Calcium signaling around Mitochondria Associated Membranes (MAMs)

A putative mitochondrial calcium uniporter in A. fumigatus contributes to mitochondrial Ca(2+) homeostasis and stress responses

Ca(2+) uptake into mitochondria plays a central role in cell physiology by stimulating ATP production, shaping cytosolic Ca(2+) transients and regulating cell survival or death. Although this system has been studied extensively in mammalian cells, the physiological implications of Ca(2+) uptake into mitochondria in fungal cells are still unknown. In this study, a bi-directional best-hit BLASTP search revealed that the genome of Aspergillus fumigatus encodes a homolog of a putative mitochondrial Ca(2+) uniporter (MCU) and a mitochondrial carrier protein AGC1/MICU1 homolog. Both putative homologs are mitochondrially localized and required for the response to azole and oxidative stress such that the loss of either McuA or AgcA results in reduced susceptibility to azole and oxidative stress, suggesting a role in environmental stress adaptation. Overexpressing mcuA restores the azole-resistance phenotype of the ΔagcA strain to wild-type levels, but not vice versa, indicating McuA plays a dominant role during these stress responses. Using a mitochondrially targeted version of the calcium-sensitive photoprotein aequorin, we found that only mcuA deletion leads to dysfunctional [Ca(2+)]mt and [Ca(2+)]c homeostasis, suggesting that McuA, but not AgcA, contributes to Ca(2+) uptake into mitochondria. Further point-mutation experiments combined with extracellular Ca(2+) chelator treatment verified that two predicted Ca(2+)-binding sites in McuA are required for Ca(2+) uptake into mitochondria and stress responses through the regulation of [Ca(2+)]c homeostasis.

Mitochondria-associated ER membranes and Alzheimer disease

The series of events underlying the pathogenesis of Alzheimer disease (AD) in unknown. The most widely accepted hypothesis is called the amyloid cascade, based on the observation that the brains of AD patients contain high levels of extracellular plaques, composed mainly of β-amyloid (Aβ), and intracellular tangles, composed of hyperphosphorylated forms of the microtubule-associated protein tau. However, AD is also characterized by other features, including aberrant cholesterol, phospholipid, and calcium metabolism, and mitochondrial dysfunction, all ostensibly unrelated to plaque and tangle formation. Notably, these 'other' aspects of AD pathology are functions related to mitochondria-associated ER membranes (MAM), a subdomain of the endoplasmic reticulum (ER) that is apposed to, and communicates with, mitochondria. Given the potential relationship between MAM and AD, we explored the possibility that perturbed MAM function might play a role in AD pathogenesis. We found that γ-secretase activity, which processes the amyloid precursor protein to generate Aβ, is located predominantly in the MAM, and that ER-mitochondrial apposition and MAM function are increased significantly in cells from AD patients. These observations may help explain not only the aberrant Aβ production, but also many of the 'other' biochemical and morphological features of the disease. Based on these, and other, data we propose that AD is fundamentally a disorder of ER-mitochondrial hyperconnectivity.

Intracellular Ca(2+) signaling and Ca(2+) microdomains in the control of cell survival, apoptosis and autophagy

The endoplasmic reticulum (ER), mitochondria and lysosomes are physically and/or functionally linked, establishing close contact sites between these organelles. As a consequence, Ca(2+) release events from the ER, the major intracellular Ca(2+)-storage organelle, have an immediate effect on the physiological function of mitochondria and lysosomes. Also, the lysosomes can act as a Ca(2+) source for Ca(2+) release into the cytosol, thereby influencing ER-based Ca(2+) signaling. Given the important role for mitochondria and lysosomes in cell survival, cell death and cell adaptation processes, it has become increasingly clear that Ca(2+) signals from or towards these organelles impact these processes. In this review, we discuss the most recent insights in the emerging role of Ca(2+) signaling in cellular survival by controlling basal mitochondrial bioenergetics and by regulating apoptosis, a mitochondrial process, and autophagy, a lysosomal process, in response to cell damage and cell stress.

Scrophularia orientalis extract induces calcium signaling and apoptosis in neuroblastoma cells

Effective neuroblastoma (NB) treatments are still limited despite treatment options available today. Therefore, this study attempted to identify novel plant extracts that have anticancer effects. Cytotoxicity and increased intracellular calcium levels were determined using the Sulforhodamine B (SRB) assay and Fluo4-AM (acetoxymethyl) staining and fluorescence microscopy in NB cells in order to screen a library of plant extracts. The current study examined the anticancer effects of a dichloromethane extract from Scrophularia orientalis L. (Scrophulariaceae), a plant that has been used in Traditional Chinese Medicine. This extract contained highly potent agents that significantly reduced cell survival and increased calcium levels in NB cells. Further analysis revealed that cell death induced by this extract was associated with intracellular calcium release, opening of the MPTP, caspase 3- and PARP-cleavage suggesting that this extract induced aberrant calcium signaling that resulted in apoptosis via the mitochondrial pathway. Therefore, agents from Scrophularia orientalis may have the potential to lead to new chemo therapeutic anticancer drugs. Furthermore, targeting intracellular calcium signaling may be a novel strategy to develop more effective treatments for NB.

Methods to Study PTEN in Mitochondria and Endoplasmic Reticulum

Although PTEN has been widely described as a nuclear and cytosolic protein, in the last 2 years, alternative organelles, such as the endoplasmic reticulum (ER), pure mitochondria, and mitochondria-associated membranes (MAMs), have been recognized as pivotal targets of PTEN activity.Here, we describe different methods that have been used to highlight PTEN subcellular localization.First, a protocol to extract nuclear and cytosolic fractions has been described to assess the "canonical" PTEN localization. Moreover, we describe a protocol for mitochondria isolation with proteinase K (PK) to further discriminate whether PTEN associates with the outer mitochondrial membrane (OMM) or resides within the mitochondria. Finally, we focus our attention on a subcellular fractionation protocol of cells that permits the isolation of MAMs containing unique regions of ER membranes attached to the outer mitochondrial membrane (OMM) and mitochondria without contamination from other organelles. In addition to biochemical fractionations, immunostaining can be used to determine the subcellular localization of proteins; thus, a detailed protocol to obtain good immunofluorescence (IF) is described. The employment of these methodological approaches could facilitate the identification of different PTEN localizations in several physiopathological contexts.

Mitochondria, calcium homeostasis and calcium signaling

Са2+ is a very important and versatile intracellular signal which controls numerous biochemical and physiological (pathophysiological) processes in the cell. Good evidence exists that mitochondria are sensors, decoders and regulators of calcium signaling. Precise regulation of calcium signaling in the cell involves numerous molecular targets, which induce and decode changes of Са2+ concentrations in the cell (pumps, channels, Са2+-binding proteins, Са2+-dependent enzymes, localized in the cytoplasm and organelles). Mitochondrial Са2+ uniporter accumulates excess of Са2+ in mitochondria, while Na+/Са2+- and H+/Са2+-antiporters extrude Са2+ in the cytoplasm. Mitochondrial Са2+ overloading results in formation of mitochondria permeability transition pores which play an important role in cell death under many pathological conditions. Mitochondria regulate Са2+ homeostasis and control important cellular functions such as metabolism, proliferation, survival. Identification of cellular and mitochondrial Ca2+ transporters and understanding their functional mechanisms open up new prospects for their using as therapeutic targets

Mitochondrial Dysfunction and Ca(2+) Overload Contributes to Hesperidin Induced Paraptosis in Hepatoblastoma Cells, HepG2

