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Mesenchymal Cell Activation by Biomechanical Stimulation and its Clinical Prospects, 2016, 3-12 3
Nahum Rosenberg (Ed)
All rights reserved-© 2016 Bentham Science Publishers
CHAPTER 1
Mechanics of Living Cells – General Aspects
Eyal Ginesin, Kamal Hamoud and Nahum Rosenberg
*
Department of Orthopaedic Surgery, Rambam Health Care Campus, POB 9602, Haifa 31096,
Israel
Abstract: Eukaryotic cells contain specific structural segments that determine the cellular
biomechanical characteristics, with the aid of numerous molecular structures. Cell
mechanics is mostly determined by the cytoskeleton dynamics in coordination with the
extra- and intra-cellular milieu. Cells can sense mechanical forces and, by signalling
system, convert them to biological response through mechanotransduction pathways. Cells
of mesenchymal origin are specially sensitive and responsive to mechanical forces because
they are involved in building of the biomechanically efficient tissues for force propagation.
Keywords: Actin, adhesion molecules, cell migration, fluid shear, intermediate
filaments, microtubules.
Cell Unit
Living cell is able to supply its own requirements of energy utilisation, molecular
synthesis, transport and maintenance through its life cycle. The cell shape is
determined mostly by the two major factors, the plasma membrane, which acts as
an anchor for proteins connecting to it, and the actin cortex, which provides a
mechanical support [1]. The actin, part of the cytoskeleton, plays a major role in
determining the structural deformation due to its ability of fast remodelling [2].
These supporting structures play an important role in controlling the cellular
shape during mitosis and cellular migration (Fig. 1) [3, 4].
The actin cell cortex cooperates with a flexible plasma membrane. They both act
as resistance to deformation and transmit forces from the extracellular
environment into the inner cell [5].
It is now clear that the actin and plasma membrane interact with each other. These
interactions influence the extracellular to intracellular biochemical and
biomechanical pathways.
*Corresponding author Rosenberg Nahum: Department of Orthopaedic Surgery, Rambam Health Care
Campus, POB 9602, Haifa 31096, Israel; E-mail: nahumrosenberg@hotmail.com
4 Biomechanical Stimulation and its Clinical Prospects Ginesin et al.
Figure 1: Schematic representation of actin fibres distribution during cellular deformation and
movement.
The direction of the migrating cells is determined by the rearrangement of the cell
adhesion molecules (CAM). The CAM-CAM integration and the cell shape are
controlled by the microtubules, another part of the cytoskeleton, bonding and the
adhesion domain centers.
Cellular Scaffold
As a hemi solid structure, the cell depends on the scaffolding. There are mainly
three types of filaments which include the skeleton of the cell, i.e. cytoskeleton: 6-
10 nm in diameter actin filaments, 7-11 nm intermediate filaments and 25 nm
microtubules. This system plays a role in forming a "road" to and away from the
nucleus. For the cell to keep its shape without collapsing, it has to maintain a
sufficient cortical pressure, which depends on F- actin [6, 7], myosin II [8, 9] and
intermediate filaments [10]. The tension of the cells also influences the cell shape
and movement [11, 12], locomotion [13, 14], and mitosis [15].
In addition to the internal forces, there is a balance with externally applied force
to regulate the cellular functions [16, 17]. Extracellular matrix (ECM) is a part of
and regulates the extracellular forces. The molecular component surrounding the
cells provides a structural support via interactions with integrin receptors.
Actin
Tension
forces
Extracellular
Actin
Adhesion
Movement
Mechanics of Living Cells – General Aspects Biomechanical Stimulation and its Clinical Prospects 5
Signalling Pathways
Cells communicate with the surrounding environment. Most of the
communication of this nature is mediated by fluid shear stress that is converted
into activation of cellular pathways and ending with gene expression [18]. These
mechanotransduction pathways have still not been determined sufficiently.
However, it is known that they induce cytoskeleton remodelling, giving a
direction to the cell movement according to the direction of the fluid flow.
Additional way of signalling is via the extracellular scaffolds anchoring into cell
with receptor clusters, i.e. focal adhesion complexes, which form channels from the
cytoskeleton to the extracellular matrix [19]. These channels also contribute to the
intracellular propagation of signalling and changing the cytoskeleton shape [20].
