Visfatin reduces gap junction mediated cell-to-cell communication in proximal tubule-derived epithelial cells

Collagen I Modifies Connexin-43 Hemichannel Activity via Integrin α2β1 Binding in TGFβ1-Evoked Renal Tubular Epithelial Cells

Chronic Kidney Disease (CKD) is associated with sustained inflammation and progressive fibrosis, changes that have been linked to altered connexin hemichannel-mediated release of adenosine triphosphate (ATP). Kidney fibrosis develops in response to increased deposition of extracellular matrix (ECM), and up-regulation of collagen I is an early marker of renal disease. With ECM remodeling known to promote a loss of epithelial stability, in the current study we used a clonal human kidney (HK2) model of proximal tubular epithelial cells to determine if collagen I modulates changes in cell function, via connexin-43 (Cx43) hemichannel ATP release. HK2 cells were cultured on collagen I and treated with the beta 1 isoform of the pro-fibrotic cytokine transforming growth factor (TGFβ1) ± the Cx43 mimetic Peptide 5 and/or an anti-integrin α2β1 neutralizing antibody. Phase microscopy and immunocytochemistry observed changes in cell morphology and cytoskeletal reorganization, whilst immunoblotting and ELISA identified changes in protein expression and secretion. Carboxyfluorescein dye uptake and biosensing measured hemichannel activity and ATP release. A Cytoselect extracellular matrix adhesion assay assessed changes in cell-substrate interactions. Collagen I and TGFβ1 synergistically evoked increased hemichannel activity and ATP release. This was paralleled by changes to markers of tubular injury, partly mediated by integrin α2β1/integrin-like kinase signaling. The co-incubation of the hemichannel blocker Peptide 5, reduced collagen I/TGFβ1 induced alterations and inhibited a positive feedforward loop between Cx43/ATP release/collagen I. This study highlights a role for collagen I in regulating connexin-mediated hemichannel activity through integrin α2β1 signaling, ahead of establishing Peptide 5 as a potential intervention.

Blocking Connexin-43 mediated hemichannel activity protects against early tubular injury in experimental chronic kidney disease

BACKGROUND

Tubulointerstitial fibrosis represents the key underlying pathology of Chronic Kidney Disease (CKD), yet treatment options remain limited. In this study, we investigated the role of connexin43 (Cx43) hemichannel-mediated adenosine triphosphate (ATP) release in purinergic-mediated disassembly of adherens and tight junction complexes in early tubular injury.

METHODS

Human primary proximal tubule epithelial cells (hPTECs) and clonal tubular epithelial cells (HK2) were treated with Transforming Growth Factor Beta1 (TGF-β1) ± apyrase, or ATPγS for 48 h. For inhibitor studies, cells were co-incubated with Cx43 mimetic Peptide 5, or purinergic receptor antagonists Suramin, A438079 or A804598. Immunoblotting, single-cell force spectroscopy and trans-epithelial electrical resistance assessed protein expression, cell-cell adhesion and paracellular permeability. Carboxyfluorescein uptake and biosensing measured hemichannel activity and real-time ATP release, whilst a heterozygous Cx43+/- mouse model with unilateral ureteral obstruction (UUO) assessed the role of Cx43 in vivo.

RESULTS

Immunohistochemistry of biopsy material from patients with diabetic nephropathy confirmed increased expression of purinergic receptor P2X7. TGF-β1 increased Cx43 mediated hemichannel activity and ATP release in hPTECs and HK2 cells. The cytokine reduced maximum unbinding forces and reduced cell-cell adhesion, which translated to increased paracellular permeability. Changes were reversed when cells were co-incubated with either Peptide 5 or P2-purinoceptor inhibitors. Cx43+/- mice did not exhibit protein changes associated with early tubular injury in a UUO model of fibrosis.

CONCLUSION

Data suggest that Cx43 mediated ATP release represents an initial trigger in early tubular injury via its actions on the adherens and tight junction complex. Since Cx43 is highly expressed in nephropathy, it represents a novel target for intervention of tubulointerstitial fibrosis in CKD. Video Abstract In proximal tubular epithelial cells (PTECs), tight junction proteins, including zona occuludens-1 (ZO-1), contribute to epithelial integrity, whilst the adherens junction protein epithelial (E)-cadherin (ECAD) maintains cell-cell coupling, facilitating connexin 43 (Cx43) gap junction-mediated intercellular communication (GJIC) and the direct transfer of small molecules and ions between cells. In disease, such as diabetic nephropathy, the pro-fibrotic cytokine transforming growth factor beta1 (TGF-β1) binds to its receptor and recruits SMAD2/3 signalling ahead of changes in gene transcription and up-regulation of Cx43-mediated hemichannels (HC). Uncoupled hemichannels permit the release of adenosine triphosphate (ATP) in to the extracellular space (↑[ATP]e), where ATP binds to the P2X7 purinoreceptor and activates the nucleotide-binding domain and leucine-rich repeat containing (NLR) protein-3 (NLRP3) inflammasome. Inflammation results in epithelial-to-mesenchymal transition (EMT), fibrosis and tubular injury. A major consequence is further loss of ECAD and reduced stickiness between cells, which can be functionally measured as a decrease in the maximum unbinding force needed to uncouple two adherent cells (Fmax). Loss of ECAD feeds forward to further lessen cell-cell coupling exacerbating the switch from GJIC to HC-mediated release of ATP. Reduction in ZO-1 impedes tight junction effectiveness and decreases trans-epithelial resistance (↓TER), resulting in increased paracellular permeability.

