Hydronephrosis and renal abnormalities in Dlgh1^–/– mice

Prenatal ultrasound detects hydronephrosis in one of 100 pregnancies; 20% to 30% fail to resolve and are clinically significant. Urine accumulation creates pressure that impairs kidney development and damages the renal parenchyma, which, in turn, can lead to renal failure.

The multidomain scaffolding protein discs-large homolog 1 (Dlgh1), which plays an important role in the establishment and maintenance of epithelial polarity in Drosophila, is expressed at the basolateral membrane of kidney epithelial cells. Previous studies (E. Naim et al., Kidney Int, 2005; 68: 955–965) using a truncated Dlgh1 protein fused to a LacZ reporter found that ureteric and mesenchyme-derived epithelial cells expressed Dlgh1 during kidney development and that truncation of Dlgh1 altered the normal basolateral localization of the protein. Importantly, this was also

Dlgh1^–/– embryos exhibit renal hypoplasia, renal agenesis, and hydronephrosis.

associated with an approximately 30% decrease in nephron number; whether this was due to a defect in ureteric bud branching and/or a delay in mesenchyme-to-epithelial transition was unclear.

Mahoney et al. now report the generation and characterization of Dlgh1-null (–/–) mice. They found that Dlgh1^–/– mice developed renal hypoplasia and severe urinary tract abnormalities, including congenital hydronephrosis (see Figure). To investigate the cause of the renal hypoplasia, they examined branching morphogenesis of the ureteric bud in vitro and found reduced branching in Dlgh1^–/– kidneys. In contrast, the condensation of the metanephric mesenchyme and its further development into glomeruli and tubules were unaffected by the loss of Dlgh1. Glomerular ultrastructure was comparable in the control and mutant kidneys. Several genes that are important for kidney development, such as Ret, glial cell line-derived neurotrophic factor, Wnt4, retinaldehyde dehydrogenase 2, Wilms tumor 1, and Pax2, were all expressed normally in the Dlgh1^–/– kidney. Thus, it appears that reduced ureteric bud branching is the primary defect in the hypoplastic Dlgh1^–/– kidneys.

Very interestingly, the authors also found that Dlgh1 was expressed in the developing ureter, and in its absence, the stromal cells that normally lie between the urothelial and smooth muscle layers were missing. Moreover, in the ureteric smooth muscle, the circular smooth muscle cells were misaligned in a longitudinal orientation. These abnormalities in the ureter led to severely impaired ureteric peristalsis. These results suggest that, besides its well-documented role in regulating epithelial polarity, Dlgh1 also regulates smooth muscle orientation. It is of interest that smooth muscle defects similar to these found in Dlgh1^–/– mice are frequently observed in patients with ureteropelvic junction obstruction, a common form of hydronephrosis. Thus, human Dlgh1 mutations may contribute to hereditary forms of hydronephrosis. (Proc Natl Acad Sci USA 2006; 103: 19872–19877)

Juan Oliver

Immunoglobulin A nephropathy-like disease in -1,4-galactosyltransferase-I-deficient mice

Human immunoglobulin A nephropathy (IgAN) is the most common form of glomerulonephritis, and a significant proportion of patients progress to renal failure. However, the pathological molecular mechanisms of IgAN are poorly understood. In humans, abnormal character of serum IgA, especially serum IgA1 with aberrant galactosylation and sialylation of O-glycans in its hinge region, is thought to

Expansion of the mesangial matrix and deposition of immune complexes in the mesangial area in 4GalT-I–/– mice.

contribute to the pathogenesis of IgAN. In addition, 4-galactosylation of glycoproteins plays an important role in protein conformation, stability, transport, and clearance from the circulation. Recent studies have revealed that aberrant glycosylation causes various human diseases. Specifically, -1,4-galactosyltransferase-I (4GalT-I) transfers galactose to the terminal N-acetylglucosamine of N- and O-linked glycans in a -1,4 linkage.

