Stefan Zschiedrich

Role of mTOR in podocyte function and diabetic nephropathy in humans and mice

Chronic glomerular diseases, associated with renal failure and cardiovascular morbidity, represent a major health issue. However, they remain poorly understood. Here we have reported that tightly controlled mTOR activity was crucial to maintaining glomerular podocyte function, while dysregulation of mTOR facilitated glomerular diseases. Genetic deletion of mTOR complex 1 (mTORC1) in mouse podocytes induced proteinuria and progressive glomerulosclerosis. Furthermore, simultaneous deletion of both mTORC1 and mTORC2 from mouse podocytes aggravated the glomerular lesions, revealing the importance of both mTOR complexes for podocyte homeostasis. In contrast, increased mTOR activity accompanied human diabetic nephropathy, characterized by early glomerular hypertrophy and hyperfiltration. Curtailing mTORC1 signaling in mice by genetically reducing mTORC1 copy number in podocytes prevented glomerulosclerosis and significantly ameliorated the progression of glomerular disease in diabetic nephropathy. These results demonstrate the requirement for tightly balanced mTOR activity in podocyte homeostasis and suggest that mTOR inhibition can protect podocytes and prevent progressive diabetic nephropathy.

FAN1 mutations cause karyomegalic interstitial nephritis, linking chronic kidney failure to defective DNA damage repair

Chronic kidney disease (CKD) represents a major health burden. Its central feature of renal fibrosis is not well understood. By exome sequencing, we identified mutations in FAN1 as a cause of karyomegalic interstitial nephritis (KIN), a disorder that serves as a model for renal fibrosis. Renal histology in KIN is indistinguishable from that of nephronophthisis, except for the presence of karyomegaly. The FAN1 protein has nuclease activity and acts in DNA interstrand cross-link (ICL) repair within the Fanconi anemia DNA damage response (DDR) pathway. We show that cells from individuals with FAN1 mutations have sensitivity to the ICL-inducing agent mitomycin C but do not exhibit chromosome breakage or cell cycle arrest after diepoxybutane treatment, unlike cells from individuals with Fanconi anemia. We complemented ICL sensitivity with wild-type FAN1 but not with cDNA having mutations found in individuals with KIN. Depletion of fan1 in zebrafish caused increased DDR, apoptosis and kidney cysts. Our findings implicate susceptibility to environmental genotoxins and inadequate DNA repair as novel mechanisms contributing to renal fibrosis and CKD.

AKT2 is essential to maintain podocyte viability and function during chronic kidney disease

In chronic kidney disease (CKD), loss of functional nephrons results in metabolic and mechanical stress in the remaining ones, resulting in further nephron loss. Here we show that Akt2 activation has an essential role in podocyte protection after nephron reduction. Glomerulosclerosis and albuminuria were substantially worsened in Akt2−/− but not in Akt1−/− mice as compared to wild-type mice. Specific deletion of Akt2 or its regulator Rictor in podocytes revealed that Akt2 has an intrinsic function in podocytes. Mechanistically, Akt2 triggers a compensatory program that involves mouse double minute 2 homolog (Mdm2), glycogen synthase kinase 3 (Gsk3) and Rac1. The defective activation of this pathway after nephron reduction leads to apoptosis and foot process effacement of the podocytes. We further show that AKT2 activation by mammalian target of rapamycin complex 2 (mTORC2) is also required for podocyte survival in human CKD. More notably, we elucidate the events underlying the adverse renal effect of sirolimus and provide a criterion for the rational use of this drug. Thus, our results disclose a new function of Akt2 and identify a potential therapeutic target for preserving glomerular function in CKD.

