Moving or stopping mitochondria: Miro as a traffic cop by sensing calcium

MIRO1 Is Required for Dynamic Increases in Mitochondria-ER Contact Sites and Mitochondrial ATP During the Cell Cycle

Mitochondria-ER contact sites (MERCS) are vital for mitochondrial dynamics, lipid exchange, Ca2+ homeostasis, and energy metabolism. We examined whether mitochondrial metabolism changes during the cell cycle depend on MERCS dynamics and are regulated by the outer mitochondrial protein mitochondrial rho GTPase 1 (MIRO1). Wound healing was assessed in mice with fibroblast-specific deletion of MIRO1. Wild-type and MIRO1-/- fibroblasts and vascular smooth muscle cells were evaluated for proliferation, cell cycle progression, number of MERCS, distance, and protein composition throughout the cell cycle. Restoration of MIRO1 mutants was used to test the role of MIRO1 domains; Ca2+ transients and mitochondrial metabolism were evaluated using biochemical, immunodetection, and fluorescence techniques. MERCS increased in number during G1/S compared with during G0, which was accompanied by a notable rise in protein-protein interactions involving VDAC1 and IP3R as well as GRP75 and MIRO1 by proximity-ligation assays. Split-GFP ER/mitochondrial contacts of 40 nm also increased. Mitochondrial Ca2+ concentration ([Ca2+]), membrane potential, and ATP levels correlated with the formation of MERCS during the cell cycle. MIRO1 deficiency blocked G1/S progression and the cell-cycle-dependent formation of MERCS and altered ER Ca2+ release and mitochondrial Ca2+ uptake. MIRO1 mutants lacking the Ca2+-sensitive EF hands or the transmembrane domain did not rescue cell proliferation or the formation of MERCS. MIRO1 controls an increase in the number of MERCS during cell cycle progression and increases mitochondrial [Ca2+], driving metabolic activity and proliferation through its EF hands.

Infection-induced peripheral mitochondria fission drives ER encapsulations and inter-mitochondria contacts that rescue bioenergetics

The dynamic regulation of mitochondria shape via fission and fusion is critical for cellular responses to stimuli. In homeostatic cells, two modes of mitochondrial fission, midzone and peripheral, provide a decision fork between either proliferation or clearance of mitochondria. However, the relationship between specific mitochondria shapes and functions remains unclear in many biological contexts. While commonly associated with decreased bioenergetics, fragmented mitochondria paradoxically exhibit elevated respiration in several disease states, including infection with the prevalent pathogen human cytomegalovirus (HCMV) and metastatic melanoma. Here, incorporating super-resolution microscopy with mass spectrometry and metabolic assays, we use HCMV infection to establish a molecular mechanism for maintaining respiration within a fragmented mitochondria population. We establish that HCMV induces fragmentation through peripheral mitochondrial fission coupled with suppression of mitochondria fusion. Unlike uninfected cells, the progeny of peripheral fission enter mitochondria-ER encapsulations (MENCs) where they are protected from degradation and bioenergetically stabilized during infection. MENCs also stabilize pro-viral inter-mitochondria contacts (IMCs), which electrochemically link mitochondria and promote respiration. Demonstrating a broader relevance, we show that the fragmented mitochondria within metastatic melanoma cells also form MENCs. Our findings establish a mechanism where mitochondria fragmentation can promote increased respiration, a feature relevant in the context of human diseases.

Miro-mediated mitochondrial transport: A new dimension for disease-related abnormal cell metabolism?

Mitochondria are versatile and highly dynamic organelles found in eukaryotic cells that play important roles in a variety of cellular processes. The importance of mitochondrial transport in cell metabolism, including variations in mitochondrial distribution within cells and intercellular transfer, has grown in recent years. Several studies have demonstrated that abnormal mitochondrial transport represents an early pathogenic alteration in a variety of illnesses, emphasizing its significance in disease development and progression. Mitochondrial Rho GTPase (Miro) is a protein found on the outer mitochondrial membrane that is required for cytoskeleton-dependent mitochondrial transport, mitochondrial dynamics (fusion and fission), and mitochondrial Ca2+ homeostasis. Miro, as a critical regulator of mitochondrial transport, has yet to be thoroughly investigated in illness. This review focuses on recent developments in recognizing Miro as a crucial molecule in controlling mitochondrial transport and investigates its roles in diverse illnesses. It also intends to shed light on the possibilities of targeting Miro as a therapeutic method for a variety of diseases.

Calcium Dyshomeostasis Drives Pathophysiology and Neuronal Demise in Age-Related Neurodegenerative Diseases

This review postulates that age-related neurodegeneration entails inappropriate activation of intrinsic pathways to enable brain plasticity through deregulated calcium (Ca2+) signalling. Ca2+ in the cytosol comprises a versatile signal controlling neuronal cell physiology to accommodate adaptive structural and functional changes of neuronal networks (neuronal plasticity) and, as such, is essential for brain function. Although disease risk factors selectively affect different neuronal cell types across age-related neurodegenerative diseases (NDDs), these appear to have in common the ability to impair the specificity of the Ca2+ signal. As a result, non-specific Ca2+ signalling facilitates the development of intraneuronal pathophysiology shared by age-related NDDs, including mitochondrial dysfunction, elevated reactive oxygen species (ROS) levels, impaired proteostasis, and decreased axonal transport, leading to even more Ca2+ dyshomeostasis. These core pathophysiological processes and elevated cytosolic Ca2+ levels comprise a self-enforcing feedforward cycle inevitably spiralling toward high levels of cytosolic Ca2+. The resultant elevated cytosolic Ca2+ levels ultimately gear otherwise physiological effector pathways underlying plasticity toward neuronal demise. Ageing impacts mitochondrial function indiscriminately of the neuronal cell type and, therefore, contributes to the feedforward cycle of pathophysiology development seen in all age-related NDDs. From this perspective, therapeutic interventions to safely restore Ca2+ homeostasis would mitigate the excessive activation of neuronal destruction pathways and, therefore, are expected to have promising neuroprotective potential.

MIRO-1 interacts with VDAC-1 to regulate mitochondrial membrane potential in Caenorhabditis elegans

Precise regulation of mitochondrial fusion and fission is essential for cellular activity and animal development. Imbalances between these processes can lead to fragmentation and loss of normal membrane potential in individual mitochondria. In this study, we show that MIRO-1 is stochastically elevated in individual fragmented mitochondria and is required for maintaining mitochondrial membrane potential. We further observe a higher level of membrane potential in fragmented mitochondria in fzo-1 mutants and wounded animals. Moreover, MIRO-1 interacts with VDAC-1, a crucial mitochondrial ion channel located in the outer mitochondrial membrane, and this interaction depends on the residues E473 of MIRO-1 and K163 of VDAC-1. The E473G point mutation disrupts their interaction, resulting in a reduction of the mitochondrial membrane potential. Our findings suggest that MIRO-1 regulates membrane potential and maintains mitochondrial activity and animal health by interacting with VDAC-1. This study provides insight into the mechanisms underlying the stochastic maintenance of membrane potential in fragmented mitochondria.

