Role of molecular charge and hydrophilicity in regulating the kinetics of crystal growth

The composition of biologic molecules isolated from biominerals suggests that control of mineral growth is linked to biochemical features. Here, we define a systematic relationship between the ability of biomolecules in solution to promote the growth of calcite (CaCO3) and their net negative molecular charge and hydrophilicity. The degree of enhancement depends on peptide composition, but not on peptide sequence. Data analysis shows that this rate enhancement arises from an increase in the kinetic coefficient. We interpret the mechanism of growth enhancement to be a catalytic process whereby biomolecules reduce the magnitude of the diffusive barrier, Ek, by perturbations that displace water molecules. The result is a decrease in the energy barrier for attachment of solutes to the solid phase. This previously unrecognized relationship also rationalizes recently reported data showing acceleration of calcite growth rates over rates measured in the pure system by nanomolar levels of abalone nacre proteins. These findings show that the growth-modifying properties of small model peptides may be scaled up to analyze mineralization processes that are mediated by more complex proteins. We suggest that enhancement of calcite growth may now be estimated a priori from the composition of peptide sequences and the calculated values of hydrophilicity and net molecular charge. This insight may contribute to an improved understanding of diverse systems of biomineralization and design of new synthetic growth modulators.

Dual roles of brushite crystals in calcium oxalate crystallization provide physicochemical mechanisms underlying renal stone formation

Calcium oxalate monohydrate (COM) crystals are the major mineral component of most kidney stones, and thus have an important role in chronic human disease. However, the physicochemical mechanisms leading to calcium oxalate (CaOx) stone disease are only partially defined. As spontaneous precipitation of CaOx is rare under renal conditions, an alternative pathway for CaOx crystallization seems necessary to resolve this central issue. We performed kinetic studies using the dual constant composition method to simultaneously analyze the crystallization of COM and brushite, the form of calcium phosphate that is most readily formed in the typical slightly acidic urinary milieu. These studies were supported by parallel analysis by scanning electron and atomic force microscopy. In these studies, mineralization of a thermodynamically stable phase (COM) was induced by the presence of brushite, a more readily precipitated inorganic phase. Furthermore, once formed, the COM crystals grew at the expense of brushite crystals causing the dissolution of the brushite crystals. These studies show that brushite may play crucial roles in the formation of COM crystals. The definition of these two roles for brushite thereby provides physicochemical explanations for the initiation of COM crystallization and also for the relative paucity of calcium phosphate detected in the majority of CaOx renal stones.

Molecular modulation of calcium oxalate crystallization by osteopontin and citrate

Calcium oxalate monohydrate (COM), which plays a functional role in plant physiology, is a source of chronic human disease, forming the major inorganic component of kidney stones. Understanding molecular mechanisms of biological control over COM crystallization is central to development of effective stone disease therapies and can help define general strategies for synthesizing biologically inspired materials. To date, research on COM modification by proteins and small molecules has not resolved the molecular-scale control mechanisms. Moreover, because proteins directing COM inhibition have been identified and sequenced, they provide a basis for general physiochemical investigations of biomineralization. Here, we report molecular-scale views of COM modulation by two urinary constituents, the protein osteopontin and citrate, a common therapeutic agent. Combining force microscopy with molecular modeling, we show that each controls growth habit and kinetics by pinning step motion on different faces through specific interactions in which both size and structure determine the effectiveness. Moreover, the results suggest potential for additive effects of simultaneous action by both modifiers to inhibit the overall growth of the crystal and demonstrate the utility of combining molecular imaging and modeling tools for understanding events underlying aberrant crystallization in disease.

Phosphorylated osteopontin peptides suppress crystallization by inhibiting the growth of calcium oxalate crystals

BACKGROUND

Osteopontin isolated from human urine [uropontin (uOPN)] is a potent inhibitor of calcium oxalate (CaOx) monohydrate (COM) crystallization. However, specific structural features responsible for its effects on CaOx crystallization were not previously known. The present studies were designed to define molecular features responsible for interactions of uOPN with COM crystals and the inhibition of crystallization.

METHODS

Peptides and phosphopeptides with sequences corresponding to potential crystal binding domains within the protein sequence of osteopontin were synthesized. Then the effects of these peptides on COM crystal growth and crystal aggregation were investigated and their secondary structures analyzed.

RESULTS

Growth of COM crystals was inhibited by approximately 50% at 1000 nmol/L concentrations by the two unmodified peptides with the closest clustering of aspartic acid residues. Growth was not inhibited by the other two unmodified peptides, with aspartic residues more evenly distributed within their sequences. Phosphorylation markedly increased inhibition of COM crystal growth, so that each of the four phosphorylated peptides inhibited growth by at least 50% at concentrations of < or =200 nmol/L. Phosphorylation of these peptides did not cause changes in secondary structure that would favor interaction with COM crystal surfaces.

CONCLUSIONS

These studies of synthetic peptides identify molecular features within the osteopontin molecule that contribute to the inhibition of one aspect of COM crystallization. The inhibition of crystal growth induced by phosphorylation appears to result from altered local patterns of charge density, since conformational changes favoring interaction with crystals were not caused by phosphorylation.

Immunomorphometric studies of proteinuria in individual deep and superficial nephrons of rats

Heterogeneity of structure and function among nephrons is a well-recognized feature of chronic renal diseases. However, only a small number of superficial nephrons per kidney are accessible for micropuncture analysis and relationships of proteinuria to structural change in individual nephrons of experimental models are not clearly established. To directly evaluate proteinuria in many individual nephrons, we developed an immunomorphometric method of analysis. This method is based on the uniformly abundant renal synthesis of Tamm-Horsfall protein (THP) in the thick ascending limb of Henle's loop (TAL). Luminal rabbit immunoglobulin G (IgG) deposits are formed in TALs of proteinuric nephrons in rats injected with heterologous IgG anti-THP antibodies. This immunomorphometric luminal deposit method of assessing proteinuria was previously validated through analysis of heterologous immune complex nephropathy. Glomerular dysfunction in several models-spontaneously hypertensive rats (SHR), aging Sprague-Dawley (SD) rats, rats with adriamycin nephropathy (ADR), and rats subjected to subtotal nephrectomy (NX)-was characterized by immunomorphometric analysis after injection of anti-THP antibodies. Luminal IgG deposits were used to identify nephrons with increased proteinuria. Nephrons were identified histologically as either long looped (LL) or short looped (SL), and frequency of luminal deposits in these nephrons was determined. Glomerular size and sclerosis in deep and superficial zones of renal cortex were determined. Luminal deposits in LL nephrons were more frequent than luminal deposits in SL nephrons in SHRs (p < .001) and aging rats (p < .001) and SL nephrons in ADR rats (p < .02). Whole kidney levels of albuminuria correlated closely with the frequency of luminal deposits in both LL and SL nephrons of SHRs and ADR rats and in LL nephrons of aging rats (p < .005). In contrast, LL and SL deposits were equal in NX rats and did not correlate with albuminuria. A majority of luminal deposits extended beyond the first medullary TAL zone of NX rats, but was confined to this zone in the other 3 models. Deep cortical glomeruli were larger with more glomerulosclerosis than superficial cortical glomeruli. Albuminuria correlated with sclerosis of both deep (p < .002) and superficial (p < .01) glomeruli in NX rats, but not in the other three models. These studies provide a detailed characterization of a new method that allows comparison of proteinuria derived from deep and superficial nephrons. They also provide evidence that pathogenesis of the glomerulosclerosis in NX rats differs from that of the other three models. Glomerulosclerosis was closely linked to the overall level of albuminuria in NX rats, but not to luminal deposits. In the other three models, albuminuria and luminal deposits were closely linked but did not correlate with glomerulosclerosis. Furthermore, LL and SL nephron proteinuria of NX rats was comparable while LL proteinuria was markedly greater than SL proteinuria in the other three models. The luminal deposit method provides a new way to analyze heterogeneity of proteinuria among nephrons and the mechanisms underlying structural change in experimental glomerular diseases.

