Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

Cell Biology of the Glomerular Podocyte

Familial idiopathic nephrotic syndromes represent a heterogeneous group of kidney disorders, and include autosomal recessive steroid-resistant nephrotic syndrome, which is characterized by early childhood onset of proteinuria, rapid progression to end-stage renal disease and focal segmental glomerulosclerosis. A causative gene for this disease, NPHS2, was mapped to 1q25-31 and we report here its identification by positional cloning. NPHS2 is almost exclusively expressed in the podocytes of fetal and mature kidney glomeruli, and encodes a new integral membrane protein, podocin, belonging to the stomatin protein family. We found ten different NPHS2 mutations, comprising nonsense, frameshift and missense mutations, to segregate with the disease, demonstrating a crucial role for podocin in the function of the glomerular filtration barrier.

NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome

Focal and segmental glomerulosclerosis (FSGS) is a common, non-specific renal lesion Although it is often secondary to other disorders, including HIV infection, obesity, hypertension and diabetes, FSGS also appears as an isolated, idiopathic condition FSGS is characterized by increased urinary protein excretion and decreasing kidney function Often, renal insufficiency in affected patients progresses to end-stage renal failure, a highly morbid state requiring either dialysis therapy or kidney transplantation Here we present evidence implicating mutations in the gene encoding alpha-actinin-4 (ACTN4; ref 2), an actin-filament crosslinking protein, as the cause of disease in three families with an autosomal dominant form of FSGS In vitro, mutant alpha-actinin-4 binds filamentous actin (F-actin) more strongly than does wild-type alpha-actinin-4 Regulation of the actin cytoskeleton of glomerular podocytes may be altered in this group of patients Our results have implications for understanding the role of the cytoskeleton in the pathophysiology of kidney disease and may lead to a better understanding of the genetic basis of susceptibility to kidney damage

https://www.childrenshospital.org/~/media/research-and-innovation/research-labs/beggs-lab/pdfs/44.ashx?la=en

Mutations in ACTN4, encoding alpha-actinin-4, cause familial focal segmental glomerulosclerosis.

Recent molecular insights have established the podocyte as a key component of the glomerular filtration barrier, and hence an important common pathway in proteinuric diseases. A conditionally immortalized human podocyte cell line has been developed by transfection with the temperature-sensitive SV40-T gene. These cells proliferate at the "permissive" temperature (33 degrees C). After transfer to the "nonpermissive" temperature (37 degrees C), they entered growth arrest and expressed markers of differentiated in vivo podocytes, including the novel podocyte proteins, nephrin, podocin, CD2AP, and synaptopodin, and known molecules of the slit diaphragm ZO-1, alpha-, beta-, and gamma-catenin and P-cadherin. The differentiation was accompanied by a growth arrest and the upregulation of cyclin-dependent kinase inhibitors, p27 and p57, as well as cyclin D(1), whereas cyclin A was downregulated. These data are consistent with cell cycle protein expression during podocyte maturation in vivo. In conclusion, the development of this cell line provides a new tool in the study of podocyte biology, which will enable accurate assessment of the behavior of these complex cells in health and disease.

A Conditionally Immortalized Human Podocyte Cell Line Demonstrating Nephrin and Podocin Expression

Nephrotic syndrome in childhood.

CD2-associated protein (CD2AP) is an 80-kilodalton protein that is critical for stabilizing contacts between T cells and antigen-presenting cells. In CD2AP-deficient mice, immune function was compromised, but the mice died at 6 to 7 weeks of age from renal failure. In the kidney, CD2AP was expressed primarily in glomerular epithelial cells. Knockout mice exhibited defects in epithelial cell foot processes, accompanied by mesangial cell hyperplasia and extracellular matrix deposition. Supporting a role for CD2AP in the specialized cell junction known as the slit diaphragm, CD2AP associated with nephrin, the primary component of the slit diaphragm.

http://science.sciencemag.org/content/sci/286/5438/312.full.pdf

Congenital Nephrotic Syndrome in Mice Lacking CD2-Associated Protein

CD2AP, an adapter protein containing multiple SH3 domains, plays a critical role in kidney function. Mice lacking CD2AP die soon after birth because of kidney failure. In the kidney, CD2AP is expressed in glomerular podocytes, which suggests that it may play a role in a specialized adhesion complex known as the slit diaphragm. One of the major components of the slit diaphragm is nephrin, a podocyte-specific protein. Here we demonstrate that CD2AP localizes to the slit diaphragm in podocytes using immunoelectron microscopy and that nephrin and CD2AP co-immunoprecipitate from a podocyte cell line. The specificity of this interaction was verified by mapping studies, which demonstrated that a novel domain at the C terminus of CD2AP interacts with the C-terminal portion of the nephrin cytoplasmic domain. These studies lend further support to the idea that CD2AP plays a role in the structural integrity of the slit diaphragm.

/pdf/cd2ap-localizes-to-the-slit-diaphragm-and-binds-to-nephrin-2mt1hx2z08.pdf

Neng-Yao Shih

Papers

Congenital Nephrotic Syndrome in Mice Lacking CD2-Associated Protein

CD2AP Localizes to the Slit Diaphragm and Binds to Nephrin via a Novel C-Terminal Domain