Severe defect in proglucagon processing in islet A-cells of prohormone convertase 2 null mice

GLP-1 and IL-6 regulates obesity in the gut and brain

Obesity is a chronic metabolic disease characterized by excessive nutrient intake leading to increased subcutaneous or visceral fat, resulting in pathological and physiological changes. The incidence rate of obesity, an important form of metabolic syndrome, is increasing worldwide. Excess appetite is a key pathogenesis of obesity, and the inflammatory response induced by obesity has received increasing attention. This review focuses on the role of appetite-regulating factor (Glucogan-like peptide 1) and inflammatory factor (Interleukin-6) in the gut and brain in individuals with obesity and draws insights from the current literature.

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Human alpha cell transcriptomic signatures of types 1 and 2 diabetes highlight disease-specific dysfunction pathways

Although glucagon secretion is perturbed in both T1D and T2D, the pathophysiological changes in individual pancreatic alpha cells are still obscure. Using recently curated single-cell RNASeq data from T1D or T2D donors and their controls, we identified alpha cell transcriptomic alterations consistent with both common and discrete pathways. Although alterations in alpha cell identity gene (ARX, MAFB) expression were conserved, cytokine-regulated genes and genes involved in glucagon biosynthesis and processing were up-regulated in T1D. Conversely, mitochondrial genes associated with ROS (COX7B, NQO2) were dysregulated in T2D. Additionally, T1D alpha cells displayed altered expression of autoimmune-induced ER stress genes (ERLEC1, HSP90), whilst those from T2D subjects showed modified glycolytic and citrate cycle gene (LDHA?, PDHB, PDK4) expression. Thus, despite conserved alterations related to loss of function, alpha cells display disease-specific gene signatures which may be secondary to the main pathogenic events in each disease, namely immune- or metabolism-mediated-stress, in T1D and T2D, respectively.

Pathways of Glucagon Secretion and Trafficking in the Pancreatic Alpha Cell: Novel Pathways, Proteins, and Targets for Hyperglucagonemia

Patients with diabetes mellitus exhibit hyperglucagonemia, or excess glucagon secretion, which may be the underlying cause of the hyperglycemia of diabetes. Defective alpha cell secretory responses to glucose and paracrine effectors in both Type 1 and Type 2 diabetes may drive the development of hyperglucagonemia. Therefore, uncovering the mechanisms that regulate glucagon secretion from the pancreatic alpha cell is critical for developing improved treatments for diabetes. In this review, we focus on aspects of alpha cell biology for possible mechanisms for alpha cell dysfunction in diabetes: proglucagon processing, intrinsic and paracrine control of glucagon secretion, secretory granule dynamics, and alterations in intracellular trafficking. We explore possible clues gleaned from these studies in how inhibition of glucagon secretion can be targeted as a treatment for diabetes mellitus.

Altered islet prohormone processing: A cause or consequence of diabetes?

Peptide hormones are first produced as larger precursor prohormones that require endoproteolytic cleavage to liberate the mature hormones. A structurally conserved but functionally distinct family of nine prohormone convertase enzymes (PCs) are responsible for cleavage of protein precursors of which PC1/3 and PC2 are known to be exclusive to neuroendocrine cells and responsible for prohormone cleavage. Differential expression of PCs within tissues define prohormone processing; whereas glucagon is the major product liberated from proglucagon via PC2 in pancreatic α-cells, proglucagon is preferentially processed by PC1/3 in intestinal L cells to produce glucagon-like peptides 1 and 2 (GLP-1, GLP-2). Beyond our understanding of processing of islet prohormones in healthy islets, there is convincing evidence that proinsulin, proIAPP, and proglucagon processing is altered during prediabetes and diabetes. There is predictive value of elevated circulating proinsulin or proinsulin : C-peptide ratio for progression to type 2 diabetes and elevated proinsulin or proinsulin : C-peptide is predictive for development of type 1 diabetes in at risk groups. After onset of diabetes, patients have elevated circulating proinsulin and proIAPP and proinsulin may be an autoantigen in type 1 diabetes. Further, preclinical studies reveal that α-cells have altered proglucagon processing during diabetes leading to increased GLP-1 production. We conclude that despite strong associative data, current evidence is inconclusive on the potential causal role of impaired prohormone processing in diabetes, and suggest that future work should focus on resolving the question of whether altered prohormone processing is a causal driver or merely a consequence of diabetes pathology.

Cell-specific expression and individual function of prohormone convertase PC1/3 in Tribolium larval growth highlights major evolutionary changes between beetle and fly neuroendocrine systems

Background

The insect neuroendocrine system acts in the regulation of physiology, development and growth. Molecular evolution of this system hence has the potential to allow for major biological differences between insect groups. Two prohormone convertases, PC1/3 and PC2, are found in animals and both function in the processing of neuropeptide precursors in the vertebrate neurosecretory pathway. Whereas PC2-function is conserved between the fly Drosophila and vertebrates, ancestral PC1/3 was lost in the fly lineage and has not been functionally studied in any protostome.

Results

In order to understand its original functions and the changes accompanying the gene loss in the fly, we investigated PC1/3 and PC2 expression and function in the beetle Tribolium castaneum. We found that PC2 is broadly expressed in the nervous system, whereas surprisingly, PC1/3 expression is restricted to specific cell groups in the posterior brain and suboesophageal ganglion. Both proteases have parallel but non-redundant functions in adult beetles’ viability and fertility. Female infertility following RNAi is caused by a failure to deposit sufficient yolk to the developing oocytes. Larval RNAi against PC2 produced moulting defects where the larvae were not able to shed their old cuticle. This ecdysis phenotype was also observed in a small subset of PC1/3 knockdown larvae and was strongest in a double knockdown. Unexpectedly, most PC1/3-RNAi larvae showed strongly reduced growth, but went through larval moults despite minimal to zero weight gain.

Conclusions

The cell type-specific expression of PC1/3 and its essential requirement for larval growth highlight the important role of this gene within the insect neuroendocrine system. Genomic conservation in most insect groups suggests that it has a comparable individual function in other insects as well, which has been replaced by alternative mechanisms in flies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-021-00179-w.

Mouse Models of Human Proprotein Convertase Insufficiency

The kexin-like proprotein convertases perform the initial proteolytic cleavages that ultimately generate a variety of different mature peptide and proteins, ranging from brain neuropeptides to endocrine peptide hormones, to structural proteins, among others. In this review, we present a general introduction to proprotein convertase structure and biochemistry, followed by a comprehensive discussion of each member of the kexin-like subfamily of proprotein convertases. We summarize current knowledge of human proprotein convertase insufficiency syndromes, including genome-wide analyses of convertase polymorphisms, and compare these to convertase null and mutant mouse models. These mouse models have illuminated our understanding of the roles specific convertases play in human disease and have led to the identification of convertase-specific substrates; for example, the identification of procorin as a specific PACE4 substrate in the heart. We also discuss the limitations of mouse null models in interpreting human disease, such as differential precursor cleavage due to species-specific sequence differences, and the challenges presented by functional redundancy among convertases in attempting to assign specific cleavages and/or physiological roles. However, in most cases, knockout mouse models have added substantively both to our knowledge of diseases caused by human proprotein convertase insufficiency and to our appreciation of their normal physiological roles, as clearly seen in the case of the furin, proprotein convertase 1/3, and proprotein convertase 5/6 mouse models. The creation of more sophisticated mouse models with tissue- or temporally-restricted expression of specific convertases will improve our understanding of human proprotein convertase insufficiency and potentially provide support for the emerging concept of therapeutic inhibition of convertases.

