Elke Neumann-Haefelin

A Dynamic Network Model of mTOR Signaling Reveals TSC-Independent mTORC2 Regulation

Modeling and testing network structures reveal a distinct mechanism of mTORC2 activation by insulin. Computing TOR Regulation Without effective genetic or specific pharmacological tools, it can be challenging to dissect regulatory mechanisms in complex, interconnected pathways. Dalle Pezze et al. combined computational modeling with biochemical analysis to test specific regulatory mechanisms of the mammalian target of rapamycin (mTOR) pathway, in particular the regulation of mTOR complex mTORC2, which has important functions in controlling cell growth. After identifying key network components that could be experimentally monitored to explicitly test specific network structures, they compared the experimental data to the modeled networks to rule out previously suggested regulatory mechanisms and postulated the existence of a previously unknown pathway. Their results led to the proposal of a distinct phosphoinositide 3-kinase–dependent pathway from the insulin receptor to mTORC2 that is independent of various components that participate in the activation of mTORC1 and thus should open new areas of research into mTOR signaling and could provide direction for developing selective regulators of mTORC1 and mTORC2 signaling. The kinase mammalian target of rapamycin (mTOR) exists in two multiprotein complexes (mTORC1 and mTORC2) and is a central regulator of growth and metabolism. Insulin activation of mTORC1, mediated by phosphoinositide 3-kinase (PI3K), Akt, and the inhibitory tuberous sclerosis complex 1/2 (TSC1-TSC2), initiates a negative feedback loop that ultimately inhibits PI3K. We present a data-driven dynamic insulin-mTOR network model that integrates the entire core network and used this model to investigate the less well understood mechanisms by which insulin regulates mTORC2. By analyzing the effects of perturbations targeting several levels within the network in silico and experimentally, we found that, in contrast to current hypotheses, the TSC1-TSC2 complex was not a direct or indirect (acting through the negative feedback loop) regulator of mTORC2. Although mTORC2 activation required active PI3K, this was not affected by the negative feedback loop. Therefore, we propose an mTORC2 activation pathway through a PI3K variant that is insensitive to the negative feedback loop that regulates mTORC1. This putative pathway predicts that mTORC2 would be refractory to Akt, which inhibits TSC1-TSC2, and, indeed, we found that mTORC2 was insensitive to constitutive Akt activation in several cell types. Our results suggest that a previously unknown network structure connects mTORC2 to its upstream cues and clarifies which molecular connectors contribute to mTORC2 activation.

Long-term outcome of ABO-incompatible living donor kidney transplantation based on antigen-specific desensitization. An observational comparative analysis

BACKGROUND ABO-incompatible living donor kidney transplantation based on specific conditioning has been successfully adopted by transplant centres worldwide. Excellent short-term results have been reported in small cohorts. However, long-term data and comparative analyses are still sparse. We report on the outcome of 40 consecutive ABO-incompatible living donor kidney transplant recipients and compare their clinical course to a control group of 43 ABO-compatible living donor transplant patients transplanted during the same time period. METHODS This is an observational single-centre analysis of 40 consecutive patients undergoing ABO-incompatible kidney grafting between April 2004 and April 2009, using a protocol of rituximab, antigen-specific immunoadsorption, intravenous immunoglobulin, basiliximab induction and oral triple immunosuppression with tacrolimus, mycophenolic acid and prednisone. Forty-three ABO-compatible kidney transplant recipients served as controls. The control group had also received basiliximab induction and an identical initial maintenance immunosuppression. The two groups were observed for an average of 39 and 19 months, respectively. RESULTS There was a significantly higher incidence of lymphoceles requiring surgical revisions in the ABO-incompatible group. However, this surgical complication did not affect patient or graft survival. Mean serum creatinine, estimated glomerular filtration rate and proteinuria did not differ between the two groups. Furthermore, ABO-incompatible and ABO-compatible patients had the same incidence of humoral and cellular rejections. Despite a more aggressive induction therapy, no differences in infectious or malignant complications were observed. CONCLUSIONS ABO-incompatible living donor kidney transplantation utilizing a combination of rituximab and antigen-specific immunoadsorption yielded results identical to ABO-compatible transplantation despite a significantly higher number of human leukocyte antigen mismatches.

