Quantitative assessment of low-level parental mosaicism of SNVs and CNVs in Waardenburg syndrome.

Waardenburg syndrome (WS) is a rare inherited autosomal dominant disorder caused by SOX10, PAX3, MITF, EDNRB, EDN3, and SNAI2. A large burden of pathogenic de novo variants is present in patients with WS, which may be derived from parental mosaicism. Previously, we retrospectively analyzed 90 WS probands with family information. And the frequency of de novo events and parental mosaicism was preliminary investigated in our previous study. In this study, we further explored the occurrence of low-level parental mosaicism in 33 WS families with de novo variants and introduced our procedure of quantifying low-level mosaicism. Mosaic single nucleotide polymorphisms (SNPs) were validated by amplicon-based next-generation sequencing (NGS); copy-number variants (CNVs) were validated by droplet-digital polymerase chain reaction (ddPCR). Molecular validation of low-level mosaicism of WS-causing variants was performed in four families (12.1%, 4/33). These four mosaic variants, comprising three SNVs and one CNV, were identified in SOX10. The rate of parental mosaicism was 25% (4/16) in WS families with de novo SOX10 variants. The lowest allele ratio of a mosaic variant was 2.0% in parental saliva. These de novo WS cases were explained by parental mosaicism conferring an elevated recurrence risk in subsequent pregnancies of parents. Considering its importance in genetic counseling, low-level parental mosaicism should be systematically investigated by personalized sensitive testing. Amplicon-based NGS and ddPCR are recommended to detect and precisely quantify the mosaicism for SNPs and CNVs.

Exchange Broadening Underlies the Enhancement of IDE-Dependent Degradation of Insulin by Anionic Membranes.

Insulin-degrading enzyme (IDE) is an evolutionarily conserved ubiquitous zinc metalloprotease implicated in the efficient degradation of insulin monomer. However, IDE also degrades monomers of amyloidogenic peptides associated with disease, complicating the development of IDE inhibitors. In this work, we investigated the effects of the lipid composition of membranes on the IDE-dependent degradation of insulin. Kinetic analysis based on chromatography and insulin's helical circular dichroic signal showed that the presence of anionic lipids in membranes enhances IDE's activity toward insulin. Using NMR spectroscopy, we discovered that exchange broadening underlies the enhancement of IDE's activity. These findings, together with the adverse effects of anionic membranes in the self-assembly of IDE's amyloidogenic substrates, suggest that the lipid composition of membranes is a key determinant of IDE's ability to balance the levels of its physiologically and pathologically relevant substrates and achieve proteostasis.

Establishment of an iPSC line (CPGHi005-A) from a patient with Waardenburg syndrome carrying a heterozygous SVA-F retrotransposon insertion into SOX10.

Mutations of SOX10 result in Waardenburg syndrome characterized by sensorineural hearing loss and pigmentary abnormalities, which can be found in association with a defect of migrating neural crest cells. The role of SINE-VNTR-Alu (SVA) retrotransposon insertions in disorders has only been minimally explored and there have been no reports of WS cases related to SVA retrotransposons. Here, we report the successful establishment and characterization of an iPSC line from a patient diagnosed with Waardenburg syndrome carrying an insertion of SVA in intron 2 of SOX10.

Differential Effects of Polyphenols on Insulin Proteolysis by the Insulin-Degrading Enzyme.

The insulin-degrading enzyme (IDE) possesses a strong ability to degrade insulin and Aß42 that has been linked to the neurodegeneration in Alzheimer's disease (AD). Given this, an attractive IDE-centric strategy for the development of therapeutics for AD is to boost IDE's activity for the clearance of Aß42 without offsetting insulin proteostasis. Recently, we showed that resveratrol enhances IDE's activity toward Aß42. In this work, we used a combination of chromatographic and spectroscopic techniques to investigate the effects of resveratrol on IDE's activity toward insulin. For comparison, we also studied epigallocatechin-3-gallate (EGCG). Our results show that the two polyphenols affect the IDE-dependent degradation of insulin in different ways: EGCG inhibits IDE while resveratrol has no effect. These findings suggest that polyphenols provide a path for developing therapeutic strategies that can selectively target IDE substrate specificity.

Helix Dipole and Membrane Electrostatics Delineate Conformational Transitions in the Self-Assembly of Amyloidogenic Peptides.

The self-assembly of amyloidogenic peptides on membrane surfaces is associated with the death of neurons and ß-cells in Alzheimer's disease and type 2 diabetes, respectively. The early events of self-assembly in vivo are not known, but there is increasing evidence for the importance of the α-helix. To test the hypothesis that electrostatic interactions involving the helix dipole play a key role in membrane-mediated peptide self-assembly, we studied IAPP[11-25(S20G)-NH2] (R11LANFLVHSGNNFGA25-NH2), which under certain conditions self-assembles in hydro to form ß-sheet assemblies through an α-helix-containing intermediate. In the presence of small unilamellar vesicles composed solely of zwitterionic lipids, the peptide does not self-assemble presumably because of the absence of stabilizing electrostatic interactions between the membrane surface and the helix dipole. In the presence of vesicles composed solely of anionic lipids, the peptide forms a long-lived α-helix presumably stabilized by dipole-dipole interactions between adjacent helix dipoles. This helix represents a kinetic trap that inhibits ß-sheet formation. Intriguingly, when the amount of anionic lipids was decreased to mimic the ratio of zwitterionic and anionic lipids in cells, the α-helix was short-lived and underwent an α-helix to ß-sheet conformational transition. Our work suggests that the helix dipole and membrane electrostatics delineate the conformational transitions occurring along the self-assembly pathway to the amyloid.

