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Cell Motility and the Cytoskeleton 34:230-245 (1996)
Microtubules Can Modulate Pseudopod
Activity From a Distance
Inside Macrophages
Gustavo R. Rosania and Joel A. Swanson
Department of Cell Biology and the Program in Biological and Biomedical
Sciences, Harvard Medical School, Boston, Massachusetts
Microtubules are thought to influence cell shape as structural components of an
integrated cytoskeletal matrix. Here we show that microtubules can affect the
dynamics of macrophage pseudopodia without being integrated into their structure. Macrophages landing on glass surfaces spread within 15 min into flattened
circular cells with radial symmetry, and the radial distribution of microtubules
reflected this symmetry. Depolymerization of microtubules using nocodazole,
colchicine, or vinblastine did not inhibit macrophage spreading or the early establishment of radial symmetry. Shortly after spreading, however, macrophages
without microtubules gradually became asymmetric, assuming irregular, lobed
profiles. The asymmetry resulted from exaggerated protrusion and retraction of
pseudopodia, with net retraction overall. This loss of radial symmetry could be
inhibited by treatment of initially spread cells with cytochalasin D, indicating that
the change in cell shape was mediated by the actin cytoskeleton. Intact microtubules suppressed the exaggerated pseudopod movements, even when they were
separated by a distance from the cell margin. In cells treated with taxol, microtubules remained clustered near the cell center after spreading, yet the dynamics
of pseudopodia at the cell margin were reduced and cells maintained a circular
profile. Similarly, in cells treated with low concentrations of nocodazole, a much
reduced microtubule cytoskeleton nonetheless suppressed pseudopod dynamics.
We propose that microtubules act to stabilize cell shape at a distance from the cell
edge via a biochemical intermediate that affects the structure or function of the
microfilament System. 0 1996 Wiley-Liss, Inc.
Key words: microtubules, macrophage pseudopodia, cell shape
INTRODUCTION
Microtubules have long been thought to play a role
in regulating cellular morphogenetic processes, including the assembly and positioning of the cleavage furrow
[Fishkind and Wang, 19951, the extension and guidance
of growth cones in neurons [Tanaka et al., 199.51, and the
establishment or maintenance of cell shape and polarity
[Dugina et al., 1995; Glasgow and Danielle, 1994;
Pletjushkina et al., 1994; Schliwa and Honer, 1993; Tomasek and Hay, 1984; Vasiliev et al., 19701. In spreading polarized fibroblasts, microtubule ends lie in the vicinity of cell attachment sites, suggesting that
microtubules stabilize focal contacts at the sites of cellsubstrate adhesion and thereby organize actin filaments
0 1996 Wiley-Liss, Inc.
[Rinnerthaler et al., 19881. Microtubules can also affect
contractile activity at the cell cortex, suggesting that they
also control the function of myosin [Bornens et al.,
19891. Nevertheless, the mechanism by which microtubules influence the organization, dynamics, or contractile activity of the microfilament system is unknown.
The effects of microtubules on microfilaments may
be studied by looking at how the two systems interact to
maintain cell shape. In most cells, shape reflects both the
Received January 25, 1996; accepted April 4, 1996.
Address reprint requests to Joel A. Swanson, Department of Anatomy
and Cell Biology, University of Michigan Medical School, Ann Arbor, MI 48109.
Microtubules Modulate Pseudopod Activity
viscoelastic properties [Janmey, 19911 and the dynamics
of the cytoskeleton. In spread cells, pseudopod extensions occur continuously at sites of actin polymerization
at the periphery of the cell [Okabe and Hirokawa, 1989;
Symons and Mitchison, 19911. Cytochalasins, which
block actin polymerization, inhibit cell spreading,
phagocytosis, ruffling, and pseudopod extensions at the
cell edge [Axline and Reaven, 19741. Counteracting
the continuous growth of actin polymer at the edge of the
cell is a continuous centripetal flow of actin, thought to
be mediated in part by myosin [Bray and White, 1988;
Fischer et al., 1988; Heath and Hollifield, 1993; Lee et
al., 19931. The depolymerization of monomers from the
“pointed” ends of actin filaments and the polymerization of monomers at the “barbed” ends of the filaments
lead to the treadmilling of subunits along the polymer
[Wang, 19851, which may also contribute to centripetal
flow. Pseudopod extension or retraction may result when
the cortical flow of actin is unbalanced by polymerization of new actin filaments at the cell edge. Microtubules
could influence cell shape by affecting any of these processes.
