The Insulin-Degrading Enzyme from Structure to Allosteric Modulation: New Perspectives for Drug Design.

The insulin-degrading enzyme (IDE) is a Zn2+ peptidase originally discovered as the main enzyme involved in the degradation of insulin and other amyloidogenic peptides, such as the ß-amyloid (Aß) peptide. Therefore, a role for the IDE in the cure of diabetes and Alzheimer's disease (AD) has been long envisaged. Anyway, its role in degrading amyloidogenic proteins remains not clearly defined and, more recently, novel non-proteolytic functions of the IDE have been proposed. From a structural point of view, the IDE presents an atypical clamshell structure, underscoring unique enigmatic enzymological properties. A better understanding of the structure-function relationship may contribute to solving some existing paradoxes of IDE biology and, in light of its multifunctional activity, might lead to novel therapeutic approaches.

Inhibition of Insulin Degrading Enzyme to Control Diabetes Mellitus and its Applications on some Other Chronic Disease: a Critical Review.

PURPOSE: This review aims to provide a precise perceptive of the insulin-degrading enzyme (IDE) and its relationship to type 2 diabetes (T2D), Alzheimer's disease (AD), obesity, and cardiovascular diseases. The purpose of the current study was to provide clear idea of treating prevalent diseases such as T2D, and AD by molecular pharmacological therapeutics rather than conventional medicinal therapy. METHODS: To achieve the aims, molecular docking was performed using several softwares such as LIGPLOT+, Python, and Protein-Ligand Interaction Profiler with corresponding tools. RESULTS: The IDE is a large zinc-metalloprotease that breakdown numerous pathophysiologically important extracellular substrates, comprising amyloid ß-protein (Aß) and insulin. Recent studies demonstrated that dysregulation of IDE leads to develop AD and T2D. Specifically, IDE regulates circulating insulin in a variety of organs via a degradation-dependent clearance mechanism. IDE is unique because it was subjected to allosteric activation and mediated via an oligomer structure. CONCLUSION: In this review, we summarised the factors that modulate insulin reformation by IDE and interaction of IDE and some recent reports on IDE inhibitors against AD and T2D. We also highlighted the latest signs of progress of the function of IDE and challenges in advancing IDE- targetted therapies against T2D and AD.

Exploring the Structural Rearrangements of the Human Insulin-Degrading Enzyme through Molecular Dynamics Simulations.

Insulin-degrading enzyme (IDE) is a ubiquitously expressed metallopeptidase that degrades insulin and a large panel of amyloidogenic peptides. IDE is thought to be a potential therapeutic target for type-2 diabetes and neurodegenerative diseases, such as Alzheimer's disease. IDE catalytic chamber, known as a crypt, is formed, so that peptides can be enclosed and degraded. However, the molecular mechanism of the IDE function and peptide recognition, as well as its conformation changes, remains elusive. Our study elucidates IDE structural changes and explains how IDE conformational dynamics is important to modulate the catalytic cycle of IDE. In this aim, a free-substrate IDE crystallographic structure (PDB ID: 2JG4) was used to model a complete structure of IDE. IDE stability and flexibility were studied through molecular dynamics (MD) simulations to witness IDE conformational dynamics switching from a closed to an open state. The description of IDE structural changes was achieved by analysis of the cavity and its expansion over time. Moreover, the quasi-harmonic analysis of the hinge connecting IDE domains and the angles formed over the simulations gave more insights into IDE shifts. Overall, our results could guide toward the use of different approaches to study IDE with different substrates and inhibitors, while taking into account the conformational states resolved in our study.

Insulin-Degrading Enzyme: Paradoxes and Possibilities.

