Proinsulin processing by the subtilisin-related proprotein convertases furin, PC2, and PC3

Cab45G trafficking through the insulin secretory pathway is altered in human type 2 diabetes

In type 2 diabetes (T2D), the rate of insulin secretory granule biogenesis can limit insulin secretion from pancreatic β-cells. Using rat insulinoma INS1 β-cells, we show that the soluble Ca2+-binding/trafficking protein, Cab45G, serves as a non-essential chaperone for insulin granule biogenesis. In β-cells, Cab45G is stored within a cis-Golgi reservoir. Cab45G deletion dysregulates Ca2+ homeostasis and leads to secretory abnormality, but insulin granule biogenesis remains intact. Increasing Cab45G biosynthesis leads to anterograde trafficking into insulin granules, stimulating their production. Using human donor islets, we identify increased anterograde Cab45G trafficking in obese humans with and without T2D, consistent with the heightened demand for granule biogenesis. However, humans with T2D demonstrate decreased Golgi Cab45G localization and increased granule Cab45G localization compared to those without T2D. Our study provides the first insight into Cab45G function in specialized secretory cells and opens avenues of investigation into mechanisms associated with β-cell compensation and failure.

Optical Nanoscopy of Cytokine-Induced Structural Alterations of the Endoplasmic Reticulum and Golgi Apparatus in Insulin-Secreting Cells

Pro-inflammatory cytokines play a role in the failure of β cells in type 1 and type 2 diabetes. While existing data from 'omics' experiments allow for some understanding of the molecular mechanisms behind cytokine-induced dysfunction in β cells, no report thus far has provided information on the direct imaging of the β cell landscape with nanoscale resolution following cytokine exposure. In this study, we use Airyscan-based optical super-resolution microscopy of Insulinoma 1E (INS-1E) cells to investigate the structural properties of two subcellular membranous compartments involved in the production, maturation and secretion of insulin-containing granules, the endoplasmic reticulum (ER) and the Golgi apparatus (GA). Our findings reveal that exposure of INS-1E cells to IL-1β and IFN-γ for 24 h leads to significant structural alterations of both compartments. In more detail, both the ER and the GA fragment and give rise to vesicle-like structures with markedly reduced characteristic area and perimeter and increased circularity with respect to the original structures. These findings complement the molecular data collected thus far on these compartments and their role in β cell dysfunction and lay the groundwork for future optical microscopy-based ex vivo and in vivo investigations.

Understanding the Genetic Landscape of Gestational Diabetes: Insights into the Causes and Consequences of Elevated Glucose Levels in Pregnancy

Background/Objectives: During pregnancy, physiological changes in maternal circulating glucose levels and its metabolism are essential to meet maternal and fetal energy demands. Major changes in glucose metabolism occur throughout pregnancy and consist of higher insulin resistance and a compensatory increase in insulin secretion to maintain glucose homeostasis. For some women, this change is insufficient to maintain normoglycemia, leading to gestational diabetes mellitus (GDM), a condition characterized by maternal glucose intolerance and hyperglycaemia first diagnosed during the second or third trimester of pregnancy. GDM is diagnosed in approximately 14.0% of pregnancies globally, and it is often associated with short- and long-term adverse health outcomes in both mothers and offspring. Although recent studies have highlighted the role of genetic determinants in the development of GDM, research in this area is still lacking, hindering the development of prevention and treatment strategies. Methods: In this paper, we review recent advances in the understanding of genetic determinants of GDM and glycaemic traits during pregnancy. Results/Conclusions: Our review highlights the need for further collaborative efforts as well as larger and more diverse genotyped pregnancy cohorts to deepen our understanding of the genetic aetiology of GDM, address research gaps, and further improve diagnostic and treatment strategies.

Predictive Prioritization of Enhancers Associated with Pancreas Disease Risk

Genetic and epigenetic variations in regulatory enhancer elements increase susceptibility to a range of pathologies. Despite recent advances, linking enhancer elements to target genes and predicting transcriptional outcomes of enhancer dysfunction remain significant challenges. Using 3D chromatin conformation assays, we generated an extensive enhancer interaction dataset for the human pancreas, encompassing more than 20 donors and five major cell types, including both exocrine and endocrine compartments. We employed a network approach to parse chromatin interactions into enhancer-promoter tree models, facilitating a quantitative, genome-wide analysis of enhancer connectivity. With these tree models, we developed a machine learning algorithm to estimate the impact of enhancer perturbations on cell type-specific gene expression in the human pancreas. Orthogonal to our computational approach, we perturbed enhancer function in primary human pancreas cells using CRISPR interference and quantified the effects at the single-cell level through RNA FISH coupled with high-throughput imaging. Our enhancer tree models enabled the annotation of common germline risk variants associated with pancreas diseases, linking them to putative target genes in specific cell types. For pancreatic ductal adenocarcinoma, we found a stronger enrichment of disease susceptibility variants within acinar cell regulatory elements, despite ductal cells historically being assumed as the primary cell-of-origin. Our integrative approach-combining cell type-specific enhancer-promoter interaction mapping, computational models, and single-cell enhancer perturbation assays-produced a robust resource for studying the genetic basis of pancreas disorders.

mTOR: Its Critical Role in Metabolic Diseases, Cancer, and the Aging Process

The mammalian target of rapamycin (mTOR) is a pivotal regulator, integrating diverse environmental signals to control fundamental cellular functions, such as protein synthesis, cell growth, survival, and apoptosis. Embedded in a complex network of signaling pathways, mTOR dysregulation is implicated in the onset and progression of a range of human diseases, including metabolic disorders such as diabetes and cardiovascular diseases, as well as various cancers. mTOR also has a notable role in aging. Given its extensive biological impact, mTOR signaling is a prime therapeutic target for addressing these complex conditions. The development of mTOR inhibitors has proven advantageous in numerous research domains. This review delves into the significance of mTOR signaling, highlighting the critical components of this intricate network that contribute to disease. Additionally, it addresses the latest findings on mTOR inhibitors and their clinical implications. The review also emphasizes the importance of developing more effective next-generation mTOR inhibitors with dual functions to efficiently target the mTOR pathways. A comprehensive understanding of mTOR signaling will enable the development of effective therapeutic strategies for managing diseases associated with mTOR dysregulation.

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

An Overview of the Role of Furin in Type 2 Diabetes

Post-translational modifications (PTMs) play important roles in regulating several human diseases, like cancer, neurodegenerative disorders, and metabolic disorders. Investigating PTMs' contribution to protein functions is critical for modern biology and medicine. Proprotein convertases (PCs) are irreversible post-translational modifiers that have been extensively studied and are considered as key targets for novel therapeutics. They cleave proteins at specific sites causing conformational changes affecting their functions. Furin is considered as a PC model in regulating growth factors and is involved in regulating many pro-proteins. The mammalian target of the rapamycin (mTOR) signaling pathway is another key player in regulating cellular processes and its dysregulation is linked to several diseases including type 2 diabetes (T2D). The role of furin in the context of diabetes has been rarely explored and is currently lacking. Moreover, furin variants have altered activity that could have implications on overall health. In this review, we aim to highlight the role of furin in T2D in relation to mTOR signaling. We will also address furin genetic variants and their potential effect on T2D and β-cell functions. Understanding the role of furin in prediabetes and dissecting it from other confounding factors like obesity is crucial for future therapeutic interventions in metabolic disorders.

