Ultrastructural evidence that apoptosis is the mechanism by which human amylin evokes death in RINm5F pancreatic islet beta-cells.

A view is emerging that human amylin (HA) kills pancreatic islet beta-cells by apoptosis. This study strengthens this view by documenting time-dependent morphological and ultrastructural changes in 10 microm HA-treated cultured RINm5F islet beta-cells. Membrane blebbing and microvilli loss were the earliest detectable apoptosis-related phenomena, already evident 1 h after HA exposure. Following 6-12 h of HA-treatment, chromatin margination became evident, consistent with detecting DNA laddering about the same time. Nuclear shrinkage, nuclear membrane convolution and prominent cytoplasmic vacuolization were clearly recognized at 22 h post-treatment. Together, these cellular changes constitute a strong case for HA-induced apoptosis, and further demonstrates that electron microscopy is a more sensitive tool for early apoptosis detection in cultured cells than classical biochemical assays like visualizing DNA laddering. The ultrastructural changes reported here contribute further evidence to be included in the ongoing dissection of molecular mechanisms underlying HA-induced apoptosis, as may occur in type-2 diabetes mellitus.

Expression, crystallization and preliminary characterization of methylmalonyl coenzyme A epimerase from Propionibacterium shermanii.

Methylmalonyl-CoA epimerase (MMCE) is an enzyme that interconverts the R and S epimers of methylmalonyl-CoA in the pathway that links propionyl-CoA with succinyl-CoA. This is used for both biosynthetic and degradative processes, including the breakdown of odd-numbered fatty acids and some amino acids. The enzyme has been expressed in Escherichia coli both as the native enzyme and as its selenomethionine (SeMet) derivative. Crystals of both forms have been obtained by vapour diffusion using monomethylether PEG 2000 as precipitant. The native MMCE crystals are orthorhombic, with unit-cell parameters a = 56.0, b = 114.0, c = 156.0 A, and the SeMet-MMCE crystals are monoclinic, with unit-cell parameters a = 43.6, b = 78.6, c = 89.4 A, beta = 92.0 degrees; both diffract to better than 2.8 A resolution.

Role of Ca2+ in apoptosis evoked by human amylin in pancreatic islet beta-cells.

The objective of these studies was to clarify the role of Ca(2+) in the mechanism of death evoked by human amylin (hA) in islet beta-cells. hA forms fibrils in vitro and islet amyloid in vivo. Here we show that pure synthetic hA aggregated in solution, formed fibrils and evoked death in cultured RINm5F islet beta-cells in a time-dependent (0-24 h) and concentration-dependent (0-20 microM) manner. Dying cells underwent shrinkage of the nucleus, with clumping and segregation of chromatin into masses that lay against the nuclear envelope, and internucleosomal DNA fragmentation. These cells therefore show many features of apoptosis, although aspects of the morphology might be characteristic of this particular cell type rather than of a general apoptotic nature. Aurintricarboxylic acid, an inhibitor of both Ca(2+)-dependent and Ca(2+)-independent nucleases, suppressed this DNA fragmentation and inhibited apoptosis at concentrations between 25 and 200 microM. Direct measurements of the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) in fura-2 acetoxymethyl ester (AM)-loaded beta-cells showed that neither hA nor its non-cytotoxic homologue, rat amylin were effective in raising [Ca(2+)](i). Modulators of Ca(2+) regulation were tested for their effects on hA-induced beta-cell apoptosis. Ca(2+) ionophore (A23187) and thapsigargin (an inhibitor of endoplasmic reticular Ca(2+)-ATPase activity) by themselves evoked apoptosis accompanied by increased [Ca(2+)](i). Neither the Ca(2+) channel blocker verapamil, the extracellular Ca(2+) chelator EGTA nor the cytosolic Ca(2+) buffer bis-(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid ('BAPTA')/AM protected beta-cells from hA-evoked apoptosis. Prolonged incubation of beta-cells with a lethal dose of hA altered neither the basal [Ca(2+)](i) nor the thapsigargin-induced release of Ca(2+) from intracellular stores. Furthermore, (45)CaCl(2) uptake by RINm5F cells did not differ in the presence or absence of hA. These results suggest that, whereas alterations in cytosolic Ca(2+) homoeostasis do have a significant role in certain forms of beta-cell death, they do not contribute to the pathway of apoptosis evoked by hA in islet beta-cells.

Induction of apoptosis by human amylin in RINm5F islet beta-cells is associated with enhanced expression of p53 and p21WAF1/CIP1.

Human amylin (10 microM) significantly inhibited RINm5F islet beta-cell proliferation and evoked apoptosis associated with typical degenerative ultrastructural changes and DNA fragmentation, whereas rat amylin did not. Time course analysis showed that human amylin elicited apoptosis in a passage-dependent manner. Expression of the apoptosis-related genes p53, bcl-2 and WAF1/CIP1 was examined using Northern blots. mRNAs corresponding to p53 and to p21WAF/CIP1 were remarkably increased following human amylin treatment, whereas no change in bcl-2 was detected. Our data suggest a role of p53 and p21 in human amylin-induced beta-cell apoptosis. Furthermore, cells with higher proliferative potential (lower passage) were found to be more susceptible to apoptosis and to induction of p53, suggesting that beta-cells with different proliferation rates respond differently to human amylin, and that human amylin may be more toxic to proliferating cells.

Polymorphic fibrillar assembly of human amylin.

Human amylin forms fibrillar amyloid between pancreatic islet cells in patients with non-insulin-dependent (type 2) diabetes mellitus. Fibrillar assemblies also form in vitro in aqueous solutions of synthetic human amylin. We now report on the structural polymorphism of these fibrils. The thinnest fibril, referred to as the protofibril, has an apparent width of 5 nm but is only rarely observed by itself. These protofibrils spontaneously assemble into higher order fibrillar structures with distinct morphologies. Prominent among these is an 8-nm fibril with a distinct 25-nm axial crossover repeat which is formed by left-handed coiling of two 5-nm protofibrils. Coiling of more than two 5-nm protofibrils results in cable-like structures of variable width depending on the number of protofibrils involved. Lateral (side-by-side) assembly of 5-nm protofibrils is also observed and produces ribbons which may contain two, three, four, or more protofibrils and occasionally large single-layered sheets. The mass-per-length (MPL) of the 5-nm protofibril is 10 kDa/nm. This has been established in two ways: first, the 8-nm fibril, which is formed by coiling two 5-nm protofibrils around each other, has an MPL of 20 kDa/nm. Second, higher order fibrils differ by increments of 10 kDa/nm. Hence, about 2.6 human amylin molecules (3904 Da) are packed in 1 nm of protofibril length. Similarities exist between amylin fibrils and those formed from other amyloid proteins, suggesting that the in vitro assembly of synthetic protein may serve as a useful model system in advancing our understanding of amyloid formation in disease.