Localisation of islet amyloid polypeptide and its carboxy terminal flanking peptide in islets of diabetic man and monkey

The β Cell in Diabetes: Integrating Biomarkers With Functional Measures

The pathogenesis of hyperglycemia observed in most forms of diabetes is intimately tied to the islet β cell. Impairments in propeptide processing and secretory function, along with the loss of these vital cells, is demonstrable not only in those in whom the diagnosis is established but typically also in individuals who are at increased risk of developing the disease. Biomarkers are used to inform on the state of a biological process, pathological condition, or response to an intervention and are increasingly being used for predicting, diagnosing, and prognosticating disease. They are also proving to be of use in the different forms of diabetes in both research and clinical settings. This review focuses on the β cell, addressing the potential utility of genetic markers, circulating molecules, immune cell phenotyping, and imaging approaches as biomarkers of cellular function and loss of this critical cell. Further, we consider how these biomarkers complement the more long-established, dynamic, and often complex measurements of β-cell secretory function that themselves could be considered biomarkers.

Amyloid in human islets of Langerhans: immunologic evidence that islet amyloid polypeptide is modified in amyloidogenesis

Amyloid derived from the beta-cell product islet amyloid polypeptide (IAPP) has been implicated for a beta-cell lesion in Type II diabetes mellitus. The pathogenesis of islet amyloid is poorly understood, and in addition to an amyloidogenic IAPP molecule and possibly increased concentration of IAPP, other unknown factors seem to be included. It was shown previously that polyclonal rabbit IAPP antisera label beta cells close to amyloid only weakly. Whether this lack of immunoreactivity depends on lack of IAPP or on hidden epitopes is in question. In the present study, we show that the IAPP immunoreactivity of these beta cells is possible to retrieve. On the other hand, the monoclonal IAPP antibody 4A5, which labels IAPP in beta cells, does not label IAPP in its native amyloid form. We show evidence that this lack of immunoreactivity is not dependent on conformational change of the IAPP molecules in the amyloidogenesis but is likely owing to glycation of IAPP in human islet amyloid deposits.

Pro islet amyloid polypeptide (ProIAPP) immunoreactivity in the islets of Langerhans

Islet amyloid is typically found in type 2 diabetes mellitus and is believed to participate in the beta cell deterioration. The islet amyloid fibril consists of the 37-amino-acid islet amyloid polypeptide (IAPP) but its pathogenesis is only partly understood. We developed several different rabbit antisera against the flanking peptides of the IAPP precursor (proIAPP) and the proIAPP processing sites in order to study the possible occurrence of unprocessed proIAPP or parts thereof in islet amyloid. We applied these antisera in an immunohistochemical study on, islet amyloid deposits present in a newly generated mouse strain that over-expresses human IAPP but is devoid of mouse IAPP. Male mice of this strain develop severe islet amyloidosis when given a high fat diet. Generally, the antisera showed no immunoreactivity with the amyloid. However, in scattered single beta cells, where amyloid could be seen intracellularly, immunoreactivity with one or more of the antisera co-localized with the amyloid. Although virtually all amyloid in human islets of Langerhans is found extracellularly, we propose that the initial amyloid formation occurs intracellularly, perhaps by not fully processed or folded (pro)IAPP. This amyloid, which may develop rapidly under certain circumstances, probably leads to cell death. If not degraded these amyloid spots may then act as nidus for further amyloid formation from fully processed IAPP, secreted from surrounding beta cells.

Processing of pro-islet amyloid polypeptide (proIAPP) by the prohormone convertase PC2

Islet amyloid polypeptide (IAPP), 'amylin', is the component peptide of islet amyloid formed in Type 2 diabetes. IAPP is expressed in islet beta-cells and is derived from a larger precursor, proIAPP, by proteolysis. An in vitro translation/translocation system was used to separately examine processing of human proIAPP by the beta-cell endopeptidases PC2, PC3 or furin. ProIAPP was converted to mature IAPP by PC2 but there was little conversion by furin or PC3. These data are consistent with processing of proIAPP in beta-cell secretory granules. Abnormal cellular proteolysis associated with type 2 diabetes could contribute to IAPP amyloidosis.

