In JoVE (1)

Other Publications (26)

Articles by Markus Waldeck-Weiermair in JoVE

 JoVE Biology

Application of Genetically Encoded Fluorescent Nitric Oxide (NO•) Probes, the geNOps, for Real-time Imaging of NO• Signals in Single Cells

1Institute of Molecular Biology and Biochemistry, Medical University of Graz

JoVE 55486

Other articles by Markus Waldeck-Weiermair on PubMed

Integrin Clustering Enables Anandamide-induced Ca2+ Signaling in Endothelial Cells Via GPR55 by Protection Against CB1-receptor-triggered Repression

Journal of Cell Science. May, 2008  |  Pubmed ID: 18445684

Although the endocannabinoid anandamide is frequently described to act predominantly in the cardiovascular system, the molecular mechanisms of its signaling remained unclear. In human endothelial cells, two receptors for anandamide were found, which were characterized as cannabinoid 1 receptor (CB1R; CNR1) and G-protein-coupled receptor 55 (GPR55). Both receptors trigger distinct signaling pathways. It crucially depends on the activation status of integrins which signaling cascade becomes promoted upon anandamide stimulation. Under conditions of inactive integrins, anandamide initiates CB1R-derived signaling, including Gi-protein-mediated activation of spleen tyrosine kinase (Syk), resulting in NFkappaB translocation. Furthermore, Syk inhibits phosphoinositide 3-kinase (PI3K) that represents a key protein in the transduction of GPR55-originated signaling. However, once integrins are clustered, CB1R splits from integrins and, thus, Syk cannot further inhibit GPR55-triggered signaling resulting in intracellular Ca2+ mobilization from the endoplasmic reticulum (ER) via a PI3K-Bmx-phospholipase C (PLC) pathway and activation of nuclear factor of activated T-cells. Altogether, these data demonstrate that the physiological effects of anandamide on endothelial cells depend on the status of integrin clustering.

The Contribution of UCP2 and UCP3 to Mitochondrial Ca(2+) Uptake is Differentially Determined by the Source of Supplied Ca(2+)

Cell Calcium. May, 2010  |  Pubmed ID: 20403634

The transmission of Ca(2+) signals to mitochondria is an important phenomenon in cell signaling. We have recently reported that the novel uncoupling proteins UCP2 and UCP3 (UCP2/3) are fundamental for mitochondrial Ca(2+) uniport (MCU). In the present study we investigate the contribution of UCP2/3 to mitochondrial accumulation of Ca(2+) either exclusively released from the ER or entering the cell via the store-operated Ca(2+) entry (SOCE) pathway. Using siRNA we demonstrate that constitutively expressed UCP2/3 are essentially involved in mitochondrial sequestration of intracellularly released Ca(2+) but not of that entering the cells via SOCE. However, overexpression of UCP2/3 yielded elevated mitochondrial Ca(2+) uptake from both sources, though it was more pronounced in case of entering Ca(2+), indicating that the expression levels of UCP2/3 are crucial for the capacity of mitochondria to sequester entering Ca(2+). Our data point to distinct UCP2/3-dependent and UCP2/3-independent modes of mitochondrial Ca(2+) sequestration, which may meet the various demands necessary for an adequate organelle Ca(2+) loading from different Ca(2+) sources in intact cells.

Mitochondrial Ca2+ Uptake and Not Mitochondrial Motility is Required for STIM1-Orai1-dependent Store-operated Ca2+ Entry

Journal of Cell Science. Aug, 2010  |  Pubmed ID: 20587595

Store-operated Ca(2+) entry (SOCE) is established by formation of subplasmalemmal clusters of the endoplasmic reticulum (ER) protein, stromal interacting molecule 1 (STIM1) upon ER Ca(2+) depletion. Thereby, STIM1 couples to plasma membrane channels such as Orai1. Thus, a close proximity of ER domains to the plasma membrane is a prerequisite for SOCE activation, challenging the concept of local Ca(2+) buffering by mitochondria as being essential for SOCE. This study assesses the impact of mitochondrial Ca(2+) handling and motility on STIM1-Orai1-dependent SOCE. High-resolution microscopy showed only 10% of subplasmalemmal STIM1 clusters to be colocalized with mitochondria. Impairments of mitochondrial Ca(2+) handling by inhibition of mitochondrial Na(+)-Ca(2+) exchanger (NCX(mito)) or depolarization only partially suppressed Ca(2+) entry in cells overexpressing STIM1-Orai1. However, SOCE was completely abolished when both NCX(mito) was inhibited and the inner mitochondrial membrane was depolarized, in STIM1- and Orai1-overexpressing cells. Immobilization of mitochondria by expression of mAKAP-RFP-CAAX, a construct that physically links mitochondria to the plasma membrane, affected the Ca(2+) handling of the organelles but not the activity of SOCE. Our observations indicate that mitochondrial Ca(2+) uptake, including reversal of NCX(mito), is fundamental for STIM1-Orai1-dependent SOCE, whereas the proximity of mitochondria to STIM1-Orai1 SOCE units and their motility is not required.

