Abstract

Urinary losses of macromolecules in nephrotic syndrome (NS) reflect a dysfunction of the highly permselective glomerular filtration barrier. Genetic studies of hereditary forms of NS have led to the identification of proteins playing a crucial role in slit-diaphragm signalling, regulation of actin cytoskeleton dynamics, maintenance of podocyte integrity and cell–matrix interactions. This review will focus on recent molecular and clinical findings in the field of genetics of NS, thereby providing a better understanding of the complex glomerular filtration barrier physiology.

INTRODUCTION

Nephrotic syndrome (NS) is a heterogeneous group of disorders characterized by heavy proteinuria with hypoalbuminemia, edema and dyslipidemia. Urinary losses of macromolecules, such as albumin, reflect a dysfunction of the normally highly permselective glomerular filtration barrier (GFB) (1). The GFB consists of three interacting layers (Fig. 1): the glomerular fenestrated endothelium, the glomerular basement membrane (GBM) and the podocytes, with their interdigitated foot processes interconnected by the slit diaphragm (SD), a multiprotein structural and signalling complex. Two additional layers, the endothelial surface layer and the subpodocyte space, are now also considered as part of the major determinants of glomerular permeability (1,2). The GFB mainstone is the podocyte, which contains a highly dynamic cytoarchitecture exhibiting enormous plasticity in response to harmful events. Indeed, the profound morphologic changes (complete foot-process effacement, dedifferentiation and focal detachment) occurring during NS may be reversible in cases without a primary podocyte defect. GFB dysfunction may be secondary to an immune disorder or due to intrinsic podocytes defects. Accumulating data suggests that steroid-sensitive NS, as well as a subset of steroid-resistant NS (SRNS), particularly those with response to immunosuppressive agents and/or relapsing after kidney transplantation, have an underlying immune defect. Indeed, immediate and iterative recurrence of proteinuria after transplantation and the favourable effect of plasma exchange (3) or immunoadsorption (4,5) support the putative role of an unrecognized circulating permeability factor (6), whose production seems to follow T cell dysfunction, among immune forms of NS (7). Instead, SRNS cases without relapse after transplantation (8–10) as well as familial forms of SRNS, are generally due to a primary defect in the GFB, are resistant to other immunosuppressive agents and almost invariably progress to end-stage kidney disease (ESKD).

Figure 1.

Glomerular filtration barrier structure. (A) Glomerular structure: (AA) afferent arteriole, (AE) efferent arteriole, (DT) distal tubule, (C) capillary loop, (P) podocyte, (FP) podocyte foot-processes, (M) mesangium, (U) urinary space, Bowman's capsule (BC) and (PT) urinary pole of the proximal tubule. (B) Details of the glomerular filtration barrier structure: (FP) podocyte foot-process, (GBM) glomerular basement membrane, (EF) endothelial fenestration and (SD) slit-diaphragm. Subpodocyte space (SP) and the endothelial surface layer (ESL) are also considered as main components of the GFB.

Figure 1.

Glomerular filtration barrier structure. (A) Glomerular structure: (AA) afferent arteriole, (AE) efferent arteriole, (DT) distal tubule, (C) capillary loop, (P) podocyte, (FP) podocyte foot-processes, (M) mesangium, (U) urinary space, Bowman's capsule (BC) and (PT) urinary pole of the proximal tubule. (B) Details of the glomerular filtration barrier structure: (FP) podocyte foot-process, (GBM) glomerular basement membrane, (EF) endothelial fenestration and (SD) slit-diaphragm. Subpodocyte space (SP) and the endothelial surface layer (ESL) are also considered as main components of the GFB.

Whereas most of the cases with SSNS exhibit minimal glomerular changes distinguished by normal glomeruli at light microscopy and diffuse podocyte foot-process effacement on electron microscopy, cases with SRNS present with either focal segmental glomerulosclerosis (FSGS) or diffuse mesangial sclerosis (DMS). In addition to sclerosis, FSGS consists of foot process effacement in absence of glomerular immune complex deposits, although DMS is characterized by mesangial matrix expansion accompanied by hypertrophy and mild cobblestone hyperplasia of podocytes. Indeed, DMS and FSGS result from a more sustained and severe injurious process which eventually leads to progressive podocyte loss and glomerular scarring.

Until the familial pattern of some glomerular diseases was recognized, little was known on hereditary NS (11). In the last decade, studies of familial cases of SRNS have led to the identification of genes encoding proteins highly expressed in podocytes, but also elsewhere in the glomerular capillary wall, unravelling the basis of NS and GFB physiology (Table 1). Structural elements of the SD (nephrin, podocin and CD2AP) and actin cytoskeleton (α-actinin-4) (12–15) control podocyte differentiation and survival (16), cell polarity (17) and cytoskeletal dynamics (18) (Fig. 2). Podocyte and glomerular development are critically regulated by the transcription factor WT1 and phospholipase C ε1 (PLCε1) mediated signals (19,20). The calcium channel TRPC6, which localizes in membrane lipids supercomplex along podocin, regulates mechanosensation sensed at the SD (21), whereas the structural component of the GBM, laminin-β2, is essential for podocyte cell–matrix interactions (22). Podocyte integrity may also be affected by derangements in proteins involved in varied subcellular processes including the mitochondrial respiratory chain, DNA restructuring and repair and lysosomal function. Finally, identification of novel genetic determinants in glomerular disease, such as high risk haplotypes in the MYH9 gene (23,24) may also explain the increased risk of some adult patients to glomerular injury.

Figure 2.

Molecular overview of the slit-diaphragm and podocyte cell–matrix interactions. (FP) foot-process, (SD) slit diaphragm, (GBM) glomerular basement membrane and (EF) fenestrated endothelium. At the SD, nephrin mediated signals that control actin cytoskeleton remodeling (NCK 1/2, WASp), cell polarity (Par3/6, aPKC) and survival (PI3K, AKT). TRPC6-podocin interactions modulate mechanosensation, whereas angiotensin II type 1 receptor (AT1) may increase TRPC6 mediated calcium influx upon stimuli by angiotensin II (AGT II). Activation of PLCε1 degrades phosphatidyl inositol-bisphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3), which leads to protein kinase C (PKC) activation and Ca2+ efflux from the endoplasmic reticulum (ER). Main component of the podocyte-matrix interaction structure include the integrin α3β1—laminin α5β2γ1 and dystroglycan—uthropin complexes which connect the GBM components (proteoglycans, nidogen, perlecan, agrin and type-IV collagen) to the cell actin cytoskeleton. Additional pathways controlling actin cytoskeleton remodelling include the podocalyxin, NHERF 1/2, ezrin (EZR) complex. Molecules and pathways included are only those relevant to GFB function as defects are related with a human or animal glomerular disease phenotype.

Figure 2.

Molecular overview of the slit-diaphragm and podocyte cell–matrix interactions. (FP) foot-process, (SD) slit diaphragm, (GBM) glomerular basement membrane and (EF) fenestrated endothelium. At the SD, nephrin mediated signals that control actin cytoskeleton remodeling (NCK 1/2, WASp), cell polarity (Par3/6, aPKC) and survival (PI3K, AKT). TRPC6-podocin interactions modulate mechanosensation, whereas angiotensin II type 1 receptor (AT1) may increase TRPC6 mediated calcium influx upon stimuli by angiotensin II (AGT II). Activation of PLCε1 degrades phosphatidyl inositol-bisphosphate (PIP2) into diacylglycerol (DAG) and inositol triphosphate (IP3), which leads to protein kinase C (PKC) activation and Ca2+ efflux from the endoplasmic reticulum (ER). Main component of the podocyte-matrix interaction structure include the integrin α3β1—laminin α5β2γ1 and dystroglycan—uthropin complexes which connect the GBM components (proteoglycans, nidogen, perlecan, agrin and type-IV collagen) to the cell actin cytoskeleton. Additional pathways controlling actin cytoskeleton remodelling include the podocalyxin, NHERF 1/2, ezrin (EZR) complex. Molecules and pathways included are only those relevant to GFB function as defects are related with a human or animal glomerular disease phenotype.

Table 1.

