GRIF-1 and OIP106, members of a novel gene family of coiled-coil domain proteins: association in vivo and in vitro with kinesin

The myriad roles of Miro in the nervous system: axonal transport of mitochondria and beyond

Mitochondrial rho GTPase (Miro) is a mitochondrial outer membrane protein containing two GTPase domains and two helix-loop-helix Ca²⁺-binding domains called EF hands. Pioneering genetic studies in Drosophila first revealed a key function of Miro in regulating the axonal transport of mitochondria, during which Miro forms a multi-protein transport complex with Milton and Kinesin heavy chain (KHC) to link trafficking mitochondria with the microtubule (MT) cytoskeleton. Recent studies showed that through binding to the EF hands of Miro and causing conformational changes of Miro and alteration of protein-protein interactions within the transport complex, Ca²⁺ can alter the engagement of mitochondria with the MT/kinesin network, offering one mechanism to match mitochondrial distribution with neuronal activity. Despite the importance of the Miro/Milton/Kinesin complex in regulating mitochondrial transport in metazoans, not all components of the transport complex are conserved in lower organisms, and transport-independent functions of Miro are emerging. Here we review the diverse functions of the evolutionarily conserved Miro proteins that are relevant to the development, maintenance, and functioning of the nervous system and discuss the potential contribution of Miro dysfunction to the pathogenesis of diseases of the nervous system.

Mitochondrial trafficking in neurons and the role of the Miro family of GTPase proteins

Correct mitochondrial dynamics are essential to neuronal function. These dynamics include mitochondrial trafficking and quality-control systems that maintain a precisely distributed and healthy mitochondrial network, so that local energy demands or Ca2+-buffering requirements within the intricate architecture of the neuron can be met. Mitochondria make use of molecular machinery that couples these organelles to microtubule-based transport via kinesin and dynein motors, facilitating the required long-range movements. These motors in turn are associated with a variety of adaptor proteins allowing additional regulation of the complex dynamics demonstrated by these organelles. Over recent years, a number of new motor and adaptor proteins have been added to a growing list of components implicated in mitochondrial trafficking and distribution. Yet, there are major questions that remain to be addressed about the regulation of mitochondrial transport complexes. One of the core components of this machinery, the mitochondrial Rho GTPases Miro1 (mitochondrial Rho 1) and Miro2 have received special attention due to their Ca2+-sensing and GTPase abilities, marking Miro an exceptional candidate for co-ordinating mitochondrial dynamics and intracellular signalling pathways. In the present paper, we discuss the wealth of literature regarding Miro-mediated mitochondrial transport in neurons and recently highlighted involvement of Miro proteins in mitochondrial turnover, emerging as a key process affected in neurodegeneration.

Loss of Miro1-directed mitochondrial movement results in a novel murine model for neuron disease

Defective mitochondrial distribution in neurons is proposed to cause ATP depletion and calcium-buffering deficiencies that compromise cell function. However, it is unclear whether aberrant mitochondrial motility and distribution alone are sufficient to cause neurological disease. Calcium-binding mitochondrial Rho (Miro) GTPases attach mitochondria to motor proteins for anterograde and retrograde transport in neurons. Using two new KO mouse models, we demonstrate that Miro1 is essential for development of cranial motor nuclei required for respiratory control and maintenance of upper motor neurons required for ambulation. Neuron-specific loss of Miro1 causes depletion of mitochondria from corticospinal tract axons and progressive neurological deficits mirroring human upper motor neuron disease. Although Miro1-deficient neurons exhibit defects in retrograde axonal mitochondrial transport, mitochondrial respiratory function continues. Moreover, Miro1 is not essential for calcium-mediated inhibition of mitochondrial movement or mitochondrial calcium buffering. Our findings indicate that defects in mitochondrial motility and distribution are sufficient to cause neurological disease.

The mitochondrial Ras-related GTPase Miro: views from inside and outside the metazoan kingdom

Miro GTPase, a member of the Ras superfamily, consists of two GTPase domains flanking a pair of EF hand motifs and a C-terminal transmembrane domain that anchors the protein to the mitochondrial outer membrane. Since the identification of Miro in humans, a series of studies in metazoans, including mammals and fruit flies, have shown that Miro plays a role in the calcium-dependent regulation of mitochondrial transport along microtubules. However, in non-metazoans, including yeasts, slime molds, and plants, Miro is primarily involved in the maintenance of mitochondrial morphology and homeostasis. Given the high level of conservation of Miro in eukaryotes and the variation in the molecular mechanisms of mitochondrial transport between eukaryotic lineages, Miro may have a common ancestral function in mitochondria, and its roles in the regulation of mitochondrial transport may have been acquired specifically by metazoans after the evolutionary divergence of eukaryotes.

Evidence that dendritic mitochondria negatively regulate dendritic branching in pyramidal neurons in the neocortex

The precise branching patterns of dendritic arbors have a profound impact on information processing in individual neurons and the brain. These patterns are established by positive and negative regulation of the dendritic branching. Although the mechanisms for positive regulation have been extensively investigated, little is known about those for negative regulation. Here, we present evidence that mitochondria located in developing dendrites are involved in the negative regulation of dendritic branching. We visualized mitochondria in pyramidal neurons of the mouse neocortex during dendritic morphogenesis using in utero electroporation of a mitochondria-targeted fluorescent construct. We altered the mitochondrial distribution in vivo by overexpressing Mfn1, a mitochondrial shaping protein, or the Miro-binding domain of TRAK2 (TRAK2-MBD), a truncated form of a motor-adaptor protein. We found that dendritic mitochondria were preferentially targeted to the proximal portion of dendrites only during dendritic morphogenesis. Overexpression of Mfn1 or TRAK2-MBD depleted mitochondria from the dendrites, an effect that was accompanied by increased branching of the proximal portion of the dendrites. This dendritic abnormality cannot be accounted for by changes in the distribution of membrane trafficking organelles since the overexpression of Mfn1 did not alter the distributions of the endoplasmic reticulum, Golgi, or endosomes. Additionally, neither did these constructs impair neuronal viability or mitochondrial function. Therefore, our results suggest that dendritic mitochondria play a critical role in the establishment of the precise branching pattern of dendritic arbors by negatively affecting dendritic branching.