Convertase-dependent regulation of membrane-tethered and secreted ligands tunes dendrite adhesion

During neural development, cellular adhesion is crucial for interactions among and between neurons and surrounding tissues. This function is mediated by conserved cell adhesion molecules, which are tightly regulated to allow for coordinated neuronal outgrowth. Here, we show that the proprotein convertase KPC-1 (homolog of mammalian furin) regulates the Menorin adhesion complex during development of PVD dendritic arbors in Caenorhabditis elegans. We found a finely regulated antagonistic balance between PVD-expressed KPC-1 and the epidermally expressed putative cell adhesion molecule MNR-1 (Menorin). Genetically, partial loss of mnr-1 suppressed partial loss of kpc-1, and both loss of kpc-1 and transgenic overexpression of mnr-1 resulted in indistinguishable phenotypes in PVD dendrites. This balance regulated cell-surface localization of the DMA-1 leucine-rich transmembrane receptor in PVD neurons. Lastly, kpc-1 mutants showed increased amounts of MNR-1 and decreased amounts of muscle-derived LECT-2 (Chondromodulin II), which is also part of the Menorin adhesion complex. These observations suggest that KPC-1 in PVD neurons directly or indirectly controls the abundance of proteins of the Menorin adhesion complex from adjacent tissues, thereby providing negative feedback from the dendrite to the instructive cues of surrounding tissues.

Berberine enhances the function of db/db mice islet β cell through GLP-1/GLP-1R/PKA signaling pathway in intestinal L cell and islet α cell

Background: The evidence on berberine stimulating the secretion of GLP-1 in intestinal L cell has been studied. However, few research has explored its role on generating GLP-1 of islet α cell. Our experiment aims to clarify the mechanism of berberine promoting the secretion of GLP-1 in intestinal L cell and islet α cell, activating GLP-1R and its downstream molecules through endocrine and paracrine ways, thus improving the function of islet β cell and treating T2DM. Methods: After confirming that berberine can lower blood glucose and improve insulin resistance in db/db mice, the identity maintenance, proliferation and apoptosis of islet cells were detected by immunohistochemistry and immunofluorescence. Then, the activation of berberine on GLP-1/GLP-1R/PKA signaling pathway was evaluated by Elisa, Western blot and PCR. Finally, this mechanism was verified by in vitro experiments on Min6 cells, STC-1 cells and aTC1/6 cells. Results: Berberine ameliorates glucose metabolism in db/db mice. Additionally, it also increases the number and enhances the function of islet β cell. This process is closely related to improve the secretion of intestinal L cell and islet α cell, activate GLP-1R/PKA signaling pathway through autocrine and paracrine, and increase the expression of its related molecule such as GLP-1, GLP-1R, PC1/3, PC2, PKA, Pdx1. In vitro, the phenomenon that berberine enhanced the GLP-1/GLP-1R/PKA signal pathway had also been observed, which confirmed the results of animal experiments. Conclusion: Berberine can maintain the identity and normal function of islet β cell, and its mechanism is related to the activation of GLP-1/GLP-1R/PKA signal pathway in intestinal L cell and islet α cell.

Inhibition of Insulin Secretion Induces Golgi Morphological Changes

Objectives

The role of autophagy in pancreatic β cells has been reported, but the relationship between autophagy and insulin metabolism is complex and is not fully understood yet.

Design

We here analyze the relationship between autophagy and insulin metabolism from a morphological aspect.

Methods

We observe the morphological changes of β cell-specific Atg7-deficient mice and Atg5-deficient MIN6 cells with electron microscopy.

Results

We find that Atg7-deficient β cells exhibit a marked expansion of the endoplasmic reticulum (ER). We also find that the inhibitory state of insulin secretion causes morphological changes in the Golgi, including ministacking and swelling. The same morphological alterations are observed when insulin secretion is suppressed in Atg5-deficient MIN6 cells.

Conclusions

The defect of autophagy induces ER expansion, and inhibition of insulin secretion induces Golgi swelling, probably via ER stress and Golgi stress, respectively.

Understanding Insulin in the Age of Precision Medicine and Big Data: Under-Explored Nature of Genomics

Insulin is amongst the human genome's most well-studied genes/proteins due to its connection to metabolic health. Within this article, we review literature and data to build a knowledge base of Insulin (INS) genetics that influence transcription, transcript processing, translation, hormone maturation, secretion, receptor binding, and metabolism while highlighting the future needs of insulin research. The INS gene region has 2076 unique variants from population genetics. Several variants are found near the transcriptional start site, enhancers, and following the INS transcripts that might influence the readthrough fusion transcript INS-IGF2. This INS-IGF2 transcript splice site was confirmed within hundreds of pancreatic RNAseq samples, lacks drift based on human genome sequencing, and has possible elevated expression due to viral regulation within the liver. Moreover, a rare, poorly characterized African population-enriched variant of INS-IGF2 results in a loss of the stop codon. INS transcript UTR variants rs689 and rs3842753, associated with type 1 diabetes, are found in many pancreatic RNAseq datasets with an elevation of the 3'UTR alternatively spliced INS transcript. Finally, by combining literature, evolutionary profiling, and structural biology, we map rare missense variants that influence preproinsulin translation, proinsulin processing, dimer/hexamer secretory storage, receptor activation, and C-peptide detection for quasi-insulin blood measurements.

G protein-coupled receptor kinase 6 (GRK6) regulates insulin processing and secretion via effects on proinsulin conversion to insulin

Recent studies identified a missense mutation in the gene coding for G protein–coupled receptor kinase 6 (GRK6) that segregates with type 2 diabetes (T2D). To better understand how GRK6 might be involved in T2D, we used pharmacological inhibition and genetic knockdown in the mouse β-cell line, MIN6, to determine whether GRK6 regulates insulin dynamics. We show inhibition of GRK5 and GRK6 increased insulin secretion but reduced insulin processing while GRK6 knockdown revealed these same processing defects with reduced levels of cellular insulin. GRK6 knockdown cells also had attenuated insulin secretion but enhanced proinsulin secretion consistent with decreased processing. In support of these findings, we demonstrate GRK6 rescue experiments in knockdown cells restored insulin secretion after glucose treatment. The altered insulin profile appears to be caused by changes in the proprotein convertases, the enzymes responsible for proinsulin to insulin conversion, as GRK6 knockdown resulted in significantly reduced convertase expression and activity. To identify how the GRK6-P384S mutation found in T2D patients might affect insulin processing, we performed biochemical and cell biological assays to study the properties of the mutant. We found that while GRK6-P384S was more active than WT GRK6, it displayed a cytosolic distribution in cells compared to the normal plasma membrane localization of GRK6. Additionally, GRK6 overexpression in MIN6 cells enhanced proinsulin processing, while GRK6-P384S expression had little effect. Taken together, our data show that GRK6 regulates insulin processing and secretion in a glucose-dependent manner and provide a foundation for understanding the contribution of GRK6 to T2D.

ZnT8 loss-of-function accelerates functional maturation of hESC-derived β cells and resists metabolic stress in diabetes

Human embryonic stem cell-derived β cells (SC-β cells) hold great promise for treatment of diabetes, yet how to achieve functional maturation and protect them against metabolic stresses such as glucotoxicity and lipotoxicity remains elusive. Our single-cell RNA-seq analysis reveals that ZnT8 loss of function (LOF) accelerates the functional maturation of SC-β cells. As a result, ZnT8 LOF improves glucose-stimulated insulin secretion (GSIS) by releasing the negative feedback of zinc inhibition on insulin secretion. Furthermore, we demonstrate that ZnT8 LOF mutations endow SC-β cells with resistance to lipotoxicity/glucotoxicity-triggered cell death by alleviating endoplasmic reticulum (ER) stress through modulation of zinc levels. Importantly, transplantation of SC-β cells with ZnT8 LOF into mice with preexisting diabetes significantly improves glycemia restoration and glucose tolerance. These findings highlight the beneficial effect of ZnT8 LOF on the functional maturation and survival of SC-β cells that are useful as a potential source for cell replacement therapies.

Immature function and fragility hinder application of hESC-derived β cells (SC-β cell) for diabetes cell therapy. Here, the authors identify ZnT8 as a gene editing target to enhance the insulin secretion and cell survival under metabolic stress by abolishing zinc transport in SC-β cells.

