title: Aestivation Induces Changes in the mRNA Expression Levels and Protein Abundance of Two Isoforms of Urea Transporters in the Gills of the African Lungfish,Protopterus annectens.
abstract: The African lungfish,Protopterus annectens, is ammonotelic in water despite being ureogenic. When it aestivates in mucus cocoon on land, ammonia is detoxified to urea. During the maintenance phase of aestivation, urea accumulates in the body, which is subsequently excreted upon arousal. Urea excretion involves urea transporters (UT/Ut). This study aimed to clone and sequence theutisoforms from the gills ofP. annectens, and to test the hypothesis that the mRNA and/or protein expression levels ofut/Ut isoforms could vary in the gills ofP. annectensduring the induction, maintenance, and arousal phases of aestivation. Two isoforms ofut, ut-a2aandut-a2b, were obtained from the gills ofP. annectens.ut-a2aconsisted of 1227 bp and coded for 408 amino acids with an estimated molecular mass of 44.7 kDa, whileut-a2bconsisted of 1392 bp and coded for 464 amino acids with an estimated molecular mass of 51.2 kDa. Ut-a2a and Ut-a2b ofP. annectenshad a closer phylogenetic relationship with Ut/UT of tetrapods than Ut of fishes. While the mRNA expression pattern ofut-a2aandut-a2bacross various tissues ofP. annectensdiffered, the transcript levels ofut-a2aandut-a2bin the gills were comparable, indicating that they might be equally important for branchial urea excretion during the initial arousal phase of aestivation. During the maintenance phase of aestivation, the transcript level ofut-a2aincreased significantly, but the protein abundance of Ut-a2a remained unchanged in the gills ofP. annectens. This could be an adaptive feature to prepare for an increase in the production of Ut-a2a upon arousal. Indeed, arousal led to a significant increase in the branchial Ut-a2a protein abundance. Although the transcript level ofut-a2bremained unchanged, there were significant increases in the protein abundance of Ut-a2b in the gills ofP. annectensthroughout the three phases of aestivation. The increase in the protein abundance of Ut-a2b during the maintenance phase could also be an adaptive feature to prepare for efficient urea excretion when water becomes available.
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: You R
authors.initials: YR
authors.lastname: Chng
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: Jasmine L Y
authors.initials: JL
authors.lastname: Ong
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: Biyun
authors.initials: B
authors.lastname: Ching
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: Xiu L
authors.initials: XL
authors.lastname: Chen
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: Kum C
authors.initials: KC
authors.lastname: Hiong
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: Wai P
authors.initials: WP
authors.lastname: Wong
authors.affiliation: Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University Singapore, Singapore.
authors.forename: Shit F
authors.initials: SF
authors.lastname: Chew
authors.affiliation: Department of Biological Sciences, National University of SingaporeSingapore, Singapore; NUS Environmental Research Institute, National University of SingaporeSingapore, Singapore.
authors.forename: Siew H
authors.initials: SH
authors.lastname: Lam
authors.affiliation: Department of Biological Sciences, National University of Singapore Singapore, Singapore.
authors.forename: Yuen K
authors.initials: YK
authors.lastname: Ip
keywords: Ut-a2a; Ut-a2b; ammonia; nitrogenous waste; urea excretion