Nephrotic Syndrome Gene TBC1D8B Is Required for Endosomal Maturation and Nephrin Endocytosis in Drosophila

BACKGROUND

Variants in TBC1D8B cause nephrotic syndrome. TBC1D8B is a GTPase-activating protein for Rab11 (RAB11-GAP) that interacts with nephrin, but how it controls nephrin trafficking or other podocyte functions remains unclear.

METHODS

We generated a stable deletion in Tbc1d8b and used microhomology-mediated end-joining for genome editing. Ex vivo functional assays utilized slit diaphragms in podocyte-like Drosophila nephrocytes. Manipulation of endocytic regulators and transgenesis of murine Tbc1d8b provided a comprehensive functional analysis of Tbc1d8b.

RESULTS

A null allele of Drosophila TBC1D8B exhibited a nephrocyte-restricted phenotype of nephrin mislocalization, similar to patients with isolated nephrotic syndrome who have variants in the gene. The protein was required for rapid nephrin turnover in nephrocytes and for endocytosis of nephrin induced by excessive Rab5 activity. The protein expressed from the Tbc1d8b locus bearing the edited tag predominantly localized to mature early and late endosomes. Tbc1d8b was required for endocytic cargo processing and degradation. Silencing Hrs, a regulator of endosomal maturation, phenocopied loss of Tbc1d8b. Low-level expression of murine TBC1D8B rescued loss of the Drosophila gene, indicating evolutionary conservation. Excessive murine TBC1D8B selectively disturbed nephrin dynamics. Finally, we discovered four novel TBC1D8B variants within a cohort of 363 patients with FSGS and validated a functional effect of two variants in Drosophila, suggesting a personalized platform for TBC1D8B-associated FSGS.

CONCLUSIONS

Variants in TBC1D8B are not infrequent among patients with FSGS. TBC1D8B, functioning in endosomal maturation and degradation, is essential for nephrin trafficking.

Selective endocytosis controls slit diaphragm maintenance and dynamics in Drosophila nephrocytes

The kidneys generate about 180 l of primary urine per day by filtration of plasma. An essential part of the filtration barrier is the slit diaphragm, a multiprotein complex containing nephrin as major component. Filter dysfunction typically manifests with proteinuria and mutations in endocytosis regulating genes were discovered as causes of proteinuria. However, it is unclear how endocytosis regulates the slit diaphragm and how the filtration barrier is maintained without either protein leakage or filter clogging. Here, we study nephrin dynamics in podocyte-like nephrocytes of Drosophila and show that selective endocytosis either by dynamin- or flotillin-mediated pathways regulates a stable yet highly dynamic architecture. Short-term manipulation of endocytic functions indicates that dynamin-mediated endocytosis of ectopic nephrin restricts slit diaphragm formation spatially while flotillin-mediated turnover of nephrin within the slit diaphragm is needed to maintain filter permeability by shedding of molecules bound to nephrin in endosomes. Since slit diaphragms cannot be studied in vitro and are poorly accessible in mouse models, this is the first analysis of their dynamics within the slit diaphragm multiprotein complex. Identification of the mechanisms of slit diaphragm maintenance will help to develop novel therapies for proteinuric renal diseases that are frequently limited to symptomatic treatment.

mTOR-Dependent Autophagy Regulates Slit Diaphragm Density in Podocyte-like Drosophila Nephrocytes

Both mTOR signaling and autophagy are important modulators of podocyte homeostasis, regeneration, and aging and have been implicated in glomerular diseases. However, the mechanistic role of these pathways for the glomerular filtration barrier remains poorly understood. We used Drosophila nephrocytes as an established podocyte model and found that inhibition of mTOR signaling resulted in increased spacing between slit diaphragms. Gain-of-function of mTOR signaling did not affect spacing, suggesting that additional cues limit the maximal slit diaphragm density. Interestingly, both activation and inhibition of mTOR signaling led to decreased nephrocyte function, indicating that a fine balance of signaling activity is needed for proper function. Furthermore, mTOR positively controlled cell size, survival, and the extent of the subcortical actin network. We also showed that basal autophagy in nephrocytes is required for survival and limits the expression of the sns (nephrin) but does not directly affect slit diaphragm formation or endocytic activity. However, using a genetic rescue approach, we demonstrated that excessive, mTOR-dependent autophagy is primarily responsible for slit diaphragm misspacing. In conclusion, we established this invertebrate podocyte model for mechanistic studies on the role of mTOR signaling and autophagy, and we discovered a direct mTOR/autophagy-dependent regulation of the slit diaphragm architecture.

