Essential regulation of cell bioenergetics by constitutive InsP3 receptor Ca2+ transfer to mitochondria

Where the endoplasmic reticulum and the mitochondrion tie the knot: the mitochondria-associated membrane (MAM)

More than a billion years ago, bacterial precursors of mitochondria became endosymbionts in what we call eukaryotic cells today. The true significance of the word "endosymbiont" has only become clear to cell biologists with the discovery that the endoplasmic reticulum (ER) superorganelle dedicates a special domain for the metabolic interaction with mitochondria. This domain, identified in all eukaryotic cell systems from yeast to man and called the mitochondria-associated membrane (MAM), has a distinct proteome, specific tethers on the cytosolic face and regulatory proteins in the ER lumen of the ER. The MAM has distinct biochemical properties and appears as ER tubules closely apposed to mitochondria on electron micrographs. The functions of the MAM range from lipid metabolism and calcium signaling to inflammasome formation. Consistent with these functions, the MAM is enriched in lipid metabolism enzymes and calcium handling proteins. During cellular stress situations, like an altered cellular redox state, the MAM alters its set of regulatory proteins and thus alters MAM functions. Notably, this set prominently comprises ER chaperones and oxidoreductases that connect protein synthesis and folding inside the ER to mitochondrial metabolism. Moreover, ER membranes associated with mitochondria also accommodate parts of the machinery that determines mitochondrial membrane dynamics and connect mitochondria to the cytoskeleton. Together, these exciting findings demonstrate that the physiological interactions between the ER and mitochondria are so bilateral that we are tempted to compare their relationship to the one of a married couple: distinct, but inseparable and certainly dependent on each other. In this paradigm, the MAM stands for the intracellular location where the two organelles tie the knot. Resembling "real life", the happy marriage between the two organelles prevents the onset of diseases that are characterized by disrupted metabolism and decreased lifespan, including neurodegeneration and cancer. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

ROS-induced DNA damage and PARP-1 are required for optimal induction of starvation-induced autophagy

In response to nutrient stress, cells start an autophagy program that can lead to adaptation or death. The mechanisms underlying the signaling from starvation to the initiation of autophagy are not fully understood. In the current study we show that the absence or inactivation of PARP-1 strongly delays starvation-induced autophagy. We have found that DNA damage is an early event of starvation-induced autophagy as measured by γ-H2AX accumulation and comet assay, with PARP-1 knockout cells displaying a reduction in both parameters. During starvation, ROS-induced DNA damage activates PARP-1, leading to ATP depletion (an early event after nutrient deprivation). The absence of PARP-1 blunted AMPK activation and prevented the complete loss of mTOR activity, leading to a delay in autophagy. PARP-1 depletion favors apoptosis in starved cells, suggesting a pro-survival role of autophagy and PARP-1 activation after nutrient deprivation. In vivo results show that neonates of PARP-1 mutant mice subjected to acute starvation, also display deficient liver autophagy, implying a physiological role for PARP-1 in starvation-induced autophagy. Thus, the PARP signaling pathway is a key regulator of the initial steps of autophagy commitment following starvation.

ATP synthesis and storage

Since 1929, when it was discovered that ATP is a substrate for muscle contraction, the knowledge about this purine nucleotide has been greatly expanded. Many aspects of cell metabolism revolve around ATP production and consumption. It is important to understand the concepts of glucose and oxygen consumption in aerobic and anaerobic life and to link bioenergetics with the vast amount of reactions occurring within cells. ATP is universally seen as the energy exchange factor that connects anabolism and catabolism but also fuels processes such as motile contraction, phosphorylations, and active transport. It is also a signalling molecule in the purinergic signalling mechanisms. In this review, we will discuss all the main mechanisms of ATP production linked to ADP phosphorylation as well the regulation of these mechanisms during stress conditions and in connection with calcium signalling events. Recent advances regarding ATP storage and its special significance for purinergic signalling will also be reviewed.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Mitochondrial Ca(2+) and apoptosis

Mitochondria are key decoding stations of the apoptotic process. In support of this view, a large body of experimental evidence has unambiguously revealed that, in addition to the well-established function of producing most of the cellular ATP, mitochondria play a fundamental role in triggering apoptotic cell death. Various apoptotic stimuli cause the release of specific mitochondrial pro-apoptotic factors into the cytosol. The molecular mechanism of this release is still controversial, but there is no doubt that mitochondrial calcium (Ca(2+)) overload is one of the pro-apoptotic ways to induce the swelling of mitochondria, with perturbation or rupture of the outer membrane, and in turn the release of mitochondrial apoptotic factors into the cytosol. Here, we review as different proteins that participate in mitochondrial Ca(2+) homeostasis and in turn modulate the effectiveness of Ca(2+)-dependent apoptotic stimuli. Strikingly, the final outcome at the cellular level is similar, albeit through completely different molecular mechanisms: a reduced mitochondrial Ca(2+) overload upon pro-apoptotic stimuli that dramatically blunts the apoptotic response.

