胰岛淀粉样多肽诱导β细胞凋亡机制的研究进展

2型糖尿病病变中由人类胰岛淀粉样多肽(hIAPP)形成的蛋白纤维沉淀被认为是引起β细胞凋亡的重要原因。目前,hIAPP诱导β细胞凋亡的确切机制尚未完全明了,很多研究显示hIAPP引起的β细胞膜破裂是hIAPP产生细胞毒性的主要原因。不仅hIAPP具有引起膜损伤,从而导致细胞淀粉样改变的细胞毒性机制,一些与错误折叠疾病(如阿尔兹海默病、帕金森综合征、朊病毒病等)相关的多肽和蛋白质也具有相同的细胞毒性机理。结合最新研究进展,讨论了hIAPP与膜的相互作用,阐述了hIAPP诱导β细胞凋亡的几种可能机制。     

2型糖尿病——一种构象疾病

构象疾病是由非天然构象蛋白异常聚集引起的具有一系列症状的疾病。既往研究认为糖尿病是一组以血葡萄糖水平增高为特征的代谢性疾病群,而最近Hayden MR等在2005年JOP．J Pancreas杂志上发表综述,根据部分β细胞蛋白、胰岛淀粉样多肽在经过三级结构的改变后,发生自联（即错误折叠）和组织沉淀（即异常聚集）等现象（self—association and tissue deposition）,他们提出了2型糖尿病亦属于一种构象疾病的观点。2型糖尿病与多种代谢紊乱有关,并可以导致过量活性氧的产生和氧化应激反应。     

β淀粉样蛋白对线粒体作用的研究进展

阿尔茨海默病的一个重要病理特征是胞外β淀粉样蛋白沉积形成的老年斑,β淀粉样蛋白可以引起氧化损伤以及神经细胞凋亡等。随着研究的深入,在细胞内也发现了β淀粉样蛋白的存在。线粒体是细胞内ATP和活性氧自由基产生的主要部位,在氧化损伤和细胞凋亡过程中起到重要的作用。近年的研究表明,β淀粉样蛋白对线粒体有很重要的作用。该文主要针对这一领域的进展,介绍了阿尔茨海默病中β淀粉样蛋白对线粒体多个生理过程的作用以及这些作用在阿尔茨海默病中产生的影响。     

纳米硒抗胰岛β细胞凋亡作用与机制的研究

旨在研究纳米硒(Se NPs)抗二型糖尿病胰岛β细胞凋亡的作用及机制。通过电子显微镜,纳米粒度仪等技术方法对Se NPs的表征进行鉴定,并运用相关分子生物学技术验证其对于胰岛β细胞凋亡模型的保护作用及机制。结果表明,壳聚糖(CS)修饰后的Se NPs表面电荷增加到24.8 mv,其保存期延长到100 d以上。另外,细胞学实验表明浓度在2μmol/L以下Se NPs不具有毒性,38.1 nmol/L Se NPs可通过激活硫氧还蛋白还原酶(Trx R)活性,从而减轻胞内氧化应激(ROS)水平,最终使胰岛β凋亡模型细胞存活率提高了31.75%。动物学实验也表明Se NPs能抗胰岛β细胞凋亡。因此,Se NPs可有效减轻二型糖尿病中胰岛β细胞凋亡,对于二型糖尿病具有潜在的治疗价值。     

雌激素对淀粉样β蛋白代谢的调节和毒性缓解

以淀粉样β蛋白为主的老年斑胞外沉积和神经元内神经原纤维缠结,是阿尔采末病(AD)特征性的病理学改变。近来,人们逐渐认可淀粉样蛋白假说,即认为淀粉样蛋白沉积是AD最初起因。研究人员正在寻找针对淀粉样β蛋白沉积的药物,雌激素是其中之一。初步的工作证明,雌激素能够调节淀粉样β蛋白前体代谢,减少淀粉样β蛋白生成,也能够减轻淀粉样B蛋白引起的免疫炎症反应、氧应激对细胞造成的损伤,和对抗细胞凋亡。     

