For our body to work properly, our kidneys play a vital role in keeping our blood composition constant. To achieve this, the kidneys' approximately one million filtration units (glomeruli) first remove excess water and waste products, then specialized structures known as the proximal tubules reabsorb the "good" molecules returning them to our bloodstream. While the reabsorptive functions of the proximal tubule can be compromised by drugs, chemicals, or genetic and blood-borne diseases, our understanding of how these effects occur is still limited.
Immunofluorescence staining of a 3D bioprinted vascularized proximal tubule with a proximal tubule epithelial marker stained in green in the proximal tubule channel and a vascular endothelial marker stained in red in the adjacent vascular channel. The magnified cross-section illustrates that the two different cell types form luminal perfusable structures in their respective channels. Credit: Wyss Institute at Harvard University
To enable the study of renal reabsorption outside the human body, Wyss Institute Core Faculty member Jennifer Lewis, Sc.D., in collaboration with the Roche Innovation Center Basel in Switzerland, created a 3D bioprinted vascularized proximal tubule model in which independently perfusable tubules and blood vessels are printed adjacent to one another within an engineered extracellular matrix. This work builds upon a continuously perfused 3D proximal tubule model reported earlier by the team that still was lacking a functional blood vessel compartment. Using their next-generation device, the team has measured the transport of glucose from the proximal tubule to the blood vessels, along with the effects of hyperglycemia, a condition associated with diabetes in patients.
"We construct these living renal devices in a few days and they can remain stable and functional for months," said Neil Lin, Ph.D., who is a Roche Fellow and Postdoctoral Fellow on Lewis' team. "Importantly, these 3D vascularized proximal tubules exhibit the desired epithelial and endothelial cell morphologies and luminal architectures, as well as the expression and correct localization of key structural and transport proteins, and factors that allow the tubular and vascular compartments to communicate with each other."
As a first step towards testing drugs and modeling diseases, the team induced "hyperglycemia", a high-glucose condition typical of diabetes and a known risk factor for vascular disease, in their model by circulating a four-fold higher than normal glucose concentration through the proximal tubule compartment. "We found that high levels of glucose transported to endothelial cells in the vascular compartment caused cell damage," said Kimberly Homan, Ph.D., Research Associate in Lewis' group at the Wyss Institute and SEAS. "By circulating a drug through the tubule that specifically inhibits a major glucose transporter in proximal tubule epithelial cells, we prevented those harmful changes from happening to the endothelial cells in the adjacent vessels."
The team's immediate focus is to further scale up these 3D printed models for use in pharmaceutical applications. "Our system could enable the screening of focused drug libraries for renal toxicity and thus help reduce animal experiments," said Annie Moisan, Ph.D., an industry collaborator on the study, and Principal Scientist at Roche Innovation Center Basel.
"Our new 3D printed kidney model is an exciting advance as it more fully recapitulates the proximal tubule segments found in native kidney tissue," said Lewis. "Beyond its immediate applications for drug screening and disease modelling, we are also exploring whether these living devices can be used to augment kidney dialysis." Currently, life-saving dialysis machines filter blood, but they are unable to retrieve precious nutrients and other species from the filtrate that the body needs for many of its functions, which can cause specific deficiencies and complications down the line. Lewis and her colleagues believe that 3D bioprinted vascularized tubules may lead to improved renal replacement therapies.
Their study is published in the Proceedings of the National Academy of Sciences (PNAS).