Synaptic mitochondria are critical for mobilization of reserve pool vesicles at Drosophila neuromuscular junctions

straightjacket is required for the synaptic stabilization of cacophony, a voltage-gated calcium channel alpha1 subunit

In a screen to identify genes involved in synaptic function, we isolated mutations in Drosophila melanogaster straightjacket (stj), an α₂δ subunit of the voltage-gated calcium channel. stj mutant photoreceptors develop normal synaptic connections but display reduced “on–off” transients in electroretinogram recordings, indicating a failure to evoke postsynaptic responses and, thus, a defect in neurotransmission. stj is expressed in neurons but excluded from glia. Mutants exhibit endogenous seizure-like activity, indicating altered neuronal excitability. However, at the synaptic level, stj larval neuromuscular junctions exhibit approximately fourfold reduction in synaptic release compared with controls stemming from a reduced release probability at these synapses. These defects likely stem from destabilization of Cacophony (Cac), the primary presynaptic α₁ subunit in D. melanogaster. Interestingly, neuronal overexpression of cac partially rescues the viability and physiological defects in stj mutants, indicating a role for the α₂δ Ca²⁺ channel subunit in mediating the proper localization of an α₁ subunit at synapses.

Erect wing regulates synaptic growth in Drosophila by integration of multiple signaling pathways

Background

Formation of synaptic connections is a dynamic and highly regulated process. Little is known about the gene networks that regulate synaptic growth and how they balance stimulatory and restrictive signals.

Results

Here we show that the neuronally expressed transcription factor gene erect wing (ewg) is a major target of the RNA binding protein ELAV and that EWG restricts synaptic growth at neuromuscular junctions. Using a functional genomics approach we demonstrate that EWG acts primarily through increasing mRNA levels of genes involved in transcriptional and post-transcriptional regulation of gene expression, while genes at the end of the regulatory expression hierarchy (effector genes) represent only a minor portion, indicating an extensive regulatory network. Among EWG-regulated genes are components of Wingless and Notch signaling pathways. In a clonal analysis we demonstrate that EWG genetically interacts with Wingless and Notch, and also with TGF-β and AP-1 pathways in the regulation of synaptic growth.

Conclusion

Our results show that EWG restricts synaptic growth by integrating multiple cellular signaling pathways into an extensive regulatory gene expression network.

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.

Gamma oscillations and spontaneous network activity in the hippocampus are highly sensitive to decreases in pO2 and concomitant changes in mitochondrial redox state

Gamma oscillations have been implicated in higher cognitive processes and might critically depend on proper mitochondrial function. Using electrophysiology, oxygen sensor microelectrode, and imaging techniques, we investigated the interactions of neuronal activity, interstitial pO2, and mitochondrial redox state [NAD(P)H and FAD (flavin adenine dinucleotide) fluorescence] in the CA3 subfield of organotypic hippocampal slice cultures. We find that gamma oscillations and spontaneous network activity decrease significantly at pO2 levels that do not affect neuronal population responses as elicited by moderate electrical stimuli. Moreover, pO2 and mitochondrial redox states are tightly coupled, and electrical stimuli reveal transient alterations of redox responses when pO2 decreases within the normoxic range. Finally, evoked redox responses are distinct in somatic and synaptic neuronal compartments and show different sensitivity to changes in pO2. We conclude that the threshold of interstitial pO2 for robust CA3 network activities and required mitochondrial function is clearly above the "critical" value, which causes spreading depression as a result of generalized energy failure. Our study highlights the importance of a functional understanding of mitochondria and their implications on activities of individual neurons and neuronal networks.

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.

The PINK1/Parkin pathway regulates mitochondrial morphology

Loss-of-function mutations in the PTEN-induced kinase 1 (PINK1) or parkin genes, which encode a mitochondrially localized serine/threonine kinase and a ubiquitin-protein ligase, respectively, result in recessive familial forms of Parkinsonism. Genetic studies in Drosophila indicate that PINK1 acts upstream of Parkin in a common pathway that influences mitochondrial integrity in a subset of tissues, including flight muscle and dopaminergic neurons. The mechanism by which PINK1 and Parkin influence mitochondrial integrity is currently unknown, although mutations in the PINK1 and parkin genes result in enlarged or swollen mitochondria, suggesting a possible regulatory role for the PINK1/Parkin pathway in mitochondrial morphology. To address this hypothesis, we examined the influence of genetic alterations affecting the machinery that governs mitochondrial morphology on the PINK1 and parkin mutant phenotypes. We report that heterozygous loss-of-function mutations of drp1, which encodes a key mitochondrial fission-promoting component, are largely lethal in a PINK1 or parkin mutant background. Conversely, the flight muscle degeneration and mitochondrial morphological alterations that result from mutations in PINK1 and parkin are strongly suppressed by increased drp1 gene dosage and by heterozygous loss-of-function mutations affecting the mitochondrial fusion-promoting factors OPA1 and Mfn2. Finally, we find that an eye phenotype associated with increased PINK1/Parkin pathway activity is suppressed by perturbations that reduce mitochondrial fission and enhanced by perturbations that reduce mitochondrial fusion. Our studies suggest that the PINK1/Parkin pathway promotes mitochondrial fission and that the loss of mitochondrial and tissue integrity in PINK1 and parkin mutants derives from reduced mitochondrial fission.

The PINK1/Parkin pathway regulates mitochondrial morphology

Loss-of-function mutations in the PTEN-induced kinase 1 (PINK1) or parkin genes, which encode a mitochondrially localized serine/threonine kinase and a ubiquitin-protein ligase, respectively, result in recessive familial forms of Parkinsonism. Genetic studies in Drosophila indicate that PINK1 acts upstream of Parkin in a common pathway that influences mitochondrial integrity in a subset of tissues, including flight muscle and dopaminergic neurons. The mechanism by which PINK1 and Parkin influence mitochondrial integrity is currently unknown, although mutations in the PINK1 and parkin genes result in enlarged or swollen mitochondria, suggesting a possible regulatory role for the PINK1/Parkin pathway in mitochondrial morphology. To address this hypothesis, we examined the influence of genetic alterations affecting the machinery that governs mitochondrial morphology on the PINK1 and parkin mutant phenotypes. We report that heterozygous loss-of-function mutations of drp1, which encodes a key mitochondrial fission-promoting component, are largely lethal in a PINK1 or parkin mutant background. Conversely, the flight muscle degeneration and mitochondrial morphological alterations that result from mutations in PINK1 and parkin are strongly suppressed by increased drp1 gene dosage and by heterozygous loss-of-function mutations affecting the mitochondrial fusion-promoting factors OPA1 and Mfn2. Finally, we find that an eye phenotype associated with increased PINK1/Parkin pathway activity is suppressed by perturbations that reduce mitochondrial fission and enhanced by perturbations that reduce mitochondrial fusion. Our studies suggest that the PINK1/Parkin pathway promotes mitochondrial fission and that the loss of mitochondrial and tissue integrity in PINK1 and parkin mutants derives from reduced mitochondrial fission.

