Calyculin A, an enhancer of myosin, speeds up anaphase chromosome movement
© Fabian et al; licensee BioMed Central Ltd. 2007
Received: 19 January 2007
Accepted: 24 March 2007
Published: 24 March 2007
Actin and myosin inhibitors often blocked anaphase movements in insect spermatocytes in previous experiments. Here we treat cells with an enhancer of myosin, Calyculin A, which inhibits myosin-light-chain phosphatase from dephosphorylating myosin; myosin thus is hyperactivated. Calyculin A causes anaphase crane-fly spermatocyte chromosomes to accelerate poleward; after they reach the poles they often move back toward the equator. When added during metaphase, chromosomes at anaphase move faster than normal. Calyculin A causes prometaphase chromosomes to move rapidly up and back along the spindle axis, and to rotate. Immunofluorescence staining with an antibody against phosphorylated myosin regulatory light chain (p-squash) indicated increased phosphorylation of cleavage furrow myosin compared to control cells, indicating that calyculin A indeed increased myosin phosphorylation. To test whether the Calyculin A effects are due to myosin phosphatase or to type 2 phosphatases, we treated cells with okadaic acid, which inhibits protein phosphatase 2A at concentrations similar to Calyculin A but requires much higher concentrations to inhibit myosin phosphatase. Okadaic acid had no effect on chromosome movement. Backward movements did not require myosin or actin since they were not affected by 2,3-butanedione monoxime or LatruculinB. Calyculin A affects the distribution and organization of spindle microtubules, spindle actin, cortical actin and putative spindle matrix proteins skeletor and titin, as visualized using immunofluorescence. We discuss how accelerated and backwards movements might arise.
Mechanisms of chromosome movements during anaphase have been investigated extensively and several models attempt to explain the forces involved [1–4]. Proteins implicated as key players in mitosis include tubulin [5–7], microtubule motors [8–12], actin [1, 13–16], myosin [1, 15–22], the elastic component titin [23–25], and matrix proteins skeletor [16, 22, 26–28], megator , chromator , EAST [31, 32], NuMA [33–37] and laminB . In this article we present data dealing with spindle myosin.
Myosin in mitotic cells generally is thought to be involved with cytokinesis, primarily with contractile ring formation and ingression [39, 40], and with positioning and orientation of the mitotic spindle . But myosin also is present in the spindle [1, 15]. Some of the early studies that showed that actin and myosin were present in the spindle also discussed a possible role for myosin in force production during anaphase chromosome movement [17, 18, 42–45], but no physiological data were presented. More recent evidences that implicate myosin function in anaphase chromosome movements are based on experiments using various inhibitors of myosin or inhibitors of myosin phosphorylation [1, 21, 22, 46]. In particular, movement of chromosomes during anaphase is stopped or slowed by the myosin inhibitor 2,3-butanedione monoxime (BDM) [1, 16, 21] or by the Rho-kinase inhibitor Y27632 . Our present experiments utilise Calyculin A (CalA), a compound which prevents myosin dephosphorylation.
In order for non-muscle and smooth muscle myosin to be functional, the regulatory light chain (RLC) of myosin must be activated by phosphorylation by specific kinases, either myosin light chain kinase (MLCK) [47–49] or Rho-kinase (Rho-K) [40, 50–52], and possibly others [e.g. [53–55]]. Myosin homeostasis is achieved by the balance between activation by phosphorylation, and inactivation by dephosphorylation, the latter being due to the action of myosin light chain phosphatase (MLCPase) [40, 56–58], a type 1 protein phosphatase (PPase1) , which, like most PPases1, is probably targeted to its site by activity of other proteins [60, 61]. Rho-K plays a double role in myosin homeostasis: it phosphorylates myosin RLC, thereby MLRC is activated [62–66], and it phosphorylates MLCPase, thereby MLCPase is inactivated. Rho-K thus regulates the degree of myosin phosphorylation and hence the activity of myosin [52, 67–69].