NLRP3炎症小体与2型糖尿病的研究进展

NLRP3炎症小体作为固有免疫系统的重要组成部分,在2型糖尿病的发病过程中起着重要作用,而白细胞介素1β(IL-1β)是介导其发挥作用的关键因子。核糖体蛋白质合成、嘌呤受体P2X7、活性氧敏感的硫氧还原蛋白相互作用蛋白(TXNIP)与NLRP3炎症小体激活密切相关。肥胖时胰岛素作用的靶组织中NLRP3、IL-1β表达均增高,其介导的炎症反应在胰岛β细胞功能障碍、凋亡以及胰岛素抵抗发生过程中起关键作用。NLRP3炎症小体被多种途径激活,从而上调胰岛和脂肪组织中IL-1β的表达,促进胰岛β细胞凋亡及胰岛素抵抗的发生发展,导致糖尿病的产生。     

促胰岛素分泌活性肽对胰岛β细胞作用的分子机制

促胰岛素分泌活性肽具有促进胰岛素分泌、增加胰岛β细胞数量和抑制胰岛β细胞凋亡等作用。研究这些活性肽功效的细胞信号转导及其分子机制,将为进一步研究及开发高效、低毒副作用的2型糖尿病治疗药物奠定理论基础。该文综述了部分促胰岛素分泌活性肽对胰岛β细胞作用的细胞分子机制研究进展,为进一步进行相关研究提供参考。     

胰高血糖素样肽-1与Ⅰ型糖尿病治疗

近年来研究表明,胰高血糖素样肽-1（GLP-1）对胰岛β细胞的分化、增殖均起重要作用,包括抑制β细胞凋亡、刺激β细胞增生、诱导干细胞分化为胰腺内分泌细胞,从而使被破坏的胰岛细胞恢复分泌胰岛素的功能,这些作用为其治疗Ⅰ型糖尿病提供了证据,使其成为Ⅰ型糖尿病治疗领域研究的热点。     

淀粉样前体蛋白（APP）相互作用及其生物学意义

淀粉样前体蛋白（amyloid precursor protein,APP）是一类与阿尔茨海默氏病（Alzheimer＇s disease,AD）的发生、发展密切相关的I型跨膜蛋白,具有膜受体样结构,但迄今人们对APP真正的生理功能仍知之甚少。近年来研究发现,APP分子间可以进行二聚化,并且反式的二聚化作用有促进细胞黏附的功能。而APP的降解产物β-淀粉样蛋白（β—amyloid protein,Aβ）反过来又可以加速APP的聚集,经过一系列反应,最终引发细胞凋亡。本文综述这一领域的研究进展,特别是APP的相互作用,以及这些相互作用对细胞状态和行为的影响。     

胰岛β细胞炎症与2型糖尿病

2型糖尿病属于代谢性疾病,它的发生发展受环境因素和多种基因的共同调控.近年来研究认为2型糖尿病属于代谢性炎症,可能是由细胞因子介导的一种慢性炎症反应性疾病.胰岛作为胰岛素的分泌器官,它的异常是2型糖尿病发病进程中的一个重要病理基础.长期的高糖,高脂及巨噬细胞浸润等因素都会刺激细胞因子的大量生成,造成胰岛β细胞的炎症反应,对胰岛β细胞分泌胰岛素的功能和细胞活力产生不同程度的损伤,导致其功能障碍和凋亡,进而促使2型糖尿病的发生发展.本文根据国内外近几年的研究进展,进一步了探讨胰岛β细胞炎症与2型糖尿病的关系.这种代谢性炎症的研究,进一步阐明了炎症的发生,引起胰岛素抵抗、功能障碍的具体机制,革新了对2型糖尿病发病机理的认识,并为2型糖尿病的防治提供了新的方向.     

Amyloid inhibitors enhance survival of cultured human islets

Background

Amyloid fibrils created by misfolding and aggregation of proteins are a major pathological feature in a variety of degenerative diseases. Therapeutic approaches including amyloid vaccines and anti-aggregation compounds in models of amyloidosis point to an important role for amyloid in disease pathogenesis. Amyloid deposits derived from the β-cell peptide islet amyloid polypeptide (IAPP or amylin) are a characteristic of type 2 diabetes and may contribute to loss of β-cells in this disease.

