Marino Asselman

Calcium Oxalate Crystal Adherence to Hyaluronan-, Osteopontin-, and CD44-Expressing Injured/Regenerating Tubular Epithelial Cells in Rat Kidneys

Retention of crystals in the kidney is an essential early step in renal stone formation. Studies with renal tubular cells in culture indicate that hyaluronan (HA) and osteopontin (OPN) and their mutual cell surface receptor CD44 play an important role in calcium oxalate (CaOx) crystal binding during wound healing. This concept was investigated in vivo by treating rats for 1, 4, and 8 d with ethylene glycol (0.5 and 0.75%) in their drinking water to induce renal tubular cell damage and CaOx crystalluria. Tubular injury was morphologically scored on periodic acid-Schiff-stained renal tissue sections and tissue repair assessed by immunohistochemical staining for proliferating cell nuclear antigen. CaOx crystals were visualized in periodic acid-Schiff-stained sections by polarized light microscopy, and renal calcium deposits were quantified with von Kossa staining. HA was visualized with HA-binding protein and OPN and CD44 immunohistochemically with specific antibodies and quantified with an image analyzer system. Already after 1 d of treatment, both concentrations of ethylene glycol induced hyperoxaluria and CaOx crystalluria. At this point, there was neither tubular injury nor crystal retention in the kidney, and expression of HA, OPN, and CD44 was comparable to untreated controls. After 4 and 8 d of ethylene glycol, however, intratubular crystals were found adhered to injured/regenerating (proliferating cell nuclear antigen positive) tubular epithelial cells, expressing HA, OPN, and CD44 at their luminal membrane. In conclusion, the expression of HA, OPN, and CD44 by injured/regenerating tubular cells seems to play a role in retention of crystals in the rat kidney.

Crystal-cell interaction in the pathogenesis of kidney stone disease.

Purpose of review Renal stone formation depends not so much on the formation of crystals, but on their retention in the kidney. Evidence has emerged that crystal retention is caused predominantly by the adherence of crystals to the epithelial cells lining the renal tubules. Understanding the mechanisms involved in crystal retention could lead to new therapeutic approaches for interfering with the renal stone-forming process in patients. Cell-culture studies have been performed to obtain insights into the susceptibility of the cell surface to crystal attachment, and to uncover cell-surface crystal-binding molecules. This review aims to put the relevant publications of the last decade in perspective. Recent findings Crystal-cell interaction has been investigated by using various renal tubular cell types in culture. Such studies have yielded several candidate crystal-binding molecules, including phosphatidylserine, sialic acid, collagen IV, osteopontin and, recently, hyaluronan. Summary Here, the results obtained in crystal-binding studies are recapitulated, compared and evaluated. Arguments are provided in support of the view that many of the proposed crystal-binding molecules could be linked in the series of events resulting in crystal retention. Under pathological conditions, pericellular matrices rich in the polysaccharide hyaluronan are proposed as the key binding substance for crystals at the surface of renal tubular cells.

Fructose intake as a risk factor for kidney stone disease

Taylor and Curhan report that consumption of fructose is independently associated with an increased risk of incident kidney stones. What could be the mechanisms underlying the relation between fructose intake and stone risk? And how should we incorporate this finding into the dietary advice that we give to our patients to prevent kidney stone formation?

Pericellular Matrix Formation by Renal Tubule Epithelial Cells in Relation to Crystal Binding

Background/Aim: Retention of crystals in the kidney ultimately leads to renal stone formation. Hyaluronan (HA) has been identified as binding molecule for calcium oxalate monohydrate crystals. The association of high molecular mass (Mr) HA with cell surface receptors such as CD44 gives rise to pericellular matrix (PCM) formation by many eukaryotic cells in culture. Here, we study the ability of several renal tubular cell lines to assemble PCMs and to synthesize high-Mr HA during proliferation in relation to crystal retention. Methods: PCM assembly by MDCK-I, MDCK-II, and LLC-PK1 cells was visualized by particle exclusion assay. Metabolic labeling studies were performed to estimate the cellular production of HA. The expression of CD44 and HA was studied using fluorescent probes, and crystal binding was quantified with radiolabeled calcium oxalate monohydrate. Results: PCMs were formed, and HA was expressed by most MDCK-I and some MDCK-II, but not by LLC-PK1 cells. All cell types expressed CD44 at their apical surface. MDCK-I and MDCK-II cells secreted, respectively, 14.7 ± 1.6 and 0.5 ± 0.2 pmol [3H]glucosamine incorporated in high-Mr HA, whereas LLC-PK1 cells did not secrete HA. Streptomyces hyaluronidase treatment significantly decreased crystal binding (µg/cm2) to MDCK-I cells (from 8.6 ± 0.4 to 3.9 ± 0.9), but hardly to MDCK-II cells (from 10.2 ± 0.2 to 9.6 ± 0.1) or LLC-PK1 cells (from 10.2 ± 0.8 to 9.9 ± 0.3). Conclusions: There are various forms of crystal binding to renal tubular cells in culture. Crystal attachment to MDCK-I and some MDCK-II cells involves PCM assembly that requires high-Mr HA synthesis. HA production and PCM formation do not play a role in crystal binding to LLC-PK1 and the majority of MDCK-II cells. It remains to be determined which form of binding is involved in renal stone disease.

Hyaluronan and Stone Disease

Kidney stones cannot be formed as long as crystals are passed in the urine. However, when crystals are retained it becomes possible for them to aggregate and form a stone. Crystals are expected to be formed not earlier than the distal tubules and collecting ducts. Studies both in vitro and in vivo demonstrate that calcium oxalate monohydrate crystals do not adhere to intact distal epithelium, but only when the epithelium is proliferating or regenerating, so that it possesses dedifferentiated cells expressing hyaluronan, osteopontin (OPN) and their mutual receptor CD44 at the apical cell membrane. The polysaccharide hyaluronan is an excellent crystal binding molecule because of its negative ionic charge. We hypothesized that the risk for crystal retention in the human kidney would be increased when tubular cells express hyaluronan at their apical cell membrane. Two different patient categories in which nephrocalcinosis frequently occurs were studied to test this hypothesis (preterm neonates and kidney transplant patients). Hyaluronan (and OPN) expression at the luminal membrane of tubular cells indeed was observed, which preceded subsequent retention of crystals in the distal tubules. Tubular nephrocalcinosis has been reported to be associated with decline of renal function and thus further studies to extend our knowledge of the mechanisms of retention and accumulation of crystals in the kidney are warranted. Ultimately, this may allow the design of new strategies for the prevention and treatment of both nephrocalcinosis and nephrolithiasis in patients.