More than seven decades have passed since the discovery of a proteolytic activity within crude tissue extracts that would become known as insulin-degrading enzyme (IDE). Certainly much has been learned about this atypical zinc-metallopeptidase; at the same time, however, many quite fundamental gaps in our understanding remain. Herein, I outline what I consider to be among the most critical unresolved questions within the field, many presenting as intriguing paradoxes. For instance, where does IDE, a predominantly cytosolic protein with no signal peptide or clearly identified secretion mechanism, interact with insulin and other extracellular substrates? Where precisely is IDE localized within the cell, and what are its functional roles in these compartments? How does IDE, a bowl-shaped protein that completely encapsulates its substrates, manage to avoid getting "clogged" and thus rendered inactive virtually immediately? Although these paradoxes are by definition unresolved, I offer herein my personal insights and informed speculations based on two decades working on the biology and pharmacology of IDE and suggest specific experimental strategies for addressing these conundrums. I also offer what I believe to be especially fruitful avenues for investigation made possible by the development of new technologies and IDE-specific reagents. It is my hope that these thoughts will contribute to continued progress elucidating the physiology and pathophysiology of this important peptidase.

Quantitative NMR Study of Insulin-Degrading Enzyme Using Amyloid-ß and HIV-1 p6 Elucidates Its Chaperone Activity.

Insulin-degrading enzyme (IDE) hydrolyzes monomeric polypeptides, including amyloid-ß (Aß) and HIV-1 p6. It also acts as a nonproteolytic chaperone to prevent Aß polymerization. Here we compare interactions of Aß and non-amyloidogenic p6 with IDE. Although both exhibited similar proteolysis rates, the binding kinetics to an inactive IDE characterized using relaxation-based NMR were remarkably different. IDE and Aß formed a sparsely populated complex with a lifetime of milliseconds in which a short hydrophobic cleavage segment of Aß was anchored to IDE. Strikingly, a second and more stable complex was significantly populated with a subsecond lifetime owing to multiple intermolecular contacts between Aß and IDE. By selectively sequestering Aß in this nonproductive complex, IDE likely increases the critical concentration required for fibrillization. In contrast, IDE and p6 formed a transient, submillisecond complex involving a single anchoring p6 motif. Modulation of intermolecular interactions, thus, allows IDE to differentiate between non-amyloidogenic and amyloidogenic substrates.

Degradation of Alzheimer's Amyloid-ß by a Catalytically Inactive Insulin-Degrading Enzyme.

It is known that insulin-degrading-enzyme (IDE) plays a crucial role in the clearance of Alzheimer's amyloid-ß (Aß). The cysteine-free IDE mutant (cf-E111Q-IDE) is catalytically inactive against insulin, but its effect on Aß degradation is unknown that would help in the allosteric modulation of the enzyme activity. Herein, the degradation of Aß(1-40) by cf-E111Q-IDE via a non-chaperone mechanism is demonstrated by NMR and LC-MS, and the aggregation of fragmented peptides is characterized using fluorescence and electron microscopy. cf-E111Q-IDE presented a reduced effect on the aggregation kinetics of Aß(1-40) when compared with the wild-type IDE. Whereas LC-MS and diffusion ordered NMR spectroscopy revealed the generation of Aß fragments by both wild-type and cf-E111Q-IDE. The aggregation propensities and the difference in the morphological phenotype of the full-length Aß(1-40) and its fragments are explained using multi-microseconds molecular dynamics simulations. Notably, our results reveal that zinc binding to Aß(1-40) inactivates cf-E111Q-IDE's catalytic function, whereas zinc removal restores its function as evidenced from high-speed AFM, electron microscopy, chromatography, and NMR results. These findings emphasize the catalytic role of cf-E111Q-IDE on Aß degradation and urge the development of zinc chelators as an alternative therapeutic strategy that switches on/off IDE's function.

The insulin-degrading enzyme as a link between insulin and neuropeptides metabolism.

We have applied a recently developed HPLC-MS enzymatic assay to investigate the cryptic peptides generated by the action of the insulin-degrading enzyme (IDE) on some neuropeptides (NPs) involved in the development of tolerance and dependence to opioids. Particularly, the tested NPs are generated from the NPFF precursor (pro-NPFF (A)): NPFF (FLFQPQRF) and NPAF (AGEGLSSPFWSLAAPQRF). The results show that IDE is able to cleave NPFF and NPAF, generating specific cryptic peptides. As IDE is also responsible for the processing of many other peptides in the brain (amyloid beta protein among the others), we have also performed competitive degradation assays using mixtures of insulin and the above mentioned NPs. Data show that insulin is able to slow down the degradation of both NPs tested, whereas, surprisingly, NPAF is able to accelerate insulin degradation, hinting IDE as the possible link responsible of the mutual influence between insulin and NPs metabolism.

