Opposite-polarity motors activate one another to trigger cargo transport in live cells

Analysis of Cooperative Behavior in Multiple Kinesins Motor Protein Transport by Varying Structural and Chemical Properties

Intracellular transport is a fundamental biological process during which cellular materials are driven by enzymatic molecules called motor proteins. Recent optical trapping experiments and theoretical analysis have uncovered many features of cargo transport by multiple kinesin motor protein molecules under applied loads. These studies suggest that kinesins cooperate negatively under typical transport conditions, although some productive cooperation could be achieved under higher applied loads. However, the microscopic origins of this complex behavior are still not well understood. Using a discrete-state stochastic approach we analyze factors that affect the cooperativity among kinesin motors during cargo transport. Kinesin cooperation is shown to be largely unaffected by the structural and mechanical parameters of a multiple motor complex connected to a cargo, but much more sensitive to biochemical parameters affecting motor-filament affinities. While such behavior suggests the net negative cooperative responses of kinesins will persist across a relatively wide range of cargo types, it is also shown that the rates with which cargo velocities relax in time upon force perturbations are influenced by structural factors that affect the free energies of and load distributions within a multiple kinesin complex. The implications of these later results on transport phenomena where loads change temporally, as in the case of bidirectional transport, are discussed.

Numerical investigation of axonal cargo rerouting in a dendrite: a three kinetic state model

The purpose of this paper is to develop a three-kinetic state model to investigate axonal cargo rerouting in a dendrite. It is assumed that axonal cargos are transported by kinesin motors; they can enter dendrites riding on plus-end-out microtubules (MTs). If this happens, cargos are rerouted by dissociating from these MTs and associating with minus-end-out MTs. The same kinesin motors then move axonal cargos out of the dendrite. The model predicts how far axonal cargos can penetrate into the dendrite, concentrations of various populations of axonal cargos, and cargo fluxes.

Directional persistence of migrating cells requires Kif1C-mediated stabilization of trailing adhesions

Directional cell migration requires the establishment and maintenance of long-term differences in structure and function between the front and back of a cell. Here, we show that the microtubule motor Kif1C contributes to persistent cell migration primarily through stabilization of an extended cell rear. Kif1C-mediated transport of α5β1-integrins is required for the proper maturation of trailing focal adhesions and resistance to tail retraction. Tail retraction precedes and induces changes in migration direction. Stabilization of cell tails through inhibition of myosin II activity suppresses the Kif1C depletion phenotype and results in longer-lived tails and higher directional stability of migrating cells. Taken together, these findings indicate that the maintenance of an extended, tense cell tail facilitates directional migration. We propose a rear drag mechanism for directional persistence of migration whereby the counterforce originating from a well-anchored tail serves to maintain directionality of the force-generating leading edge of the cell.

In vivo optical trapping indicates kinesin's stall force is reduced by dynein during intracellular transport

Kinesin and dynein are fundamental components of intracellular transport, but their interactions when simultaneously present on cargos are unknown. We built an optical trap that can be calibrated in vivo during data acquisition for each individual cargo to measure forces in living cells. Comparing directional stall forces in vivo and in vitro, we found evidence that cytoplasmic dynein is active during minus- and plus-end directed motion, whereas kinesin is only active in the plus direction. In vivo, we found outward (∼plus-end) stall forces range from 2 to 7 pN, which is significantly less than the 5- to 7-pN stall force measured in vitro for single kinesin molecules. In vitro measurements on beads with kinesin-1 and dynein bound revealed a similar distribution, implying that an interaction between opposite polarity motors causes this difference. Finally, inward (∼minus-end) stalls in vivo were 2-3 pN, which is higher than the 1.1-pN stall force of a single dynein, implying multiple active dynein.

Endogenous GSK-3/shaggy regulates bidirectional axonal transport of the amyloid precursor protein

Neurons rely on microtubule (MT) motor proteins such as kinesin-1 and dynein to transport essential cargos between the cell body and axon terminus. Defective axonal transport causes abnormal axonal cargo accumulations and is connected to neurodegenerative diseases, including Alzheimer's disease (AD). Glycogen synthase kinase 3 (GSK-3) has been proposed to be a central player in AD and to regulate axonal transport by the MT motor protein kinesin-1. Using genetic, biochemical and biophysical approaches in Drosophila melanogaster, we find that endogenous GSK-3 is a required negative regulator of both kinesin-1-mediated and dynein-mediated axonal transport of the amyloid precursor protein (APP), a key contributor to AD pathology. GSK-3 also regulates transport of an unrelated cargo, embryonic lipid droplets. By measuring the forces motors generate in vivo, we find that GSK-3 regulates transport by altering the activity of kinesin-1 motors but not their binding to the cargo. These findings reveal a new relationship between GSK-3 and APP, and demonstrate that endogenous GSK-3 is an essential in vivo regulator of bidirectional APP transport in axons and lipid droplets in embryos. Furthermore, they point to a new regulatory mechanism in which GSK-3 controls the number of active motors that are moving a cargo.

Measuring collective transport by defined numbers of processive and nonprocessive kinesin motors

Intracellular transport is thought to be achieved by teams of motor proteins bound to a cargo. However, the coordination within a team remains poorly understood as a result of the experimental difficulty in controlling the number and composition of motors. Here, we developed an experimental system that links together defined numbers of motors with defined spacing on a DNA scaffold. By using this system, we linked multiple molecules of two different types of kinesin motors, processive kinesin-1 or nonprocessive Ncd (kinesin-14), in vitro. Both types of kinesins markedly increased their processivities with motor number. Remarkably, despite the poor processivity of individual Ncd motors, the coupling of two Ncd motors enables processive movement for more than 1 μm along microtubules (MTs). This improvement was further enhanced with decreasing spacing between motors. Force measurements revealed that the force generated by groups of Ncd is additive when two to four Ncd motors work together, which is much larger than that generated by single motors. By contrast, the force of multiple kinesin-1s depends only weakly on motor number. Numerical simulations and single-molecule unbinding measurements suggest that this additive nature of the force exerted by Ncd relies on fast MT binding kinetics and the large drag force of individual Ncd motors. These features would enable small groups of Ncd motors to crosslink MTs while rapidly modulating their force by forming clusters. Thus, our experimental system may provide a platform to study the collective behavior of motor proteins from the bottom up.

