Evidence for Proteotoxicity in β Cells in Type 2 Diabetes

Abstract

The islet in type 2 diabetes mellitus (T2DM) is characterized by a deficit in ␤ cells and islet amyloid derived from islet amyloid polypeptide (IAPP), a protein coexpressed with insulin by ␤ cells. It is increasingly appreciated that the toxic form of amyloidogenic proteins is not amyloid but smaller membrane-permeant oligomers. Using an antibody specific for toxic oligomers and cryo-immunogold labeling in human IAPP transgenic mice, human insulinoma and pancreas from humans with and without T2DM, we sought to establish the abundance and sites of formation of IAPP toxic oligomers. We conclude that IAPP toxic oligomers are formed intracellularly within the secretory pathway in T2DM. Most striking , IAPP toxic oligomers appear to disrupt membranes of the secretory pathway , and then when adjacent to mitochondria , disrupt mitochondrial membranes. Toxic oligomer-induced secretory pathway and mitochondrial membrane disruption is a novel mechanism to account for cellular dysfunction and apoptosis in T2DM. Type 2 diabetes (T2DM) is characterized by a progressive deficit in ␤ cell function and mass with increased ␤ cell apoptosis.1,2 In common with several neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease, the loss of ␤ cells in T2DM is associated with accumulation of locally expressed misfolded proteins that share a propensity to form amyloid.3 Islet amyloid in T2DM is composed primarily of a 37-amino acid protein, islet amyloid polypeptide (IAPP).3 IAPP is co-expressed and secreted with insulin by pancreatic ␤ cells, and is thought to play a paracrine inhibitory role in regulation of insulin secretion. 4,5 The property of IAPP to form amyloid fibrils depends on IAPP 20-29 . This sequence is closely homologous in humans, nonhuman primates and cats, 6 all of which spontaneously develop T2DM characterized by a deficit in ␤ cell mass and islet amyloid. In contrast, rodent IAPP (mouse and rat) does not have the propensity to form amyloid fibrils due to proline substitutions in IAPP 20-29 and wild-type mice and rats do not spontaneously develop T2DM.There is accumulating evidence that the toxic form of amyloidogenic protein aggregates is distinct from amyloid fibrils. The latter tend to accumulate extracellularly where they are relatively inert.3,7 Abnormal non-fibrillar intracellular IAPP aggregates were noted in human insulinoma tissue adjacent to disrupted intracellular membranes.

Calcium-activated Calpain-2 Is a Mediator of Beta Cell Dysfunction and Apoptosis in Type 2 Diabetes

“…examined the relationship between the frequency of cleaved spectrin and the frequency of intracellular toxic IAPP oligomers detected with a specific anti-toxic oligomer antibody (A11) in pancreatic beta cells of nine human pancreas samples obtained at surgery as reported previously (15). We observed a positive correlation (r 2 ϭ 0.8, p Ͻ 0.01) between the frequency of beta cells with cleaved spectrin and the oligomer-specific immunoreactivity (Fig.…”

“…C, there was a positive correlation between frequency of beta cells with cleaved spectrin and frequency of beta cells positive for toxic oligomers detected by immunofluorescent labeling with oligomer-specific antibody A11 in surgically obtained human pancreas. These tissue samples were previously used to report the increased frequency of toxic IAPP oligomers in beta cells of individuals with T2DM compared with nondiabetic controls (15). Ⅺ, nondiabetic; f, T2DM.…”

“…It has been reported that exogenous hIAPP can activate transient receptor potential channels (43). Cytosolic Ca 2ϩ elevation induced by hIAPP overexpression may arise as a result of hIAPP induced leakage into the cytosol from the secretory pathway or mitochondria because membrane-permeant IAPP toxic oligomers are identified at each of these sites (14,15). Also, oligomeric ␤-amyloidinduced Ca 2ϩ release can be blocked by the inhibition of the ER calcium release channels (42).…”

“…One plausible explanation is that local membrane instability caused by toxic oligomers permits unregulated Ca 2ϩ surges from the ER or other intracellular Ca 2ϩ -enriched compartments. Toxic IAPP oligomers appear to form and act intracellularly (14) within the secretory pathway (15). Moreover, they escape the secretory pathway, apparently by disrupting intracellular membranes (15).…”

“…Toxic IAPP oligomers appear to form and act intracellularly (14) within the secretory pathway (15). Moreover, they escape the secretory pathway, apparently by disrupting intracellular membranes (15). Therefore, disturbance of the usually discrete cellular compartmentalization of Ca 2ϩ is a logical candidate to link membrane-permeant toxic oligomers and induction of cellular dysfunction and, under more extreme circumstances, cell death.…”