Studies using a variety of microtubule-specific
drugs indicate a role for microtubules in the maintenance
of cell shape. When elongated fibroblasts [Gail and
Boone, 1971; Middleton et al., 1988; Vasiliev et al.,
19701, PC12 cells [Joshi et al., 19851, or other polarized
cell types are incubated with colchicine or nocodazole,
microtubules depolymerize and cell shape changes from
polarized to symmetric. When drug removal allows microtubules to re-polymerize, asymmetric shape is restored [Vasiliev et al. , 19701. Fibroblasts spreading in
the presence of microtubule inhibitors form discoidal
shapes, indicating that microtubules are required for both
the establishment and maintenance of the asymmetry
[Vasiliev et al., 19701. Fibroblasts treated with taxol, a
drug that stabilizes microtubules [de Brabander et al.,
1981; Schiff and Honvitz, 19801 and disrupts the arrangement of microtubules in the cell [Pletjushkina et
al., 19941, also acquire circular, symmetric shapes
[Pletjushkina et al., 19941. In experiments with primary
fibroblasts [Middleton et al., 19891 and cells that are
normally circular or unpolarized [Domnina et al., 1985;
Middleton et al., 19881, depolymerization of microtubules showed little effect on cell shape, indicating that
the effects of microtubules are context dependent.
It has been proposed that microtubules stabilize cell
shape by acting as rigid, compression-resistant struts,
linked to the actomyosin network and mechanically
counteracting the contractile force generated by that network [Ingber, 1993; Joshi et al., 19851. Consistent with
this hypothesis, microtubule depolymerization in PC 12
cells increases tension in neurites [Dernell et al., 19881,
leading to their retraction [Joshi et al., 1985; Solomon
231
and Magendantz, 19811. Simultaneous incubation in nocodazole and cytochalasin inhibited retraction [Solomon
and Magendantz, 19811, indicating that microfilaments
are somehow linked to the microtubules and mediate the
retraction. Microtubules have also been proposed to stabilize cell shape by serving as tracks that direct components for protrusive activity toward the cell edge [Bershadsky and Vasiliev, 19931. In fibroblasts, disrupting
vesicle trafficking with antibodies that block the function
of the microtubule-dependent mechanoenzyme kinesin
[Rodionov et al., 19931, leads to changes in pseudopod
activity and cell shape similar to those observed when
microtubules are depolymerized. This is consistent with
the idea that polarized membrane insertion at the leading
edge is responsible for increased protrusive activity
[Kupfer et al., 1987; Singer and Kupfer, 19861. Finally,
microtubules could act as intracellular signal transducers
[de Brabander et al., 19771, controlling cell shape by
affecting the levels of some diffusible molecule or ion
that in turn affects the dynamics of the actin cytoskeleton. Although there is no clear evidence for a diffusible
molecule linking microtubule depolymerization with
changes in actin dynamics, a number of signaling molecules influencing protrusive activity and cytoskeletal
organization [Hall, 19941 could serve as intermediates.
In this study, time-lapse video microscopy and
quantitative morphometric image analysis were used to
examine the contribution of microtubules to the establishment and maintenance of cell shape. In macrophages,
we report that microtubules are essential for maintaining
cell symmetry, yet their effects are independent of microtubule distribution inside the cell. Microtubules modulate pseudopod dynamics at the cell margin without
necessarily extending all the way to that margin. We
conclude that macrophage microtubules do not stabilize
cell shape by mechanically supporting the cell edge, nor
are they necessary as tracks that extend to the cell margin. These observations support the hypothesis that microtubules affect cell shape globally through a chemical
intermediate influencing the structure and function of the
actin cytoskeleton.
MATERIALS AND METHODS
Materials
Fluorescent probes were obtained from Molecular
Probes (Eugene, OR). Monoclonal antibodies against vimentin (H5) and tubulin (E7) were developed by Joshua
Sanes and Michael Klymkowsky, and obtained from the
Developmental Studies Hybridoma Bank (Department of
Pharmacology and Molecular Sciences, The Johns Hopkins University School of Medicine, Baltimore, MD
21205, and the Department of Biology, University of
Iowa, Iowa City, IA 52242). Secondary antibodies were
232
Rosania and Swanson
from Vector Laboratories (Burlingame, CA). Other reagents were obtained from Sigma Chemical Co. (St.
Louis, MO).
Cell Culture
Bone marrow-derived macrophages were obtained
from femurs of C3H/HeJ mice (Jackson Laboratory, Bar
Harbor, ME) and cultured in vitro [Cannon and Swanson, 19921. Five-day macrophages were suspended in
cold PD (137 mM NaC1, 3 mM KC1, 7 mM phosphate
buffer, pH 7.4) and 2 X lo6 cells were plated in 100 mm
Lab-Tek polystyrene Petri dishes in Dulbecco-Modified
Eagle’s Medium plus 10% fetal calf serum, overnight at
37°C in 5% CO,.
BSA-Coated Cover Slips
Thirteen millimeter, circular, no. 1 cover slips
were cleaned for 2 h in concentrated sulfuric acid,
washed overnight under continuous flow of ddH20, allowed to dry, and stored in a clean, dust-free container.
