Robert A. Fenton

Osteonecrosis of the Jaw in Multiple Myeloma Patients: Clinical Features and Risk Factors

PURPOSE To describe the clinical, radiologic, and pathologic features and risk factors for osteonecrosis of the jaw (ONJ) in multiple myeloma (MM) patients. PATIENTS AND METHODS A retrospective review of 90 MM patients who had dental assessments, including 22 patients with ONJ. There were 62 men; the median age was 61 years in ONJ patients and 58 years among the rest. Prior MM therapy included thalidomide (n = 67) and stem-cell transplantation (n = 72). Bisphosphonate therapy included zoledronate (n = 34) or pamidronate (n = 17) and pamidronate followed by zoledronate (n = 33). Twenty-seven patients had recent dental extraction, including 12 patients in the ONJ group. Median time from MM diagnosis to ONJ was 8.4 years for the whole group. RESULTS Patients usually presented with pain. ONJ occurred posterior to the cuspids (n = 20) mostly in the mandible. Debridement and sequestrectomy with primary closure were performed in 14 patients; of these, four patients had major infections and four patients had recurrent ONJ. Bone histology revealed necrosis and osteomyelitis. Microbiology showed actinomycetes (n = 7) and mixed bacteria (n = 9). More than a third of ONJ patients also suffered from long bone fractures (n = 4) and/or avascular necrosis of the hip (n = 4). The variables predictive of developing ONJ were dental extraction (P = .009), treatment with pamidronate/zoledronate (P = .009), longer follow-up time (P = .03), and older age at diagnosis of MM (P = .006). CONCLUSION ONJ appears to be time-dependent with higher risk after long-term use of bisphosphonates in older MM patients often after dental extractions. No satisfactory therapy is currently available. Trials addressing the benefits/risks of continuing bisphosphonate therapy are needed.

Defective glycerol metabolism in aquaporin 9 (AQP9) knockout mice.

Aquaporin-9 (AQP9) is an aquaglyceroporin membrane channel shown biophysically to conduct water, glycerol, and other small solutes. Because the physiological role/s of AQP9 remain undefined and the expression sites of AQP9 remain incomplete and conflicting, we generated AQP9 knockout mice. In the absence of physiological stress, knockout mice did not display any visible behavioral or severe physical abnormalities. Immunohistochemical analyses using multiple antibodies revealed AQP9 specific labeling in hepatocytes, epididymis, vas deferens, and in epidermis of wild type mice, but a complete absence of labeling in AQP9−/− mice. In brain, no detectable labeling was observed. Compared with control mice, plasma levels of glycerol and triglycerides were markedly increased in AQP9−/− mice, whereas glucose, urea, free fatty acids, alkaline phosphatase, and cholesterol were not significantly different. Oral administration of glycerol to fasted mice resulted in an acute rise in blood glucose levels in both AQP9−/− and AQP9+/− mice, revealing no defect in utilization of exogenous glycerol as a gluconeogenic substrate and indicating a high gluconeogenic capacity in nonhepatic organs. Obese Leprdb/Leprdb AQP9−/− and obese Leprdb/Leprdb AQP9+/− mice showed similar body weight, whereas the glycerol levels in obese Leprdb/Leprdb AQP9−/− mice were dramatically increased. Consistent with a role of AQP9 in hepatic uptake of glycerol, blood glucose levels were significantly reduced in Leprdb/Leprdb AQP9−/− mice compared with Leprdb/Leprdb AQP9+/− in response to 3 h of fasting. Thus, AQP9 is important for hepatic glycerol metabolism and may play a role in glycerol and glucose metabolism in diabetes mellitus.