Additional file 1: Movie representing meiosis I in control crane-fly spermatocyte. Two half-bivalent pairs (out of the total three) are in focus. After the autosomes reached the poles, the two smaller univalents sex chromosomes, which stayed at the metaphase plate, start their anaphase, at about the same time as cytokinesis starts. This movie displays time-lapsed images, obtained from real-time sequences recorded on DVD, played back at 120× the recorded speed. (MPG 2 MB)
Calyculin A added in anaphase
We tested various concentrations of CalA, from 5 nM to 0.5 μM, in order to determine the lowest concentration that affects chromosome movement. Concentrations of 5 nM and 10 nM had no effect on anaphase chromosome movement, 20 nM had inconsistent effects, and 50 nM had consistent effects. Thus we used 50 nM for most of the experiments; unless noted otherwise, all descriptions are for cells treated with ≥ 50 nM CalA.
Effect of Calyculin A and Okadaic acid on anaphase chromosome movements**
# of half-bivalent pairs followed
# of cells
CalA in anaphase
Okadaic acid in anaphase
Additional file 2: Anaphase in a crane-fly spermatocyte treated with 50 nM Calyculin A at 13:03:30. Two half-bivalent pairs are in focus. Autosomes accelerated their poleward movement after Calyculin A addition. After they reached the poles, they moved backwards, toward the equator, where they joined their partners from the opposite pole and the two univalent sex chromosomes. Cell does not enter cytokinesis. This movie displays time-lapsed images, obtained from real-time sequences recorded on DVD, played back at 120× the recorded speed. (MPG 2 MB)
Velocities after Calyculin A treatment in anaphase (in μm/min)
× times faster*
Number of pairs
The equatorial sex chromosomes also are affected by CalA. In control cells, without exception, sex chromosomes remain stationary at the equator as autosomes move poleward. Immediately after adding CalA during autosomal anaphase, however, the sex chromosomes take long excursions up and down along the spindle, at high speeds and with sudden changes of direction, and they do not segregate in a normal anaphase.
Because of these dramatic effects on the pre-anaphase sex chromosomes, we added CalA to pre-anaphase cells, to see if it would affect autosomes similarly.
Calyculin A added in metaphase and prometaphase
When CalA is added during metaphase (2 cells), 2–4 minutes before anaphase, the chromosomes still entered anaphase, moved with high speeds poleward, and then moved backwards (Fig. 3B). There was no cytokinesis. Thus, the behaviour of the chromosomes in metaphase-treated spermatocytes is the same as in anaphase-treated spermatocytes.
Additional file 3: Prometaphase in a crane-fly spermatocyte treated with 50 nM Calyculin A at 13:41:00. Two autosomal bivalents are in focus. After Calyculin A addition, the chromosomes started to rotate and to move randomly in the spindle, much faster than before Calyculin A. The chromosomes do not align at the metaphase plate and the cell does not enter anaphase. This movie displays time-lapsed images, obtained from real-time sequences recorded on DVD, played back at 120× the recorded speed. (MPG 2 MB)
Calyculin A added during cytokinesis
Cytokinesis in crane-fly spermatocytes is a myosin-dependent process [1, 21], but, as with other cells [76, 89], CalA (added 4–7 minutes after the onset of cytokinesis) stopped cleavage almost immediately (it took 2–3 minutes for the effect to be visible) and the furrow regressed (Fig. 3C). Cleavage did not resume after washing out CalA.
Calyculin A and myosin
CalA might affect PPase1 or PPase2A enzymes as well as myosin phosphatase. We did several experiments to test whether the effects indeed were via myosin phosphorylation. In one set of experiments we treated anaphase cells with 25–50 nM okadaic acid, the same concentration at which we used CalA. Okadaic acid had no effect on chromosome movement or, in a small proportion of pairs (Table 1), slowed or stopped chromosome movement. Thus, since okadaic acid inhibits PPase2A at the same concentration as CalA, but requires 100 times the concentration of CalA to inhibit PPase1 , the effect of CalA would seem to be via a PPase1, such as myosin phosphatase.