The Role of Proprotein Convertases in the Regulation of the Function of Immune Cells in the Oncoimmune Response

Proprotein convertases (PC) are a family of 9 serine proteases involved in the processing of cellular pro-proteins. They trigger the activation, inactivation or functional changes of many hormones, neuropeptides, growth factors and receptors. Therefore, these enzymes are essential for cellular homeostasis in health and disease. Nine PC subtilisin/kexin genes (PCSK1 to PCSK9) encoding for PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P and PCSK9 are known. The expression of PC1/3, PC2, PC5/6, Furin and PC7 in lymphoid organs such as lymph nodes, thymus and spleen has suggested a role for these enzymes in immunity. In fact, knock-out of Furin in T cells was associated with high secretion of pro-inflammatory cytokines and autoantibody production in mice. This suggested a key role for this enzyme in immune tolerance. Moreover, Furin through its proteolytic activity, regulates the suppressive functions of Treg and thus prevents chronic inflammation and autoimmune diseases. In macrophages, Furin is also involved in the regulation of their inflammatory phenotype. Similarly, PC1/3 inhibition combined with TLR4 stimulation triggers the activation of the NF-κB signaling pathway with an increased secretion of pro-inflammatory cytokines. Factors secreted by PC1/3 KD macrophages stimulated with LPS exert a chemoattractive effect on naive auxiliary T lymphocytes (Th0) and anti-tumoral activities. The link between TLR and PCs is thus very important in inflammatory response regulation. Furin regulates TL7 and TLR8 processing and trafficking whereas PC1/3 controls TLR4 and TLR9 trafficking. Since PC1/3 and Furin are key regulators of both the innate and adaptive immune responses their inhibition may play a major role in oncoimmune therapy. The role of PCs in the oncoimmune response and therapeutic strategies based on PCs inhibition are proposed in the present review.

Mice lacking PC1/3 expression in POMC-expressing cells do not develop obesity

Pro-opiomelanocortin (POMC) neurons form an integral part of the central melanocortin system regulating food intake and energy expenditure. Genetic and pharmacological studies have revealed that defects in POMC synthesis, processing, and receptor signaling lead to obesity. It is well established that POMC is extensively processed by a series of enzymes, including prohormone convertases PC1/3 and PC2, and that genetic insufficiency of both PC1/3 and POMC is strongly associated with obesity risk. However, whether PC1/3-mediated POMC processing is absolutely tied to body weight regulation is not known. To investigate this question, we generated a Pomc-CreER  T2; Pcsk1  lox/lox mouse model in which Pcsk1 is specifically and temporally knocked out in POMC-expressing cells of adult mice by injecting tamoxifen at eight weeks of age. We then measured the impact of Pcsk1 deletion on POMC cleavage to ACTH and α-MSH, and on body weight. In whole pituitary, POMC cleavage was significantly impacted by the loss of Pcsk1, while hypothalamic POMC-derived peptide levels remained similar in all genotypes. However, intact POMC levels were greatly elevated in Pomc-CreER  T2; Pcsk1  lox/lox mice. Males expressed two-fold greater levels of pituitary PC1/3 protein than females, consistent with their increased POMC cleavage. Past studies show that mice with germline removal of PC1/3 do not develop obesity, while mice expressing mutant PC1/3 forms do develop obesity. We conclude that obesity pathways are not disrupted by PC1/3 loss solely in POMC-expressing cells, further disfavoring the idea that alterations in POMC processing underlie obesity in PCSK1 deficiency.

Protein restriction during pregnancy impairs intra-islet GLP-1 and the expansion of β-cell mass

We evaluated whether protein restriction during pregnancy alters the morphometry of pancreatic islets, the intra-islet glucagon-like peptide-1 (GLP-1) production, and the anti-apoptotic signalling pathway modulated by GLP-1. Control non-pregnant (CNP) and control pregnant (CP) rats were fed a 17% protein diet, and low-protein non-pregnant (LPNP) and low-protein pregnant (LPP) groups were fed a 6% protein diet. The masses of islets and β-cells were similar in the LPNP group and the CNP group but were higher in the CP group than in the CNP group and were equal in the LPP group and the LPNP group. Both variables were lower in the LPP group than in the CP group. Prohormone convertase 2 and GLP-1 fluorescence in α-cells was lower in the low-protein groups than in the control groups. The least PC2/glucagon colocalization was observed in the LPP group, and the most was observed in the CP group. There was less prohormone convertase 1/3/glucagon colocalization in the LPP group than in the CP group. GLP-1/glucagon colocalization was similar in the LPP, CP and CNP groups, which showed less GLP-1/glucagon colocalization than the LPNP group. The mRNA Pka, Creb and Pdx-1 contents were higher in islets from pregnant rats than in islets from non-pregnant rats. Protein restriction during pregnancy impaired the mass of β-cells and the intra-islet GLP-1 production but did not interfere with the transcription of genes of the anti-apoptotic signalling pathway modulated by GLP-1.

Proglucagon-Derived Peptides Expression and Secretion in Rat Insulinoma INS-1 Cells

Rat insulinoma INS-1 cells are widely used to study insulin secretory mechanisms. Studies have shown that a population of INS-1 cells are bi-hormonal, co-expressing insulin, and proglucagon proteins. They coined this population as immature cells since they co-secrete proglucagon-derived peptides from the same secretory vesicles similar to that of insulin. Since proglucagon encodes multiple peptides including glucagon, glucagon-like-peptide-1 (GLP-1), GLP-2, oxyntomodulin, and glicentin, their specific expression and secretion are technically challenging. In this study, we aimed to focus on glucagon expression which shares the same amino acid sequence with glicentin and proglucagon. Validation of the anti-glucagon antibody (Abcam) by Western blotting techniques revealed that the antibody detects proglucagon (≈ 20 kDa), glicentin (≈ 9 kDa), and glucagon (≈ 3 kDa) in INS-1 cells and primary islets, all of which were absent in the kidney cell line (HEK293). Using the validated anti-glucagon antibody, we showed by immunofluorescence imaging that a population of INS-1 cells co-express insulin and proglucagon-derived proteins. Furthermore, we found that chronic treatment of INS-1 cells with high-glucose decreases insulin and glucagon content, and also reduces the percentage of bi-hormonal cells. In line with insulin secretion, we found glucagon and glicentin secretion to be induced in a glucose-dependent manner. We conclude that INS-1 cells are a useful model to study glucose-stimulated insulin secretion, but not that of glucagon or glicentin. Our study suggests Western blotting technique as an important tool for researchers to study proglucagon-derived peptides expression and regulation in primary islets in response to various metabolic stimuli.

