A model for the role of integrins in flow induced mechanotransduction in osteocytes

Abstract

A fundamental paradox in bone mechanobiology is that tissue-level strains caused by human locomotion are too small to initiate intracellular signaling in osteocytes. A cellular-level strain-amplification model previously has been proposed to explain this paradox. However, the molecular mechanism for initiating signaling has eluded detection because none of the molecules in this previously proposed model are known mediators of intracellular signaling. In this paper, we explore a paradigm and quantitative model for the initiation of intracellular signaling, namely that the processes are attached directly at discrete locations along the canalicular wall by beta(3) integrins at the apex of infrequent, previously unrecognized canalicular projections. Unique rapid fixation techniques have identified these projections and have shown them to be consistent with other studies suggesting that the adhesion molecules are alpha(v)beta(3) integrins. Our theoretical model predicts that the tensile forces acting on the integrins are <15 pN and thus provide stable attachment for the range of physiological loadings. The model also predicts that axial strains caused by the sliding of actin microfilaments about the fixed integrin attachments are an order of magnitude larger than the radial strains in the previously proposed strain-amplification theory and two orders of magnitude greater than whole-tissue strains. In vitro experiments indicated that membrane strains of this order are large enough to open stretch-activated cation channels.

Fig. 1.

Transverse cross-section (A) and longitudinal cross-section (B) of TEM micrographs showing infrequent, discrete structures resembling focal adhesion complexes protruding from the bony canalicular wall, completely crossing the pericellular space to contact the cell membrane of the osteocyte process along the canaliculi. (Scale bars: A, 500 nm; B, 100 nm.)

Fig. 2.

Transverse cross-section of the idealized structural model for a cell process in a canaliculus attached to a focal attachment complex and tethered by the pericellular matrix.

Fig. 3.

Deformation diagrams for the idealized mathematical model. (A) Transverse cross-section of the idealized homogeneous elastic cylinder and the idealized plane for all of the transverse tethering elements and focal attachment. (B) Longitudinal cross-section of the deformed transverse tethering elements and sliding actin filaments. (C) Top view of the undeformed and deformed cell process membrane around the focal attachment site (not to scale). (D) Force balance on a deformed transverse tethering element. The dashed lines indicate the deformed structural elements.

Fig. 4.

Membrane strains in the vicinity of focal attachment complex. (A) The radial strain ε_r (open arrow points to the radial strain on the cell process membrane for an axisymmetric loading for a tissue loading of 10 MPa, from ref. 9). (B) The axial strain ε_a as a function of loading frequency with tissue-loading amplitude as a parameter. The dashed lines in both A and B show the physiological loadings of bone tissue based on the power-law relationship between strain amplitudes and loading frequencies observed by Fritton et al. (2).

Fig. 5.

The tension on the focal attachment T₀ as a function of loading frequency with tissue-loading amplitude as a parameter. [The dashed line shows the physiological loadings of bone tissue based on the power-law relationship between strain amplitudes and loading frequencies observed by Fritton et al. (2).]