On the day of the experiment, cover slips were placed in
24-well plates and incubated in 1 ml of 0.1 m g / d polyL-lysine in PD for 15 rnin with continuous agitation.
Afterwards, they were washed 3 X 5 min in PD, and
incubated with 10 mg/ml BSA in PD for at least 30 rnin
in continuous agitation. Immediately prior to addition of
cells, cover slips were washed 2 x 5 rnin in Ringer’s
buffer (155 mM NaC1, 5 mM KC1, 2 mM CaCl,, 1 mM
MgCl,, 2 mM NaH2P0,, 10 mM HEPES, and 10 mM
glucose) without agitation.
were incubated under permeabilization buffer for 15 rnin
at room temperature prior to fixation. Subsequent incubations were in cytoskeletal buffer. For morphometry ,
each preparation was stained for tubulin (fluorescein immunofluorescence) and for total cellular protein using
Texas Red [Coates et al., 19921.
For immunofluorescent labeling, permeabilized
cells were incubated with blocking buffer (cytoskeletal
buffer plus 2% heat-inactivated goat serum) for 15 min,
then for 1 h in hybridoma cell supernatant (E7 for tubulin
or H5 for vimentin), diluted 1:4 in cytoskeletal buffer.
After incubation with primary antibody, cells were
washed 3 X 15 min in blocking buffer. Cells were
stained with secondary antibody (fluorescein, horse antimouse IgG, Vector Laboratories, Burlingame, CA) in
blocking buffer plus 25 pg/ml Texas Red sulphonyl chloride. For F-actin staining, cells were incubated with 20
U/ml of NBD-phallicidin (400 U/ml stock, in MeOH).
For micrographs and cytoskeletal visualization, cells
were observed with a fluorescein filter set on a Zeiss
Axioskop epifluorescence microscope (Zeiss, Thornwood, NY). General cell morphology was observed with
a Texas Red filter set. For quantitative measurements,
video images of cells were collected on a Zeiss IM35
inverted microscope, with a multichannel plate intensifier (Videoscope International, Washington, DC) and a
Nuvicon video camera connected to a Metamorph image
processing system (Universal Imaging Corporation,
West Chester, PA).
Synchronized Cell Spreading
Image Processing and Cell Morphometry
Cells were removed from dishes using cold Ringer’s buffer, and were suspended in Ringer’s buffer containing either taxol, nocodazole (both from 10 mM stock
in DMSO), colchicine or vinblastine for 45 min at 4°C.
Cells (2-4 x lo4) were dropped onto BSA-coated cover
slips in 24-well plates, and left to settle for 5-15 min at
4°C. Under these conditions, cells attached to the BSAcoated surface, but they did not spread. After cells had
attached, the buffer was removed and replaced with fresh
buffer containing the different drugs. The plate was then
transferred to 37°C water bath, and cell spreading began
immediately. After various intervals, cells were fixed
and processed for immunof luorescence or image analysis. DMSO (0.2%) did not affect cell morphology.
To outline cell shape, background-subtracted images of cells labeled with Texas Red were collected using
a X63 Zeiss planapo oil immersion lens. A low pass
filter was applied to the images to remove pixelation
from the cell edge. This improved image signal-to-noise
as judged by comparing processed video images with the
cells seen through the eyepiece. Images were then
thresholded to make binary masks, and morphometric
measurements were applied to these masks using built-in
measurement routines of the Metamorph imaging system. Four shape parameters were useful: area, which
measures the surface covered by the cell; length, which
measures the longest chord through the cell; radial ratio,
which is the ratio of the distance from the geometric
center to the closest point on the perimeter of the cell,
divided by the distance from that center point to the
furthest point on the perimeter; and shape factor, which
is a ratio equal to 47r (Area)/Perimete?. Shape factor and
radial ratio measured how closely the cell profile resembled a circle. For each data point, at least 50 cells were
measured in two or three different experiments, unless
otherwise indicated.
lmmunofluorescence
Cells were fixed for 30 min in 2% paraformaldehyde in PD at room temperature, then were washed 3 x
5 rnin in PD, and permeabilized in cytoskeletal buffer
(30 mM HEPES, 10 mM EGTA, 0.5 mM EDTA, 5 mM
MgSO,, 33 mM potassium acetate, pH 7.4, and 7% PEG
400) plus 0.5% Triton X-100. For extracted cells, cells
Microtubules Modulate Pseudopod Activity
Videomicroscopy
For time-lapse movies and motion analysis, 25
mm, circular, no. 1 BSA-coated cover slips were assembled on Leiden chambers in a temperature-controlledmicroscope stage (Medical Systems Corp., Greenvale,
NY). One milliliter of Ringer's buffer, with or without
drugs, was immediately added into the chamber, covered
with silicon oil to prevent evaporation, and warmed to
37°C. Fifty microliters of cell suspension was then added
to the chamber. Individual cells were located as they
were falling onto the cover slip. Time-lapse movies of
cell spreading were recorded using a video camera
(DAGE-MTI, Inc. , Michigan City, IN) connected to the
Metamorph Imaging System. Data were stored on a Panasonic optical disc recorder.
