Ca(2+)-induced inhibition of apoptosis in human SH-SY5Y neuroblastoma cells: degradation of apoptotic protease activating factor-1 (APAF-1).

During apoptotic and excitotoxic neuron death, challenged mitochondria release the pro-apoptotic factor cytochrome c. In the cytosol, cytochrome c is capable of binding to the apoptotic protease-activating factor-1 (APAF-1). This complex activates procaspase-9 in the presence of dATP, resulting in caspase-mediated execution of apoptotic neuron death. Many forms of Ca(2+)-mediated neuron death, however, do not lead to prominent activation of the caspase cascade despite significant release of cytochrome c from mitochondria. We demonstrate that elevation of cytosolic Ca(2+) induced prominent degradation of APAF-1 in human SH-SY5Y neuroblastoma cells and in a neuronal cell-free apoptosis system. Loss of APAF-1 correlated with a reduced ability of cytochrome c to activate caspase-3-like proteases. Ca(2+) induced the activation of calpains, monitored by the cleavage of full-length alpha-spectrin into a calpain-specific 150-kDa breakdown product. However, pharmacological inhibition of calpain activity indicated that APAF-1 degradation also occurred via calpain-independent pathways. Our data suggest that Ca(2+) inhibits caspase activation during Ca(2+)-mediated neuron death by triggering the degradation of the cytochrome c-binding protein APAF-1.

Up-regulation of Bcl-xL in response to subtoxic beta-amyloid: role in neuronal resistance against apoptotic and oxidative injury.

Neuron death in Alzheimer's disease is believed to be triggered by an increased production of amyloidogenic beta-amyloid peptides, involving both increased oxidative stress and activation of a conserved death program. Bcl-xL, an anti-apoptotic protein of the Bcl-2 family, is expressed at high levels in the adult nervous system. Exposure of neuronal cultures to subtoxic concentrations of beta-amyloid peptide 1-40 (1-10microM) or the fragment 25-35 (1-10microM) up-regulated both bcl-xL mRNA and Bcl-xL protein levels, determined by reverse transcriptase-polymerase chain reaction and western blot analysis. Bcl-xL protein was also up-regulated during oxidative stress induced by exposure to hydrogen peroxide (3-100microM) or ferric ions (1-10microM). In contrast, apoptotic stimuli (exposure to staurosporine or serum withdrawal) actually decreased neuronal Bcl-xL expression. To investigate the role of Bcl-xL in cell death relevant to Alzheimer's disease, we stably overexpressed Bcl-xL in human SH-SY5Y neuroblastoma cells. Cells overexpressing Bcl-xL were significantly protected from beta-amyloid neurotoxicity and staurosporine-induced apoptosis compared to vector-transfected controls. In contrast, Bcl-xL overexpression only conferred a mild protection against oxidative injury induced by hydrogen peroxide. We conclude that up-regulation of Bcl-xL expression in response to subtoxic concentrations of beta-amyloid is a stress response that increases the resistance of neurons to beta-amyloid neurotoxicity primarily by inhibiting apoptotic processes.

Phosphorylation of the 5-hydroxytryptamine3 (5-HT3) receptor expressed in HEK293 cells.

The 5-hydroxytrptamine3 (5-HT3) receptor is a pentameric complex belonging to the family of ligand-gated ion channels. A variety of studies have suggested that phosphorylation regulates the rate of desensitisation and the size of amplitude of the receptor current. In this study we have examined the phosphorylation of the myc-tagged wild-type 5-HT3A receptor subunits from guinea-pig expressed in HEK293 cells (human embryonic kidney). Stably transfected cells were metabolically labelled with 32P-phosphoric acid. The results of immunoprecipitation and autoradiography demonstrate that both splicc variants of the 5-HT3A receptor subunit are phosphorylated in HEK293 cells. Site-specific mutagenesis revealed that phosphorylation occurs at serine 409, a potential target of protein kinase A. Thus the 5-HT3 receptor might be modulated by intracellular pathways, that allow variable 5-hydroxytryptamine action as responses to different extracellular stimuli.

