The axonal transport of mitochondria

Dopamine inhibits mitochondrial motility in hippocampal neurons

Background

The trafficking of mitochondria within neurons is a highly regulated process. In an earlier study, we found that serotonin (5-HT), acting through the 5-HT1A receptor subtype, promotes axonal transport of mitochondria in cultured hippocampal neurons by increasing Akt activity, and consequently decreasing glycogen synthase kinase (GSK3β) activity. This finding suggests a critical role for neuromodulators in the regulation of mitochondrial trafficking in neurons. In the present study, we investigate the effects of a second important neuromodulator, dopamine, on mitochondrial transport in hippocampal neurons.

Methodology/Principal Findings

Here, we show that dopamine, like 5-HT, regulates mitochondrial motility in cultured hippocampal neurons through the Akt-GSK3β signaling cascade. But, in contrast to the stimulatory effect of 5-HT, administration of exogenous dopamine or bromocriptine, a dopamine 2 receptor (D2R) agonist, caused an inhibition of mitochondrial movement. Moreover, pretreatment with bromocriptine blocked the stimulatory effect of 5-HT on mitochondrial movement. Conversely, in cells pretreated with 5-HT, no further increases in movement were observed after administration of haloperidol, a D2R antagonist. In contrast to the effect of the D2R agonist, addition of SKF38393, a dopamine 1 receptor (D1R) agonist, promoted mitochondrial transport, indicating that the inhibitory effect of dopamine was actually the net summation of opposing influences of the two receptor subtypes. The most pronounced effect of dopamine signals was on mitochondria that were already moving directionally. Western blot analysis revealed that treatment with either a D2R agonist or a D1R antagonist decreased Akt activity, and conversely, treatment with either a D2R antagonist or a D1R agonist increased Akt activity.

Conclusions/Significance

Our observations strongly suggest a role for both dopamine and 5-HT in regulating mitochondrial movement, and indicate that the integrated effects of these two neuromodulators may be important in determining the distribution of energy sources in neurons.

Creatine kinase B deficient neurons exhibit an increased fraction of motile mitochondria

BACKGROUND

Neurons require an elaborate system of intracellular transport to distribute cargo throughout axonal and dendritic projections. Active anterograde and retrograde transport of mitochondria serves in local energy distribution, but at the same time also requires input of ATP. Here we studied whether brain-type creatine kinase (CK-B), a key enzyme for high-energy phosphoryl transfer between ATP and CrP in brain, has an intermediary role in the reciprocal coordination between mitochondrial motility and energy distribution. Therefore, we analysed the impact of brain-type creatine kinase (CK-B) deficiency on transport activity and velocity of mitochondria in primary murine neurons and made a comparison to the fate of amyloid precursor protein (APP) cargo in these cells, using live cell imaging.

RESULTS

Comparison of average and maximum transport velocities and global transport activity showed that CK-B deficiency had no effect on speed of movement of mitochondria or APP cargo, but that the fraction of motile mitochondria was significantly increased by 36% in neurons derived from CK-B knockout mice. The percentage of motile APP vesicles was not altered.

CONCLUSION

CK-B activity does not directly couple to motor protein activity but cells without the enzyme increase the number of motile mitochondria, possibly as an adaptational strategy aimed to enhance mitochondrial distribution versatility in order to compensate for loss of efficiency in the cellular network for ATP distribution.

Positioning mitochondrial plasticity within cellular signaling cascades

Mitochondria evolved from alpha-proteobacteria captured within a host between two and three billion years ago. This origin resulted in the formation of a double-layered organelle resulting in four distinct sub-compartments: the outer membrane, the intermembrane space, the inner membrane and the matrix. The inner membrane is organized in cristae, harboring the respiratory chain and ATP synthase complexes responsible of the oxidative phosphorylation, the main energy-generating system of the cell. It is generally considered that the ultrastructure of the inner membrane provides a large variety of morphologies that facilitate metabolic output. This classical view of mitochondria as bean-shaped organelles was static until in the last decade when new imaging studies and genetic screens provided a more accurate description of a dynamic mitochondrial reticulum that fuse and divide continuously. Since then significant findings have been made in the study of machineries responsible for fusion, fission and motility, however the mechanisms and signals that regulate mitochondrial dynamics are only beginning to emerge. A growing body of evidence indicates that metabolic and cellular signals influence mitochondrial dynamics, leading to a new understanding of how changes in mitochondrial shape can have a profound impact on the functional output of the organelle. The mechanisms that regulate mitochondrial morphology are incompletely understood, but evidence to date suggests that the morphology machinery is modulated through the use of post-translational modifications, including nucleotide-binding proteins, phosphorylation, ubiquitination, SUMOylation, and changes in the lipid environment. This review focuses on the molecular switches that control mitochondrial dynamics and the integration of mitochondrial morphology within cellular signaling cascades.

Huntingtin phosphorylation acts as a molecular switch for anterograde/retrograde transport in neurons

The transport of vesicles in neurons is a highly regulated process, with vesicles moving either anterogradely or retrogradely depending on the nature of the molecular motors, kinesins and dynein, respectively, which propel vesicles along microtubules (MTs). However, the mechanisms that determine the directionality of transport remain unclear. Huntingtin, the protein mutated in Huntington's disease, is a positive regulatory factor for vesicular transport. Huntingtin is phosphorylated at serine 421 by the kinase Akt but the role of this modification is unknown. Here, we demonstrate that phosphorylation of wild-type huntingtin at S421 is crucial to control the direction of vesicles in neurons. When phosphorylated, huntingtin recruits kinesin-1 to the dynactin complex on vesicles and MTs. Using brain-derived neurotrophic factor as a marker of vesicular transport, we demonstrate that huntingtin phosphorylation promotes anterograde transport. Conversely, when huntingtin is not phosphorylated, kinesin-1 detaches and vesicles are more likely to undergo retrograde transport. This also applies to other vesicles suggesting an essential role for huntingtin in the control of vesicular directionality in neurons.

