Circulating factors cause proteinuria in parabiotic zebrafish

Proteinuria can be induced by impairment of any component of the glomerular filtration barrier (GFB). To determine the role of circulating permeability factors on glomerular damage, we developed a parabiosis-based zebrafish model to generate a common circulation between zebrafish larvae. A morpholino-mediated knockdown of a podocyte specific gene (nephronectin) was induced in one zebrafish larva which was then fused to an un-manipulated fish. Notably, proteinuria and glomerular damage were present in the manipulated fish and in the parabiotically-fused partner. Thus, circulating permeability factors may be induced by proteinuria even when an induced podocyte gene dysregulation is the initiating cause.

Automated segmentation and quantification of actin stress fibres undergoing experimentally induced changes

The actin cytoskeleton is a main component of cells and it is crucially involved in many physiological processes, e.g. cell motility. Changes in the actin organization can be effected by diseases or vice versa. Due to the nonuniform pattern, it is difficult to quantify reasonable features of the actin cytoskeleton for a significantly high cell number. Here, we present an approach capable to fully segment and analyse the actin cytoskeleton of 2D fluorescence microscopic images with a special focus on stress fibres. The extracted feature data include length, width, orientation and intensity distributions of all traced stress fibres. Our approach combines morphological image processing techniques and a trace algorithm in an iterative manner, classifying the segmentation result with respect to the width of the stress fibres and in nonfibre-like actin. This approach enables us to capture experimentally induced processes like the condensation or the collapse of the actin cytoskeleton. We successfully applied the algorithm to F-actin images of cells that were treated with the actin polymerization inhibitor latrunculin A. Furthermore, we verified the robustness of our algorithm by a sensitivity analysis of the parameters, and we benchmarked our algorithm against established methods. In summary, we present a new approach to segment actin stress fibres over time to monitor condensation or collapse processes.

4D in vivo imaging of glomerular barrier function in a zebrafish podocyte injury model

AIM

Zebrafish larvae with their simplified pronephros are an ideal model to study glomerular physiology. Although several groups use zebrafish larvae to assess glomerular barrier function, temporary or slight changes are still difficult to measure. The aim of this study was to investigate the potential of in vivo two-photon microscopy (2-PM) for long-term imaging of glomerular barrier function in zebrafish larvae.

METHODS

As a proof of principle, we adapted the nitroreductase/metronidazole model of targeted podocyte ablation for 2-PM. Combination with a strain, which expresses eGFP-vitamin D-binding protein in the blood plasma, led to a strain that allowed induction of podocyte injury with parallel assessment of glomerular barrier function. We used four-dimensional (4D) 2-PM to assess eGFP fluorescence over 26 h in the vasculature and in tubules of multiple zebrafish larvae (5 days post-fertilization) simultaneously.

RESULTS

By 4D 2-PM, we observed that, under physiological conditions, eGFP fluorescence was retained in the vasculature and rarely detected in proximal tubule cells. Application of metronidazole induced podocyte injury and cell death as shown by TUNEL staining. Induction of podocyte injury resulted in a dramatic decrease of eGFP fluorescence in the vasculature over time (about 50% and 90% after 2 and 12 h respectively). Loss of vascular eGFP fluorescence was paralleled by an endocytosis-mediated accumulation of eGFP fluorescence in proximal tubule cells, indicating proteinuria.

CONCLUSION

We established a microscopy-based method to monitor the dynamics of glomerular barrier function during induction of podocyte injury in multiple zebrafish larvae simultaneously over 26 h.

Mechanical stress enhances CD9 expression in cultured podocytes

Elevated glomerular pressure represents a high risk for the development of severe kidney diseases and causes an increase in mechanical load to podocytes. In this study, we investigated whether mechanical stress alters gene expression in cultured podocytes using gene arrays. We found that tetraspanin CD9 is significantly upregulated in cultured podocytes after mechanical stress. The differential expression of CD9 was confirmed by RT-PCR and Western blotting under stretched and unstretched conditions. Furthermore, mechanical stress resulted in a relocalization of CD9. To get an insight into the functional role of CD9, podocytes were transfected with pEGFP-CD9. The expression of CD9 induced the formation of substratum-attached thin arborized protrusions. Ca(2+) depletion revealed that podocytes overexpressing CD9 possess altered adhesive properties in contrast to the control transfected cells. Finally, elevated CD9 expression increased migration of podocytes in a wound assay. In summary, our results suggest that upregulation of CD9 may play an important role in podocyte morphology, adhesion, and migration.