Synaptic vesicle mobility in mouse motor nerve terminals with and without synapsin

We measured synaptic vesicle mobility using fluorescence recovery after photobleaching of FM 1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide] stained mouse motor nerve terminals obtained from wild-type (WT) and synapsin triple knock-out (TKO) mice at room temperature and physiological temperature. Vesicles were mobile in resting terminals at physiological temperature but virtually immobile at room temperature. Mobility was increased at both temperatures by blocking phosphatases with okadaic acid, decreased at physiological temperature by blocking kinases with staurosporine, and unaffected by disrupting actin filaments with latrunculin A or reducing intracellular calcium concentration with BAPTA-AM. Synapsin TKO mice showed reduced numbers of synaptic vesicles and reduced FM 1-43 staining intensity. Synaptic transmission, however, was indistinguishable from WT, as was synaptic vesicle mobility under all conditions tested. Thus, in TKO mice, and perhaps WT mice, a phospho-protein different from synapsin but otherwise of unknown identity is the primary regulator of synaptic vesicle mobility.

FM 1-43 labeling of synaptic vesicle pools at the Drosophila neuromuscular junction

To maintain transmitter release during intense stimulation, neurons need to efficiently recycle vesicles at the synapse. Following membrane fusion, vesicles are reshaped and formed from the plasma membrane by bulk or clathrin-mediated endocytosis. Most synapses, including the Drosophila neuromuscular junction (NMJ), can also recycle synaptic vesicles directly by closing the fusion pore, a process referred to as "kiss and run." While the process of clathrin-mediated vesicle retrieval is under intense investigation, the kiss-and-run phenomenon remains much less accepted. To gain better insight into the mechanisms of synaptic vesicle recycling, it is therefore critical not only to identify and characterize novel players involved in the process, but also to develop novel methods to study vesicle recycling. Although in recent years numerous techniques to study vesicle traffic have been developed (see also this volume), in this chapter we outline established procedures that use the fluorescent dye FM 1-43 or related compounds to study vesicle cycling. We describe how FM 1-43 can be used to study and visualize clathrin-mediated or bulk endocytosis from the presynaptic membrane as well as exocytosis of labeled vesicles at the Drosophila NMJ, one of the best-characterized model synapses to study synaptic function in a genetic model system.

Huntingtin-interacting protein 14, a palmitoyl transferase required for exocytosis and targeting of CSP to synaptic vesicles

Posttranslational modification through palmitoylation regulates protein localization and function. In this study, we identify a role for the Drosophila melanogaster palmitoyl transferase Huntingtin-interacting protein 14 (HIP14) in neurotransmitter release. hip14 mutants show exocytic defects at low frequency stimulation and a nearly complete loss of synaptic transmission at higher temperature. Interestingly, two exocytic components known to be palmitoylated, cysteine string protein (CSP) and SNAP25, are severely mislocalized at hip14 mutant synapses. Complementary DNA rescue and localization experiments indicate that HIP14 is required solely in the nervous system and is essential for presynaptic function. Biochemical studies indicate that HIP14 palmitoylates CSP and that CSP is not palmitoylated in hip14 mutants. Furthermore, the hip14 exocytic defects can be suppressed by targeting CSP to synaptic vesicles using a chimeric protein approach. Our data indicate that HIP14 controls neurotransmitter release by regulating the trafficking of CSP to synapses.

Bcl-xL inhibitor ABT-737 reveals a dual role for Bcl-xL in synaptic transmission

A role for BCL-xL in regulating neuronal activity is suggested by its dramatic effects on synaptic function and mitochondrial channel activity. When recombinant BCL-xL is injected into the giant presynaptic terminal of squid stellate ganglion or applied directly to mitochondrial outer membranes within the living terminal, it potentiates synaptic transmission acutely, and it produces mitochondrial channel activity. The squid, however, is a genetically intractable model, making it difficult to apply genetic tools in squid to explore the role of endogenous BCL-xL in synaptic function. Therefore the small molecule inhibitor ABT-737, a mimetic of the BH3-only protein BAD, binding to the BH3-binding domain pocket, was tested in squid, revealing a dual role for BCL-xL. ABT-737 slowed recovery of synaptic responses after repetitive synaptic activity, indicating that endogenous BCL-xL is necessary for timely recovery of rapidly firing synapses. Unexpectedly, however, ABT-737 also protected neurons from hypoxia-induced synaptic rundown and from increased permeability of the mitochondrial outer membrane during hypoxia. This implies that endogenous BCL-xL or a modified form of BCL-xL, such as the N-truncated, proteolytic, pro-apoptotic cleavage product, DeltaN BCL-xL, contributes to injurious responses of the hypoxic synapse. To determine if ABT-737 is also an inhibitor of DeltaN BCL-xL, recombinant DeltaN BCL-xL protein was injected into the synapse. ABT-737 potently inhibited synaptic rundown induced by recombinant DeltaN BCL-xL. These observations support the possibility that endogenous proteolysis or a functionally equivalent modification of BCL-xL is responsible for the deleterious effects of hypoxia on synaptic activity.

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.

Insights into amyloid-beta-induced mitochondrial dysfunction in Alzheimer disease

Amyloid-beta has long been implicated in the pathogenesis of Alzheimer disease. The focus was initially on the extracellular fibrillar deposits of amyloid-beta but more recently has shifted to intracellular oligomeric forms of amyloid-beta. Unfortunately, the mechanism(s) by which either extracellular or intracellular amyloid-beta induces neuronal toxicity remains unclear. That said, a number of recent studies indicate that mitochondria might be an important target of amyloid-beta. Neurons rely heavily on mitochondria for energy and it is well established that mitochondrial dysfunction might be an important target of amyloid-beta. Mechanistically, amyloid-beta aggregates in mitochondria to impair function, leading to energy hypometabolism and elevated reactive oxygen species production. Additionally, amyloid-beta affects the balance of mitochondrial fission/fusion and mitochondrial transport, negatively impacting a host of cellular functions of neurons. Here, we review the role that amyloid-beta plays in mitochondrial structure and function of neurons and the importance of this in the pathogenesis of Alzheimer disease.

