Steroid-resistant nephrotic syndrome is characterized by podocyte dysfunction.
garland cell nephrocytes are podocyte-like cells and thus provide a potential
model in which to study the pathogenesis ...of nephrotic syndrome. However, relevant pathomechanisms of nephrotic syndrome have not been studied in nephrocytes. Here, we discovered that two
slit diaphragm proteins, orthologs of the human genes encoding nephrin and nephrin-like protein 1, colocalize within a fingerprint-like staining pattern that correlates with ultrastructural morphology. Using RNAi and conditional CRISPR/Cas9 in nephrocytes, we found this pattern depends on the expression of both orthologs. Tracer endocytosis by nephrocytes required Cubilin and reflected size selectivity analogous to that of glomerular function. Using RNAi and tracer endocytosis as a functional read-out, we screened
orthologs of human monogenic causes of nephrotic syndrome and observed conservation of the central pathogenetic alterations. We focused on the coenzyme Q
(CoQ
) biosynthesis gene
, the silencing of which disrupted slit diaphragm morphology. Restoration of CoQ
synthesis by vanillic acid partially rescued the phenotypic and functional alterations induced by
-RNAi. Notably,
colocalized with mitochondria, and
silencing increased the formation of reactive oxygen species (ROS). Silencing of
, a subunit of the mitochondrial respiratory chain that controls ROS formation independently of CoQ
, phenocopied the effect of
-RNAi. Moreover, the ROS scavenger glutathione partially rescued the effects of
-RNAi. In conclusion,
garland cell nephrocytes provide a model with which to study the pathogenesis of nephrotic syndrome, and ROS formation may be a pathomechanism of
-nephropathy.
ATP6AP2 (also known as the prorenin receptor) is a type I transmembrane protein that can be cleaved into two fragments in the Golgi apparatus. While in Drosophila ATP6AP2 functions in the planar cell ...polarity (PCP) pathway, recent human genetic studies have suggested that ATP6AP2 could participate in the assembly of the V-ATPase in the endoplasmic reticulum (ER). Using a yeast model, we show here that the V-ATPase assembly factor Voa1 can functionally be replaced by Drosophila ATP6AP2. This rescue is even more efficient when coexpressing its binding partner ATP6AP1, indicating that these two proteins together fulfill Voa1 functions in higher organisms. Structure-function analyses in both yeast and Drosophila show that proteolytic cleavage is dispensable, while C-terminus-dependent ER retrieval is required for ATP6AP2 function. Accordingly, we demonstrate that both overexpression and lack of ATP6AP2 causes ER stress in Drosophila wing cells and that the induction of ER stress is sufficient to cause PCP phenotypes. In summary, our results suggest that full-length ATP6AP2 contributes to the assembly of the V-ATPase proton pore and that impairment of this function affects ER homeostasis and PCP signaling.
Planar cell polarity (PCP) controls the orientation of cells within tissues and the polarized outgrowth of cellular appendages. So far, six PCP core proteins including the transmembrane proteins ...Frizzled (Fz), Strabismus (Stbm) and Flamingo (Fmi) have been identified. These proteins form asymmetric PCP domains at apical junctions of epithelial cells. Here, we demonstrate that VhaPRR, an accessory subunit of the proton pump V‐ATPase, directly interacts with the protocadherin Fmi through its extracellular domain. It also shows a striking co‐localization with PCP proteins during all pupal wing stages in Drosophila. This localization depends on intact PCP domains. Reversely, VhaPRR is required for stable PCP domains, identifying it as a novel PCP core protein. VhaPRR performs an additional role in vesicular acidification as well as endolysosomal sorting and degradation. Membrane proteins, such as E‐Cadherin and the Notch receptor, accumulate at the surface and in intracellular vesicles of cells mutant for VhaPRR. This trafficking defect is shared by other V‐ATPase subunits. By contrast, the V‐ATPase does not seem to have a direct role in PCP regulation. Together, our results suggest two roles for VhaPRR, one for PCP and another in endosomal trafficking. This dual function establishes VhaPRR as a key factor in epithelial morphogenesis.