The Long Noncoding RNA Paupar Modulates PAX6 Regulatory Activities to Promote Alpha Cell Development and Function

Many studies have highlighted the role of dysregulated glucagon secretion in the etiology of hyperglycemia and diabetes. Accordingly, understanding the mechanisms underlying pancreatic islet α cell development and function has important implications for the discovery of new therapies for diabetes. In this study, comparative transcriptome analyses between embryonic mouse pancreas and adult mouse islets identified several pancreatic lncRNAs that lie in close proximity to essential pancreatic transcription factors, including the Pax6-associated lncRNA Paupar. We demonstrate that Paupar is enriched in glucagon-producing α cells where it promotes the alternative splicing of Pax6 to an isoform required for activation of essential α cell genes. Consistently, deletion of Paupar in mice resulted in dysregulation of PAX6 α cell target genes and corresponding α cell dysfunction, including blunted glucagon secretion. These findings illustrate a distinct mechanism by which a pancreatic lncRNA can coordinate glucose homeostasis by cell-specific regulation of a broadly expressed transcription factor.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Glucagon Receptor Antagonist-Stimulated α-Cell Proliferation Is Severely Restricted With Advanced Age

Glucagon-containing α-cells potently regulate glucose homeostasis, but the developmental biology of α-cells in adults remains poorly understood. Although glucagon receptor antagonists (GRAs) have great potential as antidiabetic therapies, murine and human studies have raised concerns that GRAs might cause uncontrolled α-cell growth. Surprisingly, previous rodent GRA studies were only performed in young mice, implying that the potential impact of GRAs to drive α-cell expansion in adult patients is unclear. We assessed adaptive α-cell turnover and adaptive proliferation, administering a novel GRA (JNJ-46207382) to both young and aged mice. Basal α-cell proliferation rapidly declined soon after birth and continued to drop to very low levels in aged mice. GRA drove a 2.4-fold increase in α-cell proliferation in young mice. In contrast, GRA-induced α-cell proliferation was severely reduced in aged mice, although still present at 3.2-fold the very low basal rate of aged controls. To interrogate the lineage of GRA-induced α-cells, we sequentially administered thymidine analogs and quantified their incorporation into α-cells. Similar to previous studies of β-cells, α-cells only divided once in both basal and stimulated conditions. Lack of contribution from highly proliferative "transit-amplifying" cells supports a model whereby α-cells expand by self-renewal and not via specialized progenitors.

Chronic Exposure to Palmitate Impairs Insulin Signaling in an Intestinal L-cell Line: A Possible Shift from GLP-1 to Glucagon Production

Obesity and type 2 diabetes mellitus (T2DM) are characterized by insulin resistance and impaired glucagon-like peptide-1 (GLP-1) secretion/function. Lipotoxicity, a chronic elevation of free fatty acids in the blood, could affect insulin-signaling in many peripheral tissues. To date, the effects of lipotoxicity on the insulin receptor and insulin resistance in the intestinal L-cells need to be elucidated. Moreover, recent observations indicate that L-cells may be able to process not only GLP-1 but also glucagon from proglucagon. The aim of this study was to investigate the effects of chronic palmitate exposure on insulin pathways, GLP-1 secretion and glucagon synthesis in the GLUTag L-cell line. Cells were cultured in the presence/absence of palmitate (0.5 mM) for 24 h to mimic lipotoxicity. Palmitate treatment affected insulin-stimulated GLP-1 secretion, insulin receptor phosphorylation and IRS-1-AKT pathway signaling. In our model lipotoxicity induced extracellular signal-regulated kinase (ERK 44/42) activation both in insulin stimulated and basal conditions and also up-regulated paired box 6 (PAX6) and proglucagon expression (Gcg). Interestingly, palmitate treatment caused an increased glucagon secretion through the up-regulation of prohormone convertase 2. These results indicate that a state of insulin resistance could be responsible for secretory alterations in L-cells through the impairment of insulin-signaling pathways. Our data support the hypothesis that lipotoxicity might contribute to L-cell deregulation.

Islet prohormone processing in health and disease

Biosynthesis of peptide hormones by pancreatic islet endocrine cells is a tightly orchestrated process that is critical for metabolic homeostasis. Like neuroendocrine peptides, insulin and other islet hormones are first synthesized as larger precursor molecules that are processed to their mature secreted products through a series of proteolytic cleavages, mediated by the prohormone convertases Pc1/3 and Pc2, and carboxypeptidase E. Additional posttranslational modifications including C-terminal amidation of the β-cell peptide islet amyloid polypeptide (IAPP) by peptidyl-glycine α-amidating monooxygenase (Pam) may also occur. Genome-wide association studies (GWAS) have showed genetic linkage of these processing enzymes to obesity, β-cell dysfunction, and type 2 diabetes (T2D), pointing to their important roles in metabolism and blood glucose regulation. In both type 1 diabetes (T1D) and T2D, and in the face of metabolic or inflammatory stresses, islet prohormone processing may become impaired; indeed elevated proinsulin:insulin (PI:I) ratios are a hallmark of the β-cell dysfunction in T2D. Recent studies suggest that genetic or acquired defects in proIAPP processing may lead to the production and secretion of incompletely processed forms of proIAPP that could contribute to T2D pathogenesis, and additionally that impaired processing of both PI and proIAPP may be characteristic of β-cell dysfunction in T1D. In islet α-cells, the prohormone proglucagon is normally processed to bioactive glucagon by Pc2 but may express Pc1/3 under certain conditions leading to production of GLP-1(7-36_NH2 ). A better understanding of how β-cell processing of PI and proIAPP, as well as α-cell processing of proglucagon, are impacted by genetic susceptibility and in the face of diabetogenic stresses, may lead to new therapeutic approaches for improving islet function in diabetes.

MafB Is Critical for Glucagon Production and Secretion in Mouse Pancreatic α Cells In Vivo

The MafB transcription factor is expressed in pancreatic α and β cells during development but becomes exclusive to α cells in adult rodents. Mafb-null (Mafb^-/- ) mice were reported to have reduced α- and β-cell numbers throughout embryonic development. To further analyze the postnatal function of MafB in the pancreas, we generated endocrine cell-specific (Mafb^ΔEndo ) and tamoxifen-dependent (Mafb^ΔTAM ) Mafb knockout mice. Mafb^ΔEndo mice exhibited reduced populations of insulin-positive (insulin⁺) and glucagon⁺ cells at postnatal day 0, but the insulin⁺ cell population recovered by 8 weeks of age. In contrast, the Arx⁺ glucagon⁺ cell fraction and glucagon expression remained decreased even in adulthood. Mafb^ΔTAM mice, with Mafb deleted after pancreas maturation, also demonstrated diminished glucagon⁺ cells and glucagon content without affecting β cells. A decreased Arx⁺ glucagon⁺ cell population in Mafb^ΔEndo mice was compensated for by an increased Arx⁺ pancreatic polypeptide⁺ cell population. Furthermore, gene expression analyses from both Mafb^ΔEndo and Mafb^ΔTAM islets revealed that MafB is a key regulator of glucagon expression in α cells. Finally, both mutants failed to respond to arginine, likely due to impaired arginine transporter gene expression and glucagon production ability. Taken together, our findings reveal that MafB is critical for the functional maintenance of mouse α cells in vivo, including glucagon production and secretion, as well as in development.

Gut hormone polyagonists for the treatment of type 2 diabetes

Chemical derivatives of the gut-derived peptide hormone glucagon-like peptide 1 (GLP-1) are among the best-in-class pharmacotherapies to treat obesity and type 2 diabetes. However, GLP-1 analogs have modest weight lowering capacity, in the range of 5-10%, and the therapeutic window is hampered by dose-dependent side effects. Over the last few years, a new concept has emerged: combining the beneficial effects of several key metabolic hormones into a single molecular entity. Several unimolecular GLP-1-based polyagonists have shown superior metabolic action compared to GLP-1 monotherapies. In this review article, we highlight the history of polyagonists targeting the receptors for GLP-1, GIP and glucagon, and discuss recent progress in expanding of this concept to now allow targeted delivery of nuclear hormones via GLP-1 and other gut hormones, as a novel approach towards more personalized pharmacotherapies.

