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Mitochondrial Trafficking in Neurons
Thomas L. Schwarz
F.M. Kirby Neurobiology Center, Children’s Hospital Boston, and Department of Neurobiology, Harvard Medical School, Boston, Massachusetts 02115 Correspondence: [email protected]
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 mitochon- drial 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 extraordinaryarchitecture 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 con- junction with biochemical assays in many cell types.
THE IMPORTANCE OF MITOCHONDRIAL nus ends in the cell body and the plus ends in the MOVEMENT TO NEURONS distal tips. This uniform polarity has made neu- ive imaging of mitochondria has trans- rons particularly useful for studying transport. Lformed our understanding of these organ- Moreover, in neuronal cultures, the axons lie flat elles from static lumps afloat in a cytoplasmic and are typically about a micrometer in diame- soup to animated actors that slide and fuse and ter. The mitochondria therefore move within divide (Jakobs 2006). Their dance is captivating, an easily visualized plane and, as long as the cell even when we are uncertain of its purpose. In body or growth cone of the axon can be identi- neurons, and especially in their axons and den- fied, it is easy to distinguish plus-end from drites, the trafficking of mitochondria is essen- minus-end directed movement and to follow tial and perhaps more orderly than in other mitochondria for 100 mm or more. Moreover, cells. Long-range intracellular transport in ani- whereas mitochondria in many cell types form a mal cells is accomplished primarily by microtu- complex reticulum, axonal mitochondria have bule-based motors—kinesins and dynein—and separated from the reticulum and exist as dis- this is true also for the movement of mitochon- crete organelles of typically 1–3 mm in length; dria (Ligon and Steward 2000b; Hollenbeck and for unknown reasons, those in dendrites tend to Saxton 2005). Axons contain linear arrays of be longer than those in axons (Chang et al. uniformly polarized microtubules, with the mi- 2006). In some preparations, shorter mitochon-
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T.L. Schwarz
dria are more frequently in motion than longer functions. The task is further complicated by ones (Misgeld et al. 2007), but the relationship changes in energy use that require changes in of size to motility is not yet clear. mitochondrial distribution. An interesting il- The urgency of proper mitochondrial traf- lustration of this phenomenon is the behavior ficking in neurons arises from their exceptional of mitochondria in a myelinated nerve, a phe- cellular morphology. Whereas most cells are nomenon recently analyzed in detail (Ohno measured in micrometers or tens of microme- et al. 2011). Most mitochondria in myelinated ters, neurons extend their axons and dendrites nerves reside in stationary pools in the inter- for millimeters, centimeters, and, in the case of node and juxtaparanode. Electrical activity, human peripheral nerves or corticospinal tracts, however, will slow or arrest the movement of up to a meter. Thus the neuron poses an extreme mitochondria as they traverse the node of Ran- case for mitochondrial distribution and the vier and a blockade of electrical activity increas- need to supply energy to far-flung cellular re- es the motile fraction. These modulations of gions. This challenge is probably why mutations movement probably represent a response to of motor proteins found in all cell types give rise the change in energy supply necessitated by to selectively neurological pathologies in hu- the opening of ion channels at the nodes when mans (Baloh 2007; De Voset al. 2008); the stress action potentials are firing. on the axonal transport system is such that per- Nor is the traffic of mitochondria a one-time turbations that another cell could survive cause event to populate the periphery. These postmi- degeneration in neurons. totic cells need to survive for the lifetime of the Indeed, the regions of the neuron that have organism, although the lifetime of mitochon- the highest demand for mitochondrial ATPpro- drial proteins is likely to be a matter of weeks. duction, that is, the highest energy consump- This means there must be constant turnover tion, are the synapses, and these specializations of mitochondria throughout the cell, including are often located at the extremities of the cell. the clearance of older, damaged components When a single synaptic vesicle releases a trans- and the delivery of new materials, most of which mitter at a synapse, it opens tens to hundreds of are encoded by nuclear genes. Three processes ion channels on the postsynaptic membrane, are likely to contribute to this maintenance. The which allows up to 70 million ions to enter the first is the constant movement of a population of cell at that point. This influx will require compa- mitochondria in axons and dendrites. Determi- rable millions of ATP to be hydrolyzed to pump nations of the fraction moving at any given mo- the ions back acrossthe plasma membrane. With ment vary, but typically fall in the range of 10%– hundreds or thousands of synapses impinging 40% in motion, with 60%–90% stationary (for on a cell, it is easy to see why a recent imaging examples, see Morris and Hollenbeck 1993; Li- study determined