Cytoplasmic dynein and microtubule transport in the axon: The action connection

The neuron uses two families of microtubule-based motors for fast axonal transport, kinesin, and cytoplasmic dynein. Cytoplasmic dynein moves membranous organelles from the distal regions of the axon to the cell body. Because dynein is synthesized in the cell body, it must first be delivered to the axon tip. It has recently been shown that cytoplasmic dynein is moved from the cell body along the axon by two different mechanisms. A small amount is associated with fast anterograde transport, the membranous organelles moved by kinesin. Most of the dynein is transported in slow component b, the actin-based transport compartment. Dynactin, a protein complex that binds dynein, is also transported in slow component b. The dynein in slow component b binds to microtubules in an ATP-dependent manner in vitro, suggesting that this dynein is enzymatically active. The finding that functionally active dynein, and dynactin, are associated with the actin-based transport compartment suggests a mechanism whereby dynein anchored to the actin cytoskeleton via dynactin provides the motive force for microtubule movement in the axon.     

Quantitative evaluation of the mode of microtubule transport in Xenopus neurons

Tubulin is synthesized in the cell body and must be delivered to the axon to support axonal growth. However, the exact form in which these proteins, in particular tubulin, move within the axon remains contentious. According to the "polymer transport model", tubulin is transported in the form of microtubules. In an alternative hypothesis, the "short oligomer transport model", tubulin is added to existing, stationary microtubules along the axon. In this study, we measured the translocation of microtubule plus ends in soma segments, the middle of axonal shafts and the growth cone areas, by expressing GFP-EB3 in cultured Xenopus embryonic spinal neurons. We found that none of the microtubules in the three compartments were transported rapidly as would be expected from the polymer transport model. These results suggest that microtubules are stationary in most segments of the axon, thus supporting the model according to which tubulin is transported in non-polymeric form in rapidly growing Xenopus neurons.     

Transport complexes associated with slow axonal flow

Cytoskeletal proteins-neurofilament polypeptides, tubulin and actin-are transported along axons by slow transport. How or in what form they are transported is not known. One hypothesis is that they are assembled into the cytoskeleton at the cell body and transported as intact polymers down the axon. However, recent radiolabeling and photobleaching studies have shown that tubulin and actin exist in both a mobile phase and a stationary phase in the axon. Consequently, it is more likely that cytoskeletal proteins move along the axon in some form of transport complex and are assembled into a cytoskeleton which is stationary. In this overview we discuss these topics and consider the evidence for the existence of transport complexes associated with slow axonal flow. Such evidence includes the slow transport of particulate complexes containing tubulin and neurofilament polypeptides along reconstituted microtubules in vitro, and the coordinate slow transport of actin with actin-binding in vivo.Special issue dedicated to Dr. Lawrence Austin.     

Transport and detection of unlabeled nucleotide targets by microtubules functionalized with molecular beacons

Shrinking biosensors down to microscale dimensions enables increases in sensitivity and the ability to analyze minute samples such as the contents of individual cells. The goal of the present study is to create mobile microscale biosensors by attaching molecular beacons to microtubules and using kinesin molecular motors to transport these functionalized microtubules across two-dimensional surfaces. Previous work has shown that microfluidic channels can be functionalized with kinesin motors such that microtubules can be transported and directed through these channels without the need for external power or pressure-driven pumping. In this work, we show that molecular beacons can be attached to microtubules such that both the fluorescence reporting capability of the beacon and the motility of the microtubules are retained. These molecular beacon-functionalized microtubules were able to bind ssDNA target sequences, transport them across surfaces, and report their presence by an increase in fluorescence that was detected by fluorescence microscopy. This work is an important step toward creating hybrid microdevices for sensitive virus detection or analyzing mRNA profiles of individual cells.     

One axon,many kinesins: What's the logic?