Kinetic Analysis of Pseudopod Dynamics
Cells were allowed to spread on BSA-coated cover
slips in Leiden chambers, with or without drugs, at 37°C.
To measure pseudopod extension and retraction, the areas of displacement during a 5 min interval were determined. Two phase-contrast video images of individual
cells were collected 5 min apart, 30 to 60 min after the
cells had begun spreading. Binary images of the cell
profile were generated from the video frames. Subtracting the early binary from the latter one produced a binary
image of the areas of extension, and vice versa.
Kinetic Analysis of Cell Spreading
Time lapse video images of spreading cells were
analyzed by measuring cell areas in each frame. Phase
contrast, background-subtractedimages of at least 8 cells
were collected at 15 sec intervals for 15 min. An edgedetection routine was applied to the stack using a Sobel
filter to outline the silhouette of the cell. This silhouette
was then thresholded, binarized, and eroded such that the
processed image could be satisfactorily superimposed on
the shape of the cell in the original phase contrast video
image. The area of the objects in this image-processed
stack were then measured.
RESULTS
Effects of Microtubules on Cell Shape
Macrophages treated with nocodazole, vinblastine,
or colchicine spread into highly irregular and asymmetric
forms, while control cells spread into circles (Fig. 1). By
immunofluorescence microscopy, microtubules in control cells were arranged as spokes radiating from the
centrosome to the cell margin. Longer microtubules bent
at the margin. Cells that were incubated with microtubule inhibitors and extracted before fixation were completely devoid of filamentous microtubule staining. In
233
unextracted cells, free tubulin labeled cytoplasm diffusely. Although nocodazole, colchicine, and vinblastine
are different chemically, their effects on cell shape were
similar. Thus, their effects on cell shape were most likely
a consequence of their common effect on microtubule
polymerization.
All depolymerizing drugs significantly decreased
cell area, length, radial ratio (P < .001, t-test) and shape
factor (P < .05, t-test) as determined by measuring at
least 50 cells in three separate experiments. To test
whether microtubules were affecting the establishment of
cell shape, the morphologies of cells were measured at
various times during spreading. Spreading was synchronized by allowing attachment to cover slips at 4"C, followed by warming. Cells fixed at various times after
warming were analyzed morphometrically (Fig. 2).
Within 5 minutes, 100% of control cells had microtubules while 100% of nocodazole-treated cells lacked microtubules, as determined by anti-tubulin immunofluorescence (n = 100, data not shown). Both control
and nocodazole-treated cells spread synchronously, symmetrically, and at similar rates during the first 5 min.
Both cell populations reached one-half of their maximum area within the first 5 min and about 90% of their
maximum area within the first 15 min. There were no
significant differences in the length, shape factor, and
radial ratio of cells spreading for the first 5 min. Therefore, symmetrical spreading was microtubule-independent.
Maintenance of circular profiles required microtubules. The loss of symmetry in nocodazole-treated cells
followed the initially symmetric spreading response. Although cell length remained relatively constant, the areas
of the nocodazole-treated cells decreased relative to control cells. The radial ratio and shape factors also dropped
significantly below control values after 15 min. Since
nocodazole-treated cells decreased in area but not length,
it indicated that the cells were retracting or extending
incompletely in the absence of microtubules, and that
these differences might be responsible for the changes in
morphology.
To examine the role of microtubule distribution in
the maintenance of cell shape, we analyzed spreading of
taxol-treated cells (Fig. 3). By immunofluorescence, microtubules in taxol-treated cells were shorter, detached
from the centrosome, and excluded from the cell margin.
Nonetheless, macrophages in taxol maintained their circular profiles. When cells spread in taxol plus nocodazole, microtubules were depolymerized and spreading
was irregular. This indicated that the effect of taxol on
cell symmetry was related to its effect on microtubule
stability rather than to some other biochemical effect of
taxol. Quantitative measurements of cell shape supported
these impressions (Fig. 4). The symmetry of cells in
234
Rosania and Swanson
TEXAS RED
UNEXTRACTED IgG
Fig. 1 . Microtubule depolymerization leads to changes in cell shape.
Cells were suspended in Ringer’s (A,B,C) or Ringer’s plus different
microtubule inhibitors: 20 pM colchicine (D,E,F); 20 pM nocodazole
(G,H,I); 200 nM vinblastine (J,K,L). They were then allowed to
spread for 60 min before fixing and staining with Texas Red to outline
EXTRACTED IgG
cell shape (A,D,G,J) and anti-tubulin immunofluorescence (fluorescein signal, B,C,E,F,H,I,K,L). Cells were either fixed without extraction, or were extracted before fixation, as indicated. Scale bar =
10 p m .
