4.6 Metabolism and salinity acclimation
Our analyses showed changes in the expression and methylation of genes
involved in lipid, protein and carbohydrate metabolism as well as
mitochondrial functions. The transition from a seawater- to a freshwater
physiology requires gill epithelium turnover and active ion transport
and is thus an energy consuming process. This energy is provided by
metabolites related to carbohydrates, proteins/amino acids and lipids,
transported from liver stores to the blood, towards the gill. In
addition to energy sources provided by the blood, there is also a local
energy supply at the gill level necessary for the modulation and
stimulation of gill epithelium reorganization and ion transport
mechanisms (Hwang and Lee., 2007).
Lipid metabolism in fish gills has been widely overlooked, despite the
growing number of studies in mammals indicating the importance of lipids
in ion channel regulation notably through lipid-protein interactions
(Rosenhouse-Dantsker et al., 2012). In both analyses (GO-term and KEGG
enrichment analyses), the sphingolipid synthesis and signaling pathways
were significantly enriched and are thus worth considering.
Sphingolipids are involved in multiple functions, such as immune-related
functions, cellular growth, differentiation, apoptosis and have been
argued to have second-messenger functions (Shayman, 2000). Sphingolipids
and mainly sphingomyelin, are also major components of membrane
microdomains, called lipid rafts, where they are involved in the
regulation of ion channels (Rosenhouse-Dantsker et al., 2012). Lipid
raft abundance in fish gills can in fact change upon salinity transfer.
They can be enriched in membrane transporters like
Na+/K+-ATPase, as shown in tilapiaO. mossambicus and milkfish Chanos chanos gills (Lin et
al., 2021). Hydrolysis of sphingomyelin due to a cell stress can lead to
ceramide formation, which is implicated in numerous physiological
functions. Several ion channels such as CFTR (Ramu et al., 2007) and
voltage gated K+ channels (Fan et al., 1997) have been
shown to be inhibited by ceramide. In this study, sphingomyelin
synthase (sgms1 ) was upregulated and hypomethylated in FW. We
also observed a downregulation of serine palmitoyltransferase(sptlc2 ) which is involved in the first step of sphingolipid
biosynthesis (Hanada., 2003) and ceramide synthase 5(cers5 ). Altogether, these results indicate a potential change in
the ceramide species profiles (Gault et al., 2010). However, the
potential functional link between methylation changes and expression
changes of genes involved in sphingolipid metabolism is not clear and
needs further investigation.
Studies have pointed out that the enzymes involved in methylation and
demethylation have substrates that are responsive to cellular metabolism
(Reid et al., 2017). Mitochondria provide key metabolites for epigenetic
processes (Shaughnessy et al., 2014). The availability of these
metabolites change, when fish are energetically challenged by
environmental stressors. For DNA methyltransferases, substrates and
cofactors include methylthioadenosine (MTA), S-adenosylmethionine (SAM)
and S-adenosylhomocysteine (SAH) for example whereas for DNA
demethylases (TET-family), other metabolites are used as succinate,
fumarate, … being intermediates of the TCA cycle (Reid et al.,
2017). We observed changes in the expression of genes linked to
methionine metabolism (ahcyl1, 2, mtap ) and an overall
overexpression of genes involved in the TCA cycle (dlst ,sdha, sdhd, sdhb , cs ). This clearly displays changes in
metabolic pathways and energetic status, which could directly affect the
epigenome and DNA methylation dynamics. In gills of sea bass transferred
to FW, ionocytes, previously called mitochondria-rich cells due to their
high abundance of mitochondria, are present at higher densities with
higher energy-consuming
Na+/K+-ATPase activities (Masroor et
al., 2018). We also measured higher expression of paralogs encoding for
the V-H+-ATPase indicating a high energy demand for
transepithelial ion transport in FW. In accordance with this, we
observed higher mitochondrial activities in FW. Mitochondria have a
central role in energy (ATP) production but also in metabolite
production in the TCA cycle and mitochondria-nuclear signaling. These
processes can be linked to epigenetic regulation. As chromatin-modifying
enzymes use as substrates and cofactors metabolites derived from diverse
metabolic pathways including notably the TCA cycle (Lopes, 2020),
salinity-driven changes in transcription of these genes might also
directly affect the availability of substrates for chromatin-modifying
enzymes and affect DNA methylation dynamics. Relationships between
genes, environment and epigenetic marks, and the variation of those
marks still require more investigations to gather a full understanding
of the determinism of salinity acclimation in fish.