Morphological and conformational studies of Tamm-Horsfall urinary glycoprotein

Identification of a Novel TGF-β-Binding Site in the Zona Pellucida C-terminal (ZP-C) Domain of TGF-β-Receptor-3 (TGFR-3)

The zona pellucida (ZP) domain is present in extracellular proteins such as the zona pellucida proteins and tectorins and participates in the formation of polymeric protein networks. However, the ZP domain also occurs in the cytokine signaling co-receptor transforming growth factor β (TGF-β) receptor type 3 (TGFR-3, also known as betaglycan) where it contributes to cytokine ligand recognition. Currently it is unclear how the ZP domain architecture enables this dual functionality. Here, we identify a novel major TGF-β-binding site in the FG loop of the C-terminal subdomain of the murine TGFR-3 ZP domain (ZP-C) using protein crystallography, limited proteolysis experiments, surface plasmon resonance measurements and synthetic peptides. In the murine 2.7 Å crystal structure that we are presenting here, the FG-loop is disordered, however, well-ordered in a recently reported homologous rat ZP-C structure. Surprisingly, the adjacent external hydrophobic patch (EHP) segment is registered differently in the rat and murine structures suggesting that this segment only loosely associates with the remaining ZP-C fold. Such a flexible and temporarily-modulated association of the EHP segment with the ZP domain has been proposed to control the polymerization of ZP domain-containing proteins. Our findings suggest that this flexibility also extends to the ZP domain of TGFR-3 and might facilitate co-receptor ligand interaction and presentation via the adjacent FG-loop. This hints that a similar C-terminal region of the ZP domain architecture possibly regulates both the polymerization of extracellular matrix proteins and cytokine ligand recognition of TGFR-3.

Zona pellucida domain proteins

Many eukaryotic proteins share a sequence designated as the zona pellucida (ZP) domain. This structural element, present in extracellular proteins from a wide variety of organisms, from nematodes to mammals, consists of approximately 260 amino acids with eight conserved cysteine (Cys) residues and is located close to the C terminus of the polypeptide. ZP domain proteins are often glycosylated, modular structures consisting of multiple types of domains. Predictions can be made about some of the structural features of the ZP domain and ZP domain proteins. The functions of ZP domain proteins vary tremendously, from serving as structural components of egg coats, appendicularian mucous houses, and nematode dauer larvae, to serving as mechanotransducers in flies and receptors in mammals and nonmammals. Generally, ZP domain proteins are present in filaments and/or matrices, which is consistent with the role of the domain in protein polymerization. A general mechanism for assembly of ZP domain proteins has been presented. It is likely that the ZP domain plays a common role despite its presence in proteins of widely diverse functions.

Role of polymeric Tamm-Horsfall protein in cast formation: oligosaccharide and tubular fluid ions

BACKGROUND

In acute tubular necrosis (ATN), distal tubules are obstructed by casts formed by tubular debris, cells, and Tamm-Horsfall protein (THP). Since there are Arginine-Glycine-Aspartate (RGD) and Leucine-Aspartate-Valine (LDV) adhesive sequences in human THP, there may be direct integrin-mediated binding of tubular cells to THP. Alternatively, polymerization of THP may result in entrapment of the cells in its gel.

METHODS

Adhesion of LLC-PK(1) cells to THP-coated wells was directly measured. THP concentrate was dissolved in solutions which mimic urine from ATN (ATN-S), distal convoluted tubule (DCT-S), collecting duct (CD-S), and monomeric buffer (M buffer). THP was also denatured by either boiling or N-glycanase digestion. Gel formation of THP was then measured. Inhibition of LLC-PK(1) cell adhesion to collagen type I was measured with each solution, as well as after the collagen was pretreated with either monomeric (mTHP) or polymeric (pTHP) THP. The effect of pTHP on the settling rate of LLC-PK(1) cells in suspension was also measured.

RESULTS

LLC-PK(1) cells did not directly adhere to THP, a finding against integrin-mediated binding as a mechanism for in vivo tubular cell/THP cast formation. The high electrolyte concentration of ATN-S and CD-S, however, was associated with pTHP gel formation. Moreover, cells suspended in pTHP remained in suspension. In cell adhesion studies, mTHP attenuated cell adhesion by binding to the matrix, whereas pTHP attenuated cell adhesion by trapping cells in its gel. An active process was involved since both pTHP gel formation and attenuation of cell adhesion were abolished by boiling or oligosaccharide removal with N-glycanase digestion.

CONCLUSIONS

With renal ischemia and proximal tubule cell shedding, ATN and collecting duct fluid composition enhance THP gel formation and thus favor tubular cast formation and obstruction. The present in vitro results indicate the importance of oligosaccharide residues in mediating the effect of the THP on gel formation and potential cast formation in ATN.

