Translocation of vesicles from squid axoplasm on flagellar microtubules

An ensemble of specifically targeted proteins stabilizes cortical microtubules in the human parasite Toxoplasma gondii

The human parasite Toxoplasma gondii has 22 regularly spaced microtubules associated with the cortex. This work defines the differential localization of associated proteins, explores the biophysical constraints on specific targeting along the cortical microtubules, and investigates the function of these proteins in stabilizing the polymers.

Although all microtubules within a single cell are polymerized from virtually identical subunits, different microtubule populations carry out specialized and diverse functions, including directional transport, force generation, and cellular morphogenesis. Functional differentiation requires specific targeting of associated proteins to subsets or even subregions of these polymers. The cytoskeleton of Toxoplasma gondii, an important human parasite, contains at least five distinct tubulin-based structures. In this work, we define the differential localization of proteins along the cortical microtubules of T. gondii, established during daughter biogenesis and regulated by protein expression and exchange. These proteins distinguish cortical from mitotic spindle microtubules, even though the assembly of these subsets is contemporaneous during cell division. Finally, proteins associated with cortical microtubules collectively protect the stability of the polymers with a remarkable degree of functional redundancy.

Observation of microtubule-based motor protein activity

It is possible to detect the presence of motor proteins that have the ability to translocate particles along microtubules. The two procedures described here were developed to detect microtubule-dependent motor protein activity in cell lysates or of purified proteins. In the first procedure, latex beads bound to the putative motor protein are assayed for their ability to translocate along microtubules in an ATP-dependent fashion. If motor protein activity is present, it will bind to the beads and translocate them unidirectionally along the microtubules. In the second procedure, motor proteins induce microtubule gliding over a glass coverslip surface that is coated with active motor protein. Because the mass of a microtubule is negligible compared to that of a coverslip or slide, the microtubule glides over the glass surface when the surface is coated with active motor protein. Also included here are descriptions of assays designed to determine the directionality of movement of microtubule-based motor proteins.

No conventional function for the conventional kinesin?

A paper by DeGiorgis et al. (DeGiorgis JA, Petukhova TA, Evans TA, Reese TS. Kinesin-3 is an organelle motor in the squid giant axon. Traffic 2008; DOI: 10.1111/j.1600-0854.2008.00809.x) in this issue of Traffic reports on the identification and function of a second squid kinesin, a kinesin-3 motor. As expected, the newly discovered motor associates with axoplasmic organelles in situ and powers motility along microtubules of vesicles isolated from squid axoplasm. Less expected was the finding that kinesin-3 may be the predominant motor for anterograde organelle movement in the squid axon, which challenges the so far undisputed view that this function is fulfilled by the conventional kinesin, kinesin-1. These novel findings let us wonder what the real function of kinesin-1--the most abundant motor in squid axons--actually is.

A protein methylation pathway in Chlamydomonas flagella is active during flagellar resorption

During intraflagellar transport (IFT), the regulation of motor proteins, the loading and unloading of cargo and the turnover of flagellar proteins all occur at the flagellar tip. To begin an analysis of the protein composition of the flagellar tip, we used difference gel electrophoresis to compare long versus short (i.e., regenerating) flagella. The concentration of tip proteins should be higher relative to that of tubulin (which is constant per unit length of the flagellum) in short compared with long flagella. One protein we have identified is the cobalamin-independent form of methionine synthase (MetE). Antibodies to MetE label flagella in a punctate pattern reminiscent of IFT particle staining, and immunoblot analysis reveals that the amount of MetE in flagella is low in full-length flagella, increased in regenerating flagella, and highest in resorbing flagella. Four methylated proteins have been identified in resorbing flagella, using antibodies specific for asymmetrically dimethylated arginine residues. These proteins are found almost exclusively in the axonemal fraction, and the methylated forms of these proteins are essentially absent in full-length and regenerating flagella. Because most cells resorb cilia/flagella before cell division, these data indicate a link between flagellar protein methylation and progression through the cell cycle.

Mechanisms of organelle transport and capture along proplatelets during platelet production

Megakaryocytes generate platelets by remodeling their cytoplasm into long proplatelet extensions, which serve as assembly lines for platelet production. Platelet packaging and release concludes at the tips of each proplatelet. Essential in this process is the distribution of organelles and platelet-specific granules into the nascent platelets. To investigate the mechanism of delivery of organelles into putative platelets, the distribution and dynamics of organelles/granules was monitored. Individual organelles are sent from the cell body to the proplatelets where they move bidirectionally until they are captured at proplatelet ends. Movement occurs at approximately 0.2 microm/min, but pauses and changes in direction are frequent. At any given time, approximately 30% of organelles/granules are in motion. Actin poisons do not diminish organelle motion, and vesicular structures are intimately associated with the microtubules. Therefore, movement appears to involve microtubule-based forces. Bidirectional organelle movement is conveyed by the bipolar organization of microtubules within the proplatelet, as kinesin-coated beads move bidirectionally on the microtubule arrays of permeabilized proplatelets. Movement of organelles along proplatelets involves 2 mechanisms: organelles travel along microtubules, and the linked microtubules move relative to each other. These studies demonstrate that the components that form platelets are delivered to and assembled de novo along proplatelets.

Intraflagellar transport and the flagellar tip complex

Intraflagellar transport (IFT) is the term that refers to the microtubule dependent particle motility that is common to almost all flagella and cilia and is distinct from the mechanism of flagellar beating. IFT involves the rapid, bi-directional transport of molecular motors and their cargo proteins from the base to the tip of the flagellum and back again. While the basic mechanism of IFT is well established, the varied functions of this process are continually being elucidated. For example, although IFT plays a clear role in flagellar assembly, disassembly and stability, the exact sequence of events that take place when tubulin subunit addition and loss occur during flagellar assembly and disassembly, respectively, are unknown. Key to furthering our understanding of IFT is greater knowledge of the flagellar tip complex (FTC) because it is at the FTC that flagellar assembly and disassembly, cargo loading and unloading, and motor protein regulation occur. Yet these related processes may only represent one aspect of the importance of IFT in flagellar dynamics. IFT may also provide the basic elements of a signal transduction mechanism that functions to provide the nucleus with information about the outside environment and even about the state of the flagellum itself. Thus, IFT may function as the central component of a signal transduction system that controls flagellar gene transcription.

