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Research Article
PTP1B promotes focal complex maturation, lamellar
persistence and directional migration
Juan E. Burdisso, Ángela González and Carlos O. Arregui*
Instituto de Investigaciones Biotecnológicas, Universidad de San Martı́n/CONICET, 1650 San Martı́n, Buenos Aires, Argentina
*Author for correspondence (carregui@iib.unsam.edu.ar)
Journal of Cell Science
Accepted 21 January 2013
Journal of Cell Science 126, 1820–1831
ß 2013. Published by The Company of Biologists Ltd
doi: 10.1242/jcs.118828
Summary
Previous findings established that ER-bound PTP1B targets peripheral cell–matrix adhesions and positively regulates cell adhesion to
fibronectin. Here we show that PTP1B enhances focal complex lifetime at the lamellipodium base, delaying their turnover and facilitating
a-actinin incorporation. We demonstrate the presence of catalytic PTP1BD181A–a-actinin complexes at focal complexes. Kymograph
analysis revealed that PTP1B contributes to lamellar protrusion persistence and directional cell migration. Pull-down and FRET analysis
also showed that PTP1B is required for efficient integrin-dependent downregulation of RhoA and upregulation of Rac1 during spreading. A
substrate trap strategy revealed that FAK/Src recruitment and Src activity are essential for the generation of PTP1B substrates in adhesions.
PTP1B targets the negative regulatory site of Src (phosphotyrosine 529), paxillin and p130Cas at peripheral cell–matrix adhesions. We
postulate that PTP1B modulates more than one pathway required for focal complex maturation and membrane protrusion, including aactinin-mediated cytoskeletal anchorage, integrin-dependent activation of the FAK/Src signaling pathway, and RhoA and Rac1 GTPase
activity. By doing so, PTP1B contributes to coordinated adhesion turnover, lamellar stability and directional cell migration.
Key words: PTP1B, Adhesion, Migration, Src, FAK
Introduction
Cell migration requires a regulated adhesion assembly–
disassembly cycle. Advance of the protruding cell edge occurs
concomitantly with the appearance of nascent adhesions within
the lamellipodium (Choi et al., 2008). As the leading edge moves
forward, nascent adhesions grow and turn into focal complexes,
which at the lamellipodium base may turnover or grow further
and mature into elongated focal, and fibrillar adhesions (VicenteManzanares et al., 2009; Scales and Parsons, 2011).
PTP1B is a non-receptor protein tyrosine phosphatase bound to
the cytosolic face of the endoplasmic reticulum (ER) through a
hydrophobic C-terminal tail (Frangioni et al., 1992). PTP1B is
present in complexes of b1- and b3-integrin (Arregui et al., 1998;
Arias-Salgado et al., 2005), and interacts with the adaptor protein
p130Cas, which in part localizes at focal adhesions (Harte et al.,
1996; Liu et al., 1996). As the catalytic domain of PTP1B faces
the cytosol, it has the potential for substrate dephosphorylation
throughout the extensive branching network occupied by the ER.
Indeed, PTP1B has been shown to dephosphorylate plasma
membrane receptors (Ahmad et al., 1995; Elchebly et al., 1999;
Buckley et al., 2002; Haj et al., 2002), protein adaptors (Garton
et al., 1996), and cytosolic tyrosine kinases such as Src (Arregui
et al., 1998; Bjorge et al., 2000). Identification of most PTP1B
substrates was made possible by the generation of effective
substrate trapping mutants, such as PTP1BDA, in which the
invariant catalytic aspartic acid 181 is replaced by alanine (Flint
et al., 1997). This mutation increases substantially the steadystate population of PTP1BDA–substrate complexes, allowing
their direct visualization by optical techniques. Interactions
between ER-bound PTP1BDA and endocytosed EGFR and
PDGFR have been detected as puncta by Förster resonance
energy transfer (FRET) (Haj et al., 2002), and by cryoimmunoelectron microscopy (Eden et al., 2010). PTP1BDA
interactions with targets localized at integrin and cadherin
adhesion complexes, as well as with EphA3/ephrin-mediated
cell–cell contacts, seem to occur at the cell surface (Arregui et al.,
1998; Balsamo et al., 1998; Hernández et al., 2006; Hernández
et al., 2010; Nievergall et al., 2010; Haj et al., 2012).
We previously reported that ER-bound GFP–PTP1BDA
accumulates in puncta over peripheral cell–matrix adhesions
(Hernández et al., 2006). However, the identity of substrates to
which this mutant trap binds in adhesions, and the functional
consequences of PTP1B activity, remained elusive. By time-lapse
analysis we have now directly demonstrated that focal complexes
in cell protrusions extend their lifetimes when contacted by active
ER-bound PTP1B. We demonstrate the presence of catalytic
PTP1BDA/a-actinin in focal complexes and show that a-actinin
is essentially absent from focal complexes of PTP1B null cells,
suggesting an inefficient coupling to actin cytoskeleton.
Kymograph analysis revealed that PTP1B promotes the
persistence of leading edge protrusions and the directionality of
cell migration. We also show that PTP1B modulate integrindependent regulation of RhoA and Rac1 GTPases, and present
compelling evidence suggesting that in addition to a-actinin, Src,
paxillin and p130Cas are PTP1B substrates targeted at adhesions.
Results
ER-bound PTP1B regulates adhesion lifetimes during
lamellar protrusion
Mechanisms underlying turnover or maturation of focal
complexes at the lamellipodium base are poorly understood
(Geiger et al., 2009; Vicente-Manzanares et al., 2009; Scales and
Journal of Cell Science
Role of PTP1B in adhesion dynamics and migration
Parsons, 2011). Based on previous antecedents showing that
PTP1B is positioned over peripheral paxillin adhesions by tubular
extensions of the ER (Hernández et al., 2006), and that peripheral
adhesions targeted by ER tubules subsequently grew in size
(Zhang et al., 2010), we hypothesized that PTP1B on the surface
of the ER could regulate adhesion lifetime during cell protrusion.
We tested this hypothesis in immortalized fibroblasts derived
from PTP1B-deficient mouse (KO cells) (Haj et al., 2002)
transfected with mRFP-paxillin to label adhesions, and either
GFP-PTP1B wild type (WT) or GFP-PTP1B (CS), a catalytically
inactive mutant with the essential cysteine 215 substituted by
serine (Guan and Dixon, 1991; Flint et al., 1997). WT and CS
expression did not significantly affect the spatial organization
and dynamics of the ER [(Arregui et al., 1998) and results not
shown]. Time-lapse analysis revealed that ER tubules containing
WT and CS targeted paxillin adhesions assembled at or near the
leading edge (Fig. 1A; supplementary material Fig. S1; Movies
1, 2). Target events occurred mainly during the growing phase of
adhesions (97%, n534 adhesions, 8 cells), as judged by the
increase of mRFP–paxillin fluorescence intensity over time. In
addition, adhesions targeted by WT lasted longer than those
targeted by CS (compare Fig. 1A and Fig. 1B). Quantification of
adhesion lifetime, measured as the time span from the first
appearance of a resolvable mRFP–paxillin cluster until complete
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disassembly, revealed that in KO cells expressing CS, targeted
and non targeted adhesions have similar lifetimes, ranging from
1–12 minutes, with a median of 4 minutes (Fig. 1C, CST and
CSNT). These results were similar to those of non targeted
paxillin clusters in KO cells expressing WT (Fig. 1C, WTNT).