Structural basis for recruitment of mitochondrial fission complexes by Fis1

Mitochondrial fission controls mitochondrial shape and physiology, including mitochondrial remodeling in apoptosis. During assembly of the yeast mitochondrial fission complex, the outer membrane protein Fis1 recruits the dynamin-related GTPase Dnm1 to mitochondria. Fis1 contains a tetratricopeptide repeat (TPR) domain and interacts with Dnm1 via the molecular adaptors Mdv1 and Caf4. By using crystallographic analysis of adaptor-Fis1 complexes, we show that these adaptors use two helices to bind to both the concave and convex surfaces of the Fis1 TPR domain. Fis1 therefore contains two interaction interfaces, a binding mode that, to our knowledge, has not been observed previously for TPR domains. Genetic and biochemical studies indicate that both binding interfaces are important for binding of Mdv1 and Caf4 to Fis1 and for mitochondrial fission activity in vivo. Our results reveal how Fis1 recruits the mitochondrial fission complex and will facilitate efforts to manipulate mitochondrial fission.

Dopamine selectively sensitizes dopaminergic neurons to rotenone-induced apoptosis

Among various types of neurons affected in Parkinson's disease, dopamine (DA) neurons of the substantia nigra undergo the most pronounced degeneration. Products of DA oxidation and consequent cellular damage have been hypothesized to contribute to neuronal death. To examine whether elevated intracellular DA will selectively predispose the dopaminergic subpopulation of nigral neurons to damage by an oxidative insult, we first cultured rat primary mesencephalic cells in the presence of rotenone to elevate reactive oxygen species. Although MAP2(+) neurons were more sensitive to rotenone-induced toxicity than type 1 astrocytes, rotenone affected equally both DA (TH(+)) neurons and MAP2(+) neurons. In contrast, when intracellular DA concentration was elevated, DA neurons became selectively sensitized to rotenone. Raising intracellular DA levels in primary DA neurons resulted in dopaminergic neuron death in the presence of subtoxic concentrations of rotenone. Furthermore, mitochondrial superoxide dismutase mimetic, manganese (III) meso-tetrakis (4-benzoic acid) porphyrin, blocked activation of caspase-3, and consequent cell death. Our results demonstrate that an inhibitor of mitochondrial complex I and increased cytosolic DA may cooperatively lead to conditions of elevated oxidative stress and thereby promote selective demise of dopaminergic neurons.

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.

Structural determinants of transmission at large hippocampal mossy fiber synapses

Synapses are the key elements for signal processing and plasticity in the brain. To determine the structural factors underlying the unique functional properties of the hippocampal mossy fiber synapse, the complete quantitative geometry was investigated, using electron microscopy of serial ultrathin sections followed by computer-assisted three-dimensional reconstruction. In particular, parameters relevant for transmitter release and synaptic plasticity were examined. Two membrane specializations were found: active zones (AZs), transmitter release sites, and puncta adherentia, putative adhesion complexes. Individual boutons had, on average, 25 AZs (range, 7-45) that varied in shape and size (mean, 0.1 microm2; range, 0.07-0.17 microm2). The mean distance between individual AZs was 0.45 microm. Mossy fiber boutons and their target structures were mostly ensheathed by astrocytes, but fine glial processes never reached the active zones. Two structural factors are likely to promote synaptic cross talk: the short distance between AZs and the absence of fine glial processes at AZs. Thus, synaptic cross talk may contribute to the efficacy of hippocampal mossy fiber synapses. On average, a bouton contained 20,400 synaptic vesicles; approximately 900 vesicles were located within 60 nm from the active zone, approximately 4400 between 60 and 200 nm, and the remaining beyond 200 nm, suggesting large readily releasable, recycling, and reserve pools. The organization of the different pools may be a key structural correlate of presynaptic plasticity at this synapse. Thus, the mossy fiber bouton differs fundamentally in structure and function from the calyx of Held and other central synapses.

Mitochondrial dysfunction, oxidative stress, regulation of exocytosis and their relevance to neurodegenerative diseases

A common feature in the early stages of many neurodegenerative diseases lies in mitochondrial dysfunction, oxidative stress, and reduced levels of synaptic transmission. Many genes associated with neurodegenerative diseases are now known to regulate either mitochondrial function, redox state, or the exocytosis of neurotransmitters. Mitochondria are the primary source of reactive oxygen species and ATP and control apoptosis. Mitochondria are concentrated in synapses and significant alterations to synaptic mitochondrial localization, number, morphology, or function can be detrimental to synaptic transmission. Mitochondrial by-products are capable of regulating various steps of neurotransmission and mitochondrial dysfunction and oxidative stress occur in the early stages of many neurodegenerative diseases. This mini-review will highlight the prospect that mitochondria regulates synaptic exocytosis by controlling synaptic ATP and reactive oxygen species levels and that dysfunctional exocytosis caused by mitochondrial abnormalities may be a common underlying phenomenon in the initial stages of some human neurodegenerative diseases.

Hindlimb gait defects due to motor axon loss and reduced distal muscles in a transgenic mouse model of Charcot-Marie-Tooth type 2A

Charcot-Marie-Tooth (CMT) disease type 2A is a progressive, neurodegenerative disorder affecting long peripheral motor and sensory nerves. The most common clinical sign is weakness in the lower legs and feet, associated with muscle atrophy and gait defects. The axonopathy in CMT2A is caused by mutations in Mitofusin 2 (Mfn2), a mitochondrial GTPase necessary for the fusion of mitochondria. Most Mfn2 disease alleles dominantly aggregate mitochondria upon expression in cultured fibroblasts and neurons. To determine whether this property is related to neuronal pathogenesis, we used the HB9 promoter to drive expression of a pathogenic allele, Mfn2(T105M), in the motor neurons of transgenic mice. Transgenic mice develop key clinical signs of CMT2A disease in a dosage-dependent manner. They have a severe gait defect due to an inability to dorsi-flex the hindpaws. Consequently, affected animals drag their hindpaws while walking and support themselves on the hind knuckles, rather than the soles. This distal muscle weakness is associated with reduced numbers of motor axons in the motor roots and severe reduction of the anterior calf muscles. Many motor neurons from affected animals show improper mitochondrial distribution, characterized by tight clusters of mitochondria within axons. This transgenic line recapitulates key motor features of CMT2A and provides a system to dissect the function of mitochondria in the axons of mammalian motor neurons.

