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Polymorphism of the gene encoding mucin 1 (MUC1) is associated with skeletal and dental phenotypes in human genomic studies. Animals lacking MUC1 exhibit mild reduction in bone density. These phenotypes could be a consequence of modulation of bodily Ca homeostasis by MUC1, as suggested by the previous observation that MUC1 enhances cell surface expression of the Ca2+-selective channel, TRPV5, in cultured unpolarized cells. Using biotinylation of cell surface proteins, we asked whether MUC1 influences endocytosis of TRPV5 and another Ca2+-selective TRP channel, TRPV6, in cultured polarized epithelial cells. Our results indicate that MUC1 reduces endocytosis of both channels, enhancing cell surface expression. Further, we found that mice lacking MUC1 lose apical localization of TRPV5 and TRPV6 in the renal tubular and duodenal epithelium. Females, but not males, lacking MUC1 exhibit reduced blood Ca2+. However, mice lacking MUC1 exhibited no differences in basal urinary Ca excretion or Ca retention in response to PTH receptor signaling, suggesting compensation by transport mechanisms independent of TRPV5 and TRPV6. Finally, humans with autosomal dominant tubulointerstitial kidney disease due to frame-shift mutation of MUC1 (ADTKD-MUC1) exhibit reduced plasma Ca concentrations compared to control individuals with mutations in the gene encoding uromodulin (ADTKD-UMOD), consistent with MUC1 haploinsufficiency causing reduced bodily Ca2+. In summary, our results provide further insight into the role of MUC1 in Ca2+-selective TRP channel endocytosis and the overall effects on Ca concentrations. [ABSTRACT FROM AUTHOR]

Cell-associated kidney injury molecule-1 (KIM-1) exerts an anti-inflammatory role following kidney injury by mediating efferocytosis and downregulating the NF-κB pathway. KIM-1 cleavage blunts its anti-inflammatory activities. We reported that mucin 1 (MUC1) is protective in a mouse model of ischemia-reperfusion injury (IRI). As both KIM-1 and MUC1 are induced in the proximal tubule (PT) during IRI and are a disintegrin and metalloprotease 17 (ADAM17) substrates, we tested the hypothesis that MUC1 protects KIM-1 activity. Muc1 knockout (KO) mice and wild-type (WT) littermates were subjected to IRI. KIM-1, MUC1, and ADAM17 levels (and signaling pathways) were assessed by immunoblot analysis. PT localization was assessed by confocal microscopy and an in situ proximity ligation assay. Findings were extended using human kidneys and urine as well as KIM-1-mediated efferocytosis assays in mouse PT cultures. In response to tubular injury in mouse and human kidneys, we observed induction and coexpression of KIM-1 and MUC1 in the PT. Compared with WT mice, Muc1 KO mice had higher urinary KIM-1 and lower kidney KIM-1. KIM-1 was apical in the PT of WT kidneys but predominately with luminal debris in Muc1 KO mice. Efferocytosis was reduced in Muc1 KO PT cultures compared with WT cultures, whereas inflammation was increased in Muc1 KO kidneys compared with WT kidneys. MUC1 was cleaved by ADAM17 in PT cultures and blocked KIM-1 shedding in Madin-Darby canine kidney cells. We conclude that KIM-1-mediated efferocytosis and thus anti-inflammatory activity during IRI is preserved in the injured kidney by MUC1 inhibition of KIM-1 shedding. [ABSTRACT FROM AUTHOR]

The proper function of the organs that make up the urinary tract (kidneys, ureters, bladder, and urethra) depends on their ability to sense and respond to mechanical forces, including shear stress and wall tension. However, we have limited understanding of the mechanosensors that function in these organs and the tissue sites in which these molecules are expressed. Possible candidates include stretch-activated PIEZO channels (PIEZO1 and PIEZO2), which have been implicated in mechanically regulated body functions including touch sensation, proprioception, lung inflation, and blood pressure regulation. Using reporter mice expressing a COOH-terminal fusion of Piezo1 with the sequence for the tandem-dimer Tomato gene, we found that PIEZO1 is expressed in the kidneys, ureters, bladder, and urethra as well as organs in close proximity, including the prostate, seminal vesicles and ducts, ejaculatory ducts, and the vagina. We further found that PIEZO1 expression is not limited to one cell type; it is observed in the endothelial and parietal cells of the renal corpuscle, the basolateral surfaces of many of the epithelial cells that line the urinary tract, the interstitial cells of the bladder and ureters, and populations of smooth and striated muscle cells. We propose that in the urinary tract, PIEZO1 likely functions as a mechanosensor that triggers responses to wall tension. [ABSTRACT FROM AUTHOR]

