Role of Unc104/KIF1-related motor proteins in mitochondrial transport in Neurospora crassa

Neuroprotection Exerted by Netrin-1 and Kinesin Motor KIF1A in Secondary Brain Injury following Experimental Intracerebral Hemorrhage in Rats

Binding of extracellular netrin-1 to its receptors, deleted in colorectal cancer (DCC) and uncoordinated gene 5H2 (UNC5H2), inhibits apoptosis mediated by these receptors. A neuron-specific kinesin motor protein, KIF1A, has been shown to participate in netrin-1 secretion. This study aimed to identify the roles of netrin-1 and KIF1A in secondary brain injury after intracerebral hemorrhage (ICH) and the potential mechanisms. An autologous blood ICH model was established in adult male Sprague-Dawley rats, and cultured neurons were exposed to OxyHb to mimic ICH conditions in vitro. Mouse recombinant netrin-1, expression vectors encoding KIF1A, and KIF1A-specific siRNAs were administered intracerebroventricularly. After ICH, protein levels of netrin-1, DCC, and UNC5H2 increased, while protein levels of KIF1A decreased. Levels of UNC5H2 and DCC bound to netrin-1 increased after ICH but were significantly lower than the increase in total amount of protein. Administration of recombinant netrin-1 attenuated neuronal apoptosis and degeneration in ICH rats. Moreover, KIF1A overexpression increased concentrations of netrin-1 in cerebrospinal fluid and cell culture supernatant and exerted neuroprotective effects via netrin-1 and its receptor pathways. KIF1A plays a critical role in netrin-1 secretion by neurons. An increase in protein levels of netrin-1 may be a neuroprotective strategy after ICH. However, this process is almost completely abolished by ICH-induced loss of KIF1A. An exogenous increase of KIF1A may be a potential strategy for neuroprotection via the netrin-1 pathway.

Intracellular Cargo Transport by Kinesin-3 Motors

Intracellular transport along microtubules enables cellular cargoes to efficiently reach the extremities of large, eukaryotic cells. While it would take more than 200 years for a small vesicle to diffuse from the cell body to the growing tip of a one-meter long axon, transport by a kinesin allows delivery in one week. It is clear from this example that the evolution of intracellular transport was tightly linked to the development of complex and macroscopic life forms. The human genome encodes 45 kinesins, 8 of those belonging to the family of kinesin-3 organelle transporters that are known to transport a variety of cargoes towards the plus end of microtubules. However, their mode of action, their tertiary structure, and regulation are controversial. In this review, we summarize the latest developments in our understanding of these fascinating molecular motors.

Neurospora crassa NKIN2, a kinesin-3 motor, transports early endosomes and is required for polarized growth

Biological motors are molecular nanomachines, which convert chemical energy into mechanical forces. The combination of mechanoenzymes with structural components, such as the cytoskeleton, enables eukaryotic cells to overcome entropy, generate molecular gradients, and establish polarity. Hyphae of filamentous fungi are among the most polarized cells, and polarity defects are most obvious. Here, we studied the role of the kinesin-3 motor, NKIN2, in Neurospora crassa. We found that NKIN2 localizes as fast-moving spots in the cytoplasm of mature hyphae. To test whether the spots represented early endosomes, the Rab5 GTPase YPT52 was used as an endosomal marker. NKIN2 colocalized with YPT52. Deletion of nkin2 caused strongly reduced endosomal movement. Combined, these results confirm the involvement of NKIN2 in early endosome transport. Introduction of a rigor mutation into NKIN2 labeled with green fluorescent protein (GFP) resulted in decoration of microtubules. Interestingly, NKIN2(rigor) was associated with a subpopulation of microtubules, as had been shown earlier for the Aspergillus nidulans orthologue UncA. Other kinesins did not show this specificity.

Mitochondrial DNA-depleted A549 cells are resistant to bleomycin

Alveolar epithelial cells are considered to be the primary target of bleomycin-induced lung injury, leading to interstitial fibrosis. The molecular mechanisms by which bleomycin causes this damage are poorly understood but are suspected to involve generation of reactive oxygen species and DNA damage. We studied the effect of bleomycin on mitochondrial DNA (mtDNA) and nuclear DNA (nDNA) in human alveolar epithelial A549 cells. Bleomycin caused an increase in reactive oxygen species production, DNA damage, and apoptosis in A549 cells; however, bleomycin induced more mtDNA than nDNA damage. DNA damage was associated with activation of caspase-3, cleavage of poly(ADP-ribose) polymerase, and cleavage and activation of protein kinase D1 (PKD1), a newly identified mitochondrial oxidative stress sensor. These effects appear to be mtDNA-dependent, because no caspase-3 or PKD1 activation was observed in mtDNA-depleted (ρ(0)) A549 cells. Survival rate after bleomycin treatment was higher for A549 ρ(0) than A549 cells. These results suggest that A549 ρ(0) cells are more resistant to bleomycin toxicity than are parent A549 cells, likely in part due to the depletion of mtDNA and impairment of mitochondria-dependent apoptotic pathways.

