Expression of parathyroid hormone receptors in MDCK and LLC-PK1 cells

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

The absence of a clathrin adapter confers unique polarity essential to proximal tubule function

It is well established that many cognate basolateral plasma membrane proteins are expressed apically in proximal tubule cells thus optimizing the reabsorption capacity of the kidney. The protein clathrin and its adapter proteins normally regulate basolateral polarity. Here we tested whether the unique proximal tubule polarity is dependent on an epithelial-specific basolateral clathrin adapter, AP1B, present in most other epithelia. Quantitative PCR of isolated mouse renal tubules showed that AP1B was absent in proximal tubules but present in medullary and cortical thick ascending limbs of Henle, and cortical collecting ducts. Western blot confirmed the absence of AP1B in three established proximal tubule cell lines. Knockdown of AP1B by shRNA in prototypical distal tubule MDCK cells resulted in redistribution of the basolateral parathyroid hormone receptor, the insulin-like growth factor II receptor/calcium-independent mannose-6-phosphate receptor, and the junctional adhesion molecule, JAM-C, to a proximal tubule-like nonpolar localization. Yeast two-hybrid assays detected direct interactions between the cytoplasmic tails of these plasma membrane proteins and the cargo-binding region of the AP1B complex. Hence, our results show that differential expression of AP1B contributes to normal kidney function and illustrates possible roles of this adapter protein in kidney development, physiology, and pathology.

Protein kinase C inhibits caveolae-mediated endocytosis of TRPV5

Transient receptor potential vanilloid 5 (TRPV5) constitutes the apical entry pathway for transepithelial Ca2+reabsorption in kidney. Many hormones alter renal Ca2+reabsorption at least partly by regulating TRPV5. The mechanism for acute regulation of TRPV5 by phospholipase C-coupled hormones is largely unknown. Here, we found that protein kinase C (PKC) activator 1-oleoyl-acetyl-sn-glycerol (OAG) increased TRPV5 current density and surface abundance in cultured cells. The OAG-mediated increase of TRPV5 was prevented by preincubation with specific PKC inhibitors. Coexpression with a dominant-negative dynamin increased the basal TRPV5 current density and prevented the increase by OAG. Knockdown of caveolin-1 by small interference RNA (siRNA) prevented the increase of TRPV5 by OAG. In contrast, knockdown of clathrin heavy chain had no effects. OAG had no effect on TRPV5 expressed in caveolin-1 null cells derived from caveolin-1 knockout mice. Forced expression of recombinant caveolin-1 restored the regulation of TRPV5 by OAG in caveolin-1 knockout cells. Mutations of serine-299 and/or serine-654 of TRPV5 (consensus residues for phosphorylation by PKC) abolished the regulation by OAG. Parathyroid hormone (PTH) increased TRPV5 current density in cells coexpressing TRPV5 and type 1 PTH receptor. The increase caused by PTH was prevented by PKC inhibitor, mutation of serine-299/serine-654, or by knockdown of caveolin-1. Thus, TRPV5 undergoes constitutive caveolae-mediated endocytosis. Activation of PKC increases cell surface abundance of TRPV5 by inhibiting the endocytosis. This mechanism of regulation by PKC may contribute to the acute stimulation of TRPV5 and renal Ca2+reabsorption by PTH.

Functional role of the NPxxY motif in internalization of the type 2 vasopressin receptor in LLC-PK1 cells

Interaction of the type 2 vasopressin receptor (V2R) with hormone causes desensitization and internalization. To study the role of the V2R NPxxY motif (which is involved in the clathrin-mediated endocytosis of several other receptors) in this process, we expressed FLAG-tagged wild-type V2R and a Y325F mutant V2R in LLC-PK1a epithelial cells that have low levels of endogenous V2R. Both proteins had a similar apical (35%) and basolateral (65%) membrane distribution. Substitution of Tyr325 with Phe325 prevented ligand-induced internalization of V2R determined by [3H]AVP binding and immunofluorescence but did not prevent ligand binding or signal transduction via adenylyl cyclase. Desensitization and resensitization of the V2R-Y325F mutation occurred independently of internalization. The involvement of clathrin in V2R downregulation was also shown by immunogold electron microscopy. We conclude that the NPxxY motif of the V2R is critically involved in receptor downregulation via clathrin-mediated internalization. However, this motif is not essential for the apical/basolateral sorting and polarized distribution of the V2R in LLC-PK1a cells or for adenylyl cyclase-mediated signal transduction.