Vasopressin-stimulated Increase in Phosphorylation at Ser269 Potentiates Plasma Membrane Retention of Aquaporin-2

Vasopressin controls water excretion through regulation of aquaporin-2 (AQP2) trafficking in renal collecting duct cells. Using mass spectrometry, we previously demonstrated four phosphorylated serines (Ser256, Ser261, Ser264, and Ser269) in the carboxyl-terminal tail of rat AQP2. Here, we used phospho-specific antibodies and protein mass spectrometry to investigate the roles of vasopressin and cyclic AMP in the regulation of phosphorylation at Ser269 and addressed the role of this site in AQP2 trafficking. The V2 receptor-specific vasopressin analog dDAVP increased Ser(P)269-AQP2 abundance more than 10-fold, but at a rate much slower than the corresponding increase in Ser256 phosphorylation. Vasopressin-mediated changes in phosphorylation at both sites were mimicked by cAMP addition and inhibited by protein kinase A (PKA) antagonists. In vitro kinase assays, however, demonstrated that PKA phosphorylates Ser256, but not Ser269. Phosphorylation of AQP2 at Ser269 did not occur when Ser256 was replaced by an unphosphorylatable amino acid, as seen in both S256L-AQP2 mutant mice and in Madin-Darby canine kidney cells expressing an S256A mutant, suggesting that Ser269 phosphorylation depends upon prior phosphorylation at Ser256. Immunogold electron microscopy localized Ser(P)269-AQP2 solely in the apical plasma membrane of rat collecting duct cells, in contrast to the other three phospho-forms (found in both apical plasma membrane and intracellular vesicles). Madin-Darby canine kidney cells expressing an S269D “phosphomimic” AQP2 mutant showed constitutive localization at the plasma membrane. The data support a model in which vasopressin-mediated phosphorylation of AQP2 at Ser269:(a) depends on prior PKA-mediated phosphorylation of Ser256 and (b) enhances apical plasma membrane retention of AQP2.

Angiotensin II induces phosphorylation of the thiazide-sensitive sodium chloride cotransporter independent of aldosterone

We studied here the independent roles of angiotensin II and aldosterone in regulating the sodium chloride cotransporter (NCC) of the distal convoluted tubule. We adrenalectomized three experimental and one control group of rats. Following surgery, the experimental groups were treated with either a high physiological dose of aldosterone, a non-pressor, or a pressor dose of angiotensin II for 8 days. Aldosterone and both doses of angiotensin II lowered sodium excretion and significantly increased the abundance of NCC in the plasma membrane compared with the control. Only the pressor dose of angiotensin II caused hypertension. Thiazides inhibited the sodium retention induced by the angiotensin II non-pressor dose. Both aldosterone and the non-pressor dose of angiotensin II significantly increased phosphorylation of NCC at threonine-53 and also increased the intracellular abundance of STE20/SPS1-related, proline alanine-rich kinase (SPAK). No differences were found in other modulators of NCC activity such as oxidative stress responsive protein type 1 or with-no-lysine kinase 4. Thus, our in vivo study shows that aldosterone and angiotensin II independently increase the abundance and phosphorylation of NCC in the setting of adrenalectomy; effects are likely mediated by SPAK. These results may explain, in part, the hormonal control of renal sodium excretion and the pathophysiology of several forms of hypertension.

Phosphorylation of aquaporin-2 regulates its endocytosis and protein–protein interactions

The water channel aquaporin-2 (AQP2) is essential for urine concentration. Vasopressin regulates phosphorylation of AQP2 at four conserved serine residues at the COOH-terminal tail (S256, S261, S264, and S269). We used numerous stably transfected Madin–Darby canine kidney cell models, replacing serine residues with either alanine (A), which prevents phosphorylation, or aspartic acid (D), which mimics the charged state of phosphorylated AQP2, to address whether phosphorylation is involved in regulation of (i) apical plasma membrane abundance of AQP2, (ii) internalization of AQP2, (iii) AQP2 protein–protein interactions, and (iv) degradation of AQP2. Under control conditions, S256D- and 269D-AQP2 mutants had significantly greater apical plasma membrane abundance compared to wild type (WT)-AQP2. Activation of adenylate cyclase significantly increased the apical plasma membrane abundance of all S-A or S-D AQP2 mutants with the exception of 256D-AQP2, although 256A-, 261A-, and 269A-AQP2 mutants increased to a lesser extent than WT-AQP2. Biotin internalization assays and confocal microscopy demonstrated that the internalization of 256D- and 269D-AQP2 from the plasma membrane was slower than WT-AQP2. The slower internalization corresponded with reduced interaction of S256D- and 269D-AQP2 with several proteins involved in endocytosis, including Hsp70, Hsc70, dynamin, and clathrin heavy chain. The mutants with the slowest rate of internalization, 256D- and 269D-AQP2, had a greater protein half-life (t1/2 = 5.1 h and t1/2 = 4.4 h, respectively) compared to WT-AQP2 (t1/2 = 2.9 h). Our results suggest that vasopressin-mediated membrane accumulation of AQP2 can be controlled via regulated exocytosis and endocytosis in a process that is dependent on COOH terminal phosphorylation and subsequent protein–protein interactions.

