Quercetin protects cardiomyoblasts against hypertonic cytotoxicity by abolishing intracellular Ca2+ elevations and mitochondrial depolarisation

BACKGROUND AND AIM

Osmotic changes represent a burden for the body and their limitation would be beneficial. We hypothesized that ubiquitous natural compounds could guard against cytotoxic effects of osmotic stress. We evaluated the anti-hypertonic mechanism of quercetin and 2,3-dehydrosilybin in H9c2 cells in vitro.

EXPERIMENTAL PROCEDURE

Protective effect of both compounds was determined by neutral red assay, cell apoptosis was estimated by measuring caspase-3 activity and verified by western blot and annexin V assay. Phosphorylation level of selected proteins was also detected. Mitochondrial membrane potential was evaluated using dye JC-1. Ca2+ signals were evaluated using genetically encoded fluorescent Ca2+ biosensor GCaMP7f. Formation of reactive oxygen species was measured using an oxidant-sensing probe dihydrofluorescein diacetate.

KEY RESULTS

Quercetin protected H9c2 cells against hypertonic stress-induced cell death. We observed a significant increase in intracellular Ca2+ levels ([Ca2+]cyto) when cells originally placed in a hypertonic solution were returned to a normotonic environment. Quercetin was found to prevent this increase in [Ca2+]cyto and also the depolarization of mitochondrial membrane potential.

CONCLUSIONS AND IMPLICATIONS

Quercetin, but not 2,3-dehydrosilybin, reduced adverse effects of osmotic stress mainly by dampening the elevation of [Ca2+]cyto and mitochondrial Ca2+ overload. This may consequently prevent MPTP pore opening and activation of apoptosis.

Fluid shear stress induces a shift from glycolytic to amino acid pathway in human trophoblasts

BACKGROUND

The human placenta, a tissue with a lifespan limited to the period of pregnancy, is exposed to varying shear rates by maternal blood perfusion depending on the stage of development. In this study, we aimed to investigate the effects of fluidic shear stress on the human trophoblast transcriptome and metabolism.

RESULTS

Based on a trophoblast cell line cultured in a fluidic flow system, changes caused by shear stress were analyzed and compared to static conditions. RNA sequencing and bioinformatics analysis revealed an altered transcriptome and enriched gene ontology terms associated with amino acid and mitochondrial metabolism. A decreased GLUT1 expression and reduced glucose uptake, together with downregulated expression of key glycolytic rate-limiting enzymes, hexokinase 2 and phosphofructokinase 1 was observed. Altered mitochondrial ATP levels and mass spectrometry data, suggested a shift in energy production from glycolysis towards mitochondrial oxidative phosphorylation. This shift in energy production could be supported by increased expression of glutamic-oxaloacetic transaminase variants in response to shear stress as well as under low glucose availability or after silencing of GLUT1. The shift towards amino acid metabolic pathways could be supported by significantly altered amino acid levels, like glutamic acid, cysteine and serine. Downregulation of GLUT1 and glycolytic rate-limiting enzymes, with concomitant upregulation of glutamic-oxaloacetic transaminase 2 was confirmed in first trimester placental explants cultured under fluidic flow. In contrast, high fluid shear stress decreased glutamic-oxaloacetic transaminase 2 expression in term placental explants when compared to low flow rates. Placental tissue from pregnancies with intrauterine growth restriction are exposed to high shear rates and showed also decreased glutamic-oxaloacetic transaminase 2, while GLUT1 was unchanged and glycolytic rate-limiting enzymes showed a trend to be upregulated. The results were generated by using qPCR, immunoblots, quantification of immunofluorescent pictures, padlock probe hybridization, mass spectrometry and FRET-based measurement.

CONCLUSION

Our study suggests that onset of uteroplacental blood flow is accompanied by a shift from a predominant glycolytic- to an alternative amino acid converting metabolism in the villous trophoblast. Rheological changes with excessive fluidic shear stress at the placental surface, may disrupt this alternative amino acid pathway in the syncytiotrophoblast and could contribute to intrauterine growth restriction.

MICU1 deficiency alters mitochondrial morphology and cytochrome C release

The mitochondrial inner boundary membrane harbors a protein called MICU1, which is sensitive to Ca²⁺ and binds to the MICOS components Mic60 and CHCHD2. Changes in the mitochondrial cristae junction structure and organization in MICU1^-/- cells lead to increased cytochrome c release, membrane potential rearrangement, and changes in mitochondrial Ca²⁺ uptake dynamics. These findings shed new light on the multifaceted role of MICU1, highlighting its involvement not only as an interaction partner and regulator of the MCU complex but also as a crucial determinant of mitochondrial ultrastructure and, thus, an essential player in processes initiating apoptosis.

Synergy of Uncoupling Proteins (1 and 2) with Mitochondrial Ca2+ Uptake Machinery Potentiate Mitochondrial Uncoupling

Mitochondrial uncoupling proteins UCP1 and UCP2 have a structural homology of app. 60%. They execute their mitochondria uncoupling function through different molecular mechanisms. Non-shivering thermogenesis by UCP1 is mediated through a transmembrane dissipation of the proton motive force to create heat during sympathetic stimulation. UCP2, on the other hand, modulates through the interaction with methylated MICU1 the permeability of the cristae junction, which acts as an isolator for the cristae-located mitochondrial membrane potential. In this mini-review, we discuss and compare the recently described molecular mechanism of UCP1 in brown adipose tissue and UCP2 in aged and cancer non-excitable cells that contribute to mitochondrial uncoupling, and the synergistic effects of both UCPs with the mitochondrial Ca²⁺ uptake machinery.

Monoglyceride Lipase Deficiency Is Associated with Altered Thrombogenesis in Mice

Monoglyceride lipase (MGL) hydrolyzes monoacylglycerols (MG) to glycerol and one fatty acid. Among the various MG species, MGL also degrades 2-arachidonoylglycerol, the most abundant endocannabinoid and potent activator of the cannabinoid receptors 1 and 2. We investigated the consequences of MGL deficiency on platelet function using systemic (Mgl-/-) and platelet-specific Mgl-deficient (platMgl-/-) mice. Despite comparable platelet morphology, loss of MGL was associated with decreased platelet aggregation and reduced response to collagen activation. This was reflected by reduced thrombus formation in vitro, accompanied by a longer bleeding time and a higher blood volume loss. Occlusion time after FeCl3-induced injury was markedly reduced in Mgl-/- mice, which is consistent with contraction of large aggregates and fewer small aggregates in vitro. The absence of any functional changes in platelets from platMgl-/- mice is in accordance with lipid degradation products or other molecules in the circulation, rather than platelet-specific effects, being responsible for the observed alterations in Mgl-/- mice. We conclude that genetic deletion of MGL is associated with altered thrombogenesis.

