A Mechanism for Cytoplasmic Streaming: Kinesin-Driven Alignment of Microtubules and Fast Fluid Flows.

The transport of cytoplasmic components can be profoundly affected by hydrodynamics. Cytoplasmic streaming in Drosophila oocytes offers a striking example. Forces on fluid from kinesin-1 are initially directed by a disordered meshwork of microtubules, generating minor slow cytoplasmic flows. Subsequently, to mix incoming nurse cell cytoplasm with ooplasm, a subcortical layer of microtubules forms parallel arrays that support long-range, fast flows. To analyze the streaming mechanism, we combined observations of microtubule and organelle motions with detailed mathematical modeling. In the fast state, microtubules tethered to the cortex form a thin subcortical layer and undergo correlated sinusoidal bending. Organelles moving in flows along the arrays show velocities that are slow near the cortex and fast on the inward side of the subcortical microtubule layer. Starting with fundamental physical principles suggested by qualitative hypotheses, and with published values for microtubule stiffness, kinesin velocity, and cytoplasmic viscosity, we developed a quantitative coupled hydrodynamic model for streaming. The fully detailed mathematical model and its simulations identify key variables that can shift the system between disordered (slow) and ordered (fast) states. Measurements of array curvature, wave period, and the effects of diminished kinesin velocity on flow rates, as well as prior observations on f-actin perturbation, support the model. This establishes a concrete mechanistic framework for the ooplasmic streaming process. The self-organizing fast phase is a result of viscous drag on kinesin-driven cargoes that mediates equal and opposite forces on cytoplasmic fluid and on microtubules whose minus ends are tethered to the cortex. Fluid moves toward plus ends and microtubules are forced backward toward their minus ends, resulting in buckling. Under certain conditions, the buckling microtubules self-organize into parallel bending arrays, guiding varying directions for fast plus-end directed fluid flows that facilitate mixing in a low Reynolds number regime.

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.

Kinesin-1 tail autoregulation and microtubule-binding regions function in saltatory transport but not ooplasmic streaming.

The N-terminal head domain of kinesin heavy chain (Khc) is well known for generating force for transport along microtubules in cytoplasmic organization processes during metazoan development, but the functions of the C-terminal tail are not clear. To address this, we studied the effects of tail mutations on mitochondria transport, determinant mRNA localization and cytoplasmic streaming in Drosophila. Our results show that two biochemically defined elements of the tail - the ATP-independent microtubule-binding sequence and the IAK autoinhibitory motif - are essential for development and viability. Both elements have positive functions in the axonal transport of mitochondria and determinant mRNA localization in oocytes, processes that are accomplished by biased saltatory movement of individual cargoes. Surprisingly, there were no indications that the IAK autoinhibitory motif acts as a general downregulator of Kinesin-1 in those processes. Time-lapse imaging indicated that neither tail region is needed for fast cytoplasmic streaming in oocytes, which is a non-saltatory bulk transport process driven solely by Kinesin-1. Thus, the Khc tail is not constitutively required for Kinesin-1 activation, force transduction or linkage to cargo. It might instead be crucial for more subtle elements of motor control and coordination in the stop-and-go movements of biased saltatory transport.

Dissection and Direct Imaging of Axonal Transport in Drosophila Segmental Nerves.

For neurons, especially those with long axons, the forceful transport of mitochondria, vesicles, and other cytoplasmic components by cytoskeletal motors is vital. Defects in cytoplasmic transport machinery cause a degradation of signaling capacity that is most severe for neurons with the longest axons. In humans, with motor axons up to a meter long, even a mild mutation in one copy of the gene that codes for kinesin-1, the primary anterograde axonal transport motor, can cause spastic paraplegia and other distal neuropathies.To address questions about the molecular mechanisms of organelle movement, we turned to Drosophila as a model system, because it offered rigorous genetic and molecular approaches to the identification and inhibition of specific elements of transport machinery. However, methods for direct observation of organelle transport were largely lacking. We describe here an approach that we developed for imaging the transport behaviors of specific organelles in the long motor axons of larvae. It is straightforward, the equipment is commonly available, and it provides a powerful tool for studying the contributions of specific proteins to organelle transport mechanisms.

Control of a kinesin-cargo linkage mechanism by JNK pathway kinases.