A large number of membrane-bounded organelles, protein complexes, and mRNAs are transported along microtubules to different locations within the neuronal axon. Axonal transport in the anterograde direction is carried out by members of a superfamily of specialized motor proteins, the kinesins. All kinesins contain a conserved motor domain that hydrolyses ATP to generate movement along microtubules. Regions outside the motor domain are responsible for cargo binding and regulation of motor activity. Present in a soluble, inactive form in the cytoplasm, kinesins are activated upon cargo binding. Selective targeting of different types of kinesin motors to specific cargoes is directed by amino acid sequences situated in their variable tails. Cargo proteins with specific function at their destination, bind directly to specific kinesins for transport. Whereas most kinesins move to microtubule plus-ends, a small number of them move to microtubule minus-ends, and may participate in retrograde axonal transport. Axonal transport by kinesins has a logic: Fully assembled, multisubunit, functional complexes (e.g., ion channel complexes, signaling complexes, RNA-protein complexes) are transported to their destination by kinesin motors that interact transiently (i.e., during transport only) with one of the complexes' subunits.     

Diffusion of kinesin motors on cargo can enhance binding and run lengths during intracellular transport

Cellular cargoes, including lipid droplets and mitochondria, are transported along microtubules using molecular motors such as kinesins. Many experimental and computational studies focused on cargoes with rigidly attached motors, in contrast to many biological cargoes that have lipid surfaces that may allow surface mobility of motors. We extend a mechanochemical three-dimensional computational model by adding coupled-viscosity effects to compare different motor arrangements and mobilities. We show that organizational changes can optimize for different objectives: Cargoes with clustered motors are transported efficiently but are slow to bind to microtubules, whereas those with motors dispersed rigidly on their surface bind microtubules quickly but are transported inefficiently. Finally, cargoes with freely diffusing motors have both fast binding and efficient transport, although less efficient than clustered motors. These results suggest that experimentally observed changes in motor organization may be a control point for transport.     

Effects of dynactin disruption and dynein depletion on axonal microtubules

We investigated potential roles of cytoplasmic dynein in organizing axonal microtubules either by depleting dynein heavy chain from cultured neurons or by experimentally disrupting dynactin. The former was accomplished by siRNA while the latter was accomplished by overexpressing P50-dynamitin. Both methods resulted in a persistent reduction in the frequency of transport of short microtubules. To determine if the long microtubules in the axon also undergo dynein-dependent transport, we ascertained the rates of EGFP-EB3 "comets" observed at the tips of microtubules during assembly. The rates of the comets, in theory, should reflect a combination of the assembly rate and any potential transport of the microtubule. Comets were initially slowed during P50-dynamitin overexpression, but this effect did not persist beyond the first day and was never observed in dynein-depleted axons. In fact, the rates of the comets were slightly faster in dynein-depleted axons. We conclude that the transient effect of P50-dynamitin overexpression reflects a reduction in microtubule polymerization rates. Interestingly, after prolonged dynein depletion, the long microtubules were noticeably misaligned in the distal regions of axons and failed to enter the filopodia of growth cones. These results suggest that the forces generated by cytoplasmic dynein do not transport long microtubules, but may serve to align them with one another and also permit them to invade filopodia.     

Slow axonal transport: fast motors in the slow lane.

The bulk of neuronally synthesized proteins destined for the axon is transported in a phase of transport approximately 100 times slower (1mm/day) than the vesicular traffic of fast axonal transport (100mm/day). Of late, a number of studies have shed considerable light on the controversies and mechanisms surrounding this slow phase of axonal transport. Along-standing controversy has centered on the form of the transported proteins. One major transport cargo, neurofilament protein, has now been seen in a number of contexts to be transported primarily in a polymeric form, whereas a second cargo tubulin is transported as a small oligomer. The development of techniques to visualize the slow transport process in live cells has demonstrated that instantaneous motions of transported neurofilaments, and presumably other slow transport cargoes, are fast, bidirectional and interspersed with long pauses. This and additional biochemical efforts indicate that traditional fast motors, such as conventional kinesin and dynein, are responsible for these fast motions.     

Tau-induced traffic jams reflect organelles accumulation at points of microtubule polar mismatching

It is currently accepted that tau overexpression leads to impaired organelle transport and thus to neuronal degeneration. Nevertheless, the underlying mechanisms that lead to impaired organelle transport are not entirely clear. Using cultured Aplysia neurons and online confocal imaging of human tau, microtubules (MTs), the plus-end tracking protein – end-binding protein 3, retrogradely and anterogradely transported organelles, we found that overexpression of tau generates the hallmarks of human tau pathogenesis. Nevertheless, in contrast to earlier reports, we found that the tau-induced impairment of organelle transport is because of polar reorientation of the MTs along the axon or their displacement to submembrane domains. 'Traffic jams' reflect the accumulation of organelles at points of MT polar discontinuations or polar mismatching rather than because of MT depolymerization. Our findings offer a new mechanistic explanation for earlier observations, which established that tau overexpression leads to impaired retrograde and anterograde organelle transport, while the MT skeleton appeared intact.     