Spastic paraplegia mutation N256S in the neuronal microtubule motor KIF5A disrupts axonal transport in a Drosophila HSP model

Hereditary spastic paraplegias (HSPs) comprise a group of genetically heterogeneous neurodegenerative disorders characterized by spastic weakness of the lower extremities. We have generated a Drosophila model for HSP type 10 (SPG10), caused by mutations in KIF5A. KIF5A encodes the heavy chain of kinesin-1, a neuronal microtubule motor. Our results imply that SPG10 is not caused by haploinsufficiency but by the loss of endogenous kinesin-1 function due to a selective dominant-negative action of mutant KIF5A on kinesin-1 complexes. We have not found any evidence for an additional, more generalized toxicity of mutant Kinesin heavy chain (Khc) or the affected kinesin-1 complexes. Ectopic expression of Drosophila Khc carrying a human SPG10-associated mutation (N256S) is sufficient to disturb axonal transport and to induce motoneuron disease in Drosophila. Neurofilaments, which have been recently implicated in SPG10 disease manifestation, are absent in arthropods. Impairments in the transport of kinesin-1 cargos different from neurofilaments are thus sufficient to cause HSP–like pathological changes such as axonal swellings, altered structure and function of synapses, behavioral deficits, and increased mortality.

Hereditary spastic paraplegias (HSPs) comprise a group of inherited neurological diseases. The main feature of HSP is progressive stiffness of the lower limbs due to a dysfunction of nerve cells. We study HSP type 10, which is caused by mutations in the neuronal motor protein KIF5A. HSP type 10 is inherited in an autosomal-dominant manner, which means that patients have a normal and a mutated copy of the KIF5A gene. KIF5A plays an important role in neuronal function: it transports cargos to the synapse that can be up to 1 m from the cell body. We use the fruit fly as a model to investigate the role of mutations in KIF5A. Our fly model replicates a central feature of HSP: muscles that are activated by nerve cells that have long cellular processes are more severely impaired. We now address why one mutated copy of KIF5A is sufficient to cause HSP. To date, it has been thought that patients might have HSP because they have insufficient functional KIF5A or because mutated KIF5A disturbs the function of normal KIF5A. We provide evidence for the latter possibility.

A model of axonal transport drug delivery: effects of diffusivity

This paper investigates the effects of diffusivity on retrograde dynein-driven transport of pharmaceutical agent complexes (PACs) in axons. The model is designed with two goals in mind: (1) to capture results on axonal transport drug delivery reported in recent experimental research by Filler et al. (Filler AG, Whiteside GT, Bacon M, Frederickson M, Howe FA, Rabinowitz MD, Sokoloff AJ, Deacon TW, Abell C, Munglani R, Griffiths JR, Bell BA, Lever AML. Tri-partite complex for axonal transport drug delivery achieves pharmacological effect. BMC Neuroscience 2010; 11: 8) and (2) to produce analytically tractable equations. It is shown that the inclusion of a diffusion process in the model produces equations that can still be solved by Laplace transform, although the last step of the solution, finding the inverse Laplace transform, has to be accomplished numerically, thus leading to a hybrid analytical and numerical solution technique. The effects of diffusivity and the kinetic rates describing PAC transition between the dynein-driven and accumulated states on transport of PACs toward the neuron soma are investigated.

A THREE-KINETIC-STATE MODEL OF AXONAL TRANSPORT DRUG DELIVERY

This paper develops a model of axonal transport drug delivery that includes three populations (kinetic states) of pharmaceutical agent complexes (PACs), namely PACs transported by dynein motors, PACs freely suspended in the cytosol, and PACs accumulated at the Nodes of Ranvier. The number of model parameters is minimized by recasting governing equations into the dimensionless form. The obtained equations are solved numerically. The dependencies of the three PAC concentrations as well as the diffusion-driven, motor-driven, and total PAC fluxes on the PAC diffusivity and the length of the axon are investigated. Two situations are analyzed: when all kinetic constantans are the same (in this case the dynein-driven PAC flux exceeds the diffusion flux by a large amount) and when kinetic constants describing PAC transition from the freely suspended state are small (in the this case the diffusion-driven flux is the major component of the total flux, but since the diffusion transport mechanism is highly inefficient compared to the motor-driven one for large particles, the total PAC flux is much smaller in this case).

How the interplay between mechanical and nonmechanical interactions affects multiple kinesin dynamics

Intracellular transport is supported by enzymes called motor proteins that are often coupled to the same cargo and function collectively. Recent experiments and theoretical advances have been able to explain certain behaviors of multiple motor systems by elucidating how unequal load sharing between coupled motors changes how they bind, step, and detach. However, nonmechanical interactions are typically overlooked despite several studies suggesting that microtubule-bound kinesins interact locally via short-range nonmechanical potentials. This work develops a new stochastic model to explore how these types of interactions influence multiple kinesin functions in addition to mechanical coupling. Nonmechanical interactions are assumed to affect kinesin mechanochemistry only when the motors are separated by less than three microtubule lattice sites, and it is shown that relatively weak interaction energies (~2 k(B)T) can have an appreciable influence over collective motor velocities and detachment rates. In agreement with optical trapping experiments on structurally defined kinesin complexes, the model predicts that these effects primarily occur when cargos are transported against loads exceeding single-kinesin stalling forces. Overall, these results highlight the interdependent nature of factors influencing collective motor functions, namely, that the way the bound configuration of a multiple motor system evolves under load determines how local nonmechanical interactions influence motor cooperation.