Aquaporin-2 (AQP2) is essential to maintain body water homeostasis. AQP2 traffics from intracellular vesicles to the apical membrane of kidney collecting duct principal cells in response to vasopressin [arginine vasopressin (AVP)], a hormone released with low intravascular volume, which causes decreased kidney perfusion. Decreased kidney perfusion activates AMP-activated kinase (AMPK), a metabolic sensor that inhibits the activity of several transport proteins. We hypothesized that AMPK activation also inhibits AQP2 function. These putative AMPK effects could protect interstitial ionic gradients required for urinary concentration during metabolic stress when low intravascular volume induces AVP release. Here we found that shortterm AMPK activation by treatment with 5-aminoimidazole-4-carboxamide- 1-α-D-ribofuranoside (AICAR; 75 min) in kidney tissue prevented baseline AQP2 apical accumulation in principal cells, but did not prevent AQP2 apical accumulation in response to the AVP analog desmopressin (dDAVP). Prolonged AMPK activation prevented AQP2 cell membrane accumulation in response to forskolin in mouse collecting duct mpkCCDc14 cells. Moreover, AMPK inhibition accelerated hypotonic lysis of Xenopus oocytes expressing AQP2. We performed phosphorylation assays to elucidate the mechanism by which AMPK regulates AQP2. Although AMPK weakly phosphorylated immunoprecipitated AQP2 in vitro, no direct AMPK phosphorylation of the AQP2 COOH-terminus was detected by mass spectrometry. AMPK promoted Ser-261 phosphorylation and antagonized dDAVP-dependent phosphorylation of other AQP2 COOH-terminal sites in cells. Our findings suggest an increasing, time-dependent antagonism of AMPK on AQP2 regulation with AICAR-dependent inhibition of cAMP-dependent apical accumulation and AVP-dependent phosphorylation of AQP2. This inhibition likely occurs via a mechanism that does not involve direct AQP2 phosphorylation by AMPK. [ABSTRACT FROM AUTHOR]

Extracellular proton-secreting transport systems that contribute to extracellular pH include the vacuolar H+-ATPase (V-ATPase). This pump, which mediates ATP-driven transport of H+ across membranes, is involved in metastasis. We previously showed (Alzamora R, Thali RF, Gong F, Smolak C, Li H, Baty CJ, Bertrand CA, Auchli Y, Brunisholz RA, Neumann D, Hallows KR, Pastor-Soler NM. J Biol Chem 285: 24676-24685, 2010) that V-ATPase A subunit phosphorylation at Ser-175 is important for PKA-induced V-ATPase activity at the membrane of kidney intercalated cells. However, Ser-175 is also located within a larger phosphorylation consensus sequence for Aurora kinases, which are known to phosphorylate proteins that contribute to the pathogenesis of metastatic carcinomas. We thus hypothesized that Aurora kinase A (AURKA), overexpressed in aggressive carcinomas, regulates the V-ATPase in human kidney carcinoma cells (Caki-2) via Ser-175 phosphorylation. We found that AURKA is abnormally expressed in Caki-2 cells, where it binds the V-ATPase A subunit in an AURKA phosphorylation-dependent manner. Treatment with the AURKA activator anacardic acid increased V-ATPase expression and activity at the plasma membrane of Caki-2 cells. In addition, AURKA phosphorylates the V-ATPase A subunit at Ser-175 in vitro and in Caki-2 cells. Immunolabeling revealed that anacardic acid induced marked membrane accumulation of the V-ATPase A subunit in transfected Caki-2 cells. However, anacardic acid failed to induce membrane accumulation of a phosphorylation-deficient Ser-175-to-Ala (S175A) A subunit mutant. Finally, S175A-expressing cells had decreased migration in a wound-healing assay compared with cells expressing wild-type or a phospho-mimetic Ser-175-to-Asp (S175D) mutant A subunit. We conclude that AURKA activates the V-ATPase in kidney carcinoma cells via phosphorylation of Ser-175 in the V-ATPase A subunit. This regulation contributes to kidney carcinoma V-ATPase-mediated extracellular acidification and cell migration. [ABSTRACT FROM AUTHOR]

