Starvation is defined as the short or long-term absence of nutrients, including caloric energy, below the threshold that is needed to support the life of an organism (Baugh and Hu, 2020). In its natural habitat, C. elegans leads a boom-or-bust lifestyle characterized by a prolific and rapid reproduction, typically experiencing either plentiful food environments or else a complete absence of food. Related laboratory experiments usually mimic these conditions, providing either food ad libitum or not at all (note, dietary restriction is discussed below).

C. elegans features several responses to starvation that involve different regulatory factors and that lead to distinct outcomes for the animal depending on its developmental stage at the onset of starvation. These adaptations include the dauer diapause, the adult reproductive diapause (ARD), and larval arrest [for details, see (Baugh and Hu, 2020)]. For example, the response to larval stage 1 (L1) starvation involves rewiring of energy metabolism from anabolic processes towards catabolic lipolysis. This is regulated by the transcription factor helix-loop-helix 30 (HLH-30), the homolog of the mammalian transcription factor EB (TFEB) (O’Rourke and Ruvkun, 2013; Settembre et al., 2013).

NHR-49 is an important regulator of the starvation response, with roles in several of the aforementioned stages. When food is available, nhr-49 is required for the expression of many lipid metabolism genes, including fat-7 and acs-2 (Van Gilst et al., 2005a; Van Gilst et al., 2005b). However, when nutrients are limited in L1 or L4 stage larvae, animals break down fats stored in lipid droplets to satisfy their energy requirements, and NHR-49 plays a key role in controlling this process. Specifically, following a 12 h starvation, acs-2 is highly induced by NHR-49, whereas expression of fat-7 is downregulated across all worm stages (Taubert et al., 2006; Van Gilst et al., 2005b). nhr-49 is also required to induce non-lipid metabolism related genes, including the glyoxylate cycle enzyme gene icl-1, the oxidoreductase gene sodh-1, and the flavin-containing monooxygenase gene fmo-2 (Goh et al., 2018). Although it is not clear if or how these genes all contribute to metabolic remodeling in starvation, fmo-2 is required in wild-type worms for starvation survival (Goh et al., 2018). In addition, NHR-49 acts in the intestine during short-term starvation (2 h) to limit lipid accumulation within lysosomes. NHR-49 does so by upregulating lysosomal hydrolases and phospholipases to catabolize lipids within this organelle (Huang et al., 2014). Due to its important downstream regulatory functions in starvation, nhr-49 is thus required for L1 stage starvation recovery in C. elegans (Goh et al., 2018).

NHR-49’s relationship with other starvation response regulators suggests nonredundant functions. HLH-30 is a master regulator of the starvation response that regulates lysosome biogenesis and autophagy (Lapierre et al., 2013; Settembre et al., 2013; Harvald et al., 2017). In the mammalian liver, its ortholog TFEB controls expression of the PPARα–PPARγ coactivator 1 alpha (PGC-1α) complex to regulate lipid metabolism during starvation (Settembre et al., 2013). However, no data to date support a similar interaction between HLH-30 and NHR-49 in C. elegans. In fact, HLH-30 seems to be dispensable or only partially required for the induction of nhr-49-dependent stress response genes (Leiser et al., 2015; Goh et al., 2018). This, along with the synthetic lethality seen in the attempt to make an nhr-49;hlh-30 double null mutant (Goh et al., 2018) suggests that these two factors act non-redundantly in C. elegans.

To regulate transcription of target genes, NHR-49 must localize to the nucleus. Watterson et al. observed this under starved conditions by studying animals that overexpress an NHR-49::GFP fusion protein (Watterson et al., 2022b). In ad libitum fed conditions, NHR-49 is bound to cytosolic vesicles by the small G protein RAB-11.1 (Watterson et al., 2022b). However, during starvation, loss of lipid homeostasis causes NHR-49 to be released from these vesicles, whereupon it localizes to the nucleus to activate genes such as acs-2 (Watterson et al., 2022b). Interestingly, NHR-49’s release from endocytic vesicles can also be triggered by loss of the heat shock factor hsf-1, which leads to increased NHR-49 nuclear localization and activity (Brunquell et al., 2016; Watterson et al., 2022a). Future work into this mechanism could distinguish if NHR-49 nuclear localization is controlled by similar mechanisms in other stresses to which NHR-49 provides functional adaptation (see below).

