The Role of Calcium, Lipid Membranes and Islet Amyloid Polypeptide in the Onset of Type 2 Diabetes: Innocent Bystanders or Partners in a Crime?

INTRODUCTION Type II diabetes mellitus (T2DM) is one of the most disabling age-related pathologies in developed countries (1). Its incidence is constantly growing: the number of people affected by T2DM worldwide grew from 28 million cases in 1985 to around 168 million in 2001, with an estimate of 350 million cases in 2030 (2). Despite the intense research efforts focusing on T2DM in the last 20 years, the cause of the underlying pathology is still unclear. Islet Amyloid Polypeptide (IAPP) is an amyloidogenic protein, member of the calcitonin family (3), which is known to be involved in the mechanisms lying at the root of T2DM pathogenesis (4, 5). Indeed, although the correlation between islet βcell death and amyloid fibril formation is still a matter of debate, recent studies, performed on transgenic mice, show that IAPP aggregation can mediate β-cell failure in T2DM (6). Indeed, despite human and rat variants of IAPP differing by only six amino acids, the two variants behave in a completely different way. In particular, rat IAPP (rIAPP) is unable to form amyloid aggregates under normal conditions (7) and rats do not develop T2DM in analogous conditions. Human IAPP (hIAPP) amyloid aggregates are detectable in pancreas’ extracellular space in 90% of patients (4), while only 10% of patients do not have detectable amyloid deposits. We propose that T2DM etiology might be clarified by studying the close relationships between three apparently independent issues: (i) amyloid toxicity; (ii) genetic factors; and (iii) environmental risk factors related to lifestyle and diet. MECHANISMS OF IAPP-INDUCED MEMBRANE DISRUPTION The mechanisms by which amyloid intermediates (Aβ peptide, hIAPP, and other amyloid-forming proteins) cause cytotoxicity and disease remains unsolved, although some recent studies (8) suggest that the water/membrane interface is critical for influencing amyloid aggregation. Of note, mature amyloid fibrils are presumed to be inert while small oligomeric intermediates are suggested to penetrate the cell membrane (9). It has been hypothesized that amyloid-forming peptides may cause membrane damage by: (a) changing the bilayer fluidity, (b) generating proteinstabilized pores (poration), (c) laying on one leaflet of the membrane (carpeting), or (d) removing lipid components from the bilayer by detergent-like mechanisms (10). It has also been shown that the formation of amyloid fibers occurs independently of membrane leakage and that membrane composition and the presence of ions may have a major influence on amyloidmediated membrane damage (11). In recent years, several studies have focused on the interpretation of molecular mechanisms underlying T2DM. In particular, researchers investigated the interaction between hIAPP and phospholipid membranes, a process that seems to be correlated to the loss of pancreatic β-cells (12–15). The most recent results suggest that hIAPP-induced membrane disruption could be described as a two independent step process. The first step is correlated with pore formation occurring after the insertion of monomeric or oligomeric species inside the membrane hydrophobic core. The insertion of a protein into a membrane interferes with lipid packing and, at high concentration, it might result in a local bending deformation. To minimize the energetically unfavorable deformation, proteins tend to cluster (16). The second step of membrane disruption is correlated with the growth of fibers onto the membrane surface, which causes complete membrane dissolution through a detergent-like mechanism (13, 17). Since the two steps are independent processes, they might be separately targeted by novel therapeutic approaches. The membrane lipid composition has been shown to be a critical point for the mechanism of membrane disruption mediated by hIAPP. The presence of phosphatidylethanolamine headgroup lipids, for example, can modulate the entity of the first and second step of membrane disruption. Also, the presence of cholesterol could finely regulate membrane dissolution by both inhibiting the affinity of hIAPP for the membrane as well as its insertion in the bilayer. Unfortunately, to date there are a lack of studies that have systematically investigated the role played by membrane composition (i.e., length of hydrophobic tails, percentage of cholesterol, presence of different lipid headgroups) on hIAPP/membrane interactions. Therefore, the membrane disruption mechanism can be regulated by several factors, such as metal ions balance, increased hIAPP secretion, the presence of free fatty acid, etc. which could arise from genetic predisposition, lifestyle, and diet.