Neph-Nephrin Proteins Bind the Par3-Par6-Atypical Protein Kinase C (aPKC) Complex to Regulate Podocyte Cell Polarity

The kidney filter represents a unique assembly of podocyte epithelial cells that tightly enwrap the glomerular capillaries with their foot processes and the interposed slit diaphragm. So far, very little is known about the guidance cues and polarity signals required to regulate proper development and maintenance of the glomerular filtration barrier. We now identify Par3, Par6, and atypical protein kinase C (aPKC) polarity proteins as novel Neph1-Nephrin-associated proteins. The interaction was mediated through the PDZ domain of Par3 and conserved carboxyl terminal residues in Neph1 and Nephrin. Par3, Par6, and aPKC localized to the slit diaphragm as shown in immunofluorescence and immunoelectron microscopy. Consistent with a critical role for aPKC activity in podocytes, inhibition of glomerular aPKC activity with a pseudosubstrate inhibitor resulted in a loss of regular podocyte foot process architecture. These data provide an important link between cell recognition mediated through the Neph1-Nephrin complex and Par-dependent polarity signaling and suggest that this molecular interaction is essential for establishing the three-dimensional architecture of podocytes at the kidney filtration barrier.

SHC-1/p52Shc targets the insulin/IGF-1 and JNK signaling pathways to modulate life span and stress response in C. elegans

Correlative evidence links stress, accumulation of oxidative cellular damage, and aging in several species. Genetic studies in species ranging from yeast to mammals revealed several pathways regulating stress response and life span, including caloric intake, mitochondrial respiration, insulin/IGF-1 (IIS), and JNK (c-Jun N-terminal kinase) signaling. How IIS and JNK signaling cross-talk to defend against diverse stressors contributing to aging is of critical importance but, so far, only poorly understood. In this study, we demonstrate that the adaptor protein SHC-1, the Caenorhabditis elegans homolog of human p52Shc, coordinates mechanisms of stress response and aging. Using genetic and biochemical approaches, we discover that SHC-1 not only opposes IIS but also activates JNK signaling. Loss of shc-1 function results in accelerated aging and enhanced sensitivity to heat, oxidative stress, and heavy metals, whereas expression of human p52Shc rescues the shc-1 mutant phenotype. SHC-1 acts upstream of the insulin/IGF receptor DAF-2 and the PI3 kinase AGE-1 and directly interacts with DAF-2. Moreover, SHC-1 activates JNK signaling by binding to MEK-1 kinase. Both aspects converge on controlling the nuclear translocation and activation of the FOXO transcription factor DAF-16. Our findings establish C. elegans SHC-1 as a critical scaffold that directly cross-connects the two parallel JNK and IIS pathways and help to explain how these signaling cascades cooperate to ascertain normal stress response and life span in C. elegans.

A model organism approach: defining the role of Neph proteins as regulators of neuron and kidney morphogenesis