Long-distance organelle transport toward axon terminals, critical for neuron development and function, is driven along microtubules by kinesins [1, 2]. The biophysics of force production by various kinesins is known in detail. However, the mechanisms of in vivo transport processes are poorly understood because little is known about how motor-cargo linkages are controlled. A c-Jun N-terminal kinase (JNK)-interacting protein (JIP1) has been identified previously as a linker between kinesin-1 and certain vesicle membrane proteins, such as Alzheimer's APP protein and a reelin receptor ApoER2 [3, 4]. JIPs are also known to be scaffolding proteins for JNK pathway kinases [5, 6]. Here, we report evidence that a Drosophila ubiquitin-specific hydrolase and a JNK signaling pathway that it modulates can regulate a JIP1-kinesin linkage. The JNK pathway includes a MAPKKK (Wallenda/DLK), a MAPKK (Hemipterous/MKK7), and the Drosophila JNK homolog Basket. Genetic tests indicate that those kinases are required for normal axonal transport. Biochemical tests show that activation of Wallenda (DLK) and Hemipterous (MKK7) disrupts binding between kinesin-1 and APLIP1, which is the Drosophila JIP1 homolog. This suggests a control mechanism in which an activated JNK pathway influences axonal transport by functioning as a kinesin-cargo dissociation factor.

Loss of KLP-19 polar ejection force causes misorientation and missegregation of holocentric chromosomes.

Holocentric chromosomes assemble kinetochores along their length instead of at a focused spot. The elongated expanse of an individual holocentric kinetochore and its potential flexibility heighten the risk of stable attachment to microtubules from both poles of the mitotic spindle (merotelic attachment), and hence aberrant segregation of chromosomes. Little is known about the mechanisms that holocentric species have evolved to avoid this type of error. Our studies of the influence of KLP-19, an essential microtubule motor, on the behavior of holocentric Caenorhabditis elegans chromosomes suggest that it has a major role in combating merotelic attachments. Depletion of KLP-19, which associates with nonkinetochore chromatin, allows aberrant poleward chromosome motion during prometaphase, misalignment of holocentric kinetochores, and multiple anaphase chromosome bridges in all mitotic divisions. Time-lapse movies of GFP-labeled mono- and bipolar spindles demonstrate that KLP-19 generates a force on relatively stiff holocentric chromosomes that pushes them away from poles. We hypothesize that this polar ejection force minimizes merotelic misattachment by maintaining a constant tension on pole-kinetochore connections throughout prometaphase, tension that compels sister kinetochores to face directly toward opposite poles.

A standardized kinesin nomenclature.

In recent years the kinesin superfamily has become so large that several different naming schemes have emerged, leading to confusion and miscommunication. Here, we set forth a standardized kinesin nomenclature based on 14 family designations. The scheme unifies all previous phylogenies and nomenclature proposals, while allowing individual sequence names to remain the same, and for expansion to occur as new sequences are discovered.

Kinesin-1 and Dynein are the primary motors for fast transport of mitochondria in Drosophila motor axons.

To address questions about mechanisms of filament-based organelle transport, a system was developed to image and track mitochondria in an intact Drosophila nervous system. Mutant analyses suggest that the primary motors for mitochondrial movement in larval motor axons are kinesin-1 (anterograde) and cytoplasmic dynein (retrograde), and interestingly that kinesin-1 is critical for retrograde transport by dynein. During transport, there was little evidence that force production by the two opposing motors was competitive, suggesting a mechanism for alternate coordination. Tests of the possible coordination factor P150(Glued) suggested that it indeed influenced both motors on axonal mitochondria, but there was no evidence that its function was critical for the motor coordination mechanism. Observation of organelle-filled axonal swellings ("organelle jams" or "clogs") caused by kinesin and dynein mutations showed that mitochondria could move vigorously within and pass through them, indicating that they were not the simple steric transport blockades suggested previously. We speculate that axonal swellings may instead reflect sites of autophagocytosis of senescent mitochondria that are stranded in axons by retrograde transport failure; a protective process aimed at suppressing cell death signals and neurodegeneration.

APLIP1, a kinesin binding JIP-1/JNK scaffold protein, influences the axonal transport of both vesicles and mitochondria in Drosophila.