Paraptosis is a programmed cell death which is morphologically and biochemically different from apoptosis. In this study, we have investigated the role of Ca(2+) in hesperidin-induced paraptotic cell death in HepG2 cells. Increase in mitochondrial Ca(2+) level was observed in hesperidin treated HepG2 cells but not in normal liver cancer cells. Inhibition of inositol-1,4,5-triphosphate receptor (IP3 R) and ryanodine receptor also block the mitochondrial Ca(2+) accumulation suggesting that the release of Ca(2+) from the endoplasmic reticulum (ER) may probably lead to the increase in mitochondrial Ca(2+) level. Pretreatment with ruthenium red (RuRed), a Ca(2+) uniporter inhibitor inhibited the hesperidin-induced mitochondrial Ca(2+) overload, swelling of mitochondria, and cell death in HepG2 cells. It has also been demonstrated that mitochondrial Ca(2+) influxes act upstream of ROS and mitochondrial superoxide production. The increased ROS production further leads to mitochondrial membrane loss in hesperidin treated HepG2 cells. Taken together our results show that IP3 R and ryanodine receptor mediated release of Ca(2+) from the ER and its subsequent influx through the uniporter into mitochondria contributes to hesperidin-induced paraptosis in HepG2 cells.

Mitochondrial dysfunction in inflammatory bowel disease

Inflammatory Bowel Disease (IBD) represents a group of idiopathic disorders characterized by chronic or recurring inflammation of the gastrointestinal tract. While the exact etiology of disease is unknown, IBD is recognized to be a complex, multifactorial disease that results from an intricate interplay of genetic predisposition, an altered immune response, changes in the intestinal microbiota, and environmental factors. Together, these contribute to a destruction of the intestinal epithelial barrier, increased gut permeability, and an influx of immune cells. Given that most cellular functions as well as maintenance of the epithelial barrier is energy-dependent, it is logical to assume that mitochondrial dysfunction may play a key role in both the onset and recurrence of disease. Indeed several studies have demonstrated evidence of mitochondrial stress and alterations in mitochondrial function within the intestinal epithelium of patients with IBD and mice undergoing experimental colitis. Although the hallmarks of mitochondrial dysfunction, including oxidative stress and impaired ATP production are known to be evident in the intestines of patients with IBD, it is as yet unclear whether these processes occur as a cause of consequence of disease. We provide a current review of mitochondrial function in the setting of intestinal inflammation during IBD.

Loss of Phagocytic and Antigen Cross-Presenting Capacity in Aging Dendritic Cells Is Associated with Mitochondrial Dysfunction

Impaired functionality of dendritic cells (DCs) significantly contributes to decreased adaptive immune responses in aged hosts. The expression of MHC-peptide on the DC surface is the critical first step in T cell priming, but few studies have addressed the effect of aging on Ag acquisition, processing, and presentation by DCs. In this study, we show that aged murine DCs were less efficient in the cross-presentation of cell-associated Ag and subsequently in the cross-priming of CD8(+) T cells than were their young counterparts. The decreased cross-presentation was associated with a reduction in the frequency of CD8α DCs and merocytic (CD8α(-)CD11b(-))DCs that could endocytose cell-associated Ag, as well as the number and the size of the endocytosed particles in the DC that did internalize cell-associated materials. Mechanistically, phagocytic capacity has been associated with mitochondrial activity and membrane potential (Δψm). Aged DCs exhibited profound signs of mitochondrial dysfunction, illustrated by lower Δψm, reduced ATP turnover and coupling efficiency, decreased baseline oxidative phosphorylation, and greater proton leak and reactive oxygen species (ROS) production. Mimicking the aged metabolic phenotype in young DCs by pharmacologic manipulation indicated that the reductions in Δψm and ATP impeded the phagocytic capacity whereas ROS interfered with a later step in the cross-presentation process. Conversely, in vitro scavenging of ROS partially restored cross-presentation by aged DCs. Taken together, these data suggest that improvement of aged DC functionality might be feasible in the elderly by targeting metabolic dysfunction or its downstream sequelae, thereby opening new avenues for enhancing vaccine efficiency in this population.

Ca2+ influx-mediated dilation of the endoplasmic reticulum and c-FLIPL downregulation trigger CDDO-Me-induced apoptosis in breast cancer cells

The synthetic triterpenoid 2-cyano-3, 12-dioxooleana-1, 9(11)-dien-C28-methyl ester (CDDO-Me) is considered a promising anti-tumorigenic compound. In this study, we show that treatment with CDDO-Me induces progressive endoplasmic reticulum (ER)-derived vacuolation in various breast cancer cells and ultimately kills these cells by inducing apoptosis. We found that CDDO-Me-induced increases in intracellular Ca2+ levels, reflecting influx from the extracellular milieu, make a critical contribution to ER-derived vacuolation and subsequent cell death. In parallel with increasing Ca2+ levels, CDDO-Me markedly increased the generation of reactive oxygen species (ROS). Interestingly, there exists a reciprocal positive-regulatory loop between Ca2+ influx and ROS generation that triggers ER stress and ER dilation in response to CDDO-Me. In addition, CDDO-Me rapidly reduced the protein levels of c-FLIPL (cellular FLICE-inhibitory protein) and overexpression of c-FLIPL blocked CDDO-Me-induced cell death, but not vacuolation. These results suggest that c-FLIPL downregulation is a key contributor to CDDO-Me-induced apoptotic cell death, independent of ER-derived vacuolation. Taken together, our results show that ER-derived vacuolation via Ca2+ influx and ROS generation as well as caspase activation via c-FLIPL downregulation are responsible for the potent anticancer effects of CDDO-Me on breast cancer cells.

Focus on PTEN Regulation

The role of phosphatase and tensin homolog on chromosome 10 (PTEN) as a tumor suppressor has been for a long time attributed to its lipid phosphatase activity against PI(3,4,5)P₃, the phospholipid product of the class I PI3Ks. Besides its traditional role as a lipid phosphatase at the plasma membrane, a wealth of data has shown that PTEN can function independently of its phosphatase activity and that PTEN also exists and plays a role in the nucleus, in cytoplasmic organelles, and extracellularly. Accumulating evidence has shed light on diverse physiological functions of PTEN, which are accompanied by a complex regulation of its expression and activity. PTEN levels and function are regulated transcriptionally, post-transcriptionally, and post-translationally. PTEN is also sensitive to regulation by its interacting proteins and its localization. Herein, we summarize the current knowledge on mechanisms that regulate the expression and enzymatic activity of PTEN and its role in human diseases.

Mitochondria-Associated Endoplasmic Reticulum Membranes Microenvironment: Targeting Autophagic and Apoptotic Pathways in Cancer Therapy

Autophagy is a tightly regulated catabolic pathway that terminates in the lysosomal compartment after the formation of a cytoplasmic vacuole that engulfs macromolecules and organelles. Notably, autophagy is associated with several human pathophysiological conditions, playing either a cytoprotective or cytopathic role. Many studies have investigated the role of autophagy in cancer. However, whether autophagy suppresses tumorigenesis or provides cancer cells with a rescue mechanism under unfavorable conditions remains unclear. Mitochondria-associated membranes (MAMs) are juxtaposed between the endoplasmic reticulum and mitochondria and have been identified as critical hubs in the regulation of apoptosis and tumor growth. One key function of MAMs is to provide asylum to a number of proteins with tumor suppressor and oncogenic properties. Accordingly, mechanistic studies during tumor progression suggest a strong involvement of these proteins at various steps of the autophagic process. This paper discusses the present state of our knowledge about the intimate molecular networks between MAMs and autophagy in cancer cells and addresses how these networks might be manipulated to improve anticancer therapeutics.