Advances in the study of axon-associated vesicles

The central nervous system is the most important and difficult to study system in the human body and is known for its complex functions, components, and mechanisms. Neurons are the basic cellular units realizing neural functions. In neurons, vesicles are one of the critical pathways for intracellular material transport, linking information exchanges inside and outside cells. The axon is a vital part of neuron since electrical and molecular signals must be conducted through axons. Here, we describe and explore the formation, trafficking, and sorting of cellular vesicles within axons, as well as related-diseases and practical implications. Furthermore, with deepening of understanding and the development of new approaches, accumulating evidence proves that besides signal transmission between synapses, the material exchange and vesicular transmission between axons and extracellular environment are involved in physiological processes, and consequently to neural pathology. Recent studies have also paid attention to axonal vesicles and their physiological roles and pathological effects on axons themselves. Therefore, this review mainly focuses on these two key nodes to explain the role of intracellular vesicles and extracellular vesicles migrated from cells on axons and neurons, providing innovative strategy for future researches.

Mitochondrial fragmentation and ROS signaling in wound response and repair

Mitochondria are organelles that serve numerous critical cellular functions, including energy production, Ca2+ homeostasis, redox signaling, and metabolism. These functions are intimately linked to mitochondrial morphology, which is highly dynamic and capable of rapid and transient changes to alter cellular functions in response to environmental cues and cellular demands. Mitochondrial morphology and activity are critical for various physiological processes, including wound healing. In mammals, wound healing is a complex process that requires coordinated function of multiple cell types and progresses in partially overlapping but distinct stages: hemostasis and inflammation, cell proliferation and migration, and tissue remodeling. The repair process at the single-cell level forms the basis for wound healing and regeneration in tissues. Recent findings reveal that mitochondria fulfill the intensive energy demand for wound repair and aid wound closure by cytoskeleton remodeling via morphological changes and mitochondrial reactive oxygen species (mtROS) signaling. In this review, we will mainly elucidate how wounding induces changes in mitochondrial morphology and activity and how these changes, in turn, contribute to cellular wound response and repair.

Neurobiology: Resetting the axon's batteries

Neuronal injury can cause mitochondrial damage, leading to reduced energy production, decreased Ca²⁺ storage capacity, and increased reactive oxygen species. A new study reveals a mechanism to trigger the axonal transport of previously anchored mitochondria and promote neuroprotection and axon regeneration by replacing damaged with functional mitochondria.

Intricate role of mitochondrial calcium signalling in mitochondrial quality control for regulation of cancer cell fate

Mitochondrial quality control is crucial for sustaining cellular maintenance. Mitochondrial Ca²⁺ plays an important role in the maintenance of mitochondrial quality control through regulation of mitochondrial dynamics, mitophagy and mitochondrial biogenesis for preserving cellular homeostasis. The regulation of this dynamic interlink between these mitochondrial networks and mitochondrial Ca²⁺ appears indispensable for the adaptation of cells under external stimuli. Moreover, dysregulation of mitochondrial Ca²⁺ divulges impaired mitochondrial control that results in several pathological conditions such as cancer. Hence this review untangles the interplay between mitochondrial Ca²⁺ and quality control that govern mitochondrial health and mitochondrial coordinates in the development of cancer.

Miro, a Rho GTPase genetically interacts with Alzheimer's disease-associated genes (Tau, Aβ42 and Appl) in Drosophila melanogaster

Miro (mitochondrial Rho GTPases), a mitochondrial outer membrane protein, facilitates mitochondrial axonal transport along the microtubules to facilitate neuronal function. It plays an important role in regulating mitochondrial dynamics (fusion and fission) and cellular energy generation. Thus, Miro might be associated with the key pathologies of several neurodegenerative diseases (NDs) including Alzheimer's disease (AD). In the present manuscript, we have demonstrated the possible genetic interaction between Miro and AD-related genes such as Tau, Aβ42 and Appl in Drosophila melanogaster Ectopic expression of Tau, Aβ42 and Appl induced a rough eye phenotype, defects in phototaxis and climbing activity, and shortened lifespan in the flies. In our study, we have observed that overexpression of Miro improves the rough eye phenotype, behavioral activities (climbing and phototaxis) and ATP level in AD model flies. Further, the improvement examined in AD-related phenotypes was correlated with decreased oxidative stress, cell death and neurodegeneration in Miro overexpressing AD model flies. Thus, the obtained results suggested that Miro genetically interacts with AD-related genes in Drosophila and has the potential to be used as a therapeutic target for the design of therapeutic strategies for NDs.This article has an associated First Person interview with the first author of the paper.

Altered Mitochondrial Dynamics in Motor Neuron Disease: An Emerging Perspective

Mitochondria plays privotal role in diverse pathways that regulate cellular function and survival, and have emerged as a prime focus in aging and age-associated motor neuron diseases (MNDs), such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Accumulating evidence suggests that many amyloidogenic proteins, including MND-associated RNA/DNA-binding proteins fused in sarcoma (FUS) and TAR DNA binding protein (TDP)-43, are strongly linked to mitochondrial dysfunction. Animal model and patient studies have highlighted changes in mitochondrial structure, plasticity, replication/copy number, mitochondrial DNA instability, and altered membrane potential in several subsets of MNDs, and these observations are consistent with the evidence of increased excitotoxicity, induction of reactive oxygen species, and activation of intrinsic apoptotic pathways. Studies in MND rodent models also indicate that mitochondrial abnormalities begin prior to the clinical and pathological onset of the disease, suggesting a causal role of mitochondrial dysfunction. Our recent studies, which demonstrated the involvement of specific defects in DNA break-ligation mediated by DNA ligase 3 (LIG3) in FUS-associated ALS, raised a key question of its potential implication in mitochondrial DNA transactions because LIG3 is essential for both mitochondrial DNA replication and repair. This question, as well as how wild-type and mutant MND-associated factors affect mitochondria, remain to be elucidated. These new investigation avenues into the mechanistic role of mitochondrial dysfunction in MNDs are critical to identify therapeutic targets to alleviate mitochondrial toxicity and its consequences. In this article, we critically review recent advances in our understanding of mitochondrial dysfunction in diverse subgroups of MNDs and discuss challenges and future directions.