The Aspergillus nidulans kinesin-3 tail is necessary and sufficient to recognize modified microtubules

Posttranslational microtubule modifications (PTMs) are numerous; however, the biochemical and cell biological roles of those modifications remain mostly an enigma. The Aspergillus nidulans kinesin-3 UncA uses preferably modified microtubules (MTs) as tracks for vesicle transportation. Here, we show that a positively charged region in the tail of UncA (amino acids 1316 to 1402) is necessary for the recognition of modified MTs. Chimeric proteins composed of the kinesin-1 motor domain and the UncA tail displayed the same specificity as UncA, suggesting that the UncA tail is sufficient to establish specificity. Interaction between the UncA tail and alpha-tubulin was shown using a yeast two-hybrid assay and in A. nidulans by bimolecular fluorescence complementation. This is the first demonstration of how a kinesin-3 motor protein distinguishes among different MT populations in fungal cells, and how specificity determination depends on the tail rather than the motor domain, as has been demonstrated for kinesin 1 in neuronal cells.

Mitochondrial dynamics

Mitochondrial dynamics is a key feature for the interaction of mitochondria with other organelles within a cell and also for the maintenance of their own integrity. Four types of mitochondrial dynamics are discussed: Movement within a cell and interactions with the cytoskeleton, fusion and fission events which establish coherence within the chondriome, the dynamic behavior of cristae and their components, and finally, formation and disintegration of mitochondria (mitophagy). Due to these essential functions, disturbed mitochondrial dynamics are inevitably connected to a variety of diseases. Localized ATP gradients, local control of calcium-based messaging, production of reactive oxygen species, and involvement of other metabolic chains, that is, lipid and steroid synthesis, underline that physiology not only results from biochemical reactions but, in addition, resides on the appropriate morphology and topography. These events and their molecular basis have been established recently and are the topic of this review.

Properties of the kinesin-3 NcKin3 motor domain and implications for neck function

Members of the Kinesin-3 family are microtubule motors involved in the transport of membranous cargo. NcKin3 from the fungus Neurospora crassa is dimeric but inactivates one of its motor heads to generate nonprocessive motility. To determine how one of the heads is inactivated, we investigated truncated monomeric constructs. None of the constructs generated processive single-molecule motility, and multimotor velocities depended linearly on the number of residues remaining in the neck. The kinetic analysis suggests futile ATP hydrolysis cycles, because a representative monomer showed a faster ATP turnover than the dimer while supporting slower motility. The K(0.5,MT) was 70-fold lower, the microtubule-bound portion of the kinetic cycle eight-fold longer and the microtubule detachment rate almost 15-fold slower than that of the dimer. Moreover, the monomer's microtubule-dependent ADP release occurred three-fold to four-fold faster than k(cat) (125 versus 34 s(-1)), whereas phosphate release was approximately equally fast (29 s(-1)). A dimeric construct containing a structure-breaking insert between motor head and neck showed a similar behaviour. These data suggest that the heads of the wild-type NcKin3 motor are strictly coupled via the neck domain, and that the dimeric structure is required for proper detachment after one ATPase cycle. This is the first direct comparison of a monomeric Kinesin-3 with its dimeric full-length counterpart, and the kinetic changes observed here may also apply to other Kinesin-3 motors.