Lysine 27 ubiquitination of the mitochondrial transport protein Miro is dependent on serine 65 of the Parkin ubiquitin ligase

Mitochondrial transport plays an important role in matching mitochondrial distribution to localized energy production and calcium buffering requirements. Here, we demonstrate that Miro1, an outer mitochondrial membrane (OMM) protein crucial for the regulation of mitochondrial trafficking and distribution, is a substrate of the PINK1/Parkin mitochondrial quality control system in human dopaminergic neuroblastoma cells. Moreover, Miro1 turnover on damaged mitochondria is altered in Parkinson disease (PD) patient-derived fibroblasts containing a pathogenic mutation in the PARK2 gene (encoding Parkin). By analyzing the kinetics of Miro1 ubiquitination, we further demonstrate that mitochondrial damage triggers rapid (within minutes) and persistent Lys-27-type ubiquitination of Miro1 on the OMM, dependent on PINK1 and Parkin. Proteasomal degradation of Miro1 is then seen on a slower time scale, within 2-3 h of the onset of ubiquitination. We find Miro ubiquitination in dopaminergic neuroblastoma cells is independent of Miro1 phosphorylation at Ser-156 but is dependent on the recently identified Ser-65 residue within Parkin that is phosphorylated by PINK1. Interestingly, we find that Miro1 can stabilize phospho-mutant versions of Parkin on the OMM, suggesting that Miro is also part of a Parkin receptor complex. Moreover, we demonstrate that Ser-65 in Parkin is critical for regulating Miro levels upon mitochondrial damage in rodent cortical neurons. Our results provide new insights into the ubiquitination-dependent regulation of the Miro-mediated mitochondrial transport machinery by PINK1/Parkin and also suggest that disruption of this regulation may be implicated in Parkinson disease pathogenesis.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.

DISC1 complexes with TRAK1 and Miro1 to modulate anterograde axonal mitochondrial trafficking

Disrupted-In-Schizophrenia 1 (DISC1) is a candidate risk factor for schizophrenia, bipolar disorder and severe recurrent depression. Here, we demonstrate that DISC1 associates robustly with trafficking-protein-Kinesin-binding-1 which is, in turn, known to interact with the outer mitochondrial membrane proteins Miro1/2, linking mitochondria to the kinesin motor for microtubule-based subcellular trafficking. DISC1 also associates with Miro1 and is thus a component of functional mitochondrial transport complexes. Consistent with these observations, in neuronal axons DISC1 promotes specifically anterograde mitochondrial transport. DISC1 thus participates directly in mitochondrial trafficking, which is essential for neural development and neurotransmission. Any factor affecting mitochondrial DISC1 function is hence likely to have deleterious consequences for the brain, potentially contributing to increased risk of psychiatric illness. Intriguingly, therefore, a rare putatively causal human DISC1 sequence variant, 37W, impairs the ability of DISC1 to promote anterograde mitochondrial transport. This is likely related to a number of mitochondrial abnormalities induced by expression of DISC1-37W, which redistributes mitochondrial DISC1 and enhances kinesin mitochondrial association, while also altering protein interactions within the mitochondrial transport complex.

Delineation of the TRAK binding regions of the kinesin-1 motor proteins

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TRAK2, a kinesin adaptor protein, binds the cargo binding domain of the kinesin-1 motor, KIF5A.

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Three KIF5A regions were found to contribute to the TRAK2 binding site.

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KIF5A discriminates between TRAK1 and TRAK2 with respect to binding specificity.

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These data yield insights into kinesin/kinesin adaptor protein interactions.

Understanding specific cargo distribution in differentiated cells is a major challenge. Trafficking kinesin proteins (TRAKs) are kinesin adaptors. They bind the cargo binding domain of kinesin-1 motor proteins forming a link between the motor and their cargoes. To refine the TRAK1/2 binding sites within the kinesin-1 cargo domain, rationally designed C-terminal truncations of KIF5A and KIF5C were generated and their co-association with TRAK1/2 determined by quantitative co-immunoprecipitations following co-expression in mammalian cells. Three contributory regions forming the TRAK2 binding site within KIF5A and KIF5C cargo binding domains were delineated. Differences were found between TRAK1/2 with respect to association with KIF5A.

TRAK2physically interacts with KIF5C by anti tag coimmunoprecipitation (View interaction)

TRAK1physically interacts with KIF5A by anti tag coimmunoprecipitation (View interaction)

TRAK2physically interacts with KIF5A by anti tag coimmunoprecipitation (1, 2)

Revisiting the TRAK family of proteins as mediators of GABAA receptor trafficking

γ-Aminobutyric acid type A (GABAA) receptor interacting factor-1 (GRIF-1) was originally discovered as a result of studies aiming to find the elusive GABAA receptor clustering protein. It was identified as a GABAA receptor associated protein by virtue of its specific interaction with the GABAA receptor β2 subunit intracellular loop in a yeast two-hybrid screen of a rat brain cDNA library. Further work however, established that GRIF-1, now known as trafficking kinesin protein 2 (TRAK2), is a member of the TRAK family of kinesin adaptor proteins. A pivotal role for TRAK1 and TRAK2 in the transport of mitochondria is well recognized. Notwithstanding this progress, there is a body of evidence that still supports a role for TRAKs in the intracellular transport of GABAA receptors. This is critically reviewed in this article.

