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Here, we report that interruption of NGF or BDNF signaling in hippocampal neurons rapidly activates the amyloidogenic pathway and causes neuronal apoptotic death. These events are associated with an early intracellular accumulation of PS1 N-terminal catalytic subunits and of APP C-terminal fragments and a progressive accumulation of intra- and extracellular A[beta] aggregates partly released into the culture medium. The released pool of A[beta] induces an increase of APP and PS1 holoprotein levels, creating a feed-forward toxic loop that might also cause the death of healthy neurons. These events are mimicked by exogenously added A[beta] and are prevented by exposure to [beta]- and [gamma]-secretase inhibitors and by antibodies directed against A[beta] peptides. The same cultured neurons deprived of serum die, but APP and PS1 overexpression does not occur, A[beta] production is undetectable, and cell death is not inhibited by anti-A[beta] antibodies, suggesting that hippocampal amyloidogenesis is not a simple consequence of an apoptotic trigger but is due to interruption of neurotrophic signaling. Alzheimer disease | apoptosis | APP | neurotrophin

The altered function and/or structure of tau protein is postulated to cause cell death in tauopathies and Alzheimer's disease. However, the mechanisms by which tau induces neuronal death remain unclear. Here we show that overexpression of human tau and of some of its N-terminal fragments in primary neuronal cultures leads to an N-methyl-D-aspartate receptor (NMDAR)-mediated and caspase-independent cell death. Death signaling likely originates from stimulation of extrasynaptic NR2B-subunit-containing NMDARs because it is accompanied by dephosphorylation of cAMP-response-element-binding protein (CREB) and it is inhibited by ifenprodil. Interestingly, activation of NMDAR leads to a crucial, sustained, and delayed phosphorylation of extracellular-regulated kinases 1 and 2, whose inhibition largely prevents tau-induced neuronal death. Moreover, NMDAR involvement causes the fatal activation of calpain, which, in turn, degrades tau protein into a 17-kDa peptide and possibly other highly toxic N-terminal peptides. Some of these peptides are hypothesized, on the basis of our in vitro experiments, to initiate a negative loop, ultimately leading to cell death. Thus, inhibition of calpain largely prevents tau degradation and cell death. Our findings unravel a cellular mechanism linking tau toxicity to NMDAR activation and might be relevant to Alzheimer's disease and tauopathies where NMDAR-mediated toxicity is postulated to play a pivotal role. extracellular-regulated kinase | mitogen-activated protein kinase | neurodegenerative diseases | glutamate receptors | Alzheimer's disease

Some clues suggest that neuronal damage induces a secondary change of amyloid [Beta] protein (A[Beta]) metabolism. We investigated this possibility by analyzing the secretion of A[Beta] and processing of its precursor protein (amyloid precursor protein, APP) in an in vitro model of neuronal apoptosis. Primary cultures of rat cerebellar granule neurons were metabolically labeled with [35S]methionine. Apoptosis was induced by shifting extracellular KCl concentration from 25 mM to 5 mM for 6 h. Control and apoptotic neurons were then subjected to depolarization-stimulated secretion. Constitutive and stimulated secretion media and cell lysates were immunoprecipitated with antibodies recognizing regions of A[Beta], full-length APP, [Alpha]- and [Beta]-APP secreted forms. Immunoprecipitated proteins were separated by SDS/PAGE and quantitated with a PhosphorImager densitometer. Although intracellular full-length APP was not significantly changed after apoptosis, the monomeric and oligomeric forms of 4-kDa A[Beta] were 3-fold higher in depolarization-stimulated secretion compared with control neurons. Such increments were paralleled by a corresponding increase of the [Beta]-APPs/[Alpha]-APPs ratio in apoptotic secretion. Immunofluorescence studies performed with an antibody recognizing an epitope located in the A[Beta] sequence showed that the A[Beta] signal observed in the cytoplasm and in the Golgi apparatus of control neurons is uniformly redistributed in the condensed cytoplasm of apoptotic cells. These studies indicate that neuronal apoptosis is associated with a significant increase of metabolic products derived from [Beta]-secretase cleavage and suggest that an overproduction of A[Beta] may be the consequence of neuronal damage from various causes.

The insulin-like growth factor I (IGF-I) stimulated amino acids and the sensitivity to these amino acids has been observed in the phenotype of cerebellar granule cells. Total susceptibility of IGF-I to the toxic and lethal quality of glutamate is seen, though this reaction is reversible by eliminating somatomedin. Glutamate treatment on Ca2+ entry leads to an enhanced Ca2+ permeability, occurring through N-methyl-D-aspartate (NMDA) and non-NMDA receptors.

Cultured cerebellar granule neurons are widely used as a cellular model to study mechanisms of neuronal cell death because they undergo programmed cell death when switched from a culture medium containing 25 mM to one containing 5 mM [K.sup.+]. We have found that the growth-related gene product [Beta] (GRO[Beta]) partially prevents the [K.sup.+]-depletion-induced cell death, and that the neuroprotective action of GRO[Beta] on granule cells is mediated through the [alpha]-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type of ionotropic glutamate receptors. GRO[Beta]-induced survival was suppressed by 6-cyano-7-nitroquinoxaline-2,3-dione, which is a specific antagonist of AMPA/kainate receptors; it was not affected by the inhibitor of N-methyl-D-aspartate receptors, 2-amino-5-phosphonopentanoic acid, and was comparable to the survival of granule cells induced by AMPA (10 [micro]M) treatment. Moreover, GRO[Beta]-induced neuroprotection was abolished when granule cells were treated with antisense oligonucleotides specific for the AMPA receptor subunits, which significantly reduced receptor expression, as verified by Western blot analysis with subunit-specific antibodies and by granule cell electrophysiological sensitivity to AMPA. Our data demonstrate that GRO[Beta] is neurotrophic for cerebellar granule cells, and that this activity depends on AMPA receptors. chemokines | apoptosis | cerebellar granule neurons | glutamate receptors

Features a study which found that the chemokine growth-related gene product beta protects rat cerebellar granule cells from apoptotic cell death through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors. Methodology of the study; Results and discussion; Conclusion.