Effect of double treatments on anaphase chromosome movements
# of half-bivalent pairs followed
# of cells
accelerated after first drug
slowed or stopped after first drug
no change after first drug
accelerated after second drug
slowed or stopped after second drug
no change after second drug
1st – CalA in anaphase
2nd – Y27632 in anaphase
1st – Y27632 in anaphase
2nd – CalA in anaphase
1st – CalA in anaphase
2nd – BDM in anaphase
1st – LatB in anaphase
2nd – CalA in anaphase
To further test whether the effect of CalA on anaphase is due to its action on myosin, we double treated cells with CalA followed by BDM made up in CalA solution. BDM, a myosin inhibitor that acts directly on phosphorylated myosin and inhibits its motor activity , stops or slows chromosome movement in crane-fly spermatocytes when added during anaphase [1, 21]. When we added 5 mM BDM to CalA treated cells as the chromosomes started to accelerate their poleward movement, the chromosomes slowed down or stopped (Table 3; Fig. 4C). The chromosomes resumed their accelerated movement after washing out BDM but leaving cells immersed in CalA (Fig. 4C).
Calyculin A affects the distribution of spindle proteins
Since CalA altered the distribution of p-squash, we used confocal microscopy to study the distribution of various other spindle proteins (i.e. actin, tubulin, titin, skeletor and myosin) in cells treated with 50 nM CalA, to see if CalA caused changes in their distribution as well. Crane-fly spermatocytes were treated with CalA for 3, 5, 10 or 25 minutes, according to the timing of various effects we saw in living cells. In cells treated for 25 minutes the spindle shape was not visible after staining with tubulin and actin. For shorter treatments there was not a drastic difference between 3, 5 and 10 minutes treatment, so we present them together.
Actin also is affected by CalA: actin filaments present in the spindle fibres (Fig. 6D) become more "visible" as stronger/thicker bundles of actin filaments (Fig. 6E). We identified continuous actin filaments mainly along the kinetochore spindle fibres (Fig. 6E). Actin filaments in the cortex seem to be shorter and broken, sometimes with a "knotted" appearance. Actin also accumulated at the two poles and there was less actin in the mid region of the cell. The actin filament distribution also was altered in cells in which cytokinesis started before CalA addition: there were numerous actin aggregates in the midregion of the cell or in the midbody (Fig. 6F,F').
Skeletor, which normally is arranged in beaded filaments along the spindle fibres (Fig. 6G), also was affected by CalA. In kinetochore microtubules that extended from kinetochore to the pole, the skeletor beads seemed much farther apart than in control cells (Fig. 6H). Skeletor also was associated with the "peeled off" microtubules, but, unlike in non-treated cells, skeletor was not associated with anything other than kinetochore microtubules. Furthermore, there is less skeletor along the interzone connections between half-bivalents and along the sex chromosome kinetochore microtubule bundles than in controls.
Titin also is affected by CalA. Titin still is localised primarily in the spindle, but there is less of it than in controls and it seems less well organized (Fig. 6J). Titin remains present in the interzone during anaphase (arrowheads in Fig. 6K, L), distributed between the arms of separating half-bivalent pairs (brackets in Fig 6K, L).
Myosin, which normally is present in the whole spindle area and also outside the spindle (Fig. 6M) , is affected by CalA in that there is less myosin in the spindle and in the cytoplasm, but more myosin in the chromosomes (Fig. 6N) or around the chromosomes.
Calyculin A affects myosin in actin-filament-free spindles
Effect of Calyculin A on anaphase chromosome movements in actin filament-free spindles
# of half-bivalent pairs followed
Initial anaphase movement
Effect of CalA
# of cells
accelerated after CalA
slowed or stopped after CalA
1st – LatB in prometaphase
2nd – CalA in anaphase
Effect of actin and myosin inhibitors on backwards movement in CalA-treated spermatocytes
# of half-bivalent pairs followed
# of cells
Slowed or stopped after BDM or LatB
no change after BDM or LatB
1st – CalA in anaphase
2nd – BDM or LatB in late anaphase
In this study we showed that CalA, an enhancer of myosin, added during anaphase or metaphase, speeds up anaphase chromosome movement. Subsequently chromosomes move backwards. CalA also causes sex chromosomes to make long excursions in the spindle at high speed, and to rotate; and it blocks cytokinesis. When added during prometaphase, chromosomes rotate, move up and down, and jiggle. CalA also affects the distribution of various spindle proteins.
