Generation and functions of second messengers microdomains

Myosin Va: Capturing cAMP for synaptic plasticity

The plus-end directed actin-dependent motor protein, myosin Va, is of particular relevance for outward vesicular protein trafficking and for restraining specific cargo vesicles within the actin cortex. The latter is a preferred site of cAMP production, and the specificity of cAMP signaling is largely mediated through the formation of microdomains that spatially couple localized metabotropic receptor activity and cAMP production to selected effectors and downstream targets. This review summarizes the core literature on the role of myosin Va for the creation of such a cAMP microdomain at the mammalian nerve-muscle synapse that serves the activity-dependent recycling of nicotinic acetylcholine receptors (nAChRs)-a principal ligand-gated ion channel which is imperative for voluntary muscle contraction. It is discussed that i) the nerve-muscle synapse is a site with a unique actin-dependent microstructure, ii) myosin Va and protein kinase A regulatory subunit Iα as well as nAChR and its constitutive binding partner, rapsyn, colocalize in endocytic/recycling vesicles near the postsynaptic membrane, and iii) impairment of myosin Va or displacement of protein kinase A regulatory subunit Iα leads to the loss of nAChR stability. Regulation of this signaling process and underlying basic pieces of machinery were covered in previous articles, to which the present review refers.

CaMKII inhibition due to TRIC-B loss-of-function dysregulates SMAD signaling in osteogenesis imperfecta

Ca2+ is a second messenger that regulates a variety of cellular responses in bone, including osteoblast differentiation. Mutations in trimeric intracellular cation channel B (TRIC-B), an endoplasmic reticulum channel specific for K+, a counter ion for Ca2+flux, affect bone and cause a recessive form of osteogenesis imperfecta (OI) with a still puzzling mechanism. Using a conditional Tmem38b knock out mouse, we demonstrated that lack of TRIC-B in osteoblasts strongly impairs skeleton growth and structure, leading to bone fractures. At the cellular level, delayed osteoblast differentiation and decreased collagen synthesis were found consequent to the Ca2+ imbalance and associated with reduced collagen incorporation in the extracellular matrix and poor mineralization. The impaired SMAD signaling detected in mutant mice, and validated in OI patient osteoblasts, explained the osteoblast malfunction. The reduced SMAD phosphorylation and nuclear translocation were mainly caused by alteration in Ca2+ calmodulin kinase II (CaMKII)-mediated signaling and to a less extend by a lower TGF-β reservoir. SMAD signaling, osteoblast differentiation and matrix mineralization were only partially rescued by TGF-β treatment, strengthening the impact of CaMKII-SMAD axes on osteoblast function. Our data established the TRIC-B role in osteoblasts and deepened the contribution of the CaMKII-SMAD signaling in bone.

A Perspective on the Link between Mitochondria-Associated Membranes (MAMs) and Lipid Droplets Metabolism in Neurodegenerative Diseases

Mitochondria interact with the endoplasmic reticulum (ER) through contacts called mitochondria-associated membranes (MAMs), which control several processes, such as the ER stress response, mitochondrial and ER dynamics, inflammation, apoptosis, and autophagy. MAMs represent an important platform for transport of non-vesicular phospholipids and cholesterol. Therefore, this region is highly enriched in proteins involved in lipid metabolism, including the enzymes that catalyze esterification of cholesterol into cholesteryl esters (CE) and synthesis of triacylglycerols (TAG) from fatty acids (FAs), which are then stored in lipid droplets (LDs). LDs, through contact with other organelles, prevent the toxic consequences of accumulation of unesterified (free) lipids, including lipotoxicity and oxidative stress, and serve as lipid reservoirs that can be used under multiple metabolic and physiological conditions. The LDs break down by autophagy releases of stored lipids for energy production and synthesis of membrane components and other macromolecules. Pathological lipid deposition and autophagy disruption have both been reported to occur in several neurodegenerative diseases, supporting that lipid metabolism alterations are major players in neurodegeneration. In this review, we discuss the current understanding of MAMs structure and function, focusing on their roles in lipid metabolism and the importance of autophagy in LDs metabolism, as well as the changes that occur in neurogenerative diseases.

Interorganellar Communication Through Membrane Contact Sites in Toxoplasma Gondii

Apicomplexan parasites are a group of protists that cause disease in humans and include pathogens like Plasmodium spp., the causative agent of malaria, and Toxoplasma gondii, the etiological agent of toxoplasmosis and one of the most ubiquitous human parasites in the world. Membrane contact sites (MCSs) are widespread structures within eukaryotic cells but their characterization in apicomplexan parasites is only in its very beginnings. Basic biological features of the T. gondii parasitic cycle support numerous organellar interactions, including the transfer of Ca2+ and metabolites between different compartments. In T. gondii, Ca2+ signals precede a series of interrelated molecular processes occurring in a coordinated manner that culminate in the stimulation of key steps of the parasite life cycle. Calcium transfer from the endoplasmic reticulum to other organelles via MCSs would explain the precision, speed, and efficiency that is needed during the lytic cycle of T. gondii. In this short review, we discuss the implications of these structures in cellular signaling, with an emphasis on their potential role in Ca2+ signaling.

Mitochondrial Ca2+ Signaling and Bioenergetics in Alzheimer's Disease

Alzheimer's disease (AD) is a hereditary and sporadic neurodegenerative illness defined by the gradual and cumulative loss of neurons in specific brain areas. The processes that cause AD are still under investigation and there are no available therapies to halt it. Current progress puts at the forefront the "calcium (Ca2+) hypothesis" as a key AD pathogenic pathway, impacting neuronal, astrocyte and microglial function. In this review, we focused on mitochondrial Ca2+ alterations in AD, their causes and bioenergetic consequences in neuronal and glial cells, summarizing the possible mechanisms linking detrimental mitochondrial Ca2+ signals to neuronal death in different experimental AD models.

Ultrastructural and proteomic profiling of mitochondria-associated endoplasmic reticulum membranes reveal aging signatures in striated muscle

Aging is a major risk for developing cardiac and skeletal muscle dysfunction, yet the underlying mechanism remains elusive. Here we demonstrated that the mitochondria-associated endoplasmic reticulum membranes (MAMs) in the rat heart and skeletal muscle were disrupted during aging. Using quantitative morphological analysis, we showed that the mitochondria-endoplasmic reticulum contacts (MERCs) were reduced by half over the lifespan with an early onset of accelerated thickening in the clefts. The ultrastructural changes were further validated by proteomic profiling of the MAM fractions. A combination of subcellular fractionation and quantitative mass spectrometry identified 1306 MAM-enriched proteins in both heart and skeletal muscle, with a catalog of proteins dysregulated with aging. Functional mapping of the MAM proteome suggested several aging signatures to be closely associated with the ER-mitochondria crosstalk, including local metabolic rewiring, calcium homeostasis imbalance, and impaired organelle dynamics and autophagy. Moreover, we identified a subset of highly interconnected proteins in an ER-mitochondria organization network, which were consistently down-regulated with aging. These decreased proteins, including VDAC1, SAMM50, MTX1 and MIC60, were considered as potential contributors to the age-related MAM dysfunction. This study highlights the perturbation in MAM integrity during the striated muscle aging process, and provides a framework for understanding aging biology from the perspective of organelle interactions.

