Effect of nucleotides on the cytosolic free calcium activity and inositol phosphate formation in human glomerular epithelial cells

Loss of S1P Lyase Expression in Human Podocytes Causes a Reduction in Nephrin Expression That Involves PKCδ Activation

Sphingosine 1-phosphate (S1P) lyase (SPL, Sgpl1) is an ER-associated enzyme that irreversibly degrades the bioactive lipid, S1P, and thereby regulates multiple cellular functions attributed to S1P. Biallelic mutations in the human Sglp1 gene lead to a severe form of a particular steroid-resistant nephrotic syndrome, suggesting that the SPL is critically involved in maintaining the glomerular ultrafiltration barrier, which is mainly built by glomerular podocytes. In this study, we have investigated the molecular effects of SPL knockdown (kd) in human podocytes to better understand the mechanism underlying nephrotic syndrome in patients. A stable SPL-kd cell line of human podocytes was generated by the lentiviral shRNA transduction method and was characterized for reduced SPL mRNA and protein levels and increased S1P levels. This cell line was further studied for changes in those podocyte-specific proteins that are known to regulate the ultrafiltration barrier. We show here that SPL-kd leads to the downregulation of the nephrin protein and mRNA expression, as well as the Wilms tumor suppressor gene 1 (WT1), which is a key transcription factor regulating nephrin expression. Mechanistically, SPL-kd resulted in increased total cellular protein kinase C (PKC) activity, while the stable downregulation of PKCδ revealed increased nephrin expression. Furthermore, the pro-inflammatory cytokine, interleukin 6 (IL-6), also reduced WT1 and nephrin expression. In addition, IL-6 caused increased PKCδ Thr⁵⁰⁵ phosphorylation, suggesting enzyme activation. Altogether, these data demonstrate that nephrin is a critical factor downregulated by the loss of SPL, which may directly cause podocyte foot process effacement as observed in mice and humans, leading to albuminuria, a hallmark of nephrotic syndrome. Furthermore, our in vitro data suggest that PKCδ could represent a new possible pharmacological target for the treatment of a nephrotic syndrome induced by SPL mutations.

Loss of sphingosine kinase 2 enhances Wilm's tumor suppressor gene 1 and nephrin expression in podocytes and protects from streptozotocin-induced podocytopathy and albuminuria in mice

The sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that is now appreciated as key regulatory factor for various cellular functions in the kidney, including matrix remodeling. It is generated by two sphingosine kinases (Sphk), Sphk1 and Sphk2, which are ubiquitously expressed, but have distinct enzymatic activities and subcellular localizations. In this study, we have investigated the role of Sphk2 in podocyte function and its contribution to diabetic nephropathy. We show that streptozotocin (STZ)-induced nephropathy and albuminuria in mice is prevented by genetic depletion of Sphk2. This protection correlated with an increased protein expression of the transcription factor Wilm's tumor suppressor gene 1 (WT1) and its target gene nephrin, and a reduced macrophage infiltration in immunohistochemical renal sections of STZ-treated Sphk2^-/- mice compared to STZ-treated wildtype mice. To investigate changes on the cellular level, we used an immortalized human podocyte cell line and generated a stable knockdown of Sphk2 (Sphk2-kd) by a lentiviral transduction method. These Sphk2-kd cells accumulated sphingosine as a consequence of the knockdown, and showed enhanced nephrin and WT1 mRNA and protein expressions similar to the finding in Sphk2 knockout mice. Treatment of wildtype podocytes with the highly selective Sphk2 inhibitor SLM6031434 caused a similar upregulation of nephrin and WT1 expression. Furthermore, exposing cells to the profibrotic mediator transforming growth factor β (TGFβ) resulted on the one side in reduced nephrin and WT1 expression, but on the other side, in upregulation of various profibrotic marker proteins, including connective tissue growth factor (CTGF), fibronectin (FN) and plasminogen activator inhibitor (PAI) 1. All these effects were reverted by Sphk2-kd and SLM6031434. Mechanistically, the protection by Sphk2-kd may depend on accumulated sphingosine and inhibited PKC activity, since treatment of cells with exogenous sphingosine not only reduced the phosphorylation pattern of PKC substrates, but also increased WT1 protein expression. Moreover, the selective stable knockdown of PKCδ increased WT1 expression, suggesting the involvement of this PKC isoenzyme in WT1 regulation. The glucocorticoid dexamethasone, which is a treatment option in many glomerular diseases and is known to mediate a nephroprotection, not only downregulated Sphk2 and enhanced cellular sphingosine, but also enhanced WT1 and nephrin expressions, thus, suggesting that parts of the nephroprotective effect of dexamethasone is mediated by Sphk2 downregulation. Altogether, our data demonstrated that loss of Sphk2 is protective in diabetes-induced podocytopathy and can prevent proteinuria, which is a hallmark of many glomerular diseases. Thus, Sphk2 could serve as a new attractive pharmacological target to treat proteinuric kidney diseases.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

WT1-dependent sulfatase expression maintains the normal glomerular filtration barrier

Paracrine signaling between podocytes and glomerular endothelial cells through vascular endothelial growth factor A (VEGFA) maintains a functional glomerular filtration barrier. Heparan sulfate proteoglycans (HSPGs), located on the cell surface or in the extracellular matrix, bind signaling molecules such as VEGFA and affect their local concentrations, but whether modulation of these moieties promotes normal crosstalk between podocytes and endothelial cells is unknown. Here, we found that the transcription factor Wilms' Tumor 1 (WT1) modulates VEGFA and FGF2 signaling by increasing the expression of the 6-O-endosulfatases Sulf1 and Sulf2, which remodel the heparan sulfate 6-O-sulfation pattern in the extracellular matrix. Mice deficient in both Sulf1 and Sulf2 developed age-dependent proteinuria as a result of ultrastructural abnormalities in podocytes and endothelial cells, a phenotype similar to that observed in children with WT1 mutations and in Wt1(+/-) mice. These kidney defects associated with a decreased distribution of VEGFA in the glomerular basement membrane and on endothelial cells. Collectively, these data suggest that WT1-dependent sulfatase expression plays a critical role in maintaining the glomerular filtration barrier by modulating the bioavailability of growth factors, thereby promoting normal crosstalk between podocytes and endothelial cells.

