Reversible hexadimethrine-induced alterations in glomerular structure and permeability

Characterizing molecular diffusion in the lens capsule

The lens capsule compartmentalizes the cells of the avascular lens from other ocular tissues. Small molecules required for lens cell metabolism, such as glucose, salts, and waste products, freely pass through the capsule. However, the lens capsule is selectively permeable to proteins such as growth hormones and substrate carriers which are required for proper lens growth and development. We used fluorescence recovery after photobleaching (FRAP) to characterize the diffusional behavior of various sized dextrans (3, 10, 40, 150, and 250 kDa) and proteins endogenous to the lens environment (EGF, gammaD-crystallin, BSA, transferrin, ceruloplasmin, and IgG) within the capsules of whole living lenses. We found that proteins had dramatically different diffusion and partition coefficients as well as capsule matrix binding affinities than similar sized dextrans, but they had comparable permeabilities. We also found ionic interactions between proteins and the capsule matrix significantly influence permeability and binding affinity, while hydrophobic interactions had less of an effect. The removal of a single anionic residue from the surface of a protein, gammaD-crystallin [E107A], significantly altered its permeability and matrix binding affinity in the capsule. Our data indicated that permeabilities and binding affinities in the lens capsule varied between individual proteins and cannot be predicted by isoelectric points or molecular size alone.

The lens capsule

The lens capsule is a modified basement membrane that completely surrounds the ocular lens. It is known that this extracellular matrix is important for both the structure and biomechanics of the lens in addition to providing informational cues to maintain lens cell phenotype. This review covers the development and structure of the lens capsule, lens diseases associated with mutations in extracellular matrix genes and the role of the capsule in lens function including those proposed for visual accommodation, selective permeability to infectious agents, and cell signaling.

Properties of the Glomerular Barrier and Mechanisms of Proteinuria

This review focuses on the intricate properties of the glomerular barrier. Other reviews have focused on podocyte biology, mesangial cells, and the glomerular basement membrane (GBM). However, since all components of the glomerular membrane are important for its function, proteinuria will occur regardless of which layer is affected by disease. We review the properties of endothelial cells and their surface layer, the GBM, and podocytes, discuss various methods of studying glomerular permeability, and analyze data concerning the restriction of solutes by size, charge, and shape. We also review the physical principles of transport across biological or artificial membranes and various theoretical models used to predict the fluxes of solutes and water. The glomerular barrier is highly size and charge selective, in qualitative agreement with the classical studies performed 30 years ago. The small amounts of albumin filtered will be reabsorbed by the megalin-cubulin complex and degraded by the proximal tubular cells. At present, there is no unequivocal evidence for reuptake of intact albumin from urine. The cellular components are the key players in restricting solute transport, while the GBM is responsible for most of the resistance to water flow across the glomerular barrier.

Localization of alpha-dystroglycan on the podocyte: from top to toe

alpha-Dystroglycan (DG) is a negatively charged membrane-associated glycoprotein that links the cytoskeleton to the extracellular matrix. Previously, we described that alpha-DG covers the whole podocyte cell membrane in the rat. However, our finding was challenged by the description of a strictly basolateral localization in human kidney biopsies, using a different antibody against alpha-DG. Therefore, we studied the exact localization of glomerular alpha-DG by using these two antibodies in both species. The studies were performed by using monoclonal antibodies (MoAbs) IIH6 and VIA4.1 in immunofluorescence, confocal microscopy, and immunoelectron microscopy on both rat and human kidney sections, as well as on cultured mouse podocytes. The apical localization of alpha-DG on podocytes was more dominant than the basolateral localization. The basolateral staining with MoAb VIA4.1 was more pronounced than that of MoAb IIH6. With both MoAbs, the staining in rat kidneys was more prominent, in comparison to human kidneys. We conclude that alpha-DG is expressed at both the basolateral and apical sides of the podocyte. This localization suggests that alpha-DG plays a dual role in the maintenance of the unique architecture of podocytes by its binding to the glomerular basement membrane, and in the maintenance of the integrity of the filtration slit, respectively.

Clustering-induced tyrosine phosphorylation of nephrin by Src family kinases

BACKGROUND

Nephrin is a recently discovered protein of the immunoglobulin (Ig) superfamily. In the kidney, it is located at the slit diaphragm, which forms the decisive size-selective filter of glomerular ultrafiltration barrier and locates between the interdigitating foot processes of podocytes. Nephrin is mutated in congenital nephrosis of the Finnish type (NPHS1) and has been demonstrated to be an essential component of the slit diaphragm. Based on its domain structure, nephrin is likely to be a cell-cell or cell-matrix adhesion protein that may have a signaling function. In this study, we hypothesized that the clustering of nephrin with antibodies on cell surface mimics the situation where the interaction between nephrin and its extracellular ligand(s) is altered.