CLASP regulates mitochondrial distribution in Schizosaccharomyces pombe

Movement of mitochondria in Schizosaccharomyces pombe depends on their association with the dynamic, or plus ends, of microtubules, yet the molecular basis for this interaction is poorly understood. We identified mmd4 in a screen of temperature-sensitive S. pombe strains for aberrant mitochondrial morphology and distribution. Cells with the mmd4 mutation display mitochondrial aggregation near the cell ends at elevated temperatures, a phenotype similar to mitochondrial defects observed in wild-type cells after microtubule depolymerization. However, microtubule morphology and function appear normal in the mmd4 mutant. The mmd4 lesion maps to peg1⁺, which encodes a microtubule-associated protein with homology to cytoplasmic linker protein–associated proteins (mammalian microtubule plus end–binding proteins). Peg1p localizes to the plus end of microtubules and to mitochondria and is recovered with mitochondria during subcellular fractionation. This mitochondrial-associated fraction of Peg1p displays properties of a peripherally associated protein. Peg1p is the first identified microtubule plus end–binding protein required for mitochondrial distribution and likely functions as a molecular link between mitochondria and microtubules.

Cytoskeletal induced apoptosis in yeast

The influence of the cytoskeleton reaches into almost every aspect of eukaryotic cell function. It is a little surprise therefore that links between the regulation of the cytoskeleton and apoptosis have been found in a variety of eukaryotic systems. Studies from yeast have made a significant contribution to this new field of research and have highlighted the importance of interactions between the cytoskeleton and mitochondria in determining cell fate. In yeast both the actin and microtubular cytoskeletons have been shown to influence mitochondrial function and the commitment to apoptosis. In this review we discuss the recent advances and speculate that apoptotic mechanisms that feed off the ability of the cytoskeleton to respond to environmental signals may represent a useful mechanism to remove weak or damaged individuals from a population.

High- and low-calcium-dependent mechanisms of mitochondrial calcium signalling

The Ca(2+) coupling between endoplasmic reticulum (ER) and mitochondria is central to multiple cell survival and cell death mechanisms. Cytoplasmic [Ca(2+)] ([Ca(2+)](c)) spikes and oscillations produced by ER Ca(2+) release are effectively delivered to the mitochondria. Propagation of [Ca(2+)](c) signals to the mitochondria requires the passage of Ca(2+) across three membranes, namely the ER membrane, the outer mitochondrial membrane (OMM) and the inner mitochondrial membrane (IMM). Strategic positioning of the mitochondria by cytoskeletal transport and interorganellar tethers provides a means to promote the local transfer of Ca(2+) between the ER membrane and OMM. In this setting, even >100 microM [Ca(2+)] may be attained to activate the low affinity mitochondrial Ca(2+) uptake. However, a mitochondrial [Ca(2+)] rise has also been documented during submicromolar [Ca(2+)](c) elevations. Evidence has been emerging that Ca(2+) exerts allosteric control on the Ca(2+) transport sites at each membrane, providing mechanisms that may facilitate the Ca(2+) delivery to the mitochondria. Here we discuss the fundamental mechanisms of ER and mitochondrial Ca(2+) transport, particularly the control of their activity by Ca(2+) and evaluate both high- and low-[Ca(2+)]-activated mitochondrial calcium signals in the context of cell physiology.

Mitochondrial fission and fusion dynamics: the long and short of it

Maintenance of functional mitochondria requires fusion and fission of these dynamic organelles. The proteins that regulate mitochondrial dynamics are now associated with a broad range of cellular functions. Mitochondrial fission and fusion are often viewed as a finely tuned balance within cells, yet an integrated and quantitative understanding of how these processes interact with each other and with other mitochondrial and cellular processes is not well formulated. Direct visual observation of mitochondrial fission and fusion events, together with computational approaches promise to provide new insight.

Molecular landmarks along the axonal route: axonal transport in health and disease

Axonal transport of organelles has emerged as a key process in the regulation of neuronal differentiation and survival. Several components of this specialised transport machinery, their regulators and vesicular cargoes are mutated or altered in many neurodegenerative conditions. The molecular characterisation of these mechanisms has furthered our understanding of neuronal homeostasis, providing insights into the spatio-temporal control of membrane traffic and signalling in neurons with a precision not achievable in other cellular systems. Here, we summarise the recent advances in the field of axonal trafficking of different organelles, and the essential role of motor and adaptor proteins in this process.

Pushing for answers: is myosin V directly involved in moving mitochondria?

In budding yeast, the actin-based class V myosin motors, Myo2 and Myo4, transport virtually all organelles from mother to bud during cell division. Until recently, it appeared that mitochondria may be an exception, with studies showing that the Arp2/3 complex is required for their movement. However, several recent studies have proposed that Myo2 has a direct involvement in mitochondria inheritance. In this issue, Altmann et al. (Altmann, K., M. Frank, D. Neumann, S. Jakobs, and B. Westermann. 2008. J. Cell Biol. 181:119–130) provide the strongest support yet that Myo2 and its associated light chain Mlc1 function directly and significantly in both mitochondria–actin interactions and in the movement of mitochondria from mother to bud. The conflicting functions of Arp 2/3 and Myo2 may be reconciled by the existence of multiple pathways involved in mitochondrial transport.

Spatial patterning of metabolism by mitochondria, oxygen, and energy sinks in a model cytoplasm

Metabolite gradients might guide mitochondrial localization in cells and angiogenesis in tissues. It is unclear whether they can exist in single cells, because the length scale of most cells is small compared to the expected diffusion times of metabolites. For investigation of metabolic gradients, we need experimental systems in which spatial patterns of metabolism can be systematically measured and manipulated. We used concentrated cytoplasmic extracts from Xenopus eggs as a model cytoplasm, and visualized metabolic gradients formed in response to spatial stimuli. Restriction of oxygen supply to the edge of a drop mimicked distance to the surface of a single cell, or distance from a blood vessel in tissue. We imaged a step-like increase of Nicotinamide adenine dinucleotide (NAD) reduction approximately 600 microm distant from the oxygen source. This oxic-anoxic switch was preceded on the oxic side by a gradual rise of mitochondrial transmembrane potential (Deltapsi) and reactive oxygen species (ROS) production, extending over approximately 600 microm and approximately 300 microm, respectively. Addition of Adenosine triphosphate (ATP)-consuming beads mimicked local energy sinks in the cell. We imaged Deltapsi gradients with a decay length of approximately 50-300 microm around these beads, in the first visualization of an energy demand signaling gradient. Our study demonstrates that mitochondria can pattern the cytoplasm over length scales that are suited to convey morphogenetic information in large cells and tissues and provides a versatile model system for probing of the formation and function of metabolic gradients.

