Ethylene glycol induces calcium oxalate crystal deposition in Malpighian tubules: a Drosophila model for nephrolithiasis/urolithiasis

Drosophila melanogaster as an emerging translational model of human nephrolithiasis

PURPOSE

The limitations imposed by human clinical studies and mammalian models of nephrolithiasis have hampered the development of effective medical treatments and preventive measures for decades. The simple but elegant Drosophila melanogaster is emerging as a powerful translational model of human disease, including nephrolithiasis. It may provide important information essential to our understanding of stone formation. We present the current state of research using D. melanogaster as a model of human nephrolithiasis.

MATERIALS AND METHODS

We comprehensively reviewed the English language literature using PubMed®. When necessary, authoritative texts on relevant subtopics were consulted.

RESULTS

The genetic composition, anatomical structure and physiological function of Drosophila malpighian tubules are remarkably similar to those of the human nephron. The direct effects of dietary manipulation, environmental alteration and genetic variation on stone formation can be observed and quantified in a matter of days. Several Drosophila models of human nephrolithiasis have been developed, including genetically linked and environmentally induced stones. A model of calcium oxalate stone formation is among the most recent fly models of human nephrolithiasis.

CONCLUSIONS

The ability to readily manipulate and quantify stone formation in D. melanogaster models of human nephrolithiasis presents the urological community with a unique opportunity to increase our understanding of this enigmatic disease.

The SLC26 gene family of anion transporters and channels

The phylogenetically ancient SLC26 gene family encodes multifunctional anion exchangers and anion channels transporting a broad range of substrates, including Cl(-), HCO3(-), sulfate, oxalate, I(-), and formate. SLC26 polypeptides are characterized by N-terminal cytoplasmic domains, 10-14 hydrophobic transmembrane spans, and C-terminal cytoplasmic STAS domains, and appear to be homo-oligomeric. SLC26-related SulP proteins of marine bacteria likely transport HCO3(-) as part of oceanic carbon fixation. SulP genes present in antibiotic operons may provide sulfate for antibiotic biosynthetic pathways. SLC26-related Sultr proteins transport sulfate in unicellular eukaryotes and in plants. Mutations in three human SLC26 genes are associated with congenital or early onset Mendelian diseases: chondrodysplasias for SLC26A2, chloride diarrhea for SLC26A3, and deafness with enlargement of the vestibular aqueduct for SLC26A4. Additional disease phenotypes evident only in mouse knockout models include oxalate urolithiasis for Slc26a6 and Slc26a1, non-syndromic deafness for Slc26a5, gastric hypochlorhydria for Slc26a7 and Slc26a9, distal renal tubular acidosis for Slc26a7, and male infertility for Slc26a8. STAS domains are required for cell surface expression of SLC26 proteins, and contribute to regulation of the cystic fibrosis transmembrane regulator in complex, cell- and tissue-specific ways. The protein interactomes of SLC26 polypeptides are under active investigation.

New insights into the pathogenesis of renal calculi

The pathophysiology of the various forms of urinary stone disease remains a complex topic. Epidemiologic research and the study of urine and serum chemistries have created an abundance of data to help drive the formulation of pathophysiologic theories. This article addresses the associations of urinary stone disease with hypertension, cardiovascular disease, atherosclerosis, obesity, dyslipidemia, diabetes, and other disease states. Findings regarding the impact of dietary calcium and the formation of Randall's plaques are also explored and their implications discussed. Finally, further avenues of research are explored, including genetic analyses and the use of animal models of urinary stone disease.

In vivo Drosophilia genetic model for calcium oxalate nephrolithiasis

Nephrolithiasis is a major public health problem with a complex and varied etiology. Most stones are composed of calcium oxalate (CaOx), with dietary excess a risk factor. Because of complexity of mammalian system, the details of stone formation remain to be understood. Here we have developed a nephrolithiasis model using the genetic model Drosophila melanogaster, which has a simple, transparent kidney tubule. Drosophilia reliably develops CaOx stones upon dietary oxalate supplementation, and the nucleation and growth of microliths can be viewed in real time. The Slc26 anion transporter dPrestin (Slc26a5/6) is strongly expressed in Drosophilia kidney, and biophysical analysis shows that it is a potent oxalate transporter. When dPrestin is knocked down by RNAi in fly kidney, formation of microliths is reduced, identifying dPrestin as a key player in oxalate excretion. CaOx stone formation is an ancient conserved process across >400 My of divergent evolution (fly and human), and from this study we can conclude that the fly is a good genetic model of nephrolithiasis.