Podocytes in culture: past, present, and future

Human genetic and in vivo animal studies have helped to define the critical importance of podocytes for kidney function in health and disease. However, as in any other research area, by default these approaches do not allow for mechanistic studies. Such mechanistic studies require the availability of cells grown ex vivo (i.e., in culture) with the ability to directly study mechanistic events and control the environment such that specific hypotheses can be tested. A seminal breakthrough came about a decade ago with the documentation of differentiation in culture of primary rat and human podocytes and the subsequent development of conditionally immortalized differentiated podocyte cell lines that allow deciphering the decisive steps of differentiation and function of 'in vivo' podocytes. Although this paper is not intended to provide a comprehensive review of podocyte biology, nor their role in proteinuric renal diseases or progressive glomerulosclerosis, it will focus specifically on several aspects of podocytes in culture. In particular, we will discuss the scientific and research rationale and need for cultured podocytes, how podocyte cell-culture evolved, and how cultured podocytes are currently being used to uncover novel functions of podocytes that can then be validated in vivo in animal or human studies. In addition, we provide a detailed description of how to properly culture and characterize podocytes to avoid potential pitfalls.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Cyclic GMP signaling in podocytes

Natriuretic peptides (NP), together with nitric oxide (NO) are powerful relaxing factors acting via a common second messenger, cyclic GMP (cGMP). Together with other vasoactive modulators, these vasorelaxing factors play an essential role in regulating the function of kidney glomeruli. The presence of NP receptors in podocytes has been well documented. Recently, also mRNA for soluble guanylate cyclase, the NO receptor, has been shown in these cells. Stimulation of podocytes with atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP), and NO donors results in considerable upregulation of cellular cGMP synthesis. The podocyte foot processes contain a highly organized network of microfilaments adhering to the glomerular basement membrane (GBM). Changes in podocyte cytoskeleton accompanied by detachment of the cells from the GBM are closely associated with many glomerulopathies. The contractile apparatus in the podocyte foot processes seems to be an obvious target for the cyclic GMP signaling cascade. However, little is known about implications of the cGMP synthesis in these cells. We briefly review the current art regarding generation and modulation of cyclic GMP levels in podocytes. We discuss also the possible targets for this secondary messenger as well as its functional role in podocytes.

Extracellular nucleotides regulate cellular functions of podocytes in culture

Extracellular nucleotides are assumed to be important regulators of glomerular functions. This study characterizes purinergic receptors in podocytes. The effects of purinergic agonists on electrophysiological properties and the intracellular free Ca(2+) concentration of differentiated podocytes were examined with the patch-clamp and fura 2 fluorescence techniques. mRNA expression of purinergic receptors was investigated by RT-PCR. Purinergic agonists depolarized podocytes. Purinergic agonists similarly increased intracellular free Ca(2+) concentration of podocytes. The rank order of potency of various nucleotides on membrane voltage and free cytosolic calcium concentration was UTP approximately UDP > [adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S)] > ATP > 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP) > 2'- and 3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP) > ADP-beta-S. alpha,beta-Me-ATP was without effect. In the presence of UTP, BzATP did not cause an additional depolarization of podocytes. Incubation of cells with ATP or BzATP did not induce lactate dehydrogenase release. In RT-PCR studies, mRNAs of the P2Y(1), P2Y(2), P2Y(6), and P2X(7) receptors were detected within glomeruli and podocytes. The data indicate that extracellular nucleotides modulate podocyte function mainly by an activation of both P2Y(2) and P2Y(6) receptors.

The mononucleotide-dependent, nonantisense mechanism of action of phosphodiester and phosphorothioate oligonucleotides depends upon the activity of an ecto-5'-nucleotidase

Many reports indicate different nonantisense yet sequence-specific effects of antisense phosphorothioate oligonucleotides. Products of enzymatic degradation of the oligonucleotides can also influence cell proliferation. The cytotoxic effects of deoxyribonucleoside-5'-phosphates (dNMPs) and their 5'-phosphorothioate analogs, deoxyribonucleoside-5'-monophosphorothioates (dNMPSs) on 4 human cell types (HeLa, HL-60, K-562, and endothelial cells) were examined, and the effects were correlated with the catabolism of these compounds. The results indicate that differences in cytotoxicity of dNMPs or dNMPSs in these cells depend upon different activity of an ecto-5'-nucleotidase. It has also been found that dNMPSs stimulate proliferation of human umbilical vein endothelial cells and HL-60 cells in a concentration-dependent manner. This stimulation might be caused by the binding of deoxynucleoside-5'-phosphorothioates to as-yet unidentified nucleotide receptor(s) at the cell surface. (Blood. 2001;98:995-1002)

P2 receptors in regulation of renal microvascular function

In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.

