Kinetics of the non-specific calcium leak from non-mitochondrial calcium stores in permeabilized A7r5 cells

The endoplasmic reticulum pool of Bcl-xL prevents cell death through IP3R-dependent calcium release

Apoptosis plays a role in cell homeostasis in both normal development and disease. Bcl-xL, a member of the Bcl-2 family of proteins, regulates the intrinsic mitochondrial pathway of apoptosis. It is overexpressed in several cancers. Bcl-xL has a dual subcellular localisation and is found at the mitochondria as well as the endoplasmic reticulum (ER). However, the biological significance of its ER localisation is unclear. In order to decipher the functional contributions of the mitochondrial and reticular pools of Bcl-xL, we generated genetically modified mice expressing exclusively Bcl-xL at the ER, referred to as ER-xL, or the mitochondria, referred to as Mt-xL. By performing cell death assays, we demonstrated that ER-xL MEFs show increased vulnerability to apoptotic stimuli but are more resistant to ER stress. Furthermore, ER-xL MEFs displayed reduced 1,4,5-inositol trisphosphate receptor (IP3R)-mediated ER calcium release downstream of Phospholipase C activation. Collectively, our data indicate that upon ER stress, Bcl-xL negatively regulates IP3R-mediated calcium flux from the ER, which prevents ER calcium depletion and maintains the UPR and subsequent cell death in check. This work reveals a moonlighting function of Bcl-xL at the level of the ER, in addition to its well-known role in regulating apoptosis through the mitochondria.

Homeostasis at different backgrounds: The roles of overlayed feedback structures in vertebrate photoadaptation

We have studied the resetting behavior of eight basic integral controller motifs with respect to different but constant backgrounds. We found that the controllers split symmetrically into two classes: one class, based on derepression of the compensatory flux, leads to more rapid resetting kinetics as backgrounds increase. The other class, which directly activates the compensatory flux, shows a slowing down in the resetting at increased backgrounds. We found a striking analogy between the resetting kinetics of vertebrate photoreceptors and controllers based on derepression, i.e. vertebrate rod or cone cells show decreased sensitivities and accelerated response kinetics as background illuminations increase. The central molecular model of vertebrate photoadaptation consists of an overlay of three negative feedback loops with cytosolic calcium ([Formula: see text]), cyclic guanosine monophosphate (cGMP) and cyclic nucleotide-gated (CNG) channels as components. While in one of the feedback loops the extrusion of [Formula: see text] by potassium-dependent sodium-calcium exchangers (NCKX) can lead to integral control with cGMP as the controlled variable, the expected robust perfect adaptation of cGMP is lost, because of the two other feedback loops. They avoid that [Formula: see text] levels become too high and toxic. Looking at psychophysical laws, we found that in all of the above mentioned basic controllers Weber's law is followed when a "just noticeable difference" (threshold) of 1% of the controlled variable's set-point was considered. Applying comparable threshold pulses or steps to the photoadaptation model we find, in agreement with experimental results, that Weber's law is followed for relatively high backgrounds, while Stephens' power law gives a better description when backgrounds are low. Limitations of our photoadaption model, in particular with respect to potassium/sodium homeostasis, are discussed. Finally, we discuss possible implication of background perturbations in biological controllers when compensatory fluxes are based on activation.

Ca2+ signalling system initiated by endoplasmic reticulum stress stimulates PERK activation

The accumulation of unfolded proteins within the Endoplasmic Reticulum (ER) activates a signal transduction pathway termed the unfolded protein response (UPR), which attempts to restore ER homoeostasis. If this cannot be done, UPR signalling ultimately induces apoptosis. Ca²⁺ depletion in the ER is a potent inducer of ER stress. Despite the ubiquity of Ca²⁺ as an intracellular messenger, the precise mechanism(s) by which Ca²⁺ release affects the UPR remains unknown. Tethering a genetically encoded Ca²⁺ indicator (GCamP6) to the ER membrane revealed novel Ca²⁺ signalling events initiated by Ca²⁺ microdomains in human astrocytes under ER stress, induced by tunicamycin (Tm), an N-glycosylation inhibitor, as well as in a cell model deficient in all three inositol triphosphate receptor isoforms. Pharmacological and molecular studies indicate that these local events are mediated by translocons and that the Ca²⁺ microdomains impact (PKR)-like-ER kinase (PERK), an UPR sensor, activation. These findings reveal the existence of a Ca²⁺ signal mechanism by which stressor-mediated Ca²⁺ release regulates ER stress.