From the cell membrane to the nucleus: unearthing transport mechanisms for dynein

Mutations in the motor protein cytoplasmic dynein have been found to cause Charcot-Marie-Tooth disease, spinal muscular atrophy, and severe intellectual disabilities in humans. In mouse models, neurodegeneration is observed. We sought to develop a novel model which could incorporate the effects of mutations on distance travelled and velocity. A mechanical model for the dynein mediated transport of endosomes is derived from first principles and solved numerically. The effects of variations in model parameter values are analysed to find those that have a significant impact on velocity and distance travelled. The model successfully describes the processivity of dynein and matches qualitatively the velocity profiles observed in experiments.

Functional coupling of microtubules to membranes - implications for membrane structure and dynamics

The microtubule network dictates much of the spatial patterning of the cytoplasm, and the coupling of microtubules to membranes controls the structure and positioning of organelles and directs membrane trafficking between them. The connection between membranes and the microtubule cytoskeleton, and the way in which organelles are shaped and moved by interactions with the cytoskeleton, have been studied intensively in recent years. In particular, recent work has expanded our thinking of this topic to include the mechanisms by which membranes are shaped and how cargo is selected for trafficking as a result of coupling to the cytoskeleton. In this Commentary, I will discuss the molecular basis for membrane-motor coupling and the physiological outcomes of this coupling, including the way in which microtubule-based motors affect membrane structure, cargo sorting and vectorial trafficking between organelles. Whereas many core concepts of these processes are now well understood, key questions remain about how the coupling of motors to membranes is established and controlled, about the regulation of cargo and/or motor loading and about the control of directionality.

Lis1 is an initiation factor for dynein-driven organelle transport

The dynein-associated protein Lis1 may be a ubiquitous determinant of dynein-dependent transport required primarily at the stage of motility initiation.

The molecular motor cytoplasmic dynein is responsible for most minus-end–directed, microtubule-based transport in eukaryotic cells. It is especially important in neurons, where defects in microtubule-based motility have been linked to neurological diseases. For example, lissencephaly is caused by mutations in the dynein-associated protein Lis1. In this paper, using the long, highly polarized hyphae of the filamentous fungus Aspergillus nidulans, we show that three morphologically and functionally distinct dynein cargos showed transport defects in the genetic absence of Lis1/nudF, raising the possibility that Lis1 is ubiquitously used for dynein-based transport. Surprisingly, both dynein and its cargo moved at normal speeds in the absence of Lis1 but with reduced frequency. Moreover, Lis1, unlike dynein and dynactin, was absent from moving dynein cargos, further suggesting that Lis1 is not required for dynein-based cargo motility once it has commenced. Based on these observations, we propose that Lis1 has a general role in initiating dynein-driven motility.

Herpesvirus transport to the nervous system and back again

Herpes simplex virus, varicella zoster virus, and pseudorabies virus are neurotropic pathogens of the Alphaherpesvirinae subfamily of the Herpesviridae. These viruses efficiently invade the peripheral nervous system and establish lifelong latency in neurons resident in peripheral ganglia. Primary and recurrent infections cycle virus particles between neurons and the peripheral tissues they innervate. This remarkable cycle of infection is the topic of this review. In addition, some of the distinguishing hallmarks of the infections caused by these viruses are evaluated in terms of their underlying similarities.

An exact solution describing slow axonal transport of cytoskeletal elements: the effect of a finite half-life

This paper presents an exact solution for a two kinetic state model of slow axonal transport that is based on the stop-and-go hypothesis. The model accounts for two populations of cytoskeletal elements (CEs): pausing and running. The model also accounts for a finite half-life of CEs involved in slow axonal transport. It is assumed that initially CEs are injected into the axon such that their concentration forms a rectangular pulse; initially all CEs are assumed to be in the pausing state. Kinetic processes quickly redistribute CEs between the pausing and running states. After less than a minute, equilibrium is established, forming two pulses, representing concentrations of pausing and running CEs, respectively. As these pulses propagate, their shape changes and they turn to bell-shaped waves. The amplitude of the waves decreases, and the waves spread out as they propagate down the axon. The rate of the amplitude decrease is larger for CEs with a shorter half-life, but even if CE half-life is infinitely long, some decrease of the waves' amplitudes is observed. The velocity of the waves' propagation is found to be independent of the CE half-life and is in good agreement with published experimental data for slow axonal transport of neurofilaments.

The axonal transport of mitochondria

Vigorous transport of cytoplasmic components along axons over substantial distances is crucial for the maintenance of neuron structure and function. The transport of mitochondria, which serves to distribute mitochondrial functions in a dynamic and non-uniform fashion, has attracted special interest in recent years following the discovery of functional connections among microtubules, motor proteins and mitochondria, and their influences on neurodegenerative diseases. Although the motor proteins that drive mitochondrial movement are now well characterized, the mechanisms by which anterograde and retrograde movement are coordinated with one another and with stationary axonal mitochondria are not yet understood. In this Commentary, we review why mitochondria move and how they move, focusing particularly on recent studies of transport regulation, which implicate control of motor activity by specific cell-signaling pathways, regulation of motor access to transport tracks and static microtubule-mitochondrion linkers. A detailed mechanism for modulating anterograde mitochondrial transport has been identified that involves Miro, a mitochondrial Ca(2+)-binding GTPase, which with associated proteins, can bind and control kinesin-1. Elements of the Miro complex also have important roles in mitochondrial fission-fusion dynamics, highlighting questions about the interdependence of biogenesis, transport, dynamics, maintenance and degradation.

Bidirectional transport of organelles: unity and struggle of opposing motors

Bidirectional transport along microtubules is ensured by opposing motor proteins: cytoplasmic dynein that drives cargo to the minus-ends and various kinesins that generally move to the plus-ends of microtubules. Regulation of motor proteins that are simultaneously bound to the same organelle is required to maintain directional transport and prevent pausing of cargo pulled away by motors of opposite polarity. Debates of the recent decade have been focused on two possible mechanisms of such regulation: (i) coordination, which implies that only one type of motors is active at a given time, and (ii) tug-of-war, which assumes that both motors are active at the same time and that direction of transport depends on the outcome of motor's confrontation. The initial idea of coordination has been challenged by observations of simultaneous activity of plus- and minus-end-directed motors applied to the same cargo. Analysis of the available data indicates that coordination and tug-of-war theories rather complement than contradict each other: cargo interacts with two teams of active motors, the resulting direction and the winner team are determined by coordination complexes, but the activity of the loser team is never completely inhibited and remains at some background level. Such persisting activity might enhance the overall efficiency of transport by increasing processivity or helping to overcome the obstacles on microtubule track.

