Structural organization of the gene encoding the neuroendocrine chaperone 7B2

Stress-responsive regulation of extracellular proteostasis

Genetic, environmental, and aging-related insults can promote the misfolding and subsequent aggregation of secreted proteins implicated in the pathogenesis of numerous diseases. This has led to considerable interest in understanding the molecular mechanisms responsible for regulating proteostasis in extracellular environments such as the blood and cerebrospinal fluid (CSF). Extracellular proteostasis is largely dictated by biological pathways comprising chaperones, folding enzymes, and degradation factors localized to the ER and extracellular space. These pathways limit the accumulation of nonnative, potentially aggregation-prone proteins in extracellular environments. Many reviews discuss the molecular mechanisms by which these pathways impact the conformational integrity of the secreted proteome. Here, we instead focus on describing the stress-responsive mechanisms responsible for adapting ER and extracellular proteostasis pathways to protect the secreted proteome from pathologic insults that challenge these environments. Further, we highlight new strategies to identify stress-responsive pathways involved in regulating extracellular proteostasis and describe the pathologic and therapeutic implications for these pathways in human disease.

Genetic modifications associated with ketogenic diet treatment in the BTBRT+Tf/J mouse model of autism spectrum disorder

BACKGROUND

Autism spectrum disorder (ASD) is a prevalent and heterogeneous neurodevelopmental disorder characterized by hallmark behavioral features. The spectrum of disorders that fall within the ASD umbrella encompass a distinct but overlapping symptom complex that likely results from an array of molecular and genetic aberrations rather than a single genetic mutation. The ketogenic diet (KD) is a high-fat low-carbohydrate anti-seizure and neuroprotective diet that has demonstrated efficacy in the treatment of ASD-like behaviors in animal and human studies.

METHODS

We investigated changes in mRNA and gene expression in the BTBR mouse model of ASD that may contribute to the behavioral phenotype. In addition, we sought to examine changes in gene expression following KD treatment in BTBR mice.

RESULTS

Despite significant behavioral abnormalities, expression changes in BTBR mice did not differ substantially from controls; only 33 genes were differentially expressed in the temporal cortex, and 48 in the hippocampus. Examination of these differentially expressed genes suggested deficits in the stress response and in neuronal signaling/communication. After treatment with the KD, both brain regions demonstrated improvements in ASD deficits associated with myelin formation and white matter development.

CONCLUSIONS

Although our study supports many of the previously known impairments associated with ASD, such as excessive myelin formation and impaired GABAergic transmission, the RNAseq data and pathway analysis utilized here identified new therapeutic targets for analysis, such as Vitamin D pathways and cAMP signaling. Autism Res 2017, 10: 456-471. © 2016 International Society for Autism Research, Wiley Periodicals, Inc.

Network analysis of endogenous gene expression profiles after polyethyleneimine-mediated DNA delivery

BACKGROUND

DNA delivery systems, which transport exogenous DNA to cells, have applications that include gene therapy, tissue engineering and medical devices. Although the cationic nonviral DNA carrier polyethyleneimine (PEI) has been widely studied, the molecular factors and pathways underlying PEI-mediated DNA transfer remain largely unknown, preventing the design of more efficient delivery systems.

METHODS

HEK 293 T cells were treated with polyplexes formed with PEI and pEGFPLuc encoding for green fluorescent protein (GFP). Transfected cells expressing GFP were flow-separated from treated, untransfected cells. Gene expression profiles were obtained using Affymetrix HG-U133 2.0 microarrays and differentially expressed genes were identified using R/Bioconductor. Gene network analysis using EGAN (exploratory gene association network) bioinformatics tools was then used to find interaction among genes and enriched gene ontology (GO) terms related to transfection. Genes identified by this method were perturbed using pharmacologic activators or inhibitors to assess their effect on DNA transfer.