Tourette Syndrome Risk Genes Regulate Mitochondrial Dynamics, Structure, and Function

Gilles de la Tourette syndrome (GTS) is a neurodevelopmental disorder characterized by motor and vocal tics with an estimated prevalence of 1% in children and adolescents. GTS has high rates of inheritance with many rare mutations identified. Apart from the role of the neurexin trans-synaptic connexus (NTSC) little has been confirmed regarding the molecular basis of GTS. The NTSC pathway regulates neuronal circuitry development, synaptic connectivity and neurotransmission. In this study we integrate GTS mutations into mitochondrial pathways that also regulate neuronal circuitry development, synaptic connectivity and neurotransmission. Many deleterious mutations in GTS occur in genes with complementary and consecutive roles in mitochondrial dynamics, structure and function (MDSF) pathways. These genes include those involved in mitochondrial transport (NDE1, DISC1, OPA1), mitochondrial fusion (OPA1), fission (ADCY2, DGKB, AMPK/PKA, RCAN1, PKC), mitochondrial metabolic and bio-energetic optimization (IMMP2L, MPV17, MRPL3, MRPL44). This study is the first to develop and describe an integrated mitochondrial pathway in the pathogenesis of GTS. The evidence from this study and our earlier modeling of GTS molecular pathways provides compounding support for a GTS deficit in mitochondrial supply affecting neurotransmission.

Miro2 Regulates Inter-Mitochondrial Communication in the Heart and Protects Against TAC-Induced Cardiac Dysfunction

Rationale: The constrained mitochondria in cardiomyocytes communicate with each other, through mitochondrial kissing or nanotunneling, forming a dynamically continuous network to share content and transfer signals. However, the molecular mechanism of cardiac inter-mitochondrial communication is unclear. Objective: To determine the molecular mechanism underlying the robust inter-mitochondrial communication and its pathophysiological relevance in the heart. Methods and Results: By mitochondria-targeted expressing the photoactivatable green fluorescent protein, we revealed that most mitochondrial nanotubes bridge communicating mitochondrial pairs were associated with microtubules. Miro2 (mitochondrial Rho GTPase), the outer mitochondrial membrane protein which usually mediates mitochondrial transport within cells, accompanied with mitochondrial nanotubes along microtubules in adult cardiomyocytes. Adenovirus mediated expression of Miro2 in cardiomyocytes accelerated inter-mitochondrial communication through increasing mitochondrial nanotunneling and mitochondrial kissing between adjacent mitochondrial pairs. In transverse aortic constriction-induced hypertrophic mouse hearts Miro2 protein was declined, accompanied with decreased inter-mitochondrial communication. Miro2 transgenic mice showed ameliorated cardiac function, increased mitochondrial nanotube formation and inter-mitochondrial communication, and improved mitochondrial function after transverse aortic constriction. E3 ubiquitin ligase Parkin was increased in transverse aortic constriction mouse hearts and phenylephrine stimulation-induced hypertrophic cardiomyocytes. Inhibition of proteasome blocked phenylephrine-induced decrease of Miro2, and Parkin overexpression led to the decrease of Miro2. Conclusions: Mitochondrial Miro2 expression levels regulate inter-mitochondrial communication along microtubules in adult cardiomyocytes, and degradation of Miro2 through Parkin-mediated ubiquitination contributes to impaired inter-mitochondrial communication and cardiac dysfunction during hypertrophic heart diseases.Visual Overview: An online visual overview is available for this article.

Chain length of saturated fatty acids regulates mitochondrial trafficking and function in sensory neurons

Dyslipidemia associated with T2D leads to diabetic neuropathy, a complication characterized by sensory neuronal dysfunction and peripheral nerve damage. Sensory dorsal root ganglion (DRG) neurons are dependent on axonal mitochondrial energy production facilitated by mitochondrial transport mechanisms that distribute mitochondria throughout the axon. Because long-chain saturated FAs (SFAs) damage DRG neurons and medium-chain SFAs are reported to improve neuronal function, we evaluated the impact of SFA chain length on mitochondrial trafficking, mitochondrial function, and apoptosis. DRG neurons were exposed to SFAs with C12:0-C18:0 chain lengths and evaluated for changes in mitochondrial trafficking, mitochondrial polarization, and apoptosis. DRG neurons treated with C16:0 and C18:0 SFAs showed a significant decrease in the percentage of motile mitochondria and velocity of mitochondrial trafficking, whereas C12:0 and C14:0 SFAs had no impact on motility. Treatment with C16:0 and C18:0 SFAs exhibited mitochondrial depolarization correlating with impaired mitochondrial motility; the C12:0- and C14:0-treated neurons retained mitochondrial polarization. The reduction in mitochondrial trafficking and function in C16:0- and C18:0-treated DRG neurons correlated with apoptosis that was blocked in C12:0 and C14:0 SFA treatments. These results suggest that SFA chain length plays an important role in regulating axonal mitochondrial trafficking and function in DRG neurons.

NOS3 Inhibition Confers Post-Ischemic Protection to Young and Aging White Matter Integrity by Conserving Mitochondrial Dynamics and Miro-2 Levels

White matter (WM) damage following a stroke underlies a majority of the neurological disability that is subsequently observed. Although ischemic injury mechanisms are age-dependent, conserving axonal mitochondria provides consistent post-ischemic protection to young and aging WM. Nitric oxide synthase (NOS) activation is a major cause of oxidative and mitochondrial injury in gray matter during ischemia; therefore, we used a pure WM tract, isolated male mouse optic nerve, to investigate whether NOS inhibition provides post-ischemic functional recovery by preserving mitochondria. We show that pan-NOS inhibition applied before oxygen-glucose deprivation (OGD) promotes functional recovery of young and aging axons and preserves WM cellular architecture. This protection correlates with reduced nitric oxide (NO) generation, restored glutathione production, preserved axonal mitochondria and oligodendrocytes, and preserved ATP levels. Pan-NOS inhibition provided post-ischemic protection to only young axons, whereas selective inhibition of NOS3 conferred post-ischemic protection to both young and aging axons. Concurrently, genetic deletion of NOS3 conferred long-lasting protection to young axons against ischemia. OGD upregulated NOS3 levels in astrocytes, and we show for the first time that inhibition of NOS3 generation in glial cells prevents axonal mitochondrial fission and restores mitochondrial motility to confer protection to axons by preserving Miro-2 levels. Interestingly, NOS1 inhibition exerted post-ischemic protection selectively to aging axons, which feature age-dependent mechanisms of oxidative injury in WM. Our study provides the first evidence that inhibition of glial NOS activity confers long-lasting benefits to WM function and structure and suggests caution in defining the role of NO in cerebral ischemia at vascular and cellular levels.SIGNIFICANCE STATEMENT White matter (WM) injury during stroke is manifested as the subsequent neurological disability in surviving patients. Aging primarily impacts CNS WM and mechanisms of ischemic WM injury change with age. Nitric oxide is involved in various mitochondrial functions and we propose that inhibition of glia-specific nitric oxide synthase (NOS) isoforms promotes axon function recovery by preserving mitochondrial structure, function, integrity, and motility. Using electrophysiology and three-dimensional electron microscopy, we show that NOS3 inhibition provides a common target to improve young and aging axon function, whereas NOS1 inhibition selectively protects aging axons when applied after injury. This study provides the first evidence that inhibition of glial cell NOS activity confers long-lasting benefits to WM structure and function.