In a genetic screen for Kinesin heavy chain (Khc)-interacting proteins, we identified APLIP1, a neuronally expressed Drosophila homolog of JIP-1, a JNK scaffolding protein . JIP-1 and its homologs have been proposed to act as physical linkers between kinesin-1, which is a plus-end-directed microtubule motor, and certain anterograde vesicles in the axons of cultured neurons . Mutation of Aplip1 caused larval paralysis, axonal swellings, and reduced levels of both anterograde and retrograde vesicle transport, similar to the effects of kinesin-1 inhibition. In contrast, Aplip1 mutation caused a decrease only in retrograde transport of mitochondria, suggesting inhibition of the minus-end microtubule motor cytoplasmic dynein . Consistent with dynein defects, combining heterozygous mutations in Aplip1 and Dynein heavy chain (Dhc64C) generated synthetic axonal transport phenotypes. Thus, APLIP1 may be an important part of motor-cargo linkage complexes for both kinesin-1 and dynein. However, it is also worth considering that APLIP1 and its associated JNK signaling proteins could serve as an important signaling module for regulating transport by the two opposing motors.

Abl tyrosine kinase and its substrate Ena/VASP have functional interactions with kinesin-1.

Relatively little is known about how microtubule motors are controlled or about how the functions of different cytoskeletal systems are integrated. A yeast two-hybrid screen for proteins that bind to Drosophila Enabled (Ena), an actin polymerization factor that is negatively regulated by Abl tyrosine kinase, identified kinesin heavy chain (Khc), a member of the kinesin-1 subfamily of microtubule motors. Coimmunoprecipitation from Drosophila cytosol confirmed a physical interaction between Khc and Ena. Kinesin-1 motors can carry organelles and other macromolecular cargoes from neuronal cell bodies toward terminals in fast-axonal-transport. Ena distribution in larval axons was not affected by mutations in the Khc gene, suggesting that Ena is not itself a fast transport cargo of Drosophila kinesin-1. Genetic interaction tests showed that in a background sensitized by reduced Khc gene dosage, a reduction in Abl gene dosage caused distal paralysis and axonal swellings. A concomitant reduction in ena dosage rescued those defects. These results suggest that Ena/VASP, when not inhibited by the Abl pathway, can bind Khc and reduce its transport activity in axons.

Functional analysis of cytoplasmic dynein heavy chain in Caenorhabditis elegans with fast-acting temperature-sensitive mutations.

Cytoplasmic dynein, a minus-end-directed microtubule motor, has been implicated in many cellular and developmental processes. Identification of specific cellular processes that rely directly on dynein would be facilitated by a means to induce specific and rapid inhibition of its function. We have identified conditional variants of a Caenorhabditis elegans dynein heavy chain (DHC-1) that lose function within a minute of a modest temperature upshift. Mutant embryos generated at elevated temperature show defects in centrosome separation, pronuclear migration, rotation of the centrosome/nucleus complex, bipolar spindle assembly, anaphase chromosome segregation, and cytokinesis. Our analyses of mutant embryos generated at permissive temperature and then upshifted quickly just before events of interest indicate that DHC-1 is required specifically for rotation of the centrosome/nucleus complex, for chromosome congression to a well ordered metaphase plate, and for timely initiation of anaphase. Our results do not support the view that DHC-1 is required for anaphase B separation of spindle poles and chromosomes. A P-loop mutation identified in two independent dominant temperature-sensitive alleles of dhc-1, when engineered into the DHC1 gene of Saccharomyces cerevisiae, conferred a dominant temperature-sensitive dynein loss-of-function phenotype. This suggests that temperature-sensitive mutations can be created for time-resolved function analyses of dyneins and perhaps other P-loop proteins in a variety of model systems.

Posterior localization of dynein and dorsal-ventral axis formation depend on kinesin in Drosophila oocytes.