Role of mitochondrial dysfunction and dysregulation of Ca(2+) homeostasis in insulin insensitivity of mammalian cells

Mitochondria and endoplasmic reticulum (ER) play an important role in the maintenance of intracellular Ca(2+) homeostasis, and their defects may be etiological factors contributing to insulin resistance and type 2 diabetes (T2D). Recent studies indicate that alterations of Ca(2+) levels and Ca(2+) -dependent signaling pathways can impede the insulin signaling cascade, resulting in insulin resistance of β cells and insulin-responsive cells. Mitochondria-associated ER membranes (MAMs) are essential for efficient communication between the ER and mitochondria. Thus, abnormalities in the structure and function of MAMs in affected tissue cells in T2D are an important area of study. Recently, we demonstrated that a deficiency of Cisd2, an iron-sulfur protein localized on MAMs, could lead to mitochondrial dysfunction and disturbance of intracellular Ca(2+) homeostasis. Moreover, we first elucidated that defects in the function of MAMs in Ca(2+) uptake resulted in insulin insensitivity of adipocytes, which plays an important role in the pathogenesis of diabetes in Cisd2 knockout mice. On the basis of these observations, we suggest improving the bioenergetic function of mitochondria and the function of MAMs in maintaining Ca(2+) homeostasis as a novel strategy for the development of new therapeutics aimed at preventing and treating insulin resistance and T2D.

Release of Ca2+ from the endoplasmic reticulum and its subsequent influx into mitochondria trigger celastrol-induced paraptosis in cancer cells

Celastrol, a triterpene extracted from the Chinese “Thunder of God Vine”, is known to have anticancer activity, but its underlying mechanism is not completely understood. In this study, we show that celastrol kills several breast and colon cancer cell lines by induction of paraptosis, a cell death mode characterized by extensive vacuolization that arises via dilation of the endoplasmic reticulum (ER) and mitochondria. Celastrol treatment markedly increased mitochondrial Ca²⁺ levels and induced ER stress via proteasome inhibition in these cells. Both MCU (mitochondrial Ca²⁺ uniporter) knockdown and pretreatment with ruthenium red, an inhibitor of MCU, inhibited celastrol-induced mitochondrial Ca²⁺ uptake, dilation of mitochondria/ER, accumulation of poly-ubiquitinated proteins, and cell death in MDA-MB 435S cells. Inhibition of the IP₃ receptor (IP₃R) with 2-aminoethoxydiphenyl borate (2-APB) also effectively blocked celastrol-induced mitochondrial Ca²⁺ accumulation and subsequent paraptotic events. Collectively, our results show that the IP₃R-mediated release of Ca²⁺ from the ER and its subsequent MCU-mediated influx into mitochondria critically contribute to celastrol-induced paraptosis in cancer cells.

Mutations of C19orf12, coding for a transmembrane glycine zipper containing mitochondrial protein, cause mis-localization of the protein, inability to respond to oxidative stress and increased mitochondrial Ca²⁺

Mutations in C19orf12 have been identified in patients affected by Neurodegeneration with Brain Iron Accumulation (NBIA), a clinical entity characterized by iron accumulation in the basal ganglia. By using western blot analysis with specific antibody and confocal studies, we showed that wild-type C19orf12 protein was not exclusively present in mitochondria, but also in the Endoplasmic Reticulum (ER) and MAM (Mitochondria Associated Membrane), while mutant C19orf12 variants presented a different localization. Moreover, after induction of oxidative stress, a GFP-tagged C19orf12 wild-type protein was able to relocate to the cytosol. On the contrary, mutant isoforms were not able to respond to oxidative stress. High mitochondrial calcium concentration and increased H₂O₂ induced apoptosis were found in fibroblasts derived from one patient as compared to controls. C19orf12 protein is a 17 kDa mitochondrial membrane-associated protein whose function is still unknown. Our in silico investigation suggests that, the glycine zipper motifs of C19orf12 form helical regions spanning the membrane. The N- and C-terminal regions with respect to the transmembrane portion, on the contrary, are predicted to rearrange in a structural domain, which is homologs to the N-terminal regulatory domain of the magnesium transporter MgtE, suggesting that C19orf12 may act as a regulatory protein for human MgtE transporters. The mutations here described affect respectively one glycine residue of the glycine zipper motifs, which are involved in dimerization of transmembrane helices and predicted to impair the correct localization of the protein into the membranes, and one residue present in the regulatory domain, which is important for protein-protein interaction.

Mitochondria-associated membranes: composition, molecular mechanisms, and physiopathological implications

SIGNIFICANCE

In all cells, the endoplasmic reticulum (ER) and mitochondria are physically connected to form junctions termed mitochondria-associated membranes (MAMs). This subcellular compartment is under intense investigation because it represents a "hot spot" for the intracellular signaling of important pathways, including the synthesis of cholesterol and phospholipids, calcium homeostasis, and reactive oxygen species (ROS) generation and activity.

RECENT ADVANCES

The advanced methods currently used to study this fascinating intracellular microdomain in detail have enabled the identification of the molecular composition of MAMs and their involvement within different physiopathological contexts.

CRITICAL ISSUES

Here, we review the knowledge regarding (i) MAMs composition in terms of protein composition, (ii) the relationship between MAMs and ROS, (iii) the involvement of MAMs in cell death programs with particular emphasis within the tumor context, (iv) the emerging role of MAMs during inflammation, and (v) the key role of MAMs alterations in selected neurological disorders.

FUTURE DIRECTIONS

Whether alterations in MAMs represent a response to the disease pathogenesis or directly contribute to the disease has not yet been unequivocally established. In any case, the signaling at the MAMs represents a promising pharmacological target for several important human diseases.

Dose-dependent ATP depletion and cancer cell death following calcium electroporation, relative effect of calcium concentration and electric field strength

Background

Electroporation, a method for increasing the permeability of membranes to ions and small molecules, is used in the clinic with chemotherapeutic drugs for cancer treatment (electrochemotherapy). Electroporation with calcium causes ATP (adenosine triphosphate) depletion and cancer cell death and could be a novel cancer treatment. This study aims at understanding the relationship between applied electric field, calcium concentration, ATP depletion and efficacy.

Methods

In three human cell lines — H69 (small-cell lung cancer), SW780 (bladder cancer), and U937 (leukaemia), viability was determined after treatment with 1, 3, or 5 mM calcium and eight 99 μs pulses with 0.8, 1.0, 1.2, 1.4 or 1.6 kV/cm. Fitting analysis was applied to quantify the cell-killing efficacy in presence of calcium. Post-treatment intracellular ATP was measured in H69 and SW780 cells. Post-treatment intracellular ATP was observed with fluorescence confocal microscopy of quinacrine-labelled U937 cells.

Results

Both H69 and SW780 cells showed dose-dependent (calcium concentration and electric field) decrease in intracellular ATP (p<0.05) and reduced viability. The 50% effective cell kill was found at 3.71 kV/cm (H69) and 3.28 kV/cm (SW780), reduced to 1.40 and 1.15 kV/cm (respectively) with 1 mM calcium (lower EC50 for higher calcium concentrations). Quinacrine fluorescence intensity of calcium-electroporated U937 cells was one third lower than in controls (p<0.0001).

Conclusions

Calcium electroporation dose-dependently reduced cell survival and intracellular ATP. Increasing extracellular calcium allows the use of a lower electric field.