The function of mitochondria in presynaptic development at the neuromuscular junction

Mitochondria with high membrane potential (DeltaPsi(m)) are enriched in the presynaptic nerve terminal at vertebrate neuromuscular junctions, but the exact function of these localized synaptic mitochondria remains unclear. Here, we investigated the correlation between mitochondrial DeltaPsi(m) and the development of synaptic specializations. Using mitochondrial DeltaPsi(m)-sensitive probe JC-1, we found that DeltaPsi(m) in Xenopus spinal neurons could be reversibly elevated by creatine and suppressed by FCCP. Along naïve neurites, preexisting synaptic vesicle (SV) clusters were positively correlated with mitochondrial DeltaPsi(m), suggesting a potential regulatory role of mitochondrial activity in synaptogenesis. Indicating a specific role of mitochondrial activity in presynaptic development, mitochondrial ATP synthase inhibitor oligomycin, but not mitochondrial Na(+)/Ca(2+) exchanger inhibitor CGP-37157, inhibited the clustering of SVs induced by growth factor-coated beads. Local F-actin assembly induced along spinal neurites by beads was suppressed by FCCP or oligomycin. Our results suggest that a key role of presynaptic mitochondria is to provide ATP for the assembly of actin cytoskeleton involved in the assembly of the presynaptic specialization including the clustering of SVs and mitochondria themselves.

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.

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

Synapsin maintains the reserve vesicle pool and spatial segregation of the recycling pool in Drosophila presynaptic boutons

We employed optical detection of the lipophylic dye FM1-43 and focal recordings of quantal release to investigate how synapsin affects vesicle cycling at the neuromuscular junction of synapsin knockout (Syn KO) Drosophila. Loading the dye employing high K+ stimulation, which presumably involves the recycling pool of vesicles in exo/endocytosis, stained the periphery of wild type (WT) boutons, while in Syn KO the dye was redistributed towards the center of the bouton. When endocytosis was promoted by cyclosporin A pretreatment, the dye uptake was significantly enhanced in WT boutons, and the entire boutons were stained, suggesting staining of the reserve vesicle pool. In Syn KO boutons, the same loading paradigm produced fainter staining and significantly faster destaining. When the axon was stimulated electrically, a distinct difference in dye loading patterns was observed in WT boutons at different stimulation frequencies: a low stimulation frequency (3 Hz) produced a ring-shaped staining pattern, while at a higher frequency (10 Hz) the dye was redistributed towards the center of the bouton and the fluorescence intensity was significantly increased. This difference in staining patterns was essentially disrupted in Syn KO boutons, although synapsin did not affect the rate of quantal release. Stimulation of the nerve in the presence of bafilomycin, the blocker of the transmitter uptake, produced significantly stronger depression in Syn KO boutons. These results, taken together, suggest that synapsin maintains the reserve pool of vesicles and segregation between the recycling and reserve pools, and that it mediates mobilization of the reserve pool during intense stimulation.

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.

Mitochondrial fusion protects against neurodegeneration in the cerebellum

Mutations in the mitochondrial fusion gene Mfn2 cause the human neurodegenerative disease Charcot-Marie-Tooth type 2A. However, the cellular basis underlying this relationship is poorly understood. By removing Mfn2 from the cerebellum, we established a model for neurodegeneration caused by loss of mitochondrial fusion. During development and after maturity, Purkinje cells require Mfn2 but not Mfn1 for dendritic outgrowth, spine formation, and cell survival. In vivo, cell culture, and electron microscopy studies indicate that mutant Purkinje cells have aberrant mitochondrial distribution, ultrastructure, and electron transport chain activity. In fibroblasts lacking mitochondrial fusion, the majority of mitochondria lack mitochondrial DNA nucleoids. This deficiency provides a molecular mechanism for the dependence of respiratory activity on mitochondrial fusion. Our results show that exchange of mitochondrial contents is important for mitochondrial function as well as organelle distribution in neurons and have important implications for understanding the mechanisms of neurodegeneration due to perturbations in mitochondrial fusion.

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.

Phosphorylation of synapsin domain A is required for post-tetanic potentiation

Post-tetanic potentiation (PTP) is a form of homosynaptic plasticity important for information processing and short-term memory in the nervous system. The synapsins, a family of synaptic vesicle (SV)-associated phosphoproteins, have been implicated in PTP. Although several synapsin functions are known to be regulated by phosphorylation by multiple protein kinases, the role of individual phosphorylation sites in synaptic plasticity is poorly understood. All the synapsins share a phosphorylation site in the N-terminal domain A (site 1) that regulates neurite elongation and SV mobilization. Here, we have examined the role of phosphorylation of synapsin domain A in PTP and other forms of short-term synaptic enhancement (STE) at synapses between cultured Helix pomatia neurons. To this aim, we cloned H. pomatia synapsin (helSyn) and overexpressed GFP-tagged wild-type helSyn or site-1-mutant helSyn mutated in the presynaptic compartment of C1-B2 synapses. We found that PTP at these synapses depends both on Ca2+/calmodulin-dependent and cAMP-dependent protein kinases, and that overexpression of the non-phosphorylatable helSyn mutant, but not wild-type helSyn, specifically impairs PTP, while not altering facilitation and augmentation. Our findings show that phosphorylation of site 1 has a prominent role in the expression of PTP, thus defining a novel role for phosphorylation of synapsin domain A in short-term homosynaptic plasticity.