Reduction of ER-Mitochondria Distance: a Key Feature in Alzheimer's and Parkinson's Disease, and During Cancer Treatment

One remarkable dynamic cell structure is the region between the endoplasmic reticulum (ER) and the mitochondria, termed the mitochondria-associated membranes (MAM). MAMs carry out different cellular functions such as Ca²⁺ homeostasis and lipid synthesis, which depend on an adequate distance separating the ER and mitochondria. A decreased distance has been observed in Alzheimer's disease, Parkinson's disease, and during cancer treatment. It is unclear how dysregulation of the spatial characteristics of MAMs can cause abnormal Ca²⁺ dynamics which could end in cell death. In this work, a computational model was proposed to study the relationship between a decreased ER-mitochondria distance and mitochondria-induced cell death. Our results point towards the mitochondrial permeability transition pore (mPTP) as a key cell death signaling mechanism indirectly regulated by the spatial characteristics of MAMs.Clinical Relevance- The endoplasmic reticulum-mitochondria crosstalk plays an important role in the mPTP-induced apoptosis. This process could be behind neurodegeneration in Alzheimer's and Parkinson's diseases, as well as behind the induced cell death during cancer treatment.

Knocking out TMEM38B in human foetal osteoblasts hFOB 1.19 by CRISPR/Cas9: A model for recessive OI type XIV

Osteogenesis imperfecta (OI) type XIV is a rare recessive bone disorder characterized by variable degree of severity associated to osteopenia. It is caused by mutations in TMEM38B encoding for the trimeric intracellular cation channel TRIC-B, specific for potassium and ubiquitously present in the endoplasmic reticulum (ER) membrane. OI type XIV molecular basis is largely unknown and, due to the rarity of the disease, the availability of patients’ osteoblasts is challenging. Thus, CRISPR/Cas9 was used to knock out (KO) TMEM38B in the human Foetal Osteoblast hFOB 1.19 to obtain an OI type XIV model. CRISPR/Cas9 is a powerful technology to generate in vitro and in vivo models for heritable disorders. Its limited cost and ease of use make this technique widely applicable in most laboratories. Nevertheless, to fully take advantage of this approach, it is important to be aware of its strengths and limitations. Three gRNAs were used and several KO clones lacking the expression of TRIC-B were obtained. Few clones were validated as good models for the disease since they reproduce the altered ER calcium flux, collagen I structure and impaired secretion and osteoblastic markers expression detected in patients’ cells. Impaired proliferation and mineralization in KO clones unveiled the relevance of TRIC-B in osteoblasts functionality.

Cross-Talk between Mechanosensitive Ion Channels and Calcium Regulatory Proteins in Cardiovascular Health and Disease

Mechanosensitive ion channels are widely expressed in the cardiovascular system. They translate mechanical forces including shear stress and stretch into biological signals. The most prominent biological signal through which the cardiovascular physiological activity is initiated or maintained are intracellular calcium ions (Ca²⁺). Growing evidence show that the Ca²⁺ entry mediated by mechanosensitive ion channels is also precisely regulated by a variety of key proteins which are distributed in the cell membrane or endoplasmic reticulum. Recent studies have revealed that mechanosensitive ion channels can even physically interact with Ca²⁺ regulatory proteins and these interactions have wide implications for physiology and pathophysiology. Therefore, this paper reviews the cross-talk between mechanosensitive ion channels and some key Ca²⁺ regulatory proteins in the maintenance of calcium homeostasis and its relevance to cardiovascular health and disease.

Ca2+ handling at the mitochondria-ER contact sites in neurodegeneration

Mitochondria-endoplasmic reticulum (ER) contact sites (MERCS) are morpho-functional units, formed at the loci of close apposition of the ER-forming endomembrane and outer mitochondrial membrane (OMM). These sites contribute to fundamental cellular processes including lipid biosynthesis, autophagy, apoptosis, ER-stress and calcium (Ca²⁺) signalling. At MERCS, Ca²⁺ ions are transferred from the ER directly to mitochondria through a core protein complex composed of inositol-1,4,5 trisphosphate receptor (InsP₃R), voltage-gated anion channel 1 (VDAC1), mitochondrial calcium uniporter (MCU) and adaptor protein glucose-regulated protein 75 (Grp75); this complex is regulated by several associated proteins. Deregulation of ER-mitochondria Ca²⁺ transfer contributes to pathogenesis of neurodegenerative and other diseases. The efficacy of Ca²⁺ transfer between ER and mitochondria depends on the protein composition of MERCS, which controls ER-mitochondria interaction regulating, for example, the transversal distance between ER membrane and OMM and the extension of the longitudinal interface between ER and mitochondria. These parameters are altered in neurodegeneration. Here we overview the ER and mitochondrial Ca²⁺ homeostasis, the composition of ER-mitochondrial Ca²⁺ transfer machinery and alterations of the ER-mitochondria Ca²⁺ transfer in three major neurodegenerative diseases: motor neurone diseases, Parkinson disease and Alzheimer's disease.

Investigating the landscape of intracellular [Ca2+] in live cells by rapid photoactivated cross-linking of calmodulin-protein interactions

The Ca²⁺ sensor protein calmodulin interacts in a Ca²⁺-dependent manner with a large number of proteins that among them encompass a diverse assortment of functions and subcellular localizations. A method for monitoring calmodulin-protein interactions as they occur throughout a living cell would thus uniquely enable investigations of the intracellular landscape of [Ca²⁺] and its relationship to cell function. We have developed such a method based on capture of calmodulin-protein interactions by rapid photoactivated cross-linking (t_1/2 ∼7s) in cells stably expressing a tandem affinity tagged calmodulin that have been metabolically labeled with a photoreactive methionine analog. Tagged adducts are stringently enriched, and captured calmodulin interactors are then identified and quantified based on tandem mass spectrometry data for their tryptic peptides. In this paper we show that the capture behaviors of interactors in cells are consistent with the presence of basal microdomains of elevated [Ca²⁺]. Ca²⁺ sensitivities for capture were determined, and these suggest that [Ca²⁺] levels are above ∼1 μM in these regions. Although the microdomains appear to affect capture of most proteins, capture of some is at an apparent Ca²⁺-dependent maximum, suggesting they are targeted to the domains. Removal of extracellular Ca²⁺ has both immediate (5 min) and delayed (30 min) effects on capture, implying that the microdomains are supported by a combination of Ca²⁺ influx across the cell membrane and Ca²⁺ derived from internal stores. The known properties of the presumptive microdomain targeted proteins suggestroles in a variety of Ca²⁺-dependent basal metabolism and in formation and maintenance of the domains.