Mitochondrial dynamics in cardiovascular health and disease

SIGNIFICANCE

Mitochondria are dynamic organelles capable of changing their shape and distribution by undergoing either fission or fusion. Changes in mitochondrial dynamics, which is under the control of specific mitochondrial fission and fusion proteins, have been implicated in cell division, embryonic development, apoptosis, autophagy, and metabolism. Although the machinery for modulating mitochondrial dynamics is present in the cardiovascular system, its function there has only recently been investigated. In this article, we review the emerging role of mitochondrial dynamics in cardiovascular health and disease.

RECENT ADVANCES

Changes in mitochondrial dynamics have been implicated in vascular smooth cell proliferation, cardiac development and differentiation, cardiomyocyte hypertrophy, myocardial ischemia-reperfusion injury, cardioprotection, and heart failure.

CRITICAL ISSUES

Many of the experimental studies investigating mitochondrial dynamics in the cardiovascular system have been confined to cardiac cell lines, vascular cells, or neonatal cardiomyocytes, in which mitochondria are distributed throughout the cytoplasm and are free to move. However, in the adult heart where mitochondrial movements are restricted by their tightly-packed distribution along myofibrils or beneath the subsarcolemma, the relevance of mitochondrial dynamics is less obvious. The investigation of transgenic mice deficient in cardiac mitochondrial fission or fusion proteins should help elucidate the role of mitochondrial dynamics in the adult heart.

FUTURE DIRECTIONS

Investigating the role of mitochondrial dynamics in cardiovascular health and disease should result in the identification of novel therapeutic targets for treating patients with cardiovascular disease, the leading cause of death and disability globally.

Mitochondrial trafficking in neurons

Neurons, perhaps more than any other cell type, depend on mitochondrial trafficking for their survival. Recent studies have elucidated a motor/adaptor complex on the mitochondrial surface that is shared between neurons and other animal cells. In addition to kinesin and dynein, this complex contains the proteins Miro (also called RhoT1/2) and milton (also called TRAK1/2) and is responsible for much, although not necessarily all, mitochondrial movement. Elucidation of the complex has permitted inroads for understanding how this movement is regulated by a variety of intracellular signals, although many mysteries remain. Regulating mitochondrial movement can match energy demand to energy supply throughout the extraordinary architecture of these cells and can control the clearance and replenishing of mitochondria in the periphery. Because the extended axons of neurons contain uniformly polarized microtubules, they have been useful for studying mitochondrial motility in conjunction with biochemical assays in many cell types.

Distinct functions of nuclear distribution proteins LIS1, Ndel1 and NudCL in regulating axonal mitochondrial transport

Neurons critically depend on the long-distance transport of mitochondria. Motor proteins kinesin and dynein control anterograde and retrograde mitochondrial transport, respectively in axons. The regulatory molecules that link them to mitochondria need to be better characterized. Nuclear distribution (Nud) family proteins LIS1, Ndel1 and NudCL are critical components of cytoplasmic dynein complex. Roles of these Nud proteins in neuronal mitochondrial transport are unknown. Here we report distinct functions of LIS1, Ndel1 and NudCL on axonal mitochondrial transport in cultured hippocampal neurons. We found that LIS1 interacted with kinsein family protein KIF5b. Depletion of LIS1 enormously suppressed mitochondrial motility in both anterograde and retrograde directions. Inhibition of either Ndel1 or NudCL only partially reduced retrograde mitochondrial motility. However, knocking down both Ndel1 and NudCL almost blocked retrograde mitochondrial transport, suggesting these proteins may work together to regulate retrograde mitochondrial transport through linking dynein-LIS1 complex. Taken together, our results uncover novel roles of LIS1, Ndel1 and NudCL in the transport of mitochondria in axons.

Coupling mechanical forces to electrical signaling: molecular motors and the intracellular transport of ion channels

Proper localization of various ion channels is fundamental to neuronal functions, including postsynaptic potential plasticity, dendritic integration, action potential initiation and propagation, and neurotransmitter release. Microtubule-based forward transport mediated by kinesin motors plays a key role in placing ion channel proteins to correct subcellular compartments. PDZ- and coiled-coil-domain proteins function as adaptor proteins linking ionotropic glutamate and GABA receptors to various kinesin motors, respectively. Recent studies show that several voltage-gated ion channel/transporter proteins directly bind to kinesins during forward transport. Three major regulatory mechanisms underlying intracellular transport of ion channels are also revealed. These studies contribute to understanding how mechanical forces are coupled to electrical signaling and illuminating pathogenic mechanisms in neurodegenerative diseases.

Mitochondrial involvement in neurodegenerative diseases

The classical bioenergetical view of the involvement of mitochondria in neurogeneration is based on the fact that mitochondria are the central players of ATP synthesis in neurons and their failure leads to neuronal dysfunction and eventually to cell death. Mutations in at least 39 genes in inherited neurodegenerative disorders seem to alter directly or indirectly mitochondrial function. Most of these mutations do not directly affect oxidative phosphorylation, but act through disturbed mitochondrial dynamics and quality control. This, however, does not invalidate the bioenergetic hypothesis. Neurodegeneration is not necessarily associated with a gross failure of ATP production, but might rather be a consequence of local insufficiencies of ATP supply in critical compartments of neurons, like the presynaptic terminal. We hypothesize that slow disease progression, at least in a subgroup of neurodegenerative diseases, can be explained by the parallel action of subcellular ATP insufficiency and clonal expansion of somatic mitochondrial DNA mutations, and particularly deletions.