Axonal guidance protein FEZ1 associates with tubulin and kinesin motor protein to transport mitochondria in neurites of NGF-stimulated PC12 cells

Fasciculation and elongation protein zeta-1 (FEZ1) promotes efficiently the neurite elongation of rat phaeochromocytoma PC12 cells. We here characterized FEZ1 in PC12 cells. Nerve growth factor (NGF) stimulation induces significant expression of endogenous FEZ1 in PC12 cells. Upon NGF stimulation FEZ1 localizes in both cytoplasm and neuritis, co-localizing with mitochondria. Silencing of FEZ1 by RNA interference efficiently reduces NGF-induced neurite elongation and the anterograde motility of mitochondria in PC12 cells. Immunoprecipitation and pulldown assay shows that FEZ1 interacts with kinesin superfamily protein 5 (KIF5) and tubulin. Thus, our results suggest that the FEZ1/kinesin complex functions for the transport of mitochondria along microtubules toward the extending neurites in differentiating PC12 cells.

OPA1 processing reconstituted in yeast depends on the subunit composition of the m-AAA protease in mitochondria

The morphology of mitochondria in mammalian cells is regulated by proteolytic cleavage of OPA1, a dynamin-like GTPase of the mitochondrial inner membrane. The mitochondrial rhomboid protease PARL, and paraplegin, a subunit of the ATP-dependent m-AAA protease, were proposed to be involved in this process. Here, we characterized individual OPA1 isoforms by mass spectrometry, and we reconstituted their processing in yeast to identify proteases involved in OPA1 cleavage. The yeast homologue of OPA1, Mgm1, was processed both by PARL and its yeast homologue Pcp1. Neither of these rhomboid proteases cleaved OPA1. The formation of small OPA1 isoforms was impaired in yeast cells lacking the m-AAA protease subunits Yta10 and Yta12 and was restored upon expression of murine or human m-AAA proteases. OPA1 processing depended on the subunit composition of mammalian m-AAA proteases. Homo-oligomeric m-AAA protease complexes composed of murine Afg3l1, Afg3l2, or human AFG3L2 subunits cleaved OPA1 with higher efficiency than paraplegin-containing m-AAA proteases. OPA1 processing proceeded normally in murine cell lines lacking paraplegin or PARL. Our results provide evidence for different substrate specificities of m-AAA proteases composed of different subunits and reveal a striking evolutionary switch of proteases involved in the proteolytic processing of dynamin-like GTPases in mitochondria.

Mitochondrial disruption in Drosophila apoptosis

Mitochondrial disruption is a conserved aspect of apoptosis, seen in many species from mammals to nematodes. Despite significant conservation of other elements of the apoptotic pathway in Drosophila, a broad role for mitochondrial changes in apoptosis in flies remains unconfirmed. Here, we show that Drosophila mitochondria become permeable in response to the expression of Reaper and Hid, endogenous regulators of developmental apoptosis. Caspase activation in the absence of Reaper and Hid is not sufficient to permeabilize mitochondria, but caspases play a role in Reaper- and Hid-induced mitochondrial changes. Reaper and Hid rapidly localize to mitochondria, resulting in changes in mitochondrial ultrastructure. The dynamin-related protein, Drp1, is important for Reaper- and DNA-damage-induced mitochondrial disruption. Significantly, we show that inhibition of Reaper or Hid mitochondrial localization or inhibition of Drp1 significantly inhibits apoptosis, indicating a role for mitochondrial disruption in fly apoptosis.

Cytoplasmic and mitochondrial protein translation in axonal and dendritic terminal arborization

We identified a mutation in Aats-gly (also known as gars or glycyl-tRNA synthetase), the Drosophila melanogaster ortholog of the human GARS gene that is associated with Charcot-Marie-Tooth neuropathy type 2D (CMT2D), from a mosaic genetic screen. Loss of gars in Drosophila neurons preferentially affects the elaboration and stability of terminal arborization of axons and dendrites. The human and Drosophila genes each encode both a cytoplasmic and a mitochondrial isoform. Using additional mutants that selectively disrupt cytoplasmic or mitochondrial protein translation, we found that cytoplasmic protein translation is required for terminal arborization of both dendrites and axons during development. In contrast, disruption of mitochondrial protein translation preferentially affects the maintenance of dendritic arborization in adults. We also provide evidence that human GARS shows equivalent functions in Drosophila, and that CMT2D causal mutations show loss-of-function properties. Our study highlights different demands of protein translation for the development and maintenance of axons and dendrites.

A cell biological perspective on mitochondrial dysfunction in Parkinson disease and other neurodegenerative diseases

Dysfunction of mitochondria is frequently proposed to be involved in neurodegenerative disease. Deficiencies in energy supply, free radical generation, Ca2+ buffering or control of apoptosis, could all theoretically contribute to progressive decline of the central nervous system. Parkinson disease illustrates how mutations in very different genes finally impinge directly or indirectly on mitochondrial function, causing subtle but finally fatal dysfunction of dopaminergic neurons. Neurons in general appear more sensitive than other cells to mutations in genes encoding mitochondrial proteins. Particularly interesting are mutations in genes such as Opa1, Mfn1 and Dnm1l, whose products are involved in the dynamic morphological alterations and subcellular trafficking of mitochondria. These indicate that mitochondrial dynamics are especially important for the long-term maintenance of the nervous system. The emerging evidence clearly demonstrates the crucial role of specific mitochondrial functions in maintaining neuronal circuit integrity.

Role of mitochondrial remodeling in programmed cell death in Drosophila melanogaster

The role of mitochondria in Drosophila programmed cell death remains unclear, although certain gene products that regulate cell death seem to be evolutionarily conserved. We find that developmental programmed cell death stimuli in vivo and multiple apoptotic stimuli ex vivo induce dramatic mitochondrial fragmentation upstream of effector caspase activation, phosphatidylserine exposure, and nuclear condensation in Drosophila cells. Unlike genotoxic stress, a lipid cell death mediator induced an increase in mitochondrial contiguity prior to fragmentation of the mitochondria. Using genetic mutants and RNAi-mediated knockdown of drp-1, we find that Drp-1 not only regulates mitochondrial fission in normal cells, but mediates mitochondrial fragmentation during programmed cell death. Mitochondria in drp-1 mutants fail to fragment, resulting in hyperplasia of tissues in vivo and protection of cells from multiple apoptotic stimuli ex vivo. Thus, mitochondrial remodeling is capable of modifying the propensity of cells to undergo death in Drosophila.

