Cell death signaling in Anopheles gambiae initiated by Bacillus thuringiensis Cry4B toxin involves Na + /K + ATPase

Abstract

Identifying the mechanisms by which bacterial pathogens kill host cells is fundamental to understanding how to control and prevent human and animal disease. In the case of Bacillus thuringiensis (Bt), such knowledge is critical to using the bacterium to kill insect vectors that transmit human and animal disease. For the Cry4B toxin produced by Bt, its capacity to kill Anopheles gambiae, the primary mosquito vector of malaria, is the consequence of a variety of signaling activities. We show here that Cry4B, acting as first messenger, binds specifically to the bitopic cadherin BT-R₃ G-protein-coupled receptor (GPCR) localized in the midgut of A. gambiae, activating the downstream second messenger cyclic adenosine monophosphate (cAMP). The direct result of the Cry4B–BT-R₃ binding is the release of α_s from the heterotrimeric αβγ-G-protein complex and its activation of adenylyl cyclase (AC). The upshot is an increased level of cAMP, which activates protein kinase A (PKA). The functional impact of cAMP–PKA signaling is the stimulation of Na⁺/K⁺-ATPase (NKA) which serves as an Na⁺/K⁺ pump to maintain proper gradients of extracellular Na⁺ and intracellular K⁺. Increased level of cAMP amplifies NKA and upsets normal ion concentration gradients. NKA, as a scaffolding protein, accelerates the first messenger signal to the nucleus, generating additional BT-R₃ molecules and promoting their exocytotic trafficking to the cell membrane. Accumulation of BT-R₃ on the cell surface facilitates recruitment of additional toxin molecules which, in turn, amplify the original signal in a cascade-like manner. This report provides the first evidence of a bacterial toxin using NKA via AC/PKA signaling to execute cell death.