Functional correlates of positional and gender-specific renal asymmetry in Drosophila

BACKGROUND

In humans and other animals, the internal organs are positioned asymmetrically in the body cavity, and disruption of this body plan can be fatal in humans. The mechanisms by which internal asymmetry are established are presently the subject of intense study; however, the functional significance of internal asymmetry (outside the brain) is largely unexplored. Is internal asymmetry functionally significant, or merely an expedient way of packing organs into a cavity?

METHODOLOGY/PRINCIPAL FINDINGS

Like humans, Drosophila shows internal asymmetry, with the gut thrown into stereotyped folds. There is also renal asymmetry, with the rightmost pair of renal (Malpighian) tubules always ramifying anteriorly, and the leftmost pair always sitting posteriorly in the body cavity. Accordingly, transcriptomes of anterior-directed (right-side) and posterior-directed (left-side) Malpighian (renal) tubules were compared in both adult male and female Drosophila. Although genes encoding the basic functions of the tubules (transport, signalling) were uniformly expressed, some functions (like innate immunity) showed positional or gender differences in emphasis; others, like calcium handling or the generation of potentially toxic ammonia, were reserved for just the right-side or left-side tubules, respectively. These findings correlated with the distinct locations of each tubule pair within the body cavity. Well known developmental genes (like dorsocross, dachshund and doublesex) showed continuing, patterned expression in adult tubules, implying that somatic tissues maintain both left-right and gender identities throughout life. Gender asymmetry was also noted, both in defence and in male-specific expression of receptors for neuropeptide F and sex-peptide: NPF elevated calcium only in male tubules.

CONCLUSIONS/SIGNIFICANCE

Accordingly, the physical asymmetry of the tubules in the body cavity is directly adaptive. Now that the detailed machinery underlying internal asymmetry is starting to be delineated, our work invites the investigation, not just of tissues in isolation, but in the context of their unique physical locations and milieux.

Melamine-induced urolithiasis in a Drosophila model

Melamine-tainted food can induce renal stones in both humans and animals. We have previously reported a novel Drosophila model for the study of renal stone disease. In addition to hyperoxaluria-causing agents, we also tested herein the effect of melamine on crystal formation in Drosophila . The results indicate that administration of melamine alone caused crystal formation in a dose-dependent manner. The crystals also appeared after ingestion of melamine for 3 weeks in the Malpighian tubules of Drosophila when viewed with polarized light. Administration of potassium citrate (K citrate) was found to significantly ameliorate the melamine-induced reduction of lifespan. However, administration of K citrate failed to reduce the quantity of crystals. Because calcium oxalate is not the major crystal induced by melamine, the predominant components of melamine-induced crystals and the potential crystal inhibitors warrant further investigation.

Nutritional chemoprevention of urinary tract tumors (UTT) induced by lithogenic agents: risk for UTT in children exposed to melamine-contaminated milk formulas

Urinary tract tumors are tenth in frequency, and many environmental carcinogens are excreted by urine. Interplay between chronic inflammatory urolithiasis and urothelial carcinogenesis is not well understood. Experimental evidences show that dietary melamine induce these events even at low concentrations. This is important because thousands of children were exposed to melamine through intentionally contaminated milk formula worldwide. We propose that an increased risk for urinary tumors in adult life may occur and screenings for early urinary signs may be necessary. Therefore, urothelial biology, melamine carcinogenic potential, and related epidemiology are discussed, recommending a preventive dietary polyunsaturated fatty acid-based supplementation, since they modulate such interplay in rodents.

Drosophila: a fruitful model for calcium oxalate nephrolithiasis?

Even though the prevalence of nephrolithiasis is increasing, our understanding of the pathophysiology has not kept pace and new therapeutic approaches have not emerged. The potential of a new physiological model (the fruitfly) is exciting. The model has strengths, namely the low cost of maintaining colonies and rapid deployment of new transgenic lines, but also weaknesses that may ultimately limit its usefulness, such as the mechanism of tubular fluid formation and difficulties in following plasma and urine biochemistries.