ER-Mitochondria Crosstalk during Cerebral Ischemia: Molecular Chaperones and ER-Mitochondrial Calcium Transfer

It is commonly believed that sustained elevations in the mitochondrial matrix Ca²⁺ concentration are a major feature of the intracellular cascade of lethal events during cerebral ischemia. The physical association between the endoplasmic reticulum (ER) and mitochondria, known as the mitochondria-associated ER membrane (MAM), enables highly efficient transmission of Ca²⁺ from the ER to mitochondria under both physiological and pathological conditions. Molecular chaperones are well known for their protective effects during cerebral ischemia. It has been demonstrated recently that many molecular chaperones coexist with MAM and regulate the MAM and thus Ca²⁺ concentration inside mitochondria. Here, we review recent research on cerebral ischemia and MAM, with a focus on molecular chaperones and ER-mitochondrial calcium transfer.

Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Guidelines for the use and interpretation of assays for monitoring autophagy

In 2008 we published the first set of guidelines for standardizing research in autophagy. Since then, research on this topic has continued to accelerate, and many new scientists have entered the field. Our knowledge base and relevant new technologies have also been expanding. Accordingly, it is important to update these guidelines for monitoring autophagy in different organisms. Various reviews have described the range of assays that have been used for this purpose. Nevertheless, there continues to be confusion regarding acceptable methods to measure autophagy, especially in multicellular eukaryotes. A key point that needs to be emphasized is that there is a difference between measurements that monitor the numbers or volume of autophagic elements (e.g., autophagosomes or autolysosomes) at any stage of the autophagic process vs. those that measure flux through the autophagy pathway (i.e., the complete process); thus, a block in macroautophagy that results in autophagosome accumulation needs to be differentiated from stimuli that result in increased autophagic activity, defined as increased autophagy induction coupled with increased delivery to, and degradation within, lysosomes (in most higher eukaryotes and some protists such as Dictyostelium) or the vacuole (in plants and fungi). In other words, it is especially important that investigators new to the field understand that the appearance of more autophagosomes does not necessarily equate with more autophagy. In fact, in many cases, autophagosomes accumulate because of a block in trafficking to lysosomes without a concomitant change in autophagosome biogenesis, whereas an increase in autolysosomes may reflect a reduction in degradative activity. Here, we present a set of guidelines for the selection and interpretation of methods for use by investigators who aim to examine macroautophagy and related processes, as well as for reviewers who need to provide realistic and reasonable critiques of papers that are focused on these processes. These guidelines are not meant to be a formulaic set of rules, because the appropriate assays depend in part on the question being asked and the system being used. In addition, we emphasize that no individual assay is guaranteed to be the most appropriate one in every situation, and we strongly recommend the use of multiple assays to monitor autophagy. In these guidelines, we consider these various methods of assessing autophagy and what information can, or cannot, be obtained from them. Finally, by discussing the merits and limits of particular autophagy assays, we hope to encourage technical innovation in the field.

Mitochondrial Ca(2+) signals in autophagy

Macroautophagy (autophagy) is a lysosomal degradation pathway that is conserved from yeast to humans that plays an important role in recycling cellular constituents in all cells. A number of protein complexes and signaling pathways impinge on the regulation of autophagy, with the mammalian target of rapamycin (mTOR) as the central player in the canonical pathway. Cytoplasmic Ca(2+) signaling also regulates autophagy, with both activating and inhibitory effects, mediated by the canonical as well as non-canonical pathways. Here we review this regulation, with a focus on the role of an mTOR-independent pathway that involves the inositol trisphosphate receptor (InsP(3)R) Ca(2+) release channel and Ca(2+) signaling to mitochondria. Constitutive InsP(3)R Ca(2+) transfer to mitochondria is required for autophagy suppression in cells in nutrient-replete media. In its absence, cells become metabolically compromised due to insufficient production of reducing equivalents to support oxidative phosphorylation. Absence of this Ca(2+) transfer to mitochondria results in activation of AMPK, which activates mTOR-independent pro-survival autophagy. Constitutive InsP(3)R Ca(2+) release to mitochondria is an essential cellular process that is required for efficient mitochondrial respiration, maintenance of normal cell bioenergetics and suppression of autophagy.