Phosphorylation by casein kinase 2 induces PACS-1 binding of nephrocystin and targeting to cilia

Mutations in proteins localized to cilia and basal bodies have been implicated in a growing number of human diseases. Access of these proteins to the ciliary compartment requires targeting to the base of the cilia. However, the mechanisms involved in transport of cilia proteins to this transitional zone are elusive. Here we show that nephrocystin, a ciliary protein mutated in the most prevalent form of cystic kidney disease in childhood, is expressed in respiratory epithelial cells and accumulates at the base of cilia, overlapping with markers of the basal body area and the transition zone. Nephrocystin interacts with the phosphofurin acidic cluster sorting protein (PACS)‐1. Casein kinase 2 (CK2)‐mediated phosphorylation of three critical serine residues within a cluster of acidic amino acids in nephrocystin mediates PACS‐1 binding, and is essential for colocalization of nephrocystin with PACS‐1 at the base of cilia. Inhibition of CK2 activity abrogates this interaction and results in the loss of correct nephrocystin targeting. These data suggest that CK2‐dependent transport processes represent a novel pathway of targeting proteins to the cilia.

An update on ABO-incompatible kidney transplantation

ABO‐incompatible kidney transplantation is nowadays a well‐established procedure to expand living donor transplantation to blood group incompatible donor/recipient constellations. In the last two decades, transplantation protocols evolved to more specific isohaemagglutinin elimination techniques and established competent antirejection protection protocols without the need of splenectomy. ABOi kidney transplantation associated accommodation despite isohaemagglutinin reappearance, C4d positivity of peritubular capillaries as well as the increased incidence of bleeding complications is currently under intense investigation. However, most recent data show excellent graft survival rates equivalent to ABO‐compatible kidney transplantation outcome.

Characterization of endolymphatic sac tumors and von Hippel–Lindau disease in the International Endolymphatic Sac Tumor Registry

Endolymphatic sac tumors (ELSTs) are, with a prevalence of up to 16%, a component of von Hippel–Lindau (VHL) disease. Data from international registries regarding heritable fraction and characteristics, germline VHL mutation frequency, and prevalence are lacking.

One hundred ABO-incompatible kidney transplantations between 2004 and 2014: a single-centre experience

BACKGROUND ABO-incompatible kidney transplantation (ABOi KTx) expands the living donor transplantation options. However, long-term outcome data, especially in comparison with ABO-compatible kidney transplantation (ABOc KTx), remain limited. Since the first ABOi KTx in Germany on 1 April 2004 at our centre, we have followed 100 ABOi KTx over up to 10 years. METHODS One hundred ABOi KTx and 248 ABOc KTx from 1 April 2004 until 28 October 2014 were analysed in this observational, single-centre study. Three ABOi KTx and 141 ABOc KTx were excluded because of cyclosporine A-based immunosuppression, and 1 ABOc KTx was lost to follow-up. RESULTS Median estimated 10-year patient and graft survival in ABOi KTx was 99 and 94%, respectively, and surpassed ABOc-KTx patient and graft survival of 80 and 88%, respectively. The incidence rate of antibody-mediated rejections was 10 and 8%, and that of T-cell-mediated rejections was 17 and 20% in ABOi KTx and ABOc KTx, respectively. Infectious and malignant complications in ABOi KTx were not more common than in ABOc KTx. However, postoperative lymphoceles occurred more frequently in ABOi KTx. Subgroup analysis of ABOi-KTx patients revealed that patients with high-titre isohaemagglutinins before transplantation had equal long-term results compared with low-titre isohaemagglutinin patients. CONCLUSION Taken together, long-term outcome of ABOi KTx is not inferior to ABOc KTx. Incidences of rejection episodes, infectious complications and malignancies are not increased, despite the more vigorous immunosuppression in ABOi KTx. Our data provide further evidence that ABOi KTx with living donation is a safe, successful and reasonable option to reduce the organ shortage.