Unipolar microtubule array is directly involved in nurse cell-oocyte transport

The telotrophic ovariole of Rhodnius prolixus is richly endowed with microtubules (MTs). An extensive, stable array of MTs packs the trophic core and trophic cords which link the nurse cell compartments to the growing oocytes. This system is excellent to study MT-based transport as the MTs are believed to play a role in transport of nurse cell-produced mitochondria, ribosomes, and mRNAs to the oocytes. We investigated MT polarity and molecular MT motors in this unidirectional transport system. Hook decoration revealed that the MTs of the trophic core and cords have their plus (+) ends in the tropharium and minus (-) ends in the oocytes. Video differential interference optics (DIC) microscopy showed that vesicle transport was saltatory, ATP-dependent, and had an average velocity of 0.77 micron/sec toward the oocyte. Transport was sensitive to 2 mM N-ethylmaleimide (NEM) and 50 microM vanadate and resistant to 1 mM 5'-adenylylimidodiphosphate (AMP-PNP) and 5 microM vanadate. We report that the unipolar, acetylated trophic cord MTs play a direct role in nurse cell-oocyte transport via a cytoplasmic dynein-like retrograde motor.

Transport of fragmented DNA in apical dendrites of gerbil CA1 pyramidal neurons following transient forebrain ischemia

Transport of fragmented DNA in apical dendrites of the CA1 pyramidal neurons of gerbil hippocampus is observed in the apoptotic process following transient forebrain ischemia. The time-course of specific DNA fragmentation was examined after the ischemic insult by in situ nick-end-labeling method and fluorescence detection technique by DAPI. Although the role of the fragmented DNA movement is unclear, the transport mechanism of fragmented DNA is still active in the late phase of apoptotic process.

The interaction between cytoplasmic dynein and dynactin is required for fast axonal transport

Fast axonal transport is characterized by the bidirectional, microtubule-based movement of membranous organelles. Cytoplasmic dynein is necessary but not sufficient for retrograde transport directed from the synapse to the cell body. Dynactin is a heteromultimeric protein complex, enriched in neurons, that binds to both microtubules and cytoplasmic dynein. To determine whether dynactin is required for retrograde axonal transport, we examined the effects of anti-dynactin antibodies on organelle transport in extruded axoplasm. Treatment of axoplasm with antibodies to the p150(Glued) subunit of dynactin resulted in a significant decrease in the velocity of microtubule-based organelle transport, with many organelles bound along microtubules. We examined the molecular mechanism of the observed inhibition of motility, and we demonstrated that antibodies to p150(Glued) disrupted the binding of cytoplasmic dynein to dynactin and also inhibited the association of cytoplasmic dynein with organelles. In contrast, the anti-p150(Glued) antibodies had no effect on the binding of dynactin to microtubules nor on cytoplasmic dynein-driven microtubule gliding. These results indicate that the interaction between cytoplasmic dynein and the dynactin complex is required for the axonal transport of membrane-bound vesicles and support the hypothesis that dynactin may function as a link between the organelle, the microtubule, and cytoplasmic dynein during vesicle transport.

Movement of axoplasmic organelles on actin filaments assembled on acrosomal processes: evidence for a barbed-end-directed organelle motor

The directionality of the actin-dependent motors on squid axoplasmic organelles was determined using actin filaments assembled on the barbed ends of acrosomal processes. Acrosomal processes were isolated from Limulus polyphemus sperm and incubated in monomeric actin under conditions that promoted barbed end assembly only. Newly assembled actin was stabilized and stained with rhodamine-phalloidin and the presence of filaments at the barbed ends of the acrosomal processes was verified by fluorescence microscopy and negative contrast electron microscopy. Axoplasmic organelles that dissociated from extruded axoplasm were observed by video microscopy to move along the newly assembled actin filaments at an average velocity of 1.1 +/- 0.3 microns/second. All organelles moved in the direction away from the acrosomal fragment and towards the tip of the actin filaments. Therefore, the actin-dependent organelle motor on axoplasmic organelles is a barbed-end-directed motor like other myosins analyzed. These findings support the conclusions that axoplasmic organelles are driven by a myosin-like motor along actin filaments and that these filaments as well as microtubules function in fast axonal transport.

Kinesin is bound with high affinity to squid axon organelles that move to the plus-end of microtubules

This paper addresses the question of whether microtubule-directed transport of vesicular organelles depends on the presence of a pool of cytosolic factors, including soluble motor proteins and accessory factors. Earlier studies with squid axon organelles (Schroer et al., 1988) suggested that the presence of cytosol induces a > 20-fold increase in the number of organelles moving per unit time on microtubules in vitro. These earlier studies, however, did not consider that cytosol might nonspecifically increase the numbers of moving organelles, i.e., by blocking adsorption of organelles to the coverglass. Here we report that treatment of the coverglass with casein, in the absence of cytosol, blocks adsorption of organelles to the coverglass and results in vigorous movement of vesicular organelles in the complete absence of soluble proteins. This technical improvement makes it possible, for the first time, to perform quantitative studies of organelle movement in the absence of cytosol. These new studies show that organelle movement activity (numbers of moving organelles/min/micron microtubule) of unextracted organelles is not increased by cytosol. Unextracted organelles move in single directions, approximately two thirds toward the plus-end and one third toward the minus-end of microtubules. Extraction of organelles with 600 mM KI completely inhibits minus-end, but not plus-end directed organelle movement. Upon addition of cytosol, minus-end directed movement of KI organelles is restored, while plus--end directed movement is unaffected. Biochemical studies indicate that KI-extracted organelles attach to microtubules in the presence of AMP-PNP and copurify with tightly bound kinesin. The bound kinesin is not extracted from organelles by 1 M KI, 1 M NaCl or carbonate (pH 11.3). These results suggest that kinesin is irreversibly bound to organelles that move to the plus-end of microtubules and that the presence of soluble kinesin and accessory factors is not required for movement of plus-end organelles in squid axons.