Microtubules Modulate Pseudopod Activity
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taxol indicated that microtubules could stabilize cell
morphology without extending to the cell margin.
The geometry of the microtubule system was also
manipulated with nocodazole, vinblastine, or colchicine
at concentrations that depolymerized microtubules incompletely. Under these conditions, microtubules remained attached to the centrosome but often failed to
extend all the way to the cell margin. As in taxol, the cell
extended beyond the region of cytoplasm containing microtubules (data not shown). Changes in cell shape
caused by partial microtubule depolymerization were
less pronounced than when microtubules were completely depolymerized from the cell. The effect of low
doses of nocodazole and other inhibitors on cell spreading and shape corroborates the idea that the extension of
microtubules to the edge of the cell was not necessary to
maintain a circular profile.
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Effect of Microtubules on Pseudopod Dynamics
We next measured the effects of microtubule inhibitors on pseudopod dynamics. Cells spreading in nocodazole or taxol were examined by time-lapse video microscopy. Within 5 min of attachment, control cells spread
symmetrically into discoidal shapes (Fig. 5). Once cells
spread to their maximum area, they generally held their
position and did not move much from their original landing point. Instead, cell margins began actively extending
and retracting, such that the overall dynamics were balanced and cells maintained a relatively constant area and
shape. Despite this difference, spreading was similar in
control and nocodazole-treated cells. The rate of spreading at half the maximal area, and the total extent of
spreading, calculated for five control cells and five nocodazole-treated cells, were not significantly different
(data not shown).
236
Rosania and Swanson
Fig. 3. Changes in cell shape in taxol are independent of microtubule
distribution. Cells were pre-treated with drugs, then were allowed to
spread for 60 min in buffer (A,B), 10 pM taxol (C,D), 20 pM no-
codazole (E,F), or 20 pM nocodazole plus 10 pM taxol (G,H). Cells
were stained with Texas Red (A,C,E,G) and with anti-tubulin immunofluorescence (B,D,F,H). Scale bar = 10 pm.
Microtubules Modulate Pseudopod Activity
237
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Fig. 4. In the presence of microtubules, the morphology of taxoltreated cells closely resembled that of control cells. Cells were pretreated with drugs, then were allowed to spread in 10 p M taxol (Tax),
20 pM nocodazole (Noc), or 20 pM nocodazole plus 10 pM taxol
(Noc
Tax). "Con" indicates cells in buffer only. Morphological
measurements indicate cell area (A), length (B),radial ratio (C), and
shape factor (D) after 60 min of spreading. Each data point is cumulative from two experiments, n > 50. Differences between control and
nocodazole
taxol are significant (P < .001, t-test).
Loss of radial symmetry in nocodazole began with
a retraction at one or more sites along the cell margin
(Fig. 6). As these retractions occurred, a new pseudopod
extended in a different direction. With this exaggerated
retraction and extension, cell shape became progressively asymmetric. Frequently, parts of the retracting
cell remained attached, with thin strips of cytoplasm connecting the attachment sites to the cell body. Sometimes
they broke off, leaving fragments of cytoplasm on the
cover slip.
Cell spreading in taxol was very similar to control
cells (Fig. 7). Once the cells spread to their maximum
size, pseudopod activity remained confined to the cell
margin. With time, taxol-treated cells drifted from their
original landing point, indicating that although extension
and retraction seemed to be quantitatively balanced, they
might be uncoordinated. Moreover, while control cells
ruffled mostly at their margins, taxol-treated cells ruffled
mostly in the center of the cell. These observations indicate that the spatial arrangement of the microtubules
was not critical for maintaining cell shape, although their
arrangement may have determined where ruffling occurred.
We next examined the dynamics at the cell margin
by measuring the areas covered by extension and
retraction. Two cell profiles, taken 5 min apart, were
compared morphometrically (Fig. 8). Relative to controls, the areas covered by extension and retraction
increased in cells treated with nocodazole. Taxol, on the
other hand, did not affect extension or retraction, or
the net balance of pseudopod dynamics. In low concentrations of nocodazole, extension and retraction were
intermediate between controls and 20 pM nocodazoletreated cells. However, the net change of cell area was
similar to controls, indicating that whenever cells contained microtubules, extension matched retraction. Thus,
microtubules suppressed and balanced extension and
retraction such that cell shape and area remained constant.
+
+
Role of Other Cytoskeletal Elements
To examine F-actin, spread cells were fixed and
stained with NBD-phallicidin, a fluorescent probe for
F-actin (Fig. 9). As previously reported [Amato et al.,
19831, F-actin in control cells accumulated in discrete,
238
Rosania and Swanson
Fig. 5 . Time-lapse sequence of spreading in control cells. The initial spreading was symmetrical. Thereafter, pseudopod extension and retraction were restricted to the cell margin, and were balanced such that
cell shape remained roughly circular. Labels indicate minutes after the beginning of spreading. Scale bar
= 10 pm.