Methods

We developed a cellular model of rapid amyloid deposition using cultured human islets and observed a correlation between fibril accumulation and β-cell death. A series of overlapping peptides derived from IAPP was generated.

Results

A potent inhibitor (ANFLVH) of human IAPP aggregation was identified. This inhibitory peptide prevented IAPP fibril formation in vitro and in human islet cultures leading to a striking increase in islet cell viability.

Conclusions

These findings indicate an important contribution of IAPP aggregation to β-cell death in situ and point to therapeutic applications for inhibitors of IAPP aggregation in enhancing β-cell survival.

General significance

Anti-amyloid compounds could potentially reduce the loss of β-cell mass in type 2 diabetes and maintain healthy human islet cultures for β-cell replacement therapies.     

Serum albumin impedes the amyloid aggregation and hemolysis of human islet amyloid polypeptide and alpha synuclein

Protein aggregation is a ubiquitous phenomenon underpinning the origins of a range of human diseases. The amyloid aggregation of human islet amyloid polypeptide (IAPP) and alpha synuclein (αS), specifically, is a hallmark of type 2 diabetes (T2D) and Parkinson's disease impacting millions of people worldwide. Although IAPP and αS are strongly associated with pancreatic β-cell islets and presynaptic terminals, they have also been found in blood circulation and the gut. While extensive biophysical and biochemical studies have been focused on IAPP and αS interacting with cell membranes or model lipid vesicles, the roles of plasma proteins on the amyloidosis and membrane association of these two major types of amyloid proteins have rarely been examined. Using a thioflavin T kinetic assay, transmission electron microscopy and a hemolysis assay here we show that human serum albumin, the most abundant protein in the plasma, impeded the fibrillization and mitigated membrane damage of both IAPP and αS. This study offers a new insight on the native inhibition of amyloidosis.     

The effect of Aβ on IAPP aggregation in the presence of an isolated β-cell membrane

Fibrillar aggregates of the islet amyloid polypeptide (IAPP) and amyloid-β (Aβ) are known to deposit at pancreatic β-cells and neuronal cells and are associated with the cell degenerative diseases type-2 diabetes mellitus (T2DM) and Alzheimer's disease (AD), respectively. Since IAPP is secreted by β-cells and a membrane-damaging effect of IAPP has been discussed as a reason for β-cell dysfunction and the development of T2DM, studies of the interaction of IAPP with the β-cell membrane are of high relevance for gaining a molecular-level understanding of the underlying mechanism. Recently, it has also been shown that patients suffering from T2DM exhibit an increased risk to develop AD and vice versa, and a molecular link between AD and T2DM has been suggested. In this study, membrane lipids from the rat insulinoma-derived INS-1E β-cell line were isolated, and their interaction with the amyloidogenic peptides IAPP and Aβ and a mixture of both peptides has been studied. To yield insight into the associated peptides' conformational changes and their effect on the membrane integrity during aggregation, we have carried out attenuated total reflection Fourier transform infrared spectroscopy, fluorescence microscopy, and atomic force microscopy experiments. The IAPP-Aβ heterocomplexes formed were shown to adsorb, aggregate, and permeabilize the isolated β-cell membrane significantly slower than pure IAPP, however, at a rate that is much faster than that of pure Aβ. In addition, it could be shown that isolated β-cell membranes cause similar effects on the kinetics of IAPP and IAPP-Aβ fibril formation as anionic heterogeneous model membranes.     

IAPP in type II diabetes: Basic research on structure,molecular interactions,and disease mechanisms suggests potential intervention strategies