Inhibition of endoplasmic reticulum stress signaling rescues cytotoxicity of human apolipoprotein-L1 risk variants in Drosophila

Risk variants of the apolipoprotein-L1 (APOL1) gene are associated with severe kidney disease, putting homozygous carriers at risk. Since APOL1 lacks orthologs in all major model organisms, a wide range of mechanisms frequently in conflict have been described for APOL1-associated nephropathies. The genetic toolkit in Drosophila allows unique in vivo insights into disrupted cellular homeostasis. To perform a mechanistic analysis, we expressed human APOL1 control and gain-of-function kidney risk variants in the podocyte-like garland cells of Drosophila nephrocytes and a wing precursor tissue. Expression of APOL1 risk variants was found to elevate endocytic function of garland cell nephrocytes that simultaneously showed early signs of cell death. Wild-type APOL1 had a significantly milder effect, while a control transgene with deletion of the short BH3 domain showed no overt phenotype. Nephrocyte endo-lysosomal function and slit diaphragm architecture remained unaffected by APOL1 risk variants, but endoplasmic reticulum (ER) swelling, chaperone induction, and expression of the reporter Xbp1-EGFP suggested an ER stress response. Pharmacological inhibition of ER stress diminished APOL1-mediated cell death and direct ER stress induction enhanced nephrocyte endocytic function similar to expression of APOL1 risk variants. We confirmed APOL1-dependent ER stress in the Drosophila wing precursor where silencing the IRE1-dependent branch of ER stress signaling by inhibition with Xbp1-RNAi abrogated cell death, representing the first rescue of APOL1-associated cytotoxicity in vivo. Thus, we uncovered ER stress as an essential consequence of APOL1 risk variant expression in vivo in Drosophila, suggesting a central role of this pathway in the pathogenesis of APOL1-associated nephropathies.

TBC1D8B Mutations Implicate RAB11-Dependent Vesicular Trafficking in the Pathogenesis of Nephrotic Syndrome

BACKGROUND

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.

METHODS

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 in vitro and in podocyte-like Drosophila nephrocytes.

RESULTS

We identified hemizygous missense mutations in the gene TBC1D8B in five families with nephrotic syndrome. Coimmunoprecipitation assays indicated interactions between TBC1D8B and active forms of RAB11. Silencing TBC1D8B 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 Tbc1d8b with patient-derived mutations had lower affinity for endogenous RAB11 and nephrin compared with wild-type Tbc1d8b protein. Knockdown of Tbc1d8b in Drosophila impaired function of the podocyte-like nephrocytes, and caused mistrafficking of Sns, the Drosophila ortholog of nephrin. Expression of Rab11 RNAi in nephrocytes entailed defective delivery of slit diaphragm protein to the membrane, whereas RAB11 overexpression revealed a partial phenotypic overlap to Tbc1d8b loss of function.

CONCLUSIONS

Novel mutations in TBC1D8B 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.

