The kidney purifies blood through the hard work of thousands of tiny glomeruli, the organ’s powerhouse filtration units. Many diseases target the glomeruli, but studying the long-term effects on these filters is difficult because the same glomerulus cannot be imaged in vivo over time.
In a new paper in the journal Scientific Reports, however, researchers unveil a surprising new way to observe the glomeruli: transplanting them into a mouse eye. The glomeruli not only survive, they also link up to the eye’s blood supply and filter that blood for up to six months. Researchers can then watch them working through the eye’s transparent cornea.
“We did this knowing it was a very contra-physiological way to study glomeruli, and really with very little hope that it would work,” said Alessia Fornoni at the University of Miami in Florida, co-corresponding author of the study with Andreas Kistler from the University of Zurich. “It was very surprising that after a few weeks the glomeruli were already revascularized.”
The glomerulus is a cluster of capillaries that acts as a blood microfilter, keeping proteins in the blood and filtering toxins out by passing them into the urine. In the past, researchers have imaged working glomeruli using multiphoton microscopy inside living mouse kidneys. But mice don’t survive the surgery required for this imaging, so the glomeruli cannot be repeatedly imaged.
Fornoni wanted to find a way to watch the glomeruli over a long period of time, so she collaborated with Per-Olaf Berggren at the Karolinska Institutet (and the University of Miami). In 2008, Berggren and his team transplanted human islets into the anterior chamber of the mouse eye, where the islets revascularized and secreted insulin. Fornoni isolated mouse kidney glomeruli, plucking the tiny tufts of blood vessels from the surrounding Bowman’s capsules, and then injected the glomeruli into the anterior chamber of the eye in immunodeficient mice. Many glomeruli collapsed and decomposed, but 10%-20% revascularized completely onto the iris after just 2 weeks.
To show that the glomeruli were intact, the team used donor mice that expressed cyan fluorescent protein (CFP) in the podocyte, which is a key component of the glomerulal filtration barrier. In transplanted glomeruli, researchers saw cyan fluorescence, showing that the podocytes remained alive. The glomeruli maintained their structure and function, even after repeated imaging over a period of six months. “I think this is one of the major advantages, the capability to allow in vivo longitudinal imaging of glomeruli,” said Fornoni.
Furthermore, upon injecting fluorescence-labeled dextrans of various molecular weights into the system, the researchers saw that the glomeruli did indeed filter blood, allowing low molecular weight molecules to pass and retaining high molecular weight ones. Scientists could use the technique in the future to study the mechanisms of proteinurea in vivo, as well as the role of stem cells in the regeneration of glomeruli, said Fornoni.
Since the eye is so easily accessible, Fornoni envisions it as a site for drug discovery research. Scientists can apply small molecules via eye-drops or larger molecules via micro-injection into the anterior chamber, which can show “whether a drug prevents the leakage of proteins from the blood into the urine,” said Fornoni. “You keep your mouse under the microscope and you watch it in real time.”
She’s now working on transplanting human glomeruli into the eyes of mice, which will offer scientists their first opportunity to watch functioning human glomeruli in vivo.
Kistler AD, Caicedo A, Abdulreda MH, Faul C, Kerjaschki D, Berggren PO, Reiser J, Fornoni A. In vivo imaging of kidney glomeruli transplanted into the anterior chamber of the mouse eye. Sci Rep. 2014 Jan 27;4:3872.