that a resting cortical neuron gon and Steward 2000a; Misgeld et al. 2007; Rus- consumes 4.7 billion ATP molecules per second so et al. 2009; Wangand Schwarz 2009). Of those (Zhu et al. 2012). It is not only the synapses that in motion, roughly half are moving anterograde consume energy; action potentials also require (away from the cell body) and half retrograde ATP to restore ion gradients, axonal transport is (toward the cell body). This movement likely energy intensive, and, of course, there are all the reflects a major mechanism for the delivery “normal” requirements of a cell. Thus, achiev- and exchange of newly synthesized mitochon- ing the correct distribution of mitochondria drial components. One topic of ongoing debate may be a particularly challenging biological is whether the anterograde-moving population problem for the cell. It is not a question of sim- comprises healthy, refreshed mitochondria and ply recognizing the polarity of the cell and dis- those moving retrograde represent older, dam- tinguishing an axon from a dendrite, but rather aged mitochondria with lower mitochondrial of distributing mitochondria to all regions so as membrane potentials, as determined from volt- to match their density to the local demand for age-sensing dyes. An early study found that the AT P, C a 2þ buffering, and other mitochondrial membrane potential was indeed diminished in
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Mitochondrial Trafficking in Neurons
retrogradely moving mitochondria (Miller and replication, local synthesis of mitochondrial Sheetz 2004), but a subsequent study saw no proteins is also likely; several studies have found difference (Verburg and Hollenbeck 2008). But transcripts for local synthesis of mitochondrial whether or not these populations differ, the fact proteins (Gioio et al. 2001; Aschrafi et al. 2010; that there is such extensive movement means Yoon et al. 2012). Thus local biogenesis of mi- that there is opportunity for mitochondrial pro- tochondria may assist in maintaining healthy teins synthesized in the soma to be delivered to mitochondria in axons and dendrites. More- the periphery. over, local biogenesis would have the potential The second important factor for maintain- to respond rapidly to local changes brought ing peripheral mitochondria is mitochondrial about by either mitochondrial damage or in- fission and fusion. Because these processes are creased demand. At present, it is unknown observed in axons and dendrites, as they are in whether the nucleus is informed about the state all cells, they allow the exchange of materials of the mitochondrial supply in the neuronal pe- between mitochondria (Amiri and Hollenbeck riphery, what such a signal would be, or how 2008). Even brief moments of contact can in- quickly it could resupply a particular region in volve fusion and the extensive exchange of pro- the neuronal arbor. teins in each compartment of the mitochondri- on, as has been shown in non-neuronal cells (Liu A KEY MOTOR/ADAPTOR COMPLEX FOR et al. 2009). Therefore, the mitochondria that MITOCHONDRIAL MOTILITY appear to be stationary, at least for the duration of what can be successfully imaged, may be re- Recently, a motor/adaptor complex was de- freshed in place by exchanging proteins with the scribed that mediates the transport of mito- motile fraction. Mitochondrial fission and fu- chondria in neurons and possibly most animal sion are important to the viability of neuronal cells (Fig. 1) (Stowers et al. 2002; Fransson et al. mitochondria; mutations in mitofusin give rise 2003, 2006; Brickley et al. 2005; Guo et al. 2005; to a peripheral neuropathy (Zuchner et al. 2004; Glater et al. 2006; Brickley and Stephenson Kijima et al. 2005) and cause progressive de- 2011). The core of this complex consists of three creases in the integrity of mitochondrial DNA proteins: the heavy chain of the conventional and membrane potential (Chen et al. 2007). kinesin-1, a protein anchored to the outer sur- Naturally, if there were no mitochondrial move- ment, fusions would be unlikely to occur and indeed, moving mitochondria have a higher rate of fusion than stationary mitochondria (Liu et al. 2009) and the mechanisms of move- Miro ment and fusion may be mechanistically cou- milton pled (Misko et al. 2010). The third factor is the possibility of local Dynein biogenesis of mitochondria in addition to what Kinesin occurs in the cell body. Although the extent to which local synthesis contributes to the mito- chondrial pool is unknown (much certainly Figure 1. The mitochondrial motor/adaptor com- occurs in the soma [Davis and Clayton 1996]), plex. The anterograde motor Kinesin-1 (also called there are two strong lines of evidence that it kinesin heavy chain or Kif5) and the retrograde motor occurs. In an elegant experiment, Amiri and (thedynein/dynactincomplex)areabletocarry many Hollenbeck (2008) explanted sympathetic gan- cellular cargoes but are bound to mitochondria by their interactions with two mitochondrion-specific glia and, after axons had grown out from the proteins: milton and Miro. Miro is carboxy-terminal ganglion, excised the cell bodies and observed anchored to the outer membrane of the mitochondri- BrdU incorporation into the mitochondrial on. Milton serves as an adaptor between Miro and the DNA of the severed axons. In addition to DNA motors. (Figure courtesy of Jarom Chung.)
