Laminin and type IV collagen composition of the glomerular basement membrane changes during glomerular development and maturation. Although it is known that both glomerular endothelial cells and podocytes produce different laminin isoforms at the appropriate stages of development, the cellular origins for the different type IV collagen heterotrimers that appear during development are unknown. Here, immunoelectron microscopy demonstrated that endothelial cells, mesangial cells, and podocytes of immature glomeruli synthesize collagen ␣1␣2␣1(IV). However, intracellular labeling revealed that podocytes, but not endothelial or mesangial cells, contain collagen ␣3␣4␣5(IV). To evaluate the origins of collagen IV further, we transplanted embryonic kidneys from Col4a3-null mutants (Alport mice) into kidneys of newborn, wildtype mice. Hybrid glomeruli within grafts containing numerous host-derived, wildtype endothelial cells never expressed collagen ␣3␣4␣5(IV). Finally, confocal microscopy of glomeruli from infant Alport mice that had been dually labeled with anti-collagen ␣5(IV) and the podocyte marker anti-GLEPP1 showed immunolabeling exclusively within podocytes. Together, these results indicate that collagen ␣3␣4␣5(IV) originates solely from podocytes; therefore, glomerular Alport disease is a genetic defect that manifests specifically within this cell type. Basement membranes are thin sheets of extracellular matrix that underlie epithelial cells, including the vascular endothelium, and surround all muscle cells, Schwann cells, and adipocytes. They are composed of polymers of laminin and type IV collagen, and also contain nidogen/entactin, and proteoglycans. During glomerulogenesis, a basement membrane beneath developing endothelial cells fuses with a separate basement membrane layer beneath differentiating podocytes, to produce the glomerular basement membrane (GBM) shared on opposing surfaces by both cell types. 1 Unlike most basement membranes in the body, the laminin and collagen IV composition of the GBM changes temporally as the glomerulus develops. 2 The earliest GBMs of comma-and S-shaped nephrons contain laminin ␣1␤1␥1 (laminin 111), whereas those at later developmental stages and in adulthood contain laminin ␣5␤2␥1 (laminin 521). 2,3 Previously, we showed by postfixation immunoelectron microscopy that both endothelial cells and podocytes synthesize laminin ␣1 and ␤1 initially, and both cells then undergo a laminin isoform switch and synthesize laminin ␣5 and ␤2 as glomeruli mature. 4 The mechanism and reason why laminin replacement occurs are unknown, but

Alport disease is caused by mutations in genes encoding the ␣3, ␣4, or ␣5 chains of type IV collagen, which form the collagenous network of mature glomerular basement membrane (GBM). In the absence of ␣3, ␣4, ␣5 (IV) collagen, ␣1, ␣2 (IV) collagen persists, which ordinarily is found only in GBM of developing kidney. In addition to dysregulation of collagen IV, Alport GBM contains aberrant laminins, which may contribute to the progressive GBM thickening and splitting, proteinuria, and renal failure seen in this disorder. This study sought to characterize further the laminin dysregulation in collagen ␣3(IV) knockout mice, a model of Alport disease. With the use of confocal microscopy, laminin ␣1 and ␣5 abundance was quantified, and it was found that they co-distributed in significantly large amounts in areas of GBM thickening. In addition, labeling of entire glomeruli for laminin ␣5 was significantly greater in Alport mice than in wild-type siblings. Reverse transcriptase-PCR from isolated glomeruli demonstrated significantly more laminin ␣5 mRNA in Alport mice than in wild-type controls, indicating upregulated transcription of Lama5. For testing glomerular barrier function, ferritin was injected into 2-wk-old Alport and control mice, and GBM was examined by electron microscopy. Highest ferritin levels were seen in Alport GBM thickenings beneath effaced podocyte foot processes, but morphologically normal GBM was significantly permeable as well. We concluded that (1) ultrastructurally normal Alport GBM residing beneath differentiated podocyte foot processes is inherently and abnormally permeable, and (2) upregulation of Lama5 transcription and concentration of laminin ␣1 and ␣5 within Alport GBM thickenings contribute to abnormal permeabilities.

