Transcriptome Analysis of Stinging Catfish (Heteropneustes fossilis) Kidney Reveals its Role During Adaptation to Hypertonic Environment.

The facultative air-breather stinging catfish Heteropneustes fossilis can easily adapt to anisotonic environment up to a level of 6 ppt. The teleost kidney is an essential osmoregulatory organ and plays a vital role in immune system maintenance. We took kidney samples from control and 100 mM NaCl-treated stinging catfish to study the molecular responses to salinity stress by looking at the DEGs. We used high-throughput sequencing followed by de novo transcriptome assembly to analyze the transcriptome of the fish. A total of 89,813 unigene transcripts with 47.28% GC content and 901 bp average length were produced during the final contig assembly. A BLAST search revealed that 45,855 unigenes had a considerable amount of similarity to sequences found in the Swiss-Prot and Ref-Seq databases. Comparative transcriptome analysis revealed that 1660 genes were differentially expressed (693 were upregulated and 967 downregulated) in the kidneys of fish treated with NaCl compared to control kidneys. Several GO pathway terms, including "inflammatory response" and "neutrophil chemotaxis" in the biological process category, "cytoplasm" and "integral component of membrane" in the cellular component category, and "metal ion binding" and "DNA binding," in the molecular function category, were found to be significantly over-represented by enrichment and functional analysis of the DEGs. In addition, there was an overrepresentation of KEGG pathways like "chemokine signaling pathway," "cytokine-cytokine receptor interaction," and "metabolic pathways." The current work demonstrates that tolerance to hypertonic environmental stress necessitates the activation of many molecular pathways involved in sustaining the physiological systems in air-breathing stinging catfish. Adaptation is achieved through the enrichment of genes related to metabolic pathways, immune system response, stress response such as apoptosis and protein folding, and transporter of molecules involved in ionic and osmoregulation.