An unexpected effect of risperidone reveals a nonlinear relationship between cytosolic Ca2+ and mitochondrial Ca2+ uptake

Mitochondria actively contribute to cellular Ca²⁺ homeostasis. The molecular mechanisms of mitochondrial Ca²⁺ uptake and release are well characterized and are attributed to the multi-protein assembly of the mitochondrial Ca²⁺ uniporter complex (MCUC) and the mitochondrial sodium-calcium exchanger (NCLX), respectively. Hence, Ca²⁺ transfer from the endoplasmic reticulum (ER) and store-operated Ca²⁺ entry (SOCE) into the mitochondrial matrix has been quantitatively visualized on the subcellular level using targeted fluorescent biosensors. However, a correlation between the amplitude of cytosolic Ca²⁺ elevation with that in the mitochondrial matrix has not been investigated in detail so far. In the present study, we combined the Ca²⁺-mobilizing agonist histamine with the H₁-receptor antagonist risperidone to establish a well-tunable experimental approach allowing the correlation between low, slow, high, and fast cytosolic and mitochondrial Ca²⁺ signals in response to inositol 1,4,5-trisphosphate (IP₃)-triggered ER Ca²⁺ release. Our present data confirm a defined threshold in cytosolic Ca²⁺, which is necessary for the activation of mitochondrial Ca²⁺ uptake. Moreover, our data support the hypothesis of different modes of mitochondrial Ca²⁺ uptake depending on the source of the ion (i.e., ER vs SOCE).

MFN2 mediates ER-mitochondrial coupling during ER stress through specialized stable contact sites

Endoplasmic reticulum (ER) functions critically depend on a suitable ATP supply to fuel ER chaperons and protein trafficking. A disruption of the ability of the ER to traffic and fold proteins leads to ER stress and the unfolded protein response (UPR). Using structured illumination super-resolution microscopy, we revealed increased stability and lifetime of mitochondrial associated ER membranes (MAM) during ER stress. The consequent increase of basal mitochondrial Ca2+ leads to increased TCA cycle activity and enhanced mitochondrial membrane potential, OXPHOS, and ATP generation during ER stress. Subsequently, OXPHOS derived ATP trafficking towards the ER was increased. We found that the increased lifetime and stability of MAMs during ER stress depended on the mitochondrial fusion protein Mitofusin2 (MFN2). Knockdown of MFN2 blunted mitochondrial Ca2+ effect during ER stress, switched mitochondrial F1FO-ATPase activity into reverse mode, and strongly reduced the ATP supply for the ER during ER stress. These findings suggest a critical role of MFN2-dependent MAM stability and lifetime during ER stress to compensate UPR by strengthening ER ATP supply by the mitochondria.

MICU1 controls spatial membrane potential gradients and guides Ca2+ fluxes within mitochondrial substructures

Mitochondrial ultrastructure represents a pinnacle of form and function, with the inner mitochondrial membrane (IMM) forming isolated pockets of cristae membrane (CM), separated from the inner-boundary membrane (IBM) by cristae junctions (CJ). Applying structured illumination and electron microscopy, a novel and fundamental function of MICU1 in mediating Ca²⁺ control over spatial membrane potential gradients (SMPGs) between CM and IMS was identified. We unveiled alterations of SMPGs by transient CJ openings when Ca²⁺ binds to MICU1 resulting in spatial cristae depolarization. This Ca²⁺/MICU1-mediated plasticity of the CJ further provides the mechanistic bedrock of the biphasic mitochondrial Ca²⁺ uptake kinetics via the mitochondrial Ca²⁺ uniporter (MCU) during intracellular Ca²⁺ release: Initially, high Ca²⁺ opens CJ via Ca²⁺/MICU1 and allows instant Ca²⁺ uptake across the CM through constantly active MCU. Second, MCU disseminates into the IBM, thus establishing Ca²⁺ uptake across the IBM that circumvents the CM. Under the condition of MICU1 methylation by PRMT1 in aging or cancer, UCP2 that binds to methylated MICU1 destabilizes CJ, disrupts SMPGs, and facilitates fast Ca²⁺ uptake via the CM.

ALYREF, a novel factor involved in breast carcinogenesis, acts through transcriptional and post-transcriptional mechanisms selectively regulating the short NEAT1 isoform

The RNA-binding protein ALYREF (THOC4) is involved in transcriptional regulation and nuclear mRNA export, though its role and molecular mode of action in breast carcinogenesis are completely unknown. Here, we identified high ALYREF expression as a factor for poor survival in breast cancer patients. ALYREF significantly influenced cellular growth, apoptosis and mitochondrial energy metabolism in breast cancer cells as well as breast tumorigenesis in orthotopic mouse models. Transcriptional profiling, phenocopy and rescue experiments identified the short isoform of the lncRNA NEAT1 as a molecular trigger for ALYREF effects in breast cancer. Mechanistically, we found that ALYREF binds to the NEAT1 promoter region to enhance the global NEAT1 transcriptional activity. Importantly, by stabilizing CPSF6, a protein that selectively activates the post-transcriptional generation of the short isoform of NEAT1, as well as by direct binding and stabilization of the short isoform of NEAT1, ALYREF selectively fine-tunes the expression of the short NEAT1 isoform. Overall, our study describes ALYREF as a novel factor contributing to breast carcinogenesis and identifies novel molecular mechanisms of regulation the two isoforms of NEAT1.

T3-induced enhancement of mitochondrial Ca2+ uptake as a boost for mitochondrial metabolism

Thyroid hormones act as master regulators of cellular metabolism. Thereby, the biologically active triiodothyronine (T3) induces the expression of genes to enhance mitochondrial metabolic function. Notably, Ca²⁺ ions are necessary for the activity of dehydrogenases of the tricarboxylic acid cycle and, thus, mitochondrial respiration. We investigated whether treating HeLa cells with T3 causes alterations in mitochondrial Ca²⁺ ([Ca²⁺]_mito) levels. Real-time measurements by fluorescence microscopy revealed that treatment with T3 for 3 h induces a significant increase in basal [Ca²⁺]_mito levels and [Ca²⁺]_mito uptake upon the depletion of the endoplasmic reticulum (ER) Ca²⁺ store, while cytosolic Ca²⁺ levels remained unchanged. T3 incubation was found to upregulate mRNA expression levels of uncoupling proteins 2 and 3 (UCP2, UCP3) and of protein arginine methyltransferase 1 (PRMT1). Live-cell imaging revealed that T3-induced enhancement of mitochondrial Ca²⁺ uptake depends on the mitochondrial Ca²⁺ uniporter (MCU), UCP2, and PRMT1 that are essential for increased mitochondrial ATP ([ATP]_mito) production after T3 treatment. Besides, increased [Ca²⁺]_mito and [ATP]_mito levels correlated with enhanced production of reactive oxygen species (ROS) in mitochondria. Notably, ROS scavenging causes mitochondrial Ca²⁺ elevation and outplays the impact of T3 on [Ca²⁺]_mito homeostasis. Based on these results, we assume that thyroid hormones adjust [Ca²⁺]_mito homeostasis by modulating the UCP2- and PRMT1-balanced [Ca²⁺]_mito uptake via MCU in case of physiological ROS levels to convey their impact on mitochondrial ATP and ROS production.

