Proteomics as a tool for discovery: proteins implicated in Alzheimer's disease are highly expressed in normal pancreatic islets

A proteomic analysis of islets was undertaken to determine the protein constituents of normal adult mouse islets. Unexpectedly, we identified several islet proteins that are associated with the pathogenesis of Alzheimer's disease. Some of these proteins had chaperone activity that is integral to proper protein folding. This group includes GRP78, valosin-containing protein, calreticulin, protein disulfide isomerase, DnaK, HSP70, HSP60, and TCP-1. Additionally, neuronal proteins key to coordinated neuronal guidance and survival were also identified in islets. This group includes proprotein convertase subtilisin, collapsin response mediator protein 2, ubiquinol-cytochrome c reductase core protein, L-3-hydroxyacyl-Coenzyme A dehydrogenase, glutamine synthetase, peroxiredoxin, and secretogogin. An important subset of the proteins identified here has not been reported previously in pancreatic islets. Abnormal activity of these proteins in brain may contribute to the pathogenesis of Alzheimer's disease, a neurodegenerative condition characterized by focal amyloid deposits with neurofibrillary tangles. The putative role of these proteins in Alzheimer's pathogenesis is intriguing given the possible clinical relationship and pathological similarity of Alzheimer's disease to type 2 diabetes. These findings have therefore led to the hypothesis that these proteins may also play a role in type 2 diabetes.     

A tissue-specific atlas of mouse protein phosphorylation and expression

Although most tissues in an organism are genetically identical, the biochemistry of each is optimized to fulfill its unique physiological roles, with important consequences for human health and disease. Each?tissue's unique physiology requires tightly regulated gene and protein expression coordinated by specialized, phosphorylation-dependent intracellular signaling. To better understand the role of phosphorylation in maintenance of physiological differences among tissues, we performed proteomic and phosphoproteomic characterizations of nine mouse tissues. We identified 12,039 proteins, including 6296 phosphoproteins harboring nearly 36,000 phosphorylation sites. Comparing protein abundances and phosphorylation levels revealed specialized, interconnected phosphorylation networks within each tissue while suggesting that many proteins are regulated by phosphorylation independently of their expression. Our data suggest that the "typical" phosphoprotein is widely expressed yet displays variable, often tissue-specific phosphorylation that tunes protein activity to the specific needs of each tissue. We offer this dataset as an online resource for the biological research community.     

Unraveling pancreatic islet biology by quantitative proteomics

The pancreatic islets of Langerhans play a critical role in maintaining blood glucose homeostasis by secreting insulin and several other important peptide hormones. Impaired insulin secretion due to islet dysfunction is linked to the pathogenesis underlying both Type 1 and Type 2 diabetes. Over the past 5 years, emerging proteomic technologies have been applied to dissect the signaling pathways that regulate islet functions and gain an understanding of the mechanisms of islet dysfunction relevant to diabetes. Herein, we briefly review some of the recent quantitative proteomic studies involving pancreatic islets geared towards gaining a better understanding of islet biology relevant to metabolic diseases.     

Proteomic analysis of mouse islets after multiple low-dose streptozotocin injection

The islets of Langerhans are scattered throughout the pancreas and play a major role in the control of metabolic fuel homeostasis. To get a better understanding of the mechanisms underlying type 1 diabetes mellitus, we have generated a mouse model by injections of multiple low-dose streptozotocin. The islets in the mouse pancreas were handpicked and proteins from the islets were then isolated and separated by two-dimensional gel electrophoresis. Seven proteins were found to be altered significantly at expression level. Among the seven proteins, four were up-regulated and three were down-regulated in diabetic mice as compared with controls. These proteins were successfully identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry and the changes of selected protein expression were further validated by quantitative real time PCR and Western blotting. Voltage-dependent anion-selective channel protein 1 and peroxiredoxin-4 were found for the first time to be associated with type 1 diabetes mellitus in mouse islets in the current study. These results suggest that glucose transport, beta cell proliferation/death, and oxidative stress play important roles in maintaining the balance of glucose level. Our study also provides novel insight into the mechanism of type 1 diabetes mellitus.     