The mitochondrial Na(+)/Ca(2+) exchanger

Powered by the steep mitochondrial membrane potential Ca(2+) permeates into the mitochondria via the Ca(2+) uniporter and is then extruded by a mitochondrial Na(+)/Ca(2+) exchanger. This mitochondrial Ca(2+) shuttling regulates the rate of ATP production and participates in cellular Ca(2+) signaling. Despite the fact that the exchanger was functionally identified 40 years ago its molecular identity remained a mystery. Early studies on isolated mitochondria and intact cells characterized the functional properties of a mitochondrial Na(+)/Ca(2+) exchanger, and showed that it possess unique functional fingerprints such as Li(+)/Ca(2+) exchange and that it is displaying selective sensitivity to inhibitors. Purification of mitochondria proteins combined with functional reconstitution led to the isolation of a polypeptide candidate of the exchanger but failed to molecularly identify it. A turning point in the search for the exchanger molecule came with the recent cloning of the last member of the Na(+)/Ca(2+) exchanger superfamily termed NCLX (Na(+)/Ca(2+)/Li(+) exchanger). NCLX is localized in the inner mitochondria membrane and its expression is linked to mitochondria Na(+)/Ca(2+) exchange matching the functional fingerprints of the putative mitochondrial Na(+)/Ca(2+) exchanger. Thus NCLX emerges as the long sought mitochondria Na(+)/Ca(2+) exchanger and provide a critical molecular handle to study mitochondrial Ca(2+) signaling and transport. Here we summarize some of the main topics related to the molecular properties of the Na(+)/Ca(2+) exchanger, beginning with the early days of its functional identification, its kinetic properties and regulation, and culminating in its molecular identification.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

The role of mitochondrial Ca(2+) and NAD(P)H in the control of aldosterone secretion

The mineralocorticoid hormone aldosterone is synthesized in the zona glomerulosa of the adrenal cortex. Glomerulosa cells respond to the physiological stimuli, elevated extracellular [K(+)] and angiotensin II, with an intracellular Ca(2+) signal. Cytosolic Ca(2+) facilitates the transport of the steroid-precursor cholesterol to mitochondria and, after a few hours, it also induces the transcription of aldosterone synthase. Therefore, the cytosolic Ca(2+) signal is regarded as the most important short and long-term mediator of aldosterone secretion. However, cytosolic Ca(2+) is also taken up by mitochondria and, in turn, the mitochondrial Ca(2+) response activates mitochondrial dehydrogenases resulting in stimulation of respiration and increase in reduced pyridine nucleotides. Since both cholesterol side-chain cleavage and all of the hydroxylation steps of steroid synthesis require NADPH as a cofactor, the importance of cytosolic Ca(2+) - mitochondrial Ca(2+) coupling and of appropriate NADPH supply in respect to hormone production can be assumed. However, the importance of the mitochondrial factors has been neglected so far. Here, after summarizing earlier findings we provide new results obtained through modifying mitochondrial Ca(2+) uptake by knocking down p38 MAPK or OPA1 and overexpressing S100G, supporting the notion that mitochondrial Ca(2+) and reduced pyridine nucleotides are facilitating factors for both basal and stimulated steroid production.

Nanospaces between endoplasmic reticulum and mitochondria as control centres of pancreatic β-cell metabolism and survival

Nanometre-scale spaces between organelles represent focused nodes for signal transduction and the control of cellular decisions. The endoplasmic reticulum (ER) and the mitochondria form dynamic quasi-synaptic interaction nanodomains in all cell types examined, but the functional role of these junctions in cellular metabolism and cell survival remains to be fully understood. In this paper, we review recent evidence that ER Ca(2+) channels, such as the RyR and IP(3)R, can signal specifically across this nanodomain to the adjacent mitochondria to pace basal metabolism, with focus on the pancreatic β-cell. Blocking these signals in the basal state leads to a form of programmed cell death associated with reduced ATP and the induction of calpain-10 and hypoxia-inducible factors. On the other hand, the hyperactivity of this signalling domain plays a deleterious role during classical forms of apoptosis. Thus, the nanospace between ER and mitochondria represents a critical rheostat controlling both metabolism and programmed cell death. Many aspects of the mechanisms underlying this control system remain to be uncovered, and new nanotechnologies are required understand these domains at a molecular level.

Endoplasmic reticulum: ER stress regulates mitochondrial bioenergetics

Endoplasmic reticulum (ER) stress activates an adaptive unfolded protein response (UPR) that facilitates cellular repair, however, under prolonged ER stress, the UPR can ultimately trigger apoptosis thereby terminating damaged cells. The molecular mechanisms responsible for execution of the cell death program are relatively well characterized, but the metabolic events taking place during the adaptive phase of ER stress remain largely undefined. Here we discuss emerging evidence regarding the metabolic changes that occur during the onset of ER stress and how ER influences mitochondrial function through mechanisms involving calcium transfer, thereby facilitating cellular adaptation. Finally, we highlight how dysregulation of ER-mitochondrial calcium homeostasis during prolonged ER stress is emerging as a novel mechanism implicated in the onset of metabolic disorders.

