Chemotaxis is defined as the response of an organism or cell to chemical stimuli by directed movement. The ability to discriminate between different stimuli is crucial for food detection, spatial orientation and other adaptive behaviours in both prokaryotes and eukaryotes. Furthermore, individual cells such as sperm, neurons or tumour cells rely on chemotaxis to reach the egg, their synaptic targets or to form metastasis, respectively. From March 29 - April 1, 2006 about 60 researchers met in Titisee, Germany, to discuss how cells and whole organisms can sense and move towards extracellular stimuli. As the meeting progressed, some important common themes emerged.

In her keynote lecture, Cori Bargmann (The Rockefeller University, New York), described the molecular mechanisms underlying the response of C. elegans to different odours. Thanks to the relatively simple nervous system of the nematode, the specific neurons that generate movement towards or away from certain stimuli have been defined, thus facilitating the determination of the signalling components involved. As in other animals, odours are detected by G protein-coupled odorant receptors, which lead to the activation of guanylyl cyclases and an increase in intracellular calcium. Interestingly, the Bargmann laboratory have also found a role for glutamatergic signalling in modulating the response to particular sensory inputs, and for serotonin in avoiding noxious foods. In humans, an increase in serotonin release from enterochromaffin cells has been associated with causing nausea during chemotherapy, suggesting a conserved role for serotonin in the communication between the intestine and the nervous system.

Bacteria rely on chemotaxis to respond to an ever-changing environment and one session of the meeting focused on the mechanisms that drive it. In E. coli, the protein network responsible for chemotaxis has been characterized in detail. Chemical stimuli are detected by receptor-kinase complexes clustered at the poles and subsequent phosphorylation of a diffusible response regulator protein (CheY) transduces the signal to flagellar motors. An important feature of bacterial chemotaxis is amplification; a 0.2% change in receptor occupancy leads to a 23% change in kinase activity. John S. Parkinson (University of Utah, Salt Lake City) showed how receptor clustering into trimers of dimers regulates signal transduction. Genetic analyses suggest that attractant ligands can inhibit the activity of the receptor-associated kinase by inducing a conformational change in the HAMP linker domain, which mediates communication between the sensing and signalling domains of the receptor. Interestingly, work from Victor Sourjik's lab (University of Heidelberg, Heidelberg) shows that receptor sensitivity and adaptation does not change with increased protein expression, and that while the histidine kinase CheA promotes receptor clustering, cluster formation actually depends on the adaptor protein CheW.

E. coli swim by rotating their five to eight helical flagella anticlockwise. CheY switches the direction of rotation, and thus the bacterium's swimming direction, through a yet unclear mechanism. Michael Eisenbach's laboratory (The Weizmann Institute of Science, Rehovot) has identified fumarate as a 'switching factor' able to induce abrupt direction changes, or tumbling, by binding to fumarate reductase, which associates with the motor component FliG. These findings suggest that flagellar motors receive input from other molecules besides CheY. Curiously, CheY acetylation as well as phosphorylation seems to be important for its activity. The physiological significance of this modification is unknown.

The ability of D. discoideum, a soil amoeba, to move up a gradient of an attractive chemical and form aggregates is crucial for their survival in harsh environmental conditions. Rick Firtel's talk (University of California, La Jolla) focused on the signalling feedback loops that amplify shallow extracellular gradients into steep intracellular ones. Work from his lab, and others, has shown that local activation of Ras stimulates phosphatidylinositol 3-kinase (PI3K). This leads to the activation of Rac and actin polymerization, which in turn leads to the recruitment of more PI3K and Rac guanine nucleotide exchange factors (GEFs) thus establishing a positive feedback loop at the leading edge that ensures directional cell movement. By knocking out three PI3Ks in D. discoideum, Firtel's group significantly impaired directional movement as the cells were unable to form stable pseudopods.

The session on sperm chemotaxis, a process that has been recognised in mammals in the past 15 years, highlighted the similarities between the signalling pathways triggered upon attractant binding between species, despite the chemical differences between ligands. In both mammals and non-mammalian species, calcium and voltage-dependent calcium channels are essential for the control of sperm swimming behaviour.

Christopher Wood (Universidad Nacional Autonoma, Mexico) and Benjamin Kaupp (Research Centre Jülich, Jülich) presented data on sea urchin chemotaxis triggered by the egg peptides speract and resact, respectively. Both chemoattractants activate guanylyl cyclase leading to an increase in cGMP that opens K⁺ channels, and to a rapid, transient increase in calcium. Wood's work showed how short calcium fluctuations, such as those induced by speract, create only very brief increases in flagellar asymmetry that result in the sperm alternating frequently between turns and linear swimming. Increasing the length of the calcium fluctuation with niflumic acid increases the asymmetry of the flagellum and thus, the curvature of its trajectory and the duration of turns. Benjamin Kaupp's 'turn and run' model elegantly explains how sperm are able to reach the source of chemoattractant while undergoing multiple turns. Sea urchin sperm normally swims in circles and by the time they sense the gradient the head has already turned away from it. However, the model suggests that the attractant-induced calcium fluctuation is able to rectify the swimming direction causing a 'run' which is almost perfectly directed toward the source of chemoattractant.

