Dynamic modulation of prohormone convertase 2 (PC2)-mediated precursor processing by 7B2 protein: preferential effect on glucagon synthesis

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.

FAM3D inhibits glucagon secretion via MKP1-dependent suppression of ERK1/2 signaling

Dysregulated glucagon secretion is a hallmark of type 2 diabetes (T2D). To date, few effective therapeutic agents target on deranged glucagon secretion. Family with sequence similarity 3 member D (FAM3D) is a novel gut-derived cytokine-like protein, and its secretion timing is contrary to that of glucagon. However, the roles of FAM3D in metabolic disorder and its biological functions are largely unknown. In the present study, we investigated whether FAM3D modulates glucagon production in mouse pancreatic alpha TC1 clone 6 (αTC1-6) cells. Glucagon secretion, prohormone convertase 2 (PC2) activity, and mitogen-activated protein kinase (MAPK) pathway were assessed. Exogenous FAM3D inhibited glucagon secretion, PC2 activity, as well as extracellular-regulated protein kinase 1/2 (ERK1/2) signaling and induced MAPK phosphatase 1 (MKP1) expression. Moreover, knockdown of MKP1 and inhibition of ERK1/2 abolished and potentiated the inhibitory effect of FAM3D on glucagon secretion, respectively. Taken together, FAM3D inhibits glucagon secretion via MKP1-dependent suppression of ERK1/2 signaling. These results provide rationale for developing the therapeutic potential of FAM3D for dysregulated glucagon secretion and T2D.

Pax6 Inactivation in the Adult Pancreas Reveals Ghrelin as Endocrine Cell Maturation Marker

The transcription factor Pax6 is an important regulator of development and cell differentiation in various organs. Thus, Pax6 was shown to promote neural development in the cerebral cortex and spinal cord, and to control pancreatic endocrine cell genesis. However, the role of Pax6 in distinct endocrine cells of the adult pancreas has not been addressed. We report the conditional inactivation of Pax6 in insulin and glucagon producing cells of the adult mouse pancreas. In the absence of Pax6, beta- and alpha-cells lose their molecular maturation characteristics. Our findings provide strong evidence that Pax6 is responsible for the maturation of beta-, and alpha-cells, but not of delta-, and PP-cells. Moreover, lineage-tracing experiments demonstrate that Pax6-deficient beta- and alpha-cells are shunted towards ghrelin marked cells, sustaining the idea that ghrelin may represent a marker for endocrine cell maturation.

Phosphorylation and Alternative Splicing of 7B2 Reduce Prohormone Convertase 2 Activation

FAM20C is a secretory kinase responsible for the phosphorylation of multiple secreted proteins in mammalian cells; it has been shown to phosphorylate serine residues within a variety of different bone proteins. In this work we demonstrate that FAM20C also phosphorylates threonines, specifically those within the N-terminal domain of the neuroendocrine chaperone 7B2. Analysis of the primary sequence of 7B2 revealed that three threonine residues in its N-terminal domain are located within FAM20C consensus motifs: Thr73, Thr99, and Thr111. The individual substitution of Thr73 and Thr111 residues by neutral alanines caused a marked decrease in the total phosphorylation of 7B2. Furthermore, the phosphomimetic substitution of Thr111 by Glu clearly diminished the ability of 7B2 to activate pro-prohormone convertase 2 (PC2) in 7B2-lacking SK-N-MC neuroblastoma cells, suggesting that the phosphorylation of this residue critically impacts the 7B2-proPC2 interaction. However, the phosphomimetic mutation did not alter 7B2's ability to function as an antiaggregant for human islet amyloid polypeptide. FAM20C-mediated phosphorylation of a common alternatively spliced variant of human 7B2 that lacks Ala100 (thus eliminating the Thr99 phosphorylation consensus site) was similar to the Ala-containing protein, but this variant did not activate proPC2 as efficiently as the Ala-containing protein. Although threonines within 7B2 were phosphorylated efficiently, FAM20C was incapable of performing the well-known regulatory threonine phosphorylation of the molecular chaperone binding immunoglobulin protein. Taken together, these results indicate that FAM20C plays a role in 7B2-mediated proPC2 activation by phosphorylating residue Thr111; and that 7B2 function is regulated by alternative splicing.

Biochemical and cell biological properties of the human prohormone convertase 1/3 Ser357Gly mutation: a PC1/3 hypermorph

Satiety and appetite signaling are accomplished by circulating peptide hormones. These peptide hormones require processing from larger precursors to become bioactive, often by the proprotein convertase 1/3 (PC1/3). Several subcellular maturation steps are necessary for PC1/3 to achieve its optimal enzymatic activity. Certain PC1/3 variants found in the general population slightly attenuate its enzymatic activity and are associated with obesity and diabetes. However, mutations that increase PC1/3 activity and/or affect its specificity could also have physiological consequences. We here present data showing that the known human Ser357Gly PC1/3 mutant (PC1/3(S357G)) represents a PC1/3 hypermorph. Conditioned media from human embryonic kidney-293 cells transfected with PC1/3(WT) and PC1/3(S357G) were collected and enzymatic activity characterized. PC1/3(S357G) exhibited a lower calcium dependence; a higher pH optimum (neutral); and a higher resistance to peptide inhibitors than the wild-type enzyme. PC1/3(S357G) exhibited increased cleavage to the C-terminally truncated form, and kinetic parameters of the full-length and truncated mutant enzymes were also altered. Lastly, the S357G mutation broadened the specificity of the enzyme; we detected PC2-like specificity on the substrate proCART, the precursor of the cocaine- and amphetamine regulated transcript neuropeptide known to be associated with obesity. The production of another anorexigenic peptide normally synthesized only by PC2, αMSH, was increased when proopiomelanocortin was coexpressed with PC1/3(S357G). Considering the aberrant enzymatic profile of PC1/3(S357G), we hypothesize that this enzyme possesses unusual processing activity that may significantly change the profile of circulating peptide hormones.

