Topic 23.2

ABA May Be an Ancient Stress Signal

Although long considered a plant-specific signal, several recent reports have documented a role for ABA in regulating stress responses in animals ranging from sea sponges to mammals. ABA was first detected in animals over twenty years ago, where it was found to accumulate to the highest levels (~15-20 nM) in mammalian brain tissue (Le Page-Degivry et al. 1986). Despite evidence indicating that it was unlikely that the ABA could be coming from the animals’ diets, no function for the ABA was proposed or tested.

Subsequent studies in diverse organisms (cyanobacteria, unicellular parasites, sponges, hydroids, and mammals) have suggested that ABA may serve as a “universal Ca²⁺ agonist” in a variety of stress responses. In cyanobacteria, ABA is produced in response to salt stress and promotes formation of heterocysts, thick-walled cells specialized for nitrogen fixation. The protozoan parasite Toxoplasma gondii uses ABA to control Ca²⁺ release, which stimulates egress and therefore transmission of the parasite from its host.

Over the past decade, Zocchi and coworkers have worked their way up the evolutionary tree, investigating ABA responses from sponges to mammals (reviewed in Wasilewska et al. 2008). Sponges respond to a high temperature stress by an ABA-induced transient increase in water filtration, mediated by cADPR-stimulated release of intracellular Ca²⁺. Hydroids are capable of regenerating entire sections of their bodies following damage; this process is stimulated by light, in an ABA-dependent process that is again mediated by cADPR-stimulated increases in Ca²⁺. In humans, ABA signaling has been demonstrated in pathogen response, where it acts as a pro-inflammatory cytokine to stimulate phagocytosis, cell migration, ROS, and NO production. ABA also mediates response to high glucose levels in pancreatic β-cells, resulting in insulin secretion (Bruzzone et al., 2008), and promotes stem cell proliferation and mobility in response to specific growth factors and inflammatory cytokines. In all of these examples, ABA is produced and active at nanomolar to submicromolar levels. Also similar to ABA signaling in plants is the implication of G-protein involvement and evidence of binding at the cell surface, but the specific receptors involved in animal recognition of ABA have not yet been identified.

However , the observations that ABA production by diverse organisms is sensitive to common inhibitors of carotenoid synthesis, and that ABA signaling is mediated by mechanisms involving G-protein complexes, cADPR activation of Ca²⁺ release, and in some cases ROS and NO production suggest that this is an evolutionarily ancient signal.