Techsighting. Test tube kidneys.

A nyone who has ever treated patients with end-stage renal failure will immediately recognize the value of the research described below. While most people are aware that hemodialysis will keep people with failing kidneys alive, many do not know the significant morbidity and mortality associated with this procedure. For instance, patients in their early 50s who are undergoing chronic hemodialysis have less than a 50:50 chance of surviving for 5 years. Why? The answer has to do with the exact role played by the functioning kidney cells. The kidney is an amazing organ that functions as both an endocrine gland (secreting a version of vitamin D), and a filtration unit (removing the bloodstream's many toxic waste products). Toxins such as blood urea nitrogen, ammonia, and organic ions will rise to toxic levels in the blood if not for the function of specific cells in the kidney. These key cells are located in the proximal renal tubules. A group in Michigan has now tested whether these cells could augment hemodialysis and yield a better treatment for patients with renal failure. The investigators used a coupled system that relies on standard ultrafiltration technology to remove small molecules, like blood urea nitrogen, in concert with a novel, cell-based technology to perform the metabolic processes of the kidney. The latter technology is based on the ability to grow porcine proximal renal tubules in culture after harvest from 4- to 6-week-old Yorkshire pigs. These renal cells can be grown as confluent monolayers on the inner surface of hollow fibers. These fibers can then be packed into bioreactor cartridge devices, termed renal tubule assist devices (RAD), that contain up to 2.5 × 109 cells. To test the function of a such a bioreactor kidney, they chose to use dogs that were put into renal failure by bilateral nephrectomies. By creating a closed perfusion circuit that went from jugular vein to hemofilter to RAD cartridge to jugular vein, they created a system in which to study the biochemical changes produced by the live cells. First, they did a mock experiment to find whether the porcine cells would be harmed by the toxic metabolites in the blood of human renal failure patients. They took a culture of porcine renal tubules and bathed it in the uremic ultrafiltrate obtained from patients with end-stage renal disease. After culturing these cells with the ultrafiltrate for 1 week, they did not see any significant cell death. The pig renal cells seemed well adapted to functioning in a uremic environment. Next, they looked at the stability of the porcine cells in the RAD cartridge. The uremic filtrate must flow over the cells in the system, a situation that produces some amount of shear stress on the cell layer. They investigated whether the cells could remain firmly attached during the cleansing run. During the first hour of the run there was some cell loss, but it decreased to near zero for the last part of a 24-hour run. Of the 2.5 × 109 cells, only <6 × 104 cells were lost in total. The researchers then measured critical elements in the plasma of dogs before and after treatment with the RAD system and compared the results to control dogs that were in renal failure but were left untreated. They found a significant decrease in the levels of potassium and blood urea nitrogen in the treated dogs. Potassium fell from an average of 5.7 meq/l in the untreated dogs to 3.8 meq/l in the treated dogs during the approximately 24-hour treatment procedure. The blood urea nitrogen level fell from 90 to 57 mg/dl in the same treated dogs. To ask whether metabolic activity was robust in the RAD system, they looked at the processing of ammonia in the filtrate. Ammonia constitutes about 15% of all nitrogenous waste produced in the body. It is also a key player in the removal of acid from the peripheral blood. From analysis of both acid and filtered ammonia levels, the RAD cartridge was shown to be active. In addition, the group measured the level of 1,25-(OH)2D3 an active version of vitamin D produced by functioning kidneys. Amazingly, they found that animals repeatedly treated with RAD had levels of the hormone nearly equivalent to those of normal subjects. The results of the RAD studies are encouraging, and show nicely how the fruits of cell culture and bioreactor technology can be applied to a medical problem. The next steps may involve experiments to immortalize the porcine renal tubule cells to make it easier to obtain significant supplies for large-scale clinical trials in the future. 1. 1. H. D. Humes, 2. D. A. Buffington, 3. S. M. MacKay, 4. A. J. Funke, 5. W. F. Weitzel , Nature Biotechnol. 17, 451 (1999). 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