Citrin mediated metabolic rewiring in response to altered basal subcellular Ca2+ homeostasis

In contrast to long-term metabolic reprogramming, metabolic rewiring represents an instant and reversible cellular adaptation to physiological or pathological stress. Ca2+ signals of distinct spatio-temporal patterns control a plethora of signaling processes and can determine basal cellular metabolic setting, however, Ca2+ signals that define metabolic rewiring have not been conclusively identified and characterized. Here, we reveal the existence of a basal Ca2+ flux originating from extracellular space and delivered to mitochondria by Ca2+ leakage from inositol triphosphate receptors in mitochondria-associated membranes. This Ca2+ flux primes mitochondrial metabolism by maintaining glycolysis and keeping mitochondria energized for ATP production. We identified citrin, a well-defined Ca2+-binding component of malate-aspartate shuttle in the mitochondrial intermembrane space, as predominant target of this basal Ca2+ regulation. Our data emphasize that any manipulation of this ubiquitous Ca2+ system has the potency to initiate metabolic rewiring as an instant and reversible cellular adaptation to physiological or pathological stress.

Cristae junction as a fundamental switchboard for mitochondrial ion signaling and bioenergetics

OPA1 and MICU1 are both involved in the regulation of mitochondrial Ca2+ uptake and the stabilization of the cristae junction, which separates the inner mitochondrial membrane into the interboundary membrane and the cristae membrane. In this mini-review, we focus on the synergetic control of OPA1 and MICU1 on the cristae junction that serves as a fundamental regulator of multiple mitochondrial functions. In particular, we point to the critical role of an adaptive cristae junction permeability in mitochondrial Ca2+ signaling, spatial H+ gradients and mitochondrial membrane potential, metabolic activity, and apoptosis. These characteristics bear on a distinct localization of the oxidative phosphorylation machinery, the FoF1-ATPase, and mitochondrial Ca2+uniporter (MCU) within sections of the inner mitochondrial membrane isolated by the cristae junction and regulated by proteins like OPA1 and MICU1. We specifically focus on the impact of MICU1-regulated cristae junction on the activity and distribution of MCU within the complex ultrastructure of mitochondria.

Near-UV Light Induced ROS Production Initiates Spatial Ca2+ Spiking to Fire NFATc3 Translocation

Ca2+-dependent gene regulation controls several functions to determine the fate of the cells. Proteins of the nuclear factor of activated T-cells (NFAT) family are Ca2+ sensitive transcription factors that control the cell growth, proliferation and insulin secretion in β-cells. Translocation of NFAT proteins to the nucleus occurs in a sequence of events that starts with activating calmodulin-dependent phosphatase calcineurin in a Ca2+-dependent manner, which dephosphorylates the NFAT proteins and leads to their translocation to the nucleus. Here, we examined the role of IP3-generating agonists and near-UV light in the induction of NFATc3 migration to the nucleus in the pancreatic β-cell line INS-1. Our results show that IP3 generation yields cytosolic Ca2+ rise and NFATc3 translocation. Moreover, near-UV light exposure generates reactive oxygen species (ROS), resulting in cytosolic Ca2+ spiking via the L-type Ca2+ channel and triggers NFATc3 translocation to the nucleus. Using the mitochondria as a Ca2+ buffering tool, we showed that ROS-induced cytosolic Ca2+ spiking, not the ROS themselves, was the triggering mechanism of nuclear import of NFATc3. Collectively, this study reveals the mechanism of near-UV light induced NFATc3 migration.

Sigma-1 Receptor Promotes Mitochondrial Bioenergetics by Orchestrating ER Ca2+ Leak during Early ER Stress

The endoplasmic reticulum (ER) is a complex, multifunctional organelle of eukaryotic cells and responsible for the trafficking and processing of nearly 30% of all human proteins. Any disturbance to these processes can cause ER stress, which initiates an adaptive mechanism called unfolded protein response (UPR) to restore ER functions and homeostasis. Mitochondrial ATP production is necessary to meet the high energy demand of the UPR, while the molecular mechanisms of ER to mitochondria crosstalk under such stress conditions remain mainly enigmatic. Thus, better understanding the regulation of mitochondrial bioenergetics during ER stress is essential to combat many pathologies involving ER stress, the UPR, and mitochondria. This article investigates the role of Sigma-1 Receptor (S1R), an ER chaperone, has in enhancing mitochondrial bioenergetics during early ER stress using human neuroblastoma cell lines. Our results show that inducing ER stress with tunicamycin, a known ER stressor, greatly enhances mitochondrial bioenergetics in a time- and S1R-dependent manner. This is achieved by enhanced ER Ca2+ leak directed towards mitochondria by S1R during the early phase of ER stress. Our data point to the importance of S1R in promoting mitochondrial bioenergetics and maintaining balanced H2O2 metabolism during early ER stress.

Different Roles of p62 (SQSTM1) Isoforms in Keratin-Related Protein Aggregation

p62/Sequestosome-1 (p62) is a multifunctional adaptor protein and is also a constant component of disease-associated protein aggregates, including Mallory–Denk bodies (MDBs), in steatohepatitis and hepatocellular carcinoma. We investigated the interaction of the two human p62 isoforms, p62-H1 (full-length isoform) and p62-H2 (partly devoid of PB1 domain), with keratins 8 and 18, the major components of MDBs. In human liver, p62-H2 is expressed two-fold higher compared to p62-H1 at the mRNA level and is present in slightly but not significantly higher concentrations at the protein level. Co-transfection studies in CHO-K1 cells, PLC/PRF/5 cells as well as p62⁻ total-knockout and wild-type mouse fibroblasts revealed marked differences in the cytoplasmic distribution and aggregation behavior of the two p62 isoforms. Transfection-induced overexpression of p62-H2 generated large cytoplasmic aggregates in PLC/PRF/5 and CHO-K1 cells that mostly co-localized with transfected keratins resembling MDBs or (transfection without keratins) intracytoplasmic hyaline bodies. In fibroblasts, however, transfected p62-H2 was predominantly diffusely distributed in the cytoplasm. Aggregation of p62-H2 and p62ΔSH2 as well as the interaction with K8 (but not with K18) involves acquisition of cross-β-sheet conformation as revealed by staining with luminescent conjugated oligothiophenes. These results indicate the importance of considering p62 isoforms in protein aggregation disease.