Pancreatic α Cell-Derived Glucagon-Related Peptides Are Required for β Cell Adaptation and Glucose Homeostasis

Pancreatic α cells may process proglucagon not only to glucagon but also to glucagon-like peptide-1 (GLP-1). However, the biological relevance of paracrine GLP-1 for β cell function remains unclear. We studied effects of locally derived insulin secretagogues on β cell function and glucose homeostasis using mice with α cell ablation and with α cell-specific GLP-1 deficiency. Normally, intestinal GLP-1 compensates for the lack of α cell-derived GLP-1. However, upon aging and metabolic stress, glucose tolerance is impaired. This was partly rescued with the DPP-4 inhibitor sitagliptin, but not with glucagon administration. In isolated islets from these mice, glucose-stimulated insulin secretion was heavily impaired and exogenous GLP-1 or glucagon rescued insulin secretion. These data highlight the importance of α cell-derived GLP-1 for glucose homeostasis during metabolic stress and may impact on the clinical use of systemic GLP-1 agonists versus stabilizing local α cell-derived GLP-1 by DPP-4 inhibitors in type 2 diabetes.

Functional analysis of PCSK2 coding variants: A founder effect in the Old Order Amish population

AIMS

In humans, noncoding variants of PCSK2, the gene encoding prohormone convertase 2 (PC2), have been previously associated with risk for and age of onset of type 2 diabetes (T2D). The aims of this study were to identify coding variants in PCSK2; to determine their possible association with glucose handling; and to determine functional outcomes for coding variants in biochemical studies.

METHODS

Exome-wide genotyping was performed on 1725 Old Order Amish (OOA) subjects. PCSK2 coding variants were tested for association with diabetes-related phenotypes. In vitro analyses using transfected human PC2-encoding constructs were performed to determine the impact of each mutation on PC2 activity.

RESULTS

We identified 10 rare missense coding variants in PCSK2 in various genomic databases. R430W (rs200711626) is greatly enriched in the OOA population (MAF 4.3%). This variant is almost twice as common (MAF 7.4%) in OOA individuals with T2D as in OOA individuals with normal or with normal/impaired glucose tolerance (MAF 3.9% and 2.9%, respectively; p=0.25 and p=0.10). In vitro experiments revealed a broadening of the pH optimum for the R430W variant, which may result in increased activity against PCSK2 substrates.

CONCLUSIONS

Although the association of the R430W variation with T2D in the OOA population did not reach significance, based upon the broadened pH profile of R430W PC2, we speculate that the presence of this substitution may result in altered processing of PCSK2 substrates, ultimately leading to increased conversion to diabetes.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Pancreatic α- and β-cellular clocks have distinct molecular properties and impact on islet hormone secretion and gene expression

Here, Petrenko et al. present the first integrative analysis of the molecular properties of circadian clocks in α and β pancreatic cells and provide new insights into the complex regulation of islet cell physiology at transcriptional and functional levels.

A critical role of circadian oscillators in orchestrating insulin secretion and islet gene transcription has been demonstrated recently. However, these studies focused on whole islets and did not explore the interplay between α-cell and β-cell clocks. We performed a parallel analysis of the molecular properties of α-cell and β-cell oscillators using a mouse model expressing three reporter genes: one labeling α cells, one specific for β cells, and a third monitoring circadian gene expression. Thus, phase entrainment properties, gene expression, and functional outputs of the α-cell and β-cell clockworks could be assessed in vivo and in vitro at the population and single-cell level. These experiments showed that α-cellular and β-cellular clocks are oscillating with distinct phases in vivo and in vitro. Diurnal transcriptome analysis in separated α and β cells revealed that a high number of genes with key roles in islet physiology, including regulators of glucose sensing and hormone secretion, are differentially expressed in these cell types. Moreover, temporal insulin and glucagon secretion exhibited distinct oscillatory profiles both in vivo and in vitro. Altogether, our data indicate that differential entrainment characteristics of circadian α-cell and β-cell clocks are an important feature in the temporal coordination of endocrine function and gene expression.

Visualization of glucagon secretion from pancreatic α cells by bioluminescence video microscopy: Identification of secretion sites in the intercellular contact regions

We have firstly visualized glucagon secretion using a method of video-rate bioluminescence imaging. The fusion protein of proglucagon and Gaussia luciferase (PGCG-GLase) was used as a reporter to detect glucagon secretion and was efficiently expressed in mouse pancreatic α cells (αTC1.6) using a preferred human codon-optimized gene. In the culture medium of the cells expressing PGCG-GLase, luminescence activity determined with a luminometer was increased with low glucose stimulation and KCl-induced depolarization, as observed for glucagon secretion. From immunochemical analyses, PGCG-GLase stably expressed in clonal αTC1.6 cells was correctly processed and released by secretory granules. Luminescence signals of the secreted PGCG-GLase from the stable cells were visualized by video-rate bioluminescence microscopy. The video images showed an increase in glucagon secretion from clustered cells in response to stimulation by KCl. The secretory events were observed frequently at the intercellular contact regions. Thus, the localization and frequency of glucagon secretion might be regulated by cell-cell adhesion.

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.

Alpha-, Delta- and PP-cells: Are They the Architectural Cornerstones of Islet Structure and Co-ordination?

Islet non-β-cells, the α- δ- and pancreatic polypeptide cells (PP-cells), are important components of islet architecture and intercellular communication. In α-cells, glucagon is found in electron-dense granules; granule exocytosis is calcium-dependent via P/Q-type Ca(2+)-channels, which may be clustered at designated cell membrane sites. Somatostatin-containing δ-cells are neuron-like, creating a network for intra-islet communication. Somatostatin 1-28 and 1-14 have a short bioactive half-life, suggesting inhibitory action via paracrine signaling. PP-cells are the most infrequent islet cell type. The embryologically separate ventral pancreas anlage contains PP-rich islets that are morphologically diffuse and α-cell deficient. Tissue samples taken from the head region are unlikely to be representative of the whole pancreas. PP has anorexic effects on gastro-intestinal function and alters insulin and glucagon secretion. Islet architecture is disrupted in rodent diabetic models, diabetic primates and human Type 1 and Type 2 diabetes, with an increased α-cell population and relocation of non-β-cells to central areas of the islet. In diabetes, the transdifferentiation of non-β-cells, with changes in hormone content, suggests plasticity of islet cells but cellular function may be compromised. Understanding how diabetes-related disordered islet structure influences intra-islet cellular communication could clarify how non-β-cells contribute to the control of islet function.

The FOXP1, FOXP2 and FOXP4 transcription factors are required for islet alpha cell proliferation and function in mice

AIMS/HYPOTHESIS

Several forkhead box (FOX) transcription factor family members have important roles in controlling pancreatic cell fates and maintaining beta cell mass and function, including FOXA1, FOXA2 and FOXM1. In this study we have examined the importance of FOXP1, FOXP2 and FOXP4 of the FOXP subfamily in islet cell development and function.

METHODS

Mice harbouring floxed alleles for Foxp1, Foxp2 and Foxp4 were crossed with pan-endocrine Pax6-Cre transgenic mice to generate single and compound Foxp mutant mice. Mice were monitored for changes in glucose tolerance by IPGTT, serum insulin and glucagon levels by radioimmunoassay, and endocrine cell development and proliferation by immunohistochemistry. Gene expression and glucose-stimulated hormone secretion experiments were performed with isolated islets.

