Eukaryotic Chemotaxis: Distinctions between Directional Sensing and Polarization*
Peter Devreotes
and
Chris Janetopoulos
From the
Department of Cell Biology, Johns Hopkins University, School of Medicine,
Baltimore, Maryland 21205
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ABSTRACT
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Directional sensing and polarization are fundamental cellular responses
that play a central role in health and disease. In this review we define each
process and evaluate a series of models previously proposed to explain these
phenomena. New findings show that directional sensing by G protein-coupled
receptors is localized at a discrete step in the signaling pathway downstream
of G protein activation but upstream of the accumulation of PIP3.
Local levels of PIP3, whether triggered by chemoattractants,
particle binding, or spontaneous events, determine the sites of new
actin-filled projections. Robust control of the temporal and spatial levels of
PIP3 is achieved by reciprocal regulation of PI3K and PTEN. These
observations suggest that a local excitation-global inhibition model can
account for the localization of PI3K and PTEN and thereby explain directional
sensing. However, elements of other models, including positive feedback and
the reaction of the cytoskeleton, must be invoked to account for
polarization.
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Directional Sensing Is a Fundamental Cellular Process
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Many types of cells are able to sense extracellular directional cues and
respond with asymmetric changes in cell morphology and motility. For example,
during chemotaxis a chemical gradient serves as a directional signal that
organizes cell movement. This intriguing process plays a central role in
development, immunity, and tissue homeostasis
(1,
2,
3,
4). During embryogenesis,
movements of cells in response to chemotactic stimuli bring form and
organization to tissues and organs and steer axons in the formation of the
nervous system (5,
6,
7). In the immune system, an
elaborate network of chemoattractants directs leukocytes to their correct
locations and facilitates cell-cell interactions. Chemotaxis is also central
to wound healing and has been implicated in disease states such as metastasis
and atherosclerosis (8,
9,
10,
11,
12). This review will focus on
mechanisms of directional sensing with emphasis on chemotactic systems.
By investigating model chemotactic systems such as Dictyostelium
discoideum, researchers are uncovering the general principles by which
cells sense asymmetric environmental stimuli
(13,
14,
15). Mechanisms of chemotaxis
in mammalian cells are remarkably similar to those in this genetically
tractable organism (16,
17,
18). During growth, D.
discoideum amoebae use chemotaxis to track down and phagocytose bacteria.
When starved, the cells differentiate, polarize, and migrate directionally
toward secreted 3',5'-cyclic adenosine monophosphate (cAMP). The
cAMP is detected by four serpentine receptors, designated cAR1cAR4,
coupled to a single heterotrimeric G protein
(19). A similar situation is
found in mammalian leukocytes where 20 types of chemoattractant, or chemokine,
receptors couple to the same G protein, Gi
(20,
21). Other similarities with
mammalian systems include chemoattractant-elicited transient increases in
phosphoinositides
(PIs),1 cAMP, cGMP,
inositol trisphosphate, and Ca2+ and rearrangements in
the cytoskeleton (16,
22). PIP3 has
emerged as an important intermediate in chemotactic signaling in D.
discoideum amoebae and mammalian leukocytes
(23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34).
The terms directional sensing, polarity, and chemotaxis are often used
interchangeably. We offer these definitions to more clearly distinguish these
phenomena. Directional sensing refers to the ability of a cell to detect an
asymmetric extracellular cue and generate an internal amplified response
(15). In cells exposed to
shallow gradients in chemoattractant concentration, signaling molecules
accumulate at the membrane adjacent to the higher concentration and initiate
downstream responses locally. This localized activation can be visualized, for
example, with proteins containing a PH domain fused to green fluorescent
protein (GFP) (Fig. 1). The
directional sensing response does not require the cell to be polarized.
Unpolarized, immobilized cells can also detect gradients with a similar degree
of signal amplification (Fig.
1, left). The sensitivity to chemoattractant is uniform
around the perimeter, and when the gradient is shifted, the PH domain proteins
rapidly redistribute according to the new direction
(Fig. 1, left).

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FIG. 1. Unpolarized D. discoideum cells are equally responsive at all
points on their perimeters whereas polarized amoebae have restricted
sensitivity. Cells expressing PHCrac-GFP sense a gradient of
cAMP released from a micropipette. A latrunculin-treated cell (top panel,
left) displays PHCrac-GFP binding to the membrane on the side
of the cell exposed to gradient emanating from pipette 1 (dot), and
then rapidly (within 60 s) translocates to the other side when pipette 2
(dot) is turned on. Polarized cells initially chemotax toward pipette
1 (top panel, right). When a competing gradient from pipette 2
(dot) is turned on, they either turn or continue forward. (The rear
of cell b is actually closer to pipette 2.) Time between frames in
right panels is 30 s.