The hypoxia-inducible factor (HIF)-1 and β-catenin protective pathways represent the two most significant cellular responses that are activated in response to acute kidney injury. We previously reported that murine mucin (Muc)1 protects kidney function and morphology in a mouse model of ischemia-reperfusion injury (IRI) by stabilizing HIF-1α, enhancing HIF-1 downstream signaling, and thereby preventing metabolic stress (Pastor-Soler et al. Muc1 is protective during kidney ischemia-reperfusion injury. Am J Physiol Renal Physiol 308: F1452-F1462, 2015). We asked if Muc1 regulates the β-catenin protective pathway during IRI as 1) β-catenin nuclear targeting is MUC1 dependent in cultured human cells, 2) β-catenin is found in coimmunoprecipitates with human MUC1 in extracts of both cultured cells and tissues, and 3) MUC1 prevents β-catenin phosphorylation by glycogen synthase kinase (GSK)3β and thereby β-catenin degradation. Using the same mouse model of IRI, we found that levels of active GSK3β were significantly lower in kidneys of control mice compared with Muc1 knockout (KO) mice. Consequently, β-catenin was significantly upregulated at 24 and 72 h of recovery and appeared in the nuclear fraction at 72 h in control mouse kidneys. Both β-catenin induction and nuclear targeting were absent in Muc1 KO mice. We also found downstream induction of β-catenin prosurvival factors (activated Akt, survivin, transcription factor T cell factor 4 (TCF4), and its downstream target cyclin D1) and repression of proapoptotic factors (p53, active Bax, and cleaved caspase-3) in control mouse kidneys that were absent or aberrant in kidneys of Muc1 KO mice. Altogether, the data clearly indicate that Muc1 protection during acute kidney injury proceeds by enhancing both the HIF-1 and β-catenin protective pathways. [ABSTRACT FROM AUTHOR]

Ischemia-reperfusion injury (IRI) due to hypotension is a common cause of human acute kidney injury (AKI). Hypoxia-inducible transcription factors (HIFs) orchestrate a protective response in renal endothelial and epithelial cells in AKI models. As human mucin 1 (MUC1) is induced by hypoxia and enhances HIF-1 activity in cultured epithelial cells, we asked whether mouse mucin 1 (Muc1) regulates HIF-1 activity in kidney tissue during IRI. Whereas Muc1 was localized on the apical surface of the thick ascending limb, distal convoluted tubule, and collecting duct in the kidneys of sham-treated mice, Muc1 appeared in the cytoplasm and nucleus of all tubular epithelia during IRI. Muc1 was induced during IRI, and Muc1 transcripts and protein were also present in recovering proximal tubule cells. Kidney damage was worse and recovery was blocked during IRI in Muc1 knockout mice compared with congenic control mice. Muc1 knockout mice had reduced levels of HIF-1a, reduced or aberrant induction of HIF-1 target genes involved in the shift of glucose metabolism to glycolysis, and prolonged activation of AMP-activated protein kinase, indicating metabolic stress. Muc1 clearly plays a significant role in enhancing the HIF protective pathway during ischemic insult and recovery in kidney epithelia, providing a new target for developing therapies to treat AKI. Moreover, our data support a role specifically for HIF-1 in epithelial protection of the kidney during IRI as Muc1 is present only in tubule epithelial cells. [ABSTRACT FROM AUTHOR]

The vacuolar H+-ATPase (VATPase) mediates ATP-driven H+ transport across membranes. This pump is present at the apical membrane of kidney proximal tubule cells and intercalated cells. Defects in the V-ATPase and in proximal tubule function can cause renal tubular acidosis. We examined the role of protein kinase A (PKA) and AMP-activated protein kinase (AMPK) in the regulation of the V-ATPase in the proximal tubule as these two kinases coregulate the V-ATPase in the collecting duct. As the proximal tubule V-ATPases have different subunit compositions from other nephron segments, we postulated that V-ATPase regulation in the proximal tubule could differ from other kidney tubule segments. Immunofluorescence labeling of rat ex vivo kidney slices revealed that the V-ATPase was present in the proximal tubule both at the apical pole, colocalizing with the brush-border marker wheat germ agglutinin, and in the cytosol when slices were incubated in buffer alone. When slices were incubated with a cAMP analog and a phosphodiesterase inhibitor, the V-ATPase accumulated at the apical pole of S3 segment cells. These PKA activators also increased V-ATPase apical membrane expression as well as the rate of VATPase- dependent extracellular acidification in S3 cell monolayers relative to untreated cells. However, the AMPK activator AICAR decreased PKA-induced V-ATPase apical accumulation in proximal tubules of kidney slices and decreased V-ATPase activity in S3 cell monolayers. Our results suggest that in proximal tubule the V-ATPase subcellular localization and activity are acutely coregulated via PKA downstream of hormonal signals and via AMPK downstream of metabolic stress. [ABSTRACT FROM AUTHOR]

