The first and well-established model, referred to as the pore-forming model, requires the sequential binding to two specific receptors localized at the plasma membrane of insect intestinal cells: a cadherin receptor protein (CADR) and a glycosyl-phosphatidylinositol (GPI) membrane-anchored aminopeptidase N (APN). Two different modes of action on intestinal cells have been proposed and particularly well documented for Cry1A toxins. After spore and crystal ingestion they are delivered to the insect intestinal tract where their activation occurs allowing binding to midgut epithelial cells that results in cell lysis and death of the target insect ( Raymond et al., 2010). They belong to the pore forming toxins (PFT) class of bacterial toxins ( Palma et al., 2014). Cry proteins are produced as protoxins in crystal inclusions during Bt sporulation phase. The major insecticidal weapons of Bt are two multigenic families of toxins, cry and cyt ( Crickmore et al., 1998). Together, these results provide new information about the mechanism of Cry1Ca toxicity and clues to potential resistance factors yet to discover. Indeed, Sf9 sensitive cells produced high levels of cAMP upon toxin stimulation, while Sf9 resistant cells were unable to increase their intracellular cAMP. We observed a correlation between Cry1Ca cytotoxicity and the increase of intracellular cAMP levels. This suggested a resistance mechanism different from the classical ‘loss of toxin binding’. Selected cells were highly resistant to Cry1Ca while toxin binding onto their plasma membrane was not affected. Ion chelators protected sensitive cells from Cry1Ca toxicity suggesting the necessity of both Ca 2+ and/or Mg 2+ for toxin action. frugiperda microarray we performed a comparative transcriptomic analysis between sensitive and resistant cells and revealed genes differentially expressed in resistant cells and related to cation-dependent signalling pathways. Through a selection program we developed various levels of laboratory-evolved Cry1Ca-resistant Sf9 cell lines. The Sf9 insect cell line, derived from Spodoptera frugiperda, is highly and specifically sensitive to Cry1Ca. Although Cry1Ca has been shown to form ionic pores in the plasma membrane leading to cell swelling and death, we investigated the existence of other cellular or molecular events involved in Cry1Ca toxicity. While a first model – referred to as the pore forming model – is the most widely accepted scenario, a second model proposed that toxins could trigger an Mg 2+-dependent intracellular signalling pathway leading to cell death. However, their molecular and cellular mode of action is still unclear. Bacillus thuringiensis (Bt) produces pore forming toxins that have been used for pest control in agriculture for many years.
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