The Complex Interaction of Mitochondrial Genetics and Mitochondrial Pathways in Psychiatric Disease

While accounting for only 2% of the body's weight, the brain utilizes up to 20% of the body's total energy. Not surprisingly, metabolic dysfunction and energy supply-and-demand mismatch have been implicated in a variety of neurological and psychiatric disorders. Mitochondria are responsible for providing the brain with most of its energetic demands, and the brain uses glucose as its exclusive energy source. Exploring the role of mitochondrial dysfunction in the etiology of psychiatric disease is a promising avenue to investigate further. Genetic analysis of mitochondrial activity is a cornerstone in understanding disease pathogenesis related to metabolic dysfunction. In concert with neuroimaging and pathological study, genetics provides an important bridge between biochemical findings and clinical correlates in psychiatric disease. Mitochondrial genetics has several unique aspects to its analysis, and corresponding special considerations. Here, we review the components of mitochondrial genetic analysis - nuclear DNA, mitochon-drial DNA, mitochondrial pathways, pseudogenes, nuclear-mitochondrial mismatch, and microRNAs - that could contribute to an observable clinical phenotype. Throughout, we highlight psychiatric diseases that can arise due to dysfunction in these processes, with a focus on schizophrenia and bipolar disorder.

FOXO3 promoted mitophagy via nuclear retention induced by manganese chloride in SH-SY5Y cells

OBJECTIVES

To evaluate the role of FOXO3 during the process of mitophagy induced by manganese chloride (MnCl₂), mitochondrial dysfunction and mitophagy were detected before and after FOXO3 was knocked down in SH-SY5Y cells.

METHOD

Transmission electron microscopy (TEM), flow cytometry, confocal microscopy and a western blot were used to detect mitochondrial ultrastructure and autophagy, Ca²⁺ levels, mitochondrial reactive oxygen species (ROS) and the mitochondrial membrane potential (MMP), autophagosomes and mitophagy marker proteins (p62, LC3-II/LC3-I, Beclin-1, PINK1 and P-parkin), respectively.

RESULTS

After SH-SY5Y cells were exposed to MnCl₂, the levels of cytoplasmic Ca²⁺ and mitochondrial ROS increased but the mitochondrial MMP decreased significantly compared to the control in a dose- and time-dependent manner (p < 0.05), which indicated that MnCl₂ can lead to mitochondrial dysfunction. Under TEM, mitophagy and autolysosomes were observed. The WB results also showed that mitophagy marker proteins including LC3-II/LC3-I, Beclin-1, PINK1 and P-parkin except for p62 increased in a dose- and time-dependent manner, accompanied by FOXO3 nuclear retention, which indicated that MnCl₂ can lead to mitophagy and FOXO3 nuclear translocation may be involved in this process. After FOXO3 was knocked down, the inverse results of mitophagy and the levels of mitochondrial ROS decreasing were observed, which showed that FOXO3 silencing could inhibit mitophagy and mitochondrial dysfunction induced by MnCl₂.

CONCLUSIONS

Our results indicated that Mn could induce mitophagy by enhancing FOXO3 nuclear retention, which might promote mitophagy induced by MnCl₂.

Identification of Miro as a mitochondrial receptor for myosin XIX

Mitochondrial distribution in cells is critical for cellular function and proper inheritance during cell division. In mammalian cells, mitochondria are transported predominantly along microtubules by kinesin and dynein motors that bind indirectly via TRAK1/2 to outer mitochondrial membrane proteins Miro1/2. Here, using proximity labeling, we identified Miro1/2 as potential binding partners of myosin XIX (Myo19). Interaction studies show that Miro1 binds directly to a C-terminal fragment of the Myo19 tail region and that Miro recruits the Myo19 tail in vivo. This recruitment is regulated by the nucleotide-state of the N-terminal Rho-like GTPase domain of Miro. Notably, Myo19 protein stability in cells depends on its association with Miro. Downregulation of Miro or overexpression of the adapter proteins TRAK1 and TRAK2 caused a reduction in Myo19 protein levels. Finally, Myo19 regulates the subcellular distribution of mitochondria. Downregulation, as well as overexpression, of Myo19 induces perinuclear collapse of mitochondria, phenocopying the loss of kinesin KIF5, dynein or their mitochondrial receptor Miro. These results suggest that Miro coordinates microtubule- and actin-based mitochondrial movement.

Dyslipidemia impairs mitochondrial trafficking and function in sensory neurons

Mitochondrial trafficking plays a central role in dorsal root ganglion (DRG) neuronal cell survival and neurotransmission by transporting mitochondria from the neuronal cell body throughout the bundles of DRG axons. In type 2 diabetes (T2DM), dyslipidemia and hyperglycemia damage DRG neurons and induce mitochondrial dysfunction; however, the impact of free fatty acids and glucose on mitochondrial trafficking in DRG neurons remains unknown. To evaluate the impact of free fatty acids compared to hyperglycemia on mitochondrial transport, primary adult mouse DRG neuron cultures were treated with physiologic concentrations of palmitate and glucose and assessed for alterations in mitochondrial trafficking, mitochondrial membrane potential, and mitochondrial bioenergetics. Palmitate treatment significantly reduced the number of motile mitochondria in DRG axons, but physiologic concentrations of glucose did not impair mitochondrial trafficking dynamics. Palmitate-treated DRG neurons also exhibited a reduction in mitochondrial velocity, and impaired mitochondrial trafficking correlated with mitochondrial depolarization in palmitate-treated DRG neurons. Finally, we found differential bioenergetic effects of palmitate and glucose on resting and energetically challenged mitochondria in DRG neurons. Together, these results suggest that palmitate induces DRG neuron mitochondrial depolarization, inhibiting axonal mitochondrial trafficking and altering mitochondrial bioenergetic capacity.-Rumora, A. E., Lentz, S. I., Hinder, L. M., Jackson, S. W., Valesano, A., Levinson, G. E., Feldman, E. L. Dyslipidemia impairs mitochondrial trafficking and function in sensory neurons.