Phosphoinositide signaling

"All things flow and change…even in the stillest matter there is unseen flux and movement." Attributed to Heraclitus (530-470 BC), from The Story of Philosophy by Will Durant. Heraclitus, a Greek philosopher, was thinking on a much larger scale than molecular signaling; however, his visionary comments are an important reminder for those studying signaling today. Even in unstimulated cells, signaling pathways are in constant metabolic flux and provide basal signals that travel throughout the organism. In addition, negatively charged phospholipids, such as the polyphosphorylated inositol phospholipids, provide a circuit board of on/off switches for attracting or repelling proteins that define the membranes of the cell. This template of charged phospholipids is sensitive to discrete changes and metabolic fluxes-e.g., in pH and cations-which contribute to the oscillating signals in the cell. The inherent complexities of a constantly fluctuating system make understanding how plants integrate and process signals challenging. In this review we discuss one aspect of lipid signaling: the inositol family of negatively charged phospholipids and their functions as molecular sensors and regulators of metabolic flux in plants.

Control of autophagy as a therapy for neurodegenerative disease

Autophagy is an intracellular degradation process that clears long-lived proteins and organelles from the cytoplasm. It involves the formation of double-membraned structures called autophagosomes that can engulf portions of cytoplasm containing oligomeric protein complexes and organelles, such as mitochondria. Autophagosomes fuse with lysosomes and their contents then are degraded. Failure of autophagy in neurons can result in the accumulation of aggregate-prone proteins and neurodegeneration. Pharmacological induction of autophagy can enhance the clearance of intracytoplasmic aggregate-prone proteins, such as mutant forms of huntingtin, and ameliorate pathology in cell and animal models of neurodegenerative diseases. In this Review, the autophagic machinery and the signaling pathways that regulate the induction of autophagy are described. The ways in which dysfunctions at multiple stages in the autophagic pathways contribute to numerous neurological disorders are highlighted through the use of examples of Mendelian and complex conditions, including Alzheimer disease, Parkinson disease and forms of motor neuron disease. The different ways in which autophagic pathways might be manipulated for the therapeutic benefit of patients with neurodegenerative disorders are also considered.

Energy-preserving effects of IGF-1 antagonize starvation-induced cardiac autophagy

AIMS

Insulin-like growth factor 1 (IGF-1) is known to exert cardioprotective actions. However, it remains unknown if autophagy, a major adaptive response to nutritional stress, contributes to IGF-1-mediated cardioprotection.

METHODS AND RESULTS

We subjected cultured neonatal rat cardiomyocytes, as well as live mice, to nutritional stress and assessed cell death and autophagic rates. Nutritional stress induced by serum/glucose deprivation strongly induced autophagy and cell death, and both responses were inhibited by IGF-1. The Akt/mammalian target of rapamycin (mTOR) pathway mediated the effects of IGF-1 upon autophagy. Importantly, starvation also decreased intracellular ATP levels and oxygen consumption leading to AMP-activated protein kinase (AMPK) activation; IGF-1 increased mitochondrial Ca(2+) uptake and mitochondrial respiration in nutrient-starved cells. IGF-1 also rescued ATP levels, reduced AMPK phosphorylation and increased p70(S6K) phosphorylation, which indicates that in addition to Akt/mTOR, IGF-1 inhibits autophagy by the AMPK/mTOR axis. In mice harbouring a liver-specific igf1 deletion, which dramatically reduces IGF-1 plasma levels, AMPK activity and autophagy were increased, and significant heart weight loss was observed in comparison with wild-type starved animals, revealing the importance of IGF-1 in maintaining cardiac adaptability to nutritional insults in vivo.

CONCLUSION

Our data support the cardioprotective actions of IGF-1, which, by rescuing the mitochondrial metabolism and the energetic state of cells, reduces cell death and controls the potentially harmful autophagic response to nutritional challenges. IGF-1, therefore, may prove beneficial to mitigate damage induced by excessive nutrient-related stress, including ischaemic disease in multiple tissues.

Calcium and reactive oxygen species in acute pancreatitis: friend or foe?

SIGNIFICANCE

Acute pancreatitis (AP) is a debilitating and, at times, lethal inflammatory disease, the causes and progression of which are incompletely understood. Disruption of Ca(2+) homeostasis in response to precipitants of AP leads to loss of mitochondrial integrity and cellular necrosis.

