Maud Frieden

Ca2+ Homeostasis during Mitochondrial Fragmentation and Perinuclear Clustering Induced by hFis1

Mitochondria modulate Ca2+ signals by taking up, buffering, and releasing Ca2+ at key locations near Ca2+ release or influx channels. The role of such local interactions between channels and organelles is difficult to establish in living cells because mitochondria form an interconnected network constantly remodeled by coordinated fusion and fission reactions. To study the effect of a controlled disruption of the mitochondrial network on Ca2+ homeostasis, we took advantage of hFis1, a protein that promotes mitochondrial fission by recruiting the dynamin-related protein, Drp1. hFis1 expression in HeLa cells induced a rapid and complete fragmentation of mitochondria, which redistributed away from the plasma membrane and clustered around the nucleus. Despite the dramatic morphological alteration, hFis1-fragmented mitochondria maintained a normal transmembrane potential and pH and took up normally the Ca2+ released from intracellular stores upon agonist stimulation, as measured with a targeted ratiometric pericam probe. In contrast, hFis1-fragmented mitochondria took up more slowly the Ca2+ entering across plasma membrane channels, because the Ca2+ ions reaching mitochondria propagated faster and in a more coordinated manner in interconnected than in fragmented mitochondria. In parallel cytosolic fura-2 measurements, the capacitative Ca2+ entry (CCE) elicited by store depletion was only marginally reduced by hFis1 expression. Regardless of mitochondria shape and location, disruption of mitochondrial potential with uncouplers or oligomycin/rotenone reduced CCE by ∼35%. These observations indicate that close contact to Ca2+ influx channels is not required for CCE modulation and that the formation of a mitochondrial network facilitates Ca2+ propagation within interconnected mitochondria.

Sustained Ca2+ Transfer across Mitochondria Is Essential for Mitochondrial Ca2+ Buffering, Store-operated Ca2+ Entry, and Ca2+ Store Refilling

Mitochondria have been found to sequester and release Ca2+ during cell stimulation with inositol 1,4,5-triphosphate-generating agonists, thereby generating subplasmalemmal microdomains of low Ca2+ that sustain activity of capacitative Ca2+ entry (CCE). Procedures that prevent mitochondrial Ca2+ uptake inhibit local Ca2+ buffering and CCE, but it is not clear whether Ca2+ has to transit through or remains trapped in the mitochondria. Thus, we analyzed the contribution of mitochondrial Ca2+ efflux on the ability of mitochondria to buffer subplasmalemmal Ca2+, to maintain CCE, and to facilitate endoplasmic reticulum (ER) refilling in endothelial cells. Upon the addition of histamine, the initial mitochondrial Ca2+ transient, monitored with ratio-metric-pericam-mitochondria, was largely independent of extracellular Ca2+. However, subsequent removal of extracellular Ca2+ produced a reversible decrease in [Ca2+]mito, indicating that Ca2+ was continuously taken up and released by mitochondria, although [Ca2+]mito had returned to basal levels. Accordingly, inhibition of the mitochondrial Na+/Ca2+ exchanger with CGP 37157 increased [Ca2+]mito and abolished the ability of mitochondria to buffer subplasmalemmal Ca2+, resulting in an increased activity of BKCa channels and a decrease in CCE. Hence, CGP 37157 also reversibly inhibited ER refilling during cell stimulation. These effects of CGP 37157 were mimicked if mitochondrial Ca2+ uptake was prevented with oligomycin/antimycin A. Thus, during cell stimulation a continuous Ca2+ flux through mitochondria underlies the ability of mitochondria to generate subplasmalemmal microdomains of low Ca2+, to facilitate CCE, and to relay Ca2+ from the plasma membrane to the ER.

