When flux through the vitamin-B12-dependent mitochondrial propionate breakdown pathway is limiting in the nematode C. elegans, the animal transcriptionally activates a compensatory propionate shunt pathway, and a parallel mechanism adjusts methionine/S-adenosylmethionine-cycle genes. The regulatory circuitry is now well mapped: the nuclear hormone receptors NHR-10 and NHR-68 jointly form a coherent feed-forward 'persistence detector' that activates shunt genes upon sustained propionate accumulation, and NHR-114 mediates the methionine/SAM-cycle response. Yet the sensing event itself — the identity of the causal signal molecule and whether these receptors detect it directly as a ligand or indirectly — has remained unknown since Watson et al. (2013) posed the question of how information about mitochondrial metabolic flux is relayed to nuclear transcription factors. The problem asks for this molecular mechanism: the signal and the sensing event by which flux information reaches these receptors.

Background

When the nematode Caenorhabditis elegans is fed different bacterial diets, large fractions of its genome change expression in a diet-specific way. Watson et al. (2013) combined a forward genetic screen with a genome-wide RNAi screen (RNA interference, in which feeding worms bacteria producing double-stranded RNA against a gene knocks down that gene) on a reporter strain carrying green fluorescent protein driven by the promoter of acdh-1 (an acyl-CoA dehydrogenase gene that is repressed by the Comamonas DA1877 bacterial diet but induced when mitochondrial amino-acid metabolism is perturbed). This delineated a network of 184 genes — mostly mitochondrial enzymes of branched-chain amino acid breakdown, methionine metabolism, and the TCA (tricarboxylic acid) cycle — whose perturbation converges on shared transcriptional regulators. The nuclear hormone receptors (NHRs; ligand-regulated transcription factors, expanded to about 274 members in C. elegans versus 48 in humans) NHR-10 and NHR-68 were identified as key mediators: NHR-10 directly binds the acdh-1 promoter, and both receptors are required for acdh-1 induction in metabolic mutants. Many of the dietary-sensor repressors are orthologs of human genes mutated in inborn errors of amino-acid metabolism, most of which are treated by dietary intervention. Two large coexpressed gene modules showed reciprocal 'see-saw' behavior (one repressed by the Comamonas diet but activated in metabolic mutants, the other the converse), and the authors proposed that metabolites shared among the perturbed pathways — acetyl-CoA, succinyl-CoA, pyruvate, lipoamide, and vitamin B12 — might carry the signal from mitochondria to nucleus, either by acting as nuclear hormone receptor ligands, by inactivating receptors, or by altering protein modifications. The mechanistic question of what is actually sensed, and how, is the problem below.

Subsequent work sharpened the system considerably. Watson et al. (2016) discovered a vitamin-B12-independent propionate shunt pathway that is transcriptionally activated when the canonical, B12-dependent propionate breakdown pathway is blocked by diet or mutation; vitamin B12 is an enzyme cofactor for methylmalonyl-CoA mutase (canonical propionate breakdown) and methionine synthase (the methionine/S-adenosylmethionine, or Met/SAM, one-carbon cycle). Bulcha et al. (2019) showed that shunt activation strictly requires NHR-10 and NHR-68, which operate as a type-1 coherent feed-forward loop (a circuit in which regulator A activates both a target gene and a second regulator B, which also activates the target) with AND logic, functioning as a 'persistence detector' that delays shunt activation until propionate buildup is sustained and avoids responses to transient pulses. Giese et al. (2020) delineated a second, propionate-independent mechanism ('B12-mechanism-II') in which NHR-114 adjusts Met/SAM-cycle genes when that cycle is perturbed by low B12 or mutation. Goh et al. (2023) identified the Mediator complex subunit MDT-15 (a transcriptional coregulator) as an essential physical partner of NHR-10 for shunt-gene activation. Most recently, Worm Perturb-Seq (whole-animal RNAi combined with RNA sequencing) was applied to roughly 900 metabolic genes to map a metabolic gene regulatory network, derive a compensation–repression design principle of transcriptional metabolic rewiring, and infer semi-quantitative metabolic flux wiring from transcriptional phenotypes (Li et al. 2025; Zhang et al. 2025). These studies describe the wiring of the dietary response at the circuit and network levels; the molecular event that initiates it — what the receptors, or an upstream sensor, actually detect — is still not identified.

Problem Statement

The problem is to determine the molecular mechanism by which information about mitochondrial metabolic flux is communicated to specific nuclear transcription factors in the C. elegans dietary response. In the best-mapped instance, reduced flux through the vitamin-B12-dependent propionate breakdown pathway (due to low dietary B12 or mutations in pathway enzymes such as propionyl-CoA carboxylase) causes accumulation of propionate and transcriptional activation of the alternative propionate shunt pathway; this activation strictly requires the nuclear hormone receptors NHR-10 and NHR-68, which operate as a coherent type-1 feed-forward 'persistence detector' (Bulcha et al. 2019), while a parallel B12-dependent mechanism adjusts methionine/S-adenosylmethionine-cycle genes through NHR-114 (Giese et al. 2020). What is not known is the sensing step itself: (1) the identity of the causal signal molecule(s) — propionate itself, a CoA-linked or otherwise derivatized pathway intermediate, the ratio of S-adenosylmethionine to its demethylation product S-adenosylhomocysteine (SAM/SAH), or another shared metabolite; and (2) whether NHR-10, NHR-68, and NHR-114 sense the signal by direct ligand binding (activation or inactivation of the receptor), or indirectly through metabolite-dependent protein modifications or intermediary proteins. A resolution must causally demonstrate the sensing event and its in vivo relevance — for example, direct binding of the signal to the receptor's ligand-binding domain with affinities consistent with intracellular concentrations, and loss of the transcriptional response when the interaction is disrupted — rather than only correlating metabolite levels with gene expression. Enumerating the remaining genes of the dietary response, mapping all contributing signaling pathways, and testing human translation of worm findings are related future directions raised by the source paper but are outside this problem's completion boundary.

References

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