 and c-Myc in both PC cells, all of which indicating a close interaction of CRT with Integrin/EGFR-ERK/MAPK signaling pathway in PC. In vivo, CRT silencing inhibited subcutaneous tumor growth and liver metastasis of pancreatic tumor. A positive relationship of CRT with Fibronectin, Integrinβ1, c-Myc and pERK and a negative association of CRT with E-cad was also observed in vivo and clinical samples. Meanwhile, overexpression of the above proteins was closely associated with multiple aggressive clinicopathological characteristics and the poor prognosis of PC patients. CRT promotes EGF-induced EMT in PC cells via Integrin/EGFR-ERK/MAPK signaling pathway, which would be a promising therapy target for PC.

Ca2+ involvement in activation of extracellular-signal-regulated-kinase 1/2 and m-calpain after axotomy of the sciatic nerve

Detailed mechanisms behind regeneration after nerve injury, in particular signal transduction and the fate of Schwann cells (SCs), are poorly understood. Here, we investigated axotomy-induced activation of extracellular-signal-regulated kinase-1/2 (ERK1/2; important for proliferation) and m-calpain in vitro, and the relation to Ca²⁺ deletion and Schwann cell proliferation and death after rat sciatic nerve axotomy. Nerve segments were cultured for up to 72 hours with and without ethylene glycol-bis(β-aminoethyl ether)-N, N, N’, N’-tetraacetic acid (EGTA). In some experiments, 5-bromo-2’-deoxyuridine (BrdU) was added during the last 24 hours to detect proliferating cells and propidium iodide (PI) was added at the last hour to detect dead and/or dying cells. Immunohistochemistry of sections of the cultured nerve segments was performed to label m-calpain and the phosphorylated and activated form of ERK1/2. The experiments revealed that immunoreactivity for p-ERK1/2 increased with time in organotypically cultured SCs. p-ERK1/2 and m-calpain were also observed in axons. A significant increase in the number of dead or dying SCs was observed in nerve segments cultured for 24 hours. When deprived of Ca²⁺, activation of axonal m-calpain was reduced, whereas p-ERK1/2 was increased in SCs. Ca²⁺ deprivation also significantly reduced the number of proliferating SCs, and instead increased the number of dead or dying SCs. Ca²⁺ seems to play an important role in activation of ERK1/2 in SCs and in SC survival and proliferation. In addition, extracellular Ca²⁺ levels are also required for m-calpain activation and up-regulation in axons. Thus, regulation of Ca²⁺ levels is likely to be a useful method to promote SC proliferation.

D-Aspartate Induces Proliferative Pathways in Spermatogonial GC-1 Cells

D-aspartate (D-Asp) is an endogenous amino acid present in vertebrate tissues, with particularly high levels in the testis. In vivo studies indicate that D-Asp indirectly stimulates spermatogenesis through the hypothalamic-pituitary-gonadal axis. Moreover, in vitro studies have demonstrated that D-Asp up-regulates testosterone production in Leydig cells by enhancing expression of the steroidogenic acute regulatory protein. In this study, a cell line derived from immortalized type-B mouse spermatogonia retaining markers of mitotic germ cells (GC-1) was employed to explore more direct involvement of D-Asp in spermatogenesis. Activity and protein expression of markers of cell proliferation were determined at intervals during incubation in D-Asp-containing medium. D-Asp induced phosphorylation of ERK and Akt proteins, stimulated expression of PCNA and Aurora B, and enhanced mRNA synthesis and protein expression of P450 aromatase and protein expression of Estrogen Receptor β (ERβ). These results are the first demonstration of a direct effect of D-Asp on spermatogonial mitotic activity. Considering that spermatogonia express the NR1 subunit of the N-Methyl-D-Aspartic Acid receptor (NMDAR), we suggest that their response to D-Asp depends on NMDAR-mediated activation of the ERK and Akt pathways and is further enhanced by activation of the P450 aromatase/ERβ pathway.

Ras does not contribute to the facilitation of hippocampal synaptic plasticity enabled by environmental enrichment

Environmental enrichment (EE), which mimics the wealth of sensory, motor and cognitive stimuli that arise through intense interactions with the ambient environment, results in enhanced hippocampal long-term potentiation (LTP) and spatial learning. A key molecular factor in the mediation of these changes is the brain-derived neurotrophic factor (BDNF). One of the downstream cascades that is activated by BDNF is the cascade linked to the small GTPase, Ras, that triggers mitogen-activated protein kinase (MAPK) activity and is part of the cAMP response element-binding protein (CREB) pathway that can lead to synaptic restructuring to support LTP. Here, we explored whether persistent activation of Ras in neurons further enhances LTP following EE of rodents. Immediately following weaning, transgenic mice that expressed constitutively activated neuronal Ras, or their wildtype (Wt) littermates, underwent 3weeks of constant EE. In the absence of EE, theta burst stimulation (TBS) evoked LTP in the CA1 region of transgenic mice that was not significantly different from LTP in Wts. After 3weeks of EE, hippocampal LTP was improved in Wt mice. Enriched transgenic mice showed an equivalent level of LTP to enriched Wts, but it was not significantly different from non-enriched synRas controls. Western blot analysis performed after a pull-down assay showed that non-enriched transgenic mice expressed higher Ras activity compared to non-enriched Wts. Following EE, Ras activity was reduced in transgenics to levels detected in Wts. These results show that constitutive activation of Ras does not mimic the effects of EE on LTP. In addition, EE results in an equivalent enhancement of LTP transgenics and Wts, coupled with a decrease in Ras activity to Wt levels. This suggests that permanent activation of Ras in neurons of synRas animals following EE results in an altered feedback regulation of endogenous Ras activity that is not a key factor in LTP enhancements. The maintenance of Ras within a physiological range may thus be required for the optimization of LTP in the hippocampus.