Genome-wide association meta-analyses and fine-mapping elucidate pathways influencing albuminuria

Increased levels of the urinary albumin-to-creatinine ratio (UACR) are associated with higher risk of kidney disease progression and cardiovascular events, but underlying mechanisms are incompletely understood. Here, we conduct trans-ethnic (n = 564,257) and European-ancestry specific meta-analyses of genome-wide association studies of UACR, including ancestry- and diabetes-specific analyses, and identify 68 UACR-associated loci. Genetic correlation analyses and risk score associations in an independent electronic medical records database (n = 192,868) reveal connections with proteinuria, hyperlipidemia, gout, and hypertension. Fine-mapping and trans-Omics analyses with gene expression in 47 tissues and plasma protein levels implicate genes potentially operating through differential expression in kidney (including TGFB1, MUC1, PRKCI, and OAF), and allow coupling of UACR associations to altered plasma OAF concentrations. Knockdown of OAF and PRKCI orthologs in Drosophila nephrocytes reduces albumin endocytosis. Silencing fly PRKCI further impairs slit diaphragm formation. These results generate a priority list of genes and pathways for translational research to reduce albuminuria.

Reduced sensitivity to emotional facial expressions in borderline personality disorder: effects of emotional valence and intensity

A heightened sensitivity towards negative emotional stimuli has been described for Borderline Personality Disorder (BPD). We investigated whether a faster and more accurate detection of negatively valent information in BPD can be confirmed by means of a visual search task which required subjects to detect a face with an incongruent emotional expression within a crowd of neutral faces. Twenty eight BPD patients and 28 nonpatients were asked to indicate whether a set of schematic neutral faces (3 × 3, 4 × 4 matrices) contained a happy or an angry face. Besides valence, the intensity of the target's emotion was varied in two steps. BPD patients and nonpatients both demonstrated an anger-superiority effect. However, no higher sensitivity towards negative stimuli was observed in BPD compared to nonpatients. BPD patients seem to rely to a stronger extent on controlled, i.e., serial, attention demanding processes when searching more subtle social-emotional information with positive valence.

Evaluation of a new simple and rapid enzyme-linked immunosorbent assay kit for neopterin determination

A new commercially available enzyme-linked immunosorbent assay (ELISA) kit has been evaluated for the measurement of neopterin concentrations in serum, plasma and urine. This competitive ELISA is technically simple, requires only small sample volume and is rapid to perform. The assay procedure consists of sequential 1.5 h and 10 min room temperature incubation steps. The ELISA is accurate, sensitive, specific, and precise. Linear regression analysis of neopterin concentrations measured with the new ELISA and with an established method yielded a highly significant correlation (r = 0.99). The new assay is applicable to ELISA workstations, thus enabling determination of neopterin in large series of samples. The neopterin ELISA kit has been used in routine laboratory testing of blood donations in a blood bank.

[Neopterin variations during the menstrual cycle]

During a complete estrous cycle Neopterin levels were followed up daily in sera of six healthy female volunteers below 30 years of age. Average values found in these individuals were within the normal range. Only two single data--measured on the 29th and the 30th day of one test person's estrous cycle--out of 145 were elevated (between 10 and 15 nmol/l). Therefore elevated Neopterin values of clinically healthy persons should only be taken as an indicator for pathological activation of the cellular immune system if the elevation continues after repeated confirmation in a possibly different estrous phase.

[Radioimmunological determination of neopterin in blood donors]

In 518 sera from blood donors below 30 years of age Neopterin was determined by radioimmunoassay. 21 of these patients (= 4.05%) showed elevated serum levels for Neopterin. By clinical investigation of these cases viral infections of the upper airways were found. Furthermore after elimination of elevated values significant differences in normal Neopterin serum levels could be demonstrated for female and male blood donors (p less than 0.01). Because elevated Neopterin serum levels indicate immune responses to several antigens, determination of Neopterin from serum may be useful for detection of infectious blood samples.

[Newer aspects of the electrophoretic mobility test after stimulation of human lymphocytes with PPD]

The Electrophoresis-Mobility-Test is a modified Macrophage-Electrophoretic-Mobility-Test by using sheep erythrocytes, tanned and stabilized by sulfosalicyl acid. The test is used to demonstrate specific sensibilised T-lymphocytes in cell culture after having them isolated from human venous blood and after stimulation with antigen. 34 tests were done by stimulation with antigen PPD, the lymphocytes having been taken from patients with positive Tuberculin-Tine-Test. All tests were bacteriologically and cytologically controlled and only 9 could be used finally as a base for statistical control by using x2- und t-tests and histogram. A special attention was payed to the aggregation of monocytes and their morphological changes in cell culture. Every single test is not statistically significant, but the results of the whole lot give an attest of the concept of the EM-Test, which can be reproduced with a probability of about 60%.