The Path to Therapeutic Furin Inhibitors: From Yeast Pheromones to SARS-CoV-2

The spurious acquisition and optimization of a furin cleavage site in the SARS-CoV-2 spike protein is associated with increased viral transmission and disease, and has generated intense interest in the development and application of therapeutic furin inhibitors to thwart the COVID-19 pandemic. This review summarizes the seminal studies that informed current efforts to inhibit furin. These include the convergent efforts of endocrinologists, virologists, and yeast geneticists that, together, culminated in the discovery of furin. We describe the pioneering biochemical studies which led to the first furin inhibitors that were able to block the disease pathways which are broadly critical for pathogen virulence, tumor invasiveness, and atherosclerosis. We then summarize how these studies subsequently informed current strategies leading to the development of small-molecule furin inhibitors as potential therapies to combat SARS-CoV-2 and other diseases that rely on furin for their pathogenicity and progression.

Inside the Insulin Secretory Granule

The pancreatic β-cell is purpose-built for the production and secretion of insulin, the only hormone that can remove glucose from the bloodstream. Insulin is kept inside miniature membrane-bound storage compartments known as secretory granules (SGs), and these specialized organelles can readily fuse with the plasma membrane upon cellular stimulation to release insulin. Insulin is synthesized in the endoplasmic reticulum (ER) as a biologically inactive precursor, proinsulin, along with several other proteins that will also become members of the insulin SG. Their coordinated synthesis enables synchronized transit through the ER and Golgi apparatus for congregation at the trans-Golgi network, the initiating site of SG biogenesis. Here, proinsulin and its constituents enter the SG where conditions are optimized for proinsulin processing into insulin and subsequent insulin storage. A healthy β-cell is continually generating SGs to supply insulin in vast excess to what is secreted. Conversely, in type 2 diabetes (T2D), the inability of failing β-cells to secrete may be due to the limited biosynthesis of new insulin. Factors that drive the formation and maturation of SGs and thus the production of insulin are therefore critical for systemic glucose control. Here, we detail the formative hours of the insulin SG from the luminal perspective. We do this by mapping the journey of individual members of the SG as they contribute to its genesis.

Differential Neuropeptidomes of Dense Core Secretory Vesicles (DCSV) Produced at Intravesicular and Extracellular pH Conditions by Proteolytic Processing

Neuropeptides mediate cell–cell signaling in the nervous and endocrine systems. The neuropeptidome is the spectrum of peptides generated from precursors by proteolysis within dense core secretory vesicles (DCSV). DCSV neuropeptides and contents are released to the extracellular environment where further processing for neuropeptide formation may occur. To assess the DCSV proteolytic capacity for production of neuropeptidomes at intravesicular pH 5.5 and extracellular pH 7.2, neuropeptidomics, proteomics, and protease assays were conducted using chromaffin granules (CG) purified from adrenal medulla. CG are an established model of DCSV. The CG neuropeptidome consisted of 1239 unique peptides derived from 15 proneuropeptides that were colocalized with 64 proteases. Distinct CG neuropeptidomes were generated at the internal DCSV pH of 5.5 compared to the extracellular pH of 7.2. Class-specific protease inhibitors differentially regulated neuropeptidome production involving aspartic, cysteine, serine, and metallo proteases. The substrate cleavage properties of CG proteases were assessed by multiplex substrate profiling by mass spectrometry (MSP-MS) that uses a synthetic peptide library containing diverse cleavage sites for endopeptidases and exopeptidases. Parallel inhibitor-sensitive cleavages for neuropeptidome production and peptide library proteolysis led to elucidation of six CG proteases involved in neuropeptidome production, represented by cathepsins A, B, C, D, and L and carboxypeptidase E (CPE). The MSP-MS profiles of these six enzymes represented the majority of CG proteolytic cleavages utilized for neuropeptidome production. These findings provide new insight into the DCSV proteolytic system for production of distinct neuropeptidomes at the internal CG pH of 5.5 and at the extracellular pH of 7.2.

Evidence from oyster suggests an ancient role for Pdx in regulating insulin gene expression in animals

Hox and ParaHox genes encode transcription factors with similar expression patterns in divergent animals. The Pdx (Xlox) homeobox gene, for example, is expressed in a sharp spatial domain in the endodermal cell layer of the gut in chordates, echinoderms, annelids and molluscs. The significance of comparable gene expression patterns is unclear because it is not known if downstream transcriptional targets are also conserved. Here, we report evidence indicating that a classic transcriptional target of Pdx1 in vertebrates, the insulin gene, is a likely direct target of Pdx in Pacific oyster adults. We show that one insulin-related gene, cgILP, is co-expressed with cgPdx in oyster digestive tissue. Transcriptomic comparison suggests that this tissue plays a similar role to the vertebrate pancreas. Using ATAC-seq and ChIP, we identify an upstream regulatory element of the cgILP gene which shows binding interaction with cgPdx protein in oyster hepatopancreas and demonstrate, using a cell culture assay, that the oyster Pdx can act as a transcriptional activator through this site, possibly in synergy with NeuroD. These data argue that a classic homeodomain-target gene interaction dates back to the origin of Bilateria.

In vertebrates insulin is a direct transcriptional target of Pdx: the same is true in Pacific oysters and the authors show insulin-related gene, cgILP, is co-expressed with cgPdx in oyster digestive tissue, showing this gene interaction dates back to the origin of Bilateria.

Persistence of Coxsackievirus B4 in Pancreatic β Cells Disturbs Insulin Maturation, Pattern of Cellular Proteins, and DNA Methylation

Coxsackievirus-B4 (CV-B4) can persist in pancreatic cell lines and impair the phenoytpe and/or gene expressions in these cells; however, the models used to study this phenomenon did not produce insulin. Therefore, we investigated CV-B4 persistence and its consequences in insulin-producing pancreatic β cells. The insulin-secreting rat β cell line, INS-1, was infected with CV-B4. After lysis of a large part of the cell layer, the culture was still maintained and no additional cytopathic effect was observed. The amount of insulin in supernatants of cell cultures persistently infected with CV-B4 was not affected by the infection; in fact, a larger quantity of proinsulin was found. The mRNA expression of pro-hormone convertase 2, an enzyme involved in the maturation of proinsulin into insulin and studied using real-time reverse transcription-polymerase chain reaction, was inhibited in infected cultures. Further, the pattern of 47 cell proteins analyzed using Shotgun mass spectrometry was significantly modified. The DNA of persistently infected cell cultures was hypermethylated unlike that of controls. The persistent infection of INS-1 cells with CV-B4 had a deep impact on these cells, especially on insulin metabolism. Cellular changes caused by persistent CV-B4 infection of β cells can play a role in type 1 diabetes pathogenesis.

The Probable, Possible, and Novel Functions of ERp29

The luminal endoplasmic reticulum (ER) protein of 29 kDa (ERp29) is a ubiquitously expressed cellular agent with multiple critical roles. ERp29 regulates the biosynthesis and trafficking of several transmembrane and secretory proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial sodium channel (ENaC), thyroglobulin, connexin 43 hemichannels, and proinsulin. ERp29 is hypothesized to promote ER to cis-Golgi cargo protein transport via COP II machinery through its interactions with the KDEL receptor; this interaction may facilitate the loading of ERp29 clients into COP II vesicles. ERp29 also plays a role in ER stress (ERS) and the unfolded protein response (UPR) and is implicated in oncogenesis. Here, we review the vast array of ERp29’s clients, its role as an ER to Golgi escort protein, and further suggest ERp29 as a potential target for therapies related to diseases of protein misfolding and mistrafficking.

