A High-Powered View of the Filtration Barrier

Abstract

Multiphoton excitation fluorescence microscopy is a powerful noninvasive imaging technique for the deep optical sectioning of living tissues. Its application in several intact tissues is a significant advance in our understanding of organ function, including renal pathophysiological mechanisms. The glomerulus, the filtering unit in the kidney, is one good example of a relatively inaccessible and complex structure, with cell types that are otherwise difficult to study at high resolution in their native environment. In this article, we address the application, advantages, and limitations of this imaging technology for the study of the glomerular filtration barrier and the controversy it recently generated regarding the glomerular filtration of macromolecules. More advanced and accurate multiphoton determinations of the glomerular sieving coefficient that are presented here dismiss previous claims on the filtration of nephrotic levels of albumin. The sieving coefficient of 70-kD dextran was found to be around 0.001. Using a model of focal segmental glomerulosclerosis, increased filtration barrier permeability is restricted only to areas of podocyte damage, consistent with the generally accepted role of podocytes and the glomerular origin of albuminuria. Time-lapse imaging provides new details and important in vivo confirmation of the dynamics of podocyte movement, shedding, replacement, and the role of the parietal epithelial cells and Bowman's capsule in the pathology of glomerulosclerosis.

Proximal Tubules Have the Capacity to Regulate Uptake of Albumin

“…2,12,14,22 One of the main functions of the PTC is retrieval of filtered proteins and our studies, and other recent reports, support the importance of the PTC in regulating albumin excretion rate. 2 of TR-RSA uptake in all surface PTs of control (n=3 rats, 157 fields quantified) and protein overloaded (n=8 rats, 176 fields quantified) rats.…”

“…Their superficial glomeruli have allowed our laboratory, and others, to directly observe and quantify glomerular and tubular events using intravital two-photon microscopy. 12,14 Figure 1A shows a rapid increase in plasma albumin and then recovery to baseline serum levels on days 3 and 4, as previously published, 9 when imaging was undertaken. Figure 1, B and C show IP albumin injection markedly increased 24 hour urinary protein and albumin from baseline.…”

WT1 in disease: shifting the epithelial–mesenchymal balance

“…These podocytes are complex, terminally differentiated cells that play a pivotal role in the renal glomerulus, the filtering unit of the kidney. Although traditionally thought of as an epithelial cell, the podocyte maintains some mesenchymal characteristics including motility and expression of vimentin [56][57][58].…”

Multiphoton Imaging of the Glomerular Permeability of Angiotensinogen

“…Indirectly, these studies also addressed the currently highly debated issue of the glomerular filtration of albumin. [14][15][16][17][18][19] To compare the glomerular processing of these two proteins, we employed intravital multiphoton fluorescence microscopy, which has been used by our laboratory 15,20,21 and the laboratories of others, [16][17][18][19] to directly and quantitatively visualize the glomerular permeability of macromolecules.…”