Historical background

RAGE binds preamyloid IAPP intermediates and mediates pancreatic β cell proteotoxicity

Islet amyloidosis is characterized by the aberrant accumulation of islet amyloid polypeptide (IAPP) in pancreatic islets, resulting in β cell toxicity, which exacerbates type 2 diabetes and islet transplant failure. It is not fully clear how IAPP induces cellular stress or how IAPP-induced toxicity can be prevented or treated. We recently defined the properties of toxic IAPP species. Here, we have identified a receptor-mediated mechanism of islet amyloidosis-induced proteotoxicity. In human diabetic pancreas and in cellular and mouse models of islet amyloidosis, increased expression of the receptor for advanced glycation endproducts (RAGE) correlated with human IAPP-induced (h-IAPP-induced) β cell and islet inflammation, toxicity, and apoptosis. RAGE selectively bound toxic intermediates, but not nontoxic forms of h-IAPP, including amyloid fibrils. The isolated extracellular ligand-binding domains of soluble RAGE (sRAGE) blocked both h-IAPP toxicity and amyloid formation. Inhibition of the interaction between h-IAPP and RAGE by sRAGE, RAGE-blocking antibodies, or genetic RAGE deletion protected pancreatic islets, β cells, and smooth muscle cells from h-IAPP-induced inflammation and metabolic dysfunction. sRAGE-treated h-IAPP Tg mice were protected from amyloid deposition, loss of β cell area, β cell inflammation, stress, apoptosis, and glucose intolerance. These findings establish RAGE as a mediator of IAPP-induced toxicity and suggest that targeting the IAPP/RAGE axis is a potential strategy to mitigate this source of β cell dysfunction in metabolic disease.

Amylin and the regulation of appetite and adiposity: recent advances in receptor signaling, neurobiology and pharmacology

PURPOSE OF REVIEW

This review focuses on recent advances in receptor signaling, neurobiology, and pharmacological interactions of amylin with nutritive status, as well as other metabolism-related regulatory signals.

RECENT FINDINGS

Manipulation of components of the amylin receptor complex revealed important roles for the accessory proteins of amylin receptors in energy balance. In-vitro findings point to potential novel sites of action and postreceptor signaling pathways activated by amylin. Neurobiological studies elucidated how amylin activation of hindbrain neural circuitry modulates hypothalamic signaling and responsiveness to leptin. The notion of 'amylin resistance' was addressed in several models (drug or diet-induced hyper-amylinemia). Finally, progress in the design and delivery of amylinomimetics is briefly discussed.

SUMMARY

Collectively, these mechanistic studies deepen our understanding of the role of endogenous amylin in the regulation of appetite and adiposity, and hopefully will help guide research efforts towards the development of more effective amylin-based therapies for metabolic diseases.

GLP-1R and amylin agonism in metabolic disease: complementary mechanisms and future opportunities

The discoveries of the incretin hormone glucagon-like peptide-1 (GLP-1) and the β-cell hormone amylin have translated into hormone-based therapies for diabetes. Both classes of molecules also exhibit weight-lowering effects and have been investigated for their anti-obesity potential. In the present review, we explore the mechanisms underlying the physiological and pharmacological actions of GLP-1 and amylin agonism. Despite their similarities (e.g. both molecular classes slow gastric emptying, decrease glucagon and inhibit food intake), there are important distinctions between the central and/or peripheral pathways that mediate their effects on glycaemia and energy balance. We suggest that understanding the similarities and differences between these molecules holds important implications for the development of novel, combination-based therapies, which are increasingly the norm for diabetes/metabolic disease. Finally, the future of GLP-1- and amylin agonist-based therapeutics is discussed.