Functional expression of the renin-angiotensin system in human podocytes

Experimental and clinical studies impressively demonstrate that angiotensin-converting enzyme inhibitors (ACEI) and angiotensin receptor blockers (ARB) significantly reduce proteinuria and retard progression of glomerular disease. The underlying intraglomerular mechanisms are not yet fully elucidated. As podocyte injury constitutes a critical step in the pathogenesis of glomerular proteinuria, beneficial effects of ACEI and ARB may partially result from interference with a local renin-angiotensin system (RAS) in podocytes. The knowledge of expression and function of a local RAS in podocytes is limited. In this study, we demonstrate functional expression of key components of the RAS in differentiated human podocytes: podocytes express mRNA for angiotensinogen, renin, ACE type 1, and the AT(1) and AT(2) angiotensin receptor subtypes. In Western blot experiments and immunostainings, expression of the AT(1) and AT(2) receptor was demonstrated both in differentiated human podocytes and in human kidney cortex. ANG II induced a concentration-dependent increase in cytosolic Ca(2+) concentration via AT(1) receptors in differentiated human podocytes, whereas it did not increase cAMP. Furthermore, ANG II secretion was detected, which was blocked by neither the ACEI captopril nor the renin inhibitor remikiren nor the chymase inhibitor chymostatin. ANG II secretion of podocytes was not increased by mechanical stress. Finally, ANG II was found to increase staurosporine-induced apoptosis in podocytes. We speculate that ACEI and ARB exert their beneficial effects, in part, by interfering with a local RAS in podocytes. Further experiments are required to identify the underlying molecular mechanism(s) of podocyte protection.

Expression and signaling of parathyroid hormone-related protein in cultured podocytes

Podocyte function appears to be regulated by vasoactive factors. In vivo podocytes express parathyroid hormone-related protein (PTHrP), the N-terminal fragment of which has vasoactive properties. Since the signaling pathway(s) of PTHrP(1-36) are unknown in podocytes, differentiated cells of a conditionally immortalized mouse podocyte cell line were studied. Gene expression of PTHrP and the PTH/PTHrP receptor was investigated by RT-PCR; protein distribution of PTHrP was examined by immunofluorescence. Accumulation of cAMP was determined by an enzyme immunoassay; [Ca2+]i was measured by fura-2 ratio imaging. PTHrP and PTH/PTHrP receptor mRNA was detected in differentiated podocytes. Immunoreactive PTHrP exhibited a granular distribution in the cytoplasm of differentiated podocytes. With regard to the signaling pathway(s) of PTHrP(1-36), a concentration-dependent increase of cAMP levels with an EC50 value of 4 +/- 2 nM was found. PTHrP(1-36) (1 microM) increased cAMP levels 5.5 +/- 1.1-fold above baseline as compared with a 25.4 +/- 4.2-fold increase in response to forskolin (10 microM). The PTH/PTHrP receptor antagonist PTHrP(7-34) significantly diminished the PTHrP(1-36)-induced cAMP increase. While superfusion of podocytes with bradykinin (100 nM) increased [Ca2+]i, PTHrP(1-36) (100 nM) was without effect on [Ca2+]i. However, PTHrP(1-36) attenuated the bradykinin-induced increase in [Ca2+]i. Our results suggest that PTHrP is an autocrine hormone in podocytes, which selectively activates the cAMP pathway through the PTH/PTHrP receptor.

CD2AP and p130Cas localize to different F-actin structures in podocytes

Mice lacking the 80-kDa CD2-associated protein (CD2AP) develop progressive renal failure that starts soon after birth with proteinuria and foot process effacement by unknown mechanisms. CD2AP has been identified and cloned independently by virtue of its interaction with the T cell protein CD2 and with the docking protein p130Cas. In the present study we examined the localization of CD2AP and p130Cas in the mouse glomerulus and in cultured podocytes. In glomeruli, CD2AP and p130Cas immunofluorescence were observed in podocytes, where they colocalized with F-actin in foot processes. In addition, p130Cas was strongly expressed in mesangial cells. Immunoelectron microscopy demonstrated that CD2AP was present in podocyte foot processes without a prevailing localization. In cultured podocytes, p130Cas was enriched at sites of focal adhesions, where it colocalized like vinculin with F-actin at stress fiber ends. In contrast, CD2AP colocalized with F-actin at the leading edge of lamellipodia and in small spots, which were unevenly distributed in the cytoplasm. The spot-shaped F-actin structures were also stained by antibodies against the actin nucleation Arp2/3 complex and cortactin, both contributing to dynamic actin assembly. Moreover, CD2AP spots in cultured podocytes were in close spatial association with actinin-4, but not actinin-1. Our results suggest that CD2AP and p130Cas, which both colocalize with F-actin in podocytes in situ, possess different functions. Whereas p130Cas is found in focal adhesions, CD2AP seems to be involved in the regulation of highly dynamic F-actin structures in podocyte foot processes.