RECENT ADVANCES

While oxidative stress has been implicated as a major player in the pathogenesis of this disease, its precise roles remain to be defined. Recent developments are challenging the perception of reactive oxygen species (ROS) as nonspecific cytotoxic agents, suggesting that ROS promote apoptosis that may play a vital protective role in cellular stress since necrosis is avoided.

CRITICAL ISSUES

Fresh clinical findings have indicated that antioxidant treatment does not ameliorate AP and may actually worsen the outcome. This review explores the complex links between cellular Ca(2+) signaling and the intracellular redox environment, with particular relevance to AP.

FUTURE DIRECTIONS

Recent publications have underlined the importance of both Ca(2+) and ROS within the pathogenesis of AP, particularly in the determination of cell fate. Future research should elucidate the subtle interplay between Ca(2+) and redox mechanisms that operate to modulate mitochondrial function, with a view to devising strategies for the preservation of organellar function.

Presenilin-2 modulation of ER-mitochondria interactions: FAD mutations, mechanisms and pathological consequences

Presenilin (PS) mutations are the main cause of Familial Alzheimer's Disease (FAD) and have been demonstrated to cause an imbalance of intracellular Ca(2+) homeostasis. Though PS1 and 2 are generally considered to behave similarly in terms of their effects on Ca(2+) handling, we have recently described a novel function, which is unique to PS2, i.e., the modulation of ER-mitochondria juxtaposition. Accordingly, PS2, but not PS1, affects the Ca(2+) cross-talk between these organelles, a key feature in determining cell fate. In particular, PS2 overexpression, and more drastically that of FAD-linked PS2 mutants, strongly increases the interaction between ER and mitochondria, thus facilitating mitochondrial Ca(2+) uptake. The likely mechanisms behind this phenomenon and its potential effects in cell physiology and pathology are discussed.

Leucine-rich repeat kinase 2 regulates autophagy through a calcium-dependent pathway involving NAADP

Mutations in the leucine-rich repeat kinase-2 (LRRK2) gene cause late-onset Parkinson’s disease, but its physiological function has remained largely unknown. Here we report that LRRK2 activates a calcium-dependent protein kinase kinase-β (CaMKK-β)/adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway which is followed by a persistent increase in autophagosome formation. Simultaneously, LRKR2 overexpression increases the levels of the autophagy receptor p62 in a protein synthesis-dependent manner, and decreases the number of acidic lysosomes. The LRRK2-mediated effects result in increased sensitivity of cells to stressors associated with abnormal protein degradation. These effects can be mimicked by the lysosomal Ca²⁺-mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) and can be reverted by an NAADP receptor antagonist or expression of dominant-negative receptor constructs. Collectively, our data indicate a molecular mechanism for LRRK2 deregulation of autophagy and reveal previously unidentified therapeutic targets.

Autophagy and polyglutamine diseases

In polyglutamine diseases, an abnormally elongated polyglutamine tract results in protein misfolding and accumulation of intracellular aggregates. The length of the polyglutamine expansion correlates with the tendency of the mutant protein to aggregate, as well as with neuronal toxicity and earlier disease onset. Although currently there is no effective cure to prevent or slow down the progression of these neurodegenerative disorders, increasing the clearance of mutant proteins has been proposed as a potential therapeutic approach. The ubiquitin–proteasome system and autophagy are the two main degradative pathways responsible for eliminating misfolded and unnecessary proteins in the cell. We will review some of the studies that have proposed autophagy as a strategy to reduce the accumulation of polyglutamine-expanded protein aggregates and protect against mutant protein neurotoxicity. We will also discuss some of the currently known mechanisms that induce autophagy, which may be beneficial for the treatment of these and other neurodegenerative disorders.

Calcium signals and calpain-dependent necrosis are essential for release of coxsackievirus B from polarized intestinal epithelial cells

Coxsackievirus B (CVB), a member of the enterovirus family, targets the polarized epithelial cells lining the intestinal tract early in infection. Although the polarized epithelium functions as a protective barrier, this barrier is likely exploited by CVB to promote viral entry and subsequent egress. Here we show that, in contrast to nonpolarized cells, CVB-infected polarized intestinal Caco-2 cells undergo nonapoptotic necrotic cell death triggered by inositol 1,4,5-trisphosphate receptor-dependent calcium release. We further show that CVB-induced cellular necrosis depends on the Ca(2+)-activated protease calpain-2 and that this protease is involved in CVB-induced disruption of the junctional complex and rearrangements of the actin cytoskeleton. Our study illustrates the cell signaling pathways hijacked by CVB, and perhaps other viral pathogens, to promote their replication and spread in polarized cell types.

