Light-evoked recovery from wortmannin-induced inhibition of catecholamine secretion and synaptic transmission

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Dependence of STIM1/Orai1-mediated calcium entry on plasma membrane phosphoinositides

Recent studies identified two main components of store-operated calcium entry (SOCE): the endoplasmic reticulum-localized Ca2+ sensor protein, STIM1, and the plasma membrane (PM)-localized Ca2+ channel, Orai1/CRACM1. In the present study, we investigated the phosphoinositide dependence of Orai1 channel activation in the PM and of STIM1 movements from the tubular to PM-adjacent endoplasmic reticulum regions during Ca2+ store depletion. Phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) levels were changed either with agonist stimulation or by chemically induced recruitment of a phosphoinositide 5-phosphatase domain to the PM, whereas PtdIns4P levels were decreased by inhibition or down-regulation of phosphatidylinositol 4-kinases (PI4Ks). Agonist-induced phospholipase C activation and PI4K inhibition, but not isolated PtdIns(4,5)P(2) depletion, substantially reduced endogenous or STIM1/Orai1-mediated SOCE without preventing STIM1 movements toward the PM upon Ca2+ store depletion. Patch clamp analysis of cells overexpressing STIM1 and Orai1 proteins confirmed that phospholipase C activation or PI4K inhibition greatly reduced I(CRAC) currents. These results suggest an inositide requirement of Orai1 activation but not STIM1 movements and indicate that PtdIns4P rather than PtdIns(4,5)P2 is a likely determinant of Orai1 channel activity.

Maintenance of hormone-sensitive phosphoinositide pools in the plasma membrane requires phosphatidylinositol 4-kinase IIIalpha

Type III phosphatidylinositol (PtdIns) 4-kinases (PI4Ks) have been previously shown to support plasma membrane phosphoinositide synthesis during phospholipase C activation and Ca(2+) signaling. Here, we use biochemical and imaging tools to monitor phosphoinositide changes in the plasma membrane in combination with pharmacological and genetic approaches to determine which of the type III PI4Ks (alpha or beta) is responsible for supplying phosphoinositides during agonist-induced Ca(2+) signaling. Using inhibitors that discriminate between the alpha- and beta-isoforms of type III PI4Ks, PI4KIIIalpha was found indispensable for the production of phosphatidylinositol 4-phosphate (PtdIns4P), phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)], and Ca(2+) signaling in angiotensin II (AngII)-stimulated cells. Down-regulation of either the type II or type III PI4K enzymes by small interfering RNA (siRNA) had small but significant effects on basal PtdIns4P and PtdIns(4,5)P(2) levels in (32)P-labeled cells, but only PI4KIIIalpha down-regulation caused a slight impairment of PtdIns4P and PtdIns(4,5)P(2) resynthesis in AngII-stimulated cells. None of the PI4K siRNA treatments had a measurable effect on AngII-induced Ca(2+) signaling. These results indicate that a small fraction of the cellular PI4K activity is sufficient to maintain plasma membrane phosphoinositide pools, and they demonstrate the value of the pharmacological approach in revealing the pivotal role of PI4KIIIalpha enzyme in maintaining plasma membrane phosphoinositides.

Mode of mitochondrial Ca2+ clearance and its influence on secretory responses in stimulated chromaffin cells

To study the role of mitochondrial Ca(2+) clearance in stimulated cells, changes in free Ca(2+) concentration in the cytosol, [Ca(2+)](c) and that in mitochondria, [Ca(2+)](m) along with secretory responses were observed using chromaffin cells co-loaded with Fura-2 and Rhod-2 in the perfused rat adrenal medulla. When the cells were stimulated with 40 mM K(+) in the perfusate, the duration of [Ca(2+)](m) response markedly increased with prolongation of the stimulation period, exhibiting a mean half-decay time of 21 min with 30s stimulation, whereas its amplitude was not altered with stimulations of 10-30s. A computer simulation analysis showed that such a mode of [Ca(2+)](m) response can be produced if excess Ca(2+) taken up by mitochondria precipitates as calcium phosphate (Pi) salt. In the presence of 5 microM rotenone plus 10 microM oligomycin, a decrease in the duration of [Ca(2+)](m) response and a slight but significant increase (24%) in the secretory response to 30s stimulation with 40 mM K(+) were observed. Simulation analyses suggested that this effect of rotenone may be due to reduction in mitochondrial Ca(2+) uptake induced by rotenone-elicited partial depolarization of the mitochondrial membrane potential. In chromaffin cells transsynaptically stimulated through the splanchnic nerve, the intensity of NAD(P)H autofluorescence changed with time courses similar to those of [Ca(2+)](m) responses. The temporal profiles of those two responses were prolonged in a similar manner by application of an inhibitor of mitochondrial Na(+)/Ca(2+) exchanger, CGP37157. Thus, due to the unique Ca(2+) buffering mechanism, [Ca(2+)](m) responses associated with massive mitochondrial Ca(2+) uptake may occur within a limited concentration range in which Ca(2+)-sensitive dehydrogenases are activated to control the mitochondrial redox state in stimulated chromaffin cells.