When L3 or L4 stage worms are deprived of food, they arrest in a specialized adult stage termed the ARD. The germline of these animals contains only a few quiescent stem cells. This allows the worms to extend their lifespan 3-fold, providing an opportunity to locate food (Angelo and Van Gilst, 2009). When food is available, these worms can recover and produce progeny as normal (Angelo and Van Gilst, 2009; Baugh and Hu, 2020; Gerisch et al., 2020). Although nhr-49 is not involved in ARD when beginning the starvation period during the mid-L3 stage (Gerisch et al., 2020), starvation onset in mid-L4 worms requires nhr-49 for entry into and recovery from adult reproductive diapause (Angelo and Van Gilst, 2009; Eustice et al., 2022). nhr-49 may be important for entry into ARD due to its requirement in inducing β-oxidation (Eustice et al., 2022). Thus, NHR-49 plays a major role in controlling the cellular response to several different starvation responses, likely acting both through lipid metabolism and other genes and processes.

Oxidative stress occurs when reactive oxygen species (ROS) accumulate within the cell to toxic levels. ROS are produced endogenously as obligate and ubiquitous by-products of aerobic respiration, mainly by electron transport chain (ETC) complexes I and III, when leaky electrons form superoxide. Substantial amounts of ROS are produced by peroxisomes and by enzymes such as cytochrome P450 and NADPH oxidases (Halliwell, 1991; Lenaz, 2001; Shields et al., 2021; Sies et al., 2022). ROS can also be taken up exogenously or produced by the environment. For example, ROS levels increase as a consequence of exposure to heavy metals, xenobiotics, radiation such as UV-C, and other sources. In these contexts, ROS can cause transient or irreversible damage to macromolecules such as DNA, RNA, lipids, including membrane lipids, and proteins (Shields et al., 2021; Sies et al., 2022). Cellular systems have evolved to neutralize and limit ROS accumulation, including non-enzymatic molecules glutathione and flavonoids, and enzymes such as superoxide dismutases (SOD) and catalases (Martindale and Holbrook, 2002). However, at controlled levels, ROS are important for physiological functions such as innate immune responses, development, and cytoskeletal organization (Wilson and González-Billault, 2015; Miranda-Vizuete and Veal, 2017; Oswald et al., 2018; Wilson et al., 2018; Sies and Jones, 2020; Al-Shehri, 2021). Homeostasis is achieved when the generation and removal of ROS is properly controlled, ensuring cellular function while avoiding or faithfully repairing damage caused by ROS.

The evolutionarily conserved Cap “n” collar (CNC)-basic leucine zipper (bZIP) transcription factor Nrf2, encoded by the nuclear factor, erythroid-derived 2-like 2 (NFE2L2) gene, is often considered a master regulator of oxidative stress responses (Alam et al., 1999; Ma, 2013; Blackwell et al., 2015). In C. elegans, oxidative stress responses typically require skinhead (skn-1), the homolog of Nrf2 (An and Blackwell, 2003; Blackwell et al., 2015). Indeed, skn-1 is vital for animal survival after exposure to many oxidative stressors, including paraquat, sodium arsenite, and tert-butyl hydroperoxide (tBOOH). SKN-1 is also an important regulator of longevity (An and Blackwell, 2003; An et al., 2005; Tullet et al., 2008; Oliveira et al., 2009; Blackwell et al., 2015; Steinbaugh et al., 2015).