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Polarization defines the propensity of the cell to assume an asymmetric
shape with a defined anterior and posterior. Molecules associated with the
"leading edge" include actin and actin-binding proteins Scar,
WASP, filopodin, cofilin, and coronin, whereas molecules associated with the
trailing edge include myosin II and cortexillin
(35,
36,
37,
38,
39). In polarized cells the
anterior surface is more sensitive to chemoattractants than other regions.
When the direction of chemoattractant gradient is changed, a polarized cell
generally turns toward the new highest concentration and maintains its
original anterior instead of redistributing PH domains
(Fig. 1, right). A
very steep gradient in an opposing direction can sometimes override this
asymmetry and generate a new axis in the new direction (not shown). The
localized sensitivity afforded by polarization focuses the activity of the
actin cytoskeleton at the leading edge, resulting in faster movement toward a
chemoattractant source. However, the sensing must occur within a small zone at
the front rather than across the entire cell diameter. In contrast, the
symmetrical sensitivity of the unpolarized cell means the area involved in
gradient detection is larger (Fig.
1, left).
Cells display various degrees of polarization that may also change with
conditions. In general, neutrophils are immobile until exposed to
chemoattractant. They then polarize, acquire a distinct leading edge and
uropod, and begin to move (16,
40,
41,
42). Growth stage D.
discoideum amoebae are unpolarized and move randomly without exogenous
chemoattractant. These cells can still sense direction and carry out
chemotaxis. As they differentiate, they become elongated, motile, and highly
chemotactic (43). Polarization
can also be enhanced by a period of directed movement in a gradient. Unlike
directional sensing, polarization depends critically on the actin
cytoskeleton, and inhibitors of actin polymerization convert a polarized cell
to an unpolarized one. This treatment eliminates both polarized morphology and
sensitivity, suggesting that an interaction of key signaling molecules with
the cytoskeleton stabilizes the polarized state
(44,
45). Here we focus on the
mechanisms of directional sensing and speculate on emerging models for
polarization and chemotaxis.
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Series of Models Proposed to Account for Directional Sensing and
Polarization
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Fig. 2 illustrates some of
the ideas that have been put forward to explain direction sensing and
polarization. An early proposal, based on the temporal mechanism of chemotaxis
in Escherichia coli, holds that a eukaryotic cell extends
"pilot pseudopodia" in random directions
(16,
46,
47). Those extended up the
gradient experience a positive change in chemoattractant concentration and are
reinforced whereas those projected down the gradient receive a negative signal
and are extinguished. The random walk of pseudopodia tends to move the cell
steadily toward the attractant. A second proposal reasons that a gradient
applied to a cell must first contact the cell on one side
(48). This "first
hit" triggers a rapid inhibitory response that spreads across the cell
and prevents the posterior from responding. When the gradient is repositioned,
there is again an initial contact and the direction of the response is
reset.

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FIG. 2. Salient models proposed to explain chemotaxis and polarity. Each
panel indicates the same cell at an initial and an advanced stage of
gradient sensing. In the gradient represented by the yellow shading,
the highest concentration is on the right. In the "pilot
pseudopodia" model, pseudopodia are reinforced only when they detect an
increasing concentration (+dC/dt). In the "first hit
inhibition" model, an inhibitory molecule (red line) diffuses
rapidly through the cell or along the membrane and blocks the back of the cell
from responding further. "Positive feedback loops" of internal
signaling components (green arrows) have also been proposed to
amplify the shallow gradient across the cell. "Mechanism
restriction" models invoke the cytoskeleton (red arrows) to
couple an extension at the front of the cell to a retraction in the back. The
"intermediate depletion" model proposes that binding of a limited
internal signaling component (gray dots) is highly cooperative. The
excitation-inhibition model proposes that the response depends on a balance
between rapid excitation and slower inhibition processes. Excitation (E,
green) reflects local receptor occupancy whereas an inhibition (I,
red) reflects average receptor occupancy across the cell.
