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Omar-Hmeadi, MuhmmadORCID iD iconorcid.org/0000-0001-8893-7348
Publications (9 of 9) Show all publications
Gucek, A., Gandasi, N. R., Omar-Hmeadi, M., Bakke, M., Doskeland, S. O., Tengholm, A. & Barg, S. (2019). Fusion pore regulation by cAMP/Epac2 controls cargo release during insulin exocytosis. eLIFE, 8, Article ID e41711.
Open this publication in new window or tab >>Fusion pore regulation by cAMP/Epac2 controls cargo release during insulin exocytosis
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2019 (English)In: eLIFE, E-ISSN 2050-084X, Vol. 8, article id e41711Article in journal (Refereed) Published
Abstract [en]

Regulated exocytosis establishes a narrow fusion pore as initial aqueous connection to the extracellular space, through which small transmitter molecules such as ATP can exit. Co-release of polypeptides and hormones like insulin requires further expansion of the pore. There is evidence that pore expansion is regulated and can fail in diabetes and neurodegenerative disease. Here, we report that the cAMP-sensor Epac2 (Rap-GEF4) controls fusion pore behavior by acutely recruiting two pore-restricting proteins, amisyn and dynamin-1, to the exocytosis site in insulin-secreting beta-cells. cAMP elevation restricts and slows fusion pore expansion and peptide release, but not when Epac2 is inactivated pharmacologically or in Epac2(-/-) (Rapgef4(-/-)) mice. Consistently, overexpression of Epac2 impedes pore expansion. Widely used antidiabetic drugs (GLP-1 receptor agonists and sulfonylureas) activate this pathway and thereby paradoxically restrict hormone release. We conclude that Epac2/cAMP controls fusion pore expansion and thus the balance of hormone and transmitter release during insulin granule exocytosis.

Place, publisher, year, edition, pages
ELIFE SCIENCES PUBLICATIONS LTD, 2019
National Category
Cell Biology
Identifiers
urn:nbn:se:uu:diva-389824 (URN)10.7554/eLife.41711 (DOI)000471123300001 ()31099751 (PubMedID)
Funder
Novo NordiskSwedish Research Council, 2014-02575Swedish Research Council, 2017-00956Swedish Research Council, 2018-02871Ernfors FoundationSwedish Society for Medical Research (SSMF)The Research Council of NorwaySwedish Diabetes AssociationEXODIAB - Excellence of Diabetes Research in SwedenThe Swedish Brain Foundation
Available from: 2019-07-29 Created: 2019-07-29 Last updated: 2019-07-29Bibliographically approved
Omar-Hmeadi, M. (2019). Regulation of docking and priming in pancreatic α- and β-cells. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
Open this publication in new window or tab >>Regulation of docking and priming in pancreatic α- and β-cells
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The secretion of islet hormones from endocrine cells of the pancreas plays vital roles in maintaining glucose homeostasis. Dysfunction of these cells leads to diabetes, a devastating metabolic disorder affecting millions worldwide, but underlying mechanisms remain poorly understood. In hyperglycemic conditions, β-cells secrete insulin, whereas α-cells secrete an increased amount of glucagon in hypoglycemic conditions. Both insulin and glucagon are stored in secretory granules preceding their release by regulated exocytosis. This process involves several steps, including tethering, docking, priming, and finally, a fusion of the granules with the plasma membrane. Soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) proteins and phosphoinositides (PIs) drive pancreatic hormone exocytosis and secretion, which follows a biphasic time course. Biphasic secretion is thought to reflect the vastly different release probabilities of individual granules, but direct evidence for this is still lacking.  Therefore, this thesis investigates exocytosis in the two main pancreatic cell types with a particular focus on preceding steps docking and priming, to identify rate-limiting steps in health and type-2 diabetes (T2D). Our data indicated that granule docking is critical for sustained secretion in α- and β-cells. Glucagon granule exocytosis had a U-shaped sensitivity to glucose in both healthy and T2D α-cells. However, T2D α-cells exhibited a marginal decrease in exocytosis, as well as docking, and they were markedly insensitive to somatostatin and insulin. T2D β-cells reduced exocytosis dramatically, and docking was compromised and no longer responsive to glucose, which correlated with reduced insulin secretion and elevated donor HbA1c. These results were further strengthened by the finding that expression of a group of genes that are involved explicitly in granule docking was reduced (by RNAseq of islets from over 200 human donors), and overexpression of the corresponding proteins increased granule docking in human β-cells.

We further aimed to study the basis for the recruitment of these proteins to the docking site. Here we tested the hypothesis that highly charged lipids mainly PIs act as a hotspot to interact with SNARE proteins that initiate docking. We showed the homogenous distribution of all PIs markers in the plasma membrane, with no PIs microdomains at the exocytotic site during granule docking. However, rapid and local PI(4,5)P2 signaling at fusion sites was crucial for stabilizing fusion pore by binding to proteins related to the release site. These results suggested a role of PI(4,5)P2 in priming and fusion regulation rather than docking. Overall, this work gives new insights into the mechanisms underlying pancreatic hormone secretion in both healthy and diabetic conditions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 54
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1586
Keywords
Type-2 diabetes, glucagon, insulin, somatostatin, exocytosis
National Category
Cell and Molecular Biology Cell Biology Endocrinology and Diabetes
Identifiers
urn:nbn:se:uu:diva-388354 (URN)978-91-513-0704-6 (ISBN)
Public defence
2019-09-11, B42 BMC, BMC, Husargatan 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2019-08-20 Created: 2019-08-02 Last updated: 2019-09-17
Mojtaba Ghiasi, S., Dahlby, T., Hede Andersen, C., Haataja, L., Petersen, S., Omar-Hmeadi, M., . . . Tomasz Marzec, M. (2019). The Endoplasmic Reticulum Chaperone Glucose-Regulated Protein 94 is Essential for Proinsulin Handling. Diabetes, 68(4), 747-760
Open this publication in new window or tab >>The Endoplasmic Reticulum Chaperone Glucose-Regulated Protein 94 is Essential for Proinsulin Handling
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2019 (English)In: Diabetes, ISSN 0012-1797, E-ISSN 1939-327X, Vol. 68, no 4, p. 747-760Article in journal (Refereed) Published
Abstract [en]