General Significance

This study supports the use of calcium electroporation for treatment of cancer and possibly lowering the applied electric field in future trials.

What is the function of mitochondrial networks? A theoretical assessment of hypotheses and proposal for future research

Mitochondria can change their shape from discrete isolated organelles to a large continuous reticulum. The cellular advantages underlying these fused networks are still incompletely understood. In this paper, we describe and compare hypotheses regarding the function of mitochondrial networks. We use mathematical and physical tools both to investigate existing hypotheses and to generate new ones, and we suggest experimental and modelling strategies. Among the novel insights we underline from this work are the possibilities that (i) selective mitophagy is not required for quality control because selective fusion is sufficient; (ii) increased connectivity may have non-linear effects on the diffusion rate of proteins; and (iii) fused networks can act to dampen biochemical fluctuations. We hope to convey to the reader that quantitative approaches can drive advances in the understanding of the physiological advantage of these morphological changes.

Molecular mechanisms of cell death: central implication of ATP synthase in mitochondrial permeability transition

The term mitochondrial permeability transition (MPT) is commonly used to indicate an abrupt increase in the permeability of the inner mitochondrial membrane to low molecular weight solutes. Widespread MPT has catastrophic consequences for the cell, de facto marking the boundary between cellular life and death. MPT results indeed in the structural and functional collapse of mitochondria, an event that commits cells to suicide via regulated necrosis or apoptosis. MPT has a central role in the etiology of both acute and chronic diseases characterized by the loss of post-mitotic cells. Moreover, cancer cells are often relatively insensitive to the induction of MPT, underlying their increased resistance to potentially lethal cues. Thus, intense efforts have been dedicated not only at the understanding of MPT in mechanistic terms, but also at the development of pharmacological MPT modulators. In this setting, multiple mitochondrial and extramitochondrial proteins have been suspected to critically regulate the MPT. So far, however, only peptidylprolyl isomerase F (best known as cyclophilin D) appears to constitute a key component of the so-called permeability transition pore complex (PTPC), the supramolecular entity that is believed to mediate MPT. Here, after reviewing the structural and functional features of the PTPC, we summarize recent findings suggesting that another of its core components is represented by the c subunit of mitochondrial ATP synthase.

Protein expression profiling in head fragments during planarian regeneration after amputation

Following amputation, a planarian tail fragment can regrow into a complete organism including a well-organized brain within about 2-3 weeks, thus restoring the structure and function to presurgical levels. Despite the enormous potential of these animals for regenerative medicine, our understanding of the exact mechanism of planarian regeneration is incomplete. To better understand the molecular nature of planarian head regeneration, we applied two-dimensional electrophoresis (2-DE)/matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)/time-of-flight mass spectrometry (TOF MS) technique to analyze the dynamic proteomic expression profiles over the course of 6 to 168 h post-decapitation. This approach identified a total of 141 differentially expressed proteins, 47 of which exhibited exceptionally high fold changes (≥3-fold change). Of these, Rx protein, an important regulator of head and brain development, was considered to be closely related to planarian head regeneration because of its exceptional high expression almost throughout the time course of regeneration process. Functional annotation analysis classified the 141 proteins into eight categories: (1) signaling, (2) Ca(2+) binding and translocation, (3) transcription and translation, (4) cytoskeleton, (5) metabolism, (6) cell protection, (7) tissue differentiation, and (8) cell cycle. Signaling pathway analysis indicated that Wnt1/Ca(2+) signaling pathway was activated during head regeneration. Integrating the analyses of proteome expression profiling, functional annotation, and signaling pathway, amputation-induced head reformation requires some mechanisms to promote cell proliferation and differentiation, including differential regulation of proapoptotic and antiapoptotic proteins, and the regulation of proliferation and differentiation-related proteins. Importantly, Wnt1/Ca(2+) signaling pathway upregulates Rx expression, finally facilitating the differentiation of neoblasts into various cell types. Taken together, our study demonstrated that proteomic analysis approach used by us is a powerful tool in understanding molecular process related to head regeneration of planarian.

The BH4 domain of anti-apoptotic Bcl-XL, but not that of the related Bcl-2, limits the voltage-dependent anion channel 1 (VDAC1)-mediated transfer of pro-apoptotic Ca2+ signals to mitochondria

Excessive Ca(2+) fluxes from the endoplasmic reticulum to the mitochondria result in apoptotic cell death. Bcl-2 and Bcl-XL proteins exert part of their anti-apoptotic function by directly targeting Ca(2+)-transport systems, like the endoplasmic reticulum-localized inositol 1,4,5-trisphosphate receptors (IP3Rs) and the voltage-dependent anion channel 1 (VDAC1) at the outer mitochondrial membranes. We previously demonstrated that the Bcl-2 homology 4 (BH4) domain of Bcl-2 protects against Ca(2+)-dependent apoptosis by binding and inhibiting IP3Rs, although the BH4 domain of Bcl-XL was protective independently of binding IP3Rs. Here, we report that in contrast to the BH4 domain of Bcl-2, the BH4 domain of Bcl-XL binds and inhibits VDAC1. In intact cells, delivery of the BH4-Bcl-XL peptide via electroporation limits agonist-induced mitochondrial Ca(2+) uptake and protects against staurosporine-induced apoptosis, in line with the results obtained with VDAC1(-/-) cells. Moreover, the delivery of the N-terminal domain of VDAC1 as a synthetic peptide (VDAC1-NP) abolishes the ability of BH4-Bcl-XL to suppress mitochondrial Ca(2+) uptake and to protect against apoptosis. Importantly, VDAC1-NP did not affect the ability of BH4-Bcl-2 to suppress agonist-induced Ca(2+) release in the cytosol or to prevent apoptosis, as done instead by an IP3R-derived peptide. In conclusion, our data indicate that the BH4 domain of Bcl-XL, but not that of Bcl-2, selectively targets VDAC1 and inhibits apoptosis by decreasing VDAC1-mediated Ca(2+) uptake into the mitochondria.

Dysregulated Ca2+ homeostasis in Fanconi anemia cells

Fanconi Anemia (FA) is a rare and complex inherited blood disorder associated with bone marrow failure and malignancies. Many alterations in FA physiology appear linked to red-ox unbalance including alterations in the morphology and structure of nuclei, intermediate filaments and mitochondria, defective respiration, reduced ATP production and altered ATP/AMP ratio. These defects are consistently associated with impaired oxygen metabolism indeed treatment with antioxidants N-acetylcysteine (NAC) and resveratrol (RV) does rescue FA physiology. Due to the importance of the intracellular calcium signaling and its key function in the control of intracellular functions we were interested to study calcium homeostasis in FA. We found that FANCA cells display a dramatically low intracellular calcium concentration ([Ca²⁺]_i) in resting conditions. This condition affects cellular responses to stress. The flux of Ca²⁺ mobilized by H₂O₂ from internal stores is significantly lower in FANCA cells in comparison to controls. The low basal [Ca²⁺]_i in FANCA appears to be an actively maintained process controlled by a finely tuned interplay between different intracellular Ca²⁺ stores. The defects associated with the altered Ca²⁺ homeostasis appear consistently overlapping those related to the unbalanced oxidative metabolism in FA cells underlining a contiguity between oxidative stress and calcium homeostasis.

