BOSTON & CAMBRIDGE, Mass.—Increasingly essential for modeling human functions in the lab, manufactured mini-organs called organoids are grown in a culture dish, designed to contain many of the cell types and complex microarchitectures found in human organs. As Harvard University noted, they “have the potential to transform drug discovery, allowing researchers to experiment on samples of human tissue grown directly from patients.”
But because organoids are grown outside of a body, the university adds, “they lack the blood vessel structure, or vasculature, needed to circulate oxygen and nutrients, remove waste and send messages between different cell types. This has been a major roadblock in maturing groups of cells in a dish into truly functional tissues.”
For kidney organoids, this shortcoming has prevented researchers from emulating key kidney functions, such as blood filtration, reabsorption and urine production. Stem cell scientists have been working to create vascularized kidney organoids that are robust enough to enhance renal drug toxicity testing and, ultimately, lead to new building blocks for renal replacement therapies. A new approach, published in Nature Methods and developed by a Harvard University-led team, demonstrates how organoids can be vascularized, opening the door to a flood of possibilities in stem cell research.
The work was led by Drs. Jennifer Lewis and Ryuji Morizane, with a team of scientists at the Harvard Stem Cell Institute (HSCI), the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), Brigham and Women’s Hospital and the Wyss Institute for Biologically Inspired Engineering.
Their bioengineering approach exposes stem cell-derived organoids to fluidic shear stress—that is, the frictional force of flowing biological fluids. Using this technique, researchers were able to expand organoid-derived vascular networks significantly. Compared with previous, static culturing methods, the exposure improved the maturation of kidney compartments.
In 2015, Morizane and fellow HSCI faculty member Dr. Joseph Bonventre developed a method that enabled them to derive 3D kidney organoids from human pluripotent stem cells.
“While our organoids and those generated in other laboratories contained large numbers of well-organized nephrons and primitive blood vessels, they still lacked pervasive vascular compartments with perfusable lumens,” said Morizane, who is also an assistant professor at Brigham and Women’s Hospital and Harvard Medical School.
More recently, researchers around the world have matured kidney organoids by implanting them into animals, where they can connect directly to the host’s vasculature.
“For the first time, our study demonstrates that by exposing growing organoids to fluid flow, a mechanical cue known to play an important role for tissue development in the body, we can greatly enhance their vascularization and maturation in vitro,” noted Morizane.
To accomplish this feat, the team used expertise from the Lewis lab that has pioneered strategies to create vascularized human tissues, including 3D kidney-on-chip models. These strategies use 3D bioprinting that can be perfused and sustained for long durations. Building on this work, the team explored the idea that fluid flow could also promote the formation of blood vessels from precursor endothelial cells found in growing kidney organoids.
“We determined the right combination of underlying extracellular matrix, media additives, and fluidic shear stress under which human stem-cell derived organoids would flourish when grown in our 3D-printed millifluidic chips,” said Dr. Kimberly Homan, who is a co-first author on the study with Dr. Navin Gupta.
“The vascular networks form close to the epithelial structures that build the glomerular and tubular compartments, and in turn promote epithelial maturation. This integrated process works really like a two-way street,” added Gupta.
Homan is a research associate in Lewis’ group at the Wyss Institute and SEAS, and Gupta is a clinical research fellow working on Morizane’s team at the Brigham.
The vessels growing on the 3D-printed chips formed an interconnected network with open lumens. The network could be perfused with fluids, as confirmed by directly imaging fluorescent beads moving freely through them.
“This important advance opens up new avenues for accurately testing drug toxicity in vitro in differentiated nephron compartments and modeling kidney diseases, like polycystic kidney disease, that affect specific structures and cell types using patient-derived stem cells as the starting point,” said Lewis. “Our method may pave the way to also vascularize other types of organoids, such as the liver organoids.”