Geodesic Tensegrity Gels

Actin-myosin contraction also occurs in non-muscle cells. Soon after actin and myosin were identified in muscle, Loewy found that extracts of slime mold cytoplasm responded to ATP with a decrease in viscosity. He hypothesized that ATP provided chemical energy which was converted into mechanical work required for slime mold locomotion. After H. E. Huxley and colleagues at University College London showed that muscle contraction was achieved by ATP dependent sliding of actin and myosin filaments past one another, Hatano and Osawa at Nagoya purified actin from slime mold. Subsequent investigations turned up myriads of cytomatrix proteins including myosin which were related to contractility and its regulation (Lazarides and Revel, 1979).

Using fluorescent antibodies to "light up" specific proteins, Lazarides and Revel (1979) have observed actin filaments organized into strikingly regular networks looking remarkably like geodesic domes. They describe icosahedral geodesic networks that encompass the area above and around cell nuclei and other cell regions after the cells are treated with fluorescent antibodies illuminating actin, alpha actinin and tropomyosin. Alpha actinin is localized primarily at the vertices of the geodesic network and tropomyosin is localized along the actin fibers connecting the vertices. Longer fibers attach to network vertices and extend into filopodia, lamellipodia, and membranes. The geodesic network and vertices act as organization centers involved in maintaining cell structure (Figure 5.26).

Figure 5.26: Geodesic actin cytomatrix gel surrounding cell nucleus, from Lazarides and Revel (1979). Vertices of the icosahedral nuclear dome attach filaments which extend to the cell membrane. By Paul Jablonka.

Some actin networks are transient dynamic structures which serve fleeting, but vital functions during the cell cycle. During the final stages of cell division, after duplicated chromosomes have been pulled apart by the mitotic spindle, a ring of constriction encircles the equator of the mother cell perpendicular to the spindle axis. Work by this "cleavage furrow" constricts the cytoplasm until it is divided into two daughter cells. The constricting tension has been estimated from observations in sand dollar and sea urchin eggs to be about 3 x 105 dynes per square centimeter, a value comparable to the force generated in muscle (Lazarides and Revel, 1979). The cleavage furrow contractile ring is a temporary structure that exists for only about 10 minutes; it rapidly assembles and disassembles. Although the width and thickness of the contractile ring remain constant during constriction, the volume decreases. The ring must disassemble even as it contracts, meaning that any sliding interaction of the actin and myosin filaments must be followed by disaggregation of some of the filaments. This disassembly must take place uniformly through the ring during its entire brief lifetime. Therefore, a simple sliding filament model is insufficient to explain the cell cleavage function of the contractile ring.

Steve Heidemann and colleagues (Joshi, Chu, Buxbaum and Heidemann, 1985) at Michigan State University have examined compressive and tensile properties of cytoplasm. They find that semi-rigid microtubules are under compressive forces generated by interwoven contractile actin filaments. A balance between parallel compressive and tensile forces leads to a selfsupporting property characterized by Buckminster Fuller (1975) as "tensegrity." Tensegrity can provide cell support even if the rigid parallel element are not in direct contact. Tensegrity in the cytoskeleton might explain the self-supporting structural properties of cytoplasm in which the rigid parallel elements are not in direct contact.

Robert Jarosch (1986) has proposed that contractile actin winds and unwinds microtubules by a "torque drive," causing rotational oscillations and perhaps tuning and detuning of the microtubule system. Dynamic compression/tension may also be important in the regulation of membrane receptors whose mobility are limited by anchoring MT. Conversely, contractile actin filaments can redistribute the receptors unless they are restrained.

Dynamic tensegrity (Chapter 8) may be an important mechanism in many biological functions. In the cytoplasm, complex structures assemble, perform, and vanish into soluble subunits.

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