G alpha12 is targeted to the mitochondria and affects mitochondrial morphology and motility

G alpha12 constitutes, along with G alpha13, one of the four families of alpha subunits of heterotrimeric G proteins. We found that the N terminus of G alpha12, but not those of other G alpha subunits, contains a predicted mitochondrial targeting sequence. Using confocal microscopy and cell fractionation, we demonstrated that up to 40% of endogenous G alpha12 in human umbilical vein endothelial cells colocalize with mitochondrial markers. N-terminal sequence of G alpha12 fused to GFP efficiently targeted the fusion protein to mitochondria. G alpha12 with mutated mitochondrial targeting sequence was still located in mitochondria, suggesting the existence of additional mechanisms for mitochondrial localization. Lysophosphatidic acid, one of the known stimuli transduced by G alpha12/13, inhibited mitochondrial motility, while depletion of endogenous G alpha12 increased mitochondrial motility. G alpha12Q229L variants uncoupled from RhoGEFs (but not fully functional activated G alpha12Q229L) induced transformation of the mitochondrial network into punctate mitochondria and resulted in a loss of mitochondrial membrane potential. All examined G alpha12Q229L variants reduced phosphorylation of Bcl-2 at Ser-70, while only mutants unable to bind RhoGEFs also decreased cellular levels of Bcl-2. These G alpha12 mutants were also more efficient Hsp90 interactors. These findings are the first demonstration of a heterotrimeric G protein alpha subunit specifically targeted to mitochondria and involved in the control of mitochondrial morphology and dynamics.

EMB2473/MIRO1, an Arabidopsis Miro GTPase, is required for embryogenesis and influences mitochondrial morphology in pollen

The regulation of mitochondrial biogenesis, subcellular distribution, morphology, and metabolism are essential for all aspects of plant growth and development. However, the molecular mechanisms involved are still unclear. Here, we describe an analysis of the three Arabidopsis thaliana orthologs of the evolutionarily conserved Miro GTPases. Two of the genes, MIRO1 and MIRO2, are transcribed ubiquitously throughout the plant tissues, and their gene products localize to mitochondria via their C-terminal transmembrane domains. While insertional mutations in the MIRO2 gene do not have any visible impact on plant development, an insertional mutation in the MIRO1 gene is lethal during embryogenesis at the zygote to four-terminal-cell embryo stage. It also substantially impairs pollen germination and tube growth. Laser confocal and transmission electron microscopy revealed that the miro1 mutant pollen exhibits abnormally enlarged or tube-like mitochondrial morphology, leading to the disruption of continuous streaming of mitochondria in the growing pollen tube. Our findings suggest that mitochondrial morphology is influenced by MIRO1 and plays a vital role during embryogenesis and pollen tube growth.

Mitochondrial energetics and insulin resistance

Obesity, insulin resistance, type 2 diabetes mellitus, and aging are associated with impaired skeletal muscle oxidation capacity, reduced mitochondrial content, and lower rates of oxidative phosphorylation. Several studies have reported ultrastructural abnormalities in mitochondrial morphology and reductions in mitochondrial mass in insulin-resistant individuals. From lower organisms to rodents, mitochondrial membrane structure, function, and programmed cell death are regulated in part by the balance between the opposing forces of mitochondrial fusion and fission, suggesting they may also play an important role in human physiology.

Bcl-xL induces Drp1-dependent synapse formation in cultured hippocampal neurons

Maturation of neuronal synapses is thought to involve mitochondria. Bcl-xL protein inhibits mitochondria-mediated apoptosis but may have other functions in healthy adult neurons in which Bcl-xL is abundant. Here, we report that overexpression of Bcl-xL postsynaptically increases frequency and amplitude of spontaneous miniature synaptic currents in rat hippocampal neurons in culture. Bcl-xL, overexpressed either pre or postsynaptically, increases synapse number, the number and size of synaptic vesicle clusters, and mitochondrial localization to vesicle clusters and synapses, likely accounting for the changes in miniature synaptic currents. Conversely, knockdown of Bcl-xL or inhibiting it with ABT-737 decreases these morphological parameters. The mitochondrial fission protein, dynamin-related protein 1 (Drp1), is a GTPase known to localize to synapses and affect synaptic function and structure. The effects of Bcl-xL appear mediated through Drp1 because overexpression of Drp1 increases synaptic markers, and overexpression of the dominant-negative dnDrp1-K38A decreases them. Furthermore, Bcl-xL coimmunoprecipitates with Drp1 in tissue lysates, and in a recombinant system, Bcl-xL protein stimulates GTPase activity of Drp1. These findings suggest that Bcl-xL positively regulates Drp1 to alter mitochondrial function in a manner that stimulates synapse formation.

Function and regulation of local axonal translation

An increasing body of evidence indicates that local axonal translation is required for growing axons to respond appropriately to guidance cues and other stimuli. Recent studies suggest that asymmetrical synthesis of cytoskeletal proteins mediates growth cone turning and that local translation and retrograde transport of transcription factors mediate neuronal survival. Axonal translation is regulated partly by selective axonal localization of mRNAs and by translation initiation factors and RNA-binding proteins. We discuss possible rationales for local axonal translation, including distinct properties of nascent proteins, precise localization, and axonal autonomy.

OPA1 mutations associated with dominant optic atrophy impair oxidative phosphorylation and mitochondrial fusion

Dominant optic atrophy (DOA) is characterized by retinal ganglion cell degeneration leading to optic neuropathy. A subset of DOA is caused by mutations in the OPA1 gene, encoding for a dynamin-related GTPase required for mitochondrial fusion. The functional consequences of OPA1 mutations in DOA patients are still poorly understood. This study investigated the effect of five different OPA1 pathogenic mutations on the energetic efficiency and mitochondrial network dynamics of skin fibroblasts from patients. Although DOA fibroblasts maintained their ATP levels and grew in galactose medium, i.e. under forced oxidative metabolism, a significant impairment in mitochondrial ATP synthesis driven by complex I substrates was found. Furthermore, balloon-like structures in the mitochondrial reticulum were observed in galactose medium and mitochondrial fusion was completely inhibited in about 50% of DOA fibroblasts, but not in control cells. Respiratory complex assembly and the expression level of complex I subunits were similar in control and DOA fibroblasts. Co-immunoprecipitation experiments revealed that OPA1 directly interacts with subunits of complexes I, II and III, but not IV and with apoptosis inducing factor. The results disclose a novel link between OPA1, apoptosis inducing factor and the respiratory complexes that may shed some light on the pathogenic mechanism of DOA.