SpiCee: A Genetic Tool for Subcellular and Cell-Specific Calcium Manipulation

Calcium is a second messenger crucial to a myriad of cellular processes ranging from regulation of metabolism and cell survival to vesicle release and motility. Current strategies to directly manipulate endogenous calcium signals lack cellular and subcellular specificity. We introduce SpiCee, a versatile and genetically encoded chelator combining low- and high-affinity sites for calcium. This scavenger enables altering endogenous calcium signaling and functions in single cells in vitro and in vivo with biochemically controlled subcellular resolution. SpiCee paves the way to investigate local calcium signaling in vivo and directly manipulate this second messenger for therapeutic use.

A Review of the Role of Endo/Sarcoplasmic Reticulum-Mitochondria Ca2+ Transport in Diseases and Skeletal Muscle Function

The physical contact site between a mitochondrion and endoplasmic reticulum (ER), named the mitochondria-associated membrane (MAM), has emerged as a fundamental platform for regulating the functions of the two organelles and several cellular processes. This includes Ca²⁺ transport from the ER to mitochondria, mitochondrial dynamics, autophagy, apoptosis signalling, ER stress signalling, redox reaction, and membrane structure maintenance. Consequently, the MAM is suggested to be involved in, and as a possible therapeutic target for, some common diseases and impairment in skeletal muscle function, such as insulin resistance and diabetes, obesity, neurodegenerative diseases, Duchenne muscular dystrophy, age-related muscle atrophy, and exercise-induced muscle damage. In the past decade, evidence suggests that alterations in Ca²⁺ transport from the ER to mitochondria, mediated by the macromolecular complex formed by IP₃R, Grp75, and VDAC1, may be a universal mechanism for how ER-mitochondria cross-talk is involved in different physiological/pathological conditions mentioned above. A better understanding of the ER (or sarcoplasmic reticulum in muscle)-mitochondria Ca²⁺ transport system may provide a new perspective for exploring the mechanism of how the MAM is involved in the pathology of diseases and skeletal muscle dysfunction. This review provides a summary of recent research findings in this area.

The journey of Ca2+ through the cell - pulsing through the network of ER membrane contact sites

Calcium is the third most abundant metal on earth, and the fundaments of its homeostasis date back to pre-eukaryotic life forms. In higher organisms, Ca²⁺ serves as a cofactor for a wide array of (enzymatic) interactions in diverse cellular contexts and constitutes the most important signaling entity in excitable cells. To enable responsive behavior, cytosolic Ca²⁺ concentrations are kept low through sequestration into organellar stores, particularly the endoplasmic reticulum (ER), but also mitochondria and lysosomes. Specific triggers are then used to instigate a local release of Ca²⁺ on demand. Here, communication between organelles comes into play, which is accomplished through intimate yet dynamic contacts, termed membrane contact sites (MCSs). The field of MCS biology in relation to cellular Ca²⁺ homeostasis has exploded in recent years. Taking advantage of this new wealth of knowledge, in this Review, we invite the reader on a journey of Ca²⁺ flux through the ER and its associated MCSs. New mechanistic insights and technological advances inform the narrative on Ca²⁺ acquisition and mobilization at these sites of communication between organelles, and guide the discussion of their consequences for cellular physiology.

Mitochondrial Ca2+ Dynamics in MCU Knockout C. elegans Worms

Mitochondrial [Ca²⁺] plays an important role in the regulation of mitochondrial function, controlling ATP production and apoptosis triggered by mitochondrial Ca²⁺ overload. This regulation depends on Ca²⁺ entry into the mitochondria during cell activation processes, which is thought to occur through the mitochondrial Ca²⁺ uniporter (MCU). Here, we have studied the mitochondrial Ca²⁺ dynamics in control and MCU-defective C. elegans worms in vivo, by using worms expressing mitochondrially-targeted YC3.60 yellow cameleon in pharynx muscle. Our data show that the small mitochondrial Ca²⁺ oscillations that occur during normal physiological activity of the pharynx were very similar in both control and MCU-defective worms, except for some kinetic differences that could mostly be explained by changes in neuronal stimulation of the pharynx. However, direct pharynx muscle stimulation with carbachol triggered a large and prolonged increase in mitochondrial [Ca²⁺] that was much larger in control worms than in MCU-defective worms. This suggests that MCU is necessary for the fast mitochondrial Ca²⁺ uptake induced by large cell stimulations. However, low-amplitude mitochondrial Ca²⁺ oscillations occurring under more physiological conditions are independent of the MCU and use a different Ca²⁺ pathway.

IP3 mediated global Ca2+ signals arise through two temporally and spatially distinct modes of Ca2+ release

The ‘building-block’ model of inositol trisphosphate (IP₃)-mediated Ca²⁺ liberation posits that cell-wide cytosolic Ca²⁺ signals arise through coordinated activation of localized Ca²⁺ puffs generated by stationary clusters of IP₃ receptors (IP₃Rs). Here, we revise this hypothesis, applying fluctuation analysis to resolve Ca²⁺ signals otherwise obscured during large Ca²⁺ elevations. We find the rising phase of global Ca²⁺ signals is punctuated by a flurry of puffs, which terminate before the peak by a mechanism involving partial ER Ca²⁺ depletion. The continuing rise in Ca²⁺, and persistence of global signals even when puffs are absent, reveal a second mode of spatiotemporally diffuse Ca²⁺ signaling. Puffs make only small, transient contributions to global Ca²⁺ signals, which are sustained by diffuse release of Ca²⁺ through a functionally distinct process. These two modes of IP₃-mediated Ca²⁺ liberation have important implications for downstream signaling, imparting spatial and kinetic specificity to Ca²⁺-dependent effector functions and Ca²⁺ transport.