Measurement of intracellular Ca2+ release in permeabilized cells using 45Ca2+

This protocol describes a technique to measure Ca(2+) release from the nonmitochondrial intracellular Ca(2+) stores in monolayers of saponin-permeabilized cells cultured in 12-well 4-cm(2) clusters. The (45)Ca(2+)-flux technique described here can only be applied to cell types that still adhere to the plastic after exposing them to saponin. We describe the permeabilization procedure, the loading of the nonmitochondrial Ca(2+) stores with (45)Ca(2+), and the subsequent (45)Ca(2+) efflux.

mTOR-Controlled Autophagy Requires Intracellular Ca(2+) Signaling

Autophagy is a lysosomal degradation pathway important for cellular homeostasis and survival. Inhibition of the mammalian target of rapamycin (mTOR) is the best known trigger for autophagy stimulation. In addition, intracellular Ca²⁺ regulates autophagy, but its exact role remains ambiguous. Here, we report that the mTOR inhibitor rapamycin, while enhancing autophagy, also remodeled the intracellular Ca²⁺-signaling machinery. These alterations include a) an increase in the endoplasmic-reticulum (ER) Ca²⁺-store content, b) a decrease in the ER Ca²⁺-leak rate, and c) an increased Ca²⁺ release through the inositol 1,4,5-trisphosphate receptors (IP₃Rs), the main ER-resident Ca²⁺-release channels. Importantly, buffering cytosolic Ca²⁺ with BAPTA impeded rapamycin-induced autophagy. These results reveal intracellular Ca²⁺ signaling as a crucial component in the canonical mTOR-dependent autophagy pathway.

Ins(1,4,5)P3 receptor-mediated Ca2+ signaling and autophagy induction are interrelated

The role of intracellular Ca2+ signaling in starvation-induced autophagy remains unclear. Here, we examined Ca2+ dynamics during starvation-induced autophagy and the underlying molecular mechanisms. Tightly correlating with autophagy stimulation, we observed a remodeling of the Ca2+ signalosome. First, short periods of starvation (1 to 3 h) caused a prominent increase of the ER Ca2+-store content and enhanced agonist-induced Ca2+ release. The mechanism involved the upregulation of intralumenal ER Ca2+-binding proteins, calreticulin and Grp78/BiP, which increased the ER Ca2+-buffering capacity and reduced the ER Ca2+ leak. Second, starvation led to Ins(1,4,5)P3R sensitization. Immunoprecipitation experiments showed that during starvation Beclin 1, released from Bcl-2, first bound with increasing efficiency to Ins(1,4,5)P3Rs; after reaching a maximal binding after 3 h, binding, however, decreased again. The interaction site of Beclin 1 was determined to be present in the N-terminal Ins(1,4,5)P3-binding domain of the Ins(1,4,5)P3R. The starvation-induced Ins(1,4,5)P3R sensitization was abolished in cells treated with BECN1 siRNA, but not with ATG5 siRNA, pointing toward an essential role of Beclin 1 in this process. Moreover, recombinant Beclin 1 sensitized Ins(1,4,5)P3Rs in 45Ca2+-flux assays, indicating a direct regulation of Ins(1,4,5)P3R activity by Beclin 1. Finally, we found that Ins(1,4,5)P3R-mediated Ca2+ signaling was critical for starvation-induced autophagy stimulation, since the Ca2+ chelator BAPTA-AM as well as the Ins(1,4,5)P3R inhibitor xestospongin B abolished the increase in LC3 lipidation and GFP-LC3-puncta formation. Hence, our results indicate a tight and essential interrelation between intracellular Ca2+ signaling and autophagy stimulation as a proximal event in response to starvation.