Mitochondria and Ca2+ signaling: old guests, new functions

Mitochondria are ancient endosymbiotic guests that joined the cells in the evolution of complex life. While the unique ability of mitochondria to produce adenosine triphosphate (ATP) and their contribution to cellular nutrition metabolism received condign attention, our understanding of the organelle’s contribution to Ca2+ homeostasis was restricted to serve as passive Ca2+ sinks that accumulate Ca2+ along the organelle’s negative membrane potential. This paradigm has changed radically. Nowadays, mitochondria are known to respond to environmental Ca2+ and to contribute actively to the regulation of spatial and temporal patterns of intracellular Ca2+ signaling. Accordingly, mitochondria contribute to many signal transduction pathways and are actively involved in the maintenance of capacitative Ca2+ entry, the accomplishment of Ca2+ refilling of the endoplasmic reticulum and Ca2+-dependent protein folding. Mitochondrial Ca2+ homeostasis is complex and regulated by numerous, so far, genetically unidentified Ca2+ channels, pumps and exchangers that concertedly accomplish the organelle’s Ca2+ demand. Notably, mitochondrial Ca2+ homeostasis and functions are crucially influenced by the organelle’s structural organization and motility that, in turn, is controlled by matrix/cytosolic Ca2+. This review intends to provide a condensed overview on the molecular mechanisms of mitochondrial Ca2+ homeostasis (uptake, buffering and storage, extrusion), its modulation by other ions, kinases and small molecules, and its contribution to cellular processes as fundamental basis for the organelle’s contribution to signaling pathways. Hence, emphasis is given to the structure-to-function and mobility-to-function relationship of the mitochondria and, thereby, bridging our most recent knowledge on mitochondria with the best-established mitochondrial function: metabolism and ATP production.

The Role of Mitochondria for Ca2+ Refilling of the Endoplasmic Reticulum

Endoplasmic reticulum (ER) Ca2+ refilling is an active process to ensure an appropriate ER Ca2+ content under basal conditions and to maintain or restore ER Ca2+ concentration during/after cell stimulation. The mechanisms to achieve successful ER Ca2+ refilling are multiple and built on a concerted action of processes that provide a suitable reservoir for Ca2+ sequestration into the ER. Despite mitochondria having been found to play an essential role in the maintenance of capacitative Ca2+ entry by buffering subplasmalemmal Ca2+, their contribution to ER Ca2+ refilling was not subjected to detailed analysis so far. Thus, this study was designed to elucidate the involvement of mitochondria in Ca2+ store refilling during and after cell stimulation. ER Ca2+ refilling was found to be accomplished even during continuous inositol 1,4,5-trisphosphate (IP3)-triggered ER Ca2+ release by an agonist. Basically, ER Ca2+ refilling depended on the presence of extracellular Ca2+ as the source and sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) activity. Interestingly, in the presence of an IP3-generating agonist, ER Ca2+ refilling was prevented by the inhibition of trans-mitochondrial Ca2+ flux by CGP 37157 (7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one) that precludes the mitochondrial Na+/Ca2+ exchanger as well as by mitochondrial depolarization using a mixture of oligomycin and antimycin A. In contrast, after the removal of the agonist, ER refilling was found to be largely independent of trans-mitochondrial Ca2+ flux. Under these conditions, ER Ca2+ refilling took place even without an associated Ca2+ elevation in the deeper cytosol, thus, indicating that superficial ER domains mimic mitochondrial Ca2+ buffering and efficiently sequester subplasmalemmal Ca2+ and consequently facilitate capacitative Ca2+ entry. Hence, these data point to different contribution of mitochondria in the process of ER Ca2+ refilling based on the presence or absence of IP3, which represents the turning point for the dependence or autonomy of ER Ca2+ refilling from trans-mitochondrial Ca2+ flux.

EPOXYEICOSATRIENOIC ACIDS ACTIVATE A HIGH-CONDUCTANCE, CA2+-DEPENDENT K+ CHANNEL ON PIG CORONARY ARTERY ENDOTHELIAL CELLS