In a recent issue of the American Journal of Pathology, Nishie et al. found that mice lacking 4GalT-I developed IgAN-like glomerular lesions with IgA deposition and expanded mesangial matrix (see Figure). 4GalT-I-deficient mice also showed high serum IgA levels with increased polymeric forms as in human IgAN. Mouse IgA has N-glycans but not O-glycans, and 4-galactosylation and sialylation of the N-glycans on the serum IgA from 4GalT-I-deficient mice were completely absent. This is the first study to demonstrate that genetic remodeling of protein glycosylation causes IgAN. The authors propose that carbohydrates of serum IgA are involved in the development of IgAN, whether the carbohydrates are O-glycans or N-glycans. (Am J Pathol 2007; 170: 447–456)

Detlef Schlöndorff

Hif-2 is the critical factor regulating erythropoietin and erythropoiesis in the adult

Hypoxia-inducible factors (HIFs) are transcription factors that regulate oxygen (O₂) homeostasis. They stimulate genes important for angiogenesis, erythropoiesis, and glucose metabolism. HIFs are heterodimeric factors consisting of -subunits (HIF-1, -2, and -3) and -subunits. Their activities are regulated via the labile -subunit, whereas HIF- is expressed constitutively. At normal O₂ levels (5% or 36 mmHg), HIF- subunits are hydroxylated. Thus, they are recognited by the tumor suppressor protein von Hippel-Lindau and targeted for proteosomal degradation. Under conditions of low O₂ (<5%), HIF- subunits are no longer degraded and translocate into the nucleus, where they form a heterodimeric transcription factor with HIF-1. HIF activates the transcription of more than 150 target genes. The closely related HIF-1 and HIF-2 proteins activate common target genes (for example, VEGF) but also regulate unique genes, such as TGF- and the stem-cell factor Oct-4, respectively. The regulation of different target genes might be due to distinct -subunit expression patterns. HIF-1 mRNA seems to be expressed ubiquitously, whereas HIF-2 mRNA expression is much more restricted. Immunohistochemical analysis of HIF-2 in adult rat organs demonstrated hypoxic stabilization of HIF-2 in distinct cell populations in the brain, heart, lung, liver, pancreas, intestine, and kidney. Interestingly, in the kidneys, hypoxia induces Hif-2 in renal interstitial cells.

HIF activity is essential during embryogenesis, as deletion of Hif-1, Hif-2, and the gene coding for HIF-1 leads to embryonic lethality. The phenotype of Hif-2^–/– embryos varies widely, with embryos dying from embryonic day 9.5 to perinatally, probably because of modifier loci in the different mouse strains. These results demonstrated an important role for HIF-2 during embryogenesis and shortly after birth, but the role of HIF-2 in the adult is not fully understood.

To investigate the adult role of HIF-2, Gruber et al. generated a conditional allele of Hif-2. By crossing this allele to a ubiquitously expressed, inducible Cre transgene, they ablated Hif-2 postnatally, allowing them to investigate the physiologic role of HIF-2 in the adult while excluding possible secondary effects because of the lack of HIF-2 during embryogenesis. Acute deletion of Hif-2 resulted in reduced red blood cell numbers, hemoglobin, and hematocrit values. Furthermore, the potential of bone marrow progenitors to form erythroid colonies in vitro was reduced in Hif-2 mutant animals. Gruber et al. also deleted Hif-1 in adults, and, surprisingly, they found that HIF-2, and not HIF-1, is the critical HIF complex regulating erythropoietin expression in vivo under both physiologic and stress conditions. Thus, although HIF-1 was first purified and cloned on the basis of its affinity for the human erythropoietin 3' enhancer hypoxia response element and regulates Epo expression during mouse embryogenesis, HIF-2 is the critical -isoform regulating Epo under physiologic and stress conditions in adults.

These interesting results should refocus investigators' attention on renal interstitial cells expressing HIF-2. (Proc Natl Acad Sci USA 2007; 104: 2301–2306)

Juan Oliver