SPOTLIGHT REVIEW
Cardiovascular Research (2012) 95, 215–222
doi:10.1093/cvr/cvs168
Mechanical regulation of cellular phenotype:
implications for vascular tissue regeneration
Wendy F. Liu*
Department of Biomedical Engineering and the Edwards Lifesciences Center for Advanced Cardiovascular Technology, University of California, 2412 Engineering Hall, Irvine,
CA 92697-2730, USA
Received 23 January 2012; revised 14 May 2012; accepted 16 May 2012; online publish-ahead-of-print 23 May 2012
Abstract
Cells sense a myriad of cues from their surrounding microenvironment to regulate their function. In recent years, it
has become clear that physical and mechanical cues are as critical as biochemical factors in regulating cellular function.
The geometry of the extracellular matrix (ECM), degree of cell spreading, and ECM rigidity all influence the physical
connection between cells and their microenvironment and play a major role in regulating proliferation, differentiation,
and migration. Leveraging these findings to promote specific cell behaviours will be paramount to realize the full potential of cellular therapies. In this review, I examine our current understanding of how mechanical cues—specifically,
geometric control of cell shape and matrix rigidity—are transduced by stem cells to control their stemness, proliferation, and differentiation. The implications of these findings for vascular smooth muscle cell differentiation and cardiovascular tissue engineering will be highlighted.
----------------------------------------------------------------------------------------------------------------------------------------------------------Keywords
Stem cells † Vascular smooth muscle † Mechanotransduction † Micropatterning † Rigidity
----------------------------------------------------------------------------------------------------------------------------------------------------------This article is part of the Spotlight Issue on: Smooth Muscle Cells and Vascular Diseases
1. Introduction
Vascular tissue engineering remains a promising solution for replacing
diseased blood vessels with healthy functional tissue.1,2 This approach
typically combines cells with a physical scaffold onto which the
cells can expand and grow into the appropriate tissue structure.3
However, obtaining an appropriate and available cell source remains
a major challenge in the field. Recent developments in stem cell
biology have opened up the possibility of using adult and embryonic
stem cells as a potential source of cells for tissue engineering applications.4 – 6 Stem cells are capable of expanding in culture while maintaining the ability to differentiate into multiple distinct lineages.
Moreover, inducing pluripotency from adult cell sources may circumvent issues associated with immunogenicity and transplant rejection.7
Stem cells could thus provide an abundant source of endothelial,
smooth muscle, and adventitial cells needed to engineer artificial vascular constructs.
In order to make stem cells a therapeutic reality, much needs to be
understood about how to regulate their expansion and differentiation.
Traditional studies have focused on the role of soluble factors, such as
growth factors or cytokines, as well as genetic regulators including
transcription factors. However, a growing body of evidence now suggests that physical cues in the cellular microenvironment also play
a critical role in regulating stem cell lineage commitment. Lessons
can often be learned from examining the mechanical cues present
during development, homeostasis, and disease. For example, shear
stress and mechanical stretch imparted by pulsating blood flow are
sensed by the endothelium and underlying smooth muscle layers
and are known regulators of the normal physiological function as
well as embryonic development of the cardiovascular system.8 – 11 In
addition, the biophysical properties of the tissue itself contribute to
vascular physiology, and disease processes such as atherosclerosis
are associated with changes in the physical tissue microenvironment.12 – 14 In the adult organism, many cell types including vascular
smooth muscle cells display remarkable plasticity and can undergo
phenotypic changes depending on their environment. In normal,
healthy vasculature, smooth muscle cells remain quiescent and
highly contractile; however, during vascular injury, smooth muscle
cells adopt a proliferative and non-contractile phenotype. Moreover,
differentiation of smooth muscle cells from adult stem cells contributes to vascular regeneration. Thus, understanding how biophysical
factors regulate differentiation and phenotypic switching of vascular
smooth muscle will help define the microenvironmental cues that
are necessary to direct stem cell fate. Incorporating mechanical cues
along with biochemical stimuli will likely be necessary to achieve
optimal cell phenotypes for vascular tissue engineering applications.
* Corresponding author: Tel: +1 949 824 1682; fax: +1 949 824 9968. Email: [email protected]
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2012. For permissions please email: [email protected].