In addition to its role in vesicle acidification and sorting, the V‐ATPase regulatory subunit ATP6AP2/VhaPRR is a new core component of the planar cell polarity (PCP) pathway in Drosophila. VhaPRR interacts with PCP protein Flamingo (Fmi) and regulates the asymmetric distribution of PCP components in wing epithelial cells.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
mTOR kinase is a master growth regulator that can be stimulated by multiple signals, including amino acids and the lysosomal small GTPase Rheb. Recent studies have proposed an important role for the ...V-ATPase in the sensing of amino acids in the lysosomal lumen. Using the Drosophila wing as a model epithelium, we show here that the V-ATPase is required for Rheb-dependent epithelial growth. We further uncover a positive feedback loop for the control of apical protein uptake that depends on V-ATPase/mTOR signaling. This feedback loop includes Rheb-dependent transcriptional regulation of the multiligand receptor Megalin, which itself is required for Rheb-induced endocytosis. In addition, we provide evidence that long-term mTOR inhibition with rapamycin in mice causes reduction of Megalin levels and proteinuria in the proximal tubular epithelium of the kidney. Thus, our findings unravel a homeostatic mechanism that allows epithelial cells to promote protein uptake under normal conditions and to prevent uptake in lysosomal stress conditions.
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•The V-ATPase is required for mTOR-dependent tissue growth in Drosophila•V-ATPase/mTOR signaling controls apical endocytosis•Megalin activity and apical surface area is regulated by V-ATPase/mTOR signaling•Long-term mTOR inhibition causes Megalin downregulation and proteinuria in the kidney
Epithelial growth requires the endocytic uptake of proteins, which are broken down into amino acids. Amino acids recruit the growth-regulating mTORC1 complex to the lysosomal surface, thereby causing its activation. Gleixner et al. now find that in epithelial cells apical protein uptake is controlled by the lysosomal V-ATPase/mTORC1 complex in a feedforward manner. The multiligand receptor Megalin, which internalizes proteins on the apical surface, is crucial for this regulatory pathway.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Galloway‐Mowat syndrome (GAMOS) is a phenotypically heterogeneous disorder characterized by neurodevelopmental defects combined with renal‐glomerular disease, manifesting with proteinuria. To ...identify additional monogenic disease causes, we here performed whole exome sequencing (WES), linkage analysis, and homozygosity mapping in three affected siblings of an Indian family with GAMOS. Applying established criteria for variant filtering, we identify a novel homozygous splice site mutation in the gene WDR4 as the likely disease‐causing mutation in this family. In line with previous reports, we observe growth deficiency, microcephaly, developmental delay, and intellectual disability as phenotypic features resulting from WDR4 mutations. However, the newly identified allele additionally gives rise to proteinuria and nephrotic syndrome, a phenotype that was never reported in patients with WDR4 mutations. Our data thus expand the phenotypic spectrum of WDR4 mutations by demonstrating that, depending on the specific mutated allele, a renal phenotype may be present. This finding suggests that GAMOS may occupy a phenotypic spectrum with other microcephalic diseases. Furthermore, WDR4 is an additional example of a gene that encodes a tRNA modifying enzyme and gives rise to GAMOS, if mutated. Our findings thereby support the recent observation that, like neurons, podocytes of the renal glomerulus are particularly vulnerable to cellular defects resulting from altered tRNA modifications.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Mutations in about 50 genes have been identified as monogenic causes of nephrotic syndrome, a frequent cause of CKD. These genes delineated the pathogenetic pathways and rendered significant insight ...into podocyte biology.
We used whole-exome sequencing to identify novel monogenic causes of steroid-resistant nephrotic syndrome (SRNS). We analyzed the functional significance of an SRNS-associated gene
and in podocyte-like
nephrocytes.