Enzyme kinetics from circular dichroism of insulin reveals mechanistic insights into the regulation of insulin-degrading enzyme.

Insulin-degrading enzyme (IDE) is a zinc metalloprotease that selectively degrades biologically important substrates associated with type 2 diabetes and Alzheimer's disease (AD). As such, IDE is an attractive target for therapeutic innovations. A major requirement is an understanding of how other molecules present in cells regulate the activity of the enzyme toward insulin, IDE's most important physiologically relevant substrate. Previous kinetic studies of the IDE-dependent degradation of insulin in the presence of potential regulators have used iodinated insulin, a chemical modification that has been shown to alter the biological and biochemical properties of insulin. Here, we present a novel kinetic assay that takes advantage of the loss of helical circular dichroic signals of insulin with IDE-dependent degradation. As proof of concept, the resulting Michaelis-Menten kinetic constants accurately predict the known regulation of IDE by adenosine triphosphate (ATP). Intriguingly, we found that when Mg2+ is present with ATP, the regulation is abolished. The implication of this result for the development of preventative and therapeutic strategies for AD is discussed. We anticipate that the new assay presented here will lead to the identification of other small molecules that regulate the activity of IDE toward insulin.

Molecular Determinants of Substrate Specificity in Human Insulin-Degrading Enzyme.

Insulin-degrading enzyme (IDE) is a 110 kDa chambered zinc metalloendopeptidase that degrades insulin, amyloid ß, and other intermediate-sized aggregation prone peptides that adopt ß-structures. Structural studies of IDE in complex with multiple physiological substrates have suggested a role for hydrophobic and aromatic residues of the IDE active site in substrate binding and catalysis. Here, we examine functional requirements for conserved hydrophobic and aromatic IDE active site residues that are positioned within 4.5 Å of IDE-bound insulin B chain and amyloid ß peptides in the reported crystal structures for the respective enzyme-substrate complexes. Charge, size, hydrophobicity, aromaticity, and other functional group requirements for substrate binding IDE active site residues were examined through mutational analysis of the recombinant human enzyme and enzyme kinetic studies conducted using native and fluorogenic derivatives of human insulin and amyloid ß peptides. A functional requirement for IDE active site residues F115, A140, F141, Y150, W199, F202, F820, and Y831 was established, and specific contributions of residue charge, size, and hydrophobicity to substrate binding, specificity, and proteolysis were demonstrated. IDE mutant alleles that exhibited enhanced or diminished proteolytic activity toward insulin or amyloid ß peptides and derivative substrates were identified.

Ensemble cryoEM elucidates the mechanism of insulin capture and degradation by human insulin degrading enzyme.

Insulin degrading enzyme (IDE) plays key roles in degrading peptides vital in type two diabetes, Alzheimer's, inflammation, and other human diseases. However, the process through which IDE recognizes peptides that tend to form amyloid fibrils remained unsolved. We used cryoEM to understand both the apo- and insulin-bound dimeric IDE states, revealing that IDE displays a large opening between the homologous ~55 kDa N- and C-terminal halves to allow selective substrate capture based on size and charge complementarity. We also used cryoEM, X-ray crystallography, SAXS, and HDX-MS to elucidate the molecular basis of how amyloidogenic peptides stabilize the disordered IDE catalytic cleft, thereby inducing selective degradation by substrate-assisted catalysis. Furthermore, our insulin-bound IDE structures explain how IDE processively degrades insulin by stochastically cutting either chain without breaking disulfide bonds. Together, our studies provide a mechanism for how IDE selectively degrades amyloidogenic peptides and offers structural insights for developing IDE-based therapies.

The insulin-degrading enzyme is an allosteric modulator of the 20S proteasome and a potential competitor of the 19S.