The meaning of mitochondrial movement to a neuron's life

Cells precisely regulate mitochondrial movement in order to balance energy needs and avoid cell death. Neurons are particularly susceptible to disturbance of mitochondrial motility and distribution due to their highly extended structures and specialized function. Regulation of mitochondrial motility plays a vital role in neuronal health and death. Here we review the current understanding of regulatory mechanisms that govern neuronal mitochondrial transport and probe their implication in health and disease. This article is part of a Special Issue entitled: Mitochondrial dynamics and physiology.

PINK1 and Parkin target Miro for phosphorylation and degradation to arrest mitochondrial motility

Cells keep their energy balance and avoid oxidative stress by regulating mitochondrial movement, distribution, and clearance. We report here that two Parkinson's disease proteins, the Ser/Thr kinase PINK1 and ubiquitin ligase Parkin, participate in this regulation by arresting mitochondrial movement. PINK1 phosphorylates Miro, a component of the primary motor/adaptor complex that anchors kinesin to the mitochondrial surface. The phosphorylation of Miro activates proteasomal degradation of Miro in a Parkin-dependent manner. Removal of Miro from the mitochondrion also detaches kinesin from its surface. By preventing mitochondrial movement, the PINK1/Parkin pathway may quarantine damaged mitochondria prior to their clearance. PINK1 has been shown to act upstream of Parkin, but the mechanism corresponding to this relationship has not been known. We propose that PINK1 phosphorylation of substrates triggers the subsequent action of Parkin and the proteasome.

Amyotrophic lateral sclerosis-associated mutant VAPBP56S perturbs calcium homeostasis to disrupt axonal transport of mitochondria

A proline-to-serine substitution at position 56 in the gene encoding vesicle-associated membrane protein-associated protein B (VAPB; VAPBP56S) causes some dominantly inherited familial forms of motor neuron disease, including amyotrophic lateral sclerosis (ALS) type-8. Here, we show that expression of ALS mutant VAPBP56S but not wild-type VAPB in neurons selectively disrupts anterograde axonal transport of mitochondria. VAPBP56S-induced disruption of mitochondrial transport involved reductions in the frequency, velocity and persistence of anterograde mitochondrial movement. Anterograde axonal transport of mitochondria is mediated by the microtubule-based molecular motor kinesin-1. Attachment of kinesin-1 to mitochondria involves the outer mitochondrial membrane protein mitochondrial Rho GTPase-1 (Miro1) which acts as a sensor for cytosolic calcium levels ([Ca²⁺]c); elevated [Ca²⁺]c disrupts mitochondrial transport via an effect on Miro1. To gain insight into the mechanisms underlying the VAPBP56S effect on mitochondrial transport, we monitored [Ca²⁺]c levels in VAPBP56S-expressing neurons. Expression of VAPBP56S but not VAPB increased resting [Ca²⁺]c and this was associated with a reduction in the amounts of tubulin but not kinesin-1 that were associated with Miro1. Moreover, expression of a Ca²⁺ insensitive mutant of Miro1 rescued defective mitochondrial axonal transport and restored the amounts of tubulin associated with the Miro1/kinesin-1 complex to normal in VAPBP56S-expressing cells. Our results suggest that ALS mutant VAPBP56S perturbs anterograde mitochondrial axonal transport by disrupting Ca²⁺ homeostasis and effecting the Miro1/kinesin-1 interaction with tubulin.

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.

Amplification of thymosin beta 10 and AKAP13 genes in metastatic and aggressive papillary thyroid carcinomas

Papillary thyroid carcinoma (PTC) is the most common well-differentiated thyroid cancer. Although the great majority of the cases exhibit an indolent clinical course, some of them develop local invasion with distant metastasis, and a few cases transform into undifferentiated/anaplastic thyroid carcinoma with a rapidly lethal course. To identify gene copy number alterations predictive of metastatic potential or aggressive transformation, array-based comparative genomic hybridization (CGH-array) was performed in 43 PTC cases. Formalin-fixed and paraffin-embedded samples from primary tumours of 16 cases without metastasis, 14 cases with only regional lymph node metastasis, and 13 cases with distant metastasis, recurrence or extrathyroid extension were analysed. The CGH-array and confirmatory quantitative real-time PCR results identified the deletion of the EIF4EBP3 and TRAK2 gene loci, while amplification of thymosin beta 10 (TB10) and Tre-2 oncogene regions were observed as general markers for PTC. Although there have been several studies implicating TB10 as a specific marker based on gene expression data, our study is the first to report on genomic amplification. Although no significant difference could be detected between the good and bad prognosis cases in the A-kinase anchor protein 13 (AKAP13) gene region, it was discriminative markers for metastasis. Amplification in the AKAP13 region was demonstrated in 42.9% and 15.4% of the cases with local or with distant metastasis, respectively, while no amplification was detected in non-metastatic cases. AKAP13 and TB10 regions may represent potential new genomic markers for PTC and cancer progression.