Binding of human neutrophils to cell-surface anchored Tamm-Horsfall glycoprotein in tubulointerstitial nephritis

BACKGROUND

Human Tamm-Horsfall glycoprotein (T-H) is a glycosylphosphatidylinositol-anchored protein exposed at the surface of distal nephron cells, and urinary T-H is the released soluble counterpart. The latter has been implicated in tubulointerstitial nephritis, and the proinflammatory potential has been related to its ability to bind in vitro human neutrophils (PMNs). We have examined the conditions required for the binding of neutrophils to cell-surface anchored T-H and the consequent effects.

METHODS

A HeLa cell-line derivative permanently transformed with human T-H cDNA and expressing T-H at the cell surface was used throughout the study. The adhesion of PMNs to cells expressing T-H was analyzed by immunofluorescence microscopy before and after the opsonization of cells with anti-T-H antibodies. The oxidative burst induced by adhesion of PMNs to the cells was determined by the activation of myeloperoxidase. Quantitative and qualitative changes in the release of T-H under the adhesion of activated PMNs were determined by dot-blot and Western blot analysis.

RESULTS

No binding of neutrophils to cell-surface-anchored T-H was observed. On the contrary, the opsonization of cells with anti-T-H antibodies resulted in a dramatic adhesion of neutrophils. Such an adhesion induced the oxidative burst of PMNs and a large increment in the release of T-H, as well as the release of the slightly faster migrating T-H form, which is normally retained intracellularly.

CONCLUSIONS

These results support the notion that, after the autoimmune response, the adhesion of neutrophils to cell-surface T-H contributes to the pathogenesis of tubulointerstitial nephritis, favoring a further accumulation of T-H in the interstitium and inducing the loss of cell integrity via reactive oxygen metabolites generated by activated neutrophils.

Scanning electron microscopy of human urine and purified Tamm-Horsfall's glycoprotein

Dialyzed and dehydrated human urine and purified Tamm-Horsfall's glycoprotein revealed similar morphology by scanning electron microscopy. Single filaments, with diameters between 15-45 nm, splitting off and merging with thicker fibers at irregular intervals made up a three-dimensional meshwork with submicrometer pores. The resulting "fishing net" is capable of trapping microorganisms and may facilitate their elimination from the urinary tract by micturition. Tamm-Horsfall's glycoprotein may accordingly be a factor protecting against urinary tract infections.

Uromucoid (Tamm-Horsfall glycoprotein) forms different polymeric arrangements on a filter surface under different physicochemical conditions

Normal human urine cannot be forced through a 0.2 micron filter. To investigate the reason for this phenomenon, uromucoid (Tamm-Horsfall protein) was purified from human urine and its capacity to block a 0.2 micron Millipore filter was measured under different conditions. In the presence of cations (H+, Na+, Ca2+) uromucoid blocked the filter. The blocking varied with cation concentration. Scanning electron microscopy of the filter surface revealed different arrangements of polymerized uromucoid coating the filter surface depending on ionic conditions. In the presence of 100 mmol/l NaCl or 1 mmol/l CaCl2 uromucoid polymers were present in a fibrous arrangement. In the presence of both NaCl and CaCl2 a dense mat of uromucoid polymers was present together with clumps of aggregated polymer. In the absence of ions uromucoid formed a homogeneous coat on the filter surface (as demonstrated by scanning electron microscopy, Western blotting and 125I-uromucoid binding studies) but did not block the filter. Similar fibrous and highly aggregated arrangements of uromucoid polymer were seen in hyaline casts from urine. These data are consistent with the concept that the uromucoid glycoprotein can exist in several different polymeric forms under different ionic conditions.

Procoagulant activity in normal human urine associated with subcellular particles

Procoagulant activity (PCA) in normal human urine was found to be sedimented by centrifugation at X 100,000g. Therefore, studies were done to identify the structures associated with the procoagulant activity. Transmission electron microscopy of the X 100,000g pellet revealed numerous membrane-bound vesicles as well as fibrous material. Filtration of normal urine through a 0.2-micron filter removed more than 90% of the procoagulant activity. Scanning electron microscopy of the filter surface revealed 0.1 to 1.1 micron particles and fibrous material. By centrifugation at pH 3 and 5 the fibrous material and particles were separated. The procoagulant activity remained with the particles in each case. The fibrous material was shown to be Tamm-Horsfall protein by SDS-PAGE and Western blotting using anti-Tamm-Horsfall protein serum. Purified Tamm-Horsfall protein itself was not procoagulant. Therefore, PCA in normal human urine is associated with particles 0.1 to 1.1 micron in diameter which appear to be lipid membranes in various arrangements.

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.