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A third class of models is based on powerful internal "positive
feedback" loops. Signaling molecules are selectively amplified at the
anterior of the cell and thereby localize the response
(49,
50). Several models link a
positive action at the front of the cell to an opposing action at the back. In
the "mechanical restriction" model an extension at the front of
the cell is physically coupled to a retraction at the back. In the
"intermediate depletion" model highly cooperative binding at the
front limits the availability of free signaling molecules at the back of the
cell (51). Finally, the
"local excitation-global inhibition" model proposes that
directional sensing depends on a balance between a rapid, local
"excitation" and a slower global "inhibition" process
(15,
16,
52,
53,
54). Receptor occupancy
controls the steady-state levels of each process, and the difference between
the two regulates the response. Because inhibition depends on average receptor
occupancy, its steady-state level is less than that of local excitation at the
front of the cell. At the back, the situation is reversed.
Many of these concepts are useful to our understanding of directional
sensing and polarization, but none can account for observed responses under
all experimental paradigms. The "pilot pseudopodia" model cannot
explain how a completely immobile cell that is unable to extend projections is
still able to amplify a stable external gradient
(Fig. 1, left). The
"first hit" inhibition model cannot account for the ability of a
cell to sense a gradient formed by lowering the concentration from an
initially high uniform
level.2 The
"positive feedback" models provide large amplification, but once
initiated the response becomes relatively independent of the external signal.
This property is useful for polarization but is inconsistent with the ability
of an unpolarized cell to respond to rapid shifts in directional input. The
mechanical restriction model is incompatible with the capacity of a paralyzed
cell to sense the external gradient (Fig.
1, left). The "intermediate depletion" model
requires strong cooperative binding and cannot account for the ability of the
cell to respond over a wide range of stimulus concentrations.
The "local excitation-global inhibition" model is consistent
with many features of the chemotactic responses. Cells respond to
changes in receptor occupancy and adapt when occupancy is
held constant. The model accounts for transient responses, the directional
responses to spatial gradients, and for observed responses to combinations of
temporal and spatial stimuli. It is also consistent with the ability of the
cell to respond to gradients with a wide range of midpoint concentrations.
However, the model lacks the large amplification afforded by positive feedback
and does not explain the slow reactions of polarized cells to shifts in the
external gradients (see Fig. 1, right). A comprehensive, predictive scheme for directional sensing
and polarization will likely bring together elements from a number of
these models.
Directional sensing of chemoattractants occurs within the signaling pathway
after G protein activation and before the accumulation of PIP3.
During directional sensing and polarization there is surprisingly little
redistribution of the upstream components and biochemical reactions in the
signaling pathway. In unpolarized cells, the chemoattractant receptors and G
proteins are distributed uniformly along the cell membrane, whereas receptor
occupancy closely mirrors the shallow concentration gradient of
chemoattractant (Fig. 3)
(55,
56,
57).2
Cell polarization leads to only subtle changes in these parameters; the G
protein subunits acquire a slightly asymmetric distribution toward the front
of the cell, and the on and off rates of cAMP binding are faster at the
anterior end (55,
58). G protein activation has
not been directly imaged, but its kinetics suggests that it is not sharply
confined to the front of cells whether or not they are polarized. During
chemoattractant stimulation, the G protein
- and 
-subunits
remain dissociated as long as receptors are occupied
(59). It is difficult to
envision then how the G proteins would be inactivated at the back of the cell
where receptor occupancy is only slightly lower than at the front. Rather, it
seems likely that a global inhibitory process offsets G protein activation at
the back of the cell and thereby localizes responses to the front.

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FIG. 3. Cartoon depicting the distribution and activity of signaling molecules
in unpolarized and polarized cells during temporal and spatial
stimulation. Chemoattractant receptors (cAR1), receptor occupancy,
associated and dissociated (activated) G protein
  -subunits, excitation, inhibition, PI3Ks, PTEN, PI3K and
PTEN binding sites, PIP3, F-actin, and myosin are indicated.
Upper diagram illustrates the response to a temporal stimulus. In
resting cells, PTEN is bound to the membrane and PI3Ks are in the cytosol
(t = 0 s). An increase in receptor occupancy by chemoattractant
(orange hexagons) triggers, through the heterotrimeric G proteins, a
rapid increase in excitation (green arrow), which leads to binding of
PI3Ks (light blue squares) to their binding sites (dark blue
squares) at the membrane and causes PTEN (maroon triangles) to
dissociate from binding sites (purple squares) at the membrane
(t = 5 s). The combined effect causes a large increase in
PIP3 (green lollipops). At longer times, inhibition
(red line) increases and eventually balances excitation. PI3Ks return
to the cytosol, PTEN returns to the membrane, and PIP3 returns to
prestimulus levels (t = 180 s). Lower diagram shows the
response of a cell treated with latrunculin A (left) and of a
polarized cell (right) in a spatial gradient. The appearance of the
polarized cell would be similar in a uniform concentration of attractant (see
text). The "global" inhibition (red line) is equal at
both ends of the cell. "Local" excitation is slightly higher at
the front causing the binding of PI3K to and the loss of PTEN from the
membrane at the front. This leads to a large steady-state accumulation of
PIP3 selectively at the front and, in untreated cells, actin
polymerization and directed motility.
