Page 85 - Physico-Chemical Niche Conditions for Bone Cells

INTRODUCTION
Bone adapts to mechanical loading. Osteocytes play an important role in this bone adaptation. They are highly mechanosensitive, more so than osteoblasts and fibroblasts. However, osteoblasts also clearly respond, albeit less than osteocytes, to mechanical load by pulsating fluid flow (PFF) and intermittent hydrostatic compression (IHC), e.g. with increased prostaglandin E2 (PGE2) and prostaglandin I2 (PGI2) release [1]. PFF also triggers the production of other signaling molecules by osteoblasts that regulate bone mechanical adaptation, such as Wnt signaling [2], and nitric oxide (NO) production [3]. The production of growth factors by osteoblasts may also be altered in response to PFF, such as the production of specific bone morphogenetic proteins and fibroblast growth factors (FGFs), although more research is needed. FGFs play a vital role in the regulation of bone development [4]. FGF2 is produced by osteoblasts and stored in the extracellular matrix (ECM) to control osteoblast differentiation [4].
In the absence of loading, the production of sclerostin (osteoblast inhibitor) and RANKL (osteoclast stimulator) by osteocytes increases [5]. In this regard, osteocytes can be considered stimulators of unloading-associated bone loss. Tatsumi and colleagues showed that hindlimb suspension in osteocyte-ablated mice did not lead to bone loss, demonstrating the importance of osteocytes for stimulating bone loss with unloading [6]. However, they also showed that enhanced bone formation after re-loading did not require osteocytes [6]. Osteoblasts might regulate loading-stimulated bone formation independent of osteocytes in the hindlimbs of osteocyte-less mice [7]. This is also in line with reports from the group of Donahue showing that loaded MLO-Y4 cells hardly affect osteoblast proliferation in co-culture [8]. A limitation is of course that MLO-Y4 cells do not produce sclerostin, but in the presence of loading primary osteocytes produce little sclerostin anyhow, while it is known that mechanical loading is a potent stimulator of bone (re)modeling by osteoblasts [9]. Osteoblasts may either stimulate bone formation by their neighbors through the production of signaling molecules as described above, or the mechanical stimulus directly affects osteogenic differentiation, e.g. through alterations in the cytoskeleton.
Mesenchymal stem cells (MSCs) undergo osteogenic differentiation when seeded on a hard substrate, which starts with reorganization of the cytoskeleton and the nuclear skeleton [10,11]. Physical and chemical stimulation affects the organization of the cytoskeleton, resulting in changes in cell adhesion, morphology, and differentiation [12]. Hard substrates affect MSC differentiation, even in the presence of chemical factors [12]. Mechanical loading dramatically changes the orientation of actin stress fibers in MC3T3-E1 pre-osteoblasts compared to non-treated cells [13]. The actin stress fibers become thicker, more abundant, and align roughly parallel to the long axis of the cell [13]. Focal adhesions also can be
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