1 Epoxyeicosatrienoic acids (EETs) have been described as endothelium‐derived hyper‐polarizing factors (EDHFs), based on their stimulatory effects on smooth muscle K+channels. In order to reveal a putative autocrine effect of EETs on endothelial channels, we have studied the effects of the four EET regioisomers (5,6‐EET, 8,9‐EET, 11,12‐EET and 14,15‐EET) on the high‐conductance, Ca2+‐dependent K+ (BKCa) channel recorded in inside‐out patches of primary cultured pig coronary artery endothelial cells. Currents were recorded in the presence of either 500 nm or 1 μm free Ca2+ on the cytosolic side of the membrane. 2 In 81% of experiments, EETs at < 156 nm, applied on the cytosolic side of the membrane, transiently increased BKCa channel open state probability (P0) without affecting its unitary conductance, thus providing evidence for direct action of EETs, without involvement of a cytosolic transduction pathway. 3 The four EET regioisomers appeared to be equally active, multiplying the BKCa channel Po by a mean factor of 4.3 ± 0.6 (n= 15), and involving an increase in the number and duration of openings. 4 The EET‐induced increase in BKCa channel activity was more pronounced with low initial Po. When the BKCa channel was activated by 500 nm Ca2+, application of EETs increased the initial Po value of below 0.1 by a factor of 5. When the channel was activated by 1 μm Ca2+, application of EETs increased the initial Po value by a factor of 3. 5 Our results show that EETs potentiate endothelial BKCa channel activation by Ca2+. The autocrine action of EETs on endothelial cells, which occurs in the same concentration range as their action on muscle cells, should therefore fully participate in the vasoactive effects of EETs, and thus be taken into account when considering their putative EDHF function.

Feature Article: mTOR complex 2-Akt signaling at mitochondria-associated endoplasmic reticulum membranes (MAM) regulates mitochondrial physiology.

The target of rapamycin (TOR) is a highly conserved protein kinase and a central controller of growth. Mammalian TOR complex 2 (mTORC2) regulates AGC kinase family members and is implicated in various disorders, including cancer and diabetes. Here we report that mTORC2 is localized to the endoplasmic reticulum (ER) subcompartment termed mitochondria-associated ER membrane (MAM). mTORC2 localization to MAM was growth factor-stimulated, and mTORC2 at MAM interacted with the IP3 receptor (IP3R)-Grp75–voltage-dependent anion-selective channel 1 ER-mitochondrial tethering complex. mTORC2 deficiency disrupted MAM, causing mitochondrial defects including increases in mitochondrial membrane potential, ATP production, and calcium uptake. mTORC2 controlled MAM integrity and mitochondrial function via Akt mediated phosphorylation of the MAM associated proteins IP3R, Hexokinase 2, and phosphofurin acidic cluster sorting protein 2. Thus, mTORC2 is at the core of a MAM signaling hub that controls growth and metabolism.

STIM1 Knockdown Reveals That Store-operated Ca2+ Channels Located Close to Sarco/Endoplasmic Ca2+ ATPases (SERCA) Pumps Silently Refill the Endoplasmic Reticulum

Stromal interaction molecule (STIM) proteins are putative ER Ca2+ sensors that recruit and activate store-operated Ca2+ (SOC) channels at the plasma membrane, a process triggered by the Ca2+ depletion of the endoplasmic reticulum (ER). To test whether STIM1 is required for ER refilling, we used RNA interference and measured Ca2+ signals in the cytosol, the ER, and the mitochondria of HeLa cells. Knockdown of STIM1 (mRNA levels, 73%) reduced SOC entry by 73% when sarco/endoplasmic Ca2+ ATPases (SERCA) were inhibited by thapsigargin but did not prevent Ca2+ stores refilling when cells were stimulated by physiological agonists. Stores could be fully refilled by increasing the external Ca2+ concentration above physiological values, but no cytosolic Ca2+ signals were detected during store refilling even at very high Ca2+ concentrations. [Ca2+]ER measurements revealed that the basal activity of SERCA was not affected in STIM1 knockdown cells and that [Ca2+]ER levels were restored within 2 min in physiological saline following store depletion. Mitochondrial inhibitors reduced ER refilling in wild-type but not in STIM1 knockdown cells, indicating that ER refilling does not require functional mitochondria at low STIM1 levels. Our data show that ER refilling is largely preserved at reduced STIM1 levels, despite a drastic reduction of store-operated Ca2+ entry, because Ca2+ ions are directly transferred from SOC channels to SERCA. These findings are consistent with the formation of microdomains containing not only SOC channels on the plasma membrane and STIM proteins on the ER but also SERCA pumps and mitochondria to refill the ER without perturbing the cytosol.