METHODS

Nephrin was clustered on the surface of stably transfected HEK293 cells by a monoclonal antinephrin antibody and polyclonal secondary antibody. Clusters were visualized by immunofluorescence microscopy. Changes in protein phosphorylation were studied employing immunoprecipitations and Western blot analysis. A specific inhibitor and cotransfection experiments were used to investigate role of Src family kinases in nephrin phosphorylation.

RESULTS

Clustering of nephrin induced its own tyrosine phosphorylation. This phosphorylation was inhibited by PP2, an inhibitor of Src family kinases. Several members of Src family kinases were able to induce nephrin phosphorylation when cotransfected to HEK293 cells with nephrin. Moreover, the Src family kinase Fyn was consistently found to be coimmunoprecipitated with nephrin. Interestingly, clustering of nephrin induced also tyrosine phosphorylation of a 46 kD protein that was as well found to be coimmunoprecipitated with nephrin.

CONCLUSION

Nephrin is a signaling protein phosphorylated by Src family kinases.

Heparin neutralization with methylene blue, hexadimethrine, or vancomycin after cardiopulmonary bypass

There are no clinically available alternatives for reversing heparin in protamine-allergic patients. This study examined the ability of methylene blue, hexadimethrine, and vancomycin to reverse circulating heparin so that these compounds can be carefully examined in future placebo-controlled studies in humans. Heparin activity in blood obtained from extracorporeal circuits was reversed by adding protamine (13.5, 27.0, 81.1, 135.1, and 270.3 micrograms/mL), methylene blue (13.5, 27.0, 135.1, 202.7, 270.3, 337.8, 405.4, 473.0, 540.5, and 810.8 micrograms/mL), hexadimethrine (6.8, 13.5, 20.3, 27.0, 81.1, and 135.1 micrograms/mL), or vancomycin (13.5, 27.0, 135.1, 270.3, 540.5, and 810.8 micrograms/mL), and activated clotting times (ACTs) were measured with kaolin (n = 18). Heparinase-ACT was obtained to determine complete reversal. Heparin concentrations were 3.3 +/- 0.3 U/mL with ACT values of 485 +/- 97 s. The ACT at a protamine concentration of 81.1 micrograms/mL and at hexadimethrine concentrations of 81.1 and 135.1 micrograms/mL was not statistically different from heparinase-ACT; however, methylene blue or vancomycin did not reverse the anticoagulation at any concentrations. Hexadimethrine can reverse heparin-induced anticoagulation after cardiopulmonary bypass as well as protamine, although methylene blue or vancomycin did not neutralize heparin in vitro.

Alterations in the glomerular charge barrier in human lupus nephritis

Renal tissue taken from ten patients with proteinuria and diagnosed as suffering from human lupus nephritis was examined by a post-embedding charge labelling method using cationic gold (CG). The method allows localization of the anionic charge in the glomerular epithelial cell coat and/or glomerular basement membrane (GBM). Our results demonstrate that in human lupus nephritis there are gaps in the charge barrier associated with immune deposits found in the lamina rara externa of the GBM. These data suggest that the loss of glomerular fixed anionic charges in the GBM may play an important role in the development of proteinuria in patients suffering from lupus nephritis.

Mechanisms of proteinuria in noninflammatory glomerular diseases

Neither the initiating factors nor the proximate causes of injury that produce proteinuria in nephrotic syndrome have been clearly defined. Immune mechanisms have been postulated in minimal-change nephrotic syndrome (MCNS), focal segmental glomerular sclerosis (FSGS), and glomerular sclerosis associated with human immunodeficiency virus (HIV) infection. Circulating factors have been proposed in MCNS and FSGS, although no specific mediator has been identified. Prompt remission of proteinuria following steroid treatment and the presence of altered immune responsiveness in patients with MCNS have been used to support the participation of an immune mechanism in the pathogenesis of MCNS. Both FSGS and HIV-related nephropathy are usually steroid-resistant. Immune mechanisms are postulated in FSGS because of early recurrence after transplantation, and in HIV-related nephropathy because of the numerous associated immune abnormalities. Experimental models of nephrotic syndrome based on neutralization of glomerular charge, toxic injury to podocytes, injection of antibodies to glomerular components, or abnormalities in transgenic mice have been used to define mechanisms of glomerular injury. This review summarizes physiologic and immunologic abnormalities in MCNS, FSGS, and HIV-associated nephropathy and in several experimental models of nephrotic syndrome, and outlines the immunologic mechanisms and cellular reactions that may be responsible for glomerular dysfunction in these entities.