Myoblast fusion is essential for the formation of skeletal muscle myofibres. Studies have shown that phosphatidylserine is necessary for myoblast fusion, but the underlying mechanism is not known. Here we show that the phosphatidylserine receptor stabilin-2 acts as a membrane protein for myoblast fusion during myogenic differentiation and muscle regeneration. Stabilin-2 expression is induced during myogenic differentiation, and is regulated by calcineurin/NFAT signalling in myoblasts. Forced expression of stabilin-2 in myoblasts is associated with increased myotube formation, whereas deficiency of stabilin-2 results in the formation of small, thin myotubes. Stab2-deficient mice have myofibres with small cross-sectional area and few myonuclei and impaired muscle regeneration after injury. Importantly, myoblasts lacking stabilin-2 have reduced phosphatidylserine-dependent fusion. Collectively, our results show that stabilin-2 contributes to phosphatidylserine-dependent myoblast fusion and provide new insights into the molecular mechanism by which phosphatidylserine mediates myoblast fusion during muscle growth and regeneration.

The E3 ubiquitin (Ub)-protein ligases (E3s) play a role as regulators of protein trafficking and degradation. We aimed to integrate the profile of E3s in rat kidney and examine the changes in protein abundance of the selected E3s in response to 1-deamino-8-D-arginine vasopressin (dDAVP) stimulation/withdrawal. Sprague-Dawley rats were infused with vehicle (n = 13), dDAVP for 5 days (n = 13), or dDAVP was withdrawn for periods (15 min, 30 min, 1, 3, 6, 12, or 24 h) after 5-day infusion (n = 46). Total RNA was isolated from the inner medulla (IM) for transcriptome analysis. Plasma membrane (PM)- or intracellular vesicle (ICV)-enriched fractions of whole kidney were immunoisolated for liquid chromatography-tandem mass spectrometry analysis. dDAVP infusion for 5 days (D5d) significantly increased urine osmolality, which was maintained during 3-h withdrawal of dDAVP after 5-day infusion (D5d-3h). Consistent with this, aquaporin-2 (AQP2) expression in the PM fractions of D5d and D5d-3h increased, whereas AQP2 expression in the ICV fractions of D5d-3h was further increased, indicating internalization of AQP2. Transcriptome analysis revealed 86 genes of E3s and LC-MS/MS analysis demonstrated 16 proteins of E3s. Among these, seven E3s (BRCA1, UBR4, BRE1B, UHRF1, NEDD4, CUL5, and FBX6) were shared. RT-PCR demonstrated mRNA expressions of the seven identified E3s in the kidney, and immunoblotting demonstrated changes in protein abundance of the selected E3s (BRE1B, NEDD4, and CUL5) in response to dDAVP stimulation/withdrawal or lithium-induced nephrogenic diabetes insipidus. The rate of AQP2 degradation was retarded in mpkCCDc14 cells with small interfering RNA-mediated knockdown of NEDD4 or CUL5. Taken together, identified E3s could be involved in the degradation of proteins associated with vasopressin-induced urine concentration.

AS160, a novel Akt substrate of 160 kDa, contains a Rab GTPase-activating protein (GAP) domain. The present study examined the role of Akt and AS160 in aquaporin-2 (AQP2) trafficking. The main strategy was to examine the changes in AQP2 translocation in response to small interfering RNA (siRNA)-mediated AS160 knockdown in mouse cortical collecting duct cells (M-1 cells and mpkCCDc14 cells). Short-term dDAVP treatment in M-1 cells stimulated phosphorylation of Akt (S473) and AS160, which was also seen in mpkCCDc14 cells. Conversely, the phosphoinositide 3-kinase (PI3K) inhibitor LY 294002 diminished phosphorylation of Akt (S473) and AS160. Moreover, siRNA-mediated Akt1 knockdown was associated with unchanged total AS160 but decreased phospho-AS160 expression, indicating that phosphorylation of AS160 is dependent on PI3K/Akt pathways. siRNA-mediated AS160 knockdown significantly decreased total AS160 and phospho-AS160 expression. Immunocytochemistry revealed that AS160 knockdown in mpkCCDc14 cells was associated with increased AQP2 density in the plasma membrane [135 ± 3% of control mpkCCDc14 cells (n = 65), P < 0.05, n = 64] despite the absence of dDAVP stimulation. Moreover, cell surface biotinylation assays of mpkCCDc14 cells with AS160 knockdown exhibited significantly higher AQP2 expression [150 ± 15% of control mpkCCDc14 cells (n = 3), P < 0.05, n = 3]. Taken together, PI3K/Akt pathways mediate the dDAVP-induced AS160 phosphorylation, and AS160 knockdown is associated with higher AQP2 expression in the plasma membrane. Since AS160 contains a GAP domain leading to a decrease in the active GTP-bound form of AS160 target Rab proteins for vesicle trafficking, decreased expression of AS160 is likely to play a role in the translocation of AQP2 to the plasma membrane.