The Aspergillus nidulans kinesin-3 UncA motor moves vesicles along a subpopulation of microtubules

The extremely polarized growth form of filamentous fungi imposes a huge challenge on the cellular transport machinery, because proteins and lipids required for hyphal extension need to be continuously transported to the growing tip. Recently, it was shown that endocytosis is also important for hyphal growth. Here, we found that the Aspergillus nidulans kinesin-3 motor protein UncA transports vesicles and is required for fast hyphal extension. Most surprisingly, UncA-dependent vesicle movement occurred along a subpopulation of microtubules. Green fluorescent protein (GFP)-labeled UncA(rigor) decorated a single microtubule, which remained intact during mitosis, whereas other cytoplasmic microtubules were depolymerized. Mitotic spindles were not labeled with GFP-UncA(rigor) but reacted with a specific antibody against tyrosinated alpha-tubulin. Hence, UncA binds preferentially to detyrosinated microtubules. In contrast, kinesin-1 (conventional kinesin) and kinesin-7 (KipA) did not show a preference for certain microtubules. This is the first example for different microtubule subpopulations in filamentous fungi and the first example for the preference of a kinesin-3 motor for detyrosinated microtubules.

CLASP regulates mitochondrial distribution in Schizosaccharomyces pombe

Movement of mitochondria in Schizosaccharomyces pombe depends on their association with the dynamic, or plus ends, of microtubules, yet the molecular basis for this interaction is poorly understood. We identified mmd4 in a screen of temperature-sensitive S. pombe strains for aberrant mitochondrial morphology and distribution. Cells with the mmd4 mutation display mitochondrial aggregation near the cell ends at elevated temperatures, a phenotype similar to mitochondrial defects observed in wild-type cells after microtubule depolymerization. However, microtubule morphology and function appear normal in the mmd4 mutant. The mmd4 lesion maps to peg1⁺, which encodes a microtubule-associated protein with homology to cytoplasmic linker protein–associated proteins (mammalian microtubule plus end–binding proteins). Peg1p localizes to the plus end of microtubules and to mitochondria and is recovered with mitochondria during subcellular fractionation. This mitochondrial-associated fraction of Peg1p displays properties of a peripherally associated protein. Peg1p is the first identified microtubule plus end–binding protein required for mitochondrial distribution and likely functions as a molecular link between mitochondria and microtubules.

The class V myosin motor protein, Myo2, plays a major role in mitochondrial motility in Saccharomyces cerevisiae

The actin cytoskeleton is essential for polarized, bud-directed movement of cellular membranes in Saccharomyces cerevisiae and thus ensures accurate inheritance of organelles during cell division. Also, mitochondrial distribution and inheritance depend on the actin cytoskeleton, though the precise molecular mechanisms are unknown. Here, we establish the class V myosin motor protein, Myo2, as an important mediator of mitochondrial motility in budding yeast. We found that mutants with abnormal expression levels of Myo2 or its associated light chain, Mlc1, exhibit aberrant mitochondrial morphology and loss of mitochondrial DNA. Specific mutations in the globular tail of Myo2 lead to aggregation of mitochondria in the mother cell. Isolated mitochondria lacking functional Myo2 are severely impaired in their capacity to bind to actin filaments in vitro. Time-resolved fluorescence microscopy revealed a block of bud-directed anterograde mitochondrial movement in cargo binding–defective myo2 mutant cells. We conclude that Myo2 plays an important and direct role for mitochondrial motility and inheritance in budding yeast.

Polarized growth in fungi--interplay between the cytoskeleton, positional markers and membrane domains

One kind of the most extremely polarized cells in nature are the indefinitely growing hyphae of filamentous fungi. A continuous flow of secretion vesicles from the hyphal cell body to the growing hyphal tip is essential for cell wall and membrane extension. Because microtubules (MT) and actin, together with their corresponding motor proteins, are involved in the process, the arrangement of the cytoskeleton is a crucial step to establish and maintain polarity. In Saccharomyces cerevisiae and Schizosaccharomyces pombe, actin-mediated vesicle transportation is sufficient for polar cell extension, but in S. pombe, MTs are in addition required for the establishment of polarity. The MT cytoskeleton delivers the so-called cell-end marker proteins to the cell pole, which in turn polarize the actin cytoskeleton. Latest results suggest that this scenario may principally be conserved from S. pombe to filamentous fungi. In addition, in filamentous fungi, MTs could provide the tracks for long-distance vesicle movement. In this review, we will compare the interaction of the MT and the actin cytoskeleton and their relation to the cortex between yeasts and filamentous fungi. In addition, we will discuss the role of sterol-rich membrane domains in combination with cell-end marker proteins for polarity establishment.

