Mitochondrial positions before (green) or after (red) a micromanipulator (circle) was used to exert upwards tension on a surface-bound bead.

Ingber/NAS

For 20 years the notion that stability of cell shape is based on the rules of an architectural system known as tensegrity has been raising hackles in cell biology. Now tensegrity's foremost proponent, Donald Ingber of Harvard Medical School, Boston, MA, reports additional experimental evidence supporting the hypothesis.

Tensegrity (tensional integrity), exemplified by Buckminster Fuller's goedesic dome, is a tension-dependent building system, as opposed to a compression-dependent system. An example of the latter is a conventional house made of brick stacked on brick that gets its stability from gravity. By contrast, a tensegrity structure like Fuller's gets its stability from continuous tension that is transmitted over all of its elements and is balanced by a subset of elements that cannot be compressed. Ingber has long argued that cells are not, as conventionally pictured, elastic bags of viscous gel. Instead, cells create tension in their contractile microfilaments and transmit the tension all over the cell. The tension is resisted both by external attachments and by internal elements, such as microtubules, that Ingber likens to compression-resistant tent poles.

In their most recent paper, Ingber and colleagues deal with several criticisms of tensegrity via experimental results that are also consistent with the a priori predictions from a mathematical model of tensegrity previously devised by coauthor Dimitrije Stamenovic (Boston University, Boston, MA). For example, they allowed a cell containing GFP-tagged mitochondria to bind to coated beads that stick to integrin receptors across the cell surface. They then pulled the bead away from the cell, which caused the integrins to link up to the microfilament cytoskeleton and made the fluorescent mitochondria move at a distance.

Ingber argues that this means forces are transmitted across the integrin, over the microfilaments, and through structural interconnections to the microtubules. “So it confirms that if you physically connect to the right receptors that couple to the internal cytoskeleton, you get long-distance force transfer,” he says.

Consistent with the idea of an interconnected cytoskeleton, Anne-Marie Yvon and her colleagues in Patricia Wadsworth's lab (University of Massachusetts, Amherst, MA) show that actomyosin generates tension that moves microtubules in cells, whereas cytoplasmic dynein/dynactin complexes resist that tension. ▪

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Yvon, A., et al. 2001. Proc. Natl. Acad. Sci. USA. 10.1073/pnas.141224198. http://www.pnas.org/cgi/doi/10.1073/pnas.141224198