Amyloid Beta Peptide Is an Endogenous Negative Allosteric Modulator of Leptin Receptor.

INTRODUCTION: Metabolic dysfunction is now recognized as a pivotal component of Alzheimer's disease (AD), the most common dementia worldwide. However, the precise molecular mechanisms linking metabolic dysfunction to AD remain elusive. OBJECTIVE: Here, we investigated the direct impact of soluble oligomeric amyloid beta (Aß) peptides, the main molecular hallmark of AD, on the leptin system, a major component of central energy metabolism regulation. METHODS: We developed a new time-resolved fluorescence resonance energy transfer-based Aß binding assay for the leptin receptor (LepR) and studied the effect of Aß on LepR function in several in vitro assays. The in vivo effect of Aß on LepR function was studied in an Aß-specific AD mouse model and in pro-opiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus. RESULTS: We revealed specific and high-affinity (Ki = 0.1 nM) binding of Aß to LepR. Pharmacological characterization of this interaction showed that Aß binds allosterically to the extracellular domain of LepR and negatively affects receptor function. Negative allosteric modulation of LepR by Aß was detected at the level of signaling pathways (STAT-3, AKT, and ERK) in vitroand in vivo. Importantly, the leptin-induced response of POMC neurons, key players in the regulation of metabolic function, was completely abolished in the presence of Aß. CONCLUSION: Our data indicate that Aß is a negative allosteric modulator of LepR, resulting in impaired leptin action, and qualify LepR as a new and direct target of Aß oligomers. Preventing the interaction of Aß with LepR might improve both the metabolic and cognitive dysfunctions in AD condition.

A novel leptin receptor antagonist uncouples leptin's metabolic and immune functions.

Leptin links body energy stores to high energy demanding processes like reproduction and immunity. Based on leptin's role in autoimmune diseases and cancer, several leptin and leptin receptor (LR) antagonists have been developed, but these intrinsically lead to unwanted weight gain. Here, we report on the uncoupling of leptin's metabolic and immune functions based on the cross talk with the epidermal growth factor receptor (EGFR). We show that both receptors spontaneously interact and, remarkably, that this complex can partially overrule the lack of LR activation by a leptin antagonistic mutein. Moreover, this leptin mutant induces EGFR phosphorylation comparable to wild-type leptin. Exploiting this non-canonical leptin signalling pathway, we identified a camelid single-domain antibody that selectively inhibits this LR-EGFR cross talk without interfering with homotypic LR signalling. Administration in vivo showed that this single-domain antibody did not interfere with leptin's metabolic functions, but could reverse the leptin-driven protection against starvation-induced thymic and splenic atrophy. These findings offer new opportunities for the design and clinical application of selective leptin and LR antagonists that avoid unwanted metabolic side effects.

The intracellular domain of the leptin receptor prevents mitochondrial depolarization and mitophagy.

Hypothalamic leptin receptor (LR) signaling regulates body weight by balancing food intake and energy expenditure. It is well established that the human LR undergoes ectodomain shedding, but little is known about the fate of the remaining cytosolic domain. This study demonstrates that regulated intramembrane proteolysis (RIP) releases the LR intracellular domain (LR ICD), which translocates to the mitochondria where it binds to SOCS6. This LR ICD-SOCS6 interaction stabilizes both proteins on the mitochondrial outer membrane and requires a functional BC box in SOCS6 for mitochondrial association and a central motif in the LR ICD for SOCS6 binding. The LR ICD prevents CCCP-induced mitochondrial depolarization and mitophagy as shown by lowered Parkin translocation and p62 accumulation. Strict regulation of mitochondrial dynamics in the hypothalamus is known to be essential for body weight homeostasis. This is the first study showing that the LR can directly modulate mitochondrial biology.

High-Confidence Interactome for RNF41 Built on Multiple Orthogonal Assays.

