Neuronally differentiated macula densa cells regulate tissue remodeling and regeneration in the kidney.

Tissue regeneration is limited in several organs, including the kidney, contributing to the high prevalence of kidney disease globally. However, evolutionary and physiological adaptive responses and the presence of renal progenitor cells suggest an existing remodeling capacity. This study uncovered endogenous tissue remodeling mechanisms in the kidney that were activated by the loss of body fluid and salt and regulated by a unique niche of a minority renal cell type called the macula densa (MD).

Increased tubular proliferation as an adaptive response to glomerular albuminuria.

Renal tubular atrophy accompanies many proteinuric renal diseases, suggesting that glomerular proteinuria injures the tubules. However, local or systemic inflammation and filtration of abnormal proteins known to directly injure tubules are also present in many of these diseases and animal models; therefore, whether glomerular proteinuria directly causes tubular injury is unknown. Here, we examined the renal response to proteinuria induced by selective podocyte loss.

A disintegrin and metalloprotease 10 activity sheds the ectodomain of the amyloid precursor-like protein 2 and regulates protein expression in proximal tubule cells.

A disintegrin and metalloprotease 10 (ADAM10) is a zinc protease that mediates ectodomain shedding of numerous receptors including Notch and members of the amyloid precursor protein family (APP, APLP1, and APLP2). Ectodomain shedding frequently activates a process called regulated intramembrane proteolysis (RIP) that links cellular events with gene regulation. To characterize ADAM10 in kidney and in opossum kidney proximal tubule (OKP) cells, we performed indirect immunofluorescence microscopy and immunoblotting of renal membrane fractions using specific antibodies.

Altered renal proximal tubular endocytosis and histology in mice lacking myosin-VI.

Myosin VI (Myo6) is an actin-based molecular motor involved in clathrin-mediated endocytosis that is highly expressed in the renal proximal tubule brush border. We investigated the renal physiological consequences of loss of Myo6 function by performing renal clearance and physiological measurements on Myo6 functional null Snell's waltzer (sv/sv) and control heterozygous (+/sv) mice. Sv/sv mice showed reduced body weight and elevated blood pressure compared with controls; no differences were observed for glomerular flow rate, urine volume, blood acid-base parameters, and plasma concentrations and urinary excretions of Na(+) and K(+).

The COOH terminus of megalin regulates gene expression in opossum kidney proximal tubule cells.

We recently reported that megalin is subjected to regulated intramembrane proteolysis (RIP) and includes 1) protein kinase C (PKC)-regulated, metalloprotease-mediated ectodomain shedding producing a membrane-bound megalin COOH-terminal fragment (MCTF) and 2) gamma-secretase-mediated cleavage of the MCTF producing a soluble megalin intracellular domain (MICD). Based on studies of RIP of other receptors, the MICD is predicted to target to the nucleus and regulate gene expression. To determine whether RIP of megalin regulates proximal tubule gene expression, we stably expressed the transfected MCTF (tMCTF) or transfected MICD (tMICD) in opossum kidney proximal tubule (OKP) cells and examined the resulting phenotype.

MiRP3 acts as an accessory subunit with the BK potassium channel.

MinK-related peptides (MiRPs) are single-span membrane proteins that assemble with specific voltage-gated K+ (Kv) channel alpha-subunits to establish gating kinetics, unitary conductance, expression level, and pharmacology of the mixed complex. MiRP3 (encoded by the KCNE4 gene) has been shown to alter the behavior of some Kv alpha-subunits in vitro but its natural partners and physiologic functions are unknown. Seeking in vivo partners for MiRP3, immunohistochemistry was used to localize its expression to a unique subcellular site, the apical membrane of renal intercalated cells, where one potassium channel type has been recorded, the calcium- and voltage-gated channel BK.

A role of the Lowe syndrome protein OCRL in early steps of the endocytic pathway.

Mutations in the inositol 5-phosphatase OCRL are responsible for Lowe syndrome, whose manifestations include mental retardation and renal Fanconi syndrome. OCRL has been implicated in membrane trafficking, but disease mechanisms remain unclear. We show that OCRL visits late-stage, endocytic clathrin-coated pits and binds the Rab5 effector APPL1 on peripheral early endosomes.

Immunolocalization of anion transporter Slc26a7 in mouse kidney.