Ca2+ Signaling in Exocrine Cells

Calcium (Ca²⁺) and cyclic AMP (cAMP) signaling cross talk and synergize to stimulate the cardinal functions of exocrine cells, regulated exocytosis, and fluid and electrolyte secretion. This physiological process requires the organization of the two signaling pathways into complexes at defined cellular domains and close placement. Such domains are formed by membrane contact sites (MCS). This review discusses the basic properties of Ca²⁺ signaling in exocrine cells, the role of MCS in the organization of cell signaling and in cross talk and synergism between the Ca²⁺ and cAMP signaling pathways and, finally, the mechanism by which the Ca²⁺ and cAMP pathways synergize to stimulate epithelial fluid and electrolyte secretion.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

Live Mitochondrial or Cytosolic Calcium Imaging Using Genetically-encoded Cameleon Indicator in Mammalian Cells

Calcium (Ca²⁺) imaging aims at investigating the dynamic changes in live cells of its concentration ([Ca²⁺]) in different pathophysiological conditions. Ca²⁺ is an ubiquitous and versatile intracellular signal that modulates a large variety of cellular functions thanks to a cell type-specific toolkit and a complex subcellular compartmentalization. Many Ca²⁺ sensors are presently available (chemical and genetically encoded) that can be specifically targeted to different cellular compartments. Using these probes, it is now possible to monitor Ca²⁺ dynamics of living cells not only in the cytosol but also within specific organelles. The choice of a specific sensor depends on the experimental design and the spatial and temporal resolution required. Here we describe the use of novel Förster resonance energy transfer (FRET)-based fluorescent Ca²⁺ probes to dynamically and quantitatively monitor the changes in cytosolic and mitochondrial [Ca²⁺] in a variety of cell types and experimental conditions. FRET-based sensors have the enormous advantage of being ratiometric, a feature that makes them particularly suitable for quantitative and in vivo applications.

Noise analysis of cytosolic calcium image data

Cellular Ca²⁺ signals are often constrained to cytosolic micro- or nano-domains where stochastic openings of Ca²⁺ channels cause large fluctuations in local Ca²⁺ concentration (Ca²⁺ 'noise'). With the advent of TIRF microscopy to image the fluorescence of Ca²⁺-sensitive probes from attoliter volumes it has become possible to directly monitor these signals, which closely track the gating of plasmalemmal and ER Ca²⁺-permeable channels. Nevertheless, it is likely that many physiologically important Ca²⁺ signals are too small to resolve as discrete events in fluorescence recordings. By analogy with noise analysis of electrophysiological data, we explore here the use of statistical approaches to detect and analyze such Ca²⁺ noise in images obtained using Ca²⁺-sensitive indicator dyes. We describe two techniques - power spectrum analysis and spatio-temporal correlation - and demonstrate that both effectively identify discrete, spatially localized calcium release events (Ca²⁺ puffs). Moreover, we show they are able to detect localized noise fluctuations in a case where discrete events cannot directly be resolved.

Variants in Miro1 Cause Alterations of ER-Mitochondria Contact Sites in Fibroblasts from Parkinson's Disease Patients

Background: Although most cases of Parkinson´s disease (PD) are idiopathic with unknown cause, an increasing number of genes and genetic risk factors have been discovered that play a role in PD pathogenesis. Many of the PD-associated proteins are involved in mitochondrial quality control, e.g., PINK1, Parkin, and LRRK2, which were recently identified as regulators of mitochondrial-endoplasmic reticulum (ER) contact sites (MERCs) linking mitochondrial homeostasis to intracellular calcium handling. In this context, Miro1 is increasingly recognized to play a role in PD pathology. Recently, we identified the first PD patients carrying mutations in RHOT1, the gene coding for Miro1. Here, we describe two novel RHOT1 mutations identified in two PD patients and the characterization of the cellular phenotypes. Methods: Using whole exome sequencing we identified two PD patients carrying heterozygous mutations leading to the amino acid exchanges T351A and T610A in Miro1. We analyzed calcium homeostasis and MERCs in detail by live cell imaging and immunocytochemistry in patient-derived fibroblasts. Results: We show that fibroblasts expressing mutant T351A or T610A Miro1 display impaired calcium homeostasis and a reduced amount of MERCs. All fibroblast lines from patients with pathogenic variants in Miro1, revealed alterations of the structure of MERCs. Conclusion: Our data suggest that Miro1 is important for the regulation of the structure and function of MERCs. Moreover, our study supports the role of MERCs in the pathogenesis of PD and further establishes variants in RHOT1 as rare genetic risk factors for neurodegeneration.

The role of mitochondria-associated membranes in cellular homeostasis and diseases

Mitochondria and endoplasmic reticulum (ER) are fundamental in the control of cell physiology regulating several signal transduction pathways. They continuously communicate exchanging messages in their contact sites called MAMs (mitochondria-associated membranes). MAMs are specific microdomains acting as a platform for the sorting of vital and dangerous signals. In recent years increasing evidence reported that multiple scaffold proteins and regulatory factors localize to this subcellular fraction suggesting MAMs as hotspot signaling domains. In this review we describe the current knowledge about MAMs' dynamics and processes, which provided new correlations between MAMs' dysfunctions and human diseases. In fact, MAMs machinery is strictly connected with several pathologies, like neurodegeneration, diabetes and mainly cancer. These pathological events are characterized by alterations in the normal communication between ER and mitochondria, leading to deep metabolic defects that contribute to the progression of the diseases.

Organellar Calcium Handling in the Cellular Reticular Network

Ca²⁺ is an important intracellular messenger affecting diverse cellular processes. In eukaryotic cells, Ca²⁺ is handled by a myriad of Ca²⁺-binding proteins found in organelles that are organized into the cellular reticular network (CRN). The network is comprised of the endoplasmic reticulum, Golgi apparatus, lysosomes, membranous components of the endocytic and exocytic pathways, peroxisomes, and the nuclear envelope. Membrane contact sites between the different components of the CRN enable the rapid movement of Ca²⁺, and communication of Ca²⁺ status, within the network. Ca²⁺-handling proteins that reside in the CRN facilitate Ca²⁺ sensing, buffering, and cellular signaling to coordinate the many processes that operate within the cell.

Cl- as a bona fide signaling ion

Cl^- is the major extracellular (Cl^-_out) and intracellular (Cl^-_in) anion whose concentration is actively regulated by multiple transporters. These transporters generate Cl^- gradients across the plasma membrane and between the cytoplasm and intracellular organelles. [Cl^-]_in changes rapidly in response to cell stimulation and influences many physiological functions, as well as cellular and systemic homeostasis. However, less appreciated is the signaling function of Cl^-. Cl^- interacts with multiple proteins to directly modify their activity. This review highlights the signaling function of Cl^- and argues that Cl^- is a bona fide signaling ion, a function deserving extensive exploration.