Acute regulation of aquaporin-2 phosphorylation at Ser-264 by vasopressin

By phosphoproteome analysis, we identified a phosphorylation site, serine 264 (pS264), in the COOH terminus of the vasopressin-regulated water channel, aquaporin-2 (AQP2). In this study, we examined the regulation of AQP2 phosphorylated at serine 264 (pS264–AQP2) by vasopressin, using a phospho-specific antibody (anti-pS264). Immunohistochemical analysis showed pS264–AQP2 labeling of inner medullary collecting duct (IMCD) from control mice, whereas AQP2 knockout mice showed a complete absence of labeling. In rat and mouse, pS264–AQP2 was present throughout the collecting duct system, from the connecting tubule to the terminal IMCD. Immunogold electron microscopy, combined with double-labeling confocal immunofluorescence microscopy with organelle-specific markers, determined that the majority of pS264 resides in compartments associated with the plasma membrane and early endocytic pathways. In Brattleboro rats treated with [deamino-Cys-1, d-Arg-8]vasopressin (dDAVP), the abundance of pS264–AQP2 increased 4-fold over controls. Additionally, dDAVP treatment resulted in a time-dependent change in the distribution of pS264 from predominantly intracellular vesicles, to both the basolateral and apical plasma membranes. Sixty minutes after dDAVP exposure, a proportion of pS264–AQP2 was observed in clathrin-coated vesicles, early endosomal compartments, and recycling compartments, but not lysosomes. Overall, our results are consistent with a dynamic effect of AVP on the phosphorylation and subcellular distribution of AQP2.

Vasopressin induces phosphorylation of the thiazide-sensitive sodium chloride cotransporter in the distal convoluted tubule

The thiazide-sensitive Na(+)-Cl(-) cotransporter (NCC) is important for renal electrolyte balance and its phosphorylation causes an increase in its transport activity and cellular localization. Here, we generated phospho-specific antibodies against two conserved N-terminal phosphorylation sites (Thr53, Thr58 and Thr53/Thr58) to assess the role of arginine vasopressin (AVP) in regulating NCC in rodent kidney in vivo. Immunohistochemistry showed distinct staining of phosphorylated NCC (pNCC) at the apical plasma membrane domain of distal convoluted tubule (DCT) cells. Unlike total NCC, pNCC was localized only to the apical plasma membrane as determined by immunogold electron microscopy. In AVP-deficient Brattleboro rats, acute deamino-Cys-1, d-Arg-8 vasopressin (dDAVP) exposure significantly increased pNCC abundance at the apical plasma membrane by about threefold, whereas total NCC and its cellular distribution were not affected. dDAVP significantly increased the abundance of phosphorylated STE20/SPS1-related proline-alanine-rich kinase and oxidative stress-response kinase (SPAK and OSR1), kinases implicated in NCC phosphorylation. Intracellular calcium levels in early and late DCTs were increased in response to 1 min superfusion of dDAVP, confirming that these segments are AVP responsive. In rats fed a high-salt diet with angiotensin (ANG) type 1-receptor blockade, similar increases in pNCC and active SPAK and OSR1 were detected following chronic or acute dDAVP, thus indicating the effects of AVP are independent of ANGII. Our results show that AVP is a potent regulator of NCC activity.