Interestingly, however, the transcriptional response to one oxidative stressor, tBOOH, is at least partially independent of SKN-1 (Oliveira et al., 2009; Goh et al., 2014). A substantial part of the response to tBOOH instead requires nhr-49 and its coregulator mdt-15 (Goh et al., 2014; Goh et al., 2018). Consequently, loss of nhr-49 renders worms sensitive to tBOOH; in addition, loss of nhr-49 sensitizes worms to arsenite and paraquat (Horikawa and Sakamoto, 2009; Goh et al., 2014; Goh et al., 2018), although nhr-49’s involvement in transcriptomic changes caused by these molecules has not yet been defined. In the tBOOH response, nhr-49 is required for the upregulation of dozens of genes, including fmo-2, dhs-18, icl-1, sodh-1, and nlp-25, several of which NHR-49 also induces during starvation (Goh et al., 2018). Knockdown of the oxidoreductase sodh-1 and K05B2.4, predicted to encode an enzyme with acyl-CoA hydrolase activity, rendered worms sensitive to tBOOH, suggesting that activation of these enzymes may be how NHR-49 promotes oxidative stress protection. In contrast, loss of fmo-2, a gene highly induced by tBOOH and completely dependent on nhr-49 for induction, paradoxically increased worm survival in this context (Goh et al., 2018), perhaps because FMO-2’s predicted oxidase activity leads to ROS production, exacerbating the stress.

Interestingly, NHR-49 appears to be in a regulatory relationship with SKN-1. Specifically, nhr-49 gain of function mutant strains showed induction of the Glutathione S-Transferase gst-4, a highly stress sensitive gene that requires skn-1 for activation in most contexts (An and Blackwell, 2003). Indeed, like skn-1, nhr-49 and mdt-15 are required for gst-4 induction in response to arsenite and paraquat (Hu et al., 2018), although nhr-49 was dispensable to induce several SKN-1-induced genes, including gst-4, in another study (Goh et al., 2018). Nevertheless, nhr-49 is needed to activate the expression of the SKN-1c isoform, the key isoform driving antioxidant responses, in a worm strain mutant for brap-2/BRCA1 Associated Protein homolog, and is essential for increased SKN-1 activity in the amdh-1/aminohydrolase domain containing protein mutant, although it is not clear if this is a transcriptional or posttranscriptional role of NHR-49 (Hu et al., 2018; Frankino et al., 2022). Another link between these two regulators is that they share MDT-15 as a physical interactor and functional coregulator (Taubert et al., 2006; Goh et al., 2014). Together, these studies suggest that the SKN-1 and NHR-49 pathways for oxidative stress resistance crosstalk, possibly to achieve optimal transcriptional response to pro-oxidant conditions.

NHR-49 and MDT-15 also regulate peroxisomal quality control genes and some genes involved in peroxisomal β-oxidation (Van Gilst et al., 2005a; Rackles et al., 2021). Thus, although nhr-49 promotes the oxidation of fatty acids in peroxisomes and mitochondria, processes which generate ROS, it also has an oxidoprotective role for organismal survival during oxidative stress. Perhaps these activities are coordinated, such that when NHR-49 induces fatty acid oxidation for energy production, it induces genes such as sodh-1 and skn-1 that protect against the concurrently produced ROS.

Hypoxia is a stress that occurs when cellular oxygen levels are too low for normal physiological functions. It occurs naturally in cells and tissues during development, as well as in many diseases (Powell-Coffman, 2010; Lee et al., 2020). Aerobic respiration, the principal source of energy generation in most eukaryotes, requires oxygen. As a small organism without a dedicated respiratory system, C. elegans receives oxygen in all cells of its body by diffusion. In animals, cellular damage and death through apoptosis can occur when oxygen availability drops below the physiologically required level (Carreau et al., 2011). Thus, adaptation to hypoxia is critical for maintaining cellular and organismal health.

