title: The renal Na+/phosphate cotransporter NaPi-IIa is internalized via the receptor-mediated endocytic route in response to parathyroid hormone.
abstract: The major renal Na(+)/phosphate cotransporter, NaPi-IIa, is regulated by a number of factors including parathyroid hormone (PTH), dopamine, and dietary phosphate intake. PTH induces the acute internalization of NaPi-IIa from the brush border membrane (BBM) and its routing to and subsequent degradation in lysosomes. Previous work indicated that megalin, part of the apical receptor-mediated endocytic apparatus, may play a role in the PTH-induced removal of NaPi-IIa. Here we examined in rats the time-dependent internalization route of NaPi-IIa after acute PTH application using immunohistochemistry and markers of several endocytic compartments. NaPi-IIa removal from the BBM was detectable as early as 5 min after PTH injection. After 10-15 min, NaPi-IIa was localized in subapical compartments positive for clathrin. Shortly thereafter, NaPi-IIa appeared in endosomes stained for EEA1 (early endosomal antigen 1). After 45-60 min, NaPi-IIa was found in late endosomes/lysosomes marked with lgp120. In contrast, no change in the subcellular localization of megalin and the Na(+)/H(+) exchanger NHE3 was detected up to 60 min after PTH injection. To further characterize the internalization route, insulin, as a marker for receptor-mediated endocytosis, and horseradish peroxidase (HRP) and fluorescein isothiocyanate (FITC)-dextran (10 kDa), as markers for fluid-phase mediated endocytosis, were used. NaPi-IIa colocalized with insulin 5-30 min after PTH injection but did not overlap with HRP or FITC-dextran. These results demonstrate a distinct internalization route of NaPi-IIa in response to acute PTH application that may involve the receptor-mediated endocytic pathway including clathrin-coated vesicles and EEA1-positive early endosomes, and routes NaPi-IIa to lysosomes for degradation.
authors.affiliation: Institute of Physiology and Center for Integrative Human Physiology, University of Zurich, Zurich, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Bacic
authors.forename: M
authors.initials: M
authors.lastname: Lehir
authors.forename: J
authors.initials: J
authors.lastname: Biber
authors.forename: B
authors.initials: B
authors.lastname: Kaissling
authors.forename: H
authors.initials: H
authors.lastname: Murer
authors.forename: C A
authors.initials: CA
authors.lastname: Wagner
mesh_terms: D000818:Animals; D002966:Clathrin / Q000032:analysis; D018699:Coated Vesicles / Q000737:chemistry / Q000502:physiology; D004705:Endocytosis / Q000187:drug effects* / Q000502:physiology; D011992:Endosomes / Q000737:chemistry / Q000502:physiology; D005455:Fluorescent Antibody Technique; D007150:Immunohistochemistry / Q000379:methods; D007328:Insulin / Q000032:analysis / Q000502:physiology; D007668:Kidney / Q000737:chemistry / Q000502:physiology; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000032:analysis / Q000502:physiology; D008247:Lysosomes / Q000737:chemistry / Q000502:physiology; D008297:Male; D010281:Parathyroid Hormone / Q000008:administration & dosage / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000032:analysis / Q000502:physiology; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa / Q000378:metabolism*
chemical_list: D002966:Clathrin; D007328:Insulin; D026561:Low Density Lipoprotein Receptor-Related Protein-2; D010281:Parathyroid Hormone; C493556:Slc34a1 protein, rat; C000613387:Slc9a3 protein, rat; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D050613:Sodium-Phosphate Cotransporter Proteins, Type IIa