GPR55-dependent and -independent Ion Signalling in Response to Lysophosphatidylinositol in Endothelial Cells

British Journal of Pharmacology. Sep, 2010  |  Pubmed ID: 20735417

The glycerol-based lysophospholipid lysophosphatidylinositol (LPI) is an endogenous agonist of the G-protein-coupled receptor 55 (GPR55) exhibiting cannabinoid receptor-like properties in endothelial cells. To estimate the contribution of GPR55 to the physiological effects of LPI, the GPR55-dependent and -independent electrical responses in this cell type were investigated.

Uncoupling Protein 3 Adjusts Mitochondrial Ca(2+) Uptake to High and Low Ca(2+) Signals

Cell Calcium. Nov, 2010  |  Pubmed ID: 21047682

Uncoupling proteins 2 and 3 (UCP2/3) are essential for mitochondrial Ca(2+) uptake but both proteins exhibit distinct activities in regard to the source and mode of Ca(2+) mobilization. In the present work, structural determinants of their contribution to mitochondrial Ca(2+) uptake were explored. Previous findings indicate the importance of the intermembrane loop 2 (IML2) for the contribution of UCP2/3. Thus, the IML2 of UCP2/3 was substituted by that of UCP1. These chimeras had no activity in mitochondrial uptake of intracellularly released Ca(2+), while they mimicked the wild-type proteins by potentiating mitochondrial sequestration of entering Ca(2+). Alignment of the IML2 sequences revealed that UCP1, UCP2 and UCP3 share a basic amino acid in positions 163, 164 and 167, while only UCP2 and UCP3 contain a second basic residue in positions 168 and 171, respectively. Accordingly, mutants of UCP3 in positions 167 and 171/172 were made. In permeabilized cells, these mutants exhibited distinct Ca(2+) sensitivities in regard to mitochondrial Ca(2+) sequestration. In intact cells, these mutants established different activities in mitochondrial uptake of either intracellularly released (UCP3(R171,E172)) or entering (UCP3(R167)) Ca(2+). Our data demonstrate that distinct sites in the IML2 of UCP3 effect mitochondrial uptake of high and low Ca(2+) signals.

Leucine Zipper EF Hand-containing Transmembrane Protein 1 (Letm1) and Uncoupling Proteins 2 and 3 (UCP2/3) Contribute to Two Distinct Mitochondrial Ca2+ Uptake Pathways

The Journal of Biological Chemistry. Aug, 2011  |  Pubmed ID: 21613221

Cytosolic Ca(2+) signals are transferred into mitochondria over a huge concentration range. In our recent work we described uncoupling proteins 2 and 3 (UCP2/3) to be fundamental for mitochondrial uptake of high Ca(2+) domains in mitochondria-ER junctions. On the other hand, the leucine zipper EF hand-containing transmembrane protein 1 (Letm1) was identified as a mitochondrial Ca(2+)/H(+) antiporter that achieved mitochondrial Ca(2+) sequestration at small Ca(2+) increases. Thus, the contributions of Letm1 and UCP2/3 to mitochondrial Ca(2+) uptake were compared in endothelial cells. Knock-down of Letm1 did not affect the UCP2/3-dependent mitochondrial uptake of intracellularly released Ca(2+) but strongly diminished the transfer of entering Ca(2+) into mitochondria, subsequently, resulting in a reduction of store-operated Ca(2+) entry (SOCE). Knock-down of Letm1 and UCP2/3 did neither impact on cellular ATP levels nor the membrane potential. The enhanced mitochondrial Ca(2+) signals in cells overexpressing UCP2/3 rescued SOCE upon Letm1 knock-down. In digitonin-permeabilized cells, Letm1 exclusively contributed to mitochondrial Ca(2+) uptake at low Ca(2+) conditions. Neither the Letm1- nor the UCP2/3-dependent mitochondrial Ca(2+) uptake was affected by a knock-down of mRNA levels of mitochondrial calcium uptake 1 (MICU1), a protein that triggers mitochondrial Ca(2+) uptake in HeLa cells. Our data indicate that Letm1 and UCP2/3 independently contribute to two distinct, mitochondrial Ca(2+) uptake pathways in intact endothelial cells.