The pathways that regulate the response to hypoxia are evolutionarily conserved. As in mammals, a key pathway in C. elegans involves the transcription factor hypoxia inducible factor 1 (HIF-1; the sole C. elegans homolog of mammalian HIFa), which is critical for the cellular responses to and the defence against hypoxia (Jiang et al., 2001; Choudhry and Harris, 2018). To survive hypoxia (0.3%–1% O₂ in C. elegans) (Jiang et al., 2001), worms activate the EGL-Nine homolog (egl-9)–von Hippel–Lindau (vhl-1)–hif-1 pathway. In normoxic conditions (21% O₂), HIF-1 is degraded and thus inactive. This occurs when EGL-9 adds a hydroxyl group onto a proline residue in HIF-1. The hydroxylated proline promotes binding of the E3 ubiquitin ligase VHL-1 (the C. elegans VHL homolog), leading to poly-ubiquitination and proteasomal degradation of HIF-1. However, in hypoxic conditions, EGL-9 hydroxylation is rendered inactive by the lack of oxygen; hence, HIF-1 is stabilized, allowing it to dimerize with the HIF1b homolog AHA-1 and to activate a hypoxia adaptation gene program (Epstein et al., 2001; Powell-Coffman, 2010). Accordingly, C. elegans carrying vhl-1 or egl-9 mutations show increased HIF-1 protein levels in normoxic conditions (Epstein et al., 2001), and loss of hif-1 renders worms sensitive to hypoxic exposure (Jiang et al., 2001; Shen et al., 2005).

In addition to the HIF-1 responses, several parallel transcriptional programs exist in C. elegans and mammalian organisms that are critical for protection from hypoxia (Pursiheimo et al., 2009; Li et al., 2013; Padmanabha et al., 2015; Valko et al., 2021). NHR-49 is an important contributor to hypoxia resistance in C. elegans, as its loss results in hypoxia sensitivity that is equivalent to that caused by loss of hif-1. Concomitant loss of both genes results in virtually complete lethality in hypoxia, demonstrating that nhr-49 and hif-1 act non-redundantly and in separate pathways (Doering et al., 2022).

How does nhr-49 promote protection from hypoxia? During hypoxia, damaged cellular components can be cleared or recycled via autophagy (Mazure and Pouysségur, 2010; Tan et al., 2016). C. elegans show sensitivity to hypoxia and anoxia when the autophagy pathway is disrupted (Samokhvalov et al., 2008; Doering et al., 2022), and autophagy genes as well as the formation of autophagosomes are upregulated in hypoxia and anoxia (Chapin et al., 2015; Doering et al., 2022). Critically, nhr-49 is required to upregulate both autophagy genes and autophagosome formation in hypoxia, and autophagy genes act in the same pathway, and independently of hif-1, to promote survival in hypoxia.

Activation of autophagy appears to be a key function of NHR-49 driven hypoxia adaptation but is likely not the only one. In hypoxia, NHR-49 also induces a suite of detoxification genes (Doering et al., 2022). Interestingly, a separate set of detoxification genes depend only on hif-1, and a third set of detoxification genes is independent of both nhr-49 and hif-1. This suggests that induction of detoxification genes in hypoxia is an important process that is achieved by multiple transcription factors acting in parallel in hypoxia. Autophagy and detoxification are also enriched biological processes dependent on nhr-49 for induction in oxidative stress caused by tBOOH (Goh et al., 2018). However, nhr-49 regulates some unique sets of genes involved in each process in each stress, suggesting some genes and processes regulated by nhr-49 are stress-specific, whereas some are common amongst stresses (Doering et al., 2022).

Another important role of NHR-49 in hypoxia relates to extracellular matrix remodelling. In C. elegans, hypoxia results in cuticle disorganization. The hypoxia inhibited receptor tyrosine kinase HIR-1 coordinates remodelling of the extracellular matrix, and the downstream signalling pathway is HIF-1-independent but linked to NHR-49. Although nhr-49 loss does not affect cuticle organization, nhr-49 is required to regulate the expression of many cuticle-related genes in the hir-1 mutant (Vozdek et al., 2018). NHR-49’s role in hypoxia resistance therefore may include physiological and developmental adaptations.