Multiple pathways influence mitochondrial inheritance in budding yeast

Yeast mitochondria form a branched tubular network. Mitochondrial inheritance is tightly coupled with bud emergence, ensuring that daughter cells receive mitochondria from mother cells during division. Proteins reported to influence mitochondrial inheritance include the mitochondrial rho (Miro) GTPase Gem1p, Mmr1p, and Ypt11p. A synthetic genetic array (SGA) screen revealed interactions between gem1Delta and deletions of genes that affect mitochondrial function or inheritance, including mmr1Delta. Synthetic sickness of gem1Delta mmr1Delta double mutants correlated with defective mitochondrial inheritance by large buds. Additional studies demonstrated that GEM1, MMR1, and YPT11 each contribute to mitochondrial inheritance. Mitochondrial accumulation in buds caused by overexpression of either Mmr1p or Ypt11p did not depend on Gem1p, indicating these three proteins function independently. Physical linkage of mitochondria with the endoplasmic reticulum (ER) has led to speculation that distribution of these two organelles is coordinated. We show that yeast mitochondrial inheritance is not required for inheritance or spreading of cortical ER in the bud. Moreover, Ypt11p overexpression, but not Mmr1p overexpression, caused ER accumulation in the bud, revealing a potential role for Ypt11p in ER distribution. This study demonstrates that multiple pathways influence mitochondrial inheritance in yeast and that Miro GTPases have conserved roles in mitochondrial distribution.

Genome-wide and functional annotation of human E3 ubiquitin ligases identifies MULAN, a mitochondrial E3 that regulates the organelle's dynamics and signaling

Specificity of protein ubiquitylation is conferred by E3 ubiquitin (Ub) ligases. We have annotated approximately 617 putative E3s and substrate-recognition subunits of E3 complexes encoded in the human genome. The limited knowledge of the function of members of the large E3 superfamily prompted us to generate genome-wide E3 cDNA and RNAi expression libraries designed for functional screening. An imaging-based screen using these libraries to identify E3s that regulate mitochondrial dynamics uncovered MULAN/FLJ12875, a RING finger protein whose ectopic expression and knockdown both interfered with mitochondrial trafficking and morphology. We found that MULAN is a mitochondrial protein - two transmembrane domains mediate its localization to the organelle's outer membrane. MULAN is oriented such that its E3-active, C-terminal RING finger is exposed to the cytosol, where it has access to other components of the Ub system. Both an intact RING finger and the correct subcellular localization were required for regulation of mitochondrial dynamics, suggesting that MULAN's downstream effectors are proteins that are either integral to, or associated with, mitochondria and that become modified with Ub. Interestingly, MULAN had previously been identified as an activator of NF-kappaB, thus providing a link between mitochondrial dynamics and mitochondria-to-nucleus signaling. These findings suggest the existence of a new, Ub-mediated mechanism responsible for integration of mitochondria into the cellular environment.

Cellular Zn2+ chelators cause "dying-back" neurite degeneration associated with energy impairment

Most cellular zinc is tightly associated with metalloproteins and other Zn2+-dependent proteins, which along with cellular Zn2+ compartments may coordinately regulate cytoplasmic free Zn2+ levels in the picomolar range. Moreover, Zn2+-containing endosomes or protein complexes appear to move along axons or dendrites, suggesting a dynamic mechanism for trafficking, exchanging, or scavenging Zn2+ and/or Zn2+ protein complexes in neurons. It is therefore interesting to examine whether cellular Zn2+ levels might alter neurite integrity and dynamics. Here we show that membrane-permeable zinc chelators, including 1,10-phenanthroline, N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine (TPEN), and zinquin, selectively elicit axon and dendrite degeneration but leave the cell body intact in sympathetic neurons. The process begins distally and then moves retrogradely, with a distinct "dying-back" pattern. An inactive isomer of 1,10-phenanthroline failed to cause neuite degeneration, and these chelators mediated their effects by selectively chelating Zn2+, but not other metals. Moreover, neurite degeneration was associated with a decrease in neuritic ATP levels and was caused by energy failure, because an exogenous supply of nicotinamide adenine dinucleotide (NAD) or its precursor nicotinamide suppressed the degeneration by delaying axonal ATP reduction caused by Zn2+ depletion. Blockage of autophagy by 3-methyladenine provided partial protection against degeneration of terminal axons or dendrites; there was, however, no obvious alteration in that of medial portions. Collectively, our results show that cellular Zn2+ depletion induces a "dying-back" degeneration characterized by an NAD- and autophagy-dependent process, independently of neurite elongation dynamics.

A novel role for villin in intestinal epithelial cell survival and homeostasis

Apoptosis is a key regulator for the normal turnover of the intestinal mucosa, and abnormalities associated with this function have been linked to inflammatory bowel disease and colorectal cancer. Despite this, little is known about the mechanism(s) mediating intestinal epithelial cell apoptosis. Villin is an actin regulatory protein that is expressed in every cell of the intestinal epithelium as well as in exocrine glands associated with the gastrointestinal tract. In this study we demonstrate for the first time that villin is an epithelial cell-specific anti-apoptotic protein. Absence of villin predisposes mice to dextran sodium sulfate-induced colitis by promoting apoptosis. To better understand the cellular and molecular mechanisms of the anti-apoptotic function of villin, we overexpressed villin in the Madin-Darby canine kidney Tet-Off epithelial cell line to demonstrate that expression of villin protects cells from apoptosis by maintaining mitochondrial integrity thus inhibiting the activation of caspase-9 and caspase-3. Furthermore, we report that the anti-apoptotic response of villin depends on activation of the pro-survival proteins, phosphatidylinositol 3-kinase and phosphorylated Akt. The results of our studies shed new light on the previously unrecognized function of villin in the regulation of apoptosis in the gastrointestinal epithelium.

IP3 receptors in cell survival and apoptosis: Ca2+ release and beyond

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) serve to discharge Ca(2+) from ER stores in response to agonist stimulation. The present review summarizes the role of these receptors in models of Ca(2+)-dependent apoptosis. In particular we focus on the regulation of IP(3)Rs by caspase-3 cleavage, cytochrome c, anti-apoptotic proteins and Akt kinase. We also address the evidence that some of the effects of IP(3)Rs in apoptosis may be independent of their ion-channel function. The role of IP(3)Rs in delivering Ca(2+) to the mitochondria is discussed from the perspective of the factors determining inter-organellar dynamics and the spatial proximity of mitochondria and ER membranes.

