Page 118 - Physico-Chemical Niche Conditions for Bone Cells

Regulation of bone cell mitochondrial structure and dynamics
hysteresis and memory for the external physical stimulation. This interesting phenomenon can be epigenetically passed onto the next generation following cell division [14]. Owing to cytoskeletal hysteresis and memory in general cell behavior, we were interested in determining a possible relationship between the cytoskeleton and mitochondrial network structure in bone cells treated by mechanical loading.
The cytoskeleton is composed of three major types of polymers, i.e. actin filaments, microtubules, and intermediate filaments [18]. These cytoskeletal filaments consist of an interconnected network structure with the help of stabilizers, motor proteins, and cross-linkers [15]. The structural organization and amount of these network structures determine the morphology and mechanical properties of the cell [18]. These network structures can also sense, transmit, and respond to mechanical stimuli by communicating via signaling molecules to each other or other organelles and reorganizing their network structure [19]. The process of reorganization always involves cytoskeletal polymerization and depolymerization [20], which is affected by factors, e.g. nucleation-promoting factors, capping proteins, polymerases, and depolymerizing factors [14]. All three cytoskeletal filament networks associate with many and diverse mitochondrial functions [15].
The actin filament is a dynamic network structure which participates in cell movement and function, e.g. cell motility, cell division, intracellular trafficking of organelles, and maintenance of cell polarity and morphology [21–25]. Interactions between cytoskeletal filaments and mitochondria are essential for normal morphology, distribution, and motility of mitochondria [26]. Actin filaments are known to interact with mitochondria in many cell types [26]. However, little is known about the interaction between actin filaments and mitochondria in bone cells (Fig. 1A). Microtubules form an extremely dynamic and spatially organized network structure [27]. They serve as railroad tracks along which mitochondrial clusters move within the cell with the aid of motor proteins (Fig. 1B), e.g. kinesins and dyneins, which allow mitochondrial movement toward the plus end and the minus end of a microtubule, respectively [28,29]. Microtubules can completely eliminate mitochondrial motility when they are disassembled [30]. This process is necessary for fission and fusion, as well as for the maintenance of healthy mitochondrial bioenergetics and structure [15]. Intermediate filaments, including vimentin and plectin, also involve mitochondrial structure and function (Fig. 1C) [31]. Vimentin is associated with mitochondria, since mitochondrial disorganization and fragmentation occurs in vimentin-null cells, perhaps by regulating the association of microtubules with mitochondria [32]. In fact, vimentin protects the cell against compressive stresses, suggesting that external mechanical stimuli might modulate mitochondrial structure and function [33]. Plectin, a cytoskeletal cross-linker, is associated with desmin, which is an intermediate filament and co-localizes with mitochondria along the entire length of the sarcomere in striated muscle, indicating that plectin may play a role in the shape, formation,
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