Hereditary forms of Nephrotic Syndrome

Gene Locus Inheritance Protein Functiona Phenotype or Syndrome 
Slit-Diaphragm protein complex 
NPHS1 19q13.1 AR Nephrin Main component of the SD. Anchors the SD to the actin cytoskeleton. Modulate signalling events related with actin cytoskeleton dynamics, cell polarity and survival CNS of the Finnish type. Early-onset SRNS in cases carrying at least one mild mutation 
NPHS2 1q25–31 AR Podocin Scaffold protein linking plasma membrane to the actin cytoskeleton. Modulates mechanosensation CNS. Early and late onset AR SRNS. Juvenile and adult SRNS in cases bearing the R229Q variant in compound heterozygous state with a pathogenic mutation 
PLCE1 10q23 AR Phospholipase Cε1 Involved in cell junction signalling and glomerular development Early-onset SRNS with DMS and FSGS 
CD2AP 6p12.3 AR (?) CD2 associated protein Adapter protein, may anchor the SD to the actin cytoskeleton Not precisely defined in humans, may cause early-onset SRNS and FSGS. Mice model exhibits a severe phenotype resembling CNS in humans 
TRPC6 11q21–22 AD TRPC6 Receptor-activated non-selective calcium permeant cation channel. Involved in mechanosensation Adult-onset SRNS with FSGS 
Actin cytoskeleton components 
ACTN4 19q13 AD α-actinin-4 F-actin cross-linking protein Late-onset SRNS with incomplete penetrance and slow progression to ESRD 
MYH9 22q12.3 complex NMMHC-A Cellular myosin that appears to play a role in cytokinesis and cell shape High risk haplotypes associated with increased risk of FSGS and ESKD in African-Americans 
Nuclear proteins 
LMX1B 9q34.1 AD LIM/homeobox protein LMX1B Podocyte and GBM development and maintenance Nail-patella syndrome. NS in 40% of cases 
SMARCAL1 2q35 AR hHARP ATP-dependent annealing helicase that rewind stably unwound DNA Schimke immuno-osseus dysplasia 
WT1 11p13 AD Wilms’ tumour 1 Zinc finger transcription factor that functions both as a tumour suppressor and as a critical regulator of kidney and gonadal development Denys–Drash syndrome, Frasier syndrome, WAGR syndrome, isolated FSGS and DMS 
Glomerular basement membrane proteins 
LAMB2 3p21 AR Laminin-β2 GBM component, scaffold for type IV collagen assembly. Interactions with integrin α3β1 links the GBM to the actin cytoskeleton Pierson syndrome 
ITGB4 17q25.1 AR Integrin-β4 Cell-matrix adhesion, critical structural role in the hemidesmosome of epithelial cells Epidermolysis bullosa. Anecdotic cases presenting with NS and FSGS 
Mitochondrial proteins 
COQ2 4q21–q22 AR Polyprenyltransferase CoQ10 biosynthesis, which transfers electrons from the mitochondrial respiratory chain COQ10 deficiency, early-onset SRNS, with or without encephalomyopathy 
PDSS2 6q21 AR Decaprenyl diphosphate synthase-2 CoQ10 biosynthesis, which transfers electrons from the mitochondrial respiratory chain COQ10 deficiency, Leigh syndrome and SRNS 
MTTL1 mtDNA  tRNA-LEU Mitochondrial tRNA for leucine MELAS syndrome. Mitochondrial diabetes, deafness and FSGS, with or without nephrotic syndrome 
Lysosomal proteins 
SCARB2 4q13–21 AR LIMP II May act as a lysosomal receptor Action myoclonus renal failure 
Gene Locus Inheritance Protein Functiona Phenotype or Syndrome 
Slit-Diaphragm protein complex 
NPHS1 19q13.1 AR Nephrin Main component of the SD. Anchors the SD to the actin cytoskeleton. Modulate signalling events related with actin cytoskeleton dynamics, cell polarity and survival CNS of the Finnish type. Early-onset SRNS in cases carrying at least one mild mutation 
NPHS2 1q25–31 AR Podocin Scaffold protein linking plasma membrane to the actin cytoskeleton. Modulates mechanosensation CNS. Early and late onset AR SRNS. Juvenile and adult SRNS in cases bearing the R229Q variant in compound heterozygous state with a pathogenic mutation 
PLCE1 10q23 AR Phospholipase Cε1 Involved in cell junction signalling and glomerular development Early-onset SRNS with DMS and FSGS 
CD2AP 6p12.3 AR (?) CD2 associated protein Adapter protein, may anchor the SD to the actin cytoskeleton Not precisely defined in humans, may cause early-onset SRNS and FSGS. Mice model exhibits a severe phenotype resembling CNS in humans 
TRPC6 11q21–22 AD TRPC6 Receptor-activated non-selective calcium permeant cation channel. Involved in mechanosensation Adult-onset SRNS with FSGS 
Actin cytoskeleton components 
ACTN4 19q13 AD α-actinin-4 F-actin cross-linking protein Late-onset SRNS with incomplete penetrance and slow progression to ESRD 
MYH9 22q12.3 complex NMMHC-A Cellular myosin that appears to play a role in cytokinesis and cell shape High risk haplotypes associated with increased risk of FSGS and ESKD in African-Americans 
Nuclear proteins 
LMX1B 9q34.1 AD LIM/homeobox protein LMX1B Podocyte and GBM development and maintenance Nail-patella syndrome. NS in 40% of cases 
SMARCAL1 2q35 AR hHARP ATP-dependent annealing helicase that rewind stably unwound DNA Schimke immuno-osseus dysplasia 
WT1 11p13 AD Wilms’ tumour 1 Zinc finger transcription factor that functions both as a tumour suppressor and as a critical regulator of kidney and gonadal development Denys–Drash syndrome, Frasier syndrome, WAGR syndrome, isolated FSGS and DMS 
Glomerular basement membrane proteins 
LAMB2 3p21 AR Laminin-β2 GBM component, scaffold for type IV collagen assembly. Interactions with integrin α3β1 links the GBM to the actin cytoskeleton Pierson syndrome 
ITGB4 17q25.1 AR Integrin-β4 Cell-matrix adhesion, critical structural role in the hemidesmosome of epithelial cells Epidermolysis bullosa. Anecdotic cases presenting with NS and FSGS 
Mitochondrial proteins 
COQ2 4q21–q22 AR Polyprenyltransferase CoQ10 biosynthesis, which transfers electrons from the mitochondrial respiratory chain COQ10 deficiency, early-onset SRNS, with or without encephalomyopathy 
PDSS2 6q21 AR Decaprenyl diphosphate synthase-2 CoQ10 biosynthesis, which transfers electrons from the mitochondrial respiratory chain COQ10 deficiency, Leigh syndrome and SRNS 
MTTL1 mtDNA  tRNA-LEU Mitochondrial tRNA for leucine MELAS syndrome. Mitochondrial diabetes, deafness and FSGS, with or without nephrotic syndrome 
Lysosomal proteins 
SCARB2 4q13–21 AR LIMP II May act as a lysosomal receptor Action myoclonus renal failure 

(AR) autosomal recessive, (AD) autosomal dominant. In certain cases, mutations in WT1, LAMB2, COQ2 and PDSS2 can be associated with isolated SRNS. A locus for SSNS has been mapped on chr 2p12–13.2. Two loci on chr 14q24.2 and 11q24 have been mapped in cases with SRNS and deafness of AR and AD inheritance, respectively.

aOnly protein functions directly related with podocyte physiology and NS are listed.

Advances in genetics of SRNS have significantly increased our understanding of GFB physiology providing insights into future promising therapeutic strategies. Genotype-phenotype correlations and recent molecular findings in the field of hereditary NS will be reviewed here.

GENETIC OVERVIEW

Main autosomal recessive forms of NS

Positional cloning has allowed identifying the main GFB players. Mutations in genes encoding nephrin, podocin and PLCε1 are responsible for most of the severe cases of congenital and early onset NS (Table 1, Fig. 3); however, it has been recently shown that mutations may be associated with less severe phenotypes, either given the type of mutation (mild mutation or non-silent polymorphisms) and/or because of modifier genes affecting the final phenotype (Fig. 3).

Figure 3.

Age at onset of nephrotic syndrome according to the underlying genetic defect. NS, nephrotic syndrome; CNS, congenital nephrotic syndrome; AR, autosomal recessive; AD, autosomal dominant. Light blue, light red and light green represent non-classical ages at onset of NS that have also been reported in the literature. Only one homozygous CD2AP mutation has been described in an infantile form of FSGS. Few heterozygous CD2AP mutations have also been reported in pediatric- and adult-onset FSGS with incomplete segregation data. The paucity of reports on CD2AP mutations precludes us to define a precise human phenotype.

Figure 3.

Age at onset of nephrotic syndrome according to the underlying genetic defect. NS, nephrotic syndrome; CNS, congenital nephrotic syndrome; AR, autosomal recessive; AD, autosomal dominant. Light blue, light red and light green represent non-classical ages at onset of NS that have also been reported in the literature. Only one homozygous CD2AP mutation has been described in an infantile form of FSGS. Few heterozygous CD2AP mutations have also been reported in pediatric- and adult-onset FSGS with incomplete segregation data. The paucity of reports on CD2AP mutations precludes us to define a precise human phenotype.