Cyclic GMP-dependent relaxation of isolated rat renal glomeruli induced by extracellular ATP

The relaxing effect of extracellular ATP on renal glomeruli has been investigated by applying ATP and its analogues to suspensions of angiotensin II-precontracted rat renal glomeruli. Based on changes of glomerular [3H]inulin space (GIS) the relaxation of glomeruli was analysed in the presence of agonists: ATP, ADP, AMP, UTP, 2-methylthio-ATP (P2Y agonist), beta,gamma-methylene-ATP (P2X agonist) and adenosine. ATP, 2-methylthio-ATP, ADP and UTP induced concentration-dependent relaxation whereas AMP, beta,gamma-methylene-ATP and adenosine had no effect. The rank order of relaxation potency was 2-methylthio-ATP > ATP > ADP > UTP. An inhibitor of constitutive nitric oxide synthase (NOS), Nomega-nitro-L-arginine (NNA) prevented the ATP-induced increased accumulation of L-citrulline and the relaxation effect of ATP. An inhibitor of the neuronal isoform of NOS, 7-nitroindazole, had no effect on the relaxation effect of ATP. The relaxing effect of ATP was prevented in the presence of inhibitors of cyclic guanylyl cyclase: methylene blue (MB) and the more specific inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ). ATP stimulated an accumulation of cGMP that was diminished in the presence of MB. We indicated that extracellular ATP may relax the glomeruli via activation of P2Y receptors with the subsequent activation of the endothelial isoform of nitric oxide synthase and soluble guanylyl cyclase. We suggest that, based on the described mechanism, extracellular ATP may increase the filtration surface which, in turn, may influence the glomerular filtration rate.

Roles of the podocyte in glomerular function

The podocyte is the most differentiated cell type in the glomerulum, which forms a crucial component of the glomerular filtration barrier. It has been assumed that podocyte foot processes counteract the elastic force of the glomerular basement membrane and that vasoactive hormones may regulate the contractile state of their foot processes and thereby modulate the ultrafiltration coefficient K(f). Podocyte damage leads to proteinuria, and podocyte injury occurs in many glomerular diseases, which may progress to chronic renal failure. The understanding of the regulation of physiological properties of the podocyte and the mechanisms of its cellular response to injury may thus provide a clue to the understanding of the pathogenesis of proteinuria and glomerular diseases. In the past it was difficult to study cellular functions in this cell type, because of its unique anatomic location and the difficulty in characterizing podocytes in cell culture. However, recent advances in physiological, molecular biological, and cell culture techniques have increased the knowledge of the role of the podocyte in glomerular function. The present review attempts to outline new aspects of podocyte function in the glomerulum.

Polycations induce calcium signaling in glomerular podocytes

BACKGROUND

The neutralization of the polyanionic surface of the podocyte by perfusion of kidneys with polycations, such as protamine sulfate, leads to a retraction of podocyte foot processes and proteinuria. This study investigates the effects of protamine sulfate or anionic, neutral, or cationic dextrans on the cytosolic calcium activity ([Ca2+]i) in podocytes.

METHODS

[Ca2+]i was measured in single cultured differentiated mouse podocytes with the fluorescence dye fura-2/AM.

RESULTS

Protamine sulfate caused a concentration-dependent and partially reversible increase of [Ca2+]i (EC50 approximately 1.5 micromol/liter). Pretreatment of the cells with heparin (100 U/liter) inhibited the protamine sulfate-mediated increase of [Ca2+]i. Like protamine sulfate, diethylaminoethyl dextran (DEAE-dextran) concentration dependently increased [Ca2+]i in podocytes (EC50 approximately 20 nmol/liter), whereas dextran sulfate or uncharged dextran (both 10 micromol/liter) did not influence [Ca2+]i. A reduction of the extracellular Ca2+ concentration (from 1 mmol/liter to 1 micomol/liter) partially inhibited the protamine sulfate and the DEAE-dextran-induced [Ca2+]i response. Flufenamate (100 micromol/liter) or Gd3+ (10 micromol/liter), which are known to inhibit nonselective ion channels, did not influence the [Ca2+]i increase induced by protamine sulfate. In the presence of thapsigargin (50 nmol/liter), an inhibitor of the endoplasmic reticulum Ca2+-ATPase, both protamine sulfate and DEAE-dextran increased [Ca2+]i.

CONCLUSIONS

The data indicate that polycations increase podocyte [Ca2+]i. The increase of [Ca2+]i may be an early event in the pathogenesis of protamine sulfate-mediated retraction of podocyte foot processes.

NAD(P)H oxidase activity in cultured human podocytes: effects of adenosine triphosphate

Reactive oxygen species contribute to glomerular damage and proteinuria. In this study, we show that cultured human podocytes produce superoxide in response to extracellular adenosine triphosphate (ATP), and we identified the oxidases involved in this process. Adenosine triphosphate (10-4 M for 4 hr) raised superoxide production from 1.28 +/- 0.15 to 2.67 &/- 0.34 nmol/mg protein/min. Studies with podocyte homogenates revealed activation of both nicotinamide adenine dinucleotide (NADH; from 2.65 +/- 0.23 to 7.43 +/- 0.57) and nicotinamide adenine dinucleotide phosphate (NADPH) dependent oxidases [from 1.74 +/- 0.13 to 4.05 +/- 0.12 (nmol O2/mg protein/min)] by ATP. Activity of xanthine-oxidases was low and unchanged by ATP. Activation of the plasma-membrane bound NAD(P)H oxidases by ATP was time and dose dependent. Reverse transcribed-polymerase chain reaction (RT-PCR) studies with primers derived from monocyte sequences amplified mRNA for the NADPH oxidase subunits p22phox, p47phox, gp91phox, and p67phox, and the latter was transiently increased by ATP. Experiments with actinomycin D and cycloheximide suggested that ATP modulates enzyme activity at the transcriptional and translational levels. In conclusion, NAD(P)H dependent, membrane associated oxidases represent the major superoxide source in human podocytes. Activation of NAD(P)H oxidase by ATP might be secondary to increased mRNA expression of the NADPH oxidase subunit gp67phox.