It has been reported that several proteins [heat shock protein 70 (Hsp70 and Hsc70), annexin II, and tropomyosin 5b] interact with the Ser(256) residue on the COOH terminus of aquaporin-2 (AQP2), where vasopressin-induced phosphorylation occurs for mediating AQP2 trafficking. However, it remains unknown whether these proteins, particularly Hsp70, play a role in AQP2 trafficking. Semiquantitative immunoblotting revealed that renal expression of AQP2 and Hsp70 was significantly increased in water-restricted or dDAVP-infused rats. In silico analysis of the 5'-flanking regions of AQP2, Hsp70-1, and Hsp70-2 genes revealed that transcriptional regulator binding elements associated with cAMP response were identified at both the Hsp70-1 and Hsp70-2 promoter regions, in addition to AQP2. Luciferase reporter assay demonstrated the significant increase of luminescence after dDAVP stimulation (10(-8) M, 6 h) in the LLC-PK1 cells transfected with luciferase vector containing 1 kb of the 5'-flanking region of Hsp70-2 gene. Hsp70-2 protein expression was also increased in mpkCCDc14 cells treated by dDAVP in a concentration-dependent manner. Cell surface biotinylation analysis demonstrated that forskolin (10(-5) M, 15 min)-induced AQP2 targeting to the apical plasma membrane was significantly attenuated in the mpkCCDc14 cells with Hsp70-2 knockdown. Moreover, forskolin-induced AQP2 phosphorylation (Ser(256)) was not significantly induced in the mpkCCDc14 cells with Hsp70-2 knockdown. In contrast, Hsp70-2 knockdown did not affect the dDAVP-induced AQP2 abundance. In addition, siRNA-directed knockdown of Hsp70 significantly decreased cell viability. The results suggest that Hsp70 is likely to play a role in AQP2 trafficking to the apical plasma membrane, partly through affecting AQP2 phosphorylation at Ser(256) and cell viability.

Nielsen S, Kwon TH. Tankyrase-mediated ␤-catenin activity regulates vasopressin-induced AQP2 expression in kidney collecting duct mpkCCDc14 cells. Am J Physiol Renal Physiol 308: F473-F486, 2015. First published December 17, 2014 doi:10.1152/ajprenal.00052.2014.-Aquaporin-2 (AQP2) mediates arginine vasopressin (AVP)-induced water reabsorption in the kidney collecting duct. AVP regulates AQP2 expression primarily via Gs␣/cAMP/PKA signaling. Tankyrase, a member of the poly(ADP-ribose) polymerase family, is known to mediate Wnt/␤-catenin signaling-induced gene expression. We examined whether tankyrase plays a role in AVP-induced AQP2 regulation via ADP-ribosylation of G protein-␣ (G␣) and/or ␤-catenin-mediated transcription of AQP2. RT-PCR and immunoblotting analysis revealed the mRNA and protein expression of tankyrase in mouse kidney and mouse collecting duct mpkCCDc14 cells. dDAVP-induced AQP2 upregulation was attenuated in mpkCCDc14 cells under the tankyrase inhibition by XAV939 treatment or small interfering (si) RNA knockdown. Fluorescence resonance energy transfer image analysis, however, revealed that XAV939 treatment did not affect dDAVP-or forskolin-induced PKA activation. Inhibition of tankyrase decreased dDAVP-induced phosphorylation of ␤-catenin (S552) and nuclear translocation of phospho-␤-catenin. siRNA-mediated knockdown of ␤-catenin decreased forskolin-induced AQP2 transcription and dDAVP-induced AQP2 expression. Moreover, inhibition of phosphoinositide 3-kinase/Akt, which was associated with decreased nuclear translocation of ␤-catenin, diminished dDAVP-induced AQP2 upregulation, further indicating that ␤-catenin mediates AQP2 expression. Taken together, tankyrase plays a role in AVP-induced AQP2 regulation, which is likely via ␤-catenin-mediated transcription of AQP2, but not ADP-ribosylation of G␣. The results provide novel insights into vasopressin-mediated urine concentration and homeostasis of body water metabolism.aquaporin-2; ␤-catenin; collecting duct; tankyrase; vasopressin AQUAPORINS (AQPS) ARE WATER channel proteins that transport water molecules across the biomembrane. Aquaporin-2 (AQP2) is the key water channel protein expressed in the kidney connecting tubules and collecting ducts for arginine vasopressin (AVP)-mediated water reabsorption (5,10,12,20,22). Vasopressin V2-receptor (V2R)-mediated cAMP/PKA signaling has been shown to be a principal pathway for both AQP2 trafficking and protein expression via activation of G s ␣-mediated adenylyl cyclase activity. Increased intracellular cAMP concentration and activation of PKA phosphorylate cAMP-response element binding protein, which increases transcription of the AQP2 gene (41).The AVP-induced cAMP/PKA signaling pathway interacts with other signals, such as phosphoinositide pathways (19,31) and Wnt/␤-catenin signaling (34). However, cross talk between AVP and Wnt/␤-catenin signaling in the kidney collecting ducts, particularly for the regulation of AVP-induced AQP2 expression, is unknown. Previous studies demonstrated that AVP-mediate...