Computational Modeling Reveals Optimal Strategy for Kinase Transport by Microtubules to Nerve Terminals

Intracellular transport of proteins by motors along cytoskeletal filaments is crucial to the proper functioning of many eukaryotic cells. Since most proteins are synthesized at the cell body, mechanisms are required to deliver them to the growing periphery. In this article, we use computational modeling to study the strategies of protein transport in the context of JNK (c-JUN NH2-terminal kinase) transport along microtubules to the terminals of neuronal cells. One such strategy for protein transport is for the proteins of the JNK signaling cascade to bind to scaffolds, and to have the whole protein-scaffold cargo transported by kinesin motors along microtubules. We show how this strategy outperforms protein transport by diffusion alone, using metrics such as signaling rate and signal amplification. We find that there exists a range of scaffold concentrations for which JNK transport is optimal. Increase in scaffold concentration increases signaling rate and signal amplification but an excess of scaffolds results in the dilution of reactants. Similarly, there exists a range of kinesin motor speeds for which JNK transport is optimal. Signaling rate and signal amplification increases with kinesin motor speed until the speed of motor translocation becomes faster than kinase/scaffold-motor binding. Finally, we suggest experiments that can be performed to validate whether, in physiological conditions, neuronal cells do indeed adopt such an optimal strategy. Understanding cytoskeletal-assisted protein transport is crucial since axonal and cell body accumulation of organelles and proteins is a histological feature in many human neurodegenerative diseases. In this paper, we have shown that axonal transport performance changes with altered transport component concentrations and transport speeds wherein these aspects can be modulated to improve axonal efficiency and prevent or slowdown axonal deterioration.     

Speckle microscopic evaluation of microtubule transport in growing nerve processes

Assembly of microtubules is fundamental to neuronal morphogenesis. Microtubules typically form crosslinked bundles in nerve processes, precluding resolution of single microtubules at the light microscopic level. Therefore, previous studies of microtubule transport in neurites have had to rely on indirect approaches. Here we show that individual microtubules can be visualized directly in the axonal shafts of Xenopus embryo neurons by using digital fluorescence microscopy. We find that, although the array of axonal microtubules is dynamic, microtubules are stationary relative to the substrate. These results argue against a model in which newly synthesized tubulin is transported down the axon in the form of microtubules.     

Regulation of organelle transport: Lessons from color change in fish

Organelles transported along microtubules are normally moved to precise locations within cells. For example, synaptic vesiceles are transported to the neruronal synapse, the Golgi apparatus is generally found in a perinuclear location, and the membranes of the endoplasmic reticulum are actively extended to the cell periphery. The correct positioning of these organelles depends on microtubules and microtubule motors. Melanophores provide an extreme example of organized organelle transport. These cells are specialized to transport pigment granules, which are coordinately moved towards or away from the cell center, and result in the cell appearing alternately light or dark. Melanophores have proved to be an ideal system for studying the mechanisms by which the cell controls the direction of its organelle transport. Pigment granule dispersion (the movement away from the cell center) requires protein phosphorylation, while pigment aggregation (the movement towards the cell center) requires protein dephosphorylation. The target of this phosphorylation and dephosphorylation event is a protein that interacts with the microtubule motor protein, kinesin. Thus, the direction of organelle transport along microtubules may be regulated by controlling the activity of a microtubule motor.     

Axonal protein synthesis and transport

Recent evidence has challenged our ideas about the nature of axonal protein synthesis and transport. Previous metabolic labeling evidence supported the idea that all axonal proteins were synthesized in the cell body and then transported as formed cytoplasmic structures into the axon. Recent evidence suggests that neither the synthesis nor the transport of axonal proteins is that simple. Though most axonal proteins do appear to be synthesized in the neuronal cell body, a small amount of protein appears to be synthesized intra-axonally in some axons. Though small in amount, intra-axonal protein synthesis may be important functionally in some axons. Recent experiments have also begun to identify the presence of a rich array of transport motors in axons, including many members of the kinesin, dynein and myosin families. Progress is being made in identifying which cargoes are being transported by which of these motors. Finally, recent experiments have addressed an old question about whether axoplasmic proteins are transported as filamentous polymers or as soluble components in axons. The answer is that both mechanism can be used in axons. For example, neurofilament protein can move in its particulate or polymeric state, while tubulin can move in its soluble or unpolymerized state.     