ERp29 as a regulator of Insulin biosynthesis

The environment within the Endoplasmic Reticulum (ER) influences Insulin biogenesis. In particular, ER stress may contribute to the development of Type 2 Diabetes (T2D) and Cystic Fibrosis Related Diabetes (CFRD), where evidence of impaired Insulin processing, including elevated secreted Proinsulin/Insulin ratios, are observed. Our group has established the role of a novel ER chaperone ERp29 (ER protein of 29 kDa) in the biogenesis of the Epithelial Sodium Channel, ENaC. The biogenesis of Insulin and ENaC share may key features, including their potential association with COP II machinery, their cleavage into a more active form in the Golgi or later compartments, and their ability to bypass such cleavage and remain in a less active form. Given these similarities we hypothesized that ERp29 is a critical factor in promoting the efficient conversion of Proinsulin to Insulin. Here, we confirmed that Proinsulin associates with the COP II vesicle cargo recognition component, Sec24D. When Sec24D expression was decreased, we observed a corresponding decrease in whole cell Proinsulin levels. In addition, we found that Sec24D associates with ERp29 in co-precipitation experiments and that ERp29 associates with Proinsulin in co-precipitation experiments. When ERp29 was overexpressed, a corresponding increase in whole cell Proinsulin levels was observed, while depletion of ERp29 decreased whole cell Proinsulin levels. Together, these data suggest a potential role for ERp29 in regulating Insulin biosynthesis, perhaps in promoting the exit of Proinsulin from the ER via Sec24D/COPII vesicles.

Pro-islet amyloid polypeptide in micelles contains a helical prohormone segment

Pro-islet amyloid polypeptide (proIAPP) is the prohormone precursor molecule to IAPP, also known as amylin. IAPP is a calcitonin family peptide hormone that is cosecreted with insulin, and largely responsible for hunger satiation and metabolic homeostasis. Amyloid plaques containing mixtures of mature IAPP and misprocessed proIAPP deposit on, and destroy pancreatic β-cell membranes, and they are recognized as a clinical hallmark of type 2 diabetes mellitus. In order to better understand the interaction with cellular membranes, we solved the solution NMR structure of proIAPP bound to dodecylphosphocholine micelles at pH 4.5. We show that proIAPP is a dynamic molecule with four α-helices. The first two helices are contained within the mature IAPP sequence, while the second two helices are part of the C-terminal prohormone segment (Cpro). We mapped the membrane topology of the amphipathic helices by paramagnetic relaxation enhancement, and we used CD and diffusion-ordered spectroscopy to identify environmental factors that impact proIAPP membrane affinity. We discuss how our structural results relate to prohormone processing based on the varied pH environments and lipid compositions of organelle membranes within the regulated secretory pathway, and the likelihood of Cpro survival for cosecretion with IAPP. DATABASE: The assigned resonances have been deposited in the Biological Magnetic Resonance Bank (BMRB) with accession numbers 50007 and 50019 for proIAPP and Cpro, respectively. The lowest energy structures have been deposited in the Protein Data Bank (PDB) with access codes 6UCJ and 6UCK.

A novel mutation in the mouse Pcsk1 gene showing obesity and diabetes

The proprotein convertase subtilisin/Kexin type 1 (PCSK1/PC1) protein processes inactive pro-hormone precursors into biologically active hormones in a number of neuroendocrine and endocrine cell types. Patients with recessive mutations in PCSK1 exhibit a complex spectrum of traits including obesity, diarrhoea and endocrine disorders. We describe here a new mouse model with a point mutation in the Pcsk1 gene that exhibits obesity, hyperphagia, transient diarrhoea and hyperproinsulinaemia, phenotypes consistent with human patient traits. The mutation results in a pV96L amino acid substitution and changes the first nucleotide of mouse exon 3 leading to skipping of that exon and in homozygotes very little full-length transcript. Overexpression of the exon 3 deleted protein or the 96L protein results in ER retention in Neuro2a cells. This is the second Pcsk1 mouse model to display obesity phenotypes, contrasting knockout mouse alleles. This model will be useful in investigating the basis of endocrine disease resulting from prohormone processing defects.

EIPR1 controls dense-core vesicle cargo retention and EARP complex localization in insulin-secreting cells

Dense-core vesicles (DCVs) are secretory vesicles found in neurons and endocrine cells. DCVs package and release cargoes including neuropeptides, biogenic amines, and peptide hormones. We recently identified the endosome-associated recycling protein (EARP) complex and the EARP-interacting-protein EIPR-1 as proteins important for controlling levels of DCV cargoes in Caenorhabditis elegans neurons. Here we determine the role of mammalian EIPR1 in insulinoma cells. We find that in Eipr1 KO cells, there is reduced insulin secretion, and mature DCV cargoes such as insulin and carboxypeptidase E (CPE) accumulate near the trans-Golgi network and are not retained in mature DCVs in the cell periphery. In addition, we find that EIPR1 is required for the stability of the EARP complex subunits and for the localization of EARP and its association with membranes, but EIPR1 does not affect localization or function of the related Golgi-associated retrograde protein (GARP) complex. EARP is localized to two distinct compartments related to its function: an endosomal compartment and a DCV biogenesis-related compartment. We propose that EIPR1 functions with EARP to control both endocytic recycling and DCV maturation.

Preproinsulin Designer Antigens Excluded from Endoplasmic Reticulum Suppressed Diabetes Development in NOD Mice by DNA Vaccination

DNA vaccines against autoimmune type 1 diabetes (T1D) contain a nonpredictable risk to induce autoreactive T cell responses rather than a protective immunity. Little is known if (and how) antigen expression and processing requirements favor the induction of autoreactive or protective immune responses by DNA immunization. Here, we analyzed whether structural properties of preproinsulin (ppins) variants and/or subcellular targeting of ppins designer antigens influence the priming of effector CD8⁺ T cell responses by DNA immunization. Primarily, we used H-2^b RIP-B7.1 tg mice, expressing the co-stimulator molecule B7.1 in beta cells, to identify antigens that induce or fail to induce autoreactive ppins-specific (K^b/A_12-21 and/or K^b/B_22-29) CD8⁺ T cell responses. Female NOD mice, expressing the diabetes-susceptible H-2^g7 haplotype, were used to test ppins variants for their potential to suppress spontaneous diabetes development. We showed that ppins antigens excluded from expression in the endoplasmic reticulum (ER) did not induce CD8⁺ T cells or autoimmune diabetes in RIP-B7.1 tg mice, but efficiently suppressed spontaneous diabetes development in NOD mice as well as ppins-induced CD8⁺ T cell-mediated autoimmune diabetes in PD-L1^−/− mice. The induction of a ppins-specific therapeutic immunity in mice has practical implications for the design of immune therapies against T1D in individuals expressing different major histocompatibility complex (MHC) I and II molecules.

PICK1 is essential for insulin production and the maintenance of glucose homeostasis

Protein interacting with C-kinase 1 (PICK1) is a peripheral membrane protein that controls insulin granule formation, trafficking, and maturation in INS-1E cells. However, global Pick1-knockout mice showed only a subtle diabetes-like phenotype. This raises the possibility that compensatory effects from tissues other than pancreatic beta cells may obscure the effects of insulin deficiency. To explore the role of PICK1 in pancreatic islets, we generated mice harboring a conditional Pick1 allele in a C57BL/6J background. The conditional Pick1-knockout mice exhibited impaired glucose tolerance, profound insulin deficiency, and hyperglycemia. In vitro experiments showed that the ablation of Pick1 in pancreatic beta cells selectively decreased the initial rapid release of insulin and the total insulin levels in the islets. Importantly, the specific ablation of Pick1 induced elevated proinsulin levels in the circulation and in the islets, accompanied by a reduction in the proinsulin processing enzymes prohormone convertase 1/3 (PC1/3). The deletion of Pick1 triggered the specific elimination of chromogranin B in pancreatic beta cells, which is believed to control granule formation and release. Collectively, these data demonstrate the critical role of PICK1 in secretory granule biogenesis, proinsulin processing, and beta cell function. We conclude that the beta cell-specific deletion of Pick1 in mice led to hyperglycemia and eventually to diabetes.