[Pulmonary alveolar microlithiasis (author's transl)]

The Pulmonary alveolar microlithiasis is a rare disease of unknown etiology with chronic course. Clinical symptoms are not characteristic or missing completely. The disease is diagnosed by X-ray examination and histologic examination from biopsy specimen of the lung. A report on the first case in Austria is given.

[Bronchial atresia--a rare anomalia (author's transl)]

Report on three cases of bronchial atresia. Two patients had an atresia of a segmental bronchus with consecutive bronchocele and regional hyperinflation, one patient had an atresia of a lobar bronchus. The diagnosis is made by association of radiologic and bronchologic analysis. Possible pathogenetic mechanisms are briefly discussed.

[Lipoma of bronchus (author's transl)]

Lipomas of bronchus have a special clinical importance because of their predominantly endobronchial growth. Chronic obstruction and stenosis result in irreversible changes of lung. The clinical symptomatology is untypical. The X-ray examination points to the right diagnosis. The tumour can be localized bronchoscopically and recognized macromorphologically. The histological examination of a biopsy results in the exact diagnosis. This is a report of two cases.

[Diagnosis of retroperitoneal and intrathoracic Schwannomas (author's transl)]

Retroperitoneal and intrathoracic Schwannomas are difficult to diagnose in the preoperative stage. The following methods of examination may be used: 1. X-ray examinations to exclude diseases of the internal organs and to establish displacements.--2. Computertomography to determine localisation and size of the tumour.--3. Immediate section for microscopic examination under surgery to determine the dignity of the tumour and needlebiopsy with histological and cytological examination to obtain an exact diagnosis.

[Early bronchiolar-alveolar cell cicatricial carcinoma; a case report with 2 illustrations]

Only the solitary type of bronchiolo-alveolar cell carcinoma has a chance of being healed if surgery is performed in time. This type of tumor is often found as a cicatricial carcinoma in the periphery of the lung. Small tumors do not exhibit any clinical symptoms, they are either detected by mere chance or during radiological check-up or routine medical check-up of healthy persons. In the present case a small posttuberculous scar in the lung had been known to exist for ten years. Only a repeated routine medical check-up including X-ray examination and subsequent tomographic X-ray gave an indication for the performance of thoracotomy. While surgery was being performed the malignancy and the tumor type where cytologically determined and verified by histological examination of the excised tumor.

Morphological changes in anaplastic gliomas treated with radiation and chemotherapy

The effects of antineoplastic treatment on gliomas are related to tumour cell cycle and proliferation kinetics, glioma tissue architecture, and the surrounding environment. Morphological changes induced by radiation and chemotherapy are characterized by cell necrosis and severe alterations in cell and nuclear morphology caused by changes in the cell kinetic parameters which, however, may also occur spontaneously in untreated anaplastic gliomas. Comparative studies of cytological imprints and routine histological preparations of biopsy and autopsy specimens were performed in four groups of anaplastic astrocytomas and glioblastomas (78 cases) with postsurgical irradiation, combination chemotherapy, and CCNU treatment, and without specific postsurgical treatment (control group). Following radiation and chemotherapy, in addition to increased necrosis and vascular response, a variety of characteristic but nonspecific changes were observed in cell and nuclear morphology with prominent formation of multinucleated giant and monstrous cells, irregular and hyperchromatic nuclei, and severe cytoplasmic degeneration indicating both inhibition of cell division and cell damage. Statistically significant findings were a posttreatment increase in the number of multinucleated giant and monstrous cells and a decrease in the number of mitoses. These changes were more pronounced after chemotherapy than after radiation, while no significant dissimilarities were found between combination chemotherapy and CCNU. The implications of these changes on the mechanisms of antitumour treatment in anaplastic gliomas are discussed.