Nuclear hormone receptors (NHRs) are transcription factors that are regulated by ligands, e.g. steroid hormones, lipids, and xenobiotics. C. elegans encodes 284 NHRs, an unusually large number; most have no known function. One such NHR, NHR-28, may be involved in the insulin/IGF-1-like signaling (IIS) pathway: NHR-28 targets, as determined by ChIP-seq, are enriched for genes differentially regulated in
daf-2/insulin receptor mutants in a manner dependent on its downstream effector
daf-16, but are not direct DAF-16 targets. However, the biological function of NHR-28 is still unknown.As NHR-28 may regulate genes downstream of the IIS pathway, we investigated whether
nhr-28 affected two known phenotypes of
daf-2 mutants, extended lifespan and dauer formation. A null mutation of
nhr-28 had no effect on
daf-2(
e1370) lifespan, although the
nhr-28(0) single mutant had a shorter mean but not maximum lifespan compared to wild-type.
nhr-28(0) single mutants also did not show dauer formation phenotypes. However, a
daf-2(
e1370);
nhr-28(0) double mutant showed a strong enhancement of constitutive dauer formation (Daf-c) phenotype compared to
daf-2(
e1370) alone at 22degC. We therefore pursued two lines of investigation: that
nhr-28 may act in a pathway parallel to IIS in dauer formation, e.g. the TGF-beta pathway, or that it may show complex interactions with different
daf-2 alleles e.g. in the manner of the developmental regulator DAF-12. While
nhr-28(0) did not show any genetic interaction with the TGF-beta signaling mutants
daf-7 and
daf-8, it enhanced the Daf-c phenotype of
daf-14(
m77)/Smad mutants, implying that it acts parallel to
daf-14 in dauer formation. On the other hand,
nhr-28(0) completely suppressed the Daf-c phenotype of the Class 1
daf-2 allele
daf-2(
m41), but did not interact with another Class 1 allele,
daf-2(
e1368). This differs from
daf-12 null mutant interactions with
daf-2 alleles, which suppress the Daf-c phenotype of Class 1 alleles but enhance it in Class 2 alleles e.g.
daf-2(
e1370), implying a different mechanism for
nhr-28.In conclusion,
nhr-28 genetically interacts with
daf-2 in dauer formation independently of
daf-2's role in aging. Within dauer formation pathways,
nhr-28 may act in the IIS and/or TGF-beta pathways, as it shows complex genetic interactions with different
daf-2 alleles and with
daf-14/Smad mutants; these interactions remain to be fully characterized. Future work will include further dissecting where and how
nhr-28 acts in dauer formation pathways as well as determining its downstream regulatory targets via unbiased approaches e.g. RNA-seq.