Marine sperm have the added difficulty of negotiating attractant gradients in laminar-shear flows. Work from Richard Zimmer's lab (University of California, Los Angeles) has examined how shear flow affects the chemoattractant's range of action or 'plume' around an egg, in a swirling eddy. Their data show that while low shear flow promotes sperm attraction by elongating the plume, high shear flow decreases plume size and inhibits egg-sperm interactions.

Both chemotaxis and thermotaxis have been shown to guide mammalian sperm to the egg. Mammalian spermatozoa must also undergo a maturation or capacitation process to be able to penetrate and fertilize the egg. Donner Babcock (University of Washington, Seattle) showed how motility of mouse sperm undergoes an early activation by bicarbonate and a delayed hyperactivation during capacitation. These events do not occur in cells lacking adenylyl cyclase, PKA-C2, or the sperm-specific calcium channel CatSper. These findings suggest that sperm hypermotility is strongly regulated by pH and that it is both cAMP and calcium dependent.

The steroid hormone progesterone, found in follicular fluid and cumulus cells which surround mammalian oocytes, also stimulates sperm hyperactivation. Curiously, progesterone induces chemotaxis at concentrations in the 10^-10 M range but not at higher concentrations. Laura Giojalas (Universidad de Cordoba, Cordoba, Argentina) suggested that higher progesterone concentrations could aid fertilization in other ways, such as by priming the acrosome reaction. Whether different receptors mediate these effects and whether other chemoattractants take over as the progesterone concentration increases remains to be investigated. The signalling pathways involved in progesterone-triggered sperm chemotaxis and hyperactivation are poorly understood, but work from Steve Publicover's lab (University of Birmingham, Birmingham) suggests ryanodine receptors and nitric oxide regulate the progesterone-induced oscillations of intracellular calcium.

Marc Spehr (Ruhr-University of Bochum, Germany) presented work on bourgeonal - an odorant used by the perfume industry to mimic the Lily of the Valley scent - which has been identified as a potent chemoattractant for human sperm. Bourgeonal signals through a G protein coupled odorant receptor (hOR17-4) to increase intracellular calcium and flagellar beat frequency in capacitated sperm. Despite the fact bourgeonal is unlikely to be secreted in the female genital tract, it can be used as a structural template for the identification of hOR17-4 ligands and the design of potential fertility or contraceptive drugs.

Bernhard Homey (University of Düsseldorf, Düsseldorf) brought the session to a close with surprising findings on the role of chemokines as human sperm chemoattractants. Several assays revealed that the interaction between the chemokine receptor CCR6, found in the flagellum of a subpopulation of human sperm and CCL20, a chemokine secreted by both granulosa cells and oocytes, is required for human sperm chemotaxis.

The last session of the meeting focussed on nerve cell chemotropism. During the development of the nervous system, neurons migrate and extend neurites towards their synaptic targets. The leading edge of the axon, or growth cone, functions as an exquisite sensor that detects and responds to both attractive and repulsive cues that help guide axons to their destinations. Rüdiger Klein (Max-Planck Institute for Neurobiology, Munich-Martinsried) presented work on the Eph family of tyrosine kinase receptors and ephrins, well established guidance molecules in the retina and spinal cord. Interestingly, reminiscent of data on bacterial chemotaxis, ephrin-induced Eph receptor clustering into higher order aggregates affects the local signalling output and thereby, guidance decisions.

cAMP and calcium have previously been shown to modulate the response to guidance cues in neurons. Jochen Buck's laboratory (Cornell University, New York) has identified soluble adenylyl cyclase (sAC) as a novel regulator of the netrin-induced elevation in cAMP and growth cone elaboration. Unlike transmembrane adenylyl cyclases, sAC is found in clusters inside cells, close to cAMP targets. sAC is regulated by bicarbonate and calcium, and it is able to mediate signalling in sperm and neutrophils suggesting it may function as a general regulator of chemotaxis.

James Zheng (University of Medicine and Dentistry, New Jersey) showed how asymmetric elevations of calcium in growth cones regulate axon turning. Interestingly, the actual levels of calcium determine whether the growth cone turns towards or away from a guidance cue. Examination of attractive turning induced by brain-derived neurotrophic factor (BDNF), or local uncaging of calcium, reveals that both Src tyrosine kinase activation and local synthesis of -actin are required for the turning response.

In summary, the 93^rd Boehringer Ingelheim Fonds International Titisee Conference successfully brought together scientists working on the molecular mechanisms underlying the switch from random to directed motility in bacteria, sperm, nerve cells and whole organisms. Despite the diverse nature of chemoattractant molecules and different molecular components in the systems discussed, it was possible to establish some important common themes. For example, receptor clustering, adenylyl and guanylyl cyclases and calcium emerged as general regulators of chemotaxis. This meeting not only helped researchers to put their work in a broader context, it also enabled them to discuss in detail new techniques that may be applied in different systems.

Author: Monica Hoyos-Flight