Evaluating Calmodulin-Protein Interactions by Rapid Photoactivated Cross-Linking in Live Cells Metabolically Labeled with Photo-Methionine

This work addresses the question of how the Ca²⁺ sensor protein calmodulin shapes cellular responses to Ca²⁺ signals. Proteins interacting with affinity tagged calmodulin were captured by rapid (t_1/2 ≈ 7 s) photoactivated cross-linking under basal conditions, after brief removal of extracellular Ca²⁺ and during a cytosolic [Ca²⁺] transient in cells metabolically labeled with a photoreactive methionine analog. Tagged adducts were stringently enriched, and captured proteins were identified and quantified by LC-MS/MS. A set of 489 proteins including 27 known calmodulin interactors was derived. A threshold for fractional capture was applied to define a high specificity group of 170 proteins, including 22 known interactors, and a low specificity group of 319 proteins. Capture of ∼60% of the high specificity group was affected by manipulations of Ca²⁺, compared with ∼20% of the low specificity group. This suggests that the former is likely to contain novel interactors of physiological significance. The capture of 29 proteins, nearly all high specificity, was decreased by the removal of extracellular Ca²⁺, although this does not affect cytosolic [Ca²⁺]. Capture of half of these was unaffected by the cytosolic [Ca²⁺] transient, consistent with high local [Ca²⁺]. These proteins are hypothesized to reside in or near microdomains of high [Ca²⁺] supported by the Ca²⁺ influx.

The Link of the Prion Protein with Ca2+ Metabolism and ROS Production, and the Possible Implication in Aβ Toxicity

The cellular prion protein (PrP^C) is an ubiquitous cell surface protein mostly expressed in neurons, where it localizes to both pre- and post-synaptic membranes. PrP^C aberrant conformers are the major components of mammalian prions, the infectious agents responsible for incurable neurodegenerative disorders. PrP^C was also proposed to bind aggregated misfolded proteins/peptides, and to mediate their neurotoxic signal. In spite of long-lasting research, a general consensus on the precise pathophysiologic mechanisms of PrP^C has not yet been reached. Here we review our recent data, obtained by comparing primary neurons from PrP-expressing and PrP-knockout mice, indicating a central role of PrP^C in synaptic transmission and Ca²⁺ homeostasis. Indeed, by controlling gene expression and signaling cascades, PrP^C is able to optimize glutamate secretion and regulate Ca²⁺ entry via store-operated channels and ionotropic glutamate receptors, thereby protecting neurons from threatening Ca²⁺ overloads and excitotoxicity. We will also illustrate and discuss past and unpublished results demonstrating that Aβ oligomers perturb Ca²⁺ homeostasis and cause abnormal mitochondrial accumulation of reactive oxygen species by possibly affecting the PrP-dependent downregulation of Fyn kinase activity.

Direct visualization of cAMP signaling in primary cilia reveals up-regulation of ciliary GPCR activity following Hedgehog activation

The primary cilium permits compartmentalization of specific signaling pathways, including elements of the Hedgehog (Hh) pathway. Hh transcriptional activity is thought to be negatively regulated by constitutively high ciliary cAMP maintained by the Gα(s)-coupled GPCR, GPR161. However, cilia also sequester many other Gα(s)-coupled GPCRs with unknown potential to regulate Hh. Here we used biosensors optimized for ciliary cAMP and strategies to isolate signals in the cilium from the cell body and neighboring cells. We found that ciliary cAMP was not elevated relative to cellular cAMP, inconsistent with constitutive cAMP production. Gα(s)-coupled GPCRs (e.g., the 5-HT₆ serotonin and D1R dopamine receptor) had reduced ability to generate cAMP upon trafficking to the ciliary membrane. However, activation of the Hh pathway restored or amplified GPCR function to permit cAMP elevation selectively in the cilium. Hh therefore enables its own local GPCR-dependent cAMP regulatory circuit. Considering that GPCRs comprise much of the druggable genome, these data suggest alternative strategies to modify Hh signaling.

Wild type- and mutant p53 proteins in mitochondrial dysfunction: emerging insights in cancer disease

Deregulated cell metabolism is one of the cancer hallmarks. Mitochondrial DNA mutations and enzyme defects, aberrant tumor suppressor or oncogenic activities cause mitochondrial dysfunction leading to deregulated cellular energetics. The tumor suppressor protein, p53 is a tetrameric transcription factor that in response to diverse genotoxic and non-genotoxic insults activates a plethora of target genes to preserve genome integrity. In the last two decades the discovery of cytoplasmic p53 localization focused intense research on its extra-nuclear functions. The ability of p53 to induce apoptosis acting directly at mitochondria and the related mechanisms of p53 localization and translocation in the cytoplasm have been investigated. A role of cytoplasmic p53 in autophagy, pentose phosphate pathway, fatty acid synthesis and oxidation, and drug response has been proposed. TP53 gene is mutated in more than half of human cancers. In parallel to loss of tumor suppressive functions, mutant p53 proteins often gain new tumorigenic activities (GOF, gain of function). It has been recently shown that mutant p53 proteins mediate metabolic changes thereby promoting cancer development and metastases. Here we review the contribution of either wild-type p53 or mutant p53 proteins to the fine-tuning of mitochondrial metabolism of both normal and cancer cells. Greater knowledge at the mechanistic level might provide insights to develop new cancer therapeutic approaches.

A genetically encoded toolkit of functionalized nanobodies against fluorescent proteins for visualizing and manipulating intracellular signalling

BACKGROUND

Intrabodies enable targeting of proteins in live cells, but generating specific intrabodies against the thousands of proteins in a proteome poses a challenge. We leverage the widespread availability of fluorescently labelled proteins to visualize and manipulate intracellular signalling pathways in live cells by using nanobodies targeting fluorescent protein tags.

RESULTS

We generated a toolkit of plasmids encoding nanobodies against red and green fluorescent proteins (RFP and GFP variants), fused to functional modules. These include fluorescent sensors for visualization of Ca2+, H+ and ATP/ADP dynamics; oligomerising or heterodimerising modules that allow recruitment or sequestration of proteins and identification of membrane contact sites between organelles; SNAP tags that allow labelling with fluorescent dyes and targeted chromophore-assisted light inactivation; and nanobodies targeted to lumenal sub-compartments of the secretory pathway. We also developed two methods for crosslinking tagged proteins: a dimeric nanobody, and RFP-targeting and GFP-targeting nanobodies fused to complementary hetero-dimerizing domains. We show various applications of the toolkit and demonstrate, for example, that IP3 receptors deliver Ca2+ to the outer membrane of only a subset of mitochondria and that only one or two sites on a mitochondrion form membrane contacts with the plasma membrane.

CONCLUSIONS

This toolkit greatly expands the utility of intrabodies and will enable a range of approaches for studying and manipulating cell signalling in live cells.

