Page 388 - Advances in Biomechanics and Tissue Regeneration

19.4 EFFECT OF EXTERNAL MECHANICAL SIGNALS 385

primary chondrocytes into a fibroblast morphology and the loss of chondrogenic phenotype [81]. Rho/ROCK and
Rac1 have been shown to generally inhibit MSC chondrogenesis by regulating Sox9 expression [69, 82, 83], indicating
that structural changes to the cytoskeleton and reduction of cytoskeleton tension are critical for chondrogenesis.
Nuclear morphology of MSCs, compared with terminally differentiated fibrochondrocytes, was shown to be much
sensitive to mechanical cues. Traction force generated from interaction with ECM cues that get transmitted through
the cytoskeleton actin would have translated to differences in nuclear geometry and organization that could directly
alter transcriptional events [34, 84] and regulate collagen expression [37, 77, 85]. Reduced cytoskeletal tension, when
cells are cultured on soft substrates, results in nuclear envelope proteins posttranslational modification and leads to the
modulation of transcriptional activity [33]. The different traction forces generated by actin cytoskeleton of MSCs dic-
tated by specific surface topography and material stiffness can also regulate the nuclear translocation of the mechan-
osensitive transcription factor, Yes-associated protein (YAP), through altering the permeability of the nuclear pore
complexes (NPC) [40]. YAP has been identified as sensor and mediator of mechanical cues instructed by ECM rigidity
and cell shape, which relays actomyosin cytoskeleton tension through a Rho GTPase–dependent translocation to the
nucleus [22, 86]. The phenotypic variation of chondrocytes in response to substrate stiffness was shown to be concom-
itant with the changes in YAP localization, with YAP silencing of chondrocytes on stiffer substrate reversing the loss of
chondrocyte phenotype [87]. YAP is a negative regulator of chondrogenesis in mesenchymal stem cells, which is
downregulated during MSC chondrogenesis, while overexpression of YAP inhibits chondrogenic differentiation
[88]. The nuclear translocation and total expression level of YAP/TAZ in adipose stem cells on aligned fibers was
shown to be notably elevated compared with those on random fibers, suggesting regulation of YAP/TAZ by surface
topography at transcriptional and translational levels [89]. While the exact mechanism of enhanced fibro/superficial
zone-like chondrogenic development by unidirectional topography and substrate stiffness [77, 78] remains unclear, it
is likely that the internal tension generated by the augmented stress fibers could lead to YAP nuclear translocation and
activation, which in turn regulate chondrogenesis, and modulate the phenotype of the derivative chondrocytes.

19.4 EFFECT OF EXTERNAL MECHANICAL SIGNALS

During physiologic cartilage loading, water in this highly hydrated tissue is gradually squeezed out, causing direct
strains at the tissue, cellular, and nuclear levels [7]. Concomitantly, mechanical compressive loading of cartilage gen-
erates secondary biophysical signals, such as hydrostatic pressures and shear force caused by the fluid flow. The oscil-
lating sliding motion of the articulating joint also imparts shearing tension to the surface of the articular cartilage.
Physiologic mechanical loading is a pivotal factor for the development of zonally defined cartilage in mature animals.
Cartilage tissues formed during embryonic chondrogenesis consist of neither multiple zones nor the unique Benningh-
off collagen architecture associated with articular cartilage. Such features are absent during birth and develop during
skeletal maturation in postnatal development [90, 91], shaped by articular motions exerting a combination of compres-
sive, tensile, and shear loading. Thus externally applied mechanical force can also significantly influence the chondro-
cyte ECM biosynthesis and chondrogenic differentiation of MSCs during articular cartilage repair. The type,
frequency, magnitude and duration of such cues have all been shown to affect chondrocyte response and MSC dif-
ferentiation [92]. The following subsections will summarize the response of chondrocytes and MSCs to the different
forms of mechanical signals.

19.4.1 Compression

Compression by direct loading or the nondeforming hydrostatic pressure (HP) of chondrocytes and MSCs encap-
sulated in hydrogels has been found to be a strong prochondrogenic stimulus. Chondrocytes respond to physiologic
magnitudes of dynamic compression (DC; 10%–20%) with enhanced synthesis of ECM molecules, including PGs,
collagens, and COMP [93–95]. The responses of chondrocytes to mechanical loading are highly dependent on loading
frequency, amplitude, strain rate, and loading history, with superphysiologic magnitudes of loading (>20%) failing to
enhance matrix production [93], while static or very low frequency loading [96] inhibits matrix synthesis. Chondro-
cytes respond to dynamic hydrostatic pressure similarly to DC [97, 98].
For MSCs, both DC [99–102] and HP [103–105] were shown to induce chondrogenic gene expression in MSCs in the
absence of exogenous growth factor stimulation, by increasing autocrine TGFβ1 production [104, 106], suggesting that
compression and exogenous TGFβ stimulation activate similar pathways. However, unlike the effect on chondrocytes,
there were some uncertainties as to the impact of DC/HP stimulation on MSCs, with some studies demonstrating that

II. MECHANOBIOLOGY AND TISSUE REGENERATION