Studying Mitochondrial Ca(2+) Uptake - a Revisit

Molecular and Cellular Endocrinology. Apr, 2012  |  Pubmed ID: 22100614

Mitochondrial Ca(2+) sequestration is a well-known process that is involved in various physiological and pathological mechanisms. Using isolated suspended mitochondria one unique mitochondrial Ca(2+) uniporter was considered to account ubiquitously for the transfer of Ca(2+) into these organelles. However, by applying alternative techniques for measuring mitochondrial Ca(2+) uptake evidences for molecularly distinct mitochondrial Ca(2+) carriers accumulated recently. Herein we compared different methodical approaches of studying mitochondrial Ca(2+) uptake. Patch clamp technique on mitoplasts from endothelial and HeLa cells revealed the existence of three and two mitoplast Ca(2+) currents (I(CaMito)), respectively. According to their conductance, these channels were named small (s-), intermediate (i-), large (l-) and extra-large (xl-) mitoplast Ca(2+) currents (MCC). i-MCC was found in mitoplasts of both cell types whereas s-MCC and l-MCC or xl-MCC were/was exclusively found in mitoplasts from endothelial cells or HeLa cells. The comparison of mitochondrial Ca(2+) signals, measured either indirectly by sensing extra-mitochondrial Ca(2+) or directly by recording changes of the matrix Ca(2+), showed different Ca(2+) sensitivities of the distinct mitochondrial Ca(2+) uptake routes. Subpopulations of mitochondria with different Ca(2+) uptake capacities in intact endothelial cells could be identified using Rhod-2/AM. In contrast, cells expressing mitochondrial targeted pericam or cameleon (4mtD3cpv) showed homogeneous mitochondrial Ca(2+) signals in response to cell stimulation. The comparison of different experimental approaches and protocols using isolated organelles, permeabilized and intact cells, pointed to cell-type specific and versatile pathways for mitochondrial Ca(2+) uptake. Moreover, this work highlights the necessity of the utilization of multiple technical approaches to study the complexity of mitochondrial Ca(2+) homeostasis.

Endothelial Mitochondria--less Respiration, More Integration

Pflugers Archiv : European Journal of Physiology. Jul, 2012  |  Pubmed ID: 22382745

Lining the inner surface of the circulatory system, the vascular endothelium accomplishes a vast variety of specialized functions. Even slight alterations of these functions are implicated in the development of certain cardiovascular diseases that represent major causes of morbidity and mortality in developed countries. Endothelial mitochondria are essential to the functional integrity of the endothelial cell as they integrate a wide range of cellular processes including Ca²⁺ handling, redox signaling and apoptosis, all of which are closely interrelated. Growing evidence supports the notion that impairment of mitochondrial signaling in the endothelium is an early event and a causative factor in the development of diseases such as atherosclerosis or diabetic complications. In this review, we want to outline the significance of mitochondria in both physiology and pathology of the vascular endothelium.

Inhibition of Autophagy Rescues Palmitic Acid-induced Necroptosis of Endothelial Cells

The Journal of Biological Chemistry. Jun, 2012  |  Pubmed ID: 22556413

Accumulation of palmitic acid (PA) in cells from nonadipose tissues is known to induce lipotoxicity resulting in cellular dysfunction and death. The exact molecular pathways of PA-induced cell death are still mysterious. Here, we show that PA triggers autophagy, which did not counteract but in contrast promoted endothelial cell death. The PA-induced cell death was predominantly necrotic as indicated by annexin V and propidium iodide (PI) staining, absence of caspase activity, low levels of DNA hypoploidy, and an early ATP depletion. In addition PA induced a strong elevation of mRNA levels of ubiquitin carboxyl-terminal hydrolase (CYLD), a known mediator of necroptosis. Moreover, siRNA-mediated knockdown of CYLD significantly antagonized PA-induced necrosis of endothelial cells. In contrast, inhibition and knockdown of receptor interacting protein kinase 1 (RIPK1) had no effect on PA-induced necrosis, indicating the induction of a CYLD-dependent but RIPK1-independent cell death pathway. PA was recognized as a strong and early inducer of autophagy. The inhibition of autophagy by both pharmacological inhibitors and genetic knockdown of the autophagy-specific genes, vacuolar protein sorting 34 (VPS34), and autophagy-related protein 7 (ATG7), could rescue the PA-induced death of endothelial cells. Moreover, the initiation of autophagy and cell death by PA was reduced in endothelial cells loaded with the Ca(2+) chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-(acetoxymethyl) ester (BAPTA-AM), indicating that Ca(2+) triggers the fatal signaling of PA. In summary, we introduce an unexpected mechanism of lipotoxicity in endothelial cells and provide several novel strategies to counteract the lipotoxic signaling of PA.

