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Ideas & Speculations
Insights & Perspectives
A mechanism of mechanotransduction
at the cell-cell interface
Emergence of a-catenin as the center of a force-balancing mechanism for morphogenesis
in multicellular organisms
Shigenobu Yonemura
Introduction
Cells can sense and respond to their
mechanical environment, resulting in
not only the function of sensory organs
for touch, sound or balance, but also
proper morphogenesis and differentiation during embryogenesis [1–3].
The conversion of mechanical forces
into biochemical signals is called
mechanotransduction.
Defects
in
mechanotransduction lead to diseases
such as sensory organ disorders, muscular dystrophy, atherosclerosis, osteoporosis, and polycystic kidney disease
[4, 5], and mechanotransduction is now
an emerging field in biology and medicine. Mechanotransduction has been
examined in a variety of cells or organs,
such as sensory cells, heart, blood
vessels, kidney, bone, and tumors,
and in the cell-extracellular matrix
adhesion structure (focal adhesion),
in which the extracellular matrix and
fluid are considered the mechanical
environment [6, 7]. In the morphogenesis of multicellular organisms, cells are
also important mechanical environment
elements because each cell is surrounded by adjacent cells. One type of
cell-cell junction structure, the desmosome, plays an important role in resisting mechanical stresses, and may also
work as a possible mediator of mechanical signals. Contractile forces generated
by a cell provide mechanical stimuli
both to itself and to neighboring cells
through the adherens junction (AJ), a
cell-cell junction structure responsible
for intercellular transmission of forces
[8–12]. Here, I describe a mechanism of
mechanotransduction at AJs based on
a-catenin functions, and discuss its significance and molecular mechanisms.
.
Keywords:
a-catenin; adherens junction; mechanotransduction; myosin II; vinculin
DOI 10.1002/bies.201100064
Electron Microscope Laboratory, Riken Center
for Developmental Biology, Kobe, Japan
*Corresponding author:
Shigenobu Yonemura
E-mail: [email protected]
732
www.bioessays-journal.com
Abbreviations:
AJ, adherens junction; ZA, zonula adherens; PA,
punctum adherens.
Mechanotransduction
in general
Mechanotransduction
has
been
examined at the molecular level, especially in sensory cells, with a focus on
mechanosensitive ion channels [3].
Mechanical forces applied to sensory
cells induce membrane depolarization
through the opening of ion channels.
It is also known that at focal adhesion
sites, recruitment or activity of adaptor
proteins, protein kinases or actin filaments can be promoted depending on
forces applied to it. These forces are
related to the rigidity of the extracellular
matrix, and modulate migration, proliferation and fate determination of cells
[7, 13]. In these cases, mechanotransduction starts locally, but its effects
are transferred throughout the cells,
including the nucleus, through phosphorylation cascades or diffusion of second messengers such as calcium ions
that activate complex signaling pathways and elicit global responses. It is
also possible that forces are transmitted
directly to the nucleus through the actin
cytoskeleton, leading to a modification
of transcription within the nucleus [14].
In morphogenesis involving multiple
cells, especially epithelial sheets, a
designed shape such as a tube, ball or
cup cannot be formed without proper
intercellular transmission of forces
generated by each cell through AJs or
coordinated contractility in a specific
group of cells. Therefore, cells would
need to regulate the reinforcement of
Bioessays 33: 732–736,ß 2011 WILEY Periodicals, Inc.
.....
Insights & Perspectives
S. Yonemura
Ideas & Speculations
AJs and contractility by sensing applied
forces. As focal adhesions are thought to
be capable of sensing the rigidity of the
extracellular matrix [15], AJs are likely to
be the stage where mechanotransduction at cell-cell interfaces takes place.
Adherens junction:
Molecular details suggest
that mechanotransduction
is mediated via cadherin
The AJ is a type of cell-cell junction
associated with actin filaments, and it
plays a central role in transmitting cellular contractile forces generated by
actin filament/myosin II interactions
to neighboring cells [8, 9, 11]. It does
this by intercellular interaction of its
adhesion molecule, cadherin, and by
intracellular linkage of cadherin to actomyosin. The extracellular regions of the
membrane protein cadherin, which
extend out between two adjacent cells,
bind to each other at the AJ. The cytoplasmic region of cadherin binds to
b-catenin. b-Catenin then in turn binds
to a-catenin, resulting in the formation
of the cadherin-catenin complex.
a-Catenin is thought to be essential
for linking cadherin to actin filaments
for several reasons: it has an actin filament-binding region at its C terminus; it
can bind to a variety of actin-binding
proteins such as vinculin, ZO-1, AF6/
afadin/canoe, or eplin; and because loss of a-catenin disrupts the cadherinactin linkage, resulting in defects in
AJ formation or position [8, 9, 11]. The
mode of association of actin filament
bundles with the AJ membrane can be
divided largely into two categories. One
type of AJ called punctum adherens (PA)
has an actin filament bundle oriented
perpendicularly to the plasma membrane (Fig. 1A and C). The other, called
zonula adherens (ZA), has an actin filament bundle oriented parallel to the
plasma membrane (Fig. 1B and D).