The vacuolar H+-ATPase (V-ATPase) in intercalated cells contributes to luminal acidification in the kidney collecting duct and nonvolatile acid excretion. We previously showed that the A subunit in the cytoplasmic V1 sector of the V-ATPase (ATP6V1A) is phosphorylated by the metabolic sensor AMP-activated protein kinase (AMPK) in vitro and in kidney cells. Here, we demonstrate that treatment of rabbit isolated, perfused collecting ducts with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) inhibited V-ATPase-dependent H+ secretion from intercalated cells after an acid load. We have identified by mass spectrometry that Ser-384 is a major AMPK phosphorylation site in the V-ATPase A subunit, a result confirmed by comparing AMPK-dependent phosphate labeling of wild-type A-subunit (WT-A) with that of a Ser-384- to-Ala A subunit mutant (S384A-A) in vitro and in intact HEK-293 cells. Compared with WT-A-expressing HEK-293 cells, S384A-Aexpressing cells exhibited greater steady-state acidification of HCO3- containing media. Moreover, AICAR treatment of clone C rabbit intercalated cells expressing the WT-A subunit reduced V-ATPasedependent extracellular acidification, an effect that was blocked in cells expressing the phosphorylation-deficient S384A-A mutant. Finally, expression of the S384A-A mutant prevented cytoplasmic redistribution of the V-ATPase by AICAR in clone C cells. In summary, direct phosphorylation of the A subunit at Ser-384 by AMPK represents a novel regulatory mechanism of the V-ATPase in kidney intercalated cells. Regulation of the V-ATPase by AMPK may couple V-ATPase activity to cellular metabolic status with potential relevance to ischemic injury in the kidney and other tissues. [ABSTRACT FROM AUTHOR]

Galectins (Gal) are β-galactoside-binding proteins that function in epithelial development and homeostasis. An overlapping role for Gal-3 and Gal-7 in wound repair was reported in stratified epithelia. Although Gal-7 was thought absent in simple epithelia, it was reported in a proteomic analysis of cilia isolated from cultured human airway, and we recently identified Gal-7 transcripts in Madin-Darby canine kidney (MDCK) cells (Poland PA, Rondanino C, Kinlough CL, Heimburg-Molinaro J, Arthur CM, Stowell SR, Smith DF, Hughey RP. J Biol Chem 286: 6780-6790, 2011). We now report that Gal-7 is localized exclusively on the primary cilium of MDCK, LLC-PK1 (pig kidney), and mpkCCDc14 (mouse kidney) cells as well as on cilia in the rat renal proximal tubule. Gal-7 is also present on most cilia of multiciliated cells in human airway epithelia primary cultures. Interestingly, exogenous glutathione S-transferase (GST)-Gal-7 bound the MDCK apical plasma membrane as well as the cilium, while the lectin Ulex europeaus agglutinin, with glycan preferences similar to Gal-7, bound the basolateral plasma membrane as well as the cilium. In pull-down assays, β1-integrin isolated from either the basolateral or apical/cilia membranes of MDCK cells was similarly bound by GST-Gal-7. Selective localization of Gal-7 to cilia despite the presence of binding sites on all cell surfaces suggests that intracellular Gal-7 is specifically delivered to cilia rather than simply binding to surface glycoconjugates after generalized secretion. Moreover, depletion of Gal-7 using tetracycline-induced short-hairpin RNA in mpkCCDc14 cells significantly reduced cilia length and slowed wound healing in a scratch assay. We conclude that Gal-7 is selectively targeted to cilia and plays a key role in surface stabilization of glycoconjugates responsible for integrating cilia function with epithelial repair. [ABSTRACT FROM AUTHOR]