Mitochondria: the next (neurode)generation

Adult-onset neurodegenerative disorders are disabling and often fatal diseases of the nervous system whose underlying mechanisms of cell death remain unknown. Defects in mitochondrial respiration had previously been proposed to contribute to the occurrence of many, if not all, of the most common neurodegenerative disorders. However, the discovery of genes mutated in hereditary forms of these enigmatic diseases has additionally suggested defects in mitochondrial dynamics. Such disturbances can lead to changes in mitochondrial trafficking, in interorganellar communication, and in mitochondrial quality control. These new mechanisms by which mitochondria may also be linked to neurodegeneration will likely have far-reaching implications for our understanding of the pathophysiology and treatment of adult-onset neurodegenerative disorders.

KLP6: a newly identified kinesin that regulates the morphology and transport of mitochondria in neuronal cells

Mitochondria utilize diverse cytoskeleton-based mechanisms to control their functions and morphology. Here, we report a role for kinesin-like protein KLP6, a newly identified member of the kinesin family, in mitochondrial morphology and dynamics. An RNA interference screen using Caenorhabditis elegans led us to identify a C. elegans KLP-6 involved in maintaining mitochondrial morphology. We cloned a cDNA coding for a rat homolog of C. elegans KLP-6, which is an uncharacterized kinesin in vertebrates. A rat KLP6 mutant protein lacking the motor domain induced changes in mitochondrial morphology and significantly decreased mitochondrial motility in HeLa cells, but did not affect the morphology of other organelles. In addition, the KLP6 mutant inhibited transport of mitochondria during anterograde movement in differentiated neuro 2a cells. To date, two kinesins, KIF1Bα and kinesin heavy chain (KHC; also known as KIF5) have been shown to be involved in the distribution of mitochondria in neurons. Expression of the kinesin heavy chain/KIF5 mutant prevented mitochondria from entering into neurites, whereas both the KLP6 and KIF1Bα mutants decreased mitochondrial transport in axonal neurites. Furthermore, both KLP6 and KIF1Bα bind to KBP, a KIF1-binding protein required for axonal outgrowth and mitochondrial distribution. Thus, KLP6 is a newly identified kinesin family member that regulates mitochondrial morphology and transport.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

Trafficking kinesin protein (TRAK)-mediated transport of mitochondria in axons of hippocampal neurons

In neurons, the proper distribution of mitochondria is essential because of a requirement for high energy and calcium buffering during synaptic neurotransmission. The efficient, regulated transport of mitochondria along axons to synapses is therefore crucial for maintaining function. The trafficking kinesin protein (TRAK)/Milton family of proteins comprises kinesin adaptors that have been implicated in the neuronal trafficking of mitochondria via their association with the mitochondrial protein Miro and kinesin motors. In this study, we used gene silencing by targeted shRNAi and dominant negative approaches in conjunction with live imaging to investigate the contribution of endogenous TRAKs, TRAK1 and TRAK2, to the transport of mitochondria in axons of hippocampal pyramidal neurons. We report that both strategies resulted in impairing mitochondrial mobility in axonal processes. Differences were apparent in terms of the contribution of TRAK1 and TRAK2 to this transport because knockdown of TRAK1 but not TRAK2 impaired mitochondrial mobility, yet both TRAK1 and TRAK2 were shown to rescue transport impaired by TRAK1 gene knock-out. Thus, we demonstrate for the first time the pivotal contribution of the endogenous TRAK family of kinesin adaptors to the regulation of mitochondrial mobility.

Molecular mechanisms and physiologic functions of mitochondrial dynamics

Mitochondria are highly dynamic organelles that continuously change their shape through frequent fusion, fission and movement throughout the cell, and these dynamics are crucial for the life and death of the cells as they have been linked to apoptosis, maintenance of cellular homeostasis, and ultimately to neurologic disorders and metabolic diseases. Over the past decade, a growing number of novel proteins that regulate mitochondrial dynamics have been discovered. Large GTPase family proteins and their regulators control these aspects of mitochondrial dynamics. In this review, we briefly summarize the current knowledge about molecular machineries regulating mitochondrial fusion/fission and the role of mitochondrial dynamics in cell pathophysiology.

N-acetylglucosamine transferase is an integral component of a kinesin-directed mitochondrial trafficking complex

Trafficking kinesin proteins (TRAKs) 1 and 2 are kinesin-associated proteins proposed to function in excitable tissues as adaptors in anterograde trafficking of cargoes including mitochondria. They are known to associate with N-acetylglucosamine transferase and the mitochondrial rho GTPase, Miro. We used confocal imaging, Förster resonance energy transfer and immunoprecipitations to investigate association between TRAKs1/2, N-acetylglucosamine transferase, the prototypic kinesin-1, KIF5C, and Miro. We demonstrate that in COS-7 cells, N-acetylglucosamine transferase, KIF5C and TRAKs1/2 co-distribute. Förster resonance energy transfer was observed between N-acetylglucosamine transferase and TRAKs1/2. Despite co-distributing with KIF5C and immunoprecipitations demonstrating a TRAK1/2, N-acetylglucosamine transferase and KIF5C ternary complex, no Förster resonance energy transfer was detected between N-acetylglucosamine transferase and KIF5C. KIF5C, N-acetylglucosamine transferase, TRAKs1/2 and Miro formed a quaternary complex. The presence of N-acteylglucosamine transferase partially prevented redistribution of mitochondria induced by trafficking proteins 1/2 and KIF5C. TRAK2 was a substrate for N-acetylglucosamine transferase with TRAK2 (S562) identified as a site of O-N-acetylglucosamine modification. These findings substantiate trafficking kinesin proteins as scaffolds for the formation of a multi-component complex involved in anterograde trafficking of mitochondria. They further suggest that O-glycosylation may regulate complex formation.