Paradoxical actions of exogenous and endogenous parathyroid hormone-related protein on renal vascular smooth muscle cell proliferation: reversion in the SHR model of genetic hypertension

In previous studies, added parathyroid hormone-related protein (PTHrP) inhibits whereas transfected PTHrP stimulates the proliferation of A10 aortic smooth muscle cells by nuclear translocation of the peptide. In the present studies, we asked whether these paradoxical trophic actions of PTHrP occur in smooth muscle cells (SMC) cultured from small intrarenal arteries of, and whether they are altered in, 12-wk-old spontaneously hypertensive rats (SHR) as compared to normotensive Wistar-Kyoto (WKY) rats. SHR cells grew faster than WKY cells. PTHrP transcript was increased in SHR-derived cells whereas PTH1 receptor (PTH1R) transcripts were similar in both cell lines. In both strains of cells, stable transfection with human PTHrP(1-139) cDNA did not further induce proliferation, suggesting maximal effect of endogenous PTHrP in wild cells. In contrast, transfection with antisense hPTHrP(1-139) cDNA, which abolished PTHrP mRNA, decreased WKY but increased SHR cell proliferation. Added PTHrP(1-36) (1-100 pM) decreased WKY and increased SHR cell proliferation. Additional studies indicated that the preferential coupling of PTH1-R to G-protein Gi was responsible for the proliferative effect of exogenous PTHrP in SHR cells. Moreover, PTHrP was detected in the nucleolus of a fraction of WKY and SHR renal SMC, in vitro as well as in situ, suggesting that the nucleolar translocation of PTHrP might be involved in the proliferative effects of endogenous PTHrP. In renovascular SMC, added PTHrP is antimitogenic, whereas endogenously produced PTHrP is mitogenic. These paradoxical effects of PTHrP on renovascular SMC proliferation appear to be reversed in the SHR model of genetic hypertension. A new concept emerges from these results, according to which a single molecule may have opposite effects on VSMC proliferation under physiological and pathophysiological conditions.

Culture of vascular smooth muscle cells from small arteries of the rat kidney

BACKGROUND

In contrast to arterioles, small arteries appear to be the preferential site of renal vascular smooth muscle cell (VSMC) proliferation under pathophysiological conditions. To date, techniques have been described to isolate renal arterioles and to culture VSMCs. The aim of the present study was to develop a method of culturing VSMCs from isolated small arteries of the rat kidney and to characterize their growth as compared with that of aortic VSMCs.

METHODS

Renal vascular trees were isolated from kidneys of male Wistar rats by a sieving technique. VSMCs were grown from explants of collagenase-treated renal vascular trees and thoracic aorta. Growth curves and proliferation of renal and aortic VSMCs in response to fetal bovine serum (FBS) were compared by determination of cell number and DNA synthesis, measured as incorporation of 5-bromo-2'-deoxyuridine.

RESULTS

Renal vascular trees consisted mainly of small arteries with a diameter of 80 to 400 microm (interlobar and arcuate arteries). As compared with total kidney or renal cortex, alkaline phosphatase activity was decreased by 81%, and vasopressin (10 micromol/L) was unable to stimulate adenylyl cyclase in renal vascular trees, indicating little tubular contamination. A homogenous population of spindle-shaped cells was cultured from renal vascular trees, which grew in a hill-and-valley pattern and stained positively for smooth muscle alpha-actin, according to the characteristics of VSMC phenotype. Renal VSMCs proliferated more slowly than aortic VSMCs and reached the plateau of growth at about half of the cell density of aortic VSMCs. Furthermore, proliferation of renal VSMCs depended more heavily on FBS concentration, since about threefold higher concentrations of FBS were needed for renal VSMCs to multiply at the same rate and to similarly stimulate DNA synthesis as compared with aortic VSMCs.