216
On the cellular scale, mechanotransduction—the transduction of
physical forces into biochemical signals—leads to changes in signalling
pathways that control a variety of cellular behaviours including proliferation, differentiation, and migration. While force and function have
been linked for many decades, the molecular mechanisms underlying
these processes are only currently being unravelled. The number of
studies investigating how mechanical cues regulate biological function
at the cellular and molecular level has swelled enormously in the past
several decades, in part due to the numerous technological advances
that enable the control and measurement of forces at a scale relevant
to cells. Flow chambers, flexible cell culture substrates, micromechanical devices, patterned surfaces, and optical tweezers have been
developed to deliver and sense cellular forces at the pico- to nanoNewton range.15 – 20 Using such bioengineered tools, it is now clear
that cells are intricately linked to their physical environment
through mechanosensitive proteins and that forces are transmitted
through cytoskeletal structures, which eventually lead to alterations
in intracellular signalling pathways and gene expression patterns.
The focus of this review article will be to critically examine recent
findings demonstrating the regulation of stem cell fate by mechanical
cues in the cell microenvironment. I will describe work demonstrating
how the spatial presentation of adhesive cues as well as the rigidity of
the extracellular environment regulate stem cell expansion and commitment to different lineages. Where possible, I will highlight how
similar biophysical cues have been shown to regulate the vascular
smooth muscle phenotype. While the role of haemodynamic forces
is significant, the reader is directed to other excellent reviews that
cover this topic.21 A better understanding of how physical and mechanical cues regulate stem cell biology and vascular smooth muscle
phenotype will ultimately aid in the development of new strategies
for vascular smooth muscle regeneration and tissue engineering.
2. Regulation of cell phenotype
by shape
Cell shape has long been known to play a crucial role in cell physiology.22 During development, cell shape changes often precede differentiation for a number of different cell types. For smooth muscle cells,
it has been observed that the onset of cellular elongation is immediately followed by the production of smooth muscle-specific proteins.23 Using polycarbonate surfaces with topographies that
stimulated cell rounding or elongation, Yang et al.24 demonstrated
that rounded mesenchymal stem cells remained undifferentiated
while elongated cells expressed smooth muscle markers and exhibited smooth muscle-like calcium signalling. Treatment with soluble stimulators of smooth muscle differentiation could only occur when
cells were permitted to elongate, suggesting that mesenchymal cell
shape is a downstream and critical regulator of myogenesis.
Initial studies investigating the multilineage potential of mesenchymal stem cells focused on the effects of soluble mediators, but the differentiation procedures required cells to be cultured in very specific
architectures in order to enhance differentiation potential.25 Stem
cells were cultured within sparse, confluent, or pelleted cultures,
which was a requirement for the efficient commitment of cells to different lineages. It was not until micropatterning tools were applied to
specifically control cell adhesion and spreading when it became clear
that cell shape itself can influence stem cell commitment. Using surfaces with discrete islands of adhesive proteins to control the
W.F. Liu
degree of spreading, McBeath et al.26 demonstrated that mesenchymal
stem cells that were spread onto large islands of extracellular matrix
(ECM) tended to differentiate into bone. In contrast, cells seeded on
small islands of ECM which were restricted from spreading tended to
differentiate into fat. These effects were dependent on intracellular
contractility and RhoA signalling, since expression of a dominant negative mutant of the protein led to fat differentiation while expressing a
constitutively active form of RhoA lead to differentiation into bone,
independent of the soluble factors present.
These studies suggest that intrinsically generated mechanical forces
play a role in the regulation of cell behaviour by cell shape. Such
forces were first shown by culturing cells on soft polymer substrates,
which wrinkled upon cell contraction.27 This technique was later
extended to include fluorescent beads embedded within the
polymer in order to measure cell traction forces.18 Cellular contraction leads to the bead displacement, which can be measured to calculate the amount of forces generated by cells. Taking a microfabricated
approach, Tan et al.16 demonstrated that cells cultured on top of a
microarray of needles deflect the needles depending on the amount
of force generated. Using such bioengineered tools, it has been
demonstrated that cell shape, including the degree of spreading as
well as isotropy, contributes to intracellular contractile force generation.28 Increases in the degree of cell spreading correlate directly
with an increase in traction forces, and abrogation of cytoskeleton
abolishes shape-dependent effects on force.
In addition to the degree of cell spreading, anisotropy and cell
curvature has also been shown to provide a geometric cue to
direct stem cell differentiation. Two separate groups have recently
shown that cells spread on rectangular-shaped islands exhibited a
preferential differentiation to bone, when compared with cells
spread on squares of the same area.29,30 Moreover, cells that were
engineered to have more sharp edges in a star shape differentiated
into bone, whereas cells with concave edges that were spread to
the same degree differentiated into fat. These effects were associated
with an increase in myosin II expression and thus intracellular contractility. Genomic analysis suggested that MAPK and Wnt signalling
were downstream effectors of cell shape and important regulators
of differentiation. This work demonstrated that even subtle changes
in cell geometry may have a remarkable impact on the stem cell
phenotype.
