Islet Amyloid: A Critical Entity in the Pathogenesis of Type 2 Diabetes

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Amyloid fiber formation is correlated with pathology in many diseases, including Alzheimer's, Parkinson's, and type II diabetes. Although β-sheet-rich fibrillar protein deposits define this class of disorder, increasing evidence points toward small oligomeric species as being responsible for cell dysfunction and death. The molecular mechanism by which this occurs is unknown, but likely involves the interaction of these species with biological membranes, with a subsequent loss of integrity. Here, we investigate islet amyloid polypeptide, which is implicated in the loss of insulin-secreting cells in type II diabetics. We report the discovery of oligomeric species that arise through stochastic nucleation on membranes and result in disruption of the lipid bilayer. These species are stable, result in all-or-none leakage, and represent a definable protein/lipid phase that equilibrates over time. We characterize the reaction pathway of assembly through the use of an experimental design that includes both ensemble and single-particle evaluations. Complexity in the reaction pathway could not be satisfied using a two-state description of membrane-bound monomer and oligomeric species. We therefore put forward a threestate kinetic framework, one of which we conjecture represents a non-amyloid, non-β-sheet intermediate previously shown to be a candidate therapeutic target.amylin | membrane pore | cytotoxicity | disordered protein A lzheimer's, Parkinson's, type II diabetes, and other epidemiologically important diseases are characterized, in part, by the deposition of proteinaceous plaques termed amyloid (1, 2). For each disease, a specific protein is involved in amyloid assembly via a process that is cytotoxic, ultimately leading to degeneration. Although a defining characteristic, it is now widely thought that these deposits are not the origin of cytotoxicity. Instead, it has been suggested that cell dysfunction and death are mediated by small oligomeric species that acquire the capacity to disrupt cellular membrane integrity (3, 4). The energetic and structural basis by which these states mediate toxicity, however, is unclear.Islet amyloid polypeptide (IAPP or amylin) is a 37-residue peptide hormone cosecreted with insulin by pancreatic β-cells. Unmodified, wild-type IAPP self-assembles to form amyloid in type II diabetic patients in a process that is associated with β-cell dysfunction (2, 5). Indeed, the capacity of species-based sequence variants to form amyloid is correlated with the observation of metabolic disease (6). Thus, whereas primates and cats readily form amyloid and acquire diabetes, rats and mice do not spontaneously develop diabetes, and rodent IAPP does not form amyloid or other β-sheet aggregates. Diabetic symptoms can be induced in model rodents either by using toxins or by using rodents transgenic for human IAPP (7).Previous in vitro studies have shown that the binding of human and rat IAPP to lipid bilayers is cooperative (8), an observation accounted for by the presence of oligomeric species. The pop...

Section: Discussionmentioning

confidence: 59%

mentioning

confidence: 99%

Islet amyloid polypeptide demonstrates a persistent capacity to disrupt membrane integrity

Last

Rhoades

Miranker

2011

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145

“…Proteotoxicity can be due to different etiologies and a prominent cytoplasmic feature in a number of proteotoxic diseases is the formation of large inclusion bodies visible by light microscopy. These proteinaceous aggregates have been characterized in a number of diseases, including neurodegenerative disorders (49,50) and diabetes (51,52). More recently, we have focused on protein misfolding in cardiovascular disease and uncovered a causative role for proteotoxicity in desmin-related myopathies (8,9,16,53).…”

Section: Discussionmentioning

confidence: 99%

Tubulin hyperacetylation is adaptive in cardiac proteotoxicity by promoting autophagy