Fig. 6. Time-lapse sequence of a nocodazole-treated cell. The initial
spreading was radially symmetric, but later the cell margin extended
and retracted at several points. During retraction, fragments of cyto-
plasm still attached to the cover slip were left behind. Active extension
eventually led to changes in cell shape. Labels indicate minutes after
the beginning of spreading. Scale bar = 10 pm.
punctate foci. F-actin in taxol-treated cells resembled
controls, with slightly more labeling in the center of the
cell. In nocodazole-treated cells, most of the F-actin was
concentrated in the cell body, and actin-rich foci were
absent from the cell margin.
We tested whether the effect of nocodazole on cell
shape could be blocked by cytochalasin D (Fig. 10).
Control and nocodazole-treated cells were allowed to attach to coverslips at 4°C and then warmed to 37°C for 5
min. During this interval, cells spread symmetrically to
about 50% of their maximum area. Cytochalasin D was
then added and cells were left for 60 min. Cytochalasin
Microtubules Modulate Pseudopod Activity
239
Fig. 7. Time-lapse sequence of taxol-treated cell. The initial spreading was symmetrical and protrusive
activity along the cell margin is visibly reduced. After some time, the cytoplasmic mass tended to drift.
Labels indicate minutes after the beginning of spreading. Scale bar = 10 pm.
I:
I
-400
Con
I
,
,
20 plvl Noc 20 plvl Tax 0.2 pM Noc
Fig. 8. The effect of microtubule-specific drugs on pseudopod dynamics. Bars represent the average area covered by extension (hatched
bars) or retraction (black bars) by individual cells in a 5-min time
interval. Untreated cells (Con) or cells pre-treated with different drugs
(20 pM nocodazole, 20 pM Noc; 20 p M taxol, 20 pM Tax; 0.2 pM
nocodazole, 0.2 p M Noc) were allowed to spread in the different
conditions for 30 min before the images were collected. Cumulative
data from two experiments, n = 8; error bars represent standard error
of the mean.
D inhibited further spreading by control cells, but did not
lead to changes in the shape factor or radial ratio. In
nocodazole-treated cells, cytochalasin D inhibited further cell spreading and the nocodazole-related shape
changes: cells looked radially symmetric, and their shape
factor and radial ratio reflected this (Fig. 10).
The organization of F-actin was also compared in
control or nocodazole-treated cells that were left to
spread for 5 min and then incubated with cytochalasin D
for 60 min. Under these conditions, the F-actin staining
of nocodazole-treated cells resembled that of control
cells (Fig. 9). Thus, changes in cell shape induced by
microtubule depolymerization correlated with changes in
the pattern of F-actin staining, consistent with the idea
that microtubules influenced cell shape by affecting the
dynamics or spatial organization of the actin cytoskeleton. The inhibition of shape changes by cytochalasin D
indicated that the nocodazole-induced cell shape changes
depended on the organization and dynamics of the actin
cytoskeleton.
We also examined the distribution of the intermediate filament protein vimentin. Cells were allowed to
land and spread on BSA-coated cover slips for 60 min,
then were fixed, permeabilized, and stained. In all cells,
vimentin filaments were restricted into a tightly tangled
perinuclear mass (Fig. 11). Although filaments in nocodazole were more tightly packed than in taxol or controls, the periphery of the cell was completely free of
filamentous vimentin in every condition examined. This
architecture of intermediate filaments seemed to be peculiar to cells under the conditions used in this experiment, since macrophages plated overnight on cover slips
display filamentous vimentin staining throughout the cytoplasm [Swanson et al., 19921. Because of their restricted distribution in the present circumstances, intermediate filaments did not appear to contribute directly to
the maintenance of cell shape.
240
Rosania and Swanson
Fig. 9. The effect of different drugs on F-actin organization in macrophages. Cells spread for 60 min in
Ringer’s (A) or Ringer’s plus 20 pM nocodazole (B), 20 p M taxol (C), or in Ringer’s for 5 min and then
in 10 pM cytochalasin D for 60 minutes (D).Cells were stained for F-actin with NBD-phallicidin. Scale
bar = 10 pm
DISCUSSION
Although microtubules are not necessary for macrophages to spread into radially symmetric circles, they
do contribute to the maintenance of that shape. Without
microtubules, cells spread into circles, then exaggerated
extension and retraction change their shape. Microtubule
depolymerization stimulates pseudopod activity at the
cell margin. Conversely, the presence of intact microtubules, anywhere in the cell, inhibits that activity. Here,
microtubules do not serve as part of a structural matrix
that maintains cell shape. Instead, these observations are
consistent with a model in which microtubules modulate
the level of a chemical intermediate that regulates the
actin cytoskeleton.