Flow stimulated endocytosis in the proximal tubule

PURPOSE OF REVIEW

The proximal tubule plays a critical role in the reabsorption of ions, solutes and low molecular weight proteins from the glomerular filtrate. Although the proximal tubule has long been known to acutely modulate ion reabsorption in response to changes in flow rates of the glomerular filtrate, it has only recently been discovered that proximal tubule cells can similarly adjust endocytic capacity in response to flow. This review synthesizes our current understanding of mechanosensitive regulation of endocytic capacity in proximal tubule epithelia and highlights areas of opportunity for future investigations.

RECENT FINDINGS

Recent studies have reported that the endocytic capacity of proximal tubule cells is dramatically increased upon exposure to flow and the accompanying fluid shear stress. Modulation of this pathway is dependent on increases in intracellular calcium initiated by bending of the primary cilium, and also requires purinergic receptor activation that is mediated by release of extracellular ATP. This article summarizes what is currently known about the signaling cascade that transduces changes in flow into alterations in endocytosis. We discuss the implications of this newly described regulatory pathway with respect to our understanding of protein retrieval by the kidney under normal conditions, and in diseases that present with low molecular weight proteinuria.

SUMMARY

Primary cilia act as mechanotransducers that modulate apical endocytic capacity in proximal tubule cells in response to changes in fluid shear stress.

Epithelial-to-Mesenchymal Transition in Diabetic Nephropathy: Fact or Fiction?

The pathophysiology of diabetic nephropathy (DN), one of the most serious complications in diabetic patients and the leading cause of end-stage renal disease worldwide, is complex and not fully elucidated. A typical hallmark of DN is the excessive deposition of extracellular matrix (ECM) proteins in the glomerulus and in the renal tubulointerstitium, eventually leading to glomerulosclerosis and interstitial fibrosis. Although it is obvious that myofibroblasts play a major role in the synthesis and secretion of ECM, the origin of myofibroblasts in DN remains the subject of controversial debates. A number of studies have focused on epithelial-to-mesenchymal transition (EMT) as one source of matrix-generating fibroblasts in the diseased kidney. EMT is characterized by the acquisition of mesenchymal properties by epithelial cells, preferentially proximal tubular cells and podocytes. In this review we comprehensively review the literature and discuss arguments both for and against a function of EMT in renal fibrosis in DN. While the precise extent of the contribution to nephrotic fibrosis is certainly arduous to quantify, the picture that emerges from this extensive body of literature suggests EMT as a major source of myofibroblasts in DN.

Mind the gap: connexins and cell-cell communication in the diabetic kidney

Connexins, assembled as a hexameric connexon, form a transmembrane hemichannel that provides a conduit for paracrine signalling of small molecules and ions to regulate the activity and function of adjacent cells. When hemichannels align and associate with similar channels on opposing cells, they form a continuous aqueous pore or gap junction, allowing the direct transmission of metabolic and electrical signals between coupled cells. Regulation of gap junction synthesis and channel activity is critical for cell function, and a number of diseases can be attributed to changes in the expression/function of these important proteins. Diabetic nephropathy is associated with several complex metabolic and inflammatory responses characterised by defects at the molecular, cellular and tissue level. In both type 1 and type 2 diabetes, glycaemic injury of the kidney is the leading cause of end-stage renal failure, a consequence of multiple aetiologies, including increased deposition of extracellular matrix, glomerular hyperfiltration, albuminuria and tubulointerstitial fibrosis. In diabetic nephropathy, loss of connexin mediated cell-cell communication within the nephron may represent an early sign of disease; however, our current knowledge of the role of connexins in the diabetic kidney is sparse. This review highlights recent evidence demonstrating that maintenance of connexin-mediated cell-cell communication could benefit region-specific renal function in diabetic nephropathy and suggests that these proteins should be viewed as a tantalising novel target for therapeutic intervention.