RESULTS

Only the triple-compound Foxp1/2/4 conditional knockout (cKO) mutant had an overt islet phenotype, manifested physiologically by hypoglycaemia and hypoglucagonaemia. This resulted from the reduction in glucagon-secreting alpha cell mass and function. The proliferation of alpha cells was profoundly reduced in Foxp1/2/4 cKO islets through the effects on mediators of replication (i.e. decreased Ccna2, Ccnb1 and Ccnd2 activators, and increased Cdkn1a inhibitor). Adult islet Foxp1/2/4 cKO beta cells secrete insulin normally while the remaining alpha cells have impaired glucagon secretion.

CONCLUSIONS/INTERPRETATION

Collectively, these findings reveal an important role for the FOXP1, 2, and 4 proteins in governing postnatal alpha cell expansion and function.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease

The preproglucagon gene (Gcg) is expressed by specific enteroendocrine cells (L-cells) of the intestinal mucosa, pancreatic islet α-cells, and a discrete set of neurons within the nucleus of the solitary tract. Gcg encodes multiple peptides including glucagon, glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, and glicentin. Of these, glucagon and GLP-1 have received the most attention because of important roles in glucose metabolism, involvement in diabetes and other disorders, and application to therapeutics. The generally accepted model is that GLP-1 improves glucose homeostasis indirectly via stimulation of nutrient-induced insulin release and by reducing glucagon secretion. Yet the body of literature surrounding GLP-1 physiology reveals an incompletely understood and complex system that includes peripheral and central GLP-1 actions to regulate energy and glucose homeostasis. On the other hand, glucagon is established principally as a counterregulatory hormone, increasing in response to physiological challenges that threaten adequate blood glucose levels and driving glucose production to restore euglycemia. However, there also exists a potential role for glucagon in regulating energy expenditure that has recently been suggested in pharmacological studies. It is also becoming apparent that there is cross-talk between the proglucagon derived-peptides, e.g., GLP-1 inhibits glucagon secretion, and some additive or synergistic pharmacological interaction between GLP-1 and glucagon, e.g., dual glucagon/GLP-1 agonists cause more weight loss than single agonists. In this review, we discuss the physiological functions of both glucagon and GLP-1 by comparing and contrasting how these peptides function, variably in concert and opposition, to regulate glucose and energy homeostasis.

Foxa1 and Foxa2 regulate α-cell differentiation, glucagon biosynthesis, and secretion

The Forkhead box A transcription factors are major regulators of glucose homeostasis. They show both distinct and redundant roles during pancreas development and in adult mouse β-cells. In vivo ablation studies have revealed critical implications of Foxa1 on glucagon biosynthesis and requirement of Foxa2 in α-cell terminal differentiation. In order to examine the respective role of these factors in mature α-cells, we used small interfering RNA (siRNA) directed against Foxa1 and Foxa2 in rat primary pancreatic α-cells and rodent α-cell lines leading to marked decreases in Foxa1 and Foxa2 mRNA levels and proteins. Both Foxa1 and Foxa2 control glucagon gene expression specifically through the G2 element. Although we found that Foxa2 controls the expression of the glucagon, MafB, Pou3f4, Pcsk2, Nkx2.2, Kir6.2, and Sur1 genes, Foxa1 only regulates glucagon gene expression. Interestingly, the Isl1 and Gipr genes were not controlled by either Foxa1 or Foxa2 alone but by their combination. Foxa1 and Foxa2 directly activate and bind the promoter region the Nkx2.2, Kir6.2 and Sur1, Gipr, Isl1, and Pou3f4 genes. We also demonstrated that glucagon secretion is affected by the combined effects of Foxa1 and Foxa2 but not by either one alone. Our results indicate that Foxa1 and Foxa2 control glucagon biosynthesis and secretion as well as α-cell differentiation with both common and unique target genes.

Action and therapeutic potential of oxyntomodulin

Oxyntomodulin (OXM) is a peptide hormone released from the gut in post-prandial state that activates both the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR) resulting in superior body weight lowering to selective GLP1R agonists. OXM reduces food intake and increases energy expenditure in humans. While activation of the GCGR increases glucose production posing a hyperglycemic risk, the simultaneous activation of the GLP1R counteracts this effect. Acute OXM infusion improves glucose tolerance in T2DM patients making dual agonists of the GCGR and GLP1R new promising treatments for diabetes and obesity with the potential for weight loss and glucose lowering superior to that of GLP1R agonists.

Two dipolar α-helices within hormone-encoding regions of proglucagon are sorting signals to the regulated secretory pathway

Proglucagon is expressed in pancreatic α cells, intestinal L cells, and some hypothalamic and brainstem neurons. Tissue-specific processing of proglucagon yields three major peptide hormones as follows: glucagon in the α cells and glucagon-like peptides (GLP)-1 and -2 in the L cells and neurons. Efficient sorting and packaging into the secretory granules of the regulated secretory pathway in each cell type are required for nutrient-regulated secretion of these proglucagon-derived peptides. Our previous work suggested that proglucagon is directed into granules by intrinsic sorting signals after initial processing to glicentin and major proglucagon fragment (McGirr, R., Guizzetti, L., and Dhanvantari, S. (2013) J. Endocrinol. 217, 229-240), leading to the hypothesis that sorting signals may be present in multiple domains. In the present study, we show that the α-helices within glucagon and GLP-1, but not GLP-2, act as sorting signals by efficiently directing a heterologous secretory protein to the regulated secretory pathway. Biophysical characterization of these peptides revealed that glucagon and GLP-1 each encode a nonamphipathic, dipolar α-helix, whereas the helix in GLP-2 is not dipolar. Surprisingly, glicentin and major proglucagon fragment were sorted with different efficiencies, thus providing evidence that proglucagon is first sorted to granules prior to processing. In contrast to many other prohormones in which sorting is directed by ordered prodomains, the sorting determinants of proglucagon lie within the ordered hormone domains of glucagon and GLP-1, illustrating that each prohormone has its own sorting "signature."

Nutrient regulation of glucagon secretion: involvement in metabolism and diabetes

Glucose homeostasis is precisely regulated by glucagon and insulin, which are released by pancreatic α- and β-cells, respectively. While β-cells have been the focus of intense research, less is known about α-cell function and the actions of glucagon. In recent years, the study of this endocrine cell type has experienced a renewed drive. The present review contains a summary of established concepts as well as new information about the regulation of α-cells by glucose, amino acids, fatty acids and other nutrients, focusing especially on glucagon release, glucagon synthesis and α-cell survival. We have also discussed the role of glucagon in glucose homeostasis and in energy and lipid metabolism as well as its potential as a modulator of food intake and body weight. In addition to the well-established action on the liver, we discuss the effects of glucagon in other organs, where the glucagon receptor is expressed. These tissues include the heart, kidneys, adipose tissue, brain, small intestine and the gustatory epithelium. Alterations in α-cell function and abnormal glucagon concentrations are present in diabetes and are thought to aggravate the hyperglycaemic state of diabetic patients. In this respect, several experimental approaches in diabetic models have shown important beneficial results in improving hyperglycaemia after the modulation of glucagon secretion or action. Moreover, glucagon receptor agonism has also been used as a therapeutic strategy to treat obesity.