Patterns of gene expression in Drosophila InsP3 receptor mutant larvae reveal a role for InsP3 signaling in carbohydrate and energy metabolism

Background

The Inositol 1,4,5-trisphosphate receptor (InsP₃R) is an InsP₃ gated intracellular Ca²⁺-release channel. Characterization of Drosophila mutants for the InsP₃R has demonstrated that InsP₃-mediated Ca²⁺ release is required in Drosophila larvae for growth and viability.

Methodology/Principal Findings

To understand the molecular basis of these growth defects a genome wide microarray analysis has been carried out with larval RNA obtained from a strong InsP₃R mutant combination in which 1504 independent genes were differentially regulated with a log₂ of fold change of 1 or more and P<0.05. This was followed by similar transcript analyses from InsP₃R mutants where growth defects were either suppressed by introduction of a dominant suppressor or rescued by ectopic expression of an InsP₃R transgene in the Drosophila insulin like peptide-2 (Dilp2) producing cells.

Conclusions/Significance

These studies show that expression of transcripts related to carbohydrate and amine metabolism is altered in InsP₃ receptor mutant larvae. Moreover, from a comparative analysis of genes that are regulated in the suppressed and rescued conditions with the mutant condition, it appears that the organism could use different combinations of pathways to restore a ‘normal’ growth state.

Constitutive cAMP response element binding protein (CREB) activation by Alzheimer's disease presenilin-driven inositol trisphosphate receptor (InsP3R) Ca2+ signaling

Mutations in presenilins (PS) account for most early-onset familial Alzheimer's disease (FAD). Accumulating evidence suggests that disrupted Ca(2+) signaling may play a proximal role in FAD specifically, and Alzheimer's disease (AD) more generally, but its links to the pathogenesis of AD are obscure. Here we demonstrate that expression of FAD mutant PS constitutively activates the transcription factor cAMP response element binding protein (CREB) and CREB target gene expression in cultured neuronal cells and AD mouse models. Constitutive CREB activation was associated with and dependent on constitutive activation of Ca(2+)/CaM kinase kinase β and CaM kinase IV (CaMKIV). Depletion of endoplasmic reticulum Ca(2+) stores or plasma membrane phosphatidylinositol-bisphosphate and pharmacologic inhibition or knockdown of the expression of the inositol trisphosphate receptor (InsP(3)R) Ca(2+) release channel each abolished FAD PS-associated constitutive CaMKIV and CREB phosphorylation. CREB and CaMKIV phosphorylation and CREB target gene expression, including nitric oxide synthase and c-fos, were enhanced in brains of M146V-KI and 3xTg-AD mice expressing FAD mutant PS1 knocked into the mouse locus. FAD mutant PS-expressing cells demonstrated enhanced cell death and sensitivity to Aβ toxicity, which were normalized by interfering with the InsP(3)R-CAMKIV-CREB pathway. Thus, constitutive CREB phosphorylation by exaggerated InsP(3)R Ca(2+) signaling in FAD PS-expressing cells may represent a signaling pathway involved in the pathogenesis of AD.

Sensing cellular stress through STIM proteins

In response to decreasing Ca2+ levels in the endoplasmic reticulum, STIM proteins couple with Orai channels in the plasma membrane, leading to Ca2+ influx into the cell. In addition to Ca2+-related endoplasmic reticulum stress, STIM proteins are emerging as general stress sensors that react to multiple stress signals to orchestrate Ca2+ signaling and homeostasis.

Quantitative proteomic analyses of human cytomegalovirus-induced restructuring of endoplasmic reticulum-mitochondrial contacts at late times of infection

Endoplasmic reticulum-mitochondrial contacts, known as mitochondria-associated membranes, regulate important cellular functions including calcium signaling, bioenergetics, and apoptosis. Human cytomegalovirus is a medically important herpesvirus whose growth increases energy demand and depends upon continued cell survival. To gain insight into how human cytomegalovirus infection affects endoplasmic reticulum-mitochondrial contacts, we undertook quantitative proteomics of mitochondria-associated membranes using differential stable isotope labeling by amino acids in cell culture strategy and liquid chromatography-tandem MS analysis. This is the first reported quantitative proteomic analyses of a suborganelle during permissive human cytomegalovirus infection. Human fibroblasts were uninfected or human cytomegalovirus-infected for 72 h. Heavy mitochondria-associated membranes were isolated from paired unlabeled, uninfected cells and stable isotope labeling by amino acids in cell culture-labeled, infected cells and analyzed by liquid chromatography-tandem MS analysis. The results were verified by a reverse labeling experiment. Human cytomegalovirus infection dramatically altered endoplasmic reticulum-mitochondrial contacts by late times. Notable is the increased abundance of several fundamental networks in the mitochondria-associated membrane fraction of human cytomegalovirus-infected fibroblasts. Chaperones, including HSP60 and BiP, which is required for human cytomegalovirus assembly, were prominently increased at endoplasmic reticulum-mitochondrial contacts after infection. Minimal translational and translocation machineries were also associated with endoplasmic reticulum-mitochondrial contacts and increased after human cytomegalovirus infection as were glucose regulated protein 75 and the voltage dependent anion channel, which can form an endoplasmic reticulum-mitochondrial calcium signaling complex. Surprisingly, mitochondrial metabolic enzymes and cytosolic glycolytic enzymes were confidently detected in the mitochondria-associated membrane fraction and increased therein after infection. Finally, proapoptotic regulatory proteins, including Bax, cytochrome c, and Opa1, were augmented in endoplasmic reticulum-mitochondrial contacts after infection, suggesting attenuation of proapoptotic signaling by their increased presence therein. Together, these results suggest that human cytomegalovirus infection restructures the proteome of endoplasmic reticulum-mitochondrial contacts to bolster protein translation at these junctions, calcium signaling to mitochondria, cell survival, and bioenergetics and, thereby, allow for enhanced progeny production.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