Modes of secretagogue-induced [Ca(2+)](i) responses in individual chromaffin cells of the perfused rat adrenal medulla

Chromaffin cells in the perfused rat adrenal medulla were loaded with indo-1 for confocal image analyses. Resting levels of [Ca(2+)](i) in chromaffin cells were similar and were stable with time. This is in contrast to the situation in isolated rat chromaffin cells, in which spontaneous oscillations of [Ca(2+)](i) are known to occur. When chromaffin cells were stimulated for 3-4 min by high K(+) or nicotine, [Ca(2+)](i) increased to a peak in 20-30 s and then declined rather smoothly. In contrast, chromaffin cells stimulated by muscarine or low pH (6.5) commonly exhibited irregular oscillations in [Ca(2+)](i). This provides additional evidence supporting the previous claim that muscarine and low pH evoke catecholamine secretion using partly shared mechanisms. Although muscarine and low pH were speculated to produce weaker responses in noradrenaline-secreting cells due to their selective stimulation of adrenaline secretion, no clear indications for segregation of cell types from [Ca(2+)](i) responses to these stimulants were found. The perfused adrenal medulla loaded with Indo-1 was also employed for simultaneously monitoring integrated changes in [Ca(2+)](i)(Ca responses) by conventional microfluorometry and in catecholamine secretion from a whole medulla (secretory responses). When the profiles of secretory responses were approximated by the kth power of the profiles of Ca responses, the k-values were estimated to be 2.2 and 2.3 for high-K(+)- and nicotine-elicited responses, respectively, whereas a k-value of 1.4 was obtained for both muscarine- and low-pH-elicited responses. An analysis showed that the significant difference in the k-value with these two classes of stimulants is accounted for by the stimulant-dependent patterns of [Ca(2+)](i) responses found in confocal image analysis.

Mechanism by which wortmannin and LY294002 inhibit catecholamine secretion in the rat adrenal medullary cells

The effects of wortmannin and LY294002, inhibitors of PI(3)-kinase, in secretagogue-stimulated rat adrenal chromaffin cells loaded with Calcium Green-1 were studied by simultaneously measuring changes in the fluorescence intensity of the indicator (Ca-response) and in the release of catecholamine (secretory response). Before application of these agents, the profile of the secretory response evoked by a 10-min stimulation with 30 mM K(+)] was approximated by the k th (2.6 on average) power of that of the Ca-response. Both agents dose-dependently inhibited the high-K(+)-elicited Ca-response and secretory response in a similar mode to which the k th power relation was preserved despite the occurrence of profound changes in the shapes and sizes of these two responses. The L-type Ca(2+)-channel blocker PN200-110 inhibited the high-K(+)-evoked responses in a similar fashion. Thus, it is likely that wortmannin and LY294002 inhibit high-K(+)-evoked CA secretion by inhibiting a Ca(2+)-influx through voltage-dependent Ca(2+)channels. Although regulation of L-type Ca(2+)channel activity via PI(3)-kinase has been reported in vascular myocytes, this possibility may be limited in the present case since the doses of LY294002 and wortmannin used to inhibit the secretory response are much higher than IC(50)'s for inhibition of PI(3)-kinase with these agents. Compared with the high-K(+)-elicited responses, muscarine-evoked Ca-responses and secretory responses were more strongly inhibited by wortmannin, but less affected by LY294002. The differential effects suggest that the inhibition of the muscarine-evoked secretion by these agents i s not associated with the inhibition of PI(3)-kinase.