Dynamic Control of Mitochondrial Ca2+ Levels as a Survival Strategy of Cancer Cells

Cancer cells have increased energy requirements due to their enhanced proliferation activity. This energy demand is, among others, met by mitochondrial ATP production. Since the second messenger Ca²⁺ maintains the activity of Krebs cycle dehydrogenases that fuel mitochondrial respiration, proper mitochondrial Ca²⁺ uptake is crucial for a cancer cell survival. However, a mitochondrial Ca²⁺ overload induces mitochondrial dysfunction and, ultimately, apoptotic cell death. Because of the vital importance of balancing mitochondrial Ca²⁺ levels, a highly sophisticated machinery of multiple proteins manages mitochondrial Ca²⁺ homeostasis. Notably, mitochondria sequester Ca²⁺ preferentially at the interaction sites between mitochondria and the endoplasmic reticulum (ER), the largest internal Ca²⁺ store, thus, pointing to mitochondrial-associated membranes (MAMs) as crucial hubs between cancer prosperity and cell death. To investigate potential regulatory mechanisms of the mitochondrial Ca²⁺ uptake routes in cancer cells, we modulated mitochondria–ER tethering and the expression of UCP2 and analyzed mitochondrial Ca²⁺ homeostasis under the various conditions. Hence, the expression of contributors to mitochondrial Ca²⁺ regulation machinery was quantified by qRT-PCR. We further used data from The Cancer Genome Atlas (TCGA) to correlate these in vitro findings with expression patterns in human breast invasive cancer and human prostate adenocarcinoma. ER-mitochondrial linkage was found to support a mitochondrial Ca²⁺ uptake route dependent on uncoupling protein 2 (UCP2) in cancer cells. Notably, combined overexpression of Rab32, a protein kinase A-anchoring protein fostering the ER-mitochondrial tethering, and UCP2 caused a significant drop in cancer cells' viability. Artificially enhanced ER-mitochondrial tethering further initiated a sudden decline in the expression of UCP2, probably as an adaptive response to avoid mitochondrial Ca²⁺ overload. Besides, TCGA analysis revealed an inverse expression correlation between proteins stabilizing mitochondrial-ER linkage and UCP2 in tissues of human breast invasive cancer and prostate adenocarcinoma. Based on these results, we assume that cancer cells successfully manage mitochondrial Ca²⁺ uptake to stimulate Ca²⁺-dependent mitochondrial metabolism while avoiding Ca²⁺-triggered cell death by fine-tuning ER-mitochondrial tethering and the expression of UCP2 in an inversed manner. Disruption of this equilibrium yields cancer cell death and may serve as a treatment strategy to specifically kill cancer cells.

ER-to-Golgi Transport in HeLa Cells Displays High Resilience to Ca2+ and Energy Stresses

One third of all human proteins are either transmembrane or soluble secretory proteins that first target the endoplasmic reticulum (ER). These proteins subsequently leave the ER and enter the Golgi apparatus via ER-Golgi intermediate vesicular structures. Live-cell imaging of cargos fused to fluorescent proteins (FPs) enables the high-resolution visualization and characterization of secretory transport processes. Here, we performed fluorescence time-lapse imaging to assess the Ca2+ and energy dependency of ER-to-Golgi transport in living HeLa cells, a cancer cell model which has been well investigated. Our data revealed that ER-to-Golgi transport remained highly efficient in the absence of ATP-generating substrates, despite clear reductions in cytosolic and mitochondrial ATP levels under these energy stress conditions. However, cell treatment with 2-deoxy-D-glucose (2-DG), which severely diminished subcellular ATP levels, abolished ER-to-Golgi transport. Interestingly, while 2-DG elevated cytosolic Ca2+ levels and reduced long-distance movements of glycosylphosphatidylinositol (GPI)-positive vesicles, robust short-term ER Ca2+ mobilizations, which strongly affected the motility of these vesicles, did not considerably impair ER-to-Golgi transport. In summary, we highlight that ER-to-Golgi transport in HeLa cells remains functional despite high energy and Ca2+ stress levels.

The contribution of uncoupling protein 2 to mitochondrial Ca2+ homeostasis in health and disease - A short revisit

Considering the versatile functions attributed to uncoupling protein 2 (UCP2) in health and disease, a profound understanding of the protein's molecular actions under physiological and pathophysiological conditions is indispensable. This review aims to revisit and shed light on the fundamental molecular functions of UCP2 in mitochondria, with particular emphasis on its intricate role in regulating mitochondrial calcium (Ca²⁺) uptake. UCP2's modulating effect on various vital processes in mitochondria makes it a crucial regulator of mitochondrial homeostasis in health and disease.

ATGL/CGI-58-Dependent Hydrolysis of a Lipid Storage Pool in Murine Enterocytes

As circulating lipid levels are balanced by the rate of lipoprotein release and clearance from the plasma, lipid absorption in the small intestine critically contributes to the maintenance of whole-body lipid homeostasis. Within enterocytes, excessive triglycerides are transiently stored as cytosolic lipid droplets (cLDs), and their mobilization sustains lipid supply during interprandial periods. Using mice lacking adipose triglyceride lipase (ATGL) and its coactivator comparative gene identification-58 (CGI-58) exclusively in the intestine (intestine-specific double KO [iDKO]), we show that ATGL/CGI-58 are not involved in providing substrates for chylomicron synthesis. Massive intestinal cLD accumulation in iDKO mice independent of dietary lipids together with inefficient lipid incorporation into cLDs in the early absorption phase demonstrate the existence of a secretion/re-uptake cycle, corroborating the availability of two diverse cLD pools. This study identified ATGL/CGI-58 as critical players in the catabolism of basolaterally (blood) derived lipids and highlights the necessity to modify the current model of intestinal lipid metabolism.

Endothelial lipase increases eNOS activating capacity of high-density lipoprotein

Endothelial lipase (EL) changes structural and functional properties of high-density lipoprotein (HDL). HDL is a relevant modulator of endothelial nitric oxide synthase (eNOS) activity, but the effect of EL on HDL induced eNOS-activation has not yet been investigated. Here, we examined the impact of EL-modified HDL (EL-HDL) on eNOS activity, subcellular trafficking, and eNOS- dependent vasorelaxation. EL-HDL and empty virus (EV)-HDL as control were isolated from human serum incubated with EL-overexpressing or EV infected HepG2 cells. EL-HDL exhibited higher capacity to induce eNOS phosphorylation at Ser1177 and eNOS activity in EA.hy 926 cells, as well as eNOS-dependent vasorelaxation of mouse aortic rings compared to control HDL. As revealed by confocal and structured illumination-microscopy EL-HDL-driven induction of eNOS was accompanied by an increased eNOS-GFP targeting to the plasma membrane and a lower eNOS-GFP colocalization with Golgi and mitochondria. Widefield microscopy of filipin stained cells revealed that EL-HDL lowered cellular free cholesterol (FC) and as found by thin-layer chromatography increased cellular cholesterol ester (CE) content. Additionally, cholesterol efflux capacity, acyl-coenzyme A: cholesterol acyltransferase activity, and HDL particle uptake were comparable between EL-HDL and control HDL. In conclusion, EL increases eNOS activating capacity of HDL, a phenomenon accompanied by an enrichment of the plasma membrane eNOS pool, a decreased cell membrane FC and increased cellular CE content.