Effects of microgravity on urinary osteopontin

Increased risk of renal stone formation during space flight has been linked primarily to increased calcium excretion from bone demineralization induced by space flight. Other factors contributing to increased risk include increased urinary calcium oxalate supersaturation, while urinary citrate, magnesium and volume are all decreased. The aim of this study was to increase the predictive value of stone risk profiles for crew members during space flight by evaluating the excretion of urinary protein inhibitors of calcium crystallization so that more comprehensive stone risk profiles could relate mineral saturation to the concentrations of inhibitor proteins. Levels of urinary osteopontin (uropontin) are reported in a series of 14 astronauts studied before, during, and after space flights. During space flight, a compensatory increase in uropontin excretion was not observed. However, the uropontin excretion of a majority of astronauts was increased during the period after space flight and was maximal at 2 wk after landing. The downward shift in the molecular size of uropontin observed in samples obtained during space flight was shown to result from storage at ambient temperature during flight, rather than an effect of microgravity on uropontin synthesis.

Quantitative studies of human urinary excretion of uropontin

Uropontin is the urinary form of osteopontin, an aspartic acid-rich phosphorylated glycoprotein. Uropontin has been previously shown to be a potent inhibitor of the nucleation, growth and aggregation of calcium oxalate crystals and the binding of these crystals to renal epithelial cells. Quantitative data defining the excretion of this protein are necessary to determine its role in urinary stone formation. In the present studies, we determined uropontin excretion rates of normal humans. Urine samples were obtained under conditions of known dietary intake from young adult human volunteers with no history, radiographic or laboratory evidence of renal disease. Urinary concentrations of uropontin were measured by a sensitive ELISA employing an affinity purified polyclonal antiserum to uropontin. Thirteen normal subjects ingested a constant diet providing 1 gram of calcium, 1 gram of phosphorus, 150 mEq of sodium and 1 gram of protein per kilogram of body wt per day during an eight day study period. The relationship of urinary volume to uropontin excretion was assessed by varying fluid intake on the last four days of the study to change the mean urine volume/24 hr by > 500 ml. Urine collected in six hour aliquots for eight days was analyzed for uropontin by ELISA, and for calcium, and creatinine. Daily uropontin excretion of 13 individual subjects was 3805 +/- 1805 micrograms/24 hr (mean +/- 1 SD). The mean urinary levels (1.9 micrograms/ml) detected in the present study are sufficient for inhibition of crystallization; our previous studies have demonstrated that the nucleation, growth and aggregation of calcium oxalate crystals and their binding to renal cells in vitro are inhibited by this concentration of purified uropontin. In contrast to the regular pattern of diurnal variation of calcium excretion seen in most subjects, uropontin excretion showed no regularity of diurnal variation and was not directly related to either calcium or creatinine excretion or changes in urinary volume. However, uropontin concentration varied inversely with urine volume (P < or = 0.001), so that the highest uropontin concentrations occurred when urine volume was the lowest. We conclude that the physiologic characteristic of an inverse relationship of uropontin concentration to urine volume favors protection from urinary crystallization of calcium oxalate by uropontin. Our quantitative definition of urinary uropontin excretion of normal adults provides the basis for the evaluation of uropontin excretion by individuals who have formed urinary stones.

Contribution of human uropontin to inhibition of calcium oxalate crystallization

Uropontin (UP) is known to inhibit the growth and nucleation of calcium oxalate monohydrate (COM) crystals, and it also impedes attachment of calcium oxalate crystals to cultured renal epithelial cells. However, its role in normal defense against renal crystallization, and in pathogenesis of nephrolithiasis is unclear. In this study we determined the effect of UP on aggregation of COM crystals as well as the inhibitory activity of UP on COM crystal growth and nucleation in a series of normal subjects, in order to assess the potential of UP as an important urinary inhibitor. The mean urinary excretion of UP measured by ELISA was 185 +/- 12 nmol/24 hr (mean +/- SEM) with a mean urine UP concentration of 131 +/- 13 nM. Uropontin isolated by immunoaffinity chromatography was a very potent inhibitor of COM crystal aggregation, with a mean UP concentration of 28 +/- 4 nM required for a 50% reduction in aggregation. The kDa for COM crystal growth inhibition determined from Langmuir type isotherms was 21 +/- 3 nM and the concentration required for 50% reduction in COM crystal growth rate was 16 +/- 2 nM. Inhibition of secondary nucleation was measured at a single concentration of 200 nM, which reduced the nucleation rate to 42 +/- 3% of control. Using a theoretical model of growth and aggregation inhibition at varying urine flow rates, we showed that inhibitory activity of UP would be significant for all subjects over a wide range of urine flow rates. Overall, UP is a potent inhibitor of COM aggregation as well as growth and nucleation. The urinary concentration of UP is in the range in which its contribution to inhibition of growth and aggregation are likely to be substantial. Thus, UP appears to be an important natural defense against renal crystallizations and nephrolithiasis.

Ring chromosome 4 mosaicism coincidence of oligomeganephronia and signs of Seckel syndrome

We present a patient with features suggestive of Seckel syndrome who was found to be mosaic for ring 4 chromosome. Seckel syndrome is a rare entity characterized by marked growth retardation, microcephaly, facies characterized by receding forehead and chin, large beaked nose, and severe retardation, usually thought to be inherited as an autosomal recessive condition. In addition, our patient had oligomeganephronia, a rare and usually sporadic renal malformation, previously reported in two other patients with abnormalities of chromosome 4. Besides pointing out the overlap between the Seckel phenotype and Wolf-Hirschhorn syndrome, our patient illustrates the need to consider cytogenetic studies in patients with the Seckel phenotype, so that accurate diagnoses can be given to families. Also, the case suggests that there may be a locus for oligomeganephronia distal to the Wolf-Hirschhorn critical region on 4p.

Progesterone regulates osteopontin expression in human trophoblasts: a model of paracrine control in the placenta?

Osteopontin (OPN), a matrix glycosylated phosphoprotein, has been proposed to play a role(s) in basic cellular processes, such as neovascularization and tissue remodeling, which are essential to placental morphogenesis and embryo implantation. We have shown OPN to be expressed by cytotrophoblasts of the chorionic villus, and a putative progesterone regulatory element in the OPN promoter suggests hormonal regulatory control. This led us to test the hypothesis that progesterone regulates OPN expression in human cytotrophoblasts. Cytotrophoblasts isolated from human placentas were treated with combinations of progesterone, RU486, and/or aminoglutethimide, and their expression of OPN was assessed by Northern hybridization and immunocytochemistry. The expression of OPN messenger RNA (mRNA) declined as trophoblasts aggregated, but rebounded at later times when syncytia and mononuclear cytotrophoblasts coexisted in culture. Progesterone increased OPN mRNA expression by aggregating mononuclear cytotrophoblasts. Aminoglutethimide suppression of endogenous steroidogenesis by syncytiotrophoblasts inhibited OPN expression, whereas the addition of exogenous progesterone to cells treated with aminoglutethimide reversed this inhibitory effect. These observations were confirmed at the protein level by immunocytochemistry. Treatment of cytotrophoblasts with both progesterone and RU486 inhibited the up-regulatory effect on OPN mRNA associated with exposure to progesterone alone, further confirming a direct effect of progesterone. We conclude that progesterone up-regulates OPN expression in human cytotrophoblasts, and we propose that in vivo, progesterone secretion by syncytiotrophoblasts regulates the expression of OPN by the underlying cytotrophoblasts. As the receptors for OPN, alpha(v) integrins, are expressed by syncytiotrophoblasts, we postulate that these paracrine regulatory mechanisms contribute to the adhesive and/or signaling events between the two trophoblast cell types of the chorionic villus.

Renal cell osteopontin production is stimulated by calcium oxalate monohydrate crystals

Specific anions in tubular fluid, including uropontin (UP), the urinary form of human osteopontin (OPN), block adhesion to renal tubular cells of the most common crystal in kidney stones, calcium oxalate monohydrate (COM). In this study, monkey renal epithelial cells (BSC-1 line) in monolayer culture constitutively secreted UP into the culture medium. COM crystals added to the medium avidly bound previously secreted UP, reducing its concentration by 46% one hour later. However, the net UP content of cultures after a 24-hour exposure to COM crystals was increased by 18%. Northern blotting showed that the constitutively expressed gene encoding human OPN was maximally stimulated in BSC-1 cells after exposure to COM crystals for 12 hours. Two other calcium-containing crystals, hydroxyapatite and brushite, did not alter OPN gene expression or protein production. OPN mRNA expression was enhanced in canine renal epithelial cells (MDCK line) after exposure to COM crystals for six hours, whereas the constitutive expression of murine OPN mRNA by 3T3 fibroblasts was unchanged. In vivo this glycoprotein could defend the cell against adhesion of crystals in tubular fluid, and/or promote renal interstitial fibrosis in subjects with heavy crystalluria.