RESULTS

Microarray analysis comparing transfected cells to untransfected cells revealed 215 genes to be differentially expressed, with the majority enriched to GO processes including metabolism, response to stimulus, cell cycle, biological regulation and cellular component organization or biogenesis pathways. Gene network analysis revealed a coordinated induction of RAP1A, SCG5, PGAP1, ATF3 and NEB genes implicated in cell stress, cell cycle and cytoskeletal processes. Altering pathways with pharmacologic agents confirmed the potential role of RAP1A, SCG5 and ATF3 in transfection.

CONCLUSIONS

Microarray and gene network analyses of the sorted, transfected cell population can identify potential mediators of transfection, providing a basis for the design of improved delivery systems.

Frequent epigenetic inactivation of the chaperone SGNE1/7B2 in human gliomas

In a genome-wide screen using DMH (differential methylation hybridization) we have identified a CpG island within the 5' region and untranslated first exon of the secretory granule neuroendocrine protein 1 gene (SGNE1/7B2) that showed hypermethylation in low- and high-grade astrocytomas compared to normal brain tissue. Pyrosequencing was performed to confirm the methylation status of this CpG island in 89 astrocytic gliomas of different malignancy grades and six glioma cell lines. Hypermethylation of SGNE1/7B2 was significantly more frequent in diffuse low-grade astrocytomas as well as secondary glioblastomas and anaplastic astrocytomas as compared to primary glioblastomas. mRNA expression analysis by real-time RT-PCR indicates that SGNE1/7B2 expression is downregulated in astrocytic gliomas compared to white matter samples. Treatment of glioma cells with the demethylating agent 5-aza-2'-deoxycytidine restores the transcription of SGNE1/7B2. Overexpression of SGNE1/7B2 in T98G, A172 and U373MG glioblastoma cells significantly suppressed focus formation and led to a significant increase in apoptotic cells as determined by flow cytometric analysis in T98G cells. In summary, we have identified SGNE1/7B2 as a novel target silenced by DNA methylation in astrocytic gliomas. The high incidence of this alteration and the significant effects of SGNE1/7B2 on the growth and apoptosis of glioblastoma cells provide a first proof for a functional implication of SGNE1/7B2 inactivation in the molecular pathology of gliomas.

Regulation of 7B2 mRNA translation: dissecting the role of its 5'-untranslated region

7B2 is a chaperone for the prohormone/proneuropeptide convertase PC2. Its mRNA is readily detectable in most neuronal and endocrine cells; the protein, in contrast, is often found at relatively low levels, suggesting that translation of the corresponding mRNA may be repressed. Because the 5' untranslated region (5'-UTR) of this mRNA is relatively long and burdened with multiple AUGs, it has been speculated that it contributes to this repression. In this report, the influence of this region was assessed using in vitro and ex vivo approaches. The results showed that, in a cell-free system, full-length 7B2 mRNA was a poor template for translation. Its translatability dramatically improved when its 5'-UTR was truncated or when it was replaced with the 5'-UTR of carboxypeptidase E mRNA. These observations were confirmed in transfected mouse insulinoma MIN6 cells and human embryonic kidney HEK293 cells. Acute exposure of MIN6 cells to high glucose increased endogenous 7B2 biosynthesis without affecting the levels of its mRNA, suggesting that translation repression of this mRNA can be relieved by physiological stimuli.

Pax6 regulates the proglucagon processing enzyme PC2 and its chaperone 7B2

Pax6 is important in the development of the pancreas and was previously shown to regulate pancreatic endocrine differentiation, as well as the insulin, glucagon, and somatostatin genes. Prohormone convertase 2 (PC2) is the main processing enzyme in pancreatic alpha cells, where it processes proglucagon to produce glucagon under the spatial and temporal control of 7B2, which functions as a molecular chaperone. To investigate the role of Pax6 in glucagon biosynthesis, we studied potential target genes in InR1G9 alpha cells transfected with Pax6 small interfering RNA and in InR1G9 clones expressing a dominant-negative form of Pax6. We now report that Pax6 controls the expression of the PC2 and 7B2 genes. By binding and transactivation studies, we found that Pax6 indirectly regulates PC2 gene transcription through cMaf and Beta2/NeuroD1 while it activates the 7B2 gene both directly and indirectly through the same transcription factors, cMaf and Beta2/NeuroD1. We conclude that Pax6 is critical for glucagon biosynthesis and processing by directly and indirectly activating the glucagon gene through cMaf and Beta2/NeuroD1, as well as the PC2 and 7B2 genes.