As a nuclear receptor, NHR-49 likely functions in the nucleus to regulate gene expression, and some NHRs can enter the nucleus when bound to their cognate ligand. However, this may not be the case for NHR-49 in hypoxia, as Vozdek et al. found that the subcellular localization of a NHR-49::Venus fusion protein did not change after hypoxia exposure (Vozdek et al., 2018). Doering et al. did not observe a change in localization during hypoxia, either, but found that an overexpressed, fluorescently tagged NHR-49::GFP fusion protein was mildly induced by hypoxia (Vozdek et al., 2018; Doering et al., 2022).

Changes in overall levels and/or subcellular localization of transcription factors is often governed by upstream kinases via phosphorylation. Although no kinase has yet been shown to directly target NHR-49, evidence for such regulation has begun to emerge. Homeodomain-interacting protein kinase-1 (HPK-1), the only C. elegans HIPK homolog, is a nuclear kinase that participates in stress response (Rinaldo et al., 2007; Berber et al., 2013; Berber et al., 2016; Das et al., 2017). nhr-49 coordinates the hypoxia response with hpk-1, which is required to promote accumulation of NHR-49 in low oxygen and to induce autophagy genes and autophagosome formation (Doering et al., 2022). HPK-1 also regulates autophagosome formation during dietary restriction (Das et al., 2017), another context regulated by NHR-49 (see below). Future work could determine if these two factors work together in the response to additional stresses and whether NHR-49 is a direct target of HPK-1 kinase activity.

To date, nhr-49’s role in hypoxia response has been studied in the HIF-1-depedenent range of oxygen concentration (i.e., 1%–0.3% O₂). However, the responses to severe hypoxia (<0.3%) and anoxia (0%) involve additional, hif-1-independent pathways (Powell-Coffman, 2010). Whether nhr-49 is essential in these conditions is unknown. In 0.5% oxygen, RNA-sequencing analysis showed that approximately 26% (83 of 315) of nhr-49-dependent genes are hif-1-independent, including autophagy genes and detoxification genes (Doering et al., 2022). This may suggest that nhr-49, having some hif-1-independent functions at 0.5% oxygen, may also regulate these genes in 0.1% or 0% oxygen. In particular, autophagy genes are essential for survival in anoxia (Samokhvalov et al., 2008), so nhr-49 may be required in these conditions to promote survival via autophagy. Further research into the role of NHR-49 in hypoxia adaption will be interesting.

A recent report revealed that NHR-49 may also play a role in the heat shock and proteostasis response of C. elegans (Sala et al., 2023). Specifically, although nhr-49 loss did not render wild-type background animals sensitive to acute heat shock, it did cause such sensitivity in several contexts that feature enhanced thermotolerance. Moreover, the nhr-49(et7) gain of function mutation promoted thermotolerance, which required the conserved hsf-1 master regulator. Mechanistically, nhr-49 gain caused the activation of heat shock protein family chaperones. This interplay of NHR-49 with HSF-1 resembles that of NHR-49 with SKN-1 in some oxidative stress responses and generally suggests that NHR-49’s interactions with other critical stress response pathways may reveal interesting regulatory paradigms.

In their natural habitat, C. elegans encounters many pathogens. While C. elegans lacks an adaptive immune system and mobile immune cells, it has an innate immune system to activate sophisticated responses to escape from or resist and survive infection (Ermolaeva and Schumacher, 2014; Kim and Ewbank, 2018; Martineau et al., 2021; Tran and Luallen, 2023). Innate immunity is the first line of protection against pathogens and refers to the non-specific defense systems. As a bacterivore, C. elegans is an excellent model to study the process of and the response to infection, allowing for in vivo study of pathogenesis and response to intestinal infections simply via feeding. Interestingly, upon infection with common human pathogenic bacteria such as P. aeruginosa, S. aureus, and Enterococcus faecalis, many upregulated genes including icl-1, fmo-2, and acs-2, are regulated by NHR-49 (Dasgupta et al., 2020; Naim et al., 2021; Wani et al., 2021), revealing a role for this transcription factor in the innate immune response.