Ring finger protein 41 (RNF41) is an E3 ubiquitin ligase involved in the ubiquitination and degradation of many proteins including ErbB3 receptors, BIRC6, and parkin. Next to this, RNF41 regulates the intracellular trafficking of certain JAK2-associated cytokine receptors by ubiquitinating and suppressing USP8, which, in turn, destabilizes the ESCRT-0 complex. To further elucidate the function of RNF41 we used different orthogonal approaches to reveal the RNF41 protein complex: affinity purification-mass spectrometry, BioID, and Virotrap. We combined these results with known data sets for RNF41 obtained with microarray MAPPIT and Y2H screens. This way, we establish a comprehensive high-resolution interactome network comprising 175 candidate protein partners. To remove potential methodological artifacts from this network, we distilled the data into a high-confidence interactome map by retaining a total of 19 protein hits identified in two or more of the orthogonal methods. AP2S1, a novel RNF41 interaction partner, was selected from this high-confidence interactome for further functional validation. We reveal a role for AP2S1 in leptin and LIF receptor signaling and show that RNF41 stabilizes and relocates AP2S1.

TYK2-induced phosphorylation of Y640 suppresses STAT3 transcriptional activity.

STAT3 is a pleiotropic transcription factor involved in homeostatic and host defense processes in the human body. It is activated by numerous cytokines and growth factors and generates a series of cellular effects. Of the STAT-mediated signal transduction pathways, STAT3 transcriptional control is best understood. Jak kinase dependent activation of STAT3 relies on Y705 phosphorylation triggering a conformational switch that is stabilized by intermolecular interactions between SH2 domains and the pY705 motif. We here show that a second tyrosine phosphorylation within the SH2 domain at position Y640, induced by Tyk2, negatively controls STAT3 activity. The Y640F mutation leads to stabilization of activated STAT3 homodimers, accelerated nuclear translocation and superior transcriptional activity following IL-6 and LIF stimulation. Moreover, it unlocks type I IFN-dependent STAT3 signalling in cells that are normally refractory to STAT3 transcriptional activation.

RNF41 interacts with the VPS52 subunit of the GARP and EARP complexes.

RNF41 (Ring Finger Protein 41) is an E3 ubiquitin ligase involved in the intracellular sorting and function of a diverse set of substrates. Next to BRUCE and Parkin, RNF41 can directly ubiquitinate ErbB3, IL-3, EPO and RARα receptors or downstream signaling molecules such as Myd88, TBK1 and USP8. In this way it can regulate receptor signaling and routing. To further elucidate the molecular mechanism behind the role of RNF41 in intracellular transport we performed an Array MAPPIT (Mammalian Protein-Protein Interaction Trap) screen using an extensive set of proteins derived from the human ORFeome collection. This paper describes the identification of VPS52, a subunit of the GARP (Golgi-Associated Retrograde Protein) and the EARP (Endosome-Associated Recycling Protein) complexes, as a novel interaction partner of RNF41. Through interaction via their coiled coil domains, RNF41 ubiquitinates and relocates VPS52 away from VPS53, a common subunit of the GARP and EARP complexes, towards RNF41 bodies.

The Leptin Receptor Complex: Heavier Than Expected?

Under normal physiological conditions, leptin and the leptin receptor (ObR) regulate the body weight by balancing food intake and energy expenditure. However, this adipocyte-derived hormone also directs peripheral processes, including immunity, reproduction, and bone metabolism. Leptin, therefore, can act as a metabolic switch connecting the body's nutritional status to high energy consuming processes. We provide an extensive overview of current structural insights on the leptin-ObR interface and ObR activation, coupling to signaling pathways and their negative regulation, and leptin functioning under normal and pathophysiological conditions (obesity, autoimmunity, cancer, … ). We also discuss possible cross-talk with other receptor systems on the receptor (extracellular) and signaling cascade (intracellular) levels.

Reciprocal cross-regulation between RNF41 and USP8 controls cytokine receptor sorting and processing.

