title: Gut microbial regulation of organismal health through Tachykinin in Drosophila
Creators: name: Marcu, Diana Elena
Publisher: name: University of Glasgow
Year: 2024

description: The complex relationship between the gut microbiota and host physiology is a multifaceted area of investigation with profound implications for systemic health and ageing. Despite residing predominantly in the gut, the microbiota holds the potential to systemically impact overall host health, including complex processes like ageing. This prompts the question, by what mechanisms does the gut microbiota systematically influence the host? Host-derived hormones, particularly gut peptides secreted by enteroendocrine cells, emerge as potential mediators for conveying the microbiota's influence on lifespan and metabolism. However, the exact molecular mechanisms through which microbiota regulate host enteroendocrine signalling, and the relevance of this in systemic host health, is unknown.

Drosophila melanogaster was used as an in vivo model to study the impact of microbiota on host via enteroendocrine signalling. To address this, I used a unique approach, integrating germ-free and gnotobiotic conditions with targeted genetic manipulations. This strategy provided a platform to unravel the specific roles of enteroendocrine peptides in the context of microbial influence. RNAseq analysis and fluorescence staining showed that the microbiota shapes the expression levels of gut peptides, and the number EE cells present in the gut. In particular, the differentially expressed host derived gut peptide, tachykinin (TK), proved to be a strong candidate to mediate the influence of microbiota on host health. Germ-free and conventional flies were used to determine if TK responds to microbial cues to regulate complex host phenotypes such lipid metabolism, lifespan, starvation resistance, feeding behaviour and fecundity. The focus was specifically on two phenotypes: lifespan and lipid metabolism. In the presence of microbiota, ubiquitous RNAi against TK extended lifespan, but eliminating the microbiota had no additive effect. TK knockdown also increased lipid levels in conventional flies, but this effect was reversed in germ-free flies, demonstrating that the microbiota regulates complex host traits through a TK mediary. To refine which members of the microbiota interact through TK, gnotobiotic flies mono-colonised by either the gut symbiont Acetobacter pomorum, or Lactobacillus brevis were used. A. pomorum was found to strongly modulate lifespan and lipid levels via TK, while L. brevis had a marginal impact. It was further determined that in order to achieve lifespan modulation, A. pomorum regulated TK expression in the gut, which then targets its receptor TKR99D in the brain. In terms of potential mechanisms mediating the impact of the interaction between A. pomorum and TK - feeding and egg laying assays suggest that nutrient restriction and reduced reproduction can be excluded but impacts on 4E-BP and Akt expression suggest roles for the IIS/TOR signalling network. In support of this, ablation of insulin producing cells phenocopies the TK knockdown lifespan phenotype. However, knockdown of TK in null-dFOXO mutants showed that, while dFOXO is required for TK to modulate lifespan, it is not required for microbial lifespan regulation, suggesting that other interacting mechanisms are likely to be involved. 

In conclusion, this thesis implicates TK as a pivotal mediator of the effect of microbiota on host lifespan, setting the stage for innovative approaches to delay ageing and improve healthspan.