SGNE1/7B2 is epigenetically altered and transcriptionally downregulated in human medulloblastomas

In a genome-wide screen using differential methylation hybridization (DMH), we have identified a CpG island within the 5' region and untranslated first exon of the secretory granule neuroendocrine protein 1 gene (SGNE1/7B2) that showed hypermethylation in medulloblastomas compared to fetal cerebellum. Bisulfite sequencing and combined bisulfite restriction assay were performed to confirm the methylation status of this CpG island in primary medulloblastomas and medulloblastoma cell lines. Hypermethylation was detected in 16/23 (70%) biopsies and 7/8 (87%) medulloblastoma cell lines, but not in non-neoplastic fetal (n=8) cerebellum. Expression of SGNE1 was investigated by semi-quantitative competitive reverse transcription-polymerase chain reaction and found to be significantly downregulated or absent in all, but one primary medulloblastomas and all cell lines compared to fetal cerebellum. After treatment of medulloblastoma cell lines with 5-aza-2'-deoxycytidine, transcription of SGNE1 was restored. No mutation was found in the coding region of SGNE1 by single-strand conformation polymorphism analysis. Reintroduction of SGNE1 into the medulloblastoma cell line D283Med led to a significant growth suppression and reduced colony formation. In summary, we have identified SGNE1 as a novel epigenetically silenced gene in medulloblastomas. Its frequent inactivation, as well as its inhibitory effect on tumor cell proliferation and focus formation strongly argues for a significant role in medulloblastoma development.

Different profiles of neuroendocrine cell differentiation evolve in the PC-310 human prostate cancer model during long-term androgen deprivation

BACKGROUND

Neuroendocrine (NE) cells are androgen-independent cells and secrete growth-modulating peptide hormones via a regulated secretory pathway (RSP). We studied NE differentiation after long-term androgen withdrawal in the androgen-dependent human prostate cancer xenograft PC-310.

METHODS

Tumor-bearing nude mice were killed at 0, 2, 5, 7, 14, 21, 47, 84, and 154 days after castration. The half-life of the PC-310 tumor was 10 days, with a stable residual tumor volume of 30--40% after 21 days and longer periods of androgen deprivation.

RESULTS

Proliferative activity and prostate-specific antigen serum levels decreased to zero after castration, whereas cell-cycle arrest was manifested by increased p27(kip1) expression. A temporary downregulation of androgen receptor (AR) expression was noted after androgen deprivation. The expression of chromogranin A, secretogranin III, and secretogranin V (7B2) increased 5 days after castration and later. Subsequently, pro-hormone convertase 1 and peptidyl alpha--amidating monooxygenase as well as vascular endothelial growth factor were expressed from 7 days after castration on. Finally, such growth factors as gastrin-releasing peptide and serotonin were expressed in a small part of the NE cells 21 days after castration, but strong expression was induced late during androgen deprivation, that is, 84 and 154 days after castration, respectively.