Urban planning of the endoplasmic reticulum (ER): how diverse mechanisms segregate the many functions of the ER

The endoplasmic reticulum (ER) is the biggest organelle in most cell types, but its characterization as an organelle with a continuous membrane belies the fact that the ER is actually an assembly of several, distinct membrane domains that execute diverse functions. Almost 20 years ago, an essay by Sitia and Meldolesi first listed what was known at the time about domain formation within the ER. In the time that has passed since, additional ER domains have been discovered and characterized. These include the mitochondria-associated membrane (MAM), the ER quality control compartment (ERQC), where ER-associated degradation (ERAD) occurs, and the plasma membrane-associated membrane (PAM). Insight has been gained into the separation of nuclear envelope proteins from the remainder of the ER. Research has also shown that the biogenesis of peroxisomes and lipid droplets occurs on specialized membranes of the ER. Several studies have shown the existence of specific marker proteins found on all these domains and how they are targeted there. Moreover, a first set of cytosolic ER-associated sorting proteins, including phosphofurin acidic cluster sorting protein 2 (PACS-2) and Rab32 have been identified. Intra-ER targeting mechanisms appear to be superimposed onto ER retention mechanisms and rely on transmembrane and cytosolic sequences. The crucial roles of ER domain formation for cell physiology are highlighted with the specific targeting of the tumor metastasis regulator gp78 to ERAD-mediating membranes or of the promyelocytic leukemia protein to the MAM.

Autophagy as a therapeutic target in cardiovascular disease

The epidemic of heart failure continues apace, and development of novel therapies with clinical efficacy has lagged. Now, important insights into the molecular circuitry of cardiovascular autophagy have raised the prospect that this cellular pathway of protein quality control may be a target of clinical relevance. Whereas basal levels of autophagy are required for cell survival, excessive levels - or perhaps distinct forms of autophagic flux - contribute to disease pathogenesis. Our challenge will be to distinguish mechanisms that drive adaptive versus maladaptive autophagy and to manipulate those pathways for therapeutic gain. Recent evidence suggests this may be possible. Here, we review the fundamental biology of autophagy and its role in a variety of forms of cardiovascular disease. We discuss ways in which this evolutionarily conserved catabolic mechanism can be manipulated, discuss studies presently underway in heart disease, and provide our perspective on where this exciting field may lead in the future. This article is part of a special issue entitled ''Key Signaling Molecules in Hypertrophy and Heart Failure.''

Bcl-2 interaction with the inositol 1,4,5-trisphosphate receptor: role in Ca(2+) signaling and disease

The Bcl-2 protein, best known for its ability to inhibit apoptosis, interacts with the inositol 1,4,5-trisphosphate receptor (IP(3)R) Ca(2+) channel to regulate IP(3)-mediated Ca(2+) release from the endoplasmic reticulum. This review summarizes the current state of knowledge regarding the interaction of Bcl-2, and also its homologue Bcl-xl, with the IP(3)R and how these interactions regulate Ca(2+) signaling. The dual role of these interactions in promoting prosurvival Ca(2+) signals, while at the same time inhibiting proapoptotic Ca(2+) signals, is discussed. Moreover, this review will elucidate the recently recognized importance of the Bcl-2-IP(3)R interaction in human disease.

