Ca2+ entry through store-operated Ca2+ channels involves the interaction at ER–PM (endoplasmic reticulum–plasma membrane) junctions of STIM (stromal interaction molecule) and Orai. STIM proteins are sensors of the luminal ER Ca2+ concentration and, following depletion of ER Ca2+, they oligomerize and translocate to ER–PM junctions where they form STIM puncta. Direct binding to Orai proteins activates their Ca2+ channel function. It has been suggested that an additional interaction of the C-terminal polybasic domain of STIM1 with PM phosphoinositides could contribute to STIM1 puncta formation prior to binding to Orai. In the present study, we investigated the role of phosphoinositides in the formation of STIM1 puncta and SOCE (store-operated Ca2+ entry) in response to store depletion. Treatment of HeLa cells with inhibitors of PI3K (phosphatidylinositol 3-kinase) and PI4K (phosphatidylinositol 4-kinase) (wortmannin and LY294002) partially inhibited formation of STIM1 puncta. Additional rapid depletion of PtdIns(4,5)P2 resulted in more substantial inhibition of the translocation of STIM1–EYFP (enhanced yellow fluorescent protein) into puncta. The inhibition was extensive at a concentration of LY294002 (50 μM) that should primarily inhibit PI3K, consistent with a major role for PtdIns(4,5)P2 and PtdIns(3,4,5)P3 in puncta formation. Depletion of phosphoinositides also inhibited SOCE based on measurement of the rise in intracellular Ca2+ concentration after store depletion. Overexpression of Orai1 resulted in a recovery of translocation of STMI1 into puncta following phosphoinositide depletion and, under these conditions, SOCE was increased to above control levels. These observations support the idea that phosphoinositides are not essential but contribute to STIM1 accumulation at ER–PM junctions with a second translocation mechanism involving direct STIM1–Orai interactions.

Amyloidogenic processing of the amyloid precursor protein (APP) results in the generation of ␤-amyloid, the main constituent of Alzheimer plaques, and the APP intracellular domain (AICD).Recently, it has been demonstrated that AICD has transactivation potential; however, the targets of AICD-dependent gene regulation and hence the physiological role of AICD remain largely unknown. We analyzed transcriptome changes during AICD-dependent gene regulation by using a human neural cell culture system inducible for expression of AICD, its coactivator FE65, or the combination of both. Induction of AICD was associated with increased expression of genes with known function in the organization and dynamics of the actin cytoskeleton, including ␣2-Actin and Transgelin (SM22). AICD target genes were also found to be differentially regulated in the frontal cortex of Alzheimer's disease patients compared with controls as well as in AICD/FE65 transiently transfected murine cortical neurons. Confocal image analysis of neural cells and cortical neurons expressing both AICD and FE65 confirmed pronounced changes in the organization of the actin cytoskeleton, including the destabilization of actin fibers and clumping of actin at the sites of cellular outgrowth. Our data point to a role of AICD in developmental and injury-related cytoskeletal dynamics in the nervous system. INTRODUCTIONThe amyloid precursor protein (APP) is a type-1 transmembrane protein composed of a large extracellular and a small intracellular domain and has been widely implicated in the pathogenesis of Alzheimer's disease (AD). APP can undergo proteolytic cleavage by ␤-and ␥-secretase activities, resulting in the production of ␤-amyloid (A␤) (Selkoe, 1994). A␤ is the main constituent of amyloid plaques and is thought to be neurotoxic by inducing oxidative stress, inflammation, and neurodegeneration (Mattson, 2004). The intraneuronal accumulation of A␤ protofibrils is thought to cause progressive neurotoxicity in cortical neurons (Hartley et al., 1999). In addition, the secretion of soluble A␤ oligomers inhibits hippocampal long-term potentiation and alters the memory of complex learned behavior (Walsh et al., 2005).The APP intracellular domain (AICD), a small 6-kDa protein, originates from APP cleavage mediated by ␥-secretase activity (Octave et al., 2000). This cleavage can occur after Val636 (AICD59), Ala638 (AICD57), or Leu645 (AICD50) corresponding to the ␥-or -cleavage site ( Figure 1A) of the ␥-secretase complex (Sastre et al., 2001;Yu et al., 2001). More recently, AICD was shown to have transactivation potential (Cao and Sudhof, 2001). AICD levels are detectable in membrane fractions of murine total brain homogenates, and they increase significantly in mice overexpressing the Swedish mutation of human APP (Ryan and Pimplikar, 2005). Moreover, Hirano bodies found in the degenerating neurons of Alzheimer's patients stain positive with antisera raised against the cytoplasmic domain of APP (Munoz et al., 1993), suggesting an accumulation of AICD during Alzheimer...

