This article is reprinted from the "DXY Academic" WeChat Official Account.
Olfaction is an important sense in animals, responsible for detecting odor signals from the external environment, including tens of thousands of volatile chemical molecules.
In Drosophila, odors are detected by olfactory receptor neurons located in the head, antennae, and maxillary palps. The Drosophila olfactory system shares a similar organizational structure with that of mammals but is comparatively simpler, making it an ideal model for studying olfaction.
Previous studies have shown that fruit flies experience a progressive decline in olfactory function as they age. Furthermore, this loss of olfactory ability may serve as an early indicator of neurodegeneration. Concurrently, aging fruit flies also exhibit a sharp increase in inflammation. For instance, gut microbiota dysbiosis leads to the accumulation of intestinal epithelial cell damage, which in turn promotes the release of inflammatory cytokines. The overactivation of these inflammatory factors may also be associated with neurodegenerative pathologies.
Although scientists have confirmed that olfactory function in fruit flies declines with age, the mechanism underlying this decline remains unclear. Additionally, whether and how this olfactory decline is associated with the increased inflammation experienced by the flies remains unknown.
On July 21, 2021, a research team from Genentech, Inc. (California, USA), the Buck Institute for Research on Aging in Novato, and other institutions published an article titled "Gut cytokines modulate olfaction through metabolic reprogramming of glia" online in the top international journal *Nature*.
Image source: Nature
Studies have shown that following intestinal bacterial infection, glial cells and neurons in the *Drosophila* brain suppress olfaction to prevent the animal from ingesting additional pathogens. This research reveals a mechanism linking gut bacteria to animal behavior at the genetic, neuronal, and organismal levels, which may also represent one of the fundamental ways in which gut microbiota modulate the central nervous system.
Main Research Content
Gut Infection Modulates Drosophila Olfaction
First, researchers used a modified capillary feeder assay to assess the preference of fruit flies for food with or without *Erwinia carotovora carotovora* 15 (Ecc15), a non-lethal pathogen that induces intestinal inflammation.
It was found that uninfected fruit flies consumed more food containing Ecc15; however, fruit flies orally infected with Ecc15 for 24 hours prior to the feeding assay exhibited a pronounced aversion to Ecc15-containing food, indicating that infection leads to a non-specific impairment in olfactory discrimination.
Further research revealed that this olfactory preference is transient, as olfactory recognition recovers 5 days post-infection, consistent with the clearance of gut bacteria and the regeneration of epithelial cells. These observations indicate that Drosophila exhibit aversion to gut pathogens through the coordinated activation of taste and immune receptors, coupled with the suppression of olfaction.
Image source: Nature
Subsequently, the researchers investigated the underlying molecular mechanisms. Following Ecc15 infection in Drosophila, damaged intestinal enterocytes produce the inflammatory IL-6-like cytokines Unpaired 2 and 3 (Upd2 and Upd3), which stimulate intestinal stem cell proliferation and epithelial cell regeneration. Furthermore, Upd family proteins can also activate the JAK-STAT signaling pathway via the Dome receptor.
Subsequent experiments further confirmed this: they used the 2xSTAT::GFP reporter system to monitor JAK-STAT pathway activity and found that 4 hours post-Ecc15 infection, the intestinal epithelium was disrupted and GFP expression was upregulated in the brain. JAK-STAT activity was observed in a sparse population of glial marker-positive cells within the brain.
Further research revealed that, among the five subtypes of Drosophila glial cells (astrocytes, ensheathing glia, perineurial glia, subperineurial glia, and cortex glia 17), ensheathing glia (EG) are the primary population that upregulates STAT activity in response to Ecc15 infection.
Image source: Nature
Gut-Derived Upd Protein Regulates the Drosophila Olfactory System
Subsequently, to test whether gut-derived Upd protein is directly involved in the activation of JAK-STAT signaling in infection-induced EGs, they used Mex1::Gal4 (an enterocyte driver that is not expressed in the brain) to perform enterocyte-specific knockdown.
The results showed that JAK-STAT activation in glia could be triggered in uninfected Drosophila or blocked in infected Drosophila through overexpression or knockdown of Upd2 and Upd3 in enterocytes. Therefore, enterocyte-derived Upd2 and Upd3 are sufficient and necessary for the regulation of infection-induced olfactory discrimination.
Image credit: Nature
Lipid Metabolic Remodeling in Glial Cells
To further investigate how infection modulates olfaction, researchers performed transcriptome sequencing analysis on central nervous system glial cells in Drosophila expressing the 10xSTAT::GFP reporter gene.
The results indicate that, compared with the transcriptome of STAT::GFP− glial cells, the transcriptomes of STAT::GFP+ glial cells from Ecc15-infected and uninfected flies are more similar, suggesting that JAK-STAT induction exerts a stronger impact on the glial transcriptome than other infection-related changes.
Differentially expressed genes were primarily enriched in pathways associated with lipid metabolism and carbohydrate transmembrane transport, including lipid-binding proteins and glycerol O-acyltransferases. These results indicate that activation of the JAK-STAT pathway induces the reprogramming of lipid metabolism.
Image source: Nature
Finally, to demonstrate that infection- or aging-induced changes in EG metabolism can affect the metabolic coupling between Drosophila antennal neurons and glial cells, they used GH146::Gal4 to directly manipulate projection neurons. The results revealed that overexpressing lipase 4 (Lip-4) or knocking down the neuronal lipid-binding protein in projection neurons significantly improved the olfactory discrimination ability of infected or aged flies.
Image source: Nature
Research Summary
In summary, this study demonstrates that gut-derived inflammatory cytokines modulate glial-neuronal metabolic coupling. In the olfactory bulb, glial cells activate STAT, leading to altered expression of genes involved in lactate metabolism. This results in a transient halt in lactate metabolism and lipid accumulation in glial cells, thereby impairing olfaction. However, this impairment is reversible once inflammation subsides.
Age-related intestinal inflammation also triggers the same JAK-STAT pathway, propagating from the gut to glial cells, which leads to permanent changes in olfaction in aging fruit flies.
Image source: Nature
In conclusion, this study provides further compelling evidence for the communication between the gut microbiota and the nervous system. The upregulation of gut-derived Upd2 and Upd3 remodels lipid metabolism in EG, thereby enhancing lactate and lipid transport between glial cells and olfactory neurons.
The detailed characterization of this metabolic reprogramming, along with further exploration of the role of lipid synthesis in projection neurons in glial lipid accumulation and olfactory recognition, will be the focus of future research. Meanwhile, whether this protective mechanism present in juvenile animals—specifically, infection-induced olfactory alterations that drive them to avoid food containing these bacteria—is conserved across other animal species remains to be investigated in greater depth.
References:
1.Cai, X.T., Li, H., Borch Jensen, M. et al. Gut cytokines modulate olfaction through metabolic reprogramming of glia. Nature .
2.Wang, A. et al. Opposing effects of fasting metabolism on tissue tolerance in bacterial and viral inflammation. Cell 166, 1512–1525 .
3.SoriaGómez, E. et al. The endocannabinoid system controls food intake via olfactory processes. Nat. Neurosci. 17, 407–415 (2014).
4.Soni, K. G. et al. Coatomer-dependent protein delivery to lipid droplets. J. Cell Sci. 122,1834–1841.[5] Chakrabarti, S. et al. Remote control of intestinal stem cell activity by haemocytes in Drosophila. PLoS Genet. 12, e1006089 (2016).