CONCLUSIONS

We present a method to culture renal VSMCs from small arteries of the rat kidney, which possess distinct growth characteristics as compared with aortic VSMCs.

Parathyroid hormone-related protein in rat penis: expression, localization, and effect on cavernosal pressure

Although PTH-related protein-(1-36) [PTHrP-(1-36)] is known to be expressed in smooth muscle and to exert potent myorelaxant effects, its tonic effects on cavernosal smooth muscle has not yet been explored. Using the RT-PCR technique, the present study establishes that PTHrP messenger RNA is present in microdissected corpus cavernosa in the rat. In immunohistochemical studies using affinity-purified antibodies to middle regions of PTHrP, immunostaining was localized throughout the penile structures, including vessels, cavernosal smooth muscle, and trabecular fibroblasts. Strong immunostaining for PTHrP was also detected in the dorsal nerve bundles. In anesthetized rats, intracavernosally injected boluses of increasing doses of PTHrP-(1-36) (0.3-30 pmol in 100 microl saline) had little effect on intracavernosal pressure. However, they markedly potentiated the dilatory response to papaverine (8-800 nmol), increasing the papaverine-induced intracavernous pressure by 2.5-fold, close to the mean arterial pressure. In conclusion, the cavernosal expression of PTHrP messenger RNA, the distribution of immunoreactive PTHrP throughout the structuro-functional components of the erectile apparatus and its strong potentiating action on papaverine-induced cavernosal relaxation, collectively suggest that PTHrP participates in the control of cavernosal tone.

Renovascular parathyroid hormone-related protein in spontaneously hypertensive rats: dilator or trophic factor?

Parathyroid hormone-related protein (PTHrP) is expressed throughout the renovascular system, and it dilates renal vessels, increases renal blood flow and glomerular filtration rate, and stimulates renin release. Mechanical forces and experimental hypertension have been shown to stimulate PTHrP expression in smooth muscles, suggesting a negative feedback control of vascular tone by PTHrP in hypertension. In this study, we compared the impact of a PTHrP receptor antagonist, PTHrP (7-34), and a PTHrP receptor agonist, PTHrP (1-36), on the vascular resistance of perfused kidneys isolated from spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY). Endogenous PTHrP appears not to act as a renal vasodilator in either WKY or SHR. However, the vasodilation following infused PTHrP (1-36) is blunted markedly in SHR, possibly due to desensitization or down-regulation of PTH/PTHrP receptors. Negative feedback control of vascular tone by PTHrP in SHR thus appears unlikely. The results raise the question of whether endogenous renovascular PTHrP behaves rather as a growth factor than as a vasodilator.

Observation and manipulation of different structural variants of individual cation-DNA complexes in the light microscope

Using different preparation protocols, different structures consisting of individual cation-DNA complexes are obtained. Vigorous mixing provides globular complexes, with different degrees of packing when poly-L-lysine or histone H1 is the cation. When hydrodynamic shearing is minimized by gentle handling, molecular networks or single cable like structures can be obtained. After 4',6-diamidino-2-phenyl indole (DAPI) fluorescence staining as well as with bright-field microscopy, the cables can be directly visualized in the light microscope. Using a pulsed ultraviolet laser coupled into the microscope (a laser microbeam) stretches of complex can be cut out from networks of poly-L-lysine-DNA complexes which may serve as models for highly extended chromosomes.

Painting of defined chromosomal regions by in situ suppression hybridization of libraries from laser-microdissected chromosomes

"Painting" of defined chromosomal regions provides a powerful tool for cytogenetic analyses. Here, we demonstrate that chromosomal in situ suppression (CISS)-hybridization of DNA libraries derived by microcloning laser-microdissected chromosomal regions can be applied to achieve this goal. As an example, we used unbanded metaphase spreads from a female patient carrying a balanced translocation. t(1;7)(1qter----1p36::7q11----7qter). Fragments from the long arms of 130 translocation chromosomes were microdissected. After microcloning, human inserts with an average size of about 3 kb were pooled from 400 recombinant bacteriophage DNA clones and used as a complex probe set in CISS-hybridization experiments. This resulted in painting of the translocation chromosome along the region 7q35 to 1p31. Painted chromosomal subregions in normal chromosomes 1 and 7 were consistent with this finding. This approach may be used to perform painting of any chromosome regions for which microlibraries can be established. Possible applications include the definition of marker chromosomes in clinical and tumor cytogenetics and studies of chromosomal evolution, as well as studies of nuclear chromosome topography in animal and plant species.