Activation of calpain I converts excitotoxic neuron death into a caspase-independent cell death.

Glutamate receptor overactivation contributes to neuron death after stroke, trauma, and epileptic seizures. Exposure of cultured rat hippocampal neurons to the selective glutamate receptor agonist N-methyl-d-aspartate (300 microm, 5 min) or to the apoptosis-inducing protein kinase inhibitor staurosporine (300 nm) induced a delayed neuron death. In both cases, neuron death was preceded by the mitochondrial release of the pro-apoptotic factor cytochrome c. Unlike staurosporine, the N-methyl-d-aspartate-induced release of cytochrome c did not lead to significant activation of caspase-3, the main caspase involved in the execution of neuronal apoptosis. In contrast, activation of the Ca(2+)-activated neutral protease calpain I was readily detectable after the exposure to N-methyl-d-aspartate. In a neuronal cell-free apoptosis system, calpain I prevented the ability of cytochrome c to activate the caspase cascade by inhibiting the processing of procaspase-3 and -9 into their active subunits. In the hippocampal neuron cultures, the inhibition of calpain activity restored caspase-3-like protease activity after an exposure to N-methyl-d-aspartate. Our data demonstrate the existence of signal transduction pathways that prevent the entry of cells into a caspase-dependent cell death program after the mitochondrial release of cytochrome c.

Molecular cloning, functional expression, and pharmacological characterization of 5-hydroxytryptamine3 receptor cDNA and its splice variants from guinea pig.

Polymerase chain reaction and rapid amplification of cDNA ends were used to isolate cDNAs encoding a 5-hydroxytryptamine3 (5-HT3) receptor subunit and its splice variants from guinea pig intestine. The amino acid sequence predicted from this cDNA is 81% homologous to the murine 5-HT3 receptor subunits cloned from NCB20 and N1E-115 cells. The splice variants code for two proteins differing by a deletion of six amino acids located in the large intracellular loop between transmembrane domains M3 and M4. For characterization, the cloned 5-HT3 cDNA was expressed in HEK 293 cells, and the electrophysiological and pharmacological properties of the recombinant ion/channel/receptor complex were investigated by patch clamping. Our data reveal that the cloned cDNAs code for guinea pig 5-HT3 receptors, which functionally assemble as homo-oligomers. The kinetic behavior of the ion channel and its sensitivity to several agonists and antagonists were markedly different from those of the cloned 5-HT3 receptors from mouse and human under similar experimental conditions. The agonists used were 5-hydroxytryptamine, 2-methyl-5-hydroxytryptamine, 1-phenylbiguanide (PBG), m-chlorophenylbiguanide, and the antagonists tropisetron and metoclopramide. In addition, 5-HT, PBG, and tropisetron were investigated through radioligand binding to isolated membranes. Compared with the human and murine 5-HT3 receptors, the guinea pig receptor showed prolonged desensitization kinetics. In addition, the guinea pig 5-HT3 receptor did not respond to the selective 5-HT3 receptor agonist PBG. Construction of chimeric receptors between guinea pig and human 5-HT3 receptor sequences localized the differences in desensitization kinetics to the carboxyl-terminal domain and the ligand binding site to the amino-terminal domain of the receptor protein. Molecular determinants of the PBG binding site of the human 5-HT3 receptor were localized to a 28-amino-acid spanning region adjacent to the M1 region.

Enhanced RACE method using specific enrichment by biotinylated oligonucleotides bound to streptavidin coated magnetic particles.

RACE (rapid amplification of cDNA ends) is commonly used for identification and isolation of 3'and 5'termini of cDNA. We developed an improvement of the RACE-method that allows the enrichment of wanted fragments. The important new feature is the purification of the amplified products by biotinylated oligonucleotides that hybridize internally. Hybrids are isolated by streptavidin coated magnetic particles.