However, adhesions targeted with WT increased dramatically
their lifetimes, ranging from 11 to 36 minutes, with a median of
20 minutes (Fig. 1C, WTT). We also analyzed the lifetime of
peripheral mRFP–paxillin adhesions in KO cells and KO cells
stably reconstituted with PTP1BWT (WT cells) (Haj et al., 2002).
We found a wide range of adhesion lifetimes, with a median of
24 minutes for WT cells and 18 minutes for KO cells.
Interestingly, the number of adhesions with lifetime
#7 minutes was 2.5-fold higher in KO cells compared to WT
cells (Fig. 1D). However, the number of adhesions with longer
lifetimes was similar or higher in WT cells implicating that
PTP1B contributes to stabilize focal complexes assembled during
lamellar protrusion.
PTP1B regulates paxillin turnover and promotes
incorporation of a-actinin in focal complexes
To further investigate the function of PTP1B in adhesions we
determined the paxillin assembly/disassembly rate constants in
WT and KO cells transfected with mRFP-paxillin, using methods
Fig. 1. ER-bound PTP1B regulates adhesion lifetimes during lamellar
protrusions. KO cells expressing mRFP–paxillin and either (A) GFP–
PTP1BWT (WT) or (B) GFP–PTP1BCS (CS) were analyzed by time-lapse
microscopy. A protruding lamella, marked by a dashed yellow box in the
first frame, was magnified (46) to illustrate targeting events. A targeting
event is defined as an event in which the tip of a single ER tubule is found
juxtaposed to one mRFP–paxillin cluster. Numbers in each frame indicate
minutes elapsed since the beginning of the experiment. Arrows point to a
single targeting event in the first and subsequent frames in which they were
observed. (A) Adhesions targeted with WT (color arrows) persisted and
grew in size. (B) Four different targeting events of CS occurred at 10, 12,
14 and 17 minutes (white arrows). The first three targeted foci disappeared
at 11, 13, and 17 minutes, respectively (yellow arrows). (C) Quantification
of adhesion lifetimes targeted (WTT, CST) and non-targeted (WTNT,
CSNT). Each box in the plot encloses 50% of the data and the line marks the
median value. Lines extending from the top and bottom of each box mark
the minimum and maximum values within the data set. *Significant
difference when compared to the other conditions (P,0.0001, one-way
ANOVA followed by a Tukey’s HSD post-hoc test). (D) Frequency
distribution plot (numbers on the abscissa mark the center of the bin size;
5 minutes) of lifetime adhesions in WT and KO cells. Data were analyzed
by a Wilcoxon–Mann–Whitney non-parametric test. Note that short-lived
adhesions (#7 minutes) in KO cells duplicated those in WT cells. Scale
bar: 25 mm.
Journal of Cell Science
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Journal of Cell Science 126 (8)
previously described (Franco et al., 2004; Webb et al., 2004). In
both cell lines paxillin assembly and disassembly rate constants
correlated inversely with adhesion lifetimes (Fig. 2A). Assembly
rate constants did not differ significantly between WT and KO
cells (WT, 0.5060.04 versus KO, 0.6660.05). However,
disassembly rates were significantly higher in KO cells
compared to WT cells (WT, 0.4460.03 versus KO, 0.7060.06;
Fig. 2A). To test if this result could be due to more abundant
short-lived adhesions in KO cells, we sorted a similar number of
short-lived adhesions (#10 minutes, n528 adhesions/5 cells) for
each cell line. Although these pools displayed similar lifetimes
(WT mean: 7.1 minutes versus KO mean: 6.5 minutes), paxillin
disassembly rates were significantly higher in KO cells compared
to WT cells (WT, 0.7760.07 versus KO, 1.1760.10), while
assembly rate constants did not differ (WT, 0.8560.08 versus
KO, 1.0460.10). These results suggest that the prevalence of
short-lived paxillin adhesions in KO cells is most likely a
consequence of their higher disassembly rates. Adhesions with
longer lifetimes (.15 minutes), did not show significant
differences in paxillin disassembly rates between WT and KO
cells (WT, 0.27 versus KO, 0.24, P50.4), suggesting that a major
impact of PTP1B activity is on the newly born population.
As the leading edge advances, some adhesions grow and
elongate centripetally, often accompanied by the incorporation of
a-actinin (Laukaitis et al., 2001). Alpha actinin mediates integrinactin cytoskeleton linkages in a process that is negatively
regulated by tyrosine phosphorylation of a-actinin (Izaguirre
et al., 2001; von Wichert et al., 2003). Since PTP1B
dephosphorylates a-actinin (Zhang et al., 2006), we predicted a
failure in a-actinin incorporation to focal complexes in KO cells.
WT- and KO cells were co-transfected with a-actinin-GFP and
mRFP-paxillin and analyzed by time-lapse microscopy. During
protrusion, a-actinin–GFP strongly labeled the lamellipodium,
which contained small clusters of mRFP–paxillin (Fig. 2B,C). As
the lamellipodium moved forward, paxillin clusters remained
stationary and grew in size. About 98.763.7% of paxillin clusters
in WT cells incorporated a-actinin in a polar fashion and grew
centripetally compared to only 22.267% in KO cells
(Fig. 2B,D,F; supplementary material Movies 3, 4). When
lamellae retracted, paxillin foci in WT and KO cells
incorporated a-actinin in similar manner, suggesting that
PTP1B contributes to anchor new adhesions to the actin
cytoskeleton during protrusions. To directly visualize catalytic
PTP1B–a-actinin complexes in adhesions of intact WT cells we
performed bimolecular fluorescence complementation (BiFC)
analysis (Hu et al., 2002). Catalytic PTP1B–substrate complexes
can be visualized only when their steady state concentration is
significantly increased by using substrate trapping mutants of
PTP1B, such as the PTP1B D181A (DA) (Haj et al., 2002; Boute
et al., 2003; Monteleone et al., 2012). Indeed, co-expression of
YN-PTP1BWT/a-actinin-YC (YN, amino acids 1–154; YC,
amino acids 155–238 of EYFP) did not reveal a detectable
BiFC signal (supplementary material Fig. S2). In contrast, coexpression of YN-PTP1BDA/a-actinin-YC exhibited bright BiFC
Fig. 2. PTP1B regulates paxillin turnover and incorporation of
a-actinin in cell protrusions. (A) mRFP–paxillin assembly and
disassembly rate constants are inversely correlated with adhesion lifetimes
in both WT and KO cells. Global disassembly rates in KO cells were
higher than in WT cells (P50.01), a difference that is magnified in a subset
of short-lived adhesions (#10 minutes, marked by dashed lines; WT, 28
adhesions; KO, 43 adhesions; P50.006). Data were analyzed by the
Wilcoxon–Mann–Whitney non-parametric test. N.S.: P.0.05. (B) In WT
cells, most mRFP–paxillin adhesions (in green) at the lamellipodium base
(arrowheads) incorporated a-actinin–GFP (in red). (C) In KO cells,
mRFP–paxillin quickly turned over without incorporation of a-actinin–
GFP. (D,E) Plot profiles of mRFP–paxillin- and a-actinin–GFP-integrated
fluorescence intensities over time, in representative WT and KO cells. The
first fluorescence peak is within the lamellipodium (La). Note that in the
WT cell both plot profiles overlap spatially and temporally (D), but those
in the KO cell only overlap within the lamellipodium (E).
