title: Transcriptomic analysis reveals specific osmoregulatory adaptive responses in gill mitochondria-rich cells and pavement cells of the Japanese eel.
abstract: BACKGROUND
  Homeostasis of ions and water is important for the maintenance of cellular functions. The regulation of the homeostasis is particularly important in euryhaline fish that migrate between freshwater (FW) and seawater (SW) environments. The fish gill, the major tissue that forms an interface separating the extracellular fluids and external water environment, has an effective transport system to maintain and regulate a constant body osmolality. In fish gills, the two major epithelial cells, pavement cells (PVCs) and mitochondria-rich cells (MRCs), are known to play key and complementary roles in ion transport at the interface. Discovering the robust mechanisms underlying the two cell types' response to osmotic stress would benefit our understanding of the fundamental mechanism allowing PVCs and MRCs to handle osmotic stress. Owing to the limited genomic data available on estuarine species, existing knowledge in this area is slim. In this study, transcriptome analyses were conducted using PVCs and MRCs isolated from Japanese eels adapted to FW or SW environments to provide a genome-wide molecular study to unravel the fundamental processes at work.
  
  
  RESULTS
  The study identified more than 12,000 transcripts in the gill cells. Interestingly, remarkable differential expressed genes (DEGs) were identified in PVCs (970 transcripts) instead of MRCs (400 transcripts) in gills of fish adapted to FW or SW. Since PVCs cover more than 90 % of the gill epithelial surface, the greater change in gene expression patterns in PVCs in response to external osmolality is anticipated. In the integrity pathway analysis, 19 common biological functions were identified in PVCs and MRCs. In the enriched signaling pathways analysis, most pathways differed between PVCs and MRCs; 14 enriched pathways were identified in PVCs and 12 in MRCs. The results suggest that the osmoregulatory responses in PVCs and MRCs are cell-type specific, which supports the complementary functions of the cells in osmoregulation.
  
  
  CONCLUSIONS
  This is the first study to provide transcriptomic analysis of PVCs and MRCs in gills of eels adapted to FW or SW environments. It describes the cell-type specific transcriptomic network in different tonicity. The findings consolidate the known osmoregulatory pathways and provide molecular insight in osmoregulation. The presented data will be useful for researchers to select their targets for further studies.
authors.affiliation: School of Biological Sciences, Kadoorie Biological Sciences Building, The University of Hong Kong, Pokfulam Road, Pok Fu Lam, Hong Kong.
authors.forename: Keng Po
authors.initials: KP
authors.lastname: Lai
authors.affiliation: State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, Hong Kong.|Faculty of Medicine, The Chinese University of Hong Kong, Shatin, Hong Kong.
authors.forename: Jing-Woei
authors.initials: JW
authors.lastname: Li
authors.affiliation: Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong.
authors.forename: Je
authors.initials: J
authors.lastname: Gu
authors.affiliation: State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, Hong Kong.
authors.forename: Ting-Fung
authors.initials: TF
authors.lastname: Chan
authors.affiliation: Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong. kftse@hkbu.edu.hk.
authors.forename: William Ka Fai
authors.initials: WK
authors.lastname: Tse
authors.affiliation: Department of Biology, Hong Kong Baptist University, Kowloon Tong, Hong Kong. ckcwong@hkbu.edu.hk.|Croucher Institute for Environmental Sciences, Hong Kong Baptist University, Pok Fu Lam, Hong Kong. ckcwong@hkbu.edu.hk.
authors.forename: Chris Kong Chu
authors.initials: CK
authors.lastname: Wong
mesh_terms: D000220:Adaptation, Biological / Q000235:genetics*; D000818:Animals; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation; D005880:Gills / Q000378:metabolism*; D059014:High-Throughput Nucleotide Sequencing; D008928:Mitochondria / Q000378:metabolism*; D064587:Osmoregulation / Q000235:genetics*; D009997:Osmotic Pressure; D015398:Signal Transduction; D059467:Transcriptome