Mitochondrial trafficking and the provision of energy and calcium buffering at excitatory synapses

Neuronal postsynaptic currents consume most of the brain's energy supply. Delineating how neurons control the distribution, morphology and function of the energy-producing mitochondria that fuel synaptic communication is therefore important for our understanding of nervous system function and pathology. Here we review recent insights into the molecular mechanisms that control activity-dependent regulation of mitochondrial trafficking, morphology and activity at excitatory synapses. We also consider some implications of this regulation for synaptic function and plasticity and discuss how this may contribute to synaptic dysfunction and signalling in neurological disease, with a focus on Alzheimer's disease.

Kinesin's light chains inhibit the head- and microtubule-binding activity of its tail

Kinesin-1 is a microtubule-based motor comprising two heavy chains (KHCs) and two light chains (KLCs). Motor activity is precisely regulated to avoid futile ATP consumption and to ensure proper intracellular localization of kinesin-1 and its cargoes. The KHC tail inhibits ATPase activity by interacting with the enzymatic KHC heads, and the tail also binds microtubules. Here, we present a role for the KLCs in regulating both the head- and microtubule-binding activities of the kinesin-1 tail. We show that KLCs reduce the affinity of the head-tail interaction over tenfold and concomitantly repress the tail's regulatory activity. We also show that KLCs inhibit tail-microtubule binding by a separate mechanism. Inhibition of head-tail binding requires steric and electrostatic factors. Inhibition of tail-microtubule binding is largely electrostatic, pH dependent, and mediated partly by a highly negatively charged linker region between the KHC-interacting and cargo-binding domains of the KLCs. Our data support a model wherein KLCs promote activation of kinesin-1 for cargo transport by simultaneously suppressing tail-head and tail-microtubule interactions. KLC-mediated inhibition of tail-microtubule binding may also influence diffusional movement of kinesin-1 on microtubules, and kinesin-1's role in microtubule transport/sliding.

Control of mitochondrial transport and localization in neurons

Mitochondria play an essential role in ATP generation, calcium buffering and apoptotic signalling. In neurons, the transport of mitochondria to specific locations where they are needed has emerged as an important process for correct nerve cell function. Recent studies have shed light on the mechanisms that control mitochondrial transport and localization in neurons. We describe the machinery that is important for constitutive transport of mitochondria throughout the cell, and highlight recent advances in our understanding of how signalling pathways can converge on this machinery and allow for rapid activity-dependent control of mitochondrial trafficking and localization. Regulation of mitochondrial trafficking might work in concert with mitochondrial tethering systems to give precise control of mitochondrial delivery and localization to regions of high energy and calcium buffering requirements within neurons.

Traffic control: regulation of kinesin motors

Kinesins are a family of molecular motors that use the energy of ATP hydrolysis to move along the surface of, or destabilize, microtubule filaments. Much progress has been made in understanding the mechanics and functions of the kinesin motors that play important parts in cell division, cell motility, intracellular trafficking and ciliary function. How kinesins are regulated in cells to ensure the temporal and spatial fidelity of their microtubule-based activities is less well understood. Recent work has revealed molecular mechanisms that control kinesin autoinhibition and subsequent activation, binding to cargos and microtubule tracks, and localization at specific sites of action.

Mitochondrial dysfunction in amyotrophic lateral sclerosis

The etiology of motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains to be better understood. Based on the studies from ALS patients and transgenic animal models, it is believed that ALS is likely to be a multifactorial and multisystem disease. Many mechanisms have been postulated to be involved in the pathology of ALS, such as oxidative stress, glutamate excitotoxicity, mitochondrial damage, defective axonal transport, glia cell pathology and aberrant RNA metabolism. Mitochondria, which play crucial roles in excitotoxicity, apoptosis and cell survival, have shown to be an early target in ALS pathogenesis and contribute to the disease progression. Morphological and functional defects in mitochondria were found in both human patients and ALS mice overexpressing mutant SOD1. Mutant SOD1 was found to be preferentially associated with mitochondria and subsequently impair mitochondrial function. Recent studies suggest that axonal transport of mitochondria along microtubules and mitochondrial dynamics may also be disrupted in ALS. These results also illustrate the critical importance of maintaining proper mitochondrial function in axons and neuromuscular junctions, supporting the emerging "dying-back" axonopathy model of ALS. In this review, we will discuss how mitochondrial dysfunction has been linked to the ALS variants of SOD1 and the mechanisms by which mitochondrial damage contributes to the disease etiology.

The hexosamine signaling pathway: O-GlcNAc cycling in feast or famine

The enzymes of O-GlcNAc cycling couple the nutrient-dependent synthesis of UDP-GlcNAc to O-GlcNAc modification of Ser/Thr residues of key nuclear and cytoplasmic targets. This series of reactions culminating in O-GlcNAcylation of targets has been termed the hexosamine signaling pathway (HSP). The evolutionarily ancient enzymes of O-GlcNAc cycling have co-evolved with other signaling effecter molecules; they are recruited to their targets by many of the same mechanisms used to organize canonic kinase-dependent signaling pathways. This co-recruitment of the enzymes of O-GlcNAc cycling drives a binary switch impacting pathways of anabolism and growth (nutrient uptake) and catabolic pathways (nutrient sparing and salvage). The hexosamine signaling pathway (HSP) has thus emerged as a versatile cellular regulator modulating numerous cellular signaling cascades influencing growth, metabolism, cellular stress, circadian rhythm, and host-pathogen interactions. In mammals, the nutrient-sensing HSP has been harnessed to regulate such cell-specific functions as neutrophil migration, and activation of B-cells and T-cells. This review summarizes the diverse approaches being used to examine O-GlcNAc cycling. It will emphasize the impact O-GlcNAcylation has upon signaling pathways that may be become deregulated in diseases of the immune system, diabetes mellitus, cancer, cardiovascular disease, and neurodegenerative diseases.

