Wall Tension and Tubular Resistance in Kidney Cystic Conditions

Decoupling shear stress and pressure effects in the biomechanics of autosomal dominant polycystic kidney disease using a perfused kidney-on-chip

Kidney tubular cells are submitted to two distinct mechanical forces generated by the urine flow: shear stress and hydrostatic pressure. In addition, the mechanical properties of the surrounding extracellular matrix modulate tubule deformation under constraints. These mechanical factors likely play a role in the pathophysiology of kidney as exemplified by autosomal dominant polycystic kidney disease, in which pressure, flow and matrix stiffness have been proposed to modulate the cystic dilation of tubules with PKD1 mutations. The lack of in vitro systems recapitulating the mechanical environment of kidney tubules impedes our ability to dissect the role of these mechanical factors. Here we describe a perfused kidney-on-chip with tunable extracellular matrix mechanical properties and hydrodynamic constraints, that allows a decoupling of shear stress and flow. We used this system to dissect how these mechanical cues affect Pkd1-/- tubule dilation. We investigated cell behavior for a flow shear stress of 1 dyn/cm², combined or not with a 10-mbar intraluminal pressure. Our results showed two distinct mechanisms leading to tubular dilation. For Pkd1-/- PCT cells (proximal tubule), overproliferation mechanically leads to tubular dilation of 1.5-2-fold in 5 days, regardless of the mechanical context. For mIMCD-3 cells (collecting duct), tube dilation was associated with a squamous cell morphology but not with overproliferation and was highly sensitive to extracellular matrix properties, with suppression of the dilation when switching extracellular matrix composition from 6 to 9 mg/ml collagen. Contrary to PCT, mIMCD-3 tube dilation was highly sensitive to the nature of hydrodynamic constraint. Surprisingly, flow alone suppressed Pkd1-/- mIMCD-3 tubule dilation observed in static conditions, while the addition of luminal pressure restored it. Our in vitro model emulating nephron geometrical and mechanical organization sheds light on the roles of mechanical constraints in ADPKD and demonstrates the importance of controlling intraluminal pressure in kidney tubule models. STATEMENT OF SIGNIFICANCE: In autosomal dominant polycystic kidney disease, the development of numerous renal cysts leads to renal failure, with no curative therapy available. The initial stage of cyst formation, local tubule dilation, remains poorly understood. Although mechanical cues may be decisive, there is a lack of biomimetic systems recapitulating them. Here, an innovative kidney-on-a-chip was designed to decouple different hydrodynamic cues. We observed disease-specific tube dilation, driven by distinct mechanisms based or not on proliferation, in proximal tubule or collecting duct cell lines. Strikingly in the latter case, dilation, highly dependent on mechanical conditions, was suppressed by flow but restored by luminal pressure. Our model highlights the role of mechanical constraints in ADPKD and the importance of pressure control in renal models.

SGLT2 Inhibitors: The First Endothelial-Protector for Diabetic Nephropathy

Sodium-glucose co-transporter type 2 inhibitors (SGLT2i) have emerged as a class of agents relevant for managing diabetic nephropathy and cardiopathy. In a previous report, we noticed that these drugs share, with other drugs with "nephroprotective" effects, the ability to reduce the glomerular filtration rate (GFR), thus suggesting the kidney hemodynamic effect as a proxy for optimal drug dosage. We also noticed that all known nephroprotective drugs exert cardioprotective functions, suggesting the possibility of activities not mediated by the kidney. Finally, we observe that nephroprotective drugs can be grouped according to their effects on hemoglobin levels, thus suggesting their mechanism of action. While the primary mechanism of SGLT2i involves glycosuria and natriuria, growing evidence suggests broader therapeutic effects beyond hemodynamic modulation. Specifically, the evidence that SGLT2 can be expressed in several atypical regions under pathological conditions, supports the possibility that its inhibition has several extratubular effects. Evidence supports the hypothesis that SGLT2i influence mitochondrial function in various cell types affected by diabetes, particularly in the context of diabetic nephropathy. Notably, in SGLT2i-treated patients, the extent of albumin-creatinine ratio (ACR) reduction post-treatment may be correlated with mitochondrial staining intensity in glomerular endothelial cells. This implies that the anti-proteinuric effects of SGLT2i could involve direct actions on glomerular endothelial cell. Our investigation into the role of SGLT2 inhibitors (SGLT2i) in endothelial function suggests that the aberrant expression of SGLT2 in endothelial cells in T2DM would lead to intracellular accumulation of glucose; therefore, SGLT2i are the first type of endothelial protective drugs available today, with potential implications for ageing-related kidney disease. The review reveals two major novel findings: SGLT2 inhibitors are the first known class of endothelial-protective drugs, due to their ability to prevent glucose accumulation in endothelial cells where SGLT2 is aberrantly expressed in Type 2 Diabetes. Additionally, the research demonstrates that SGLT2 inhibitors share a GFR-reducing effect with other nephroprotective drugs, suggesting both a mechanism for optimal drug dosing and potential broader applications in ageing-related kidney disease through their effects on mitochondrial function and glomerular endothelial cells.

