Hydroxylases involved in vitamin D metabolism are differentially expressed in murine embryonic kidney: application of whole mount in situ hybridization

Incubation of metanephroi with vitamin d(3) increases numbers of glomeruli

To characterize actions of vitamin D3 on metanephroi transplanted from rat embryos to adult recipients, we incubated metanephroi with or without 0.01, 0.1 or 1 ug/ml vitamin D3, 25-hydroxyvitamin D(3) [25(OH)D(3)] or 1, 25-hydroxyvitamin D(3) [1,25(OH)2D(3)] prior to implantation. The number of glomeruli in developed metanephroi three weeks post-transplantation that had been incubated with 1.0 ug/ml vitamin D(3) was increased relative to the number in metanephroi that were not incubated with vitamin D(3) (control), an effect that was not recapitulated by administration of vitamin D(3) directly to hosts at the time of transplantation. Incubation of metanephroi with 1.0 ug/ml vitamin D(3) also enhanced inulin clearances of metanephroi measured at 12 weeks post-transplantation. The hydroxylated derivative of vitamin D(3), 25(OH)D(3), increased glomerulus number when applied at 0.01 ug/ml but not at higher concentrations, while the twice-hydroxylated derivative 1,25(OH)(2)D(3), failed to increase glomerulus number at any concentration tested. We conclude that incubation with vitamin D(3) prior to implantation enhances inulin clearance possibly by increasing the number of glomeruli that develop post-transplantation.Our findings suggest the vitamin D(3) effect is mediated locally.

A novel, low-volume method for organ culture of embryonic kidneys that allows development of cortico-medullary anatomical organization

Here, we present a novel method for culturing kidneys in low volumes of medium that offers more organotypic development compared to conventional methods. Organ culture is a powerful technique for studying renal development. It recapitulates many aspects of early development very well, but the established techniques have some disadvantages: in particular, they require relatively large volumes (1–3 mls) of culture medium, which can make high-throughput screens expensive, they require porous (filter) substrates which are difficult to modify chemically, and the organs produced do not achieve good cortico-medullary zonation. Here, we present a technique of growing kidney rudiments in very low volumes of medium–around 85 microliters–using silicone chambers. In this system, kidneys grow directly on glass, grow larger than in conventional culture and develop a clear anatomical cortico-medullary zonation with extended loops of Henle.

Screening of chondrogenic factors with a real-time fluorescence-monitoring cell line ATDC5-C2ER: identification of sorting nexin 19 as a novel factor

OBJECTIVE

To establish a cell culture system for noninvasive and real-time monitoring of chondrogenic differentiation in order to screen for chondrogenic factors.

METHODS

The optimum reporter construct transfected into chondrogenic ATDC5 cells was selected by a luciferase reporter assay and fluorescence analysis during cultures with insulin. The established cell line was validated according to its fluorescence following stimulation with SOX proteins, bone morphogenetic protein 2 (BMP-2), or transforming growth factor beta (TGFbeta) and was compared with the level of messenger RNA for COL2A1 as well as with the degree of Alcian blue staining. Screening of chondrogenic factors was performed by expression cloning using a retroviral expression library prepared from human tracheal cartilage. The expression pattern of the identified molecule was examined by in situ hybridization and immunohistochemistry. Functional analysis was performed by transfection of the identified gene, the small interfering RNA, and the mutated gene.

RESULTS

We established an ATDC5 cell line with 4 repeats of a highly conserved enhancer ligated to a COL2A1 basal promoter and the DsRed2 reporter (ATDC5-C2ER). Fluorescence was induced under the stimulations with SOX proteins, BMP-2, or TGFbeta, showing good correspondence to the chondrogenic markers. Screening using the ATDC5-C2ER system identified several chondrogenic factors, including sorting nexin 19 (SNX19). SNX19 was expressed in the limb cartilage of mouse embryos and in the degraded cartilage of adult mouse knee joints during osteoarthritis progression. The gain-of-function and loss-of-function analyses revealed a potent chondrogenic activity of SNX19.

CONCLUSION

We established the ATDC5-C2ER system for efficient monitoring of chondrogenic differentiation by fluorescence analysis, and we identified a novel chondrogenic factor (SNX19) using this system. This system will be useful for elucidating the molecular network of chondrogenic differentiation.

