Megalin is involved in angiotensinogen-induced, angiotensin II-mediated ERK1/2 signaling to activate Na+-H+ exchanger 3 in proximal tubules

Abstract

Background: Kidney angiotensin (Ang) II is produced mainly from liver-derived, glomerular-filtered angiotensinogen (AGT). Podocyte injury has been reported to increase the kidney Ang II content and induce Na+ retention depending on the function of megalin, a proximal tubular endocytosis receptor. However, how megalin regulates the renal content and action of Ang II remains elusive. Methods: We used a mass spectrometry-based, parallel reaction-monitoring assay to quantitate Ang II in plasma, urine, and kidney homogenate of kidney-specific conditional megalin knockout (MegKO) and control (Ctl) mice. We also evaluated the pathophysiological changes in both mouse genotypes under the basal condition and under the condition of increased glomerular filtration of AGT induced by administration of recombinant mouse AGT (rec-mAGT). Results: Under the basal condition, plasma and kidney Ang II levels were comparable in the two mouse groups. Ang II was detected abundantly in fresh spot urine in conditional MegKO mice. Megalin was also found to mediate the uptake of intravenously administered fluorescent Ang II by PTECs. Administration of rec-mAGT increased kidney Ang II, exerted renal extracellular signal-regulated kinase 1/2 (ERK1/2) signaling, activated proximal tubular Na+-H+ exchanger 3 (NHE3), and decreased urinary Na+ excretion in Ctl mice, whereas these changes were suppressed but urinary Ang II was increased in conditional MegKO mice. Conclusion: Increased glomerular filtration of AGT is likely to augment Ang II production in the proximal tubular lumen. Thus, megalin-dependent Ang II uptake should be involved in the ERK1/2 signaling that activates proximal tubular NHE3 in vivo, thereby causing Na+ retention.