Deciphering the role of aquaporin 1 in the adaptation of the stinging catfish Heteropneustes fossilis to environmental hypertonicity by molecular dynamics simulation studies.

A thorough investigation of the water permeability of H. fossilis aquaporin 1 (hfAQP1) in a hypertonic environment can provide a useful insight into the understanding of the underlying molecular mechanism of its high tolerance to salinity. Here, we constructed a 3 D homology model of hfAQP1 by taking Bos taurus AQP1, AQP0, and human AQP2 as templates using I-TASSER. The model obtained has similar structural organizations with mammalian AQP1s in all aspects. We investigated the water permeability of the modeled hfAQP1 in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane under neutral and 100 mM hypersalinity by subjecting each system to a 100 ns molecular dynamics simulation. Our results show that hypersalinity hinders water permeation across the membrane through the hfAQP1 channel. A change in the intermolecular distance between key residues of the ar/R selectivity filter along with charge redistribution resulted in the accommodation of only 2-6 water molecules inside the channel at once under hypersaline conditions. We investigated the mRNA expression pattern of hfaqp1 in osmoregulatory organs of H. fossilis in response to 100 mM hypertonicity by using qPCR analysis. The transcript was downregulated in kidney and GI tract, but upregulated in the Gills. Thus, the catfish survive in a hypertonic environment by reducing the transport of water in its cellular systems and downregulating the expression of the hfaqp1 gene. The results observed in our study can shed more light on the functionality of AQP1 in catfishes under salinity stress and aid in future researches on solving more gating mechanisms involved in its regulation.Communicated by Ramaswamy H. Sarma.

Transcriptome analysis of gills reveals novel insights into the molecular response of stinging catfish (Heteropneustes fossilis) to environmental hypertonicity.

The stinging catfish Heteropneustes fossilis is a champion survivor under hypertonic stress and is suggested to be a profitable candidate for culture in slightly saline water in coastal regions. Fish gills are an essential site of osmoregulation and other physiological processes. To investigate the stress responses and mechanisms of salinity tolerance in stinging catfish, we sampled gills tissues from control and hypertonicity (100 mM NaCl solution) treated adult catfish and assessed for transcriptomic profiling by high throughput sequencing. The raw data generated was filtered and assembled for de novo transcriptome assembly. The final contig assembly produced a total of 1,71,478 unigene transcripts with an average transcript length of 898 bp and a GC content of 45%. A total of 22,231 transcripts matched with Chordata with BLAST search and were functionally annotated, out of which 21,814 were best-hit transcripts aligned with the UniProt database. Comparative transcriptomic analysis revealed that a total of 1951 genes were differentially expressed in the gills of NaCl-treated fish compared to the control. Functional and enrichment analysis of the Differentially expressed genes demonstrated that several GO pathway terms were significantly over-represented, such as 'catalytic activity', 'hydrolase activity' in molecular function category, 'membrane', 'integral component of membrane' in cellular component category and 'metabolic process', 'regulation of transcription' in biological process category. The functional analysis study of DEGs demonstrated that tolerance to hypertonic stress by stinging catfish is associated with a few pathways related to stress response, immune response, biosynthesis, metabolism, molecular transport, cytoskeleton remodeling, apoptosis, cell signaling, transcriptional regulation, etc. The present study provides a novel insight into the molecular responses of the air-breathing stinging catfish against salinity stress, which could elucidate the underlying mechanisms of adaptation of this stenohaline species under various environmental constraints.

Differential expression of aquaporin genes and the influence of environmental hypertonicity on their expression in juveniles of air-breathing stinging catfish (Heteropneustes fossilis).

Aquaporins (AQPs) are a superfamily of transmembrane channel proteins that are responsible for the transport of water and some other molecules to and from the cell, mainly for osmoregulation under anisotonicity. We investigated here the expression patterns of different AQP isoforms and also during exposure to hypertonicity (300 mOsmol/L) for 48 h in juvenile stages of air-breathing stinging catfish (Heteropneustes fossilis). A total of 8 mRNA transcripts for different isoforms of AQPs and their translated proteins could be detected in the anterior and posterior regions of S1, S2, and S3 stages of juveniles of stinging catfish at variable levels. In general, more expression of mRNAs for different aqp genes was seen in the S2 and S3 juveniles than in the S1 juveniles. Most interestingly, exposure to hypertonicity of S2 juveniles for a period of 48 h led to increased expression of most of the aqp genes both at transcriptional and translational levels, except for aqp3 in the anterior and posterior regions and aqp1 in the anterior region, showing maximum expression at later stages of hypertonic exposure. Thus, it is evident that AQPs play crucial roles in maintaining the water and ionic balances under anisotonic conditions even at the early developmental stages of stinging catfish as a biochemical adaptational strategy to survive and grow in anisotonic environment.