The Na+/Ca2+ exchanger NCX1 plays a fundamental role in regulating nuclear Ca2+ homeostasis during neuronal differentiation and in anoxic conditions

Calcium signalling, through specific transductional pathways, regulates several relevant cellular functions including gene transcription, differentiation, growth and apoptosis. Several lines of evidence indicate that increases in nuclear calcium concentrations ([Ca2+]n), through independent Ca2+ signals, may occur independently from cytosolic events. The electrogenic Na+-Ca2+ Exchanger (NCX) couples the efflux/influx of Ca2+ ions to the influx/efflux of Na+ by operating in a bidirectional way. Three different gene products of this exchanger have been cloned: NCX1, which is ubiquitously expressed in several tissues, NCX2, and NCX3, both widely expressed in the brain and in the skeletal tissue. Recent data showed a specific localization of NCX1 also at nuclear level but its role remains still unrevealed. Our study was focused on the role played by NCX1 in neuronal differentiation and anoxia in differentiated PC12 cells. In these cells and in their isolated intact nuclei, the expression of NCX1 has been detected by Western Blot analysis and Immunocytochemistry. In addition, the exchanger activity was evaluated by means of conventional and confocal microscopy. NCX1 expression and activity were studied in PC12 at different days of NGF treatment. These parameters were increased after 3 days of treatment, an early phase of the neuronal process, thus promoting an increase in [Ca2+]n. Interestingly, the activation of the exchanger was able to induced Akt/PKB phosphorylation in isolated intact nuclei. This activation was significantly prevented by knocking down NCX1 by a specific siRNA approach. In addition, our data suggested that chemical hypoxia induced a significant reduction in nuclear NCX1 activity in neuronal cells. Concomitantly, we showed that an increase in [Ca2+]n occurred. This effect was counteract by replenish ATP content with pyruvate. Interestingly, this drug was ineffective when nuclear NCX1 expression was reduced by a specific siRNA approach.