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The accumulation of PIP3 at the cell anterior is an early point
where strong asymmetric activation of the signaling pathway is observed
(Fig. 1). This was first shown
in D. discoideum by visualization of these PIs with a variety of
GFP-tagged PH domain-containing proteins
(44,
60). Similar asymmetric
localizations of PH domains occur in leukocytes exposed to gradients of
chemoattractants (33). PI
accumulations are transient in cells exposed to uniform chemoattractant,
whereas the G protein subunit dissociation is not
(Fig. 3, top). In
cells lacking functional G proteins, chemoattractants do not elevate
PIP3 levels (45,
61), whereas inhibitors of the
cytoskeleton, such as latrunculin A, do not interfere with the response (see
Fig. 1, left). Taken
together, these observations and others suggest that gradient detection
becomes sharply localized at a step downstream of G protein activation and
upstream of the generation of the PIs
(14,
59).
Further evidence for a key role of PIP3 in directional sensing
has come from studies of the subcellular distribution of PI3Ks and PTEN in
D. discoideum (62,
63). These enzymes are
reciprocally regulated in response to chemotactic stimulation
(Fig. 3). In resting cells, the
PI3Ks are cytosolic whereas a fraction of PTEN is bound to the plasma
membrane. Uniform addition of chemoattractant results in the translocation of
PI3Ks to the membrane while PTEN rapidly dissociates. Then, PI3Ks return to
the cytosol and PTEN reassociates with the membrane. The PTEN reassociation
results in a higher than prestimulus level of membrane-bound PTEN. In a
gradient, the PI3Ks are recruited to the front of the cell, and PTEN
associates with the membrane at the back
(Fig. 3, bottom).
Interestingly, this spatial asymmetry in the distributions of the two enzymes
is greater in polarized versus unpolarized cells.
The movements of PI3K and PTEN and the changes in PIP3 levels
suggest that the enzyme activities are reciprocally regulated during the
response. Indeed, there is an extremely rapid increase in PI3K activity
following an increase in chemoattractant
(64). Cell lysates prepared
within 5 s of addition of a stimulus incorporate 32P-labeled
-ATP into [32P]PIP3 about 6-fold higher than
lysates from unstimulated cells. This activation is transient; PI3K activity
in lysates of cells pretreated for 30 s or more returns to a plateau level
that is slightly elevated. Thus, receptor-mediated activation of PI3K
contributes to the transient increases in PIP3. It is expected that
the rapid loss of PTEN from the membrane enhances the accumulation of the PIs,
and the return of the enzyme to the membrane helps terminate the response.
Cells lacking PTEN display changes in PI3K activity essentially identical to
those in wild-type cells, yet increases in PIP3 are higher and
prolonged (64). The parallel
regulation of PI synthesis and degradation provides a robust system that is
resistant to perturbation
(14). Since changes in both
enzymes contribute to the accumulation of the PIs, partial inhibition of
either is unlikely to completely impair the response.
These observations focus attention on the membrane binding sites and
activators of PI3Ks and PTEN. These regulatory events create the initial
asymmetry in signaling that leads to directional sensing. The movements and
regulation of PI3K and PTEN can be explained by the excitation-inhibition
model described above (see Figs.
2 and
3). We propose that the balance
between an excitation and an inhibition process controls the membrane binding
and activity of each enzyme. For PI3K, excitation reflecting local levels of
receptor occupancy leads to recruitment and activation of the enzyme whereas
global inhibition, determined by the average receptor occupancy, counteracts
these effects. For PTEN, local excitation decreases its association with the
membrane whereas global inhibition restores binding. Recent structural
information has shed some light on the membrane binding and activation of
these enzymes. For the PI3Ks, the N-terminal hydrophilic regions can target
GFP to the membrane whereas a Ras binding domain is not required for enzyme
recruitment but might be important for activation
(62). PTEN contains an
N-terminal PI(4,5)P2 binding motif, and its deletion completely
redistributes the enzyme to the cytosol. This mutated PTEN, when expressed in
pten cells, is unable to rescue their chemotactic
defects, suggesting that membrane association is crucial for function
(63).