Effects of amylin on eating and adiposity

Amylin's best investigated function is to reduce eating via a meal size effect by promoting meal-ending satiation. This effect seems to depend on an activation of specific area postrema neurons. Brain areas that convey the neural signal to the forebrain include the nucleus of the solitary tract and the lateral parabrachial nucleus. Acute application of amylin modulates the activity of hypothalamic areas involved in the control of eating, namely, the lateral hypothalamic area and possibly the ventromedial hypothalamic nucleus. Amylin also interacts with other satiating signals, such as cholecystokinin, presumably in the brainstem. Interestingly, amylin also exhibits characteristics of adiposity signals; plasma levels of amylin are higher in obese individuals, chronic infusion of amylin into the brain reduces body weight gain and adiposity, and infusion of amylin antagonists increases adiposity. Furthermore, amylin maintains energy expenditure at higher levels than would be expected considering its body weight-lowering effect. However, much less is known (e.g., site of action, signaling pathways, differential activation of brain sites, and, most importantly, physiological relevance) with respect to its role as adiposity signal and regulator of energy expenditure than about its satiating action. Notwithstanding, and perhaps because amylin resistance does not seem to be a general and prohibitive concomitant of obesity, animal data and recent clinical data in humans indicate that amylin is a very promising candidate for the treatment of obesity. Amylin seems to be particularly effective when combined with other hormones such as leptin.

Gut microbiota in health and disease

Gut microbiota is an assortment of microorganisms inhabiting the length and width of the mammalian gastrointestinal tract. The composition of this microbial community is host specific, evolving throughout an individual's lifetime and susceptible to both exogenous and endogenous modifications. Recent renewed interest in the structure and function of this "organ" has illuminated its central position in health and disease. The microbiota is intimately involved in numerous aspects of normal host physiology, from nutritional status to behavior and stress response. Additionally, they can be a central or a contributing cause of many diseases, affecting both near and far organ systems. The overall balance in the composition of the gut microbial community, as well as the presence or absence of key species capable of effecting specific responses, is important in ensuring homeostasis or lack thereof at the intestinal mucosa and beyond. The mechanisms through which microbiota exerts its beneficial or detrimental influences remain largely undefined, but include elaboration of signaling molecules and recognition of bacterial epitopes by both intestinal epithelial and mucosal immune cells. The advances in modeling and analysis of gut microbiota will further our knowledge of their role in health and disease, allowing customization of existing and future therapeutic and prophylactic modalities.

Interaction of membrane-bound islet amyloid polypeptide with soluble and crystalline insulin

Islet amyloid polypeptide (IAPP, also known as amylin) is the major protein component of pancreatic amyloid fibers in type II diabetes and is normally cosecreted with insulin from the beta-cells of the pancreas. IAPP forms amyloid fibrils rapidly at concentrations well below those found in vivo, yet progression of type II diabetes occurs over many years. Insulin, a known inhibitor of IAPP fibrillogenesis, exists as a dense crystalline or near-crystalline core in the secretory vesicle, while IAPP localizes to the region between the crystal and the secretory vesicle membrane. In vitro, IAPP fibrillogenesis is both accelerated by lipid membranes and inhibited by monomeric insulin. In this work, we investigate insulin-IAPP-lipid interactions in vitro under conditions chosen to approximate native secretory vesicle physiology and the amyloid disease state. The effect of insulin on IAPP fibrillogenesis is investigated using fluorescence spectrometry. Additionally, interactions of IAPP and lipids with crystalline insulin are studied using fluorescence microscopy. We find that, while soluble states of insulin and IAPP do not interact significantly, large assemblies of either insulin (crystals) or IAPP (fibers) can lead to stable IAPP-insulin interactions. The results raise the possibility of multiple physiological interactions between these two beta-cell hormones.

The sensory circumventricular organs: brain targets for circulating signals controlling ingestive behavior

Sensory circumventricular organs (CVOs) are specialized areas of the brain that lack a normal blood-brain barrier, and therefore are in constant contact with signaling molecules circulating in the bloodstream. Neurons of the CVOs are well endowed with a wide spectrum of receptors for hormones and other signaling molecules, and they have strong connections to hypothalamic and brainstem nuclei. Therefore, lying at the blood-brain interface, the sensory CVOs are in a unique position of being able to detect and integrate humoral and neural information and relay the resulting signals to autonomic control centers of the hypothalamus and medulla. This review focuses primarily on the roles played by the sensory CVOs in fluid balance and energy metabolism.