ILK and cytoskeletal architecture: an important determinant of AQP2 recycling and subsequent entry into the exocytotic pathway

Quantitative image mean squared displacement (iMSD) analysis of the dynamics of Aquaporin 2 within the membrane of live cells

Nanodomains are a biological membrane phenomenon which have a large impact on various cellular processes. They are often analysed by looking at the lateral dynamics of membrane lipids or proteins. The localization of the plasma membrane protein aquaporin-2 in nanodomains has so far been unknown. In this study, we use total internal reflection fluorescence microscopy to image Madin-Darby Canine Kidney (MDCK) cells expressing aquaporin-2 tagged with mEos 3.2. Then, image mean squared displacement (iMSD) approach was used to analyse the diffusion of aquaporin-2, revealing that aquaporin-2 is confined within membrane nanodomains. Using iMSD analysis, we found that the addition of the drug forskolin increases the diffusion of aquaporin-2 within the confined domains, which is in line with previous studies. Finally, we observed an increase in the size of the membrane domains and the extent of trapping of aquaporin-2 after stimulation with forskolin.

Actin-related protein 2/3 complex plays a critical role in the aquaporin-2 exocytotic pathway

The trafficking of proteins such as aquaporin-2 (AQP2) in the exocytotic pathway requires an active actin cytoskeleton network, but the mechanism is incompletely understood. Here, we show that the actin-related protein (Arp)2/3 complex, a key factor in actin filament branching and polymerization, is involved in the shuttling of AQP2 between the trans-Golgi network (TGN) and the plasma membrane. Arp2/3 inhibition (using CK-666) or siRNA knockdown blocks vasopressin-induced AQP2 membrane accumulation and induces the formation of distinct AQP2 perinuclear patches positive for markers of TGN-derived clathrin-coated vesicles. After a 20°C cold block, AQP2 formed perinuclear patches due to continuous endocytosis coupled with inhibition of exit from TGN-associated vesicles. Upon rewarming, AQP2 normally leaves the TGN and redistributes into the cytoplasm, entering the exocytotic pathway. Inhibition of Arp2/3 blocked this process and trapped AQP2 in clathrin-positive vesicles. Taken together, these results suggest that Arp2/3 is essential for AQP2 trafficking, specifically for its delivery into the post-TGN exocytotic pathway to the plasma membrane.NEW & NOTEWORTHY Aquaporin-2 (AQP2) undergoes constitutive recycling between the cytoplasm and plasma membrane, with an intricate balance between endocytosis and exocytosis. By inhibiting the actin-related protein (Arp)2/3 complex, we prevented AQP2 from entering the exocytotic pathway at the post-trans-Golgi network level and blocked AQP2 membrane accumulation. Arp2/3 inhibition, therefore, enables us to separate and target the exocytotic process, while not affecting endocytosis, thus allowing us to envisage strategies to modulate AQP2 trafficking and treat water balance disorders.

Advances in aquaporin-2 trafficking mechanisms and their implications for treatment of water balance disorders

In mammals, conservation of body water is critical for survival and is dependent on the kidneys' ability to minimize water loss in the urine during periods of water deprivation. The collecting duct water channel aquaporin-2 (AQP2) plays an essential role in this homeostatic response by facilitating water reabsorption along osmotic gradients. The ability to increase the levels of AQP2 in the apical plasma membrane following an increase in plasma osmolality is a rate-limiting step in water reabsorption, a process that is tightly regulated by the antidiuretic hormone arginine vasopressin (AVP). In this review, the focus is on the role of the carboxyl-terminus of AQP2 as a key regulatory point for AQP2 trafficking. We provide an overview of AQP2 structure, disease-causing mutations in the AQP2 carboxyl-terminus, the role of posttranslational modifications such as phosphorylation and ubiquitylation in the tail domain, and their implications for balanced trafficking of AQP2. Finally, we discuss how various modifications of the AQP2 tail facilitate selective protein-protein interactions that modulate the AQP2 trafficking mechanism.

Integrin-Linked Kinase Deficiency in Collecting Duct Principal Cell Promotes Necroptosis of Principal Cell and Contributes to Kidney Inflammation and Fibrosis

BACKGROUND

Necroptosis is a newly discovered cell death pathway that plays a critical role in AKI. The involvement of integrin-linked kinase (ILK) in necroptosis has not been studied.

METHODS

We performed experiments in mice with an Ilk deletion in collecting duct (CD) principal cells (PCs), and cultured tubular epithelial cells treated with an ILK inhibitor or ILK siRNA knockdown.