Inhibition of Miro1 disturbs mitophagy and pancreatic β-cell function interfering insulin release via IRS-Akt-Foxo1 in diabetes

Mitochondrial function is essential to meet metabolic demand of pancreatic beta cells respond to high nutrient stress. Mitophagy is an essential component to normal pancreatic β-cell function and has been associated with β-cell failure in Type 2 diabetes (T2D). Our previous studies have indicated that mitochondrial Rho (Miro) GTPase-mediated mitochondrial dysfunction under high nutrient stress leads to NOD-like receptor 3 (NLRP3)-dependent proinflammatory responses and subsequent insulin resistance. However, the in vivo mechanism by which Miro1 underlies mitophagy has not been identified. Here we show firstly that the expression of Miro is reduced in human T2D and mouse db/db islets and in INS-1 cell line exposed to high glucose and palmitate. β-cell specific ablation of Miro1 (Miro1f/f: Rip-cre mice, or (IKO) under high nutrient stress promotes the development of hyperglycemia. β-cells from IKO mice display an inhibition of mitophagy under oxidative stress and induces mitochondrial dysfunction. Dysfunctional mitophagy in IKO mice is represented by damaged islet beta cell mitochondrial and secretory capacity, unbalanced downstream MKK-JNK signalling without affecting the levels of MEK, ERK or p38 activation and subsequently, impaired insulin secretion signaling via inhibition IRS-AKT-Foxo1 pathway, leading to worsening glucose tolerance in these mice. Thus, these data suggest that Miro1 may be responsible for mitophagy deficiency and β-cell dysfunction in T2D and that strategies target Miro1 in vivo may provide a therapeutic target to enhance β-cell mitochondrial quality and insulin secretion to ameliorate complications associated with T2D.

An emerging role for mitochondrial dynamics in schizophrenia

Abnormal brain development has long been thought to contribute to the pathophysiology of schizophrenia. Impaired dendritic arborization, synaptogenesis, and long term potentiation and memory have been demonstrated in animal models of schizophrenia. In addition to aberrant nervous system development, altered brain metabolism and mitochondrial function has long been observed in schizophrenic patients. Single nucleotide polymorphisms in the mitochondrial genome as well as impaired mitochondrial function have both been associated with increased risk for developing schizophrenia. Mitochondrial function in neurons is highly dependent on fission, fusion, and transport of the organelle, collectively referred to as mitochondrial dynamics. Indeed, there is mounting evidence that mitochondrial dynamics strongly influences neuron development and synaptic transmission. While there are a few studies describing altered mitochondrial shape in schizophrenic patients, as well as in animal and in vitro models of schizophrenia, the precise role of mitochondrial dynamics in the pathophysiology of schizophrenia is all but unexplored. Here we discuss the influence of mitochondrial dynamics and mitochondrial function on nervous system development, and highlight recent work suggesting a link between aberrant mitochondrial dynamics and schizophrenia.

Regulation of mitochondrial dynamics by DISC1, a putative risk factor for major mental illness

Mitochondria are dynamic organelles that are essential to power the process of neurotransmission. Neurons must therefore ensure that mitochondria maintain their functional integrity and are efficiently transported along the full extent of the axons and dendrites, from soma to synapses. Mitochondrial dynamics (trafficking, fission and fusion) co-ordinately regulate mitochondrial quality control and function. DISC1 is a component of the mitochondrial transport machinery and regulates mitochondrial dynamics. DISC1's role in this is adversely affected by sequence variants connected to brain structure/function and disease risk, and by mutant truncation. The DISC1 interactors NDE1 and GSK3β are also involved, indicating a convergence of putative risk factors for psychiatric illness upon mitochondrial dynamics.

Axonal transport deficits in multiple sclerosis: spiraling into the abyss

The transport of mitochondria and other cellular components along the axonal microtubule cytoskeleton plays an essential role in neuronal survival. Defects in this system have been linked to a large number of neurological disorders. In multiple sclerosis (MS) and associated models such as experimental autoimmune encephalomyelitis (EAE), alterations in axonal transport have been shown to exist before neurodegeneration occurs. Genome-wide association (GWA) studies have linked several motor proteins to MS susceptibility, while neuropathological studies have shown accumulations of proteins and organelles suggestive for transport deficits. A reduced effectiveness of axonal transport can lead to neurodegeneration through inhibition of mitochondrial motility, disruption of axoglial interaction or prevention of remyelination. In MS, demyelination leads to dysregulation of axonal transport, aggravated by the effects of TNF-alpha, nitric oxide and glutamate on the cytoskeleton. The combined effect of all these pathways is a vicious cycle in which a defective axonal transport system leads to an increase in ATP consumption through loss of membrane organization and a reduction in available ATP through inhibition of mitochondrial transport, resulting in even further inhibition of transport. The persistent activity of this positive feedback loop contributes to neurodegeneration in MS.

Mitochondrial fission is an acute and adaptive response in injured motor neurons

Successful recovery from neuronal damage requires a huge energy supply, which is provided by mitochondria. However, the physiological relevance of mitochondrial dynamics in damaged neurons in vivo is poorly understood. To address this issue, we established unique bacterial artificial chromosome transgenic (BAC Tg) mice, which develop and function normally, but in which neuronal injury induces labelling of mitochondria with green fluorescent protein (GFP) and expression of cre recombinase. GFP-labelled mitochondria in BAC Tg mice appear shorter in regenerating motor axons soon after nerve injury compared with mitochondria in non-injured axons, suggesting the importance of increased mitochondrial fission during the early phase of nerve regeneration. Crossing the BAC Tg mice with mice carrying a floxed dynamin-related protein 1 gene (Drp1), which is necessary for mitochondrial fission, ablates mitochondrial fission specifically in injured neurons. Injury-induced Drp1-deficient motor neurons show elongated or abnormally gigantic mitochondria, which have impaired membrane potential and axonal transport velocity during the early phase after injury, and eventually promote neuronal death. Our in vivo data suggest that acute and prominent mitochondrial fission during the early stage after nerve injury is an adaptive response and is involved in the maintenance of mitochondrial and neuronal integrity to prevent neurodegeneration.