Cell Adhesion
The mechanism of cell adhesion, including its mechanics and kinetics, is of high
interest in the research of cellular biomechanics. Cell adhesion is a good example
of interweaving of cell signalling and changing of cellular shape. Many
mathematical models have analysed the cell processes which are part of this
process. It appears that the overall process of cell adhesion is based on kinetic,
thermodynamic and mechanical properties of cellular membrane and of the
cytoskeleton. The adhesion mechanism involves receptor-ligand bonds. The
adhesion domains are the areas of biochemical reactions, which control the
enzymes that govern the adhesion process. The adhesion domain is
thermodynamically controlled by cohesive forces from the cells surrounding
which pass through the actin anchorage at the cellular membrane [21-25]. The
order of the adhesion bonds starts from the stronger initial bond followed by
weaker successor bond, etc. This order is essential to generate local efficient
adhesions. Even for a temporary adhesion this order of bond events is essential
[26, 27]. However, in several experimental settings the temporary cellular
adhesions are generated by a random order of events of the unrelated bonds [26,
28, 29].
In the normally adhering cells, slow dissociation of bonds is also in the precise
successor order, according to the adhesion surface properties. This order is
essential for cell functioning as a unit and not as an unorganised cluster.
The cellular adhesion micro domains act by biochemical reactions which are
controlled in situ by the activator and inhibitor molecules [30]. The cellular
6 Biomechanical Stimulation and its Clinical Prospects Ginesin et al.
adhesion process is usually initiated in the micro domains with a rapid
biochemical turnover [31, 32]. The stabilisation of adhesion domains is
determined by the interrelations between the receptors on the cell membrane, the
CAMs and the actin cortex [33].
The control of the cell tissue adhesion is determined by the interactions of local
ligands with blocking sites on CAMs and on the surface of cellular membrane.
This theory on the dynamics of cellular adhesion is supported by the classic
experimental model of Erich Sackmann et al. [34] who showed that after forming
the steady state of CAM – ligand - receptor bonding, addition of soluble blocking
antibodies caused shrinking of the adhesion domains. Due to the fact that the
antibody had stronger affinity to the domain site, the ligand could not attach to the
domain and the domain became unstable and the adhesion of the cell became
ineffective.
Deformation of Molecules
The three dimensional structure of molecules is highly interconnected with their
biological activities and plays a crucial role in mechanotrasduction. Commonly
the proteins change between active and not active state when subjected to
different mechanical forces. The interconnection forces, e.g. Van der Waals
forces, electrostatic forces and hydrophobic interactions, determine the volumetric
three dimensional shape of a molecule in order to create a stable fit with the
specific ligand and/or receptor. For example the hydrogen bonds, which are
generally weak, contribute to the molecular stability when accumulated
simultaneously in large numbers. The three dimensional shape of the receptor and
ligand molecules determines their bonding abilities in the chain of force
propagation through the cell. Thus the effect of the forces acting through the chain
of cellular mechanotransductionon pathways can be “switched on” or “switched
off” according to the three dimensional shape of the key molecules following the
effect of forces that were mentioned above. These cellular signalling pathways are
regulated by the changing in the shape of molecules. In this process the ligand –
receptor interactions are the key points in the chain of pathways of transmitting
the mechanical force from ECM into cells via the binding domain [35].
Cellular Mechanosensing
The actin cytoskeleton acts with several components, i.e. myosin II motor, actin
filament and actin crosslinks which bind to the membrane via anchoring proteins.
Mechanics of Living Cells – General Aspects Biomechanical Stimulation and its Clinical Prospects 7
The external forces are transmitted to the cytoskeletal components. Tianzhi Lou
et al. [36] described a model of signalling during force propagation in cells (Fig. 2).
Figure 2: Cellular mechanosensing and signaling.
The cytoskeleton - membrane interconnection acts as dynamic elastic component.
An extracellular mechanical force passes via the anchoring proteins on the
membrane and transmitted to the parallel chains of actin and myosin II
interconnected by the actin crosslinker (AC). The further transmission of force
can be reduced, inducing less mechanosensitive signals, if the ACs are
disconnected from the actin chains. The opposite process occurs when all the ACs
are occupied and the force redistributed to the cytoskeleton, with higher
accumulation of myosin II. This interrelation of the cytoskeletal components has a
regulatory role in cellular force transduction, when the interactions between actin,
myosin II and ACs are force dependent.
