Expression of aquaporins in the renal connecting tubule

Abstract

The renal connecting tubule (CNT) is a distinct segment that occurs between the distal convoluted tubule (DCT) and the cortical collecting duct. On the basis of its characterization in rabbit it is widely believed that connecting tubule cells have a low permeability to water and do not respond to vasopressin. Here we utilize segment-specific markers and specific aquaporin antibodies to characterize expression of water channels in CNT of the rat by immunocytochemistry. Colocalization of aquaporin 2 (AQP2), AQP3, and AQP4 with Na(+), Ca(2+) exchanger (NCX), a transporter characteristic of the connecting tubule, gave heterogeneous labeling. There was aquaporin labeling in many but not all regions labeled by NCX. Colocalization of AQP2 with AQP3 and with AQP4 showed that AQP3 and AQP4 labeling were always accompanied by AQP2. Immunogold labeling and electron microscopy showed that NCX-labeled cells with AQP2 labeling had the morphology of CNT cells, whereas NCX-labeled cells without AQP2 labeling were DCT cells. The latter regions were identified as the late region of the DCT known as DCT2. Additionally, regions of CNT lacking AQP2 labeling could be identified in Brattleboro rats not treated with vasopressin but not in such animals chronically treated with deamino-Cys(1),D-Arg(8)-vasopressin (dDAVP). Quantitative analysis of labeling was consistent with expression of AQP2 over a longer region of CNT after dDAVP exposure.

Sodium and Potassium Balance Depends on αENaC Expression in Connecting Tubule

“…19 Moreover, in TRPV5::EGFP transgenic mice, some tubule segments were enhanced green fluorescent protein and calbindin positive but negative for AQP2 and TSC, thus likely representing early CNT. 20 We observed that the Cre recombinase protein was expressed along the CD, consistent with CNT/CD-specific MR KO mice.…”

Vasopressin-independent mechanisms in controlling water homeostasis

“…It is most abundant in principal cells of the collecting duct and less so in tubule cells of the connecting tubule or inner medullary collecting duct (IMCD; IMCD cells; Nielsen et al 1993, Loffing et al 2000. Within these cells, AQP2 is primarily found embedded in the apical PM and subapical vesicles, while it is also present in the basolateral PM, particularly in IMCD cells , Breton & Brown 1998, Coleman et al 2000.…”

Two isoforms of aquaporin 2 responsive to hypertonic stress in bottlenose dolphin

“…Under hypertonic conditions, TonEBP is expressed and binds to the tonicity-responsive enhancer cis element upstream of AQP2 to initiate its expression (Miyakawa et al, 1999). TonEBP also participates in compatible osmolyte accumulation by upregulating relevant genes (Coleman et al, 2000;Miyakawa et al, 1998;Rim et al, 1998). The DNA sequence of the TonEBP binding site in the AQP2 promoter is GGAAA (Halterman et al, 2012); the bottlenose dolphin AQP2 proximal promoter includes this sequence at −139 and −327 bp upstream of the start codon (turTru1: GeneScaffold_1570, Ensembl Genome Blowser).…”

“…In terrestrial mammals, AQP2 expression is limited to the principal cells of the renal collecting duct, where it functions in the reabsorption of water from primitive urine by trafficking through cells from the endosomal compartments to the apical membrane under the control of vasopressin (Coleman et al, 2000;Fushimi et al, 1997;Nielsen et al, 1999). In the principal cells of the renal inner medulla, AQP2 expression is induced by hypertonicity (Hasler, 2009;Kasono et al, 2005;Storm et al, 2003).…”