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T.L. Schwarz
face of the mitochondrion called Miro (also region, which includes a significant region of known as RhoT1 and RhoT2), and milton homology with Huntingtin-associated protein (also known as TRAK1 and TRAK2), a protein 1 (HAP-1) (Stowers et al. 2002), a protein also that links kinesin and Miro. Dynein, as will implicated in organelle traffic (Gauthier et al. be discussed below, interacts with milton and 2004). The ability of milton to associate with Miro. The stoichiometry of the complex is not mitochondria when expressed in cells, to alter yet known, but the properties of the individual their distribution, and to bind KHC is shared proteins and their relevance to mitochondrial by the fly and mammalian homologs (Stowers transport have been elucidated through genetic, et al. 2002; Brickley et al. 2005, 2011; Glater et al. biochemical, and imaging studies. 2006; Smith et al. 2006). The interaction of The kinesin heavy chain (KHC) or kinesin- TRAK1 with KHC is direct (Smith et al. 2006) 1 motor has three isoforms in mammals: Kif5A, and uses the cargo-binding region of KHC near B, and C. All have an amino-terminal motor the carboxyl terminus.Conventionally,the KHC domain that moves toward the plus end of the binds cargo via the kinesin light chain (KLC), microtubule and a large coiled-coil tail that me- but milton is in competition with KLC for bind- diates its dimerization. Cargo binding occurs ing to this region of KHC (Glater et al. 2006). via the carboxy-terminal portion (reviewed in The role of KLC in mitochondrial trafficking is Vale 2003). The significance of KHC for mito- therefore unclear. Although it has been reported chondrial movement is evident from the phe- to be present in mitochondrial fractions (Khod- notypes of mutations in mouse (Tanaka et al. jakov et al. 1998), it is not detected in the mil- 1998) and Drosophila (Hurd and Saxton 1996) ton/Miro/KHC complex and is dispensable and from its association with the other proteins for mitochondrial transport in Drosophila pho- of the motor/adaptor complex (Stowers et al. toreceptors (Glater et al. 2006) and cultured 2002; Brickley et al. 2005; Glater et al. 2006). mammalian epithelial cells (Trejo et al. 2010). The adaptor protein milton was named for It seems likely that, for mitochondria, milton the blind seventeenth century poet, John Mil- will replace KLC as the linker between cargo ton, because it was identified in a Drosophila and motor. mutant screen for blind flies (Stowers et al. Miro (Fig. 2) serves as the membrane an- 2002). The significance of milton for mitochon- chor that links milton, and thereby kinesin, to drial transport was established by the phenotype the mitochondrial surface (Glater et al. 2006). of these mutants. In photoreceptors homozy- Miro has two domains of homology with small gous for the mutation, axons and terminals GTPases and those domains are separated by a were completely devoid of mitochondria, al- linker with two EF-hand motifs. At the carboxyl though functional and morphologically normal terminus there is a trans-membrane domain mitochondria persisted in the somata of these that tethers it to the outer surface of the outer cells (Stowers et al. 2002; Gorska-Andrzejak et al. 2003). Moreover, milton cofractionated IMM with mitochondria, coprecipitated with KHC, and could cause the redistribution of mitochon- OMM dria when overexpressed in COS cells (Stowers
et al. 2002; Glater et al. 2006). The two mamma- GTPase GTPase Miro lian isoforms of milton are called TRAK1 and EFEF TRAK2 (trafficking protein kinesin-binding) and sometimes GRIF1 and OIP106 for historical Figure 2. Domain structure of Miro (RhoT1/2). Miro reasons, as discussed below (Beck et al. 2002; has two GTPase domains that are distantly related to the small GTPases of the ras superfamily. These do- Iyer et al. 2003; Brickley et al. 2005), but for mains are separated by a pair of Ca2þ-binding EF simplicity’s sake will be discussed in aggregate hands. At the carboxyl terminus of the protein, a as milton. Milton lacks an obvious domain short trans-membrane domain anchors Miro in the structure other than an extensive coiled-coil outer mitochondrial membrane (OMM).
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Mitochondrial Trafficking in Neurons
mitochondrial membrane (Fransson et al. 2003, milton/Miro complex has been obtained (van 2006). There is a single fly Miro (Guo et al. Spronsen et al. 2013). 2005), but two mammalian isoforms that are The abundant evidence linking milton and sometimes called RhoT1 and RhoT2 (Aspen- Miro to mitochondrial traffic does not preclude strom et al. 2007). A more distant homolog in their involvement in the movement of other yeast, GEM1p, is necessary for proper mito- cargo and there are some indications that other chondrial morphology and inheritance (Freder- cargos exist. Mammalian milton, for example, ick et al. 2004). Thus, the importance of the was also identified as a GABA receptor interact- protein for regulating mitochondrial distribu- ing factor (and consequently also referred to as tion appears to be very ancient, even though GRIF-1) in a yeast two-hybrid screen (Beck et al. yeast does not use kinesin and microtubules 2002); but in vivo evidence that it is required for for mitochondrial movement (Frederick and GABA receptor transport is presently lacking. It Shaw 2007). The importance of Miro for mito- has also been reported to interact with the en- chondrial movement was hinted at bychanges in dosomal protein Hrs (Kirk et al. 2006) and with mitochondrial distribution in overexpression aKþ channel (Grishin et al. 2006). studies in cells (Fransson et al. 2003), but estab- lished more directly by two parallel approaches. OTHER MECHANISMS FOR The first, a Drosophila mutant screen very sim- MITOCHONDRIAL MOVEMENT ilar to that which isolated milton alleles, iso- lated alleles of miro and found that they, too, Is the milton/Miro/kinesin-heavy-chain com- lacked axonal and synaptic mitochondria (Guo plex the only mechanism by which mitochon- et al. 2005). The untransported mitochondria dria are transported? On the one hand, the phe- formed unusual aggregates in the neuronal so- notypes of Drosophila null alleles of milton and mata. In parallel, biochemical studies identified miro are extreme. In the cell types examined, milton and Miro as direct binding partners mitochondria were absent from axons and den- (Fransson et al. 2006; Glater et al. 2006) and drites. This finding, however, does not preclude found that a truncated Miro lacking its mem- the involvement of other motors and adaptors in brane anchorcould serve as a dominant negative other species, other cell types, or even within the construct that prevented milton from associat- cell bodies of neurons and there is some evidence ing with mitochondria (Glater et al. 2006). for additional transport mechanisms. These in- Biochemical data originally linked milton clude alternative kinesin-based mechanisms. and Miro only to kinesin, but Drosophila phe- Kif1Ba, for example, is a Kinesin-3 motor and notypes indicated that they may also be needed was reported to associate with mitochondria for dynein-mediated transport. The polarity of and to