Localization of amylin-like immunoreactivity in the striped velvet gecko pancreas

Immunohistochemical techniques were employed to investigate the distribution of amylin-like immunoreactive cells in the pancreas of gecko Homopholis fasciata. Four types of endocrine cells were distinguished: insulin immunoreactive (B cells), pancreatic polypeptide immunoreactive (PP cells), glucagon and pancreatic polypeptide immunoreactive (A/PP cells) and somatostatin immunoreactive cells (D cells). Pancreatic islets contained B, A/PP and D cells, whereas extrainsular regions contained B, D and PP cells. In the pancreatic islets, amylin-like immunoreactive cells corresponded to B cells, but not to A/PP or D cells. In the extrainsular regions, amylin-like immunoreactive cells corresponded to either B or PP cells. Amylin secreted from intrainsular B cells may regulate pancreatic hormone secretion in an autocrine and/or a paracrine fashion. On the other hand, amylin secreted from extrainsular PP and B cells, and/or intrainsular B cells may participate in the modulation of calcium homoeostasis in an endocrine fashion.

Proteolytic Cleavage-Mechanisms, Function, and "Omic" Approaches for a Near-Ubiquitous Posttranslational Modification

Proteases enzymatically hydrolyze peptide bonds in substrate proteins, resulting in a widespread, irreversible posttranslational modification of the protein's structure and biological function. Often regarded as a mere degradative mechanism in destruction of proteins or turnover in maintaining physiological homeostasis, recent research in the field of degradomics has led to the recognition of two main yet unexpected concepts. First, that targeted, limited proteolytic cleavage events by a wide repertoire of proteases are pivotal regulators of most, if not all, physiological and pathological processes. Second, an unexpected in vivo abundance of stable cleaved proteins revealed pervasive, functionally relevant protein processing in normal and diseased tissue-from 40 to 70% of proteins also occur in vivo as distinct stable proteoforms with undocumented N- or C-termini, meaning these proteoforms are stable functional cleavage products, most with unknown functional implications. In this Review, we discuss the structural biology aspects and mechanisms of catalysis by different protease classes. We also provide an overview of biological pathways that utilize specific proteolytic cleavage as a precision control mechanism in protein quality control, stability, localization, and maturation, as well as proteolytic cleavage as a mediator in signaling pathways. Lastly, we provide a comprehensive overview of analytical methods and approaches to study activity and substrates of proteolytic enzymes in relevant biological models, both historical and focusing on state of the art proteomics techniques in the field of degradomics research.

Loss of mTORC1 signalling impairs β-cell homeostasis and insulin processing

Deregulation of mTOR complex 1 (mTORC1) signalling increases the risk for metabolic diseases, including type 2 diabetes. Here we show that β-cell-specific loss of mTORC1 causes diabetes and β-cell failure due to defects in proliferation, autophagy, apoptosis and insulin secretion by using mice with conditional (βraKO) and inducible (MIP-βraKO^f/f) raptor deletion. Through genetic reconstitution of mTORC1 downstream targets, we identify mTORC1/S6K pathway as the mechanism by which mTORC1 regulates β-cell apoptosis, size and autophagy, whereas mTORC1/4E-BP2-eIF4E pathway regulates β-cell proliferation. Restoration of both pathways partially recovers β-cell mass and hyperglycaemia. This study also demonstrates a central role of mTORC1 in controlling insulin processing by regulating cap-dependent translation of carboxypeptidase E in a 4EBP2/eIF4E-dependent manner. Rapamycin treatment decreases CPE expression and insulin secretion in mice and human islets. We suggest an important role of mTORC1 in β-cells and identify downstream pathways driving β-cell mass, function and insulin processing.

Deregulation of mTORC1 pathway has been associated with several human diseases including diabetes, neurodegeneration and cancer. Here Blandino-Rosano et al. show that mTORC1 signalling controls insulin secretion and β-cell maintenance by regulation of β-cell proliferation, apoptosis and autophagy and insulin processing.

Molecular regulation of insulin granule biogenesis and exocytosis

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by hyperglycemia, insulin resistance and hyperinsulinemia in early disease stages but a relative insulin insufficiency in later stages. Insulin, a peptide hormone, is produced in and secreted from pancreatic β-cells following elevated blood glucose levels. Upon its release, insulin induces the removal of excessive exogenous glucose from the bloodstream primarily by stimulating glucose uptake into insulin-dependent tissues as well as promoting hepatic glycogenesis. Given the increasing prevalence of T2DM worldwide, elucidating the underlying mechanisms and identifying the various players involved in the synthesis and exocytosis of insulin from β-cells is of utmost importance. This review summarizes our current understanding of the route insulin takes through the cell after its synthesis in the endoplasmic reticulum as well as our knowledge of the highly elaborate network that controls insulin release from the β-cell. This network harbors potential targets for anti-diabetic drugs and is regulated by signaling cascades from several endocrine systems.

Islet amyloid polypeptide: Another key molecule in Alzheimer's pathogenesis?

Recent epidemiological evidence reveals that patients suffering from type 2 diabetes mellitus (T2DM) often experience a significant decline in cognitive function, and approximately 70% of those cases eventually develop Alzheimer's disease (AD). Although several pathological processes are shared by AD and T2DM, the exact molecular mechanisms connecting these two diseases are poorly understood. Aggregation of human islet amyloid polypeptide (hIAPP), the pathological hallmark of T2DM, has also been detected in brain tissue and is associated with cognitive decline and AD development. In addition, hIAPP and amyloid β protein (Aβ) share many biophysical and physiological properties as well as exert similar cytotoxic mechanisms. Therefore, it is important to examine the possible role of hIAPP in the pathogenesis of AD. In this article, we introduce the basics on this amyloidogenic protein. More importantly, we discuss the potential mechanisms of hIAPP-induced AD development, which will be beneficial for proposing novel and feasible strategies to optimize AD prevention and/or treatment in diabetics.

The Proprotein Convertases in Hypercholesterolemia and Cardiovascular Diseases: Emphasis on Proprotein Convertase Subtilisin/Kexin 9

The secretory proprotein convertase (PC) family comprises nine members, as follows: PC1/3, PC2, furin, PC4, PC5/6, paired basic amino acid cleaving enzyme 4, PC7, subtilisin kexin isozyme 1/site 1 protease (SKI-1/S1P), and PC subtilisin/kexin type 9 (PCSK9). The first seven PCs cleave their substrates at single/paired basic residues and exhibit specific and often essential functions during development and/or in adulthood. The essential SKI-1/S1P cleaves membrane-bound transcription factors at nonbasic residues. In contrast, PCSK9 cleaves itself once, and the secreted inactive protease drags the low-density lipoprotein receptors (LDLR) and very LDLR (VLDLR) to endosomal/lysosomal degradation. Inhibitory PCSK9 monoclonal antibodies are now prescribed to treat hypercholesterolemia. This review focuses on the implication of PCs in cardiovascular functions and diseases, with a major emphasis on PCSK9. We present a phylogeny of the PCs and the analysis of PCSK9 haplotypes in modern and archaic human species. The absence of PCSK9 in mice led to the discovery of a sex- and tissue-specific subcellular distribution of the LDLR and VLDLR. PCSK9 inhibition may have other applications because it reduces inflammation and sepsis in a LDLR-dependent manner. Our present understanding of the cellular mechanism(s) that enables PCSK9 to induce the degradation of receptors is reviewed, as well as the consequences of its key natural mutations. The PCSK9 ongoing clinical trials are reviewed. Finally, how the other PCs may impact cardiovascular disease and the metabolic syndrome, and become relevant targets, is discussed.

