A MODEL FOR THE ROLE OF INTEGRINS IN FLOW INDUCED MECHANOTRANSDUCTION IN OSTEOCYTES
B
METHODS: Our idealized structural model showing the suspension of
a cell process in canalicular cross-sections with a local collagen hillock
and its associated integrin complex is sketched in Fig. 2. Structurally,
one conical collagen hillock, one integrin molecule and one integrin
intracellular anchor protein complex locally replace one local transverse
tethering element and its associated transmembrane molecule and crossfilament in the axisymmetric model in (5), in which only transverse
tethering elements are present. The hydrodynamic loading on the
transverse elements and the resulting deformation of the cell process is
asymmetric because of the asymmetry of the local collagen hillock.
Y
Canalicular Wall B
Transverse
Element
T6
T4 Cell Process
Membrane
T3 X
Fimbrin
Intracellular
Anchor Protein
Complex
T6
T5
Actin
Filament
Transmembrane
Molecule
O
A
O’
O’’
Collagen
Hillock
T0
20
0
10
20
DISCUSSION: Although the precise mechanism by which bone cells
sense mechanical signals still remains elusive, several candidate
mechanotransducers, including the integrin-cytoskeleton structure,
mechanically activated channels, G protein-dependent pathways and
caveolin, have been proposed (9). Furthermore, links are emerging
between integrins and other putative membrane mechanotransducers
through the cytoskeleton or intermediate linkers (10). Therefore,
mechanical consequences due to the integrin attachment could initiate
other signaling pathways directly or indirectly and integrins would be
the primary mediator in mechantransduction.
β1 and β3 integrin subunits have been observed along the cell
process (7). Results in Fig. 3A imply a rate of increase in T0 of
approximately 20pN/s for a sinusoidal tissue loading of 20MPa at 1Hz.
In this low applied loading rate limit, the typical force for detachment of
a single α5β1 integrin-fibronectin pair would be 40-60pN (11), and that
of a single β3 integrin roughly 20pN based on the measurements in (12,
13) and an analogy of the rupture force patterns between β1 and β3
integrins. Our model predicts that T0 is approximately 12pN at this tissue
loading rate. Thus, the integrin attachment can resist rupture even if the
focal attachment involves only a single integrin molecule.
The large axial strains result from the large axial displacements of
the free ends of the tethering elements about a fixed integrin site and fall
into the range that could significantly excite bone cells in vitro (14).
CONCLUSION: The prediction of our model provides a quantitatively
feasible hypothesis that integrin attachments between the cell process
membrane and the collagen hillocks could serve as mechanical
transducer sites to directly or indirectly excite osteocytes via the flow
induced tensions and strains in the vicinity of such sites.
T2
T1
10
1 MPa
0
Frequency (Hz)
Frequency (Hz)
Fig.3. Mechanical consequences of the integrin attachment. A) The
tensile force at integrin attachment site and B) the axial strain of the
cell membrane around the integrin attachment site as a function of
loading frequency with loading amplitude as a parameter.
Fig.1. EM micrographs showing collagen hillocks with attachments
and transverse tethering elements spanning the pericellular space
between the cell process membrane and the canalicular wall. A)
Longitudinal cross-section (bar=500nm). B) Transverse crosssection (bar=100nm).
Pericellular
Space
Homogenous
Elastic Cylinder
0
0
Collagen Hillock
A
Axial Strain (%)
Transverse
Tethering
Element
A
Tensile Force (pN)
*Wang, Y; *Weinbaum, S; **McNamara, LM; +**Schaffler, MB
*Department of Biomedical Engineering, The City College of New York and CUNY Graduate Center, New York, NY;
+**Leni and Peter W. May Department of Orthopedics, Mount Sinai School of Medicine, New York, NY
[email protected]
tethering filaments and their associated cross-filaments are assumed to
INTRODUCTION: Osteocytes are generally believed to be the primary
act in the same cross-sectional plane in treating the radial force balance
mechanosensory cells in bone tissue (1). A growing body of theoretical
shown in Fig. 2A.
and experimental studies (2-5) suggests that the interstitial fluid flow in
the lacunar-canalicular porosity provides stress-induced mechanical
RESULTS: In Fig. 3, we plot the tensile force T0 acting at the integrin
signals to excite osteocytes. However, the mechanoreceptors for the
initial detection and the transduction of such extracellular mechanical
attachment site and the axial strain εa of the cell membrane in the
signals into intracellular biological signals remain to be determined.
vicinity of the integrin attachment site as a function of loading frequency
Integrins are ubiquitous transmembrane adhesion proteins that
with tissue loading amplitude as a parameter. T0 increases as the
couple the cytoskeleton to the extracellular matrix, and are essential for
frequency or the amplitude of the tissue loading increases. Surprisingly,
cellular responses to mechanical stimuli in many cell types (6). Recently,
εa is approximately one order of magnitude larger than the radial strain
it has been observed that conical collagen hillocks occasionally project
of the cell membrane (data not shown) for the same tissue loading. T0
from the bony wall completely across the pericellular space to contact
could reach 15pN, and εa 7%, for physiological tissue loading rates (8).
the cell membrane of the osteocyte process as shown in Fig.1 and that
A
B
such local asymmetric attachments are integrin-based (7). The primary
40
15
20 MPa
goal of this study is to determine the mechanical consequence of such
30
attachments and its role in mechanotransduction. Of particular interest
10
10 MPa
20
are the tensile forces generated at the integrin attachment sites and the
5 MPa
5
resulting strains on the cell process membrane.
10
Cross Filament
Integrin Molecule
Fig.2. The idealized structural model showing the suspension of a
cell process in canalicular cross-sections with a locally
asymmetric collagen hillock and its associated integrin complex
(solid line—before deformed; dash line—after deformed). A)
Transverse cross-section. B) Longitudinal cross-section.
To simplify the mechanical analysis of the cell process, two
idealizations have been made. First, the central actin bundle in the core
of the cell process is replaced by a homogenous elastic cylinder of the
same size and overall radial elastic coefficient as the original cross-inked
structure when dealing with the radial deformations due to the radial
tensile forces. Second, the tensile forces on the integrin attachment, the
ACKNOWLEDGMENTS: Funded by NIH AR41210 and AR48699.
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53rd Annual Meeting of the Orthopaedic Research Society
Poster No: 1362