Switching Cell Polarity, and Regulating Gene Expression to Build an Organ

A. Dale Kaiser, PhD
Biochemistry and Developmental Biology

Myxobacteria build multicellular fruiting bodies, and sporulate within them. When a dense population of cells has exhausted its food supply, the cells stop swarming outward to hunt food, and return to the center of their colony. In the center they form several fruiting bodies, each containing 100,000 cells. A fraction of the cells in a fruiting body become spores. Fruiting body shape is species specific, and about 50 species are known. Because the fruiting body disperses its spores in nature, ie the soil, and because it has a specific shape, the fruiting body can be considered a rudimentary organ. As a tool for experiments, the genome of the myxobacterium, Myxococcus xanthus is haploid; it has been sequenced; its genes can readily be manipulated, and individual cells can be tracked using GFP.

Myxobacterial cells only move on surfaces, they glide. They have pili at the front that pull cells forward. They secrete slime through jets at the rear that push cells forward. Early in the process of building fruiting bodies, waves of high cell density sweep across the swarm, and those waves nucleate aggregation foci. The foci enlarge as cells stream over and around them in circular orbits, building concentric spherical shells of cells, like the skins of an onion. Both the waves and aggregation depend on a morphogen, a 17 kDa, surface-bound protein called the C-signal (1). Several mathematical models of cell movement and cell contact signaling in the traveling waves and in the aggregates have been developed that agree with recorded cell trajectories (2,3). The C-signal transduction circuit also controls gene expression that is both temporally and spatially specific. C-signal is not diffusible, the model depends on signaling by cell-cell contact.

(1) Kaiser D and Welch R (2004) Dynamics of fruiting body morphogenesis. J Bacteriol. 186: 919-27.
(2) Igoshin O, Welch R, Kaiser D, and Oster G (2004) Waves and aggregation patterns in Myxobacteria. Proc. Natl. Acad. Sci. USA 101 15760-15765.
(3) Sozinova, O., Jang, Y., Kaiser, D., and Alber, M.S. (2005) Three-dimensional model of myxobacterial aggregation by contact-mediated interaction. Proc Natl Acad Sci U S A 102: 11308-11312

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