The metabolic sensor Sirt1 and the hypothalamus: Interplay between peptide hormones and pro-hormone convertases

The last decade had witnessed a tremendous progress in our understanding of the causes of metabolic diseases including obesity. Among the contributing factors regulating energy balance are nutrient sensors such as sirtuins. Sirtuin1 (Sirt1), a NAD + - dependent deacetylase is affected by diet, environmental stress, and also plays a critical role in metabolic health by deacetylating proteins in many tissues, including liver, muscle, adipose tissue, heart, endothelium, and in the complexity of the hypothalamus. Because of its dependence on NAD+, Sirt1 also functions as a nutrient/redox sensor, and new novel data show a function of this enzyme in the maturation of hypothalamic peptide hormones controlling energy balance either through regulation of specific nuclear transcription factors or by regulating specific pro-hormone convertases (PCs) involved in the post-translational processing of pro-hormones. The post-translational processing mechanism of pro-hormones is critical in the pathogenesis of obesity as recently shown that metabolic and physiological triggers affect the biosynthesis and processing of many peptides hormones. Specific regulation of pro-hormone processing is likely another key step where final amounts of bioactive peptides can be tightly regulated. Different factors stimulate or inhibit pro-hormones biosynthesis in concert with an increase in the PCs involved in the maturation of bioactive hormones. Adding more complexity to the system, the new studies describe here suggest that Sirt1 could also regulate the fate of peptide hormone biosynthesis. The present review summarizes the recent progress in hypothalamic SIRT1 research with a particular emphasis on the tissue-specific control of neuropeptide hormone maturation. The series of studies done in mouse and rat models strongly advocate for the first time that a deacetylating enzyme could be a regulator in the maturation of peptide hormones and their processing enzymes. These discoveries are the culmination of the first in-depth understanding of the metabolic role of Sirt1 in the brain. It suggests that Sirt1 behaves differently in the brain than in organs such as the liver and pancreas, where the enzyme has been more commonly studied.

HID-1 is required for homotypic fusion of immature secretory granules during maturation

Secretory granules, also known as dense core vesicles, are generated at the trans-Golgi network and undergo several maturation steps, including homotypic fusion of immature secretory granules (ISGs) and processing of prehormones to yield active peptides. The molecular mechanisms governing secretory granule maturation are largely unknown. Here, we investigate a highly conserved protein named HID-1 in a mouse model. A conditional knockout of HID-1 in pancreatic β cells leads to glucose intolerance and a remarkable increase in the serum proinsulin/insulin ratio caused by defective proinsulin processing. Large volume three-dimensional electron microscopy and immunofluorescence imaging reveal that ISGs are much more abundant in the absence of HID-1. We further demonstrate that HID-1 deficiency prevented secretory granule maturation by blocking homotypic fusion of immature secretory granules. Our data identify a novel player during the early maturation of immature secretory granules.

PCSK1 Variants and Human Obesity

PCSK1, encoding prohormone convertase 1/3 (PC1/3), was one of the first genes linked to monogenic early-onset obesity. PC1/3 is a protease involved in the biosynthetic processing of a variety of neuropeptides and prohormones in endocrine tissues. PC1/3 activity is essential for the activating cleavage of many peptide hormone precursors implicated in the regulation of food ingestion, glucose homeostasis, and energy homeostasis, for example, proopiomelanocortin, proinsulin, proglucagon, and proghrelin. A large number of genome-wide association studies in a variety of different populations have now firmly established a link between three PCSK1 polymorphisms frequent in the population and increased risk of obesity. Human subjects with PC1/3 deficiency, a rare autosomal-recessive disorder caused by the presence of loss-of-function mutations in both alleles, are obese and display a complex set of endocrinopathies. Increasing numbers of genetic diagnoses of infants with persistent diarrhea has recently led to the finding of many novel PCSK1 mutations. PCSK1-deficient infants experience severe intestinal malabsorption during the first years of life, requiring controlled nutrition; these children then become hyperphagic, with associated obesity. The biochemical characterization of novel loss-of-function PCSK1 mutations has resulted in the discovery of new pathological mechanisms affecting the cell biology of the endocrine cell beyond simple loss of enzyme activity, for example, dominant-negative effects of certain mutants on wild-type PC1/3 protein, and activation of the cellular unfolded protein response by endoplasmic reticulum-retained mutants. A better understanding of these molecular and cellular pathologies may illuminate possible treatments for the complex endocrinopathy of PCSK1 deficiency, including obesity.

A Novel GLP1 Receptor Interacting Protein ATP6ap2 Regulates Insulin Secretion in Pancreatic Beta Cells

GLP1 activates its receptor, GLP1R, to enhance insulin secretion. The activation and transduction of GLP1R requires complex interactions with a host of accessory proteins, most of which remain largely unknown. In this study, we used membrane-based split ubiquitin yeast two-hybrid assays to identify novel GLP1R interactors in both mouse and human islets. Among these, ATP6ap2 (ATPase H(+)-transporting lysosomal accessory protein 2) was identified in both mouse and human islet screens. ATP6ap2 was shown to be abundant in islets including both alpha and beta cells. When GLP1R and ATP6ap2 were co-expressed in beta cells, GLP1R was shown to directly interact with ATP6ap2, as assessed by co-immunoprecipitation. In INS-1 cells, overexpression of ATP6ap2 did not affect insulin secretion; however, siRNA knockdown decreased both glucose-stimulated and GLP1-induced insulin secretion. Decreases in GLP1-induced insulin secretion were accompanied by attenuated GLP1 stimulated cAMP accumulation. Because ATP6ap2 is a subunit required for V-ATPase assembly of insulin granules, it has been reported to be involved in granule acidification. In accordance with this, we observed impaired insulin granule acidification upon ATP6ap2 knockdown but paradoxically increased proinsulin secretion. Importantly, as a GLP1R interactor, ATP6ap2 was required for GLP1-induced Ca(2+) influx, in part explaining decreased insulin secretion in ATP6ap2 knockdown cells. Taken together, our findings identify a group of proteins that interact with the GLP1R. We further show that one interactor, ATP6ap2, plays a novel dual role in beta cells, modulating both GLP1R signaling and insulin processing to affect insulin secretion.

Tequila Regulates Insulin-Like Signaling and Extends Life Span in Drosophila melanogaster

The aging process is a universal phenomenon shared by all living organisms. The identification of longevity genes is important in that the study of these genes is likely to yield significant insights into human senescence. In this study, we have identified Tequila as a novel candidate gene involved in the regulation of longevity in Drosophila melanogaster. We have found that a hypomorphic mutation of Tequila (Teq(f01792)), as well as cell-specific downregulation of Tequila in insulin-producing neurons of the fly, significantly extends life span. Tequila deficiency-induced life-span extension is likely to be associated with reduced insulin-like signaling, because Tequila mutant flies display several common phenotypes of insulin dysregulation, including reduced circulating Drosophila insulin-like peptide 2 (Dilp2), reduced Akt phosphorylation, reduced body size, and altered glucose homeostasis. These observations suggest that Tequila may confer life-span extension by acting as a modulator of Drosophila insulin-like signaling.

An efficient two-step subcellular fractionation method for the enrichment of insulin granules from INS-1 cells

Insulin is one of the key regulators for blood glucose homeostasis. More than 99% of insulin is secreted from the pancreatic β-cells. Within each β-cell, insulin is packaged and processed in insulin secretary granules (ISGs) before its exocytosis. Insulin secretion is a complicated but well-organized dynamic process that includes the budding of immature ISGs (iISGs) from the trans-Golgi network, iISG maturation, and mature ISG (mISG) fusion with plasma membrane. However, the molecular mechanisms involved in this process are largely unknown. It is therefore crucial to separate and enrich iISGs and mISGs before determining their distinct characteristics and protein contents. Here, we developed an efficient two-step subcellular fractionation method for the enrichment of iISGs and mISGs from INS-1 cells: OptiPrep gradient purification followed by Percoll solution purification. We demonstrated that by using this method, iISGs and mISGs can be successfully distinguished and enriched. This method can be easily adapted to investigate SGs in other cells or tissues, thereby providing a useful tool for elucidating the mechanisms of granule maturation and secretion.