A major finding of our study is that anaphase chromosome movements were accelerated by a factor of two after addition of CalA. We think it likely that this acceleration is due to hyper-phosphorylation of myosin. An alternate possibility is that the acceleration is due to CalA action on other possible targets. Okadaic acid blocks PPase2A activity at the same concentration as CalA , and since okadaic acid did not cause either acceleration or backwards movements, this would seem to eliminate the possibility that PPase2A is a CalA target. It is possible that CalA acts on a PPase1 other than on the myosin phosphatase, but several lines of evidence speak against this. For one, the general effect of inhibiting random PPases is that cells do not proceed past metaphase , or are much delayed , whereas crane-fly spermatocytes treated in metaphase with CalA entered anaphase normally and chromosome movement was accelerated, arguing that the CalA effect is a more specific one. For another, staining with antibodies to p-squash indicates that phosphorylated myosin is located in the spindle, along the kinetochore microtubule bundles; staining with antibodies against various PPase1 isoforms indicates that one such enzyme also is located along spindle fibres . CalA causes hyper-phosphorylation of myosin, as indicated by increased staining of cleavage furrow myosin and of chromosomes. While none of these arguments is airtight, we nonetheless favour the interpretation that acceleration is due to increased myosin activity because it is consistent with other experiments in which myosin and actin inhibitors block chromosome movement [1, 15]. Whereas the interpretation of each individual study or of the action of each individual drug might be debated, the overall fact that inhibitors of actin and myosin generally inhibit movement and a myosin enhancer speeds up movement makes a strong case for involvement of myosin in anaphase chromosome movements and supports our interpretation that the acceleration is caused by myosin hyper-activation. Thus, while we cannot definitively rule out interpretations in which other PPase1 enzymes are involved, we think it likely that chromosome acceleration is due to enhanced myosin activity.
Our interpretation that CalA effects are due to blocking MLCPase would seem to be negated by our experiments using Y27632. Y27632, a specific inhibitor of Rho-K, slowed chromosome movement in anaphase. This effect presumably is due to reduced myosin phosphorylation, and therefore myosin is less active after Y27632 addition. CalA was not expected to accelerate these chromosomes, since myosin was not phosphorylated, but it did. The interpretation of these results is ambiguous, however, because the same result was obtained in studying smooth muscle contraction, which is known to be due to myosin activity . It seems that inhibition of MLCPase by CalA unmasks another phosphorylation pathway, separate from Rho-K pathway and therefore not inhibited by Y-27632 . This is confirmed in other experiments in which inhibition of smooth muscle Rho-K by Y27632 or H-1152 unmasked an integrin-linked kinase which then phosphorylated myosin . Thus, because of such potential redundant phosphorylation pathways, this particular experiment is ambiguous and is not a clear test of how CalA causes chromosome acceleration.
The only previous data on CalA effects on mitosis that we know of is by Hamaguchi and Kuriyama . The authors concluded that anaphase chromosome movements in sand dollar eggs were blocked by okadaic acid and CalA. With respect to CalA, they state in the text that chromosome movements were inhibited by CalA at concentrations ≥ 1–2 μM. We generally used concentrations 20 times lower than that, but we found that concentrations of 0.5 μM caused chromosomes to accelerate. Thus there appears to be a discrepancy between the two sets of results. However, from the description in Figure 10 of , it would seem that the authors derived their conclusion from fluorescence micrographs of chromosomes positions after injection of CalA. Their description in Figure 10 that "chromosome movement did not occur, although the chromosomes aggregated into two clusters", does not necessarily have to be due to blocked movements, though; the clusters could have arisen from backwards movements of the kind we described. With respect to the apparent discrepancy between our "no effect" of okadaic acid (Table 1) and the blockage of movement by okadaic acid reported by Hamaguchi and Kuriyama , their blockage of movement occurred at concentrations of 1–2 μM, which is 20–40 times higher than the concentration we used (50 nM). Indeed, they found that 50 nM okadaic acid had little or no effect on anaphase in their cells, so there is no discrepancy between the two sets of results.