Slow axonal transport and the genesis of neuronal morphology

The classic view of slow axonal transport maintains that microtubules, neurofilaments, and actin filaments move down the axon relatively coherently at rates significantly slower than those characteristic of known motor proteins. Recent studies indicate that the movement of these cytoskeletal polymers is actually rapid, asynchronous, intermittent, and most probably fueled by familiar motors such as kinesins, myosins, and cytoplasmic dynein. This new view, which is supported by both live-cell imaging and mechanistic analyses, suggests that slow axonal transport is both rapid and plastic, and hence could underlie transformations in neuronal morphology.     

CRMP-2 directly binds to cytoplasmic dynein and interferes with its activity

The active transport of proteins and organelles is critical for cellular organization and function in eukaryotic cells. A substantial portion of long-distance transport depends on the opposite polarity of the kinesin and dynein family molecular motors to move cargo along microtubules. It is increasingly clear that many cargo molecules are moved bi-directionally by both sets of motors; however, the regulatory mechanism that determines the directionality of transport remains unclear. We previously reported that collapsin response mediator protein-2 (CRMP-2) played key roles in axon elongation and neuronal polarization. CRMP-2 was also found to associate with the anterograde motor protein Kinesin-1 and was transported with other cargoes toward the axon terminal. In this study, we investigated the association of CRMP-2 with a retrograde motor protein, cytoplasmic dynein. Immunoprecipitation assays showed that CRMP-2 interacted with cytoplasmic dynein heavy chain. Dynein heavy chain directly bound to the N-terminus of CRMP-2, which is the distinct side of CRMP-2's kinesin light chain-binding region. Furthermore, over-expression of the dynein-binding fragments of CRMP-2 prevented dynein-driven microtubule transport in COS-7 cells. Given that CRMP-2 is a key regulator of axon elongation, this interference with cytoplasmic dynein function by CRMP-2 might have an important role in axon formation, and neuronal development.     

Myosin Va movements in normal and dilute-lethal axons provide support for a dual filament motor complex.

To investigate the role that myosin Va plays in axonal transport of organelles, myosin Va-associated organelle movements were monitored in living neurons using microinjected fluorescently labeled antibodies to myosin Va or expression of a green fluorescent protein-myosin Va tail construct. Myosin Va-associated organelles made rapid bi-directional movements in both normal and dilute-lethal (myosin Va null) neurites. In normal neurons, depolymerization of microtubules by nocodazole slowed, but did not stop movement. In contrast, depolymerization of microtubules in dilute-lethal neurons stopped movement. Myosin Va or synaptic vesicle protein 2 (SV2), which partially colocalizes with myosin Va on organelles, did not accumulate in dilute-lethal neuronal cell bodies because of an anterograde bias associated with organelle transport. However, SV2 showed peripheral accumulations in axon regions of dilute-lethal neurons rich in tyrosinated tubulin. This suggests that myosin Va-associated organelles become stranded in regions rich in dynamic microtubule endings. Consistent with these observations, presynaptic terminals of cerebellar granule cells in dilute-lethal mice showed increased cross-sectional area, and had greater numbers of both synaptic and larger SV2 positive vesicles. Together, these results indicate that myosin Va binds to organelles that are transported in axons along microtubules. This is consistent with both actin- and microtubule-based motors being present on these organelles. Although myosin V activity is not necessary for long-range transport in axons, myosin Va activity is necessary for local movement or processing of organelles in regions, such as presynaptic terminals that lack microtubules.     

Molecular motors in axonal transport

Neurons require a large amount of intracellular transport. Cytoplasmic polypeptides and membrane-bounded organelles move from the perikaryon, down the length of the axon, and to the synaptic terminals. This movement occurs at distinct rates and is termed axonal transport. Axonal transport is divided into the slow transport of cytoplasmic proteins including glycolytic enzymes and cytoskeletal structures and the fast transport of membrane-bounded organelles along linear arrays of microtubules. The polypeptide compositions of the rate classes of axonal transport have been well characterized, but the underlying molecular mechanisms of this movement are less clear. Progress has been particularly slow toward understanding force-generation in slow transport, but recent developments have provided insight into the molecular motors involved in fast axonal transport. Recent advances in the cellular and molecular biology of one fast axonal transport motor, kinesin, have provided a clearer understanding of organelle movement along microtubules. The availability of cellular and molecular probes for kinesin and other putative axonal transport motors have led to a reevaluation of our understanding of intracellular motility.     