Localization and regulation of the rat renal Na(+)-K(+)-2Cl- cotransporter, BSC-1

To investigate the role of the thick ascending limb (TAL) Na(+)-K(+)-2Cl- cotransporter in regulation of water excretion, we have prepared a peptide-derived polyclonal antibody based on the cloned cDNA sequence of the rat type 1 bumetanide-sensitive cotransporter, BSC-1 (also termed "NKCC-2"). Immunoblots revealed a single broad 161-kDa band in membrane fractions of rat renal outer medulla and cortex but not from rat colon or parotid gland. A similar protein was labeled in mouse kidney. Immunoperoxidase immunohistochemistry in rat kidney revealed labeling restricted to the medullary and cortical TAL segments. Because long-term regulation of urinary concentrating ability may depend on regulation of Na(+)-K(+)-2Cl- cotransporter abundance, we used immunoblotting to evaluate the effects of several in vivo factors on expression levels of BSC-1 protein in rat kidney outer medulla. Chronic oral saline loading with 0.16 M NaCl markedly increased BSC-1 abundance. However, long-term vasopressin infusion or thirsting of rats did not affect BSC-1 abundance. Chronic furosemide infusion caused a 9-kDa upward shift in apparent molecular mass and an apparent increase in expression level. These results support the previous identification of BSC-1 as the TAL Na(+)-K(+)-2Cl- transporter and demonstrate that the expression of this transporter is regulated.

Cell differentiation and endogenous cyclic adenosine 3',5'-monophosphate regulate osteopontin expression in human trophoblasts

Integrin receptors and their extracellular matrix ligands have been implicated in the molecular and cellular mechanisms of trophoblast adhesion and migration. In the present series of experiments, the regulation of expression of osteopontin (OPN), a secretory extracellular matrix protein that mediates cell adhesion by binding to members of the alpha V family of integrins was investigated. Human chorionic villi were obtained during the mid and late first trimester, early second trimester, and late third trimester of pregnancy. In addition, cytotrophoblasts were isolated from chorionic villi, and the effects of exposure of cultured cytotrophoblasts to cAMP agonists or antagonists were assessed. OPN messenger RNA (mRNA) was expressed by human trophoblasts in the first trimester and throughout pregnancy. Using immunolocalization in prepared tissue sections, cytotrophoblasts stained intensely for OPN, but syncytial trophoblasts did not. This differentiation-dependent expression was confirmed in vitro by demonstrating that freshly isolated mononuclear cytotrophoblasts exhibited a high level of OPN mRNA, but as the cells aggregated and fused to form multinucleated syncytia in vitro, mRNA levels decreased dramatically. 8-Bromo-cAMP inhibited the expression of OPN mRNA, whereas the cAMP antagonist Rp-cAMP inhibited the OPN mRNA decrease during the in vitro differentiation of the cells. This study demonstrates the regulated expression of OPN by human trophoblasts. In addition, our data suggest that this expression is dependent upon the state of cellular differentiation of the trophoblasts and is regulated by endogenous cAMP. We speculate that binding of OPN to its alpha V beta 3-integrin receptor may be a critical signaling pathway contributing to the integrity of the chorionic villus and may also play a role in maternal-embryonic communication during the process of placentation.

A biochemical characterization of the binding of osteopontin to integrins alpha v beta 1 and alpha v beta 5

Osteopontin (OPN) is an extracellular matrix protein that binds to integrin alpha v beta 3. Here we demonstrate that two other integrins, alpha v beta 1 and alpha v beta 5, are also receptors for OPN. Human embryonic kidney 293 cells adhere to human recombinant osteopontin (glutathione S-transferase-osteopontin; GST-OPN) using integrin alpha v beta 1. When the 293 cells are transfected with the beta 5 subunit, they can also adhere to GST-OPN using integrin alpha v beta 5. Divalent cations regulate the binding of GST-OPN to both alpha v beta 1 and alpha v beta 5. Mg2+ and Mn2+ support the binding of GST-OPN to these integrins but Ca2+ does not. The highest affinity is observed in Mn2+. In the presence of this ion, the affinity of GST-OPN for alpha v beta 1 is 18 nM and the affinity for alpha v beta 5 is 48 nM. The antibody 8A2, which is an agonist for beta 1, promotes the adhesion of 293 cells to GST-OPN even when Ca2+ is present. This observation suggests that cellular events could modulate the affinity of alpha v beta 1 for OPN. Collectively, these findings prove that integrins alpha v beta 1, alpha v beta 3, and alpha v beta 5 have similar affinity for OPN. Therefore, all three integrins must be considered when evaluating the biological affects of OPN.

Ca2+ suppresses cell adhesion to osteopontin by attenuating binding affinity for integrin alpha v beta 3

Osteopontin (OPN) is an extracellular matrix protein that supports osteoclast adhesion to the bone by binding to integrin alpha v beta 3. We measured the binding between OPN and integrin alpha v beta 3 with recombinant human OPN and the urinary form of human OPN, uropontin. Recombinant OPN was expressed in Escherichia coli as a fusion protein with glutathione S-transferase and cleaved from glutathione S-transferase with Factor Xa. The mass of this form of OPN (rOP27) is 27,046 Da. rOP27 is truncated at arginine residue 228, 69 amino acids short of the native carboxyl terminus. Uropontin and rOP27 support RGD-dependent cell adhesion and to bind purified integrin alpha v beta 3 with similar affinities. Further study showed that OPN is the only known naturally occurring RGD-containing protein with a much greater affinity for alpha v beta 3 than for the platelet integrin alpha IIb beta 3. Most importantly, we find that physiologic levels of Ca2+ block cell adhesion to OPN. Measurement of binding constants between rOPN and purified integrin alpha v beta 3 with surface plasmon resonance showed that the affinity between rOPN and alpha v beta 3 is 26-fold lower in Ca2+ (Kd = 1.1 x 10(-8) M) than in Mn2+ (Kd = 4.3 x 10(-10) M) and 9-fold lower than in Mg2+ (Kd = 1.3 x 10(-9) M). In bone, the resorbing osteoclast generates elevated levels of extracellular Ca2+, therefore the findings presented here suggest a previously unappreciated mechanism for the modulation of bone resorption by extracellular Ca2+.

Soluble alpha v beta 3-integrin ligands raise [Ca2+]i in rat osteoclasts and mouse-derived osteoclast-like cells

We evaluated the possible involvement of intracellular Ca2+ concentration ([Ca2+]i) changes in the action of alpha v beta 3-ligands, known to regulate osteoclast function. Rat osteoclasts or mouse osteoclast-like cells, as examined by microfluorimetry and fura 2, showed a transient [Ca2+]i increase when perfused with (all 0.1 microM) vitronectin, osteopontin, polypeptide echistatin, fibronectin, and Arg-Gly-Asp-Asp and Arg-Gly-Asp-Ser peptides (10(-4) M) but not with laminin, collagen I, collagen IV, or [Ala24]echistatin, in which Ala was substituted for Arg in the Arg-Gly-Asp complex. The threshold for echistatin was 10 pM, the 50% effective concentration was 1 nM, and the median [Ca2+]i increase was 420 nM above the resting level (217 +/- 22 nM) at saturating concentration of 0.1 microM. Echistatin did not cause Mn2+ influx, and 10 microM nifedipine, 10 nM omega-conotoxin, 5 mM Ni2+, or Cd2+ did not prevent [Ca2+]i change. However, extracellular Ca2+ was needed for the [Ca2+]i increase, probably enabling ligand-integrin interaction. Polyclonal and monoclonal (LM609) antibody as well as depletion of [Ca2+]i stores with 5 microM thapsigargin and Ca(2+)-free medium abolished the [Ca2+]i increase, after restoring extracellular Ca2+. Furthermore, the LM609 antibody induced a Ca2+ signal in the presence or absence of extracellular Ca2+, suggesting that the alpha v beta 3-ligand interaction is mediated at least partially by Ca2+ mobilized from intracellular stores.