Comparative conformational analysis of human choriomammotropin and somatotropin from several species

The conformations of porcine somatotropin and human choriomammotropin have been studied using circular dichroism (CD) and the results compared with spectra of human, murine, ovine, and bovine somatotropin. The far ultraviolet CD spectra of the six proteins were similar, and each spectrum was analyzed using constrained linear least squares. The following average percentages of alpha-helicity, beta-structure, and aperiodic (nonhelical) conformation were obtained: 57, 6, and 37, respectively, based on a standard protein reference set, and 42, 22, and 36, respectively, based on poly-L-lysine as reference. Thus, the estimated secondary structure is strongly dependent upon the reference data used. Interestingly for these similar proteins, it appears that over 60% of the residues are part of ordered secondary structure and less that 40% are in an aperiodic conformation. The near ultraviolet CD spectra of these hormones were similar in many respects, although certain significant differences were observed, particularly in the sign of various extrema. These spectral differences probably reflect non-identical microenvironments of the aromatics and disulfides, arising from differences both in amino acid sequence and local conformation.

Physical properties of Tamm-Horsfall glycoprotein and its glycopolypeptide

The molecular weight of the constituent glycopolypeptide chain of T-H glycoprotein was determined by sedimentation equilibrium under two entirely different sets of denaturing conditions. For both sets of denaturing conditions, the average molecular weight estimated for T-H glycopolypeptide was 74,000. The gel chromatographic behavior in 6M guanidium chloride of T-H glycopolypeptide with disulfide bonds intact as compared with its gel chromatographic behavior with disulfide bonds broken indicated that the glycopolypeptide is highly constrained by intrachain disulfide bonds.

Circular dichroism of human urinary Tamm-Horsfall glycoprotein

Human urinary Tamm-Horsfall glycoprotein, which contains 28% carbohydrate, has a monomeric molecular weight of about 80,000 but is isolated from urine in the form of intertwining helical suprastructures with molecular weights greater than 10(7). The native glycoprotein was dissociated and denatured with 6 M guanidinium chloride and was subsequently renatured by dialysis against a Tris-HCl buffer. Using sedimetation equilibrium, the renatured glycoprotein was characterized by a Mw cell of 256,800 and a Mz cell of 356,000. The ratio, Mz/Mw, of 1.39 indicates some polydispersity with regard to molecular size. There was no evidence of helical suprastructures in the renatured glycoprotein as judged by electron microscopy. Ca2+ concentrations of up to 50 mM failed to precipitate the renatured glycoprotein; in contrast, the native glycoprotein is precipitated by Ca2+ concentrations between 5-10 mM. The circular dichroic spectrum of renatured Tamm-Horsfall glycoprotein was obtained, resolved, and tentative band assignments made. The spectrum, which is quite similar to that of native Tamm-Horsfall glycoprotein, exhibited negative extrema at 269 nm (due in large part to disulfides and tyrosines) and at 215 nm (due to protein beta-structure and the N-acetylated hexosamines). The alpha-helical content of the glycoprotein was estimated to be no more than 10% and the amount of beta-structure to be about 33%; these values were not affected by the presence of Ca2+ (1 mM). A glcopeptide fraction (ca. 90% carbohydrate), prepared by extensive pronase digestion of the reduced, S-carboxymethylated glycoprotein, exhibited an ellipticity extremum at 212 nm of + 4,750 deg-cm2/dmole, referred to the concentration of (N-acetylated) hexosamines and neuraminic acid.

Acid phosphomonoesterase of human prostate. Carbohydrate content and optical properties

Human prostatic acid phosphatase I, a glycoprotein, has been analyzed with respect to its quantiative carbohydrate composition and its fluorescence, optical rotatory and circular dichroic spectra. The protein of 89 000 molecular weight has 38 to 41 carbohydrate residues attached, of which 3 residues are fucose, 4 are galactose, 11 mannose, 15 glucosamine and there are 7 to 8 residues of sialic acid. The native glycoprotein contains about 30% alpha-helix as estimated from the rotational and dichroic spectra. Upon removal of sialic acid by neuraminidase treatment, there is a small increase in this value, while the fluorescence intensity at he emission maximum (357 nm) is distinctly increased. These effects suggest that an interaction exists of the sialic acid group with parts of the native protein. Allowance is made for the contribution of the carbohydrate components in interpreting the spectra in structural terms.

Autoantibodies to Tamm-Horsfall protein, a tool for diagnosing the level of urinary-tract infection

In sera from ten girls with acute pyelonephritis, antibodies of the IgG-class to Tamm-Horsfall protein were found in significantly increased titres; in contrast, titres in sera from six girls with acute cystitis did not differ from those of healthy controls. The results suggest that determination of Tamm-Horsfall antibodies may prove useful for differentiating between upper and lower urinary-tract infection.