Spatial Arrangement of Microtubules in Relation
to Pseudopod Dynamics and Cell Shape
in Macrophages
The stabilizing effect of microtubules on cell shape
and pseudopod dynamics is independent of microtubule
distribution. As in fibroblasts [de Brabander et al., 1981;
Schiff and Honvitz, 19801, microtubules of taxol-treated
macrophages polymerized independently of the centrosome, such that the radial geometry of the microtubule
array was disrupted. Detached microtubules formed bundles around the nucleus and were generally removed
from the periphery of the cell. In other respects, however, taxol-treated macrophages resembled control cells.
Macrophages allowed to spread in nocodazole plus taxol
acquired irregular shapes, indicating that taxol did not
stabilize cell shape by a microtubule-independent mechanism. Accordingly, the inhibitory effect of microtubules on cell shape and pseudopod dynamics in macrophages depended on the polymerization state of
microtubules, but not on the exact position of microtubules in the cell. Thus, microtubules appear to act at a
distance from the cell edge, inhibiting movements without necessarily extending out to the edge.
Interaction of Microtubules With Other
Cytoskeletal Systems
The effects of microtubules on cell shape were mediated by the actin cytoskeleton. In untreated and taxoltreated macrophages , F-actin accumulated in discrete,
Microtubules Modulate Pseudopod Activity
A
B
14007
1300
N
1200
1100
;
lo00
45 1
h
E
5
5.
6
6
900
800
700
J
D
C
‘-
:-.3
2
35
30
600
.0
40
v
9,
2
241
0.8
0.75
0.7
0.65
F.
5
2
%
m
0.6
0.55
0.5
0.45
5
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
Con
Cyt
Noc
Cyt+Noc
Fig. 10. Cell spreading and nocodazole-induced changes in cell shape
are dependent on the actin cytoskeleton. Cells were suspended in
Ringer’s (Con, Cyt) or Ringer’s plus 20 JLMnocodazole (Noc, Cyt +
Noc) and allowed to spread for 5 min. Cytochalasin D was then added
to cells in Ringer’s (Cyt) or nocodazole (Noc) and cells were allowed
to spread for 60 min. Morphological measurements were made to
assay changes in cell area (A), length (B), radial ratio (C), and shape
factor (D). Each data point is cumulative from two experiments, n >
50. Differences in radial ratio and shape factor between Noc and Cyt
Noc are significant ( P < ,001; t-test).
symmetrically distributed foci adjacent to the plasma
membrane, consistent with previous reports of microfilament structure in these cells [Amato et al., 1983; Yin
and Hartwig, 19881. In nocodazole-treated cells, F-actin
staining in the pseudopod was generally diffuse and
asymmetric, with most of the F-actin accumulating in the
cell body. In addition, cytochalasin D, which blocks the
polymerization of actin by capping actin filaments, inhibited cell spreading, as expected, but did not cause
cells to lose their circular profiles. Nevertheless, cytochalasin D inhibited the changes in cell shape induced by
nocodazole, indicating that the effect of microtubule depolymerization on cell shape was mediated by the actin
cytoskeleton.
Actin-rich foci associated with the macrophage
plasma membrane are likely to be involved in pseudopod
extension, attachment, and the establishment of cell
shape, by analogy with other systems [Burridge et al.,
1988; Clark and Brugge, 1995; Okabe and Hirokawa,
1989; Symons and Mitchison, 19911. Under the conditions used in this study, vimentin filaments were absent
from the cell margin, making it unlikely that they are
directly involved in the changes in protrusive activity,
cell area, or shape. Thus, microfilaments are responsible
for the changes in pseudopod dynamics and cell morphology observed when microtubules are depolymerized .
+
Microtubules as Regulators of
lntracellular Signaling
Based on the inhibitory effects of microtubules on
pseudopod dynamics, it is possible that microtubules
suppress some intracellular signal which would otherwise lead to exaggerated protrusive activity. In macrophages, as well as in other cell types, pseudopod dynamics are under the control of signal transduction pathways
[Hall, 19941. Ligation of the M-CSF receptor triggers a
signaling pathway leading to pronounced ruffling at the
cell margins [Racoosin and Swanson, 19921. Microinjection of ras protein into fibroblasts [Bar Sagi and Feramisco, 19861 or activation of protein kinase C in macrophages [Swanson, 19891 also increases pseudopod
dynamics. Microtubule depolymerization can activate
protein kinases [Manie et al., 1993; Shinohara-Gotoh et
a]., 19911 and transcription factors [Rosette and Karin,
19951; it can affect mRNA levels in the cell and stimulate
DNA synthesis [Pavlath et al., 19931.
