Regulation of Mitochondrial Structure and Dynamics by the Cytoskeleton and Mechanical Factors

Abstract

Mitochondria supply cells with energy in the form of ATP, guide apoptosis, and contribute to calcium buffering and reactive oxygen species production. To support these diverse functions, mitochondria form an extensive network with smaller clusters that are able to move along microtubules aided by motor proteins. Mitochondria are also associated with the actin network, which is involved in cellular responses to various mechanical factors. In this review, we discuss mitochondrial structure and function in relation to the cytoskeleton and various mechanical factors influencing cell functions. We first summarize the morphological features of mitochondria with an emphasis on fission and fusion as well as how network properties govern function. We then review the relationship between the mitochondria and the cytoskeletal structures, including mechanical interactions. We also discuss how stretch and its dynamic pattern affect mitochondrial structure and function. Finally, we present preliminary data on how extracellular matrix stiffness influences mitochondrial morphology and ATP generation. We conclude by discussing the more general role that mitochondria may play in mechanobiology and how the mechanosensitivity of mitochondria may contribute to the development of several diseases and aging.

Keywords: bioenergetics; fission; fusion; network; stiffness.

Figure 1

The mitochondria are the powerhouses of the cell. Left: A cartoon of a mitochondrion showing its outer and inner membranes, the cristae, and the matrix. Fat and sugar enter the mitochondria through channels of the outer membrane. Right: The Krebs, or citric acid, cycle feeds the chain of respiratory complexes I through IV which create an electrical and proton (H⁺) gradient, the electromotive force across the inner membrane. ATP synthase utilizes the electromotive force to generate ATP from ADP and inorganic phosphate (P_i) (Right image from Wikipedia [24]).

Figure 2

Intracellular and extracellular processes contributing to mitochondrial structure and function. Intracellular mitochondrial (orange) dynamics include the processes of fusion, fission, mitophagy, and biogenesis. See text for explanation for how the various molecules such as OPA1, Mfn1, Mfn2, and DRP1 govern these processes. The grey mitochondrion is damaged and degraded by mitophagy. Green lines represent microtubules, along which small mitochondrial clusters can travel with the aid of motor proteins. The dashed line represents the site of fission and the red arrows indicate the cyclic nature of fission and fusion. Note that biogenesis increases mitochondrial volume and is regulated by the peroxisome proliferator-activated receptor γ coactivator (PGC-1α). Cells are connected to the ECM (extracellular matrix) and exposed to external mechanical forces (F) at focal adhesions (FA), involving integrin receptors (Int) on the cell surface and Arg-Gly-Asp (RGD) binding sites on collagens fibers (Col-I). The cytoskeleton is linked to FAs and therefore mechanical forces from the ECM are transmitted to the mitochondria.

Figure 3

Cells were cultured on elastic membranes that could be stretched equibiaxially in a stretching device. (A) A cell labeled for cytosol (green), mitochondria (red: tetramethylrhodamine methyl ester, TMRM), and nucleus (blue) at 0% (left) and 14% (right) strains. (B) The top row shows the mitochondrial network of an entire cell imaged during constant strain application at increments of 0, 7, 14, and 30% change in membrane surface area. The bottom row shows the zoomed-in details of an individual cluster (green rectangle in top row) changing shape as higher strains are applied (green arrow), as well as a cluster undergoing fission and splitting into two smaller clusters (red arrow) [77].

Figure 4

Relationship between complexity, measured by the fractal dimension D_f, and function, assessed by a fluorescent dye (TMRM, see text) intensity that is related to ATP production rate in VSMCs. There is a linear relation between the log of ATP production and D_f. The dots represent binned data from about 2000 cells showing unstretched control cells (US), 4 h of monotonously stretched (MS) cells (10% area strain at 1 Hz), and 4 h of stretching cells with a variable stretch (VS) pattern in which every cycle is different with the amplitudes uniformly distributed between 7.5% and 12.5% area strain. The US cells are in the lower left corner. These cells produce little energy and their mitochondrial fractal organization is the least complex. MS cells produce somewhat more energy and their D_f is also higher, whereas VS cells produce the most ATP and have the highest complexity in terms of their fractal organization. The images show mitochondrial networks corresponding to US, MS, and VS cells. ATP production rate is related to the intensity of red color. The results were obtained by reanalyzing the data from [14].

Figure 5

Mitochondrial structure-function relations as a function of substrate stiffness. VSMCs were seeded on substrates of different stiffness, labeled with TMRM, and cluster sizes and mean intensities were measured. The horizontal lines in the boxes are the median, the box represents the 25th percentile, and the horizontal bars are the 75th percentile of the data. The symbols are data outside the 75th percentile. (A) Cluster sizes were stiffness dependent, but only the clusters on 12.5 kPa stiffness were different from the rest. (B) Intensities were also stiffness dependent, and again only data on 12.5 kPa stiffness were different from the rest.