Dictyostelium

How do the amoebae know where to go? Amoebae are chemotactic: they can sense gradients of certain chemicals and move along them. Dicty is known to chemotax to two chemicals: folic acid, which is released by bacteria and used in the hunt for food, and cAMP, which is released by amoebae during starvation and used to find each other during aggregation. Cells have evolved a relay mechanism in which cAMP stimulates its own release, thus forming waves that can propagate through a field of responsive amoebae (Figure 2). Amoebae respond to cAMP gradients by polarising: creating a leading edge and a rear with different sets of lipids and proteins defining each pole. A classic example of this is the accumulation of PI(3,4,5)P3 at the leading edge. After polarising, amoebae begin to move up the gradient of cAMP and are extremely sensitive to even shallow concentration changes — they can detect as little as 2% difference across their length. How cells are able to sense and interpret a gradient is a major question in biology. It is widely accepted that the core features of the chemotactic signalling process and machinery are conserved from Dicty to mammals. Dicty has therefore become a very popular model for studying chemotaxis, because findings in Dicty often translate to the directed migration seen during the immune response, wound healing, embryogenesis and in tumour cell metastasis.