Heterogeneity of osteopontin expression among nephrons in mouse kidneys and enhanced expression in sclerotic glomeruli

BACKGROUND

Osteopontin (OPN) is a secreted Ca(2+)-binding phosphoprotein able to mediate cell attachment to bone via an RGD sequence and the alpha v beta 3 integrin. OPN mRNA is found at high levels in the kidney, and the protein is found in the urine. Because published reports of where the protein is produced conflict, we undertook a comprehensive study to localize OPN expression.

EXPERIMENTAL DESIGN

In situ hybridization with a mouse cDNA probe and immunohistochemical staining with three different antisera to mouse OPN were used to identify those cells that contained significant levels of mRNA and protein, respectively.

RESULTS

Both methods of analysis revealed that OPN expression in the normal mouse kidney was primarily restricted to the thick ascending limbs of the loop of Henle and to the distal convoluted tubules. Protein was detected predominantly at the apical surface of cells lining the lumen of a subset of tubules. The alpha v beta 3 integrin, which is a receptor for vitronectin and osteopontin, was uniformly localized by immunostaining not on the apical surface but rather to the baso-lateral surface of cells in the distal part of the tubule. OPN expression was not detected in healthy glomeruli, proximal tubules, thin limbs of the loop of Henle, collecting ducts, or interstitial fibroblasts. In contrast to the localization of Tamm-Horsfall protein expression, in all distal nephrons, expression of OPN was detected by both methods of analysis in only some nephrons. OPN expression (relative to male mice) was somewhat increased in female, pregnant and lactating mice and markedly increased in the parietal epithelium of glomeruli undergoing sclerosis in aging mice. OPN was also detected in the macula densa.

CONCLUSIONS

OPN is synthesized and secreted into the tubule fluid by the luminal epithelia of the distal portions of a subset of kidney nephrons. As animals age expression is found in more proximal portions of the tubule. OPN may contribute to, or be a consequence of, glomerular sclerosis, and may be an indicator of subclinical injury or infection.

Immunomorphometric studies of proteinuria in individual nephrons of rats

BACKGROUND

Heterogeneity of proteinuria among nephrons has been directly shown by micropuncture analysis of experimental models. Since the number of nephrons per kidney that are accessible for micropuncture is very small, we have developed a new immunomorphometric method for directly studying proteinuria in a much larger number of individual nephrons. This new method is based on the luminal surfaces of thick ascending limb of the loop of Henle (TAL) cells collectively functioning as an immunoabsorbent column for anti-Tamm-Horsfall protein antibodies that enter the urinary filtrate of that nephron.

EXPERIMENTAL DESIGN

The distribution of luminal IgG deposits formed in TALs after injection of anti-Tamm-Horsfall protein antibodies was studied in three models of experimental proteinuria in rats to define the relationship between the formation of luminal deposits and the overall level of albuminuria and to test the utility of this method in analyzing the heterogeneity of luminal deposits among nephrons in other models.

RESULTS

A close relationship between the magnitude of albuminuria and the distances that luminal IgG deposits extended along the straight course of TALs in individual nephrons of rats injected with rabbit anti-Tamm-Horsfall protein antibodies was established in a model of proteinuria with a uniform pattern of glomerular hemodynamics, heterologous immune complex nephropathy in rats. In models with more heterogeneous glomerular hemodynamics, autologous immune complex nephropathy and aminonucleoside nephrosis, greater heterogeneity in luminal IgG deposit formation among nephrons was demonstrated. The distances that luminal IgG deposits extended along TALs was more variable in these models than in heterologous immune complex nephropathy rats with comparable levels of albuminuria.

CONCLUSIONS

Variability in the distances that luminal IgG deposits extend along TALs reflects heterogeneity among nephrons. The luminal deposit technique provides a new means for direct analysis of variability of dysfunction in many individual nephrons not accessible to micropuncture.

Attenuated expression of epithelial cell adhesion molecules in murine polycystic kidney disease

Polycystic kidney disease is an inherited disorder of parenchymal structure that leads to renal failure. Cysts begin as focal dilations in proximal tubules and collecting ducts, giving rise to cyst walls lined by a phenotypically disturbed epithelium that expresses dysfunctional transport and matrix proteins. We used an mRNA search protocol to probe efficiently for tissue-specific disturbances that might underlie the formation of cysts. This search assessed the relative abundance of transcripts encoding a variety of growth factors (transforming growth factor-beta 1, interleukin-6, tumor necrosis factor, and endothelin-1), structural proteins (collagen IV, nidogen, fibronectin, and laminins A and B1), and cell adhesion molecules (CAMs; E-cadherin, N-CAM, laminin receptor, and fibronectin receptor) in the cystic kidneys of cpk/cpk mice and uncovered a previously unrecognized early reduction in mRNA encoding N-CAM (54%) and E-cadherin (56%) (n = 5; P less than 0.001). Levels of transcripts for growth factors, structural proteins, and for fibronectin and laminin receptors in normal and cystic kidneys were generally similar. The reduction in transcripts for N-CAM and E-cadherin in kidneys from cystic mice was not observed in autologous liver. The immunofluorescent staining of cystic kidneys confirmed that the decrease in N-CAM and E-cadherin was generally confined to regions abundant in developing cystic epithelium. The presence of both N-CAM and E-cadherin appears to guide the sequential differentiation and polarization of normal renal epithelium, and their attenuated expression in the kidney of cpk/cpk mice may be a material factor contributing to the pathogenesis of cyst formation.

Tubular antigen-binding proteins repress transcription of type IV collagen in the autoimmune target epithelium of experimental interstitial nephritis

We have been studying immune interactions with somatic cells using a tubular antigen-binding protein (ThF) secreted by helper T lymphocytes harvested from mice that have an autoimmune form of interstitial nephritis called anti-tubular basement membrane disease. This ThF, although characterized originally because of its ability to induce effector T cells, additionally recognizes the nephritogenic 3M-1 antigen expressed by its target renal tubular epithelium. We believe these proteins, in general, may modulate directly some homeostatic functions in organ-derived cells, and now report that our ThF represses specifically the cellular transcription and secretion of basement membrane type IV collagen in tubular epithelium. These in vitro findings of reduced levels of mRNA encoding type IV collagen correlate well with in situ hybridization studies performed on kidneys expressing early autoimmune lesions, and predict a progressive drop in the expression of type IV collagen in the interstitium. Such a novel and unexpected repression of transcription of type IV collagen might easily impart or facilitate permanent change in the infrastructure of kidney architecture during autoimmune injury and, perhaps, contributes to the process of tubular atrophy attendant to prolonged renal inflammation.

Quantitative studies of tubular immune complex formation and clearance in rats

Tubular antibody deposition and clearance was quantitatively studied using affinity-purified rabbit antibodies to rat Tamm-Horsfall protein (TH), a surface membrane glycoprotein of the tubular cells of the thick ascending limb of the loop of Henle. Immune complexes are formed in situ at the base of these cells in rats injected with antisera to TH. The renal binding of I125-anti-TH was determined in pair label studies. Kidneys and other organs were removed from groups of rats for isotope counting at four hours to 14 days after an injection of I125-anti-TH and I131-normal rabbit IgG. The greatest total renal anti-TH binding after injection of 500 micrograms of anti-TH was observed at 24 hours in normal rats (18.55 +/- 1.6 micrograms). During the period of most rapid clearance (day 2 to day 7) the half life of renal anti-TH binding (84.2 hours) and the half life of anti-TH in the serum (68.5 hours) were shorter than that of IgG in the serum (117.8 hours). There was no substantial uptake of anti-TH by other organs. A close relationship between serum levels and renal uptake of anti-TH at 24 hours was also observed in rats given from 50 to 6000 micrograms of anti-TH; renal saturation was evident only at the highest dose. This close relationship was also present during the clearance phase in rats injected with 3700 micrograms of anti-TH; the half life of anti-TH was 96.2 hours in kidneys and 110 hours in serum while the half life of rabbit IgG in serum was 151.8 hours. Markedly increased renal uptake of anti-TH was observed in protein-uric rats with passive Heymann nephritis. In very proteinuric rats, 14.1% of the injected dose was bound to kidneys at 24 hours. In these rats, serum anti-TH levels decreased very rapidly to 4% of control serum levels by five days. Throughout the period of study, the serum levels of anti-TH determined by direct radiometric assay corresponded very closely to those obtained by enzyme-linked immunosorbent assay (ELISA). Urinary excretion was a major mechanism for the clearance of anti-TH in proteinuric rats; more than 10% of the injected I125-anti-TH was recovered intact (that is, protein bound) during the first day after injection. During the clearance phase for renal deposits, urinary clearance of anti-TH exceeded urinary clearance of IgG due to release of renal bound antibody into urine.(ABSTRACT TRUNCATED AT 400 WORDS)