Stress Coping Strategies in the Heart: An Integrated View

The heart is made up of an ordered amalgam of cardiac cell types that work together to coordinate four major processes, namely energy production, electrical conductance, mechanical work, and tissue remodeling. Over the last decade, a large body of information has been amassed regarding how different cardiac cell types respond to cellular stress that affect the functionality of their elaborate intracellular membrane networks, the cellular reticular network. In the context of the heart, the manifestations of stress coping strategies likely differ depending on the coping strategy outcomes of the different cardiac cell types, and thus may underlie the development of distinct cardiac disorders. It is not clear whether all cardiac cell types have similar sensitivity to cellular stress, how specific coping response strategies modify their unique roles, and how their metabolic status is communicated to other cells within the heart. Here we discuss our understanding of the roles of specialized cardiac cells that together make the heart function as an organ with the ability to pump blood continuously and follow a regular rhythm.

Calcium, mitochondria and cell metabolism: A functional triangle in bioenergetics

The versatility of mitochondrial metabolism and its fine adjustments to specific physiological or pathological conditions regulate fundamental cell pathways, ranging from proliferation to apoptosis. In particular, Ca²⁺ signalling has emerged as a key player exploited by mitochondria to tune their activity according with cell demand. The functional interaction between mitochondria and endoplasmic reticulum (ER) deeply impacts on the correct mitochondrial Ca²⁺ signal, thus modulating cell bioenergetics and functionality. Indeed, Ca²⁺ released by the ER is taken up by mitochondria where, both in the intermembrane space and in the matrix, it regulates the activity of transporters, enzymes and proteins involved in organelles' metabolism. In this review, we will briefly summarize Ca²⁺-dependent mechanisms involved in the regulation of mitochondrial activity. Moreover, we will discuss some recent reports, in which alterations in mitochondrial Ca²⁺ signalling have been associated with specific pathological conditions, such as neurodegeneration and cancer.

Metabolic Plasticity of Tumor Cell Mitochondria

Mitochondria are dynamic organelles that exchange a multiplicity of signals with other cell compartments, in order to finely adjust key biological routines to the fluctuating metabolic needs of the cell. During neoplastic transformation, cells must provide an adequate supply of the anabolic building blocks required to meet a relentless proliferation pressure. This can occur in conditions of inconstant blood perfusion leading to variations in oxygen and nutrient levels. Mitochondria afford the bioenergetic plasticity that allows tumor cells to adapt and thrive in this ever changing and often unfavorable environment. Here we analyse how mitochondria orchestrate the profound metabolic rewiring required for neoplastic growth.

Beyond Intracellular Signaling: The Ins and Outs of Second Messengers Microdomains

A typical characteristic of eukaryotic cells compared to prokaryotes is represented by the spatial heterogeneity of the different structural and functional components: for example, most of the genetic material is surrounded by a highly specific membrane structure (the nuclear membrane), continuous with, yet largely different from, the endoplasmic reticulum (ER); oxidative phosphorylation is carried out by organelles enclosed by a double membrane, the mitochondria; in addition, distinct domains, enriched in specific proteins, are present in the plasma membrane (PM) of most cells. Less obvious, but now generally accepted, is the notion that even the concentration of small molecules such as second messengers (Ca²⁺ and cAMP in particular) can be highly heterogeneous within cells. In the case of most organelles, the differences in the luminal levels of second messengers depend either on the existence on their membrane of proteins that allow the accumulation/release of the second messenger (e.g., in the case of Ca²⁺, pumps, exchangers or channels), or on the synthesis and degradation of the specific molecule within the lumen (the autonomous intramitochondrial cAMP system). It needs stressing that the existence of a surrounding membrane does not necessarily imply the existence of a gradient between the cytosol and the organelle lumen. For example, the nuclear membrane is highly permeable to both Ca²⁺ and cAMP (nuclear pores are permeable to solutes up to 50 kDa) and differences in [Ca²⁺] or [cAMP] between cytoplasm and nucleoplasm are not seen in steady state and only very transiently during cell activation. A similar situation has been observed, as far as Ca²⁺ is concerned, in peroxisomes.

Divalent metal ions control activity and inhibition of protein kinases

Protein kinases are key enzymes in the regulation of eukaryotic signal transduction. As metalloenzymes they employ divalent cations for catalysis and regulation. We used the catalytic (C) subunit of cAMP-dependent protein kinase (PKA) as a model protein to investigate the role of a variety of physiologically or pathophysiologically relevant divalent metal ions in distinct steps within the catalytic cycle. It is established that divalent metal ions play a crucial role in co-binding of nucleotides and also assist in catalysis. Our studies reveal that besides the physiologically highly relevant magnesium, metals like zinc and manganese can assist in phosphoryl transfer, however, only a few support efficient substrate turnover (turnover catalysis). Those trace metals allow for substrate binding and phosphotransfer but hamper product release. We further established the unique role of magnesium as the common biologically relevant divalent metal ideally enabling (co-) substrate binding and orientation. Magnesium allows stable substrate binding and, on the other hand accelerates product release, thus being extremely efficient in turnover catalysis. We extended our studies to non-catalytic functions of protein kinases looking at pseudokinases, a subfamily of protein kinases inherently lacking critical residues for catalysis. Recently, pseudokinases have been linked to human diseases. Some pseudokinases are still capable of binding metal ions, yet have lost the ability to transfer the phosphoryl group from ATP to a given substrate. Here metal ions stabilize an active like, though catalytically unproductive conformation and are therefore crucial to maintain non-catalytic function. Finally, we demonstrate for the canonical kinase PKA that the trace metal manganese alone can stabilize protein kinases in an active like conformation allowing them to bind substrates even in the absence of nucleotides.

Intracellular Calcium Mobilization Is Required for Sonic Hedgehog Signaling

Graded Shh signaling across fields of precursor cells coordinates patterns of gene expression, differentiation, and morphogenetic behavior as precursors form complex structures, such as the nervous system, the limbs, and craniofacial skeleton. Here we discover that intracellular calcium mobilization, a process tightly controlled and readily modulated, regulates the level of Shh-dependent gene expression in responding cells and affects the development of all Shh-dependent cell types in the zebrafish embryo. Reduced expression or modified activity of ryanodine receptor (RyR) intracellular calcium release channels shifted the allocation of Shh-dependent cell fates in the somitic muscle and neural tube. Mosaic analysis revealed that RyR-mediated calcium mobilization is required specifically in Shh ligand-receiving cells. This work reveals that RyR channels participate in intercellular signal transduction events. As modulation of RyR activity modifies tissue patterning, we hypothesize that alterations in intracellular calcium mobilization contribute to both birth defects and evolutionary modifications of morphology.

