Submitted on October 27, 2007
Accepted on August 11, 2008
Mechanical loading by fluid shear is sufficient to alter the cytoskeletal composition of osteoblastic cells
Wesley M Jackson1*, Michael J Jaasma2, Raymond S Tang2, and Tony M Keaveny2
1 Orthopaedic Biomechanics Laboratory, Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States; UCSF/UCB Joint Graduate Program in Bioengineering, Universities of California, San Francisco and Berkeley, Berkeley, California, United States
2 Orthopaedic Biomechanics Laboratory, Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States
* To whom correspondence should be addressed. E-mail: jacksonw2{at}mail.nih.gov.
Many structural modifications have been observed as a part of the cellular response to mechanical loading in a variety of cell types. Although changes in morphology and cytoskeletal rearrangement have been widely reported, few studies have investigated the change in cytoskeletal composition. Measuring how the amounts of specific structural proteins in the cytoskeleton change in response to mechanical loading will help to elucidate cellular mechanisms of functional adaptation to the applied forces. Therefore, the overall hypothesis of this study was that osteoblasts respond to fluid shear stress by altering the amount of specific crosslinking proteins in the composition of the cytoskeleton. MC3T3-E1 and hFOB osteoblasts were exposed to 2 Pa of steady fluid shear for 2 hours in a parallel plate flow chamber, and then the amount of actin, vimentin, alpha-actinin, filamin and talin in the cytoskeleton was measured using western blots. After mechanical loading, there was no change in the amount of actin monomers in the cytoskeleton, but the crosslinking proteins alpha-actinin and filamin that co-fractionated with the cytoskeleton increased by 29% (p < 0.01) and 18% (p < 0.02), respectively. Localization of the crosslinking proteins by fluorescent microscopy revealed that they were more widely distributed throughout the cell after exposure to fluid shear. The amount of vimentin in the cytoskeleton also increased by 15% (p < 0.01). These results indicate that osteoblasts responded to mechanical loading by altering the cytoskeletal composition, which included an increase in specific proteins that would likely enhance the mechanical resistance of the cytoskeleton.