Cell Migration
Cell migration depends on the formation of traction forces which are opposite to
the forces arising from cytoskeleton and from adhesions to the extracellular
environment. The migration mechanism involves deformation of the cellular
structure. Thus, a cell moves in the direction of the vector of traction force if it
exceeds the opposite vector of the internal resisting stress (Fig. 3).
Cytoskeleton
signaling
Force generating
Mechanosensitive protiens
protiens
Signaling
Extracellular
Actin
and myosin
II
8 Biomechanical Stimulation and its Clinical Prospects Ginesin et al.
Figure 3: Intracellular and extracellular forces acting together giving rise to cell migration.
Cells are subjected to passive and active mechanics. The passive mechanics are
dependent on the relation between the mechanical stress and strain. Cells are
viscoelastic; therefore a deformation occurs and gradually increases when a
constant stress is applied [1]. The active mechanics depend on internal actin –
myosin interactions which generate contractile forces. These forces are important
to the migration because cell movement occurs when the cellular adhesion to the
surface is followed by cellular contraction in a repetitive sequence. Therefore, the
cellular movement depends on the regulation of the cellular adhesion and
mechanotransduction signalling which are described above, i.e. the mechanical
force from the surrounding surface or ECM is transformed to the internal stress
via cytoskeleton. If the regulatory cellular signal causes cytoskeletal stiffness, the
adherent cell stops to move. Thus, the level of the cellular stiffness dictates its
migration ability. There is also signalling interactions between the neighbouring
cells that determine their level of stiffness and, as a consequence, their ability to
migrate [37, 38].
Bone Cells
One of the types of mesenchymal that is highly sensitive to mechanical
stimulation is the bone matrix generating cells, i.e. osteoblasts and osteocytes. In
these cells mechanical stimulation regulates bone matrix elaboration and
eventually whole bone structure, composition and shape [39, 40]. Therefore, bone
undergoes deformation as a result of damage at micro and macro level [41]. The
osteoblasts act as mechnosensors of external mechanical force [39, 42]. Calcium
flux is the main mediator of mechanotransduction in bone cells [43, 44]. Bone
cells react to external mechanical stimulation originating from external
hydrostatic pressure and three dimensional fluid shears [45].
Nucleus
Signal
conversion
Extracellular
Plopagation of cell
Mechanics of Living Cells – General Aspects Biomechanical Stimulation and its Clinical Prospects 9
Primary calcium signalling following osteoblast stimulation is transferred to the
cytoskeleton and gene expression [46] for matrix elaboration and cellular
differentiation [47].
Mesenchymal Stem Cells
Multipotent mesenchymal stem cells (MSCs) are a more general example of cells
of mesenchymal origin that are very sensitive to external mechanical stimulation.
As progenitor cells have the potential to be differentiated to the most type of the
connective tissue cells. Mechanical force induces activation and differentiation of
these cells into specific cellular lineage. Among the mechanical factors that
influence the differentiation of MSCs are the rigidity and topology of the
extracellular matrix, adhesion factors, mechanical loading and shear stresses [1].
The connective tissue contains MSCs with lineage potential to osteogenic,
chondrogenic and adipose line by mechanical loading [48, 49]. The MSC-like
cells present in blood vessels and heart valves [50, 51], contribute to tissue
renewal according to the surface rigidity [52, 53], cell-surface adhesion geometry
[54-56] and the magnitude of mechanical forces.
MSC maturation into specific lineage starts with initial proliferation and then
maturation of cells for matrix elaboration [57]. These stages of the cell cycle are
regulated by the interaction between MSC’s and ECM. These interactions rely
upon signalling molecules, adhesion clusters, and myosin contractility.
All these cellular interactions are under control of dynamic mechanical forces that
eventually contribute and determine the growth, maintenance and function of the
connective tissues [58]. There might be a single mechanical unidirectional force
or complex multidirectional forces, constant or intermittent, which affects the
lineage determination.
This phenomenon was demonstrated, among others, by Arnsdorf et al. [59] using
oscillatory fluid flow, which in turn induces epigenetic changes that control
osteogenic cell fate with generation of stable osteoblastic phenotype. Similarly, a
cyclic mechanical strain applied on MSCs induces the synthesis of potent growth
factors that regulate specific mature cell lineage characteristics [60, 61].
CONFLICT OF INTEREST
The author confirms that author has no conflict of interest to declare for this
publication.
10 Biomechanical Stimulation and its Clinical Prospects Ginesin et al.
ACKNOWLEDGEMENTS
Declared None.
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