Feedback regulation of extracellular ATP-stimulated phosphoinositide hydrolysis by protein kinase C-alpha in bovine glomerular endothelial cells

In glomerular endothelial cells, extracellular ATP stimulates a phospholipase C with subsequent hydrolysis of polyphosphoinositides and an increase in cytosolic free Ca2+ concentration ([Ca2+]i). Short-term (30 min) pretreatment of endothelial cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), a potent activator of protein kinase C (PKC), decreases the ATP-stimulated phosphoinositide degradation and Ca2+ mobilization. However, this inhibition was lost after incubating the cells for four hours with TPA. Longer-term pretreatment (10 to 48 hr) even potentiated ATP-induced phosphoinositide breakdown and Ca2+ mobilization. In addition, pretreating the cells for 30 minutes with the specific PKC inhibitor Ro 31-8220 dose-dependently increased ATP-stimulated phosphoinositide hydrolysis, thus clearly indicating a regulatory role for PKC in the inositol lipid signaling pathway in glomerular endothelial cells. By using specific antibodies recognizing the different PKC isoenzymes, it is observed that glomerular endothelial cells express five isoenzymes: PKC-alpha, -delta, -epsilon, -zeta and -theta. No PKC-beta, -gamma, -eta and -mu isoenzymes were detected. On exposure to TPA, a complete depletion of PKC-alpha is observed within four hours. In contrast, PKC-epsilon was more resistant to phorbol ester, and even after 48 hours of TPA treatment, only 60% of PKC-epsilon was down-regulated. PKC-theta decreased very slowly from the cytosol (47% left after 24 hr of phorbol ester treatment) and translocated to the Triton X100-insoluble fraction. Moreover, PKC-delta and PKC-zeta were not significantly affected by 48 hours of phorbol ester incubation. Thus, only PKC-alpha is depleted with a kinetic that corresponds to the loss of feedback inhibition of ATP-stimulated phosphoinositide turnover. In the next step, [Ca2+]i changes were measured in single cells loaded with Fura-2 after microinjection of neutralizing PKC isoenzyme-specific antibodies. Injection of antibodies specific for PKC-alpha potently increased Ca2+ mobilization in response to ATP stimulation when compared to cells injected with buffer only or antibodies specific for PKC-epsilon. These results provide evidence that PKC-alpha mediates feedback inhibition of ATP-stimulated phosphoinositide hydrolysis in glomerular endothelial cells.

Extracellular ATP regulates glomerular endothelial cell function

1. Glomerular endothelial cells form the inner part of the filtration barrier and are involved in pathophysiological processes in the glomerulum. New techniques for culturing glomerular endothelial cells have been established recently. The effect of extracellular ATP on membrane voltage and intracellular calcium activity was examined in bovine glomerular endothelial cells (GEC) in culture. 2. Membrane voltage was measured with the patch clamp technique in the fast whole cell configuration. GEC possess a stable membrane voltage of -88 mV. ATP induced a small transient hyperpolarization, which was followed by a depolarization. The ATP-induced depolarization was significantly inhibited by flufenamate, a blocker of non-selective ion channels. 3. The intracellular calcium activity [Ca2+]i was measured in single cells with the fura-2 technique. ATP stimulated an increase of [Ca2+]i. The increase of [Ca2+]i was biphasic with an initial peak followed by a sustained plateau. The [Ca2+]i peak was still present in an extracellular Ca(2+)-free solution, whereas the plateau was inhibited. 4. The order of potency of different purine nucleotides in stimulating [Ca2+]i and inositol formation was UTP = ATP > ATP-gamma-S > 2-methylthio ATP > [alpha,beta-CH2]ATP. 5. The data indicate that ATP regulates membrane voltage and [Ca2+]i in glomerular endothelial cells by a P2y2 receptor.

Extracellular ATP increases cytosolic calcium in cultured rat renal arterial smooth muscle cells

1. Experiments were conducted on cultured renal arterial smooth muscle cells to determine the ability of extracellular ATP to alter cytosolic calcium concentration and to determine the mechanisms by which this effect occurs. 2. ATP (100 mumol/L) caused the fluorescence ratio of fura-2 to increase from a control value of 1.06 +/- 0.05 to 2.06 +/- 0.13 (P < 0.01) before stabilizing at a sustained level of 1.35 +/- 0.04 (n = 8; P < 0.05). 3. Removal of extracellular calcium from the bathing medium resulted in an attenuation of the initial response to 100 mumol/L ATP with cell fluorescence increasing from 1.16 +/- 0.18 to 1.44 +/- 0.18 ratio units (n = 5). Furthermore, the initial increase in fluorescence ratio rapidly declined to 1.02 +/- 0.06, indicating that an influx of extracellular calcium is required to sustain the increase in fura-2 fluorescence. 4. Depletion of intracellular calcium pools with thapsigargin prevented the increase in fura-2 fluorescence evoked by ATP. 5. These data suggest that ATP-mediated increases in cytosolic calcium in cultured renal arterial smooth muscle cells involve calcium release from the thapsigargin-sensitive, intracellular pool in conjunction with calcium influx from the extracellular medium.