To establish the major body axes, late Drosophila oocytes localize determinants to discrete cortical positions: bicoid mRNA to the anterior cortex, oskar mRNA to the posterior cortex, and gurken mRNA to the margin of the anterior cortex adjacent to the oocyte nucleus (the "anterodorsal corner"). These localizations depend on microtubules that are thought to be organized such that plus end-directed motors can move cargoes, like oskar, away from the anterior/lateral surfaces and hence toward the posterior pole. Likewise, minus end-directed motors may move cargoes toward anterior destinations. Contradicting this, cytoplasmic dynein, a minus-end motor, accumulates at the posterior. Here, we report that disruption of the plus-end motor kinesin I causes a shift of dynein from posterior to anterior. This provides an explanation for the dynein paradox, suggesting that dynein is moved as a cargo toward the posterior pole by kinesin-generated forces. However, other results present a new transport polarity puzzle. Disruption of kinesin I causes partial defects in anterior positioning of the nucleus and severe defects in anterodorsal localization of gurken mRNA. Kinesin may generate anterodorsal forces directly, despite the apparent preponderance of minus ends at the anterior cortex. Alternatively, kinesin I may facilitate cytoplasmic dynein-based anterodorsal forces by repositioning dynein toward microtubule plus ends.

KLP-18, a Klp2 kinesin, is required for assembly of acentrosomal meiotic spindles in Caenorhabditis elegans.

The proper segregation of chromosomes during meiosis or mitosis requires the assembly of well organized spindles. In many organisms, meiotic spindles lack centrosomes. The formation of such acentrosomal spindles seems to involve first assembly or capture of microtubules (MTs) in a random pattern around the meiotic chromosomes and then parallel bundling and bipolar organization by the action of MT motors and other proteins. Here, we describe the structure, distribution, and function of KLP-18, a Caenorhabditis elegans Klp2 kinesin. Previous reports of Klp2 kinesins agree that it concentrates in spindles, but do not provide a clear view of its function. During prometaphase, metaphase, and anaphase, KLP-18 concentrates toward the poles in both meiotic and mitotic spindles. Depletion of KLP-18 by RNA-mediated interference prevents parallel bundling/bipolar organization of the MTs that accumulate around female meiotic chromosomes. Hence, meiotic chromosome segregation fails, leading to haploid or aneuploid embryos. Subsequent assembly and function of centrosomal mitotic spindles is normal except when aberrant maternal chromatin is present. This suggests that although KLP-18 is critical for organizing chromosome-derived MTs into a parallel bipolar spindle, the order inherent in centrosome-derived astral MT arrays greatly reduces or eliminates the need for KLP-18 organizing activity in mitotic spindles.

Two kinesins drive anterograde neuropeptide transport.

Motor-dependent anterograde transport, a process that moves cytoplasmic components from sites of biosynthesis to sites of use within cells, is crucial in neurons with long axons. Evidence has emerged that multiple anterograde kinesins can contribute to some transport processes. To test the multi-kinesin possibility for a single vesicle type, we studied the functional relationships of axonal kinesins to dense core vesicles (DCVs) that were filled with a GFP-tagged neuropeptide in the Drosophila nervous system. Past work showed that Unc-104 (a kinesin-3) is a key anterograde DCV motor. Here we show that anterograde DCV transport requires the well-known mitochondrial motor Khc (kinesin-1). Our results indicate that this influence is direct. Khc mutations had specific effects on anterograde run parameters, neuron-specific inhibition of mitochondrial transport by Milton RNA interference had no influence on anterograde DCV runs, and detailed colocalization analysis by superresolution microscopy revealed that Unc-104 and Khc coassociate with individual DCVs. DCV distribution analysis in peptidergic neurons suggest the two kinesins have compartment specific influences. We suggest a mechanism in which Unc-104 is particularly important for moving DCVs from cell bodies into axons, and then Unc-104 and kinesin-1 function together to support fast, highly processive runs toward axon terminals.

Three routes to suppression of the neurodegenerative phenotypes caused by kinesin heavy chain mutations.

Kinesin-1 is a motor protein that moves stepwise along microtubules by employing dimerized kinesin heavy chain (Khc) subunits that alternate cycles of microtubule binding, conformational change, and ATP hydrolysis. Mutations in the Drosophila Khc gene are known to cause distal paralysis and lethality preceded by the occurrence of dystrophic axon terminals, reduced axonal transport, organelle-filled axonal swellings, and impaired action potential propagation. Mutations in the equivalent human gene, Kif5A, result in similar problems that cause hereditary spastic paraplegia (HSP) and Charcot-Marie-Tooth type 2 (CMT2) distal neuropathies. By comparing the phenotypes and the complementation behaviors of a large set of Khc missense alleles, including one that is identical to a human Kif5A HSP allele, we identified three routes to suppression of Khc phenotypes: nutrient restriction, genetic background manipulation, and a remarkable intramolecular complementation between mutations known or likely to cause reciprocal changes in the rate of microtubule-stimulated ADP release by kinesin-1. Our results reveal the value of large-scale complementation analysis for gaining insight into protein structure-function relationships in vivo and point to possible paths for suppressing symptoms of HSP and related distal neuropathies.