Internalization of receptor proteins after interacting with specific ligands has been proposed to facilitate siRNA delivery into the target cells via receptor-mediated siRNA transduction. In this study, we demonstrated a novel method of vasopressin V2 receptor (V2R)-mediated siRNA delivery against AQP2 in primary cultured inner medullary collecting duct (IMCD) cells of rat kidney. We synthesized the dDAVP conjugated with nine D-arginines (dDAVP-9r) as a peptide carrier for siRNA delivery. The structure of synthetic peptide carrier showed two regions (i.e., ligand domain to V2R (dDAVP) and siRNA carrying domain (nine D-arginine)) bisected with a spacer of four glycines. The results revealed that 1) synthesized dDAVP-9r peptides formed a stable polyplex with siRNA; 2) siRNA/dDAVP-9r polyplex could bind to the V2R of IMCD cells and induced AQP2 phosphorylation (Ser 256); 3) siRNA/dDAVP-9r polyplex was stable in response to the wide range of different osmolalities, pH levels, or to the RNases; 4) fluorescein-labeled siRNA was delivered into V2R-expressing MDCK and LLC-PK1 cells by siRNA/dDAVP-9r polyplex, but not into the V2R-negative Cos-7 cells; and 5) AQP2-siRNA/dDAVP-9r polyplex effectively delivered siRNA into the IMCD cells, resulting in the significant decrease of protein abundance of AQP2, but not AQP4. Therefore, for the first time to our knowledge, we demonstrated that V2R-mediated siRNA delivery could be exploited to deliver specific siRNA to regulate abnormal expression of target proteins in V2R-expressing kidney cells. The methods could be potentially used in vivo to regulate abnormal expression of proteins associated with disease conditions in the V2R-expressing kidney cells.

Lee YJ, Choi HJ, Lim JS, Earm JH, Lee BH, Kim IS, Frøkiaer J, Nielsen S, Kwon TH. A novel method of ligand peptidomics to identify peptide ligands binding to AQP2-expressing plasma membranes and intracellular vesicles of rat kidney. Am J Physiol Renal Physiol 295: F300 -F309, 2008. First published May 14, 2008 doi:10.1152/ajprenal.00006.2008, the vasopressin-regulated water channel in collecting duct principal cells, plays a key role in the regulation of body water balance. We aimed to isolate high-affinity peptide ligands that bind to immunoisolated AQP2-expressing plasma membrane (PM) or intracellular vesicle (ICV) preparations from rat kidney by the in vitro phage display technique. Immunoblotting revealed that AQP2 was exclusively expressed in the immunoisolated AQP2 membrane fractions (PM and ICV), compared with the nonimmunoisolated or preimmune IgG pulldown rat kidney samples. Moreover, AQP1 or H ϩ -ATPase (B1 subunit) expression was minimal in the immunoisolated AQP2 membrane fractions, indicating the specificity of AQP2 membrane isolation. A phage peptide library based on T7 415-1b phage vector displaying CX 7C was constructed. After three rounds of biopanning, seven phage clones of high frequency were selected, which showed high affinity to the AQP2-containing PM or ICV fractions compared with a nonrecombinant T7 insertless phage clone. In contrast, these phage clones showed lower affinity to H ϩ -ATPase-containing fractions. Fluorescein-conjugated peptide labeling was associated with intracellular compartment and PM of primary cultured inner medullary collecting duct cells, relative to absent or very weak labeling with fluorescein-conjugated control peptide. Library analyses demonstrated proteins that had motifs homologous to the peptide ligands, albeit with a high probability of a random match due to short peptide sequences. In summary, we applied the in vitro phage display technique to identify high-affinity peptide ligands to AQP2-expressing membranes. Library analyses identified proteins having homologous motifs, which need to be examined for involvement in AQP2 trafficking and regulation. aquaporin; phage display; urine concentration IN VITRO PHAGE DISPLAY represents an emerging and innovative technology for the rapid isolation of high-affinity peptide ligands (15, 39). Phage display technologies using phages comprising a vast library of peptides have become fundamental to the isolation of high-affinity binding ligands for diagnostic and therapeutic applications, e.g., ligand proteomics, discovery of novel protein-protein interactions, antibody engineering, targeted delivery of therapeutic agents, and development of imaging probes (23,35,36,38). Phage display makes use of bacteriophages that propagate in Escherichia coli to express a wide variety of peptide ligands with high specificity and affinity in vitro. These ligands, displayed on the surface of the phage particle, can be selected against any given target (36, 38).Regulation of the water permeability of the apical plasma membrane of kidney...

No abstract