Complementation between mouse Mfn1 and Mfn2 protects mitochondrial fusion defects caused by CMT2A disease mutations

Mfn2, an oligomeric mitochondrial protein important for mitochondrial fusion, is mutated in Charcot-Marie-Tooth disease (CMT) type 2A, a peripheral neuropathy characterized by axonal degeneration. In addition to homooligomeric complexes, Mfn2 also associates with Mfn1, but the functional significance of such heterooligomeric complexes is unknown. Also unknown is why Mfn2 mutations in CMT2A lead to cell type–specific defects given the widespread expression of Mfn2. In this study, we show that homooligomeric complexes formed by many Mfn2 disease mutants are nonfunctional for mitochondrial fusion. However, wild-type Mfn1 complements mutant Mfn2 through the formation of heterooligomeric complexes, including complexes that form in trans between mitochondria. Wild-type Mfn2 cannot complement the disease alleles. Our results highlight the functional importance of Mfn1–Mfn2 heterooligomeric complexes and the close interplay between the two mitofusins in the control of mitochondrial fusion. Furthermore, they suggest that tissues with low Mfn1 expression are vulnerable in CMT2A and that methods to increase Mfn1 expression in the peripheral nervous system would benefit CMT2A patients.

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.

Mutations in dynamin-related protein result in gross changes in mitochondrial morphology and affect synaptic vesicle recycling at the Drosophila neuromuscular junction

Mitochondria are the primary source of ATP needed for the steps of the synaptic vesicle cycle. Dynamin-related protein (DRP) is involved in the fission of mitochondria and peroxisomes. To assess the role of mitochondria in synaptic function, we characterized a Drosophila DRP mutant combination that shows an acute temperature-sensitive paralysis. Sequencing of the mutant reveals a single amino acid change in the guanosine triphosphate hydrolysing domain (GTPase domain) of DRP. The synaptic mitochondria in these mutants are remarkably elongated, suggesting a role for DRP in mitochondrial fission in Drosophila. There is a loss of neuronal transmission at restrictive temperatures in electroretinogram (ERG) recordings. Like stress-sensitive B (sesB), a mitochondrial adenosine triphosphate (ATP) translocase mutant we studied earlier for its effects on synaptic vesicle recycling, an allele-specific reduction in the temperature of paralysis of Drosophila synaptic vesicle recycling mutant shibire was seen in the DRP mutant background. These data, in addition to depletion of vesicles observed in electron microscopic sections of photoreceptor synapses at restrictive temperatures, suggest a block in synaptic vesicle recycling due to reduced mitochondrial function.

The lysosome or lysosome-related organelle may serve as a Ca2+ store in the boutons of hippocampal pyramidal cells

Boutons are specialised presynaptic compartments that lie along the axons of central neurons. Release of neurotransmitter from boutons is tightly regulated by the level of intracellular calcium [Ca2+]i. A rise in Ca2+ level may be generated in several ways; entry of extracellular Ca2+ via voltage gated calcium channels (VGCCs), entry via ligand-operated channels (LOCs) or the release of Ca2+ from intracellular Ca2+ stores. The role of Ca2+ stores in boutons remains poorly understood, despite recent work indicating that the release of Ca2+ from the endoplasmic reticulum (ER) may contribute to transmitter release. In this study we assess whether the lysosome or a closely related organelle functions as a Ca2+ store in the boutons of hippocampal pyramidal neurones. Lysosomes are small acidic organelles more commonly known for their role in degrading redundant cellular constituents. Using a fluorescent lysosomal marker, we show that lysosomes are located in the axons of hippocampal CA3 neurones. Selective pharmacological lysis of the lysosomes with glycyl-phenylalanine 2-naphthylamide (GPN) generates rapid, highly focal Ca2+ transients within the axon and increases the frequency of spontaneous miniature excitatory post-synaptic currents (mEPSCs), revealing that the organelle contains Ca2+ at a concentration sufficient to evoke transmitter release. Confocal laser scanning microscopy, combined with electrophysiology is used to monitor the action potential evoked increases in [Ca2+]i in boutons. We show that disruption of lysosomes compromises action potential evoked [Ca2+]i but this effect is occluded if the ER is discharged. Conversely, disruption of the lysosome does not appear to impact on the capacity of the ER to release Ca2+. These results suggest that the lysosome may serve as a Ca2+ store within hippocampal boutons, with a Ca2+ signalling role that is unique from that of the ER.

Mitochondrial fusion and fission in mammals

Eukaryotic cells maintain the overall shape of their mitochondria by balancing the opposing processes of mitochondrial fusion and fission. Unbalanced fission leads to mitochondrial fragmentation, and unbalanced fusion leads to mitochondrial elongation. Moreover, these processes control not only the shape but also the function of mitochondria. Mitochondrial dynamics allows mitochondria to interact with each other; without such dynamics, the mitochondrial population consists of autonomous organelles that have impaired function. Key components of the mitochondrial fusion and fission machinery have been identified, allowing initial dissection of their mechanisms of action. These components play important roles in mitochondrial function and development as well as programmed cell death. Disruption of the fusion machinery leads to neurodegenerative disease.

Fasciculation and elongation protein zeta-1 (FEZ1) participates in the polarization of hippocampal neuron by controlling the mitochondrial motility

The fasciculation and elongation protein zeta-1 (FEZ1), a mammalian orthologue of Caenorhabditis elegans UNC-76 protein, is a 45-kDa protein with four coiled-coiled domains and efficiently promotes the neurite elongation in the rat phaeochromocytoma PC12 cells. UNC-76 proteins of C. elegans and Drosophila have been genetically demonstrated to be involved in the axonal guidance. We here show that FEZ1 RNA interference (RNAi) represses the formation of axon in rat embryo hippocampal neurons. An anterograde mitochondrial movement is also retarded in neurites of the RNAi-treated hippocampal neurons. Moreover, the size of mitochondria is considerably elongated by the RNAi treatment. The transport of mitochondria from soma to axon or dendrites is essential for the neuronal differentiation. Therefore, our results strongly suggest that FEZ1 participates in the establishment of neuronal polarity by controlling the mitochondrial motility along axon.

Orchestration of lymphocyte chemotaxis by mitochondrial dynamics

Lymphocyte traffic is required to maintain homeostasis and perform appropriate immunological reactions. To migrate into inflamed tissues, lymphocytes must acquire spatial and functional asymmetries. Mitochondria are highly dynamic organelles that distribute in the cytoplasm to meet specific cellular needs, but whether this is essential to lymphocyte functions is unknown. We show that mitochondria specifically concentrate at the uropod during lymphocyte migration by a process involving rearrangements of their shape. Mitochondrial fission facilitates relocation of the organelles and promotes lymphocyte chemotaxis, whereas mitochondrial fusion inhibits both processes. Our data substantiate a new role for mitochondrial dynamics and suggest that mitochondria redistribution is required to regulate the motor of migrating cells.

