XB-ART-7578J Cell Biol March 4, 2002; 156 (5): 855-65.
Interactions and regulation of molecular motors in Xenopus melanophores.
Many cellular components are transported using a combination of the actin- and microtubule-based transport systems. However, how these two systems work together to allow well-regulated transport is not clearly understood. We investigate this question in the Xenopus melanophore model system, where three motors, kinesin II, cytoplasmic dynein, and myosin V, drive aggregation or dispersion of pigment organelles called melanosomes. During dispersion, myosin V functions as a "molecular ratchet" to increase outward transport by selectively terminating dynein-driven minus end runs. We show that there is a continual tug-of-war between the actin and microtubule transport systems, but the microtubule motors kinesin II and dynein are likely coordinated. Finally, we find that the transition from dispersion to aggregation increases dynein-mediated motion, decreases myosin V--mediated motion, and does not change kinesin II--dependent motion. Down-regulation of myosin V contributes to aggregation by impairing its ability to effectively compete with movement along microtubules.
PubMed ID: 11864991
PMC ID: PMC2173315
Article link: J Cell Biol
Genes referenced: actl6a dnai1 dync1li1 foxm1 klc1 pnp wars1
Article Images: [+] show captions
|Figure 1. Tracking organelles in cells with a limited number of melanosomes. (A) Cultured Xenopus melanophore. The large number of melanosomes makes tracking individual organelles impossible. (B) Cell after 4 wk of culture in 1 mM PTU. No melanosomes are seen in the cytoplasm. (C) Melanophore after 24 h recovery from PTU has a small number of melanosomes. (D and E) Melanophore after 24 h recovery from PTU before (D) and 1000 s after (E) addition of MSH, showing that the melanosomes in PTU-treated cells disperse in response to MSH. (F) Sequence of images taken during pigment dispersion of the cell shown in D and E. Arrows show positions of two melanosomes in subsequent frames. Time after addition of MSH (in seconds) is shown on each frame. Bright-field microscopy (A and D–F); phase–contrast microscopy (B and C). Bars: (A–E), 10 μm; (F) 2.5 μm.|
|Figure 2. Distribution of microtubules and actin filaments in Xenopus melanophores. (A) Microtubules stained by a tubulin antibody. Note radial pattern of distribution. (B) Actin filament distribution as revealed by rhodamine-phalloidin staining. In addition to thin bundles, most of the staining is distributed diffusely. Since rhodamine-phalloidin does not bind nonpolymerized actin, diffuse staining suggests random arrangement of actin filaments. Bar, 20 μm.|
|Figure 3. Linear melanosome tracks correlate with the position of microtubules. Movement of melanosomes in cells transfected with dominant negative myosin V (A) or treated with nocodazole (B). Note that in A melanosomes move along linear tracks, whereas movement in B is more random. Position of melanosomes was registered at 30 Hz, but for clarity only every sixth frame is displayed. Bar, 1 μm. (C) Overlay of melanosome tracks and fluorescently labeled microtubules. Microtubules were labeled by injecting cells with Alexa 488 tubulin. Movement of melanosomes was tracked using a series of bright-field images taken every 3 s. The resulting tracks were overlayed onto a fluorescent image showing the microtubule distribution. Note that linear movements correlate with the position of microtubules. The track labeled with an asterisk does not exactly overlap with the image of the underlying microtubule because this microtubule moved substantially in the course of the tracking. Bar, 5 μm.|
|Figure 4. Quantification of displacement generated by myosin V. The r2(t) plots show the square of the average displacement of the pigment granules as a function of time. Each curve represents an average of ∼100 plots for individual moving granules (∼25 melanosomes chosen at random in four different cells). The indicated error bars are the standard error of the mean. Nocodazole was used to depolymerize microtubules in order to quantify myosin V (M-V)–driven motion. The decrease in motion in aggregating versus dispersing cells indicates that myosin V–driven motion is down-regulated as part of the dispersion to aggregation transition. The lowest curve shows that in the absence of microtubules expression of the myosin V dominant negative construct completely eliminates motion, demonstrating the efficacy of the construct. The magnitude of dispersion in a wild-type background driven by kinesin II, dynein, and myosin V (top curve) is similar to dispersion driven only by myosin V (second curve).|
|Figure 5. Regulation of microtubule-based motion. To investigate regulation of microtubule motors, their activity was studied in the absence of functional myosin V. Shown are (modified) averages of travel distances (as described in Materials and methods) and velocities of plus and minus end runs. A run was defined as a period of uninterrupted motion along the microtubule (as described in Materials and methods). (A) The change from dispersion to aggregation coincides with a change in the length of (long-type) minus end runs, whereas the length of plus end runs remains constant. (B) The average velocity of (short-type) plus and minus end runs. There is no significant change in run velocity in either direction due to the change from dispersion to aggregation. These data show an average of ∼400 individual runs from four or more cells. M-V, myosin V. The error bars are the standard error of the mean.|