An exact solution of transient equations describing slow axonal transport

An exact analytical solution of equations describing slow axonal transport of cytoskeletal elements (CEs) injected in an axon is presented. The equations modelling slow axonal transport are based on the stop-and-go hypothesis. The simplest model implementing this hypothesis postulates that CEs switch between pausing and running kinetic states, and that the probabilities of CE transition between these two states are described by first-order rate constants. It is assumed that initially CEs are injected such that they form a uniform pulse of a given width. All injected CEs are initially attributed to the pausing state. It is shown that within 30 s kinetic processes redistribute CEs between pausing and running states; after that the process occurs under quasi-equilibrium conditions. The parameter accessible to experiments is the total concentration of CEs (pausing plus running). As the initial rectangular-shaped pulse moves, it changes its shape to become a bell-shaped wave that spreads out as it propagates. The wave's amplitude is decreasing during the wave's propagation. It is also shown that the system forgets its initial condition, meaning that if one starts with pulses of different widths, after sometime they converge to the same bell-shaped wave.

Molecular motor function in axonal transport in vivo probed by genetic and computational analysis in Drosophila

Amyloid precursor protein (APP) vesicle movement by kinesin-1 and cytoplasmic dynein exhibits kinesin-1–dependent velocity. Our data also suggest that kinesin-1 and cytoplasmic dynein motors assemble in stable mixtures on APP vesicles and that their direction and velocity are controlled at least in part by dynein IC.

Bidirectional axonal transport driven by kinesin and dynein along microtubules is critical to neuronal viability and function. To evaluate axonal transport mechanisms, we developed a high-resolution imaging system to track the movement of amyloid precursor protein (APP) vesicles in Drosophila segmental nerve axons. Computational analyses of a large number of moving vesicles in defined genetic backgrounds with partial reduction or overexpression of motor proteins enabled us to test with high precision existing and new models of motor activity and coordination in vivo. We discovered several previously unknown features of vesicle movement, including a surprising dependence of anterograde APP vesicle movement velocity on the amount of kinesin-1. This finding is largely incompatible with the biophysical properties of kinesin-1 derived from in vitro analyses. Our data also suggest kinesin-1 and cytoplasmic dynein motors assemble in stable mixtures on APP vesicles and their direction and velocity are controlled at least in part by dynein intermediate chain.

Mitochondrial dynamics and bioenergetic dysfunction is associated with synaptic alterations in mutant SOD1 motor neurons

Mutations in Cu,Zn superoxide dismutase (SOD1) cause familial amyotrophic lateral sclerosis (FALS), a rapidly fatal motor neuron disease. Mutant SOD1 has pleiotropic toxic effects on motor neurons, among which mitochondrial dysfunction has been proposed as one of the contributing factors in motor neuron demise. Mitochondria are highly dynamic in neurons; they are constantly reshaped by fusion and move along neurites to localize at sites of high-energy utilization, such as synapses. The finding of abnormal mitochondria accumulation in neuromuscular junctions, where the SOD1-FALS degenerative process is though to initiate, suggests that impaired mitochondrial dynamics in motor neurons may be involved in pathogenesis. We addressed this hypothesis by live imaging microscopy of photo-switchable fluorescent mitoDendra in transgenic rat motor neurons expressing mutant or wild-type human SOD1. We demonstrate that mutant SOD1 motor neurons have impaired mitochondrial fusion in axons and cell bodies. Mitochondria also display selective impairment of retrograde axonal transport, with reduced frequency and velocity of movements. Fusion and transport defects are associated with smaller mitochondrial size, decreased mitochondrial density, and defective mitochondrial membrane potential. Furthermore, mislocalization of mitochondria at synapses among motor neurons, in vitro, correlates with abnormal synaptic number, structure, and function. Dynamics abnormalities are specific to mutant SOD1 motor neuron mitochondria, since they are absent in wild-type SOD1 motor neurons, they do not involve other organelles, and they are not found in cortical neurons. Together, these results suggest that impaired mitochondrial dynamics may contribute to the selective degeneration of motor neurons in SOD1-FALS.

High-resolution imaging reveals indirect coordination of opposite motors and a role for LIS1 in high-load axonal transport

High-resolution particle tracking shows a specific role for the dynein regulatory factor LIS1 in high-load axonal transport of large vesicles but no evidence for mechanical activation of opposite-directed motors.

The specific physiological roles of dynein regulatory factors remain poorly understood as a result of their functional complexity and the interdependence of dynein and kinesin motor activities. We used a novel approach to overcome these challenges, combining acute in vivo inhibition with automated high temporal and spatial resolution particle tracking. Acute dynein inhibition in nonneuronal cells caused an immediate dispersal of diverse forms of cargo, resulting from a sharp decrease in microtubule minus-end run length followed by a gradual decrease in plus-end runs. Acute LIS1 inhibition or LIS1 RNA interference had little effect on lysosomes/late endosomes but severely inhibited axonal transport of large, but not small, vesicular structures. Our acute inhibition results argue against direct mechanical activation of opposite-directed motors and offer a novel approach of potential broad utility in the study of motor protein function in vivo. Our data also reveal a specific but cell type–restricted role for LIS1 in large vesicular transport and provide the first quantitative support for a general role for LIS1 in high-load dynein functions.

Bidirectional intracellular transport: utility and mechanism

Bidirectional transport of intracellular cargo along microtubule tracks is the subject of intense debate in the motility field. In the present review, we provide an overview of the models describing the possible mechanisms driving intracellular saltatory transport, taking into account current experimental results that may at first seem contradictory. We examine the phenomenon of saltatory motion, in an attempt to interpret the mechanistic debate in terms of the utility of saltatory motion.