Tubulin protofilaments and kinesin-dependent motility

Microtubules are built of tubulin subunits assembled into hollow cylinders which consist of parallel protofilaments. Thus, motor molecules interacting with a microtubule could do so either with one or several tubulin subunits. This makes it difficult to determine the structural requirements for the interaction. One way to approach the problem is to alter the surface lattice. This can be done in several ways. Proto-filaments can be exposed on their inside (C-tubules or "sheets"), they can be made antiparallel (zinc sheets), or they can be rolled up (duplex tubules). We have exploited this polymorphism to study how the motor protein kinesin attached to a glass surface interacts and moves the various tubulin assemblies. Microtubules glide over the surface along straight paths and with uniform velocities. In the case of C-tubules, approximately 40% glide similarly to microtubules, but a major fraction do not glide at all. This indicates (a) that a full cylindrical closure is not necessary for movement, and (b) that the inside surface of microtubules does not support gliding. With zinc sheets, up to 70% of the polymers move, but the movement is discontinuous, has a reduced speed, and follows along a curved path. Since zinc sheets have protofilaments alternating in orientation and polarity, this result suggests that in principle a single protofilament can produce movement, even when its neighbors cannot. Duplex microtubules do not move because they are covered with protofilaments coiled inside out, thus preventing the interaction with kinesin. The data can be explained by assuming that the outside of one protofilament represents the minimal track for kinesin, but smooth gliding requires several parallel protofilaments. Finally, we followed the motion of kinesin-coated microbeads on sea-urchin sperm flagella, from the flagellar outer doublet microtubules to the singlet microtubule tips extending from the A-tubules. No change in behavior was detected during the transition. This indicates that even if these microtubules differ in surface lattice, this does not affect the motility.

Molecular genetics of kinesin light chains: generation of isoforms by alternative splicing

Movement of membrane-bounded organelles to intracellular destinations requires properly oriented microtubules and force-generating enzymes, such as the microtubule-stimulated ATPase kinesin. Kinesin is a heterotetramer with two heavy chain (approximately 124-kDa) and two light chain (approximately 64-kDa) subunits. Kinesin heavy chains contain both ATP- and microtubule-binding domains and are capable of force generation in vitro. Functions of the light chains are undetermined, although evidence suggests they interact with membrane surfaces. We have used molecular genetic approaches to dissect the kinesin light chain structure. Three distinct kinesin light chain cDNAs were cloned and sequenced from rat brain, and they were found to result from alternative splicing of a single gene. Polypeptides encoded by these cDNAs are identical except for their carboxyl ends. Synthesis of multiple light chains, differing from one another in primary structure, could provide a means of generating multiple, functionally specialized forms of the kinesin holoenzyme.

Microtubules have special physical associations with smooth endoplasmic reticula and mitochondria in axons

Ultrastructural morphometry was used to document the non-random spatial distributions of organelles within the compact myelinated region of avian oculomotor axons. These regions contain large numbers of loosely packed neurofilaments (NFs) (241/microns 2) and only a relatively small number of microtubules (MTs) (4/microns 2), mitochondria (0.6/microns 2), and smooth endoplasmic reticulum (SER) (1.6/microns 2). Random co-occurrences between the relatively sparsely distributed MTs, mitochondria, and SER are probably infrequent in these axons. The actual co-occurrences of MTs, mitochondria, and SER with MTs were counted and compared to the co-occurrences expected in a random Poisson distribution. At long distances (200 nm), the co-occurrences were random. At shorter distances (40 nm and less), MTs were still randomly associated with other MTs. However, at these shorter distances, the spatial associations of mitochondria with MTs and of SER with MTs were not random; such preferential stable associations may be produced by specific MT associated cross-bridging proteins. In axons, MTs tend to be clustered together, giving the appearance of MT bundles. We propose that the MT-MT bundling is an indirect result of MT concentration along the continuous intra-axonal SER network, to which the MTs are apparently tied directly by dynamic molecular cross-bridges.

Secondary lysosomes as an integral part of the cytoskeleton: a morphological study in rat Kupffer cells

Rat Kupffer cells contain the three major cytoskeletal components: microfilaments (MF), microtubules (MT), and intermediate filaments (IF) of the vimentin type. Previous cytomagnetometric data obtained from alveolar macrophages and rat Kupffer cells in culture provided evidence that actin filaments contribute to the movements of lysosomes. The lysosomal transport in living cells was affected, when the MFs were selectively disturbed, whereas the depolymerization of the MTs had no effect on the lysosomal movement measured by cytomagnetometric means. Immunofluorescence and ultrastructural studies of isolated and cultured rat Kupffer cells, presented in this paper, will investigate the relationship between lysosomes and the cytoskeleton. The principal filamentous structure in the peripheral cytoplasm of Kupffer cells in a dense meshwork of actin filaments. The dimension of the meshes combined with the dimensions of lysosomes implies the necessity of either (i) disintegration of the actin filament cross-links, (ii) depolarymerization and redistribution of MF's, or (iii) a displacement of actin filaments by the lysosomes during the organelle transport. The presence of microtubules in cytoplasmic protrusions and their track from the periphery to the perinuclear region during interphase might play a role in the transport mechanism of lysosomes, the more so because microtubules could often be demonstrated in closest association with lysosomes even in the first phase of endocytosis. The distribution pattern of vimentin, found as a dense interconnected framework surrounding the lysosomes like a basket, could play a role in positioning the organelles. The dynamic functions of MF's and MT's and their multifunctionality led to an adaptive and flexible organization of these filaments which may both be involved in lysosomal motion.(ABSTRACT TRUNCATED AT 250 WORDS)