CalA alters the distribution of various spindle proteins in crane-fly spermatocytes. P-squash staining relocates in the cell, from being associated with the kinetochore microtubules to being at the poles, at the kinetochores, and around the chromosomes. Similar results occur in other systems. For example, there are increased levels of phosphorylated myosin after CalA treatment in various cells [68, 69, 72, 74, 75, 84] and CalA causes relocation of actin, myosin and other cytoskeletal components [83, 98, 99]. In crane-fly spermatocytes the microtubule bundles become thinner, presumably due to splitting, narrowing and disappearance of microtubules from the bundles. This is consistent with results in other studies that showed that microtubules are disassembled after CalA [98, 100]. In crane-fly spermatocytes actin filaments become more visible in spindle fibres possibly due to actin filament stabilization by inhibition of MLCPase, similar to results in sea urchin eggs [69, 76].
When CalA is added in prometaphase, chromosomes in crane-fly spermatocytes lose their attachment to microtubules (Fig. 6C). In our experiments, the unattached chromosomes rotated and moved rapidly up and down in the spindle, similar to the rapid movements displayed by the sex chromosomes after CalA treatment during anaphase. The longitudinal movements might be due to chromosomes capturing and sliding along remnant microtubules, but we can only speculate on forces that might be producing the rotations. All these extraordinary movements, though, seem to indicate that there is loss of equilibrium and lack of coordination between different force producers.
CalA caused late anaphase chromosomes to move backwards. Backward chromosome movements in anaphase have been seen previously in crane-fly spermatocytes, but only rarely and only when the poleward force was blocked by UV microbeam irradiation [101, 102]. However, chromosome arms, severed with a laser beam, regularly moved backwards . Backward movements were seen also in silkworm spermatocytes after UV irradiation of a spindle pole  : the chromosomes associated with the irradiated pole moved across the equator to the opposite pole. In grasshopper spermatocytes  there are fast backwards movements after UV irradiation of the kinetochore in early anaphase. Ilagan and Forer , LaFountain et al.  and Wong and Forer  all considered that there are mechanical connections between the arms of separating half-bivalents in crane-fly spermatocytes and that these connections, or "tethers", elastically cause the backwards movements. Even though the tethers act on both partner half-bivalents, the backwards movements need not be symmetric, with both partners moving equally, because in all previous observations only one chromosome (or arm) moved, the one no longer attached to the pole. We think the same applies to CalA treated cells: when the connection to the pole is lost, then that chromosome moves to the equator independent of the partner at the other pole. We assume that the accelerated movements to the pole are associated (for some chromosomes) with pre-mature release of attachment to the pole, which allows the backward movements to be driven by the elastic tethers between partners. In our experiments, backwards movements in CalA treated spermatocytes were not altered by LatB or BDM, suggesting that the movements are not dependent on actin or myosin. It is possible that actin filaments remaining in the interzone after CalA treatment (e.g., Fig. 6E) are resistant to LatB by virtue of the bundling induced by Cal A, and that the backwards movements might require these actin filaments. Two experiments speak against this possibility, however. For one, we know that LatB removes actin filaments from normal cells  yet backwards movements still take place when cells pre-treated with LatB are treated with CalA (Table 3, 4). This indicates that actin filaments are not necessary for backwards movements to take place. For another, BDM has no effect on backwards movements, indicating that those movements do not require myosin or actomyosin. Thus our results indicate that backwards movements require neither actin nor myosin. On the other hand, titin, the protein responsible for muscle elasticity [105–107] is present between the arms of separating half-bivalents in control crane-fly spermatocytes as well as in CalA treated spermatocytes and could provide the necessary elasticity. Thus, our interpretation is that backwards movements are due to titin filaments which pull the chromosomes back together. As a corollary, we suggest that the regularly organized spindle matrix and cytoskeletal components prevent the backwards movement in non-treated cells, but the disorganization that ensues after CalA treatment reduces or removes attachment to the pole and allows titin to pull the chromosomes back together, to near the equator.