Islet amyloid in type 2 (non-insulin-dependent) diabetes

Amyloid deposits are found in pancreatic islets of 90% of type 2 (non-insulin-dependent) diabetic subjects at postmortem. Islet amyloid is formed from islet amyloid polypeptide (IAPP). IAPP is a 37 amino acid peptide which is a normal constituent of beta cells and is co-secreted with insulin in animals and in man. The causative factors for fibrillogenesis of IAPP are unclear, but could be related to the sequence of IAPP and abnormal production of the peptide. The lack of islet amyloid in rodent models of diabetes is due to proline substitutions in the amyloidogenic region of IAPP. Amyloid fibrils are deposited between beta cells and islet capillaries: fibrils in invaginations of the plasma membrane may interfere with membrane signalling and insulin release. Amyloid fibrils are formed within 2 days in culture in islets isolated from transgenic mice expressing the gene for human IAPP, but not in vivo. Overexpression and decreased clearance of human IAPP from islet spaces may be important factors. Progressive deposition of IAPP fibrils combined with the associated reduction in the insulin-secreting beta cells is likely to contribute to deterioration of islet function in the course of type 2 diabetes.

Islet amyloid polypeptide and its N-terminal and C-terminal flanking peptides' immunoreactivity in islet amyloid of diabetic patients

We determined immunohistochemically whether the islet amyloid polypeptide (IAPP)/amylin precursor is one component of islet amyloid, using polyclonal antibodies specific for human IAPP8-17 and amino (N)-terminal and carboxy (C)-terminal flanking peptides. To enhance immunostaining of the amyloid, we pretreated the pancreatic tissue sections with 100% formic acid. In three non-diabetic subjects, pancreatic islet cells were immunoreactive to anti-IAPP8-17 and anti-N-terminal and C-terminal flanking peptide antibodies and the reactivity was enhanced with formic acid pretreatment. In six type 2 diabetic subjects and a subject with type A insulin resistance, islet amyloid deposits were reactive to anti-IAPP8-17 antibody, but not to anti-N-terminal and C-terminal flanking peptide antibodies. Formic acid pretreatment markedly enhanced the reactivity to anti-IAPP8-17 antibody; however, it failed to show the reactivity to anti-N-terminal and C-terminal flanking peptide antibodies. Formic acid pretreatment of pancreatic tissue sections prepared for immunostaining is useful for visualization of buried epitopes of mature IAPP and its precursor molecules, either in islet amyloid deposits or in the islet cells. We conclude that the IAPP precursor and N-terminal and C-terminal flanking peptides are not constituents of human islet amyloid.

Islet amyloid polypeptide is expressed in endocrine cells of the gastric mucosa in the rat and mouse

BACKGROUND/AIMS

Islet amyloid polypeptide (IAPP) or amylin is a novel islet hormone candidate with a suggested role in the regulation of glucose homeostasis and the pathogenesis of non-insulin dependent diabetes mellitus. Occurrence of IAPP in the gastrointestinal tract of rats and humans has also been shown. The expression of IAPP in the stomach of the rat and mouse and the possible colocalization of IAPP and known gastric hormones were investigated in this study.

METHODS

In situ hybridization, immunofluorescence, and combined in situ hybridization and immunocytochemistry were used.

RESULTS

IAPP messenger RNA and IAPP-like immunoreactivity were shown in the same endocrine cells in the antrum and fundus of the rat and in the antrum of the mouse. IAPP was expressed in a major population of somatostatin-immunoreactive cells as well as in small populations of gastrin- and peptide YY-immunoreactive cells.

CONCLUSIONS

Our results establish the synthesis and storage of IAPP in gastric endocrine cells in the rat and mouse. The extensive colocalization of IAPP with somatostatin and to a lesser extent with gastrin and peptide YY suggests that IAPP may modulate endocrine activity in the gastric mucosa in a paracrine and/or autocrine mode.

Immunochemical investigation of insulinomas for islet amyloid polypeptide and insulin: evidence for differential synthesis and storage

An affinity purified antibody to fragment 14-29 of islet amyloid polypeptide (IAPP) has been prepared. This antibody, which does not cross-react with the related molecule calcitonin gene-related peptide, was used to investigate immunochemically the presence of IAPP in normal and neoplastic human pancreatic endocrine tissue. The pattern of IAPP staining in normal pancreas mirrors that of insulin, although slight differences were observed. In neoplastic tissue, IAPP was found in 16 out of 19 tumours that were positive for insulin, and was absent from one tumour negative for insulin. In some cases there were differences in the staining patterns of IAPP and insulin. These results suggest that the synthesis and secretion of IAPP and insulin are not inter-dependent and support the concept that IAPP has a discrete biological function. Islet amyloid polypeptide was found in six out of six insulinoma amyloid deposits, suggesting that the peptide is an invariable component of these deposits. Over-expression of IAPP, with aberrant processing and/or secretion, may be the causative factor for amyloid deposition in insulinomas and in the islets of type 2 (non-insulin dependent) diabetic patients. Investigation of patients with insulinomas and of insulin cells in culture and tissue sections may help to clarify the biological function of IAPP.