Mitochondrial trafficking and morphology in healthy and injured neurons

Mitochondria are the primary generators of ATP and are important regulators of intracellular calcium homeostasis. These organelles are dynamically transported along lengthy neuronal processes, presumably for appropriate distribution to cellular regions of high metabolic demand and elevated intracellular calcium, such as synapses. The removal of damaged mitochondria that produce harmful reactive oxygen species and promote apoptosis is also thought to be mediated by transport of mitochondria to autophagosomes. Mitochondrial trafficking is therefore important for maintaining neuronal and mitochondrial health while cessation of movement may lead to neuronal and mitochondrial dysfunction. Mitochondrial morphology is also dynamic and is remodeled during neuronal injury and disease. Recent studies reveal different manifestations and mechanisms of impaired mitochondrial movement and altered morphology in injured neurons. These are likely to cause varied courses toward neuronal degeneration and death. The goal of this review is to provide an appreciation of the full range of mitochondrial function, morphology and trafficking, and the critical role these parameters play in neuronal physiology and pathophysiology.

P[acman]: a BAC transgenic platform for targeted insertion of large DNA fragments in D. melanogaster

We describe a transgenesis platform for Drosophila melanogaster that integrates three recently developed technologies: a conditionally amplifiable bacterial artificial chromosome (BAC), recombineering, and bacteriophage PhiC31-mediated transgenesis. The BAC is maintained at low copy number, facilitating plasmid maintenance and recombineering, but is induced to high copy number for plasmid isolation. Recombineering allows gap repair and mutagenesis in bacteria. Gap repair efficiently retrieves DNA fragments up to 133 kilobases long from P1 or BAC clones. PhiC31-mediated transgenesis integrates these large DNA fragments at specific sites in the genome, allowing the rescue of lethal mutations in the corresponding genes. This transgenesis platform should greatly facilitate structure/function analyses of most Drosophila genes.

Myosin light chain kinase is not a regulator of synaptic vesicle trafficking during repetitive exocytosis in cultured hippocampal neurons

The mechanism by which synaptic vesicles (SVs) are recruited to the release site is poorly understood. One candidate mechanism for trafficking of SVs is the myosin-actin motor system. Myosin activity is modulated by myosin light chain kinase (MLCK), which in turn is activated by calmodulin. Ca(2+) signaling in presynaptic terminals, therefore, may serve to regulate SV mobility along actin filaments via MLCK. Previous studies in different types of synapses have supported such a hypothesis. Here, we further investigated the role of MLCK in neurotransmitter release at glutamatergic synapses in cultured hippocampal neurons by examining the effects of two MLCK inhibitors, 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine.HCl (ML-7) and wortmannin. Bath application of ML-7 enhanced short-term depression of EPSCs to repetitive stimulation, whereas it reduced presynaptic release probability. However, ML-7 also inhibited action potential amplitude and voltage-gated Ca(2+) channel currents. These effects were not mimicked by wortmannin, suggesting that ML-7 was not specific to MLCK in hippocampal neurons. When SV exocytosis was directly triggered by a Ca(2+) ionophore, calcimycin, to bypass voltage-gated Ca(2+) channels, ML-7 had no effect on neurotransmitter release. Furthermore, when SV exocytosis elicited by electrical field stimulation was monitored by styryl dye, FM1-43 [N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide], the unloading kinetics of the dye was not altered in the presence of wortmannin. These data indicate that MLCK is not a major regulator of presynaptic SV trafficking during repetitive exocytosis at hippocampal synapses.

Exercise affects energy metabolism and neural plasticity-related proteins in the hippocampus as revealed by proteomic analysis

Studies were conducted to evaluate the effect of a brief voluntary exercise period on the expression pattern and post-translational modification of multiple protein classes in the rat hippocampus using proteomics. An analysis of 80 protein spots of relative high abundance on two-dimensional gels revealed that approximately 90% of the proteins identified were associated with energy metabolism and synaptic plasticity. Exercise up-regulated proteins involved in four aspects of energy metabolism, i.e. glycolysis, ATP synthesis, ATP transduction and glutamate turnover. Specifically, we found increases in fructose-bisphosphate aldolase C, phosphoglycerate kinase 1, mitochondrial ATP synthase, ubiquitous mitochondrial creatine kinase and glutamate dehydrogenase 1. Exercise also up-regulated specific synaptic-plasticity-related proteins, the cytoskeletal protein alpha-internexin and molecular chaperones (chaperonin-containing TCP-1, neuronal protein 22, heat shock 60-kDa protein 1 and heat shock protein 8). Western blot was used to confirm the direction and magnitude of change in ubiquitous mitochondrial creatine kinase, an enzyme essential for transducing mitochondrial-derived ATP to sites of high-energy demand such as the synapse. Protein phosphorylation visualized by Pro-Q Diamond fluorescent staining showed that neurofilament light polypeptide, glial fibrillary acidic protein, heat shock protein 8 and transcriptional activator protein pur-alpha were more intensely phosphorylated with exercise as compared with sedentary control levels. Our results, together with the fact that most of the proteins that we found to be up-regulated have been implicated in cognitive function, support a mechanism by which exercise uses processes of energy metabolism and synaptic plasticity to promote brain health.

ER vesicles and mitochondria move and communicate at synapses

Endoplasmic reticulum (ER) and mitochondria are multifunctional cell organelles and their involvement in Ca2+ handling is important in various neural activities. In the respiratory neurons, we observed ER as continuous reticulum in the soma and as isolated vesicles in dendrites. The vesicles moved bidirectionally with intermittent stops and decreased their velocity near exocytotic sites. ER vesicles and mitochondria that resided in these regions changed lumenal Ca2+ and mitochondrial potential in concert with synaptic activity. Ca2+ release from ER or mitochondria evoked exocytosis. ER vesicles and mitochondria bidirectionally exchanged Ca2+, the efficacy of which depended on the distance between organelles. Depolarisation-evoked exocytosis had different kinetics, depending on whether functional ER vesicles and mitochondria were present in perisynaptic regions and able to exchange Ca2+ or only one organelle type was available. Transfer of Ca2+ from ER to mitochondria produced long-lasting elevations of residual Ca2+ that increased the duration of exocytosis. In slice preparations, synaptic currents in inspiratory neurons were suppressed after disengagement of ER vesicles and mitochondria, and the activity was potentiated after stimulation of Ca2+ exchange between the organelles. We propose that communication between perisynaptic ER vesicles and mitochondria can shape intracellular Ca2+ signals and modulate synaptic and integrative neural activities.