Docking of axonal mitochondria by syntaphilin controls their mobility and affects short-term facilitation

Proper distribution of mitochondria within axons and at synapses is critical for neuronal function. While one-third of axonal mitochondria are mobile, a large proportion remains in a stationary phase. However, the mechanisms controlling mitochondrial docking within axons remain elusive. Here, we report a role for axon-targeted syntaphilin (SNPH) in mitochondrial docking through its interaction with microtubules. Axonal mitochondria that contain exogenously or endogenously expressed SNPH lose mobility. Deletion of the mouse snph gene results in a substantially higher proportion of axonal mitochondria in the mobile state and reduces the density of mitochondria in axons. The snph mutant neurons exhibit enhanced short-term facilitation during prolonged stimulation, probably by affecting calcium signaling at presynaptic boutons. This phenotype is fully rescued by reintroducing the snph gene into the mutant neurons. These findings demonstrate a molecular mechanism for controlling mitochondrial docking in axons that has a physiological impact on synaptic function.

Towards a cure for dementia: the role of axonal transport in Alzheimer's disease

Alzheimer's disease is an incurable, fatal illness characterised by years of progressive mental decline. It afflicts half a million people in the UK--more than any other dementia. The primary risk factor is old age so this number is rising as we live longer. Current treatment is palliative while more potent drugs have encountered problems during clinical trials. It is known that the disease results from brain deterioration associated with the formation of microscopic lesions. Genetic mutations cause a small minority of cases but our knowledge of the underlying biological mechanisms is limited. The key to improved understanding may be a process vital to brain cells called axonal transport. Disruption of axonal transport seems to be an early event in the progression of the disease and is linked to lesion formation and brain dysfunction so a full investigation of this process should lead to a cure, if not prevention.

Identification of an axonal kinesin-3 motor for fast anterograde vesicle transport that facilitates retrograde transport of neuropeptides

A screen for genes required in Drosophila eye development identified an UNC-104/Kif1 related kinesin-3 microtubule motor. Analysis of mutants suggested that Drosophila Unc-104 has neuronal functions that are distinct from those of the classic anterograde axonal motor, kinesin-1. In particular, unc-104 mutations did not cause the distal paralysis and focal axonal swellings characteristic of kinesin-1 (Khc) mutations. However, like Khc mutations, unc-104 mutations caused motoneuron terminal atrophy. The distributions and transport behaviors of green fluorescent protein-tagged organelles in motor axons indicate that Unc-104 is a major contributor to the anterograde fast transport of neuropeptide-filled vesicles, that it also contributes to anterograde transport of synaptotagmin-bearing vesicles, and that it contributes little or nothing to anterograde transport of mitochondria, which are transported primarily by Khc. Remarkably, unc-104 mutations inhibited retrograde runs by neurosecretory vesicles but not by the other two organelles. This suggests that Unc-104, a member of an anterograde kinesin subfamily, contributes to an organelle-specific dynein-driven retrograde transport mechanism.

Arrest of cytoplasmic streaming induces algal proliferation in green paramecia

A green ciliate Paramecium bursaria, bearing several hundreds of endosymbiotic algae, demonstrates rotational microtubule-based cytoplasmic streaming, in which cytoplasmic granules and endosymbiotic algae flow in a constant direction. However, its physiological significance is still unknown. We investigated physiological roles of cytoplasmic streaming in P. bursaria through host cell cycle using video-microscopy. Here, we found that cytoplasmic streaming was arrested in dividing green paramecia and the endosymbiotic algae proliferated only during the arrest of cytoplasmic streaming. Interestingly, arrest of cytoplasmic streaming with pressure or a microtubule drug also induced proliferation of endosymbiotic algae independently of host cell cycle. Thus, cytoplasmic streaming may control the algal proliferation in P. bursaria. Furthermore, confocal microscopic observation revealed that a division septum was formed in the constricted area of a dividing paramecium, producing arrest of cytoplasmic streaming. This is a first report to suggest that cytoplasmic streaming controls proliferation of eukaryotic cells.

Cytotoxic T lymphocytes overcome Bcl-2 inhibition: target cells contribute to their own demise

Cytotoxic T lymphocytes (CTLs) eliminate pathogenic cells in large part through the activity of the serine protease granzyme B (grB). However, while the apoptotic activity of grB is blocked by over-expression of Bcl-2, CTLs can still kill target cells through an ill-defined Bcl-2-independent pathway. In this report, we have identified key modulators of this Bcl-2-independent cell-death pathway, which is induced by CTLs and not purified components. Surprisingly, activation of this pathway is reliant on grB. Furthermore, this novel pathway requires mitochondrial contribution through triggering of permeability transition and generation of reactive oxygen species, yet is functional in the absence of Bax/Bak. This pathway stimulates movement of target cell mitochondria toward the point of contact with the CTLs and importantly, inhibition of this directed movement attenuates killing. Therefore, we propose that CTLs initiate a target cell response that activates multiple mitochondrial pathways. This ensures that CTLs can eliminate those target cells that have compromised apoptotic potential due to overexpression of Bcl-2.

Human immunodeficiency virus type 1 Vpr inhibits axonal outgrowth through induction of mitochondrial dysfunction

Human immunodeficiency virus type 1 (HIV-1)-infected macrophages damage mature neurons in the brain, although their effect on neuronal development has not been clarified. In this study, we show that HIV-1-infected macrophages produce factors that impair the development of neuronal precursor cells and that soluble viral protein R (Vpr) is one of the factors that has the ability to suppress axonal growth. Cell biological analysis revealed that extracellularly administered recombinant Vpr (rVpr) clearly accumulated in mitochondria where a Vpr-binding protein adenine nucleotide translocator localizes and also decreased the mitochondrial membrane potential, which led to ATP synthesis. The depletion of ATP synthesis reduced the transportation of mitochondria within neurites. This mitochondrial dysfunction inhibited axonal growth even when the frequency of apoptosis was not significant. We also found that point mutations of arginine (R) residues to alanine (A) residues at positions 73, 77, and 80 rendered rVpr incapable of causing mitochondrial membrane depolarization and axonal growth inhibition. Moreover, the Vpr-induced inhibition was suppressed after treatment with a ubiquinone analogue (ubiquinone-10). Our results suggest that soluble Vpr is a major viral factor that causes a disturbance in neuronal development through the induction of mitochondrial dysfunction. Since ubiquinone-10 protects the neuronal plasticity in vitro, it may be a therapeutic agent that can offer defense against HIV-1-associated neurological disease.

