Toxin Binding Receptors and the Mode of Action of Bacillus thuringiensis subsp. israelensis Cry Toxins

Cadherins play pivotal roles in the toxicity of Bacillus thuringiensis Cry proteins. Here I tested and showed that an Aedes cadherin (AAEL007478) and an N-cadherin (AAEL000597) are involved in the in vivo toxicity of Cry11A toxin to Aedes aegypti. Aedes cadherin was stably expressed in a mosquito cell line and these cells showed increased sensitivity (37% death) to Cry11Aa toxin. These results show Aedes cadherin mediates Cry11Aa toxicity, but since high toxicity was not obtained, an additional secondary receptor may be needed for manifestation of full toxicity. Using a whole genome screen to identify genes that are altered during Cry11Aa intoxication, I identified an N-cadherin gene (AAEL000597) that was significantly down-regulated. An EGF-LamG fragment from this N-cadherin bound Cry11Aa with high affinity and competed with Cry11Aa binding to mosquito midgut membranes. Moreover, N-cadherin-silenced mosquitoes showed tolerance to Cry11Aa, implying that this cadherin is involved in mediating Cry11Aa toxicity. I also showed that this N-cadherin interacts with an amino acid on loop α-8 of Cry11Aa, which is different from that which interacts with the Aedes cadherin binding region. These data suggest that Cry11Aa probably has two different pathogenic pathways that act through two different cadherins in Ae. aegypti.I also established a Cry11A-resistant strain to determine which mode of action is involved in Aedes Cry11Aa resistance. Brush border membranes from this strain (G30) bound Cry11Aa less compared to the binding in the wild type (WT), implying Cry11Aa resistance results from altered receptor binding affinity, but not proteolytic activity since no change in the latter was observed. Using RNA-seq analyses, immunoblot assays and mass spectrometry, we found the N-cadherin (AAEL000597) and an alkaline phosphatase (ALP, AAEL003298) were down-regulated in Cry11A-resistant larvae midgut. These results strongly suggest that N-cadherin and ALP are associated with Cry11Aa resistance in Ae. aegypti.In summary, based on data in the literature and my work, I demonstrate that two different pathways of Cry11Aa toxicity are possible; one involving an N-cadherin and the other a combination of Aedes cadherin and an ALP in Ae. aegypti. Moreover, N-cadherin and ALP not only mediate Cry11Aa toxicity, but they were appear to be associated with Cry11Aa resistance. ALP has been proposed as a secondary receptor mediating Cry11Aa toxicity with Aedes cadherin, and hence attenuation of its expression can lead to Cry11A resistance