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Local PIP3 Increases Lead to Directional Actin
Polymerization Responses
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Evidence suggests that PIP3 plays a central role in directing
where and when sites of actin-filled projections form in a variety of cellular
responses. First, the stimulus-induced accumulation of PIP3 as
assessed by binding of specific PH domains to the membrane co-localizes with
sites of new actin filament formation
(14,
30,
62). In D. discoideum
the PH domains label the surface membranes of pseudopodia, ruffles, filopods,
macropinosomes, phagosomes, and sites of cell-to-cell contact
(Fig. 4)
(65,
66,
67,
68,
69). Interestingly, many of
these events occur spontaneously in the absence of functional G proteins,
implying G protein-independent activators can also lead to local accumulations
of PIP3. Second, interference with PI3K alters actin polymerization
and inhibits many of these actin-based events. In macrophages the later stages
of phagocytosis are blocked by PI3K inhibitors
(66). D. discoideum
cells lacking PI3Ks or treated with PI3K inhibitors display profound defects
in ruffling, macropinocytosis, and phagocytosis.2 Third, elevation
of PIP3 by disruption of PTEN induces excess actin polymerization
(63). In wild-type cells,
chemoattractant stimulation typically triggers a biphasic actin polymerization
response. In pten cells, the second phase of actin
polymerization is 6-fold that in wild-type cells. Attempts to reduce
PIP3 and thereby block actin polymerization triggered by
chemoattractant led to a surprising observation. Inhibitors of PI3K and gene
disruptions, which reduce increases in PIP3 by over 90%, completely
block the second phase of actin polymerization but do not affect the initial
rapid phase. The first phase of the elicited actin polymerization response may
be independent of or require only very slight increases in these PIs. It is
not clear whether the effects of PIP3 on actin polymerization
require Akt-mediated phosphorylation events. Alternatively, the PIs may
activate an exchange factor for a Rac family protein that is recruited to the
membrane. There is a recent report of synergistic activation of exchange
factor by G protein 
-subunits and PIP3
(70).

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FIG. 4. Cells expressing PHCrac-GFP show localization
of this marker for PIP3 on a variety of membrane
structures. The cartoons on the left illustrate PIP3
(green labeling) in a number of events such as pseudopodia extension,
membrane ruffling, filopod extension, macropinocytosis, phagocytosis, and
cell-cell contact that require cytoskeletal remodeling. Examples on the right
include PHCrac-GFP localization on ruffles, which mediate random
movements of D. discoideum cells, and on macropinosomes (top
panel). Bottom two frames show a phagocytic cup during
phagocytosis of a yeast cell by growing cells. PHCrac-GFP signal at
the membrane appears with the initial encounter of the yeast cell and usually
disappears from the phagosome after engulfment.
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Positive Feedback and Actin Cytoskeleton May Stabilize Directional
Sensing and Establish Polarity
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Recent investigations have advanced our understanding of directional
sensing, and these findings allow us to speculate on mechanisms of polarity.
Although the local excitation global inhibition scheme is sufficient to
explain directional sensing by an unpolarized cell, we believe that polarity
must involve many of the concepts outlined in earlier models such as
mechanical restriction and positive feedback loops. Although directional
sensing does not require actin polymerization, polarity depends critically on
a signal input as well as a reorganization of the cytoskeleton. We therefore
propose that establishment of polarization involves a dynamic, coordinated
interaction of directional sensing events with the activities of the
cytoskeleton (see Fig. 3,
bottom). How might the components involved in directional sensing,
together with the cytoskeleton, bring about gradient-induced or even
"spontaneous" polarization? We suggest that an essential role of
the actin cytoskeleton is to stabilize the asymmetric distribution of key
components of the directional response apparatus. Possibly, in a polarized
cell, the associations of PI3K and PTEN with the membrane at the front and
back of the cell, respectively, are reinforced by interactions with elements
of the cytoskeleton localized to these regions. Because PIP3
promotes actin polymerization, were components of the anterior cytoskeleton to
stabilize the interaction of PI3K with the membrane, a positive feedback loop
would result and reinforce the initial asymmetry. Similarly, a connection of
PTEN to components of the cytoskeleton such as myosin II or Pak A, which are
known to be modulated by chemoattractant at the back, might create a second
feedback loop at the rear. With sensitive feedback loops, small perturbations
would be expected to trigger the cell to acquire polarized morphology and
sensitivity even in the presence of a uniform concentration of
chemoattractant. This would lead to persistent generation of PIP3
at the leading edge, which, in turn, would maintain the asymmetry in the
cytoskeleton in the absence of a gradient.
