Page 58 - Physico-Chemical Niche Conditions for Bone Cells

Shear stress modulates osteoblast cell and nucleus shape
via several actin-associated proteins, e.g., a-actinin, talin, tensin, paxillin, and vinculin [14]. The signaling transduction adaptor paxillin activates cytoplasmic signals (e.g., Wnts) and directly changes the cytoskeletal structure via recombination or reorganization of microtubules and microfilaments [15]. Although not yet shown for osteoblasts, in many cell types, the cytoskeleton is connected to molecular complexes embedded in the nuclear envelope, thereby forming a continuous connection from the extracellular matrix all the way to the nucleus [16]. Therefore, mechanical stimuli can affect cell cytoskeletal structure and morphology, nuclear structure, nuclear shape, chromatin structure, and thereby perhaps, cell behavior [16–21]. After biochemical coupling, cells, such as osteocytes and osteoblasts can alter their activity, but also that of neighboring cells, via the production of signaling molecules [10,22,23].
Bone cell shape (e.g., round and flat) is closely related to cell function [24]. In vivo, osteoblasts undergo several stages of terminal differentiation, i.e., mature osteoblast, osteoblastic osteocyte, osteoid osteocyte, and mature osteocyte, each with their own function [25]. During these stages, a shift in cell volume distribution occurs, changing the osteoblast shape from cuboidal, via round to stellate or dendritic-shaped, resulting in 30% volume- reduction in the nascent osteocyte cell body, and 70% volume-reduction in the mature osteocyte cell body compared with the volume of an osteoblast cell body [25]. Interestingly, the nuclear shape also can be changed by mechanical stress. For example, hypo-osmotic stress increases the nuclear volume (round shape), whereas hyper-osmotic stress decreases nuclear volume (convoluted shape) [26]. Cell and nuclear volume and function may, thus, be related.
Little is known about the volume of cells of the osteoblast lineage and their nuclei in response to mechanical stimuli. Therefore, we aimed to investigate whether pulsating fluid flow (PFF) induces changes in three-dimensional (3D) cell and nucleus morphology. Moreover, the function of osteoblasts. We focused on the morphology (F-actin, paxillin, integrin-a5, and a-tubulin) and volume of cell and nucleus, since these can change by PFF- treatment. We analyzed the cellular depolymerization/polymerization status after mechanical loading by PFF based on changes in morphology and function (gene and protein expression) through the determination of (intracellular and extracellular) structures.
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