Variable effects of gender and Western diet on lipid and glucose homeostasis in aged PCSK9-deficient C57BL/6 mice CSK9PC57BL/6

BACKGROUND

Proprotein convertase subtilisin/kexin-type 9 (PCSK9) downregulates clearance of plasma cholesterol by liver. Its inactivation increases this clearance, reducing cardiovascular risk. However, a lack of PCSK9 could also lead to cholesterol accumulation in pancreatic islet beta cells, impairing insulin secretion. We reported earlier that 4-month-old male PCSK9-deficient (KO) C57BL/6 mice were hyperglycemic and insulin-insufficient relative to their wild-type (WT) counterparts. Here, we examined how gender and diet affect lipid and glucose homeostasis in these mice at 8 months of age.

METHODS

After being fed a normal diet or a Western diet for over 6 months, KO mice were compared with same-gender WT mice for fasting plasma levels of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), glucose, and insulin; for glucose disposal and glucose-stimulated insulin secretion (GSIS); and for pancreatic islet morphology.

RESULTS

A. Females: On normal diet, KO mice showed lower plasma TC, HDL-C, and LDL-C, higher plasma glucose, and normal glucose disposal despite impaired GSIS. On Western diet, they showed comparable plasma TC and HDL-C, but lower LDL-C, higher plasma glucose, and normal glucose disposal despite impaired GSIS. B. Males: On normal and Western diets, KO mice showed lower plasma TC, HDL-C, and LDL-C, similarly elevated plasma glucose, glucose intolerance, and impaired GSIS. C. Both: KO mice on either diet showed pancreatic islet dysmorphism, with larger, possibly immature secretory granules.

CONCLUSIONS

Lower LDL-C and impaired GSIS are two major phenotypes in aged PCSK9-deficient C57BL/6 mice. These phenotypes are modulated by gender and diet.

Glucose-regulated insulin production in the liver improves glycemic control in type 1 diabetic mice

OBJECTIVE

Type 1 diabetes results from autoimmune destruction of beta-cells in the pancreas. Our objective is to reconstitute a glucose-responsive system in the liver to regulate hepatic insulin production for improving glycemic control in type 1 diabetes.

METHODS

We have cloned the glucose-responsive element (GRE) from the promoter of acetyl-CoA carboxylase (ACC), an enzyme that catalyzes the rate-limiting step in fatty acid synthesis in the liver in response to glucose. To increase the amplitude of glucose induction, we quadruplicated the GRE DNA by gene duplication. The resulting GRE multimer (4×GRE) was tested for its ability to drive rat proinsulin cDNA expression in hepatocytes and insulin-deficient diabetic mice.

RESULTS

We showed that this GRE multimer-directed glucose-responsive system produced insulin in hepatocytes in a glucose-dependent manner. When delivered into the liver by adenovirus-mediated gene transfer, this glucose-responsive insulin production system was able to reverse hyperglycemia to a normal range without causing hypoglycemia after glucose challenge or overnight fasting. Insulin vector-treated diabetic mice exhibited significantly improved blood glucose profiles in response to glucose tolerance, correlating with insulin production in the liver. We recapitulated these findings in streptozotocin-induced diabetic CD1 mice and autoimmune non-obese diabetic mice.

CONCLUSION

Our data characterized the GRE motif from the ACC promoter as a potent glucose-responsive element, and provided proof-of-concept that the 4×GRE-mediated hepatic insulin production is capable of correcting insulin deficiency and improving glycemic control in type 1 diabetes.

GLP-1 analog liraglutide enhances proinsulin processing in pancreatic β-cells via a PKA-dependent pathway

Hyperproinsulinemia has gained increasing attention in the development of type 2 diabetes. Clinical studies have demonstrated that glucagon-like peptide-1 (GLP-1)-based therapies significantly decrease plasma proinsulin/insulin ratio in patients with type 2 diabetes. However, the underlying mechanism remains unclear. Prohormone convertase (PC)-1/3 and PC2 are primarily responsible for processing proinsulin to insulin in pancreatic β-cells. We have recently reported that Pax6 mutation down-regulated PC1/3 and PC2 expression, resulting in defective proinsulin processing in Pax6 heterozygous mutant (Pax6(m/+)) mice. In this study, we investigated whether and how liraglutide, a novel GLP-1 analog, modulated proinsulin processing. Our results showed that liraglutide significantly up-regulated PC1/3 expression and decreased the proinsulin to insulin ratio in both Pax6(m/+) and db/db diabetic mice. In the cultured mouse pancreatic β-cell line, Min6, liraglutide stimulated PC1/3 and PC2 expression and lowered the proinsulin to insulin ratio in a dose- and time-dependent manner. Moreover, the beneficial effects of liraglutide on PC1/3 and PC2 expression and proinsulin processing were dependent on the GLP-1 receptor-mediated cAMP/protein kinase A signaling pathway. The same mechanism was recapitulated in isolated mouse islets. In conclusion, liraglutide enhanced PC1/3- and PC2-dependent proinsulin processing in pancreatic β-cells through the activation of the GLP-1 receptor/cAMP/protein kinase A signaling pathway. Our study provides a new mechanism for improvement of pancreatic β-cell function by the GLP-1-based therapy.

Group X secretory phospholipase A2 regulates insulin secretion through a cyclooxygenase-2-dependent mechanism

Group X secretory phospholipase A2 (GX sPLA2) potently hydrolyzes membrane phospholipids to release arachidonic acid (AA). While AA is an activator of glucose-stimulated insulin secretion (GSIS), its metabolite prostaglandin E2 (PGE2) is a known inhibitor. In this study, we determined that GX sPLA2 is expressed in insulin-producing cells of mouse pancreatic islets and investigated its role in beta cell function. GSIS was measured in vivo in wild-type (WT) and GX sPLA2-deficient (GX KO) mice and ex vivo using pancreatic islets isolated from WT and GX KO mice. GSIS was also assessed in vitro using mouse MIN6 pancreatic beta cells with or without GX sPLA2 overexpression or exogenous addition. GSIS was significantly higher in islets isolated from GX KO mice compared with islets from WT mice. Conversely, GSIS was lower in MIN6 cells overexpressing GX sPLA2 (MIN6-GX) compared with control (MIN6-C) cells. PGE2 production was significantly higher in MIN6-GX cells compared with MIN6-C cells and this was associated with significantly reduced cellular cAMP. The effect of GX sPLA2 on GSIS was abolished when cells were treated with NS398 (a COX-2 inhibitor) or L-798,106 (a PGE2-EP3 receptor antagonist). Consistent with enhanced beta cell function, GX KO mice showed significantly increased plasma insulin levels following glucose challenge and were protected from age-related reductions in GSIS and glucose tolerance compared with WT mice. We conclude that GX sPLA2 plays a previously unrecognized role in negatively regulating pancreatic insulin secretion by augmenting COX-2-dependent PGE2 production.

Genetics of the first seven proprotein convertase enzymes in health and disease

Members of the substilisin/kexin like proprotein convertase (PCSK) protease family cleave and convert immature pro-proteins into their biologically active forms. By cleaving for example prohormones, cytokines and cell membrane proteins, PCSKs participate in maintaining the homeostasis in a healthy human body. Conversely, erratic enzymatic function is thought to contribute to the pathogenesis of a wide variety of diseases, including obesity and hypercholestrolemia. The first characterized seven PCSK enzymes (PCSK1-2, FURIN, PCSK4-7) process their substrates at a motif made up of paired basic amino acid residues. This feature results in a variable degree of biochemical redundancy in vitro, and consequently, shared substrate molecules between the different PCSK enzymes. This redundancy has confounded our understanding of the specific biological functions of PCSKs. The physiological roles of these enzymes have been best illustrated by the phenotypes of genetically engineered mice and patients that carry mutations in the PCSK genes. Recent developments in genome-wide methodology have generated a large amount of novel information on the genetics of the first seven proprotein convertases. In this review we summarize the reported genetic alterations and their associated phenotypes.

