Hither and yon: a review of bi-directional microtubule-based transport

Quantum dots monitor TrkA receptor dynamics in the interior of neural PC12 cells

Can quantum dots (QDs) serve as physiologically relevant receptor probes in the interior of cells? We directly visualize endocytosis, redistribution, and shuttling of QD bound-TrkA receptors to PC12 neural processes and far-reaching growth cone tips. Internalized QDs are contained in microtubule-associated vesicles and possess transport properties that reflect TrkA receptor dynamics. This opens up new possibilities for the development of QD platforms as molecular tools to image biochemical signaling and transport cargo in the cell interior.

Milton controls the early acquisition of mitochondria by Drosophila oocytes

Mitochondria in many species enter the young oocyte en mass from interconnected germ cells to generate the large aggregate known as the Balbiani body. Organelles and germ plasm components frequently associate with this structure. Balbiani body mitochondria are thought to populate the germ line, ensuring that their genomes will be inherited preferentially. We find that milton, a gene whose product was previously shown to associate with Kinesin and to mediate axonal transport of mitochondria, is needed to form a normal Balbiani body. In addition, germ cells mutant for some milton or Kinesin heavy chain (Khc) alleles transport mitochondria to the oocyte prematurely and excessively, without disturbing Balbiani body-associated components. Our observations show that the oocyte acquires the majority of its mitochondria by competitive bidirectional transport along microtubules mediated by the Milton adaptor. These experiments provide a molecular explanation for Balbiani body formation and, surprisingly, show that viable fertile offspring can be obtained from eggs in which the normal program of mitochondrial acquisition has been severely perturbed.

Microtubule motors at the intersection of trafficking and transport

Molecular motors drive the transport of vesicles and organelles within the cell. Traditionally, these transport processes have been considered separately from membrane trafficking events, such as regulated budding and fusion. However, recent progress has revealed mechanistic links that integrate these processes within the cell. Rab proteins, which function as key regulators of intracellular trafficking, have now been shown to recruit specific motors to organelle membranes. Rab-independent recruitment of motors by adaptor or scaffolding proteins is also a key mechanism. Once recruited to vesicles and organelles, these motors can then drive directed transport; this directed transport could in turn affect the efficiency of trafficking events. Here, we discuss this coordinated regulation of trafficking and transport, which provides a powerful mechanism for temporal and spatial control of cellular dynamics.

Guidance of bidirectional motor complexes by mRNA cargoes through control of dynein number and activity

During asymmetric cytoplasmic mRNA transport, cis-acting localization signals are widely assumed to tether a specific subset of transcripts to motor complexes that have intrinsic directionality. Here we provide evidence that mRNA transcripts control their sorting by regulating the relative activities of opposing motors on microtubules. We show in Drosophila embryos that all mRNAs undergo bidirectional transport on microtubules and that cis-acting elements produce a range of polarized transcript distributions by regulating the frequency, velocity, and duration of minus-end-directed runs. Increased minus-end motility is dependent on the dosage of RNA elements and the proteins Egalitarian (Egl) and Bicaudal-D (BicD). We show that these proteins, together with the dynein motor, are recruited differentially to different RNA signals. Cytoplasmic transfer experiments reveal that, once assembled, cargo/motor complexes are insensitive to reduced cytoplasmic levels of transport proteins. Thus, the concentration of these proteins is only critical at the onset of transport. This work suggests that the architecture of RNA elements, through Egl and BicD, regulates directional transport by controlling the relative numbers of opposite polarity motors assembled. Our data raise the possibility that recruitment of different numbers of motors and regulatory proteins is a general strategy by which microtubule-based cargoes control their sorting.

Building complexity: an in vitro study of cytoplasmic dynein with in vivo implications

BACKGROUND

Cytoplasmic dynein is the molecular motor responsible for most retrograde microtubule-based vesicular transport. In vitro single-molecule experiments suggest that dynein function is not as robust as that of kinesin-1 or myosin-V because dynein moves only a limited distance (approximately 800 nm) before detaching and can exert a modest (approximately 1 pN) force. However, dynein-driven cargos in vivo move robustly over many microns and exert forces of multiple pN. To determine how to go from limited single-molecule function to robust in vivo transport, we began to build complexity in a controlled manner by using in vitro experiments.

RESULTS

We show that a single cytoplasmic dynein motor frequently transitions into an off-pathway unproductive state that impairs net transport. Addition of a second (and/or third) dynein motor, so that cargos are moved by two (or three) motors rather than one, is sufficient to recover several properties of in vivo motion; such properties include long cargo travels, robust motion, and increased forces. Part of this improvement appears to arise from selective suppression of the unproductive state of dynein rather than from a fundamental change in dynein's mechanochemical cycle.