The entangled ER-mitochondrial axis as a potential therapeutic strategy in neurodegeneration: A tangled duo unchained

Endoplasmic reticulum (ER) and mitochondrial function have both been shown to be critical events in neurodegenerative diseases. The ER mediates protein folding, maturation, sorting as well acts as calcium storage. The unfolded protein response (UPR) is a stress response of the ER that is activated by the accumulation of misfolded proteins within the ER lumen. Although the molecular mechanisms underlying ER stress-induced apoptosis are not completely understood, increasing evidence suggests that ER and mitochondria cooperate to signal cell death. Similarly, calcium-mediated mitochondrial function and dynamics not only contribute to ATP generation and calcium buffering but are also a linchpin in mediating cell fate. Mitochondria and ER form structural and functional networks (mitochondria-associated ER membranes [MAMs]) essential to maintaining cellular homeostasis and determining cell fate under various pathophysiological conditions. Regulated Ca(2+) transfer from the ER to the mitochondria is important in maintaining control of pro-survival/pro-death pathways. In this review, we summarize the latest therapeutic strategies that target these essential organelles in the context of neurodegenerative diseases.

Mitochondrial dynamics in astrocytes

Astrocytes exhibit cellular excitability through variations in their intracellular calcium (Ca²⁺) levels in response to synaptic activity. Astrocyte Ca²⁺ elevations can trigger the release of neuroactive substances that can modulate synaptic transmission and plasticity, hence promoting bidirectional communication with neurons. Intracellular Ca²⁺ dynamics can be regulated by several proteins located in the plasma membrane, within the cytosol and by intracellular organelles such as mitochondria. Spatial dynamics and strategic positioning of mitochondria are important for matching local energy provision and Ca²⁺ buffering requirements to the demands of neuronal signalling. Although relatively unresolved in astrocytes, further understanding the role of mitochondria in astrocytes may reveal more about the complex bidirectional relationship between astrocytes and neurons in health and disease. In the present review, we discuss some recent insights regarding mitochondrial function, transport and turnover in astrocytes and highlight some important questions that remain to be answered.

Matrix as an interstitial transport system

The extracellular matrix (ECM) is best known for its function as a structural scaffold for the tissue and more recently as a microenvironment to sequester growth factors and cytokines allowing for rapid and localized changes in their activity in the absence of new protein synthesis. In this review, we explore this and additional new aspects of ECM function in mediating cell-to-cell communications. Fibrillar and nonfibrillar components of ECM can limit and facilitate the transport of molecules through the extracellular space while also regulating interstitial hydrostatic pressure. In turn, transmembrane communications via molecules, such as ECM metalloproteinase inducer, thrombospondins, and integrins, can further mediate cell response to extracellular cues and affect ECM composition and tissue remodeling. Other means of cell-to-cell communication include extracellular microRNA transport and its contribution to gene expression in target cells and the nanotube formation between distant cells, which has recently emerged as a novel conduit for intercellular organelle sharing thereby influencing cell survival and function. The information summarized and discussed here are not limited to the cardiovascular ECM but encompass ECM in general with specific references to the cardiovascular system.

Miro-1 links mitochondria and microtubule Dynein motors to control lymphocyte migration and polarity

The recruitment of leukocytes to sites of inflammation is crucial for a functional immune response. In the present work, we explored the role of mitochondria in lymphocyte adhesion, polarity, and migration. We show that during adhesion to the activated endothelium under physiological flow conditions, lymphocyte mitochondria redistribute to the adhesion zone together with the microtubule-organizing center (MTOC) in an integrin-dependent manner. Mitochondrial redistribution and efficient lymphocyte adhesion to the endothelium require the function of Miro-1, an adaptor molecule that couples mitochondria to microtubules. Our data demonstrate that Miro-1 associates with the dynein complex. Moreover, mitochondria accumulate around the MTOC in response to the chemokine CXCL12/SDF-1α; this redistribution is regulated by Miro-1. CXCL12-dependent cell polarization and migration are reduced in Miro-1-silenced cells, due to impaired myosin II activation at the cell uropod and diminished actin polymerization. These data point to a key role of Miro-1 in the control of lymphocyte adhesion and migration through the regulation of mitochondrial redistribution.

Mitochondrial fission: regulation and ER connection

Fission and fusion of mitochondrial tubules are the main processes determining mitochondrial shape and size in cells. As more evidence is found for the involvement of mitochondrial morphology in human pathology, it is important to elucidate the mechanisms of mitochondrial fission and fusion. Mitochondrial morphology is highly sensitive to changing environmental conditions, indicating the involvement of cellular signaling pathways. In addition, the well-established structural connection between the endoplasmic reticulum (ER) and mitochondria has recently been found to play a role in mitochondrial fission. This minireview describes the latest advancements in understanding the regulatory mechanisms controlling mitochondrial morphology, as well as the ER-mediated structural maintenance of mitochondria, with a specific emphasis on mitochondrial fission.

Kinesin-1-syntaphilin coupling mediates activity-dependent regulation of axonal mitochondrial transport

Syntaphilin mediates the activity-dependent immobilization of axonal mitochondria by physically displacing KIF5 from the Miro–Trak transport complex.

Axonal mitochondria are recruited to synaptic terminals in response to neuronal activity, but the mechanisms underlying activity-dependent regulation of mitochondrial transport are largely unknown. In this paper, using genetic mouse model combined with live imaging, we demonstrate that syntaphilin (SNPH) mediates the activity-dependent immobilization of axonal mitochondria through binding to KIF5. In vitro analysis showed that the KIF5–SNPH coupling inhibited the motor adenosine triphosphatase. Neuronal activity further recruited SNPH to axonal mitochondria. This motor-docking interplay was induced by Ca²⁺ and synaptic activity and was necessary to establish an appropriate balance between motile and stationary axonal mitochondria. Deleting snph abolished the activity-dependent immobilization of axonal mitochondria. We propose an “Engine-Switch and Brake” model, in which SNPH acts both as an engine off switch by sensing mitochondrial Rho guanosine triphosphatase-Ca²⁺ and as a brake by anchoring mitochondria to the microtubule track. Altogether, our study provides new mechanistic insight into the molecular interplay between motor and docking proteins, which arrests axonal mitochondrial transport in response to changes in neuronal activity.

Drosophila as a model to study mitochondrial dysfunction in Parkinson's disease

Identification of single gene mutations that lead to inherited forms of Parkinson's disease (PD) has provided strong impetus for the use of animal models to study normal functions of these "PD genes" and the cellular defects that occur in the presence of pathogenic PD mutations. Drosophila has emerged as an effective model in PD-related gene studies. Important insights into the cellular basis of PD pathogenesis include the demonstration that two PD genes, PINK1 and parkin, function in a common pathway, with PINK1 positively regulating parkin, to control mitochondrial integrity and maintenance. This is accomplished through regulation of mitochondrial fission/fusion dynamics. Subsequent observations in both fly and mammalian systems showed that these proteins are important for sensing mitochondrial damage and recruiting damaged mitochondria to the quality-control machinery for subsequent removal. Here, I begin by reviewing the opportunities and challenges to understanding PD pathogenesis and developing new therapies. I then review the unique tools and technologies available in Drosophila for studying PD genes. Subsequently, I review lessons that we have learned from studies in Drosophila, emphasizing the PINK1/parkin pathway, as well as studies of DJ-1 and Omi/HtrA2, two additional genes associated with PD implicated in regulation of mitochondrial function. I end by discussing how Drosophila can be used to further probe the functions of PINK1 and parkin, and the regulation of mitochondrial quality more generally. In additional to PD, defects in mitochondrial function are associated with normal aging and with many diseases of aging. Thus, insights gained from the studies of mitochondrial dynamics and quality control in Drosophila are likely to be of general significance.

