Podocyte Flattening and Disorder of Glomerular Basement Membrane Are Associated with Splitting of Dystroglycan-Matrix Interaction

Abstract

Abstract. The transmembrane component of the dystroglycan complex, a heterodimer of ␣-and ␤-dystroglycan, was recently localized at the basal cell membrane domain of podocytes, and it was speculated that it serves as a device of the podocyte for maintaining the complex podocyte foot process architecture, and for regulating the exact position of its ligands, the matrix proteins laminin and agrin, in the glomerular basement membrane (GBM). The redistribution of dystroglycan in two experimental rat models of foot process flattening and proteinuria-i.e., podocyte damage induced by polycationic protamine sulfate perfusion, and reactive oxygen species (ROS)-associated puromycin aminonucleoside nephrosiswas examined. In both experimental diseases, aggregation and reduced density of ␣-dystroglycan by endocytosis by podocytes was observed. In in vitro solid-phase binding assays, protamine and ROS competed with the binding of ␣-dystroglycan with purified laminin and a recombinant C-terminal fragment of agrin that contains the dystroglycan-binding domain. These changes were associated with disorder of the fibrillar components of the lamina rara externa of the GBM, as confirmed quantitatively by fractal analysis. These results indicate that both polycation and ROS induce similar changes in the distribution of podocyte ␣-dystroglycan that involve competitive disruption of ␣-dystroglycan/matrix protein complexes, endocytosis of the liberated receptor by podocytes, and disorganization of the matrix protein arrangement in the lamina rara externa. This links functional damage of the dystroglycan complex with structural changes in the GBM.Extensive flattening of podocyte foot processes is the major pathologic feature of minimal change nephrotic syndrome and focal segmental glomerulosclerosis (FSGS), and is thought to be obligatorily associated with increased glomerular permeability and proteinuria (1-3). Experimentally, these pathologic podocyte changes are induced within minutes by kidney perfusion of polycations, such as protamine sulfate, in a complex energy-and actin-dependent fashion (4,5). Injection of puromycin aminonucleoside into rats induces foot process flattening within days that depends on the overproduction of reactive oxygen species (ROS) within the glomerulus (6 -8). Both experimental approaches also induce increased glomerular permeability and proteinuria (6,8,9).Podocyte proteins of potential importance for maintaining the complex podocyte architecture and their interactions and functional hierarchies are currently under intense investigation, and it appears, primarily from genetic knockout experiments, that several podocyte membrane and cytoskeletal proteins are critical for the development of mature foot process organization (reviewed in 10,11). These comprise membrane proteins of the luminal domain of podocytes, such as the sialoglycoprotein podocalyxin (12,13), several membrane proteins of the slit diaphragm domain (14 -17), the cytoskeletal protein ␣-actinin-4 (18), and the scaffolding protein CD2AP (19,20)...

Expression of dystroglycan correlates with tumor grade and predicts survival in renal cell carcinoma

“…The membrane staining observed in normal glomeruli likely reflects the expression of DG in podocytes. 30 On the other hand, the significance of the cytoplasmic staining observed in tubuli is not clear and it cannot be excluded that it is due to a non-specific cross-reaction.…”

“…Moreover, it has been demonstrated that the DG complex undergoes changes in human glomerular diseases in which flattening of foot processes is directly associated with dissociation of laminin-DG complexes. 30,31 Thus, it was of interest to investigate whether changes occur in the expression level of this molecule during renal tumorigenesis.…”

Proteinuria with and without Renal Glomerular Podocyte Effacement

“…Renal glomerular filtration apparatus is composed of three different components: The fenestrated endothelium, the glomerular basement membrane (GBM), and the epithelial cells with characteristic foot processes, known as podocytes (1)(2)(3)(4)(5)(6)(7)(8)(9). Filtration through this barrier depends on the charge and the size of the molecules present in the blood.…”

Contribution of Proteoglycans Towards the Integrated Functions of Renal Glomerular Capillaries

“…This is highlighted by the experiments in which reactive oxygen speciesmediated disruption of the ␣-dystroglycan/matrix transmembrane complex led to a disordered organization of the fibrillar components of lamina rara externa as well as foot process effacement, which would conceivably result in detachment of the podocytes and ensuing proteinuria. 26 Such may be the case since neuraminidase-induced detachment in situ leads to increased permeability of ferritin ( Figure 3B). 27 This notion is further supported by the fact that its intravenous administration results in an increased urinary albumin excretion.…”