Regulation of plasma membrane calcium fluxes by mitochondria

The role of mitochondria in cell signaling is becoming increasingly apparent, to an extent that the signaling role of mitochondria appears to have stolen the spotlight from their primary function as energy producers. In this chapter, we will review the ionic basis of calcium handling by mitochondria and discuss the mechanisms that these organelles use to regulate the activity of plasma membrane calcium channels and transporters.

Subplasmalemmal mitochondria modulate the activity of plasma membrane Ca2+-ATPases.

Mitochondria are dynamic organelles that modulate cellular Ca2+ signals by interacting with Ca2+ transporters on the plasma membrane or the endoplasmic reticulum (ER). To study how mitochondria dynamics affects cell Ca2+ homeostasis, we overexpressed two mitochondrial fission proteins, hFis1 and Drp1, and measured Ca2+ changes within the cytosol and the ER in HeLa cells. Both proteins fragmented mitochondria, decreased their total volume by 25-40%, and reduced the fraction of subplasmalemmal mitochondria by 4-fold. The cytosolic Ca2+ signals elicited by histamine were unaltered in cells lacking subplasmalemmal mitochondria as long as Ca2+ was present in the medium, but the signals were significantly blunted when Ca2+ was removed. Upon Ca2+ withdrawal, the free ER Ca2+ concentration decreased rapidly, and hFis1 cells were unable to respond to repetitive histamine stimulations. The loss of stored Ca2+ was due to an increased activity of plasma membrane Ca2+-ATPase (PMCA) pumps and was associated with an increased influx of Ca2+ and Mn2+ across store-operated Ca2+ channels. The increased Ca2+ influx compensated for the loss of stored Ca2+, and brief Ca2+ additions between successive agonist stimulations fully corrected subsequent histamine responses. We propose that the lack of subplasmalemmal mitochondria disrupts the transfer of Ca2+ from plasma membrane channels to the ER and that the resulting increase in subplasmalemmal [Ca2+] up-regulates the activity of PMCA. The increased Ca2+ extrusion promotes ER depletion and the subsequent activation of store-operated Ca2+ channels. Cells thus adapt to the lack of subplasmalemmal mitochondria by relying on external rather than on internal Ca2+ for signaling.

An intercellular regenerative calcium wave in porcine coronary artery endothelial cells in primary culture

1 A regenerative calcium wave is an increase in cytosolic free calcium concentration ([Ca2+]i) which extends beyond the stimulated cells without decrement of amplitude, kinetics of [Ca2+]i increase and speed of propagation. 2 The aim of the present study was to test the hypothesis that such a wave could be evoked by bradykinin stimulation and by scraping cultured endothelial cells from porcine coronary arteries. 3 Calcium imaging was performed using the calcium‐sensitive dye fura‐2. A wound or a delivery of bradykinin to two to three cells on growing clusters of ≈300 cells caused an increase in [Ca2+]i which was propagated throughout the cluster in a regenerative manner over distances up to 400 μm. This wave spread through gap junctions since it was inhibited by the cell uncoupler palmitoleic acid. 4 The same experiments performed in confluent cultures caused a rise in [Ca2+]i which failed to propagate in a regenerative way. The wave propagation probably failed because the confluent cells were less dye coupled than the growing cells. This was confirmed by immunohistology which detected a dramatic decrease in the number of connexin 40 gap junctions in the confluent cultures. 5 The regenerative propagation of the wave was blocked by inhibitors of calcium‐induced calcium release (CICR) and phospholipase C (PLC), and by suppression of extracellular calcium, but not by clamping the membrane potential with high‐potassium solution. 6 We conclude that regenerative intercellular calcium waves exist in cultured islets but not in confluent cultures of endothelial cells. An increase in [Ca2+]i is not sufficient to trigger a regenerative propagation. The PLC pathway, CICR and extracellular calcium are all necessary for a fully regenerated propagation.