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FOOTNOTES
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* This minireview will be reprinted in the 2003 Minireview Compendium, which
will be available in January, 2004. Work from our laboratory has been
supported by the National Institutes of Health and the American Cancer
Society. 
To whom correspondence should be addressed: Dept. of Cell Biology, Johns
Hopkins University, School of Medicine, 725 N. Wolfe St., 114 WBSB, Baltimore,
MD 21205.
1 The abbreviations used are: PI, phosphoinositide; PIP3,
phosphatidylinositol 3,4,5-trisphosphate; GFP, green fluorescent protein; PH,
pleckstrin homology; PI(4,5)P2, phosphatidylinositol
4,5-bisphosphate; PI3K, phosphatidylinositol 3-kinase; PTEN, PI
3-phosphatase. 
2 P. Devreotes and C. Janetopoulos, unpublished observations. 
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REFERENCES
|
---|
- Carlos, T. M. (2001) J. Leukocyte
Biol. 70,
171184[Abstract/Free Full Text]
- Lalor, P. F., Shields, P., Grant, A., and Adams, D. H.
(2002) Immunol. Cell Biol.
80,
5264[CrossRef][Medline]
[Order article via Infotrieve]
- Patel, D. D., and Haynes, B. F. (2001)
Curr. Dir. Autoimmun. 3,
133167[Medline]
[Order article via Infotrieve]
- Thelen, M. (2001) Nat.
Immunol. 2,
129134[CrossRef][Medline]
[Order article via Infotrieve]
- Biber, K., Zuurman, M. W., Dijkstra, I. M., and Boddeke, H. W.
(2002) Curr. Opin. Pharmacol.
2,
6368[CrossRef][Medline]
[Order article via Infotrieve]
- Fernandis, A. Z., and Ganju, R. K. (2001) Science's STKE
http://stke.sciencemag.org/cgi/content/full/oc_sigtrans;2001/91/pe1
- Rubel, E. W., and Cramer K. S. (2002) J.
Comp. Neurol. 448,
15[CrossRef][Medline]
[Order article via Infotrieve]
- Condeelis, J. S., Wyckoff, J. B., Bailly, M., Pestell, R.,
Lawrence, D., Backer, J., and Segall, J. E. (2001)
Semin. Cancer Biol. 11,
119128[CrossRef][Medline]
[Order article via Infotrieve]
- Gangur, V., Birmingham, N. P., and Thanesvorakul, S.
(2002) Vet. Immunol. Immunopathol.
86,
127136[CrossRef][Medline]
[Order article via Infotrieve]
- Moore, M. A. (2001) Bioessays
8,
674676[CrossRef]
- Murphy, P. M. (2001) N. Engl. J.
Med. 345,
833835[Free Full Text]
- Worthley, S. G., Osende, J. I., Helft, G., Badimon, J. J., and
Fuster, V. (2001) Mt. Sinai J. Med.
68,
167181[Medline]
[Order article via Infotrieve]
- Chung, C. Y., Funamoto, S., and Firtel, R. A. (2001)
Trends Biochem. Sci. 26,
557566[CrossRef][Medline]
[Order article via Infotrieve]
- Iijima, M., Huang, E., and Devreotes, P. N. (2002)
Dev. Cell 3,
469478[Medline]
[Order article via Infotrieve]
- Parent, C., and Devreotes, P. N. (1999)
Science 284,
765770[Abstract/Free Full Text]
- Devreotes, P. N., and Zigmond, S. H. (1988)
Annu. Rev. Cell Biol. 4,
649686[CrossRef][Medline]
[Order article via Infotrieve]
- Stephens, L., Ellson, C., and Hawkins, P. (2002)
Curr. Opin. Cell Biol.
14,
203213[CrossRef][Medline]
[Order article via Infotrieve]
- van Es, S., and Devreotes, P. N. (1999)
Cell. Mol. Life Sci. 55,
13411351[CrossRef][Medline]
[Order article via Infotrieve]
- Parent, C., and Devreotes, P N. (1996)
Annu. Rev. Biochem. 65,
411440[CrossRef][Medline]
[Order article via Infotrieve]
- Klinker, J. F., Wenzel-Seifert, K., and Seifert, R.
(1996) Gen. Pharmacol.
27,
3354[CrossRef][Medline]
[Order article via Infotrieve]
- Murphy, P. M. (1994) Annu. Rev.
Immunol. 12,
593633[CrossRef][Medline]
[Order article via Infotrieve]
- Maghazachi, A. A. (2000) Int. J. Biochem.