We identified hemizygous missense mutations in the gene
in five families with nephrotic syndrome. Coimmunoprecipitation assays indicated interactions between TBC1D8B and active forms of RAB11. Silencing
in HEK293T cells increased basal autophagy and exocytosis, two cellular functions that are independently regulated by RAB11. This suggests that TBC1D8B plays a regulatory role by inhibiting endogenous RAB11. Coimmunoprecipitation assays showed TBC1D8B also interacts with the slit diaphragm protein nephrin, and colocalizes with it in immortalized cell lines. Overexpressed murine
with patient-derived mutations had lower affinity for endogenous RAB11 and nephrin compared with wild-type Tbc1d8b protein. Knockdown of
in
impaired function of the podocyte-like nephrocytes, and caused mistrafficking of Sns, the
ortholog of nephrin. Expression of
RNAi in nephrocytes entailed defective delivery of slit diaphragm protein to the membrane, whereas
overexpression revealed a partial phenotypic overlap to
loss of function.
Novel mutations in
are monogenic causes of SRNS. This gene inhibits RAB11. Our findings suggest that RAB11-dependent vesicular nephrin trafficking plays a role in the pathogenesis of nephrotic syndrome.
Galloway-Mowat syndrome is a neurorenal syndrome that has been linked with defective transfer RNA and protein translation caused by variants in the multiprotein complex KEOPS. In the kidney, this ...syndrome seems to primarily affect the podocytes, but the pathogenesis has remained unclear. In this issue of Kidney International, Krausel et al. use Drosophila nephrocytes to link endoplasmic reticulum stress with defects in the slit diaphragm.
Glomerular disorders are a major cause of end-stage renal disease and effective therapies are often lacking. Nephrocytes are considered to be part of the
excretory system and form slit diaphragms ...across cellular membrane invaginations. Nehphrocytes have been shown to share functional, morphological, and molecular features with podocytes, which form the glomerular filter in vertebrates. Here, we report the progress and the evolving tool-set of this model system. Combining a functional, accessible slit diaphragm with the power of the genetic tool-kit in
, the nephrocyte has the potential to greatly advance our understanding of the glomerular filtration barrier in health and disease.
Abstract
Background
Nephrotic syndrome (NS), a chronic kidney disease, is characterized by significant loss of protein in the urine causing hypoalbuminemia and edema. In general, ∼15% of ...childhood-onset cases do not respond to steroid therapy and are classified as steroid-resistant NS (SRNS). In ∼30% of cases with SRNS, a causative mutation can be detected in one of 44 monogenic SRNS genes. The gene LAMA5 encodes laminin-α5, an essential component of the glomerular basement membrane. Mice with a hypomorphic mutation in the orthologous gene Lama5 develop proteinuria and hematuria.
Methods
To identify additional monogenic causes of NS, we performed whole exome sequencing in 300 families with pediatric NS. In consanguineous families we applied homozygosity mapping to identify genomic candidate loci for the underlying recessive mutation.
Results
In three families, in whom mutations in known NS genes were excluded, but in whom a recessive, monogenic cause of NS was strongly suspected based on pedigree information, we identified homozygous variants of unknown significance (VUS) in the gene LAMA5. While all affected individuals had nonsyndromic NS with an early onset of disease, their clinical outcome and response to immunosuppressive therapy differed notably.
Conclusion
We here identify recessive VUS in the gene LAMA5 in patients with partially treatment-responsive NS. More data will be needed to determine the impact of these VUS in disease management. However, familial occurrence of disease, data from genetic mapping and a mouse model that recapitulates the NS phenotypes suggest that these genetic variants may be inherited factors that contribute to the development of NS in pediatric patients.
Immunotherapy using immune checkpoint inhibitors revolutionized therapies for a variety of malignancies. Nivolumab, an antibody blocking programmed cell death 1 protein, and ipilimumab that blocks ...cytotoxic T-lymphocyte-associated protein 4 effectively target tumor cells by disinhibiting the endogenous immune response. At the same time, unrestrained T-cell activation may trigger a range of immune-mediated side effects including kidney injury. Steroid therapy constitutes the mainstay of treatment of these adverse events, but dosage, route of administration, and approach to nivolumab re-exposure remain unclear. Here, we report the case of a 72-year-old male patient who developed severe nivolumab/ipilimumab-associated acute kidney injury while on oral steroid therapy for immune-mediated colitis. Acute interstitial nephritis was confirmed by renal biopsy. Administration of high-dose intravenous steroid doses was required to revert declining renal function.
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