NPHS1 encodes nephrin, the principal component of the SD, and has been identified as the major gene involved in congenital nephrotic syndrome (CNS) of the Finnish type, with the Finmajor (c.121delCT; p.L41fs) and Finminor (c.3325C>T; p.R1109X) mutations accounting respectively for 78 and 16% of the mutated alleles in Finnish patients (12). NPHS2, encoding podocin, is responsible for most of infantile SRNS cases. Recessive mutations in this gene account for 42% of familial and 10% of sporadic cases of childhood-onset SRNS (8,25) and have also been found in 39% of patients with CNS (26). Recessive truncating and missense mutations in PLCE1, encoding PLCε1, have been detected in early-onset NS showing DMS or FSGS on renal histology, respectively (20). Subsequent mutational analysis among children with DMS demonstrated that 28.6% of cases had PLCE1 mutations (27), whereas no mutations had been found among adults with FSGS (28).

Interestingly, some NPHS1 mutations have been associated with a milder clinical course characterized by preserved renal function and proteinuria reduction towards adolescence; usually cases are females, suggesting a gender modifier effect (29). Recently, NPHS1 mutations have also been identified in patients with childhood-onset SRNS (30). Affected cases were compound heterozygotes for at least one mild missense mutation which exhibited normal trafficking to the plasma membrane. This contrasts with most of the nephrin and podocin missense mutants found in cases with very early-onset of hereditary NS (31,32), which are retained in the endoplasmic reticulum, thus acting as a null allele, and consequently leading to a severe phenotype. Therapeutic strategy relying on chemical chaperones allowing targeting of the mutant protein to the SD (33) may eventually ameliorate the clinical course of the disease. Intriguingly, compound heterozygous missense mutations were identified in two cases presenting subnephrotic persistent proteinuria and self-limited intermittent nephrotic flares triggered by upper respiratory tract infections (34). In vitro studies showed that one of these mutants, which was predominantly targeted to the plasma membrane, was unable to assemble into functioning membrane microdomains (35), thus probably hampering nephrin signalling and SD complex anchoring to the actin cytoskeleton. This observation raises the possibility that mild structural defects in the SD complex may predispose to progressive glomerular disease following intermittent environmental injurious events.

Similarly, genotype–phenotype correlations among cases with NPHS2 mutations revealed that compound heterozygotes for one pathogenic NPHS2 mutation and the p.R229Q variant (most frequent non-synonymous NPHS2 polymorphism among European derived populations) may cause juvenile or adult-onset NS (36). Moreover, the renal phenotype variability among patients bearing podocin mutations suggests a role for modifiers genes. Analysis of Nphs2 null mice phenotype revealed that glomerular disease progression and severity of histological lesions strongly depend on the genetic background and the maternal environment in which mice are nourished (37). In addition, it has recently been shown in mice that the transcript level of the Nphs2 gene is heritable and controlled by an ancestral cis-eQTL (38). These novel findings highlight the importance of putative modifier genes within podocyte-expressed genes in glomerular disorders and progression towards ESKD.

Milder phenotypes have also been described with PLCE1 mutations, including sustained complete remission in two cases with truncating mutations (20,39). A striking observation has been the identification of unaffected siblings bearing inactivating mutations in both alleles as the corresponding index case (20). We have also identified three asymptomatic adults from three unrelated families bearing homozygous mutations with at least one affected sibling haploidentical to the unaffected cases (Antignac, unpublished). These observations open the question of whether incomplete penetrance or therapy response particularly at critical phases during podocyte development may be due to the modifier role of podocyte specific genes or to individual differences due to the compensatory effect of other PLCs highly expressed on podocytes.

The undefined phenotype of mutations in CD2AP

CD2AP encodes the CD2 adaptor protein (CD2AP). In spite of a clear association of CD2AP defects with glomerular disease in animal models (40), little is known about the human phenotype associated with CD2AP mutations. Cd2ap knockout mice have a severe phenotype, in contrast to Cd2ap haploinsufficient mice which develop minor glomerular changes, without proteinuria (14). One heterozygous splice-site mutation in two patients with primary FSGS leading to a reduced expression of CD2AP in lymphocytes has been described (14). A recent study found three unrelated cases bearing heterozygous mutations associated with defective CD2–CD2AP interaction in T-lymphocytes as well as down-regulation of CD2AP, podocin and nephrin glomerular expression on renal biopsies (41). Interestingly, a homozygous mutation has been found in a 10-month-old patient with FSGS, resulting in a decreased F-actin binding efficiency in vitro and no expression of the mutated allele in lymphocytes. Both heterozygous parents were clinically unaffected (42). Because CD2AP mutations reports are scarce and complete segregation data are not available in cases with CD2AP heterozygous mutations, the phenotype and pattern of inheritance of CD2AP mutations or its putative role in the susceptibility to develop glomerular disease in humans remain uncertain.

Autosomal dominant forms of FSGS

Familial forms of NS of autosomal dominant (AD) inheritance are rare, occurring mostly among juvenile and adult cases. So far, mutations in the ACTN4 and TRPC6 genes, encoding α-actinin-4 and the transient receptor potential cation channel 6 (TRPC6), respectively, have been involved in this form of NS (15,43). Nevertheless, most families with AD FSGS do not have mutations in these two genes (21,44). Recently, an additional locus to AD FSGS and deafness has been mapped to chromosome 11q24 (45). Individuals bearing ACTN4 missense mutations have FSGS with an incompletely penetrant phenotype: proteinuria develops within the second decade of life and disease progresses to ESKD by 50 years of age (15,44,46,47). On the other hand, patients with TRPC6 mutations also exhibit an incomplete penetrant phenotype, but present with FSGS in their third or fourth decade of life (48).

FUNCTIONAL ROLES OF THE MAIN PODOCYTE PROTEINS

Slit-diaphragm signalling complex and regulation of the actin cytoskeleton dynamics

Previously seen as a static molecular sieve, the SD is now known as a dynamic signalling complex interacting with the submembranous cytoskeleton for maintaining the podocyte architecture and the function of the glomerular filter of the kidney (Fig. 2). Nephrin, podocin and CD2AP are considered as the main structural elements of the SD. Nephrin, a single-pass transmembrane protein of the immunoglobulin superfamily, homodimerizes and forms heterodimers with its homolog NEPH1, thus connecting adjacent foot-processes and transducing signals that control glomerular permeability (49). Nephrin interacts through its C-terminal part with podocin, a transmembrane harpin-like scaffolding protein, and with CD2AP, an adapter protein also found on the surface of T-cells and natural killer cells. The nephrin/NEPH1 complex transduces phosphorylation-mediated signals that assemble an actin polymerization complex at the podocyte intercellular junction. Indeed, nephrin/NEPH1 complex recruits Grb2 and Nck1/2 adaptor proteins (18), which mediate downstream activation of the cytoskeletal regulators N-WASp and Pak (50). In addition, nephrin phosphorylation by Fyn kinase increases its interaction with phosphatidylinositol 3-kinase (PI3K) and the subsequent PI3K-dependent activation of Akt and Rac modifies actin cytoskeleton (51), confirming the determinant role of nephrin signalling on podocyte morphology. Similarly, CD2AP has been implicated in the PI3K/AKT survival pathway (52) and in dynamic actin remodelling (53). Another function of the nephrin/NEPH1 complex is the regulation of podocyte cell polarity, via its binding with Par3, Par6 and atypical protein kinase C (aPKC) complex (17). The aPKC signalling is fundamental to glomerular maintenance and development, as shown in mice with podocyte-specific deletion of aPKCλ/ι, resulting in mislocalization of the SD and NS (54).

The critical importance of an intact podocyte actin cytoskeleton has been highlighted by the fact that mutations in the actin-bundling protein α-actinin-4 lead to AD FSGS. Exposure of a buried actin-binding site in mutant α-actinin-4 causes an increase in its binding affinity for F-actin and diverts its normal localization from actin stress fibres and focal adhesions. Actin filaments crosslinked by the mutant α-actinin-4 exhibit profound changes of structural and biomechanical properties, thereby perturbating the podocyte cytoskeleton (55). Finally, α-actinin-4 is required for normal podocyte adhesion, as it interacts with integrins strengthening the podocyte–GBM interaction (56,57).

Unexpectedly, PLCε1 also appears to be a crucial podocyte protein, highly expressed in embryonic and mature glomeruli, which catalyzes the hydrolysis of phosphatidylinositol-4,5-bispohosphate and generates the second messengers inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG) (20,58). IP3 releases Ca2+ from intracellular stores and DAG stimulates PKC, initiating a cascade of signalling events resulting in cell growth and differentiation (58). Additionally, PLCε1 interacts with IQGAP-1 (20), a podocyte cell junction-associated protein and interacting partner of nephrin implicated in cell adhesion (59). The absence of PLC-ε1 is associated with a significant reduction in nephrin and podocin expression and may arrest glomerular development at the capillary loop stage (20) suggesting a potential role of PLCε1 not only in cell junction and signalling events, but in development, as well (20).