Researching glutamate - induced cytotoxicity in different cell lines: a comparative/collective analysis/study

Although glutamate is one of the most important excitatory neurotransmitters of the central nervous system, its excessive extracellular concentration leads to uncontrolled continuous depolarization of neurons, a toxic process called, excitotoxicity. In excitotoxicity glutamate triggers the rise of intracellular Ca²⁺ levels, followed by up regulation of nNOS, dysfunction of mitochondria, ROS production, ER stress, and release of lysosomal enzymes. Excessive calcium concentration is the key mediator of glutamate toxicity through over activation of ionotropic and metabotropic receptors. In addition, glutamate accumulation can also inhibit cystine (CySS) uptake by reversing the action of the CySS/glutamate antiporter. Reversal of the antiporter action reinforces the aforementioned events by depleting neurons of cysteine and eventually glutathione’s reducing potential. Various cell lines have been employed in the pursuit to understand the mechanism(s) by which excitotoxicity affects the cells leading them ultimately to their demise. In some cell lines glutamate toxicity is exerted mainly through over activation of NMDA, AMPA, or kainate receptors whereas in other cell lines lacking such receptors, the toxicity is due to glutamate induced oxidative stress. However, in the greatest majority of the cell lines ionotropic glutamate receptors are present, co-existing to CySS/glutamate antiporters and metabotropic glutamate receptors, supporting the assumption that excitotoxicity effect in these cells is accumulative. Different cell lines differ in their responses when exposed to glutamate. In this review article the responses of PC12, SH-SY5Y, HT-22, NT-2, OLCs, C6, primary rat cortical neurons, RGC-5, and SCN2.2 cell systems are systematically collected and analyzed.

Rit subfamily small GTPases: regulators in neuronal differentiation and survival

Ras family small GTPases serve as binary molecular switches to regulate a broad array of cellular signaling cascades, playing essential roles in a vast range of normal physiological processes, with dysregulation of numerous Ras-superfamily G-protein-dependent regulatory cascades underlying the development of human disease. However, the physiological function for many "orphan" Ras-related GTPases remain poorly characterized, including members of the Rit subfamily GTPases. Rit is the founding member of a novel branch of the Ras subfamily, sharing close homology with the neuronally expressed Rin and Drosophila Ric GTPases. Here, we highlight recent studies using transgenic and knockout animal models which have begun to elucidate the physiological roles for the Rit subfamily, including emerging roles in the regulation of neuronal morphology and cellular survival signaling, and discuss new genetic data implicating Rit and Rin signaling in disorders such as cancer, Parkinson's disease, autism, and schizophrenia.

Neuropeptide Y modulates calcium channels in hamster submandibular ganglion neurons

It is established that neuropeptide Y (NPY) is a transmitter of parasympathetic secretory impulses in submandibular gland. The neuropeptides substance P, vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP) are likely mediators of secretory parasympathetic responses of the gland. Previously, we have shown that substance P, VIP and CGRP modulate voltage-dependent Ca(2+) channels (VDCCs) in hamster submandibular ganglion (SMG) neurons. In this study, we attempt to characterize the effect of NPY on VDCCs current using Ba(2+) (I(Ba)) in SMG neurons. Application of NPY caused both facilitation and inhibition of L-type and N/P/Q-type I(Ba), respectively. Intracellular dialysis of the Gα(s)-protein antibody attenuated the NPY-induced facilitation of I(Ba). The adenylate cyclase (AC) inhibitor, as well as protein kinase A (PKA) inhibitor attenuated the NPY-induced facilitation of I(Ba). Intracellular dialysis of the Gα(i)-protein antibody attenuated the NPY-induced inhibition of I(Ba). Application of a strong depolarizing voltage prepulse attenuated the NPY-induced inhibition of I(Ba). These results indicate that NPY facilitates L-type VDCCs via Gα(s)-protein involving AC and PKA. On the other hand, NPY also inhibits N/P/Q-type VDCCs via Gα(i)-protein βγ subunits in the SMG neurons.

Physiology and pathology of calcium signaling in the brain

Calcium (Ca(2+)) plays fundamental and diversified roles in neuronal plasticity. As second messenger of many signaling pathways, Ca(2+) as been shown to regulate neuronal gene expression, energy production, membrane excitability, synaptogenesis, synaptic transmission, and other processes underlying learning and memory and cell survival. The flexibility of Ca(2+) signaling is achieved by modifying cytosolic Ca(2+) concentrations via regulated opening of plasma membrane and subcellular Ca(2+) sensitive channels. The spatiotemporal patterns of intracellular Ca(2+) signals, and the ultimate cellular biological outcome, are also dependent upon termination mechanism, such as Ca(2+) buffering, extracellular extrusion, and intra-organelle sequestration. Because of the central role played by Ca(2+) in neuronal physiology, it is not surprising that even modest impairments of Ca(2+) homeostasis result in profound functional alterations. Despite their heterogeneous etiology neurodegenerative disorders, as well as the healthy aging process, are all characterized by disruption of Ca(2+) homeostasis and signaling. In this review we provide an overview of the main types of neuronal Ca(2+) channels and their role in neuronal plasticity. We will also discuss the participation of Ca(2+) signaling in neuronal aging and degeneration.

Ras family small GTPase-mediated neuroprotective signaling in stroke

Selective neuronal cell death is one of the major causes of neuronal damage following stroke, and cerebral cells naturally mobilize diverse survival signaling pathways to protect against ischemia. Importantly, therapeutic strategies designed to improve endogenous anti-apoptotic signaling appear to hold great promise in stroke treatment. While a variety of complex mechanisms have been implicated in the pathogenesis of stroke, the overall mechanisms governing the balance between cell survival and death are not well-defined. Ras family small GTPases are activated following ischemic insults, and in turn, serve as intrinsic switches to regulate neuronal survival and regeneration. Their ability to integrate diverse intracellular signal transduction pathways makes them critical regulators and potential therapeutic targets for neuronal recovery after stroke. This article highlights the contribution of Ras family GTPases to neuroprotective signaling cascades, including mitogen-activated protein kinase (MAPK) family protein kinase- and AKT/PKB-dependent signaling pathways as well as the regulation of cAMP response element binding (CREB), Forkhead box O (FoxO) and hypoxiainducible factor 1(HIF1) transcription factors, in stroke.

Activation of glycine site and GluN2B subunit of NMDA receptors is necessary for ERK/CREB signaling cascade in rostral anterior cingulate cortex in rats: implications for affective pain

OBJECTIVE

The rostral anterior cingulate cortex (rACC) is implicated in processing the emotional component of pain. N-methyl-D-aspartate receptors (NMDARs) are highly expressed in the rACC and mediate pain-related affect by activating a signaling pathway that involves cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and/or extracellular regulated kinase (ERK)/cAMP-response element-binding protein (CREB). The present study investigated the contributions of the NMDAR glycine site and GluN2B subunit to the activation of ERK and CREB both in vitro and in vivo in rat rACC.

METHODS

Immunohistochemistry and Western blot analysis were used to separately assess the expression of phospho-ERK (pERK) and phospho-CREB (pCREB) in vitro and in vivo. Double immunostaining was also used to determine the colocalization of pERK and pCREB.

