Mitochondrial dysfunction has been implicated in the pathogenesis of acute kidney injury due to ischemia and toxic drugs. Methods for imaging mitochondrial function in cells using confocal microscopy are well established; more recently, it was shown that these techniques can be utilized in ex vivo kidney tissue using multiphoton microscopy. We extended this approach in vivo and found that kidney mitochondrial structure and function can be imaged in anesthetized rodents using multiphoton excitation of endogenous and exogenous fluorophores. Mitochondrial nicotinamide adenine dinucleotide increased markedly in rat kidneys in response to ischemia. Following intravenous injection, the mitochondrial membrane potential–dependent dye TMRM was taken up by proximal tubules; in response to ischemia, the membrane potential dissipated rapidly and mitochondria became shortened and fragmented in proximal tubules. In contrast, the mitochondrial membrane potential and structure were better maintained in distal tubules. Changes in mitochondrial structure, nicotinamide adenine dinucleotide, and membrane potential were found in the proximal, but not distal, tubules after gentamicin exposure. These changes were sporadic, highly variable among animals, and were preceded by changes in non-mitochondrial structures. Thus, real-time changes in mitochondrial structure and function can be imaged in rodent kidneys in vivo using multiphoton excitation of endogenous and exogenous fluorophores in response to ischemia–reperfusion injury or drug toxicity.

Different laboratories recently reported incongruous results describing the quantification of albumin filtration using two-photon microscopy. We investigated the factors that influence the glomerular sieving coefficient for albumin (GSC A ) in an effort to explain these discordant reports and to develop standard operating procedures for determining GSC A . Multiple factors influenced GSC A , including the kidney depth of image acquisition (10-20 mm was appropriate), the selection of fluorophore (probes emitting longer wavelengths were superior), the selection of plasma regions for fluorescence measurements, the size and molecular dispersion characteristics of dextran polymers if used, dietary status, and the genetic strain of rat. Fasting reduced the GSC A in Simonsen Munich Wistar rats from 0.03560.005 to 0.01660.004 (P,0.01). Frömter Munich Wistar rats had a much lower GSC A in both the fed and the fasted states. Finally, we documented extensive albumin transcytosis with vesicular and tubular delivery to and fusion with the basolateral membrane in S1 proximal tubule cells. In summary, these results help explain the previously conflicting microscopy and micropuncture data describing albumin filtration and highlight the dynamic nature of glomerular albumin permeability. Over the past several years, the roles the glomerular filtration barrier and the proximal tubule play in the development of albuminuria have been debated. 1,2 The present textbook model, in which albumin filtration across a normal glomerulus is thought to be minimal, has recently been challenged. A wide array of genetic, molecular, biochemical, and imaging studies are consistent with proximal tubule cells (PTCs) having a role in reclaiming filtered proteins, including albumin, and thus minimizing proteinuria. These studies include the following: knockout mice lacking Na+-H+ exchanger isoform 3 or chloride channel-5; 3 megalin-cubilin complex defects; 4 Rab 38-mediated tubular proteinuria and albuminuria; 5 statin-mediated inhibition of guanosine triphosphatase prenylation with reduced proximal tubule (PT) endocytosis; 4,6-8 mice lacking FcRn, the IgG and albumin receptor; 9,10 and selective PTC injury using D-serine 11 or the selective expression of diphtheria toxin receptor on PTC. 12 In a preliminary communication, Menzel and colleagues showed that PT reabsorption and transcytosis of podocyte produced and secreted albumin labeled with V5 and hemagglutinin tags. 13 Finally, recently published data 14 using advanced scanning electron microscope imaging techniques indicate that the podocyte slit diaphragm has pores that are much larger than previously determined and are of the size required for albumin filtration.The development of in vivo two-photon microscopy has enabled the direct visualization and quantitation of fluorescent compounds as they filter through the glomerulus and are endocytosed by

Glomerular injury is often characterized by the effacement of podocytes, loss of slit diaphragms, and proteinuria. Renal ischemia or the loss of blood flow to the kidneys has been widely associated with tubular and endothelial injury but rarely has been shown to induce podocyte damage and disruption of the slit diaphragm. In this study, we have used an in vivo rat ischemic model to demonstrate that renal ischemia induces podocyte effacement with loss of slit diaphragm and proteinuria. Biochemical analysis of the ischemic glomerulus shows that ischemia induces rapid loss of interaction between slit diaphragm junctional proteins Neph1 and ZO-1. To further understand the effect of ischemia on molecular interactions between slit diaphragm proteins, a cell culture model was employed to study the binding between Neph1 and ZO-1. Under physiologic conditions, Neph1 co-localized with ZO-1 at cell-cell contacts in cultured human podocytes. Induction of injury by ATP depletion resulted in rapid loss of Neph1 and ZO-1 binding and redistribution of Neph1 and ZO-1 proteins from cell membrane to the cytoplasm. Recovery resulted in increased Neph1 tyrosine phosphorylation, restoring Neph1 and ZO-1 binding and their localization at the cell membrane. We further demonstrate that tyrosine phosphorylation of Neph1 mediated by Fyn results in significantly increased Neph1 and ZO-1 binding, suggesting a critical role for Neph1 tyrosine phosphorylation in reorganizing the Neph1-ZO-1 complex. This study documents that renal ischemia induces dynamic changes in the molecular interactions between slit diaphragm proteins, leading to podocyte damage and proteinuria.

Corridon PR, Rhodes GJ, Leonard EC, Basile DP, Gattone VH II, Bacallao RL, Atkinson SJ. A method to facilitate and monitor expression of exogenous genes in the rat kidney using plasmid and viral vectors.