Periodic Reporting for period 1 - BRAIN (Bacterial regulation of Apis neurophysiology)
Période du rapport: 2018-07-01 au 2020-06-30
The bacterial communities harboured by the gastrointestinal tract of animals play key roles in host nutrition and immunity, and are increasingly recognized as regulators of brain function and consequent animal behaviour. The evolution of the brain itself is likely intertwined with that of microbial symbionts, having co-evolved through evolutionary time. However, such relationships have remained largely obscure due to lack of investigation in experimental models of the gut microbiota – brain axis beyond those traditionally used in pharmacology (i.e. rodents).
This project aimed to achieve a first characterization of the effects of the gut microbiota on the brain physiology and social behaviour of the honey bee, a social insect for which the gut microbiota is well-characterized and experimental protocols to produce microbiota-depleted bees and bees harbouring any desired combination of gut microbes have previously been established. The gut microbiota – brain axis has drawn tremendous amounts of scientific and lay-public attention in recent years as an increasing number of studies are pinpointing microbial roles in neurodegenerative diseases and behavioural dysfunction. However, an integrated approach to unravel the proximate mechanisms of host-brain-symbiont interaction had not yet been pursued in social insects, whose complex social behaviours are second only to human behaviours.
This project aimed to achieve a first characterization of the effects of the gut microbiota on the brain physiology and social behaviour of the honey bee, a social insect for which the gut microbiota is well-characterized and experimental protocols to produce microbiota-depleted bees and bees harbouring any desired combination of gut microbes have previously been established. The gut microbiota – brain axis has drawn tremendous amounts of scientific and lay-public attention in recent years as an increasing number of studies are pinpointing microbial roles in neurodegenerative diseases and behavioural dysfunction. However, an integrated approach to unravel the proximate mechanisms of host-brain-symbiont interaction had not yet been pursued in social insects, whose complex social behaviours are second only to human behaviours.
We coupled microbiome manipulations with RNA-sequencing of brain and gut tissues, cuticular hydrocarbon (CHC) profiling by GC-MS, and automated behavioural tracking to unravel the host responses to microbial colonization along the honey bee gut-brain axis. To gain a better understanding of the brain regions involved in the putative interactions between the gut microbiota and the host nervous system, we dissected the brains into three macro-regions that are responsible for learning and memory, perception of olfactory and gustatory stimuli, and visual processing, respectively. We verified that microbial manipulations were successful by 16S rRNA gene amplicon sequencing and qPCR with universal bacterial primers, assessing both the composition and total load of the gut community in gut tissues.
The analyses of these large RNA-sequencing (involving 160 samples) and CHC datasets demonstrated for the first time that the gut bacterial community affects the physiology of the brain in a eusocial animal, without changing the CHC profiles that social insects use to recognize their nestmates and project their fertility status. Our results produced a short list of candidates for further functional studies into the proximate mechanisms of host-brain-symbiont interactions. They also highlighted that the brain region most affected by the gut microbiota was the one involved in the perception of olfactory and gustatory stimuli. The candidate genes include several with known involvement in brain development in other organisms, and the functional categories to which they belong show some overlap with those associated with the gut microbiota – brain axis of vertebrate models. We further developed an automated behavioural tracking approach to investigate the social behavioural phenotypes of the bees while keeping them in a microbiota-depleted state, which allowed us to assess the effects of the gut microbiota on the honey bee social network structure. These large datasets have been successfully collected, and the analyses are ongoing and will be finalized in the next few months.
As part of this project, a review on the gut microbiota – brain axis of insects was published in Current Opinion in Insect Science.
The analyses of these large RNA-sequencing (involving 160 samples) and CHC datasets demonstrated for the first time that the gut bacterial community affects the physiology of the brain in a eusocial animal, without changing the CHC profiles that social insects use to recognize their nestmates and project their fertility status. Our results produced a short list of candidates for further functional studies into the proximate mechanisms of host-brain-symbiont interactions. They also highlighted that the brain region most affected by the gut microbiota was the one involved in the perception of olfactory and gustatory stimuli. The candidate genes include several with known involvement in brain development in other organisms, and the functional categories to which they belong show some overlap with those associated with the gut microbiota – brain axis of vertebrate models. We further developed an automated behavioural tracking approach to investigate the social behavioural phenotypes of the bees while keeping them in a microbiota-depleted state, which allowed us to assess the effects of the gut microbiota on the honey bee social network structure. These large datasets have been successfully collected, and the analyses are ongoing and will be finalized in the next few months.
As part of this project, a review on the gut microbiota – brain axis of insects was published in Current Opinion in Insect Science.
Accumulating evidence suggests that microbial signals are important for healthy neurodevelopment and programming of social behaviors. However, from an evolutionary perspective it is unclear how and when relationships between bacterial symbionts and the social brain arose. Through this project, we have shown that the honey bee represents a suitable model to advance our understanding of the gut microbiota – brain axis. We provided the very first characterization of gut microbiota effects on brain physiology and social behaviour in a social insect, as well as an efficient methodology to track social interactions of gnotobiotic bees. Honey bees represent a flagship species of universal importance both in terms of economic value and for global pollinator services. Our work has contributed a first proof of concept for the potential of probiotics for behavioural trait selection, which in the future may become instrumental to improve honey bee health.