Mitochondrial Ca2+ Uptake 1 (MICU1) and Mitochondrial Ca2+ Uniporter (MCU) Contribute to Metabolism-secretion Coupling in Clonal Pancreatic β-cells

The Journal of Biological Chemistry. Oct, 2012  |  Pubmed ID: 22904319

In pancreatic β-cells, uptake of Ca(2+) into mitochondria facilitates metabolism-secretion coupling by activation of various matrix enzymes, thus facilitating ATP generation by oxidative phosphorylation and, in turn, augmenting insulin release. We employed an siRNA-based approach to evaluate the individual contribution of four proteins that were recently described to be engaged in mitochondrial Ca(2+) sequestration in clonal INS-1 832/13 pancreatic β-cells: the mitochondrial Ca(2+) uptake 1 (MICU1), mitochondrial Ca(2+) uniporter (MCU), uncoupling protein 2 (UCP2), and leucine zipper EF-hand-containing transmembrane protein 1 (LETM1). Using a FRET-based genetically encoded Ca(2+) sensor targeted to mitochondria, we show that a transient knockdown of MICU1 or MCU diminished mitochondrial Ca(2+) uptake upon both intracellular Ca(2+) release and Ca(2+) entry via L-type channels. In contrast, knockdown of UCP2 and LETM1 exclusively reduced mitochondrial Ca(2+) uptake in response to either intracellular Ca(2+) release or Ca(2+) entry, respectively. Therefore, we further investigated the role of MICU1 and MCU in metabolism-secretion coupling. Diminution of MICU1 or MCU reduced mitochondrial Ca(2+) uptake in response to d-glucose, whereas d-glucose-triggered cytosolic Ca(2+) oscillations remained unaffected. Moreover, d-glucose-evoked increases in cytosolic ATP and d-glucose-stimulated insulin secretion were diminished in MICU1- or MCU-silenced cells. Our data highlight the crucial role of MICU1 and MCU in mitochondrial Ca(2+) uptake in pancreatic β-cells and their involvement in the positive feedback required for sustained insulin secretion.

Spatiotemporal Correlations Between Cytosolic and Mitochondrial Ca(2+) Signals Using a Novel Red-Shifted Mitochondrial Targeted Cameleon

PloS One. 2012  |  Pubmed ID: 23029314

The transfer of Ca(2+) from the cytosol into the lumen of mitochondria is a crucial process that impacts cell signaling in multiple ways. Cytosolic Ca(2+) ([Ca(2+)](cyto)) can be excellently quantified with the ratiometric Ca(2+) probe fura-2, while genetically encoded Förster resonance energy transfer (FRET)-based fluorescent Ca(2+) sensors, the cameleons, are efficiently used to specifically measure Ca(2+) within organelles. However, because of a significant overlap of the fura-2 emission with the spectra of the cyan and yellow fluorescent protein of most of the existing cameleons, the measurement of fura-2 and cameleons within one given cell is a complex task. In this study, we introduce a novel approach to simultaneously assess [Ca(2+)](cyto) and mitochondrial Ca(2+) ([Ca(2+)](mito)) signals at the single cell level. In order to eliminate the spectral overlap we developed a novel red-shifted cameleon, D1GO-Cam, in which the green and orange fluorescent proteins were used as the FRET pair. This ratiometric Ca(2+) probe could be successfully targeted to mitochondria and was suitable to be used simultaneously with fura-2 to correlate [Ca(2+)](cyto) and [Ca(2+)](mito) within same individual cells. Our data indicate that depending on the kinetics of [Ca(2+)](cyto) rises there is a significant lag between onset of [Ca(2+)](cyto) and [Ca(2+)](mito) signals, pointing to a certain threshold of [Ca(2+)](cyto) necessary to activate mitochondrial Ca(2+) uptake. The temporal correlation between [Ca(2+)](mito) and [Ca(2+)](cyto) as well as the efficiency of the transfer of Ca(2+) from the cytosol into mitochondria varies between different cell types. Moreover, slow mitochondrial Ca(2+) extrusion and a desensitization of mitochondrial Ca(2+) uptake cause a clear difference in patterns of mitochondrial and cytosolic Ca(2+) oscillations of pancreatic beta-cells in response to D-glucose.

Molecularly Distinct Routes of Mitochondrial Ca2+ Uptake Are Activated Depending on the Activity of the Sarco/endoplasmic Reticulum Ca2+ ATPase (SERCA)

The Journal of Biological Chemistry. May, 2013  |  Pubmed ID: 23592775

The transfer of Ca(2+) across the inner mitochondrial membrane is an important physiological process linked to the regulation of metabolism, signal transduction, and cell death. While the definite molecular composition of mitochondrial Ca(2+) uptake sites remains unknown, several proteins of the inner mitochondrial membrane, that are likely to accomplish mitochondrial Ca(2+) fluxes, have been described: the novel uncoupling proteins 2 and 3, the leucine zipper-EF-hand containing transmembrane protein 1 and the mitochondrial calcium uniporter. It is unclear whether these proteins contribute to one unique mitochondrial Ca(2+) uptake pathway or establish distinct routes for mitochondrial Ca(2+) sequestration. In this study, we show that a modulation of Ca(2+) release from the endoplasmic reticulum by inhibition of the sarco/endoplasmatic reticulum ATPase modifies cytosolic Ca(2+) signals and consequently switches mitochondrial Ca(2+) uptake from an uncoupling protein 3- and mitochondrial calcium uniporter-dependent, but leucine zipper-EF-hand containing transmembrane protein 1-independent to a leucine zipper-EF-hand containing transmembrane protein 1- and mitochondrial calcium uniporter-mediated, but uncoupling protein 3-independent pathway. Thus, the activity of sarco/endoplasmatic reticulum ATPase is significant for the mode of mitochondrial Ca(2+) sequestration and determines which mitochondrial proteins might actually accomplish the transfer of Ca(2+) across the inner mitochondrial membrane. Moreover, our findings herein support the existence of distinct mitochondrial Ca(2+) uptake routes that might be essential to ensure an efficient ion transfer into mitochondria despite heterogeneous cytosolic Ca(2+) rises.