A squid dynein isoform promotes axoplasmic vesicle translocation

Axoplasmic vesicles that translocate on isolated microtubules in an ATP-dependent manner have an associated ATP-binding polypeptide with a previously estimated relative molecular mass of 292 kD (Gilbert, S. P., and R. D. Sloboda. 1986. J. Cell Biol. 103:947-956). Here, data are presented showing that this polypeptide (designated H1) and another high molecular mass polypeptide (H2) can be isolated in association with axoplasmic vesicles or optic lobe microtubules. The H1 and H2 polypeptides dissociate from microtubules in the presence of MgATP and can be further purified by gel filtration chromatography. The peak fraction thus obtained demonstrates MgATPase activity and promotes the translocation of salt-extracted vesicles (mean = 0.87 microns/s) and latex beads (mean = 0.92 microns/s) along isolated microtubules. The H1 polypeptide binds [alpha 32P]8-azidoATP and is thermosoluble, but the H2 polypeptide does not share these characteristics. In immunofluorescence experiments with dissociated squid axoplasm, affinity-purified H1 antibodies yield a punctate pattern that corresponds to vesicle-like particles, and these antibodies inhibit the bidirectional movement of axoplasmic vesicles. H2 is cleaved by UV irradiation in the presence of MgATP and vanadate to yield vanadate-induced peptides of 240 and 195 kD, yet H1 does not cleave under identical conditions. These experiments also demonstrate that the actual relative molecular mass of the H1 and H2 polypeptides is approximately 435 kD. On sucrose density gradients, H1 and H2 sediment at 19-20 S, and negatively stained samples reveal particles comprised of two globular heads with stems that contact each other and extend to a common base. The results demonstrate that the complex purified is a vesicle-associated ATPase whose characteristics indicate that it is a squid isoform of dynein. Furthermore, the data suggest that this vesicle-associated dynein promotes membranous organelle motility during fast axoplasmic transport.

Cytoplasmic dynein is a minus end-directed motor for membranous organelles

The role of cytoplasmic dynein in microtubule-based organelle transport was examined using a reconstituted assay developed from chick embryo fibroblasts. Factors present in a high-speed cytosol caused the movement of purified organelles on microtubules predominantly in the minus end direction. Inactivation of cytoplasmic dynein in the high-speed cytosol by vanadate-mediated UV photocleavage inhibited minus end-directed organelle motility by over 90%. Addition of purified cytoplasmic dynein to the inactive cytosol restored minus end-directed organelle motility, although purified cytoplasmic dynein by itself did not support organelle movement. We propose that cytoplasmic dynein is the motor for minus end-directed organelle movement, but that additional cytosolic factors are also required to produce organelle motility.

Mechanisms of the inhibition of fast axonal transport by local anesthetics

The present study attempted to clarify the mechanism(s) by which local anesthetics inhibit fast axonal transport. Spinal nerves of the bullfrog were incubated with local anesthetics under conditions known to inhibit transport and the effects of these exposures to local anesthetics on the content of adenosine triphosphate and creatine phosphate in nerves and on the density of microtubules in unmyelinated axons were examined. Lidocaine, at concentrations of 14 or 20 mM, did not reduce significantly the content of adenosine triphosphate (although significant reductions in creatine phosphate were observed); the density of microtubules was also not affected by 14 mM lidocaine. Some mechanism other than inhibition of oxidative metabolism or disruption of microtubules must therefore be responsible for the inhibition of fast axonal transport by 14 mM lidocaine. Significant reductions in the content of adenosine triphosphate were observed with 1 or 2 mM tetracaine and with 0.5 or 1 mM dibucaine (this latter concentration of dibucaine also reduced the content of creatine phosphate); however, comparison with the effects of 2,4-dinitrophenol indicated that these inhibitions of oxidative metabolism were insufficient to inhibit transport in the case of 0.5 mM dibucaine or could at best only partly explain the inhibition of transport in the other cases. Since the density of microtubules was not affected by 1 mM tetracaine and was not sufficiently reduced by 0.5 mM dibucaine to inhibit transport, some other effect must again largely contribute to or be solely responsible for the inhibition of fast axonal transport by these concentrations of dibucaine and tetracaine.

The molecular structure of adrenal medulla kinesin

The molecular structure of bovine adrenal kinesin was studied by electron microscopy using the low-angle rotary shadowing technique. Adrenal kinesin exhibited either a folded or an extended configuration; the ratio of the two is dependent on the salt concentration. Almost all adrenal kinesin molecules were folded in a low-ionic solution, and the ratio of extended molecules increased to 40-50% in a solution containing 1 M ammonium acetate. Kinesin in the extended configuration displayed a rod-shaped structure with a mean length of about 80 nm. The morphologies of the ends were different; one end was composed of two globular particles, similar to the two-headed structure of myosin, while the other end had a more ill-defined structure, appearing either as a globular particle, an aggregate of two to four small granules, or a frayed, fan-like structure. The folded kinesin molecule possessed a hinge region in the middle of the rod, at about 32 nm from the neck of the two heads. In our preparations, the majority of adrenal kinesin molecules were folded at physiological salt concentrations. Adrenal kinesin bound to microtubules in the presence of adenylyl imidodiphosphate (AMP-PNP) also displayed a folded morphology.