Podocyte mechanosensation and modulation of signals leading to glomerular disease

The identification of mutations in TRPC6 among cases with AD FSGS (43), unexpectedly added an ion channel to the list of SD signalling molecules and provided new insight into mechanosensation. In addition to its role in ion homeostasis, cell growth and PLC dependent calcium entry into cells (60), TRPC6 is a sensor of mechanically and osmotically induced membrane stretch (61) and is regulated by a podocin–lipid complex that might translate mechanical tension to ion channel action (62). As both TRPC6 and nephrin are a target of the tyrosine kinase Fyn, which increases the TRPC6 channel activity, it has also been suggested that TRPC6 is assembled in a complex together with nephrin and Fyn at the SD (63). In vitro experiments revealed that several TRPC6 mutants show increased current amplitudes; in addition, TRPC6P112Q exhibited an augmented angiotensin II-dependent calcium influx (43), leading to the hypothesis that mutations may disrupt podocyte cell function by amplifying calcium signals. Interestingly, TRPC6 mutations also enhances basal NFAT-mediated transcription in cultured podocytes, a pathway that can be blocked by inhibitors of calcineurin, calmodulin-dependent kinase II, and PI3K (64). Thus, the calcineurin-NFAT pathway may be a potential mediator of FSGS and calcineurin inhibitors may have a therapeutic role by blocking TRPC6 downstream signalling events.

DERANGEMENTS OF PODOCYTE CELL–MATRIX INTERACTIONS

Laminins are heterotrimeric extracellular matrix proteins consisting of α, β and γ subunits that provide the basic scaffold for assembly of type IV collagen, nidogen/entactin and sulphated proteoglycans in GBM (65). Laminin-521 (α5, β2 and γ1) is the most important β2-containing laminin isoform and is specifically expressed in the GBM and at some other sites, including intraocular muscles (66). Mutations in LAMB2 gene cause Pierson syndrome (67), an autosomal recessive (AR) disorder initially described as a severe lethal phenotype, characterized by CNS with DMS, rapidly progressing renal dysfunction and ocular malformations with microcoria as the leading feature. Blindness and neurodevelopmental deficits were noted in patients surviving infancy (68,69). The clinical spectrum of Pierson syndrome has been extended to milder phenotypes, including infantile- or childhood-onset NS (70,71), variable or even lacking ocular abnormalities (71,72) and renal survival at 16 years of age (71). This suggests that genetic modifiers may play a role in the phenotypic variability of cases with Pierson syndrome. Laminins receptors expressed on the basal side of podocytes foot processes include integrin α3β1 and dystroglycan which link the GBM to the intracellular actin cytoskeleton through a set of integrin and actin-associated proteins that include paxillin, talin, vinculin, α-actinin and filamin (22) (Fig. 2). The importance of podocyte cell–matrix interactions, although extensively confirmed by the severe disease phenotype of LAMB2 mutations in humans, has been emphasized by a conditional mouse model exhibiting podocyte-specific deletion of integrin-linked kinase (ILK), which is a downstream mediator of integrin β1 activity (73). In this model, GBM alterations preceded podocyte damage and the development of glomerulosclerosis, suggesting that alteration in matrix assembly subsequent to ILK deletion via modified integrin is responsible for the renal phenotype (73).

INHERITED DEFECTS OF MITOCHONDRIAL AND LYSOSOMAL COMPONENTS LEADING TO PROFOUND PODOCYTE DYSFUNCTION

Renal dysfunction due to mitochondropathies is infrequent, and may be a consequence of mutations in the mitochondrial or nuclear genomes (Table 1). Coenzyme Q10 (CoQ10) is a lipophilic molecule that transfers electrons from mitochondrial respiratory chain complexes I and II to complex III (74). Deficiency of CoQ10 has been associated with encephalomyopathy and multisystemic involvement, including NS (75–77). The COQ2 gene encodes the para-hydroxybenzoate-polyprenyl-transferase enzyme, which is part of the CoQ10 pathway (74,78). COQ2 mutations have been identified in patients presenting with early-onset NS, severe oliguric renal failure, collapsing glomerulopathy with or without neuromuscular symptoms (79). The PDSS2 gene encodes a subunit of decaprenyl diphosphate synthase, the first enzyme of the CoQ10 biosynthetic pathway (80). Mutations in the PDSS2 gene have been recently described in a patient with Leigh syndrome, CoQ10 deficiency and NS (80). CoQ10 supplementation rescues the renal disease in Pdss2kd/kd mice (81). Thus, early CoQ10 supplementation may be crucial for renal symptoms resolution and prevention of neurological damage, as recently demonstrated in patients with CoQ10 deficiency due to COQ2 mutations (82). Finally, mutations in the tRNALeu(UUR) gene are associated with MELAS syndrome (83), which may include FSGS nephropathy (84,85).

Homozygous truncating SCARB2 mutations have been associated with action myoclonus-renal failure syndrome (86), an AR disorder presenting in young adults and characterized by collapsing FSGS and progressive myoclonic epilepsy. SCARB2 encodes the lysosomal integral membrane protein LIMP-2, which is a receptor for lysosomal mannose-6-phosphate-independent targeting of β-glucocerebrosidase (βGC) (87). βGC is a lysosomal enzyme deficient in most cases of Gaucher disease, leading to the accumulation of its substrate, glucosylceramide (GlcCer). Although decreased βGC activity and protein expression have been shown in LIMP-2 deficient mice (87), residual βGC activity may be sufficient to prevent GlcCer accumulation to levels at which more significant Gaucher-like pathologies might be seen. The pathophysiologic events leading to glomerular disease in cases with SCARB2 mutations remain unknown.

COMPLEX GENETIC DETERMINANTS OF GLOMERULAR DISEASE: THE ROLE OF MYH9

An exceptional example of the genetic complexity of NS was shown by two independent studies demonstrating a strong association of common genetic variants in the MYH9 gene with FSGS and non-diabetic ESKD (23,24). Pathogenic mutations in the MYH9 gene, encoding the molecular motor protein non-muscle myosin heavy chain type II isoform A (NMMHC-A), are associated with the AD giant-platelet disorders which may include sensorineural deafness and glomerular disease. Recently, multiple linked non-coding SNPs in MYH9 were found to confer two to four times increased risk of ESKD in African Americans compared with European Americans (23). Moreover, the presence of the same risk haplotype was associated with almost a 5-fold increased risk of FSGS (24), accounting for a large proportion of the excess risk of ESKD and FSGS observed in African compared with European Americans. It has been demonstrated that NMMHC-A acts as a component of the podocyte cytoskeleton, contributing to its contractile functions (88,89); however, the underlying pathophysiologic events occurring at the podocyte cytoskeleton associated with MYH9 high-risk haplotypes remain unknown.

CONCLUSION

In the last decade, it has been shown that the vast majority of patients with congenital onset of NS and a major proportion of those presenting in early childhood have an underlying Mendelian disorder. Among adults, hereditary forms of NS are uncommon, although oligogenic and complex inheritance may account for a significant percentage of cases previously regarded as idiopathic, as recently shown in African-Americans with FSGS and non-diabetic ESKD. Moreover, cases thought to be of immune origin may have a primary underlying genetic defect, as suggested by individuals presenting with familial forms of steroid-sensitive NS (90–92), as well as animal models exhibiting AR FSGS and relapsing proteinuria after kidney transplantation (93,94). Independent of the subjacent etiology of NS, unravelling the complexity of the pathophysiologic events altering the stability of the glomerular permselectivity barrier may elucidate potential strategies to treat this devastating syndrome; such is the case of chemical chaperones experimentally used in vitro to redirect nephrin and podocin missense mutants to the plasma membrane which are abnormally retained in the endoplasmic reticulum.

ACKNOWLEDGEMENTS

This work has been made possible through an International Society of Nephrology Fellowship awarded to E.M and a Fonds de la Recherche en Santé Québec (FRSQ) grant accorded to G.B.

Conflict of Interest statement. All the authors state that they do not have any personal or professional potential conflict of interest. The Institut national de la santé et de la recherche médicale (INSERM) has submitted a French patent (INPI) n° FR00 00709 and an international patent (FR01/00188), entitled “Identification of the NPHS2 gene implicated in the steroid-resistant nephrotic syndrome an its potential applications” filed on the 20.01.2000. A “License Agreement” (99301) was reached with Athena Diagnostics on 18.11.2001.

