Calcium/CaM-sensitive adenylyl cyclase and its role in neuronal plasticity during learning in Aplysia

Abstract

Calmodulin-sensitive adenylyl cyclase (AC) in sensory neurons (SNs) in the marine sea snail, Aplysia, has been proposed as a molecular coincidence detector during conditioning. I identified 4 putative adenylyl cyclases expressed in Aplysia CNS. Calmodulin binds to a sequence in the C1b domain of AC-AplA that resembles the calmodulin-binding sequence in the C1b domain of AC1 in mammals, and also the C1b domain of Ca<super>2+</super>/calmodulin-sensitive rutabaga adenylyl cyclase in Drosophila. rAC-AplA was stimulated by Ca<super>2+</super>/calmodulin. AC-AplC is most similar to the Ca<super>2+</super>-inhibited AC5 and AC6 in mammals. rAC-AplC is directly inhibited by Ca<super>2+</super>, independent of calmodulin. AC-AplA and AC-AplC are expressed in sensory neurons, whereas AC-AplB and AC-AplD are not. Knockdown of AC-AplA demonstrated that serotonin stimulation of cAMP-dependent plasticity in SNs is predominantly mediated by this calmodulin-sensitive adenylyl cyclase. I suggest that the co-expression of a Ca<super>2+</super>-inhibited adenylyl cyclase in SNs, together with a calmodulin-sensitive adenylyl cyclase, would enhance the requirement for coincident Ca<super>2+</super> influx and serotonin for effective stimulation of cAMP levels. When the calmodulin-sensitive adenylyl cyclase is activated by paired Ca<super>2+</super> and serotonin stimuli, activation is most effective when the Ca<super>2+</super> stimulus begins first. This integration of sequential stimuli requires that the modulation of adenylyl cyclase by Ca<super>2+</super>/calmodulin persist after Ca<super>2+</super> levels have declined. Using modeling of calmodulin interactions with adenylyl cyclase, I explored the mechanism that underlies this persistent activation of calmodulin-sensitive adenylyl cyclase. Biochemical assays confirmed predictions of one model in which a target-dependent decrease in the Ca<super>2+</super> dissociation rate of calmodulin underlies to the persistent response of adenylyl cyclase to transient Ca<super>2+</super> stimuli. This is the first suggestion that allosteric effects on the kinetics of Ca<super>2+</super> binding to calmodulin plays a key role in the integration of transient stimuli.