be capable of transporting them in an microtubules in Drosophila dendrites is the op- in vitro assay (Nangaku et al. 1994), but in posite of that in axons; dynein delivers cargo vivo evidence for their involvement remains into dendrites and kinesins mediate movement scant and null alleles of the Drosophila homolog, back to the soma. In milton mutant flies, how- immaculate connections, has normal transport ever, the neuropil regions of the central nervous of mitochondria (Pack-Chung et al. 2007). This system, although 50% composed of dendrites, family of motors is, however, critical for the are lacking in mitochondria (Stowers et al. transport of synaptic vesicle precursors (Hall 2002), and their absence indicates that dynein- and Hedgecock 1991; Yonekawa et al. 1998; dependent movement of mitochondria was also Pack-Chung et al. 2007; Barkus et al. 2008). A disrupted. In addition, close examination of re- distantly related member of the Kinesin-3 fam- sidual mitochondria in miro mutant axons in- ily, kinesin-like protein 6 (KLP6), was found to dicated that both anterograde and retrograde alter mitochondrial shape in Caenorhabditis el- movement was severely impaired (Guo et al. egans and mitochondrial motility in HeLa cells 2005). Recently, direct evidence for a biochem- and a neural cell line (Tanaka et al. 2011) when ical interaction of the retrograde motor with the mutated or knocked down with RNA interfer-
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T.L. Schwarz
ence (RNAi). Whereas loss of the conventional cells may be to remove mitochondria from mi- kinesin heavy chain prevented mitochondria crotubules and potentially tether them to the from entering axons at all, mutant forms of actin cytoskeleton to create a stationary pool KLP6 and Kif1Ba altered the velocity of axonal (Pathak et al. 2010). How the myosins bind to mitochondria (Tanakaet al. 2011). Both of these mitochondria and the mechanisms by which Kinesin-3 motors have been shown to bind to a they influence mitochondrial movement, how- candidate adaptor protein, the kinesin binding ever, remain unknown. protein (KBP). Loss of KBP can alter the distri- bution of mitochondria in cultured cells (Woz- REGULATION OF MITOCHONDRIAL niak et al. 2005) and in zebrafish (Lyons et al. MOVEMENT: THE STATIONARY POOL 2008), although the phenotype in the latter in- dicated that the mitochondrial phenotype was Movement alone cannot achieve the correct likely to be secondary to disruption of the mi- distribution of mitochondria within the neu- crotubule cytoskeleton. Three additional bind- ronal arbor—mitochondrial movement must ing partners for KHC bear mentioning also: be closely regulated. Elucidating the machinery FEZ1, syntabulin, and Ran-binding protein 2 that moves mitochondria has begun to yield (Cai et al. 2005; Cho et al. 2007; Fujita et al. insights into how that movement is regulated, 2007; Ikuta et al. 2007). Interfering with the but many fundamental questions remain. One function of each of these can also alter mito- of the most striking features of mitochondria in chondrial distribution, although the mecha- axons is that they generally fall into three classes: nism of that effect is unclear. The function of those that move primarily anterograde (15%), these additional kinesins and binding partners, those that move primarily retrograde (15%), therefore, and their relationship to the milton/ and those that appear to be stationary for Miro/KHC complex remain an intriguing issue. an extended period (70%). What establishes In addition to microtubule-based motility, these three categories or determines when a mi- actin-based mechanisms may have a role in dis- tochondrion will switch its behavior remains tributing mitochondria (Morris and Hollen- largely mysterious. One possible mechanism beck 1993). In yeast, this is the primary mode would be that motors bind and unbind from of mitochondrial motility (Frederick and Shaw the surface of the organelle and thereby deter- 2007), and in neurons it may mediate movement mine whether and how it moves. At present, this in regions where microtubules are interrupt- mechanism seems unlikely. Neurons transfected ed or absent. A study of locust photoreceptors with fluorescently tagged KHC, together with found that mitochondria moved toward and milton and Miro, revealed that KHC was pres- away from the phototransduction apparatus of ent, at least under these circumstances, on the the cell in alight-dependent manner.This move- entire population of axonal mitochondria and ment was prevented by disruption of the actin at roughly equal levels, whether they were sta- cytoskeleton (Sturmer et al. 1995). More recent- tionary, moving anterograde, or moving in the ly, Myo19 was shown to be a mitochondria-as- dynein-driven retrograde direction (Wang and sociated myosin and its overexpression in a neu- Schwarz 2009). But some indications of how ronal cell line increased mitochondrial motility, these populations are determined have been whereas expression of a dominant-negative tail found. The above-mentioned studies of myosin construct decreased their movement by 40% Vand VI in Drosophila, for example, suggest that (Quintero et al. 2009). In contrast, Myosin V tethering to the actin cytoskeleton may be one and Myosin VI may have the opposite role, re- means of halting microtubule-based transport sisting microtubule-based mechanisms for mi- and creating a stationary pool (Pathak et al. tochondrial movement (Pathak et al. 2010). In 2010) (Fig. 3A). cultured Drosophila neurons, RNAi knockdown Another likely anchoring mechanism may of these proteins increased net movement and predominate in species that, unlike Drosophila run lengths. The function of the myosins in these (Goldstein and Gunawardena 2000), contain
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Mitochondrial Trafficking in Neurons
A Anchor to actin cytoskeleton (e.g., Myosin V) B Anchor to microtubule cytoskeleton (e.g., Syntaphilin) Mitochondrion Mitochondrion
milton milton Myosin V Miro Miro Kinesin heavy Kinesin heavy Syntaphilin chain chain
Microtubule Microtubule
C Reversible disengagement from microtubules (Ca2+) D Irreversible removal of complex from mitochondrion (e.g., PINK1/Parkin)
Mitochondrion Mitochondrion
Ca2+ milton EFEF milton Miro Kinesin heavy chain Kinesin heavy chain
Microtubule Microtubule
Figure 3. Four ways to stop a mitochondrion. (A) Myosin on mitochondria can inhibit their transport by tethering mitochondria to the axonal actin cytoskeleton (Pathak et al. 2010). (B) Syntaphilin is present on stationary axonal mitochondria and anchors them through interactions with microtubules (Kang et al. 2008). (C)Ca2þ binding to Miro causes a rearrangement of the complex such that the motor domain of kinesin directly binds to Miro and is blocked from binding to microtubules (Wang and Schwarz 2009). When cytosolic Ca2þ is lowered, this rearrangement can be reversed to permit continued kinesin-powered movement. (D) The PINK1/ Parkin pathway causes an irreversible dissociation of the motors from the mitochondrial surface by causing Miro to be degraded. Miro degradation by the proteasome is triggered by PINK1 phosphorylation of Miro and Parkin ubiquitinylation of Miro (Wang et al. 2011).
