A mechanism of cell death involving an adenylyl cyclase/PKA signaling pathway is induced by the Cry1Ab toxin of Bacillus thuringiensis

Abstract

Many pathogenic organisms and their toxins target host cell receptors, the consequence of which is altered signaling events that lead to aberrant activity or cell death. A significant body of literature describes various molecular and cellular aspects of toxins associated with bacterial invasion, colonization, and host cell disruption. However, there is little information on the molecular and cellular mechanisms associated with the insecticidal action of Bacillus thuringiensis (Bt) Cry toxins. Recently, we reported that the Cry1Ab toxin produced by Bt kills insect cells by activating a Mg 2؉ -dependent cytotoxic event upon binding of the toxin to its receptor BT-R1. Here we show that binding of Cry toxin to BT-R1 provokes cell death by activating a previously undescribed signaling pathway involving stimulation of G protein (G␣s) and adenylyl cyclase, increased cAMP levels, and activation of protein kinase A. Induction of the adenylyl cyclase͞protein kinase A pathway is manifested by sequential cytological changes that include membrane blebbing, appearance of ghost nuclei, cell swelling, and lysis. The discovery of a toxin-induced cell death pathway specifically linked to BT-R1 in insect cells should provide insights into how insects evolve resistance to Bt and into the development of new, safer insecticides.Cry toxin ͉ protein kinase A ͉ cadherin receptor ͉ cAMP ͉ signal transduction

The pre-pore from Bacillus thuringiensis Cry1Ab toxin is necessary to induce insect death in Manduca sexta

“…Cry toxins insert into lipid rafts membranes present in the apical microvilli membrane of midgut cells of susceptible larvae, causing osmotic lysis and subsequent death of the larvae [3,10,39,46]. A second model proposed that these toxins activate a signal pathway through receptor interaction, resulting in insect cell death without the participation of lytic-pores [45]. It is important to note that this alternative model was proposed based on the effect of Cry1Ab toxin to cultured Trichoplusia ni H5 insect cells expressing the Manduca sexta toxin-receptor, Bt-R 1 , not in insect larvae.…”

“…In the pore forming model, the pre-pore oligomer is responsible for initiating cell death [3]. In contrast, in the signal transduction model [45], only the binding of the monomeric form of the Cry toxin to Bt-R 1 receptor is responsible for toxicity, and interaction with the GPI-anchored receptors and pore formation are not relevant for Cry toxicity. It was proposed in the signal transduction model that binding of Cry toxin to Bt-R 1 receptor activates a G protein and adenyl cyclase, which promotes production of intracellular cAMP.…”

“…It was proposed in the signal transduction model that binding of Cry toxin to Bt-R 1 receptor activates a G protein and adenyl cyclase, which promotes production of intracellular cAMP. The increased levels of cAMP activate Protein Kinase A that causes cell death [45].…”

“…It has been proposed that in order to exert their toxic effect, a pre-pore oligomeric intermediate is formed leading finally to membrane inserted oligomeric pores [3,35]. However, an alternative model proposed that the toxicity of Cry proteins is mainly due to the interaction of monomeric toxin with the Bt-R 1 receptor activating a PKA signaling pathway that results in cell death [45,35].…”

More wrinkles to Bt susceptibility

“…1,2 The activated toxin then binds to specific receptors on gut cells, which results in either cell membrane pores leading to cell lysis or the activation of intracellular signaling pathways resulting in ischemic cell death. 3,4 The selective activity of Bt toxin has led it to become the most widely used biological insecticide worldwide. Since the discovery of its insecticidal activity over 100 y ago, Bt spore and crystal preparations have been used to control crop pests and vectors of human and animal diseases.…”