Enhanced ER Ca2+ store filling by overexpression of SERCA2b promotes IP3-evoked puffs

Liberation of Ca(2+) from the endoplasmic reticulum (ER) through inositol trisphosphate receptors (IP(3)R) is modulated by the ER Ca(2+) content, and overexpression of SERCA2b to accelerate Ca(2+) sequestration into the ER has been shown to potentiate the frequency and amplitude of IP(3)-evoked Ca(2+) waves in Xenopus oocytes. Here, we examined the effects of SERCA overexpression on the elementary IP(3)-evoked puffs to elucidate whether ER [Ca(2+)] may modulate IP(3)R function via luminal regulatory sites in addition to simply determining the size of the available store and electrochemical driving force for Ca(2+) release. SERCA2b and Ca(2+) permeable nicotinic plasmalemmal channels were expressed in oocytes, and hyperpolarizing pulses were delivered to induce Ca(2+) influx and thereby load ER stores. Puffs evoked by photoreleased IP(3) were significantly potentiated in terms of numbers of responding sites, frequency and amplitude following transient Ca(2+) influx in SERCA-overexpressing cells, whereas little change was evident with SERCA overexpression alone or following Ca(2+) influx in control cells not overexpressing SERCA. Intriguingly, we observed the appearance of a new population of puffs that arose after long latencies and had prolonged durations supporting the notion of luminal regulation of IP(3)R gating kinetics.

ULK1, mammalian target of rapamycin, and mitochondria: linking nutrient availability and autophagy

A fundamental function of autophagy conserved from yeast to mammals is mobilization of macromolecules during times of limited nutrient availability, permitting organisms to survive under starvation conditions. In yeast, autophagy is initiated following nitrogen or carbon deprivation, and autophagy mutants die rapidly under these conditions. Similarly, in mammals, autophagy is upregulated in most organs following initiation of starvation, and is critical for survival in the perinatal period following abrupt termination of the placental nutrient supply. The nutrient-sensing kinase, mammalian target of rapamycin, coordinates cellular proliferation and growth with nutrient availability, at least in part by regulating protein synthesis and autophagy-mediated degradation. This review focusses on the regulation of autophagy by Tor, a mammalian target of rapamycin, and Ulk1, a mammalian homolog of Atg1, in response to changes in nutrient availability. Given the importance of mitochondria in maintaining bioenergetic homestasis, and potentially as a source of membrane for autophagosomes during starvation, possible roles for mitochondria in this process are also discussed.

Bioenergetic homeostasis decides neuroprotection or neurotoxicity induced by volatile anesthetics: a uniform mechanism of dual effects

The commonly used volatile anesthetic isoflurane or sevoflurane has been shown to be both neuroprotective and neurotoxic in various cell cultures and animal models. Some possible mechanisms have been raised to elucidate volatile anesthetics-induced neuroprotection or neurotoxicity, respectively. However, none of these can reconcile the linkage between their dual effects. Similar to volatile anesthetics, some drugs and nonpharmacological factors also can produce neuroprotection and neurotoxicity, which is associated with bioenergetic metabolism of neuronal cells. Here we present a uniform mechanism, bioenergetic homeostasis hypothesis, to explain neuroprotection and neurotoxicity induced by volatile anesthetics. The numerous evidences have shown that volatile anesthetics could affect mitochondrial electron transport complexes and glycolysis related pathways in cells, which could alter intracellular calcium homeostasis, ROS production and adenosine triphosphate (ATP) synthesis. Duration and concentration of exposure to volatile anesthetics could play a role on severity of bioenergy inhibition. Mild bioenergetic metabolism inhibition trigger signaling events involving preconditioning on neurons, and further bioenergy impairment could lead to neuronal cellular apoptosis, inhibition of neurogenesis and elevated β-Secretase, which drive pathogenesis of neurodegeneration.

Mitochondrial matrix calcium is an activating signal for hormone secretion

Mitochondrial Ca(2+) signals have been proposed to accelerate oxidative metabolism and ATP production to match Ca(2+)-activated energy-consuming processes. Efforts to understand the signaling role of mitochondrial Ca(2+) have been hampered by the inability to manipulate matrix Ca(2+) without directly altering cytosolic Ca(2+). We were able to selectively buffer mitochondrial Ca(2+) rises by targeting the Ca(2+)-binding protein S100G to the matrix. We find that matrix Ca(2+) controls signal-dependent NAD(P)H formation, respiration, and ATP changes in intact cells. Furthermore, we demonstrate that matrix Ca(2+) increases are necessary for the amplification of sustained glucose-dependent insulin secretion in β cells. Through the regulation of NAD(P)H in adrenal glomerulosa cells, matrix Ca(2+) also acts as a positive signal in reductive biosynthesis, which stimulates aldosterone secretion. Our dissection of cytosolic and mitochondrial Ca(2+) signals reveals the physiological importance of matrix Ca(2+) in energy metabolism required for signal-dependent hormone secretion.