Dynamic Tensegrity Heidemann and Jarosch

Buckminister Fuller proposed an interesting architecture constructed from components which may have a recursive or fractal structure (Fuller, 1975). Its macro level structure, a "tensegrity mast," is a rigid structure constructed from an assembly of tension and compression members. The compression members of solid struts are isolated from each other, held together by the tension members. In one of Fuller's variations, he notes that in the macro tensegrity mast, each individual solid strut may be replaced by a miniaturized version of the macro tensegrity mast. And then each one of the miniature solid struts may itself be replaced by a still smaller subminiature tensegrity mast, and so on down to the atomic level. Thus tensegrity structures may have a fractal substructure.

Joshi, Chu, Buxbaum and Heidemann (1985) have shown that cytoplasm has both compressive and tensile elements. Semi-rigid microtubules are under compression presumably due to tension generated by actin filaments and the microtrabecular lattice or "cytomusculature" (Chapter 5). In general, MT do not contact each other so that the self-supporting capability of cytoplasm may stem from tensegrity.

Robert Jarosch (1986) has published a series of papers describing actin-MT interactions which suggest that 1) contractile actin filaments are spirally wound around microtubules, 2) coordinated contraction of the actin filaments imparts a rotational torque to MT, somewhat like a spinning top, 3) actin filaments wound in opposite directions on the same MT can cause rotational oscillations of the MT. These two models fit together to provide a picture of a dynamic cytoplasmic tensegrity network in which the cytoskeleton may be twisting back and forth, even "rockin' and rollin'!" Perturbation of any part of such a tensegrity network could have dynamic consequences throughout its domain. Transient changes in tension, compression, or oscillatory rhythm caused by a variety of factors would be detectable and possibly amplified throughout the cytoskeleton.

8.2.10 Dynamic MT Probing/Kirschner and Mitchison

Figure 8.5: Robert Jarosch has proposed that actin filaments wind around microtubules and contract, causing MT to rotate and oscillate around their long axis. With permission from Robert Jarosch (1986).

Strong evidence supports the concept of dynamic instability in microtubule assembly (Chapter 5). Many MT exist in either growing or shrinking phases and MT stabilized at merely one end by centriole based microtubule organizing centers (MTOC) tend to predominate. Selective retention of MTOC based MT establishes cell polarity important in extension of axons and dendrites, elongation of cells in embryological development, and formation of lamellipodia and filopodia in locomotory cells. All are examples of the "Indian rope trick" in which cells somehow choose their direction of growth, and then grow in that direction. A cue presented at the cell periphery can lead to rearrangements of cell symmetry and polarity. Kirschner and Mitchison (1986) ask: "how can a peripheral clue lead to reorganization deep within a cell?" One possibility is that a signal is relayed to the microtubule organizing center leading to a change in its structure, orientation, and directed nucleation of microtubules. This would be consistent with a primary role for information integration and decision making within the MTOC. A simpler "non-hierarchical" idea is that a signal at the periphery affects distribution directly. Since the whole cytoskeletal array is very dynamic, it would only be necessary to stabilize or destabilize a particular subset of microtubules for the entire cytoskeleton to rapidly transform. Preferential stabilization of a microtubule (MTOC, GTP capping, binding to membrane related structures etc.) could be mediated by interactions at the cell periphery close to the site receiving environmental information. Kirschner and Mitchison (1986) have proposed that the dynamics of the microtubule array results in probing many regions at random and, by stabilizing certain conformations as they arise, the cell "can arrive at a structure that is not precisely defined by genetic information but fulfills a particular functional role as dictated by environmental factors." Dynamically unstable MT offer many possibilities for controlling the distribution of MT by selective stabilization. Kirschner and Mitchison suggest that the tendency for probing and transformation is a fundamental advantage which favored the evolution of dynamically active microtubules.

0 0

Post a comment