Identification of an axonal kinesin-3 motor for fast anterograde vesicle transport that facilitates retrograde transport of neuropeptides.

A screen for genes required in Drosophila eye development identified an UNC-104/Kif1 related kinesin-3 microtubule motor. Analysis of mutants suggested that Drosophila Unc-104 has neuronal functions that are distinct from those of the classic anterograde axonal motor, kinesin-1. In particular, unc-104 mutations did not cause the distal paralysis and focal axonal swellings characteristic of kinesin-1 (Khc) mutations. However, like Khc mutations, unc-104 mutations caused motoneuron terminal atrophy. The distributions and transport behaviors of green fluorescent protein-tagged organelles in motor axons indicate that Unc-104 is a major contributor to the anterograde fast transport of neuropeptide-filled vesicles, that it also contributes to anterograde transport of synaptotagmin-bearing vesicles, and that it contributes little or nothing to anterograde transport of mitochondria, which are transported primarily by Khc. Remarkably, unc-104 mutations inhibited retrograde runs by neurosecretory vesicles but not by the other two organelles. This suggests that Unc-104, a member of an anterograde kinesin subfamily, contributes to an organelle-specific dynein-driven retrograde transport mechanism.

The axonal transport of mitochondria.

Organelle transport is vital for the development and maintenance of axons, in which the distances between sites of organelle biogenesis, function, and recycling or degradation can be vast. Movement of mitochondria in axons can serve as a general model for how all organelles move: mitochondria are easy to identify, they move along both microtubule and actin tracks, they pause and change direction, and their transport is modulated in response to physiological signals. However, they can be distinguished from other axonal organelles by the complexity of their movement and their unique functions in aerobic metabolism, calcium homeostasis and cell death. Mitochondria are thus of special interest in relating defects in axonal transport to neuropathies and degenerative diseases of the nervous system. Studies of mitochondrial transport in axons are beginning to illuminate fundamental aspects of the distribution mechanism. They use motors of one or more kinesin families, along with cytoplasmic dynein, to translocate along microtubules, and bidirectional movement may be coordinated through interaction between dynein and kinesin-1. Translocation along actin filaments is probably driven by myosin V, but the protein(s) that mediate docking with actin filaments remain unknown. Signaling through the PI 3-kinase pathway has been implicated in regulation of mitochondrial movement and docking in the axon, and additional mitochondrial linker and regulatory proteins, such as Milton and Miro, have recently been described.

Dynein and the actin cytoskeleton control kinesin-driven cytoplasmic streaming in Drosophila oocytes.

Mass movements of cytoplasm, known as cytoplasmic streaming, occur in some large eukaryotic cells. In Drosophila oocytes there are two forms of microtubule-based streaming. Slow, poorly ordered streaming occurs during stages 8-10A, while pattern formation determinants such as oskar mRNA are being localized and anchored at specific sites on the cortex. Then fast well-ordered streaming begins during stage 10B, just before nurse cell cytoplasm is dumped into the oocyte. We report that the plus-end-directed microtubule motor kinesin-1 is required for all streaming and is constitutively capable of driving fast streaming. Khc mutations that reduce the velocity of kinesin-1 transport in vitro blocked streaming yet still supported posterior localization of oskar mRNA, suggesting that streaming is not essential for the oskar localization mechanism. Inhibitory antibodies indicated that the minus-end-directed motor dynein is required to prevent premature fast streaming, suggesting that slow streaming is the product of a novel dynein-kinesin competition. As F-actin and some associated proteins are also required to prevent premature fast streaming, our observations support a model in which the actin cytoskeleton triggers the shift from slow to fast streaming by inhibiting dynein. This allows a cooperative self-amplifying loop of plus-end-directed organelle motion and parallel microtubule orientation that drives vigorous streaming currents and thorough mixing of oocyte and nurse-cell cytoplasm.