MICU3 is a tissue-specific enhancer of mitochondrial calcium uptake

The versatility and universality of Ca²⁺ as intracellular messenger is guaranteed by the compartmentalization of changes in [Ca²⁺]. In this context, mitochondrial Ca²⁺ plays a central role, by regulating both specific organelle functions and global cellular events. This versatility is also guaranteed by a cell type-specific Ca²⁺ signaling toolkit controlling specific cellular functions. Accordingly, mitochondrial Ca²⁺ uptake is mediated by a multimolecular structure, the MCU complex, which differs among various tissues. Its activity is indeed controlled by different components that cooperate to modulate specific channeling properties. We here investigate the role of MICU3, an EF-hand containing protein expressed at high levels, especially in brain. We show that MICU3 forms a disulfide bond-mediated dimer with MICU1, but not with MICU2, and it acts as enhancer of MCU-dependent mitochondrial Ca²⁺ uptake. Silencing of MICU3 in primary cortical neurons impairs Ca²⁺ signals elicited by synaptic activity, thus suggesting a specific role in regulating neuronal function.

TOM70 Sustains Cell Bioenergetics by Promoting IP3R3-Mediated ER to Mitochondria Ca2+ Transfer

The mitochondrial translocase of the outer membrane (TOM) is a protein complex that is essential for the post-translational import of nuclear-encoded mitochondrial proteins. Among its subunits, TOM70 and TOM20 are only transiently associated with the core complex, suggesting their possible additional roles within the outer mitochondrial membrane (OMM). Here, by using different mammalian cell lines, we demonstrate that TOM70, but not TOM20, clusters in distinct OMM foci, frequently overlapping with sites in which the endoplasmic reticulum (ER) contacts mitochondria. Functionally, TOM70 depletion specifically impairs inositol trisphosphates (IP3)-linked ER to mitochondria Ca²⁺ transfer. This phenomenon is dependent on the capacity of TOM70 to interact with IP3-receptors and favor their functional recruitment close to mitochondria. Importantly, the reduced constitutive Ca²⁺ transfer to mitochondria, observed in TOM70-depleted cells, dampens mitochondrial respiration, affects cell bioenergetics, induces autophagy, and inhibits proliferation. Our data reveal a hitherto unexpected role for TOM70 in pro-survival ER-mitochondria communication, reinforcing the view that the ER-mitochondria signaling platform is a key regulator of cell fate.

The ins and outs of Ca2+ in plant endomembrane trafficking

Trafficking of proteins and lipids within the plant endomembrane system is essential to support cellular functions and is subject to rigorous regulation. Despite this seemingly strict regulation, endomembrane trafficking needs to be dynamically adjusted to ever-changing internal and environmental stimuli, while maintaining cellular integrity. Although often overlooked, the versatile second messenger Ca²⁺ is intimately connected to several endomembrane-associated processes. Here, we discuss the impact of electrostatic interactions between Ca²⁺ and anionic phospholipids on endomembrane trafficking, and illustrate the direct role of Ca²⁺ sensing proteins in regulating endomembrane trafficking and membrane integrity preservation. Moreover, we discuss how Ca²⁺ can control protein sorting within the plant endomembrane system. We thus highlight Ca²⁺ signaling as a versatile mechanism by which numerous signals are integrated into plant endomembrane trafficking dynamics.

Permeability and calcium signaling in lung endothelium: unpack the box…

This brief review assesses the role of Ca²⁺ signaling in lung endothelium in regulation of endothelial permeability. The disconnect between experimental and clinical outcomes to date may be due, in part, to the use of tools which yield information about aggregate permeability or Ca²⁺ responses in lung or in endothelial monolayers. The teaching point of this review is to “unpack the box,” i.e. consider the many potential issues which could impact interpretation of outcomes. These include phenotypic heterogeneity and resultant segment-specific permeability responses, methodologic issues related to permeability measures, contributions from Ca²⁺ channels in cells other than endothelium—such as alveolar macrophages or blood leukocytes), Ca²⁺ dynamic patterns, rather than averaged Ca²⁺ responses to channel activation, and the background context, such as changes in endothelial bioenergetics with sepsis. Any or all of these issues might color interpretation of permeability and Ca²⁺ signaling in lung.

β-Amyloid and the Pathomechanisms of Alzheimer's Disease: A Comprehensive View

Protein dyshomeostasis is the common mechanism of neurodegenerative diseases such as Alzheimer’s disease (AD). Aging is the key risk factor, as the capacity of the proteostasis network declines during aging. Different cellular stress conditions result in the up-regulation of the neurotrophic, neuroprotective amyloid precursor protein (APP). Enzymatic processing of APP may result in formation of toxic Aβ aggregates (β-amyloids). Protein folding is the basis of life and death. Intracellular Aβ affects the function of subcellular organelles by disturbing the endoplasmic reticulum-mitochondria cross-talk and causing severe Ca²⁺-dysregulation and lipid dyshomeostasis. The extensive and complex network of proteostasis declines during aging and is not able to maintain the balance between production and disposal of proteins. The effectivity of cellular pathways that safeguard cells against proteotoxic stress (molecular chaperones, aggresomes, the ubiquitin-proteasome system, autophagy) declines with age. Chronic cerebral hypoperfusion causes dysfunction of the blood-brain barrier (BBB), and thus the Aβ-clearance from brain-to-blood decreases. Microglia-mediated clearance of Aβ also declines, Aβ accumulates in the brain and causes neuroinflammation. Recognition of the above mentioned complex pathogenesis pathway resulted in novel drug targets in AD research.

The prion protein regulates glutamate-mediated Ca2+ entry and mitochondrial Ca2+ accumulation in neurons

The cellular prion protein (PrP^C) whose conformational misfolding leads to the production of deadly prions, has a still-unclarified cellular function despite decades of intensive research. Following our recent finding that PrP^C limits Ca²⁺ entry via store-operated Ca²⁺ channels in neurons, we investigated whether the protein could also control the activity of ionotropic glutamate receptors (iGluRs). To this end, we compared local Ca²⁺ movements in primary cerebellar granule neurons and cortical neurons transduced with genetically encoded Ca²⁺ probes and expressing, or not expressing, PrP^C Our investigation demonstrated that PrP^C downregulates Ca²⁺ entry through each specific agonist-stimulated iGluR and after stimulation by glutamate. We found that, although PrP-knockout (KO) mitochondria were displaced from the plasma membrane, glutamate addition resulted in a higher mitochondrial Ca²⁺ uptake in PrP-KO neurons than in their PrP^C-expressing counterpart. This was because the increased Ca²⁺ entry through iGluRs in PrP-KO neurons led to a parallel increase in Ca²⁺-induced Ca²⁺ release via ryanodine receptor channels. These data thus suggest that PrP^C takes part in the cell apparatus controlling Ca²⁺ homeostasis, and that PrP^C is involved in protecting neurons from toxic Ca²⁺ overloads.