(F) Percentages of mRFP–paxillin foci that incorporated a-actinin during
the time-lapse assay. Bars represent means 6 s.e.m. of 8–9 cells and 36–47
adhesions per condition (Student’s t-test, P,0.0001). Scale bar: 12 mm.
(G,H) WT cell co-transfected with YN-PTP1BDA/a-actinin-YC BiFC
pairs and immunolabeled for vinculin (in red). H is an enlarged view of the
boxed region in G. (I) A surface reflection interference contrast image. A
representative polarized cell depicting a protruding lamella (right lower
corner) is shown. Note that BiFC is strong and punctate in peripheral
adhesions, although the overlapping area varies (white open arrowheads).
Scale bars: 25 mm (G), magnifications in H and I are at 200% of the
original image.
Role of PTP1B in adhesion dynamics and migration
signal in peripheral clusters containing the YN-PTP1BDA
construct (supplementary material Fig. S2). This distribution is
prevented by pre-incubation with pervanadate, which inactivates
the active site of the enzyme. The estimated expression levels of
YN-PTP1BDA (and a-actinin-YC, not shown) associated to this
BiFC signal was ,1.5–2-fold of the endogenous proteins. Each
BiFC pair transfected individually showed the expected
subcellular distribution and did not display detectable
fluorescence in the BiFC channel (supplementary material Fig.
S2) (Monteleone et al., 2012). We analyzed the presence of the
BiFC signal in adhesions by immunofluorescence detection of
vinculin and by surface reflection interference contrast (SRIC)
imaging. The BiFC signal significantly overlapped with vinculin
and (low reflection) dark peripheral adhesions in protruding
regions of the cell (Fig. 2G–I).
Journal of Cell Science
PTP1B promotes lamellar protrusion persistence and
directional cell migration
To determine if unstable focal complexes in KO-cell protrusions
affected lamellar dynamics, we performed kymograph analysis of
the leading edge in WT and KO cells transfected with a-actinin–
GFP. Visual inspection of kymographs showed persistent
protrusions in WT cells and fast protrusion/retraction cycles in
KO cells (Fig. 3A,B; supplementary material Movies 5, 6).
Quantifications revealed an approximately fivefold higher
frequency of protrusion/retraction switching in KO cells
compared with WT cells (WT cells, 1.3660.24/hour versus KO
Fig. 3. PTP1B regulates lamellar dynamics. WT and KO cells expressing
a-actinin–GFP were analyzed by kymography. (A,B) Individual frames at t0
and kymographs (insets) of representative cells. The fluorescence intensity
along line scans (1-pixel wide) drawn normal to the border of the protruding
lamella (yellow lines) was recorded every 30 seconds for 50 minutes. Note
the smooth advance of the leading edge in the WT cell compared to the
serrated, discontinuous advance of the leading edge in the KO cell. ‘t’ and ‘d’
indicate time and space dimensions, respectively. (C) Plot showing higher
frequencies of protrusion/retraction cycles in KO cells compared to WT cells,
n510 cells, P,0.0001. (D) Persistence time of protrusion phases was
markedly reduced (approximately threefold) in KO cells compared to WT
cells (P,0.0001) whereas persistence time did not differ. WT510 cells, 53
kymographs; KO511 cells, 106 kymographs. (E) Velocities of protrusion and
retraction phases were calculated from the slopes. Cell protrusions: n568
WT, n569 KO; retractions: n522 WT, n550 KO. P,0.0001. Data were
analyzed by the Wilcoxon–Mann–Whitney non-parametric test. N.S. indicates
a P.0.05. Scale bar: 35 mm.
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cells, 7.2060.70/hour; Fig. 3C). In addition, KO cells exhibited
an approximately threefold reduction in protrusion
persistence times compared to WT cells (WT cells,
mean 16.8261.46 minutes versus KO cells, mean 5.226
0.49 minutes), while retraction persistence times was
marginally affected (WT cells, 4.6360.44 minutes versus KO
cells, 3.8660.20 minutes; Fig. 3D). Protrusion speed increased
approximately threefold in KO cells (WT cells, 40.862.60 mm/
hour versus KO cells, 12568.00 mm/hour), and retraction speed
approximately twofold (WT cells, 48.7064.10 mm/hour, KO
cells, 115.2066.90 mm/hour), compared to WT cells (Fig. 3E).
These results suggest that PTP1B contributes to the steady
protrusion of the leading edge.
We also analyzed the directionality, speed and migratory
patterns of WT and KO cells under two-dimensional isotropic
conditions. Under these conditions, single cells follow an almost
straight path over short time intervals, yet exhibiting Brownianlike motion over long time intervals, which can be
mathematically characterized as a persistent random walk (Gail
and Boone, 1970; Dunn, 1983; Othmer et al., 1988). We collected
time-lapse phase contrast videos of cells moving on fibronectin
during a 10 hour period and reconstructed their trajectories. A
representative set of cell trajectories revealed that WT cells
exhibit longer and more directional paths than KO cells
(Fig. 4A). WT cells developed a large lamellar extension at the
front edge while KO cells produced transient lamellar extensions
in multiple directions (Fig. 4B; supplementary material Movies
7, 8). Long trailing tails were frequent in KO cells and rare in WT
cells. We quantified migration directionality as the ratio between
the shortest linear distance from the starting point of a time-lapse
recording to the end point (D), and the total distance (T) traversed
by the cell (Gu et al., 1999). This D/T ratio equals to one in the
case of ballistic motion. D/T ratios were significantly lower in
KO cells compared to WT cells (WT cells, 0.6460.02 versus KO
cells, 0.4260.04), suggesting a decrease of directionality
(Fig. 4C). KO cells also had slower migration speed (WT cells,
12.860.18 mm/hour versus KO cells, 9.660.13 mm/hour;
Fig. 4C). These results were confirmed by fitting the mean
square displacements (MSD) of cell paths over time to the
random walk equation. The MSD was calculated and plotted
against time using the Cell Motility software (Martens et al.,
2006). In pure random movement MSD variations would appear
as a straight line passing through the origin while in a ballistic
motion it would fit to an exponential curve. The averaged MSD
of .29 cells plotted against time showed higher displacements
and a more curved line in WT cells compared to KO cells
(Fig. 4C). To extract speed (S) and persistence (P) parameters,
MSD data were fitted to the random walk model using a Nelder–
Mead simplex non-linear regression algorithm (Martens et al.,
2006). S and P were significantly reduced in KO cells compared
to WT cells (S, WT cells, 15.161.9 mm/hour versus KO cells,
10.460.30 mm/hour; P, WT cells, 6.060.91 hours versus KO
cells, 3.360.66 hours). Since KO cells displayed persistent
trailing tails and unstable leading edges, we predicted an
alteration of the migration pattern. Thus, we examined arrays
of color-coded advance and pause phases, arranged as they occur
within the time-lapse series. Arrays revealed a seemingly higher
prevalence of pauses in KO cells compared to WT cells (Fig. 4D;
WT cells, 11.3561.40%, n528 versus KO cells, 21.1661.73%,
n531, P,0.0001).