RESULTS

Ilk deletion in CD PCs resulted in acute tubular injury and early mortality in mice. Progressive interstitial fibrosis and inflammation associated with the activation of the canonical TGF-β signaling cascade were detected in the kidneys of the mice lacking ILK in the CD PCs. In contrast to the minimal apoptosis detected in the animals' injured CDs, widespread necroptosis was present in ILK-deficient PCs, characterized by cell swelling, deformed mitochondria, and rupture of plasma membrane. In addition, ILK deficiency resulted in increased expression and activation of necroptotic proteins MLKL and RIPK3, and membrane translocation of MLKL in CD PCs. ILK inhibition and siRNA knockdown reduced cell survival in cultured tubular cells, concomitant with increased membrane accumulation of MLKL and/or phospho-MLKL. Administration of a necroptosis inhibitor, necrostatin-1, blocked cell death in vitro and significantly attenuated inflammation, interstitial fibrosis, and renal failure in ILK-deficient mice.

CONCLUSIONS

The study demonstrates the critical involvement of ILK in necroptosis through modulation of the RIPK3 and MLKL pathway and highlights the contribution of CD PC injury to the development of inflammation and interstitial fibrosis of the kidney.

Involvement of PDZ-SAP97 interactions in regulating AQP2 translocation in response to vasopressin in LLC-PK1 cells

Arginine-vasopressin (AVP)-mediated translocation of aquaporin-2 (AQP2) protein-forming water channels from storage vesicles to the membrane of renal collecting ducts is critical for the renal conservation of water. The type-1 PDZ-binding motif (PBM) in AQP2, "GTKA," is a critical barcode for its translocation, but its precise role and that of its interacting protein partners in this process remain obscure. We determined that synapse-associated protein-97 (SAP97), a membrane-associated guanylate kinase protein involved in establishing epithelial cell polarity, was an avid binding partner to the PBM of AQP2. The role of PBM and SAP97 on AQP2 redistribution in response to AVP was assessed in LLC-PK1 renal collecting cells by confocal microscopy and cell surface biotinylation techniques. These experiments indicated that distribution of AQP2 and SAP97 overlapped in the kidneys and LLC-PK1 cells and that knockdown of SAP97 inhibited the translocation of AQP2 in response to AVP. Binding between AQP2 and SAP97 was mediated by specific interactions between the second PDZ of SAP97 and PBM of AQP2. Mechanistically, inactivation of the PBM of AQP2, global delocalization of PKA, or knockdown of SAP97 inhibited AQP2 translocation as well as AVP- and forskolin-mediated phosphorylation of Ser256 in AQP2, which serves as the major translocation barcode of AQP2. These results suggest that the targeting of PKA to the microdomain of AQP2 via SAP97-AQP2 interactions in association with cross-talk between two barcodes in AQP2, namely, the PBM and phospho-Ser256, plays an important role in the translocation of AQP2 in the kidney.

Quantitative proteomics reveals TMOD1-related proteins associated with water balance regulation

The distal tubule and collecting duct in kidney regulate water homeostasis. TMOD1 is an actin capping protein that plays an important role in controlling the organization of actin filaments. In this study, we found TMOD1 was specifically expressed in distal tubules and collecting ducts. To investigate the role of TMOD1, we created Tmod1^flox/flox mice and bred them with Ksp-Cre mice to generate tubule-specific Tmod1 knockout mice, Tmod1^flox/flox/Ksp-Cre⁺ (designated as TFK). As compared with control mice, TFK mice showed oliguria, hyperosmolality urine, and high blood pressure. To determine the mechanisms underlying this phenotype, we performed label-free quantitative proteomics on kidneys of TFK and control mice. Total of 83 proteins were found differentially expressed. Bioinformatic analysis indicated that biological processes, including protein phosphorylation and metabolic process, were involved in TMOD1 regulatory network. Gene set enrichment analysis showed that multiple pathways, such as phosphatidylinositol signaling system and GnRH signaling pathway, were strongly associated with Tmod1 knockout. Western blot validated the down-regulation of three proteins, TGFBR2, SLC25A11, and MTFP1, in kidneys of TFK mice. Our study provides valuable information on the molecular functions and the regulatory network of Tmod1 gene in kidney, as well as the new mechanisms for the regulation of water balance.

