A new concept: A/β 1-42 generates a hyper functional proteolytic NCX3 fragment that delays caspase-12 activation and neuronal death

Abstract

Although the amyloid-/β 1-42 (A/β 1- 42) peptide involved in Alzheimer's disease is known to cause a dysregulation of intracellular Ca 2+ homeostasis, its molecular mechanisms still remain unclear. We report that the extracellular-dependent early increase (30 min) in intracellular calcium concentration ([Ca 2+]i), following A/β 1- 42 exposure, caused the activation of calpain that in turn elicited a cleavage of the Na +/Ca 2 + exchanger isoform NCX3. This cleavage generated a hyper functional form of the antiporter and increased NCX currents (I NCX) in the reverse mode of operation. Interestingly, this NCX3 calpain-dependent cleavage was essential for the A/β 1- 42-dependent I NCX increase. Indeed, the calpain inhibitor calpeptin and the removal of the calpain-cleavage recognition sequence, via site-directed mutagenesis, abolished this effect. Moreover, the enhanced NCX3 activity was paralleled by an increased Ca 2+ content in the endoplasmic reticulum (ER) stores. Remarkably, the silencing in PC-12 cells or the knocking-out in mice of the ncx3 gene prevented the enhancement of both I NCXand Ca 2+ content in ER stores, suggesting that NCX3 was involved in the increase of ER Ca 2+ content stimulated by A/β 1-42. By contrast, in the late phase (72 h), when the NCX3 proteolytic cleavage abruptly ceased, the occurrence of a parallel reduction in ER Ca 2+ content triggered ER stress, as revealed by caspase-12 activation. Concomitantly, the late increase in [Ca 2+]i coincided with neuronal death. Interestingly, NCX3 silencing caused an earlier activation of A/β 1-42-induced caspase-12. Indeed, in NCX3-silenced neurons, A/β 1-42 exposure hastened caspase-dependent apoptosis, thus reinforcing neuronal cell death. These results suggest that A/β 1-42, through Ca 2+-dependent cal pain activation, generates a hyperfunctional form ofNCX3 that, by increasing Ca 2+ content into ER, delays caspase-12 activation and thus neuronal death. © 2012 the authors.