|Figure 6. Microtubule motion as a function of time and as altered by the expression of the dominant negative constructs. Average travel distances (modified as described in Materials and methods) and velocities were measured during dispersion (A and B) and aggregation (C and D). Run lengths, dispersion (A); velocities, dispersion (B); run lengths, aggregation (C); velocities, aggregation (D). The myosin V and kinesin II constructs both alter the length of minus end runs (A), but myosin V does not alter the length of plus end runs. Blocking myosin V function results in increased velocities in both plus and minus end travel (B), but decreasing kinesin II function does not alter minus end velocities (B). During aggregation, blocking myosin V activity has no effect on either run lengths (C) or velocities (D). M-V, myosin V. Error bars are the standard error of the mean.|
|Figure 7. Distribution of run lengths for minus end motion in wild-type (A) and myosin V (B) cells stimulated with MSH. Individual run lengths were determined as described in Materials and methods. In all cases examined, the distribution was well described by the sum of two decaying exponentials (Table I). The original histogram (bars) together with the double-decaying exponential fit (solid line) is shown in the inserts. Note that there are relatively more long runs in the myosin V cells (B) than in wild-type cells (A). The full graphs show the natural logarithm of the number of runs versus run length. The solid line is the complete fit, whereas the dotted and dashed lines indicate the contributions of the short and long runs, respectively. The long runs (tail of the distribution) are significantly altered due to myosin V activity, but the short runs are not (compare the slope of the long-runs line in A and B).|
|Figure 8. The ability of motors to interact with melanosomes and microtubules does not change between aggregation and dispersion. (A) Melanosomes were purified from cells treated with the aggregating stimulus, melatonin, or the dispersing stimulus, MSH, and Western blots were performed with antibodies against myosin V (M-V), kinesin II (K-II), and cytoplasmic dynein (Dy). Note that the amount of microtubule motors on melanosomes does not change due to aggregation (A) or dispersion (D), whereas the amount of myosin V is higher on melanosomes purified from cells dispersing the pigment than from cells aggregating the pigment. (B) Binding of kinesin II (K-II) or cytoplasmic dynein (Dy) to microtubules in the presence of AMP-PNP. Bovine brain microtubules were added to extracts from cells aggregating (A) or dispersing (D) pigment, the mixture was spun through a glycerol cushion, and the pellets were analyzed for the presence of motors by Western blotting (experimental protocol from Reese and Haimo (2000).|
|Figure 9. Model for transport of melanosomes by actin- and microtubule-based motors. A pigment organelle can be moved in three different ways: along the microtubule by kinesin II (A and D) or dynein (B and E), or along the actin filaments by myosin V (C and F). Direction of movement is shown by double arrows. Although at any moment motion is dominated by one of the two transport systems (i.e., microtubules or actin), the motors of the other transport system are transiently active and interacting with their substrate (i.e., actin or microtubules). During dispersion, these transient interactions (indicated by T) are significant and allow myosin V to reduce the velocity of microtubule-based transport (A and B). The interactions also play a role in the switch between the two transport systems, that is, between B and C. Because myosin V activity predominantly decreases the length of minus end microtubule-based motion, we hypothesize that the switch from microtubule- to actin-based transport occurs only between state B and C (from dynein movement to myosin V movement) and that transitions from kinesin II movement (A) to myosin V movement (C) are rare. Because kinesin II but not dynein appears to win in tug-of-wars with myosin V, we suggest that the C to A transition is possible but not the C to B transition; however, we have no direct evidence on this point. During aggregation, myosin V activity is decreased, which results in reduced weak transient interactions (indicated by weak T) in D and E, and myosin V is no longer able to interfere with microtubule-based motion. Due to the weakness of the interactions, any time the microtubule motors are in contact with the microtubules they win the tug-of-war with myosin V and there is a transfer (F to D or F to E). Similarly, the reverse transfer from microtubules to actin-based transport (D to F or E to F) does not occur. M-V, myosin V; K-II, kinesin II; Dy, dynein; Ms,melanosome.|
References [+] :
Bridgman, Myosin Va movements in normal and dilute-lethal axons provide support for a dual filament motor complex. 1999, Pubmed