For the control of smooth muscle cells, it appears that elongation
of cell shape is also a critical determinant of the phenotype. Using
micropatterned substrates to control the adhesion of vascular
smooth muscle cells in vitro, Alford et al.31 demonstrated that cellular
elongation helps to maintain the smooth muscle phenotype. Culture of
vascular smooth muscle cells on patterned lines of ECM caused cells
to elongate. Cell and nuclear morphology correlate with contractile
force output: cells on 20 mm wide lines exhibited the most elongated
cell bodies and nuclei and exerted the highest amount of force
when compared with cells cultured on wider (up to 180 mm) lines
(Figure 1A and B). In a complementary approach, smooth muscle
cells cultured on microfabricated grooves also elongate along the direction of the groove.32 This surface geometry allows the smooth
muscle cells to form sheets, similar to what is observed in vivo. In
this configuration, smooth muscle cells generated similar amounts
of ECM protein, and the failure stress and stiffness of the elongated
cells was significantly higher than that of unpatterned cells. Changes
in contractile force have been associated with an increase in smoothelin and smooth muscle myosin heavy chain expression, suggesting
217
Mechanotransduction and cell phenotype
Figure 1 Microfabricated techniques are used to demonstrate a role for cell elongation in vascular smooth muscle cell function. (A) Schematic of
technique to micropattern cells. (B) Vascular smooth muscle cells patterned onto lines of various widths (20– 180 mm) demonstrated a higher degree
of cell contraction with increasing nuclear eccentricity. Adapted from Alford et al. with permission of The Royal Society of Chemistry.31 (C) Schematic
of technique to culture cells on microfabricated grooves. (D) Sheets of vascular smooth muscle cells were cultured on microfabricated grooves and
showed reduced proliferation with elongation. Adapted from Thakar et al. with permission from Elsevier.33
a link between cell shape, the cytoskeleton, and the nucleus for the
regulation of the vascular smooth muscle phenotype. Another study
by Thakar et al.33 demonstrated that cell and nuclear elongation
caused a decrease in vascular smooth muscle cell proliferation;
however, these investigators showed that the effects were independent of changes in spread cell area and smooth muscle contractile
markers (Figure 1C and D). Instead, a decrease in neuron-derived
orphan receptor-1 (NOR-1), a transcription factor known to be
involved in vascular remodelling, was involved.
Taken together, these studies suggest that cell shape, and in particular cellular elongation, is a major regulator of stem cell differentiation
and the vascular smooth muscle phenotype. Geometric patterning of
adhesion or topology of scaffolds may be a powerful way to encourage stem cell differentiation towards vascular smooth muscle lineages.
Indeed, a recent study demonstrated that strain in combination with
microfabricated grooves together encouraged differentiation of
neural crest stem cells to a smooth muscle lineage, as indicated by
an increase in expression of smooth muscle myosin.34 While these
initial results are promising, a greater fundamental understanding of
how surface topographies and cell shape regulate differentiation
towards a smooth muscle lineage will undoubtedly be needed in
order to develop successful strategies for engineering vascular tissues.
3. Regulation of cell phenotype
by the rigidity of the
microenvironment
To study cells in vitro, biologists have traditionally cultured cells on
very stiff plastic or glass substrates. However, cells in the body exist
within environments of varied mechanical properties, which is now
known to be a critical regulator of cellular behaviour. Leveraging a
previously developed technique that utilized polyacrylamide gels to
modulate the mechanical properties of the underlying substrate,35
Engler et al.36 demonstrated in a seminal study that mesenchymal
stem cells (MSCs) cultured on substrates of varied rigidities become
committed towards different lineages. By examining cell-type-specific
markers and genome-wide expression patterns, they found that MSCs
cultured on very soft substrates (0.1– 1 kPa) tended to differentiate
into neurons, whereas cells cultured on very stiff (25–40 kPa)
tended to differentiate into bone. Cells cultured on substrates of
intermediate stiffness (8–17 kPa) differentiated into muscle cells.
Interestingly, the mechanical properties of the substrate recapitulated
the properties of the native tissue to which the cells differentiated.