p53 at the endoplasmic reticulum regulates apoptosis in a Ca2+-dependent manner

The tumor suppressor p53 is a key protein in preventing cell transformation and tumor progression. Activated by a variety of stimuli, p53 regulates cell-cycle arrest and apoptosis. Along with its well-documented transcriptional control over cell-death programs within the nucleus, p53 exerts crucial although still poorly understood functions in the cytoplasm, directly modulating the apoptotic response at the mitochondrial level. Calcium (Ca(2+)) transfer between the endoplasmic reticulum (ER) and mitochondria represents a critical signal in the induction of apoptosis. However, the mechanism controlling this flux in response to stress stimuli remains largely unknown. Here we show that, in the cytoplasm, WT p53 localizes at the ER and at specialized contact domains between the ER and mitochondria (mitochondria-associated membranes). We demonstrate that, upon stress stimuli, WT p53 accumulates at these sites and modulates Ca(2+) homeostasis. Mechanistically, upon activation, WT p53 directly binds to the sarco/ER Ca(2+)-ATPase (SERCA) pump at the ER, changing its oxidative state and thus leading to an increased Ca(2+) load, followed by an enhanced transfer to mitochondria. The consequent mitochondrial Ca(2+) overload causes in turn alterations in the morphology of this organelle and induction of apoptosis. Pharmacological inactivation of WT p53 or naturally occurring p53 missense mutants inhibits SERCA pump activity at the ER, leading to a reduction of the Ca(2+) signaling from the ER to mitochondria. These findings define a critical nonnuclear function of p53 in regulating Ca(2+) signal-dependent apoptosis.

A family with paroxysmal nonkinesigenic dyskinesias (PNKD): evidence of mitochondrial dysfunction

INTRODUCTION

Paroxysmal nonkinesigenic dyskinesia (PNKD) is a rare movement disorder characterized by sudden attacks of involuntary movements. Familial PNKD is an autosomal dominant trait, caused by mutations in the myofibrillogenesis regulator 1 (MR-1) gene on chromosome 2q35. Three different mutations have been described; all of them reside in the N-terminal region common to isoforms L and S, that has been suggested to code for a mitochondrial targeting sequence, necessary for the correct sub-cellular localization of the protein into mitochondria.

METHODS

We report on four patients of the same family, affected by PNKD. Skin fibroblasts were used to analysed oxygen consumption and to measure mitochondrial matrix calcium response after agonist stimulation. Mitotracker-based visualization was also used to assess fragmentation of the mitochondrial network.

RESULTS

the paroxysmal movements were dystonic in two patients and dystonic/choreiform in the other ones; in three cases the symptoms started in one limb and then generalized, while in one case remained focal. Three had a very early onset, within the first two years of life. The frequency of episodes showed a great variability, ranging from 2 times a day to 3 times a year, while the duration of the attacks ranged from 2 min to 1,5 h, always with sudden onset and end and complete recover in between. All affected subjects harbored a heterozygous C to T substitution in MR-1, causing an Ala9Val amino acid change in the N-terminal region. A significant reduction of oxygen consumption and altered calcium homeostasis were found in mutant fibroblasts compared to controls, while no difference was detected in mitochondrial network.

CONCLUSIONS

The data on reduced oxygen consumption and altered calcium homeostasis obtained on mutant fibroblasts are the first evidences, in physiological conditions, of a mitochondrial dysfunction in PNKD.

The mitochondrial permeability transition pore and cancer: molecular mechanisms involved in cell death

Since its discovery in the 1970s, the mitochondrial permeability transition (MPT) has been proposed to be a strategic regulator of cell death. Intense research efforts have focused on elucidating the molecular components of the MPT because this knowledge may help to better understand and treat various pathologies ranging from neurodegenerative and cardiac diseases to cancer. In the case of cancer, several studies have revealed alterations in the activity of the mitochondrial permeability transition pore (mPTP) and have determined its regulatory mechanism; these studies have also suggested that suppression of the activity of the mPTP, rather than its inactivation, commonly occurs in solid neoplasms. This review focuses on the most recent advances in understanding mPTP regulation in cancer and highlights the ability of the mPTP to impede the mechanisms of cell death.

Role of Mitofusin-2 in mitochondrial localization and calcium uptake in skeletal muscle

As muscle contraction requires ATP and Ca(2+), skeletal muscle function is highly dependent on communication between two major intracellular organelles: mitochondria and sarcoplasmic reticulum (SR). In adult skeletal muscle, mitochondria located within the I-band of the sarcomere are connected to the SR by small ∼10 nm electron dense tethers that bridge the outer mitochondrial membrane to the region of SR that is ∼130 nm from the site of Ca(2+) release. However, the molecular composition of tethers and their precise impact on mitochondrial Ca(2+) uptake in skeletal muscle is unclear. Mitofusin-2 (Mfn2) is a transmembrane GTPase present in both mitochondria and ER/SR membranes that forms trans-dimers and participates in mitochondrial fusion. Here we evaluated the role Mfn2 plays in mitochondrial morphology, localization, and functional SR-mitochondrial Ca(2+) crosstalk in adult skeletal muscle. Compared to a non-targeting (CTRL) siRNA, in vivo electroporation of 400 nM Mfn2 siRNA (Mfn2 KD) into mouse footpads resulted in a marked acute reduction (67±3%) in Mfn2 protein levels in flexor digitorum brevis (FDB) muscles that occurred without a change in other key Ca(2+) regulatory proteins. Electron microscopy analyses revealed that Mfn2 knockdown resulted in a change in mitochondria morphology and mis-localization of some mitochondria from the I-band to the A-band region of the sarcomere. To assess the role of Mfn2 in SR-mitochondrial crosstalk, we measured mitochondrial Ca(2+) uptake and myoplasmic Ca(2+) transients with rhod-2 and mag-fluo-4, respectively, during repetitive high frequency tetanic stimulation (5×100 Hz tetani, 500 ms/tetani, 0.2 duty cycle) in CTRL and Mfn2 KD fibers. Mitochondrial Ca(2+) uptake during repetitive tetanic stimulation was significantly reduced (40%) in Mfn2 KD FDB fibers, which was accompanied by a parallel elevation in the global electrically evoked myoplasmic Ca(2+) transient. Mfn2 KD also resulted in a reduction of the mitochondrial membrane potential, which contributed to the observed decrease in activity-dependent mitochondrial Ca(2+) uptake. Consistent with this idea, a similar decrease in mitochondrial Ca(2+) uptake was also observed in wild type fibers following a comparable reduction in mitochondrial membrane potential induced by acute exposure to a low concentration (50 nM) of carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP). In addition, both global and mitochondrial Ca(2+) transients during repetitive tetanic stimulation were similarly reduced by both slow (EGTA) and fast (BAPTA) Ca(2+) chelating agents. Together, these results indicate that Mfn2 promotes proper mitochondrial morphology, localization, and membrane potential required for optimal activity-dependent mitochondrial Ca(2+) uptake and buffering of the global myoplasmic Ca(2+) transient in adult skeletal muscle.

Dietary fat, fatty acid saturation and mitochondrial bioenergetics

Fat intake alters mitochondrial lipid composition which can affect function. We used novel methodology to assess bioenergetics, including simultaneous ATP and reactive oxygen species (ROS) production, in liver and heart mitochondria of C57BL/6 mice fed diets of variant fatty acid content and saturation. Our methodology allowed us to clamp ADP concentration and membrane potential (ΔΨ) at fixed levels. Mice received a control diet for 17–19 weeks, a high-fat (HF) diet (60% lard) for 17–19 weeks, or HF for 12 weeks followed by 6–7 weeks of HF with 50% of fat as menhaden oil (MO) which is rich in n-3 fatty acids. ATP production was determined as conversion of 2-deoxyglucose to 2-deoxyglucose phosphate by NMR spectroscopy. Respiration and ATP production were significantly reduced at all levels of ADP and resultant clamped ΔΨ in liver mitochondria from mice fed HF compared to controls. At given ΔΨ, ROS production per mg mitochondrial protein, per unit respiration, or per ATP generated were greater for liver mitochondria of HF-fed mice compared to control or MO-fed mice. Moreover, these ROS metrics began to increase at a lower ΔΨ threshold. Similar, but less marked, changes were observed in heart mitochondria of HF-fed mice compared to controls. No changes in mitochondrial bioenergetics were observed in studies of separate mice fed HF versus control for only 12 weeks. In summary, HF feeding of sufficient duration impairs mitochondrial bioenergetics and is associated with a greater ROS “cost” of ATP production compared to controls. These effects are, in part, mitigated by MO.