Tyrphostin inhibition of ATP-stimulated DNA synthesis, cell proliferation and fos-protein expression in vascular smooth muscle cells

1. We and others have shown that extracellular ATP (adenosine triphosphate), released from sympathetic nerves and platelets, stimulates growth of vascular smooth muscle cells (SMC). To study the importance of tyrosine kinases for ATP-mediated proliferation in vascular smooth muscle cells we used tyrphostins, a recently developed group of highly specific inhibitors of tyrosine kinases. 2. ATP induced a powerful concentration-dependent increase in DNA synthesis measured by [3H]-thymidine incorporation in rat aorta SMC (RASMC) and an increase in total cell number after 72 h of incubation as measured by an enzymatic cell proliferation assay. Tyrphostin 25 (10(-5) M) had no effect per se on basal DNA synthesis but reduced ATP-stimulated DNA synthesis and increase in cell number in a dose-dependent manner. Higher concentrations of ATP could not reverse the inhibitory effect of tyrphostin 25. The potency of several (six) other tyrphostins was also examined and found to be slightly greater than tyrphostin 25 with equal efficacy. 3. When RASMC were incubated with 10(-5) M ATP for 2 h, nearly all of the cells (87 +/- 5%) were intensely stained with an antibody to the Fos protein while in the controls only 1 +/- 2% of the cells were weakly stained. Tyrphostin 25 greatly reduced the Fos-protein staining (14 +/- 2%). 4. ATP induced a concentration-dependent increase in 45Ca(2+)-influx and formation of inositol phosphates (IPtotal) in RASMC. These effects were not inhibited by tyrphostin 25. 5. Tyrphostin 25 did not alter ATP-induced contraction in ring segments of rat aorta. 6. In conclusion, tyrphostin 25 inhibited ATP-induced DNA synthesis, cell proliferation and Fos-protein expression, but not ATP-induced 45Ca(2+)-influx, inositolphosphate-production or vasoconstriction. This indicates that the mitogenic effect of ATP on vascular smooth muscle cells is dependent on tyrosine kinases in contrast to the contractile effect of ATP in blood vessels.

Interaction of native and oxidized lipoprotein(a) with human mesangial cells and matrix

The trapping of apolipoprotein(a) and apolipoprotein B-100 in glomeruli of patients with the nephrotic syndrome seems to be linked to a less favorable course of renal disease. To evaluate the potential role of lipoprotein(a) as a mediator of glomerular injury, we measured uptake of native lipoprotein(a) [Lp(a)] and oxidatively modified Lp(a) by cultured human mesangial cells and matrix and studied the effects of Lp(a) on mesangial cell DNA-synthesis and cellular proliferation. Uptake of Lp(a) by mesangial cells occurred at a significantly lower affinity (KD 16 micrograms/ml vs. 39 micrograms/ml) and a lower maximum degradative capacity (6.7-fold) than for LDL. Specificity of receptor mediated uptake was 50% for Lp(a) compared to 84% for LDL. Oxidative modification of both Lp(a) and LDL was accompanied by a significant decrease in uptake and degradative capacities. Due to the limited uptake, native and oxidatively modified Lp(a) had only marginal effects on intracellular cholesterol metabolism, which was measured as inhibition of sterol synthesis and stimulation of cholesterol esterification. However, binding of Lp(a), oxidized Lp(a) and oxidized LDL to extracellular mesangial matrix was enhanced compared to LDL. Furthermore, incubation of mesangial cells with Lp(a) and oxLp(a) in concentrations of 2.5 micrograms/ml and higher resulted in a decrease of DNA synthesis. Regardless of the oxidative status, a maximal suppression of DNA synthesis was observed at 20 micrograms/ml Lp(a). Native Lp(a) also blunted the stimulatory effects of PDGF on mesangial cell DNA-synthesis. Lp(a) did not alter basal TGF-beta transcription in human mesangial cells. The avid interaction of Lp(a) and modified lipoproteins with mesangial matrix provides a concept for the enhanced entrapment of these lipoproteins in the diseased glomerulum. Native Lp(a) is a poor ligand for the LDL receptor; oxidation of Lp(a) even lowers the affinity towards this receptor. Further studies must be carried out to clarify the pathophysiological significance of Lp(a) trapping in the mesangial matrix.

Mechanisms of intracellular calcium regulation in adult astrocytes

Microfluorimetric techniques were used to measure changes in intracellular calcium in astrocytes cultured from the forebrain of the adult rat. Application of ATP consistently raised intracellular calcium. The response persisted in the absence of extracellular calcium, but then quickly declined upon repeated agonist application. Thapsigargin abolished responses to nucleotides following depletion of the endoplasmic reticular calcium stores. Calcium release was inhibited by caffeine, but was dramatically increased through inositol phosphate receptor sensitization by the sulphydryl reagent thimerosal. Responses to repeated nucleotide applications resulted in a gradual decline of peak calcium concentrations, suggesting a (post)receptor-mediated desensitization or gradual depletion of the internal calcium stores. Subsequent application of ionomycin suggested intracellular calcium depletion as the relevant mechanism. Depletion of the internal calcium stores with ATP, ionomycin or thapsigargin failed to reveal a calcium influx pathway. These results suggest that the capacitative mechanism of calcium entry does not operate in response to nucleotide receptor activation in these cells, and that the immediate refilling of the internal calcium stores is primarily determined by re-uptake of cytosolic calcium into the endoplasmic reticulum. A complete refilling of this calcium store by extracellular calcium may be a much slower process. Control of these signal transduction pathways is crucial to the maintenance of the calcium/energy homeostasis of the adult astrocyte in the central nervous system.

Structure and function of podocytes: an update

Glomerular visceral epithelial cells, also termed podocytes, are highly specialized epithelial cells that cover the outer aspect of the glomerular basement membrane. Recent studies point to an important role of podocytes in the physiology and pathophysiology of the glomerulus. This review summarizes the structure-function relationships of podocytes. Following a description of the general morphology of podocytes, the technical problems associated with studying these cells are discussed. A survey of podocyte function forms the center of this review. Finally, selected aspects of podocyte development and cell division are discussed.

