Surprising Results following Conditional Podocyte Inactivation
Authors: Martin R. Pollak
Journal: Journal of the American Society of Nephrology (2009)

The genetic contribution to steroid-resistant nephrotic syndrome and related disorders of the podocyte has been widely appreciated only in the past decade. In fact, for at least 50 yr, nephrosis has been observed occasionally in multiple members of the same family. In a 1957 study, Farquhar et al.1 performed a detailed electron microscopic analysis in kidneys from four siblings all with manifestations of nephrosis, noting a loss of the normal intricate glomerular epithelial cell architecture. This intricate and delicate—and perhaps even beautiful—glomerular epithelial cell structure has become the focus of intense investigation, accelerated in part by the discovery of genes underlying monogenic forms of podocyte disease. In 1995, Fuchshuber et al.2 in Antignac's group3 identified a locus for what they termed steroid-resistant nephrotic syndrome on chromosome 1. Five years later, these investigators identified the responsible gene, NPHS2, encoding a novel transmembrane protein they named podocin. Since the positional cloning of NPHS2, understanding of the role of podocin in glomerular function has grown tremendously, as has our understanding of the role of variation in NPHS2 in human disease. We have learned that podocin is an integral membrane protein that localizes to the glomerular slit diaphragm and interacts with nephrin, the protein mutated in congenital nephrotic syndrome of the Finnish type.4,5 We know that podocin facilitates nephrin-mediated cell signaling, and mutations in podocin disrupt nephrin trafficking to the cell membrane.6,7 Worm biology has also contributed to our understanding: Podocin shares certain structural and functional characteristics with the homologous touch-sensitive Caenorhabditis elegans protein MEC-2, binding cholesterol and regulating associated ion channels.8 Of the various genes that when mutated lead to human phenotypes, NPHS2-associated disease gives rise to the widest spectrum of clinical disease. Although we typically think of the nephrin gene NPHS1 as the congenital nephrotic syndrome gene, NPHS2 mutations are also a frequent cause of this neonatal syndrome.9 At the other end of the spectrum, mutations in both NPHS2 alleles can also cause a clinical presentation much later in life as adult-onset FSGS.10 Understanding the in vivo role of podocin in podocyte function and disease is clinically important: NPHS2 mutations are responsible for a large fraction of steroid-resistant nephrotic syndrome and FSGS.10 Several years ago, Antignac's group11 developed a mouse model lacking expression of the podocin gene NPHS2. These mice develop severe nephrosis before birth and massive podocyte foot process effacement, as well as mesangial sclerosis and vascular lesions of variable severity. Much of this variability depends on genetic background. These investigators went on to develop a mouse with a genetically engineered R138Q point mutation in podocin to model one of the most common disease-causing human mutations. The homozygous mice died at an early age, showing mislocalization of podocin as well as nephrin. Although these mice also express severe nephrosis, mesangiolysis, and mesangial sclerosis, the kidneys show a different pattern of altered gene expression than did the knockout mice.12 In this issue of JASN, Mollet et al.13 report a new mouse model of podocin-mediated disease. This model takes nice advantage of Cre/Lox technology that allows targeted inactivation of a gene of interest. Using a Cre transgenic mouse in which the podocin promoter drives expression of Cre recombinase in a tamoxifen-inducible, podocyte-specific manner, Mollet et al. inactivate podocin in podocytes. When administered to 6-wk-old transgenic mice, tamoxifen led to the inactivation of podocin in approximately 70% of podocytes. This in turn led to massive albuminuria, hyperlipidemia, slowly progressive loss of glomerular filtration, and eventually death from kidney failure. We now know with certainty that podocin is not only essential for the development of normal glomerular architecture but also critical for maintaining normal glomerular, podocyte, and slit diaphragm function. The absence of overt vascular and mesangial lesions in this model also suggests that podocin has different, albeit related, functions in development and in adult physiology. Despite that this model leads to expression of Cre in 70% of podocytes (and a 50% reduction in podocin expression after 1 wk), these mice progress rapidly to overt kidney failure. This raises new questions about podocyte–podocyte and glomerular–glomerular communication: Why does loss of 50% of podocin lead to a much more severe phenotype than, say, losing 50% of nephrons from nephrectomy? Too many knockout models fail to answer the question asked. Like the old joke about the grasshopper (the distinguished professor demonstrates that the grasshopper's legs are required for hearing, because once its legs are cut off, it fails to jump in response to a loud noise), absence of a gene from the beginning of development (from a knockout experiment) may not accurately answer questions of mature physiology. The study by Mollet et al. illustrates the power of current mouse genetic tools to address such questions. Disclosures None.