The mechanisms controlling the steady-state cell surface levels of cytokine receptors, and consequently the cellular response to cytokines, remain poorly understood. The number of surface-exposed receptors is a dynamic balance of de novo synthesis, transport to the plasma membrane, internalization, recycling, degradation and ectodomain shedding. We previously reported that the E3 ubiquitin ligase RING finger protein 41 (RNF41) inhibits basal lysosomal degradation and enhances ectodomain shedding of JAK2-associated cytokine receptors. Ubiquitin-specific protease 8 (USP8), an RNF41-interacting deubiquitylating enzyme (DUB) stabilizes RNF41 and is involved in trafficking of various transmembrane proteins. The present study identifies USP8 as a substrate of RNF41 and reveals that loss of USP8 explains the aforementioned RNF41 effects. RNF41 redistributes and ubiquitylates USP8, and reduces USP8 levels. In addition, USP8 knockdown functionally matches the effects of RNF41 ectopic expression on the model leptin and leukemia inhibitory factor (LIF) receptors. Moreover, RNF41 indirectly destabilizes the ESCRT-0 complex through suppression of USP8. Collectively, our findings demonstrate that RNF41 controls JAK2-associated cytokine receptor trafficking by acting as a key regulator of USP8 and ESCRT-0 stability. Balanced reciprocal cross-regulation of RNF41 and USP8 thus determines whether receptors are sorted for lysosomal degradation or recycling, this way regulating basal cytokine receptor levels.

Selective inhibition of Cx43 hemichannels by Gap19 and its impact on myocardial ischemia/reperfusion injury.

Connexin-43 (Cx43), a predominant cardiac connexin, forms gap junctions (GJs) that facilitate electrical cell-cell coupling and unapposed/nonjunctional hemichannels that provide a pathway for the exchange of ions and metabolites between cytoplasm and extracellular milieu. Uncontrolled opening of hemichannels in the plasma membrane may be deleterious for the myocardium and blocking hemichannels may confer cardioprotection by preventing ionic imbalance, cell swelling and loss of critical metabolites. Currently, all known hemichannel inhibitors also block GJ channels, thereby disturbing electrical cell-cell communication. Here we aimed to characterize a nonapeptide, called Gap19, derived from the cytoplasmic loop (CL) of Cx43 as a hemichannel blocker and examined its effect on hemichannel currents in cardiomyocytes and its influence in cardiac outcome after ischemia/reperfusion. We report that Gap 19 inhibits Cx43 hemichannels without blocking GJ channels or Cx40/pannexin-1 hemichannels. Hemichannel inhibition is due to the binding of Gap19 to the C-terminus (CT) thereby preventing intramolecular CT-CL interactions. The peptide inhibited Cx43 hemichannel unitary currents in both HeLa cells exogenously expressing Cx43 and acutely isolated pig ventricular cardiomyocytes. Treatment with Gap19 prevented metabolic inhibition-enhanced hemichannel openings, protected cardiomyocytes against volume overload and cell death following ischemia/reperfusion in vitro and modestly decreased the infarct size after myocardial ischemia/reperfusion in mice in vivo. We conclude that preventing Cx43 hemichannel opening with Gap19 confers limited protective effects against myocardial ischemia/reperfusion injury.

Leptin receptor signaling: pathways to leptin resistance.

The identification of spontaneous mutations in the leptin- and leptin receptor (ObR)-encoding ob and db gene, respectively, opened up a new field in obesity research. Leptin, an adipocyte-derived hormone, mirrors the body's fat stores and thereby informs the brain about the body's energy status. In the hypothalamus, leptin triggers specific neuronal subpopulations, like POMC and AgRP/NPY neurons, and activates several intracellular signaling events, including the JAK/STAT, MAPK, PI3K and mTOR pathway, which eventually translates into decreased food intake and increased energy expenditure. Leptin is also involved in the regulation of other physiological processes including reproduction, bone homeostasis and immune function. Here, we review the pathways that are activated upon ObR activation, how ObR expression is controlled and the molecular mechanisms leading to leptin resistance, i.e. the inability to adequately respond to elevated leptin levels and therefore a primary risk factor for obesity.