Study of PTEN subcellular localization

The tumor suppressor PTEN is a key regulator of a plethora of cellular processes that are crucial in cancer development. Through its lipid phosphatase activity PTEN suppresses the PI3K/AKT pathway to govern cell proliferation, growth, migration, energy metabolism and death. The repertoire of roles fulfilled by PTEN has recently been expanded to include crucial functions in the nucleus, where it favors genomic stability and restrains cell cycle progression, as well as protein phosphatase dependent activity at the endoplasmic reticulum (ER) and mitochondria-associated membranes (MAMs), where PTEN interacts with the inositol 1,4,5-trisphosphate receptors (IP3Rs) and regulates Ca²⁺ release from the ER and sensitivity to apoptosis. Indeed, PTEN is present in definite subcellular locations where it performs distinct functions acting on specific effectors. In this review, we summarize recent advantages in methods to study PTEN subcellular localization and the distinct biological functions of PTEN in different cellular compartments. A deeper understanding of PTEN’s compartmentalized-functions will guide the rational design of novel therapies.

Mitochondria-associated endoplasmic reticulum membrane (MAM) integrity is required for insulin signaling and is implicated in hepatic insulin resistance

Mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) are functional domains between both organelles involved in Ca(2+) exchange, through the voltage-dependent anion channel (VDAC)-1/glucose-regulated protein 75 (Grp75)/inositol 1,4,5-triphosphate receptor (IP3R)-1 complex, and regulating energy metabolism. Whereas mitochondrial dysfunction, ER stress, and altered Ca(2+) homeostasis are associated with altered insulin signaling, the implication of MAM dysfunctions in insulin resistance is unknown. Here we validated an approach based on in situ proximity ligation assay to detect and quantify VDAC1/IP3R1 and Grp75/IP3R1 interactions at the MAM interface. We demonstrated that MAM integrity is required for insulin signaling and that induction of MAM prevented palmitate-induced alterations of insulin signaling in HuH7 cells. Disruption of MAM integrity by genetic or pharmacological inhibition of the mitochondrial MAM protein, cyclophilin D (CypD), altered insulin signaling in mouse and human primary hepatocytes and treatment of CypD knockout mice with metformin improved both insulin sensitivity and MAM integrity. Furthermore, ER-mitochondria interactions are altered in liver of both ob/ob and diet-induced insulin-resistant mice and improved by rosiglitazone treatment in the latter. Finally, increasing organelle contacts by overexpressing CypD enhanced insulin action in primary hepatocytes of diabetic mice. Collectively, our data reveal a new role of MAM integrity in hepatic insulin action and resistance, providing a novel target for the modulation of insulin action.

ER-mitochondrial calcium flow underlies vulnerability of mechanosensory hair cells to damage

Mechanosensory hair cells are vulnerable to environmental insult, resulting in hearing and balance disorders. We demonstrate that directional compartmental flow of intracellular Ca(2+) underlies death in zebrafish lateral line hair cells after exposure to aminoglycoside antibiotics, a well characterized hair cell toxin. Ca(2+) is mobilized from the ER and transferred to mitochondria via IP3 channels with little cytoplasmic leakage. Pharmacological agents that shunt ER-derived Ca(2+) directly to cytoplasm mitigate toxicity, indicating that high cytoplasmic Ca(2+) levels alone are not cytotoxic. Inhibition of the mitochondrial transition pore sensitizes hair cells to the toxic effects of aminoglycosides, contrasting with current models of excitotoxicity. Hair cells display efficient ER-mitochondrial Ca(2+) flow, suggesting that tight coupling of these organelles drives mitochondrial activity under physiological conditions at the cost of increased susceptibility to toxins.

Effect of mitochondrial calcium uniporter blocking on human spermatozoa

Calcium (Ca(2+) ) regulates a number of essential processes in spermatozoa. Ca(2+) is taken up by mitochondria via the mitochondrial calcium uniporter (mCU). Oxygen-bridged dinuclear ruthenium amine complex (Ru360) has been used to study mCU because it is a potent and specific inhibitor of this channel. In bovine spermatozoa, it has been demonstrated that mitochondrial calcium uptake inhibition adversely affects the capacitation process. It has been demonstrated in human spermatozoa that mCU blocking, through Ru360, prevents apoptosis; however, the contribution of the mCU to normal human sperm function has not been studied. Therefore, the aim of this study was to evaluate the effect of mCU blocking on human sperm function. Spermatozoa obtained from apparently healthy donors were incubated with 5 and 10 μm Ru360 for 4 h at 37 °C. Viability was assessed using propidium iodide staining; motility was determined by computer-aided sperm analysis, adenosine triphosphate (ATP) levels using a luminescence-based method, mitochondrial membrane potential (ΔΨm) using JC-1 staining and reactive oxygen species (ROS) production using dihydroethidium dye. Our results show that mCU blocking significantly reduced total sperm motility and ATP levels without affecting sperm viability, ΔΨm and ROS production. In conclusion, mCU contributes to the maintenance of sperm motility and ATP levels in human spermatozoa.

The mitochondrial permeability transition pore is a dispensable element for mitochondrial calcium efflux

The mitochondrial permeability transition pore (mPTP) has long been known to have a role in mitochondrial calcium (Ca(2+)) homeostasis under pathological conditions as a mediator of the mitochondrial permeability transition and the activation of the consequent cell death mechanism. However, its role in the context of mitochondrial Ca(2+) homeostasis is not yet clear. Several studies that were based on PPIF inhibition or knock out suggested that mPTP is involved in the Ca(2+) efflux mechanism, while other observations have revealed the opposite result. The c subunit of the mitochondrial F1/FO ATP synthase has been recently found to be a fundamental component of the mPTP. In this work, we focused on the contribution of the mPTP in the Ca(2+) efflux mechanism by modulating the expression of the c subunit. We observed that forcing mPTP opening or closing did not impair mitochondrial Ca(2+) efflux. Therefore, our results strongly suggest that the mPTP does not participate in mitochondrial Ca(2+) homeostasis in a physiological context in HeLa cells.