Intracellular Ca2+ storage in health and disease: a dynamic equilibrium

Homeostatic control of the endoplasmic reticulum (ER) both as the site for protein handling (synthesis, folding, trafficking, disaggregation and degradation) and as a Ca2+ store is of crucial importance for correct functioning of the cell. Disturbance of the homeostatic control mechanisms leads to a vast array of severe pathologies. The Ca2+ content of the ER is a dynamic equilibrium between active uptake via Ca2+ pumps and Ca2+ release by a number of highly regulated Ca2+-release channels. Regulation of the Ca2+-release channels is very complex and several mechanisms are still poorly understood or controversial. There is increasing evidence that a number of unrelated proteins, either by themselves or in association with other Ca2+ channels, can provide additional Ca2+-leak pathways. The ER is a dynamic organelle and changes in its size and components have been described, either as a result of (de)differentiation processes affecting the secretory capacity of cells, or as a result of adaptation mechanisms to diverse stress conditions such as the unfolded protein response and autophagy. In this review we want to give an overview of the current knowledge of the (short-term) regulatory mechanisms that affect Ca2+-release and Ca2+-leak pathways and of the (long-term) adaptations in ER size and capacity. Understanding of the consequences of these mechanisms for cellular Ca2+ signaling could provide a huge therapeutic potential.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Both translocon and a cation channel are involved in the passive Ca2+ leak from the endoplasmic reticulum: A mechanistic study on rat liver microsomes

Steady-state levels of calcium ions in endoplasmic reticulum reflect a balance between active inward transport, mediated by MgATP-dependent Ca(2+) pumps, and passive backflux of the ions, through putative "leak channels". We have investigated the efflux of Ca(2+) from rat liver microsomal vesicles, passively pre-equilibrated in the presence radiolabelled Ca(2+). Similarly, we have also evaluated the efflux of a low-Mwt uncharged compound, i.e., sucrose. The results show that two major passive Ca(2+) efflux pathways exist. One appeared to involve the translocon pore, since it was stimulated by the translocon opener puromycin, and also allowed the passage of sucrose. Putative channels likely mediated the other one, since it required counter ion influx and was inhibited by Gd(3+) and La(3+). The latter pathway did not appear to involve inactive Ca(2+) pumps, Bcl2 proteins, or known channels, such as the InsP3 and ryanodine receptors. While sucrose efflux was highly represented in a rough microsomal subfraction--enriched in the translocon component Sec61alpha--the efflux of Ca(2+) was represented both in smooth and in rough microsomes. We conclude that the passive efflux of Ca(2+) from the (liver) ER could be mediated by both the translocon pore and putative Ca(2+) leak channels. However, the relative role of these Ca(2+) efflux pathways in the intact cell as well as the molecular nature of the Ca(2+) leak channel(s) remain to be clarified.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Calcium leak from intracellular stores--the enigma of calcium signalling

Wherever you travel through the cytoplasm of the cells you will find organelles with internal [Ca(2+)] levels higher than in the surrounding cytosol. This is particularly true of the endoplasmic reticulum (ER) (or sarcoplasmic reticulum (SR) in muscle cells); such organelles serve as the main sources of releasable Ca(2+) for cytosolic cellular signalling. Calcium pumps of the SERCA family (sarcoplasmic and endoplasmic reticulum calcium ATP-ases) import calcium into the organelle lumen. The other mechanism that is responsible for the steady state calcium level within the lumen of ER or SR is a calcium leak that balances the influx created by the pumps. The leak remains the most enigmatic of the processes involved in calcium regulation. The molecular nature of the leak mechanism is not known. The basal leak is a relatively slow process, which is difficult to investigate and which is easily outmatched (both in the amplitude of calcium responses and in attractiveness to experimenters) by substantially faster second messenger-induced release. Nevertheless, information on the properties of the calcium leak, although thinly scattered through the pages of PubMed, has been slowly accumulating. In this review we will discuss the properties of the calcium leak and speculate about possible mechanisms, which could mediate this process.

Basal and physiological Ca(2+) leak from the endoplasmic reticulum of pancreatic acinar cells. Second messenger-activated channels and translocons