The Concerted Action of Mitochondrial Dynamics and Positioning: New Characters in Cancer Onset and Progression

Mitochondria are dynamic organelles whose morphology and activity are extremely variable, depending on the metabolic state of the cell. In particular, their shape and movements within the cell are finely regulated by an increasing number of proteins, which take part in the process of mitochondrial fission/fusion and connect the organelles to the cytoskeleton. As to their activities, mitochondria are considered to be at the crossroad between cell life and death since, on the one hand, they are essential in ATP production and in multiple metabolic pathways but, on the other, they are involved in the intrinsic apoptotic cascade, triggered by different stress conditions. Importantly, the process of mitochondrial Ca²⁺ uptake, as well as the morphology and the dynamics of these organelles, is known to deeply impact on both pro-survival and pro-death mitochondrial activities. Recently, increasing evidence has accrued on a central role of deregulated mitochondrial functionalities in the onset and progression of different pathologies, ranging from neurodegenerative diseases to cancer. In this contribution, we will present the latest findings connecting alterations in the machineries that control mitochondrial dynamics and localization to specific cancer hallmarks, highlighting the importance of mitochondria for the viability of cancer cells and discussing their role as promising targets for the development of novel anticancer therapies.

Ca2+ influx at the ER/PM junctions

Ca²⁺ influx across the plasma membrane is a key component of the receptor-evoked Ca²⁺ signaling that mediate numerous cell functions and reload the ER after partial or full ER Ca²⁺ store depletion. Ca²⁺ influx is activated in response to Ca²⁺ release from the ER, a concept developed by Jim Putney, and the channels mediating the influx are thus called store-operated Ca²⁺ influx channels, or SOCs. The molecular identity of the SOCs has been determined with the identification of the TRPC channels, STIM1 and the Orai channels. These channels are targeted to, operate and are regulated when at the ER/PM junctions. ER/PM junctions are a form of membrane contact sites (MCSs) that are present in all parts of the cells, where the ER makes contacts with cellular membranes and organelles. MCSs have many cellular functions, and are the sites of lipid and Ca²⁺ transport and delivery between organelles. This short review discusses aspects of MCSs in the context of Ca²⁺ transport.

The endoplasmic reticulum-mitochondria coupling in health and disease: Molecules, functions and significance

The close apposition between endoplasmic reticulum (ER) and mitochondria represents a key platform, capable to regulate different fundamental cellular pathways. Among these, Ca²⁺ signaling and lipid homeostasis have been demonstrated over the last years to be deeply modulated by ER-mitochondria cross-talk. Given its importance in cell life/death decisions, increasing evidence suggests that alterations of the ER-mitochondria axis could be responsible for the onset and progression of several diseases, including neurodegeneration, cancer and obesity. However, the molecular identity of the proteins controlling this inter-organelle apposition is still debated. In this review, we summarize the main cellular pathways controlled by ER-mitochondria appositions, focusing on the principal molecules reported to be involved in this interplay and on those diseases for which alterations in organelles communication have been reported.

The coming of age of the mitochondria-ER contact: a matter of thickness

The sites of near-contact between the mitochondrion and the endoplasmic reticulum (ER) have earned a lot of attention due to their key role in the maintenance of lipid and calcium (Ca(2+)) homeostasis, in the initiation of autophagy and mitochondrial division, and in sensing metabolic shifts. At these sites, typically called MAMs (mitochondria-associated ER membranes) or MERCs (mitochondria-ER contacts), the organelles juxtapose at a distance that can range from ~10 to ~50 nm. The multifunctional role of this subcellular compartment is puzzling; further, recent studies have shown that mitochondria-ER contacts are highly plastic structures that remodel upon metabolic transitions and that their activity in controlling lipid homeostasis could be involved in Alzheimer's disease pathogenesis. This review aims at integrating the functions of this subcellular compartment to its most characterizing and unexplored structural parameter, their 'thickness': that is, the width of the cleft that separates the cytosolic face of the outer mitochondrial membrane from that of the ER. We describe and discuss the reasons why the thickness of a MERC should be considered a regulated structural parameter of the cell that defines and controls its function. Further, we propose a MERC classification that will help organize the expanding field of MERCs biology and of their role in cell physiology and human disease.

Subcellular ROS imaging methods: Relevance for the study of calcium signaling

Recent advances in genetically encoded fluorescent probes have dramatically increased the toolkit available for imaging the intracellular environment. Perhaps the biggest improvements have been made in sensing specific reactive oxygen species (ROS) and redox changes under physiological conditions. The new generation of probes may be targeted to a wide range of subcellular environments. By targeting such probes to compartments and organelle surfaces they may be exposed to environments, which support local signal transduction and regulation. The close apposition of the endoplasmic reticulum (ER) with mitochondria and other organelles forms such a local environment where Ca(2+) dynamics are greatly enhanced compared to the bulk cytosol. We describe here how newly developed genetically encoded redox indicators (GERIs) might be used to monitor ROS and probe their interaction with Ca(2+) at both global and local level.

Intracellular tortuosity underlies slow cAMP diffusion in adult ventricular myocytes

Aims

3′,5′-Cyclic adenosine monophosphate (cAMP) signals in the heart are often confined to concentration microdomains shaped by cAMP diffusion and enzymatic degradation. While the importance of phosphodiesterases (degradative enzymes) in sculpting cAMP microdomains is well established in cardiomyocytes, less is known about cAMP diffusivity (D_cAMP) and factors affecting it. Many earlier studies have reported fast diffusivity, which argues against sharply defined microdomains.

Methods and results

[cAMP] dynamics in the cytoplasm of adult rat ventricular myocytes were imaged using a fourth generation genetically encoded FRET-based sensor. The [cAMP]-response to the addition and removal of isoproterenol (β-adrenoceptor agonist) quantified the rates of cAMP synthesis and degradation. To obtain a read out of D_cAMP, a stable [cAMP] gradient was generated using a microfluidic device which delivered agonist to one half of the myocyte only. After accounting for phosphodiesterase activity, D_cAMP was calculated to be 32 µm²/s; an order of magnitude lower than in water. Diffusivity was independent of the amount of cAMP produced. Saturating cAMP-binding sites with the analogue 6-Bnz-cAMP did not accelerate D_cAMP, arguing against a role of buffering in restricting cAMP mobility. cAMP diffused at a comparable rate to chemically unrelated but similar sized molecules, arguing for a common physical cause of restricted diffusivity. Lower mitochondrial density and order in neonatal cardiac myocytes allowed for faster diffusion, demonstrating the importance of mitochondria as physical barriers to cAMP mobility.

Conclusion

In adult cardiac myocytes, tortuosity due to physical barriers, notably mitochondria, restricts cAMP diffusion to levels that are more compatible with microdomain signalling.