Familial amyotrophic lateral sclerosis-linked SOD1 mutants perturb fast axonal transport to reduce axonal mitochondria content

Amyotrophic lateral sclerosis (ALS) is a late-onset neurological disorder characterized by death of motoneurons. Mutations in Cu/Zn superoxide dismutase-1 (SOD1) cause familial ALS but the mechanisms whereby they induce disease are not fully understood. Here, we use time-lapse microscopy to monitor for the first time the effect of mutant SOD1 on fast axonal transport (FAT) of bona fide cargoes in living neurons. We analyzed FAT of mitochondria that are a known target for damage by mutant SOD1 and also of membrane-bound organelles (MBOs) using EGFP-tagged amyloid precursor protein as a marker. We studied FAT in motor neurons derived from SOD1G93A transgenic mice that are a model of ALS and also in cortical neurons transfected with SOD1G93A and three further ALS-associated SOD1 mutants. We find that mutant SOD1 damages transport of both mitochondria and MBOs, and that the precise details of this damage are cargo-specific. Thus, mutant SOD1 reduces transport of MBOs in both anterograde and retrograde directions, whereas mitochondrial transport is selectively reduced in the anterograde direction. Analyses of the characteristics of mitochondrial FAT revealed that reduced anterograde movement involved defects in anterograde motor function. The selective inhibition of anterograde mitochondrial FAT enhanced their net retrograde movement to deplete mitochondria in axons. Mitochondria in mutant SOD1 expressing cells also displayed features of damage. Together, such changes to mitochondrial function and distribution are likely to compromise axonal function. These alterations represent some of the earliest pathological features so far reported in neurons of mutant SOD1 transgenic mice.

Switching of membrane organelles between cytoskeletal transport systems is determined by regulation of the microtubule-based transport

Intracellular transport of membrane organelles occurs along microtubules (MTs) and actin filaments (AFs). Although transport along each type of the cytoskeletal tracks is well characterized, the switching between the two types of transport is poorly understood because it cannot be observed directly in living cells. To gain insight into the regulation of the switching of membrane organelles between the two major transport systems, we developed a novel approach that combines live cell imaging with computational modeling. Using this approach, we measured the parameters that determine how fast membrane organelles switch back and forth between MTs and AFs (the switching rate constants) and compared these parameters during different signaling states. We show that regulation involves a major change in a single parameter: the transferring rate from AFs onto MTs. This result suggests that MT transport is the defining factor whose regulation determines the choice of the cytoskeletal tracks during the transport of membrane organelles.

Functions and dysfunctions of mitochondrial dynamics

Recent findings have sparked renewed appreciation for the remarkably dynamic nature of mitochondria. These organelles constantly fuse and divide, and are actively transported to specific subcellular locations. These dynamic processes are essential for mammalian development, and defects lead to neurodegenerative disease. But what are the molecular mechanisms that control mitochondrial dynamics, and why are they important for mitochondrial function? We review these issues and explore how defects in mitochondrial dynamics might cause neuronal disease.

Moving mitochondria: establishing distribution of an essential organelle

Mitochondria form a dynamic network responsible for energy production, calcium homeostasis and cell signaling. Appropriate distribution of the mitochondrial network contributes to organelle function and is essential for cell survival. Highly polarized cells, including neurons and budding yeast, are particularly sensitive to defects in mitochondrial movement and have emerged as model systems for studying mechanisms that regulate organelle distribution. Mitochondria in multicellular eukaryotes move along microtubule tracks. Actin, the primary cytoskeletal component used for transport in yeast, has more subtle functions in other organisms. Kinesin, dynein and myosin isoforms drive motor-based movement along cytoskeletal tracks. Milton and syntabulin have recently been identified as potential organelle-specific adaptor molecules for microtubule-based motors. Miro, a conserved GTPase, may function with Milton to regulate transport. In yeast, Mmr1p and Ypt11p, a Rab GTPase, are implicated in myosin V-based mitochondrial movement. These potential adaptors could regulate motor activity and therefore determine individual organelle movements. Anchoring of stationary mitochondria also contributes to organelle retention at specific sites in the cell. Together, movement and anchoring ultimately determine mitochondrial distribution throughout the cell.

Swimming against the tide: mobility of the microtubule-associated protein tau in neurons

Long-haul transport along microtubules is crucial for neuronal polarity, and transport defects cause neurodegeneration. Tau protein stabilizes microtubule tracks, but in Alzheimer's disease it aggregates and becomes missorted into the somatodendritic compartment. Tau can inhibit axonal transport by obstructing motors on microtubules, yet tau itself can still move into axons. We therefore investigated tau movement by live-cell fluorescence microscopy, FRAP (fluorescence recovery after photobleaching), and FSM (fluorescence speckle microscopy). Tau is highly dynamic, with diffusion coefficients of approximately 3 microm2/s and microtubule dwell times of approximately 4 s. This facilitates the entry of tau into axons over distances of millimeters and periods of days. For longer distances and times, two mechanisms of tau transport are observed. At low near-physiological levels, tau is cotransported with microtubule fragments from cell bodies into axons, moving at instantaneous velocities approximately 1 microm/s. At high concentrations, tau forms local accumulations moving bidirectionally at approximately 0.3 microm/s. These clusters first appear at distal endings of axons and may indicate an early stage of neurite degeneration.

Mitochondrial Dynamics and Peripheral Neuropathy

Peripheral neuropathy is perhaps the archetypal disease of axonal degeneration, characteristically involving degeneration of the longest axons in the body. Evidence from both inherited and acquired forms of peripheral neuropathy strongly supports that the primary pathology is in the axons themselves and points to disruption of axonal transport as an important disease mechanism. Recent studies in human genetics have further identified abnormalities in mitochondrial dynamics—the fusion, fission, and movement of mitochondria— as a player in the pathogenesis of inherited peripheral neuropathy. This review provides an update on the mechanisms of mitochondrial trafficking in axons and the emerging relationship between the disruption of mitochondrial dynamics and axonal degeneration. Evidence suggests mitochondria are a “critical cargo” whose transport is necessary for proper axonal and synaptic function. Importantly, understanding the regulation of mitochondrial movement and the consequences of decreased axonal mitochondrial function may define new paths for therapeutic agents in peripheral neuropathy and other neurodegenerative diseases. NEUROSCIENTIST 14(1):12—18, 2008. DOI: 10.1177/1073858407307354

Mitochondria on the move

Interactions of mitochondria with the cytoskeleton are crucial for normal mitochondrial function and for localization of the organelle at its sites of action within cells. Early studies revealed a role for microtubule motors in mitochondrial motility in neurons and other cell types. Here, we describe advances in our understanding of mitochondrial movement and distribution. Specifically, we review recent studies on proteins that mediate or regulate the interaction between motor molecules and the organelle, motor-independent mechanisms for mitochondrial motility, anchorage of mitochondria at cortical sites within cells and links between mitochondria-cytoskeleton interactions and mitochondrial plasticity.