Blocking non-muscle myosin II inhibited rigidity-directed cell
218
W.F. Liu
Figure 2 Substrate rigidity regulates vascular smooth muscle phenotype. (A) Vascular smooth muscle cells cultured on PEG-fibrinogen gels varying
from 448 to 5408 kPa show a modest increase in F-actin with increasing substrate stiffness. Adapted from Peyton et al.45 with permission from Elsevier.
(B) On a stiffer range of polyacrylamide surfaces (19– 84 kPa), vascular smooth muscle cells exhibited increased cell area and proliferation with increasing substrate stiffness. Adapted from Brown et al. with permission from John Wiley and Sons.47
commitment, suggesting that the physical microenvironment is linked
to stem cell function through mechanisms involving intracellular contractility. Although previous work had demonstrated that matrix rigidity plays a key role in the maintenance of the phenotype for numerous
cell types including neurons37 and muscle cells,38 this study was the
first to show a role for substrate rigidity on pluripotent stem cell
lineage commitment.
More recently, these findings were extended from two-dimensions
to three-dimensions by Huebsch et al.,39 who cultured stem cells
within alginate hydrogels modified with the integrin-binding peptide
RGD to foster cell adhesion. In agreement with previous work, this
study confirmed that murine mesenchymal stem cells tended to differentiate into fat when cultured within soft hydrogels (2.5 –5 kPa) and
into bone when cultured within stiff hydrogels (11–30 kPa) in threedimensional culture. In contrast to the studies performed on twodimensional flat substrates, which showed a direct correlation
between cell shape and matrix rigidity, cells cultured within threedimensional gels of varied stiffness all exhibited relatively similar cell
and nuclear geometries. Rather, the authors demonstrated a biphasic
relationship between matrix rigidity and integrin clustering and
binding. Moreover, the disruption of integrin binding using blocking
antibodies abrogated stiffness-dependent lineage commitment. The
clustering of cell adhesion proteins and the strength of
integrin-cytoskeletal linkage have indeed been shown to be regulated
by mechanical force.17,40,41 While alginate gels modified with the
integrin-binding peptide RGD provide a useful model threedimensional culture system that circumvents issues associated with
enzymatic degradation of matrix proteins, translation of these findings
to physiologically relevant ECM conditions remains to be examined.
Substrate rigidity not only provides signals that regulate stem cell
commitment and differentiation, but also their expansion and maintenance of stemness.42,43 When muscle stem cells, or adult myogenic
precursor cells that reside within muscle tissue, are cultured on
hydrogel substrates mimicking the mechanical properties of muscle
tissue (12 kPa), they exhibit a greater capacity to self-renew when
compared with cells cultured on very stiff or very soft hydrogels.42
Furthermore, the cells expanded on the gels of medium compliance
are better able to contribute to muscle regeneration after
Mechanotransduction and cell phenotype
219
Figure 3 Mechanotransduction from the ECM to the cell nucleus. (A) The actin cytoskeleton links focal adhesion at the cell membrane to the
protein complexes located on the nuclear membrane. Adapted from Eyckmans et al.73 and Wang et al.61 (B) Focal adhesions are sites of integrin
clustering and adhesion to ECM. (C) Actin is bound to nestin proteins, which are directly connected to the KASP/SUN complex located on the
nuclear membrane. Nuclear shape changes are associated with alterations in the structure of nuclear lamins and chromatin. (D) Localization of transcription factor YAP is dependent on spread cell area. Adapted from Dupont et al. by permission from Macmillan Publishers Ltd.69
transplantation into a mouse. This study was the first to demonstrate
a therapeutically useful consequence of culturing cells within a defined
mechanical environment in an animal model. In a different study, it was
shown that the mechanical properties of the underlying substrate
regulate the expansion of haematopoietic stem and progenitor cells.
Bone marrow-derived stem cells were cultured on tropoelastin, a naturally derived elastic protein, which provided signals for the expansion
of haemopoietic stem and progenitor cells leading to expansion of undifferentiated cells.44
Thus, matrix rigidity plays a pivotal role in maintaining pluripotent
stem cell expansion and commitment to various lineages. Moreover,
matrix stiffness can also regulate the phenotype of vascular smooth
muscle cells. Peyton et al.45 showed that the regulation of vascular
smooth muscle contractility is dependent on substrate rigidity.