Antagonistic forces generated by cytoplasmic dynein and myosin-II during growth cone turning and axonal retraction

Cytoplasmic dynein transports short microtubules down the axon in part by pushing against the actin cytoskeleton. Recent studies have suggested that comparable dynein-driven forces may impinge upon the longer microtubules within the axon. Here, we examined a potential role for these forces on axonal retraction and growth cone turning in neurons partially depleted of dynein heavy chain (DHC) by small interfering RNA. While DHC-depleted axons grew at normal rates, they retracted far more robustly in response to donors of nitric oxide than control axons, and their growth cones failed to efficiently turn in response to substrate borders. Live cell imaging of dynamic microtubule tips showed that microtubules in DHC-depleted growth cones were largely confined to the central zone, with very few extending into filopodia. Even under conditions of suppressed microtubule dynamics, DHC depletion impaired the capacity of microtubules to advance into the peripheral zone of the growth cone, indicating a direct role for dynein-driven forces on the distribution of the microtubules. These effects were all reversed by inhibition of myosin-II forces, which are known to underlie the retrograde flow of actin in the growth cone and the contractility of the cortical actin during axonal retraction. Our results are consistent with a model whereby dynein-driven forces enable microtubules to overcome myosin-II-driven forces, both in the axonal shaft and within the growth cone. These dynein-driven forces oppose the tendency of the axon to retract and permit microtubules to advance into the peripheral zone of the growth cone so that they can invade filopodia.     

Motor Reattachment Kinetics Play a Dominant Role in Multimotor-Driven Cargo Transport

Kinesin-based cargo transport in cells frequently involves the coordinated activity of multiple motors, including kinesins from different families that move at different speeds. However, compared to the progress at the single-molecule level, mechanisms by which multiple kinesins coordinate their activity during cargo transport are poorly understood. To understand these multimotor coordination mechanisms, defined pairs of kinesin-1 and kinesin-2 motors were assembled on DNA scaffolds and their motility examined in vitro. Although less processive than kinesin-1 at the single-molecule level, addition of kinesin-2 motors more effectively amplified cargo run lengths. By applying the law of total expectation to cargo binding durations in ADP, the kinesin-2 microtubule reattachment rate was shown to be fourfold faster than that of kinesin-1. This difference in microtubule binding rates was also observed in solution by stopped-flow. High-resolution tracking of a gold-nanoparticle-labeled motor with 1 ms and 2 nm precision revealed that kinesin-2 motors detach and rebind to the microtubule much more frequently than does kinesin-1. Finally, compared to cargo transported by two kinesin-1, cargo transported by two kinesin-2 motors more effectively navigated roadblocks on the microtubule track. These results highlight the importance of motor reattachment kinetics during multimotor transport and suggest a coordinated transport model in which kinesin-1 motors step effectively against loads whereas kinesin-2 motors rapidly unbind and rebind to the microtubule. This dynamic tethering by kinesin-2 maintains the cargo near the microtubule and enables effective navigation along crowded microtubules.     

Axonal Transport: How High Microtubule Density Can Compensate for?Boundary Effects in Small-Caliber Axons

Long-distance intracellular axonal transport is predominantly microtubule-based, and its impairment is linked to neurodegeneration. In this study, we present theoretical arguments that suggest that near the axon boundaries (walls), the effective viscosity can become large enough to impede cargo transport in small (but not large) caliber axons. Our theoretical analysis suggests that this opposition to motion increases rapidly as the cargo approaches the wall. We find that having parallel microtubules close enough together to enable a cargo to simultaneously engage motors on more than one microtubule dramatically enhances motor activity, and thus minimizes the effects of any opposition to transport. Even if microtubules are randomly placed in axons, we find that the higher density of microtubules found in small-caliber axons increases the probability of having parallel microtubules close enough that they can be used simultaneously by motors on a cargo. The boundary effect is not a factor in transport in large-caliber axons where the microtubule density is lower.