RESULTS

Both bath application of NMDA in brain slices in vitro and intraplantar injection of formalin into the rat hindpaw in vivo induced significant up-regulation of pERK and pCREB in the rACC, which was inhibited by the NMDAR antagonist DL-2-amino-5-phospho-novaleric acid. Selective blockade of the NMDAR GluN2B subunit and the glycine-binding site, or degradation of endogenous D-serine, a co-agonist for the glycine site, significantly decreased the up-regulation of pERK and pCREB expression in the rACC. Further, the activated ERK predominantly colocalized with CREB.

CONCLUSION

Either the glycine site or the GluN2B subunit of NMDARs participates in the phosphorylation of ERK and CREB induced by bath application of NMDA in brain slices or hindpaw injection of 5% formalin in rats, and these might be fundamental molecular mechanisms underlying pain affect.

Role of electrical activity of neurons for neuroprotection

Neurons of the central nervous system (CNS) of adult mammals can be damaged in a variety of ways. Most neurons rapidly die after injury. Even if the injured CNS neurons do not die in a short time, the neurons eventually die because they are not able to regenerate their axons to reconnect with their normal targets. In addition, neurons are normally not replaced. Therefore, much work has been directed toward understanding of the molecular regulation of the CNS degeneration following injury, and different experimental strategies are being used to try to protect the damaged neurons. Following axonal lesion, the neurons not only need to survive but also to reconnect to be functionally relevant, and efforts are directed toward not only survival but also axonal regeneration and proper rewiring of injured neurons. Recent experimental data suggest that electrical activity, endogenous or exogenous, can enhance neuronal survival and regeneration in vitro and in vivo. This chapter reviews the evidence that have been obtained on the role of neuronal electrical activity on neuroprotection. We will develop perspectives toward neuroprotection and regeneration of adult lesioned CNS neurons based on electrical activity-dependent cell survival that may be applicable to various diseases of the CNS.

Excitation-transcription coupling in skeletal muscle: the molecular pathways of exercise

Muscle fibres have different properties with respect to force, contraction speed, endurance, oxidative/glycolytic capacity etc. Although adult muscle fibres are normally post-mitotic with little turnover of cells, the physiological properties of the pre-existing fibres can be changed in the adult animal upon changes in usage such as after exercise. The signal to change is mainly conveyed by alterations in the patterns of nerve-evoked electrical activity, and is to a large extent due to switches in the expression of genes. Thus, an excitation-transcription coupling must exist. It is suggested that changes in nerve-evoked muscle activity lead to a variety of activity correlates such as increases in free intracellular Ca²⁺ levels caused by influx across the cell membrane and/or release from the sarcoplasmatic reticulum, concentrations of metabolites such as lipids and ADP, hypoxia and mechanical stress. Such correlates are detected by sensors such as protein kinase C (PKC), calmodulin, AMP-activated kinase (AMPK), peroxisome proliferator-activated receptor δ (PPARδ), and oxygen dependent prolyl hydroxylases that trigger intracellular signaling cascades. These complex cascades involve several transcription factors such as nuclear factor of activated T-cells (NFAT), myocyte enhancer factor 2 (MEF2), myogenic differentiation factor (myoD), myogenin, PPARδ, and sine oculis homeobox 1/eyes absent 1 (Six1/Eya1). These factors might act indirectly by inducing gene products that act back on the cascade, or as ultimate transcription factors binding to and transactivating/repressing genes for the fast and slow isoforms of various contractile proteins and of metabolic enzymes. The determination of size and force is even more complex as this involves not only intracellular signaling within the muscle fibres, but also muscle stem cells called satellite cells. Intercellular signaling substances such as myostatin and insulin-like growth factor 1 (IGF-1) seem to act in a paracrine fashion. Induction of hypertrophy is accompanied by the satellite cells fusing to myofibres and thereby increasing the capacity for protein synthesis. These extra nuclei seem to remain part of the fibre even during subsequent atrophy as a form of muscle memory facilitating retraining. In addition to changes in myonuclear number during hypertrophy, changes in muscle fibre size seem to be caused by alterations in transcription, translation (per nucleus) and protein degradation.

Strong synaptic transmission impact by copy number variations in schizophrenia

Schizophrenia is a psychiatric disorder with onset in late adolescence and unclear etiology characterized by both positive and negative symptoms, as well as cognitive deficits. To identify copy number variations (CNVs) that increase the risk of schizophrenia, we performed a whole-genome CNV analysis on a cohort of 977 schizophrenia cases and 2,000 healthy adults of European ancestry who were genotyped with 1.7 million probes. Positive findings were evaluated in an independent cohort of 758 schizophrenia cases and 1,485 controls. The Gene Ontology synaptic transmission family of genes was notably enriched for CNVs in the cases (P = 1.5 x 10(-7)). Among these, CACNA1B and DOC2A, both calcium-signaling genes responsible for neuronal excitation, were deleted in 16 cases and duplicated in 10 cases, respectively. In addition, RET and RIT2, both ras-related genes important for neural crest development, were significantly affected by CNVs. RET deletion was exclusive to seven cases, and RIT2 deletions were overrepresented common variant CNVs in the schizophrenia cases. Our results suggest that novel variations involving the processes of synaptic transmission contribute to the genetic susceptibility of schizophrenia.

A new genetic model of activity-induced Ras signaling dependent pre-synaptic plasticity in Drosophila

Techniques to induce activity-dependent neuronal plasticity in vivo allow the underlying signaling pathways to be studied in their biological context. Here, we demonstrate activity-induced plasticity at neuromuscular synapses of Drosophila double mutant for comatose (an NSF mutant) and Kum (a SERCA mutant), and present an analysis of the underlying signaling pathways. comt; Kum (CK) double mutants exhibit increased locomotor activity under normal culture conditions, concomitant with a larger neuromuscular junction synapse and stably elevated evoked transmitter release. The observed enhancements of synaptic size and transmitter release in CK mutants are completely abrogated by: a) reduced activity of motor neurons; b) attenuation of the Ras/ERK signaling cascade; or c) inhibition of the transcription factors Fos and CREB. All of which restrict synaptic properties to near wild type levels. Together, these results document neural activity-dependent plasticity of motor synapses in CK animals that requires Ras/ERK signaling and normal transcriptional activity of Fos and CREB. Further, novel in vivo reporters of neuronal Ras activation and Fos transcription also confirm increased signaling through a Ras/AP-1 pathway in motor neurons of CK animals, consistent with results from our genetic experiments. Thus, this study: a) provides a robust system in which to study activity-induced synaptic plasticity in vivo; b) establishes a causal link between neural activity, Ras signaling, transcriptional regulation and pre-synaptic plasticity in glutamatergic motor neurons of Drosophila larvae; and c) presents novel, genetically encoded reporters for Ras and AP-1 dependent signaling pathways in Drosophila.