CONCLUSIONS

Androgen deprivation of the NE-differentiated PC-310 model induced the formation of NE-differentiated AR(minus sign) and non-NE AR(+) tumor residues. The NE-differentiated cells actively produced growth factors via an RSP that may lead to hormone-refractory disease. The dormant non-NE AR(+) tumor cells were shown to remain androgen sensitive even after long-term androgen deprivation. In the PC-310 xenograft, time-dependent NE differentiation and subsequent maturation were induced after androgen depletion. The androgen-dependent PC-310 xenograft model constitutes an excellent model for studying the role of NE cells in the progression of clinical prostate cancer.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

Differential gene expression in the human ciliary epithelium

The generation of expression and subtractive libraries from the ocular ciliary body and cultured ciliary epithelial cells has been instrumental in the cloning, identification and characterization of many genes which, overall reflect a representative profile of transcripts expressed in ciliary nonpigmented, ciliary pigmented and ciliary muscle cells. The cell-specific expression of some of these genes (i.e. a neurotrophic factor, a gene associated with juvenile open glaucoma, and a visual component) reveal a degree of cell differentiation with a diversity of functions and properties higher than previously thought. The protection from light-induced oxidative reactions, free radicals and detoxification, may be partially attributed to the high level of expression in the ciliary epithelium of antioxidative enzymes (i.e., glutathione S-transferase, glutathione peroxidases, selenoprotein-P). The expression of genes encoding plasma proteins (i.e., complement component C4, alpha2-macroglobulin, apolipoprotein D) is in contrast with the view that plasma proteins in aqueous humor are synthesized outside the eye (i.e., liver). The identification of neuropeptide-processing enzymes (i.e., prohormone convertases, carboxypeptidase E, peptidyl-glycine-alpha-amidating monoxigenase), neuropeptides (i.e., secretogranin II, neurotensin) and regulatory peptides (i.e., atrial natriuretic peptide and angiotensinogen) with hypertensive and hypotensive activities provide the molecular basis to support the view that the ciliary epithelium is a neuroepithelium with neuroendocrine functions. We propose a working model to demonstrate that aqueous humor and intraocular pressure are under neuroendocrine control through regulatory peptides synthesized and released by the ciliary epithelium and targeting the peptide producing cells at the inflow system by an autocrine mechanism and/or cells at the outflow system (i.e., trabecular meshwork cells) by a paracrine mechanism. Finally, we hypothesize that these mechanisms could be entrained in the light-dark cycle following the circadian rhythm of aqueous humor and intraocular pressure.

The neuroendocrine protein 7B2 is required for peptide hormone processing in vivo and provides a novel mechanism for pituitary Cushing's disease

The neuroendocrine protein 7B2 has been implicated in activation of prohormone convertase 2 (PC2), an important neuroendocrine precursor processing endoprotease. To test this hypothesis, we created a null mutation in 7B2 employing a novel transposon-facilitated technique and compared the phenotypes of 7B2 and PC2 nulls. 7B2 null mice have no demonstrable PC2 activity, are deficient in processing islet hormones, and display hypoglycemia, hyperproinsulinemia, and hypoglucagonemia. In contrast to the PC2 null phenotype, these mice show markedly elevated circulating ACTH and corticosterone levels, with adrenocortical expansion. They die before 9 weeks of severe Cushing's syndrome arising from pituitary intermediate lobe ACTH hypersecretion. We conclude that 7B2 is indeed required for activation of PC2 in vivo but has additional important functions in regulating pituitary hormone secretion.

Kinetics of neuroendocrine differentiation in an androgen-dependent human prostate xenograft model