Properties of the Kinesin-1 motor DdKif3 from Dictyostelium discoideum

The amoeba Dictyostelium discoideum possesses genes for 13 different kinesins. Here we characterize DdKif3, a member of the Kinesin-1 family. Kinesin-1 motors form homodimers that can move micrometer-long distances on microtubules using the energy derived from ATP hydrolysis. We expressed recombinant motors in Escherichia coli and tested them in different in vitro assays. Full-length and truncated Kif3 motors were active in gliding and ATPase assays. They showed a strong dependence on ionic strength. Like the full-length motor, the truncated DdKif3-592 motor (aa 1-592; comprising motor domain, neck, and partial stalk) reached its maximum speed of around 2.0micrcom s(-1) at a potassium acetate concentration of 200mM. The shortened DdKif3-342 motor (aa 1-342; comprising motor domain, partial neck) showed a high ATP turnover, comparable to that of the fungal Kinesin-1, Nkin. Results from the duty cycle calculations and gliding assays indicate that DdKif3 is a processive motor. A GFP-fusion protein revealed a mainly cytoplasmic localization of DdKif3. Immunofluorescence staining makes an association with the endoplasmic reticulum or mitochondria unlikely. Despite a similar phylogenetic distance to both metazoa and fungi, in terms of its biochemical properties DdKif3 revealed a closer similarity to fungal than animal kinesins.

Moving mitochondria: establishing distribution of an essential organelle

Mitochondria form a dynamic network responsible for energy production, calcium homeostasis and cell signaling. Appropriate distribution of the mitochondrial network contributes to organelle function and is essential for cell survival. Highly polarized cells, including neurons and budding yeast, are particularly sensitive to defects in mitochondrial movement and have emerged as model systems for studying mechanisms that regulate organelle distribution. Mitochondria in multicellular eukaryotes move along microtubule tracks. Actin, the primary cytoskeletal component used for transport in yeast, has more subtle functions in other organisms. Kinesin, dynein and myosin isoforms drive motor-based movement along cytoskeletal tracks. Milton and syntabulin have recently been identified as potential organelle-specific adaptor molecules for microtubule-based motors. Miro, a conserved GTPase, may function with Milton to regulate transport. In yeast, Mmr1p and Ypt11p, a Rab GTPase, are implicated in myosin V-based mitochondrial movement. These potential adaptors could regulate motor activity and therefore determine individual organelle movements. Anchoring of stationary mitochondria also contributes to organelle retention at specific sites in the cell. Together, movement and anchoring ultimately determine mitochondrial distribution throughout the cell.

Mitochondria on the move

Interactions of mitochondria with the cytoskeleton are crucial for normal mitochondrial function and for localization of the organelle at its sites of action within cells. Early studies revealed a role for microtubule motors in mitochondrial motility in neurons and other cell types. Here, we describe advances in our understanding of mitochondrial movement and distribution. Specifically, we review recent studies on proteins that mediate or regulate the interaction between motor molecules and the organelle, motor-independent mechanisms for mitochondrial motility, anchorage of mitochondria at cortical sites within cells and links between mitochondria-cytoskeleton interactions and mitochondrial plasticity.

Preparing the way: fungal motors in microtubule organization

Fungal growth, development and pathogenicity require hyphal tip growth, which is supported by polar exocytosis at the expanding growth region. It is assumed that molecular motors transport growth supplies along the fibrous elements of the cytoskeleton, such as microtubules, to the hyphal apex. Recent advances in live-cell imaging of fungi revealed additional roles for motors in organizing their own tracks. These unexpected roles of the molecular motors are modifying microtubule dynamics directly, targeting stability-determining factors to microtubule plus ends, and transporting and arranging already-assembled microtubules.

Kinetic and mechanistic basis of the nonprocessive Kinesin-3 motor NcKin3

Kinesin-3 motors have been shown to transport cellular cargo along microtubules and to function according to mechanisms that differ from the conventional hand-over-hand mechanism. To find out whether the mechanisms described for Kif1A and CeUnc104 cover the full spectrum of Kinesin-3 motors, we characterize here NcKin3, a novel member of the Kinesin-3 family that localizes to mitochondria of ascomycetes. We show that NcKin3 does not move in a K-loop-dependent way as Kif1A or in a cluster-dependent way as CeUnc104. Its in vitro gliding velocity ranges between 0.30 and 0.64 mum/s and correlates positively with motor density. The processivity index (k(bi,ratio)) of approximately 3 reveals that not more than three ATP molecules are hydrolyzed per productive microtubule encounter. The NcKin3 duty ratio of 0.03 indicates that the motor spends only a minute fraction of the ATPase cycle attached to the filament. Unlike other Kinesin-3 family members, NcKin3 forms stable dimers, but only one subunit releases ADP in a microtubule-dependent fashion. Together, these data exclude a processive hand-over-hand mechanism of movement and suggest a power-stroke mechanism where nucleotide-dependent structural changes in a single motor domain lead to displacement of the motor along the filament. Thus, NcKin3 is the first plus end-directed kinesin motor that is dimeric but moves in a nonprocessive fashion to its destination.

