Chemotaxis adaptation

Adaptation—the process of recovery from a stimulated behavior when the stimulus is still present—is essential for every behavioral system. It allows detection of small changes in the stimulus level on top of existing, constant level of stimulation. In the case of bacterial chemotaxis, adaptation enables bacteria to respond to new stimuli in the presence of constant levels of chemoattractants and/or chemorepellents. 

Figure 24.10 Integral control feedback circuit representation of chemotaxis (adapted from Yi et al., 2000).

CRAMP Murine Immature neutrophils, spleen, respiratory and intestinal epithelia, keratinocytes, testis Cell proliferation/differentiation chemotaxis antiendotoxic adaptive immune polarization in vivo protection cytokine/chemokine induction FPRL1, murine FPRL2... 

Yi, T. M., Huang, Y., Simon, M. I., and Doyle, J., Robust perfect adaptation in bacterial chemotaxis through integral feedback control, Proc. Natl. Acad. Sci. USA, 97, 4649, 2000. 

As in the case of bacterial chemotaxis, where receptor methylation acts as a counterbalance to changed external conditions and thus allows the cells to adapt to constant stimulation (Koshland, 1980), phosphorylation of the cAMP receptor provides the counterbalance thanks to which D. discoideum amoebae adapt to constant cAMP stimuli. 

Goldbeter, A. D.E. Koshland Jr. 1982b. Simple molecular model for sensing and adaptation based on receptor modification, with application to bacterial chemotaxis. J. Mol. Biol. 161 395-416. 

Chemokines induce directed chemotaxis in nearby responsive cells. They are released from various cells in response to bacteria and viruses infection and in response to agents that cause physical damage such as silica or urate crystals. The main functions of chemokines are chemoattractants for leukocytes. They help to recruit monocytes, neutrophils, and other effector cells from the blood to the sites of infection or damage. They serve to guide cells involved in innate immunity and in the adaptive immune system. Some chemokines have other roles in the development of lymphocytes, migration, and the growth of new blood vessels. 

Tracking free-swimming bacteria under a microscope provides direct information not only on the motility of the bacteria but also on their chemotactic behavior. In the case of aberrant chemotaxis, this technique can provide information about whether the excitation or the adaptation step of chemotaxis is defective. Two main approaches have been used ... 

Figure 13. Schematic presentation of the structure of the best-characterized chemotaxis-specific receptor, Tar. The regions of interaction with the ligands and the proteins in the receptor complex are shown. For simplicity, helix superooiling is omitted, and the a-helices are represented by cylinders. Components that dock to the receptor in the assembled complex are shown schematically as ellipsoids and spheres. Cytoplasmic sites of methylation and demethylation (adaptation sites, residues 295, 3Q2, 309, and 491] are shown as small ovals. (Taken with permission from Falke and Hazelbauer (223], with slight modifications made by J.J. Falke.]...

The MCPs fulfill at least three functions in bacterial chemotaxis they bind the ligands (or sense changes in the ambient temperature and pH), they transduce the chemotactic signal across the cytoplasmic membrane, and, as part of their involvement in adaptation, they undergo methylation or demethylation. The focus here will be on Tar, which is the best-characterized MCP. 

Falke, J.J., Bass, R.B., Butler, S.L., Chervitz, S A. and Danielson, M.A. (1997). The two-component signaling pathway of bacterial chemotaxis A molecular view of signal transduction by receptors, kinases, and adaptation enzymes. Annu. Bev. Cell Dev. Biol 13, 457-512. 

Kehry, M.R. and Dahlquist, F.W. (1982). Adaptation in bacterial chemotaxis cheB-dependent modification permits additional methylations of sensory transducer proteins. Cell 29, 761-772. 

Surette, M.G. and Stock, J.B. (1996). Role of a-heHcal coiled-coil interactions in receptor dimerization, signaling, and adaptation during bacterial chemotaxis. /. Biol Chem. 271, 17966-17 973. 

Thus, for amoeboid chemotaxis, the spatial aspects of the signaling pathways need to be examined as well when considering excitation and adaptation pathways. These issues will be discussed in more detail in the section on signal transduction. 

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