Bacillus thuringiensis toxins divert progenitor cells toward enteroendocrine fate by decreasing cell adhesion with intestinal stem cells in Drosophila.

Bacillus thuringiensis subsp. kurstaki (Btk) is a strong pathogen toward lepidopteran larvae thanks to specific Cry toxins causing leaky gut phenotypes. Hence, Btk and its toxins are used worldwide as microbial insecticide and in genetically modified crops, respectively, to fight crop pests. However, Btk belongs to the B. cereus group, some strains of which are well known human opportunistic pathogens. Therefore, ingestion of Btk along with food may threaten organisms not susceptible to Btk infection. Here we show that Cry1A toxins induce enterocyte death and intestinal stem cell (ISC) proliferation in the midgut of Drosophila melanogaster, an organism non-susceptible to Btk. Surprisingly, a high proportion of the ISC daughter cells differentiate into enteroendocrine cells instead of their initial enterocyte destiny. We show that Cry1A toxins weaken the E-Cadherin-dependent adherens junction between the ISC and its immediate daughter progenitor, leading the latter to adopt an enteroendocrine fate. Hence, although not lethal to non-susceptible organisms, Cry toxins can interfere with conserved cell adhesion mechanisms, thereby disrupting intestinal homeostasis and endocrine functions.

Differential side-effects of Bacillus thuringiensis bioinsecticide on non-target Drosophila flies.

Bioinsecticides based on Bacillus thuringiensis (Bt) spores and toxins are increasingly popular alternative solutions to control insect pests, with potential impact of their accumulation in the environment on non-target organisms. Here, we tested the effects of chronic exposure to commercial Bt formulations (Bt var. kurstaki and israelensis) on eight non-target Drosophila species present in Bt-treated areas, including D. melanogaster (four strains). Doses up to those recommended for field application (~ 106 Colony Forming Unit (CFU)/g fly medium) did not impact fly development, while no fly emerged at ≥ 1000-fold this dose. Doses between 10- to 100-fold the recommended one increased developmental time and decreased adult emergence rates in a dose-dependent manner, with species-and strain-specific effect amplitudes. Focusing on D. melanogaster, development alterations were due to instar-dependent larval mortality, and the longevity and offspring number of adult flies exposed to bioinsecticide throughout their development were moderately influenced. Our data also suggest a synergy between the formulation compounds (spores, cleaved toxins, additives) might induce the bioinsecticide effects on larval development. Although recommended doses had no impact on non-target Drosophila species, misuse or local environmental accumulation of Bt bioinsecticides could have side-effects on fly populations with potential implications for their associated communities.

Bacillus thuringiensis Bioinsecticides Induce Developmental Defects in Non-Target Drosophila melanogaster Larvae.

Bioinsecticides made from the bacterium Bacillus thuringiensis (Bt) are the bestselling bioinsecticide worldwide. Among Bt bioinsecticides, those based on the strain Bt subsp. kurstaki (Btk) are widely used in farming to specifically control pest lepidopteran larvae. Although there is much evidence of the lack of acute lethality of Btk products for non-target animals, only scarce data are available on their potential non-lethal developmental adverse effects. Using a concentration that could be reached in the field upon sprayings, we show that Btk products impair growth and developmental time of the non-target dipteran Drosophila melanogaster. We demonstrate that these effects are mediated by the synergy between Btk bacteria and Btk insecticidal toxins. We further show that Btk bioinsecticides trigger intestinal cell death and alter protein digestion without modifying the food intake and feeding behavior of the larvae. Interestingly, these harmful effects can be mitigated by a protein-rich diet or by adding the probiotic bacterium Lactobacillus plantarum into the food. Finally, we unravel two new cellular mechanisms allowing the larval midgut to maintain its integrity upon Btk aggression: First the flattening of surviving enterocytes and second, the generation of new immature cells arising from the adult midgut precursor cells. Together, these mechanisms participate to quickly fill in the holes left by the dying enterocytes.

Apoptosis restores cellular density by eliminating a physiologically or genetically induced excess of enterocytes in the Drosophila midgut.

Using pathogens or high levels of opportunistic bacteria to damage the gut, studies in Drosophila have identified many signaling pathways involved in gut regeneration. Dying cells emit signaling molecules that accelerate intestinal stem cell proliferation and progenitor differentiation to replace the dying cells quickly. This process has been named 'regenerative cell death'. Here, mimicking environmental conditions, we show that the ingestion of low levels of opportunistic bacteria was sufficient to launch an accelerated cellular renewal program despite the brief passage of bacteria in the gut and the absence of cell death and this is is due to the moderate induction of the JNK pathway that stimulates stem cell proliferation. Consequently, the addition of new differentiated cells to the gut epithelium, without preceding cell loss, leads to enterocyte overcrowding. Finally, we show that a couple of days later, the correct density of enterocytes is promptly restored by means of a wave of apoptosis involving Hippo signaling and preferential removal of old enterocytes.

Protease Activity Assay in Fly Intestines.

The intestine is a central organ required for the digestion of food, the absorption of nutrients and for fighting against aggressors ingested along with the food. Impairment of gut physiology following mucosal damages impacts its digestive capacities that consequently will affect growth, wellbeing or even survival of the individual. Hence, the assessment of intestinal functions encompasses, among others, the monitoring of its integrity, its cellular renewing, its immune defenses, the production of enteroendocrine hormones and its digestive capacities. Here, we describe in detail how to assess the activity of the proteases secreted in the intestinal lumen of adult Drosophila melanogaster flies. This method can also be used for larval intestines. The present protocol is adapted and improved from the Sigma-Aldrich's protocol proposed in the 'Protease Fluorescent Detection Kit' (Product code PF0100).