Kidney glomerular basement membranes (GBMs) undergo laminin and type IV collagen isoform substitutions during glomerular development, which are believed to be required for maturation of the filtration barrier. Specifically, GBMs of earliest glomeruli contain laminin α1β1γ1 and collagen α1α2α1(IV), whereas mature glomeruli contain laminin α5β2γ1 and collagen α3α4α5(IV). Here, we used confocal microscopy to simultaneously evaluate expression of different laminin and collagen IV isoforms in newborn mouse GBMs. Our results show loss of laminin α1 from GBMs in early capillary loop stages and continuous linear deposition of laminin bearing the α5 chain thereafter. In contrast, collagen α1α2α1(IV) persisted in linear patterns into late capillary loop stages, when collagen α3α4α5(IV) first appeared in discontinuous, non-linear patterns. This patchy pattern for collagen α3α4α5(IV) continued into maturing glomeruli where there were lengths of linear, laminin α5-positive GBM entirely lacking either isoform of collagen IV. Relative abundance of laminin and collagen IV mRNAs in newborn and 5-week-old mouse kidneys also differed, with those encoding laminin α1, α5, β1, β2, and γ1, and collagen α1(IV) and α2(IV) chains all significantly declining at 5 weeks, but α3(IV) and α4(IV) were significantly upregulated. We conclude that different biosynthetic mechanisms control laminin and type IV collagen expression in developing glomeruli.

The hypoxia-inducible transcription factor-2 (HIF2), a heterodimer composed of HIF2␣ and HIF1␤ subunits, drives expression of genes essential for vascularization, including vascular endothelial growth factor (VEGF) and VEGF receptor-2 (VEGFR-2, Flk-1). Here, we used a HIF2␣/LacZ transgenic mouse to define patterns of HIF2␣ transcription during kidney development and maturation. Our results from embryonic heterozygotes showed HIF2␣/LacZ expression by apparently all renal endothelial cells. At 4 weeks of age, glomerular mesangial and vascular smooth muscle cells were also positive together with endothelial cells. These expression patterns were confirmed by electron microscopy using Bluo-gal as a ␤-galactosidase substrate. Small numbers of glomerular and tubular epithelial cells were also positive at all stages examined. Light and electron microscopic examination of kidneys from HIF2␣ null embryos showed no defects in renal vascular development or nephrogenesis. Similarly, the same amounts of Flk-1 protein were seen on Western blots of kidney extracts from homozygous and heterozygous HIF2␣ mutants. To examine responsiveness of HIF2␣ null kidneys to hypoxia, embryonic day 13.5 metanephroi were cultured in room air or in mild (5% O 2 ) hypoxia. For both heterozygous and null samples, VEGF mRNA levels doubled when metanephroi were cultured in mild hypoxia. Anterior chamber grafts of embryonic HIF2␣ knockouts were morphologically indistinguishable from heterozygous grafts. Endothelial markers, platelet endothelial cell adhesion molecule and BsLB4, as well as glomerular epithelial markers, GLEPP1 and WT-1, were all expressed appropriately. Finally, we undertook quantitative real-time polymerase chain reaction of kidneys from HIF2␣ null embryos and wild-type siblings and found no compensatory up-regulation of HIF1␣ or -3␣. Our results show that, although HIF2␣ was widely transcribed by kidney endothelium and vascular smooth muscle, knockouts displayed no detectable deficits in vessel development or VEGF or Flk-1 expression.

Vascular endothelial growth factor, which is critical for blood vessel formation, is regulated by hypoxia inducible transcription factors (HIFs). A component of the E3 ubiquitin ligase complex, von Hippel-Lindau (VHL) facilitates oxygen-dependent polyubiquitination and proteasomal degradation of HIF␣ subunits. Hypothesizing that deletion of podocyte VHL would result in HIF␣ hyperstabilization, we crossed podocin promoter-Cre transgenic mice, which express Cre recombinase in podocytes beginning at the capillary loop stage of glomerular development, with floxed VHL mice. Vascular patterning and glomerular development appeared unaltered in progeny lacking podocyte VHL. However, urinalysis showed increased albumin excretion by 4 weeks when compared with wild-type littermates with several sever cases (>1000 g/ml). Many glomerular ultrastructural changes were seen in mutants, including focal subendothelial delamination and widespread podocyte foot process broadening, and glomerular basement membranes (GBMs) were significantly thicker in 16-week-old mutants compared with controls. Moreover, immunoelectron microscopy showed ectopic deposition of collagen ␣1␣2␣1(IV) in GBM humps beneath podocytes. Significant increases in the number of Ki-67-positive mesangial cells were also found, but glomerular WT1 expression was significantly decreased, signifying podocyte death and/or de-differentiation. Indeed, expression profiling of mutant glomeruli suggested a negative regulatory feedback loop involving the HIF␣ prolyl hydroxylase, Egln3. In addition, the brain oxygen-binding protein, Neuroglobin, was induced in mutant podocytes. We conclude that podocyte VHL is required for normal maintenance of podocytes, GBM composition and ultrastructure, and glomerular barrier properties.