Synaptic mitochondrial pathology in Alzheimer's disease

SIGNIFICANCE

Synaptic degeneration, an early pathological feature in Alzheimer's disease (AD), is closely correlated to impaired cognitive function and memory loss. Recent studies suggest that involvement of amyloid-beta peptide (Aβ) in synaptic mitochondrial alteration underlies these synaptic lesions. Thus, to understand the Aβ-associated synaptic mitochondrial perturbations would fortify our understanding of synaptic stress in the pathogenesis of AD.

RECENT ADVANCES

Increasing evidence suggests that synaptic mitochondrial dysfunction is strongly associated with synaptic failure in many neurodegenerative diseases including AD. Based on recent findings in human AD subjects, AD animal models, and AD cellular models, synaptic mitochondria undergo multiple malfunctions including Aβ accumulation, increased oxidative stress, decreased respiration, and compromised calcium handling capacity, all of which occur earlier than changes seen in nonsynaptic mitochondria before predominant AD pathology. Of note, the impact of Aβ on mitochondrial motility and dynamics exacerbates synaptic mitochondrial alterations.

CRITICAL ISSUES

Synaptic mitochondria demonstrate early deficits in AD; in combination with the role that synaptic mitochondria play in sustaining synaptic functions, deficits in synaptic mitochondria may be a key factor involved in an early synaptic pathology in AD.

FUTURE DIRECTIONS

The importance of synaptic mitochondria in supporting synapses and the high vulnerability of synaptic mitochondria to Aβ make them a promising target of new therapeutic strategy for AD.

The Eutherian Armcx genes regulate mitochondrial trafficking in neurons and interact with Miro and Trak2

Brain function requires neuronal activity-dependent energy consumption. Neuronal energy supply is controlled by molecular mechanisms that regulate mitochondrial dynamics, including Kinesin motors and Mitofusins, Miro1-2 and Trak2 proteins. Here we show a new protein family that localizes to the mitochondria and controls mitochondrial dynamics. This family of proteins is encoded by an array of armadillo (Arm) repeat-containing genes located on the X chromosome. The Armcx cluster is unique to Eutherian mammals and evolved from a single ancestor gene (Armc10). We show that these genes are highly expressed in the developing and adult nervous system. Furthermore, we demonstrate that Armcx3 expression levels regulate mitochondrial dynamics and trafficking in neurons, and that Alex3 interacts with the Kinesin/Miro/Trak2 complex in a Ca(2+)-dependent manner. Our data provide evidence of a new Eutherian-specific family of mitochondrial proteins that controls mitochondrial dynamics and indicate that this key process is differentially regulated in the brain of higher vertebrates.

Mitochondrial transport in neurons: impact on synaptic homeostasis and neurodegeneration

Mitochondria have a number of essential roles in neuronal function. Their complex mobility patterns within neurons are characterized by frequent changes in direction. Mobile mitochondria can become stationary or pause in regions that have a high metabolic demand and can move again rapidly in response to physiological changes. Defects in mitochondrial transport are implicated in the pathogenesis of several major neurological disorders. Research into the mechanisms that regulate mitochondrial transport is thus an important emerging frontier.

Regulation of axonal mitochondrial transport and its impact on synaptic transmission

Mitochondria are essential organelles for neuronal survival and play important roles in ATP generation, calcium buffering, and apoptotic signaling. Due to their extreme polarity, neurons utilize specialized mechanisms to regulate mitochondrial transport and retention along axons and near synaptic terminals where energy supply and calcium homeostasis are in high demand. Axonal mitochondria undergo saltatory and bidirectional movement and display complex mobility patterns. In cultured neurons, approximately one-third of axonal mitochondria are mobile, while the rest remain stationary. Stationary mitochondria at synapses serve as local energy stations that produce ATP to support synaptic function. In addition, axonal mitochondria maintain local Ca²+ homeostasis at presynaptic boutons. The balance between mobile and stationary mitochondria is dynamic and responds quickly to changes in axonal and synaptic physiology. The coordination of mitochondrial mobility and synaptic activity is crucial for neuronal function synaptic plasticity. In this update article, we introduce recent advances in our understanding of the motor-adaptor complexes and docking machinery that mediate mitochondrial transport and axonal distribution. We will also discuss the molecular mechanisms underlying the complex mobility patterns of axonal mitochondria and how mitochondrial mobility impacts the physiology and function of synapses.

Matching mitochondria to metabolic needs at nodes of Ranvier

Myelinated axons conduct nerve impulses at high speed using a unique mode of excitation, referred to as saltatory conduction, which is enabled structurally by the narrowing of the site of action potentials to a tiny gap in the axon called the node of Ranvier. With this structural specialization comes an interesting metabolic matching problem. How do mitochondria find and supply energy to these tiny nodes of Ranvier distributed sparsely along a myelinated axon? Does the intense Na(+) influx at the node, which is produced by the highest known sodium channel density in all excitable membranes, help guide where mitochondria stop? Evidence suggests that during excitation in the peripheral nervous system, Na(+) influx recruits mitochondria to the node by triggering Ca(2+) elevation and activating Na(+) pumps. Intriguingly, indirect evidence suggests that in the central nervous system, activity recruits mitochondria to the internode (myelin-covered portion of the axon). Metabolic dysfunction thus might produce spatially distinct lesions in PNS and CNS myelinated fibers. Future dissection of regional variation in mitochondrial biology in myelinated axons using live imaging will likely yield surprises about sites of vulnerability in demyelinating diseases and clues for therapeutic intervention strategy.