Real-Time Imaging of Mitochondrial ATP Dynamics Reveals the Metabolic Setting of Single Cells

Reprogramming of metabolic pathways determines cell functions and fate. In our work, we have used organelle-targeted ATP biosensors to evaluate cellular metabolic settings with high resolution in real time. Our data indicate that mitochondria dynamically supply ATP for glucose phosphorylation in a variety of cancer cell types. This hexokinase-dependent process seems to be reversed upon the removal of glucose or other hexose sugars. Our data further verify that mitochondria in cancer cells have increased ATP consumption. Similar subcellular ATP fluxes occurred in young mouse embryonic fibroblasts (MEFs). However, pancreatic beta cells, senescent MEFs, and MEFs lacking mitofusin 2 displayed completely different mitochondrial ATP dynamics, indicative of increased oxidative phosphorylation. Our findings add perspective to the variability of the cellular bioenergetics and demonstrate that live cell imaging of mitochondrial ATP dynamics is a powerful tool to evaluate metabolic flexibility and heterogeneity at a single-cell level.

Visualization of Sirtuin 4 Distribution between Mitochondria and the Nucleus, Based on Bimolecular Fluorescence Self-Complementation

Mitochondrial sirtuins (Sirts) control important cellular processes related to stress. Despite their regulatory importance, however, the dynamics and subcellular distributions of Sirts remain debatable. Here, we investigate the subcellular localization of sirtuin 4 (Sirt4), a sirtuin variant with a mitochondrial targeting sequence (MTS), by expressing Sirt4 fused to the superfolder green fluorescent protein (Sirt4-sfGFP) in HeLa and pancreatic β-cells. Super resolution fluorescence microscopy revealed the trapping of Sirt4-sfGFP to the outer mitochondrial membrane (OMM), possibly due to slow mitochondrial import kinetics. In many cells, Sirt4-sfGFP was also present within the cytosol and nucleus. Moreover, the expression of Sirt4-sfGFP induced mitochondrial swelling in HeLa cells. In order to bypass these effects, we applied the self-complementing split fluorescent protein (FP) technology and developed mito-STAR (mitochondrial sirtuin 4 tripartite abundance reporter), a tripartite probe for the visualization of Sirt4 distribution between mitochondria and the nucleus in single cells. The application of mito-STAR proved the importation of Sirt4 into the mitochondrial matrix and demonstrated its localization in the nucleus under mitochondrial stress conditions. Moreover, our findings highlight that the self-complementation of split FP is a powerful technique to study protein import efficiency in distinct cellular organelles.

Presenilin-1 Established ER-Ca2+ Leak: a Follow Up on Its Importance for the Initial Insulin Secretion in Pancreatic Islets and β-Cells upon Elevated Glucose

BACKGROUND/AIMS

In our recent work, the importance of GSK3β-mediated phosphorylation of presenilin-1 as crucial process to establish a Ca²⁺ leak in the endoplasmic reticulum and, subsequently, the pre-activation of resting mitochondrial activity in β-cells was demonstrated. The present work is a follow-up and reveals the importance of GSK3β-phosphorylated presenilin-1 for responsiveness of pancreatic islets and β-cells to elevated glucose in terms of cytosolic Ca²⁺ spiking and insulin secretion.

METHODS

Freshly isolated pancreatic islets and the two pancreatic β-cell lines INS-1 and MIN-6 were used. Cytosolic Ca²⁺ was fluorometrically monitored using Fura-2/AM and cellular insulin content and secretion were measured by ELISA.

RESULTS

Our data strengthened our previous findings of the existence of a presenilin-1-mediated ER-Ca²⁺ leak in β-cells, since a reduction of presenilin-1 expression strongly counteracted the ER Ca²⁺ leak. Furthermore, our data revealed that cytosolic Ca²⁺ spiking upon administration of high D-glucose was delayed in onset time and strongly reduced in amplitude and frequency upon siRNA-mediated knock-down of presenilin-1 or the inhibition of GSK3β in the pancreatic β-cells. Moreover, glucose-triggered initial insulin secretion disappeared by depletion from presenilin-1 and inhibition of GSK3β in the pancreatic β-cells and isolated pancreatic islets, respectively.

CONCLUSION

These data complement our previous work and demonstrate that the sensitivity of pancreatic islets and β-cells to glucose illustrated as glucose-triggered cytosolic Ca²⁺ spiking and initial but not long-lasting insulin secretion crucially depends on a strong ER Ca²⁺ leak that is due to the phosphorylation of presenilin-1 by GSK3β, a phenomenon that might be involved in the development of type 2 diabetes.

Tracking intra- and inter-organelle signaling of mitochondria

Mitochondria are as highly specialized organelles and masters of the cellular energy metabolism in a constant and dynamic interplay with their cellular environment, providing adenosine triphosphate, buffering Ca²⁺ and fundamentally contributing to various signaling pathways. Hence, such broad field of action within eukaryotic cells requires a high level of structural and functional adaptation. Therefore, mitochondria are constantly moving and undergoing fusion and fission processes, changing their shape and their interaction with other organelles. Moreover, mitochondrial activity gets fine-tuned by intra- and interorganelle H⁺ , K⁺ , Na⁺ , and Ca²⁺ signaling. In this review, we provide an up-to-date overview on mitochondrial strategies to adapt and respond to, as well as affect, their cellular environment. We also present cutting-edge technologies used to track and investigate subcellular signaling, essential to the understanding of various physiological and pathophysiological processes.

Development and Application of Sub-Mitochondrial Targeted Ca2 + Biosensors

Mitochondrial Ca2+ uptake into the mitochondrial matrix is a well-established mechanism. However, the sub-organellar Ca2+ kinetics remain elusive. In the present work we identified novel site-specific targeting sequences for the intermembrane space (IMS) and the cristae lumen (CL). We used these novel targeting peptides to develop green- and red- Ca2+ biosensors targeted to the IMS and to the CL. Based on their distinctive spectral properties, and comparable sensitivities these novel constructs were suitable to visualize Ca2+-levels in various (sub) compartments in a multi-chromatic manner. Functional studies that applied these new biosensors revealed that knockdown of MCU and EMRE yielded elevated Ca2+ levels inside the CL but not the IMS in response to IP3-generating agonists. Knockdown of VDAC1, however, strongly impeded the transfer of Ca2+ through the OMM while the cytosolic Ca2+ signal remained unchanged. The novel sub-mitochondrially targeted Ca2+ biosensors proved to be suitable for Ca2+ imaging with high spatial and temporal resolution in a multi-chromatic manner allowing simultaneous measurements. These informative biosensors will facilitate efforts to dissect the complex sub-mitochondrial Ca2+ signaling under (patho)physiological conditions.