We have studied the Ca(2+) leak pathways in the endoplasmic reticulum of pancreatic acinar cells by directly measuring Ca(2+) in the endoplasmic reticulum ([Ca(2+)](ER)). Cytosolic Ca(2+) ([Ca(2+)](C)) was clamped to the resting level by a BAPTA-Ca(2+) mixture. Administration of cholecystokinin within the physiological concentration range caused a graded decrease of [Ca(2+)](ER), and the rate of Ca(2+) release generated by 10 pm cholecystokinin is at least 3x as fast as the basal Ca(2+) leak revealed by inhibition of the endoplasmic reticulum Ca(2+)-ATPase. Acetylcholine also evokes a dose-dependent decrease of [Ca(2+)](ER), with an EC(50) of 0.98 +/- 0.06 microm. Inhibition of receptors for inositol 1,4,5-trisphosphate (IP(3)) by heparin or flunarizine blocks the effect of acetylcholine but only partly blocks the effect of cholecystokinin. 8-NH(2) cyclic ADP-ribose (20 microm) inhibits the action of cholecystokinin, but not of acetylcholine(.) The basal Ca(2+) leak from the endoplasmic reticulum is not blocked by antagonists of the IP(3) receptor, the ryanodine receptor, or the receptor for nicotinic acid adenine dinucleotide phosphate. However, treatment with puromycin (0.1-1 mm) to remove nascent polypeptides from ribosomes increases Ca(2+) leak from the endoplasmic reticulum by a mechanism independent of the endoplasmic reticulum Ca(2+) pumps and of the receptors for IP(3) or ryanodine.

Washing out of lipophilic compounds induces a transient increase in the passive Ca(2+) leak in permeabilized A7r5 cells

We have investigated how the immunosuppressant drug FK506 affected the basal Ca(2+) leak in permeabilized A7r5 cells. Non-mitochondrial Ca(2+) stores loaded to steady state with Ca(2+) slowly lost their accumulated Ca(2+) during incubation in a Ca(2+)-free efflux medium. FK506 up to 100 microM had no effect on the basal Ca(2+) leak. In contrast, the rate of Ca(2+) release proceeded much faster immediately after washing out FK506. The increase in rate of Ca(2+) release after washing out of this compound depended on both its initial concentration and on the time of pre-incubation. A similar effect was also observed after removing another immunosuppressant drug (rapamycin) and after removing the inositol 1,4,5-trisphosphate receptor inhibitor xestospongin C. Since all these substances have a high octanol/H(2)O partition coefficient and accumulate in the endoplasmic reticulum membrane, we suggest that the transient increase in the basal Ca(2+) leak is due to the sudden removal of these lipophilic substances from the membrane.

Ca2+ uptake and release properties of a thapsigargin-insensitive nonmitochondrial Ca2+ store in A7r5 and 16HBE14o- cells

In a previous study we overexpressed the thapsigargin (tg)-insensitive Pmr1 Ca(2+) pump of the Golgi apparatus of Caenorhabditis elegans in COS-1 cells and studied the properties of the Ca(2+) store into which it was integrated. Here we assessed the properties of an endogenous tg-insensitive nonmitochondrial Ca(2+) store in A7r5 and 16HBE14o- cells, which express a mammalian homologue of Pmr1. The tg-insensitive Ca(2+) store was considerably less leaky for Ca(2+) than the sarco(endo)plasmic-reticulum Ca(2+)-ATPase (SERCA)-containing Ca(2+) store. Moreover like for the worm Pmr1 Ca(2+) pump expressed in COS-1 cells, Ca(2+) accumulation into the endogenous tg-insensitive store showed a 2 orders of magnitude lower sensitivity to cyclopiazonic acid than the SERCA-mediated transport. 2,5-Di-(tert-butyl)-1,4-benzohydroquinone was only a very weak inhibitor of the tg-insensitive Ca(2+) uptake in A7r5 and 16HBE14o- cells and in COS-1 cells overexpressing the worm Pmr1. Inositol 1,4,5-trisphosphate released 11% of the Ca(2+) accumulated in permeabilized A7r5 cells pretreated with tg with an EC(50) that was 5 times higher than for the SERCA-containing Ca(2+) store but failed to release Ca(2+) in 16HBE14o- cells. In the presence of tg, 15% of intact A7r5 cells responded to 10 microm arginine-vasopressin with a small rise in cytosolic Ca(2+) concentration after a long latency. In conclusion, A7r5 and 16HBE14o- cells express a Pmr1-containing Ca(2+) store with properties that differ substantially from the SERCA-containing Ca(2+) store.

