The study sits alongside work on how the nematode nervous system assembles itself day by day, but it deals not with development, rather with the behavior of the adult worm. It is already known that NHR-49 governs lipid metabolism, lifespan and pathogen resistance, and its role in bacterial avoidance and aversive memory has been explained through fatty acids. What remained unclear was whether this receptor can influence neurons directly, without the mediation of lipid metabolism. Our work is the first to test this using classic attractive chemotaxis and mechanosensory habituation.
We used C. elegans nematodes carrying the null mutation nhr-49(nr2041) and tissue-specific transgenic lines expressing NHR-49::GFP under different promoters. Chemotaxis was assessed in Petri dishes with diacetyl, pyrazine, isoamyl alcohol and benzaldehyde by calculating a chemotaxis index. To test the role of lipids, we added oleic acid and tested mutants for the genes nhr-80, pmp-4 and nhr-66. Gene expression was measured by RT-qPCR, and activity of the odr-10 promoter was visualized with a GFP reporter. Mechanosensory responses and habituation were analyzed on a Multi-Worm Tracker system with 30 consecutive stimuli and one test stimulus after a pause.
The nhr-49(nr2041) mutants showed a significant reduction in attraction to diacetyl and pyrazine, but not to isoamyl alcohol or benzaldehyde. Adding oleic acid did not rescue the defect, and the nhr-80 and pmp-4 mutants behaved like the wild type; the nhr-66 mutant showed only a slight reduction. Expression of NHR-49::GFP under its own, a pan-neuronal or a muscle promoter restored chemotaxis, whereas intestinal or hypodermal expression did not help. Depleting NHR-49 in neurons using auxin-inducible degradation also reduced chemotaxis. In the mutants, the proportion of animals with a GFP signal from odr-10p::gfp decreased, although the overall level of odr-10 mRNA did not change. In the habituation tests, the mutants were smaller and slower, but their initial reversal probability did not differ; however, the difference in reversal probability between the first and last stimuli was smaller than in the wild type, which indicates weakened habituation.
The work differs from neighboring studies in that it separates the behavioral function of NHR-49 from its canonical role in fatty acid metabolism. Whereas pathogen avoidance and aversive memory had previously been explained through desaturation and beta-oxidation, here it is shown that attractive chemotaxis does not depend on these pathways. This is the first indication that NHR-49 may act in neurons through other mechanisms, possibly involving sphingolipids or direct regulation of genes such as odr-10. Such a result changes the picture of the boundaries of this receptor's functions and opens a new direction for studying the coordination of metabolism and behavior.
The topic could develop toward mapping single-neuron transcriptomes under NHR-49 control, especially in AWA, in order to identify direct targets. It would be promising to investigate the role of sphingolipids and their inter-tissue signaling in chemotaxis, and also to test whether NHR-49 forms heterodimers with ODR-7. Another path is a detailed study of mechanosensory circuits and determining in which neurons exactly NHR-49 is required for habituation.
The results will influence invertebrate neurobiology, research on nuclear receptors and lipid signaling in the nervous system.
Next steps include RNA sequencing of isolated AWA neurons from nhr-49 mutants, analysis of sphingolipid composition and testing of the interaction between NHR-49 and ODR-7.
The work touches on a fundamental problem in neuroscience: how metabolic regulators directly influence neuronal functions, bypassing systemic effects. For the C. elegans field, it is a step toward understanding how a single transcription factor coordinates feeding behavior and energy metabolism.
Despite its tiny nervous system of 302 neurons, C. elegans distinguishes odors using specialized sensory neurons: AWA handles attractive aromas, AWC handles other attractants, and ASH handles repulsion. NHR-49 turned out to be a "conductor" only for AWA-dependent responses.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.