Inhibition of the Self-Assembly of Aß and of Tau by Polyphenols: Mechanistic Studies.

The amyloid-ß (Aß) peptide and tau protein are thought to play key neuropathogenic roles in Alzheimer's disease (AD). Both Aß and tau self-assemble to form the two major pathological hallmarks of AD: amyloid plaques and neurofibrillary tangles, respectively. In this review, we show that naturally occurring polyphenols abundant in fruits, vegetables, red wine, and tea possess the ability to target pathways associated with the formation of assemblies of Aß and tau. Polyphenols modulate the enzymatic processing of the amyloid-ß precursor protein and inhibit toxic Aß oligomerization by enhancing the clearance of Aß42 monomer, modulating monomer-monomer interactions and remodeling oligomers to non-toxic forms. Additionally, polyphenols modulate tau hyperphosphorylation and inhibit tau ß-sheet formation. The anti-Aß-self-assembly and anti-tau-self-assembly effects of polyphenols increase their potential as preventive or therapeutic agents against AD, a complex disease that involves many pathological mechanisms.

Resveratrol Sustains Insulin-Degrading Enzyme Activity toward Aß42.

Alzheimer's disease (AD), the most common cause of dementia in the elderly, is the sixth leading cause of death in the United States. We hypothesize that the impaired clearance of Aß42 from the brain is partly responsible for the onset of sporadic AD. In this work, we evaluated the activity of insulin-degrading enzyme (IDE) toward Aß42 in the presence of resveratrol, a polyphenol found in red wine and grape juice. By liquid chromatography/mass spectrometry, we identified initial cleavage sites in the absence and presence of resveratrol that carry biological relevance connected to the amyloidogenic properties of Aß42. Incubation with resveratrol results in a substantial increase in Aß42 fragmentation compared to the control, signifying that the polyphenol sustains IDE-dependent degradation of Aß42 and its fragments. Our findings suggest that therapeutic and/or preventative approaches combining resveratrol and IDE may hold promise for sporadic AD.

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.

The Longest Amyloid-ß Precursor Protein Intracellular Domain Produced with Aß42 Forms ß-Sheet-Containing Monomers That Self-Assemble and Are Proteolyzed by Insulin-Degrading Enzyme.

Alzheimer's disease (AD) is the most common neurodegenerative disease resulting in dementia. It is characterized pathologically by extracellular amyloid plaques composed mainly of deposited Aß42 and intracellular neurofibrillary tangles formed by hyperphosphorylated tau protein. Recent clinical trials targeting Aß have failed, suggesting that other polypeptides produced from the amyloid-ß precursor protein (APP) may be involved in AD. An attractive polypeptide is AICD57, the longest APP intracellular domain (AICD) coproduced with Aß42. Here, we show that AICD57 forms micelle-like assemblies that are proteolyzed by insulin-degrading enzyme (IDE), indicating that AICD57 monomers are in dynamic equilibrium with AICD57 assemblies. The N-terminal part of AICD57 monomer is not degraded, but its C-terminal part is hydrolyzed, particularly in the YENPTY motif that has been associated with the hyperphosphorylation of tau. Therefore, sustaining IDE activity well into old age holds promise for regulating levels of not only Aß but also AICD in the aging brain.

Mechanistic Studies of the Inhibition of Insulin Fibril Formation by Rosmarinic Acid.

The self-assembly of insulin to form amyloid fibrils has been widely studied because it is a significant problem in the medical management of diabetes and is an important model system for the investigation of amyloid formation and its inhibition. A few inhibitors of insulin fibrillation have been identified with potencies that could be higher. Knowledge of how these work at the molecular level is not known but important for the development of more potent inhibitors. Here we show that rosmarinic acid completely inhibits amyloid formation by dimeric insulin at pH 2 and 60 °C. In contrast to other polyphenols, rosmarinic acid is soluble in water and does not degrade at elevated temperatures, and thus we were able to decipher the mechanism of inhibition by a combination of solution-state 1H NMR spectroscopy and molecular docking. On the basis of 1H chemical shift perturbations, intermolecular nuclear Overhauser effect enhancements between rosmarinic acid and specific residues of insulin, and slowed dynamics of rosmarinic acid in the presence of insulin, we show that rosmarinic acid binds to a pocket found on the surface of each insulin monomer. This results in the formation of a mixed tetramolecular aromatic network on the surface of insulin dimer, resulting in increased resistance of the amyloidogenic protein to thermal unfolding. This finding opens new avenues for the design of potent inhibitors of amyloid formation and provides strong experimental evidence for the role of surface aromatic clusters in increasing the thermal stability of proteins.