MICU1 controls cristae junction and spatially anchors mitochondrial Ca2+ uniporter complex

Recently identified core proteins (MICU1, MCU, EMRE) forming the mitochondrial Ca²⁺ uniporter complex propelled investigations into its physiological workings. Here, we apply structured illumination microscopy to visualize and localize these proteins in living cells. Our data show that MICU1 localizes at the inner boundary membrane (IBM) due to electrostatic interaction of its polybasic domain. Moreover, this exclusive localization of MICU1 is important for the stability of cristae junctions (CJ), cytochrome c release and mitochondrial membrane potential. In contrast to MICU1, MCU and EMRE are homogeneously distributed at the inner mitochondrial membrane under resting conditions. However, upon Ca²⁺ elevation MCU and EMRE dynamically accumulate at the IBM in a MICU1-dependent manner. Eventually, our findings unveil an essential function of MICU1 in CJ stabilization and provide mechanistic insights of how sophistically MICU1 controls the MCU-Complex while maintaining the structural mitochondrial membrane framework.

pH-Lemon, a Fluorescent Protein-Based pH Reporter for Acidic Compartments

Distinct subcellular pH levels, especially in lysosomes and endosomes, are essential for the degradation, modification, sorting, accumulation, and secretion of macromolecules. Here, we engineered a novel genetically encoded pH probe by fusing the pH-stable cyan fluorescent protein (FP) variant, mTurquoise2, to the highly pH-sensitive enhanced yellow fluorescent protein, EYFP. This approach yielded a ratiometric biosensor-referred to as pH-Lemon-optimized for live imaging of distinct pH conditions within acidic cellular compartments. Protonation of pH-Lemon under acidic conditions significantly decreases the yellow fluorescence while the cyan fluorescence increases due to reduced Förster resonance energy transfer (FRET) efficiency. Because of its freely reversible and ratiometric responses, pH-Lemon represents a fluorescent biosensor for pH dynamics. pH-Lemon also shows a sizable pH-dependent fluorescence lifetime change that can be used in fluorescence lifetime imaging microscopy as an alternative observation method for the study of pH in acidic cellular compartments. Fusion of pH-Lemon to the protein microtubule-associated protein 1A/1B-light chain 3B (LC3B), a specific marker of autophagic membranes, resulted in its targeting within autolysosomes of HeLa cells. Moreover, fusion of pH-Lemon to a glycophosphatidylinositol (GPI) anchor allowed us to monitor the entire luminal space of the secretory pathway and the exoplasmic leaflet of the plasma membrane. Utilizing this new pH probe, we revealed neutral and acidic vesicles and substructures inside cells, highlighting compartments of distinct pH throughout the endomembrane system. These data demonstrate, that this novel pH sensor, pH-Lemon, is very suitable for the study of local pH dynamics of subcellular microstructures in living cells.

Glycogen Synthase Kinase 3 Beta Controls Presenilin-1-Mediated Endoplasmic Reticulum Ca²⁺ Leak Directed to Mitochondria in Pancreatic Islets and β-Cells

Background/Aims

In pancreatic β-cells, the intracellular Ca²⁺ homeostasis is an essential regulator of the cells’ major functions. The endoplasmic reticulum (ER) as interactive intracellular Ca²⁺ store balances cellular Ca²⁺. In this study basal ER Ca²⁺ homeostasis was evaluated in order to reveal potential β-cell-specificity of ER Ca²⁺ handling and its consequences for mitochondrial Ca²⁺, ATP and respiration.

Methods

The two pancreatic cell lines INS-1 and MIN-6, freshly isolated pancreatic islets, and the two non-pancreatic cell lines HeLA and EA.hy926 were used. Cytosolic, ER and mitochondrial Ca²⁺ and ATP measurements were performed using single cell fluorescence microscopy and respective (genetically-encoded) sensors/dyes. Mitochondrial respiration was monitored by respirometry. GSK3β activity was measured with ELISA.

Results

An atypical ER Ca²⁺ leak was observed exclusively in pancreatic islets and β-cells. This continuous ER Ca²⁺ efflux is directed to mitochondria and increases basal respiration and organellar ATP levels, is established by GSK3β-mediated phosphorylation of presenilin-1, and is prevented by either knockdown of presenilin-1 or an inhibition/knockdown of GSK3β. Expression of a presenlin-1 mutant that mimics GSK3β-mediated phosphorylation established a β-cell-like ER Ca²⁺ leak in HeLa and EA.hy926 cells. The ER Ca²⁺ loss in β-cells was compensated at steady state by Ca²⁺ entry that is linked to the activity of TRPC3.

Conclusion

Pancreatic β-cells establish a cell-specific ER Ca²⁺ leak that is under the control of GSK3β and directed to mitochondria, thus, reflecting a cell-specific intracellular Ca²⁺ handling for basal mitochondrial activity.

Genetic biosensors for imaging nitric oxide in single cells

Over the last decades a broad collection of sophisticated fluorescent protein-based probes was engineered with the aim to specifically monitor nitric oxide (NO), one of the most important signaling molecules in biology. Here we report and discuss the characteristics and fields of applications of currently available genetically encoded fluorescent sensors for the detection of NO and its metabolites in different cell types.

LONG ABSTRACT

Because of its radical nature and short half-life, real-time imaging of NO on the level of single cells is challenging. Herein we review state-of-the-art genetically encoded fluorescent sensors for NO and its byproducts such as peroxynitrite, nitrite and nitrate. Such probes enable the real-time visualization of NO signals directly or indirectly on the level of single cells and cellular organelles and, hence, extend our understanding of the spatiotemporal dynamics of NO formation, diffusion and degradation. Here, we discuss the significance of NO detection in individual cells and on subcellular level with genetic biosensors. Currently available genetically encoded fluorescent probes for NO and nitrogen species are critically discussed in order to provide insights in the functionality and applicability of these promising tools. As an outlook we provide ideas for novel approaches for the design and application of improved NO probes and fluorescence imaging protocols.

