High-efficiency type II cell-enhanced green fluorescent protein expression facilitates cellular identification, tracking, and isolation.

We have developed a transgenic mouse expressing enhanced green fluorescent protein (EGFP) in virtually all type II (TII) alveolar epithelial cells. The CBG mouse (SPC-BAC-EGFP) contains a bacterial artificial chromosome modified to express EGFP within the mouse surfactant protein (SP)-C gene 3' untranslated region. EGFP mRNA expression is limited to the lung. EGFP fluorescence is both limited to and exhibited by all cells expressing pro-SP-C; fluorescence is uniform throughout all lobes of the lung and does not change as mice age. EGFP(+) cells also express SP-B but do not express podoplanin, a type I (TI) cell marker. CBG mice show no evidence of lung disease with aging. In 3 hours, TII cells can be isolated in >99% purity from CBG mice by FACS; the yield of 3.7 ± 0.6 × 10(6) cells represents approximately 25 to 60% of the TII cells in the lung. By FACS analysis, approximately 0.9% of TII cells are in mitosis in uninjured lungs; after bleomycin injury, 4.1% are in mitosis. Because EGFP fluorescence can be detected for >14 days in culture, at a time that SP-C mRNA expression is essentially nil, this line may be useful for tracking TII cells in culture and in vivo. When CBG mice are crossed to transgenic mice expressing rat podoplanin, TI and TII cells can be easily simultaneously identified and isolated. When bred to other strains of mice, EGFP expression can be used to identify TII cells without the need for immunostaining for SP-C. These mice should be useful in models of mouse pulmonary disease and in studies of TII cell biology, biochemistry, and genetics.

Urea-selective concentrating defect in transgenic mice lacking urea transporter UT-B.

Urea transporter UT-B has been proposed to be the major urea transporter in erythrocytes and kidney-descending vasa recta. The mouse UT-B cDNA was isolated and encodes a 384-amino acid urea-transporting glycoprotein expressed in kidney, spleen, brain, ureter, and urinary bladder. The mouse UT-B gene was analyzed, and UT-B knockout mice were generated by targeted gene deletion of exons 3-6. The survival and growth of UT-B knockout mice were not different from wild-type littermates. Urea permeability was 45-fold lower in erythrocytes from knockout mice than from those in wild-type mice. Daily urine output was 1.5-fold greater in UT-B- deficient mice (p < 0.01), and urine osmolality (U(osm)) was lower (1532 +/- 71 versus 2056 +/- 83 mosM/kg H(2)O, mean +/- S.E., p < 0.001). After 24 h of water deprivation, U(osm) (in mosM/kg H(2)O) was 2403 +/- 38 in UT-B null mice and 3438 +/- 98 in wild-type mice (p < 0.001). Plasma urea concentration (P(urea)) was 30% higher, and urine urea concentration (U(urea)) was 35% lower in knockout mice than in wild-type mice, resulting in a much lower U(urea)/P(urea) ratio (61 +/- 5 versus 124 +/- 9, p < 0.001). Thus, the capacity to concentrate urea in the urine is more severely impaired than the capacity to concentrate other solutes. Together with data showing a disproportionate reduction in the concentration of urea compared with salt in homogenized renal inner medullas of UT-B null mice, these data define a novel "urea-selective" urinary concentrating defect in UT-B null mice. The UT-B null mice generated for these studies should also be useful in establishing the role of facilitated urea transport in extrarenal organs expressing UT-B.