Regulation of the myo-inositol pathway during salinity
stress
Many teleosts, including O. niloticus , rely onmyo -inositol as a compatible osmolyte during salinity stress. The
link between myo -inositol synthesis and environmental salinity is
well established in Oreochromis species (Cramb et al., 2013;
Gardell et al., 2013; Kalujnaia, McVee, Kasciukovic, Stewart, & Cramb,
2010; Sacchi, Gardell, Chang, & Kültz, 2014; Sacchi, Li, Villarreal,
Gardell, & Kültz, 2013). Myo -inositol is an organic osmolyte
which is used to maintain osmotic pressure within the cell to overcome
the negative effects of small and charged inorganic ions. Our data are
in agreement with the previous literature as they indicate up-regulation
of renal myo -inositol-3-phospate synthase (MIPS) during BW
acclimation of fish (FC=3.35, p=.042). Interestingly, in O.
mossambicus myo -inositol production is consistently increased
through up-regulation of IMPase1.1, which is not differentially
regulated in O. niloticus kidney (FC=1.001, p=.99, Supplementary
Table 2). On the other hand, one of the most highly significant proteins
in O. niloticus kidney is down-regulated myo -inositol
oxygenase (MIOX, FC=-6.22, p=.0058), which is a key enzyme for the
degradation of myo -inositol. MIOX has been shown to be important
in determining myo -inositol levels in vivo (Arner et al.,
2001). Transcripts of MIOX have been found to be differentially
regulated in response to salinity acclimation in ayu (Plecoglossus
altivelis ) larvae (Lu, Zhang, Yang, Li, & Chen, 2016) and turbot
(Scophthalmus maximus ) (Cui et al., 2020). Nonetheless, MIOX
regulation has not been previously noted in Oreochromis species.
The most highly regulated protein in our study is major facilitator
superfamily domain-containing protein 4A (MFSD4A) (FC=8.44, p=.001), a
transmembrane glucose transporter. Glucose, in addition to being an
important source of energy, is the substrate of MIPS (in the form of
glucose 6-phosphate), indicating that upregulation of MFSD4A facilitatesmyo -inositol production. Another indication that MFSD4A is linked
to myo -inositol production is that it is one of the few proteins
that has good correlation between protein and mRNA regulation, along
with the other important myo -inositol related proteins MIPS and
MIOX (Figure 3).
Our network and pathway analysis of DIA assay data demonstrates that
MIOX is connected to many of the other highly salinity-regulated
proteins. The most highly enriched KEGG pathway in BW acclimated fish
containing significantly regulated proteins is the ascorbate and
alderate metabolism pathway. This finding indicates the compensatory
upregulation of UGT and ALDH enzymes to replace the products ofmyo -inositol degradation by MIOX. MIOX and ALDH3 (FC=4.65,
p=.024) were both found in STRING network 3, and MIOX was also connected
to a (non-significant) isoform of UGT in network 2 (Figure 6). ALDH and
UGT also function in antioxidant metabolism and detoxification of
xenobiotics during environmental pollution.