Cytoplasmic dynein

The organization and function of eukaryotic cells rely on the action of many different molecular motor proteins. Cytoplasmic dynein drives the movement of a wide range of cargoes towards the minus ends of microtubules, and these events are needed, not just at the single-cell level, but are vital for correct development. In the present paper, I review recent progress on understanding dynein's mechanochemistry, how it is regulated and how it binds to such a plethora of cargoes. The importance of a number of accessory factors in these processes is discussed.

An analytical solution describing the propagation of positive injury signals in an axon: effect of dynein velocity distribution

A model describing the propagation of positive injury signals from the lesion site in an axon towards the neuron soma is described. It is assumed that these signals are driven by dynein molecular motors. An analytical solution that accounts for the probability density function (pdf) of a dynein velocity distribution is obtained. Two examples of pdf of dynein velocity distributions that follow from the results published in Ross et al. (2006, Processive bidirectional motion of dynein-dynactin complexes in vitro. Nat Cell Biol. 8:562-570) and Deinhardt et al. (2006, Rab5 and Rab7 control endocytic sorting along the axonal retrograde transport pathway. Neuron 52:293-305) are considered. The effect of dynein velocity distribution on the rate of spreading of the signal wave is discussed. It is demonstrated that the obtained solution can be applied to the problem of how neurons measure the distance between the lesion site and the neuron soma.

Ndel1-derived peptides modulate bidirectional transport of injected beads in the squid giant axon

Bidirectional transport is a key issue in cellular biology. It requires coordination between microtubule-associated molecular motors that work in opposing directions. The major retrograde and anterograde motors involved in bidirectional transport are cytoplasmic dynein and conventional kinesin, respectively. It is clear that failures in molecular motor activity bear severe consequences, especially in the nervous system. Neuronal migration may be impaired during brain development, and impaired molecular motor activity in the adult is one of the hallmarks of neurodegenerative diseases leading to neuronal cell death. The mechanisms that regulate or coordinate kinesin and dynein activity to generate bidirectional transport of the same cargo are of utmost importance. We examined how Ndel1, a cytoplasmic dynein binding protein, may regulate non-vesicular bidirectional transport. Soluble Ndel1 protein, Ndel1-derived peptides or control proteins were mixed with fluorescent beads, injected into the squid giant axon, and the bead movements were recorded using time-lapse microscopy. Automated tracking allowed for extraction and unbiased analysis of a large data set. Beads moved in both directions with a clear bias to the anterograde direction. Velocities were distributed over a broad range and were typically slower than those associated with fast vesicle transport. Ironically, the main effect of Ndel1 and its derived peptides was an enhancement of anterograde motion. We propose that they may function primarily by inhibition of dynein-dependent resistance, which suggests that both dynein and kinesin motors may remain engaged with microtubules during bidirectional transport.

Cooperative responses of multiple kinesins to variable and constant loads

Microtubule-dependent transport is most often driven by collections of kinesins and dyneins that function in either a concerted fashion or antagonistically. Several lines of evidence suggest that cargo transport may not be influenced appreciably by the combined action of multiple kinesins. Yet, as in previous optical trapping experiments, the forces imposed on cargos will vary spatially and temporally in cells depending on a number of local environmental factors, and the influence of these conditions has been largely overlooked. Here, we characterize the dynamics of structurally defined complexes containing multiple kinesins under the controlled loads of an optical force clamp. While demonstrating that there are generic kinetic barriers that restrict the ability of multiple kinesins to cooperate productively, the spatial and temporal properties of applied loads is found to play an important role in the collective dynamics of multiple motor systems. We propose this dependence has implications for intracellular transport processes, especially for bidirectional transport.

Mechanical stochastic tug-of-war models cannot explain bidirectional lipid-droplet transport

Intracellular transport via the microtubule motors kinesin and dynein plays an important role in maintaining cell structure and function. Often, multiple kinesin or dynein motors move the same cargo. Their collective function depends critically on the single motors' detachment kinetics under load, which we experimentally measure here. This experimental constraint--combined with other experimentally determined parameters--is then incorporated into theoretical stochastic and mean-field models. Comparison of modeling results and in vitro data shows good agreement for the stochastic, but not mean-field, model. Many cargos in vivo move bidirectionally, frequently reversing course. Because both kinesin and dynein are present on the cargos, one popular hypothesis explaining the frequent reversals is that the opposite-polarity motors engage in unregulated stochastic tugs-of-war. Then, the cargos' motion can be explained entirely by the outcome of these opposite-motor competitions. Here, we use fully calibrated stochastic and mean-field models to test the tug-of-war hypothesis. Neither model agrees well with our in vivo data, suggesting that, in addition to inevitable tugs-of-war between opposite motors, there is an additional level of regulation not included in the models.

Productive cooperation among processive motors depends inversely on their mechanochemical efficiency

Subcellular cargos are often transported by teams of processive molecular motors, which raises questions regarding the role of motor cooperation in intracellular transport. Although our ability to characterize the transport behaviors of multiple-motor systems has improved substantially, many aspects of multiple-motor dynamics are poorly understood. This work describes a transition rate model that predicts the load-dependent transport behaviors of multiple-motor complexes from detailed measurements of a single motor's elastic and mechanochemical properties. Transition rates are parameterized via analyses of single-motor stepping behaviors, load-rate-dependent motor-filament detachment kinetics, and strain-induced stiffening of motor-cargo linkages. The model reproduces key signatures found in optical trapping studies of structurally defined complexes composed of two kinesin motors, and predicts that multiple kinesins generally have difficulties in cooperating together. Although such behavior is influenced by the spatiotemporal dependence of the applied load, it appears to be directly linked to the efficiency of kinesin's stepping mechanism, and other types of less efficient and weaker processive motors are predicted to cooperate more productively. Thus, the mechanochemical efficiencies of different motor types may determine how effectively they cooperate together, and hence how motor copy number contributes to the regulation of cargo motion.