The round-shaped, Gram-negative bacterium P. aeruginosa colonizes the C. elegans intestine. Here, infection causes virulence-related membrane vesicles to accumulate and forms an extracellular biofilm matrix similar to infection in the mammalian lung. As the infection progresses, the intestine becomes distended, intracellular invasion begins, and abnormal autophagosomes form, eventually leading to animal death (Powell and Ausubel, 2008; Irazoqui et al., 2010). The Gram-positive bacteria S. aureus and E. faecalis also colonize the intestine of C. elegans (Irazoqui et al., 2010). Although both bacteria cause intestinal distention, S. aureus infection results in thinning and destruction of the intestine, anal deformations, and degradation of other organs in the worm (Garsin et al., 2001; Irazoqui et al., 2010).

nhr-49 is required for C. elegans survival upon infection with P. aeruginosa, S. aureus, and E. faecalis (Sim and Hibberd, 2016; Dasgupta et al., 2020; Naim et al., 2021; Wani et al., 2021); similarly, the compound LK56, which stimulates innate immunity, uses NHR-49 and its coregulator MDT-15 to confer resistance to these bacteria (Hummell et al., 2021). Although the intestine is the main organ affected by infection with the aforementioned bacteria, the tissue-specific roles NHR-49 plays in response to each bacteria vary. Although re-expression of NHR-49 in any of the intestine, neuron, muscle, or hypodermis is sufficient to rescue the infection survival defect of nhr-49 mutant worms on P. aeruginosa, NHR-49 expression only in the intestine or neurons rescues survival back to wild-type levels after infection with S. aureus (Naim et al., 2021; Wani et al., 2021). Additionally, overexpressing NHR-49 in the neurons enhances resistance during E. faecalis infection compared to wild type, whereas intestinal overexpression has no effect on animal survival (Dasgupta et al., 2020). This suggests that, like in other stresses, NHR-49 can regulate C. elegans innate immune responses in both cell autonomous and cell non-autonomous fashions.

How does NHR-49 promote innate immune responses? Work to date has largely focused on two NHR-49 regulated genes, acs-2 and fmo-2, and has suggested an immunometabolic role for NHR-49. acs-2 is induced during E. faecalis and P. aeruginosa infection but is only required for worm survival after infection with E. faecalis (Dasgupta et al., 2020; Naim et al., 2021). fmo-2 is induced by and required for survival after infection with S. aureus and E. faecalis, but is downregulated after infection with P. aeruginosa (Dasgupta et al., 2020; Naim et al., 2021; Wani et al., 2021). nhr-49 is also required to induce some C-type lectin genes in response to S. aureus infection (Wani et al., 2021). C-type lectins, which can bind to carbohydrates and pathogens, are another group of genes with roles in immune response in vertebrates and invertebrates. The C. elegans genome encodes 283 C-type lectins, many of which are induced by only one or two specific pathogens, highlighting a potential role in immune response specificity (Schulenburg et al., 2008; Pees et al., 2021). Future work could determine if NHR-49 regulates different subsets of C-type lectin genes in response to distinct pathogens, and the roles they may play. Additional genes that are induced by NHR-49 during infection and require further investigation into their immune-specific role include icl-1 and the lipase lipl-3 on E. faecalis, and fmo-5 in S. aureus (Dasgupta et al., 2020; Wani et al., 2021). This highlights that C. elegans features pathogen-specific responses and suggests that NHR-49 regulates different subsets of response genes depending on which pathogen it is infected by.