neurofilaments. Mitochondria have been seen via the protein syntaphilin (Kang et al. 2008) by electron microscopy to be associated with (Fig. 3B). Syntaphilin is anchored in the outer neurofilaments in frozen, deep-etched axonal membrane of the mitochondrion and also con- tissue (Hirokawa 1982). This interaction may tains a microtubule-binding domain. Syntaphi- be mediated by the direct binding to mito- lin-GFP was observed preferentially on station- chondria of the phosphorylated side arms of ary mitochondria, but not if the microtubule- the neurofilament heavy chain (Wagner et al. binding region was deleted. The dynein light 2003). Indeed, a substantial literature associates chain LC8 also appears to be involved with syn- mitochondria with intermediate filament pro- taphilin-based docking (Chen et al. 2009). Mice teins in many vertebrate cell types (e.g., Milner lacking syntaphilin were found to have substan- et al. 2000; Taoet al. 2009; Nekrasova et al. 2011). tially more mitochondria in the mobile frac- An important factor in this association may be tion (Kang et al. 2008). However, although the intermediate-filament binding protein plec- most mitochondria are stationary in both axons tin (Reipert et al. 1999; Rezniczek et al. 2003; and dendrites, syntaphilin is exclusively axonal Winter et al. 2008). (Kang et al. 2008). Moreover, syntaphilin is ab- One of the most intriguing anchoring mech- sent from the genome of Drosophila, although anisms that have been proposed involves the flies have a stationary pool similar to mammals. association of mitochondria with microtubules Thus, although several potential mechanisms
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for creating a stationary pool have been report- The mechanism underlying this regulation ed, the relative importance of each may vary and has been examined in some detail. The presence their interplay is still unclear. of EF hands in Miro suggested a likely locus for the Ca2þ-binding site and these EF hands could be disrupted by the substitution of lysine resi- REGULATING MITOCHONDRIAL dues for two key glutamates (MiroKK [Fransson MOVEMENT: CYTOSOLIC Ca2þ et al. 2006]). Overexpression in neurons of Cytosolic Ca2þ is one of the best-studied regu- MiroKK had little effect on the behavior of mi- lators of mitochondrial movement. Elevation of tochondria in conditions of resting Ca2þ, but cytosolic Ca2þ stops both the anterograde and potently disrupted the ability of elevated Ca2þ retrograde movement of mitochondria in neu- to arrest their movement (Saotome et al. 2008; rons and in many cell lines (Rintoul et al. 2003; Macaskill et al. 2009; Wang and Schwarz 2009). Yi et al. 2004; Chang et al. 2006; Szabadkai et al. A surprise from these studies was that both the 2006). From simultaneous measurements of kinesin-dependent anterograde movement and free Ca2þ levels and mitochondrial dynamics, the dynein-dependent retrograde movement it has been determined that 50% reductions in lost their regulation by Ca2þ when Miro was movement will occur at concentrations of ap- mutated. Biochemical evidence has now con- proximately 400 nM Ca2þ and complete arrest firmed that the dynein motor indeed interacts in the low micromolar range in H9c2 cells (Yi with the milton/Miro complex (van Spronsen et al. 2004; Saotome et al. 2008). Elevations in et al. 2013). Ca2þ can arise from a variety of physiological How then does Ca2þ binding to the EF stimuli, including activation of glutamate recep- hands of Miro stop the kinesin-mediated mo- tors in dendrites, action potentials in axons, and tility of the mitochondrion? The anterograde neuromodulators that trigger Ca2þ release from movement is driven by the interaction of the intracellular stores (Rintoul et al. 2003; Yi et al. head domain of KHC with microtubules, but 2004; Saotome et al. 2008; Ohno et al. 2011). The Miro binds to milton and milton binds to the purpose of this regulation is not firmly estab- opposite end of KHC. A possible mechanism lished, but is widely believed to be a homeostatic would involve the Ca2þ triggering the dissocia- response to local Ca2þ influx: if passing mito- tion of the complex by a conformational change chondria are arrested in areas where there is high (Macaskill et al. 2009) that would uncouple the cytosolic Ca2þ, they will be able to provide both kinesin from the mitochondrial surface. This Ca2þ buffering capacity and ATP for the active could either be a disruption of the KHC/milton transport of Ca2þ from the cell. In addition, interaction or the milton/Miro interaction. subcellular areas with inadequate cytosolic ATP Though this may occur under some circum- are likely to have local elevations of Ca2þ, and stances, there is substantial evidence against the arrest of mitochondria in such areas will such a model: the three proteins continue to correct the lack of ATP (Yang and Steele 2000; coprecipitate as a complex even in the presence Wang and Schwarz 2009). Tests of these pos- of very high Ca2þ (2 mM); isolated mitochon- sibilities have recently been made possible by dria washed in high Ca2þ do not lose KHC from manipulations of the motor/adaptor com- their surface; and in living neurons transfected plex that selectively remove the ability of Ca2þ with fluorescently tagged KHC, equivalent to regulate mitochondrial movement. Under amounts of KHC are present on mitochondria these circumstances, mitochondrial will no lon- before and after their arrest by elevated Ca2þ ger stop at activated synapses (Macaskill et al. (Wang and Schwarz 2009). The latter experi- 2009) and neurons become more susceptible ment contained another surprise: KHC was to excitotoxic cell death (Wang and Schwarz present on all mitochondria, including those 2009), presumably because they can no longer that were in the stationary pool or moving ret- cope with the Ca2þ that entered when glutamate rograde, in addition to those that moved anter- receptors were activated. ograde. Apparently, the choice of direction and