It was previously shown in the PC-295 xenograft that the number of chromogranin A (CgA)-positive neuroendocrine (NE) cells increased after androgen withdrawal. NE cells did not proliferate and differentiated from G0-phase-arrested cells. Here we further characterized NE differentiation, androgen receptor status, and apoptosis-associated Bcl-2 expression in the PC-295 model after androgen withdrawal to assess the origin of NE cells. PC-295 tumor volumes decreased by 50% in 4 days. Intraperitoneal bromodeoxyuridine (BrdU) incorporation and MIB-1 labeling decreased to 0%, and the apoptosis was maximal at day 4. Androgen receptor expression and prostate-specific antigen (PSA) serum levels decreased rapidly within 2 days. The number of NE cells increased 6-fold at day 4 and 30-fold at day 7. Five and ten percent of the CgA-positive cells were BrdU positive after continuous BrdU labeling for 2 and 4 days, respectively. However, no MIB-1 expression was observed in CgA-positive cells. NE cells expressed the regulated secretory pathway marker secretogranin III but were negative for androgen receptor and Bcl-2. Bcl-2 expression did increase in the non-NE tumor cells. In conclusion, androgen withdrawal leads to a rapid PC-295 tumor regression and a proliferation-independent induction of NE differentiation. The strictly androgen-independent NE cells that were still present after 21 days differentiated mainly from G0-phase-arrested cells.

Neuroendocrine protein 7B2 is essential for proteolytic conversion and activation of proprotein convertase 2 in vivo

The 7B2 protein is widely distributed in neural and endocrine tissues. Its biological function was found to be related to the processing enzyme proprotein convertase 2 (PC2), a mammalian subtilisin/kexin-like endoproteinase that cleaves at specific single or multiple basic amino-acid residues. In order to examine the proposed function of 7B2 on PC2 in in vivo models, we first compared the distribution of 7B2 and PC2 mRNAs in the rat brain. Expression of 7B2 mRNA was found to be pan-neuronal, but additionally, we observed 7B2 mRNA in ependymal cells and in the subcommissural organ. Although the expression of PC2 mRNA was exclusively neuronal, it was more restricted, sparing some regions expressing high levels of 7B2. This finding suggests that 7B2 has an additional function in non-PC2-expressing cells. No evidence of PC2-positive/7B2-negative cells could be obtained in the adult rat brain. However, in the developing rat brain (E17), such regions were easily observed, showing higher levels of pro-PC2 (75 kD). Similarly, in the animal model of insulin-induced hypoglycemic shock, where adrenomedullary 7B2 expression is decreased, the ratio of pro-PC2 to mature PC2 (75 kD:68 kD) was observed to be increased. Finally, the human neuroepithelioma SK-N-MCIXC expresses PC2 but not 7B2. Accordingly, only inactive pro-PC2 forms were observed: 75-kD intracellular and 71-kD extracellular. After stable transfection of SK-N-MCIXC cells with 27-kD pro-7B2, mature and active (68-kD) PC2 was secreted into the medium. Our data demonstrate a critical role of 7B2 in the proteolytic conversion and activation of PC2 in vivo.

Transposon-generated 'knock-out' and 'knock-in' gene-targeting constructs for use in mice

The conventional technique for targeted mutation of mouse genes entails placing a genomic DNA fragment containing the gene of interest into a vector for fine mapping, followed by cloning of two genomic arms around a selectable neomycin-resistance cassette in a vector containing thymidine kinase [1]; this generally requires 1-2 months of work for each construct. The single 'knock-out' construct is then transfected into mouse embryonic stem (ES) cells, which are subsequently subjected to positive selection (using G418 to select for neomycin-resistance) and negative selection (using FIAU to exclude cells lacking thymidine kinase), allowing the selection of cells which have undergone homologous recombination with the knockout vector. This approach leads to inactivation of the gene of interest [2]. Recently, an in vitro reaction was developed, on the basis of the yeast Ty transposon, as a useful technique in shotgun sequencing [3]. An artificial transposable element, integrase enzyme and the target plasmid are incubated together to engender transposition. The DNA is then purified, and subsequently electroporated into bacteria. The transposon and the target plasmid bear distinct antibiotic resistance markers (trimethoprim and ampicillin, respectively), allowing double selection for transposition events. In the present study, we have modified this system to allow the rapid, simultaneous generation of a palette of potential gene targeting constructs. Our approach led from genomic clone to completed construct ready for transfection in a matter of days. The results presented here indicate that this technique should also be applicable to the generation of gene fusion constructs [4-8], simplifying this technically demanding method.