4.5 Paracellular permeability-related genes are hypomethylated
and upregulated
In clinical studies on mammal cancer where disruption of cell-cell
junctions is frequently found, investigations have shown a link between
claudin overexpression and DNA hypomethylation for several claudins (Li
et al., 2018), including cldn4 (Kwon et al., 2011). In fish
gills, the tightening of the gill epithelium is a natural and essential
process in fresh water in order to avoid excess water entry and limit
passive ion loss. Euryhaline species have to constantly adjust gill
permeability through the expression of tight or leaky junctions. A
striking result of this study is the consistent promoter hypomethylation
and upregulation of genes involved in paracellular permeability and the
formation of tight junctions. In both analyses (KEGG pathway and GO-term
enrichment analyses), tight junctions appeared among the most
significant pathways. In sticklebacks, Metzger and Schulte (2018) also
showed that the category cell-cell-junction appeared among the top ten
enriched cellular component GO categories comparing differentially
methylated cytosines in different salinity conditions, which highly
suggests that methylation changes control the expression of these genes
in fish. In another study comparing slow-twitch and fast-twitch muscles
in the teleost Pseudocaranx dentex, Li et al. (2022) also
identified a correlation between DNA methylation and gene expression
levels in genes involved in cell junctions (tight and adherens
junctions). Tight junctions are composed of multiple membrane-spanning
proteins such as occludins, claudins and several junctional adhesion
molecules (Chasiotis et al., 2012). Claudins are very diverse in fish
and mammals (Engelund et al., 2012) with a total of 61 cldn genes
in European sea bass including all paralogs. Among the different genes
that show differential expression and methylation levels, cldn4and cldn8 are supposed to be involved in increasing the
epithelial tightness (Bagherie-Lachidan et al., 2009) which is essential
in freshwater environments. We found two genes encoding for Claudin 8
with a significant upregulation (log2FC: 1.44 and 0.68) and
hypomethylation. In gills of puffer fish (Tetraodon
nigroviridis ), one claudin 8 paralog (Tncldn8d ) also
showed an increased expression in FW, but not the 3 others
(Bagherie-Lachidan et al., 2009). The overexpression of cldn 4 in
FW was shown in other teleost species, like tilapia (Oreochromis
mossambicus ) (Tipsmark et al., 2008), killifish (Whitehead et al.,
2012), and rainbow trout (Oncorhynchus mykiss ) (Leguen et al.,
2015). We identified two genes encoding for D. labrax Claudin 4
paralogs that were both upregulated in FW (log2FC: 0.87 and 1.47) and
hypomethylated in promoters as well as other key genes involved in tight
junction assembling (cgn , f11r, marveld3 ). Given the
enrichment of the tight junction pathway for upregulation and
hypomethylation, there is some evidence that claudin promoters, first
exons or introns are a preferential target for differential methylation
in changing salinity environments in euryhaline D. labrax .
Studies in other teleost species on the effect of methylation on the
tight junction pathway are required to confirm this trend in fish.