Mitochondrial Ca(2+) Uniporter (MCU)-dependent and MCU-independent Ca(2+) Channels Coexist in the Inner Mitochondrial Membrane

Pflugers Archiv : European Journal of Physiology. Jul, 2014  |  Pubmed ID: 24162235

A protein referred to as CCDC109A and then renamed to mitochondrial calcium uniporter (MCU) has recently been shown to accomplish mitochondrial Ca(2+) uptake in different cell types. In this study, we investigated whole-mitoplast inward cation currents and single Ca(2+) channel activities in mitoplasts prepared from stable MCU knockdown HeLa cells using the patch-clamp technique. In whole-mitoplast configuration, diminution of MCU considerably reduced inward Ca(2+) and Na(+) currents. This was accompanied by a decrease in occurrence of single channel activity of the intermediate conductance mitochondrial Ca(2+) current (i-MCC). However, ablation of MCU yielded a compensatory 2.3-fold elevation in the occurrence of the extra large conductance mitochondrial Ca(2+) current (xl-MCC), while the occurrence of bursting currents (b-MCC) remained unaltered. These data reveal i-MCC as MCU-dependent current while xl-MCC and b-MCC seem to be rather MCU-independent, thus, pointing to the engagement of at least two molecularly distinct mitochondrial Ca(2+) channels.

ATP Increases Within the Lumen of the Endoplasmic Reticulum Upon Intracellular Ca2+ Release

Molecular Biology of the Cell. Feb, 2014  |  Pubmed ID: 24307679

Multiple functions of the endoplasmic reticulum (ER) essentially depend on ATP within this organelle. However, little is known about ER ATP dynamics and the regulation of ER ATP import. Here we describe real-time recordings of ER ATP fluxes in single cells using an ER-targeted, genetically encoded ATP sensor. In vitro experiments prove that the ATP sensor is both Ca(2+) and redox insensitive, which makes it possible to monitor Ca(2+)-coupled ER ATP dynamics specifically. The approach uncovers a cell type-specific regulation of ER ATP homeostasis in different cell types. Moreover, we show that intracellular Ca(2+) release is coupled to an increase of ATP within the ER. The Ca(2+)-coupled ER ATP increase is independent of the mode of Ca(2+) mobilization and controlled by the rate of ATP biosynthesis. Furthermore, the energy stress sensor, AMP-activated protein kinase, is essential for the ATP increase that occurs in response to Ca(2+) depletion of the organelle. Our data highlight a novel Ca(2+)-controlled process that supplies the ER with additional energy upon cell stimulation.

IP3-mediated STIM1 Oligomerization Requires Intact Mitochondrial Ca2+ Uptake

Journal of Cell Science. Jul, 2014  |  Pubmed ID: 24806964

Mitochondria contribute to cell signaling by controlling store-operated Ca(2+) entry (SOCE). SOCE is activated by Ca(2+) release from the endoplasmic reticulum (ER), whereupon stromal interacting molecule 1 (STIM1) forms oligomers, redistributes to ER-plasma-membrane junctions and opens plasma membrane Ca(2+) channels. The mechanisms by which mitochondria interfere with the complex process of SOCE are insufficiently clarified. In this study, we used an shRNA approach to investigate the direct involvement of mitochondrial Ca(2+) buffering in SOCE. We demonstrate that knockdown of either of two proteins that are essential for mitochondrial Ca(2+) uptake, the mitochondrial calcium uniporter (MCU) or uncoupling protein 2 (UCP2), results in decelerated STIM1 oligomerization and impaired SOCE following cell stimulation with an inositol-1,4,5-trisphosphate (IP3)-generating agonist. Upon artificially augmented cytosolic Ca(2+) buffering or ER Ca(2+) depletion by sarcoplasmic or endoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitors, STIM1 oligomerization did not rely on intact mitochondrial Ca(2+) uptake. However, MCU-dependent mitochondrial sequestration of Ca(2+) entering through the SOCE pathway was essential to prevent slow deactivation of SOCE. Our findings show a stimulus-specific contribution of mitochondrial Ca(2+) uptake to the SOCE machinery, likely through a role in shaping cytosolic Ca(2+) micro-domains.