Islets of Langerhans from prohormone convertase-2 knockout mice show α-cell hyperplasia and tumorigenesis with elevated α-cell neogenesis

Antagonism of the effects of glucagon as an adjunct therapy with other glucose-lowering drugs in the chronic treatment of diabetes has been suggested to aggressively control blood glucose levels. Antagonism of glucagon effects, by targeting glucagon secretion or disabling the glucagon receptor, is associated with α-cell hyperplasia. We evaluated the influence of total glucagon withdrawal on islets of Langerhans using prohormone convertase-2 knockout mice (PC2-ko), in which α-cell hyperplasia is present from a young age and persists throughout life, in order to understand whether or not sustained glucagon deficit would lead to islet tumorigenesis. PC2-ko and wild-type (WT) mice were maintained drug-free, and cohorts of these groups sampled at 3, 12 and 18 months for plasma biochemical and morphological (histological, immunohistochemical, electron microscopical and image analytical) assessments. WT mice showed no islet tumours up to termination of the study, but PC2-ko animals displayed marked changes in islet morphology from α-cell hypertrophy/hyperplasia/atypical hyperplasia, to adenomas and carcinomas, these latter being first encountered at 6-8 months. Islet hyperplasias and tumours primarily consisted of α-cells associated to varying degrees with other islet endocrine cell types. In addition to substantial increases in islet neoplasia, increased α-cell neogenesis associated primarily with pancreatic duct(ule)s was present. We conclude that absolute blockade of the glucagon signal results in tumorigenesis and that the PC2-ko mouse represents a valuable model for investigation of islet tumours and pancreatic ductal neogenesis.

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.

α-Cells are dispensable in postnatal morphogenesis and maturation of mouse pancreatic islets

Glucagon-producing α-cells are the second-most abundant cell type in the islet. Whereas α-cells make up less than 20% of the cells in a mature mouse islet, they occupy a much larger proportion of the pancreatic endocrine cell population during the early postnatal period, the time when morphological and functional maturation occurs to form adult islets. To determine whether α-cells have a role in postnatal islet development, a diphtheria toxin-mediated α-cell ablation mouse model was established. Rapid and persistent depletion of α-cells was achieved by daily injection of the toxin for 2 wk starting at postnatal day 1 (P1). Total pancreatic glucagon content in the α-cell-ablated mice was undetectable at P14 and still less than 0.3% of that of the control mice at 4 mo of age. Histological analyses revealed that formation of spherical islets occurred normally, and the islet size distribution was not changed despite the near-total lack of α-cells. Furthermore, there were no differences in expression of β-cell maturation marker proteins, including urocortin 3 and glucose transporter 2, in the α-cell-ablated islets at P14. Mice lacking α-cells grew normally and appeared healthy. Both glucose and insulin tolerance tests demonstrated that the α-cell-ablated mice had normal glucose homeostasis. These results indicate that α-cells do not play a critical role in postnatal islet morphogenesis or functional maturation of β-cells.

The multifaceted proprotein convertases: their unique, redundant, complementary, and opposite functions

The secretory proprotein convertase (PC) family comprises nine members: PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9. The first seven PCs cleave their substrates at single or paired basic residues, and SKI-1/S1P cleaves its substrates at non-basic residues in the Golgi. PCSK9 cleaves itself once, and the secreted inactive protease escorts specific receptors for lysosomal degradation. It regulates the levels of circulating LDL cholesterol and is considered a major therapeutic target in phase III clinical trials. In vivo, PCs exhibit unique and often essential functions during development and/or in adulthood, but certain convertases also exhibit complementary, redundant, or opposite functions.

Identification of optimal reference genes for RT-qPCR in the rat hypothalamus and intestine for the study of obesity

BACKGROUND

Obesity has a complicated metabolic pathology, and defining the underlying mechanisms of obesity requires integrative studies with molecular end points. Real-time quantitative PCR (RT-qPCR) is a powerful tool that has been widely utilized. However, the importance of using carefully validated reference genes in RT-qPCR seems to have been overlooked in obesity-related research. The objective of this study was to select a set of reference genes with stable expressions to be used for RT-qPCR normalization in rats under fasted vs re-fed and chow vs high-fat diet (HFD) conditions.

DESIGN

Male long-Evans rats were treated under four conditions: chow/fasted, chow/re-fed, HFD/fasted and HFD/re-fed. Expression stabilities of 13 candidate reference genes were evaluated in the rat hypothalamus, duodenum, jejunum and ileum using the ReFinder software program. The optimal number of reference genes needed for RT-qPCR analyses was determined using geNorm.

RESULTS

Using geNorm analysis, we found that it was sufficient to use the two most stably expressed genes as references in RT-qPCR analyses for each tissue under specific experimental conditions. B2M and RPLP0 in the hypothalamus, RPS18 and HMBS in the duodenum, RPLP2 and RPLP0 in the jejunum and RPS18 and YWHAZ in the ileum were the most suitable pairs for a normalization study when the four aforementioned experimental conditions were considered.

CONCLUSIONS

Our study demonstrates that gene expression levels of reference genes commonly used in obesity-related studies, such as ACTB or RPS18, are altered by changes in acute or chronic energy status. These findings underline the importance of using reference genes that are stable in expression across experimental conditions when studying the rat hypothalamus and intestine, because these tissues have an integral role in the regulation of energy homeostasis. It is our hope that this study will raise awareness among obesity researchers on the essential need for reference gene validation in gene expression studies.

The sorting of proglucagon to secretory granules is mediated by carboxypeptidase E and intrinsic sorting signals

Proglucagon is expressed in pancreatic alpha cells, intestinal L cells and brainstem neurons. Tissue-specific processing of proglucagon yields the peptide hormones glucagon in the alpha cell and glucagon-like peptide (GLP)-1 and GLP-2 in L cells. Both glucagon and GLP-1 are secreted in response to nutritional status and are critical for regulating glycaemia. The sorting of proglucagon to the dense-core secretory granules of the regulated secretory pathway is essential for the appropriate secretion of glucagon and GLP-1. We examined the roles of carboxypeptidase E (CPE), a prohormone sorting receptor, the processing enzymes PC1/3 and PC2 and putative intrinsic sorting signals in proglucagon sorting. In Neuro 2a cells that lacked CPE, PC1/3 and PC2, proglucagon co-localised with the Golgi marker p115 as determined by quantitative immunofluorescence microscopy. Expression of CPE, but not of PC1/3 or PC2, enhanced proglucagon sorting to granules. siRNA-mediated knockdown of CPE disrupted regulated secretion of glucagon from pancreatic-derived alphaTC1-6 cells, but not of GLP-1 from intestinal cell-derived GLUTag cells. Mutation of the PC cleavage site K70R71, the dibasic R17R18 site within glucagon or the alpha-helix of glucagon, all significantly affected the sub-cellular localisation of proglucagon. Protein modelling revealed that alpha helices corresponding to glucagon, GLP-1 and GLP-2, are arranged within a disordered structure, suggesting some flexibility in the sorting mechanism. We conclude that there are multiple mechanisms for sorting proglucagon to the regulated secretory pathway, including a role for CPE in pancreatic alpha cells, initial cleavage at K70R71 and multiple sorting signals.