Calcium dependent modulation of fast axonal transport

The highly differentiated structure of the neuron poses special problems for the intracellular movement of molecules throughout the cell. Molecular transport distances from the synthesizing neuron cell body along the axon (which has no substantial synthetic capabilities) to the axon terminal are very great. The transported substances, transport support structures, translocator motors, and control elements are currently the focus of intense research. Interruption of this flow of molecules could have disastrous effects upon the cell and ultimately the organism resulting in neuropathological conditions. Calcium plays a critical role in modulating fast-axonal transport (FAT) speeds. Before discussing the effect of calcium on FAT, we summarize our broad perspective on the role of axonal transport in neurologic disease.

The role of kinesin and other soluble factors in organelle movement along microtubules

Kinesin is a force-generating ATPase that drives the sliding movement of microtubules on glass coverslips and the movement of plastic beads along microtubules. Although kinesin is suspected to participate in microtubule-based organelle transport, the exact role it plays in this process is unclear. To address this question, we have developed a quantitative assay that allows us to determine the ability of soluble factors to promote organelle movement. Salt-washed organelles from squid axoplasm exhibited a nearly undetectable level of movement on purified microtubules. Their frequency of movement could be increased greater than 20-fold by the addition of a high speed axoplasmic supernatant. Immunoadsorption of kinesin from this supernatant decreased the frequency of organelle movement by more than 70%; organelle movements in both directions were markedly reduced. Surprisingly, antibody purified kinesin did not promote organelle movement either by itself or when it was added back to the kinesin-depleted supernatant. This result suggested that other soluble factors necessary for organelle movement were removed along with kinesin during immunoadsorption of the supernatant. A high level of organelle motor activity was recovered in a high salt eluate of the immunoadsorbent that contained only little kinesin. On the basis of these results we propose that organelle movement on microtubules involves other soluble axoplasmic factors in addition to kinesin.

The microtubule-dependent formation of a tubulovesicular network with characteristics of the ER from cultured cell extracts

The formation of a dynamic tubulovesicular membrane network that resembles the endoplasmic reticulum (ER) has been observed in extracts of cultured chick embryo fibroblasts (CEF cells) using video-enhanced differential interference contrast microscopy. Initially, membranes in the CEF extracts appeared amorphous and aggregated, but with time, membrane tubules moved out along stationary microtubules. The membrane tubules formed new branches on intersecting microtubules and fused with other branches to form a network of interconnected polygons. The tubulovesicular network was solubilized by detergent and took on a beaded morphology in a hypotonic buffer. Formation of the tubulovesicular network required ATP and microtubules. The network did not contain elements of the plasma membrane, Golgi apparatus, or mitochondria but could be labeled with ER markers. We suggest that the tubulovesicular network contains components from the ER and is formed by membrane associated motors moving upon microtubules in a process we call microtubule-dependent tethering.

Tracking kinesin-driven movements with nanometre-scale precision

Several enzyme complexes drive cellular movements by coupling free energy-liberating chemical reactions to the production of mechanical work. A key goal in the study of these systems is to characterize at the molecular level mechanical events associated with individual reaction steps in the catalytic cycles of single enzyme molecules. Ideally, one would like to measure movements driven by single (or a few) enzyme molecules with sufficient temporal resolution and spatial precision that these events can be directly observed. Kinesin, a force-generating ATPase involved in microtubule-based intracellular organelle transport, will drive the unidirectional movement of microscopic plastic beads along microtubules in vitro. Under certain conditions, a few (less than or equal to 10) kinesin molecules may be sufficient to drive either bead movement or organelle transport. Here we describe a method for determining precise positional information from light-microscope images. The method is applied to measure kinesin-driven bead movements in vitro with a precision of 1-2 nm. Our measurements reveal basic mechanical features of kinesin-driven movements along the microtubule lattice, and place significant constraints on possible molecular mechanisms of movement.

Vesikin, a vesicle associated ATPase from squid axoplasm and optic lobe, has characteristics in common with vertebrate brain MAP 1 and MAP 2

Vesikin, a protein that can associate with squid axoplasmic vesicles or optic lobe microtubules, has been implicated as a force-generating molecule involved in microtubule-dependent vesicle transport [Gilbert and Sloboda, 1986, 1988]. Because vesikin crossreacts with an antibody to porcine brain microtubule associated protein 2 (MAP 2), studies were conducted to compare squid vesikin and brain MAPs. When taxol stabilized microtubules containing vesikin as a microtubule associated protein were incubated in the presence of ATP, vesikin dissociated from the microtubule subunit lattice. This behavior would be expected for an ATP-dependent, force generating molecule that serves as a crossbridge between vesicles and microtubules. When chick brain microtubules were treated under the same conditions, MAP 2 remained bound to the microtubules while MAP 1 dissociated in a manner similar to vesikin. One dimensional peptide mapping procedures revealed that, although digestion of vesikin and MAP 2 generated several peptides common to both proteins, vesikin and MAP 2 are clearly not identical. Furthermore, the addition of vesikin or MAPS 1 and 2 to purified tubulin stimulated microtubule assembly in a manner dependent on the concentration of added protein. These findings demonstrate that brain MAPs share characteristics common to squid vesikin and support the suggestion that brain MAPs 1 and 2 might act as a force generating complex for vesicle transport in higher organisms.

