Early Glomerular Filtration Defect and Severe Renal Disease in Podocin-Deficient Mice

Abstract

Podocytes are specialized epithelial cells covering the basement membrane of the glomerulus in the kidney. The molecular mechanisms underlying the role of podocytes in glomerular filtration are still largely unknown. We generated podocin-deficient (Nphs2 ؊/؊ ) mice to investigate the function of podocin, a protein expressed at the insertion of the slit diaphragm in podocytes and defective in a subset of patients with steroid-resistant nephrotic syndrome and focal and segmental glomerulosclerosis. Nphs2 ؊/؊ mice developed proteinuria during the antenatal period and died a few days after birth from renal failure caused by massive mesangial sclerosis. Electron microscopy revealed the extensive fusion of podocyte foot processes and the lack of a slit diaphragm in the remaining foot process junctions. Using real-time PCR and immunolabeling, we showed that the expression of other slit diaphragm components was modified in Nphs2 ؊/؊ kidneys: the expression of the nephrin gene was downregulated, whereas that of the ZO1 and CD2AP genes appeared to be upregulated. Interestingly, the progression of the renal disease, as well as the presence or absence of renal vascular lesions, depends on the genetic background. Our data demonstrate the crucial role of podocin in the establishment of the glomerular filtration barrier and provide a suitable model for mapping and identifying modifier genes involved in glomerular diseases caused by podocyte injuries.Glomeruli are specialized structures responsible for blood filtration in the kidney and are targets of injury in a number of human diseases. Plasma ultrafiltration occurs through the glomerular filtration barrier, which is composed of highly specialized visceral epithelial cells called podocytes, a fenestrated capillary endothelium, and an intervening glomerular basement membrane (GBM) (28). Podocytes are octopus-like cells (18), comprising a cell body and cytoplasmic extensions called major processes, which divide into actin-rich foot processes interdigitating over each capillary loop and counteracting the distensive forces. Each foot process is attached to its neighbor along its length by an intercellular adherens-type junction (46), the slit diaphragm (SD), which is located just above the GBM. The recent discovery of several novel podocyte proteins and the description of their physical and functional interactions highlighted the critical role of the SD complex and of the actin cytoskeleton in the maintenance of the glomerular filtration barrier (34, 39; reviewed in references 31 and 41).Glomerular diseases are associated with leakage of proteins across the filter into the urine and with disappearance (effacement) of podocyte foot processes. Whether effacement of foot processes is the cause or consequence of the glomerular filter alterations is uncertain. However, in renal diseases progressing toward focal and segmental glomerulosclerosis (FSGS), podocytes are now thought to be the primary targets, responsible for the development of the lesion. FSGS is characterized by segmental ...

Recent advances of animal model of focal segmental glomerulosclerosis

“…Myosin 1e (Myo1e) is one of the two Src homology 3 domain-containing "long-tailed" type I myosins in Actin filament cross-linking protein/Interacts with integrins and strengthens the podocyte-GBM interaction Atypical protein kinase C [68][69][70][71] Tight junctions/Formation of Par complex and interacts with slit diaphragm Rhophilin-1 [80] Rho GTPase activating protein 24 [80] Cytoplasm/Rho GTPase-interacting protein, integrity of glomerular filtration barrier Angiotensin II receptor [55][56][57] Angiotensin-converting enzyme [55][56][57] Membrane/pseudocyst formation at podocyte CD2-associated protein [65][66][67] CD2-associated protein [64] Insertion site of the slit diaphragm/Formation SD complex with podocin and nephrin Laminin subunit beta-2 [81][82][83][84] Laminin subunit beta-2 [81][82][83][84] Podocyte anchoring and differentiation in GBM microRNA 193α [74,75] Cytoplasm/Inhibition of expression of WT-1 Myosin 1e [49,50] Myosin 1e [49,50] Actin binding long-tailed motor protein/Regulation of actin cytoskeleton Nuclear factor of activated T cells [76,77] Transient receptor potential 6 [53,54] Membrane/the activation of calcineurin-NFAT/Wnt signaling via the increased calcium influx Podocin [46] Podocin [58,59] Insertion site of the SD/SD assembly and maintaining the signaling of nephrin Shroom family member 3 …”

“…Patients with mutations in the podocin gene develop familial and sporadic forms of FSGS. Since podocin-KO mice die within a few days of birth, the effect of its deficiency on kidney function could not be investigated until recently [58]. Utilization of inducible Cre recombinase technology enabled the creation of a mouse model of podocinrelated NS through elective inactivation of podocin in mature mouse kidneys.…”

Podocyte Mitosis – A Catastrophe

“…Ten different podocin ( NPHS2 ) mutations, comprising nonsense, frameshift and missense mutations, segregate with autosomal-recessive steroid resistant NS [27]. NPHS2 -null mice develop severe proteinuria during the antenatal period and die a few days after birth from renal failure [29]. Podocalyxin/NHERF2/ezrin/actin interactions are disrupted in pathologic conditions associated with changes in podocyte foot process, providing evidence of its relevance in vivo [11].…”

Defining cell-type specificity at the transcriptional level in human disease

“…Here we use kidney disease as a proof of concept application, focusing on the podocytes, the highly differentiated glomerular epithelial cells responsible for most hereditary and acquired glomerular disease (Gerstein 2001;Kim et al 2003;Roselli et al 2004;Groop et al 2009;D'Agati et al 2011;Niewold 2011). As with most other differentiated cell lineages in solid tissues, discovering human podocyte-specific genes on a whole-genome scale has remained infeasible due to the challenge of obtaining pure ex vivo populations of sufficient size for highthroughput evaluation, making this important cell lineage an ideal proof of concept application for nanodissection.…”