Synergy between the inositol phosphate responses to transfected human adenosine A1-receptors and constitutive P2-purinoceptors in CHO-K1 cells

1. The effect of adenosine A1-receptor and P2-purinoceptor agonists on [3H]-inositol phosphate accumulation has been investigated in CHO-K1 cells transfected with the human adenosine A1-receptor. 2. Adenosine receptor agonists stimulated [3H]-inositol phosphate accumulation in CHO-K1 cells with a rank potency order of N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > N6-2-(4-aminophenyl) ethyladenosine (APNEA). The responses to both CPA and APNEA were antagonized by the A1 selective antagonist, 1,3-dipropylcyclopentylxanthine (DPCPX) yielding KD values of 1.2 nM and 4.3 nM respectively. 3. ATP, UTP and ATP gamma S were also able to stimulate [3H]-inositol phosphate accumulation in these cells with EC50 values of 1.9 microM, 1.3 microM and 5.0 microM respectively. 2-Methyl-thio-ATP was a weak agonist of this response (EC50 > 100 microM). 4. The [3H]-inositol phosphate response to CPA was completely attenuated by pertussis toxin treatment (24 h; 100 ng ml-1). In contrast, the responses to ATP, UTP and ATP gamma S were only reduced by circa 30% in pertussis toxin-treated cells. 5. The simultaneous addition of CPA and either ATP, UTP or ATP gamma S produced a large augmentation of [3H]-inositol phospholipid hydrolysis. This was due to an increase in the maximal response and was significantly greater than the predicted additive response for activation of these two receptor systems. The synergy was not observed in pertussis toxin-treated cells. 6. No synergy was observed between the [3H]-inositol phosphate responses to histamine and ATP in CHO-K1 cells transfected with the bovine histamine H1-receptor. In these cells the response to histamine was completely resistant to inhibition by pertussis toxin treatment. 7. This study provides a clear demonstration of a synergy between pertussis toxin-sensitive and insensitive receptor systems in a model cell system which is an ideal host for transfected cDNA sequences. This model system should provide a unique opportunity to unravel the mechanisms underlying this example of receptor cross-talk involving phospholipase C.

Characterisation of an ATP receptor mediating mitogenesis in vascular smooth muscle cells

Adenosine triphosphate (ATP), a co-transmitter in sympathetic nerves and released from platelets, has recently been shown to stimulate growth of vascular smooth muscle cells. It might therefore contribute to the development of vascular hypertrophy seen in hypertension and atherosclerosis. We aimed at characterising the receptor mediating this mitogenic effect in rat aorta smooth muscle cells. The potency of agonists indicates a P2 purinoceptor since ATP > or = ADP >> AMP, adenosine. The P2x-receptor subtype, which is responsible for ATP induced vasoconstriction in rat aorta, does not mediate the mitogenic effect since alpha, beta-methyleneATP had no effect and beta, gamma-methyleneATP had lower potency than ATP. The P2Y-receptor subtype was excluded since the selective agonist 2-methylthioATP had weak effect with lower potency than ATP. When we studied the involvement of other nucleotides similar effects were seen of the purines ATP, GTP and ITP; also the pyrimidine UTP had powerful mitogenic effects (Emax = 52% of ATP) with similar potency. Nucleotides with fewer phosphate groups showed a stepwise fall in mitogenic effect. This indicates involvement of a nucleotide-receptor (P2U). Ap4A were of equal potency and effect as ATP. There was strong correlation between the mitogenic effects of the nucleotides and analogues with both 45Ca(2+)-influx and inositol phosphate (IP) production, indicating that they may participate in mediating the mitogenic response. This is the first study describing the potencies for the mitogenic effects of the selective ATP-analogues and other nucleotides in vascular smooth muscle cells. The receptor characterisation indicates a nucleotide-receptor similar to the receptor which stimulates 45Ca(2+)-influx and inositol phosphate-formation in rat aorta smooth muscle cells. Substances related to ATP such as GTP, ITP, UTP and Ap4A which also can be released extracellularly in vivo stimulate mitogenesis of rat aorta smooth muscle cells through the same receptor.

Purine- and pyrimidine-stimulated phosphoinositide breakdown and intracellular calcium mobilisation in astrocytes

Phosphoinositide breakdown in cultured cortical astrocytes was assessed by measuring the accumulation of [3H]inositol phosphates (IP's) following incubations with various purines and pyrimidines. Dose-response relationships gave the following order of potency: 2-methylthioadenosine triphosphate (2-MeSATP) > uridine 5'-triphosphate (UTP) > ATP = ADP > inosine 5' triphosphate (ITP). However, 2-MeSATP and UTP were only half as effective as either ATP or ADP in stimulating [3H]IP production. Astrocytes were also challenged with combined additions of maximally effective concentrations of agonists. Responses to ADP plus UTP and 2-MeSATP plus UTP were essentially additive whilst ATP plus UTP evoked a response which was only partially additive. ATP-stimulated [3H]IP accumulation was markedly reduced in the presence of 2-MeSATP suggesting that the latter may be a partial agonist at these receptors. We also examined the ability of ATP and UTP to increase intracellular Ca2+ concentrations in these cells. Greater than 90% of all cells tested responded to ATP with a release from internal Ca2+ stores but less than half of these responded similarly when challenged with UTP. Our results indicate that astrocytes possess both P2Y-purinoceptors and a population of receptors which are also coupled to phosphoinositide metabolism and intracellular Ca2+ mobilisation but recognise ATP and the pyrimidine nucleotide UTP.