Targeting mTOR Signaling Can Prevent the Progression of FSGS

Mammalian target of rapamycin (mTOR) signaling is involved in a variety of kidney diseases. Clinical trials administering mTOR inhibitors to patients with FSGS, a prototypic podocyte disease, led to conflicting results, ranging from remission to deterioration of kidney function. Here, we combined complex genetic titration of mTOR complex 1 (mTORC1) levels in murine glomerular disease models, pharmacologic studies, and human studies to precisely delineate the role of mTOR in FSGS. mTORC1 target genes were significantly induced in microdissected glomeruli from both patients with FSGS and a murine FSGS model. Furthermore, a mouse model with constitutive mTORC1 activation closely recapitulated human FSGS. Notably, the complete knockout of mTORC1 by induced deletion of both Raptor alleles accelerated the progression of murine FSGS models. However, lowering mTORC1 signaling by deleting just one Raptor allele ameliorated the progression of glomerulosclerosis. Similarly, low-dose treatment with the mTORC1 inhibitor rapamycin efficiently diminished disease progression. Mechanistically, complete pharmacologic inhibition of mTOR in immortalized podocytes shifted the cellular energy metabolism toward reduced rates of oxidative phosphorylation and anaerobic glycolysis, which correlated with increased production of reactive oxygen species. Together, these data suggest that podocyte injury and loss is commonly followed by adaptive mTOR activation. Prolonged mTOR activation, however, results in a metabolic podocyte reprogramming leading to increased cellular stress and dedifferentiation, thus offering a treatment rationale for incomplete mTOR inhibition.

Secreted frizzled-related protein 4 predicts progression of autosomal dominant polycystic kidney disease

BACKGROUND Autosomal dominant polycystic kidney disease (ADPKD) is a common autosomal dominant condition associated with renal cysts and development of renal failure. With the availability of potential therapies, one major obstacle remains the lack of readily available parameters that identify patients at risk for disease progression and/or determine the efficacy of therapeutic interventions within short observation periods. Increased total kidney volume (TKV) correlates with disease progression, but it remains unknown how accurate this parameter can predict disease progression at early stages. METHODS To identify additional parameters that help to stratify ADPKD patients, we measured secreted frizzled-related protein 4 (sFRP4) serum concentrations at baseline and over the course of 18 months in 429 ADPKD patients. RESULTS Serum creatinine and sFRP4 as well as TKV increased over time, and were significantly different from baseline values within 1 year. CONCLUSION Elevated sFRP4 levels at baseline predicted a more rapid decline of renal function at 2, 3 and 5 years suggesting that sFRP4 serum levels may provide additional information to identify ADPKD patients at risk for rapid disease progression.

Preventive medicine for von Hippel-Lindau disease-associated pancreatic neuroendocrine tumors.

Pancreatic neuroendocrine tumors (PanNETs) are rare in von Hippel-Lindau disease (VHL) but cause serious morbidity and mortality. Management guidelines for VHL-PanNETs continue to be based on limited evidence, and survival data to guide surgical management are lacking. We established the European-American-Asian-VHL-PanNET-Registry to assess data for risks for metastases, survival and long-term outcomes to provide best management recommendations. Of 2330 VHL patients, 273 had a total of 484 PanNETs. Median age at diagnosis of PanNET was 35 years (range 10-75). Fifty-five (20%) patients had metastatic PanNETs. Metastatic PanNETs were significantly larger (median size 5 vs 2 cm; P < 0.001) and tumor volume doubling time (TVDT) was faster (22 vs 126 months; P = 0.001). All metastatic tumors were ≥2.8 cm. Codons 161 and 167 were hotspots for VHL germline mutations with enhanced risk for metastatic PanNETs. Multivariate prediction modeling disclosed maximum tumor diameter and TVDT as significant predictors for metastatic disease (positive and negative predictive values of 51% and 100% for diameter cut-off ≥2.8 cm, 44% and 91% for TVDT cut-off of ≤24 months). In 117 of 273 patients, PanNETs >1.5 cm in diameter were operated. Ten-year survival was significantly longer in operated vs non-operated patients, in particular for PanNETs <2.8 cm vs ≥2.8 cm (94% vs 85% by 10 years; P = 0.020; 80% vs 50% at 10 years; P = 0.030). This study demonstrates that patients with PanNET approaching the cut-off diameter of 2.8 cm should be operated. Mutations in exon 3, especially of codons 161/167 are at enhanced risk for metastatic PanNETs. Survival is significantly longer in operated non-metastatic VHL-PanNETs.