These two types of AJs are intimately
related because the PA often develops
into the ZA during the process of junction maturation in epithelial cells. In the
PA, actin bundles of adjacent cells are
aligned in a straight line, suggesting
that considerable tension is applied to
the bundles connected at the AJ. The ZA
circumscribes each cell together with
Figure 1. Structures of adherens junctions (AJs) and a model of mechanotransduction at the
AJ. Immunofluorescence micrographs (A, B) and models illustrating the mode of association
of actin filaments with AJs (C, D). Arrows show actin filament orientations. In the punctum
adherens (PA), actin filament bundles are associated with the plasma membrane perpendicularly (A, C). NRK cells are stained for P-cadherin (magenta) showing the position of PAs and
actin filaments (green). In the zonula adherens (ZA), actin filament bundles run parallel to the
plasma membrane (B, D). EpH4 cells are stained for vinculin (magenta) showing the position
of ZAs and actin filaments (green). Bar, 10 mm. Simplified molecular constituents of AJs (E,
F). Under low tension, the vinculin-binding region of a-catenin is masked by its inhibitory
region (E). Under high tension, a-catenin is stretched both from the adjacent cell through
cadherin binding and from the associated actin filament pulled by myosin II, unmasking the
vinculin-binding region. Orientations of forces are shown in arrows. The N and C termini of
a-catenin are shown as ‘‘N’’ and ‘‘C,’’ respectively.
associated actin bundles near the apical
region in a polarized epithelial cell
sheet. Contours of the lateral membranes are linear at the ZA region and
are curved or zigzagged at the middle
and basal regions, which also suggest
that intercellular forces are applied
mainly at the ZA in epithelial cell sheets.
Is the AJ a mechanosensitive cellular
structure? Myosin II activity reportedly
regulates cadherin-catenin functions.
Knocking out myosin II-A disrupts cell
adhesion due to a loss of E-cadherin at
cell adhesion sites [16]. Experiments
using tissue culture cells show that
the area of cadherin-mediated adhesions between two cells increases,
depending on cadherin-catenin binding, when the strength of myosin IIderived tugging forces between the
two cells is increased [17]. Further-
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more, studies using cadherin-coated
beads or surfaces have revealed that
the strength of cadherin-mediated
adhesion is regulated by these forces
[18, 19]. Compared with focal adhesions,
however, the appearance/disappearance of specific molecules that respond
to forces quickly have not been detected
in AJs until recently, preventing further
examination at the molecular level.
Molecular mechanism
of mechanotransduction
at AJs: Stretching of
a-catenin at AJs reveals
the binding site for vinculin
To analyze mechanotransduction at AJs
molecularly, we should identify a cen-
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Insights & Perspectives
Ideas & Speculations
S. Yonemura
Figure 2. Possible molecular mechanisms for mechanotransduction at the AJ, focusing on
a-catenin-actin interaction. A: The structure and functional domains of a-catenin, showing bcatenin, vinculin, and F-actin (actin filament) binding regions and an inhibitory region for vinculin binding. The vinculin-binding region is masked by the inhibitory region and can be
unmasked when the C terminus is pulled by actomyosin forces. B: Free a-catenin (monomer
and/or dimmer) contains the C-terminal actin-binding region with relatively high affinity for Factin. Although free a-catenin reportedly binds to vinculin in vitro, a-catenin is never found in
focal adhesions where vinculin accumulates. C: Upon binding to the cadherin-b-catenin
complex, a-catenin loses its high affinity for F-actin. Simultaneously, the vinculin-binding
region becomes masked by the inhibitory region unless a-catenin is stretched. D: The existence of actin-binding proteins associated with the C terminus of a-catenin can explain the
cadherin-actin linkage and the force-dependent conformational changes of a-catenin. E: A
possible idea that the C terminus of a-catenin in the cadherin-catenin complex can transiently bind to F-actin only when it is pulled in a ratchet-like manner. F: When a-catenin is
stretched by actomyosin forces, the vinculin-binding region is unmasked and the recruited
vinculin then serves as an additional F-actin-binding site to reinforce the cadherin-actin linkage. This stretching might simultaneously change the conformation of the C terminus, resulting in higher affinity to F-actin.
tral molecule that senses forces and biochemical signals produced through
sensing forces. Vinculin is an actinbinding protein that accumulates at
both focal adhesions and AJs. Its recruitment to focal adhesions is force dependent, and more recently, vinculin
recruitment to AJs was also found to
be force dependent; reducing myosin
II-derived forces causes vinculin to disappear at AJs and increasing local myosin II-derived forces causes vinculin to
accumulate at AJs where the forces are
applied [20–22]. Furthermore, magnetic
twisting cytometry revealed that vinculin potentiates E-cadherin mechanosensing [21]. Since vinculin is an actin-
734
binding protein required for early development and formation of PAs in cardiac
muscles [23], its recruitment should
increase the number of actin filaments
associated with AJs, resulting in a structural and functional strengthening of
AJs. Thus, recruitment of vinculin is a
typical response to a mechanical
stimulus to AJs and is likely involved
in mechanotransduction process at AJs.