GABA(A) receptors and their associated proteins: implications in the etiology and treatment of schizophrenia and related disorders

Gamma-aminobutyric acid type A (GABA(A)) receptors play an important role in mediating fast synaptic inhibition in the brain. They are ubiquitously expressed in the CNS and also represent a major site of action for clinically relevant drugs. Recent technological advances have greatly clarified the molecular and cellular roles played by distinct GABA(A) receptor subunit classes and isoforms in normal brain function. At the same time, postmortem and genetic studies have linked neuropsychiatric disorders including schizophrenia and bipolar disorder with GABAergic neurotransmission and various specific GABA(A) receptor subunits, while evidence implicating GABA(A)R-associated proteins is beginning to emerge. In this review we discuss the mounting genetic, molecular, and cellular evidence pointing toward a role for GABA(A) receptor heterogeneity in both schizophrenia etiology and therapeutic development. Finally, we speculate on the relationship between schizophrenia-related disorders and selected GABA(A) receptor associated proteins, key regulators of GABA(A) receptor trafficking, targeting, clustering, and anchoring that often carry out these functions in a subtype-specific manner.

HUMMR, a hypoxia- and HIF-1alpha-inducible protein, alters mitochondrial distribution and transport

Mitochondrial transport is critical for maintenance of normal neuronal function. Here, we identify a novel mitochondria protein, hypoxia up-regulated mitochondrial movement regulator (HUMMR), which is expressed in neurons and is markedly induced by hypoxia-inducible factor 1 α (HIF-1α). Interestingly, HUMMR interacts with Miro-1 and Miro-2, mitochondrial proteins that are critical for mediating mitochondrial transport. Interestingly, knockdown of HUMMR or HIF-1 function in neurons exposed to hypoxia markedly reduces mitochondrial content in axons. Because mitochondrial transport and distribution are inextricably linked, the impact of reduced HUMMR function on the direction of mitochondrial transport was also explored. Loss of HUMMR function in hypoxia diminished the percentage of motile mitochondria moving in the anterograde direction and enhanced the percentage moving in the retrograde direction. Thus, HUMMR, a novel mitochondrial protein induced by HIF-1 and hypoxia, biases mitochondria transport in the anterograde direction. These findings have broad implications for maintenance of neuronal viability and function during physiological and pathological states.

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.

Miro1 is a calcium sensor for glutamate receptor-dependent localization of mitochondria at synapses

Energy use, mainly to reverse ion movements in neurons, is a fundamental constraint on brain information processing. Trafficking of mitochondria to locations in neurons where there are large ion fluxes is essential for powering neural function. Mitochondrial trafficking is regulated by Ca²⁺ entry through ionotropic glutamate receptors, but the underlying mechanism is unknown. We show that the protein Miro1 links mitochondria to KIF5 motor proteins, allowing mitochondria to move along microtubules. This linkage is inhibited by micromolar levels of Ca²⁺ binding to Miro1. With the EF hand domains of Miro1 mutated to prevent Ca²⁺ binding, Miro1 could still facilitate mitochondrial motility, but mitochondrial stopping induced by glutamate or neuronal activity was blocked. Activating neuronal NMDA receptors with exogenous or synaptically released glutamate led to Miro1 positioning mitochondria at the postsynaptic side of synapses. Thus, Miro1 is a key determinant of how energy supply is matched to energy usage in neurons.

O-GlcNAc cycling: implications for neurodegenerative disorders

The dynamic post-translational modification of proteins by O-linked N-acetylglucosamine (O-GlcNAc), termed O-GlcNAcylation, is an important mechanism for modulating cellular signaling pathways. O-GlcNAcylation impacts transcription, translation, organelle trafficking, proteasomal degradation and apoptosis. O-GlcNAcylation has been implicated in the etiology of several human diseases including type-2 diabetes and neurodegeneration. This review describes the pair of enzymes responsible for the cycling of this post-translational modification: O-GlcNAc transferase (OGT) and beta-N-acetylglucosaminidase (OGA), with a focus on the function of their structural domains. We will also highlight the important processes and substrates regulated by these enzymes, with an emphasis on the role of O-GlcNAc as a nutrient sensor impacting insulin signaling and the cellular stress response. Finally, we will focus attention on the many ways by which O-GlcNAc cycling may affect the cellular machinery in the neuroendocrine and central nervous systems.

Pink1 forms a multiprotein complex with Miro and Milton, linking Pink1 function to mitochondrial trafficking

Recessive mutations in Pink1 lead to a selective degeneration of dopaminergic neurons in the substantia nigra that is characteristic of Parkinson disease. Pink1 is a kinase that is targeted in part to mitochondria, and loss of Pink1 function can alter mitochondrial morphology and dynamics, thus supporting a link between mitochondrial dysfunction and Parkinson disease etiology. Here, we report the unbiased identification and confirmation of a mitochondrial multiprotein complex that contains Pink1, the atypical GTPase Miro, and the adaptor protein Milton. Our screen also identified an interaction between Pink1 and Mitofilin. Based on previously established functions for Miro and Milton in the trafficking of mitochondria along microtubules, we postulate here a role for Pink1 in mitochondrial trafficking. Using subcellular fractionation, we show that the overexpression of Miro and Milton, both of which are known to reside at the outer mitochondrial membrane, increases the mitochondrial Pink1 pool, suggesting a function of Pink1 at the outer membrane. Further, we document that Pink1 expressed without a mitochondrial targeting sequence can still be targeted to a mitochondria-enriched subcellular fraction via Miro and Milton. The latter finding is important for the interpretation of a previously reported protective effect of Pink1 expressed without a mitochondrial targeting sequence. Finally, we find that Miro and Milton expression suppresses altered mitochondrial morphology induced by loss of Pink1 function in cell culture. Our findings suggest that Pink1 functions in the trafficking of mitochondria in cells.