Kinesin assembly and movement in cells

Long-distance transport in eukaryotic cells is driven by molecular motors that move along microtubule tracks. Molecular motors of the kinesin superfamily contain a kinesin motor domain attached to family-specific sequences for cargo binding, regulation, and oligomerization. The biochemical and biophysical properties of the kinesin motor domain have been widely studied, yet little is known about how kinesin motors work in the complex cellular environment. We discuss recent studies on the three major families involved in intracellular transport (kinesin-1, kinesin-2, and kinesin-3) that have begun to bridge the gap in knowledge between the in vitro and in vivo behaviors of kinesin motors. These studies have increased our understanding of how kinesin subunits assemble to produce a functional motor, how kinesin motors are affected by biochemical cues and obstacles present on cellular microtubules, and how multiple motors on a cargo surface can work collectively for increased force production and travel distance.

Roles of dynein and dynactin in early endosome dynamics revealed using automated tracking and global analysis

Microtubule-dependent movement is crucial for the spatial organization of endosomes in most eukaryotes, but as yet there has been no systematic analysis of how a particular microtubule motor contributes to early endosome dynamics. Here we tracked early endosomes labeled with GFP-Rab5 on the nanometer scale, and combined this with global, first passage probability (FPP) analysis to provide an unbiased description of how the minus-end microtubule motor, cytoplasmic dynein, supports endosome motility. Dynein contributes to short-range endosome movement, but in particular drives 85-98% of long, inward translocations. For these, it requires an intact dynactin complex to allow membrane-bound p150(Glued) to activate dynein, since p50 over-expression, which disrupts the dynactin complex, inhibits inward movement even though dynein and p150(Glued) remain membrane-bound. Long dynein-dependent movements occur via bursts at up to ∼8 µms(-1) that are linked by changes in rate or pauses. These peak speeds during rapid inward endosome movement are still seen when cellular dynein levels are 50-fold reduced by RNAi knock-down of dynein heavy chain, while the number of movements is reduced 5-fold. Altogether, these findings identify how dynein helps define the dynamics of early endosomes.

Coupling viruses to dynein and kinesin-1

It is now clear that transport on microtubules by dynein and kinesin family motors has an important if not critical role in the replication and spread of many different viruses. Understanding how viruses hijack dynein and kinesin motors using a limited repertoire of proteins offers a great opportunity to determine the molecular basis of motor recruitment. In this review, we discuss the interactions of dynein and kinesin-1 with adenovirus, the α herpes viruses: herpes simplex virus (HSV1) and pseudorabies virus (PrV), human immunodeficiency virus type 1 (HIV-1) and vaccinia virus. We highlight where the molecular links to these opposite polarity motors have been defined and discuss the difficulties associated with identifying viral binding partners where the basis of motor recruitment remains to be established. Ultimately, studying microtubule-based motility of viruses promises to answer fundamental questions as to how the activity and recruitment of the dynein and kinesin-1 motors are coordinated and regulated during bi-directional transport.

Kinesin-3 and dynein cooperate in long-range retrograde endosome motility along a nonuniform microtubule array

The polarity of microtubules (MTs) determines the motors for intracellular motility, with kinesins moving to plus ends and dynein to minus ends. In elongated cells of Ustilago maydis, dynein is thought to move early endosomes (EEs) toward the septum (retrograde), whereas kinesin-3 transports them to the growing cell tip (anterograde). Occasionally, EEs run up to 90 μm in one direction. The underlying MT array consists of unipolar MTs at both cell ends and antipolar bundles in the middle region of the cell. Cytoplasmic MT-organizing centers, labeled with a γ-tubulin ring complex protein, are distributed along the antipolar MTs but are absent from the unipolar regions. Dynein colocalizes with EEs for 10-20 μm after they have left the cell tip. Inactivation of temperature-sensitive dynein abolishes EE motility within the unipolar MT array, whereas long-range motility is not impaired. In contrast, kinesin-3 is continuously present, and its inactivation stops long-range EE motility. This indicates that both motors participate in EE motility, with dynein transporting the organelles through the unipolar MT array near the cell ends, and kinesin-3 taking over at the beginning of the medial antipolar MT array. The cooperation of both motors mediates EE movements over the length of the entire cell.

Myosin Va and myosin VI coordinate their steps while engaged in an in vitro tug of war during cargo transport

Myosin Va (myoV) and myosin VI (myoVI) are processive molecular motors that transport cargo in opposite directions on actin tracks. Because these motors may bind to the same cargo in vivo, we developed an in vitro "tug of war" to characterize the stepping dynamics of single quantum-dot-labeled myoV and myoVI motors linked to a common cargo. MyoV dominates its myoVI partner 79% of the time. Regardless of which motor wins, its stepping rate slows due to the resistive load of the losing motor (myoV, 2.1 pN; myoVI, 1.4 pN). Interestingly, the losing motor steps backward in synchrony with the winning motor. With ADP present, myoVI acts as an anchor to prevent myoV from stepping forward. This model system emphasizes the physical communication between opposing motors bound to a common cargo and highlights the potential for modulating this interaction by changes in the cell's ionic milieu.

Bi-directional transport of the nucleus by dynein and kinesin-1

Proper transport and positioning of cell organelles often depends on the antagonistic activities of dynein and kinesin-1, two microtubule motors with opposite directionality.1 One of the largest known transport cargoes is the cell nucleus. Both dynein and kinesin-1 participate in positioning of the nucleus through binding to the nuclear envelope (NE).2-9 Surprisingly, both dynein and kinesin-1 can be recruited to the NE through multiple pathways, one involving SUN-KASH domain containing proteins and the other involving nuclear pore complexes (NPCs). Here, we discuss the molecular mechanisms of dynein and kinesin recruitment to the NE through NPCs, as well as the functional implications of dynein and kinesin-1 activity at the NE in mammalian cells. Finally, we discuss how motor activities at the NE might be controlled during the cell cycle.