CONCLUSIONS

Multiple dyneins working together suppress shortcomings of a single motor and generate robust motion under in vitro conditions. There appears to be no need for additional cofactors (e.g., dynactin) for this improvement. Because cargos are often driven by multiple dyneins in vivo, our results show that changing the number of dynein motors could allow modulation of dynein function from the mediocre single-dynein limit to robust in vivo-like dynein-driven motion.

A dynein loading zone for retrograde endosome motility at microtubule plus-ends

In the fungus Ustilago maydis, early endosomes move bidirectionally along microtubules (MTs) and facilitate growth by local membrane recycling at the tip of the infectious hypha. Here, we set out to elucidate the molecular mechanism of this process. We show that endosomes travel by Kinesin-3 activity into the hyphal apex, where they reverse direction and move backwards in a dynein-dependent manner. Our data demonstrate that dynein, dynactin and Lis1 accumulate at MT plus-ends within the hyphal tip, where they provide a reservoir of inactive motors for retrograde endosome transport. Consistently, endosome traffic is abolished after depletion of the dynein activator Lis1 and in Kinesin-1 null mutants, which was due to a defect in targeting of dynein and dynactin to the apical MT plus-ends. Furthermore, biologically active GFP-dynein travels on endosomes in retrograde and not in anterograde direction. Surprisingly, a CLIP170 homologue was neither needed for dynein localization nor for endosome transport. These results suggest an apical dynein loading zone in the hyphal tip, which ensure that endosomes reach the expanding growth region before they reverse direction.

Finite-particle tracking reveals submicroscopic-size changes of mitochondria during transport in mitral cell dendrites

The mechanisms of molecular motor regulation during bidirectional organelle transport are still uncertain. There is, for instance, the unsettled question of whether opposing motor proteins can be engaged in a tug-of-war. Clearly, any non-synchronous activation of the molecular motors of one cargo can principally lead to changes in the cargo's shape and size; the cargo's size and shape parameters would certainly be observables of such changes. We therefore set out to measure position, shape and size parameters of fluorescent mitochondria (during their transport) in dendrites of cultured neurons using a finite-particle tracking algorithm. Our data clearly show transport-related submicroscopic-size changes of mitochondria. The observed displacements of the mitochondrial front and rear ends are consistent with a model in which microtubule plus- and minus-end-directed motor proteins or motors of the same type but moving along anti-parallel microtubules are often out-of-phase and occasionally engaged in a tug-of-war. Mostly the leading and trailing ends of mitochondria undergo similar characteristic movements but with a substantial time delay between the displacements of both ends, a feature reminiscent of an inchworm-like motility mechanism. More generally, we demonstrate that observing the position, shape and size of actively transported finite objects such as mitochondria can yield information on organelle transport that is generally not accessible by tracking the organelles' centroid alone.

Active diffusion of motor particles

The movement of motor particles consisting of one or several molecular motors bound to a cargo particle is studied theoretically. The particles move on patterns of immobilized filaments. Several patterns are described for which the motor particles undergo nondirected but enhanced diffusion. Depending on the walking distance of the particles and the mesh size of the patterns, the active diffusion coefficient exhibits three different regimes. For micrometer-sized motor particles in water, e.g., this diffusion coefficient can be enhanced by 2 orders of magnitude.

Nonlinear dynamics of microtubules: biophysical implications

A recently developed model of nonlinear dynamics for microtubules is further expanded based on the biophysical arguments involving the secondary structure of the constitutive protein tubulin and on the ferroelectric properties of microtubules. It is demonstrated that kink excitations arise due to GTP hydrolysis that causes a dynamical transition in the structure of tubulin. The presence of an intrinsic electric field associated with the structure of a microtubule leads to unidirectional propagation of the kink excitation along the microtubule axis. This mechanism offers an explanation of the dynamic instability phenomenon in terms of the electric field effects. Moreover, a possible elucidation of the unidirectional transport of cargo via motor proteins such as kinesin and dynein is proposed within the model developed in this paper.

Direct observation demonstrates that Liprin-alpha is required for trafficking of synaptic vesicles

Axonal transport is required for the elaboration and maintenance of synaptic morphology and function. Liprin-alphas are scaffolding proteins important for synapse structure and electrophysiology. A reported interaction with Kinesin-3 (Kif1a) suggested Liprin-alpha may also be involved in axonal transport. Here, at the light and ultrastructural levels, we discover aberrant accumulations of synaptic vesicle markers (Synaptotagmin and Synaptobrevin-GFP) and clear-core vesicles along Drosophila Liprin-alpha mutant axons. Analysis of presynaptic markers reveals reduced levels at Liprin-alpha synapses. Direct visualization of Synaptobrevin-GFP transport in living animals demonstrates a decrease in anterograde processivity in Liprin-alpha mutants but also an increase in retrograde transport initiation. Pull-down assays reveal that Liprin-alpha interacts with Drosophila Kinesin-1 (Khc) but not dynein. Together, these findings suggest that Liprin-alpha promotes the delivery of synaptic material by a direct increase in kinesin processivity and an indirect suppression of dynein activation. This work is the first to use live observation in Drosophila mutants to demonstrate the role of a scaffolding protein in the regulation of bidirectional transport. It suggests the synaptic strength and morphology defects linked to Liprin-alpha may in part be due to a failure in the delivery of synaptic-vesicle precursors.