Global analysis of cellular proteolysis by selective enzymatic labeling of protein N-termini

Proteolysis is a critical modification leading to alteration of protein function with important outcomes in many biological processes. However, for the majority of proteases, we have an incomplete understanding of both cellular substrates and downstream effects. Here, we describe detailed protocols and applications for using the rationally engineered peptide ligase, subtiligase, to specifically label and capture protein N-termini generated by proteases either induced or added to complex biological samples. This method allows identification of the protein targets as well as their precise cleavage locations. This approach has revealed >8000 proteolytic sites in healthy and apoptotic cells including >1700 caspase cleavages. One can further determine substrate preferences through rate analysis with quantitative mass spectrometry, physiological substrate specificities, and even infer the identity of proteases operating in the cell. In this chapter, we also describe how this experimental method can be generalized to investigate proteolysis in any biological sample.

Zinc and its transporters, pancreatic β-cells, and insulin metabolism

Zinc is an essential trace metal for life. Two families of zinc transporters, SLC30A (ZNT) and SLC39A (ZIP) are required for maintaining cellular zinc homeostasis. ZNTs function to decrease cytoplasmic zinc concentrations whereas ZIPs do the opposite. Expression of zinc transporters can be tissue/cell-type specific or ubiquitous. Zinc transporters that are limited in tissue/cell distributions usually perform specialized tasks to satisfy biological processes in a given cell. For example, ZNT8 is mainly expressed in β-cells and functions to deliver zinc into granules for insulin maturation and secretion. Many other zinc transporters are also expressed in β-cells. Defects in these zinc transporters have been associated with abnormalities in insulin synthesis, maturation, and secretion and subsequent glucose metabolism. This review focuses on the specific roles of zinc and its transporters in insulin metabolism and describes the current knowledge of the function of zinc transporters in β-cell health in animal knockout mouse models with respect to diabetes development in humans.

Mutant PAX6 downregulates prohormone convertase 2 expression in mouse islets

Transcriptional factor paired box 6 (PAX6) is very important for the development of the eyes, central nervous system, and pancreas. PAX6 mutations are associated with a diabetic phenotype and abnormal glucose metabolism. Our previous study showed that PAX6 directly bound to and activated the prohormone convertase 1/3 (Pc1/3) gene promoter and subsequently regulated proinsulin processing. Prohormone convertase 2 (PC2) is the essential enzyme for pancreatic proinsulin processing. To study the regulation of PAX6 in Pc2 expression, we did research on the pancreas of Pax6 R266Stop mutant mice, where truncated mutations happened in the C-terminal of the PAX6 protein. Our studies showed that the mutant PAX6 protein was stable and regulated the activity of Pc2 promoter as shown by luciferase activity assays. We found that the wild-type PAX6 protein imparts a transcriptional effect, and the mutant PAX6 can also regulate the downstream molecules. The results provide new insights into the mechanism of truncated PAX6 in regulating the functions of the pancreas and endocrine system.

Severe obesity and diabetes insipidus in a patient with PCSK1 deficiency

Non-synonymous mutations affecting both alleles of PCSK1 (proprotein convertase 1/3) are associated with obesity and impaired prohormone processing. We report a proband who was compound heterozygous for a maternally inherited frameshift mutation and a paternally inherited 474kb deletion that encompasses PCSK1, representing a novel genetic mechanism underlying this phenotype. Although pro-vasopressin is not a known physiological substrate of PCSK1, the development of central diabetes insipidus in this proband suggests that PCSK1 deficiency can be associated with impaired osmoregulation.

A missing PD-L1/PD-1 coinhibition regulates diabetes induction by preproinsulin-specific CD8 T-cells in an epitope-specific manner

Coinhibitory PD-1/PD-L1 (B7-H1) interactions provide critical signals for the regulation of autoreactive T-cell responses. We established mouse models, expressing the costimulator molecule B7.1 (CD80) on pancreatic beta cells (RIP-B7.1 tg mice) or are deficient in coinhibitory PD-L1 or PD-1 molecules (PD-L1^−/− and PD-1^−/− mice), to study induction of preproinsulin (ppins)-specific CD8 T-cell responses and experimental autoimmune diabetes (EAD) by DNA-based immunization. RIP-B7.1 tg mice allowed us to identify two CD8 T-cell specificities: pCI/ppins DNA exclusively induced K^b/A_12–21-specific CD8 T-cells and EAD, whereas pCI/ppinsΔA_12–21 DNA (encoding ppins without the COOH-terminal A_12–21 epitope) elicited K^b/B_22–29-specific CD8 T-cells and EAD. Specific expression/processing of mutant ppinsΔA_12–21 (but not ppins) in non-beta cells, targeted by intramuscular DNA-injection, thus facilitated induction of K^b/B_22–29-specific CD8 T-cells. The A_12–21 epitope binds K^b molecules with a very low avidity as compared with B_22–29. Interestingly, immunization of coinhibition-deficient PD-L1^−/− or PD-1^−/− mice with pCI/ppins induced K^b/A_12–21-monospecific CD8 T-cells and EAD but injections with pCI/ppinsΔA_12–21 did neither recruit K^b/B_22–29-specific CD8 T-cells into the pancreatic target tissue nor induce EAD. PpinsΔA_12–21/(K^b/B_22–29)-mediated EAD was efficiently restored in RIP-B7.1⁺/PD-L1^−/− mice, differing from PD-L1^−/− mice only in the tg B7.1 expression in beta cells. Alternatively, an ongoing beta cell destruction and tissue inflammation, initiated by ppins/(K^b/A_12–21)-specific CD8 T-cells in pCI/ppins+pCI/ppinsΔA_12–21 co-immunized PD-L1^−/− mice, facilitated the expansion of ppinsΔA_12–21/(K^b/B_22–29)-specific CD8 T-cells. CD8 T-cells specific for the high-affinity K^b/B_22–29- (but not the low-affinity K^b/A_12–21)-epitope thus require stimulatory ´help from beta cells or inflamed islets to expand in PD-L1-deficient mice. The new PD-1/PD-L1 diabetes models may be valuable tools to study under well controlled experimental conditions distinct hierarchies of autoreactive CD8 T-cell responses, which trigger the initial steps of beta cell destruction or emerge during the pathogenic progression of EAD.

Expression and secretion of human proinsulin-B10 from mouse salivary glands: implications for the treatment of type I diabetes mellitus

Adenovirus (Ad) mediated expression of therapeutic proteins from salivary glands can result in the delivery of biologically active proteins into the circulation where they impart their physiological function. In recent years, Ad vector delivery to salivary glands (SGs) has emerged as a viable option for gene therapy. Here, we engineered a variant of human proinsulin (hProinsulin-B10) into an Ad vector and demonstrated its ability to transduce cell lines, and express a bioactive protein that induces the phosphorylation of AKT, a key insulin signaling molecule. We also examined its expression in mice following delivery of the vector to the parotid gland (PTG), the submandibular gland (SMG) or to the liver via the tail vein and assessed transgenic protein expression and vector containment for each delivery method. In all cases, hProinsulin-B10 was expressed and secreted into the circulation. Lower levels of circulating hProinsulin-B10 were obtained from the PTG while higher levels were obtained from the tail vein and the SMG; however, vector particle containment was best when delivered to the SMG. Expression of hProinsulin-B10 in the SMG of chemically induced diabetic mice prevented excessive hyperglycemia observed in untreated mice. These results demonstrate that hProinsulin-B10 can be expressed and secreted into the circulation from SGs and can function physiologically in vivo. The ability to remediate a diabetic phenotype in a model of type 1 diabetes mellitus is the first step in an effort that may lead to a possible therapy for diabetes.