Cytoskeleton as a potential target in the neuropathology of maple syrup urine disease: insight from animal studies

In this short review we provide evidence that the branched-chain keto acids accumulating in the neurometabolic disorder maple syrup urine disease disturb rat cerebral cytoskeleton in a developmentally regulated manner. Alterations of protein phosphorylation leading to brain cytoskeletal misregulation and neural cell death caused by these metabolites are associated with energy deprivation, oxidative stress and excitotoxicity that may ultimately disrupt normal cell function and viability.

Mutant ubiquitin found in Alzheimer's disease causes neuritic beading of mitochondria in association with neuronal degeneration

A dinucleotide deletion in human ubiquitin (Ub) B messenger RNA leads to formation of polyubiquitin (UbB)+1, which has been implicated in neuronal cell death in Alzheimer's and other neurodegenerative diseases. Previous studies demonstrate that UbB+1 protein causes proteasome dysfunction. However, the molecular mechanism of UbB+1-mediated neuronal degeneration remains unknown. We now report that UbB+1 causes neuritic beading, impairment of mitochondrial movements, mitochondrial stress and neuronal degeneration in primary neurons. Transfection of UbB+1 induced a buildup of mitochondria in neurites and dysregulation of mitochondrial motor proteins, in particular, through detachment of P74, the dynein intermediate chain, from mitochondria and decreased mitochondria-microtubule interactions. Altered distribution of mitochondria was associated with activation of both the mitochondrial stress and p53 cell death pathways. These results support the hypothesis that neuritic clogging of mitochondria by UbB+1 triggers a cascade of events characterized by local activation of mitochondrial stress followed by global cell death. Furthermore, UbB+1 small interfering RNA efficiently blocked expression of UbB+1 protein, attenuated neuritic beading and preserved cellular morphology, suggesting a potential neuroprotective strategy for certain neurodegenerative disorders.

Mitochondria–endoplasmic reticulum choreography: structure and signaling dynamics

Mitochondria and endoplasmic reticulum (ER) have different roles in living cells but they interact both physically and functionally. A key aspect of the mitochondria-ER relationship is the modulation of Ca(2+) signaling during cell activation, which thus affects a variety of physiological processes. We focus here on the molecular aspects that control the dynamics of the organelle-organelle interaction and their relationship with Ca(2+) signals, also discussing the consequences that these phenomena have, not only for cell physiology but also in the control of cell death.

The machines that divide and fuse mitochondria

Mitochondria are derived from eubacteria; however, in most eukaryotes, novel mechanisms for the propagation of this organelle and its genome have evolved. This review focuses on what is currently known about the novel molecular machines that divide and fuse mitochondria.

Axon viability and mitochondrial function are dependent on local protein synthesis in sympathetic neurons

(1) Axons contain numerous mRNAs and a local protein synthetic system that can be regulated independently of the cell body. (2) In this study, cultured primary sympathetic neurons were employed, to assess the effect of local protein synthesis blockade on axon viability and mitochondrial function. (3) Inhibition of local protein synthesis reduced newly synthesized axonal proteins by 65% and resulted in axon retraction after 6 h. Acute inhibition of local protein synthesis also resulted in a significant decrease in the membrane potential of axonal mitochondria. Likewise, blockade of local protein transport into the mitochondria by transfection of the axons with Hsp90 C-terminal domain decreased the mitochondrial membrane potential by 65%. Moreover, inhibition of the local protein synthetic system also reduced the ability of mitochondria to restore axonal levels of ATP after KCl-induced depolarization. (4) Taken together, these results indicate that the local protein synthetic system plays an important role in mitochondrial function and the maintenance of the axon.

Local influence of mitochondrial calcium transport in retinal amacrine cells

Ca2+-dependent synaptic transmission from retinal amacrine cells is thought to be initiated locally at dendritic processes. Hence, understanding the spatial and temporal impact of Ca2+ transport is fundamental to understanding how amacrine cells operate. Here, we provide the first examination of the local effects of mitochondrial Ca2+ transport in neuronal processes. By combining mitochondrial localization with measurements of cytosolic Ca2+, the local impacts of mitochondrial Ca2+ transport for two types of Ca2+ signals were investigated. Disruption of mitochondrial Ca2+ uptake with carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP) produces cytosolic Ca2+ elevations. The amplitudes of these elevations decline with distance from mitochondria suggesting that they are related to mitochondrial Ca2+ transport. The time course of the FCCP-dependent Ca2+ elevations depend on the availability of ER Ca2+ and we provide evidence that Ca2+ is released primarily via nearby ryanodine receptors. These results indicate that interactions between the ER and mitochondria influence cytosolic Ca2+ in amacrine cell processes and cell bodies. We also demonstrate that the durations of glutamate-dependent Ca2+ elevations are dependent on their proximity to mitochondria in amacrine cell processes. Consistent with this observation, disruption of mitochondrial Ca2+ transport alters the duration of glutamate-dependent Ca2+ elevations near mitochondria but not at sites more than 10 microm away. These results indicate that mitochondria influence local Ca2+-dependent signaling in amacrine cell processes.