Role of NGF in spared DRG following partial dorsal rhizotomy in cats

Neuroplasticity occurs in the spinal cord in response to lesions, but less is known about the underlying mechanism. This investigation explored the role of intrinsic NGF in axonal sprouting of dorsal root ganglia (DRG) in cats subjected to unilateral removal of L1-L5, L7-S2 DRG, but leaving the L6 DRG (spared DRG) undamaged. The expression of mRNA and protein for NGF and TrkA increased significantly by using in situ hybridization histochemistry and immunohistochemistry. ELISA assay showed that the level of NGF was up-regulated in the spared DRG, compared to the control side. In vitro studies showed that cultured neurons prepared from DRG explants of cats that received partial ganglionectomy had greater neurite growth compared to those prepared from untreated controls, and that such increase in neurite was not observed in explants from cats that received partial ganglionectomy and NGF antibody treatment. Taken together, the present findings provided crucial evidence that NGF in DRG might be involved in axonal sprouting in deafferentated cats.

Activation of extracellular signal-regulated kinase in the anterior cingulate cortex contributes to the induction and expression of affective pain

The anterior cingulate cortex (ACC) is implicated in the affective response to noxious stimuli. However, little is known about the molecular mechanisms involved. The present study demonstrated that extracellular signal-regulated kinase (ERK) activation in the ACC plays a crucial role in pain-related negative emotion. Intraplantar formalin injection produced a transient ERK activation in laminae V-VI and a persistent ERK activation in laminae II-III of the rostral ACC (rACC) bilaterally. Using formalin-induced conditioned place avoidance (F-CPA) in rats, which is believed to reflect the pain-related negative emotion, we found that blockade of ERK activation in the rACC with MEK inhibitors prevented the induction of F-CPA. Interestingly, this blockade did not affect formalin-induced two-phase spontaneous nociceptive responses and CPA acquisition induced by electric foot-shock or U69,593, an innocuous aversive agent. Upstream, NMDA receptor, adenylyl cyclase (AC) and phosphokinase A (PKA) activators activated ERK in rACC slices. Consistently, intra-rACC microinjection of AC or PKA inhibitors prevented F-CPA induction. Downstream, phosphorylation of cAMP response element binding protein (CREB) was induced in the rACC by formalin injection and by NMDA, AC and PKA activators in brain slices, which was suppressed by MEK inhibitors. Furthermore, ERK also contributed to the expression of pain-related negative emotion. Thus, when rats were re-exposed to the conditioning context for retrieval of pain experience, ERK and CREB were reactivated in the rACC, and inhibiting ERK activation blocked the expression of F-CPA. All together, our results demonstrate that ERK activation in the rACC is required for the induction and expression of pain-related negative affect.

D-Aspartate affects secretory activity in rat Harderian gland: molecular mechanism and functional significance

In this paper, the role of D-aspartate in the rat Harderian gland (HG) was investigated by histochemical, ultrastructural, and biochemical analyses. In this gland, substantial amounts of endogenous D-Asp were detected, along with aspartate racemases that convert D-Asp to L-Asp and vice versa. We found that the gland was capable of uptaking and accumulating exogenously administered D-Asp. D-Asp acute treatment markedly increased lipid and porphyrin secretion and induced a powerful hyperaemia in inter-acinar interstitial tissue. Since D-Asp is known to be recognized by NMDA receptors, the expression of such receptors in rat HG led us to the hypothesis that D-Asp acute treatment induced the activation of the extracellular signal-regulated protein kinase (ERK) and nitric oxide synthase (NOS) pathways mediated by NMDA. Interestingly, as a result of enhanced oxidative stress due to increased porphyrin secretion, the revealed activation of the stress-activated protein kinase/c-jun N-terminal kinase (SAPK/JNK) pro-apoptotic pathway was probably triggered by the gland itself to preserve its cellular integrity.

Green fluorescent protein in the study of neuronal signaling pathways

In recent years, techniques have been established for transiently co-transfecting cells with cDNA of the jellyfish green fluorescent protein (GFP), a reporter gene that encodes a non-toxic marker. This approach can be applied to primary neurons where it has become especially useful for the study of neuronal second messenger pathways. This unit describes procedures for transfecting neurons in primary culture: transfection with GFP DNA, including co-transfecting with separate GFP and gene-of-interest constructs, transfecting with a single construct containing the gene of interest fused to a GFP gene, and transfecting with a single construct containing separate gene-of-interest and GFP cassettes. Also included is a method for the rapid, large-scale preparation of a nearly homogeneous population of neurons from rat cerebellum. The Commentary provides several examples of how this approach can be applied to specific biological questions on neuronal signaling pathways.

ROS-independent preconditioning in neurons via activation of mitoK(ATP) channels by BMS-191095

Previously, we have shown that the selective mitochondrial ATP-sensitive potassium (mitoK(ATP)) channel opener BMS-191095 (BMS) induces neuronal preconditioning (PC); however, the exact mechanism of BMS-induced neuroprotection remains unclear. In this study, we have identified key components of the cascade resulting in delayed neuronal PC with BMS using isolated rat brain mitochondria and primary cultures of rat cortical neurons. BMS depolarized isolated mitochondria without an increase in reactive oxygen species (ROS) generation and induced rapid phosphorylation of Akt and glycogen synthase kinase-3beta. Long-term (3 days) treatment of neurons with BMS resulted in sustained mitochondrial depolarization, decreased basal ROS generation, and elevated ATP levels. This treatment also elicited almost complete protection against glutamate excitotoxicity, which could be abolished using the phosphoinositide 3-kinase (PI3K) inhibitor wortmannin, but not with the superoxide dismutase (SOD) mimetic M40401. Long-term BMS treatment induced a PI3K-dependent increase in the expression and activity of catalase without affecting manganese SOD and copper/zinc-dependent SOD. Finally, the catalase inhibitor 3-aminotriazole dose-dependently antagonized the neuroprotective effect of BMS-induced PC. In summary, BMS depolarizes mitochondria without ROS generation, activates the PI3K-Akt pathway, improves ATP content, and increases catalase expression. These mechanisms appear to play important roles in the neuroprotective effect of BMS.