1α-Hydroxylase gene ablation and Pisupplementation inhibit renal calcification in mice homozygous for the disruptedNpt2agene

Disruption of the major renal Na-phosphate (Pi) cotransporter gene Npt2a in mice leads to a substantial decrease in renal brush-border membrane Na-Picotransport, hypophosphatemia, and appropriate adaptive increases in renal 25-hydroxyvitamin D3-1α-hydroxylase (1αOHase) activity and the serum concentration of 1,25-dihydroxyvitamin D3[1,25(OH)2D]. The latter is associated with increased intestinal Ca absorption, hypercalcemia, hypercalciuria, and renal calcification in Npt2-/-mice. To determine the contribution of elevated serum 1,25(OH)2D levels to the development of hypercalciuria and nephrocalcinosis in Npt2-/-mice, we examined the effects of 1α OHase gene ablation and long-term Pisupplementation on urinary Ca excretion and renal calcification by microcomputed tomography. We show that the urinary Ca/creatinine ratio is significantly decreased in Npt2-/-/1α OHase-/-mice compared with Npt2-/-mice. In addition, renal calcification, determined by estimating the calcified volume to total renal volume (CV/TV), is reduced by ∼80% in Npt2-/-/1α OHase-/-mice compared with that in Npt2-/-mice. In Npt2-/-mice derived from dams fed a 1% Pidiet and maintained on the same diet, we observed a significant decrease in urinary Ca/creatinine that was also associated with ∼80% reduction in CV/TV when compared with counterparts fed a 0.6% diet. Taken together, the present data demonstrate that both 1α OHase gene ablation and Pisupplementation inhibit renal calcification in Npt2-/-mice and that 1,25(OH)2D is essential for the development of hypercalciuria and nephrocalcinosis in the mutant strain.

Renal calcification in mice homozygous for the disrupted type IIa Na/Pi cotransporter gene Npt2

Mice homozygous for the disrupted renal type IIa sodium/phosphate (Na/Pi) cotransporter gene (Npt2-/-) exhibit renal Pi wasting, hypophosphatemia, and an adaptive increase in the serum concentration of 1,25-dihydroxyvitamin D with associated hypercalcemia and hypercalciuria. Because hypercalciuria is a risk factor for nephrocalcinosis, we determined whether Npt2-/- mice form renal stones. Analysis of renal sections by von Kossa staining and intact kidneys by microcomputed tomography revealed renal calcification in adult Npt2-/- mice but not in Npt2+/+ littermates. Energy-dispersive spectroscopy and selected-area electron diffraction indicated that the calcifications are comprised of calcium and Pi with an apatitic mineral phase. To determine the age of onset of nephrocalcinosis, we examined renal sections of newborn and weanling mice. At both ages, mutant but not wild-type mice display renal calcification, which is associated with renal Pi wasting and hypercalciuria. Immunohistochemistry revealed that osteopontin co-localizes with the calcifications. Furthermore, renal osteopontin messenger RNA abundance is significantly elevated in Npt2-/- mice compared with Npt2+/+ mice. The onset of renal stones correlated developmentally with the absence of Npt2 expression and the expression of the genes responsible for the renal production (1alpha-hydroxylase) and catabolism (24-hydroxylase) of 1,25-dihydroxyvitamin D. In summary, we show that Npt2 gene ablation is associated with renal calcification and suggest that mutations in the NPT2 gene may contribute to nephrocalcinosis in a subset of patients with familial hypercalciuria.

Characterization of transgenic rats constitutively expressing vitamin D-24-hydroxylase gene

Vitamin D-24-hydroxylase (CYP24) is one of the enzymes responsible for vitamin D metabolism. CYP24 catalyzes the conversion of 25-hydroxyvitamin D(3) [25(OH)D(3)] to 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)] in the kidney. CYP24 is also involved in the breakdown of 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], the active form of vitamin D(3). In this study, we generated transgenic (Tg) rats constitutively expressing CYP24 gene to investigate the biological role of CYP24 in vivo. Surprisingly, the Tg rats showed a significantly low level of plasma 24,25(OH)(2)D(3). Furthermore, the Tg rats developed albuminuria and hyperlipidemia shortly after weaning. The plasma lipid profile revealed that all lipoprotein fractions were elevated in the Tg rats. Also, the Tg rats showed atherosclerotic lesions in the aorta, which greatly progressed with high-fat and high-cholesterol feeding. These unexpected results suggest that CYP24 is involved in functions other than the regulation of vitamin D metabolism.