Organelle redox of CF and CFTR-corrected airway epithelia

In cystic fibrosis reduced CFTR function may alter redox properties of airway epithelial cells. Redox-sensitive GFP (roGFP1) and imaging microscopy were used to measure the redox potentials of the cytosol, endoplasmic reticulum (ER), mitochondria, and cell surface of cystic fibrosis nasal epithelial cells and CFTR-corrected cells. We also measured glutathione and cysteine thiol redox states in cell lysates and apical fluids to provide coverage over a range of redox potentials and environments that might be affected by CFTR. As measured with roGFP1, redox potentials at the cell surface (approx -207+/-8 mV) and in the ER (approx -217+/-1 mV) and rates of regulation of the apical fluid and ER lumen after DTT treatment were similar for CF and CFTR-corrected cells. CF and CFTR-corrected cells had similar redox potentials in mitochondria (-344+/-9 mV) and cytosol (-322+/-7 mV). Oxidation of carboxydichlorodihydrofluorescein diacetate and of apical Amplex red occurred at equal rates in CF and CFTR-corrected cells. Glutathione and cysteine redox couples in cell lysates and apical fluid were equal in CF and CFTR-corrected cells. These quantitative estimates of organelle redox potentials combined with apical and cell measurements using small-molecule couples confirmed there were no differences in the redox properties of CF and CFTR-corrected cells.

Imaging axonal transport of mitochondria in vivo

Neuronal mitochondria regulate synaptic physiology and cellular survival, and disruption of their function or transport causes neurological disease. We present a fluorescence method to selectively image mitochondrial dynamics in the mouse nervous system, in both live mice and acute explants. We show that axon damage and recovery lead to early and sustained changes in anterograde and retrograde transport. In vivo imaging of mitochondria will be a useful tool to analyze this essential organelle.

Mitochondrial optic neuropathies: how two genomes may kill the same cell type?

Ocular involvement is a prevalent feature in mitochondrial diseases. Leber's hereditary optic neuropathy (LHON) and dominant optic atrophy (DOA) are both non-syndromic optic neuropathies with a mitochondrial etiology. LHON is associated with point mutations in the mitochondrial DNA (mtDNA), which affect subunit genes of complex I. The majority of DOA patients harbor mutations in the nuclear-encoded protein OPA1, which is targeted to mitochondria and participates to cristae organization and mitochondrial network dynamics. In both disorders the retinal ganglion cells (RGCs) are specific cellular targets of the degenerative process. We here review the clinical features and the genetic bases, and delineate the possible common pathomechanism for both these disorders.

Missorting of tau in neurons causes degeneration of synapses that can be rescued by the kinase MARK2/Par-1

Early hallmarks of Alzheimer's disease include the loss of synapses, which precedes the loss of neurons and the pathological phosphorylation and aggregation of tau protein. Mitochondrial dysfunction has been suggested as a reason, but evidence on the role of tau was lacking. Here, we show that transfection of tau in mature hippocampal neurons leads to an improper distribution of tau into the somatodendritic compartment with concomitant degeneration of synapses, as seen by the disappearance of spines and of presynaptic and postsynaptic markers. This is accompanied by transport inhibition of vesicles and organelles, concomitant with an increase and bundling of microtubules. Mitochondria degenerate, thus causing ATP levels to decrease. The tau-induced synaptic decay can be relieved by the activation of the kinase MARK2 (microtubule-associated protein/microtubule affinity regulating kinase 2)/Par-1 (protease-activated receptor 1), which can remove tau from the microtubule tracks and reverses the transport block. This leads to the rescue of dendritic spines, synapses, mitochondrial transport and ATP levels.

Mitochondrial redistribution: adding new players to the chemotaxis game

Leukocyte polarization and chemotaxis have a key role in the homeostasis of the immune system and in inflammation. Recent work shows that chemoattractants induce the redistribution of mitochondria towards the uropod of polarized migrating leukocytes through a mechanism involving microtubules and mitochondrial fission. These findings underscore the key role this organelle can have in leukocyte chemotaxis by fuelling motor proteins at their trailing edge.

Asymmetric distribution of mitochondria and of spindle microtubules in opposite directions in differential mitosis of germ line cells in Acricotopus

Additional chromosomes present only in the germ line are a specific feature of the Orthocladiinae, a subfamily of the Chironomidae. During the complex chromosome cycle in the orthocladiid Acricotopus lucidus, about half of the germ-line-limited chromosomes (Ks) are eliminated in the first division of the primary germ cells. Following normal gonial mitoses, the reduction in the number of Ks is compensated for, in the last mitosis prior to meiosis, by a monopolar movement of the unseparated Ks, while the somatic chromosomes (Ss) segregate equally. This differential mitosis produces daughter cells with different chromosome constitutions and diverse developmental fates. A preferential segregation of mitochondria occurs to one pole associated with an asymmetric formation of the mitotic spindle. This has been detected in living gonial cells in both sexes by using MitoTracker probes and fluorochrome-labelled paclitaxel (taxol). In males, the resulting unequal partitioning of mitochondria to the daughter cells is equalised by the transport of mitochondria through a permanent cytoplasmic bridge from the aberrant spermatocyte to the primary spermatocyte. This asymmetry in the distribution and in the segregation of cytoplasmic components in differential gonial mitosis in Acricotopus may be involved in the process of cell-fate determination.

Calcium-dependent trapping of mitochondria near plasma membrane in stimulated astrocytes

Growing evidence suggests that astrocytes are the active partners of neurons in many brain functions. Astrocytic mitochondria are highly motile organelles which regulate the temporal and spatial patterns of Ca( 2+ ) dynamics, in addition to being a major source of ATP and reactive oxygen species. Previous studies have shown that mitochondria translocate to endoplasmic reticulum during Ca( 2+ ) release from internal stores, but whether a similar spatial interaction between mitochondria and plasma membrane occurs is not known. Using total internal reflection fluorescence (TIRF) microscopy we show that a fraction of mitochondria became trapped near the plasma membrane of cultured hippocampal astrocytes during exposure to the transmitters glutamate or ATP, resulting in net translocation of the mitochondria to the plasma membrane. This translocation was dependent on the intracellular Ca( 2+ ) rise because it was blocked by pre-incubation with BAPTA AM and mimicked by application of the Ca( 2+ ) ionophore ionomycin. Transmembrane Ca( 2+ ) influx induced by raising external Ca( 2+ ) also caused mitochondrial trapping, which occurred more rapidly than that produced by glutamate or ATP. In astrocytes treated with the microtubule-disrupting agent nocodazole, intracellular Ca( 2+ ) rises failed to induce trapping of mitochondria near plasma membrane, suggesting a role for microtubules in this phenomenon. Our data reveal the Ca( 2+ )-dependent trapping of mitochondria near the plasma membrane as a novel form of mitochondrial regulation, which is likely to control the perimembrane Ca( 2+ ) dynamics and regulate signaling by mitochondria-derived reactive oxygen species.