Mitochondria and neuronal activity

Mitochondria are central for various cellular processes that include ATP production, intracellular Ca(2+) signaling, and generation of reactive oxygen species. Neurons critically depend on mitochondrial function to establish membrane excitability and to execute the complex processes of neurotransmission and plasticity. While much information about mitochondrial properties is available from studies on isolated mitochondria and dissociated cell cultures, less is known about mitochondrial function in intact neurons in brain tissue. However, a detailed description of the interactions between mitochondrial function, energy metabolism, and neuronal activity is crucial for the understanding of the complex physiological behavior of neurons, as well as the pathophysiology of various neurological diseases. The combination of new fluorescence imaging techniques, electrophysiology, and brain slice preparations provides a powerful tool to study mitochondrial function during neuronal activity, with high spatiotemporal resolution. This review summarizes recent findings on mitochondrial Ca(2+) transport, mitochondrial membrane potential (DeltaPsi(m)), and energy metabolism during neuronal activity. We will first discuss interactions of these parameters for experimental stimulation conditions that can be related to the physiological range. We will then describe how mitochondrial and metabolic dysfunction develops during pathological neuronal activity, focusing on temporal lobe epilepsy and its experimental models. The aim is to illustrate that 1) the structure of the mitochondrial compartment is highly dynamic in neurons, 2) there is a fine-tuned coupling between neuronal activity and mitochondrial function, and 3) mitochondria are of central importance for the complex behavior of neurons.

Complete dissociation of motor neuron death from motor dysfunction by Bax deletion in a mouse model of ALS

The death of cranial and spinal motoneurons (MNs) is believed to be an essential component of the pathogenesis of amyotrophic lateral sclerosis (ALS). We tested this hypothesis by crossing Bax-deficient mice with mice expressing mutant superoxide dismutase 1 (SOD1), a transgenic model of familial ALS. Although Bax deletion failed to prevent neuromuscular denervation and mitochondrial vacuolization, MNs were completely rescued from mutant SOD1-mediated death. However, Bax deficiency extended lifespan and delayed the onset of motor dysfunction of SOD1 mutants, suggesting that Bax acts via a mechanism distinct from cell death activation. Consistent with this idea, Bax elimination delayed the onset of neuromuscular denervation, which began long before the activation of cell death proteins in SOD1 mutants. Additionally, we show that denervation preceded accumulation of mutant SOD1 within MNs and astrogliosis in the spinal cord, which are also both delayed in Bax-deficient SOD1 mutants. Interestingly, MNs exhibited mitochondrial abnormalities at the innervated neuromuscular junction at the onset of neuromuscular denervation. Additionally, both MN presynaptic terminals and terminal Schwann cells expressed high levels of mutant SOD1 before MNs withdrew their axons. Together, these data support the idea that clinical symptoms in the SOD1 G93A model of ALS result specifically from damage to the distal motor axon and not from activation of the death pathway, and cast doubt on the utility of anti-apoptotic therapies to combat ALS. Furthermore, they suggest a novel, cell death-independent role for Bax in facilitating mutant SOD1-mediated motor denervation.

Proteolytic processing of OPA1 links mitochondrial dysfunction to alterations in mitochondrial morphology

Many muscular and neurological disorders are associated with mitochondrial dysfunction and are often accompanied by changes in mitochondrial morphology. Mutations in the gene encoding OPA1, a protein required for fusion of mitochondria, are associated with hereditary autosomal dominant optic atrophy type I. Here we show that mitochondrial fragmentation correlates with processing of large isoforms of OPA1 in cybrid cells from a patient with myoclonus epilepsy and ragged-red fibers syndrome and in mouse embryonic fibroblasts harboring an error-prone mitochondrial mtDNA polymerase gamma. Furthermore, processed OPA1 was observed in heart tissue derived from heart-specific TFAM knock-out mice suffering from mitochondrial cardiomyopathy and in skeletal muscles from patients suffering from mitochondrial myopathies such as myopathy encephalopathy lactic acidosis and stroke-like episodes. Dissipation of the mitochondrial membrane potential leads to fast induction of proteolytic processing of OPA1 and concomitant fragmentation of mitochondria. Recovery of mitochondrial fusion depended on protein synthesis and was accompanied by resynthesis of large isoforms of OPA1. Fragmentation of mitochondria was prevented by overexpressing OPA1. Taken together, our data indicate that proteolytic processing of OPA1 has a key role in inducing fragmentation of energetically compromised mitochondria. We present the hypothesis that this pathway regulates mitochondrial morphology and serves as an early response to prevent fusion of dysfunctional mitochondria with the functional mitochondrial network.

Mitochondria: more than just a powerhouse

Pioneering biochemical studies have long forged the concept that the mitochondria are the 'energy powerhouse of the cell'. These studies, combined with the unique evolutionary origin of the mitochondria, led the way to decades of research focusing on the organelle as an essential, yet independent, functional component of the cell. Recently, however, our conceptual view of this isolated organelle has been profoundly altered with the discovery that mitochondria function within an integrated reticulum that is continually remodeled by both fusion and fission events. The identification of a number of proteins that regulate these activities is beginning to provide mechanistic details of mitochondrial membrane remodeling. However, the broader question remains regarding the underlying purpose of mitochondrial dynamics and the translation of these morphological transitions into altered functional output. One hypothesis has been that mitochondrial respiration and metabolism may be spatially and temporally regulated by the architecture and positioning of the organelle. Recent evidence supports and expands this idea by demonstrating that mitochondria are an integral part of multiple cell signaling cascades. Interestingly, proteins such as GTPases, kinases and phosphatases are involved in bi-directional communication between the mitochondrial reticulum and the rest of the cell. These proteins link mitochondrial function and dynamics to the regulation of metabolism, cell-cycle control, development, antiviral responses and cell death. In this review we will highlight the emerging evidence that provides molecular definition to mitochondria as a central platform in the execution of diverse cellular events.