Molecular effects of volume expansion on the renal sodium phosphate cotransporter.
BACKGROUND
Volume Expansion (VE) results in both natriuresis and a phosphaturia. In previous studies, Sprague-Dawley rats were infused with a modified saline solution. The expansion procedure resulted in a 70% increase in the phosphorylation of a 72 kDa proximal tubular brush border membrane (BBM) protein. In recent experiments, Sprague-Dawley rats were subjected to the same short term VE. For both control and VE animals, brush border membrane vesicles (BBMV) were obtained.


RESULTS
Mass spectrometry of 3 proteins in the size range of our phosphoprotein resulted in the identification of ezrin/villin2, moesin, and PDZ domain-containing 1 (PDZ-dc1). Diphor-1 (currently renamed PDZ-dc1) is involved in regulation of the type II Na/Pi cotransporter. Ezrin and moesin are membrane-cytoskeletal linking proteins that are involved in the regulation of the sodium-hydrogen exchanger (NHE3) via interactions with another PDZ protein identified as sodium-hydrogen exchanger regulatory factor (EBP50, NHERF). Ezrin, moesin, and PDZ-dc1 protein levels were not increased following short term VE. Two-dimensional electrophoresis of our phosphorylated BBM proteins, followed by MALDI/MS analysis resulted in the identification of a protein mixture containing ezrin/moesin, alkaline phosphatase, and an unknown protein. Based on Western and immunoprecipitation data for ezrin, moesin, and PDZ-dc1 we believe that it is unlikely that our phosphoprotein is any of these 3 proteins. Parallels between NHE3 regulation (through EBP50/ERM proteins) and Na/Pi cotransporter regulation (through PDZ-dc1/ERM proteins) may be drawn.


CONCLUSIONS
These changes in proximal Na/Pi cotransport may involve a signal transduction cascade including PDZ-dc1, ezrin, moesin, our phosphoprotein, and possibly other proteins.
Puschett
Jules B
JB
Department of Medicine, Tulane University School of Medicine, New Orleans, Louisiana, USA. jpusche@tulane.edu
Whitbred
Joy
J
Ianosi-Irimie
Monica
M
Vu
Hop V
HV
Rabon
Edd
E
Robinson
James
J
Deininger
Prescott
P
D000595:Amino Acid Sequence; D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D017124:Conserved Sequence; D003598:Cytoskeletal Proteins; D005260:Female; D007668:Kidney / Q000378:metabolism*; D051379:Mice; D008840:Microfilament Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D010750:Phosphoproteins / Q000378:metabolism; D010953:Plasma Volume / Q000502:physiology*; D017434:Protein Structure, Tertiary; D051381:Rats; D017207:Rats, Sprague-Dawley; D016415:Sequence Alignment; D015398:Signal Transduction; D050608:Sodium-Phosphate Cotransporter Proteins; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D027981:Symporters / Q000378:metabolism*; D000099149:Ezrin
D002352:Carrier Proteins; D003598:Cytoskeletal Proteins; D008840:Microfilament Proteins; D010750:Phosphoproteins; D050608:Sodium-Phosphate Cotransporter Proteins; D050610:Sodium-Phosphate Cotransporter Proteins, Type II; D027981:Symporters; D000099149:Ezrin; C109245:Pdzk1 protein, rat; C071058:moesin