Stochastically determined directed movement explains the dominant small-scale mitochondrial movements within non-neuronal tissue culture cells

The apparently stationary phase of mitochondrial motion was investigated in epithelial cells by spinning disk confocal light microscopy combined with image correlation based single particle tracking using custom software producing sub-pixel accuracy measurements (approximately 5 nm) at 10-12 Hz frame-rates. The analysis of these data suggests that the previously described stationary, or anchored phase, in mitochondrial movement actually comprise Brownian diffusion, interspersed with frequent and brief motor-driven events whose duration are stochastically determined. We have therefore discovered a new aspect of mitochondrial behavior, which we call stochastically determined, directed movement.

The mechanism of Ca2+ -dependent regulation of kinesin-mediated mitochondrial motility

Mitochondria are mobile organelles and cells regulate mitochondrial movement in order to meet the changing energy needs of each cellular region. Ca(2+) signaling, which halts both anterograde and retrograde mitochondrial motion, serves as one regulatory input. Anterograde mitochondrial movement is generated by kinesin-1, which interacts with the mitochondrial protein Miro through an adaptor protein, milton. We show that kinesin is present on all axonal mitochondria, including those that are stationary or moving retrograde. We also show that the EF-hand motifs of Miro mediate Ca(2+)-dependent arrest of mitochondria and elucidate the regulatory mechanism. Rather than dissociating kinesin-1 from mitochondria, Ca(2+)-binding permits Miro to interact directly with the motor domain of kinesin-1, preventing motor/microtubule interactions. Thus, kinesin-1 switches from an active state in which it is bound to Miro only via milton, to an inactive state in which direct binding to Miro prevents its interaction with microtubules. Disrupting Ca(2+)-dependent regulation diminishes neuronal resistance to excitotoxicity.

Mitochondrial transport dynamics in axons and dendrites

Mitochondrial dynamics and transport have emerged as key factors in the regulation of neuronal differentiation and survival. Mitochondria are dynamically transported in and out of axons and dendrites to maintain neuronal and synaptic function. Transport proceeds through a controlled series of plus- and minus-end directed movements along microtubule tracks (MTs) that are often interrupted by short stops. This bidirectional motility of mitochondria is facilitated by plus end-directed kinesin and minus end-directed dynein motors, and may be coordinated and controlled by a number of mechanisms that integrate intracellular signals to ensure efficient transport and targeting of mitochondria. In this chapter, we discuss our understanding of mechanisms that facilitate mitochondrial transport and delivery to specific target sites in dendrites and axons.

Identification of TRAK1 (Trafficking protein, kinesin-binding 1) as MGb2-Ag: a novel cancer biomarker

The present study aimed to describe the characterization of an antibody MGb2 that reacts with an epitope on gastric cancer cells, and identification of MGb2 antigen (MGb2-Ag). Immunostaining revealed its distribution in human tissues and demonstrated that the positive rate of MGb2-Ag was 81.48% in gastric cancer, 100% in gastric signet-ring cell carcinoma and mucinous adenocarcinoma, 13.16% in precancerous conditions, and 0% in chronic superficial gastritis. Using Western blotting, immunoprecipitation and MALDI-TOF MS (matrix assisted laser desorption/ionization time-of-flight mass spectrometry), MGb2-Ag was identified as TRAK1 (Trafficking protein, kinesin-binding 1), a new molecular gained limited recognition. Both MGb2 and commercial anti-TRAK1 Ab recognized prokaryotic expressed TRAK1. Immunostaining characteristics of TRAK1 were identical with MGb2-Ag in continuous sections of paraffin-embedded tissues of gastric tissues. This is the first report that TRAK1/MGb2-Ag is a promising diagnostic marker for gastric cancer and may help to detect signet-ring cell carcinoma and mucinous adenocarcinoma.

Hypertonia-associated protein Trak1 is a novel regulator of endosome-to-lysosome trafficking

Hypertonia, which is characterized by stiff gait, abnormal posture, jerky movements, and tremor, is associated with a number of neurological disorders, including cerebral palsy, dystonia, Parkinson's disease, stroke, and spinal cord injury. Recently, a spontaneous mutation in the gene encoding trafficking protein, kinesin-binding 1 (Trak1), was identified as the genetic defect that causes hypertonia in mice. The subcellular localization and biological function of Trak1 remain unclear. Here we report that Trak1 interacts with hepatocyte-growth-factor-regulated tyrosine kinase substrate (Hrs), an essential component of the endosomal sorting and trafficking machinery. Double-label immunofluorescence confocal studies show that the endogenous Trak1 protein partially colocalizes with Hrs on early endosomes. Like Hrs, both overexpression and small-interfering-RNA-mediated knockdown of Trak1 inhibit degradation of internalized epidermal growth factor receptors through a block in endosome-to-lysosome trafficking. Our findings support a role for Trak1 in the regulation of Hrs-mediated endosomal sorting and have important implications for understanding hypertonia associated with neurological disorders.