Mutations of the immunoglobulin superfamily proteins nephrin and Neph1 lead to congenital nephrotic syndrome in humans or mice. Neph proteins are three closely related molecules that are evolutionarily conserved and mediate cell recognition. Their importance for morphogenetic processes including the formation of the kidney filtration barrier in vertebrates and synaptogenesis in Caenorhabditis elegans has recently been uncovered. However, the individual morphogenetic function of mammalian Neph1-3 isoforms remained elusive. We demonstrate now that the Neph/nephrin family proteins can form cell-cell adhesion modules across species. Expression of all three mammalian Neph isoforms partially rescued mutant C. elegans lacking their Neph homolog syg-1 and restored synapse formation, suggesting a functional redundancy between the three isoforms. Strikingly, the rescue of defective synaptic connectivity was prevented by deletion of the highly conserved cytoplasmic PSD95/Dlg/ZO-1-binding motif of SYG-1/Neph proteins, indicating the critical role of this intracellular signaling motif for SYG-1/Neph-dependent morphogenetic events. To determine the significance of Neph isoform redundancy for vertebrate kidney development, we analyzed the expression pattern and the functional role of Neph proteins in zebrafish. In situ hybridizations identified zNeph1 and zNeph2 as glomerular proteins. Morpholino knockdown of either zNeph1 or zNeph2 resulted in loss of slit diaphragms and leakiness of the glomerular filtration barrier. This is the first report utilizing C. elegans to study mammalian Neph/nephrin protein function and to demonstrate a functional overlap of Neph1-3 proteins. Furthermore, we identify Neph2 as a novel critical regulator of glomerular function, indicating that both Neph1 and Neph2 are required for glomerular maintenance and development.

mTORC2-SGK-1 acts in two environmentally responsive pathways with opposing effects on longevity

The nematode worm Caenorhabditis elegans provides a powerful system for elucidating how genetic, metabolic, nutritional, and environmental factors influence aging. The mechanistic target of rapamycin (mTOR) kinase is important in growth, disease, and aging and is present in the mTORC1 and mTORC2 complexes. In diverse eukaryotes, lifespan can be increased by inhibition of mTORC1, which transduces anabolic signals to stimulate protein synthesis and inhibit autophagy. Less is understood about mTORC2, which affects C. elegans lifespan in a complex manner that is influenced by the bacterial food source. mTORC2 regulates C. elegans growth, reproduction, and lipid metabolism by activating the SGK‐1 kinase, but current data on SGK‐1 and lifespan seem to be conflicting. Here, by analyzing the mTORC2 component Rictor (RICT‐1), we show that mTORC2 modulates longevity by activating SGK‐1 in two pathways that affect lifespan oppositely. RICT‐1/mTORC2 limits longevity by directing SGK‐1 to inhibit the stress‐response transcription factor SKN‐1/Nrf in the intestine. Signals produced by the bacterial food source determine how this pathway affects SKN‐1 and lifespan. In addition, RICT‐1/mTORC2 functions in neurons in an SGK‐1‐mediated pathway that increases lifespan at lower temperatures. RICT‐1/mTORC2 and SGK‐1 therefore oppose or accelerate aging depending upon the context in which they are active. Our findings reconcile data on SGK‐1 and aging, show that the bacterial microenvironment influences SKN‐1/Nrf, mTORC2 functions, and aging, and identify two longevity‐related mTORC2 functions that involve SGK‐regulated responses to environmental cues.

Cell-Nonautonomous Signaling of FOXO/DAF-16 to the Stem Cells of Caenorhabditis elegans

In Caenorhabditis elegans (C. elegans), the promotion of longevity by the transcription factor DAF-16 requires reduced insulin/IGF receptor (IIR) signaling or the ablation of the germline, although the reason for the negative impact of germ cells is unknown. FOXO/DAF-16 activity inhibits germline proliferation in both daf-2 mutants and gld-1 tumors. In contrast to its function as a germline tumor suppressor, we now provide evidence that somatic DAF-16 in the presence of IIR signaling can also result in tumorigenic activity, which counteracts robust lifespan extension. In contrast to the cell-autonomous IIR signaling, which is required for larval germline proliferation, activation of DAF-16 in the hypodermis results in hyperplasia of the germline and disruption of the surrounding basement membrane. SHC-1 adaptor protein and AKT-1 kinase antagonize, whereas AKT-2 and SGK-1 kinases promote, this cell-nonautonomous DAF-16 function. Our data suggest that a functional balance of DAF-16 activities in different tissues determines longevity and reveals a novel, cell-nonautonomous role of FOXO/DAF-16 to affect stem cells.