TRPV1 Mediates Cellular Uptake of Anandamide and Thus Promotes Endothelial Cell Proliferation and Network-formation

Biology Open. Nov, 2014  |  Pubmed ID: 25395667

Anandamide (N-arachidonyl ethanolamide, AEA) is an endogenous cannabinoid that is involved in various pathological conditions, including cardiovascular diseases and tumor-angiogenesis. Herein, we tested the involvement of classical cannabinoid receptors (CBRs) and the Ca(2+)-channel transient receptor potential vanilloid 1 (TRPV1) on cellular AEA uptake and its effect on endothelial cell proliferation and network-formation. Uptake of the fluorescence-labeled anandamide (SKM4-45-1) was monitored in human endothelial colony-forming cells (ECFCs) and a human endothelial-vein cell line (EA.hy926). Involvement of the receptors during AEA translocation was determined by selective pharmacological inhibition (AM251, SR144528, CID16020046, SB366791) and molecular interference by TRPV1-selective siRNA-mediated knock-down and TRPV1 overexpression. We show that exclusively TRPV1 contributes essentially to AEA transport into endothelial cells in a Ca(2+)-independent manner. This TRPV1 function is a prerequisite for AEA-induced endothelial cell proliferation and network-formation. Our findings point to a so far unknown moonlighting function of TRPV1 as Ca(2+)-independent contributor/regulator of AEA uptake. We propose TRPV1 as representing a promising target for development of pharmacological therapies against AEA-triggered endothelial cell functions, including their stimulatory effect on tumor-angiogenesis.

Assessment of Mitochondrial Ca²⁺ Uptake

Methods in Molecular Biology (Clifton, N.J.). 2015  |  Pubmed ID: 25631032

Mitochondrial Ca(2+) uptake regulates mitochondrial function and contributes to cell signaling. Accordingly, quantifying mitochondrial Ca(2+) signals and elaborating the mechanisms that accomplish mitochondrial Ca(2+) uptake are essential to gain our understanding of cell biology. Here, we describe the benefits and drawbacks of various established old and new techniques to assess dynamic changes of mitochondrial Ca(2+) concentration ([Ca(2+)]mito) in a wide range of applications.

Generation of Red-Shifted Cameleons for Imaging Ca²⁺ Dynamics of the Endoplasmic Reticulum

Sensors (Basel, Switzerland). Jun, 2015  |  Pubmed ID: 26053751

Cameleons are sophisticated genetically encoded fluorescent probes that allow quantifying cellular Ca2+ signals. The probes are based on Förster resonance energy transfer (FRET) between terminally located fluorescent proteins (FPs), which move together upon binding of Ca2+ to the central calmodulin myosin light chain kinase M13 domain. Most of the available cameleons consist of cyan and yellow FPs (CFP and YFP) as the FRET pair. However, red-shifted versions with green and orange or red FPs (GFP, OFP, RFP) have some advantages such as less phototoxicity and minimal spectral overlay with autofluorescence of cells and fura-2, a prominent chemical Ca2+ indicator. While GFP/OFP- or GFP/RFP-based cameleons have been successfully used to study cytosolic and mitochondrial Ca2+ signals, red-shifted cameleons to visualize Ca2+ dynamics of the endoplasmic reticulum (ER) have not been developed so far. In this study, we generated and tested several ER targeted red-shifted cameleons. Our results show that GFP/OFP-based cameleons due to miss-targeting and their high Ca2+ binding affinity are inappropriate to record ER Ca2+ signals. However, ER targeted GFP/RFP-based probes were suitable to sense ER Ca2+ in a reliable manner. With this study we increased the palette of cameleons for visualizing Ca2+ dynamics within the main intracellular Ca2+ store.

UCP2 Modulates Single-channel Properties of a MCU-dependent Ca(2+) Inward Current in Mitochondria

Pflugers Archiv : European Journal of Physiology. Dec, 2015  |  Pubmed ID: 26275882