Although NHR-49 and HLH-30 act in parallel pathways during starvation and lifespan (Goh et al., 2018; Wani et al., 2021), an interesting connection between these transcription factors exists in innate immune responses. hlh-30 is required to express many genes following infection with S. aureus, but a subset of infection-response genes is hlh-30-independent, and instead requires nhr-49 for induction (Visvikis et al., 2014; Wani et al., 2021). Interestingly, NHR-49 and HLH-30 contribute to each other’s expression in response to S. aureus infection and thus may cooperate, perhaps to achieve optimal response after infection (Wani et al., 2021). Furthermore NHR-80, which can dimerize with NHR-49 (Pathare et al., 2012), is also required for C. elegans survival after infection with P. aeruginosa (Naim et al., 2021). Future work could determine if these two factors function together, potentially as dimerization partners, to control the innate immune response.

Although much of the work on NHR-49’s role in innate immunity has focused on bacterial pathogens, C. elegans are infected by many microorganisms. Examples include the fungal pathogens Drechmeria coniospora and Harposporium sp., the microsporidia genus Nematocida, and the Orsay virus (Gammon, 2017; Schulenburg and Félix, 2017). Future work may determine if the immunometabolic role NHR-49 plays may be broader and not limited to bacterial infection.

NHR-49 plays an important role in regulating the lifespan of C. elegans. At 20°C, a common temperature to cultivate C. elegans in the laboratory, nhr-49 loss shortens lifespan, whereas nhr-49 gain (overexpression) extends lifespan in wild type. In loss of function studies, the commonly used nhr-49 deletion allele, nr2041, decreases the mean lifespan of wild-type animals from 20 days to approximately 14–15 days at 20°C (Van Gilst et al., 2005a; Ratnappan et al., 2014). Another deletion allele, gk405, shortens lifespan to a similar extent (Lee et al., 2019). RNAi knockdown of nhr-49 also reduces the lifespan of wild-type worms, albeit to a lesser extent than nhr-49 mutation (Van Gilst et al., 2005a; Khan et al., 2013). Interestingly, the effect of nhr-49 loss on wild-type lifespan is temperature-dependent, as the nhr-49(gk405) mutant shows a wild-type lifespan at 25°C, but a substantially reduced lifespan at 15°C, which extends worm lifespan compared to 20°C and 25°C (Lee et al., 2019). Conversely, overexpressing NHR-49 from its own promoter rescues the short lifespan of the nhr-49 null mutant, and, importantly, increases the lifespan of wild-type worms to a mean of 26 days at 20°C (Ratnappan et al., 2014), demonstrating sufficiency. More ambiguous are the effects of the nhr-49 gain of function alleles, nhr-49(et7) (P479L), nhr-49(et8) (S432F), and nhr-49(et13) (V411E). Specifically, et7 increases lifespan, et8 decreases lifespan, and et13 has no effects (Lee et al., 2019). The reason for this discrepancy is unclear; these alleles were identified in a screen for mutants that suppress the cold sensitivity caused by the loss of the mammalian adiponectin transmembrane receptor homolog paqr-2 (Svensk et al., 2013), not for lifespan related phenotypes. However, the paqr-2–nhr-49 axis is important for longevity in some contexts such as dietary restriction ((Jeong et al., 2023); see below). Determining how et7 promotes lifespan extension would be an interesting future research direction.

The previous section describes the role of nhr-49 in the regulation in longevity, but what biological processes and pathways does it regulate that extend lifespan? Below we summarize the current level of understanding and highlight some future areas of research.

As noted, NHR-49 was originally identified as a regulator of lipid metabolism (Van Gilst et al., 2005a; Van Gilst et al., 2005b). Given the important role that lipid metabolism plays in health and lifespan it was therefore logical to consider that NHR-49’s contribution to longevity would entail modulation of these processes, and this indeed is an important contribution of NHR-49. nhr-49 is required to express genes involved in fatty acid β-oxidation and in fatty acid desaturation, and both lipid storage and saturation levels are important contributors to lifespan extension (Van Gilst et al., 2005a; Taubert et al., 2006; O’Rourke and Ruvkun, 2013; Ratnappan et al., 2014; Han et al., 2017) (Figure 3).