Alpha cells come of age

The alpha cells that coinhabit the islets with the insulin-producing beta cells have recently captured the attention of diabetes researchers because of new breakthrough findings highlighting the importance of these cells in the maintenance of beta cell health and functions. In normal physiological conditions alpha cells produce glucagon but in conditions of beta cell injury they also produce glucagon-like peptide-1 (GLP-1), a growth and survival factor for beta cells. In this review we consider these new findings on the functions of alpha cells. Alpha cells remain somewhat enigmatic inasmuch as they now appear to be important in the maintenance of the health of beta cells, but their production of glucagon promotes diabetes. This circumstance prompts an examination of approaches to coax alpha cells to produce GLP-1 instead of glucagon.

Unraveling oxyntomodulin, GLP1's enigmatic brother

Oxyntomodulin (OXM) is a peptide secreted from the L cells of the gut following nutrient ingestion. OXM is a dual agonist of the glucagon-like peptide-1 receptor (GLP1R) and the glucagon receptor (GCGR) combining the effects of GLP1 and glucagon to act as a potentially more effective treatment for obesity than GLP1R agonists. Injections of OXM in humans cause a significant reduction in weight and appetite, as well as an increase in energy expenditure. Activation of GCGR is classically associated with an elevation in glucose levels, which would be deleterious in patients with T2DM, but the antidiabetic properties of GLP1R agonism would be expected to counteract this effect. Indeed, OXM administration improved glucose tolerance in diet-induced obese mice. Thus, dual agonists of the GCGR and GLP1R represent a new therapeutic approach for diabetes and obesity with the potential for enhanced weight loss and improvement in glycemic control beyond those of GLP1R agonists.

Impact of glucagon-like peptide-1 (7-36) amide, isosteviol and 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside on leucine-mediated α-cell dysfunction

AIM

To investigate the acute and chronic effects of l-leucine on pancreatic α-cell function in vitro. Furthermore, we wanted to explore if glucagon-like peptide-1 (GLP-1), isosteviol (ISV) and 5-aminoimidazole-4-carboxamide 1-β-d-ribofuranoside (AICAR) counteract changes in α-cell function induced by chronic exposure to leucine.

METHODS

Isolated mice islets were incubated with 10 mM leucine for 2 or 72 h. We investigated glucagon and insulin secretion at 2 mM and 16.7 mM glucose. In addition, we cultured clonal α-TC1-6 cells with 5 mM leucine, 5 mM leucine plus GLP-1 (10(-6) M), or ISV (10(-6) M) or AICAR (10(-5) M) at high glucose for 72 h. We measured the glucagon secretion, cholesterol (CHO) and triglyceride (TG) content, cell proliferation as well as gene expression.

RESULTS

Ten millimolar of leucine for 2 h significantly stimulated glucagon and insulin secretion both at 2 and 16.7 mM glucose in mice islets. After 72 h incubation with 10 mM leucine the glucagon secretion was enhanced at both 2 and 16.7 mM glucose, whereas the glucose-stimulated insulin secretion (16.7 mM glucose) was inhibited. Chronic exposure to 5 mM leucine increased glucagon secretion, CHO and TG content, cell proliferation and Pcsk2 (p < 0.001), MafB (p < 0.05), Gcg (p < 0.001), Prkaa1 (p < 0.01), Hmgcr (p < 0.001), Srebf2 (p < 0.001), Acaca (p < 0.001), Mtor (p < 0.05) mRNA expression in clonal α-TC1-6 cells. While GLP-1 was cable of reducing glucagon hypersecretion and Pcsk2 (p < 0.05) mRNA expression. ISV and AICAR had no effect on leucine-induced glucagon hypersecretion.

CONCLUSIONS

Long-term exposure to leucine induces hypersecretion of glucagon secretion, that is, aminoacidotoxicity and influences some key genes of pancreatic α-cells. Interestingly, GLP-1 counteracts the leucine-induced α-cell dysfunction.

Phenotype of entero-endocrine L cells becomes restricted during development

BACKGROUND

Glucagon-like peptide (GLP)-1 and glucose-dependent insulinotropic polypeptide (GIP) are hormones secreted by L and K cells, respectively, and by LK cells. To characterize L and K cells during development, we examined ileum from embryonic (e)- 12 to e-17.

RESULTS

GLP-1 cells were first seen at e-15 and their number increased at e-17. At e-17, most GLP-1 cells co-expressed GIP. The transcription factors Pax6 and Pdx-1 are required for GIP expression, while Pax6 activates the expression of GLP-1. At e-17, the mucosa has GIP+ Pax6+, GIP+ Pdx-1+, GLP-1+ Pax6+, and GLP-1+ Pdx-1+ cells. Unlike ileal L cells of postnatal and adult mice, a subset of ileal L cells of e-17 embryos co-expressed GLP-1 and glucagon (Glu). Glu-positive cells contain proprotein-convertase 2 (PC2) and PC3/1, the enzymes responsible for Glu and GLP-1 synthesis, respectively.

CONCLUSIONS

Our findings indicate that most GLP-1+ cells of ileum of e-17 embryos co-express GIP and, therefore, are LK cells. In addition, a subset of GLP-1+ cells of embryos but not of neonates co-express glucagon, indicating that the expression of Glu in GLP-1+ cells disappears after birth.

The biology and therapeutic targeting of the proprotein convertases

The mammalian proprotein convertases constitute a family of nine secretory serine proteases that are related to bacterial subtilisin and yeast kexin. Seven of these (proprotein convertase 1 (PC1), PC2, furin, PC4, PC5, paired basic amino acid cleaving enzyme 4 (PACE4) and PC7) activate cellular and pathogenic precursor proteins by cleavage at single or paired basic residues, whereas subtilisin kexin isozyme 1 (SKI-1) and proprotein convertase subtilisin kexin 9 (PCSK9) regulate cholesterol and/or lipid homeostasis via cleavage at non-basic residues or through induced degradation of receptors. Proprotein convertases are now considered to be attractive targets for the development of powerful novel therapeutics. In this Review, we summarize the physiological functions and pathological implications of the proprotein convertases, and discuss proposed strategies to control some of their activities, including their therapeutic application and validation in selected disease states.

The potential role of glutamate in the current diabetes epidemic

In the present article, we propose the perspective that abnormal glutamate homeostasis might contribute to diabetes pathogenesis. Previous reports and our recent data indicate that chronically high extracellular glutamate levels exert direct and indirect effects that might participate in the progressive loss of β-cells occurring in both T1D and T2D. In addition, abnormal glutamate homeostasis may impact all the three accelerators of the "accelerator hypothesis" and could partially explain the rising frequency of T1D and T2D.

α-cell role in β-cell generation and regeneration

This review considers the role of α-cells in β-cell generation and regeneration. We present recent evidence obtained from lineage-tracing studies showing that α-cells can serve as progenitors of β-cells and present a hypothetical model how injured β-cells might activate α-cells in adult islets to promote β-cell regeneration. β-cells appear to arise by way of their trans-differentiation from undifferentiated α progenitor cells, pro-α-cells, both during embryonic development of the islets and in the adult pancreas in response to β-cell injuries. Plasticity of α-cells is endowed by the expression of the gene encoding proglucagon, a prohormone that can give rise to glucagon and glucagon-like peptides (GLPs). The production of glucagon from proglucagon is characteristic of fully-differentiated α-cells whereas GLP-1 is a product of undifferentiated α-cells. GLP-1, a cell growth and survival factor, is proposed to promote the expansion of neurogenin3-expressing, undifferentiated pro-α-cells during development. β-cells arise from pro-α-cells by a change in the relative amounts of the transcription factors Arx and Pax4, master regulators of the α- and β-cell lineages, respectively. A paracrine/autocrine model is proposed whereby injuries of β-cells in adult islets induce the production and release of factors, such as stromal cell-derived factor-1, that cause the de-differentiation of adjacent α-cells into pro-α-cells. Pro-α-cells produce GLP-1 and its receptor that renders them competent to trans-differentiate into β-cells. The trans-differentiation of pro-α-cells into β-cells provides a potentially exploitable mechanism for the regeneration of β-cells in individuals with type 1 diabetes.