The mitochondrial Ca(2+) uniporter is a highly Ca(2+)-selective protein complex that consists of the pore-forming mitochondrial Ca(2+) uniporter protein (MCU), the scaffolding essential MCU regulator (EMRE), and mitochondrial calcium uptake 1 and 2 (MICU1/2), which negatively regulate mitochondrial Ca(2+) uptake. We have previously reported that uncoupling proteins 2 and 3 (UCP2/3) are also engaged in the activity of mitochondrial Ca(2+) uptake under certain conditions, while the mechanism by which UCP2/3 facilitates mitochondrial Ca(2+) uniport remains elusive. This work was designed to investigate the impact of UCP2 on the three distinct mitochondrial Ca(2+) currents found in mitoplasts isolated from HeLa cells, the intermediate- (i-), burst- (b-) and extra-large (xl-) mitochondrial/mitoplast Ca(2+) currents (MCC). Using the patch clamp technique on mitoplasts from cells with reduced MCU and EMRE unveiled a very high affinity of MCU for xl-MCC that succeeds that for i-MCC, indicating the coexistence of at least two MCU/EMRE-dependent Ca(2+) currents. The manipulation of the expression level of UCP2 by either siRNA-mediated knockdown or overexpression changed exclusively the open probability (NPo) of xl-MCC by approx. 38% decrease or nearly a 3-fold increase, respectively. These findings confirm a regulatory role of UCP2 in mitochondrial Ca(2+) uptake and identify UCP2 as a selective modulator of just one distinct MCU/EMRE-dependent mitochondrial Ca(2+) inward current.

Rearrangement of MICU1 Multimers for Activation of MCU is Solely Controlled by Cytosolic Ca(2.)

Scientific Reports. Oct, 2015  |  Pubmed ID: 26489515

Mitochondrial Ca(2+) uptake is a vital process that controls distinct cell and organelle functions. Mitochondrial calcium uptake 1 (MICU1) was identified as key regulator of the mitochondrial Ca(2+) uniporter (MCU) that together with the essential MCU regulator (EMRE) forms the mitochondrial Ca(2+) channel. However, mechanisms by which MICU1 controls MCU/EMRE activity to tune mitochondrial Ca(2+) signals remain ambiguous. Here we established a live-cell FRET approach and demonstrate that elevations of cytosolic Ca(2+) rearranges MICU1 multimers with an EC50 of 4.4 μM, resulting in activation of mitochondrial Ca(2+) uptake. MICU1 rearrangement essentially requires the EF-hand motifs and strictly correlates with the shape of cytosolic Ca(2+) rises. We further show that rearrangements of MICU1 multimers were independent of matrix Ca(2+) concentration, mitochondrial membrane potential, and expression levels of MCU and EMRE. Our experiments provide novel details about how MCU/EMRE is regulated by MICU1 and an original approach to investigate MCU/EMRE activation in intact cells.

Development of Novel FP-based Probes for Live-cell Imaging of Nitric Oxide Dynamics

Nature Communications. Feb, 2016  |  Pubmed ID: 26842907

Nitric oxide () is a free radical with a wide range of biological effects, but practically impossible to visualize in single cells. Here we report the development of novel multicoloured fluorescent quenching-based probes by fusing a bacteria-derived -binding domain close to distinct fluorescent protein variants. These genetically encoded probes, referred to as geNOps, provide a selective, specific and real-time read-out of cellular dynamics and, hence, open a new era of bioimaging. The combination of geNOps with a Ca(2+) sensor allowed us to visualize and Ca(2+) signals simultaneously in single endothelial cells. Moreover, targeting of the probes was used to detect signals within mitochondria. The geNOps are useful new tools to further investigate and understand the complex patterns of signalling on the single (sub)cellular level.

Filling a GAP-An Optimized Probe for ER Ca(2+) Imaging In Vivo

Cell Chemical Biology. Jun, 2016  |  Pubmed ID: 27341431

In this issue of Cell Chemical Biology, Navas-Navarro et al. (2016) demonstrate that fusion of engineered derivatives of the long-known jellyfish proteins green fluorescent protein (GFP) and aequorin yield optimized genetically encoded fluorescent probes for detecting Ca(2+) signals within the endoplasmic reticulum (ER) of living animals.

Resveratrol Specifically Kills Cancer Cells by a Devastating Increase in the Ca2+ Coupling Between the Greatly Tethered Endoplasmic Reticulum and Mitochondria

Cellular Physiology and Biochemistry : International Journal of Experimental Cellular Physiology, Biochemistry, and Pharmacology. 2016  |  Pubmed ID: 27606689

Resveratrol and its derivate piceatannol are known to induce cancer cell-specific cell death. While multiple mechanisms of actions have been described including the inhibition of ATP synthase, changes in mitochondrial membrane potential and ROS levels, the exact mechanisms of cancer specificity of these polyphenols remain unclear. This paper is designed to reveal the molecular basis of the cancer-specific initiation of cell death by resveratrol and piceatannol.