Journal of Cell Science
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Journal of Cell Science 126 (8)
Fig. 4. PTP1B promotes persistent migration. WT and KO cells were
analyzed by time-lapse phase-contrast microscopy. (A) Randomly selected
individual tracks of WT and KO cells were copied and combined into single
panels to avoid empty spaces. Scale bar: 100 mm. (B) Phase-contrast image
sequences of representative WT and KO cells. WT cells form broad and
persistent lamellar extensions in the direction of migration (red arrowheads)
whereas KO cells form narrow and low persistence lamellar extensions in
several directions (yellow arrowheads), and frequently have long trailing tails
(yellow arrows). Numbers indicate time in minutes. Scale bar: 50 mm.
(C) Quantification of migration speed and directionality. Averaged speed,
calculated with Metamorph software, was significantly reduced in KO cells
(P,0.0001). Migration directionality was determined by calculating the D/T
ratio, and by fitting the MSD to the persistent random walk model (RWM)
equation. Note that D/T in WT cells is significantly higher than that in KO
cells (P50.001). The MSD over time was used to extract values of speed
(S) and persistence time (P). Data were analyzed by the Wilcoxon–Mann–
Whitney non-parametric test. Bars represent the means 6 s.e.m. of 29–32
cells. (D) Time plots of the migratory pattern of WT and KO cells. Advance
and pause phases of each time-lapse series were color-coded (black
square5advance, white square5pause; each square5time interval of
8 minutes) and arranged in a horizontal row as they occurred. Intervals with a
net advance lower to 1.3 mm were considered as pauses.
Integrin-dependent regulation of RhoA and Rac1 is impaired
in PTP1B null cells
Stimulation of integrins and growth factor receptors regulate
RhoA and Rac1 GTPases (Bar-Sagi and Hall, 2000; Schwartz
and Shattil, 2000; Burridge and Wennerberg, 2004). In turn,
GTPase activity coordinates the dynamics of the actin
cytoskeleton and cell–matrix adhesions during cell migration
(Vicente-Manzanares et al., 2009; Scales and Parsons, 2011).
Serum starved WT cells plated on fibronectin for 15 minutes
showed a well-defined F-actin-rich lamellipodium and few actin
stress fibers. In contrast, KO cells did not produce a
lamellipodium; instead they exhibited spiky margins and
prominent actin stress fibers (Fig. 5A). These observations
suggest impaired Rac1 and RhoA activities in KO cells. We
quantified the active GTP-bound RhoA and Rac1 by pull-down
assays. RhoA–GTP levels in WT cells were downregulated after
plating on fibronectin, reaching ,20% of the value of
cells in suspension by 30 minutes post-plating (Fig. 5B).
Downregulation of RhoA–GTP levels did not occur in KO
cells. We also quantified the Rac1–GTP levels. Soon after
seeding on fibronectin (10 minutes), WT cells increased Rac1–
GTP levels which almost doubled those of cells in suspension
(Fig. 5B). In contrast, KO cells showed no response. We
confirmed these results in individual cells by FRET analysis.
WT and KO cells were transfected with single-chain FRET
pRaichu biosensors, which monitor RhoA and Rac1 activities at
the cell membrane (Itoh et al., 2002; Yoshizaki et al., 2003).
GTPase activity, expressed as a FRET/CFP ratio for each pixel,
was visualized using the intensity modulation display mode,
which associates color hue with ratio values and the intensity of
each hue with the source image brightness (Tsien and
Harootunian, 1990). Red and blue colors in images represent
the spatial distribution of high and low GTPase activity,
respectively. In WT cells plated on fibronectin RhoA activity
was downregulated while Rac1 activity was induced. The
opposite was observed in KO cells (Fig. 5C). RhoA activity in
KO cells and Rac1 activity in WT cells were maximal at the cell
margin and decrease gradually towards the cell center. To
quantify these observations we calculated the FRET/CFP ratios
along line scans traced normal to the cell border. Adhesion to
polylysine, as a non specific substrate, showed a similar increase
of RhoA and Rac1 activity in WT cells and KO cells. However,
adhesion to fibronectin showed higher RhoA in KO cells, and
Rac1 in WT cells (Fig. 5C). These results suggest that PTP1B
promotes integrin-dependent Rac1 activation and RhoA
repression.
PTP1B targets substrata of the Src/FAK signaling pathway
in adhesions
Our previous time-lapse studies demonstrated that GFP–
PTP1BDA form fluorescent puncta over the distal pole of
peripheral mRFP–paxillin adhesions in a time-dependent manner
(Hernández et al., 2006). BiFC and FRET studies also have
observed PTP1BDA/substrate complexes as fluorescent clusters
(Haj et al., 2002; Anderie et al., 2007; Nievergall et al., 2010;
Monteleone et al., 2012). In the present study the BiFC signal for
YN-PTP1BDA/a-actinin-YC were also observed as puncta in
more peripheral adhesions. These puncta were absent in cells preincubated with pervanadate (a general inhibitor of PTPs) or
transfected with the wild-type enzyme. Thus, we assumed that
GFP–PTP1BDA fluorescent puncta associated with peripheral
adhesions reflect the accumulation of enzyme/substrate
complexes, and predicted that removal of those substrates from
adhesion sites should prevent the formation of puncta. We
screened cell lines knockout for proteins that fulfilled three
conditions: (1) that were previously identified as PTP1B
substrates; (2) that were detected in cell-matrix adhesion sites;
and (3) that were involved in integrin-dependent signaling
pathways regulating RhoA and Rac1 GTPases. These included
Journal of Cell Science
Role of PTP1B in adhesion dynamics and migration
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Fig. 5. PTP1B modulates fibronectin-dependent regulation of
RhoA and Rac1 activity. (A) Serum-starved WT and KO cells were
seeded on polylysine + fibronectin-coated coverslips for 15 minutes
and labeled with TRITC–phalloidin. WT cells showed a strong Factin staining at lamellipodia (yellow arrowheads). KO cells showed
F-actin in stress fibers (yellow arrows) and peripheral spikes (yellow
arrowheads). Scale bar: 25 mm. (B) Results of pull-down assays to
quantify GTP-bound RhoA and Rac1 in cells in suspension or plated
on fibronectin-coated dishes. Values of active GTPases were
normalized to total GTPases. Plots show the means 6 s.e.m. of three
independent experiments and are expressed relative to the
suspension value. Representative blots are also shown.
(C) Spatiotemporal distribution of RhoA and Rac1 activities, by
FRET. Cells were seeded on polylysine and polylysine + fibronectin
(FN) for the indicated times to determine the FRET signal.