Islet amyloid polypeptide (a.k.a. IAPP, amylin) is a 37 amino acid hormone that has long been associated with the progression of type II diabetes mellitus (TIIDM) disease. The endocrine peptide hormone aggregatively misfolds to form amyloid deposits in and around the pancreatic islet β-cells that synthesize both insulin and IAPP, leading to a decrease in β-cell mass in patients with the disease. Extracellular IAPP amyloids induce β-cell death through the formation of reactive oxygen species, mitochondrial dysfunction, chromatin condensation, and apoptotic mechanisms, although the precise roles of IAPP in TIIDM are yet to be established. Here we review aspects of the normal physiological function of IAPP in glucose regulation together with insulin, and its misfolding which contributes to TIIDM, and may also play roles in other pathologies such as Alzheimer's and heart disease. We summarize information on expression of the IAPP gene, the regulation of the hormone by post-translational modifications, the structural properties of the peptide in various states, the kinetics of misfolding to amyloid fibrils, and the interactions of the peptide with insulin, membranes, glycosaminoglycans, and nanoparticles. Finally, we describe how basic research is starting to have a positive impact on the development of approaches to circumvent IAPP amyloidogenesis. These include therapeutic strategies aimed at stabilizing non-amyloidogenic states, inhibition of amyloid growth or disruption of amyloid fibrils, antibodies directed towards amyloid structures, and inhibition of interactions with cofactors that facilitate aggregation or stabilize amyloids.     

Neprilysin Impedes Islet Amyloid Formation by Inhibition of Fibril Formation Rather Than Peptide Degradation

Deposition of islet amyloid polypeptide (IAPP) as islet amyloid in type 2 diabetes contributes to loss of β-cell function and mass, yet the mechanism for its occurrence is unclear. Neprilysin is a metallopeptidase known to degrade amyloid in Alzheimer disease. We previously demonstrated neprilysin to be present in pancreatic islets and now sought to determine whether it plays a role in degrading islet amyloid. We used an in vitro model where cultured human IAPP (hIAPP) transgenic mouse islets develop amyloid and thereby have increased β-cell apoptosis. Islet neprilysin activity was inhibited or up-regulated using a specific inhibitor or adenovirus encoding neprilysin, respectively. Following neprilysin inhibition, islet amyloid deposition and β-cell apoptosis increased by 54 and 75%, respectively, whereas when neprilysin was up-regulated islet amyloid deposition and β-cell apoptosis both decreased by 79%. To determine if neprilysin modulated amyloid deposition by cleaving hIAPP, analysis of hIAPP incubated with neprilysin was performed by mass spectrometry, which failed to demonstrate neprilysin-induced cleavage. Rather, neprilysin may act by reducing hIAPP fibrillogenesis, which we showed to be the case by fluorescence-based thioflavin T binding studies and electron microscopy. In summary, neprilysin decreases islet amyloid deposition by inhibiting hIAPP fibril formation, rather than degrading hIAPP. These findings suggest that targeting the role of neprilysin in IAPP fibril assembly, in addition to IAPP cleavage by other peptidases, may provide a novel approach to reduce and/or prevent islet amyloid deposition in type 2 diabetes.     

Localized amyloids important in diseases outside the brain--lessons from the islets of Langerhans and the thoracic aorta

It has long been understood that amyloids can be lethal in systemic diseases. More recently, it has been accepted that local cerebral aggregation of the small peptide Aβ is involved in the pathogenesis of Alzheimer's disease. Protein aggregation, with the generation of small amyloid deposits in specific organs, also occurs outside the central nervous system and often is associated with increased cell death. In this review, we discuss two lesser known but common localized amyloid fibril-forming proteins: the polypeptide hormone islet amyloid polypeptide (IAPP) and the lactadherin-derived peptide medin. IAPP aggregates and induces the depletion of islet β-cells in type 2 diabetes and in islets transplanted into type 1 diabetic subjects. Initial amyloid deposition occurs intracellularly and parts of this amyloid consist of proIAPP. Medin derived from lactadherin expressed by smooth muscle cells aggregates into amyloid in certain arteries, particularly the thoracic aortic media layer, and may have a role in the generation of the potentially lethal conditions of thoracic aortic aneurysm and dissection.     