"Intact nephrons" as the primary origin of proteinuria in chronic renal disease. Study in the rat model of subtotal nephrectomy

Single nephron filtration rate of albumin (SNGFRAlb) was measured in remnant nephrons of Munich-Wistar rats 4-6 wk after subtotal nephrectomy (NPX). Serial thin-section histological analysis was then conducted on the same glomeruli by light microscopy. SNGFRAlb ranged from 1 to 15 times normal. However, a direct relationship between abnormalities of structure and function was not seen, e.g. the glomeruli with the fewest structural abnormalities and marked hyperfiltration often had the highest SNGFRAlb. Moreover, the majority of glomeruli had minimal structural abnormalities. Normalization of the markedly elevated glomerular capillary pressure (PGC) in these glomeruli was accomplished by acute intravenous infusion of verapamil, which decreased SNGFRAlb by 9-83% without affecting the single nephron filtration rate of water (SNGFRH2O). 1-2 wk after subtotal NPX, all glomeruli were hyperfiltering and had elevated PGC. The fractional clearance of larger (greater than 36 A) dextrans was selectively increased in these glomeruli that lacked discernible damage by light microscopy. Verapamil normalized PGC, reduced proteinuria to 48 +/- 4% of baseline, and improved glomerular size selectivity without altering SNGFRH2O. Proteinuria after subtotal NPX thus originates largely from glomeruli with minimal structural abnormalities. The defect in size selectivity is largely attributed to the prevailing high PGC, producing large, nonselective channels on the glomerular capillary wall. The observations raise the possibility that in chronic renal diseases, the reduction in proteinuria often seen after therapeutic measures, including antihypertensive medication, may reflect their functional effect on the relatively intact glomeruli rather than their structure-sparing effect on severely damaged glomeruli, which contribute little to the proteinuria.

Extratubular Tamm-Horsfall protein deposits induced by ureteral obstruction in mice

The effects of unilateral ureteral obstruction were studied in mice. Obstruction for 24 hr led to the formation of extratubular Tamm-Horsfall protein (TH) aggregates within the renal interstitium and at the base of distal convoluted tubular (DCT) cells. These DCT deposits were shown by ultrastructural analysis to be entirely extracellular. They had the fibrillar substructure characteristic of TH and had not been seen after urinary obstruction in other species. As a consequence of retrograde flow of urine to glomeruli, obstruction also caused TH aggregates to form within Bowman's spaces. These glomerular casts of TH were detected throughout the 3-week period of study after the release of unilateral obstruction. High serum titers of IgG antibodies to TH developed in mice intradermally immunized with TH but were not observed after obstruction alone. Circulating anti-TH antibodies combined with TH present on the basal surfaces of the thick ascending limb of the loop of Henle cells and DCT cells to form immune complexes in situ. Interstitial inflammation in the areas surrounding subepithelial tubular immune deposits was not present in the kidneys of immunized mice and was not selectively induced by temporary obstruction. However, foci of inflammation were seen in all obstructed kidneys. At later times, inflammatory foci in previously obstructed kidneys were associated with progressive scarring, primarily in polar regions. The location and severity of these changes within kidneys produced by obstruction in immunized mice did not differ from those in unimmunized mice. The titers of anti-TH antibodies in immunized mice were not enhanced or depressed as a consequence of unilateral ureteral obstruction. These studies demonstrate that complete obstruction of urinary flow in the mouse for periods as short as 24 hr may lead to progressive segmental renal scarring. These studies further indicate that increasing the quantities of extracellular TH by obstruction does not facilitate inflammatory responses to TH immune complexes formed in situ. While exposure of renal tissue to highly toxic components of extravasated urine may play a crucial role in inflammatory responses, autoimmunity to TH was not implicated as a contributing factor by the present studies in mice.

Renal tubular immune complex formation in mice immunized with Tamm-Horsfall protein

Mice were given injections of rat Tamm-Horsfall protein (TH) in order to study immune complex formation in the distal nephron. After immunization, all mice had high serum levels of antibodies to TH; immune deposits consisting of IgG antibodies to TH and TH formed at the base of cells of the thick ascending limb of the loop of Henle (ALH) and distal convoluted tubule. These basal complexes were maximal in number and size within the cortical ALH, where they often radiated toward the luminal surfaces. Although the highest anti-TH antibody titers were found in animals with the most extensive deposits, antibody levels were not directly proportional to IgG deposits or to the time after immunization. The larger immune deposits were visualized by light microscopy as PAS-positive deposits. The quantity of these deposits was proportional to the time after immunization and was directly related to anti-TH antibody levels. Electron microscopy showed that these immune deposits were present within the basal and lateral intercellular spaces of the cells of the ALH. The distribution and localization of TH within the normal mouse kidney was very similar to that in rats. However, the distribution of immune complexes within the distal nephron in mice differed from rats similarly treated with TH and indicates either species differences in the distribution and/or organization of TH associated with tubular cell surfaces or that the accessibility of distal tubules to antibodies is species-dependent.

Nephronophthisis. A primary tubular basement membrane defect

In order to characterize abnormalities in nephronophthisis, renal tissues from four patients were studied by light and electron microscopies and immunofluorescence using antibodies to laminin, type IV collagen, and tubular basement membranes (TBM). There were constant morphological alterations affecting TBM of all segments of the nephron, with or without cysts. These included extreme thinning and attenuation, layering, and thickening of these structures which ranged in size from 36 nm to 2000 nm. A combination of these features often affected the same TBM simultaneously, with abrupt transitions between different lesions. Although the ultrastructural TBM aberrations were observed in a wide variety of other chronic and acute renal disorders, they rarely occurred to the extent as in nephronophthisis or with abrupt transitions, both suggesting diagnostic significance. Laminin and type IV collagen were present in normal intensity and distribution, however, anti-TBM antibody staining was inconstantly reduced, perhaps signifying lack of a normal antigenic component in the TBM. These findings may well indicate the fundamental defect in nephronophthisis to be production of abnormal TBM, similar to the glomerular basement membrane lesions and consequences in Alport's syndrome.

Studies of glomerular mesangial uptake and processing of macromolecules. I. Effect of polyvinyl alcohol-induced macrophages on uptake of iron dextran

The influence of prior glomerular mesangial uptake of a macromolecule that induces the infiltration of macrophages (M phi) into the mesangium on the uptake of a second macromolecule by the mesangium was studied in inbred Lewis rats. Renal transplantation of kidneys from rats previously injected with polyvinyl alcohol (PVA) was performed to avoid the potential influences of ongoing uptake of PVA and altered host milieu on the glomerular uptake of iron dextran (ID), a macromolecule that localizes primarily in the intrinsic mesangial cells of unmodified rats. In contrast to that in the recipient's native kidney, the uptake of ID was markedly increased in the glomeruli in kidneys previously exposed to PVA. This enhanced uptake was the consequence of the phagocytic activity of mesangial M phi elicited by and containing PVA since the site of increased ID content was shown by ultrastructural studies to be within mesangial M phi. Isogeneic renal transplantation per se did not influence mesangial function since the glomerular uptake of ID in donor and recipient kidneys was the same when donors were normal rats. In addition to the enhanced uptake of ID into lysosomes by mesangial M phi these cells were also much more active in the further processing of ID to ferritin particles within the cytoplasm than were intrinsic mesangial cells. These studies demonstrate that M phi attracted to the mesangium by a stimulus such as PVA may have important effects on the consequences of additional challenges to the mesangium.