Using gels composed of polyethylene glycol and fibrinogen, it was
demonstrated that smooth muscle cell contractility was directly correlated with the stiffness of their surroundings, as measured by the
visualization of F-actin (Figure 2A) as well as the expression of contractile markers including a-actin and calponin. Interestingly, the proliferation of smooth muscle cells appeared to be independent of
substrate stiffness. In contrast, other groups have demonstrated that
the proliferation of smooth muscle cells increases with the stiffness
of the substrate using dextran/gelatin-based hydrogel46 and polyacrylamide gels47 (Figure 2B). While these in vitro studies have suggested
220
that a proliferative phenotype and a contractile phenotype are both
enhanced by increases in substrate stiffness, it is unclear how these
results translate to a physiological role for stiffness as these distinct
phenotypes are typically mutually exclusive. Thus, further studies
will be needed to better understand how stiffness regulates smooth
muscle behaviour in vivo.
Although it is evident that substrate stiffness is critical in the regulation of both stem cell and vascular smooth muscle biology, it has yet
to be demonstrated how stiffness might be leveraged to enhance stem
cell differentiation towards vascular smooth muscle lineages. Scaffolds
designed to have elastic moduli that could enhance stem cell proliferation and then differentiation towards a contractile vascular smooth
muscle phenotype will ultimately be required.
4. Molecular signals are transduced
from mechanical signals
Based on the growing body of evidence described above, it is clear that
biophysical signals present in the cellular microenvironment, including
geometric cues that regulate cell shape, ECM rigidity, and applied mechanical forces, all play a major role in modulating the cell phenotype.
However, the molecular mediators that regulate how these mechanical
cues are transduced to biochemical signalling events and thus changes in
cell behaviour are still currently being unravelled. At the surface of the
cell, numerous receptors have been shown to exhibit mechanosensitivity (Figure 3A and B). Deformations of the plasma membrane lead to activation of stretch-activated ion channels and changes in ion flux.48 The
glycocalyx and growth factor receptors can also be directly impacted by
mechanical forces, leading to changes in downstream signalling
events.49,50 However, perhaps the most thoroughly investigated cell
surface receptors implicated in mechanotransduction are integrins,
which cluster to form focal adhesions and tether the intracellular
actin cytoskeleton to the ECM. Focal adhesions are not only important
for the formation of cohesive tissue structure, but are also critical for
cells to sensing their physical environment.51,52
Experiments using micropipettes or magnetic microparticles coated
with ECM ligands have shown that deformations of integrins lead to
conformational changes that unmask the cytoplasmic tail promoting
actin binding and the formation of large multi-protein complexes
called focal adhesions.17,40,53,54 These complexes include talin,
which binds directly to actin, and vinculin, which binds actin as well
as the actin cross-linking protein a-actinin. In addition, integrincontaining focal adhesion complexes also recruit scaffold and signalling proteins including paxillin and focal adhesion kinase (FAK).55,56
Formation of focal adhesions requires mechanical force, and accumulating evidence suggests that the composition, size, and structure of
focal adhesion is dependent on the mechanical properties of the
matrix as well as the contractility of the cell.35,57 Cells cultured on
softer surfaces typically contain smaller and more dynamic adhesions
when compared with cells on stiffer surfaces, which have larger and
more stable adhesions. In three-dimensional culture, adhesive structures are more similar to those of cells on soft surfaces.58 Formation
of larger adhesion leads to increased actin polymerization, stress fibre
formation, and actin– myosin contractility.