Activation of Ca(2+)-dependent K+ and Cl- currents by UTP and ATP in CFPAC-1 cells

Activation of Cl- and K+ conductances by nucleotide receptor-operated mobilization of intracellular Ca2+ was investigated in CFPAC-1 cells with the perforated-patch technique. Adenosine 5'-triphosphate (ATP) and uridine 5'-triphosphate (UTP) caused a dose-dependent fast and transient membrane hyperpolarization. UTP was more effective than ATP. In voltage-clamped cells, two currents with different ionic permeability and kinetics were activated by the nucleotides. The first one was carried by Cl- ions, peaked in the first few seconds after addition of nucleotides, and lasted for 1 +/- 0.3 min. Its amplitude was about 2.7 nA at -100 mV with 100 mumol/l of either ATP or UTP. The second current was carried by K+ ions and was blocked by Cs+. This current peaked more slowly and had a mean duration of 4.6 +/- 0.7 min. Its amplitude was 0.9 nA and 0.5 nA at -20 mV with 100 mumol/l UTP and ATP, respectively. Activation of the nucleotide receptor caused a transient increase in intracellular Ca2+ concentration ([Ca2+]i) that was similar in the presence or absence of extracellular Ca2+. The ED50 for UTP was 24 mumol/l and that for ATP was 94 mumol/l. Depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ store by thapsigargin prevented both the nucleotide-induced [Ca2+]i increase and the activation of membrane currents. Addition of 2 mmol/l Ca2+ to thapsigargin-treated cells produced a sustained increase of Cl- and K+ currents, which was reversed by Ca2+ removal. The present study demonstrates that CFPAC-1 cells respond to nucleotide receptor activation with a transient increase in [Ca2+]i that stimulates Ca(2+)-dependent Cl- and K+ currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Increases in intracellular calcium via activation of an endogenous P2-purinoceptor in cultured CHO-K1 cells

1. Increases in intracellular calcium ([Ca2+]i) were measured in chinese hamster cultured ovary cells (clone, CHO-K1), by use of the fluorescent, calcium-sensitive dye, fura-2. 2. Addition of both ATP and UTP elicited rapid increases in [Ca2+]i due to mobilization from intracellular stores and calcium entry across the plasma membrane. 3. Omission of calcium from the extracellular medium and pre-incubation with the inorganic calcium channel blocker, nickel (Ni2+) prevented the calcium entry components of the responses. 4. Investigation of the concentration-response relationships of various analogues of ATP suggests the presence of a purinoceptor which cannot be characterized as P2X or P2Y. In addition, there appears to be a sub-population of P2Y-purinoceptors which do not cross-react with the 'nucleotide' receptor population. 5. Cross-desensitization and additivity experiments suggest that both ATP and UTP activate the same receptor. 6. Pre-incubation with the tumour-promoting agent, beta-phorbol-12,13 dibutyrate (PDBu), caused a reduction in the increases in [Ca2+]i, suggesting a role for protein kinase C in feedback inhibition of purinoceptor responses in this cell line. 7. In summary, we present evidence for the existence of an endogenous P2U-purinoceptor (or 'nucleotide receptor') which is linked to increases in [Ca2+]i in CHO-K1 cells.

Receptor-mediated uptake of IDL and LDL from nephrotic patients by glomerular epithelial cells

Although hyperlipidemia is a well-recognized complication of the nephrotic syndrome, the precise interaction of human glomerular cells and human lipoproteins, abnormal in lipid and protein composition, has not been clearly defined. This study examines receptor mediated binding, internalization and degradation as well as intracellular cholesterol metabolism of apoB-100 containing LDL and apoB,E containing IDL, isolated from patients with the nephrotic syndrome (N = 6), in human glomerular epithelial cells and skin fibroblasts. In the patients, serum LDL cholesterol level was increased threefold and IDL elevenfold as compared to healthy subjects. IDL of nephrotic patients contained 72% more cholesterol than IDL of healthy controls. No difference in lipid/protein composition was found in the LDL density range. Therefore, nephrotic and control LDL showed identical affinities for receptor mediated binding, internalization and degradation. Furthermore, inhibition of intracellular sterol synthesis and cholesteryl ester formation after incubation with LDL was comparable. In contrast, cholesterol-rich IDL of nephrotic patients was taken up by glomerular epithelial cells with higher affinity than LDL and control IDL, as well as intracellular sterol synthesis was suppressed more effectively than by control IDL. The cholesterol esterification rate of IDL from patients was enhanced 3.5-fold as compared to control IDL. In comparison to fibroblasts, glomerular epithelial cells showed about 15% of the maximal capacity for LDL uptake, but 31% for IDL from nephrotic patients. The data indicate that hypercholesterolemia of nephrotic origin cannot be explained by reduced ligand binding for LDL. ApoE containing IDL, which accumulate in nephrotic patients, were avidly taken up by glomerular epithelial cells via receptor dependent pathway. These lipoproteins could therefore play the predominant role in glomerular lipid accumulation and development of glomerulosclerosis.