Influence of renal transplantation in rats on glomerular and tubular immune complexes

Kidneys were transplanted from rats with immune complex nephritis to normal rats to determine the role of circulating antibodies to renal antigens in the maintenance of immune complexes. Glomerular subepithelial immune complexes in nine proteinuric donor rats, actively or passively immunized with proximal tubular antigen Fx1A (active and passive Heymann nephritis), were studied at the time of transplantation and two to four and one-half months after transplantation to normal unilaterally nephrectomized recipient rats. These subepithelial IgG deposits and proteinuria persisted throughout the four-month period after transplantation. Tubular subepithelial immune complexes in the kidneys of nine rats, actively immunized with a distal renal tubular antigen, Tamm-Horsfall protein, were studied at the time of transplantation when donors had high circulating IgG antibody titers to Tamm-Horsfall protein and at one to eight weeks after transplantation. The abundant granular and nodular immune complexes of rat IgG and Tamm-Horsfall protein at the base of tubular cells were very rapidly cleared during the first two weeks after transplantation and were virtually absent by four weeks. The rapid clearance of tubular immune complexes after transplantation of kidneys from actively immunized rats followed a time course similar to that after passive immunization with antisera to Tamm-Horsfall protein. These findings support the concept that high titers of circulating antibodies to Tamm-Horsfall protein are required to prevent the rapid clearance of tubular immune complexes, a process that is facilitated by conditions of antigen excess at the surface of tubular cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Sequential studies of glomerular crescent formation in rats with antiglomerular basement membrane-induced glomerulonephritis and the role of coagulation factors

Injection of presensitized rats with goat antiserum to rat glomerular basement membrane (GBM) results in a crescentic glomerulonephritis with typical linear deposition of goat IgG and host IgG and C3 along the GBM. Sequential renal biopsies from rats with this model of anti-GBM nephritis were studied by light, electron, and immunofluorescence microscopy to investigate the mechanisms of crescent formation. The localization of fibrin-related antigens (FRA) and factor VIII-related antigens (VIIIAGN) at different stages of disease was specifically studied since earlier studies of human glomerulonephritis had shown deposits of FRA without VIIIAGN in crescents and suggested that either a thrombin-independent mechanism might cause fibrin deposition or, alternatively, that glomerular fibrinolysis was relatively deficient within Bowman's space. Separation of the endothelium from the GBM or glomerular endothelial loss was the first change noted, a lesion that promotes intracapillary FRA deposition. Subsequently, intracapillary hypercellularity and GBM damage became progressively more severe. During the second and third weeks, coincident with early crescent formation, increasing amounts of FRA were noted within Bowman's space; FRA in crescents persisted throughout the study, whereas FRA within the glomerular tuft decreased over time. In early lesions, granular deposits of IgM and factor VIIIAGN were found diffusely within crescents. However, at later times, extracapillary deposits of these proteins and IgG were limited to the periphery of crescents within the markedly thickened and altered Bowman's capsule. Persistence of IgM and factor VIIIAGN at the periphery of older crescents appears to result from entrapment in areas less subject to cellular degradation and transport. Preferential clearance of FRA from the glomerular tuft suggests that fibrinolysis is less effective in Bowman's space than clearance mechanisms within glomerular capillaries.

Influence of altered glomerular permeability on renal tubular immune complex formation and clearance

Proteinuria was induced in rats to determine whether intravenously injected antibodies to a distal tubular antigen would bind to the luminal surfaces of distal tubular cells in vivo. Rats with proteinuria induced by an intravenous injection of sheep antisera to Fx1A (passive Heymann model) were injected 10 days later with rabbit antisera to Tamm-Horsfall protein. Linear rabbit IgG deposits along the luminal cell surfaces in the initial portion of the thick ascending limb of the loop of Henle (ALH) were demonstrated by immunofluorescence microscopy at 4 hours and were maximal 1 to 3 days after injection of antibodies to Tamm-Horsfall protein. The distance that these immune deposits extended along the ALH was directly proportional to the magnitude of proteinuria. Light microscopy showed periodic acid-Schiff-positive luminal deposits and an increased number of mitoses confined to the early ALH. Ultrastructural studies revealed continuous very electron-dense deposits initially covering the luminal surfaces of ALH cells. During the clearance phase, these deposits were surrounded and separated from ALH cell surfaces by a less electron-dense fibrillar material with the ultrastructural characteristics of Tamm-Horsfall protein. The mechanism of immune complex formation in the present study appears to involve the in situ combination of rabbit antibodies to Tamm-Horsfall protein in the glomerular filtrate with a tubular surface membrane antigen, Tamm-Horsfall protein.

Role of macrophages in the glomerular mesangial uptake of polyvinyl alcohol in rats

Glomerular mesangial cell populations were characterized through sequential morphologic studies after injection of polyvinyl alcohol (PVA), a synthetic polysaccharide previously shown to accumulate in the mesangium. Renal tissue was obtained from groups of rats, 1 day to 28 days after initial subcutaneous injection of PVA, and studied by transmission electron microscopy and fluorescence microscopy using monoclonal antibodies to rat Ia. PVA was initially present in areas of attenuated mesangial matrix. Subsequently, only a small fraction of PVA was associated with "classical" mesangial cells. By 3 days, variable degrees of glomerular hypercellularity were noted, and PVA appeared within phagocytic vacuoles of immature macrophages in the mesangium and in occasional endocytic vacuoles of endothelial and mesangial cells. At later time points, increasing quantities of PVA were found within the large confluent phagolysosomes of more mature mesangial macrophages. By 2 weeks, features of epithelioid transformation were evident. By 3 and 4 weeks many glomeruli contained several mesangial microgranulomas, i.e., collections of epithelioid cells of variable degree of maturation, including giant cells. An increase in Ia-positive cells in glomeruli corresponded closely to the increase in numbers of cells with ultrastructural characteristics of macrophages and epithelioid cells within mesangial regions. These studies demonstrate a primary role for macrophages in the uptake of PVA, a process ultimately leading to granuloma formation in the mesangium.

Uptake of polyvinyl alcohol by macrophages in the glomerular mesangium of rats. Histologic and functional studies

Rats received daily subcutaneous injections of the synthetic polysaccharide polyvinyl alcohol (PVA) for 1-28 days. The amount of PVA localized in the glomerular mesangium increased progressively during this time. By 28 days, all glomeruli showed extensive intracellular mesangial sequestrations of PVA, causing marked widening of mesangial areas, while the peripheral capillary loops were unaltered. Overall glomerular hypercellularity was mild to moderate, occurring mainly in areas of PVA deposition. Follow-up studies after 6, 12, 26, and 40 weeks revealed partial reduction of glomerular PVA masses. The PVA deposits were frequently associated with nonspecific esterase (NSE)-positive cells. The number of NSE-positive cells per glomerular tuft section increased from 0.1 in controls, to 2.1, 4.0, and 4.5 after 3, 14, and 28 days of PVA treatment, respectively. Similarly, glomerular counts for Ia-antigen-bearing cells rose from 2.1 in controls to 4.8 on Day 3 and showed further increases at later time periods with confluent staining of clusters of Ia-positive cells. In glomeruli, Ia-bearing cells were mainly noted in PVA-positive mesangial areas. These results indicate that PVA is taken up in the glomerulus primarily by cells that are NSE- and Ia-antigen-positive, suggesting that these cells are activated blood-borne monocyte-macrophages that sequester this polysaccharide. Clearance studies revealed that the glomerular filtration rate and effective renal plasma flow remained normal after 4 weeks of PVA injections. PVA-treated rats showed only mild elevations of urinary protein excretion. These findings indicate that confinement of marked structural and cellular alterations to the mesangium, even including the presence of infiltrating monocyte-macrophages, is compatible with absent or minimal dysfunction of the glomerular ultrafilter.