The interaction of insulin-degrading enzyme (IDE) with the main intracellular proteasome assemblies (i.e, 30S, 26S and 20S) was analyzed by enzymatic activity, mass spectrometry and native gel electrophoresis. IDE was mainly detected in association with assemblies with at least one free 20S end and biochemical investigations suggest that IDE competes with the 19S in vitro. IDE directly binds the 20S and affects its proteolytic activities in a bimodal fashion, very similar in human and yeast 20S, inhibiting at (IDE) ≤ 30 nM and activating at (IDE) ≥ 30 nM. Only an activating effect is observed in a yeast mutant locked in the "open" conformation (i.e., the α-3ΔN 20S), envisaging a possible role of IDE as modulator of the 20S "open"-"closed" allosteric equilibrium. Protein-protein docking in silico proposes that the interaction between IDE and the 20S could involve the C-term helix of the 20S α-3 subunit which regulates the gate opening of the 20S.

Inositol phosphates and phosphoinositides activate insulin-degrading enzyme, while phosphoinositides also mediate binding to endosomes.

Insulin-degrading enzyme (IDE) hydrolyzes bioactive peptides, including insulin, amylin, and the amyloid ß peptides. Polyanions activate IDE toward some substrates, yet an endogenous polyanion activator has not yet been identified. Here we report that inositol phosphates (InsPs) and phosphatdidylinositol phosphates (PtdInsPs) serve as activators of IDE. InsPs and PtdInsPs interact with the polyanion-binding site located on an inner chamber wall of the enzyme. InsPs activate IDE by up to ∼95-fold, affecting primarily Vmax The extent of activation and binding affinity correlate with the number of phosphate groups on the inositol ring, with phosphate positional effects observed. IDE binds PtdInsPs from solution, immobilized on membranes, or presented in liposomes. Interaction with PtdInsPs, likely PtdIns(3)P, plays a role in localizing IDE to endosomes, where the enzyme reportedly encounters physiological substrates. Thus, InsPs and PtdInsPs can serve as endogenous modulators of IDE activity, as well as regulators of its intracellular spatial distribution.

Toward Allosterically Increased Catalytic Activity of Insulin-Degrading Enzyme against Amyloid Peptides.

The physiological role of insulin-degrading enzyme (IDE) in the intracytosolic clearance of amyloid ß (Aß) and other amyloid-like peptides supports a hypothesis that human IDE hyperactivation could be therapeutically beneficial for the treatment of late-onset Alzheimer's disease (AD). The major challenge standing in the way of this goal is increasing the specific catalytic activity of IDE against the Aß substrate. There were previous indications that the allosteric mode of IDE activity regulation could potentially provide a highly specific path toward degradation of amyloid-like peptides, while not dramatically affecting activity against other substrates. Recently developed theoretical concepts are used here to explore potential allosteric modulation of the IDE activity as a result of single-residue mutations. Five candidates are selected for experimental follow-up and allosteric free energy calculations: Ser137Ala, Lys396Ala, Asp426Ala, Phe807Ala, and Lys898Ala. Our experiments show that three mutations (Ser137Ala, Phe807Ala, and Lys898Ala) decrease the Km of the Aß substrate. Mutation Lys898Ala results in increased catalytic activity of IDE; on the other hand, Lys364Ala does not change the activity and Asp426Ala diminishes it. Quantifying effects of mutations in terms of allosteric free energy, we show that favorable mutations lead to stabilization of the catalytic sites and other function-relevant distal sites as well as increased dynamics of the IDE-N and IDE-C halves that allow efficient substrate entrance and cleavage. A possibility for intramolecular upregulation of IDE activity against amyloid peptides via allosteric mutations calls for further investigations in this direction. Ultimately, we are hopeful it will lead to the development of IDE-based drugs for the treatment of the late-onset form of AD characterized by an overall impairment of Aß clearance.

Mechanisms of peptide hydrolysis by aspartyl and metalloproteases.