In this review we will attempt to summarise the complex and sometimes contradictory effects that mitochondria have on different forms of calcium signalling. Mitochondria can influence Ca(2+) signalling indirectly by changing the concentration of ATP, NAD(P)H, pyruvate and reactive oxygen species - which in turn modulate components of the Ca(2+) signalling machinery i.e. buffering, release from internal stores, influx from the extracellular solution, uptake into cellular organelles and extrusion by plasma membrane Ca(2+) pumps. Mitochondria can directly influence the calcium concentration in the cytosol of the cell by importing Ca(2+) via the mitochondrial Ca(2+) uniporter or transporting Ca(2+) from the interior of the organelle into the cytosol by means of Na+/Ca(2+) or H+/Ca(2+) exchangers. Considerable progress in understanding the relationship between Ca(2+) signalling cascades and mitochondrial physiology has been accumulated over the last few years due to the development of more advanced optical techniques and electrophysiological approaches.

SOCCs (store-operated Ca2+ channels) are highly selective ion channels that are activated upon release of Ca2+ from intracellular stores to regulate a multitude of diverse cellular functions. It was reported previously that Golli-BG21, a member of the MBP (myelin basic protein) family of proteins, regulates SOCE (store-operated Ca2+ entry) in T-cells and oligodendrocyte precursor cells, but the underlying mechanism for this regulation is unknown. In the present study we have discovered that Golli can directly interact with the ER (endoplasmic reticulum) Ca2+-sensing protein STIM1 (stromal interaction molecule 1). Golli interacts with the C-terminal domain of STIM1 in both in vitro and in vivo binding assays and this interaction may be modulated by the intracellular Ca2+ concentration. Golli also co-localizes with full-length STIM1 and Orai1 complexes in HeLa cells following Ca2+ store depletion. Overexpression of Golli reduces SOCE in HeLa cells, but this inhibition is overcome by overexpressing STIM1. We therefore suggest that Golli binds to STIM1–Orai1 complexes to negatively regulate the activity of SOCCs.

Depletion of the endoplasmic reticulum (ER) calcium store triggers translocation of stromal interacting molecule one (STIM1) to the sub-plasmalemmal region and formation of puncta-structures in which STIM1 interacts and activates calcium channels. ATP depletion induced the formation of STIM1 puncta in PANC1, RAMA37, and HeLa cells. The sequence of events triggered by inhibition of ATP production included a rapid decline of ATP, depletion of phosphatidylinositol 4,5-bisphosphate (PI(4,5) P 2 ) and a slow calcium leak from the ER followed by formation of STIM1 puncta. STIM1 puncta induced by ATP depletion were co-localized with clusters of ORAI1 channels. STIM1-ORAI1 clusters that developed as a result of ATP depletion were very poor mediators of Ca 2+ influx. Re-translocation of STIM1 from puncta back to the ER was observed during total ATP depletion. We can therefore conclude that STIM1 translocation and re-translocation as well as formation of STIM1-ORAI1 clusters occur in an ATP-independent fashion and under conditions of PI(4,5)P 2 depletion.

J. Neurochem. (2010) 10.1111/j.1471‐4159.2010.06615.x Abstract The amyloid precursor protein (APP) is critically involved in the pathogenesis of Alzheimer’s disease, and is strongly up‐regulated in response to traumatic, metabolic, or toxic insults to the nervous system. The processing of APP by γ/ε‐secretase activity results in the generation of the APP intracellular domain (AICD). Previously, we have shown that AICD induces the expression of genes (transgelin, α2‐actin) with functional roles in actin organization and dynamics and demonstrated that the induction of AICD and its co‐activator Fe65 (AICD/Fe65) resulted in a loss of organized filamentous actin structures within the cell. As mitochondrial function is thought to be reliant on ordered actin dynamics, we examined mitochondrial function in human SHEP neuroblastoma cells inducibly expressing AICD/Fe65. Confocal analysis of the mitochondrial membrane potential (Δψm) identified a significant decrease in the Δψm in the AICD50/Fe65 over‐expressing cells. This was paralleled by significantly reduced ATP levels and decreased basal superoxide production. Over‐expression of the proposed AICD target gene transgelin in SHEP‐SF parental cells and primary neurons was sufficient to destabilize actin filaments, depolarize Δψm, and significantly alter mitochondrial disrtibution and morphology. Our data demonstrate that the induction of AICD/Fe65 or transgelin significantly alters actin dynamics and mitochondrial function in neuronal cells.

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