2-Aminoethoxydiphenyl borate affects the inositol 1,4,5-trisphosphate receptor, the intracellular Ca2+ pump and the non-specific Ca2+ leak from the non-mitochondrial Ca2+ stores in permeabilized A7r5 cells

2-Aminoethoxydiphenyl borate (2APB) is a membrane-permeable blocker of the inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release in bi-directional Ca2+ -flux conditions. We have now studied the effects of 2APB on the 45Ca2+ uptake into, and on the basal and IP(3)-stimulated unidirectional 45Ca2+ efflux from the non-mitochondrial Ca2+ stores in permeabilized A7r5 smooth-muscle cells. 2APB inhibited the IP3 -induced Ca2+ release, with a half maximal inhibition at 36 microM 2APB, without affecting [3H]IP3 binding to the receptor. This inhibition did not depend on the IP3, ATP or free Ca2+ concentration. The Ca2+ pumps of the non-mitochondrial Ca2+ stores were half-maximally inhibited at 91microM 2APB. Higher concentrations of 2APB increased the non-specific leak of Ca2+ from the stores. We conclude that 2APB can not be considered as a selective blocker of the IP3 -induced Ca2+ release. Our results can explain the various effects of 2APB observed in intact cells.

Luminal Ca2+ regulates passive Ca2+ efflux from the intracellular stores of hepatocytes

Ca2+ uptake into the intracellular stores of permeabilized hepatocytes was entirely dependent on ATP and substantially inhibited by either ionomycin or thapsigargin, although both were required for complete inhibition. Unidirectional efflux of 45Ca2+ after removal of ATP from cells loaded to steady state (1.60+/-0.12 nmol/10(6) cells) was monoexponential and occurred with a half-time of 103+/-10 s. However, the 45Ca2+ content of the stores did not return to their pre-ATP level, but reached a plateau at 0.12+/-0.04 nmol/10(6) cells. A similar amount of Ca2+ was trapped within the stores when Ca2+ uptake was prevented by thapsigargin and chelation of Ca2+; at all temperatures between 2 degreesC and 37 degreesC; and after stores had first been loaded with unlabelled Ca2+. Simultaneous addition of inositol 1,4,5-trisphosphate (InsP3) and inhibition of Ca2+ uptake reduced the amount of trapped Ca2+ to a level consistent with InsP3 rapidly and more completely emptying a fraction of the stores that could be only partially emptied by the passive leak. After dilution of the specific activity of the 45Ca2+ under conditions that maintained the steady-state activities of the pumps and leaks, the stores rapidly lost their entire 45Ca2+ content. We conclude that the channel responsible for mediating the leak of Ca2+ abruptly closes when the luminal [Ca2+] of the intracellular stores falls below a critical threshold corresponding to about 7% of their steady-state loading. Whereas InsP3 is capable of completely emptying a fraction of the stores, regulation of the passive leak by luminal [Ca2+] is likely to prevent it from completely emptying them; such regulation may ensure that the many other functions of Ca2+ within the endoplasmic reticulum are not compromised.

Rapid activation and partial inactivation of inositol trisphosphate receptors by inositol trisphosphate

During superfusion of permeabilized hepatocytes, submaximal concentrations of inositol 1,4,5-trisphosphate (InsP3) evoked quantal Ca2+ mobilization: a rapid acceleration in the rate of 45Ca2+ release abruptly followed by a biphasic decline to the basal rate before the InsP3-sensitive stores had fully emptied. During the fast component of the decay, the Ca2+ permeability of the stores fell rapidly by 40% (t1/2 = 250 ms) to a state indistinguishable from that evoked by preincubation with InsP3 under conditions that prevented Ca2+ mobilization. This change was accompanied by a decrease in the InsP3 dissociation rate: the response declined more quickly when InsP3 was removed during the initial stages of a response than later. We suggest that InsP3 directly causes its receptor to rapidly switch (t1/2 = 250 ms) between a low-affinity (Kd approximately 1 microM) active, and a higher-affinity (Kd approximately 100 nM) less active, conformation, and that this transition underlies the fast component of the decaying phase of Ca2+ release. Ca2+ continues to leak through the unchanging less active state of the receptor until those stores that responded initially are completely empty, accounting for the slow phase of the response. The requirements for activation of InsP3 receptors are more stringent (InsP3 and then Ca2+ binding) than those for partial inactivation (InsP3 binding); rapid inactivation is therefore likely to determine whether the cytosolic [Ca2+] reaches the threshold for regenerative Ca2+ signals.