Brain-derived neurotrophic factor activation of extracellular signal-regulated kinase is autonomous from the dominant extrasynaptic NMDA receptor extracellular signal-regulated kinase shutoff pathway

NMDA receptors bidirectionally modulate extracellular signal-regulated kinase (ERK) through the coupling of synaptic NMDA receptors to an ERK activation pathway that is opposed by a dominant ERK shutoff pathway thought to be coupled to extrasynaptic NMDA receptors. In the present study, synaptic NMDA receptor activation of ERK in rat cortical cultures was partially inhibited by the highly selective NR2B antagonist Ro25-6981 (Ro) and the less selective NR2A antagonist NVP-AAM077 (NVP). When Ro and NVP were added together, inhibition appeared additive and equal to that observed with the NMDA open-channel blocker MK-801. Consistent with a selective coupling of extrasynaptic NMDA receptors to the dominant ERK shutoff pathway, pre-block of synaptic NMDA receptors with MK-801 did not alter the inhibitory effect of bath-applied NMDA on ERK activity. Lastly, in contrast to a complete block of synaptic NMDA receptor activation of ERK by extrasynaptic NMDA receptors, activation of extrasynaptic NMDA receptors had no effect upon ERK activation by brain-derived neurotrophic factor. These results suggest that the synaptic NMDA receptor ERK activation pathway is coupled to both NR2A and NR2B containing receptors, and that the extrasynaptic NMDA receptor ERK inhibitory pathway is not a non-selective global ERK shutoff.

An essential role for the integrin-linked kinase-glycogen synthase kinase-3 beta pathway during dendrite initiation and growth

Multiple cues, including growth factors and circuit activity, signal to regulate the initiation and growth of mammalian dendrites. In this study, we have asked how these environmental cues regulate dendrite formation, and in particular, whether dendrite initiation and growth requires integrin-linked kinase (ILK) or its downstream effector, glycogen synthase kinase-3beta (GSK-3beta). In cultured sympathetic neurons, NGF and neuronal depolarization activated ILK and promoted dendrite initiation and growth, and inhibition of ILK (either pharmacologically, with a dominant-negative form of ILK, or by genetic knockdown) reduced depolarization-induced dendrite formation. In sympathetic neurons, ILK phosphorylated and inhibited GSK-3beta, and inhibition of GSK-3beta (either pharmacologically, with dominant-negative GSK-3beta, or by genetic knockdown) caused robust dendrite initiation. GSK-3beta inhibition also caused dendrite initiation in cultured cortical neurons and growth of hippocampal neurons in slice cultures. GSK-3beta functioned downstream of ILK to regulate dendrite formation, because inhibition of GSK-3beta promoted dendrite initiation even when ILK was simultaneously inhibited. Moreover, GSK-3beta promoted dendrite formation in sympathetic neurons by regulating the activity of a key dendrite formation effector, the MAP (microtubule-associated protein) kinase kinase (MEK)-extracellular signal-regulated protein kinase (ERK) pathway. Specifically, inhibition of GSK-3beta led to increased ERK phosphorylation, and inhibition of MEK completely blocked the effects of GSK-3beta inhibition on dendrite initiation and growth. Thus, the ILK-GSK-3beta pathway plays a key role in regulating dendrite formation in developing mammalian neurons.

Chronic DHEAS administration facilitates hippocampal long-term potentiation via an amplification of Src-dependent NMDA receptor signaling

Dehydroepiandrosterone sulfate (DHEAS) has well characterized effects on memory and cognitive performances. Recently we have reported that repetitive administration of DHEAS lowers the threshold pulse number in inducing activity-dependent long-term potentiation (LTP) in rat hippocampal Schaffer collateral-CA1 synapses, in which a sub-threshold high frequency stimulation (HFS, 30 pulses at 100 Hz) for normal rats could induce robust LTP in DHEAS-treated rats (Chen et al., 2006). Here we report that the sub-threshold HFS could trigger the phosphorylation of Src and ERK2 in the DHEAS-treated rats, but not in control rats. We found in slices obtained from the DHEAS-treated rats that NMDA-induced intracellular Ca2+([Ca2+]i) transients in CA1 pyramidal neurons were significantly potentiated, which was essential for the Src and ERK2 phosphorylations. The activation of ERK2, a downstream factor of Src family kinase, was required for the DHEAS-facilitated LTP. The Src family kinase inhibitor PP2, but not its inactive homologue PP3, attenuated the NMDA-induced [Ca2+]i increase and abolished the DHEAS-facilitated LTP. These findings suggest that the chronic administration of DHEAS brings the NMDA receptor (NMDAr) to a potentiated state that causes an enough level of [Ca2+]i increase for LTP induction even by the sub-threshold HFS. The potentiated [Ca2+]i transient by the sub-threshold HFS may trigger the Src phosphorylation that will further potentiate NMDAr followed by an activation of ERK2 and LTP induction. This novel postsynaptic NMDAr/Src-mediated signal amplification through "NMDAr-Ca2+-->Src-->NMDAr-Ca2+" cycle may play a pivotal role in the DHEAS-facilitated LTP induction.

Control of dendritic arborization by the phosphoinositide-3'-kinase-Akt-mammalian target of rapamycin pathway

The molecular mechanisms that determine the size and complexity of the neuronal dendritic tree are unclear. Here, we show that the phosphoinositide-3' kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) signaling pathway promotes the growth and branching of dendrites in cultured hippocampal neurons. Constitutively active mutants of Ras, PI3K, and Akt, or RNA interference (RNAi) knockdown of lipid phosphatase PTEN (phosphatase and tensin homolog deleted on chromosome Ten), induced growth and elaboration of dendrites that was blocked by mTOR inhibitor rapamycin and/or by overexpression of eIF-4E binding protein 1 (4E-BP1), which inhibits translation of 5' capped mRNAs. The effect of PI3K on dendrites was lost in more mature neurons (>14 d in vitro). Dendritic complexity was reduced by inhibition of PI3K and by RNAi knockdown of mTOR or p70 ribosomal S6 kinase (p70S6K, an effector of mTOR). A rapamycin-resistant mutant of mTOR "rescued" the morphogenetic effects of PI3K in the presence of rapamycin. By regulating global and/or local protein translation, and as a convergence point for multiple signaling pathways, mTOR could play a central role in the control of dendrite growth and branching during development and in response to activity.