Extracellular ATP and UTP activation of phospholipase D is mediated by protein kinase C-epsilon in rat renal mesangial cells

1. We have studied whether a nucleotide receptor mediates the effects of extracellular ATP and UTP on phosphatidylcholine metabolism in rat cultured glomerular mesangial cells. 2. ATP and UTP stimulated a biphasic 1,2-diacylglycerol (DAG) formation in [3H]-arachidonic acid-labelled mesangial cells. In contrast, in cells labelled with [3H]-myristic acid, a tracer that preferentially marks phosphatidylcholine, both nucleotides induced a delayed monophasic production of DAG with a concomitant increase in phosphatidic acid and choline formation. 3. A phospholipase D-mediated phosphatidylcholine hydrolysis was further suggested by the observation that ATP and UTP stimulate the accumulation of phosphatidylethanol, when ethanol was added to mesangial cells. 4. The rank order of potency of a series of nucleotide analogues for stimulation of phosphatidylethanol formation was UTP = ATP > ITP > ATP gamma S > beta gamma-imido-ATP = ADP > 2-methylthio-ATP = beta gamma-methylene-ATP = ADP beta S, while AMP, adenosine, CTP and GTP were inactive, indicating the presence of a nucleotide receptor. 5. Elevation of cytosolic free Ca2+ by the calcium ionophore A23187 (1 microM) or the Ca(2+)-ATPase inhibitor, thapsigargin (200 nM) slightly increased phosphatidylethanol formation. However, chelation of cytosolic Ca2+ with high concentrations of Quin 2 did not attenuate ATP- and UTP-induced phosphatidylethanol production, thus suggesting that Ca2+ is not crucially involved in agonist-stimulated phospholipase D activation. 6. The protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), but not the biologically inactive 4 alpha-phorbol 12,13-didecanoate, increased phospholipase D activity in mesangial cells, suggesting that PKC may mediate nucleotide-induced phosphatidylcholine hydrolysis. 7. Down-regulation of PKC-alpha and -delta isoenzymes by 8 h PMA treatment still resulted in full phospholipase D activation. In contrast, a 24 h treatment of mesangial cells with PMA, a regimen that also causes depletion of PKC-epsilon, markedly attenuated nucleotide-evoked phosphatidylethanol formation. In addition, the selective PKC inhibitor, calphostin C attenuated ATP- and UTP-induced phosphatidylethanol production.8. In summary, these data suggest that extracellular ATP and UTP use a common nucleotide receptor to activate phospholipase D-mediated phosphatidylcholine hydrolysis. Stimulation of phospholipase D appears to involve the PKC-epsilon isoenzyme, activated by DAG derived from phosphoinositide hydrolysis by phospholipase C.

Effect of extracellular ATP on contraction, cytosolic calcium activity, membrane voltage and ion currents of rat mesangial cells in primary culture

1. The effects of extracellular ATP on contraction, membrane voltage (Vm), ion currents and intracellular calcium activity [Ca2+]i were studied in rat mesangial cells (MC) in primary culture. 2. Addition of extracellular ATP (10(-5) and 10(-4) M) to MC led to a cell contraction which was independent of extracellular calcium. 3. Membrane voltage (Vm) and ion currents were measured with the nystatin patch clamp technique. ATP induced a concentration-dependent transient depolarization of Vm (ED50: 2 x 10(-6) M). During the transient depolarization ion currents were monitored simultaneously and showed an increase of the inward- and outward current. 4. In a buffer with a reduced extracellular chloride concentration (from 145 to 30 mM) ATP induced a depolarization augmented to -4 +/- 4 mV. 5. ATP-gamma-S and 2-methylthio-ATP depolarized Vm to the same extent as ATP, whereas alpha,beta-methylene-ATP (all 10(-5) M) had no effect on Vm. 6. The Ca2+ ionophore, A23187, depolarized Vm transiently from -51 +/- 2 to -28 +/- 4 mV and caused an increase of the inward current. 7. The intracellular calcium activity [Ca2+]i was measured with the fura-2 technique. ATP stimulated a concentration-dependent increase of [Ca2+]i (ED50: 5 x 10(-6) M). The increase of [Ca2+]i was biphasic with an initial peak followed by a sustained plateau. 8. The [Ca2+]i peak was still present in an extracellular Ca(2+)-free buffer, whereas the plateau was abolished. Verapamil (10(-4) M) did not inhibit the [Ca2+]i increase induced by ATP. 9. The data indicate that extracellular ATP contracts MC and is able to increase [Ca2+]i by the release of Ca2+ from intracellular stores and recruitment from the extracellular space. In addition ATP depolarizes Vm of MC by activating a Cl- conductance. The ATP-induced depolarization is mediated by a P2y receptor.

Effect of bradykinin and histamine on the membrane voltage, ion conductances and ion channels of human glomerular epithelial cells (hGEC) in culture

The effects of bradykinin (BK) and histamine (Hist) on the membrane voltage (Vm), ion conductances and ion channels of cultured human glomerular epithelial cells (hGEC) were examined with the nystatin patch clamp technique. Cells were studied between passage 3 and 20 in a bath rinsed with Ringer-like solution at 37 degrees C. The mean value of Vm was -41 +/- 0.5 mV (n = 189). BK (10(-6) mol/l, n = 29) and Hist (10(-5) mol/l, n = 55) induced a rapid transient hyperpolarization by 15 +/- 1 mV and 18 +/- 1 mV, respectively. The hyperpolarization was followed by a long lasting depolarization by 6 +/- 1 mV (BK 10(-6) mol/l) and 7 +/- 1 mV (Hist 10(-5) mol/l). The ED50 was about 5 x 10(-8) mol/l for BK and 5 x 10(-7) mol/l for Hist. In the presence of both agonists, increases of outward and inward currents were observed. A change in the extracellular K+ concentration from 3.6 to 30 mmol/l depolarized Vm by 8 +/- 1 mV and completely inhibited the hyperpolarizing effect of both agents (n = 11). Reduction of extracellular Cl- concentration from 145 to 30 mmol/l led to a depolarization by 2 +/- 1 mV (n = 25). In 30 mmol/l Cl- the depolarizations induced by BK (10(-7) mol/l) and Hist (10(-6) mol/l) were augmented to 9 +/- 2 mV (n = 14) and to 10 +/- 2 mV (n = 11), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)