Diversity of mitochondrial Ca²⁺ signaling in rat neonatal cardiomyocytes: evidence from a genetically directed Ca²⁺ probe, mitycam-E31Q

I(Ca)-gated Ca(2+) release (CICR) from the cardiac SR is the main mechanism mediating the rise of cytosolic Ca(2+), but the extent to which mitochondria contribute to the overall Ca(2+) signaling remains controversial. To examine the possible role of mitochondria in Ca(2+) signaling, we developed a low affinity mitochondrial Ca(2+) probe, mitycam-E31Q (300-500 MOI, 48-72h) and used it in conjunction with Fura-2AM to obtain simultaneous TIRF images of mitochondrial and cytosolic Ca(2+) in cultured neonatal rat cardiomyocytes. Mitycam-E31Q staining of adult feline cardiomyocytes showed the typical mitochondrial longitudinal fluorescent bandings similar to that of TMRE staining, while neonatal rat cardiomyocytes had a disorganized tubular or punctuate appearance. Caffeine puffs produced rapid increases in cytosolic Ca(2+) while simultaneously measured global mitycam-E31Q signals decreased more slowly (increased mitochondrial Ca(2+)) before decaying to baseline levels. Similar, but oscillating mitycam-E31Q signals were seen in spontaneously pacing cells. Withdrawal of Na(+) increased global cytosolic and mitochondrial Ca(2+) signals in one population of mitochondria, but unexpectedly decreased it (release of Ca(2+)) in another mitochondrial population. Such mitochondrial Ca(2+) release signals were seen not only during long lasting Na(+) withdrawal, but also when Ca(2+) loaded cells were exposed to caffeine-puffs, and during spontaneous rhythmic beating. Thus, mitochondrial Ca(2+) transients appear to activate with a delay following the cytosolic rise of Ca(2+) and show diversity in subpopulations of mitochondria that could contribute to the plasticity of mitochondrial Ca(2+) signaling.

Amyloid-beta disrupts calcium and redox homeostasis in brain endothelial cells

In Alzheimer's disease, the accumulation of amyloid-beta (Aβ) in the brain occurs in the parenchyma and cerebrovasculature. Several evidences support that the neuronal demise is potentiated by vascular alterations in the early stages of the disease, but the mechanisms responsible for the dysfunction of brain endothelial cells that underlie these cerebrovascular changes are unknown. Using rat brain microvascular endothelial cells, we found that short-term treatment with a toxic dose of Aβ1-40 inhibits the Ca(2+) refill and retention ability of the endoplasmic reticulum and enhances the mitochondrial and cytosolic response to adenosine triphosphate (ATP)-stimulated endoplasmic reticulum Ca(2+) release. Upon prolonged Aβ1-40 exposure, Ca(2+) homeostasis was restored concomitantly with a decrease in the levels of proteins involved in its regulation operating at the plasma membrane, endoplasmic reticulum, and mitochondria. Along with perturbations in Ca(2+) regulation, an early increase in the levels of oxidants and a decrease in the ratio between reduced and oxidized glutathione were observed in Aβ1-40-treated endothelial cells. Under these conditions, the nuclear levels of oxidative stress-related transcription factors, namely, hypoxia-inducible factor 1α and nuclear factor (erythroid-derived 2)-related factor 2, were enhanced as well as the protein levels of target genes. In conclusion, Aβ1-40 affects several mechanisms involved in Ca(2+) homeostasis and impairs the redox homeostasis simultaneously with stimulation of protective stress responses in brain endothelial cells. However, the imbalance between cell death and survival pathways leads to endothelial dysfunction that in turn contributes to cerebrovascular impairment in Alzheimer's disease.

Mitochondrial oxidant stress in locus coeruleus is regulated by activity and nitric oxide synthase

Loss of noradrenergic locus coeruleus (LC) neurons is a prominent feature of aging–related neurodegenerative diseases, like Parkinson’s disease (PD). The basis of this vulnerability is not understood. To explore possible physiological determinants, LC neurons were studied using electrophysiological and optical approaches in ex vivo mouse brain slices. These studies revealed that autonomous activity in LC neurons was accompanied by oscillations in dendritic Ca²⁺ concentration attributable to opening of L–type Ca²⁺ channels. This oscillation elevated mitochondrial oxidant stress and was attenuated by inhibition of nitric oxide synthase. The relationship between activity and stress was malleable, as arousal and carbon dioxide, each increased the spike rate, but differentially affected mitochondrial oxidant stress. Oxidant stress also was increased in an animal model of PD. Thus, our results point to activity–dependent Ca²⁺ entry and a resulting mitochondrial oxidant stress as factors contributing to the vulnerability of LC neurons.

Cisd2 modulates the differentiation and functioning of adipocytes by regulating intracellular Ca2+ homeostasis

CISD2 is a causative gene associated with Wolfram syndrome (WFS). However, it remains a mystery as to how the loss of CISD2 causes metabolic defects in patients with WFS. Investigation on the role played by Cisd2 in specific cell types may help us to resolve these underlying mechanisms. White adipose tissue (WAT) is central to the maintenance of energy metabolism and glucose homeostasis in humans. In this study, adipocyte-specific Cisd2 knockout (KO) mice showed impairment in the development of epididymal WAT (eWAT) in the cell autonomous manner. A lack of Cisd2 caused defects in the biogenesis and function of mitochondria during differentiation of adipocytes in vitro. Insulin-stimulated glucose uptake and secretion of adiponectin by the Cisd2 KO adipocytes were decreased. Moreover, Cisd2 deficiency increased the cytosolic level of Ca(2+) and induced Ca(2+)-calcineurin-dependent signaling that inhibited adipogenesis. Importantly, Cisd2 was found to interact with Gimap5 on the mitochondrial and ER membranes and thereby modulate mitochondrial Ca(2+) uptake associated with the maintenance of intracellular Ca(2+) homeostasis in adipocytes. Thus, it would seem that Cisd2 plays an important role in intracellular Ca(2+) homeostasis, which is required for the differentiation and functioning of adipocytes as well as the regulation of glucose homeostasis in mice.

A dual role for the anti-apoptotic Bcl-2 protein in cancer: mitochondria versus endoplasmic reticulum

Anti-apoptotic Bcl-2 contributes to cancer formation and progression by promoting the survival of altered cells. Hence, it is a prime target for novel specific anti-cancer therapeutics. In addition to its canonical anti-apoptotic role, Bcl-2 has an inhibitory effect on cell-cycle progression. Bcl-2 acts at two different intracellular compartments, the mitochondria and the endoplasmic reticulum (ER). At the mitochondria, Bcl-2 via its hydrophobic cleft scaffolds the Bcl-2-homology (BH) domain 3 (BH3) of pro-apoptotic Bcl-2-family members. Small molecules (like BH3 mimetics) can disrupt this interaction, resulting in apoptotic cell death in cancer cells. At the ER, Bcl-2 modulates Ca(2+) signaling, thereby promoting proliferation while increasing resistance to apoptosis. Bcl-2 at the ER acts via its N-terminal BH4 domain, which directly binds and inhibits the inositol 1,4,5-trisphosphate receptor (IP3R), the main intracellular Ca(2+)-release channel. Tools targeting the BH4 domain of Bcl-2 reverse Bcl-2's inhibitory action on IP3Rs and trigger pro-apoptotic Ca(2+) signaling in cancer B-cells, including chronic lymphocytic leukemia (CLL) cells and diffuse large B-cell lymphoma (DLBCL) cells. The sensitivity of DLBCL cells to BH4-domain targeting tools strongly correlated with the expression levels of the IP3R2 channel, the IP3R isoform with the highest affinity for IP3. Interestingly, bio-informatic analysis of a database of primary CLL patient cells also revealed a transcriptional upregulation of IP3R2. Finally, this review proposes a model, in which cancer cell survival depends on Bcl-2 at the mitochondria and/or the ER. This dependence likely will have an impact on their responses to BH3-mimetic drugs and BH4-domain targeting tools. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

NRIP1/RIP140 siRNA-mediated attenuation counteracts mitochondrial dysfunction in Down syndrome

