Chemotactic migration of Dictyostelium in a cAMP gradient.

Image courtesy of Dr. Peter N. Devreotes, John Hopkins University, Baltimore, USA.

Chemotaxis — the capacity of cells to move in response to gradients of extracellular molecules — plays an important role in embryogenesis, cancer metastasis and in the migration of immune cells towards sites of infection. Despite its importance in both physiological and pathological conditions, chemotactic regulation is still poorly understood. Levels of the second messenger phosphatidylinositol-3,4,5-trisphosphate (PIP₃) are known to be important in chemotaxis and are regulated by PI3-kinases (PI3Ks) and phosphatases such as PTEN and SHIP. Peter Devreotes and colleagues now report the identification of a patatin-like phospholipase A₂ (PLA₂) homologue that functions in parallel with PI3K pathways during chemotaxis in Dictyostelium discoideum.

The authors screened for mutants exhibiting selectively impaired chemotaxis upon inhibition of PI3K, and isolated cells carrying a mutation in the plaA gene, which encodes PLA₂A. During starvation, plaA^- cells failed to migrate to form large cell aggregates in the absence of PI3K activity. Similarly, mutants lacking PI3K1 and PI3K2 activity required functional PLA₂ in order to migrate and aggregate.

The authors further examined the migration properties of mutant cells when exposed to a gradient of cAMP. Both plaA^- and pi3k1^-/pi3k2^- cells were able to polarise and migrate towards the source of chemoattractant, but their chemotactic capacities were strongly impaired when combined with an inhibition of PI3K or PLA₂A activity, respectively. Similarly, severe defects in migration were observed in plaA^-/pi3k1^-/pi3k2^- triple mutants, suggesting that PI3K and PLA₂A function via parallel pathways. Moreover, PIP₃ accumulated at the same rate in plaA^- and wild-type cells upon chemoattractant stimulation, indicating that PLA₂A does not interfere with the PI3K pathway.

Cells lacking both PLA₂A and PI3Ks activity were unable to maintain their shape, did not follow the chemoattractant gradient and moved randomly. These observations led the authors to suggest that PLA₂A confers the capacity to respond to a chemoattractant gradient, but may not affect motility.

PIP₃ regulates actin polymerization, an important component of directed cell movement. When exposed to a chemoattractant, D. discoideum cells respond with a rapid and a slow phase of actin polymerization to change shape. Cells initially freeze and round up, and then extend multiple new pseudopodia before migrating. Devreotes had previously reported that PI3K activity is essential for the second phase, now they show that the first phase of actin polymerization can be supported by either PI3K or PLA₂ pathways.

Finally, the authors confirmed that PLA₂A has Ca²⁺-independent phospholipase activity and is able to release an arachidonic acid derivative. The levels of this compound were reduced by more than 70% in plaA^- cells. Exogenous arachidonic acid was able to partially rescue the defects observed in plaA^- cells, showing its specific requirement for chemotaxis. Targets of this metabolite remain to be identified.

Devreotes and colleagues provide evidence that the PI3K and PLA₂ pathways function in parallel to mediate chemotaxis in D. discoideum. As the role of PIP₃ in polarity and directional sensing is conserved throughout evolution, it will be interesting to determine whether PLA₂–mediated pathways are also involved in chemotaxis in other organisms.

Author: Kim Baumann