Representative cells on FN are shown. Graphs show the
quantification of the FRET/CFP ratio values along line scans drawn
from the cell margin, as shown in the WT FN RhoA image (n515–
20 cells per condition). Scale bar: 25 mm.
cells knockout for the adaptor proteins p130Cas and paxillin, and
for the protein tyrosine kinases FAK and Src. As control, we
examined a cell line derived from a littermate wild-type mouse.
On average, ,62.062.4% of vinculin peripheral adhesions in
control cells displayed associated GFP–PTP1BDA puncta
(Fig. 6A,H). This percentage was reduced in cells null for Src
(44.963.2%), paxillin (46.362.4%) and p130Cas (37.862.7%;
Fig. 7B–D,H). However, the most dramatic effect was observed
in SYF cells, which lack the expression of Src, Fyn and Yes
members of the Src family (1.860.5%) and in FAK KO cells
(2.760.9%) (Fig. 6E,F,H). A strong effect was also observed in
wild-type cells transfected with FRNK, a FAK-related non kinase
protein which displaces FAK from adhesions (supplementary
material Fig. S3) (Richardson et al., 1997). GFP–PTP1BDA did
not develop puncta in peripheral adhesions containing FRNK
(Fig. 6G,H). These results suggest that FAK expression and
localization at peripheral adhesions are both essential for the
generation of PTP1B substrates.
Downstream of integrin and growth factor stimulation, FAK is
autophosphorylated at 2397, providing a binding site for Src family
kinases (Schaller et al., 1994). Reconstitution of FAK KO cells with
GFP–FAK induced formation of mRFP–PTP1BDA puncta at
adhesions (52.465%) while reconstitution with FAK Y397F did
not (0.360.2%; Fig. 7B,I). These results suggest that Src recruitment
to adhesions by FAK Tyr397 is essential for trapping PTP1BDA.
Reconstitution of SYF cells with Src-HA rescued the
formation of puncta at adhesions (73.663.8%; Fig. 7C,I), and a
similar result was observed for Fyn-HA and Yes-HA
(supplementary material Fig. S4). These results suggest
compensatory roles among Src family members. SYF cells
reconstituted with a kinase-deficient (KD) Src mutant (Src
K297R), which as expected, was unable to phosphorylate FAK
Tyr925 (supplementary material Fig. S5) (Brunton et al., 2005),
showed a marginal effect (14.763.4%) to induce the formation of
GFP–PTP1BDA puncta associated with adhesions (Fig. 7D,I). In
contrast, reconstitution of SYF cells with Src-Y529F, a
constitutive active mutant that preferentially localizes in cell–
matrix adhesions and increase the phosphotyrosine content of
several substrata (Kaplan et al., 1994; Cary et al., 2002), induced
the accumulation of large puncta (69.564.1%; Fig. 7E,I). These
results demonstrate that localization and activity of Src at cell–
matrix adhesions are required for recruitment of GFP–PTP1BDA
puncta. Reconstitution of SYF cells with the double mutant Src
KD/Y529F, which localizes to adhesions but lacks both the
catalytic activity and the PTP1B target site pY529 (Bjorge et al.,
2000; Monteleone et al., 2012), was unable to induce the
formation of puncta (1.560.6%; Fig. 7F,I). Thus, the residual
fraction of puncta induced by Src KD expression is likely due to
the binding of the pTyr529 by PTP1BDA, in particular at high
expression levels of Src KD (supplementary material Fig. S5).
Consistent with this notion, no residual puncta of GFP–
PTP1BDA were observed in SYF cells at high expression
levels of the double mutant KD/Y529F (supplementary material
Fig. S6).
1826
Journal of Cell Science 126 (8)
reduction of puncta formation (,20%) by Y418F/Y529F compared
to Src Y418F is similar to that between Src KD and Src KD/Y529F
suggesting that PTP1BDA is capable of recognizing the
phosphorylated Src Tyr529 in peripheral adhesions.
Discussion
Journal of Cell Science
PTP1B regulates focal complex dynamics
Fig. 6. PTP1B targets Src/FAK, paxillin and p130Cas in cell–matrix
adhesions. (A) WT control fibroblast cell, (B) Src null cell, (C) Paxillin null
cell, (D) p130Cas null cell, (E) SYF cell, (F) FAK null cell. Cell lines were
transfected with GFP-PTP1BDA. (G) WT control co-transfected with GFPPTP1BDA and c-myc FRNK. Cells were fixed and processed for
immunofluorescence microscopy to detect vinculin (A–F) or c-myc
(G). Immune complexes were revealed with Alexa-Fluor-568-conjugated
secondary antibodies. Insets are 26 magnifications of part of each cell. GFP–
PTP1BDA puncta associated with peripheral adhesions are indicated by white
arrowheads. (H) Quantification of the percentage of peripheral adhesions
containing GFP–PTP1BDA puncta. Puncta were significantly reduced in Src,
Pax, and Cas knockout cells, and essentially absent in SYF, FAK KO cells
and WT cells + FRNK. Statistical significance was determined using one-way
ANOVA followed by the Dunnett’s multiple comparison post-hoc test, using
the WT cell line as the control (a,bP50.0002; a,cP,0.0001). Scale bar:
25 mm.
We also examined the effect of reconstituting SYF cells with
Src Y418F, which cannot autophosphorylate but could still be
activated by integrin stimulation (Kaplan et al., 1995; Cary et al.,
2002; Roskoski, 2005). Src Y418F induced a maximal effect, as
did Src wild type and Src Y529F (7062.4%; Fig. 7G,I).
However, the double mutant Src Y418F/Y529F induced a
reduced effect (49.665.3%) compared to Src wild type and the
individual mutants (Fig. 7H,I). Src Y418F (not shown) and Src
Y418F/Y529F phosphorylate FAK-Tyr925 at similar levels as
control Src (compare supplementary material Fig. S6 and Fig.
S5). Thus, basal or partly activated Src may be sufficient to trap
PTP1BDA at peripheral vinculin adhesions. The magnitude of
PTP1B anchored to the ER, and with the catalytic domain facing
the cytosol, has the potential for substrate dephosphorylation
throughout the extensive branching network occupied by the ER.
Here we demonstrated that paxillin adhesions targeted with ER
tubules bearing active PTP1B extended by approximately
fivefold their lifetimes compared to those not targeted, or
targeted with ER tubules bearing inactive PTP1B. Consistently,
we also found that short-lived adhesions (#10 minutes) were
significantly more abundant in KO cells compared to WT cells.
We propose that during lamellar protrusion, ER-bound PTP1B
targets newly formed adhesions and as a consequence bias their
fate towards maturation. During retraction phases adhesions grew
in size independently of PTP1B expression, suggesting that this
bias is overrode when additional mechanisms, e.g. contractile
forces, come into play. Consistent with this notion, lack of
PTP1B strongly affected protrusion persistence times but not
retraction times. Our results provide mechanistic insights to
previous findings suggesting a positive regulation of cell matrix–
adhesion and spreading by PTP1B in many cell types (Arregui
et al., 1998; Cheng et al., 2001; Pathre et al., 2001; Arias-Salgado
et al., 2005; Liang et al., 2005; Fuentes and Arregui, 2009).