Cell Biol. 32,
931943[CrossRef][Medline]
[Order article via Infotrieve]
- Curnock, A. P., Logan, M. K., and Ward, S. G. (2002)
Immunology 105,
125136[CrossRef][Medline]
[Order article via Infotrieve]
- Funamato, S., Milan, K., Meili, R., and Firtel, R.
(2001) J. Cell Biol.
153,
795810[Abstract/Free Full Text]
- Li, Z., Jiang, H., Xie, W., Zhang, Z., Smrcka, A. V., and Wu, D.
(2000) Science
287,
10461049[Abstract/Free Full Text]
- Niggli, V., and Keller, H. (1997) Eur. J.
Pharmacol. 335,
4352[CrossRef][Medline]
[Order article via Infotrieve]
- Rickert, P., Weiner, O. D., Wang, F., Bourne, H. R., and Servant,
G. (2000) Trends Cell Biol.
10,
466473[CrossRef][Medline]
[Order article via Infotrieve]
- Hannigan, M., Zhan, L., Li, Z., Ai, Y., Wu, D., and Huang, C. K.
(2002) Proc. Natl. Acad. Sci. U. S. A.
99,
36033608[Abstract/Free Full Text]
- Hirsch, E., Katanaev, V. L., Garlanda, C., Azzolino, O., Pirola,
L., Silengo, L., Sozzani, S., Mantovani, A., Altruda, F., and Wymann, M. P.
(2000) Science
287,
10491053[Abstract/Free Full Text]
- Insall, R. H., and Weiner, O. D. (2001)
Dev. Cell 1,
743747[Medline]
[Order article via Infotrieve]
- Sasaki, T., Irie-Sasaki, J., Jones, R. G., Oliveira-dos-Santos, A.
J., Stanford, W. L., Bolon, B., Wakeham, A., Itie, A., Bouchard, D.,
Kozieradzki, I., Joza, N., Mak, T. W., Ohashi, P. S., Suzuki, A., and
Pennigner, J. M. (2000) Science
287,
4046[CrossRef]
- Sotsios, Y., and Ward, S. G. (2000)
Immunol. Rev. 177,
217235[CrossRef][Medline]
[Order article via Infotrieve]
- Wang, F., Herzmark, P., Weiner, O. D., Srinivasan, S., Servant, G.,
and Bourne, H. R. (2002) Nat. Cell Biol.
4,
513518[CrossRef][Medline]
[Order article via Infotrieve]
- Wymann, M. P., Sozzani, S., Altruda, F., Mantovani, A., and Hirsch,
E. (2000) Immunol. Today
21,
260264[CrossRef][Medline]
[Order article via Infotrieve]
- Aizawa, H., Sutoh, K., Tsubuki, S., Kawashima, S., Ishii, A., and
Yahara, I. (1995) J. Biol. Chem.
270,
1092310932[Abstract/Free Full Text]
- Clow, P. A., and McNally, J. G. (1999) Mol.
Biol. Cell 10,
13091323[Abstract/Free Full Text]
- Gerisch, G., Albrecht, R., De Hostos, E., Wallraff, E., Heizer, C.,
Kreitmeier, M., and Muller-Taubenberger, A. (1993)
Symp. Soc. Exp. Biol.
47,
297315[Medline]
[Order article via Infotrieve]
- Mishima, M., and Nishida, E. (1999) J. Cell
Sci. 112,
28332842[Abstract/Free Full Text]
- Weber, I., Neujahr, R., Du, A., Kohler, J., Faix, J., and Gerisch
G. (2000) Curr. Biol.
10,
501506[CrossRef][Medline]
[Order article via Infotrieve]
- Niggli, V. (2000) FEBS Lett.
473,
217221[CrossRef][Medline]
[Order article via Infotrieve]
- Weiner, O. D., Neilsen, P. O., Prestwich, G. D., Kirschner, M. W.,
Cantley, L. C., and Bourne, H. R. (2002) Nat. Cell
Biol. 4,
509513[CrossRef][Medline]
[Order article via Infotrieve]
- Zigmond, S. H. (1978) J. Cell
Biol. 77,
269287[Medline]
[Order article via Infotrieve]
- Gerisch, G. (1987) Annu. Rev.
