Glomerular endothelial cell fenestrations: an integral component of the glomerular filtration barrier

Abstract

Glomerular endothelial cell (GEnC) fenestrations are analogous to podocyte filtration slits, but their important contribution to the glomerular filtration barrier has not received corresponding attention. GEnC fenestrations are transcytoplasmic holes, specialized for their unique role as a prerequisite for filtration across the glomerular capillary wall. Glomerular filtration rate is dependent on the fractional area of the fenestrations and, through the glycocalyx they contain, GEnC fenestrations are important in restriction of protein passage. Hence, dysregulation of GEnC fenestrations may be associated with both renal failure and proteinuria, and the pathophysiological importance of GEnC fenestrations is well characterized in conditions such as preeclampsia. Recent evidence suggests a wider significance in repair of glomerular injury and in common, yet serious, conditions, including diabetic nephropathy. Study of endothelial cell fenestrations is challenging because of limited availability of suitable in vitro models and by the requirement for electron microscopy to image these sub-100-nm structures. However, extensive evidence, from glomerular development in rodents to in vitro studies in human GEnC, points to vascular endothelial growth factor (VEGF) as a key inducer of fenestrations. In systemic endothelial fenestrations, the intracellular pathways through which VEGF acts to induce fenestrations include a key role for the fenestral diaphragm protein plasmalemmal vesicle-associated protein-1 (PV-1). The role of PV-1 in GEnC is less clear, not least because of controversy over existence of GEnC fenestral diaphragms. In this article, the structure-function relationships of GEnC fenestrations will be evaluated in depth, their role in health and disease explored, and the outlook for future study and therapeutic implications of these peculiar structures will be approached.

Fig. 1.

A: transmission electron micrograph of the fenestrated area of a whole-mount liver sinusoidal endothelial cell in vitro, showing fenestrae-associated cytoskeleton rings (large arrows) delineating the fenestrations. Note microtubules (small arrows) closely running along the sieve plate. Scale bar, 200 nm. [Adapted from Braet et al. (18).] B: scanning electron micrograph showing en face view of glomerular endothelial cell fenestrations in a mouse glomerular capillary ex vivo. Fenestrated areas or sieve plates are separated by ridges of cytoplasm. Scale bar 500 nm. [Adapted from Kamba et al. (61). Used with permission.] C: transmission electron micrograph of a transverse section through the wall of a normal rat glomerular capillary ex vivo. P, podocyte foot process; SD, slit diaphragm; BM, glomerular basement membrane; F, endothelial fenestration. Magnification: approximately ×48,000. [Adapted from Pavenstadt et al. (77). Used with permission.] D: transmission electron micrograph of the glomerular capillary wall employing specialized techniques to preserve the glycocalyx. Glycocalyx (marked with asterisk) can be seen overlying fenestrae and interfenestral domains of the glomerular endothelium. L, capillary lumen. Scale bar 1 μm. [Adapted from Microvasc Res 53, Rostgaard and Qvortrup, Electron microscopic demonstrations of filamentous molecular sieve plugs in capillary fenestrae, 1–13, copyright 1997, with permission from Elsevier (88).]

Fig. 2.

Diagram showing proposed intracellular pathways of vascular endothelial growth factor (VEGF)-induced fenestration formation in glomerular endothelial cells (GEnC). VEGF-A binds to and activates VEGF receptor (VEGFR) 2, which leads, via small protein GTPases, to actin rearrangement required for formation of a fenestral cytoskeletal ring. VEGFR2 activation also leads to recruitment of plasmalemmal vesicle-associated protein-1 (PV-1) and PV-1 multimer assembly in the forming fenestration. Fenestral diaphragms and PV-1 disappear as the fenestration matures.

Fig. 3.

Diagram showing three possible routes to fenestration formation. For a fenestration to form in the peripheral cytoplasm of GEnC, both attenuation of the cytoplasm and hole formation must occur. A: a pore forms first, and then the pore enlarges as the cytoplasm thins. B: the cytoplasm thins to a greater extent in particular regions where the opposing cell membranes eventually meet and fuse. C: as the cytoplasm thins, cell membranes fuse with membranes of preexisting intracellular organelles such as caveolae. These pathways are not necessarily mutually exclusive, and it may be that fenestrations form through a combination of pathways.