The increased paxillin disassembly kinetics observed in KO cells
may contribute to shorten the lifetime of focal complexes. This view
agrees with results showing that adhesion lifetime correlates
positively with adhesion strength and inversely with paxillin
disassembly (Gupton and Waterman-Storer, 2006). Adhesion
strength and inhibition of paxillin adhesion disassembly at the
lamellipodium base were also positively correlated with the
incorporation of a-actinin and zyxin (Laukaitis et al., 2001; von
Wichert et al., 2003; Zaidel-Bar et al., 2003; Yoshigi et al., 2005;
Choi et al., 2008; Hirata et al., 2008). We previously demonstrated
that zyxin incorporation in focal complexes was significantly
impaired in KO cells (Hernández et al., 2006). Here we show that in
WT cells most focal complexes at the lamellipodium base
incorporated a-actinin–GFP and grow centripetally, while in KO
cells these processes were impaired. Our BiFC analysis demonstrate
the presence of catalytic PTP1B/a-actinin complexes in adhesions,
strongly suggesting that PTP1B dephosphorylates a-actinin and
promotes focal complex maturation. During lamellar retractions,
adhesions incorporate a-actinin similarly regardless of PTP1B
expression, suggesting that PTP1B did not play a major role during
this phase. Whether other PTPs, e.g. SHP-1 and SHP-2 (von Wichert
et al., 2003; Lin et al., 2004) may dephosphorylate a-actinin during
retractions remains to be determined.
PTP1B modulates integrin-dependent regulation of RhoA
and Rac1
We show that fibronectin stimulated the downregulation of RhoA
activity in WT- but not in KO cells. In contrast, Rac1 activity was
upregulated in WT cells but not in KO cells. WT cells response to
fibronectin was consistent with previous studies (Ren et al., 1999;
Arthur et al., 2000; del Pozo et al., 2000; Danen et al., 2002; Lim
et al., 2008). Spatial information provided by FRET analysis
showed maximal Rac1 and RhoA activity at the cell periphery, as
Role of PTP1B in adhesion dynamics and migration
1827
impairment of cell–matrix adhesion in CHO.K1 cells reduced the
stability of protrusions and migration directionality (Harms et al.,
2005). In a reciprocal manner, forced integrin clustering in
fibroblasts promoted focal adhesion development and lamellar
persistence (Cavalcanti-Adam et al., 2007). Our results suggest
that PTP1B is seemingly implicated in the regulation of processes
occurring at different time scales, like adhesion turnover,
lamellar dynamics and directional migration. Mechanistically,
PTP1B may facilitate integrin/cytoskeleton coupling by
dephosphorylation of a-actinin. PTP1B may also promote
integrin-dependent signaling regulating RhoA and Rac1
GTPases and the actin cytoskeleton, such as the Src/FAK
signaling pathway. Strikingly, several morphological and motility
alterations shown by KO cells resemble those reported in
fibroblasts deficient in FAK expression (Tilghman et al., 2005).
Journal of Cell Science
PTP1B substrates in adhesions
Fig. 7. FAK/Src activity is required to trap PTP1BDA in adhesions.
(A,B) FAK KO cells were transfected with mRFP-PTP1BDA and
reconstituted with (A) GFP–FAK or (B) GFP–FAK-Y397F. Colors were
inverted for better visualization. (C–H) SYF cells co-transfected with GFPPTP1BDA and Src (C), Src KD (D), Src Y529F (E), Src KD/Y529F (F), Src
Y418F (G) or Src Y418F/Y529F (H). In all these cells vinculin was detected
by immunofluorescence. GFP–PTP1BDA puncta associated with peripheral
vinculin adhesions (in red) are indicated by white arrowheads in the insets (26
magnification). (I) Plot shows percentages of peripheral adhesions containing
GFP–PTP1B puncta. Statistical significance was determined using one-way
ANOVA followed by the Tukey’s HSD post-hoc test (a,bP,0.0001;
c,d; c,e
P,0.0001; c,fP50.001). Scale bar: 25 mm.
described in other cell types (Nakamura et al., 2005; Pertz, 2010).
The high RhoA and low Rac1 activities in KO cells could explain
the lack of a lamellipodium and the development of actin stress
fibers shown by phalloidin staining. Unbalanced contractile
forces generated in the lamella could explain the low protrusion
persistence during migration.
PTP1B stabilizes lamellar protrusions and promotes
directional migration
The increased focal complex turnover in KO cells may be
causally related to the lower persistence of the leading edge and
the loss of migration directionality. Consistent with this notion
Our BiFC results strongly suggest that PTP1B dephosphorylate
a-actinin at cell–matrix adhesions. This event may facilitate
cytoskeletal coupling and focal complex maturation, as
previously reported (Laukaitis et al., 2001; Rajfur et al., 2002;
von Wichert et al., 2003; Choi et al., 2008). A previous model
proposed that phosphorylated a-actinin could form a complex
with Src and compete for the binding of Src to FAK (Zhang
et al., 2006). In accordance with this model, a-actinin
dephosphorylation by PTP1B would facilitate the assembly of
Src/FAK complexes and likely the phosphorylation of
downstream substrates. We showed that the substrate trap
mutant GFP–PTP1BDA formed puncta associated to peripheral
adhesions in control cells, but not in FAK KO cells, in cells
expressing FRNK, and in SYF cells. In addition, reconstitution
with catalytically inactive mutants were unable to recover the
formation of puncta. These results clearly indicate that FAK and
Src activity are essential for generating tyrosine phosphorylated
substrates of PTP1B in adhesions. We recently have shown that
PTP1B targets the negative regulatory site of Src, Tyr529, at the
plasma membrane/substrate interface (Monteleone et al., 2012).
This result suggests PTP1B could activate Src and FAK, and
initiate phosphorylation of downstream targets, including aactinin, paxillin and p130Cas. Paxillin and p130Cas are
phosphorylated by FAK and Src, and their phosphorylated
species localize in focal adhesions (Bellis et al., 1995; Schaller
and Parsons, 1995; Schaller et al., 1999; Sakai et al., 1997;
Tachibana et al., 1997; Fonseca et al., 2004). We observed a
modest, but significant decrease of GFP–PTP1BDA puncta in
paxillin and p130Cas KO cells, suggesting a direct
dephosphorylation of paxillin and p130Cas by PTP1B in
adhesions. Our results agree with previous biochemical data
proposing that paxillin and p130Cas are direct PTP1B substrates
(Liu et al., 1996; Takino et al., 2003; Dubé et al., 2004). Since
phosphorylation of paxillin and p130Cas are required for
adhesion turnover (Webb et al., 2004), PTP1B may negatively
modulate this process.