Tau/PTL-1 associates with kinesin-3 KIF1A/UNC-104 and affects the motor's motility characteristics in C. elegans neurons

Tauopathies are neurodegenerative diseases based on pathological tau-aggregation including Alzheimer's disease, frontotemporal dementia (FTD) and Pick's disease. In general, cargo (e.g., β-amyloid precursor protein, tau, neurofilaments) accumulation is a commonly observed phenomenon in degenerated neurons. Therefore, it is crucial to investigate the interaction between cargo, microtubule-binding proteins and molecular motors. We report the effect of tau/PTL-1 (protein with tau-like repeats) on the transport characteristics of the major axonal transporter kinesin-3 KIF1A/UNC-104 in the nervous system of Caenorhabditis elegans. Using confocal spinning disk time-lapse imaging we analyzed the motility of UNC-104::mRFP in ptl-1 knockout worms and found that predominantly retrograde moving characteristics are affected (rather than the motor's anterograde displacements). A similar motility pattern was observed for synaptobrevin-1-containing vesicles, a major cargo of UNC-104. Moreover, UNC-104 and PTL-1 colocalize and occasionally co-migrate. We further confirmed physical interactions between PTL-1 and UNC-104 in living animals using the bimolecular fluorescence complementation assay (BiFC) as well as in co-immunoprecipitation experiments. Though this study focuses on PTL-1/UNC-104 interactions, we extended our research on monitoring conventional kinesin-1 (UNC-116) as well as dynein motility pattern and found that in ptl-1 mutants retrograde displacements were also affected for UNC-116, while for dynein, interestingly, its anterograde movements were affected.

Molecular motors: directing traffic during RNA localization

RNA localization, the enrichment of RNA in a specific subcellular region, is a mechanism for the establishment and maintenance of cellular polarity in a variety of systems. Ultimately, this results in a universal method for spatially restricting gene expression. Although the consequences of RNA localization are well-appreciated, many of the mechanisms that are responsible for carrying out polarized transport remain elusive. Several recent studies have illuminated the roles that molecular motor proteins play in the process of RNA localization. These studies have revealed complex mechanisms in which the coordinated action of one or more motor proteins can act at different points in the localization process to direct RNAs to their final destination. In this review, we discuss recent findings from several different systems in an effort to clarify pathways and mechanisms that control the directed movement of RNA.

Stable kinesin and dynein assemblies drive the axonal transport of mammalian prion protein vesicles

Kinesin and dynein are opposite-polarity microtubule motors that drive the tightly regulated transport of a variety of cargoes. Both motors can bind to cargo, but their overall composition on axonal vesicles and whether this composition directly modulates transport activity are unknown. Here we characterize the intracellular transport and steady-state motor subunit composition of mammalian prion protein (PrP(C)) vesicles. We identify Kinesin-1 and cytoplasmic dynein as major PrP(C) vesicle motor complexes and show that their activities are tightly coupled. Regulation of normal retrograde transport by Kinesin-1 is independent of dynein-vesicle attachment and requires the vesicle association of a complete Kinesin-1 heavy and light chain holoenzyme. Furthermore, motor subunits remain stably associated with stationary as well as with moving vesicles. Our data suggest a coordination model wherein PrP(C) vesicles maintain a stable population of associated motors whose activity is modulated by regulatory factors instead of by structural changes to motor-cargo associations.

The dynactin complex enhances the speed of microtubule-dependent motions of adenovirus both towards and away from the nucleus

Unlike transport vesicles or organelles, human adenovirus (HAdV) directly binds to the microtubule minus end-directed motor dynein for transport to the nucleus. The dynein cofactor dynactin enhances nuclear transport of HAdV and boosts infection. To determine if dynactin has a specific role in cytoplasmic trafficking of incoming HAdV on microtubules, we used live cell spinning disc confocal microscopy at 25 Hz acquisition frequency and automated tracking of single virus particles at 20–50 nm spatial resolution. Computational dissection by machine-learning algorithms extracted specific motion patterns of viral trajectories. We found that unperturbed cells supported two kinds of microtubule-dependent motions, directed motions (DM) and fast drifts (FD). DM had speeds of 0.2 to 2 μm/s and run lengths of 0.4 up to 7 μm, while FD were slower and less extensive at 0.02 to 0.4 μm/s and 0.05 to 2.5 μm. Dynactin interference by overexpression of p50/dynamitin or a coiled-coil domain of p150/Glued reduced the speeds and amounts of both center- and periphery-directed DM but not FD, and inhibited infection. These results indicate that dynactin enhances adenovirus infection by increasing the speed and efficiency of dynein-mediated virus motion to the nucleus, and, surprisingly, also supports a hereto unknown motor activity for virus transport to the cell periphery.

Transient binding of dynein controls bidirectional long-range motility of early endosomes

In many cell types, bidirectional long-range endosome transport is mediated by the opposing motor proteins dynein and kinesin-3. Here we use a fungal model system to investigate how both motors cooperate in early endosome (EE) motility. It was previously reported that Kin3, a member of the kinesin-3 family, and cytoplasmic dynein mediate bidirectional motility of EEs in the fungus Ustilago maydis. We fused the green fluorescent protein to the endogenous dynein heavy chain and the kin3 gene and visualized both motors and their cargo in the living cells. Whereas kinesin-3 was found on anterograde and retrograde EEs, dynein motors localize only to retrograde organelles. Live cell imaging shows that binding of retrograde moving dynein to anterograde moving endosomes changes the transport direction of the organelles. When dynein is leaving the EEs, the organelles switch back to anterograde kinesin-3-based motility. Quantitative photobleaching and comparison with nuclear pores as an internal calibration standard show that single dynein motors and four to five kinesin-3 motors bind to the organelles. These data suggest that dynein controls kinesin-3 activity on the EEs and thereby determines the long-range motility behavior of the organelles.

CK1 activates minus-end-directed transport of membrane organelles along microtubules

This study shows that the signal transduction pathway responsible for the initiation of minus-end–directed movement of membrane-bounded pigment granules in melanophores involves sequential activation of protein phosphatase 2A and casein kinase 1 and that this activation correlates with increased phosphorylation of the dynein intermediate chain.