Because it is already known that acatenin binds to vinculin and that it is
required for vinculin accumulation at
AJs, the a-catenin molecule has recently
become a focus of close examination to
unravel the mechanism of mechanotransduction at AJs [22]. Molecular dis-
.....
section of a-catenin by cell biological
and biochemical analyses revealed that
it has a vinculin-binding region in the
central part of the molecule and also a
region inhibiting this binding (Fig. 2A).
The C-terminal actin-binding region of
a-catenin, actin filaments, myosin IIderived forces, and cadherin homophilic binding are required to release
this inhibition within cells [22],
suggesting that vinculin binding to acatenin is force dependent and that acatenin is probably stretched to unmask
the vinculin-binding region (Fig. 1E and
F). Although there are no data showing
that a-catenin in the cadherin-catenin
complex within cells binds directly to
actin filaments, a-catenin is likely being
stretched through actomyosin forces
because even when its actin-binding
C-terminal region was exchanged with
an actin-binding region of another
actin-binding protein, vinculin, the chimeric molecule showed a clear force
sensitivity in vinculin recruitment [22].
Furthermore, a monoclonal antibody to
a-catenin, a18, binds preferentially to acatenin when the vinculin-binding
region is unmasked [22]. This suggests
that the antibody recognizes the ‘‘open’’
conformation of a-catenin.
So a molecular mechanism of
mechanotransduction at Ajs may be
the stretching of a-catenin at Ajs, which
reveals the binding site for vinculin that
localizes to AJs in a stretch-dependent
manner.
Significance of
a-catenin as a key
mechanotransducer at
AJs: Transmission of
precise temporal,
quantitative and spatial
information without
diffusible factors
The recently revealed mechanotransduction at AJs shows a distinct characteristic compared with those often found
in other systems, and leads to an understanding of the development of AJ at a
precise position on the cell-cell interface. As described above, a-catenin is
involved in the linkage between cadherin and actin, playing roles not only
as a mechanosensor but also a mecha-
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Insights & Perspectives
adjacent cells, the cell would recognize
adjacent cells as a similar type of living
cells and make functional junctions to
form and maintain a cell sheet.
Further issues to be solved:
Regulation of actin filament
binding and detailed
understanding of the
mechanotransduction
The current model can thus expand our
understanding of mechanotransduction
at AJs. However, there are still unsolved
issues especially on how actin filaments
are involved. Although free a-catenin
can bind to actin filaments, direct binding of a-catenin in the cadherin-catenin
complex and actin filaments has never
been detected in vitro [24, 25]
(Fig. 2B and C). This clearly indicates
that the actin-binding ability of a-catenin at its C terminus is highly suppressed upon binding to the cadherinb-catenin complex. For the a-catenin
molecule to be stretched by actin filaments, it may be linked indirectly to
actin filaments through actin-binding
proteins bound to a-catenin such as
eplin [26] (Fig. 2D). Alternatively, a-catenin in the cadherin-catenin complex
may be able to bind to actin filaments
under a special condition that cannot be
reproduced in ordinary binding assays
in vitro. This is a reasonable consideration because the rapid remodeling of
AJs with their associated actin filaments
seen during development and wound
repair would not be easily accomplished
if cadherin-actin linkage is stable.
Endocytosis and recycling of cadherin
through vesicle transport and fusion
would be also inhibited by tightly
associated actin filaments. It is therefore
essential that the cadherin-actin interaction be dynamic. As described previously [27], a-catenin in the complex
may create an efficient linkage with an
actin filament transiently only when the
filament pulls the a-catenin in a ratchetlike manner (Fig. 2E). Stretched a-catenin may then release the suppression of
the actin-binding ability of the C terminus (Fig. 2F). These ideas should be
tested by in vitro binding assays where
the dynamic association of a-catenin
and the actin filament is considered.
Biophysical rather than biochemical
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methods would be required to detect a
dynamic interaction.
The current model explains AJ development in a force-dependent manner.
However, we do not have enough quantitative data on the forces applied to AJs
and the position of AJs to understand
the regulatory mechanism in detail.
Further analysis incorporating live
imaging with high temporal and spatial
resolution [28] will also be necessary to
understand this mechanism. The molecular details of this mechanotransduction should also be examined by
structural analyses.
Conclusion
Recent studies on mechanotransduction
at cell-cell interfaces revealed that local
sensing and local, but not global,
responses lead to reinforcement of
cell-cell junction structures responsible
for force transmission, probably to
maintain cell-cell adhesion and force
balance within cell sheets. This mechanotransduction uses the a-catenin molecule involved in the junction structure
as a central mechanotransducer.
Although intercellular mechanotransduction may also elicit global cellular
responses such as gene expression, the
local responses will likely be revealed as
important events for understanding of
morphogenesis, including embryogenesis, organ formation and tissue repair.
Acknowledgments
I thank Hazuki Hiraga for proof reading.
This work is supported by Grants-in-Aid
for Scientific Research (C) (22570195)
and Scientific Research on Innovative
Areas (23111534) from the Ministry of
Education, Culture, Sports, Science
and Technology of Japan.
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