Manganese promotes intracellular accumulation of AQP2 via modulating F-actin polymerization and reduces urinary concentration in mice

Aquaporin-2 (AQP2) is a water channel protein expressed in principal cells (PCs) of the kidney collecting ducts (CDs) and plays a critical role in mediating water reabsorption and urine concentration. AQP2 undergoes both regulated trafficking mediated by vasopressin (VP) and constitutive recycling, which is independent of VP. For both pathways, actin cytoskeletal dynamics is a key determinant of AQP2 trafficking. We report here that manganese chloride (MnCl₂) is a novel and potent regulator of AQP2 trafficking in cultured cells and in the kidney. MnCl₂ treatment promoted internalization and intracellular accumulation of AQP2. The effect of MnCl₂ on the intracellular accumulation of AQP2 was associated with activation of RhoA and actin polymerization without modification of AQP2 phosphorylation. Although the level of total and phosphorylated AQP2 did not change, MnCl₂ treatment impeded VP-induced phosphorylation of AQP2 at its serine-256, -264, and -269 residues and dephosphorylation at serine 261. In addition, MnCl₂ significantly promoted F-actin polymerization along with downregulation of RhoA activity and prevented VP-induced membrane accumulation of AQP2. Finally, MnCl₂ treatment in mice resulted in significant polyuria and reduced urinary concentration, likely due to intracellular relocation of AQP2 in the PCs of kidney CDs. More importantly, the reduced urinary concentration caused by MnCl₂ treatment in animals was not corrected by VP. In summary, our study identified a novel effect of MnCl₂ on AQP2 trafficking through modifying RhoA activity and actin polymerization and uncovered its potent impact on water diuresis in vivo.

Peripheral insulin resistance in ILK-depleted mice by reduction of GLUT4 expression

The development of insulin resistance is characterized by the impairment of glucose uptake mediated by glucose transporter 4 (GLUT4). Extracellular matrix changes are induced when the metabolic dysregulation is sustained. The present work was devoted to analyze the possible link between the extracellular-to-intracellular mediator integrin-linked kinase (ILK) and the peripheral tissue modification that leads to glucose homeostasis impairment. Mice with general depletion of ILK in adulthood (cKD-ILK) maintained in a chow diet exhibited increased glycemia and insulinemia concurrently with a reduction of the expression and membrane presence of GLUT4 in the insulin-sensitive peripheral tissues compared with their wild-type littermates (WT). Tolerance tests and insulin sensitivity indexes confirmed the insulin resistance in cKD-ILK, suggesting a similar stage to prediabetes in humans. Under randomly fed conditions, no differences between cKD-ILK and WT were observed in the expression of insulin receptor (IR-B) and its substrate IRS-1 expressions. The IR-B isoform phosphorylated at tyrosines 1150/1151 was increased, but the AKT phosphorylation in serine 473 was reduced in cKD-ILK tissues. Similarly, ILK-blocked myotubes reduced their GLUT4 promoter activity and GLUT4 expression levels. On the other hand, the glucose uptake capacity in response to exogenous insulin was impaired when ILK was blocked in vivo and in vitro, although IR/IRS/AKT phosphorylation states were increased but not different between groups. We conclude that ILK depletion modifies the transcription of GLUT4, which results in reduced peripheral insulin sensitivity and glucose uptake, suggesting ILK as a molecular target and a prognostic biomarker of insulin resistance.

Integrin linked kinase regulates the transcription of AQP2 by NFATC3

Two processes are associated with progressive loss of renal function: 1) decreased aquaporin-2 (AQP2) expression and urinary concentrating capacity (Nephrogenic Diabetes Insipidus, NDI); and 2) changes in extracellular matrix (ECM) composition, e.g. increased collagen I (Col I) deposition, characteristic of tubule-interstitial fibrosis. AQP2 expression is regulated by both the ECM-to-intracellular scaffold protein integrin-linked kinase (ILK) by NFATc/AP1 and other transcription factors. In the present work, we used in vivo and in vitro approaches to examine ILK participation in NFATc3/AP-1-mediated increases in AQP2 gene expression. Both NFATc3 knock-out mice and ILK conditional-knockdown mice (cKD-ILK) display symptoms of NDI (polyuria and reduced AQP2 expression). NFATc3 is upregulated in the renal medulla tubular cells of cKD-ILK mice but with reduced nuclear localization. Inner medullary collecting duct mIMCD3 cells were subjected to ILK depletion and transfected with reporter plasmids. Pharmacological activators or inhibitors determined the effect of ILK activity on NFATc/AP-1-dependent increases in transcription of AQP2. Finally, mIMCD3 cultured on Col I showed reduced activity of the ILK/GSK3β/NFATc/AQP2 axis, suggesting this pathway is a potential target for therapeutic treatment of NDI.