Fast axonal transport alterations in amyotrophic lateral sclerosis (ALS) and in parathyroid hormone (PTH)-treated axons

Video-enhanced contrast techniques have been used to study fast axonal transport of organelles in diseased and normal human axons. A broad perspective on the importance of axonal transport in the pathogenesis of human neurological disorders is presented and problems in dealing with human nerve summarized. Results from analysis of organelle traffic in axons from motor nerve in patients with amyotrophic lateral sclerosis (ALS) show: 1) higher mean speed of anterograde organelles, 2) lower mean speed of retrograde organelles, and 3) lower retrograde organelle traffic density. Hyperparathyroidism, another human clinical syndrome, can mimic ALS. The effect of parathyroid hormone (PTH) on axons in vitro is to increase the mean speed of both anterograde and retrograde organelle traffic. The dose response curve and time course of the PTH effect are delineated. Dihydropyridine calcium channel antagonists block the PTH effect, implicating extracellular calcium in the alteration of organelle traffic speed. The results are discussed in relation to neuronal function and the regulation of fast axonal transport.

Microtubule-associated organelle and vesicle transport in fibroblasts

Allen Video-enhanced contrast/differential interference contrast (AVEC-DIC) microscopy was used in conjunction with video intensification immunofluorescence microscopy to demonstrate that organelles and vesicle (particles) can move in either direction along microtubular linear elements in fibroblasts [Hayden et al., 1983]. Since it is not possible to determine the number of microtubules making up a linear element with light microscopy alone, AVEC-DIC microscopy was used in conjunction with whole-mount electron microscopy to show bidirectional transport along a single microtubule [Hayden and Allen, 1984]. These studies demonstrate that the structural polarity of the microtubule does not determine the direction of particle motion, and since dynein is an asymetric molecule, a simple microtubule-dynein-particle hypothesis cannot explain bidirectional transport along a single microtubule. Very little is known about regulation of particle transport in most cell types. Human embryonic lung fibroblasts grown on glass coverslips were serum-deprived for 24 hours and re-fed with serumless medium; the particle translocations/5 minutes were then determined. The cells were then re-fed with either serumless medium, serum-containing medium, or serumless medium containing some bioactive factor, and the particle translocations/5 minutes were again determined for the same cells. Medium containing 10% fetal bovine serum inhibited particle translocation by 51.8%. Of the bioactive factors tested, only vasopressin produced a significant reduction in particle translocations (38%). This suggests that protein kinase C or calcium/calmodulin kinase could be involved in regulating particle transport.

Calcium regulation of pigment transport in vitro

Calcium has been implicated in the regulation of many cellular motility events. In this study we have examined the role of different Ca2+ concentrations on the in vitro transport of pigment within cultured chromatophores. Cells treated with Brij detergent for 1-2 min were stripped of their plasma membranes, leaving their cytoskeleton and associated pigment granules exposed to the external milieu. We found that retrograde pigment transport (aggregation) is induced upon addition of 1 mM MgATP2- with 10(-7) M free Ca2+, while an orthograde transport (redispersal) of pigment results from lowering the concentration of free Ca2+ to 10(-8) M while maintaining 1 mM MgATP2-. These Ca2+-regulated movements are ATP dependent but are apparently independent of cAMP and insensitive to calmodulin inhibitors. The observations reported here provide novel evidence that the concentration of free Ca2+ acts to regulate the direction of intracellular organelle transport.

Cultured cell extracts support organelle movement on microtubules in vitro

Directed movements of organelles have been observed in a variety of cultured cells. To study the regulation and molecular basis of intracellular organelle motility, we have prepared extracts from cultured chick embryo fibroblasts (CEF cells) which support the movement of membraneous organelles along microtubules. The velocity, frequency and characteristics of organelle movements in vitro were similar to those within intact cells. Organelles and extract-coated anionic beads moved predominantly (80%) toward the minus ends of microtubules that had been regrown from centrosomes, corresponding to retrograde translocation. Similar microtubule-dependent organelle movements were observed in extracts prepared from other cultured cells (African green monkey kidney and 3T3 cells). Organelle motility was ATP and microtubule dependent. The frequency of organelle movement was inhibited by acidic (pH less than 7) or alkaline (pH greater than 8) solutions, high ionic strength ([ KCl] = 0.1 M), and the chelation of free magnesium ions. Treatment of the extracts with adenylyl imidodiphosphate (AMP-PNP, 7 mM), sodium orthovanadate (vanadate; Na3VO4, 20 microM), or N-ethylmaleimide (NEM, 2 mM) blocked all organelle motility. The decoration of microtubules with organelles was observed in the presence of AMP-PNP or vanadate. Motility was not affected by cytochalasin D (2 microM) or cAMP (1 mM). Kinesin (Mr = 116,000), an anterograde microtubule-based motor, was partially purified from the CEF extract by microtubule affinity purification in the presence of AMP-PNP, and was able to drive the movement of microtubule on glass coverslips. A similar preparation made in the presence of vanadate contained a different subset of proteins and did not support motility. These results demonstrate that intracellular organelle motility can be reproduced in vitro and provide the basis for investigating the roles of individual molecular components involved in the organelle motor complex.

Long-term analysis of organelle translocation in isolated axoplasm of Myxicola infundibulum

Moving intra-axonal organelles demonstrate frequent variations in speed when viewed over several seconds. To evaluate these and other motion variations, a long-term analysis of organelle motion in isolated axoplasm of Myxicola infundibulum was carried out using differential interference contrast optics and analog and digital image enhancement techniques. Motion characteristics of individual organelles were analyzed for periods of up to 58 minutes. Three principle observations on organelle motion were made: 1) Classes of organelles of the same size demonstrated a 5- to 25-fold variation of speed, with the slowest speeds occurring most frequently; 2) organelle speeds over individual translocations (motion without stopping) are inversely proportional to their size, but the speeds calculated for the long-term analysis of organelle motion (total distance travelled/total observation time, including pauses) did not reflect this observation; and 3) organelles displayed variable trip lengths, durations, mean speeds, and pause durations, and the relationships between these variations showed no repetitive patterns. In contrast to reported observations of uniform velocities of organelles moving on isolated microtubule preparations, these observations suggest that a variety of factors must play a role in organelle translocation in Myxicola axoplasm.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