Increased axonal mitochondrial mobility does not slow amyotrophic lateral sclerosis (ALS)-like disease in mutant SOD1 mice

Reduced axonal mitochondrial transport has been observed in major neurodegenerative diseases, including fALS patients and SOD1(G93A) mice. However, it is unclear whether this defect plays a critical role in axonal degeneration or simply reflects sequelae of general transport alteration. Using genetic mouse models combined with time-lapse imaging of live neurons, we previously discovered that axon-targeted syntaphilin (SNPH) acts as a docking receptor specific for axonal mitochondria. Deletion of the snph gene in mice results in a substantially higher proportion of axonal mitochondria in the mobile state without any effect on the transport of other axonal organelles. Here we address whether increased (rescued) axonal mitochondrial mobility changes the disease course by crossing fALS-linked transgenic SOD1(G93A) and snph(-/-) knock-out mice. We found that a 2-fold increase in axonal mitochondrial mobility in SOD1(G93A)/snph(-/-) mice did not affect the onset of ALS-like symptoms. Both SOD1(G93A) and SOD1(G93A)/snph(-/-) mice exhibit similar weight loss, deterioration in motor function and motor neuron loss, significant gliosis, and a lifespan of 152-154 days. Thus, for the first time, our study provides genetic and pathological evidence that the impairment of mitochondrial transport seen in SOD1(G93A) mice plays a minimal role in the rapid-onset of fALS-linked pathology.

Activity-dependent regulation of mitochondrial motility by calcium and Na/K-ATPase at nodes of Ranvier of myelinated nerves

The node of Ranvier is a tiny segment of a myelinated fiber with various types of specializations adapted for generation of high-speed nerve impulses. It is ionically specialized with respect to ion channel segregation and ionic fluxes, and metabolically specialized in ionic pump expression and mitochondrial density augmentation. This report examines the interplay of three important parameters (calcium fluxes, Na pumps, mitochondrial motility) at nodes of Ranvier in frog during normal nerve activity. First, we used calcium dyes to resolve a highly localized elevation in axonal calcium at a node of Ranvier during action potentials, and showed that this calcium elevation retards mitochondrial motility during nerve impulses. Second, we found, surprisingly, that physiologic activation of the Na pumps retards mitochondrial motility. Blocking Na pumps alone greatly prevents action potentials from retarding mitochondrial motility, which reveals that mitochondrial motility is coupled to Na/K-ATPase. In conclusion, we suggest that during normal nerve activity, Ca elevation and activation of Na/K-ATPase act, possibly in a synergistic manner, to recruit mitochondria to a node of Ranvier to match metabolic needs.

A tail-anchored myotonic dystrophy protein kinase isoform induces perinuclear clustering of mitochondria, autophagy, and apoptosis

Background

Studies on the myotonic dystrophy protein kinase (DMPK) gene and gene products have thus far mainly concentrated on the fate of length mutation in the (CTG)n repeat at the DNA level and consequences of repeat expansion at the RNA level in DM1 patients and disease models. Surprisingly little is known about the function of DMPK protein products.

Methodology/Principal Findings

We demonstrate here that transient expression of one major protein product of the human gene, the hDMPK A isoform with a long tail anchor, results in mitochondrial fragmentation and clustering in the perinuclear region. Clustering occurred in a variety of cell types and was enhanced by an intact tubulin cytoskeleton. In addition to morphomechanical changes, hDMPK A expression induces physiological changes like loss of mitochondrial membrane potential, increased autophagy activity, and leakage of cytochrome c from the mitochondrial intermembrane space accompanied by apoptosis. Truncation analysis using YFP-hDMPK A fusion constructs revealed that the protein's tail domain was necessary and sufficient to evoke mitochondrial clustering behavior.

Conclusion/Significance

Our data suggest that the expression level of the DMPK A isoform needs to be tightly controlled in cells where the hDMPK gene is expressed. We speculate that aberrant splice isoform expression might be a codetermining factor in manifestation of specific DM1 features in patients.

KIF5B motor adaptor syntabulin maintains synaptic transmission in sympathetic neurons

Newly synthesized synaptic proteins and mitochondria are transported along lengthy neuronal processes to assist in the proper assembly of developing synapses and activity-dependent remodeling of mature synapses. Neuronal transport is mediated by motor proteins that associate with their cargoes via adaptors and travel along the cytoskeleton within neuronal processes. Our previous studies in developing hippocampal neurons revealed that syntabulin acts as a KIF5B motor adaptor and mediates anterograde transport of presynaptic cargoes and mitochondria, presynaptic assembly, and activity-induced plasticity. Here, using cultured superior cervical ganglion neurons combined with manipulation of syntabulin expression or interference with its interaction with KIF5B, we uncover a crucial role for syntabulin in the maintenance of presynaptic function. Syntabulin loss-of-function delayed the appearance of synaptic activity in developing neurons and impaired synaptic transmission in mature neurons, including reduced basal activity, accelerated synaptic depression under high-frequency firing, slowed recovery rates after synaptic vesicle depletion, and impaired presynaptic short-term plasticity. These defects correlated with reduced mitochondrial distribution along neuronal processes and were rescued by the application of ATP within presynaptic neurons. These results suggest that syntabulin supports the axonal transport of mitochondria and concomitant ATP production at presynaptic terminals. ATP supply from locally stationed mitochondria is in turn necessary for the efficient mobilization of synaptic vesicles into the readily releasable pool. These findings emphasize the critical role of KIF5B-syntabulin-mediated axonal transport in the maintenance of presynaptic function and regulation of synaptic plasticity.

Ca2+-dependent regulation of mitochondrial dynamics by the Miro-Milton complex

Calcium oscillations control mitochondrial motility along the microtubules and in turn, support on-demand distribution of mitochondria. However, the mechanism mediating the Ca(2+) effect remained a mystery. Recently, several papers reported on the Ca(2+)-dependent regulation of mitochondrial dynamics by a Miro-Milton complex linking mitochondria to kinesin motors. Both mitochondrial motility and fusion-fission dynamics seem to be sensitive to a Ca(2+)-dependent switch by this complex. Evidence is emerging that calcium signaling through Miro-Milton is central to coordination of the local oxidative metabolism with the energy demands and protection against Ca(2+)-induced cell injury.

Mitochondrial transport and docking in axons

Proper transport and distribution of mitochondria in axons and at synapses are critical for the normal physiology of neurons. Mitochondria in axons display distinct motility patterns and undergo saltatory and bidirectional movement, where mitochondria frequently stop, start moving again, and change direction. While approximately one-third of axonal mitochondria are mobile in mature neurons, a large proportion remains stationary. Their net movement is significantly influenced by recruitment to stationary or motile states. In response to the diverse physiological states of axons and synapses, the mitochondrial balance between motile and stationary phases is a possible target of regulation by intracellular signals and synaptic activity. Efficient control of mitochondrial retention (docking) at particular stations, where energy production and calcium homeostasis capacity are highly demanded, is likely essential for neuronal development and function. In this review, we introduce the molecular and cellular mechanisms underlying the complex mobility patterns of axonal mitochondria and discuss how motor adaptor complexes and docking machinery contribute to mitochondrial transport and distribution in axons and at synapses. In addition, we briefly discuss the physiological evidence how axonal mitochondrial mobility impacts synaptic function.