PRMT1-mediated Methylation of MICU1 Determines the UCP2/3 Dependency of Mitochondrial Ca(2+) Uptake in Immortalized Cells

Nature Communications. Sep, 2016  |  Pubmed ID: 27642082

Recent studies revealed that mitochondrial Ca(2+) channels, which control energy flow, cell signalling and death, are macromolecular complexes that basically consist of the pore-forming mitochondrial Ca(2+) uniporter (MCU) protein, the essential MCU regulator (EMRE), and the mitochondrial Ca(2+) uptake 1 (MICU1). MICU1 is a regulatory subunit that shields mitochondria from Ca(2+) overload. Before the identification of these core elements, the novel uncoupling proteins 2 and 3 (UCP2/3) have been shown to be fundamental for mitochondrial Ca(2+) uptake. Here we clarify the molecular mechanism that determines the UCP2/3 dependency of mitochondrial Ca(2+) uptake. Our data demonstrate that mitochondrial Ca(2+) uptake is controlled by protein arginine methyl transferase 1 (PRMT1) that asymmetrically methylates MICU1, resulting in decreased Ca(2+) sensitivity. UCP2/3 normalize Ca(2+) sensitivity of methylated MICU1 and, thus, re-establish mitochondrial Ca(2+) uptake activity. These data provide novel insights in the complex regulation of the mitochondrial Ca(2+) uniporter by PRMT1 and UCP2/3.

Formation of Nitric Oxide by Aldehyde Dehydrogenase-2 Is Necessary and Sufficient for Vascular Bioactivation of Nitroglycerin

The Journal of Biological Chemistry. Nov, 2016  |  Pubmed ID: 27679490

Aldehyde dehydrogenase-2 (ALDH2) catalyzes vascular bioactivation of the antianginal drug nitroglycerin (GTN), resulting in activation of soluble guanylate cyclase (sGC) and cGMP-mediated vasodilation. We have previously shown that a minor reaction of ALDH2-catalyzed GTN bioconversion, accounting for about 5% of the main clearance-based turnover yielding inorganic nitrite, results in direct NO formation and concluded that this minor pathway could provide the link between vascular GTN metabolism and activation of sGC. However, lack of detectable NO at therapeutically relevant GTN concentrations (≤1 μm) in vascular tissue called into question the biological significance of NO formation by purified ALDH2. We addressed this issue and used a novel, highly sensitive genetically encoded fluorescent NO probe (geNOp) to visualize intracellular NO formation at low GTN concentrations (≤1 μm) in cultured vascular smooth muscle cells (VSMC) expressing an ALDH2 mutant that reduces GTN to NO but lacks clearance-based GTN denitration activity. NO formation was compared with GTN-induced activation of sGC. The addition of 1 μm GTN to VSMC expressing either wild-type or C301S/C303S ALDH2 resulted in pronounced intracellular NO elevation, with maximal concentrations of 7 and 17 nm, respectively. Formation of GTN-derived NO correlated well with activation of purified sGC in VSMC lysates and cGMP accumulation in intact porcine aortic endothelial cells infected with wild-type or mutant ALDH2. Formation of NO and cGMP accumulation were inhibited by ALDH inhibitors chloral hydrate and daidzin. The present study demonstrates that ALDH2-catalyzed NO formation is necessary and sufficient for GTN bioactivation in VSMC.

Intact Mitochondrial Ca(2+) Uniport is Essential for Agonist-induced Activation of Endothelial Nitric Oxide Synthase (eNOS)

Free Radical Biology & Medicine. Jan, 2017  |  Pubmed ID: 27923677

Mitochondrial Ca(2+) uptake regulates diverse endothelial cell functions and has also been related to nitric oxide (NO(•)) production. However, it is not entirely clear if the organelles support or counteract NO(•) biosynthesis by taking up Ca(2+). The objective of this study was to verify whether or not mitochondrial Ca(2+) uptake influences Ca(2+)-triggered NO(•) generation by endothelial NO(•) synthase (eNOS) in an immortalized endothelial cell line (EA.hy926), respective primary human umbilical vein endothelial cells (HUVECs) and eNOS-RFP (red fluorescent protein) expressing human embryonic kidney (HEK293) cells. We used novel genetically encoded fluorescent NO(•) probes, the geNOps, and Ca(2+) sensors to monitor single cell NO(•) and Ca(2+) dynamics upon cell treatment with ATP, an inositol 1,4,5-trisphosphate (IP3)-generating agonist. Mitochondrial Ca(2+) uptake was specifically manipulated by siRNA-mediated knock-down of recently identified key components of the mitochondrial Ca(2+) uniporter machinery. In endothelial cells and the eNOS-RFP expressing HEK293 cells we show that reduced mitochondrial Ca(2+) uptake upon the knock-down of the mitochondrial calcium uniporter (MCU) protein and the essential MCU regulator (EMRE) yield considerable attenuation of the Ca(2+)-triggered NO(•) increase independently of global cytosolic Ca(2+) signals. The knock-down of mitochondrial calcium uptake 1 (MICU1), a gatekeeper of the MCU, increased both mitochondrial Ca(2+) sequestration and Ca(2+)-induced NO(•) signals. The positive correlation between mitochondrial Ca(2+) elevation and NO(•) production was independent of eNOS phosphorylation at serine(1177). Our findings emphasize that manipulating mitochondrial Ca(2+) uptake may represent a novel strategy to control eNOS-mediated NO(•) production.

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