Identification of human hepatoma-defined cell surface molecules

BALB/c mouse splenocytes from mice immunized with cells of the human hepatoma line Hep G2 were fused with SP2/0-Ag 14 mouse myeloma cells. Two monoclonal antibodies recognizing antigenic determinants (Hag-1, Hag-2) of hepatoma cell surface molecules were investigated. Analysis of immunoprecipitates by sodium dodecyl sulfate (SDS) gel electrophoresis revealed that the Hag-1 antigenic determinant is born ona 115, kD MW glycoprotein, and that the second antibody immunoprecipitates a group of surface proteins with MW of 230 kD, 79 kD, 23 kD, and 20 kD from human hepatoma cells. These antigenic determinants are present on cell lines derived from other human tumors, thus neither of the antibodies is hepatoma-specific; cross-reactivity with human colorectal carcinoma and some mammary carcinoma cell lines is notable. Using indirect immunofluorescence on frozen sections Hag-1 was detected in one of three liver biopsies tested whereas Hag-2 was demonstrated in all three. Both antigens were detected in sections of human kidney with Hag-2 localized to the proximal tubules.

Role for intrarenal mechanisms in the impaired salt excretion of experimental nephrotic syndrome

A unilateral model of puromycin aminonucleoside (PAN)-induced albuminuria was produced in Munich-Wistar rats to examine the mechanisms responsible for renal salt retention. 2 wk after selective perfusion of left kidneys with PAN (n = 8 rats) or isotonic saline (control, n = 7 rats), increases in albumin excretion and decreases in sodium excretion were demonstrated in PAN-perfused but not in nonperfused kidneys of PAN-treated rats although systemic plasma protein concentration remained at control level. Total kidney glomerular filtration rate (GFR) and superficial single nephron (SN) GFR were also reduced selectively in PAN-perfused kidneys, on average by approximately 30%, due primarily to a marked decline in the glomerular capillary ultrafiltration coefficient (Kf), which was also confined to PAN-perfused kidneys. Values for absolute proximal reabsorption (APR) were also selectively depressed in PAN-perfused kidneys, in keeping with a similarly selective decline in peritubular capillary oncotic pressure measured in these kidneys, the latter also a consequence of the fall in Kf. In a separate group of seven PAN-treated rats, however, no differences were detected between PAN-perfused and nonperfused kidneys in the absolute amount of sodium reaching the early (0.77 +/- 0.09 neq/min vs. 0.74 +/- 0.08, P greater than 0.40) and late portions of superficial distal tubules (0.31 +/- 0.02) neq/min vs. 0.32 +/- 0.05, P greater than 0.50), despite the lesser filtered load of sodium in PAN-perfused kidneys. Suppressed sodium reabsorption in both proximal convoluted tubules and short loops of Henle of PAN-perfused kidneys contributed to this equalization of sodium delivery rates to the late distal tubule, as did comparable reabsorption along distal convolutions. In two additional groups of PAN-treated rats, infusion of saralasin (0.3 mg/kg per h, i.v.) led to substantial increases in total kidney GFR and SNGFR in PAN-perfused but not in nonperfused kidneys. Despite these increases in total and SNGFR, urinary sodium excretion by PAN-perfused kidneys remained at a level far below that for nonperfused kidneys, again indicating that the antinatriuresis characterizing the PAN-perfused kidney is due to alterations in sodium handling by the tubules rather than changes in GFR. These results therefore indicate (a) that reductions in Kf and depressed sodium reabsorption by proximal tubules and Henle's loop segments in this model are brought about by intrarenal rather than circulating or systemic factors, and (b) assuming that superficial nephrons are representative of the entire nephron population, renal salt retention in this model is due primarily to intrarenal factor(s) acting beyond the distal convolution.

Renal localization of Tamm-horsfall protein in unilateral obstructive uropathy in rats

The distribution of Tamm-horsfall protein (THP) within nephrons and in the renal interstitium of the kidney was examined in rats after unilateral ureteral ligation. Urinary casts containing THP were detected not only distal to the site of THP synthesis in cells of the ascending limbs of the loop of Henle, but also in more proximal portions of the nephron, suggesting retrograde intratubular movement of urine. THP-positive casts within Bowman's space of glomeruli were found by 6 hours and were maximal at 2 weeks after obstruction. At this time, THP was present in 25 per cent of all glomeruli, predominantly in central zones of the outer cortex. No morphologic evidence suggesting passage of THP across Bowman's capsule of these glomeruli was found. By contrast, interstitial THP aggregates resulting from urinary extravasation from tubules were detected throughout the study. These occurred earlier and were more numerous, albeit smaller, in the cortex where they were concentrated in periarterial and periglomerular sites. Multiple pathways for escape of urinary THP from tubules were demonstrated and included tubular ruptures and necrosis, forniceal tears, and venous polyps. Although occasional masses of THP in the interstitium were surrounded by inflammatory cells, the overall time course and distribution of interstitial THP deposits did not correlate closely with the development of widespread interstitial hypercellularity and scarring after obstruction. Although not directly linked to the pathogenesis of tissue injury in obstructive uropathy, THP provides an excellent marker of urinary extravasation and of the pathways of urine flow within the kidney under pathologic conditions.

Formation and clearance of tubulointerstitial immune complexes in kidney of rats immunized with heterologous antisera to Tamm-Horsfall protein

Tubulointerstitial immune complex nephritis was produced by passive immunization of rats with antisera to rat Tamm-Horsfall protein (TH), a surface membrane glycoprotein of the cells of the thick ascending limb of Henle's loop. Circulating anti-TH antibodies were deposited in the kidney after an intravenous injection of rabbit antisera to TH. These anti-TH antibodies combined with TH at the base of tubular cells in the thick ascending limb of Henle's loop and formed granular immune complexes in situ in the space between basal cell surface membranes and tubular basement membranes. Immune complexes were also selectively formed in this site during perfusion of isolated kidneys with antisera to TH. Tubular immune complexes containing immunoglobulin, complement, and TH were maximal during the first week after an intravenous injection while high circulating anti-TH antibody titers were present. As the antibody titers subsequently fell to undetectable levels, tubular immune complexes were rapidly cleared and were virtually absent 4 weeks after the injection. During this clearance phase, rabbit IgG and rat TH were detected in the renal interstitium and in renal hilar lymph nodes. The rapid clearance of subepithelial TH immune complexes contrasts with the prolonged persistance of both glomerular subepithelial immune complexes and basement membrane deposits formed after injection of heterologous antisera to other renal components. The process of rapid clearance of tubulointerstitial immune complexes may allow rapid reversibility of immune injury in tubulointerstitial nephritis.

Mechanism of glomerulotubular balance in the setting of heterogeneous glomerular injury. Preservation of a close functional linkage between individual nephrons and surrounding microvasculature

Autologous immune complex nephropathy (AICN), an experimental model for human membranous glomerulopathy, is characterized by marked heterogeneity in function from glomerulus to glomerulus. However, the fraction of the filtered load of fluid reabsorbed by the proximal tubule remains nearly constant from nephron to nephron, despite wide variation in single nephron glomerular filtration rate (SNGFR). To define the physiological mechanisms responsible for this marked variation in SNGFR values within a given kidney and for the remarkable preservation of glomerulotubular balance, the various determinants of fluid exchange across glomerular and peritubular capillary networks were evaluated in Munich-Wistar rats with AICN. For comparison, similar measurements were obtained in rats with the functionally more homogeneous lesion of heterologous immune complex nephropathy. In AICN rats studied approximately 5 mo after injection of renal tubule epithelial antigen (Fx1A), a high degree of glomerulus-proximal tubule balance was found, despite marked variations in SNGFR values within a single kidney. These changes were associated with marked heterogeneity in immunoglobulin and complement deposition within and among glomeruli. Although mean capillary hydraulic pressure and Bowman's space hydraulic pressure ranged widely from glomerulus to glomerulus, the mean glomerular transcapillary hydraulic pressure difference was remarkably uniform among these functionally diverse glomeruli and could not, therefore, be implicated as the cause of the dispersion in SNGFR values. The two remaining determinants of SNGFR, namely, glomerular plasma flow rate (Q(A)) and ultrafiltration coefficient (K(f)), varied markedly from glomerulus to glomerulus, but always in direct proportion to SNGFR, and proved to be responsible for the marked variation in SNGFR. The mean net peritubular capillary reabsorptive force ( P(r)) correlated closely with the absolute rate of fluid reabsorption in adjacent proximal tubules (APR) in AICN. Of the factors determining P(r), peritubular capillary hydraulic pressure was essentially constant in a given AICN kidney, whereas peritubular capillary plasma protein concentration and oncotic pressure varied directly with APR and largely accounted for the observed tight correlation between P(r) and APR. ON THE BASIS OF THESE OBSERVED CORRELATIONS, WE SUGGEST THAT THE CLOSE QUANTITATIVE COUPLING BETWEEN SNGFR AND APR IN INDIVIDUAL NEPHRONS IN AICN IS DUE TO THE FUNCTIONAL RESPONSE OF INDIVIDUAL GLOMERULI: those with the most pronounced declines in SNGFR are characterized by the most pronounced declines in Q(A) and K(f). The resultant low peritubular capillary oncotic pressure favors a decline in APR, thus favoring nearly perfect glomerulotubular balance. In glomeruli with higher SNGFR values, Q(A) and K(f) values are also higher. These changes in K(f) once again are capable of establishing the conditions in downstream peritubular capillaries, this time favoring augmented APR (i.e., high intracapillary oncotic pressure), again leading to nearly perfect glomerulotubular balance.

