Discovery of aquaporin water channel proteins has provided insight into the molecular mechanism of membrane water permeability. The distribution of known mammalian aquaporins predicts roles in physiology and disease. Aquaporin-1 mediates proximal tubule fluid reabsorption, secretion of aqueous humor and cerebrospinal fluid, and lung water homeostasis. Aquaporin-2 mediates vasopressin-dependent renal collecting duct water permeability; mutations or downregulation can cause nephrogenic diabetes insipidus. Aquaporin-3 in the basolateral membrane of the collecting duct provides an exit pathway for reabsorbed water. Aquaporin-4 is abundant in brain and probably participates in reabsorption of cerebrospinal fluid, osmoregulation, and regulation of brain edema. Aquaporin-5 mediates fluid secretion in salivary and lacrimal glands and is abundant in alveolar epithelium of the lung. Specific regulation of membrane water permeability will likely prove important to understanding edema formation and fluid balance in both normal physiology and disease.

Pathophysiology of the aquaporin water channels

Aquaporin water channels: unanswered questions and unresolved controversies

Twenty-one families of secondary [proton-motive force (PMF)-driven] carriers are herein described, which, in addition to 13 of the currently recognized 48 families within the ATP-dependent ATP-binding casette (ABC) superfamily and three families of channel proteins, largely account for the transport of amino acids and their derivatives (excluding proteins) into and out of all living cells on Earth. Family characteristics as well as structural and functional properties of the family constituents are described. By reference to our web site (http :}}www-biology.ucsd.edu}Cmsaier}transport}), sequences, phylogenetic relationships, detailed substrate-specificity information and the distribution of members of each family in 20 fully sequenced genomes of bacteria, archaea and eukaryotes are available. This review provides a comprehensive guide to the diversity of carriers that mediate the transport of amino acids and their derivatives across cell membranes.

/pdf/families-of-transmembrane-transporters-selective-for-amino-pi3qp4k5si.pdf

Families of transmembrane transporters selective for amino acids and their derivatives.

Urea-selective Concentrating Defect in Transgenic Mice Lacking Urea Transporter UT-B

Cloning and functional expression of a urea transporter from human bone marrow cells.

https://hal.archives-ouvertes.fr/hal-02376408/document

Kidd Blood Group and Urea Transport Function of Human Erythrocytes Are Carried by the Same Protein

The effects of urea structural analogues on the ureafacilitated diffusion system were examined in human red cell membranes (pink ghosts) and in antidiuretic hormone(ADH)-stimulated frog urinary bladder epithelia. In both tissues, urea permeability (Purea) was dramatically but reversibly inhibited by a number of urea analogues, such as 1-(3,4-dichlorophenyl)-2-thiourea (DCPTU). This urea derivative reduced the urea flux in a dose-dependent manner (90% inhibition of Purea at 0.5 mM concentration of DCPTU). With the aim of obtaining irreversible markers of red cell and urinary bladder urea transport systems, urea derivatives were modified by addition of an azido residue (N3) and preliminary experiments of photoaffinity labelling were carried out. Two synthetic urea derivatives: 1-(3-azido-4-chlorophenyl)-2-thiourea (ACPTU) and 1-(3-azido-4-chlorophenyl)-3-methyl-2-thiourea (Me-ACPTU) were shown to be very potent inhibitors of Purea when used in the absence of light, with IC50 values 60.3 μM and 31.6 μM respectively, as measured in frog urinary bladder. Both these molecules appeared to bind covalently to the urea carrier in both frog urinary bladder and human pink red cell ghosts, when illuminated in the presence of the tissue: the urea flux, which fell to 30–70% of the value obtained in the presence of ADH after inhibitor addition, remained low after the preparation had been illuminated for 30 min and the inhibitor removed. These results provide an interesting approach to the urea carrier analysis, particularly to the urea or urea analogue binding site on the transport protein.

Urea derivatives as tools for studying the urea-facilitated transport system

The tritiated urea analogue 1-(3-azido-4-chlorophenyl)-3methyl-2-thiourea ([3H]MeACPTU) was used as a probe to photolabel the human red-blood-cell membrane facilitated urea transporter. On irradiation, [3H]MeACPTU incorporated irreversibly into white ghost membranes. SDS/gel electrophoresis of membranes revealed radioactive incorporation in five major bands of 200, 110, 60, 40 and 14 kDa. The labeling of the 40-kDa and 60-kDa bands was partly prevented by the presence of a high concentration of other urea analogues such as thiourea and 1-(3,4-dichlorophenyl) 2-thiourea (DCPTU). The photolabeling pattern obtained with white ghosts of the Kidd blood-group type Jk(a−,b−) showed no labeling of the 40-kDa polypeptide. Protecting experiments carried out with anti-Jka, anti-Jkb and anti-Jk3 sera prevented radioactive incorporation in the 60-kDa band and in the 110-kDa band. Urea permeability of pink ghosts of blood type Jk(a+,b+) measured in the presence of Jk3 antibodies was 19% lower than the control values. However, urea permeability of frog urinary bladder epithelial cells was not affected by the presence of Jk-reactive antibodies.



These results support the hypothesis that the Kidd antigen and the facilitated urea transporter are the same protein. Our estimation of the number of copies in each cell is close to that of the previously published value of 14000.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1993.tb18395.x

Photoaffinity labeling of the human red-blood-cell urea-transporter polypeptide components : possible homology with the Kidd blood group antigen

Starting from commercial 4-chloro-3-nitroaniline, through a 5 step synthesis, was prepared 3-azido-4-chlorophenylisothiocyanate 5 which was reacted with [3H]-methylamine. The latter was obtained by three methods: 

i-
[3H]-LiAlT4 reduction of benzylcarbamate gave rise to [3H]-methylamine (S.A.: >70 Ci/mmol).

ii-
Catalytic reduction of HCN with 3H2 lead to [3H]-CH3NH2 (S.A.: 0.7 Ci/mmol).

iii-
Schmidt rearrangement of [3H]-sodium acetate gave [3H]-CH3NH2 (S.A.: 29 Ci/mmol).





Compound 7 at the highest specific activity had a self radiolysis rate precluding its practical use in biological studies whilst 29 Ci/mmol [3H]-7 was satisfactory.

Philippe Neau

Papers

Cloning and functional expression of a urea transporter from human bone marrow cells.

Kidd Blood Group and Urea Transport Function of Human Erythrocytes Are Carried by the Same Protein

Urea derivatives as tools for studying the urea-facilitated transport system

Photoaffinity labeling of the human red-blood-cell urea-transporter polypeptide components : possible homology with the Kidd blood group antigen

Synthesis of N‐(3‐azido‐4‐chlorophenyl)‐N′‐[3H‐methyl] thiourea, an efficient photoaffinity probe for the urea carrier