REFERENCES

1
Haraldsson
B.
Nystrom
J.
Deen
W.M.
Properties of the glomerular barrier and mechanisms of proteinuria
Physiol. Rev.
 , 
2008
, vol. 
88
 (pg. 
451
-
487
)
2
Neal
C.R.
Crook
H.
Bell
E.
Harper
S.J.
Bates
D.O.
Three-dimensional reconstruction of glomeruli by electron microscopy reveals a distinct restrictive urinary subpodocyte space
J. Am. Soc. Nephrol.
 , 
2005
, vol. 
16
 (pg. 
1223
-
1235
)
3
Artero
M.L.
Sharma
R.
Savin
V.J.
Vincenti
F.
Plasmapheresis reduces proteinuria and serum capacity to injure glomeruli in patients with recurrent focal glomerulosclerosis
Am. J. Kidney Dis.
 , 
1994
, vol. 
23
 (pg. 
574
-
581
)
4
Dantal
J.
Bigot
E.
Bogers
W.
Testa
A.
Kriaa
F.
Jacques
Y.
Hurault de Ligny
B.
Niaudet
P.
Charpentier
B.
Soulillou
J.P.
Effect of plasma protein adsorption on protein excretion in kidney-transplant recipients with recurrent nephrotic syndrome
N. Engl. J. Med.
 , 
1994
, vol. 
330
 (pg. 
7
-
14
)
5
Dantal
J.
Godfrin
Y.
Koll
R.
Perretto
S.
Naulet
J.
Bouhours
J.F.
Soulillou
J.P.
Antihuman immunoglobulin affinity immunoadsorption strongly decreases proteinuria in patients with relapsing nephrotic syndrome
J. Am. Soc. Nephrol.
 , 
1998
, vol. 
9
 (pg. 
1709
-
1715
)
6
Savin
V.J.
Sharma
R.
Sharma
M.
McCarthy
E.T.
Swan
S.K.
Ellis
E.
Lovell
H.
Warady
B.
Gunwar
S.
Chonko
A.M.
, et al.  . 
Circulating factor associated with increased glomerular permeability to albumin in recurrent focal segmental glomerulosclerosis
N. Engl. J. Med.
 , 
1996
, vol. 
334
 (pg. 
878
-
883
)
7
Le Berre
L.
Herve
C.
Buzelin
F.
Usal
C.
Soulillou
J.P.
Dantal
J.
Renal macrophage activation and Th2 polarization precedes the development of nephrotic syndrome in Buffalo/Mna rats
Kidney Int.
 , 
2005
, vol. 
68
 (pg. 
2079
-
2090
)
8
Weber
S.
Gribouval
O.
Esquivel
E.L.
Moriniere
V.
Tete
M.J.
Legendre
C.
Niaudet
P.
Antignac
C.
NPHS2 mutation analysis shows genetic heterogeneity of steroid-resistant nephrotic syndrome and low post-transplant recurrence
Kidney Int.
 , 
2004
, vol. 
66
 (pg. 
571
-
579
)
9
Ruf
R.G.
Lichtenberger
A.
Karle
S.M.
Haas
J.P.
Anacleto
F.E.
Schultheiss
M.
Zalewski
I.
Imm
A.
Ruf
E.M.
Mucha
B.
, et al.  . 
Patients with mutations in NPHS2 (podocin) do not respond to standard steroid treatment of nephrotic syndrome
J. Am. Soc. Nephrol.
 , 
2004
, vol. 
15
 (pg. 
722
-
732
)
10
Winn
M.P.
2007 Young Investigator Award: TRP'ing into a new era for glomerular disease
J. Am. Soc. Nephrol.
 , 
2008
, vol. 
19
 (pg. 
1071
-
1075
)
11
Conlon
P.J.
Butterly
D.
Albers
F.
Rodby
R.
Gunnells
J.C.
Howell
D.N.
Clinical and pathologic features of familial focal segmental glomerulosclerosis
Am. J. Kidney Dis.
 , 
1995
, vol. 
26
 (pg. 
34
-
40
)
12
Kestila
M.
Lenkkeri
U.
Mannikko
M.
Lamerdin
J.
McCready
P.
Putaala
H.
Ruotsalainen
V.
Morita
T.
Nissinen
M.
Herva
R.
, et al.  . 
Positionally cloned gene for a novel glomerular protein–nephrin–is mutated in congenital nephrotic syndrome
Mol. Cell.
 , 
1998
, vol. 
1
 (pg. 
575
-
582
)
13
Boute
N.
Gribouval
O.
Roselli
S.
Benessy
F.
Lee
H.
Fuchshuber
A.
Dahan
K.
Gubler
M.C.
Niaudet
P.
Antignac
C.
NPHS2, encoding the glomerular protein podocin, is mutated in autosomal recessive steroid-resistant nephrotic syndrome
Nat. Genet.
 , 
2000
, vol. 
24
 (pg. 
349
-
354
)
14
Kim
J.M.
Wu
H.
Green
G.
Winkler
C.A.
Kopp
J.B.
Miner
J.H.
Unanue
E.R.
Shaw
A.S.
CD2-associated protein haploinsufficiency is linked to glomerular disease susceptibility
Science
 , 
2003
, vol. 
300
 (pg. 
1298
-
1300
)
15
Kaplan
J.M.
Kim
S.H.
North
K.N.
Rennke
H.
Correia
L.A.
Tong
H.Q.
Mathis
B.J.
Rodriguez-Perez
J.C.
Allen
P.G.
Beggs
A.H.
, et al.  . 
Mutations in ACTN4, encoding alpha-actinin-4, cause familial focal segmental glomerulosclerosis
Nat. Genet.
 , 
2000
, vol. 
24
 (pg. 
251
-
256
)
16
Patrakka
J.
Tryggvason
K.
Nephrin–a unique structural and signaling protein of the kidney filter
Trends Mol. Med.
 , 
2007
, vol. 
13
 (pg. 
396
-
403
)
17
Hartleben
B.
Schweizer
H.
Lubben
P.
Bartram
M.P.
Moller
C.C.
Herr
R.
Wei
C.
Neumann-Haefelin
E.
Schermer
B.
Zentgraf
H.
, et al.  . 
Neph-Nephrin proteins bind the Par3-Par6-atypical protein kinase C (aPKC) complex to regulate podocyte cell polarity
J. Biol. Chem.
 , 
2008
, vol. 
283
 (pg. 
23033
-
23038
)
18
Garg
P.
Verma
R.
Nihalani
D.
Johnstone
D.B.
Holzman
L.B.
Neph1 cooperates with nephrin to transduce a signal that induces actin polymerization
Mol. Cell. Biol.
 , 
2007
, vol. 
27
 (pg. 
8698
-
8712
)
19
Pritchard-Jones
K.
Fleming
S.
Davidson
D.
Bickmore
W.
Porteous
D.
Gosden
C.
Bard
J.
Buckler
A.
Pelletier
J.
Housman
D.
, et al.  . 
The candidate Wilms’ tumour gene is involved in genitourinary development
Nature
 , 
1990
, vol. 
346
 (pg. 
194
-
197
)
20
Hinkes
B.
Wiggins
R.C.
Gbadegesin
R.
Vlangos
C.N.
Seelow
D.
Nurnberg
G.
Garg
P.
Verma
R.
Chaib
H.
Hoskins
B.E.
, et al.  . 
Positional cloning uncovers mutations in PLCE1 responsible for a nephrotic syndrome variant that may be reversible
Nat. Genet.
 , 
2006
, vol. 
38
 (pg. 
1397
-
1405
)
21
Reiser
J.
Polu
K.R.
Moller
C.C.
Kenlan
P.
Altintas
M.M.
Wei
C.
Faul
C.
Herbert
S.
Villegas
I.
Avila-Casado
C.
, et al.  . 
TRPC6 is a glomerular slit diaphragm-associated channel required for normal renal function
Nat. Genet.
 , 
2005
, vol. 
37
 (pg. 
739
-
744
)
22
Faul
C.
Asanuma
K.
Yanagida-Asanuma
E.
Kim
K.
Mundel
P.
Actin up: regulation of podocyte structure and function by components of the actin cytoskeleton
Trends Cell. Biol.
 , 
2007
, vol. 
17
 (pg. 
428
-
437
)
23
Kao
W.H.
Klag
M.J.
Meoni
L.A.
Reich
D.
Berthier-Schaad
Y.
Li
M.
Coresh
J.
Patterson
N.
Tandon
A.
Powe
N.R.
, et al.  . 
MYH9 is associated with nondiabetic end-stage renal disease in African Americans
Nat. Genet.
 , 
2008
, vol. 
40
 (pg. 
1185
-
1192
)
24
Kopp
J.B.
Smith
M.W.
Nelson
G.W.
Johnson
R.C.
Freedman
B.I.
Bowden
D.W.
Oleksyk
T.
McKenzie
L.M.
Kajiyama
H.
Ahuja
T.S.
, et al.  . 
MYH9 is a major-effect risk gene for focal segmental glomerulosclerosis
Nat. Genet.
 , 
2008
, vol. 
40
 (pg. 