Biochem. 56,
853879[CrossRef][Medline]
[Order article via Infotrieve]
- Parent, C., Blacklock, B., Froelich, W., Murphy, D., and Devreotes,
P. N. (1998) Cell
95,
8191[Medline]
[Order article via Infotrieve]
- Servant, G., Weiner, O. D., Herzmark, P., Balla, T., Sedat, J. W.,
and Bourne, H. R. (2000) Science
287,
10371040[Abstract/Free Full Text]
- Gerisch, G., and Malchow, D. (1976) Adv.
Cyclic Nucleotide Res. 7,
4968[Medline]
[Order article via Infotrieve]
- Caterina, M. J., and Devreotes, P. N. (1991)
FASEB J. 5,
30783085[Abstract/Free Full Text]
- Rappel, W. J., Thomas, P. J., Levine, H., and Loomis, W. F.
(2002) Biophys. J.
83,
13611367[Abstract/Free Full Text]
- Narang, A., Subramanian, K. K., and Lauffenburger, D. A.
(2001) Ann. Biomed. Eng.
29,
677691[CrossRef][Medline]
[Order article via Infotrieve]
- Meinhart, H. (1999) J. Cell
Sci. 112,
28672874[Abstract/Free Full Text]
- Postma, M., and Van Haastert, P. J. (2001)
Biophys. J. 81,
13141323[Abstract/Free Full Text]
- Levchenko, A., and Iglesias, P. A. (2002)
Biophys. J. 82,
5063[Abstract/Free Full Text]
- Iglesias, P. A., and Levchenko, A. (2002) Science's STKE
http://stke.sciencemag.org/cgi/content/full/oc_sigtrans;2002/148/re12
- Fisher, P. R. (1990) Semin. Cell
Biol. 1,
8797[Medline]
[Order article via Infotrieve]
- Jin, T., Zhang, N., Long, Y., Parent, C. A., and Devreotes, P. N.
(2000) Science
287,
10341036[Abstract/Free Full Text]
- Servant, G., Weiner, O. D., Neptune, E. R., Sedat, J. W., and
Bourne, H. R. (1999) Mol. Biol. Cell
10,
11631178[Abstract/Free Full Text]
- Xiao, Z., Zhang, N., Murphy, D. B., and Devreotes, P. N.
(1997) J. Cell Biol.
139,
365374[Abstract/Free Full Text]
- Ueda, M., Sako, Y., Tanaka, T., Devreotes, P., and Yanagida, T.
(2001) Science
294,
864867[Abstract/Free Full Text]
- Janetopoulos, C., Jin, T., and Devreotes, P. N. (2001)
Science 291,
24082411[Abstract/Free Full Text]
- Meile, R., Ellsworth, C., Lee, S., Reddy, T. B., Ma, H., and
Firtel, R. A. (1999) EMBO J.
18,
20922105[Abstract/Free Full Text]
- Lilly, P. J., and Devreotes, P. N. (1995)
J. Cell Biol. 129,
16591665[Abstract]
- Funamoto, S., Meili, R., Lee, S., Parry, L., and Firtel, R. A.
(2002) Cell
109,
611623[Medline]
[Order article via Infotrieve]
- Iijima, M., and Devreotes, P. N. (2002)
Cell 109,
599610[Medline]
[Order article via Infotrieve]
- Huang, E., Iijima, M., Funamoto, S., Firtel, R., and Devreotes, P.
N. (2003) Mol. Biol. Cell, in
press
- Wennstrom, S., Hawkins, P., Cooke, F., Hara, K., Yonezawa, K.,
Kasuga, M., Jackson, T., Claesson-Welsh, L., and Stephens, L.
(1994) Curr. Biol.
4,
385393[Medline]
[Order article via Infotrieve]
- Diakonova, M., Bokoch, G., and Swanson, J. A. (2002)
Mol. Biol. Cell 13,
402411[Abstract/Free Full Text]
- Amyere, M., Mettlen, M., Van, D., Platek, A., Payrastre, B.,
Veithen, A., and Courtoy, P. J. (2002) Int. J. Med.
Microbiol. 291,
487494[Medline]
[Order article via Infotrieve]
- Costello, P. S., Gallagher, M., and Cantrell, D.
(2002) Nat. Immunol.
3,
10821089[CrossRef][Medline]
[Order article via Infotrieve]
- Harriague, J., and Bismuth, G. (2002) Nat.
Immunol. 3,
10901096[CrossRef][Medline]
[Order article via Infotrieve]
- Welch, H. C., Coadwell, W. J., Ellson, C. D., Ferguson, G. J.,
Andrews, S. R., Erdjument-Bromage, H., Tempst, P., Hawkins, P. T., and
Stephens, L. R. (2002) Cell
108,
809821[Medline]
[Order article via Infotrieve]