Functional and Spatial Analysis of C. elegans SYG-1 and SYG-2, Orthologs of the Neph/Nephrin Cell Adhesion Module Directing Selective Synaptogenesis

The assembly of specific synaptic connections represents a prime example of cellular recognition. Members of the Ig superfamily are among the most ancient proteins represented in the genomes of both mammalian and invertebrate organisms, where they constitute a trans-synaptic adhesion system. The correct connectivity patterns of the highly conserved immunoglobulin superfamily proteins nephrin and Neph1 are crucial for the assembly of functional neuronal circuits and the formation of the kidney slit diaphragm, a synapse-like structure forming the filtration barrier. Here, we utilize the nematode C. elegans model for studying the requirements of synaptic specificity mediated by nephrin-Neph proteins. In C. elegans, the nephrin/Neph1 orthologs SYG-2 and SYG-1 form intercellular contacts strictly in trans between epithelial guidepost cells and neurons specifying the localization of synapses. We demonstrate a functional conservation between mammalian nephrin and SYG-2. Expression of nephrin effectively compensated loss of syg-2 function in C. elegans and restored defective synaptic connectivity further establishing the C. elegans system as a valuable model for slit diaphragm proteins. Next, we investigated the effect of SYG-1 and SYG-2 trans homodimerization respectively. Strikingly, synapse assembly could be induced by homophilic SYG-1 but not SYG-2 binding indicating a critical role of SYG-1 intracellular signalling for morphogenetic events and pointing toward the dynamic and stochastic nature of extra- and intracellular nephrin-Neph interactions to generate reproducible patterns of synaptic connectivity.

Hantavirus Infection With Severe Proteinuria and Podocyte Foot-Process Effacement

Nephropathia epidemica, a zoonosis caused by Hantavirus infection (most commonly subtype Puumala) is associated with flu-like symptoms and acute kidney failure. Kidney manifestations are characterized predominantly by tubulointerstitial nephritis, hemorrhage into medullary tissues, interstitial edema, and tubular cell necrosis. Kidney failure is accompanied by proteinuria, and in some cases, nephrotic-range proteinuria may occur. However, the cellular mechanisms of proteinuria remain to be elucidated. We describe a Hantavirus (Puumala) infection in a 27-year-old man with acute kidney failure and severe and rapidly reversible proteinuria. Light microscopy of a kidney biopsy specimen showed only minor changes of glomeruli. However, transmission electron microscopy revealed podocyte foot-process effacement. Immunofluorescence staining of the slit diaphragm protein podocin and the tight junction protein ZO-1 revealed a partial mislocalization of these proteins. Together, these findings highlight that Hantavirus infection may perturb podocyte integrity, resulting in glomerular proteinuria. These alterations of podocytes and consequently the glomerular filtration barrier may be transient and resolve within weeks.

The BAR Domain Protein PICK1 Regulates Cell Recognition and Morphogenesis by Interacting with Neph Proteins

ABSTRACT Neph proteins are evolutionarily conserved membrane proteins of the immunoglobulin superfamily that control the formation of specific intercellular contacts. Cell recognition through these proteins is essential in diverse cellular contexts such as patterning of the compound eye in Drosophila melanogaster, neuronal connectivity in Caenorhabditis elegans, and the formation of the kidney filtration barrier in mammals. Here we identify the PDZ and BAR domain protein PICK1 (protein interacting with C-kinase 1) as a Neph-interacting protein. Binding required dimerization of PICK1, was dependent on PDZ domain protein interactions, and mediated stabilization of Neph1 at the plasma membrane. Moreover, protein kinase C (PKCα) activity facilitated the interaction through releasing Neph proteins from their binding to the multidomain scaffolding protein zonula occludens 1 (ZO-1), another PDZ domain protein. In Drosophila, the Neph homologue Roughest is essential for sorting of interommatidial precursor cells and patterning of the compound eye. RNA interference-mediated knockdown of PICK1 in the Drosophila eye imaginal disc caused a Roughest destabilization at the plasma membrane and a phenotype that resembled rst mutation. These data indicate that Neph proteins and PICK1 synergistically regulate cell recognition and contact formation.