Membrane permeabilization by Islet Amyloid Polypeptide

Membrane permeabilization by Islet Amyloid Polypeptide (IAPP) is suggested to be the main mechanism for IAPP-induced cytotoxicity and death of insulin-producing β-cells in type 2 diabetes mellitus (T2DM). The insoluble fibrillar IAPP deposits (amyloid) present in the pancreas of most T2DM patients are not the primary suspects responsible for permeabilization of β-cell membranes. Instead, soluble IAPP oligomers are thought to be cytotoxic by forming membrane channels or by inducing bilayer disorder. In addition, the elongation of IAPP fibrils at the membrane, but not the fibrils themselves, could cause membrane disruption. Recent reports substantiate the formation of an α-helical, membrane-bound IAPP monomer as possible intermediate on the aggregation pathway. Here, the structures and membrane interactions of various IAPP species will be reviewed, and the proposed hypotheses for IAPP-induced membrane permeabilization and cytotoxicity will be discussed.     

Human IAPP amyloidogenic properties and pancreatic β-cell death

A hallmark of type 2 diabetes mellitus (T2DM) is the presence of extracellular amyloid deposits in the islets of Langerhans. These deposits are formed by the human islet amyloid polypeptide, hIAPP (or amylin), which is a hormone costored and cosecreted with insulin. Under normal conditions, the hormone remains in solution but, in the pancreas of T2DM individuals, it undergoes misfolding giving rise to oligomers and cross-β amyloid fibrils. Accumulating evidence suggests that the amyloid deposits that accompany type 2 diabetes mellitus are not just a trivial epiphenomenon derived from the disease progression. Rather, hIAPP aggregation induces processes that impair the functionality and viability of β-cells and may lead to apoptosis. The present review article aims to summarize a few aspects of the current knowledge of this amyloidogenic polypeptide. In the first place, the physicochemical properties which condition its propensity to misfold and form aggregates. Secondly, how these properties confer hIAPP the capacity to interfere with some signaling of the pancreatic β-cell, interact with membranes, form channels or affect natural ion channels, including calcium channels. Finally, how misfolded hIAPP cytotoxicity results in apoptosis. A number of pathophysiological changes of the T2DM islet can be related to the amyloidogenic properties of hIAPP. However, in a certain way, the in vivo aggregation of the polypeptide also reflects a failure of chaperones and, in general, of cellular proteostasis, supporting the view that T2DM may also be considered as a conformational disorder.     

Concentration-dependent transitions govern the subcellular localization of islet amyloid polypeptide

Islet amyloid polypeptide (IAPP) is a peptide hormone cosecreted with insulin by pancreatic β-cells. In type II diabetes, IAPP aggregates in a process that is associated with β-cell dysfunction and loss of β-cell mass. The relationship between IAPP's conformational landscape and its capacity to mediate cell death remains poorly understood. We have addressed these unknowns by comparing the cytotoxic effects of sequence variants with differing α-helical and amyloid propensities. IAPP was previously shown to oligomerize cooperatively on binding to lipid bilayers. Here, comparable transitions are evident in cell culture and are associated with a change in subcellular localization to the mitochondria under toxic conditions. Notably, we find that this toxic gain of function maps to IAPP's capacity to adopt aggregated membrane-bound α-helical, and not β-sheet, states. Our findings suggest that upon α-helical mediated oligomerization, IAPP acquires cell-penetrating peptide (CPP) properties, facilitating access to the mitochondrial compartment, resulting in its dysfunction.     

Amyloidogenicity of naturally occurring full‐length animal IAPP variants

The aggregation of the 37‐amino acid polypeptide human islet amyloid polypeptide (hIAPP), as either insoluble amyloid or as small oligomers, appears to play a direct role in the death of human pancreatic β‐islet cells in type 2 diabetes. hIAPP is considered to be one of the most amyloidogenic proteins known. The quick aggregation of hIAPP leads to the formation of toxic species, such as oligomers and fibers, that damage mammalian cells (both human and rat pancreatic cells). Whether this toxicity is necessary for the progression of type 2 diabetes or merely a side effect of the disease remains unclear. If hIAPP aggregation into toxic amyloid is on‐path for developing type 2 diabetes in humans, islet amyloid polypeptide (IAPP) aggregation would likely need to play a similar role within other organisms known to develop the disease. In this work, we compared the aggregation potential and cellular toxicity of full‐length IAPP from several diabetic and nondiabetic organisms whose aggregation propensities had not yet been determined for full‐length IAPP.