Peptide hydrolysis has been involved in a wide range of biological, biotechnological, and industrial applications. In this perspective, the mechanisms of three distinct peptide bond cleaving enzymes, beta secretase (BACE1), insulin degrading enzyme (IDE), and bovine lens leucine aminopeptidase (BILAP), have been discussed. BACE1 is a catalytic Asp dyad [Asp, Asp-] containing aspartyl protease, while IDE and BILAP are mononuclear [Zn(His, His, Glu)] and binuclear [Zn1(Asp, Glu, Asp)-Zn2(Lys, Glu, Asp, Asp)] core possessing metallopeptidases, respectively. Specifically, enzyme-substrate interactions and the roles of metal ion(s), the ligand environment, second coordination shell residues, and the protein environment in the functioning of these enzymes have been elucidated. This information will be useful to design small inhibitors, activators, and synthetic analogues of these enzymes for biomedical, biotechnological, and industrial applications.

Multiple allosteric sites are involved in the modulation of insulin-degrading-enzyme activity by somatostatin.

Somatostatin is a cyclic peptide, released in the gastrointestinal system and the central nervous system, where it is involved in the regulation of cognitive and sensory functions, motor activity and sleep. It is a substrate of insulin-degrading enzyme (IDE), as well as a modulator of its activity and expression. In the present study, we have investigated the modulatory role of somatostatin on IDE activity at 37 °C and pH 7.3 for various substrates [i.e. insulin, ß-amyloid (Aß)1-40 and bradykinin], aiming to quantitatively characterize the correlation between the specific features of the substrates and the regulatory mechanism. Functional data indicate that somatostatin, in addition to the catalytic site of IDE (being a substrate), is also able to bind to two additional exosites, which play different roles according to the size of the substrate and its binding mode to the IDE catalytic cleft. In particular, one exosite, which displays high affinity for somatostatin, regulates only the interaction of IDE with larger substrates (such as insulin and Aß1-40 ) in a differing fashion according to their various modes of binding to the enzyme. A second exosite, which is involved in the regulation of enzymatic processing by IDE of all substrates investigated (including a 10-25 amino acid long amyloid-like peptide, bradykinin and somatostatin itself, which had been studied previously), probably acts through the alteration of an 'open-closed' equilibrium.

Identification and functional characterization of a putative IDE, C28F5.4 (ceIDE-1), in Caenorhabditis elegans: Implications for Alzheimer's disease.

Insulin-degrading enzyme (IDE) is a zinc metalloprotease, known to degrade insulin peptide and amyloid-beta (Aß); the key protein involved in Alzheimer's disease (AD). Considering the important role played by IDE in disease progression of AD and type 2 diabetes mellitus (T2DM), we endeavored to identify the Caenorhabditis elegans (C. elegans) IDE orthologous genes and test them for their role in AD related outcomes. We employed bioinformatics, reverse genetics and molecular biology approaches towards identification and functional characterization of putative IDE candidates in C. elegans. Using in-silico analysis we have identified seven C. elegans genes that possess HXXEH motif, an identifying marker of IDE. We further carried out functional analysis of the identified genes in Aß expressing C. elegans strain CL4176 [myo-3/Aß1-42 long 3'-UTR] via studying effect on Aß induced toxicity, cholinergic neuroanatomy, content of acetylcholine/acetylcholine-esterase, extent of reactive oxygen species and expression of FOXO transcription factor DAF-16. Our findings reveal that amongst the identified putative IDE orthologs, a functionally uncharacterized gene C28F5.4 had a profound effect on the tested endpoints. Knocking down C28F5.4 modulated the AD associated conditions by decreasing Aß induced toxicity, severely compromising cholinergic neuroanatomy, reducing expression of acetylcholine-transporter, decreasing acetylcholine content, elevating ROS, with no effect on DAF-16 stress-response protein. These studies provide crucial insight into the structural/functional orthology of IDEs across human and nematode species and further our understanding of the involvement of these proteins and insulin pathway in AD. Further studies could aid in identifying novel drug-targets and in understanding the common modulating factors between AD and T2DM.

Impact of Insulin Degrading Enzyme and Neprilysin in Alzheimer's Disease Biology: Characterization of Putative Cognates for Therapeutic Applications.