Proteomic analysis of lithium-induced nephrogenic diabetes insipidus: mechanisms for aquaporin 2 down-regulation and cellular proliferation.

Lithium is a commonly prescribed mood-stabilizing drug. However, chronic treatment with lithium induces numerous kidney-related side effects, such as dramatically reduced aquaporin 2 (AQP2) abundance, altered renal function, and structural changes. As a model system, inner medullary collecting ducts (IMCD) isolated from rats treated with lithium for either 1 or 2 weeks were subjected to differential 2D gel electrophoresis combined with mass spectrometry and bioinformatics analysis to identify (i) signaling pathways affected by lithium and (ii) unique candidate proteins for AQP2 regulation. After 1 or 2 weeks of lithium treatment, we identified 6 and 74 proteins with altered abundance compared with controls, respectively. We randomly selected 17 proteins with altered abundance caused by lithium treatment for validation by immunoblotting. Bioinformatics analysis of the data indicated that proteins involved in cell death, apoptosis, cell proliferation, and morphology are highly affected by lithium. We demonstrate that members of several signaling pathways are activated by lithium treatment, including the PKB/Akt-kinase and the mitogen-activated protein kinases (MAPK), such as extracellular regulated kinase (ERK), c-Jun NH2-terminal kinase (JNK), and p38. Lithium treatment increased the intracellular accumulation of β-catenin in association with increased levels of phosphorylated glycogen synthase kinase type 3β (GSK3β). This study provides a comprehensive analysis of the proteins affected by lithium treatment in the IMCD and, as such, provides clues to potential lithium targets in the brain.

Serine 269 phosphorylated aquaporin-2 is targeted to the apical membrane of collecting duct principal cells.

Trafficking of the water channel aquaporin-2 to the apical plasma membrane of the collecting duct is mediated by arginine vasopressin, rendering the cell permeable to water. We recently identified a novel form of aquaporin-2 that is phosphorylated at serine-269 (pS269-AQP2). Using antibodies specific for this form of the water channel, we detected rat and mouse pS269-AQP2 in the connecting tubule and throughout the collecting duct system. Using confocal immunofluorescence microscopy with organelle-specific markers and immunogold electron microscopy, we found that pS269-AQP2 was found only on the apical plasma membrane of principal cells. In vasopressin-deficient Brattleboro rats, pS269-AQP2 was undetectable but dramatically increased in abundance after these rats were treated with [deamino-Cys-1, d-Arg-8]vasopressin (dDAVP). This increase occurred only at the apical plasma membrane, even after long-term dDAVP treatment. Following dDAVP there was a time-dependent redistribution of total aquaporin-2 from predominantly intracellular vesicles to the apical plasma membrane, clathrin-coated vesicles, early endosomal compartments, and lysosomes. However, pS269-AQP2 was found only on the apical plasma membrane at any time. Our results show that S269 phosphorylated aquaporin-2 is exclusively associated with the apical plasma membrane, where it escapes endocytosis to remain at the cell surface.

Regulation of the water channel aquaporin-2 by posttranslational modification

The cellular functions of many eukaryotic membrane proteins, including the vasopressin-regulated water channel aquaporin-2 (AQP2), are regulated by posttranslational modifications. In this article, we discuss the experimental discoveries that have advanced our understanding of how posttranslational modifications affect AQP2 function, especially as they relate to the role of AQP2 in the kidney. We review the most recent data demonstrating that glycosylation and, in particular, phosphorylation and ubiquitination are mechanisms that regulate AQP2 activity, subcellular sorting and distribution, degradation, and protein interactions. From a clinical perspective, posttranslational modification resulting in protein misrouting or degradation may explain certain forms of nephrogenic diabetes insipidus. In addition to providing major insight into the function and dynamics of renal AQP2 regulation, the analysis of AQP2 posttranslational modification may provide general clues as to the role of posttranslational modification for regulation of other membrane proteins.