Biology Reference
In-Depth Information
dauer larvae constitutively and have gonadal outgrowth defects, similar to
daf
-
12
LBD mutants. Nematodes do not synthesize cholesterol and depriv-
ing wild type animals of cholesterol phenocopies
daf
-
9
mutants (
Gerisch
et al., 2001; Matyash et al., 2004
). Similar phenotypes are seen in mutants
of the Niemann-Pick type C1 homologs (
Li, Brown, Ailion, Lee, &
Thomas, 2004
), which affect cholesterol transport, suggesting that the
DAF-12 ligands are derived from dietary cholesterol. Additional genes
involved in hormone biosynthesis were found based on genetic screens
for similar phenotypes (
Patel, Fang, Svy, Ruvkun, & Li, 2008; Rottiers
et al., 2006; Wollam et al., 2012
).
Of course a critical understanding of steroidogenic pathways came with
the discovery of the DAs and their structure (
Motola et al., 2006
). They are
3-keto bile acid-like steroids with a 25S-carboxylic acid moiety at the end of
the cholesterol sidechain (
Fig. 7.2
). X-ray crystal structures reveal that they
bind within the DAF-12 ligand-binding pocket much like the bile acids bind
to mammalian FXR (
Zhi et al., 2012
). Of the two known DAs,
D
-4 and
D
-7-DA, the latter is the more prevalent and potent form. Both ligands acti-
vate the receptor in the nM range, are absent in
daf
-
9
mutants, and supple-
mentation rescues all known
daf
-
9
phenotypes. Whether the different DAs
have distinct transcriptomes or functions, remains unknown.
With these structures in mind, a model for DA biosynthesis was deduced
through a combination of genetics, sterol feeding, and biochemical
approaches. It has been proposed that cholesterol is converted through
branched biosynthetic pathways, dubbed the
D
-7 and the
D
-4 branches, into
the two DAs. The first committed step in the
D
-7 biosynthetic branch is
introduction of the
D
-7 double bond by the DAF-36/Rieske oxygenase,
converting cholesterol to 7-dehydrocholesterol (
Wollam et al., 2011
)
(
Fig. 7.2
). Similarly, the
Drosophila
ortholog,
neverland
, carries out the same
first step in ecdysteroid biosynthesis (
Yoshiyama-Yanagawa et al., 2011
),
while the mammalian cytochrome P450, CYP7A1, carries out analogous
chemistry hydroxylating the 7-position of the cholesterol backbone at the
first step of bile acid synthesis (
Russell, 2003
). An unknown
D
-5 reductase
is surmised to convert 7-dehydrocholesterol to lathosterol. Thereafter the
3
b
-hydrosteroid dehydrogenase, DHS-16, converts lathosterol to the
3-keto steroid lathosterone (
Wollam et al., 2012
). Finally, the last step is car-
ried out by DAF-9/CYP27A1, which oxidizes the cholesterol sidechain to a
carboxylic acid moiety, with chemistry orthologous to the mammalian
CYP27A1, which is also involved in bile acid biosynthetic pathways
(
Motola et al., 2006
). HSD-1 was suggested to work in the production of
D
-4-DA by catalyzing oxidation of cholesterol to 4-cholesten-3-one, but
