Urine proteomics: the present and future of measuring urinary protein components in disease

Abstract

For centuries, physicians have attempted to use the urine for noninvasive assessment of disease. Today, urinalysis, in particular the measurement of proteinuria, underpins the routine assessment of patients with renal disease. More sophisticated methods for assessing specific urinary protein losses have emerged; however, albumin is still the principal urinary protein measured. Changes in the pattern of urinary protein excretion are not necessarily restricted to nephrourological disease; for instance, the appearance of beta-human chorionic gonadotropin in the urine of pregnant women is the basis for all commercially available pregnancy kits. Similarly, microalbuminuria is a clinically important marker not only of early diabetic nephropathy but also of concomitant cardiovascular disease. With the emergence of newer technologies, in particular mass spectrometry, it has become possible to study urinary protein excretion in even more detail. A variety of techniques have been used both to characterize the normal complement of urinary proteins and also to identify proteins and peptides that may facilitate earlier detection of disease, improve assessment of prognosis and allow closer monitoring of response to therapy. Such proteomics-based approaches hold great promise as the basis for new diagnostic tests and as the means to better understand disease pathogenesis. In this review, we summarize the currently available methods for urinary protein analysis and describe the newer approaches being taken to identify urinary biomarkers.

Figure 1: Two-dimensional gel electrophoresis of normal human urine. Proteins are first separated by isoelectric focusing, which separates on the basis of isoelectric point (horizontal separation). This is followed by sodium dodecyl sulfate — polyacrylamide gel electrophoresis, which separates on the basis of molecular weight (vertical separation). Individual spots of interest can be cut from the gel and identified by mass spectrometry. (Gel courtesy of Jan Novak, G. Robinson, H. Kim and Bruce A. Julian, University of Alabama at Birmingham, Birmingham, Ala.).

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Figure 2: Spectra and virtual gels generated by surface-enhanced laser desorption and ionization, followed by time-of-flight mass spectrometry of urine samples from a healthy subject (A) and a patient with glomerular disease (B). Mass-to-charge values for selected peptides and proteins are shown in red. Some proteins and peptides are present in the urine of both subjects (e.g., mass-to-charge ratios 7359.29 and 7356.92, 5426.81 and 5426.55, and 2624.20 and 2625.57). Other proteins are apparent only in the urine of the patient with glomerular disease (e.g., mass-to-charge ratio 4936.23).