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Mitochondrial Trafficking in Neurons
whether or not to be stationary is not governed of Parkin. In a manner still poorly understood, by the association and dissociation of the motor. ubiquitination of proteins on the mitochondrial An alternative mechanism for Ca2þ-dependent surface initiates mitophagy. Whether as a pre- arrest has been proposed (Fig. 3C) (Wang and lude to mitophagy or perhaps just as an inde- Schwarz 2009) in which the kinesin switches be- pendent action, the PINK1/Parkin pathway also tween an active state that interacts with the mi- regulates mitochondrial morphology (Greene crotubules and an inhibited state where the mo- et al. 2003; Clark et al. 2006; Park et al. 2006; tor “head” domain of kinesin binds to the Ca2þ- Yang et al. 2006; Poole et al. 2008) and this is bound state of Miro and is thereby prevented mediated, at least in part, by targeting mitofusin from interacting with the microtubules. The iso- for degradation (Poole et al. 2010; Tanaka et al. lated KHC head domain will indeed bind to 2010; Chan et al. 2011). PINK1 is only effective wild-type Miro, but only when Ca2þ is present, in bringing about these changes when Parkin is and not to MiroKK, and this interaction prevents present, but in some cases Parkin overexpression the head domain from cosedimenting with mi- can alter mitochondria even in the absence of crotubules (Wang and Schwarz 2009). Two as- PINK1 (Clark et al. 2006; Yang et al. 2006), as pects of this mechanism are noteworthy. The might be expected in a pathway where PINK1 is first is that it is readily reversible and will allow upstream of Parkin. movement to begin again as soon as Ca2þ is The potential involvement of mitochondrial unbound from the EF hands. The second is movement in this pathway was first indicated that it explains the selective arrest of mitochon- by the observation of a biochemical associa- drial motility because the regulation by Ca2þ is tion between PINK1 and the Miro/milton com- not intrinsic to the motor domain, but instead is plex (Weihofen et al. 2009). It was subsequently based on the interaction of that domain with the found that Parkin can also bind to this complex mitochondrion-specific adaptor Miro. and that, although this association may be scant in healthy cells, it can be greatly increased if mitochondria are depolarized with carbon- REGULATING MITOCHONDRIAL yl cyanide m-chlorophenylhydrazone (CCCP) MOVEMENT: PINK1 AND PARKIN (Wang et al. 2011). PINK1 can phosphorylate Recent studies have elucidated an additional Miro on several sites, at least one of which, S156, means of arresting mitochondria—one that is is important for the subsequent stimulation of very different from the readily reversible arrest the Parkin function and mitochondrial arrest by Ca2þ. This mechanism involves two genes, (Wang et al. 2011). Miro is also a substrate of PINK1 and Parkin, which are intensively studied Parkin (Liu et al. 2012). The consequence of because mutations in them are responsible for activating this PINK1/Parkin pathway is the hereditary forms of Parkinsonism (Kitada et al. proteasome-dependent degradation of Miro 1998; Valente et al. 2004). PINK1 (PTEN-in- and the consequent release of kinesin and mil- duced putative kinase 1) is a protein kinase and ton from the mitochondrial surface (Chan et al. Parkin is an E3 ubiquitin ligase, and together 2011; Wang et al. 2011) (Fig. 3D). The loss of they form apathway that initiatesthe autophagic Miro explains why overexpression of PINK1 or clearance of mitochondriawith poor membrane Parkin stops mitochondrial motion in both potentials (reviewed elsewhere and see Whit- mammalian and Drosophila neurons and, as in worth and Pallanck 2009; Youle and Narendra the case of mitophagy, PINK1 acts upstream of 2011; Narendra et al. 2012). In brief, PINK1 is Parkin and in a manner that completely de- normally inactivated by import into mito- pends on Parkin activity (Wang et al. 2011). chondria and subsequent proteolytic cleavage. Although mitochondrial movement is normal When mitochondria are damaged or poisoned in Parkin2/2 mice, there is more movement in and do not have a sufficient membrane poten- Drosophila axons expressing either PINK1 or tial, PINK1 instead accumulates on the mito- Parkin RNAi relative to controls. Thus, whereas chondrial surface and causes the recruitment mammalian neurons may use this pathway only
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T.L. Schwarz