In addition to being a structure upon which force can be exerted
and sensed, focal adhesions are a major hub for signalling molecules
including FAK, MAPK proteins, Src, and Ras and Raf. As such,
changes in intracellular signalling pathways are a direct consequence
W.F. Liu
of changes in the adhesive environment. Central to the regulation
of both adhesion dynamics and the actin cytoskeleton organization
is the Rho GTPase family of signalling proteins, the activity of which
modulates cell proliferation, differentiation, and migration.59 Rho signalling modulators including guanine nucleotide exchange factors
and GTPase-activating proteins are recruited to and regulated by
focal adhesions. Interestingly, manipulation of Rho signalling has
been shown to modulate shape and stiffness effects on stem cell
commitment.26,60
5. Transmission of mechanical
signals to the nucleus
How mechanical forces are transmitted to the nucleus to exert
changes in gene expression and thus cell behaviour is currently an
intense area of research.61 Changes in nuclear shape induced by
microenvironmental cues, such as the reduction of size observed
during cell crowding or cellular compaction, have long been observed,
but the effects of these changes on nuclear function remain to be
understood. The shape of the nucleus appears to directly correlate
with the shape of the cell in most instances: as cells become more
spread or constrained, their nuclear area and volume also expand
and contract accordingly. Structurally, the nucleus is directly connected to the actin cytoskeleton through nestin proteins, which
bind to Klarsicht-Anc-Sine1 homology (KASH) and Sad1-UNC-84
homology (SUN) domains present on the nuclear membrane
(Figure 3C).62 Moreover, the KASH/SUN complex is connected structurally to proteins within the nucleus. Analogous to the cytoskeleton,
the interior of the nucleus contains a meshwork of structural proteins
including intermediate filaments and nuclear lamins.63 Dynamic
changes in the structure and interactions of these proteins with chromatin may indeed modulate the chromatin structure, unwinding, and
interactions with DNA-binding proteins.64
Nuclear shape changes and stresses exerted on nuclear lamins are
thought to alter the chromatin structure, thus controlling gene expression patterns. Several recent studies have demonstrated that
elongation of the nucleus as induced by geometry or surface topology
results in changes in the clustering of intranuclear lamins as well as
histone acetylation.65 – 67 These structural changes were associated
modifications in cellular behaviours including cell proliferation. Although the mechanisms are not fully understood, it is likely that the
compression or expansion of the nucleus may directly lead to exposure of transcription binding sites or DNA regulatory motifs and thus
control the binding of DNA-associated proteins and changes in gene
transcription. Moreover, it is thought that compressive physical forces
caused by aligned actin cytoskeletal filaments are required. As molecular tools and imaging techniques to probe the cell nucleus
become more advanced,68 many of the unanswered questions
about how forces are transduced and influence on the nuclear structure and function will begin to be understood.
Recent experiments have begun to elucidate how mechanical
signals lead to changes in gene expression. Most strikingly, an important role for the Yorkie-homologues YAP (Yes-associated protein) and
TAZ (transcriptional coactivator with PDZ-binding motif) in relaying
mechanical signals by cell shape and substrate rigidity to the nucleus
has been identified.69,70 Changes in cell morphology induced by cell
density, micropatterning, or matrix stiffness were associated with
the modulation of Yap phosphorylation and localization to the
Mechanotransduction and cell phenotype
nucleus as well as its distribution within the nucleus (Figure 3D). In well
spread cells and cells cultured on stiff surfaces, transcription of genes
downstream of YAP/TAZ including those involved in proliferation and
differentiation were observed. While Dupont et al.69 found that YAP/
TAZ signalling was not dependent on the Hippo pathway, Wada
et al.70 demonstrated that Hippo signalling is required. Both studies
found that the regulation of YAP/TAZ required RhoA GTPase activity
and actin–myosin contractility. Interestingly, Yap1 has been also implicated in the regulation of the phenotypic plasticity of smooth muscle
cells, as down-regulation of YAP leads to activation of the contractile
phenotype through a complex involving serum response factor and
myocardin.71 How shape and mechanics regulate YAP/TAZ and
Hippo signalling in smooth muscle differentiation from stem cell populations will be an important future area of investigation.
6. Future directions: engineering
cell phenotype to promote vascular
regeneration
Understanding the development of vascular smooth muscle cells from
stem cell populations is important not only for achieving a fundamental biological understanding, but also for engineering new vascular
cells and tissues for therapeutic applications. While the notion that
mechanics are critical for the regulation of stem and vascular
smooth muscle cell behaviour has existed for some time, a full understanding of how mechanical cues are transduced into biochemical
signals is far from complete. Future advancement of this field will
rely on tools with increasing complexity. For example, the integration
of multiple different mechanical cues (tunable adhesive geometry, rigidity, and applied mechanical forces) will be important to elucidate
how multiple signals are integrated by cells to elicit a single response.
Recently, an organ-on-a-chip model was developed to mimic lung
physiology, where endothelial and epithelial cells were cultured on
a stretchable membrane on which cells were stretched and fluid
flow was applied.72 Recreating other organ systems including blood
vessels or heart tissue will likely also require integration. Ultimately,
tools with high temporal and spatial resolution will be necessary to
probe and evaluate how dynamic mechanical cues are integrated by
cells and influence their decisions to become certain fates or adopt
specific phenotypes.
Funding
The author would like to acknowledge the Henry Samueli School of
Engineering and the Department of Biomedical Engineering, University
of California Irvine, and The Edwards Lifesciences Center for Advanced
Cardiovascular Technology for support of this work.
Conflict of interest: none declared.
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