Collectively, our results suggest a complex interplay of PTP1B
effects. The effect on Src activation could initiate downstream
protein phosphorylation in adhesions, mounting an early integrindependent signaling response channeled to GTPases. PTP1B
could also dephosphorylate specific Src/FAK substrates,
modulating the timing of complex events, like cell–matrix
turnover, lamellar dynamics, and directional migration. A
1828
Journal of Cell Science 126 (8)
Seattle, WA), mouse Src (S. Shattil, University of California at San Diego, San
Diego). Chicken c-Src Y416 was subcloned into BamHI/HindIII sites of
pcDNA3.1/zeo. Mouse Src, and Fyn were subcloned into XhoI/EcoRI sites of
phCMV3 (Genlantis), and Yes into KpnI/XmaI sites. This produces Src, Fyn and
Yes with an HA epitope at the C-terminus. GFP-FAK Y397F and mutants, Src
Y529F and Src Y418F/Y529F were constructed using the QuikChange sitedirected mutagenesis kit (Stratagene, La Jolla, CA). Src KD (K297R) and Src KD/
Y529F were generated by recombinant PCR. Preparation of YN-PTP1B WT/DA
constructs for BiFC was previously described (Monteleone et al., 2012). Alpha
actinin-YC was made by replacing GFP in a-actinin–GFP with YC (amino acid
155–239) obtained by PCR, using AgeI/XbaI restriction sites. The DNA sequence
of all constructs were verified by sequencing. Transient transfections were
performed using Lipofectamine 2000 (Invitrogen), as described (Hernández et al.,
2006).
Pull-down assays
Journal of Cell Science
RhoA and Rac1 activities were performed using pull-down assay kits
(Cytoskeleton, Inc., Denver, CO). About 350 mg of protein (0.5 mg/ml) of cells
in suspension or plated at 50–60% confluence on fibronectin-coated tissue culture
dishes were used to isolate GTP-bound RhoA and Rac1 using 50 mg RhotekinRBD beads, and 20 mg PAK-PBD beads, respectively. Total RhoA and Rac1
proteins (from 10% of cell lysate) and the isolated active forms were detected in
Western blots using the Super Signal West Femto Substrate kit (Thermo
Scientific). For stripping, blots were incubated (30 minutes, 55 ˚C) with TBS
containing 5% 2-mercaptoethanol and 2% SDS, blocked and re-probed. Integrated
optical densities of bands in scanned films were determined using ImageJ (Wayne
Rasband, NIH, Bethesda, MD, USA). Active RhoA and Rac1 proteins were
normalized to total GTPase.
Fluorescence microscopy
Fig. 8. Schematic representation of PTP1B functions in adhesion and cell
migration. (A) Adhesions near the leading edge (brown circles) are targeted
by ER-bound PTP1B (green), extending their lifetimes, incorporating aactinin (red oval), and coupling to actin filaments (in blue). (B) Within cell
adhesion complexes, PTP1B targets Src and also modulates the
phosphorylation level of downstream substrates. (C) PTP1B is required for
integrin-dependent FAK/Src signaling that regulates Rho GTPases.
scenario representing the interpretation of our data are shown
schematically (Fig. 8).
Materials and Methods
Cell lines, materials and treatments
The following cell lines were kindly provided by colleagues: KO and WT cells (B.
Neel, University Health Network, Toronto, Ontario; Haj et al., 2002), Src knockout
and wild type fibroblasts (P. Soriano, Fred Hutchinson Research Center, Seattle,
WA; Klinghoffer et al., 1999), paxillin wild-type and knockout cells (S. M.
Thomas, Harvard Medical School, Boston; Hagel et al., 2002), wild-type and
p130Cas knockout cells (H. Honda, Hiroshima University, Japan; Honda et al.,
1998). SYF and FAK knockout cell lines were purchased from ATCC (Manassas,
VA). All cell lines were cultured in high glucose DMEM containing L-glutamine
plus 10% fetal bovine serum and antibiotics (Invitrogen, Carlsbad, CA). Polylysine
and fibronectin were from Sigma-Aldrich (St. Louis, MO). For microscopy, cells
were on coverslips (Marienfeld GmbH & Co, Lauda-Königshofen, Germany) or
custom-made coverslip-bottom dishes. When indicated, cells were preincubated
for 30 minutes with 0.1 mM sodium pervanadate before processing, as described
(Hernández et al., 2010).
Antibodies and other labeling reagents
TRITC-conjugated phalloidin, and monoclonal anti-vinculin (clone hVIN-1), antiHA (clone HA-7), and anti-c-myc (clone 9E10) were from Sigma-Aldrich (St
Louis, MO, USA). Polyclonal antibodies against GFP, FAK-pY397, FAK-pY925,
Src-pY529, Src-pY418, and Alexa-Fluor-488- and 568-conjugated antibodies were
from Invitrogen. HRP-conjugated antibodies were from Jackson ImmunoResearch
(West Grove, PA).
DNA constructs and transfections
GFP-PTP1B, D181A and C215S, and mRFP-paxillin were previously described
(Arregui et al., 1998; Hernández et al., 2006). The following plasmids were
provided by colleagues: pRaichu-Rac1 (10266) and pRaichu-RhoA (12376) (M.
Matsuda, Osaka University, Osaka, Japan), chicken GFP-FAK and c-myc-FRNK
(J. T. Parsons, University of Virginia, Charlottesville, VA), a-actinin-GFP (C. A.
Otey, University of North Carolina at Chapel Hill, Chapel Hill), chicken c-Src
Y416F, mouse Fyn and Yes (J. Cooper, Fred Hutchinson Cancer Research Center,
Cells attached on fibronectin-coated (10 mg/ml) coverslips were fixed with 4%
paraformaldehyde (20 minutes), permeabilized with 0.5% Triton X-100
(5 minutes), and blocked with 5% BSA (60 minutes), all diluted in PBS
(137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, 1.8 mM KH2PO4, pH 7.4).
Primary and secondary antibodies were diluted in PBS/BSA and incubated in a
humid chamber (60 minutes). Cells were mounted in Vectashield (Vector
Laboratories, Burlingame, CA) and imaged using a Nikon TE 2000-U inverted
microscope (Melville, NY) equipped with a 606/1.4 NA objective, and an OrcaAG cooled CCD camera (Hamamatsu Photonics, Hamamatsu, Japan). For timelapse experiments, cells were kept at 37 ˚C in Phenol-Red-free DMEM with high
glucose, supplemented with 4 mM L-glutamine and 25 mM Hepes buffer, 10%
fetal bovine serum, antibiotics and 0.5 U/ml oxyfluor (Oxyrase, Inc., Mansfield,
OH). Culture medium was overlaid with mineral oil to prevent evaporation. EGFP
and mRFP were detected using Nikon B-2E/C and G-2E/C filter sets. The
excitation light was attenuated using neutral density filters and shuttered using a
SmartShutter and a Lambda 10-B controller (Sutter Instrument, Novato, CA). All
peripherals were controlled with Metamorph 6.1 software (Molecular Devices,
Downingtown, PA). Image stacks were built using ImageJ. For display purposes
images were processed for unsharp masking.