Human Islets Have Fewer Blood Vessels than Mouse Islets and the Density of Islet Vascular Structures Is Increased in Type 2 Diabetes

Human and rodent islets differ substantially in several features, including architecture, cell composition, gene expression and some aspects of insulin secretion. Mouse pancreatic islets are highly vascularized with interactions between islet endothelial and endocrine cells being important for islet cell differentiation and function. To determine whether human islets have a similar high degree of vascularization and whether this is altered with diabetes, we examined the vascularization of islets from normal human subjects, subjects with type 2 diabetes (T2D), and normal mice. Using an integrated morphometry approach to quantify intra-islet capillary density in human and mouse pancreatic sections, we found that human islets have five-fold fewer vessels per islet area than mouse islets. Islets in pancreatic sections from T2D subjects showed capillary thickening, some capillary fragmentation and had increased vessel density as compared with non-diabetic controls. These changes in islet vasculature in T2D islets appeared to be associated with amyloid deposition, which was noted in islets from 8/9 T2D subjects (and occupied 14% ± 4% of islet area), especially around the intra-islet capillaries. The physiological implications of the differences in the angioarchitecture of mouse and human islets are not known. Islet vascular changes in T2D may exacerbate β cell/islet dysfunction and β cell loss.     

Protein profiling of human pancreatic islets by two-dimensional gel electrophoresis and mass spectrometry

Completion of the human genome sequence has provided scientists with powerful resources with which to explore the molecular events associated with disease states such as diabetes. Understanding the relative levels of expression of gene products, especially of proteins, and their post-translational modifications will be critical. However, though the pancreatic islets play a key role in glucose homeostasis, global protein expression data in human are decidedly lacking. We here report the two-dimensional protein map and database of human pancreatic islets. A high level of reproducibility was obtained among the gels and a total of 744 protein spots were detected. We have successfully identified 130 spots corresponding to 66 different protein entries and generated a reference map of human islets. The functionally characterized proteins include enzymes, chaperones, cellular structural proteins, cellular defense proteins, signaling molecules, and transport proteins. A number of proteins identified in this study (e.g., annexin A2, elongation factor 1-alpha 2, histone H2B.a/g/k, heat shock protein 90 beta, heat shock 27 kDa protein, cyclophilin B, peroxiredoxin 4, cytokeratins 7, 18, and 19) have not been previously described in the database of mouse pancreatic islets. In addition, altered expression of several proteins, like GRP78, GRP94, PDI, calreticulin, annexin, cytokeratins, profilin, heat shock proteins, and ORP150 have been associated with the development of diabetes. The data presented in this study provides a first-draft reference map of the human islet proteome, that will pave the way for further proteome analysis of pancreatic islets in both healthy and diabetic individuals, generating insights into the pathophysiology of this condition.     

Discovery of a Human Peptide Sequence Signaling Islet Neogenesis

ObjectiveTo identify triggers for islet neogenesis in humans that may lead to new treatments that address the underlying mechanism of disease for patients with type 1 or type 2 diabetes.MethodsIn an effort to identify bioactive human peptide sequences that might trigger islet neogenesis, we evaluated amino acid sequences within a variety of mammalian pancreas-specific REG genes. We evaluated GenBank, the Basic Local Alignment Search Tool algorithm, and all available proteomic databases and developed large-scale protein-to-protein interaction maps. Studies of peptides of interest were conducted in human pancreatic ductal tissue, followed by investigations in mice with streptozocin-induced diabetes.ResultsOur team has defined a 14-amino acid bioactive peptide encoded by a portion of the human REG3a gene we termed Human proIslet Peptide (HIP), which is well conserved among many mammals. Treatment of human pancreatic ductal tissue with HIP stimulated the production of insulin. In diabetic mice, administration of HIP improved glycemic control and significantly increased islet number. Bioinformatics analysis, coupled with biochemical interaction studies in a human pancreatic cell line, identified the human exostoses-like protein 3 (EXTL3) as a HIP-binding protein. HIP enhanced EXTL3 translocation from the membrane to the nucleus, in support of a model whereby EXTL3 mediates HIP signaling for islet neogenesis.ConclusionOur data suggest that HIP may be a potential stimulus for islet neogenesis and that the differentiation of new islets is a process distinct from beta cell proliferation within existing islets. Human clinical trials are soon to commence to determine the effect of HIP on generating new islets from one’s own pancreatic progenitor cells. (Endocr Pract. 2008;14:1075-1083)     