In this study we showed that CalA, an enhancer of myosin, has multiple effects on chromosome movements during anaphase and alters the distribution of spindle proteins in crane-fly spermatocytes. CalA speeds up anaphase chromosome movement when added during anaphase or metaphase. It blocks the formation of the new nuclei in the daughter cells, due to the chromosomes moving backwards, toward the equator, after they reached the poles. When added during prometaphase, chromosomes rotate, move up and down, and jiggle. Sex chromosomes are also affected by CalA, in such that they make long excursions in the spindle at high speed, and rotate. And finally, CalA blocks cytokinesis initiation and completion in crane-fly spermatocytes. We suggest that these CalA effects are via myosin, as indicated by several lines of evidence we discussed.
Materials and methods
Living crane-fly spermatocytes (Nephrotoma suturalis Loew), held in place in a fibrin clot prepared as described in , were observed using phase-contrast microscopy with an objective of NA = 1.3, and images were recorded on DVDs in real time. Cells were perfused with insect Ringer's solution or with Calyculin A (LC Laboratories, MA) in insect Ringer's solution, at final concentrations of 0.5 μM, 100 nM, 50 nM, 20 nM, 10 nM or 5 nM, prepared from a 1 mM or a 50 μM Calyculin A stock in DMSO or with okadaic acid (Calbiochem) at final concentrations of 20 nM or 50 nM prepared from a 50 μM stock in DMSO. In addition to CalA and okadaic acid we perfused cells with 50 μM Y27632 (Calbiochem), 5 mM or 20 mM BDM (Sigma), and 1.5 μM LatrunculinB (Calbiochem), each one diluted in a solution containing 50 nM Calyculin A. BDM and Y27632 stocks were made in insect Ringer's solution and LatB stock was made in DMSO. In no experiment was the DMSO concentration greater than 0.2%, which has been shown previously to have no effect on chromosome velocities [14, 96, 109, 110].
For immunostaining we followed the protocol described in detail in  using the following solutions: 2.2 μM Alexa 488 phalloidin (Molecular Probes) for filamentous actin; 1:4000 YL1/2 rat antibody against tyrosinated tubulin , followed by 1:200 Alexa 594 goat anti rat (GAR); 1:200 My21 mouse IgM antibody against myosin RLC (Sigma), followed by 1:200 Alexa 488 GA mouse IgM; 1:100 1A1 mouse IgM antibody against skeletor , followed by 1:200 Alexa 488 GA mouse IgM; 1:500 LP06352 rabbit antibody, followed by 1:200 Alexa 568 GA rabbit; 1:500 α-KZ rat antibody against D-titin, followed by 1:200 Alexa 594 GAR (primary antibodies against titin are a gift from Dr. Debbie Andrew, Johns Hopkins University, MD)  ; and 1:400 rabbit anti-phosphorylated myosin squash , followed by 1:200 Alexa 568 GA rabbit. All secondary antibodies are from Molecular Probes. For preparations using triple-channel staining, the three secondary antibodies had the following excitation/emission peaks: Alexa 488/519, Alexa 568/602 and Alexa 633/647. Cells were examined with an Olympus Fluoview 300 confocal microscope, the images were collected with Fluoview (Olympus) software, and were processed further using Image J . Illustrations presented in this paper were obtained using Adobe Photoshop and were adjusted for brightness and contrast only.
We thank Drs. Julie Brill (University of Toronto) and Kristen Johansen (Iowa State University) for helpful discussions and useful comments in interpretation of various effects of Calyculin A. The work was supported in part by grants from the NSERC to A.F., by a NSERC Summer Studentship to J.T. and by an Ontario Graduate Scholarship to L.F.
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