Glucose Absorption May Drive Cyst Formation in Polycystic Kidney Disease

Researchers have identified many of the genes that cause autosomal-dominant polycystic kidney disease (ADPKD) and other forms of PKD, which are characterized by fluid-filled cysts that arise from tubules in kidneys and other organs. They have developed human kidney organoids to model these conditions, but very little is known about the mechanisms underlying cyst formation in affected patients. By applying a microfluidic chip to the organoids, investigators recently uncovered new insights into how the flow of fluid within the kidney contributes to PKD. The work, which is published in Nature Communications, points to the importance of aberrant glucose absorption in cyst formation (1).

“The results…are significant because there is a whole class of molecules that block sugar uptake in the kidneys and are attractive therapeutics for a number of conditions,” said co-senior author Benjamin Freedman, PhD, an assistant professor of medicine in the Division of Nephrology at the University of Washington School of Medicine in Seattle.

Combining kidney organoids with microfluidic chips allowed a mixture of water, sugar, amino acids, and other nutrients to flow over the organoids, which were derived from human pluripotent stem cells and contained podocyte, proximal tubule, and distal tubule segments in contiguous, nephron-like arrangements. In organoids that had been genetically edited to mimic PKD, the process of cyst swelling involved the absorption of fluid inward through cells from outside the cyst. This discovery was surprising, as the team expected cysts to form by pushing fluid outward through cells. Therefore, although the research did not rule out secretion as a causative mechanism in PKD cyst formation, it revealed that absorption also appears to play a critical role.

To observe the process of cyst formation in real time, the scientists collected time-lapse images of young PKD organoids undergoing cyst formation in culture. Cysts formed from the peripheral epithelium of the organoids that faced outward toward the media, rather than from internal regions.

Also, increasing the levels of glucose in dish cultures augmented cyst swelling, which was blocked in the presence of the sodium-glucose co-transporter inhibitors phloridzin or dapagliflozin. “Sugar uptake is something that kidneys do all the time,” Freedman said. “We found that increasing the levels of sugar in the dish cultures caused cysts to swell. And when we employed drugs known to block sugar absorption in the kidneys, it blocked this swelling. But I think it relates less to blood sugar level and more to how kidney cells take in sugar—which in this process seemed to go rogue and give rise to cysts.”

The cell experiments were supported by additional experiments conducted in a mouse model of PKD. When the scientists injected fluorescent glucose into mice with PKD, they found that the mouse cysts also took up glucose.

“Organoids are a powerful tool for disease modeling, but organoid culture systems are typically static. By combining a flow-based system with PKD organoids, the Freedman group has elegantly demonstrated that it is the transport of glucose which drives cystogenesis,” said Edward Kelly, PhD, who is an associate professor in the Department of Pharmaceutics at the University of Washington School of Medicine and was not involved with this research.

Numerous glucose inhibitors are under investigation for the treatment of various kidney diseases. This work suggests that patients with PKD are also likely to benefit from such drugs.

Gopi Rangan, PhD, FRACP, MBBS, MBA, a professor of genetic kidney disease at The University of Sydney and the director of the Michael Stern Laboratory for Polycystic Kidney Disease at the Westmead Institute for Medical Research, stated that “it is a fascinating study which has provided novel insights into disease mechanisms, verifying the importance of fluid flow in mediating cyst growth and (converse to previous understanding) that cystic cells have an absorptive phenotype. In addition, the organoid model provides a superior high-throughput method for screening and selecting compounds for further preclinical evaluation.”

But Rangan stressed that more work is needed to determine whether the findings will be translatable to human disease. “The organoids do not replicate [the] human microcyst environment. They are derived from proximal and distal tubular segments, and collecting ducts are not included; and PKD is induced by bi-allelic mutations, rather than heterozygous in human ADPKD. In humans, it is hypothesized that cysts are primarily derived from distal nephron segments, rather than proximal segments,” he said. “It is also not clear if the concentrations of phloretin or dapagliflozin are relevant to humans. In this regard, only the highest concentration of phloretin reduced cyst growth, and the highest concentration of dapagliflozin reduced the live/dead ratio of cysts but not cyst growth.”

The research may also help advance the study of organ systems by showing that simulating fluid flow with microfluidic chips can provide a more realistic environment for analyzing human organoids in the lab. “Coupling the structural and functional characteristics of organoids with the controlled, microfluidic microenvironments of organ-on-a-chip devices is a promising approach to in vitro disease modeling,” Freedman and his co-authors wrote.