Peptidome analysis reveals the involvement of endogenous peptides in mouse pancreatic dysfunction with aging

Type 2 diabetes (T2D) is a glucose regulation disorder that has significantly enhanced mortality and the global disease burden. The prevalence of T2D has increased worldwide and is higher in the elderly. The function of pancreatic islets decreases with age, which is one important reason for the occurrence of diabetes in the elderly. Recently, peptidome analysis has attracted attention. However, the role of age-related peptides in pancreatic dysfunction has not been investigated extensively. Here, we conducted a comparison of endogenous peptides between pancreas from adult and aging mice by liquid chromatography tandem mass spectrometry (LC-MS/MS). A total of 2,089 peptides originating from 1,280 protein precursors were identified, of which 232 were upregulated and 183 were downregulated in the aging mice (fold change ≥ 2 and p < 0.05), suggesting that the expression of pancreatic peptides in mice varied with age. The molecular weight of most peptides was <3.0 kDa, and the isoelectric point distribution had a bimodal characteristic. Further analysis of cleavage site patterns indicated that proteases cleaved pancreatic proteins according to their rules. Moreover, Gene Ontology and pathway analyses showed that the differentially expressed peptides potentially had specific effects on pancreatic dysfunction. Some differential peptides were located within the domains of precursor proteins that were closely associated with the development of diabetes. We believe that our research may advance the current understanding of pancreas-derived peptides and that certain peptides may be involved in the etiology of diabetes.     

Investigation of the mouse serum proteome

With the rapid assimilation of genomic information and the equally impressive developments in the field of proteomics, there is an unprecedented interest in biomarker discovery. Although human biofluids represent increasingly attractive samples from which new and more accurate disease biomarkers may be found, the intrinsic person-to-person variability in these samples complicates their discovery. One of the most extensively used animal models for studying human disease is mouse because, unlike humans, they represent a highly controllable experimental model system. Unfortunately, very little is known about the proteomic composition of mouse serum. In this study, a multidimensional fractionation approach on both the protein and the peptide level that does not require depletion of highly abundant serum proteins was combined with tandem mass spectrometry to characterize proteins within mouse serum. Over 12 300 unique peptides that originate from 4567 unique proteins-approximately 16% of all known mouse proteins-were identified. The results presented here represent the broadest proteome coverage in mouse serum and provide a foundation from which quantitative comparisons can be made in this important animal model.     

In vivo monitoring of pancreatic beta-cells in a transgenic mouse model

We generated a transgenic mouse model (RIP-luc) for the in vivo monitoring of pancreatic islet mass and function in response to metabolic disease. Using the rat insulin promoter fused to firefly luciferase, and noninvasive technology to detect luciferase activity, we tracked changes in reporter signal during metabolic disease states and correlated the changes in luciferase signal with metabolic status of the mouse. Transgene expression was found to be specific to the pancreatic islets in this transgenic model. Basal transgene expression was tracked in male and female mice fed either a chow or a high-fat diet and in response to treatment with streptozotocin. Pancreatic bioluminescent signal increased in mice fed a high-fat diet compared with chow-fed animals. In a model of chemically induced diabetes, the bioluminescent signal decreased in accordance with the onset of diabetes and reduction of islet beta-cell number. Preliminary studies using islets transplanted from this transgenic model suggest that in vivo image analysis can also be used to monitor transplanted islet viability and survival in the host. This transgenic model is a useful tool for in vivo studies of pancreatic beta-cells and as a donor for islet transplantation studies.     

Inosine protects against the development of diabetes in multiple-low-dose streptozotocin and nonobese diabetic mouse models of type 1 diabetes

