Water reabsorption capacity of the proximal convoluted tubule: a microperfusion study on rat kidney

SARS-CoV-2 spike protein inhibits megalin-mediated albumin endocytosis in proximal tubule epithelial cells

Patients with COVID-19 have high prevalence of albuminuria which is used as a marker of progression of renal disease and is associated with severe COVID-19. We hypothesized that SARS-CoV-2 spike protein (S protein) could modulate albumin handling in proximal tubule epithelial cells (PTECs) and, consequently contribute to the albuminuria observed in patients with COVID-19. In this context, the possible effect of S protein on albumin endocytosis in PTECs was investigated. Two PTEC lines were used: HEK-293A and LLC-PK1. Incubation of both cell types with S protein for 16 h inhibited albumin uptake at the same magnitude. This effect was associated with canonical megalin-mediated albumin endocytosis because: (1) DQ-albumin uptake, a marker of the lysosomal degradation pathway, was reduced at a similar level compared with fluorescein isothiocyanate (FITC)-albumin uptake; (2) dextran-FITC uptake, a marker of fluid-phase endocytosis, was not changed; (3) cell viability and proliferation were not changed. The inhibitory effect of S protein on albumin uptake was only observed when it was added at the luminal membrane, and it did not involve the ACE2/Ang II/AT1R axis. Although both cells uptake S protein, it does not seem to be required for modulation of albumin endocytosis. The mechanism underlying the inhibition of albumin uptake by S protein encompasses a decrease in megalin expression without changes in megalin trafficking and stability. These results reveal a possible mechanism to explain the albuminuria observed in patients with COVID-19.

Combinational effect of curcumin and metformin against gentamicin-induced nephrotoxicity: Involvement of antioxidative, anti-inflammatory and antiapoptotic pathway

Gentamicin (GM) is an antibiotic related to aminoglycoside group that is used in treating Gram-negative bacterial infections. However, treatment with gentamicin is considered to be limited as it induces an oxidative stress-mediated apoptosis in kidney which causes a nephrotoxicity. Metformin is a well-known biguanide that is used for treating diabetes mellitus, especially type 2. Supplement with plant metabolites or natural antioxidants produce a protective activity against many types of diseases in vivo. Curcumin is a main medicinal constituent of Curcuma longa, has reported for number of biological effects, such as antioxidant, anti-inflammatory, and antitumor. The study aims at evaluating the metformin and curcumin alone or in combination on nephrotoxicity induced by GM. The outcome of the study shows that both metformin and curcumin, when used unaided, were effectively decreasing GM-induced nephrotoxicity. The two drugs combination was showed synergistic effect in ameliorating a GM-induced kidney injury, as supported by expressively improved renal dysfunction. Metformin and curcumin showed strong protection against oxidative stress in GM treated animals through decreasing the activities and expression of various antioxidative enzymes. Moreover, combination of two drugs showed an anti-inflammatory response through reducing a level of pro-inflammatory cytokines including tumor necrosis factor-alpha, interleukin 1-beta, and interleukin 6 in GM intoxicated group of animals. Furthermore, GM agitated apoptosis was affectedly diminished by the combinational treatment of metformin and curcumin via down-regulating activity of cleaved Caspase-3 and pro-apoptotic factor Bax, whereas increasing anti-apoptotic factor Bcl-2 signaling pathways. The above results suggested that combinational treatment of metformin and curcumin might be have a synergizing effect and substantial potential against nephrotoxicity induced by GM. PRACTICAL APPLICATIONS: Curcumin and metformin combination exhibited substantial synergistic effect against GM-induced nephrotoxicity through reducing oxidative stress, inflammation, as well as apoptosis in kidney cells. Therefore, the method of combination of curcumin and metformin might be functional to treat or inhibit GM prompted nephrotoxicity in future.

Discerning the role of mechanosensors in regulating proximal tubule function

All cells in the body experience external mechanical forces such as shear stress and stretch. These forces are sensed by specialized structures in the cell known as mechanosensors. Cells lining the proximal tubule (PT) of the kidney are continuously exposed to variations in flow rates of the glomerular ultrafiltrate, which manifest as changes in axial shear stress and radial stretch. Studies suggest that these cells respond acutely to variations in flow by modulating their ion transport and endocytic functions to maintain glomerulotubular balance. Conceptually, changes in the axial shear stress in the PT could be sensed by three known structures, namely, the microvilli, the glycocalyx, and primary cilia. The orthogonal component of the force produced by flow exhibits as radial stretch and can cause expansion of the tubule. Forces of stretch are transduced by integrins, by stretch-activated channels, and by cell-cell contacts. This review summarizes our current understanding of flow sensing in PT epithelia, discusses challenges in dissecting the role of individual flow sensors in the mechanosensitive responses, and identifies potential areas of opportunity for new study.

Dependence of water movement on sodium transport in kidney proximal tubule: a microperfusion study substituting lithium for sodium

The relationship between water and sodium movements through the mammalian proximal convoluted tubule was investigated by substituting lithium for sodium. Proximal convoluted rat Kidney tubules were perfused in vivo with a Ringer solution containing 107 meq/liter lithium and 42 meq/liter sodium. Several micropunctures were made along the same nephron, and [3H] inulin, [14C] glucose, 22Na, osmolality, Na, Mg and Cl were determined on each sample. Measurements of 22Na showed that sodium and lithium diffusion rates were practically identical throughout the entire epithelium. A one- for-one exchange of sodium for lithium induced a negative transepithelial net flux of Na from plasma to lumen. However, despite this negative flux, a positive net water movement was measured from lumen to plasma. This movement was proportional both to glucose reabsorption and to the rise in the chloride concentration, two mechanisms known to be dependent on the transcellular movement of sodium. It was therefore concluded that the net water flux was a function of the unidirectional transcellular net flux of Na. Rabbit proximal convoluted tubules were perfused in vitro with solution containing 75 meq/liter Li and 75 meq/liter Na on both the luminal and peritubular sides. Under these conditions, the water reabsorption rate dropped to half its control value. Water movement was therefore a function of the external sodium concentration, which in turn probably regulates the intracellular Na concentration.