Regulation of Lipid-Droplet Transport by the Perilipin Homolog LSD2

BACKGROUND

Motor-driven transport along microtubules is a primary mechanism for moving and positioning organelles. How such transport is regulated remains poorly understood. For lipid droplets in Drosophila embryos, three distinct phases of transport can be distinguished. To identify factors regulating this transport, we biochemically purified droplets from individual phases and used 2D gel analysis to search for proteins whose amount on droplets changes as motion changes.

RESULTS

By mass spectrometry, we identified one such protein as LSD2. Similar to its mammalian counterpart Perilipin, LSD2 is responsible for regulating lipid homeostasis. Using specific antibodies, we confirmed that LSD2 is present on embryonic lipid droplets. We find that lack of LSD2 causes a specific transport defect: Droplet distribution fails to undergo the dramatic changes characteristic of the wild-type. This defect is not due to a complete failure of the core transport machinery--individual droplets still move bidirectionally along microtubules with approximately normal velocities and kinetics. Rather, detailed biophysical analysis suggests that developmental control of droplet motion is lost. We show that LSD2 is multiply phosphorylated in a developmentally controlled manner. LSD2 phosphorylation depends on the transacting signal Halo, and LSD2 can physically interact with the lipid-droplet-associated coordinator Klar, identifying LSD2 as a central player in the mechanisms that control droplet motion.

CONCLUSIONS

LSD2 appears to represent a new class of regulators, a protein that transduces regulatory signals to a separable core motor machinery. In addition, the demonstration that LSD2 regulates both transport and lipid metabolism suggests a link between lipid-droplet motion and lipid homeostasis.

Organelle-specific control of intracellular transport: distinctly targeted isoforms of the regulator Klar

Microtubule-based transport in cells is powered by a small set of distinct motors, yet timing and destination of transport can be controlled in a cargo-specific manner. The mechanistic basis for this specificity is not understood. To address this question, we analyzed the Drosophila Klarsicht (Klar) protein that regulates distinct microtubule-based transport processes. We find that localization of Klar to its cargoes is crucial for Klar function. Using mutations, we identify functionally important regions of Klar that confer distinct cargo specificity. In ovaries, Klar is present on the nuclear envelope, a localization that requires the C-terminal KASH domain. In early embryos, Klar is attached to lipid droplets, a localization mediated by a novel C-terminal domain encoded by an alternatively spliced exon. In cultured cells, these two domains are sufficient for targeting to the correct intracellular location. Our analysis disentangles Klar's modular organization: we propose that a core region integral to motor regulation is attached to variable domains so that the cell can target regulators with overlapping, yet distinct functions to specific cargoes. Such isoform variation may be a general strategy for adapting a common regulatory mechanism to specifically control motion and positioning of multiple organelles.

Partially asymmetric exclusion models with quenched disorder

We consider the one-dimensional partially asymmetric exclusion process with random hopping rates, in which a fraction of particles (or sites) have a preferential jumping direction against the global drift. In this case, the accumulated distance traveled by the particles, x, scales with the time, t, as x approximately t(1/z), with a dynamical exponent z>0. Using extreme value statistics and an asymptotically exact strong disorder renormalization group method, we exactly calculate z(PW) for particlewise disorder, which is argued to be related as z(SW)=z(PW)/2 for sitewise disorder. In the symmetric case with zero mean drift, the particle diffusion is ultraslow, logarithmic in time.

Molecular Motors: Strategies to Get Along

The majority of active transport in the cell is driven by three classes of molecular motors: the kinesin and dynein families that move toward the plus-end and minus-end of microtubules, respectively, and the unconventional myosin motors that move along actin filaments. Each class of motor has different properties, but in the cell they often function together. In this review we summarize what is known about their single-molecule properties and the possibilities for regulation of such properties. In view of new results on cytoplasmic dynein, we attempt to rationalize how these different classes of motors might work together as part of the intracellular transport machinery. We propose that kinesin and myosin are robust and highly efficient transporters, but with somewhat limited room for regulation of function. Because cytoplasmic dynein is less efficient and robust, to achieve function comparable to the other motors it requires a number of accessory proteins as well as multiple dyneins functioning together. This necessity for additional factors, as well as dynein's inherent complexity, in principle allows for greatly increased control of function by taking the factors away either singly or in combination. Thus, dynein's contribution relative to the other motors can be dynamically tuned, allowing the motors to function together differently in a variety of situations.