Pharmacological characterization of inhibitory effects of postsynaptic opioid and cannabinoid receptors on calcium currents in neonatal rat nucleus tractus solitarius

1. The profile of opioid and cannabinoid receptors in neurons of the nucleus tractus solitarius (NTS) has been studied using the whole-cell configuration of the patch clamp technique. 2. Experiments with selective agonists and antagonists of opioid, ORL and cannabinoid receptors indicated that mu-opioid, kappa-opioid, ORL-1 and CB1, but not delta-opioid, receptors inhibit VDCCs in NTS. 3. Application of [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO; mu-opioid receptor agonist), Orphanin FQ (ORL-1 receptor agonist) and WIN55,122 (CB1 receptor agonist) caused inhibition of I(Ba) in a concentration-dependent manner, with IC50's of 390 nM, 220 nM and 2.2 microM, respectively. 4. Intracellular dialysis of the G(i)-protein antibody attenuated DAMGO-, Orphanin FQ- and WIN55,122-induced inhibition of I(Ba). 5. Both pretreatment with adenylate cyclase inhibitor and intracellular dialysis of the protein kinase A (PKA) inhibitor attenuated WIN55,122-induced inhibition of I(Ba) but not DAMGO- and Orphanin FQ-induced inhibition. 6. Mainly N- and P/Q-type VDCCs were inhibited by both DAMGO and Orphanin FQ, while L-type VDCCs were inhibited by WIN55,122. 7. These results suggest that mu- and kappa-opioid receptors and ORL-1 receptor inhibit N- and P/Q-type VDCCs via G alpha(i)-protein betagamma subunits, whereas CB1 receptors inhibit L-type VDCCs via G alpha(i)-proteins involving PKA in NTS.

Age-dependent participation of Ras-GRF proteins in coupling calcium-permeable AMPA glutamate receptors to Ras/Erk signaling in cortical neurons

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors (AMPARs) are ligand-gated sodium channels. Through their ability to mediate the majority of rapid excitatory transmission in the central nervous system, these neurotransmitter receptors have been shown to influence synaptic plasticity. Some of these receptors are also calcium-permeable (CP), and they also have been implicated in regulating synaptic plasticity, particularly in interneurons where their concentration is highest. However, the biochemical pathways emanating from CP-AMPARs that mediate these effects have not been well characterized. In this paper, we show that CP-AMPARs are the predominant AMPAR class responsible for activating the Ras/Erk kinase signaling cascade and the cAMP-response element-binding protein (CREB) transcription factor in the cortex of mature mice. Activation of Ras and Erk, but not CREB, occurs through the calcium/calmodulin regulated Ras-GRF1 and Ras-GRF2 exchange factors, which form AMPA-induced complexes with CP-AMPARs but not calcium-impermeable (CI) AMPARs in vivo. Furthermore, we show that CP-AMPARs are also the major AMPAR type to activate Ras/Erk signaling in pubescent mice; however, at this developmental stage Ras-GRF (guanine nucleotide-releasing factor) proteins are not involved. Finally, in neonatal animals CI-AMPARs, but not CP-AMPARs, are the predominant AMPAR type that activates Ras-Erk signaling and CREB in cortical neurons. This occurs indirectly through activation of L-type voltage-dependent calcium channels, an event that is also Ras-GRF-independent. Thus, Ras/Erk signaling and CREB activity induced by AMPARs occur through age-dependent mechanisms that likely make unique developmentally dependent contributions to synaptic function.

Involvement of Src tyrosine kinase and mitogen-activated protein kinase in the facilitation of calcium channels in rat nucleus of the tractus solitarius by angiotensin II

It is recognized that brain contains all the components of the renin-angiotensin systems (RAS). The nucleus of the tractus solitarius (NTS) is known to play a major role in the regulation of cardiovascular, respiratory, gustatory, hepatic and swallowing functions. Voltage-dependent Ca2+ channels (VDCCs) serve as crucial mediators of membrane excitability and Ca2+-dependent functions such as neurotransmitter release, enzyme activity and gene expression. The purpose of this study was to investigate the effects of angiotensin II (Ang II) on VDCC currents (I(Ca)) in the NTS using patch-clamp recording methods. An application of Ang II caused facilitation of L-type I(Ca) in a concentration-dependent manner with an EC50 of 167 nm and a Hill coefficient of 1.73. AT1 receptor antagonist losartan antagonized the Ang II-induced facilitation of I(Ca). Intracellular dialysis of the Galpha(i)-protein antibody attenuated the Ang II-induced facilitation of I(Ca). Both Src tyrosine kinase inhibitor and mitogen-activated protein kinase (MAPK) inhibitor attenuated the Ang II-induced facilitation of I(Ca). p38 MAPK inhibitor also attenuated the Ang II-induced facilitation of I(Ca). These results indicate that Ang II facilitates L-type VDCCs via Galpha(i)-proteins involving Src tyrosine kinase and p38 MAPK kinase mediated by AT1 receptors in NTS.

Phosphatidylinositol 3-kinase activates ERK in primary sensory neurons and mediates inflammatory heat hyperalgesia through TRPV1 sensitization

Although the PI3K (phosphatidylinositol 3-kinase) pathway typically regulates cell growth and survival, increasing evidence indicates the involvement of this pathway in neural plasticity. It is unknown whether the PI3K pathway can mediate pain hypersensitivity. Intradermal injection of capsaicin and NGF produce heat hyperalgesia by activating their respective TRPV1 (transient receptor potential vanilloid receptor-1) and TrkA receptors on nociceptor sensory nerve terminals. We examined the activation of PI3K in primary sensory DRG neurons by these inflammatory agents and the contribution of PI3K activation to inflammatory pain. We further investigated the correlation between the PI3K and the ERK (extracellular signal-regulated protein kinase) pathway. Capsaicin and NGF induce phosphorylation of the PI3K downstream target AKT (protein kinase B), which is blocked by the PI3K inhibitors LY294002 and wortmannin, indicative of the activation of PI3K by both agents. ERK activation by capsaicin and NGF was also blocked by PI3K inhibitors. Similarly, intradermal capsaicin in rats activated PI3K and ERK in C-fiber DRG neurons and epidermal nerve fibers. Injection of PI3K or MEK (ERK kinase) inhibitors into the hindpaw attenuated capsaicin- and NGF-evoked heat hyperalgesia but did not change basal heat sensitivity. Furthermore, PI3K, but not ERK, inhibition blocked early induction of hyperalgesia. In acutely dissociated DRG neurons, the capsaicin-induced TRPV1 current was strikingly potentiated by NGF, and this potentiation was completely blocked by PI3K inhibitors and primarily suppressed by MEK inhibitors. Therefore, PI3K induces heat hyperalgesia, possibly by regulating TRPV1 activity, in an ERK-dependent manner. The PI3K pathway also appears to play a role that is distinct from ERK by regulating the early onset of inflammatory pain.