1175
-
1184
)
25
Hinkes
B.
Vlangos
C.
Heeringa
S.
Mucha
B.
Gbadegesin
R.
Liu
J.
Hasselbacher
K.
Ozaltin
F.
Hildebrandt
F.
Specific podocin mutations correlate with age of onset in steroid-resistant nephrotic syndrome
J. Am. Soc. Nephrol.
 , 
2008
, vol. 
19
 (pg. 
365
-
371
)
26
Hinkes
B.G.
Mucha
B.
Vlangos
C.N.
Gbadegesin
R.
Liu
J.
Hasselbacher
K.
Hangan
D.
Ozaltin
F.
Zenker
M.
Hildebrandt
F.
Nephrotic syndrome in the first year of life: two thirds of cases are caused by mutations in 4 genes (NPHS1, NPHS2, WT1 and LAMB2)
Pediatrics
 , 
2007
, vol. 
119
 (pg. 
e907
-
e919
)
27
Gbadegesin
R.
Hinkes
B.G.
Hoskins
B.E.
Vlangos
C.N.
Heeringa
S.F.
Liu
J.
Loirat
C.
Ozaltin
F.
Hashmi
S.
Ulmer
F.
, et al.  . 
Mutations in PLCE1 are a major cause of isolated diffuse mesangial sclerosis (IDMS)
Nephrol. Dial. Transplant.
 , 
2008
, vol. 
23
 (pg. 
1291
-
1297
)
28
Gbadegesin
R.
Bartkowiak
B.
Lavin
P.J.
Mukerji
N.
Wu
G.
Bowling
B.
Eckel
J.
Damodaran
T.
Winn
M.P.
Exclusion of homozygous PLCE1 (NPHS3) mutations in 69 families with idiopathic and hereditary FSGS
Pediatr. Nephrol.
 , 
2009
, vol. 
24
 (pg. 
281
-
285
)
29
Koziell
A.
Grech
V.
Hussain
S.
Lee
G.
Lenkkeri
U.
Tryggvason
K.
Scambler
P.
Genotype/phenotype correlations of NPHS1 and NPHS2 mutations in nephrotic syndrome advocate a functional inter-relationship in glomerular filtration
Hum. Mol. Genet.
 , 
2002
, vol. 
11
 (pg. 
379
-
388
)
30
Philippe
A.
Nevo
F.
Esquivel
E.L.
Reklaityte
D.
Gribouval
O.
Tete
M.J.
Loirat
C.
Dantal
J.
Fischbach
M.
Pouteil-Noble
C.
, et al.  . 
Nephrin mutations can cause childhood-onset steroid-resistant nephrotic syndrome
J. Am. Soc. Nephrol.
 , 
2008
, vol. 
19
 (pg. 
1871
-
1878
)
31
Liu
L.
Done
S.C.
Khoshnoodi
J.
Bertorello
A.
Wartiovaara
J.
Berggren
P.O.
Tryggvason
K.
Defective nephrin trafficking caused by missense mutations in the NPHS1 gene: insight into the mechanisms of congenital nephrotic syndrome
Hum. Mol. Genet.
 , 
2001
, vol. 
10
 (pg. 
2637
-
2644
)
32
Roselli
S.
Moutkine
I.
Gribouval
O.
Benmerah
A.
Antignac
C.
Plasma membrane targeting of podocin through the classical exocytic pathway: effect of NPHS2 mutations
Traffic
 , 
2004
, vol. 
5
 (pg. 
37
-
44
)
33
Ohashi
T.
Uchida
K.
Uchida
S.
Sasaki
S.
Nihei
H.
Intracellular mislocalization of mutant podocin and correction by chemical chaperones
Histochem. Cell Biol.
 , 
2003
, vol. 
119
 (pg. 
257
-
264
)
34
Kitamura
A.
Tsukaguchi
H.
Hiramoto
R.
Shono
A.
Doi
T.
Kagami
S.
Iijima
K.
A familial childhood-onset relapsing nephrotic syndrome
Kidney Int.
 , 
2007
, vol. 
71
 (pg. 
946
-
951
)
35
Shono
A.
Tsukaguchi
H.
Kitamura
A.
Hiramoto
R.
Qin
X.S.
Doi
T.
Iijima
K.
Predisposition to relapsing nephrotic syndrome by a nephrin mutation that interferes with assembly of functioning microdomains
Hum. Mol. Genet.
 , 
2009
, vol. 
18
 (pg. 
2943
-
2956
)
36
Machuca
E.
Hummel
A.
Nevo
F.
Dantal
J.
Martinez
F.
Al-Sabban
E.
Baudouin
V.
Abel
L.
Grunfeld
J.P.
Antignac
C.
Clinical and epidemiological assessment of steroid-resistant nephrotic syndrome associated with the NPHS2 R229Q variant
Kidney Int.
 , 
2009
, vol. 
75
 (pg. 
727
-
735
)
37
Ratelade
J.
Lavin
T.A.
Muda
A.O.
Morisset
L.
Mollet
G.
Boyer
O.
Chen
D.S.
Henger
A.
Kretzler
M.
Hubner
N.
, et al.  . 
Maternal environment interacts with modifier genes to influence progression of nephrotic syndrome
J. Am. Soc. Nephrol.
 , 
2008
, vol. 
19
 (pg. 
1491
-
1499
)
38
Papeta
N.
Chan
K.T.
Prakash
S.
Martino
J.
Kiryluk
K.
Ballard
D.
Bruggeman
L.A.
Frankel
R.
Zheng
Z.
Klotman
P.E.
, et al.  . 
Susceptibility loci for murine HIV-associated nephropathy encode trans-regulators of podocyte gene expression
J. Clin. Invest.
 , 
2009
, vol. 
119
 (pg. 
1178
-
1188
)
39
Quaggin
S.E.
A new piece in the nephrotic puzzle
Nat. Genet.
 , 
2006
, vol. 
38
 (pg. 
1360
-
1361
)
40
Shih
N.Y.
Li
J.
Karpitskii
V.
Nguyen
A.
Dustin
M.L.
Kanagawa
O.
Miner
J.H.
Shaw
A.S.
Congenital nephrotic syndrome in mice lacking CD2-associated protein
Science
 , 
1999
, vol. 
286
 (pg. 
312
-
315
)
41
Gigante
M.
Pontrelli
P.
Montemurno
E.
Roca
L.
Aucella
F.
Penza
R.
Caridi
G.
Ranieri
E.
Ghiggeri
G.M.
Gesualdo
L.
CD2AP mutations are associated with sporadic nephrotic syndrome and focal segmental glomerulosclerosis (FSGS)
Nephrol. Dial. Transplant.
 , 
2009
, vol. 
24
 (pg. 
1858
-
1864
)
42
Lowik
M.M.
Groenen
P.J.
Pronk
I.
Lilien
M.R.
Goldschmeding
R.
Dijkman
H.B.
Levtchenko
E.N.
Monnens
L.A.
van den Heuvel
L.P.
Focal segmental glomerulosclerosis in a patient homozygous for a CD2AP mutation
Kidney Int.
 , 
2007
, vol. 
72
 (pg. 
1198
-
1203
)
43
Winn
M.P.
Conlon
P.J.
Lynn
K.L.
Farrington
M.K.
Creazzo
T.
Hawkins
A.F.
Daskalakis
N.
Kwan
S.Y.
Ebersviller
S.
Burchette
J.L.
, et al.  . 
A mutation in the TRPC6 cation channel causes familial focal segmental glomerulosclerosis
Science
 , 
2005
, vol. 
308
 (pg. 
1801
-
1804
)
44
Weins
A.
Kenlan
P.
Herbert
S.
Le
T.C.
Villegas
I.
Kaplan
B.S.
Appel
G.B.
Pollak
M.R.
Mutational and Biological Analysis of alpha-actinin-4 in focal segmental glomerulosclerosis
J. Am. Soc. Nephrol.
 , 
2005
, vol. 
16
 (pg. 
3694
-
3701
)
45
Prakash
S.
Chung
K.W.
Sinha
S.
Barmada
M.
Ellis
D.
Ferrell
R.E.
Finegold
D.N.
Randhawa
P.S.
Dinda
A.
Vats
A.
Autosomal dominant progressive nephropathy with deafness: linkage to a new locus on chromosome 11q24
J. Am. Soc. Nephrol.
 , 
2003
, vol. 
14
 (pg. 
1794
-
1803
)
46
Vats
A.
Nayak
A.
Ellis
D.
Randhawa
P.S.
Finegold
D.N.
Levinson
K.L.
Ferrell
R.E.
Familial nephrotic syndrome: clinical spectrum and linkage to chromosome 19q13
Kidney Int.
 , 
2000
, vol. 
57
 (pg. 
875
-
881
)
47
Pollak
M.R.
Alexander
M.P.
Henderson
J.M.
A case of familial kidney disease
Clin. J. Am. Soc. Nephrol.
 , 
2007
, vol. 
2
 (pg. 
1367
-
1374
)
48
Winn
M.P.
Conlon
P.J.
Lynn
K.L.
Howell
D.N.
Slotterbeck
B.D.
Smith
A.H.
Graham
F.L.
Bembe
M.
Quarles
L.D.
Pericak-Vance
M.A.
, et al.  . 
Linkage of a gene causing familial focal segmental glomerulosclerosis to chromosome 11 and further evidence of genetic heterogeneity