Previous studies have indicated that a major fraction of the filtered Cl(-) is reabsorbed via apical membrane Cl(-)/base exchange in the proximal tubule. Recent studies in Slc26a6 null mice have suggested that this transporter mediates only a portion of proximal tubule Cl(-)/base exchange, raising the possibility that one or more unidentified apical membrane transporters may additionally contribute. Recent studies have identified Slc26a7 as another Cl(-)/base exchanger expressed in the kidney.

Use of phospho-specific antibodies to determine the phosphorylation of endogenous Na+/H+ exchanger NHE3 at PKA consensus sites.

Transfection studies using mutant constructs have implicated one or both protein kinase A (PKA) consensus phosphorylation sites [serines 552 and 605 in rat Na(+)/H(+) exchanger type 3 (NHE3)] as critical for mediating inhibition of NHE3 in response to several stimuli including dopamine. However, whether one or both of these sites is actually phosphorylated in endogenous NHE3 in proximal tubule cells is unknown. The purpose of this study was to generate phosphospecific antibodies so that the state of phosphorylation of these serine residues in endogenous NHE3 could be assessed in vitro and in vivo.

Linking receptor-mediated endocytosis and cell signaling: evidence for regulated intramembrane proteolysis of megalin in proximal tubule.

Megalin, a member of the low density lipoprotein receptor gene family, is required for efficient protein absorption in the proximal tubule. Recent studies have shown that the low density lipoprotein receptor-related protein, another member of this gene family, is proteolytically processed by gamma-secretase implying a role for low density lipoprotein receptor-related protein in a Notchlike signaling pathway. This pathway has been shown to involve: 1) metalloprotease-mediated ectodomain shedding and gamma-secretase-mediated intramembrane proteolysis of some receptors.

Does membrane trafficking play a role in regulating the sodium/hydrogen exchanger isoform 3 in the proximal tubule?

Purpose Of Review: The proximal tubule sodium/hydrogen exchanger continuously reabsorbs the bulk of the filtered sodium, controlling salt delivery to the distal nephron which is critical for tubuloglomerular feedback autoregulation and for fine control of salt excretion in the distal nephron. This review focuses on recent studies of the mechanisms of regulation of sodium transport in the proximal tubule, and addresses whether results from studies in proximal tubule cell lines are applicable to the proximal tubule in situ.

Recent Findings: Recent in-vivo studies provided evidence that sodium/hydrogen exchanger isoform 3 can move into and out of the apical microvilli accompanied by parallel changes in renal sodium transport: the exchanger is retracted from the microvilli in response to hypertension, parathyroid hormone or dopamine treatment and moved into the microvilli in response to sympathetic nervous system stimulation, puromycin aminonucleoside induced nephritic syndrome, and insulin treatment.

Expression of KCNA10, a voltage-gated K channel, in glomerular endothelium and at the apical membrane of the renal proximal tubule.

Potassium (K) channels regulate cell membrane potential and modulate a number of important cellular functions. KCNA10 is a cyclic nucleotide-gated, voltage-activated K channel that is detected in kidney, heart, and aorta by Northern blot and postulated to participate in renal K metabolism and to regulate vascular tone. The aim of this study was to establish the cellular and subcellular localization of KCNA10 in kidney and vascular tissues.

Expression of myosin VI within the early endocytic pathway in adult and developing proximal tubules.

Myosin VI is a reverse-direction molecular motor implicated in membrane transport events. Because myosin VI is most highly expressed in the kidney, we investigated its renal localization by using high-resolution immunocytochemical and biochemical methods. Indirect immunofluorescence microscopy revealed myosin VI at the base of the brush border in proximal tubule cells.

NHE3 and NHERF are targeted to the basolateral membrane in proximal tubules of colchicine-treated rats.

Background: Depolymerization of microtubules in proximal tubule (PT) cells of colchicine-treated rats causes disruption of vesicle recycling and redistribution of some brush-border membrane (BBM) transporters into cytoplasmic vesicles. NHE3, an isoform of the Na+/H+ exchanger in the PT cell BBM, is acutely regulated by a variety of mechanisms, including protein trafficking and interaction with the PDZ protein, NHERF. The effects of microtubule disruption by colchicine on NHE3 trafficking in PT and the potential role of NHERF in this process have not been studied.