Cell migration and kymographs
WT and KO cells (10,000 cells) were seeded in 24-well tissue-culture plates coated
with fibronectin and blocked with BSA. After overnight incubation, cell
movements were monitored under phase contrast using a 106 objective every
8 minutes during 10 hours. Light was attenuated by ND4 filters and shuttered
between acquisitions. Cells that divided or made contacts with others were not
analyzed. To reconstruct cell trajectories, positions of cell nuclei were determined
using the track object function of Metamorph. Velocities and persistence of
migratory directionality (D/T) were extracted from recorded data. ‘D’ refers to the
linear distance from the starting point to the end point of a time-lapse recording
and ‘T’ refers to the total distance traversed by the cell. A list of ‘x’ and ‘y’ pixel
coordinates for each cell was fed into the Cell Motility Suite software (Martens
et al., 2006) to calculate the MSD over time and extract the values of speed (S) and
persistence time (P) by fitting the MSD to the persistent random walk model
(RWM) equation (Dunn, 1983; Othmer et al., 1988).
For kymographs, cells were transfected with a-actinin-GFP and imaged every
30 seconds during 50 minutes, using 262 binning. Excitation light was attenuated
by ND4 filters. We used ImageJ to draw three lines (1-pixel-wide, 0.22 mm) per
cell in the direction of lamellar protrusion. Leading edge protrusion and retraction
rates, frequencies of switching between phases, and time of protrusion or retraction
persistence were calculated from kymographs using the kymograph plugin for
ImageJ (J. Rietdorf, FMI Basel, and A. Seitz, EMBL Heidelberg).
BiFC and FRET
Two a-actinin constructs were made, one fused to YN (amino acid 1–154) and
other fused to YC (155–239), both EYFP fragments were located at the C-terminus
of a-actinin. The correspondent PTP1B BiFC pairs were previously described
Role of PTP1B in adhesion dynamics and migration
(Monteleone et al., 2012). Only the pair a-actinin-YC/YN-PTP1B DA gave a
strong positive BiFC signal and were used in the present paper. BiFC was analyzed
with an excitation filter of 500/20 nm, an emission filter of 535/30 nm and a
86002v2bs dicroic mirror (Chroma Technology, Brattleboro, VT). In cells
detecting BiFC and immunolabeled with red fluorescents the following Nikon
filter sets were used: for BiFC, excitation 480/30 nm, emission 535/40 nm, 505
(LP) dicroic mirror; for Alexa Fluor 568 nm, excitation 540/25 nm, emission 620/
60 nm, 565 (LP) dicroic mirror. To visualize cells by SRIC, a cube (Nikon) with a
green excitation filter, a UV dicroic mirror and without barrier filter was set in
place in the epifilter rotating turret.
Spatiotemporal activities of RhoA and Rac1 in WT and KO cells were
determined by FRET analysis using pRaichu-Rac1 (10266) and pRaichu-RhoA
(12946) probes (Itoh et al., 2002; Nakamura et al., 2005). Transfected cells were
starved 4 hours before plating on coverslips coated with 150 mg polylysine or
polylysine plus 10 mg/ml fibronectin, and blocked with 1 mg/ml BSA. Incident
light was attenuated using ND8 filters. Filters used for dual-emission ratio imaging
(CFP excitation 430/25, CFP emission 470/30; YFP emission 535/30) were placed
in filter wheels and combined with the dual dichroic mirror 86002v2bs (Chroma).
CFP and YFP (FRET) images were acquired using 262 binning and exposure
times ranging 0.5–1 seconds. After shade correction and background subtraction,
FRET/CFP ratio images were generated with Metamorph and used to represent
changes in the FRET efficiency by intensity modulated display (IMD). For
quantification, pixel values for FRET and CFP along three equidistant line scans
(3-pixels wide) perpendicular to the cell border were obtained, FRET/CFP ratios
were calculated and averaged per cell.
Journal of Cell Science
Additional quantitative procedures
ER targeting events (defined in Fig. 1 legend) on paxillin adhesion lifetimes
were determined in KO cells co-transfected with mRFP-paxillin and either
GFP-PTP1BWT or GFP-PTP1BCS. Cells seeded on fibronectin and grown in
complete DMEM were imaged every 30 seconds using 262 binning. The
contrast of image stacks was improved by unsharp masking using ImageJ.
Adhesion lifetimes were determined measuring the time elapsed between the
first and last frame in which each adhesion appeared. About 50 adhesions of 6–
8 cells were analyzed.
To determine adhesion lifetimes and paxillin kinetics of assembly and
disassembly WT and KO cells were transfected with GFP- or mRFP-tagged
paxillin and 24 hours post-transfection were resuspended and seeded at ,30%
confluence on coverslip-bottom dishes coated with fibronectin. After 16 hours
cells were analyzed using time-lapse acquired images every 0.5, 1 and 3 minutes.
Different time intervals allowed a better sampling of short- and long-lived
adhesions. Fluorescence intensities of individual adhesions from backgroundsubtracted images were measured over time using Metamorph as previously
described (Webb et al., 2004). For rate constant measurements, periods of
assembly (increasing fluorescence intensity) and disassembly (decreasing
fluorescence intensity) of adhesions were plotted over time. Semi-logarithmic
plots of fluorescence intensities as a function of time were generated using the
formulas, Ln([I]/[I0]) for assembly and Ln([I0]/[I]) for disassembly, where I0 is
the initial fluorescence intensity and I is the fluorescence intensity at various time
points. The slopes of linear regression trend lines fitted to the semi-logarithmic
plots were then calculated to determine apparent rate constants of assembly and
disassembly. At least 95 individual adhesions in 20 cells were analyzed per cell
line.
To determine a-actinin–GFP incorporation in mRFP–paxillin clusters cells were
imaged every 0.5 and 1 minute. After building the image stacks, plots of
fluorescence intensity over time were obtained for paxillin clusters using ImageJ.
Incorporation of a-actinin–GFP was considered positive when the signal intensity
is at least twice higher than the background and overlaps with that of the mRFP–
paxillin. Eight or nine cells and 36–47 adhesions were analyzed per condition.
To quantify the percentage of peripheral vinculin adhesions with GFP–
PTP1BDA puncta we generated a mask image containing puncta with
fluorescence intensity at least twice the average fluorescence intensity of the ER
in a peripheral flat region of the cell, and merged it with the image showing
vinculin. More than 20 cells from four independent experiments (average 26
peripheral adhesions per cell) were analyzed.
Acknowledgements
We thank Drs B. Neel, P. Soriano, S. M. Thomas, H. Honda for
sharing cell lines, and M. Matsuda, J.T. Parsons, C.A. Otey, J.
Cooper, S. Shattil for providing DNA constructs.
Author contributions
J.E.B. and A.G. performed the experiments; J.E.B., A.G. and C.O.A.
designed the experiments and analyzed the data; C.O.A. wrote the
paper.
1829
Funding
This work was supported by Consejo Nacional de Investigaciones
Cientı́ficas y Técnicas [PhD fellowship to J.E.B. and Research
membership to C.O.A.]; and Agencia Nacional de Promoción
Cientı́fica y Tecnológica [PhD fellowship to A.G. and grant
numbers 31939 and 1363 to C.O.A.]
Supplementary material available online at
http://jcs.biologists.org/lookup/suppl/doi:10.1242/jcs.118828/-/DC1
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