Differential expression of adenylyl cyclases in the rat nephron

BACKGROUND

Adenylyl cyclases (ACs) are a family of enzymes that catalyze the formation of the second-messenger cyclic adenosine 3',5'-monophosphate (cAMP). At least nine isoforms of AC have been cloned. These isoforms differ in their tissue distribution and basal activity. AC isoforms also differ in their capacity to be stimulated or inhibited by G protein alpha(i), alpha(s) and beta/gamma subunits, protein kinase C, and intracellular calcium. The distribution of ACs in the kidney is only partially known, although it is known that ACs play important roles in kidney signal transduction. Several receptors are known to couple to AC, but their linkage to individual AC isoforms in the kidney is not known.

METHODS

This study investigated the tissue distribution of AC isoforms along the nephron of Wistar-Kyoto rats using reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry, and immunoblotting.

RESULTS

While AC VI and IX mRNA were found in all nephron segments, there was no expression of AC VIII mRNA. ACs II through V and VII mRNA were variably found in specific nephron segments. mRNA for AC isoforms II, III, VI, VII, and IX were expressed in renal proximal tubules. All of the AC isoforms studied, except VIII, were found in glomeruli. Immunoblotting and immunohistochemistry confirmed the mRNA results. AC isoforms II, III, IV, and IX were expressed in luminal rather than in basolateral membranes. However, immunohistochemical studies were not feasible for the other isoforms that could be expressed in basolateral membranes.

CONCLUSION

Knowledge of the distribution of ACs may help establish the linkage between receptors and specific AC isoforms and define their functions.

NaPO(4) cotransport type III (PiT1) expression in human embryonic kidney cells and regulation by PTH

The aim of the present study was to characterize the type(s) of NaPO(4) cotransporter expressed in the human renal cell line HEK-293 and its regulation by parathyroid hormone (PTH) in wild-type cells and in cells transfected by the PTH/PTH-related protein (PTHrP) receptor. The results showed that human embryonic kidney HEK-293 cells expressed NaPO(4) cotransporter type III (PiT1) mRNA and protein. In contrast, type I (NPT1) or II (NPT2) cotransporter mRNA were not expressed. Na(+)-dependent phosphate uptake followed a Michaelis-Menten model (apparent maximal transport rate and affinity constant: 23.32 +/- 0.69 nmol PO(4). mg protein(-1). 10 min(-1) and 0.147 +/- 0.014 mM KH(2)PO(4), respectively), was stimulated by phosphate deprivation (maximal increase 24.5 +/- 0.8%, P < 0.001, after 15 h of phosphate deprivation), and was inhibited by increasing pH (3.6 +/- 0.2-fold decrease at pH 8.5, P < 0.0001). It was inhibited in a time- and concentration-dependent fashion by PTH in HEK-293 cells stably transfected by PTH/PTHrP receptors but not in parental HEK-293 cells. Maximal inhibition of Na(+)-dependent phosphate transport was observed at 30 min after the addition of 72 nM PTH-(1-34) (31.5 +/- 2.4% inhibition, P < 0.01). PTH inhibition of phosphate transport was maintained in phosphate-deprived cells and reversed by both GF109203X (10(-6) M) or staurosporine (5.5 nM), two protein kinase C inhibitors. Na(+)-dependent phosphate uptake was also significantly inhibited by phorbol 12-myristate 13-acetate (20.9 +/- 3.9% inhibition, P < 0.001) but not by dibutyril-cAMP (10(-4) M) or forskolin (50 microM). The physiological role played by type III NaPO(4) cotransport expression in the overall renal regulation of phosphate homeostasis remains to be established.