Ionotropic and metabotropic receptors, protein kinase A, protein kinase C, and Src contribute to C-fiber-induced ERK activation and cAMP response element-binding protein phosphorylation in dorsal horn neurons, leading to central sensitization

Molecular mechanisms underlying C-fiber stimulation-induced ERK (extracellular signal-regulated kinase) activation in dorsal horn neurons and its contribution to central sensitization have been investigated. In adult rat spinal slice preparations, activation of C-fiber primary afferents by a brief exposure of capsaicin produces an eightfold to 10-fold increase in ERK phosphorylation (pERK) in superficial dorsal horn neurons. The pERK induction is reduced by blockade of NMDA, AMPA/kainate, group I metabotropic glutamate receptor, neurokinin-1, and tyrosine receptor kinase receptors. The ERK activation produced by capsaicin is totally suppressed by inhibition of either protein kinase A (PKA) or PKC. PKA or PKC activators either alone or more effectively together induce pERK in superficial dorsal horn neurons. Inhibition of calcium calmodulin-dependent kinase (CaMK) has no effect, but pERK is reduced by inhibition of the tyrosine kinase Src. The induction of cAMP response element binding protein phosphorylation (pCREB) in spinal cord slices in response to C-fiber stimulation is suppressed by preventing ERK activation with the MAP kinase kinase inhibitor 2-(2-diamino-3-methoxyphenyl-4H-1-benzopyran-4-one (PD98059) and by PKA, PKC, and CaMK inhibitors. Similar signaling contributes to pERK induction after electrical stimulation of dorsal root C-fibers. Intraplantar injection of capsaicin in an intact animal increases expression of pCREB, c-Fos, and prodynorphin in the superficial dorsal horn, changes that are prevented by intrathecal injection of PD98059. Intrathecal PD98059 also attenuates capsaicin-induced secondary mechanical allodynia, a pain behavior reflecting hypersensitivity of dorsal horn neurons (central sensitization). We postulate that activation of ionotropic and metabotropic receptors by C-fiber nociceptor afferents activates ERK via both PKA and PKC, and that this contributes to central sensitization through post-translational and CREB-mediated transcriptional regulation in dorsal horn neurons.

N-Methyl-d-aspartate Attenuates CXCR2-Mediated Neuroprotection through Enhancing the Receptor Phosphorylation and Blocking the Receptor Recycling

Abnormal extracellular accumulations of beta-amyloid, a major component of the senile plaques, and of the excitatory amino acid glutamate are both believed to be associated with degeneration of nerve cells in the central nervous system of patients with Alzheimer's disease. The chemokine receptor CXCR2 has been shown to play a role in protecting neurons against beta-amyloid-induced injury in vitro, but it remains unclear whether CXCR2-mediated neuroprotection is affected by glutamate. We demonstrated that pretreatment of hippocampal neurons with a sublethal concentration of N-methyl-d-aspartate (NMDA) attenuated the macrophage inflammatory protein 2 (MIP2)-induced protection against beta-amyloid-induced neuronal death. The NMDA induced inhibition was blocked by (+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), a noncompetitive NMDA receptor antagonist, indicating the involvement of NMDA receptors in this process. A sublethal dose of NMDA pretreatment induced CXCR2 phosphorylation, although to a lesser extent than the receptor phosphorylation induced by MIP2, and differential serine residues were involved in NMDA- and MIP2-induced CXCR2 phosphorylation. Moreover, NMDA treatment reduced the CXCR2-mediated Ca(2+) mobilization, suggesting that NMDA induces cross-desensitization of CXCR2. CXCR2 underwent dephosphorylation after removal of the extracellular ligand, but the dephosphorylation rate was significantly reduced in the cells pretreated with NMDA. Treatment of the neuronal cells with NMDA retarded the recycling of CXCR2. In view of the critical role of receptor phosphorylation and recycling in the functional responsiveness of the chemokine receptor, these observations indicate a novel pathway through which glutamate may interfere with the neuroprotective function of chemokines.

Tyrosine phosphatase SHP-2 is a mediator of activity-dependent neuronal excitotoxicity

Calcium influx can promote neuronal differentiation and survival, at least in part by activating Ras and its downstream targets, including the Erk pathway. However, excessive calcium influx can initiate molecular signals leading to neuronal death during excitotoxicity or in neurodegenerative diseases. Here we describe a new signaling pathway associated with calcium influx that contributes to neuronal cell death in cerebellar neurons. Influx of calcium, mediated either by L-type voltage-sensitive calcium channels or glutamate receptors, is associated with the suppression of brain-derived neurotrophic factor (BDNF) activation of Ras and its effectors Erk and Akt. This is the result of enhanced association of the tyrosine phosphatase Shp-2 with TrkB receptors, which inhibits BDNF-induced TrkB autophosphorylation and activation. Deletion of the Shp2 gene in neuronal cultures reverses inhibition of TrkB function and increases neuronal survival after extended depolarization or glutamate treatment. These findings implicate Shp-2 in a feedback system initiated by calcium that negatively regulates neurotrophin signaling and sensitizes neurons to excitotoxicity.

D-aspartate modulates transcriptional activity in Harderian gland of frog,Rana esculenta: Morphological and molecular evidence

In the green frog, Rana esculenta, a substantial amount of D-aspartate (D-Asp) is found endogenously within the Harderian gland (HG) following its synthesis from L-aspartate (L-Asp) by an aspartate racemase. The frog HG is an orbital seromucoid gland that displays seasonal changes in secretory activity. Our in vivo experiments, consisting of i.p. injection of 2.0 mumol/g b.w. D-Asp in frogs collected during two periods of differing glandular activity (high or medium-low secretory activity), revealed that HG can to take up and accumulate D-Asp and that this amino acid may modulate the exocrine secretion through a kinase pathway. At a time when the gland shows relatively low secretory activity, i.p. administration of D-Asp rapidly induced activation of ERK1 and an increase in cells active in RNA synthesis. This increase in transcriptional activity was followed by a significant increase in mucous secretion. By contrast, administration of exogenous D-Asp when HG was showing high activity rapidly induced inhibition of both ERK1 and transcriptional activity. Since D-Asp is known to be recognized by receptors for N-methyl-D-aspartic acid (NMDA), it is possible that in the HG, D-Asp mediated NMDA activation may enhance the kinase pathway. The above activation of opposing stimulatory and inhibitory processes could reflect different levels of NMDA-receptor activity, which could vary as a function of the level of gland activity. This study provides the first evidence of a role for this excitatory amino acid in exocrine secretion. The effects of D-Asp in HG appear to be specific since they were not seen in frogs treated with other D- or L-amino acids with known excitatory effects on neurosecretion.