Direction of force generated by the inner row of dynein arms on flagellar microtubules

Our goal was to determine the direction of force generation of the inner dynein arms in flagellar axonemes. We developed an efficient means of extracting the outer row of dynein arms in demembranated sperm tail axonemes, leaving the inner row of dynein arms structurally and functionally intact. Sperm tail axonemes depleted of outer arms beat at half the beat frequency of sperm tails with intact arms over a wide range of ATP concentrations. The isolated, outer arm-depleted axonemes were induced to undergo microtubule sliding in the presence of ATP and trypsin. Electron microscopic analysis of the relative direction of microtubule sliding (see Sale, W. S. and P. Satir, 1977, Proc. Natl. Acad. Sci. USA, 74:2045-2049) revealed that the doublet microtubule with the row of inner dynein arms, doublet N, always moved by sliding toward the proximal end of the axoneme relative to doublet N + 1. Therefore, the inner arms generate force such that doublet N pushes doublet N + 1 tipward. This is the same direction of microtubule sliding induced by ATP and trypsin in axonemes having both inner and outer dynein arms. The implications of this result for the mechanism of ciliary bending and utility in functional definition of cytoplasmic dyneins are discussed.

Dynamic shape changes of cytoplasmic organelles translocating along microtubules

Transient shape changes of organelles translocating along microtubules are directly visualized in thinly spread cytoplasmic processes of the marine foraminifer. Allogromia laticollaris, by a combination of high-resolution video-enhanced microscopy and fast-freezing electron microscopy. The interacting side of the organelle flattens upon binding to a microtubule, as if to maximize contact with it. Organelles typically assume a teardrop shape while moving, as if they were dragged through a viscous medium. Associated microtubules bend around attachments of the teardrop-shaped organelles, suggesting that they too are acted on by the forces deforming the organelles. An 18-nm gap between the organelles and the microtubules is periodically bridged by 10-nm-thick cross-bridge structures that may be responsible for the binding and motive forces deforming organelles and microtubules.

Movements of vesicles on microtubules

Many cytoplasmic vesicles are observed to move along microtubules. Often, bidirectional movement of particles is observed on a single microtubule. We have isolated one cytoplasmic motor, kinesin, and defined another, the axoplasmic retrograde factor, which are capable of powering anionic latex beads toward the plus and minus ends of microtubules, respectively. Observations of vesicle movements show that vesicles have a defined direction of movement and that vesicles copurify with a kinesin motor activity. Current evidence suggests the hypothesis that kinesin and the retrograde motors power vesicle movements in vivo by attachment to the appropriate vesicle.

A unique tubulin antibody which disrupts particle movement in squid axoplasm

Microtubules have been demonstrated to be a substrate for organelle transport and particle translocation in vitro and in vivo. Subsequent to a previous report of inhibition of axonal transport of exogenous tracers in vivo using antiserum NS-20 against tubulin (Johnston et al: Brain Res. 1986), we now show disruption of particle movement in extruded squid axoplasm using this unique immunological probe. Using video-enhanced contract-differential interference contrast (AVEC-DIC) microscopy, we examined the properties of particle movement along microtubules and demonstrated that both the velocity of particle movement and the numbers of particles moving are decreased in the presence of NS-20 antiserum or NS-20 affinity-purified antibodies but not in the presence of another antiserum against tubulin. The amount of microtubule substrate does not change in the presence of any of the antisera. In conclusion, we suggest that NS-20 antibodies bind near or at a site on the tubulin molecule which is critical in the mechanism of particle transport, and provide a direct immunological probe to examine the mechanism of microtubule involvement in axonal transport.

Substructure of sidearms on squid axoplasmic vesicles and microtubules visualized by negative contrast electron microscopy

We present a high-resolution electron microscopic study of the sidearms on microtubules and vesicles that are suggested to form the crossbridges which produce the microtubule-based vesicle transport in squid axoplasm. The sidearms were found attached to the surfaces of the anterogradely transported vesicles in the presence of ATP. These sidearms were made of one to three filaments of uniform diameter. Each filament measured 5-6 nm in width and 30-35 nm in length. The filaments in some of the sidearms had splayed apart by pivoting at their base, thereby assuming a "V" shape. The spread configuration illustrated the independence of the individual filaments. The filaments in other sidearms were closely spaced and oriented parallel to each other, a pattern called the compact configuration. In axoplasmic buffer containing AMP-PNP, structures indistinguishable from the filaments of the sidearms on the vesicles were observed attached to microtubules. Pairs of filaments, thought to represent the basic functional unit, were observed attached to adjacent protofilaments of the microtubules by their distal tips. These data support a model of vesicle movement in which a pair of filaments within a sidearm forms two crossbridges and moves a vesicle by "walking" along the protofilaments of the microtubule.

Reactivated melanophore motility: differential regulation and nucleotide requirements of bidirectional pigment granule transport

To study the molecular basis for organized pigment granule transport, procedures were developed to lyse melanophores of Tilapia mossambica under conditions in which pigment granule movements could be reactivated. Gentle lysis of the melanophores resulted in a permeabilized cell model, which, in the absence of exogenous ATP, could undergo multiple rounds of pigment granule aggregation and dispersion when sequentially challenged with epinephrine and cAMP. Both directions of transport required ATP, since aggregation or dispersion in melanophores depleted of nucleotides could be reactivated only upon addition of MgATP or MgATP plus cAMP, respectively. Differences between the nucleotide sensitivities for aggregation and dispersion were demonstrated by observations that aggregation had a lower apparent Km for ATP than did dispersion and could be initiated at a lower ATP concentration. Moreover, aggregation could be initiated by ADP, but only dispersion could be reactivated by the thiophosphate ATP analog, ATP gamma S. The direction of pigment transport was determined solely by cAMP, since pigment granules undergoing dispersion reaggregated when cAMP was removed, and those undergoing aggregation dispersed when cAMP was added. These results provide evidence that pigment granule motility may be based on two distinct mechanisms that are differentially activated and regulated to produce bidirectional movements.