under conditions of mitochondrial damage, in stop in their transit and thereby account for the Drosophila it may be part of the ongoing control accumulation. NGF activation of TrkAreceptors of mitochondrial dynamics. It is also perhaps is responsible for this effect and the pathway noteworthy that Miro and milton bind to mito- appears to involve PI3 kinase and the actin cy- fusin (Misko et al. 2010), and thus the degrada- toskeleton. In light of the evidence mentioned tion of these key regulators of mitochondrial above, that myosin interactions with actin may dynamics may be coordinated. derail mitochondria from microtubule tracks As noted above, the association of PINK1 and thereby anchor them (Pathak et al. 2010), and Parkin with Miro is stimulated by depola- it is possible that the NGF is increasing this set of rization of the mitochondrion. Mitochondrial cytoskeletal interactions. depolarization also arrests mitochondrial trans- A potentially related pathway that regulates port in axons and this arrest is accompanied by mitochondrial motility is activated by the the recruitment of cytosolic Parkin to the mito- growth factor lysophosphatidic acid (LPA) and chondrion. Moreover, the degradation of Miro mediated by the small GTPase RhoA (Minin proceeds before the wholesale loss of mitochon- et al. 2006). Either LPA or a constitutively active drial proteins by mitophagy (Wang et al. 2011). RhoA will inhibit mitochondrial movement in It is therefore likely, although as yet undemon- cultured cells, including a neuron-like Droso- strated, that halting mitochondria in some phila cell line. RhoA can regulate interactions manner facilitates their clearance by mitophagy. with the actin cytoskeleton through at least One attractive model is that the degradation two pathways. RhoA, via Rho-kinase, can acti- of Miro and mitofusin are both important as vate myosin motors (Kimura et al. 1996) and an early step to quarantine damaged mitochon- thereby potentially remove mitochondria from dria and prevent them from fusing with other their microtubule tracks. In addition, RhoA can healthy mitochondria, thereby mixing their activate the actin-regulatory formin-like protein damaged components in with the healthy pop- mDia (diaphanous in the fly) (Watanabe et al. ulation. Moving mitochondria are much more 1997). The latter pathway is both necessary for likely to fuse than stationary mitochondria (Liu and sufficient to explain the inhibitory effects et al. 2009) and therefore, together with the of RhoA and LPA and entails the anchoring of degradation of mitofusin, the loss of Miro will mitochondria to the actin cytoskeleton (Minin produce a population of stationary, fragmented et al. 2006). Despite the apparent importance of mitochondria that are ready for engulfment by the actin cytoskeleton for these mechanisms of autophagosomes. Because it is a step toward mitochondrial anchoring, the intermediate fil- clearance of the organelle, it is appropriate that ament protein vimentin can also interact with the mechanism of mitochondrial arrest is by mitochondria and influence their movement proteolysis and therefore not readily reversible. (Nekrasova et al. 2011). Other proteins that associate with the mil- ton/Miro complex may be significant regula- REGULATING MITOCHONDRIAL tors. These include HUMMR, a protein whose MOVEMENT: OTHER FACTORS expression is induced by hypoxic conditions and The Ca2þ and PINK1/Parkin pathways are very that biases mitochondrial movement in the an- far from the totality of regulatory influences, terograde direction by an unknown means (Li although for many others mechanistic informa- et al. 2009), and O-GlcNAc transferase (OGT), tion remains scant. One of the most intriguing is an enzyme that catalyzes the transferof single N- the influence of the nerve growh factor (NGF), acetylglucosamine sugars to serine and threo- which can cause a local accumulation of mito- nine residues (Iyer et al. 2003). Although this chondria where an NGF-coupled bead makes enzyme typically has only low-affinity interac- contact with a sensory axon (Chada and Hollen- tions with its many substrates, it binds strongly beck 2003, 2004). Mitochondria that enter the to milton, which is also a substrate (Iyer et al. portion of the axon closest to the bead tend to 2003). The association of OGT and milton is
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Mitochondrial Trafficking in Neurons
conserved in Drosophila (Glater et al. 2006) mammalian milton are found in multiple splice and, by an unknown mechanism, OGTexpres- isoforms. Although there is no functional infor- sion can alter the distribution of mitochondria mation at present on the significance of the in cultured cells (Brickley et al. 2011). Both mammalian splice variants, the splicing of Dro- HUMMR, as a sensor of hypoxic stress, and sophila milton appears to be highly significant. OGT, as a likely sensor of metabolic state (Laza- In particular, of four known forms of the amino- rus et al. 2009), may help mitochondria redis- terminal portion, one, milton-C, does not bind tribute themselves in response to changing en- to kinesin heavy chain when expressed in cul- vironmental and energetic conditions. In this tured cells and does not recruit KHC to mito- regard, they resemble the Ca2þ-sensing mecha- chondria, although it contains the KHC-bind- nism described above that can arrest mitochon- ing sequence (Glater et al. 2006). This amino dria in regions of high-energy demand. A paral- terminus, therefore, may be a regulatory domain lel mechanism forcontrolling the distribution of that can influence the association and dissocia- mitochondria entails the ability of ADP to de- tion of the motor from the complex in vivo. crease mitochondrial motility (Mironov 2007, Selective loss of splice isoforms in a Drosophila 2009). Because ADP levels will be high where allele also suggested