Inosine, a naturally occurring purine, was long considered to be an inactive metabolite of adenosine. However, recently inosine has been shown to be an immunomodulator and anti-inflammatory agent. The aim of this study was to determine whether inosine influences anti-inflammatory effects and affects the development of type 1 diabetes in murine models. Type 1 diabetes was induced either chemically by streptozotocin or genetically using the nonobese diabetic mouse (NOD) model. Mice were treated with inosine (100 or 200 mg kg(-1)d(-1)d) and diabetes incidence was monitored. The effect of inosine on pancreas immune cell infiltration, oxidative stress, and cytokine profile also was determined. For the transplantation model islets were placed under the renal capsule of NOD mice and inosine (200 mg kg(-1)d d(-1)d) treatment started the day of islet transplantation. Graft rejection was diagnosed by return of hyperglycemia accompanied by glucosuria and ketonuria. Inosine reduced the incidence of diabetes in both streptozotocin-induced diabetes and spontaneous diabetes in NOD mice. Inosine decreased pancreatic leukocyte infiltration and oxidative stress in addition to switching the cytokine profile from a Th1 to a Th2 profile. Inosine prolonged pancreatic islet graft survival, increased the number of surviving beta cells, and reduced the number of infiltrating leukocytes. Inosine protects against both the development of diabetes and against the rejection of transplanted islets. The purine exerts anti-inflammatory effects in the pancreas, which is its likely mode of action. The use of inosine should be considered as a potential preventative therapy in humans susceptible to developing Type 1 diabetes and as a possible antirejection therapy for islet transplant recipients.     

Differential proteomic profiling identifies novel molecular targets of pterostilbene against experimental diabetes

Pterostilbene (PTS), a naturally occurring stilbene, confers protection against oxidative and cytokine stress induced pancreatic β-cell apoptosis in vitro and in vivo. To provide insights into the molecular mechanism, we performed a proteomic study on the pancreas of PTS-treated diabetic mice using electrospray ionization tandem–mass spectrometry (LC–MS/MS). A total of 1,260 proteins were detected in triplicate samples. Of which, 359 proteins were found to be differentially regulated in streptozotocin-induced diabetic mice pancreas with two fold difference ( P < 0.05, two or more peptides) and on PTS treatment 315 proteins were normalized to control levels. Gene ontology (GO) indicated that majority of the differentially regulated proteins are involved in cellular functions such as metabolism, cellular structure, oxidative stress, endoplasmic-reticulum-associated protein degradation (ERAD) pathway and several stress sensors. Protein–protein interaction network analysis of these differentially expressed proteins showed clustering of proteins involved in protein processing in endoplasmic reticulum (protein synthesis machinery and protein folding), oxidative phosphorylation/oxidative stress proteins, oligosaccharide metabolic process, and antioxidant activity. Our results highlighted that PTS administration rehabilitated the defective metabolic process and redox imbalance, and also suppressed the unfolded protein response and ERAD pathways. The effects on targeting ER machinery and suppressing oxidative stress suggest the great potential of PTS for diabetes management.     

Glucose-induced, calcium-mediated protein phosphorylation in intact pancreatic islets

Conditions for studying protein phosphorylation in intact pancreatic islets were developed in order to study the effects of glucose and other effectors. Islets were incubated in Krebs-Ringer bicarbonate buffer containing 5 mM malate and 5 mM pyruvate (metabolic fuels that are not insulin secretagogues) for 150 min to permit incorporation of 32Pi into islet phosphate pools. Glucose or other effectors were then added, and the incubation was terminated after 10 to 30 min. Glucose increased phosphorylation of four islet peptides with molecular weights of 20,000, 33,000, 43,000 and 57,000. The calcium channel blockers, verapamil and D-600, inhibited phosphorylation of each of the four proteins, and trifluoperazine inhibited phosphorylation of the proteins with molecular weights of 20,000 and 57,000. The results indicate that glucose-induced insulin release may be mediated in part by protein phosphorylation, and that calcium may act as an intracellular messenger in coupling the glucose stimulus to the secretory process.     

Polyclonal origin of pancreatic islets in aggregation mouse chimaeras.

In the present study, we have examined the origin and growth pattern of the beta cells in pancreatic islets, to determine whether a single progenitor cell gave rise to all the precursors of the islets, or if each of a few progenitor cells is the founder of a different islet, or if each islet is a mixture of cells originating from a pool of progenitor cells. Aggregation mouse chimaeras where the pancreatic beta cells derived from each embryo can be identified in the islets on histological sections were analyzed. In two chimaeras, all the islets contained cells from both the aggregated embryo. This clearly demonstrates that each islet resulted from several independent cells. In addition, the beta cells derived from either embryo component were in very small clusters in the islets, suggesting that in situ cell division did not account significantly for islet growth.