Interactions of mitochondria with the actin cytoskeleton

Interactions between mitochondria and the cytoskeleton are essential for normal mitochondrial morphology, motility and distribution. While microtubules and their motors have been established as important factors for mitochondrial transport, emerging evidence indicates that mitochondria interact with the actin cytoskeleton in many cell types. In certain fungi, such as the budding yeast and Aspergillus, or in plant cells mitochondrial motility is largely actin-based. Even in systems such as neurons, where microtubules are the primary means of long-distance mitochondrial transport, the actin cytoskeleton is required for short-distance mitochondrial movements and for immobilization of the organelle at the cell cortex. The actin cytoskeleton is also involved in the immobilization of mitochondria at the cortex in cultured tobacco cells and in budding yeast. While the exact nature of these immobilizations is not known, they may be important for retaining mitochondria at sites of high ATP utilization or at other cellular locations where they are needed. Recent findings also indicate that mutations in actin or actin-binding proteins can influence mitochondrial pathways leading to cell death. Thus, mitochondria-actin interactions contribute to apoptosis.

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.

The role of tip-localized mitochondria in hyphal growth

Hyphal tip-growing organisms have a high density of tip-localized mitochondria which maintain a membrane potential based on Rhodamine 123 fluorescence, but do not produce ATP based on the absence of significant oxygen consumption. Two possible roles of these mitochondria in tip growth were examined: Calcium sequestration and biogenesis, because tip-high cytoplasmic calcium gradients are a common feature of tip-growing organisms, and the volume expansion as the tip extends would require a continuous supply of additional mitochondria. Co-localization of calcium-sensitive fluorescent dye and mitochondria-specific fluorescent dyes showed that the tip-localized mitochondria do contain calcium, and therefore, may function in calcium clearance from the cytoplasm. Short-term inhibition of DNA synthesis or mitochondrial protein synthesis did not affect either tip growth, or mitochondrial shape or distribution. Therefore, mitochondrial biogenesis may not occur from the tip-localized mitochondria in hyphal organisms.

Stalk region of kinesin-related protein Unc104 has moderate ability to form coiled-coil dimer

Unc104/KIF1A, a kinesin family member, is reported to be monomeric in solution, though its polypeptide has regions that potentially form coiled coils. For a better understanding of the mechanism underlying Unc104/KIF1A's motility, it is important to evaluate the dimerization ability of this protein. The CD measurement of relevant segments of Caenorhabditis elegans Unc104 indicated that peptides having a common region (N358-K379) showed spectra characteristic to an alpha-helix. Dimerization by coiled-coil formation was confirmed by analytical ultracentrifugation. By analyzing the concentration dependence of the CD spectra, the monomer-dimer dissociation constant, Kd, of (N354-E388) was estimated to be about 5 microM, which is considerably larger than that of the corresponding segment of human kinesin (62 nM). Though its dimerization ability is rather moderate, Unc104/KIF1A could nonetheless dimerize and therefore could move by the same mechanism as human kinesin when the concentration of Unc104 is high due to, e.g., local crowding. This suggests that the motility could be controlled by the concentration of the motor protein.

Systematic analysis of genes required for synapse structure and function

Chemical synapses are complex structures that mediate rapid intercellular signalling in the nervous system. Proteomic studies suggest that several hundred proteins will be found at synaptic specializations. Here we describe a systematic screen to identify genes required for the function or development of Caenorhabditis elegans neuromuscular junctions. A total of 185 genes were identified in an RNA interference screen for decreased acetylcholine secretion; 132 of these genes had not previously been implicated in synaptic transmission. Functional profiles for these genes were determined by comparing secretion defects observed after RNA interference under a variety of conditions. Hierarchical clustering identified groups of functionally related genes, including those involved in the synaptic vesicle cycle, neuropeptide signalling and responsiveness to phorbol esters. Twenty-four genes encoded proteins that were localized to presynaptic specializations. Loss-of-function mutations in 12 genes caused defects in presynaptic structure.