The binding of hydrogen and calcium ions by Tamm-Horsfall glycoprotein

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.

Tamm-Horsfall glycoprotein--inhibitor or promoter of calcium oxalate monohydrate crystallization processes?

The processes of calcium oxalate monohydrate (COM) crystal nucleation, growth and aggregation (agglomeration) generally have been studied using a wide variety of assay systems/conditions. This paper reviews the apparently conflicting data on the effects of Tamm-Horsfall glycoprotein (THP) on COM crystallization processes in vitro, with the main emphasis on crystal aggregation. According to its well-known physico-chemical properties, THP has a dual role in modifying crystal aggregation: at high pH and low ionic strength (IS). THP is a powerful crystal aggregation inhibitor. Upon lowering pH and raising IS, THP viscosity increases, leading to reduced crystal aggregation inhibition. In the presence of additional calcium ions, some THPs even become strong promoters of crystal aggregation. This phenomenon seems to be more pronounced in THPs isolated from recurrent calcium stone formers whose proteins exhibit an abnormally high tendency of polymerization. Recent studies suggest an inherited molecular abnormality of THP among some severe recurrent calcium stone formers.

Rapid isolation of Tamm-Horsfall glycoprotein (uromodulin) from human urine

An isolation method for Tamm-Horsfall protein is described which is based on the observation that a diatomaceous earth filter is able to retain most of the glycoprotein present in urine and that the glycoprotein is easily desorbed from the filter by deionized water. This behaviour depends on the tendency of Tamm-Horsfall glycoprotein at normal urinary concentrations to form a gel in a solution containing mono- and divalent ions. By means of two-step filtration, the glycoprotein was purified to homogeneity. The yield was of about 20 mg/l of urine, and the time required for the isolation was approximately 5-6 h. This procedure should be particularly useful for preparing large amounts of Tamm-Horsfall glycoprotein oligosaccharides in order to investigate their potential use as immunosuppressive agents both in vitro and in vivo.

Ultrastructural localization of Tamm-Horsfall protein in human kidney using immunogold electron microscopy

The ultrastructural localization of Tamm-Horsfall protein (THP) was studied in paraformaldehyde-fixed human renal biopsies. Pre-embedding and post-embedding immunogold labelling techniques were developed utilizing a monoclonal antibody specific for human urinary THP. With the pre-embedding technique, membrane contrast was enhanced by osmification thus allowing precise localization of gold particles. Reasonable tissue penetration of antibodies was achieved without compromising ultrastructural detail. The hydrophilic resin LR White was used for post-embedding labelling to ensure maximum penetration of antibodies. However, sections had only mild osmification and consequently localization of label was less certain. Both labelling techniques gave similar results. THP was found to be associated with two renal cell types. Epithelial cells lining the thick ascending limb of Henle's loop had gold label closely associated with the whole cell plasmalemma, with some of these cells having an apparently random distribution of label throughout the cytoplasm. Only the luminal plasmalemma of epithelial cells lining distal convoluted tubules were found to be labelled. Basolateral membranes and the cytoplasm of these cells were negative. The use of a monoclonal antibody of defined specificity combined with the two immunolabelling procedures represents a precise reliable method for studying ultrastructural localization of THP in the human kidney.

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.

Isolation and characterisation of glycopeptides from digests of human Tamm-Horsfall glycoprotein

Glycopeptides were isolated from pronase digests of human Tamm-Horsfall glycoprotein and its asialo derivative. The carbohydrate moiety of the major glycopeptide preparation isolated from the former digests had an apparent molecular weight of 4300, and those of two glycopeptides isolated from the latter digests had molecular weights of 3600 and 2300. These data, together with the compositions of the glycopeptides, indicate that the Tamm-Horsfall glycoprotein has at least five asparagine residues substituted by complex carbohydrate moieties, three being of one type, relatively rich in galactose, and two containing more sialic acid but less galactose. A small amount of a mannose-rich glycopeptide was also recovered from the digests of the Tamm-Horsfall glycoprotein.

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.

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.

Determination of the Tamm and Horsfall glycoprotein in human urine

A method for the routine determination of the Tamm and Horsfall glycoprotein in human urine is presented. The method is based on quantitative electroimmunoassay. It was demonstrated that the solubility of the Tamm and Horsfall glycoprotein was dependent on urinary ionic strength and pH. To minimize these variables all urines were buffered to pH 5.5 and then preincubated at 37 degrees C in 0.3% sodium dodecyl sulphate. This procedure solubilized and maintained the Tamm and Horsfall glycoprotein in solution. Excess sodium dodecyl sulphate was removed by gel filtration before the quantities of the Tamm and Horsfall glycoprotein were determined by electroimmunoassay. The method is simple, reproducible, has high precision and compares favourably with earlier published methods for the determinations of the Tamm and Horsfall glycoprotein.