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.

Hexa-D-arginine treatment increases 7B2•PC2 activity in hyp-mouse osteoblasts and rescues the HYP phenotype

Inactivating mutations of the "phosphate regulating gene with homologies to endopeptidases on the X chromosome" (PHEX/Phex) underlie disease in patients with X-linked hypophosphatemia (XLH) and the hyp-mouse, a murine homologue of the human disorder. Although increased serum fibroblast growth factor 23 (FGF-23) underlies the HYP phenotype, the mechanism(s) by which PHEX mutations inhibit FGF-23 degradation and/or enhance production remains unknown. Here we show that treatment of wild-type mice with the proprotein convertase (PC) inhibitor, decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone (Dec), increases serum FGF-23 and produces the HYP phenotype. Because PC2 is uniquely colocalized with PHEX in osteoblasts/bone, we examined if PC2 regulates PHEX-dependent FGF-23 cleavage and production. Transfection of murine osteoblasts with PC2 and its chaperone protein 7B2 cleaved FGF-23, whereas Signe1 (7B2) RNA interference (RNAi) transfection, which limited 7B2 protein production, decreased FGF-23 degradation and increased Fgf-23 mRNA and protein. The mechanism by which decreased 7B2•PC2 activity influences Fgf-23 mRNA was linked to reduced conversion of the precursor to bone morphogenetic protein 1 (proBMP1) to active BMP1, which resulted in limited cleavage of dentin matrix acidic phosphoprotein 1 (DMP1), and consequent increased Fgf-23 mRNA. The significance of decreased 7B2•PC2 activity in XLH was confirmed by studies of hyp-mouse bone, which revealed significantly decreased Sgne1 (7B2) mRNA and 7B2 protein, and limited cleavage of proPC2 to active PC2. The expected downstream effects of these changes included decreased FGF-23 cleavage and increased FGF-23 synthesis, secondary to decreased BMP1-mediated degradation of DMP1. Subsequent Hexa-D-Arginine treatment of hyp-mice enhanced bone 7B2•PC2 activity, normalized FGF-23 degradation and production, and rescued the HYP phenotype. These data suggest that decreased PHEX-dependent 7B2•PC2 activity is central to the pathogenesis of XLH.

The neuroendocrine protein 7B2 suppresses the aggregation of neurodegenerative disease-related proteins

Neurodegenerative diseases such as Alzheimer (AD) and Parkinson (PD) are characterized by abnormal aggregation of misfolded β-sheet-rich proteins, including amyloid-β (Aβ)-derived peptides and tau in AD and α-synuclein in PD. Correct folding and assembly of these proteins are controlled by ubiquitously expressed molecular chaperones; however, our understanding of neuron-specific chaperones and their involvement in the pathogenesis of neurodegenerative diseases is limited. We here describe novel chaperone-like functions for the secretory protein 7B2, which is widely expressed in neuronal and endocrine tissues. In in vitro experiments, 7B2 efficiently prevented fibrillation and formation of Aβ_1–42, Aβ_1–40, and α-synuclein aggregates at a molar ratio of 1:10. In cell culture experiments, inclusion of recombinant 7B2, either in the medium of Neuro-2A cells or intracellularly via adenoviral 7B2 overexpression, blocked the neurocytotoxic effect of Aβ_1–42 and significantly increased cell viability. Conversely, knockdown of 7B2 by RNAi increased Aβ_1–42-induced cytotoxicity. In the brains of APP/PSEN1 mice, a model of AD amyloidosis, immunoreactive 7B2 co-localized with aggregation-prone proteins and their respective aggregates. Furthermore, in the hippocampus and substantia nigra of human AD- and PD-affected brains, 7B2 was highly co-localized with Aβ plaques and α-synuclein deposits, strongly suggesting physiological association. Our data provide insight into novel functions of 7B2 and establish this neural protein as an anti-aggregation chaperone associated with neurodegenerative disease.

Global Approaches to the Role of miRNAs in Drug-Induced Changes in Gene Expression

Neurons modulate gene expression with subcellular precision through excitation-coupled local protein synthesis, a process that is regulated in part through the involvement of microRNAs (miRNAs), a class of small non-coding RNAs. The biosynthesis of miRNAs is reviewed, with special emphasis on miRNA families, the subcellular localization of specific miRNAs in neurons, and their potential roles in the response to drugs of abuse. For over a decade, DNA microarrays have dominated genome-wide gene expression studies, revealing widespread effects of drug exposure on neuronal gene expression. We review a number of recent studies that explore the emerging role of miRNAs in the biochemical and behavioral responses to cocaine. The more powerful next-generation sequencing technology offers certain advantages and is supplanting microarrays for the analysis of complex transcriptomes. Next-generation sequencing is unparalleled in its ability to identify and quantify low-abundance transcripts without prior sequence knowledge, facilitating the accurate detection and quantification of miRNAs expressed in total tissue and miRNAs localized to postsynaptic densities (PSDs). We previously identified cocaine-responsive miRNAs, synaptically enriched and depleted miRNA families, and confirmed cocaine-induced changes in protein expression for several bioinformatically predicted target genes. The miR-8 family was found to be highly enriched and cocaine-regulated at the PSD, where its members may modulate expression of cell adhesion molecules. An integrative approach that combines mRNA, miRNA, and protein expression profiling in combination with focused single gene studies and innovative behavioral paradigms should facilitate the development of more effective therapeutic approaches to treat addiction.