McLendon

Ferguson

Osińska

et al. 2014

106

Proteinopathy causes cardiac disease, remodeling, and heart failure but the pathological mechanisms remain obscure. Mutated αB-crystallin (CryAB R120G ), when expressed only in cardiomyocytes in transgenic (TG) mice, causes desmin-related cardiomyopathy, a protein conformational disorder. The disease is characterized by the accumulation of toxic misfolded protein species that present as perinuclear aggregates known as aggresomes. Previously, we have used the CryAB R120G model to determine the underlying processes that result in these pathologic accumulations and to explore potential therapeutic windows that might be used to decrease proteotoxicity. We noted that total ventricular protein is hypoacetylated while hyperacetylation of α-tubulin, a substrate of histone deacetylase 6 (HDAC6) occurs. HDAC6 has critical roles in protein trafficking and autophagy, but its function in the heart is obscure. Here, we test the hypothesis that tubulin acetylation is an adaptive process in cardiomyocytes. By modulating HDAC6 levels and/or activity genetically and pharmacologically, we determined the effects of tubulin acetylation on aggregate formation in CryAB R120G cardiomyocytes. Increasing HDAC6 accelerated aggregate formation, whereas siRNA-mediated knockdown or pharmacological inhibition ameliorated the process. HDAC inhibition in vivo induced tubulin hyperacetylation in CryAB R120G TG hearts, which prevented aggregate formation and significantly improved cardiac function. HDAC6 inhibition also increased autophagic flux in cardiomyocytes, and increased autophagy in the diseased heart correlated with increased tubulin acetylation, suggesting that autophagy induction might underlie the observed cardioprotection. Taken together, our data suggest a mechanistic link between tubulin hyperacetylation and autophagy induction and points to HDAC6 as a viable therapeutic target in cardiovascular disease.P roteotoxicity is an important yet understudied mechanism in cardiac pathobiology (1), as maintaining tight control of protein homeostasis is critical for proper cell function. This is especially important in the unique context of the heart, as it is under constant mechanical and oxidative stress, and cardiomyocytes appear to be largely postmitotic soon after birth and are unable to readily regenerate (2, 3). Cellular stress events, including normal physiologic stimuli, can lead to altered cardiomyocyte function if the pathways of protein quality control (PQC) are compromised. Pathological cardiac stress, including pressure overload-induced hypertrophy and ischemia-reperfusion (I/R) injury, can alter protein degradation pathways (4-6). Our laboratory has developed a model of cardiac proteotoxicity based upon transgenically mediated cardiomyocyte expression of a mutated αB-crystallin (CryAB R120G ), which causes desminrelated cardiomyopathy in humans (7-9).CryAB is a molecular chaperone for desmin, an intermediate filament protein expressed in myocytes. Desmin is a crucial cardiomyocyte protein with vital signaling and structural ro...

“…IAPP, also known as amylin, is a hormone cosecreted with insulin by the ␤-cells of the pancreas. In type II diabetes, IAPP forms amyloid deposits that are correlated with ␤-cell death (16). Recent transgenic models, e.g., the HIP rat, strongly support a role for IAPP in diabetic pathology (17).…”

mentioning

confidence: 99%

Fiber-dependent amyloid formation as catalysis of an existing reaction pathway

Ruschak¹,

Miranker

2007

204

251

A central component of a number of degenerative diseases is the deposition of protein as amyloid fibers. Self-assembly of amyloid occurs by a nucleation-dependent mechanism that gives rise to a characteristic sigmoidal reaction profile. The abruptness of this transition is a variable characteristic of different proteins with implications to both chemical mechanism and the aggressiveness of disease. Because nucleation is defined as the rate-limiting step, we have sought to determine the nature of this step for a model system derived from islet amyloid polypeptide. We show that nucleation occurs by two pathways: a fiber-independent (primary) pathway and a fiber-dependent (secondary) pathway. We first show that the balance between primary and secondary contributions can be manipulated by an external interface. Specifically, in the presence of this interface, the primary mechanism dominates, whereas in its absence, the secondary mechanism dominates. Intriguingly, we determine that both the reaction order and the enthalpy of activation of the two nucleation processes are identical. We interrogate this coincidence by global analysis using a simplified model generally applicable to protein polymerization. A physically reasonable set of parameters can be found to satisfy the coincidence. We conclude that primary and secondary nucleation need not represent different processes for amyloid formation. Rather, they are alternative manifestations of the same, surfacecatalyzed nucleation event.amylin ͉ fibers ͉ islet amyloid polypeptide ͉ nucleation T he noncovalent, fibrous self-assembly of proteins, including hemoglobin S, actin, microtubules, and amyloid fibers, occurs by a nucleation-dependent mechanism (1-5). If polymer product formation is monitored as a function of time in a reaction where all of the protein is initially monodisperse, there is a lag phase in which little product is formed, followed by a period of rapid growth (2). Such observations are consistent with a rate-limiting step in which change occurs to a high-energy intermediate known as the nucleus. In many systems such as actin, collagen, and microtubules, the nucleus is modeled as an oligomeric species that is in a highly unfavorable equilibrium with monomer (1, 3-6). The nucleus may also be a high-energy conformation of monomer, such as that reported for polyglutamine (7). Regardless, the rate-limiting step involves a nucleus because it is defined as the species with the highest free energy and therefore lowest population (1, 4).This model of nucleation alone cannot account for the high apparent cooperativity of conversion observed in many systems. Historically, hemoglobin S was the first biological polymerization reaction to demonstrate a transition time that is much shorter than its preceding lag phase (8). This same phenomenon is commonly reported for amyloid systems including islet amyloid polypeptide (IAPP) from type II diabetes (9), A␤ from Alzheimer's (10), PrP from the mammalian prion (11), and Sup35 from the yeast prion (12, 13), as well as mode...