Enhanced inter-compartmental Ca2+ flux modulates mitochondrial metabolism and apoptotic threshold during aging

Background

Senescence is characterized by a gradual decline in cellular functions, including changes in energy homeostasis and decreased proliferation activity. As cellular power plants, contributors to signal transduction, sources of reactive oxygen species (ROS) and executors of programmed cell death, mitochondria are in a unique position to affect aging-associated processes of cellular decline. Notably, metabolic activation of mitochondria is tightly linked to Ca²⁺ due to the Ca²⁺ -dependency of several enzymes in the Krebs cycle, however, overload of mitochondria with Ca²⁺ triggers cell death pathways. Consequently, a machinery of proteins tightly controls mitochondrial Ca²⁺ homeostasis as well as the exchange of Ca²⁺ between the different cellular compartments, including Ca²⁺ flux between mitochondria and the endoplasmic reticulum (ER).

Methods

In this study, we investigated age-related changes in mitochondrial Ca²⁺ homeostasis, mitochondrial-ER linkage and the activity of the main ROS production site, the mitochondrial respiration chain, in an in vitro aging model based on porcine aortic endothelial cells (PAECs), using high-resolution live cell imaging, proteomics and various molecular biological methods.

Results

We describe that in aged endothelial cells, increased ER-mitochondrial Ca²⁺ crosstalk occurs due to enhanced ER-mitochondrial tethering. The close functional inter-organelle linkage increases mitochondrial Ca²⁺ uptake and thereby the activity of the mitochondrial respiration, but also makes senescent cells more vulnerable to mitochondrial Ca²⁺-overload-induced cell death. Moreover, we identified the senolytic properties of the polyphenol resveratrol, triggering cell death via mitochondrial Ca²⁺ overload exclusively in senescent cells.

Conclusion

By unveiling aging-related changes in the inter-organelle tethering and Ca²⁺ communications we have advanced the understanding of endothelial aging and highlighted a potential basis to develop drugs specifically targeting senescent cells.

Intracellular Ca2+ release decelerates mitochondrial cristae dynamics within the junctions to the endoplasmic reticulum

Mitochondria are multifunctional organelles that essentially contribute to cell signaling by sophisticated mechanisms of communications. Live cell imaging studies showed that mitochondria are dynamic and complex structures that form ramified networks by directed movements, fission, and fusion events. There is emerging evidence that the morphology of mitochondria determines cellular functions and vice versa. Several intracellular signaling pathways and messengers including Ca²⁺ dynamically influence the architecture of mitochondria. Because electron microscopy cannot be utilized for an assessment of dynamics of mitochondrial morphology in intact cells, most studies were performed using wide-field or laser confocal fluorescence microscopies that, due to limitations of their spatial resolution, do not allow investigating sub-mitochondrial structures. Accordingly, our understanding of the dynamics of substructures of mitochondria is quite limited. Here, we present a robust super-resolution method to quantify the dynamics of mitochondrial cristae, the main substructures of the inner mitochondrial membrane, exploiting structured illumination microscopy (SIM). We observed that knockdown of the dynamin-like 120-kDa protein, which is encoded by the OPA1 gene, specifically reduces the dynamics of the mitochondrial cristae membranes (CM), while the inner boundary membrane (IBM) remained flexible. We further used dual color SIM to quantify the dynamics of CM in the junction between mitochondria and the endoplasmic reticulum (ER; mitochondrial associated membranes, MAMs). Intracellular Ca²⁺ release spatially reduced CM-dynamics in MAMs. Moreover, CM-dynamics was independent from matrix Ca²⁺ signal. Our data suggest that local Ca²⁺ signals specifically control CM-dynamics and structure to facilitate a well-balanced functional (Ca²⁺) interplay between mitochondria and the ER.

Lysosomal acid lipase regulates fatty acid channeling in brown adipose tissue to maintain thermogenesis

Lysosomal acid lipase (LAL) is the only known enzyme, which hydrolyzes cholesteryl esters and triacylglycerols in lysosomes of multiple cells and tissues. Here, we explored the role of LAL in brown adipose tissue (BAT). LAL-deficient (Lal-/-) mice exhibit markedly reduced UCP1 expression in BAT, modified BAT morphology with accumulation of lysosomes, and mitochondrial dysfunction, consequently leading to regular hypothermic events in mice kept at room temperature. Cold exposure resulted in reduced lipid uptake into BAT, thereby aggravating dyslipidemia and causing life threatening hypothermia in Lal-/- mice. Linking LAL as a potential regulator of lipoprotein lipase activity, we found Angptl4 mRNA expression upregulated in BAT. Our data demonstrate that LAL is critical for shuttling fatty acids derived from circulating lipoproteins to BAT during cold exposure. We conclude that inhibited lysosomal lipid hydrolysis in BAT leads to impaired thermogenesis in Lal-/- mice.

2-Chlorohexadecanoic acid induces ER stress and mitochondrial dysfunction in brain microvascular endothelial cells

Peripheral leukocytes induce blood-brain barrier (BBB) dysfunction through the release of cytotoxic mediators. These include hypochlorous acid (HOCl) that is formed via the myeloperoxidase-H₂O₂-chloride system of activated phagocytes. HOCl targets the endogenous pool of ether phospholipids (plasmalogens) generating chlorinated inflammatory mediators like e.g. 2-chlorohexadecanal and its conversion product 2-chlorohexadecanoic acid (2-ClHA). In the cerebrovasculature these compounds inflict damage to brain microvascular endothelial cells (BMVEC) that form the morphological basis of the BBB. To follow subcellular trafficking of 2-ClHA we synthesized a ‘clickable’ alkyne derivative (2-ClHyA) that phenocopied the biological activity of the parent compound. Confocal and superresolution structured illumination microscopy revealed accumulation of 2-ClHyA in the endoplasmic reticulum (ER) and mitochondria of human BMVEC (hCMEC/D3 cell line). 2-ClHA and its alkyne analogue interfered with protein palmitoylation, induced ER-stress markers, reduced the ER ATP content, and activated transcription and secretion of interleukin (IL)−6 as well as IL-8. 2-ClHA disrupted the mitochondrial membrane potential and induced procaspase-3 and PARP cleavage. The protein kinase R-like ER kinase (PERK) inhibitor GSK2606414 suppressed 2-ClHA-mediated activating transcription factor 4 synthesis and IL-6/8 secretion, but showed no effect on endothelial barrier dysfunction and cleavage of procaspase-3. Our data indicate that 2-ClHA induces potent lipotoxic responses in brain endothelial cells and could have implications in inflammation-induced BBB dysfunction.

Novel genetically encoded fluorescent probes enable real-time detection of potassium in vitro and in vivo

Changes in intra- and extracellular potassium ion (K⁺) concentrations control many important cellular processes and related biological functions. However, our current understanding of the spatiotemporal patterns of physiological and pathological K⁺ changes is severely limited by the lack of practicable detection methods. We developed K⁺-sensitive genetically encoded, Förster resonance energy transfer-(FRET) based probes, called GEPIIs, which enable quantitative real-time imaging of K⁺ dynamics. GEPIIs as purified biosensors are suitable to directly and precisely quantify K⁺ levels in different body fluids and cell growth media. GEPIIs expressed in cells enable time-lapse and real-time recordings of global and local intracellular K⁺ signals. Hitherto unknown Ca²⁺-triggered, organelle-specific K⁺ changes were detected in pancreatic beta cells. Recombinant GEPIIs also enabled visualization of extracellular K⁺ fluctuations in vivo with 2-photon microscopy. Therefore, GEPIIs are relevant for diverse K⁺ assays and open new avenues for live-cell K⁺ imaging.

