How to Build a Tight but Permeable Glomerular Junction
Authors: Sebastian Martini, Matthias Kretzler
Journal: Journal of the American Society of Nephrology (2009)

The filtering glomerulus is a highly specialized capillary bed that simultaneously provides maximal permeability for low molecular weight solutes and highly efficient retention of plasma macromolecules under a decreasing gradient of arteriolar pressure. Fenestrated endothelia, glomerular basement membrane, and interdigitating foot processes from podocytes are the primary structural elements of the filtration barrier. Slit diaphragms establish cell–cell contact between foot processes as the final element of the filtration barrier and are a decisive factor in modulating size selectivity.1 In the past decade, the discovery of genes mutated in inherited forms of nephrotic syndrome has expanded our understanding of the molecular composition of the slit diaphragm.2 The discovery of these framework molecules were quickly followed by identification of a series of associated intracellular signaling and adaptor proteins3,4 (for reviews, see references5,6). The molecular mechanisms of cell–cell interaction among podocytes and their associated signaling events demonstrate several unique features. Podocytes during glomerulogenesis evolve from primordial columnar epithelial cells linked together by apical junction complexes containing classical elements of tight and adherens junctions. These intercellular junctions migrate basally during the capillary loop stage to initiate glomerular slit diaphragm formation.7 In the mature slit diaphragm, tight junction–associated proteins include zonula occludens 1 (ZO-1), membrane-associated guanylate kinase 1/2, and calcium/calmodulin-dependent serine protein kinase.7,8 The mature glomerular slit diaphragm demonstrates a functionality usually associated with tight junctions in other cells: Its main role is to provide a barrier function9 with selective permeability,10 it defines domains with distinct cell membrane properties and cell polarity,11 and it offers an array of cell–cell signaling functions.5 The intercellular filtration slit is 25 to 45 nm wide12 and somewhat resembles other adherens junctions that show similarly sized intercellular gaps.13 Typical adherens junction proteins, such as P-cadherin and α-, β-, and γ-cathenin, are found in the vicinity of the slit diaphragm and co-localize with tight junction–associated ZO-1.14 The study published in this issue of JASN by Fukasawa et al.15 provides additional evidence to support a slit diaphragm model of a modified adherens and a modified tight junction, with the tight junction proteins more closely associated with slit diaphragm–specific molecules. This is the latest update on five decades of research by Dr. Farquhar and her group to define better the junctional complexes of the glomerular slit diaphragm. This longitudinal effort reflects a remarkable expansion of knowledge starting from pioneering work on the ultrastructural definition of the slit using sophisticated cell biologic approaches and now integrating and combining cell biology with a systems biology approach. In this study, cell fractionation, immunofluorescence, and immunoelectron microscopy help to define the association of typical tight junction proteins with the slit diaphragm; in particular, junctional adhesion molecule A (JAM-A), occludin, and cingulin are found in the slit diaphragm and co-localize with nephrin in intact glomeruli. Tight junctions in the slit diaphragm network with signaling cascades (aPKC, Rab3b, and Rab13) and complexes responsible for maintaining cell polarity (PAR-3, PAR-6, and Crumbs). In a puromycin model of heavy proteinuria, these proteins, together with ZO-1, increase, whereas the “core” slit diaphragm proteins, nephrin and podocin, decrease. The authors speculate that slit diaphragm–specific proteins interact with classical tight junction proteins to facilitate maturation of the slit diaphragms from classical tight junctions in glomerular development followed by the reversal of these mature arrangements during puromycin nephrosis. The concept that developmental processes are reawakened in disease states is widely known in cancer research.16 Finally, Fukasawa's work suggests the relationship between slit diaphragms, tight junction proteins, and the actin cytoskeleton in podocytes is a critical connecter for renal diseases associated with foot process effacement. By contrasting our knowledge from traditional epithelial tight junctions with the unique situation in podocytes,17 Farquhar's group establishes a model for linking junctional physiology with podocyte cell biology, leading to a better understanding of what makes this junction so unique. Their data support an emerging picture of the slit diaphragm as being not simply a static scaffold for the filtration barrier but rather a highly dynamic and vulnerable process involving complex signaling cascades. Several challenges to this concept remain, mostly why the transition from slit diaphragm to tight junction in nephrosis produces an increase and not, as one would predict, a decrease in permeability for macromolecules. For nephrologists, understanding these events and their associated signaling cascades is clinically relevant, because novel approaches and innovative treatment strategies for glomerular failure may be deduced from this knowledge. Five decades after the first work by Dr. Farquhar and her colleagues, the podocyte slit diaphragm remains a fascinating and promising area of research. DISCLOSURES None. This work is supported in part by National Institutes of Health grant U54 DA021519 “National Center for Integrative Biomedical Informatics” to M.K. We thank Dr. Jeffrey Hodgin for helpful discussion and Chrysta Lienczewski for proofreading of the manuscript.