Genomics
 , 
1999
, vol. 
58
 (pg. 
113
-
120
)
49
Liu
G.
Kaw
B.
Kurfis
J.
Rahmanuddin
S.
Kanwar
Y.S.
Chugh
S.S.
Neph1 and nephrin interaction in the slit diaphragm is an important determinant of glomerular permeability
J. Clin. Invest.
 , 
2003
, vol. 
112
 (pg. 
209
-
221
)
50
Jones
N.
Blasutig
I.M.
Eremina
V.
Ruston
J.M.
Bladt
F.
Li
H.
Huang
H.
Larose
L.
Li
S.S.
Takano
T.
, et al.  . 
Nck adaptor proteins link nephrin to the actin cytoskeleton of kidney podocytes
Nature
 , 
2006
, vol. 
440
 (pg. 
818
-
823
)
51
Zhu
J.
Sun
N.
Aoudjit
L.
Li
H.
Kawachi
H.
Lemay
S.
Takano
T.
Nephrin mediates actin reorganization via phosphoinositide 3-kinase in podocytes
Kidney Int.
 , 
2008
, vol. 
73
 (pg. 
556
-
566
)
52
Huber
T.B.
Hartleben
B.
Kim
J.
Schmidts
M.
Schermer
B.
Keil
A.
Egger
L.
Lecha
R.L.
Borner
C.
Pavenstadt
H.
, et al.  . 
Nephrin and CD2AP associate with phosphoinositide 3-OH kinase and stimulate AKT-dependent signaling
Mol. Cell. Biol.
 , 
2003
, vol. 
23
 (pg. 
4917
-
4928
)
53
Lehtonen
S.
Zhao
F.
Lehtonen
E.
CD2-associated protein directly interacts with the actin cytoskeleton
Am. J. Physiol. Renal Physiol.
 , 
2002
, vol. 
283
 (pg. 
F734
-
F743
)
54
Huber
T.B.
Hartleben
B.
Winkelmann
K.
Schneider
L.
Becker
J.U.
Leitges
M.
Walz
G.
Haller
H.
Schiffer
M.
Loss of Podocyte aPKC{lambda}/{iota} Causes Polarity Defects and Nephrotic Syndrome
J. Am. Soc. Nephrol.
 , 
2009
, vol. 
20
 (pg. 
798
-
806
)
55
Weins
A.
Schlondorff
J.S.
Nakamura
F.
Denker
B.M.
Hartwig
J.H.
Stossel
T.P.
Pollak
M.R.
Disease-associated mutant alpha-actinin-4 reveals a mechanism for regulating its F-actin-binding affinity
Proc. Natl. Acad. Sci. U.S.A.
 , 
2007
, vol. 
104
 (pg. 
16080
-
16085
)
56
Otey
C.A.
Pavalko
F.M.
Burridge
K.
An interaction between alpha-actinin and the beta 1 integrin subunit in vitro
J. Cell. Biol.
 , 
1990
, vol. 
111
 (pg. 
721
-
729
)
57
Dandapani
S.V.
Sugimoto
H.
Matthews
B.D.
Kolb
R.J.
Sinha
S.
Gerszten
R.E.
Zhou
J.
Ingber
D.E.
Kalluri
R.
Pollak
M.R.
Alpha-actinin-4 is required for normal podocyte adhesion
J. Biol. Chem.
 , 
2007
, vol. 
282
 (pg. 
467
-
477
)
58
Wing
M.R.
Bourdon
D.M.
Harden
T.K.
PLC-epsilon: a shared effector protein in Ras-, Rho-, and G alpha beta gamma-mediated signaling
Mol. Interv.
 , 
2003
, vol. 
3
 (pg. 
273
-
280
)
59
Lehtonen
S.
Ryan
J.J.
Kudlicka
K.
Iino
N.
Zhou
H.
Farquhar
M.G.
Cell junction-associated proteins IQGAP1, MAGI-2, CASK, spectrins, and alpha-actinin are components of the nephrin multiprotein complex
Proc. Natl. Acad. Sci. U.S.A.
 , 
2005
, vol. 
102
 (pg. 
9814
-
9819
)
60
Nilius
B.
Owsianik
G.
Voets
T.
Peters
J.A.
Transient receptor potential cation channels in disease
Physiol. Rev.
 , 
2007
, vol. 
87
 (pg. 
165
-
217
)
61
Spassova
M.A.
Hewavitharana
T.
Xu
W.
Soboloff
J.
Gill
D.L.
A common mechanism underlies stretch activation and receptor activation of TRPC6 channels
Proc. Natl. Acad. Sci. U.S.A.
 , 
2006
, vol. 
103
 (pg. 
16586
-
16591
)
62
Huber
T.B.
Schermer
B.
Benzing
T.
Podocin organizes ion channel-lipid supercomplexes: implications for mechanosensation at the slit diaphragm
Nephron. Exp. Nephrol.
 , 
2007
, vol. 
106
 (pg. 
e27
-
e31
)
63
Hisatsune
C.
Kuroda
Y.
Nakamura
K.
Inoue
T.
Nakamura
T.
Michikawa
T.
Mizutani
A.
Mikoshiba
K.
Regulation of TRPC6 channel activity by tyrosine phosphorylation
J. Biol. Chem.
 , 
2004
, vol. 
279
 (pg. 
18887
-
18894
)
64
Schlondorff
J.
Del Camino
D.
Carrasquillo
R.
Lacey
V.
Pollak
M.R.
TRPC6 mutations associated with focal segmental glomerulosclerosis cause constitutive activation of NFAT-dependent transcription
Am. J. Physiol. Cell Physiol.
 , 
2009
, vol. 
296
 (pg. 
C558
-
C569
)
65
Sasaki
T.
Fassler
R.
Hohenester
E.
Laminin: the crux of basement membrane assembly
J. Cell. Biol.
 , 
2004
, vol. 
164
 (pg. 
959
-
963
)
66
Miner
J.H.
Patton
B.L.
Laminin-11
Int. J. Biochem. Cell Biol.
 , 
1999
, vol. 
31
 (pg. 
811
-
816
)
67
Zenker
M.
Aigner
T.
Wendler
O.
Tralau
T.
Muntefering
H.
Fenski
R.
Pitz
S.
Schumacher
V.
Royer-Pokora
B.
Wuhl
E.
, et al.  . 
Human laminin beta2 deficiency causes congenital nephrosis with mesangial sclerosis and distinct eye abnormalities
Hum. Mol. Genet.
 , 
2004
, vol. 
13
 (pg. 
2625
-
2632
)
68
Zenker
M.
Tralau
T.
Lennert
T.
Pitz
S.
Mark
K.
Madlon
H.
Dotsch
J.
Reis
A.
Muntefering
H.
Neumann
L.M.
Congenital nephrosis, mesangial sclerosis, and distinct eye abnormalities with microcoria: an autosomal recessive syndrome
Am. J. Med. Genet. A
 , 
2004
, vol. 
130
 (pg. 
138
-
145
)
69
Wuhl
E.
Kogan
J.
Zurowska
A.
Matejas
V.
Vandevoorde
R.G.
Aigner
T.
Wendler
O.
Lesniewska
I.
Bouvier
R.
Reis
A.
, et al.  . 
Neurodevelopmental deficits in Pierson (microcoria-congenital nephrosis) syndrome
Am. J. Med. Genet. A
 , 
2007
, vol. 
143
 (pg. 
311
-
319
)
70
Choi
H.J.
Lee
B.H.
Kang
J.H.
Jeong
H.J.
Moon
K.C.
Ha
I.S.
Yu
Y.S.
Matejas
V.
Zenker
M.
Choi
Y.
, et al.  . 
Variable phenotype of Pierson syndrome
Pediatr. Nephrol.
 , 
2008
, vol. 
23
 (pg. 
995
-
1000
)
71
Matejas
V.
Al-Gazali
L.
Amirlak
I.
Zenker
M.
A syndrome comprising childhood-onset glomerular kidney disease and ocular abnormalities with progressive loss of vision is caused by mutated LAMB2
Nephrol. Dial. Transplant.
 , 
2006
, vol. 
21
 (pg. 
3283
-
3286
)
72
Hasselbacher
K.
Wiggins
R.C.
Matejas
V.
Hinkes
B.G.
Mucha
B.
Hoskins
B.E.
Ozaltin
F.
Nurnberg
G.
Becker
C.
Hangan
D.
, et al.  . 
Recessive missense mutations in LAMB2 expand the clinical spectrum of LAMB2-associated disorders
Kidney Int.
 , 
2006
, vol. 
70
 (pg. 
1008
-
1012
)
73
El-Aouni
C.
Herbach
N.
Blattner
S.M.
Henger
A.
Rastaldi
M.P.
Jarad
G.
Miner
J.H.
Moeller
M.J.
St-Arnaud
R.
Dedhar
S.
, et al.  . 
Podocyte-specific deletion of integrin-linked kinase results in severe glomerular basement membrane alterations and progressive glomerulosclerosis
J. Am. Soc. Nephrol.
 , 
2006
, vol. 
17
 (pg. 
1334
-
1344
)
74
Tran
U.C.
Clarke
C.F.
Endogenous synthesis of coenzyme Q in eukaryotes
Mitochondrion
 , 
2007
, vol. 
7
 
