Osteoblasts and osteocytes respond differently to oscillatory and unidirectional fluid flow profiles

Abstract

Bone cells subjected to mechanical loading by fluid shear stress undergo significant architectural and biochemical changes. The models of shear stress used to analyze the effects of loading bone cells in vitro include both oscillatory and unidirectional fluid shear profiles. Although the fluid flow profile experienced by cells within bone is most likely oscillatory in nature, to date there have been few direct comparisons of how bone cells respond to these two fluid flow profiles. In this study we evaluated morphologic and biochemical responses to a time course of unidirectional and oscillatory fluid flow in two commonly used bone cell lines, MC3T3-E1 osteoblasts and MLO-Y4 osteocytes. We determined that stress fibers formed and aligned within osteoblasts after 1 h of unidirectional fluid flow, but this response was not observed until greater than 5 h of oscillatory fluid flow. Despite the delay in stress fiber formation, oscillatory and unidirectional fluid flow profiles elicited similar temporal effects on the induction of both cyclooxygenase-2 (Cox-2) and osteopontin protein expression in osteoblasts. Interestingly, MLO-Y4 osteocytes formed organized stress fibers after exposure to 24 h of unidirectional shear stress, while the number of dendritic processes per cell increased along with Cox-2 protein levels after 24 h of oscillatory shear stress. Despite these differences, both flow profiles significantly altered osteopontin levels in MLO-Y4 osteocytes. Together these results demonstrate that the profile of fluid shear can induce significantly different responses from osteoblasts and osteocytes.

Two-dimensional clinorotation influences cellular morphology, cytoskeleton and secretion of MLO-Y4 osteocyte-like cells

“…MLO-Y4 cells subjected to fluid flow shear stress have shown an increase in the length of dendritic processes (Zhang et al 2006). Ponik et al (2007) have shown that MLO-Y4 osteocyte-like cells have a more dendritic morphology in response to oscillatory fluid flow. Fourty-four percent of cells have more than 3 dendrites/cell, while that is only 31% when MLO-Y4 cells held in static 258 H. Xu et al condition (Ponik et al 2007).…”

Protein Kinase G and Focal Adhesion Kinase Converge on Src/Akt/β-Catenin Signaling Module in Osteoblast Mechanotransduction

“…Osteoblasts and osteocytes differ in their sensitivity to FSS or stretching and demonstrate different kinetics of ␤-catenin nuclear translocation, but both cell types respond to mechanical stimulation with an increase in intracellular calcium and NO production (53)(54)(55)(56)(57). Nuclear ␤-catenin together with its binding partner Lef1 regulates genes in osteoblasts and osteocytes necessary for maintenance of normal bone mass in vivo, including cox-2, and control osteoblast as well as osteoclast differentiation and survival (27-30, 41, 58).…”

Osteocytes, mechanosensing and Wnt signaling

“…Regardless of how difficult or tedious the isolation, several investigators used primary osteoblasts and osteocytes to show significant differences in markers, function, sensitivity and response to mechanical strain (89)(90)(81)(82). For example, it has been shown the primary osteoblasts are less sensitive than primary osteocytes to shear stress (39)(64) that shear stress induced less of a calcium response in a single primary osteocyte than a single primary osteoblast, (60), that shear stress induced prostaglandin production in osteocytes is inhibited by cyoskeletal disruption, but not in osteoblasts (79), different ion channels are involved in osteoblast as compared to osteocytes' response to strain (101), stress fiber organization is delayed in osteocytes compared to osteoblast-like cells in response to fluid flow (98), and osteocytes have a gene expression profile distinct from osteoblasts (45).…”