Ultrastructural studies of tubulointerstitial immune complex nephritis in rats immunized with Tamm-Horsfall protein

Rats immunized with Tamm-Horsfall protein, a surface membrane glycoprotein of cells of the ascending thick limb of Henle's loop, develop autoantibodies to Tamm-Horsfall protein and a tubulointerstitial nephritis selectively involving the thick ascending limb of Henle's loop. The localization of the immune deposits formed in this model was studied by transmission electron microscopy. The frequency, size, and distribution of electron-dense deposits closely paralleled that of granular and nodular deposits of rat IgG, C3, and Tamm-Horsfall protein at the base of the cells of the ascending thick limb of Henle's loop. The earliest electron-dense deposits were present in the extracellular space between basal infoldings of cell membranes and the tubular basement membranes. In the more advanced lesions, the larger electron-dense deposits were also present within the tubular basement membranes and between tubular cell membranes displacing tubular structures. Selective mononuclear cell infiltration around these tubules with deposits in this model of tubulointerstitial nephritis suggests that antibodies to Tamm-Horsfall protein cross tubular basement membranes and form immune complexes in situ by combining with the Tamm-Horsfall protein molecules of basal cell surface membranes.

Renal function in rats with unilateral proteinuria produced by renal perfusion with aminonucleoside

Left (L) renal perfusion with an aminonucleoside of puromycin (PA), was used to produce unilateral proteinuria in 15 rats to examine the mechanisms responsible for renal salt retention in the nephrotic syndrome. Thirteen control rats underwent L renal perfusion with isotonic saline. Animals were studied 8 (group I) or 13 (group II) days after perfusion. Renal perfusion with saline per se did not change the glomerular filtration rate, renal plasma flow, or absolute and fractional excretion of sodium (Na) from the perfused kidney. PA animals showed a significant decrease in glomerular filtration rate from the perfused kidney and a proportional decrease in the absolute excretion of Na from the PA perfused kidney as compared to the right kidney. The fractional excretion of Na was equivalent in the L and R kidneys of the PA animals. The mean absolute Na excretion from the nonproteinuric R kidney of PA rats was almost twice that of the R kidney of the controls. The increased Na excretion by the nonproteinuric kidney of the PA animals compensated for the sodium retention by the proteinuric kidney.

Familial nephrotic syndrome and focal segmental glomerulosclerosis

Three of five siblings developed a steroid-resistant nephrotic syndrome with focal segmental glomerulosclerosis within a four-month period. Two of the siblings with nephrotic syndrome (Patients 1 and 2) also have sickle cell anemia; the third (Patient 3) carries the thalassemia trait. The dizygotic twin brother of Patient 2 has sickle cell anemia, but does not have the nephrotic syndrome. The nephrotic syndrome of patient 1 was resistant to corticosteroid and cyclophosphamide therapy and she developed severe renal failure 14 months after onset. The nephrotic syndrome of Patients 2 and 3 was steroid resistant but was partially responsive to cyclophosphamide therapy. They have persistent proteinuria with mild elevation of serum creatinine concentration and hypertension 5 1/2 years after diagnosis. In this family, the nephrotic syndrome appeared unrelated to the specific hemoglobinopathy, HLA type or mixed lymphocyte culture responsiveness despite the similarity of the renal disease.

Recurrent nephrotic syndrome with three successive renal allografts

A girl with idiopathic steroid-resistant nephrotic syndrome was studied at onset of the disease at 6 years of age and during recurrences of the nephrotic syndrome after successive transplantations of three renal allografts. Focal glomerulosclerosis was demonstrated shortly after onset of the original disease with progression to renal insufficiency over 14 months. Recurrence of the nephrotic syndrome was documented 4 weeks following transplantation of the first renal allograft and a biopsy demonstrated focal segmental glomerulosclerosis without evidence of rejection. By 13 months, recurrent disease had caused renal failure requiring dialysis. After transplantation of the next long-functioning graft, the nephrotic syndrome rapidly recurred with urinary protein excretion of 8 g/day and normal glomerular filtration rate at 8 weeks. After the 5th month, progressive renal insufficiency developed and dialysis was resumed at 10 months after transplantation. The nephrectomy specimen showed advanced focal segmental glomerulosclerosis and chronic rejection. Early after the last renal allograft, she had mild persistent proteinuria and then developed nephrotic syndrome at 10 months which was attributed to chronic rejection. The graft was lost to delayed accelerated rejection at 17 months. The rapid recurrence of nephrotic syndrome after transplantation of each of the first two long-functioning allografts to this patient is most consistent with recurrence of original disease. However, observations in a subsequent graft demonstrated that rapid recurrence of the original disease is not inevitable.

Tubulointerstitial immune complex nephritis in rats immunized with Tamm-Horsfall protein

Rats immunized with Tamm-Horsfall protein (TH) developed autoantibodies to TH and tubulointerstitial nephritis with granular deposits of IgG, C3, and TH at the base of tubular cells of the thick ascending limb of Henle's loop and selective infiltration of leukocytes around this distal nephron segment. Selective IgG deposition at the base of cells in the tubular segment containing TH and the direct relationships of the amount of immune deposits and the severity of histologic lesions to serum levels of anti-TH antibodies suggest a pathogenetic role for the autoantibodies to TH. The immune deposits in this model of tubulointerstitial nephritis appear to be the consequence of in situ formation of immune complexes. This mechanism involves the diffusion of circulating antibodies to TH across tubular basement membranes and their combination with TH molecules associated with tubular cell surface membranes.

Pathophysiology of Tamm-Horsfall protein

Tamm-Horsfall protein, a renal glycoprotein present in normal urine, is the primary constituent of urinary casts. Immunoelectron microscopy has shown that this protein is localized selectively along surface membranes of the thick ascending loop of Henle. In this surface membrane site, the unique aggregation and gel formation of Tamm-Horsfall protein in response to increasing concentrations of electrolytes within physiologic ranges may influence the permeability characteristics of this nephron segment. These aggregation characteristics also play a role in pathologic conditions and lead to the prolonged persistence of interstitial Tamm-Horsfall protein deposits in several tubulointerstitial diseases. Recent studies have demonstrated immunologic responses to this protein, including an immune complex tubulointerstitial nephritis in rats mediated by autoantibodies to Tamm-Horsfall protein.

Tamm-Horsfall glycoprotein: ultrastructural immunoperoxidase localization in rat kidney

Tamm-Horsfall urinary glycoprotein (TH) is the primary constituent of urinary casts. The intracellular distribution of TH in normal rat kidney was determined by immunoelectron microscopy using horseradish peroxidase-labeled antibodies and compared with morphologic localization of TH by immunofluorescence and light immunoperoxidase microscopy. Electron microscopy revealed an intracellular localization of TH restricted to the thick ascending limb of the loop of Henle (ALH). In this nephron segment, TH was distributed on and between adjacent intercellular membranes and infolding intracellular membranes at the base of these cells, within Golgi vacuoles, apical vesicles, and on the luminal membranes. Macula densa cells were negative, although typical ALH cells across the lumen of the same tubular segment were positive. Other renal segments were negative for intracellular TH. The unique distribution of TH is consistent with the known function of the ALH as the diluting segment of the nephron. We speculate that the aggregation and gel formation of TH on and between ALH surface membranes may restrict water movement across the ALH. This influence on permeability would be an important role for TH in the generation of concentration gradients for the countercurrent multiplier system of the kidney.