Isosteviol has beneficial effects on palmitate-induced α-cell dysfunction and gene expression

BACKGROUND

Long-term exposure to high levels of fatty acids impairs insulin secretion and exaggerates glucagon secretion. The aim of this study was to explore if the antihyperglycemic agent, Isosteviol (ISV), is able to counteract palmitate-induced α-cell dysfunction and to influence α-cell gene expression.

METHODOLOGY/PRINCIPAL FINDINGS

Long-term incubation studies with clonal α-TC1-6 cells were performed in the presence of 0.5 mM palmitate with or without ISV. We investigated effects on glucagon secretion, glucagon content, cellular triglyceride (TG) content, cell proliferation, and expression of genes involved in controlling glucagon synthesis, fatty acid metabolism, and insulin signal transduction. Furthermore, we studied effects of ISV on palmitate-induced glucagon secretion from isolated mouse islets. Culturing α-cells for 72-h with 0.5 mM palmitate in the presence of 18 mM glucose resulted in a 56% (p<0.01) increase in glucagon secretion. Concomitantly, the TG content of α-cells increased by 78% (p<0.01) and cell proliferation decreased by 19% (p<0.05). At 18 mM glucose, ISV (10(-8) and 10(-6) M) reduced palmitate-stimulated glucagon release by 27% (p<0.05) and 27% (p<0.05), respectively. ISV (10(-6) M) also counteracted the palmitate-induced hypersecretion of glucagon in mouse islets. ISV (10(-6) M) reduced α-TC1-6 cell proliferation rate by 25% (p<0.05), but ISV (10(-8) and 10(-6) M) had no effect on TG content in the presence of palmitate. Palmitate (0.5 mM) increased Pcsk2 (p<0.001), Irs2 (p<0.001), Fasn (p<0.001), Srebf2 (p<0.001), Acaca (p<0.01), Pax6 (p<0.05) and Gcg mRNA expression (p<0.05). ISV significantly (p<0.05) up-regulated Insr, Irs1, Irs2, Pik3r1 and Akt1 gene expression in the presence of palmitate.

CONCLUSIONS/SIGNIFICANCE

ISV counteracts α-cell hypersecretion and apparently contributes to changes in expression of key genes resulting from long-term exposure to palmitate. ISV apparently acts as a glucagonostatic drug with potential as a new anti-diabetic drug for the treatment of type 2 diabetes.

Disruption of proprotein convertase 1/3 (PC1/3) expression in mice causes innate immune defects and uncontrolled cytokine secretion

The proprotein convertase 1/3 is expressed in the regulated secretory pathway of neural and endocrine cells. Its major function is in the post-translational processing and activation of precursor proteins. The PC1/3 knock-out (KO) mouse model has allowed us to elucidate its physiological functions in studies focused primarily on neuroendocrine tissues. However, PC1/3 is also expressed in cells of the immune system, mainly in macrophages. The present study explores the effects of innate immune challenge in the PC1/3 KO mouse. PC1/3 KO mice have an enlarged spleen with marked disorganization of the marginal zone and red pulp. Immunohistochemical studies using various markers demonstrate a depletion of dendritic cells in PC1/3 KO spleens. When challenged with lipopolysaccharide, PC1/3 KO mice are more susceptible to septic shock than wild-type controls or other PC KO mice, such as PC2 and PC7 null mice. Plasma levels of proinflammatory cytokines (IL-6, IL-1β, and TNF-α) were very significantly elevated in PC1/3 KO mice, consistent with a hypercytokinemia, i.e. indicative of a major systemic uncontrolled inflammatory response or cytokine storm. Peritoneal macrophages isolated from PC1/3 KO mice also demonstrate elevated cytokine secretion when treated with LPS. Electron micrographs show morphological features indicating a prolonged activation of these cells following LPS stimulation. We also present evidence that the proinflammatory T(h)1 pathway is dominant in the PC1/3 KO mouse model. We conclude that aside from its important role in neuroendocrine functions PC1/3 also has an important role in the regulation of the innate immune system, most likely through the regulation of cytokine secretion in macrophages.

Characterization of a novel functional protein in the pancreatic islet: islet homeostasis protein regulation of glucagon synthesis in α cells

OBJECTIVE

We have identified a novel protein in bone marrow-derived insulin-producing cells. Here we characterize this protein, hereby named islet homeostasis protein (IHoP), in the pancreatic islet.

METHODS

Detection of IHoP mRNA and protein was performed using reverse transcriptase-polymerase chain reaction, immunocytochemistry, and in situ hybridization. Islet homeostasis protein functions were utilizing proliferation, insulin secretion by in vitro assays, and following small interfering RNA protocols for suppression of IHoP.

RESULTS

We found that IHoP did not homolog with known pancreatic hormones. Islet homeostasis protein expression was seen in both bone marrow-derived insulin-producing cells and isolated pancreatic islets. Immunohistochemistry on pancreatic islet revealed that IHoP localized to the glucagon-synthesizing α cells. Inhibition of IHoP by small interfering RNA resulted in the loss of glucagon expression, which induced low blood glucose levels (63-85 mg/dL). Subsequently, cellular apoptosis was observed throughout the islet, including the insulin-producing β cells. Islets of preonset diabetic patients showed normal expression of IHoP and glucagon; however, IHoP was lost upon onset of the disease.

CONCLUSIONS

These data suggest that IHoP could be a new functional protein in the islet and may play a role in islet homeostasis.

Remodeling of hepatic metabolism and hyperaminoacidemia in mice deficient in proglucagon-derived peptides

Glucagon is believed to be one of the most important peptides for upregulating blood glucose levels. However, homozygous glucagon-green fluorescent protein (gfp) knock-in mice (Gcg(gfp/gfp): GCGKO) are normoglycemic despite the absence of proglucagon-derived peptides, including glucagon. To characterize metabolism in the GCGKO mice, we analyzed gene expression and metabolome in the liver. The expression of genes encoding rate-limiting enzymes for gluconeogenesis was only marginally altered. On the other hand, genes encoding enzymes involved in conversion of amino acids to metabolites available for the tricarboxylic acid cycle and/or gluconeogenesis showed lower expression in the GCGKO liver. The expression of genes involved in the metabolism of fatty acids and nicotinamide was also altered. Concentrations of the metabolites in the GCGKO liver were altered in manners concordant with alteration in the gene expression patterns, and the plasma concentrations of amino acids were elevated in the GCGKO mice. The insulin concentration in serum and phosphorylation of Akt protein kinase in liver were reduced in GCGKO mice. These results indicated that proglucagon-derived peptides should play important roles in regulating various metabolic pathways, especially that of amino acids. Serum insulin concentration is lowered to compensate the impacts of absent proglucagon-derived peptide on glucose metabolism. On the other hand, impacts on other metabolic pathways are only partially compensated by reduced insulin action.