Mucin 1 (or MUC1) is a heterodimeric transmembrane glycoprotein expressed on the apical surface of polarized epithelial cells in several tissues including the kidney. Recent studies have revealed several novel roles for MUC1 in the kidney, potentially including bacterial infection, mineral balance, and genetic interstitial kidney disease, even though MUC1 levels are reduced not only in the kidney but in all tissues due to MUC1 mutations. A careful localization of MUC1 in discrete segments of the nephron is a first step in understanding the multiple functional roles of MUC1 in the kidney. Most published reports of MUC1 expression to date have been largely confined to cultured cells, tumor tissues, selective nephron segments of experimental rodents, and very few studies have been performed using human kidney tissues. Given the rising attention to the role of MUC1 in differing components of renal physiology, we carefully examined the kidney distribution of MUC1 in both human and mouse kidney sections using well-defined markers for different nephron segments or cell types. We further extended our investigation to include sections of early stages of mouse kidney development and upon injury in humans. We included staining for MUC1 in urothelial cells, the highly specialized epithelial cells lining the renal pelvis and bladder. These data implicate a role for MUC1 in antimicrobial defense. Our study provides the groundwork to test the physiological relevance of MUC1 in the urinary tract.

The vacuolar H(+)-ATPase (V-ATPase) mediates ATP-driven H(+) transport across membranes. This pump is present at the apical membrane of kidney proximal tubule cells and intercalated cells. Defects in the V-ATPase and in proximal tubule function can cause renal tubular acidosis. We examined the role of protein kinase A (PKA) and AMP-activated protein kinase (AMPK) in the regulation of the V-ATPase in the proximal tubule as these two kinases coregulate the V-ATPase in the collecting duct. As the proximal tubule V-ATPases have different subunit compositions from other nephron segments, we postulated that V-ATPase regulation in the proximal tubule could differ from other kidney tubule segments. Immunofluorescence labeling of rat ex vivo kidney slices revealed that the V-ATPase was present in the proximal tubule both at the apical pole, colocalizing with the brush-border marker wheat germ agglutinin, and in the cytosol when slices were incubated in buffer alone. When slices were incubated with a cAMP analog and a phosphodiesterase inhibitor, the V-ATPase accumulated at the apical pole of S3 segment cells. These PKA activators also increased V-ATPase apical membrane expression as well as the rate of V-ATPase-dependent extracellular acidification in S3 cell monolayers relative to untreated cells. However, the AMPK activator AICAR decreased PKA-induced V-ATPase apical accumulation in proximal tubules of kidney slices and decreased V-ATPase activity in S3 cell monolayers. Our results suggest that in proximal tubule the V-ATPase subcellular localization and activity are acutely coregulated via PKA downstream of hormonal signals and via AMPK downstream of metabolic stress.

Galectins (Gal) are β-galactoside-binding proteins that function in epithelial development and homeostasis. An overlapping role for Gal-3 and Gal-7 in wound repair was reported in stratified epithelia. Although Gal-7 was thought absent in simple epithelia, it was reported in a proteomic analysis of cilia isolated from cultured human airway, and we recently identified Gal-7 transcripts in Madin-Darby canine kidney (MDCK) cells (Poland PA, Rondanino C, Kinlough CL, Heimburg-Molinaro J, Arthur CM, Stowell SR, Smith DF, Hughey RP. J Biol Chem 286: 6780-6790, 2011). We now report that Gal-7 is localized exclusively on the primary cilium of MDCK, LLC-PK(1) (pig kidney), and mpkCCD(c14) (mouse kidney) cells as well as on cilia in the rat renal proximal tubule. Gal-7 is also present on most cilia of multiciliated cells in human airway epithelia primary cultures. Interestingly, exogenous glutathione S-transferase (GST)-Gal-7 bound the MDCK apical plasma membrane as well as the cilium, while the lectin Ulex europeaus agglutinin, with glycan preferences similar to Gal-7, bound the basolateral plasma membrane as well as the cilium. In pull-down assays, β1-integrin isolated from either the basolateral or apical/cilia membranes of MDCK cells was similarly bound by GST-Gal-7. Selective localization of Gal-7 to cilia despite the presence of binding sites on all cell surfaces suggests that intracellular Gal-7 is specifically delivered to cilia rather than simply binding to surface glycoconjugates after generalized secretion. Moreover, depletion of Gal-7 using tetracycline-induced short-hairpin RNA in mpkCCD(c14) cells significantly reduced cilia length and slowed wound healing in a scratch assay. We conclude that Gal-7 is selectively targeted to cilia and plays a key role in surface stabilization of glycoconjugates responsible for integrating cilia function with epithelial repair.