Gene Regulation in Spermatogenesis

Mammalian spermatogenesis is a complex hormone-dependent developmental program in which a myriad of events must take place to ensure that germ cells reach their proper stage of development at the proper time. Many of these events are controlled by cell type- and stage-specific transcription factors. The regulatory mechanisms involved provide an intriguing paradigm for the field of developmental biology and may lead to the development of new contraceptives an and innovative routs to treat male infertility. In this review, we address three aspects of the genetic regulatory mechanism that drive spermatogenesis. First, we detail what is known about how steroid hormones (both androgens and estrogens) and their cognate receptors initiate and maintain mammalian spermatogenesis. Steroids act through three mechanistic routes: (i) direct activation of genes through hormone-dependent promoter elements, (ii) secondary transcriptional responses through activation of hormone-dependent transcription factors, and (iii) rapid, transcription-independent (nonclassical) events induced by steroid hormones. Second, we provide a survey of transcription factors that function in mammalian spermatogenesis, including homeobox, zinc-finger, heat-shock, and cAMP-response family members. Our survey is not intended to cover all examples but to give a flavor for the gamut of biological roles conferred by transcription factors in the testis, particularly those defined in knockout mice. Third, we address how testis-specific transcription is achieved. In particular, we cover the evidence for and against the idea that some testis-specific genes are transcriptionally silent in somatic tissues as a result of DNA methylation.

Src in synaptic transmission and plasticity

In the central nervous system (CNS), Src and other Src family kinases are widely expressed and are abundant in neurons. Src has been implicated in proliferation and differentiation during the development of the CNS. But Src is highly expressed in fully differentiated neurons in the developed CNS, implying additional functions of this kinase. Over the past decade, a large body of evidence has accumulated showing that a main function of Src is to upregulate the activity of N-methyl-D-aspartate (NMDA) receptors and other ion channels. NMDA receptors (NMDARs) are a principal subtype of glutamate receptors, which mediate fast excitatory transmission at most central synapses. In this review, we focus on Src as a regulator of NMDARs and on the role of Src in NMDAR-dependent synaptic plasticity. We also describe recent studies that give insights into the regulation of Src itself at glutamatergic synapses. By upregulating the function of NMDARs, Src gates the production of NMDAR-dependent synaptic potentiation and plasticity. Thus, Src may be critical for processes underlying physiological plasticity, including learning and memory, and pathological plasticity, such as pain and epilepsy.

The biochemical activation of T-type Ca2+ channels in HEK293 cells stably expressing α1G and Kir2.1 subunits

In order to investigate the currently unknown cellular signaling pathways of T-type Ca(2+) channels, we decided to construct a new cell line which would stably express alpha(1G) and Kir2.1 subunits in HEK293 cells (HEK293/alpha(1G)/Kir2.1). Compared to cells which only expressed alpha(1G) (HEK293/alpha(1G)), HEK293/alpha(1G)/Kir2.1 cells produced an enormous inward rectifying current which was blocked by external Ba(2+) and Cs(+) in a concentration-dependent manner. The expression of Kir2.1 channels contributed significantly to the shift of membrane potential from -12.2+/-2.8 to -57.3+/-3.7mV. However, biophysical and pharmacological properties of alpha(1G)-mediated Ca(2+) channels remained unaffected by the expression of Kir2.1 subunits, except for the enlarging of the window current region. Biochemical activation of alpha(1G) channels using 150mM KCl brought about an increase in [Ca(2+)](i), which was blocked by mibefradil, the T-type Ca(2+) channel blocker. These data suggest that the HEK293/alpha(1G)/Kir2.1 cell line would have potential uses in the study of T-type Ca(2)(+) channel-mediated signaling pathways and possibly useful in the development of new therapeutic drugs associated with T-type Ca(2)(+) channels.

Glutamate signaling to Ras-MAPK in striatal neurons: mechanisms for inducible gene expression and plasticity

Extracellular signals can regulate mitogen-activated protein kinase (MAPK) cascades through a receptor-mediated mechanism in postmitotic neurons of adult mammalian brain. Both ionotropic and metabotropic glutamate receptors (mGluRs) are found to possess such an ability in striatal neurons. NMDA and AMPA receptor signals seem to share a largely common route to MAPK phosphorylation which involves first activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) via Ca2+ influx, followed by subsequent induction of phosphoinositide 3-kinase (PI3-kinase). Through its lipid and protein kinase activity, active PI3-kinase may transduce signals to Ras-MAPK cascades via at least two distinct pathways. A novel, Ca(2+)-independent pathway is believed to mediate mGluR signals to Ras-MAPK activation. As an information superhighway between the surface membrane and the nucleus, Ras-MAPK cascades, through activating their specific nuclear transcription factor targets, are actively involved in the regulation of gene expression. Emerging evidence shows that MAPK-mediated genomic responses in striatal neurons to drug exposure contribute to the development of neuroplasticity related to addictive properties of drugs of abuse.

NMDA and AMPA receptor expression and cortical neuronal death are associated with p38 in glutamate-induced excitotoxicity in vivo

Early overstimulation of ionotropic glutamate receptors (iGluRs), such as the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, produces excitotoxicity in several brain regions. The molecular composition of those receptors and their regulation by intracellular signaling systems could be determinants in the development of progressive neurodegenerative mechanisms in the central nervous system (CNS). Studies of p38 mitogen-activated protein kinase (MAPK) activation, morphologic changes including cell number, and the expression of the NR1 and GluR2 subunits, by reverse transcriptase-PCR were evaluated at early postnatal ages (postnatal day [PD]8-14) in cerebral cortex of rats treated with monosodium glutamate (MSG; 4 mg/g body weight) administered subcutaneously on PD1, 3, 5, and 7. An important increase in p38 activity at PD8 and loss of cortical cell number were observed from PD8-14 in animals treated with MSG, together with significant morphologic changes characterized by cell shrinkage, nuclear hyperchromatism, and cytoplasmic vacuolation. These morphologic changes were prevented by SB203580, an inhibitor of p38 signaling, at PD8-14. No change in cerebral cortex thickness was observed among experimental or control rats. A significant increase in NR1 subunit expression was observed in response to MSG from PD8-14. GluR2 expression increased from PD8-12, but at PD14, its expression was reduced to 54% with respect to controls. SB203580 prevented alone the decreased in GluR2 expression induced by MSG. These results suggest that initial neuronal death (at PD8 and 10) in cerebral cortex may be due to an excessive Ca2+ influx through NMDA receptors, whereas the further damage process could be mediated by AMPA receptors through p38 signaling. This could represent a determinant mechanism to decide whether nerve cells survive or die.