Microtubule (MT)-based organelle transport is driven by MT motor proteins that move cargoes toward MT minus-ends clustered in the cell center (dyneins) or plus-ends extended to the periphery (kinesins). Cells are able to rapidly switch the direction of transport in response to external cues, but the signaling events that control switching remain poorly understood. Here, we examined the signaling mechanism responsible for the rapid activation of dynein-dependent MT minus-end–directed pigment granule movement in Xenopus melanophores (pigment aggregation). We found that, along with the previously identified protein phosphatase 2A (PP2A), pigment aggregation signaling also involved casein kinase 1ε (CK1ε), that both enzymes were bound to pigment granules, and that their activities were increased during pigment aggregation. Furthermore we found that CK1ε functioned downstream of PP2A in the pigment aggregation signaling pathway. Finally, we discovered that stimulation of pigment aggregation increased phosphorylation of dynein intermediate chain (DIC) and that this increase was partially suppressed by CK1ε inhibition. We propose that signal transduction during pigment aggregation involves successive activation of PP2A and CK1ε and CK1ε-dependent phosphorylation of DIC, which stimulates dynein motor activity and increases minus-end–directed runs of pigment granules.

Towards a quantitative understanding of mitotic spindle assembly and mechanics

The 'simple' view of the mitotic spindle is that it self-assembles as a result of microtubules (MTs) randomly searching for chromosomes, after which the spindle length is maintained by a balance of outward tension exerted by molecular motors on the MTs connecting centrosomes and chromosomes, and compression generated by other motors on the MTs connecting the spindle poles. This picture is being challenged now by mounting evidence indicating that spindle assembly and maintenance rely on much more complex interconnected networks of microtubules, molecular motors, chromosomes and regulatory proteins. From an engineering point of view, three design principles of this molecular machine are especially important: the spindle assembles quickly, it assembles accurately, and it is mechanically robust--yet malleable. How is this design achieved with randomly interacting and impermanent molecular parts? Here, we review recent interdisciplinary studies that have started to shed light on this question. We discuss cooperative mechanisms of spindle self-assembly, error correction and maintenance of its mechanical properties, speculate on analogy between spindle and lamellipodial dynamics, and highlight the role of quantitative approaches in understanding the mitotic spindle design.

KIF1A is the primary anterograde motor protein required for the axonal transport of dense-core vesicles in cultured hippocampal neurons

Dense-core vesicles (DCVs) are responsible for transporting, processing, and secreting neuropeptide cargos that mediate a wide range of biological processes, including neuronal development, survival, and learning and memory. DCVs are synthesized in the cell body and are transported by kinesin motor proteins along microtubules to pre- and postsynaptic release sites. Due to the dependence on kinesin-based transport, we sought to determine if the kinesin-3 family member, KIF1A, transports DCVs in primary cultured hippocampal neurons, as has been described for invertebrate neurons. Two-color, live-cell imaging showed that the DCV markers, chromogranin A-RFP and BDNF-RFP, move together with KIF1A-GFP in both the anterograde and retrograde directions. To demonstrate a functional role for KIF1A in DCV transport, motor protein expression in neurons was reduced using RNA interference (shRNA). Fluorescently tagged DCV markers showed a significant reduction in organelle flux in cells expressing shRNA against KIF1A. The transport of cargo driven by motors other than KIF1A, including mitochondria and the transferrin receptor, was unaffected in KIF1A shRNA expressing cells. Taken together, these data support a primary role for KIF1A in the anterograde transport of DCVs in mammalian neurons, and also provide evidence that KIF1A remains associated with DCVs during retrograde DCV transport.

Probing intracellular motor protein activity using an inducible cargo trafficking assay

Although purified cytoskeletal motor proteins have been studied extensively with the use of in vitro approaches, a generic approach to selectively probe actin and microtubule-based motor protein activity inside living cells is lacking. To examine specific motor activity inside living cells, we utilized the FKBP-rapalog-FRB heterodimerization system to develop an in vivo peroxisomal trafficking assay that allows inducible recruitment of exogenous and endogenous kinesin, dynein, and myosin motors to drive specific cargo transport. We demonstrate that cargo rapidly redistributes with distinct dynamics for each respective motor, and that combined (antagonistic) actions of more complex motor combinations can also be probed. Of importance, robust cargo redistribution is readily achieved by one type of motor protein and does not require the presence of opposite-polarity motors. Simultaneous live-cell imaging of microtubules and kinesin or dynein-propelled peroxisomes, combined with high-resolution particle tracking, revealed that peroxisomes frequently pause at microtubule intersections. Titration and washout experiments furthermore revealed that motor recruitment by rapalog-induced heterodimerization is dose-dependent but irreversible. Our assay directly demonstrates that robust cargo motility does not require the presence of opposite-polarity motors, and can therefore be used to characterize the motile properties of specific types of motor proteins.

Kinesin-1 and dynein at the nuclear envelope mediate the bidirectional migrations of nuclei

Kinesin-1 and dynein are recruited to the nuclear envelope by the Caenorhabditis elegans klarsicht/ANC-1/Syne homology (KASH) protein UNC-83 to move nuclei. The mechanisms of how these motors are coordinated to mediate nuclear migration are unknown. Time-lapse differential interference contrast and fluorescence imaging of embryonic hypodermal nuclear migration events were used to characterize the kinetics of nuclear migration and determine microtubule dynamics and polarity. Wild-type nuclei display bidirectional movements during migration and are also able to roll past cytoplasmic granules. unc-83, unc-84, and kinesin-1 mutants have severe nuclear migration defects. Without dynein, nuclear migration initiates normally but lacks bidirectional movement and shows defects in nuclear rolling, implicating dynein in resolution of cytoplasmic roadblocks. Microtubules are highly dynamic during nuclear migration. EB1::green fluorescence protein imaging demonstrates that microtubules are polarized in the direction of nuclear migration. This organization of microtubules fits with our model that kinesin-1 moves nuclei forward and dynein functions to move nuclei backward for short stretches to bypass cellular roadblocks.