isoform-specific functions ATP consumption is high, the arrest of mito- (Cox and Spradling 2006). The two mammalian chondria by elevated ADP will cause ATP-pro- genes, TRAK1 and TRAK2, encode proteins that ducing mitochondria to concentrate where they similarlydiffer in their interactions with the mo- are most needed. tors: TRAK1 will bind both kinesin and dynein, The most recent addition to the list of mo- but TRAK2 preferentially interacts with dynein tility regulators are the proteins of the Armcx (van Spronsen et al. 2013). family, including Alex3 (Lopez-Domenech et al. Other regulatory signals may impinge on 2012). This family, characterized by multiple the motors themselves or on the microtubules armadillo-like repeats, is unique to the placental and their associated proteins. These include mi- mammals, localizes to mitochondria, and when crotubule-affinity regulating kinases (MARKs) overexpressed in cultured cells, including neu- (Mandelkow et al. 2004), JNK1 (Morfini et al. rons, causes the mitochondria to aggregate near 2006, 2009), histone deacetylase 6 (Chen et al. the nucleus. Alex3 binds to the Miro/milton 2010), and cyclin-dependent kinases (Ratner et complex and, when overexpressed, substantial- al. 1998; Morfini et al. 2004; Holzbaur 2010). ly decreases axonal transport of mitochondria These influences, however, are unlikely to be se- (Lopez-Domenech et al. 2012). Also, as previ- lective for mitochondrial motility but rather will ously mentioned, mitofusins bind to the Miro/ influence axonal transport broadly. milton complex. This association could allow Miro to regulate mitochondrial fusion or allow mitofusin to regulate mitochondrial motility SUMMARY: DISTRIBUTING MITOCHONDRIA, COMPLEX SOLUTIONS and, indeed, knockdown of the mitofusin2 ex- TO A COMPLEX PROBLEM pression decreased mitochondrial motility in axons (Misko et al. 2010). Mitochondrial move- Given the complexity of the task facing the neu- ment and fusion are likely to be coordinated ron—distributing mitochondria over long dis- processes. tances and matching their distribution to an Avery different form of regulation is likely to ever-changing demand for energy—perhaps it arise from the selection of the particular iso- is not surprising that such a rich and complex forms of milton and Miro that are found in a regulatory machinery has evolved. This appara- cell or on a subset of mitochondria. In mam- tus needs to contend not only with the deliv- mals, there are two genes apiece for both milton ery of healthy mitochondria to the appropri- and Miro (often called TRAK1 and TRAK2 and ate regions, but also to their retention in areas RhoT1 and RhoT2). This diversity is augmented with a high requirement for their Ca2þ-buffer- by alternative splicing. Both Drosophila and ing capacity and energy production. What has
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T.L. Schwarz
emerged so far is a core apparatus of milton and coil domain proteins: Association in vivo and in vitro Miro that will link the kinesin and dynein mo- with kinesin. J Biol Chem 280: 14723–14732. Brickley K, Pozo K, Stephenson FA. 2011. N-acetylglucos- tors to the mitochondrial surface and an excep- amine transferase is an integral component of a kinesin- tional number of signaling pathways and regu- directed mitochondrial trafficking complex. Biochim Bi- latory proteins that will interact with that core ophys Acta 1813: 269–281. and with the other cytoskeletal elements, partic- Cai Q, Gerwin C, Sheng ZH. 2005. Syntabulin-mediated anterograde transport of mitochondria along neuronal ularly the actin network, which can stabilize mi- processes. J Cell Biol 170: 959–969. tochondria. When this machinery is compro- Chada SR, Hollenbeck PJ. 2003. Mitochondrial movement mised there are pathological consequences that and positioning in axons: The role of growth factor sig- may include the impaired ability of neurons to naling. J Exp Biol 206: 1985–1992. survive excitotoxic stimuli or to be cleared from Chada SR, Hollenbeck PJ. 2004. Nerve growth factor signal- ing regulates motility and docking of axonal mitochon- cells when damaged. There is much still to deci- dria. Curr Biol 14: 1272–1276. pher. The stoichiometry of the motor/adaptor Chan NC, Salazar AM, Pham AH, Sweredoski MJ, Kolawa complex is unknown and these proteins cry out NJ, Graham RL, Hess S, Chan DC. 2011. Broad activation for greater structural information. The signifi- of the ubiquitin-proteasome system by Parkin is critical for mitophagy. Hum Mol Genet 20: 1726–1737. cance of distinct isoforms and splice variants Chang DT, Honick AS, Reynolds IJ. 2006. Mitochondrial remains unclear and many modulatory path- trafficking to synapses in cultured primary cortical neu- ways still lack a detailed mechanistic under- rons. J Neurosci 26: 7035–7045. standing. Most pressingly perhaps, the in vivo Chen H, McCaffery JM, Chan DC. 2007. Mitochondrial relevance of the regulatory pathways remains fusion protects against neurodegeneration in the cerebel- lum. Cell 130: 548–562. unclear. Nevertheless, studies of motility in an- Chen YM, Gerwin C, Sheng ZH. 2009. Dynein light chain imal cells have uncovered a remarkably complex LC8 regulates syntaphilin-mediated mitochondrial dock- and dynamic life for the mitochondrion. ing in axons. J Neurosci 29: 9429–9438. Chen S, Owens GC, Makarenkova H, Edelman DB. 2010. HDAC6 regulates mitochondrial transport in hippocam- REFERENCES pal neurons. PLoS ONE 5: e10848.