suppl
(pg. 
S62
-
S71
)
75
Rotig
A.
Appelkvist
E.L.
Geromel
V.
Chretien
D.
Kadhom
N.
Edery
P.
Lebideau
M.
Dallner
G.
Munnich
A.
Ernster
L.
, et al.  . 
Quinone-responsive multiple respiratory-chain dysfunction due to widespread coenzyme Q10 deficiency
Lancet
 , 
2000
, vol. 
356
 (pg. 
391
-
395
)
76
Salviati
L.
Sacconi
S.
Murer
L.
Zacchello
G.
Franceschini
L.
Laverda
A.M.
Basso
G.
Quinzii
C.
Angelini
C.
Hirano
M.
, et al.  . 
Infantile encephalomyopathy and nephropathy with CoQ10 deficiency: a CoQ10-responsive condition
Neurology
 , 
2005
, vol. 
65
 (pg. 
606
-
608
)
77
Rahman
S.
Hargreaves
I.
Clayton
P.
Heales
S.
Neonatal presentation of coenzyme Q10 deficiency
J. Pediatr.
 , 
2001
, vol. 
139
 (pg. 
456
-
458
)
78
Forsgren
M.
Attersand
A.
Lake
S.
Grunler
J.
Swiezewska
E.
Dallner
G.
Climent
I.
Isolation and functional expression of human COQ2, a gene encoding a polyprenyl transferase involved in the synthesis of CoQ
Biochem. J.
 , 
2004
, vol. 
382
 (pg. 
519
-
526
)
79
Diomedi-Camassei
F.
Di Giandomenico
S.
Santorelli
F.M.
Caridi
G.
Piemonte
F.
Montini
G.
Ghiggeri
G.M.
Murer
L.
Barisoni
L.
Pastore
A.
, et al.  . 
COQ2 nephropathy: a newly described inherited mitochondriopathy with primary renal involvement
J. Am. Soc. Nephrol.
 , 
2007
, vol. 
18
 (pg. 
2773
-
2780
)
80
Lopez
L.C.
Schuelke
M.
Quinzii
C.M.
Kanki
T.
Rodenburg
R.J.
Naini
A.
Dimauro
S.
Hirano
M.
Leigh syndrome with nephropathy and CoQ10 deficiency due to decaprenyl diphosphate synthase subunit 2 (PDSS2) mutations
Am. J. Hum. Genet.
 , 
2006
, vol. 
79
 (pg. 
1125
-
1129
)
81
Saiki
R.
Lunceford
A.L.
Shi
Y.
Marbois
B.
King
R.
Pachuski
J.
Kawamukai
M.
Gasser
D.L.
Clarke
C.F.
Coenzyme Q10 supplementation rescues renal disease in Pdss2kd/kd mice with mutations in prenyl diphosphate synthase subunit 2
Am. J. Physiol. Renal Physiol.
 , 
2008
, vol. 
295
 (pg. 
F1535
-
F1544
)
82
Montini
G.
Malaventura
C.
Salviati
L.
Early coenzyme Q10 supplementation in primary coenzyme Q10 deficiency
N. Engl. J. Med.
 , 
2008
, vol. 
358
 (pg. 
2849
-
2850
)
83
Goto
Y.
Nonaka
I.
Horai
S.
A mutation in the tRNA(Leu)(UUR) gene associated with the MELAS subgroup of mitochondrial encephalomyopathies
Nature
 , 
1990
, vol. 
348
 (pg. 
651
-
653
)
84
Jansen
J.J.
Maassen
J.A.
van der Woude
F.J.
Lemmink
H.A.
van den Ouweland
J.M.
t’ Hart
L.M.
Smeets
H.J.
Bruijn
J.A.
Lemkes
H.H.
Mutation in mitochondrial tRNA(Leu(UUR)) gene associated with progressive kidney disease
J. Am. Soc. Nephrol.
 , 
1997
, vol. 
8
 (pg. 
1118
-
1124
)
85
Kurogouchi
F.
Oguchi
T.
Mawatari
E.
Yamaura
S.
Hora
K.
Takei
M.
Sekijima
Y.
Ikeda
S.
Kiyosawa
K.
A case of mitochondrial cytopathy with a typical point mutation for MELAS, presenting with severe focal-segmental glomerulosclerosis as main clinical manifestation
Am. J. Nephrol.
 , 
1998
, vol. 
18
 (pg. 
551
-
556
)
86
Berkovic
S.F.
Dibbens
L.M.
Oshlack
A.
Silver
J.D.
Katerelos
M.
Vears
D.F.
Lullmann-Rauch
R.
Blanz
J.
Zhang
K.W.
Stankovich
J.
, et al.  . 
Array-based gene discovery with three unrelated subjects shows SCARB2/LIMP-2 deficiency causes myoclonus epilepsy and glomerulosclerosis
Am. J. Hum. Genet.
 , 
2008
, vol. 
82
 (pg. 
673
-
684
)
87
Reczek
D.
Schwake
M.
Schroder
J.
Hughes
H.
Blanz
J.
Jin
X.
Brondyk
W.
Van Patten
S.
Edmunds
T.
Saftig
P.
LIMP-2 is a receptor for lysosomal mannose-6-phosphate-independent targeting of beta-glucocerebrosidase
Cell
 , 
2007
, vol. 
131
 (pg. 
770
-
783
)
88
Arrondel
C.
Vodovar
N.
Knebelmann
B.
Grunfeld
J.P.
Gubler
M.C.
Antignac
C.
Heidet
L.
Expression of the nonmuscle myosin heavy chain IIA in the human kidney and screening for MYH9 mutations in Epstein and Fechtner syndromes
J. Am. Soc. Nephrol.
 , 
2002
, vol. 
13
 (pg. 
65
-
74
)
89
Drenckhahn
D.
Franke
R.P.
Ultrastructural organization of contractile and cytoskeletal proteins in glomerular podocytes of chicken, rat, and man
Lab. Invest.
 , 
1988
, vol. 
59
 (pg. 
673
-
682
)
90
Fuchshuber
A.
Gribouval
O.
Ronner
V.
Kroiss
S.
Karle
S.
Brandis
M.
Hildebrandt
F.
Clinical and genetic evaluation of familial steroid-responsive nephrotic syndrome in childhood
J. Am. Soc. Nephrol.
 , 
2001
, vol. 
12
 (pg. 
374
-
378
)
91
Landau
D.
Oved
T.
Geiger
D.
Abizov
L.
Shalev
H.
Parvari
R.
Familial steroid-sensitive nephrotic syndrome in Southern Israel: clinical and genetic observations
Pediatr. Nephrol.
 , 
2007
, vol. 
22
 (pg. 
661
-
669
)
92
Ruf
R.G.
Fuchshuber
A.
Karle
S.M.
Lemainque
A.
Huck
K.
Wienker
T.
Otto
E.
Hildebrandt
F.
Identification of the first gene locus (SSNS1) for steroid-sensitive nephrotic syndrome on chromosome 2p
J. Am. Soc. Nephrol.
 , 
2003
, vol. 
14
 (pg. 
1897
-
1900
)
93
Nakamura
T.
Oite
T.
Shimizu
F.
Matsuyama
M.
Kazama
T.
Koda
Y.
Arakawa
M.
Sclerotic lesions in the glomeruli of Buffalo/Mna rats
Nephron
 , 
1986
, vol. 
43
 (pg. 
50
-
55
)
94
Le Berre
L.
Godfrin
Y.
Perretto
S.
Smit
H.
Buzelin
F.
Kerjaschki
D.
Usal
C.
Cuturi
C.
Soulillou
J.P.
Dantal
J.
The Buffalo/Mna rat, an animal model of FSGS recurrence after renal transplantation
Transplant. Proc.
 , 
2001
, vol. 
33
 (pg. 
3338
-
3340
)

Author notes

The authors wish it to be known that, in their opinion, the first two authors should be regarded as joint First Authors.