K⁺ plays an important role in physiology and disease, but the lack of high specificity K⁺ sensors limits our understanding of its spatiotemporal dynamics. Here the authors develop genetically-encoded FRET-based probes able to quantify K⁺ concentration in body fluids, cells and specific organelles.

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

Nitric Oxide (NO•) is a small radical, which mediates multiple important cellular functions in mammals, bacteria and plants. Despite the existence of a large number of methods for detecting NO• in vivo and in vitro, the real-time monitoring of NO• at the single-cell level is very challenging. The physiological or pathological effects of NO• are determined by the actual concentration and dwell time of this radical. Accordingly, methods that allow the single-cell detection of NO• are highly desirable. Recently, we expanded the pallet of NO• indicators by introducing single fluorescent protein-based genetically encoded nitric oxide (NO•) probes (geNOps) that directly respond to cellular NO• fluctuations and, hence, addresses this need. Here we demonstrate the usage of geNOps to assess intracellular NO• signals in response to two different chemical NO•-liberating molecules. Our results also confirm that freshly prepared 3-(2-hydroxy-1-methyl-2-nitrosohydrazino)-N-methyl-1-propanamine (NOC-7) has a much higher potential to evoke change in intracellular NO• levels as compared with the inorganic NO• donor sodium nitroprusside (SNP). Furthermore, dual-color live-cell imaging using the green geNOps (G-geNOp) and the chemical Ca2+ indicator fura-2 was performed to visualize the tight regulation of Ca2+-dependent NO• formation in single endothelial cells. These representative experiments demonstrate that geNOps are suitable tools to investigate the real-time generation and degradation of single-cell NO• signals in diverse experimental setups.

PRMT1-mediated methylation of MICU1 determines the UCP2/3 dependency of mitochondrial Ca(2+) uptake in immortalized cells

Recent studies revealed that mitochondrial Ca²⁺ channels, which control energy flow, cell signalling and death, are macromolecular complexes that basically consist of the pore-forming mitochondrial Ca²⁺ uniporter (MCU) protein, the essential MCU regulator (EMRE), and the mitochondrial Ca²⁺ uptake 1 (MICU1). MICU1 is a regulatory subunit that shields mitochondria from Ca²⁺ 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 Ca²⁺ uptake. Here we clarify the molecular mechanism that determines the UCP2/3 dependency of mitochondrial Ca²⁺ uptake. Our data demonstrate that mitochondrial Ca²⁺ uptake is controlled by protein arginine methyl transferase 1 (PRMT1) that asymmetrically methylates MICU1, resulting in decreased Ca²⁺ sensitivity. UCP2/3 normalize Ca²⁺ sensitivity of methylated MICU1 and, thus, re-establish mitochondrial Ca²⁺ uptake activity. These data provide novel insights in the complex regulation of the mitochondrial Ca²⁺ uniporter by PRMT1 and UCP2/3.

MICU1 is a regulatory subunit of mitochondrial Ca²⁺ channels that shields mitochondria from Ca²⁺ overload. Here the authors show that MICU1 methylation by PRMT1 reduces Ca²⁺ sensitivity, which is normalized by UCP2/3, re-establishing mitochondrial Ca²⁺ uptake activity.

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

Background/Aims

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.

Methods

The two cancer cell lines EA.hy926 and HeLa, and somatic short-term cultured HUVEC were used. Cell viability and caspase 3/7 activity were tested. Mitochondrial, cytosolic and endoplasmic reticulum Ca²⁺ as well as cytosolic and mitochondrial ATP levels were measured using single cell fluorescence microscopy and respective genetically-encoded sensors. Mitochondria-ER junctions were analyzed applying super-resolution SIM and ImageJ-based image analysis.

Results

Resveratrol and piceatannol selectively trigger death in cancer but not somatic cells. Hence, these polyphenols strongly enhanced mitochondrial Ca²⁺ uptake in cancer exclusively. Resveratrol and piceatannol predominantly affect mitochondrial but not cytosolic ATP content that yields in a reduced SERCA activity. Decreased SERCA activity and the strongly enriched tethering of the ER and mitochondria in cancer cells result in an enhanced MCU/Letm1-dependent mitochondrial Ca²⁺ uptake upon intracellular Ca²⁺ release exclusively in cancer cells. Accordingly, resveratrol/piceatannol-induced cancer cell death could be prevented by siRNA-mediated knock-down of MCU and Letm1.

Conclusions

Because their greatly enriched ER-mitochondria tethering, cancer cells are highly susceptible for resveratrol/piceatannol-induced reduction of SERCA activity to yield mitochondrial Ca²⁺ overload and subsequent cancer cell death.

Development of novel FP-based probes for live-cell imaging of nitric oxide dynamics

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.

Active autophagy but not lipophagy in macrophages with defective lipolysis

During autophagy, autophagosomes fuse with lysosomes to degrade damaged organelles and misfolded proteins. Breakdown products are released into the cytosol and contribute to energy and metabolic building block supply, especially during starvation. Lipophagy has been defined as the autophagy-mediated degradation of lipid droplets (LDs) by lysosomal acid lipase. Adipose triglyceride lipase (ATGL) is the major enzyme catalyzing the initial step of lipolysis by hydrolyzing triglycerides (TGs) in cytosolic LDs. Consequently, most organs and cells, including macrophages, lacking ATGL accumulate TGs, resulting in reduced intracellular free fatty acid concentrations. Macrophages deficient in hormone-sensitive lipase (H0) lack TG accumulation albeit reduced in vitro TG hydrolase activity. We hypothesized that autophagy is activated in lipase-deficient macrophages to counteract their energy deficit. We therefore generated mice lacking both ATGL and HSL (A0H0). Macrophages from A0H0 mice showed 73% reduced neutral TG hydrolase activity, resulting in TG-rich LD accumulation. Increased expression of cathepsin B, accumulation of LC3-II, reduced expression of p62 and increased DQ-BSA dequenching suggest intact autophagy and functional lysosomes in A0H0 macrophages. Markedly decreased acid TG hydrolase activity and lipid flux independent of bafilomycin A1 treatment, however, argue against effective lysosomal degradation of LDs in A0H0 macrophages. We conclude that autophagy of proteins and cell organelles but not of LDs is active as a compensatory mechanism to circumvent and balance the reduced availability of energy substrates in A0H0 macrophages.