How could microtubules affect signal transduction
242
Rosania and Swanson
Fig. 11. Intermediate filaments are not present in the pseudopod and are restricted to the cell body. Cells
spread for 60 min in Ringer’s (A,B) or Ringer’s plus 20 p M nocodazole (C,D) or 20 p M taxol (E,F) were
stained with Texas Red (A,C,E) and with anti-vimentin immunofluorescence (B,D,F). Scale bar = 10
Ilm.
pathways? One possibility is that intact microtubules, but
not soluble tubulin, bind signaling proteins like protein
kinase A [Glantz et al., 19931 or protein kinase C [Kiley
and Parker, 19951, and allosterically affect their enzymatic activity. One candidate second messenger, known
to affect both actin dynamics and pseudopod extension,
is calcium. Calcium stimulates myosin-dependent contractile activity and affects a variety of actin-associated
proteins including gelsolin, cap Z, L-plastin, and fodrin
[Maxfield, 19933. Treatments that raise intracellular calcium interfere with cell shape, cell migration, and
chemotaxis [Gilbert et al., 19941. Interestingly, mitochondria and the endoplasmic reticulum both serve as
intracellular calcium stores and are also typically associated with microtubules [Vale, 19871. Microtubule depolymerization could affect the signal transduction
mechanisms regulating intracellular calcium levels or the
ability of cellular organelles like mitochondria to sequester intracellular calcium.
Another way microtubules might influence signal
transduction pathways controlling the actin cytoskeleton
is by affecting focal contacts [Rinnerthaler et al., 19881.
Microtubules Modulate Pseudopod Activity
243
Focal contacts are complexes of proteins at sites of cell normal segregation of the cell margin into active and
attachment [Bumdge et al., 19881, and are thought to inactive zones [Gail and Boone, 1971; Vasiliev et al.,
transduce signals that organize actin [Clark and Brugge, 19701. In fibroblasts, microtubules are necessary to sus19951. Focal contact proteins include a variety of phos- tain high levels of protrusive activity seen at the leading
phoproteins that interact with the actin directly, such as edge of polarized cells [Bershadsky et al., 19911, alfimbrin, protein kinases, and other proteins [Clark and though as in macrophages, microtubule depolymerizaBrugge, 1995; Messier et al., 1993; Miyamoto et al., tion in fibroblasts induces all margins to become active
19951. The association of actin filaments with focal con- [Gail and Boone, 1971; Vasiliev et al., 19701. Taxoltacts stabilizes cell adhesions and permits continued treatment affects cell shape in fibroblasts, causing cells
pseudopod extension [Miyamoto et al., 19951. Changes to lose protrusive activity at the leading edge and to
in cytoskeletal tension induced by microtubule depoly- increase protrusive activity at the cell margin [Pletjushmerization might destabilize cell attachments and focal kina et al., 19941. The presence of actin stress fibers in
contacts, and influence the associated signaling events fibroblasts and the absence of intermediate filaments in
affecting the recruitment of actin filaments to the sites of the macrophage pseudopod may be responsible for difadhesion.
ferences between these two cell types.
Microtubules as Orchestrators of
Localized Functions
In addition to the global inhibitory effect on protrusive activity described in this study, microtubules
have been suggested to stimulate pseudopod activity locally by transiently interacting and stabilizing structures
whose assembly is rate-limiting for translocation, by polarizing the Golgi near sites of protrusive activity, and by
serving as tracks mediating the net transport of vesicles
towards the leading edge of the cell [Bershadsky and
Vasiliev, 19931. Recent evidence indicates that localized
stimulatory activity is dependent on kinesin-based transport [Rodionov et al., 19931 and microtubule dynamics
[Tanaka et al., 1995; Liao et al., 19951. Nevertheless,
microtubules could also send diffusible signals that localize cellular morphogenetic processes to specific sites
in the cell. For example, to position the cleavage furrow
[Fishkind and Wang, 19951, microtubules may provide a
localized positive signal that stimulates the contractile
activity along the cleavage plane of the cell, and a diffuse
negative signal that prevents cortical contraction from
occumng elsewhere in the cell. Microtubules might also
influence localized cellular functions by mechanically
exerting anisotropic effects on cytoskeletal tension. For
example, to promote axonal growth in a particular direction [Tanaka et al., 19951, microtubules could bundle in
parallel, such that they are better able to resist compressive forces in one direction. Mechanically, microtubules
could guide the recruitment and polymerization of actin
filaments in the growth cone by preferentially stabilizing
focal contacts in the direction of alignment.
Cell Type-Specific Differences of
Microtubule Function
In contrast to their role in macrophages, microtubules in fibroblasts help establish and maintain cell
asymmetry. Microtubule depolymerization induces elongated cells to become more circular, and inhibits the
ACKNOWLEDGMENTS
The authors thank Paul Janmey, Claude Lechene,
Rong Li, and Yu-Li Wang for insightful discussions, and
George McNamara and Melissa Johnson for help with
the image processing system. This study was supported
by NIH grant A135950 to J.S.
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