Researchers at Harvard's Wyss Institute for Biologically Inspired Engineering have achieved the differentiation of human induced pluripotent stem (iPS) cells into mature podocytes, cells important for filtration within the kidneys, with greater than 90% efficiency. The cells were then combined with organ-on-a-chip technology to yield an in vitro model of the human glomerulus, a key component of the kidney. The model exhibited the ability to selectively filter blood proteins and was used to reveal the toxic effects of a chemotherapy drug on the podocytes, both in vitro.
The kidney controls the fluid content of the body and removes waste products from blood cells that are then excreted in urine. Each kidney is made up of approximately one million units, each based on a 'glomerulus', which is composed of podocyte cells wrapped around a “tuft” of capillaries with a thin membrane in between the cells and capillaries. Slits between the cells form the filtration barrier within the kidney. Podocytes often play a role in kidney diseases and can be damaged by different types of drugs.
Many scientists have sought to engineer human stem cells that could differentiate themselves into functional podocytes in order to construct an in vitro model of the human glomerulus for investigation of its properties and behavior, but failed to obtain cells with sufficient purity.
A team led by Donald Ingber, the Wyss Institute's Founding Director, the Judah Folkman Professor of Vascular Biology at Harvard Medical School (HMS) and the Vascular Biology Program at Boston Children's Hospital, as well as Professor of Bioengineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), generated nearly pure populations of human podocytes in cell culture. This was done by using factors that guide kidney development in the embryo, growing and differentiating stem cells on extracellular matrix components, that are also contained in the membrane, separating the glomerular blood and urinary systems. As a result, they were able to closely mimic the natural environment in which podocytes are induced and mature.
The maturation level was then improved by performing the process in the channel of a microfluidic chip along with the application of cyclical suction to create motions that mimic the rhythmic deformations generated by a beating heart.
The system is composed of a clear polymer “chip” with two microchannels separated by a porous, extracellular matrix-coated membrane. Glomerular endothelial cells are grown in one channel to mimic the blood microvessel compartment of glomeruli. The pluripotent stem cells are cultured in the other channel, which represents the urinary compartment, and induced to form mature podocytes. Cyclic suction applied to hollow chambers near the microchannels stretches and relaxes the channels at one heartbeat per second. Overall the system looks like the three-dimensional cross section of the human glomerular wall.
"The development of a functional human kidney glomerulus chip opens up an entire new experimental path to investigate kidney biology, carry out highly personalized modeling of kidney diseases and drug toxicities, and the stem cell-derived kidney podocytes we developed could even offer a new injectable cell therapy approach for regenerative medicine in patients with life-threatening glomerulopathies in the future," said Ingber.
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