Decoupling shear stress and pressure effects in the biomechanics of autosomal dominant polycystic kidney disease using a perfused kidney-on-chip

Kidney tubular cells are submitted to two distinct mechanical forces generated by the urine flow: shear stress and hydrostatic pressure. In addition, the mechanical properties of the surrounding extracellular matrix modulate tubule deformation under constraints. These mechanical factors likely play a role in the pathophysiology of kidney diseases as exemplified by autosomal dominant polycystic kidney disease, in which pressure, flow and matrix stiffness have been proposed to modulate the cystic dilation of tubules with PKD1 mutations. The lack of in vitro systems recapitulating the mechanical environment of kidney tubules impedes our ability to dissect the role of these mechanical factors. Here we describe a perfused kidney-on-chip with tunable extracellular matrix mechanical properties and hydrodynamic constraints, that allows a decoupling of shear stress and flow. We used this system to dissect how these mechanical cues affect Pkd1 -/- tubule dilation. Our results show two distinct mechanisms leading to tubular dilation. For PCT cells (proximal tubule), overproliferation mechanically leads to tubular dilation, regardless of the mechanical context. For mIMCD-3 cells (collecting duct), tube dilation is associated with a squamous cell morphology but not with overproliferation and is highly sensitive to extracellular matrix properties and hydrodynamic constraints. Surprisingly, flow alone suppressed Pkd1 -/- mIMCD-3 tubule dilation observed in static conditions, while the addition of luminal pressure restored it. Our in vitro model emulating nephron geometrical and mechanical organization sheds light on the roles of mechanical constraints in ADPKD and demonstrates the importance of controlling intraluminal pressure in kidney tubule models.

The interplay between sodium/glucose cotransporter type 2 and mitochondrial ionic environment

Mitochondrial volume is maintained through the permeability of the inner mitochondrial membrane by a specific aquaporin and the osmotic balance between the mitochondrial matrix and cellular cytoplasm. Various electrolytes, such as calcium and hydrogen ions, potassium, and sodium, as well as other osmotic substances, affect the swelling of mitochondria. Intracellular glucose levels may also affect mitochondrial swelling, although the relationship between mitochondrial ion homeostasis and intracellular glucose is poorly understood. This article reviews what is currently known about how the Sodium-Glucose transporter (SGLT) may impact mitochondrial sodium (Na+) homeostasis. SGLTs regulate intracellular glucose and sodium levels and, therefore, interfere with mitochondrial ion homeostasis because mitochondrial Na+ is closely linked to cytoplasmic calcium and sodium dynamics. Recently, a large amount of data has been available on the effects of SGLT2 inhibitors on mitochondria in different cell types, including renal proximal tubule cells, endothelial cells, mesangial cells, podocytes, neuronal cells, and cardiac cells. The current evidence suggests that SGLT inhibitors (SGLTi) may affect mitochondrial dynamics regarding intracellular Sodium and hydrogen ions. Although the regulation of mitochondrial ion channels by SGLTs is still in its infancy, the evidence accumulated thus far of the effect of SGLTi on mitochondrial functions certainly will foster further research in this direction.