Renal Function

Phenotyping renal function in genetically engineered mice consists of clinical pathology assessment as well as gross and histopathology monitoring. Many of the parameters identified by urinalysis can give a good indication of the status of the kidneys and their ability to filter the blood supply and produce urine. Gross pathology and histopathology will allow a researcher to evaluate the anatomy of the kidney and urinary tract, in effect determining if a specific transgene creates a change in anatomical structure or in ultrastructure that could, in turn, lead to a change in physiology.

In order to perform a complete urinalysis, collection of urine from the mouse is required. Many mice will urinate due to the stress of handling, and a free catch specimen can be collected in a sterile blood or culture tube. Alternatively, mice can be placed on a square of Parafilm or can be voided by gently applying abdominal pressure in a sweeping motion from midabdomen down to above the bladder.16 If more than a few drops of urine is required for analysis, multiple collections are needed from the same mouse over a series of several days. In this case, all urine collected should be kept in the refrigerator for storage. A microscopic exam of the urine can identify formed elements such as red and white blood cells and cellular casts. Multiple parameters of urine can be measured using a urine dipstick, namely urine protein, urine blood, and urine glucose. For experiments requiring daily urine sampling over an extended period of time or those in which large volumes are needed, the proper method of urine collection is with a metabolism cage. These cages have wire-bottom flooring and a funnel-shaped base. As the mice in the cage urinate and defecate, excretions drop into the funnel and are passed into a collection vessel at the base of the funnel.35 Samples taken in this manner are not sterile and cannot be used for culture, though they can be used to determine the integrity of renal physiology. Urine volume can be used to evaluate renal concentrating ability, the loss of which is one of the first signs of renal disease. Less frequently measured urine elements can indicate certain types of renal pathology. Measurement of specific proteins in the urine via SDS-PAGE or nondenaturing gel electrophoresis can differentiate between glomerular proteinuria, where there is loss of large and small proteins, and tubular proteinuria in which only low-molecular weight proteins are lost. Presence in the urine of several proximal tubular cell enzymes, including #-acetylglucosaminidase and lysozyme, is indicative of tubular cell injury. Glomerular filtration rate can be assessed by measuring inulin clearance.16

A blood serum chemistry screen and complete blood count can be used to assist in evaluating renal function, namely the parameters BUN, creatinine, glucose, total protein, and bilirubin concentration. Elevations or depressions in these values can mean various changes in urinary metabolism and can indicate changes in ultrastructure best seen with histopathology evaluations. A loss in numbers of red blood cells, or anemia, can be due to chronic renal failure, as can abnormalities in blood electrolytes such as calcium and potassium.

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