Mitochondrial dysfunction, which is consistently observed in Down syndrome (DS) cells and tissues, might contribute to the severity of the DS phenotype. Our recent studies on DS fetal hearts and fibroblasts have suggested that one of the possible causes of mitochondrial dysfunction is the downregulation of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1α or PPARGC1A)--a key modulator of mitochondrial function--and of several nuclear-encoded mitochondrial genes (NEMGs). Re-analysis of publicly available expression data related to manipulation of chromosome 21 (Hsa21) genes suggested the nuclear receptor interacting protein 1 (NRIP1 or RIP140) as a good candidate Hsa21 gene for NEMG downregulation. Indeed, NRIP1 is known to affect oxidative metabolism and mitochondrial biogenesis by negatively controlling mitochondrial pathways regulated by PGC-1α. To establish whether NRIP1 overexpression in DS downregulates both PGC-1α and NEMGs, thereby causing mitochondrial dysfunction, we used siRNAs to decrease NRIP1 expression in trisomic human fetal fibroblasts. Levels of PGC-1α and NEMGs were increased and mitochondrial function was restored, as shown by reactive oxygen species decrease, adenosine 5'-triphosphate (ATP) production and mitochondrial activity increase. These findings indicate that the Hsa21 gene NRIP1 contributes to the mitochondrial dysfunction observed in DS. Furthermore, they suggest that the NRIP1-PGC-1α axe might represent a potential therapeutic target for restoring altered mitochondrial function in DS.

Changes in mitochondrial function are pivotal in neurodegenerative and psychiatric disorders: how important is BDNF?

The brain is at the very limit of its energy supply and has evolved specific means of adapting function to energy supply, of which mitochondria form a crucial link. Neurotrophic and inflammatory processes may not only have opposite effects on neuroplasticity, but also involve opposite effects on mitochondrial oxidative phosphorylation and glycolytic processes, respectively, modulated by stress and glucocorticoids, which also have marked effects on mood. Neurodegenerative processes show marked disorders in oxidative metabolism in key brain areas, sometimes decades before symptoms appear (Parkinson's and Alzheimer's diseases). We argue that brain-derived neurotrophic factor couples activity to changes in respiratory efficiency and these effects may be opposed by inflammatory cytokines, a key factor in neurodegenerative processes.

Oncogenic and oncosuppressive signal transduction at mitochondria-associated endoplasmic reticulum membranes

The different mechanisms employed by proto-oncogenes and tumor suppressors to regulate cell death pathways are strictly linked to their localization. In addition to the canonical control of apoptosis at a transcriptional/nuclear level, intracellular zones are emerging as pivotal sites for the activities of several proapoptotic and antiapoptotic factors. Here, we review the function of the endoplasmic reticulum-mitochondria interface as a primary platform for decoding danger signals as well as a structural accommodation for several regulator or effector proteins.

Calcium signaling and mitochondrial destabilization in the triggering of the NLRP3 inflammasome

The NLRP3 inflammasome is a cytosolic complex that activates Caspase-1, leading to maturation of interleukin-1β (IL-1β) and IL-18 and induction of proinflammatory cell death in sentinel cells of the innate immune system. Diverse stimuli have been shown to activate the NLRP3 inflammasome during infection and metabolic diseases, implicating the pathway in triggering both adaptive and maladaptive inflammation in various clinically important settings. Here I discuss the emerging model that signals associated with mitochondrial destabilization may critically activate the NLRP3 inflammasome. Together with studies indicating an important role for Ca2+ signaling, these findings suggest that many stimuli engage Ca2+ signaling as an intermediate step to trigger mitochondrial destabilization, generating the mitochondrion-associated ligands that activate the NLRP3 inflammasome.

Inositol 1,4,5-trisphosphate receptor-isoform diversity in cell death and survival

Cell-death and -survival decisions are critically controlled by intracellular Ca(2+) homeostasis and dynamics at the level of the endoplasmic reticulum (ER). Inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) play a pivotal role in these processes by mediating Ca(2+) flux from the ER into the cytosol and mitochondria. Hence, it is clear that many pro-survival and pro-death signaling pathways and proteins affect Ca(2+) signaling by directly targeting IP3R channels, which can happen in an IP3R-isoform-dependent manner. In this review, we will focus on how the different IP3R isoforms (IP3R1, IP3R2 and IP3R3) control cell death and survival. First, we will present an overview of the isoform-specific regulation of IP3Rs by cellular factors like IP3, Ca(2+), Ca(2+)-binding proteins, adenosine triphosphate (ATP), thiol modification, phosphorylation and interacting proteins, and of IP3R-isoform specific expression patterns. Second, we will discuss the role of the ER as a Ca(2+) store in cell death and survival and how IP3Rs and pro-survival/pro-death proteins can modulate the basal ER Ca(2+) leak. Third, we will review the regulation of the Ca(2+)-flux properties of the IP3R isoforms by the ER-resident and by the cytoplasmic proteins involved in cell death and survival as well as by redox regulation. Hence, we aim to highlight the specific roles of the various IP3R isoforms in cell-death and -survival signaling. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

MitoSatPlant: mitochondrial microsatellites database of viridiplantae

Microsatellites also known as simple sequence repeats (SSRs) consist of 1-6 nucleotide long repeating units. The importance of mitochondrial SSRs (mtSSRs) in fields like population genetics, plant phylogenetics and genome mapping motivated us to develop MitoSatPlant, a repository of plant mtSSRs. It contains information for perfect, imperfect and compound SSRs mined from 92 mitochondrial genomes of green plants, available at NCBI (as of 1 Feb 2014). A total of 72,798 SSRs were found, of which PCR primers were designed for 72,495 SSRs. Among all sequences, tetranucleotide repeats (26,802) were found to be most abundant whereas hexanucleotide repeats (2751) were detected with least frequency. MitoSatPlant was developed using SQL server 2008 and can be accessed through a front end designed in ASP.Net. It is an easy to use, user-friendly database and will prove to be a useful resource for plant scientists. To the best of our knowledge MitoSatPlant is the only database available for plant mtSSRs and can be freely accessed at http://compubio.in/mitosatplant/.

FigA, a putative homolog of low-affinity calcium system member Fig1 in Saccharomyces cerevisiae, is involved in growth and asexual and sexual development in Aspergillus nidulans

Calcium-mediated signaling pathways are widely employed in eukaryotes and are implicated in the regulation of diverse biological processes. In Saccharomyces cerevisiae, at least two different calcium uptake systems have been identified: the high-affinity calcium influx system (HACS) and the low-affinity calcium influx system (LACS). Compared to the HACS, the LACS in fungi is not well known. In this study, FigA, a homolog of the LACS member Fig1 from S. cerevisiae, was functionally characterized in the filamentous fungus Aspergillus nidulans. Loss of figA resulted in retardant hyphal growth and a sharp reduction of conidial production. Most importantly, FigA is essential for the homothallic mating (self-fertilization) process; further, FigA is required for heterothallic mating (outcrossing) in the absence of HACS midA. Interestingly, in a figA deletion mutant, adding extracellular Ca(2+) rescued the hyphal growth defects but could not restore asexual and sexual reproduction. Furthermore, quantitative PCR results revealed that figA deletion sharply decreased the expression of brlA and nsdD, which are known as key regulators during asexual and sexual development, respectively. In addition, green fluorescent protein (GFP) tagging at the C terminus of FigA (FigA::GFP) showed that FigA localized to the center of the septum in mature hyphal cells, to the location between vesicles and metulae, and between the junctions of metulae and phialides in conidiophores. Thus, our findings suggest that FigA, apart from being a member of a calcium uptake system in A. nidulans, may play multiple unexplored roles during hyphal growth and asexual and sexual development.