RNF41 (Nrdp1) controls type 1 cytokine receptor degradation and ectodomain shedding.

Cytokines, such as interferons, erythropoietin, leptin and most interleukins, signal through type 1 cytokine receptors and activate the canonical JAK-STAT pathway. Aberrant cytokine signalling underlies numerous pathologies and adequate, temporary receptor activation is therefore under tight control. Negative-feedback mechanisms are very well studied, but cellular sensitivity also depends on the number of receptors exposed at the cell surface. This is determined by the equilibrium between receptor synthesis and transport to the plasma membrane, internalisation and recycling, degradation and ectodomain shedding, but the molecular basis of how cells establish steady state receptor levels is poorly understood. Here, we report that ring finger protein 41 (RNF41, also known as E3 ubiquitin-protein ligase Nrdp1) interacts with JAK2-associated cytokine receptor complexes and modulates their cell surface exposure and signalling. Moreover, ectopic expression of RNF41 affected turnover of leptin, leukaemia inhibitory factor and interleukin-6 receptor in a dual way: it blocked intracellular cathepsin-L-dependent receptor cleavage and concomitantly enhanced receptor shedding by metalloproteases of the ADAM family. Receptor degradation and shedding are thus interconnected phenomena with a single protein, RNF41, determining the balance.

Insulin receptor substrate 4 couples the leptin receptor to multiple signaling pathways.

Leptin is an adipokine that regulates food intake and energy expenditure by activating its hypothalamic leptin receptor (LR). Members of the insulin receptor substrate (IRS) family serve as adaptor proteins in the signaling pathways of several cytokines and hormones and a role for IRS2 in central leptin physiology is well established. Using mammalian protein-protein interaction trap (MAPPIT), a cytokine receptor-based two-hybrid method, in the N38 hypothalamic cell line, we here demonstrate that also IRS4 interacts with the LR. This recruitment is leptin dependent and requires phosphorylation of the Y1077 motif of the LR. Domain mapping of IRS4 revealed the critical role of the pleckstrin homology domain for full interaction. In line with its function as an adaptor protein, IRS4 interacted with the regulatory p85 subunit of the phosphatidylinositol 3-kinase, phospholipase Cgamma, and the suppressor of cytokine signaling (SOCS) family members SOCS2, SOCS6, and SOCS7 and thus can modulate LR signaling.

Functional cross-modulation between SOCS proteins can stimulate cytokine signaling.

SOCS (suppressors of cytokine signaling) proteins are negative regulators of cytokine signaling that function primarily at the receptor level. Remarkably, in vitro and in vivo observations revealed both inhibitory and stimulatory effects of SOCS2 on growth hormone signaling, suggesting an additional regulatory level. In this study, we examined the possibility of direct cross-modulation between SOCS proteins and found that SOCS2 could interfere with the inhibitory actions of other SOCS proteins in growth hormone, interferon, and leptin signaling. This SOCS2 effect was SOCS box-dependent, required recruitment of the elongin BC complex, and coincided with degradation of target SOCS proteins. Detailed mammalian protein-protein interaction trap (MAPPIT) analysis indicated that SOCS2 can interact with all members of the SOCS family. SOCS2 may thus function as a molecular bridge between a ubiquitin-protein isopeptide ligase complex and SOCS proteins, targeting them for proteasomal turnover. We furthermore extended these observations to SOCS6 and SOCS7. Our findings point to a unique regulatory role for SOCS2, SOCS6, and SOCS7 within the SOCS family and provide an explanation for the unexpected phenotypes observed in SOCS2 and SOCS6 transgenic mice.