Alzheimer's disease (AD) is a neurodegenerative process primarily characterized by amyloid-ß (Aß) agglomeration, neuroinflammation, and cognitive dysfunction. The prominent cause for dementia is the deposition of Aß plaques and tau-neurofibrillary tangles that hamper the neuronal organization and function. Aß pathology further affects numerous signaling cascades that disturb the neuronal homeostasis. For instance, Aß deposition is responsible for altered expression of insulin encoding genes that lead to insulin resistance, and thereby affecting insulin signaling pathway and glucose metabolism in the brain. As a result, the common pathology of insulin resistance between Type-2 diabetes mellitus and AD has led AD to be proposed as a form of diabetes and termed 'Type-3 diabetes'. Since accumulation of Aß is the prominent cause of neuronal toxicity in AD, its clearance is the prime requisite for therapeutic prospects. This purpose is expertly fulfilled by the potential role of Aß degrading enzymes such as insulin degrading enzyme (IDE) and Neprilysin (NEP). Therefore, their molecular study is important to uncover the proteolytic and regulatory mechanism of Aß degradation. Herein, (i) In silico sequential and structural analysis of IDE and NEP has been performed to identify the molecular entities for proteolytic degradation of Aß in the AD brain, (ii) to analyze their catalytic site to demonstrate the enzymatic action played by IDE and NEP, (iii) to identify their structural homologues that could behave as putative partners of IDE and NEP with similar catalytic action and (iv) to illustrate various IDE- and NEP-mediated therapeutic approaches and factors for clearing Aß in AD.

Selective Targeting of Extracellular Insulin-Degrading Enzyme by Quasi-Irreversible Thiol-Modifying Inhibitors.

Many therapeutically important enzymes are present in multiple cellular compartments, where they can carry out markedly different functions; thus, there is a need for pharmacological strategies to selectively manipulate distinct pools of target enzymes. Insulin-degrading enzyme (IDE) is a thiol-sensitive zinc-metallopeptidase that hydrolyzes diverse peptide substrates in both the cytosol and the extracellular space, but current genetic and pharmacological approaches are incapable of selectively inhibiting the protease in specific subcellular compartments. Here, we describe the discovery, characterization, and kinetics-based optimization of potent benzoisothiazolone-based inhibitors that, by virtue of a unique quasi-irreversible mode of inhibition, exclusively inhibit extracellular IDE. The mechanism of inhibition involves nucleophilic attack by a specific active-site thiol of the enzyme on the inhibitors, which bear an isothiazolone ring that undergoes irreversible ring opening with the formation of a disulfide bond. Notably, binding of the inhibitors is reversible under reducing conditions, thus restricting inhibition to IDE present in the extracellular space. The identified inhibitors are highly potent (IC50(app) = 63 nM), nontoxic at concentrations up to 100 µM, and appear to preferentially target a specific cysteine residue within IDE. These novel inhibitors represent powerful new tools for clarifying the physiological and pathophysiological roles of this poorly understood protease, and their unusual mechanism of action should be applicable to other therapeutic targets.

The insulin degrading enzyme activates ubiquitin and promotes the formation of K48 and K63 diubiquitin.

We report an ATP-dependent ubiquitin conjugation with IDE which, in turn, promotes Ub-Ub linkages in tube tests. We propose a novel function for IDE as a non-canonical ubiquitin activating enzyme.

Insulin-degrading enzyme prevents α-synuclein fibril formation in a nonproteolytical manner.

The insulin-degrading enzyme (IDE) degrades amyloidogenic proteins such as Amyloid ß (Αß) and Islet Amyloid Polypeptide (IAPP), i.e. peptides associated with Alzheimer's disease and type 2 diabetes, respectively. In addition to the protease activity normally associated with IDE function an additional activity involving the formation of stable, irreversible complexes with both Αß and α-synuclein, an amyloidogenic protein involved in Parkinson's disease, was recently proposed. Here, we have investigated the functional consequences of IDE-α-synuclein interactions in vitro. We demonstrate that IDE in a nonproteolytic manner and at sub-stoichiometric ratios efficiently inhibits α-synuclein fibril formation by binding to α-synuclein oligomers making them inert to amyloid formation. Moreover, we show that, within a defined range of α-synuclein concentrations, interaction with α-synuclein oligomers increases IDE's proteolytic activity on a fluorogenic substrate. We propose that the outcomes of IDE-α-synuclein interactions, i.e. protection against α-synuclein amyloid formation and stimulated IDE protease activity, may be protective in vivo.