Inhibition of fast axonal transport in bullfrog nerves by dibenzazepine and dibenzocycloheptadiene calmodulin inhibitors

The effects of the calmodulin inhibitors amitriptyline, desipramine, imipramine, and clomipramine on fast axonal transport, oxidative metabolism, and density of axonal microtubules were measured in bullfrog spinal nerves in vitro. The four drugs tested inhibited the fast orthograde transport of [3H]leucine-labelled proteins and the fast retrograde transport of acetylcholinesterase at a concentration of 0.2 mM. Amitriptyline, desipramine, and imipramine were equipotent inhibitors of transport, and clomipramine was a more potent inhibitor than imipramine. The adenosine triphosphate content of the nerves was reduced by at most 19% by the compounds under study; such a reduction cannot account for the inhibition of fast axonal transport. Desipramine and imipramine had no significant effect on the density of microtubules in unmyelinated axons, whereas amitriptyline only reduced it by 18%; the inhibition of axonal transport by these three drugs can therefore not be explained by microtubule disruption. Clomipramine reduced microtubular density by 40%, and this effect may have contributed to the inhibition of fast axonal transport. The inhibition of fast axonal transport by desipramine, imipramine, and amitriptyline may be related to the inhibition of calmodulin function by these drugs. The similar potency of these three drugs as inhibitors of fast axonal transport goes in parallel with their known similar potency as calmodulin antagonists.

Inhibition of axonal transport 'in vivo' by a tubulin-specific antibody

We have used antibodies against the major proteins of the cytoskeleton-tubulin, the neurofilament triplet proteins and actin-as in vivo probes to determine the contribution of separate components of the cytoskeleton in axonal transport. The injection of either Fast Blue or wheat germ agglutinin conjugated horseradish peroxidase into the caudate nucleus of adult rats resulted in the retrograde transport of these tracers to the neuronal cell bodies in the substantia nigra pars compacta. In experimental animals these tracer injections were immediately preceded by injections of antiserum against tubulin, neurofilament triplet protein or actin, into multiple sites in the caudate. Preimmune serum injection preceded tracer injection as a control in the contralateral caudate of the same animal. One antiserum against electrophoretically purified pig brain tubulin (NS-20) produced a dramatic decrease in the normal retrograde and anterograde transport of both tracers to the SN. Other antisera against tubulin, as well as neurofilament and actin antisera, had no effect on the axonal transport of the tracers. Affinity purified antibodies prepared from the NS-20 antitubulin serum also blocked axonal transport of the tracers. These results provide further support for a critical role of microtubules in axonal transport in vivo. Moreover, an antigenic determinant on tubulin that is uniquely recognized by the NS-20 antibodies may provide us with a way to define the site of association of transfer vesicles with microtubules.

Microtubule polarity confers direction to pigment transport in chromatophores

The cellular mechanisms used to direct translocating organelles are poorly understood. It is believed that the intrinsic structural polarity of microtubules may play a role in this process. We have examined the effects that differently oriented microtubules have upon the direction of pigment transport in surgically severed melanophore arms. In a previous paper (McNiven, M. A., M. Wang, and K. R. Porter, 1984, Cell, 37:753-765) we reported that after isolation, arms repolarized and reoriented their microtubules outward from their centers as if to form new "microcells." Pigment aggregation in these arms was toward a new focal point located at the arm centers. In this study we monitored pigment movement in isolated arms containing taxol-stabilized microtubules to test if the reversal in direction of pigment transport is dependent upon the repolarization of microtubules. We report that taxol delays both the microtubule reorientation and reversal in transport direction in a concentration-dependent manner. These and other presented data suggest that the polarity of the microtubule population within a melanophore confers direction on pigment transport.

Identification of a MAP 2-like ATP-binding protein associated with axoplasmic vesicles that translocate on isolated microtubules

Axoplasmic vesicles were purified and observed to translocate on isolated microtubules in an ATP-dependent, trypsin-sensitive manner, implying that ATP-binding polypeptides essential for force generation were present on the vesicle surface. To identify these proteins [alpha 32P]8-azidoadenosine 5'-triphosphate ([alpha 32P]8-N3ATP), a photoaffinity analogue of ATP, was used. The results presented here identify and characterize a vesicle-associated polypeptide having a relative molecular mass of 292 kD that bound [alpha 32P]8-N3ATP. The incorporation of label is ultraviolet light-dependent and ATP-sensitive. Moreover, the 292-kD polypeptide could be isolated in association with vesicles or microtubules, depending on the conditions used, and the data indicate that the 292-kD polypeptide is similar to mammalian brain microtubule-associated protein 2 (MAP 2) for the following reasons: The 292-kD polypeptide isolated from either squid axoplasm or optic lobe cross-reacts with antiserum to porcine brain MAP 2. Furthermore, it purifies with taxol-stabilized microtubules and is released with salt. Based on these characteristics, the 292-kD polypeptide is distinct from the known force-generating molecules myosin and flagellar dynein, as well as the 110-130-kD kinesin-like polypeptides that have recently been described (Brady, S. T., 1985, Nature (Lond.), 317:73-75; Vale, R. D., T. S. Reese, and M. P. Sheetz, 1985b, Cell, 42:39-50; Scholey, J. M., M. E. Porter, P. M. Grissom, and J. R. McIntosh, 1985, Nature (Lond.), 318:483-486). Because the 292-kD polypeptide binds ATP and is associated with vesicles that translocate on purified MAP-free microtubules in an ATP-dependent fashion, it is therefore believed to be involved in vesicle-microtubule interactions that promote organelle motility.