Volume expansion-induced changes in renal tubular membrane protein phosphorylation

The role of excessive volume expansion in the pathogenesis of preeclampsia

Preeclampsia is a disorder which is responsible for significant maternal morbidity and mortality as well as fetal wastage. Its pathogenesis remains obscure and its only treatment is the delivery of the placenta and the fetus. Over time it has become clear that this syndrome is not a single disease but a disorder with, most likely, multiple etiologic factors that have a common (or similar) phenotype(s). A leading hypothesis, first developed in the early 1970s, is that the hypertension, proteinuria and intrauterine growth restriction are the result of hypoperfusion of the maternal-fetal unit. However, the early events leading to this deranged circulatory event have not been extensively studied. We hypothesize that at least one of the early pathogenetic events is excessive expansion of the extracellular fluid volume. This leads to persistent elaboration of (a) circulating factor(s) that interfere(s) with remodeling of the decidual vasculature preventing normal placentation from occurring. Our experiments have dealt largely with the role that an endogenous bufadienolide, marinobufagenin (MBG), plays in this pathogenetic process. In this report, we provide evidence for this thesis and point to future studies aimed at testing this hypothesis. These will include evaluating large groups of preeclamptic patients to determine their blood and urinary levels of MBG. Efforts will also be made to determine if there are differences in sodium handling in those patients with elevated levels of MBG, compared to other preeclamptic patients and to normal pregnant subjects.

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.

METHODS AND 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.

CONCLUSION

These changes in proximal Na/Pi cotransport may involve a signal transduction cascade including PDZ-dc1, ezrin, moesin, our phosphoprotein, and possibly other proteins.

Proximal tubular phosphate reabsorption: molecular mechanisms

Renal proximal tubular reabsorption of P(i) is a key element in overall P(i) homeostasis, and it involves a secondary active P(i) transport mechanism. Among the molecularly identified sodium-phosphate (Na/P(i)) cotransport systems a brush-border membrane type IIa Na-P(i) cotransporter is the key player in proximal tubular P(i) reabsorption. Physiological and pathophysiological alterations in renal P(i) reabsorption are related to altered brush-border membrane expression/content of the type IIa Na-P(i) cotransporter. Complex membrane retrieval/insertion mechanisms are involved in modulating transporter content in the brush-border membrane. In a tissue culture model (OK cells) expressing intrinsically the type IIa Na-P(i) cotransporter, the cellular cascades involved in "physiological/pathophysiological" control of P(i) reabsorption have been explored. As this cell model offers a "proximal tubular" environment, it is useful for characterization (in heterologous expression studies) of the cellular/molecular requirements for transport regulation. Finally, the oocyte expression system has permitted a thorough characterization of the transport characteristics and of structure/function relationships. Thus the cloning of the type IIa Na-P(i )cotransporter (in 1993) provided the tools to study renal brush-border membrane Na-P(i) cotransport function/regulation at the cellular/molecular level as well as at the organ level and led to an understanding of cellular mechanisms involved in control of proximal tubular P(i) handling and, thus, of overall P(i) homeostasis.