Although endoplasmic reticulum (ER) chaperone binding to mutant proinsulin has been reported, the role of protein chaperones in the handling of wild-type proinsulin is under-investigated. Here, we have explored the importance of glucose regulated protein 94 (GRP94), a prominent ER chaperone known to fold insulin-like growth factors, in proinsulin handling within β-cells. We found that GRP94 co-immunoprecipitated with proinsulin and that inhibition of GRP94 function and/or expression reduced glucose-dependent insulin secretion, shortened proinsulin half-life and lowered intracellular proinsulin and insulin levels. This phenotype was accompanied by post-ER proinsulin misprocessing and higher numbers of enlarged insulin granules that contained amorphic material with reduced immunogold staining for mature insulin. Insulin granule exocytosis was two-fold accelerated but the secreted insulin had diminished bioactivity. Moreover, GRP94 knockdown or knockout in β-cells selectively activated Protein Kinase R-like Endoplasmic Reticulum Kinase (PERK), without increasing apoptosis levels. Finally, GRP94 mRNA was overexpressed in islets from T2D patients. We conclude that GRP94 is a chaperone crucial for proinsulin handling and insulin secretion.

National Category
Cell and Molecular Biology Endocrinology and Diabetes
Identifiers
urn:nbn:se:uu:diva-378915 (URN)10.2337/db18-0671 (DOI)000462053100007 ()30670477 (PubMedID)
Funder
Swedish Research Council
Available from: 2019-03-08 Created: 2019-03-08 Last updated: 2019-05-06Bibliographically approved
Gandasi, N. R., Yin, P., Omar-Hmeadi, M., Ottosson Laakso, E., Vikman, P. & Barg, S. (2018). Glucose-Dependent Granule Docking Limits Insulin Secretion and Is Decreased in Human Type 2 Diabetes. Cell Metabolism, 27(2), 470-478
Open this publication in new window or tab >>Glucose-Dependent Granule Docking Limits Insulin Secretion and Is Decreased in Human Type 2 Diabetes
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2018 (English)In: Cell Metabolism, ISSN 1550-4131, E-ISSN 1932-7420, Vol. 27, no 2, p. 470-478Article in journal (Refereed) Published
Abstract [en]

Glucose-stimulated insulin secretion is biphasic, with a rapid first phase and a slowly developing sustained second phase; both are disturbed in type 2 diabetes (T2D). Biphasic secretion results from vastly different release probabilities of individual insulin granules, but the morphological and molecular basis for this is unclear. Here, we show that human insulin secretion and exocytosis critically depend on the availability of membrane-docked granules and that T2D is associated with a strong reduction in granule docking. Glucose accelerated granule docking, and this effect was absent in T2D. Newly docked granules only slowly acquired release competence; this was regulated by major signaling pathways, but not glucose. Gene expression analysis indicated that key proteins involved in granule docking are downregulated in T2D, and overexpression of these proteins increased granule docking. The findings establish granule docking as an important glucose-dependent step in human insulin secretion that is dysregulated in T2D.

Keywords
GLP-1, biphasic secretion, dense core vesicle, docking, exocytosis, genome-wide association, insulin secretion, priming, somatostatin, type 2 diabetes
National Category
Cell and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-341518 (URN)10.1016/j.cmet.2017.12.017 (DOI)000424465200021 ()29414688 (PubMedID)
Funder
Swedish Research CouncilSwedish Diabetes AssociationSwedish Society for Medical Research (SSMF)The Swedish Brain FoundationNovo NordiskErnfors Foundation
Available from: 2018-02-09 Created: 2018-02-09 Last updated: 2019-08-02Bibliographically approved
Omar-Hmeadi, M., Gandasi, N. R. & Barg, S. (2018). PtdIns(4,5)P2 is not required for secretory granule docking. Traffic: the International Journal of Intracellular Transport, 19(6), 436-445
Open this publication in new window or tab >>PtdIns(4,5)P2 is not required for secretory granule docking
2018 (English)In: Traffic: the International Journal of Intracellular Transport, ISSN 1398-9219, E-ISSN 1600-0854, Vol. 19, no 6, p. 436-445Article in journal (Refereed) Published
Abstract [en]

Phosphoinositides (PtdIns) play important roles in exocytosis and are thought to regulate secretory granule docking by co-clustering with the SNARE protein syntaxin to form a docking receptor in the plasma membrane. Here we tested this idea by high-resolution total internal reflection imaging of EGFP-labeled PtdIns markers or syntaxin-1 at secretory granule release sites in live insulin-secreting cells. In intact cells, PtdIns markers distributed evenly across the plasma membrane with no preference for granule docking sites. In contrast, syntaxin-1 was found clustered in the plasma membrane, mostly beneath docked granules. We also observed rapid accumulation of syntaxin-1 at sites where granules arrived to dock. Acute depletion of plasma membrane phosphatidylinositol (4,5) bisphosphate (PtdIns(4,5)P-2) by recruitment of a 5-phosphatase strongly inhibited Ca2+-dependent exocytosis, but had no effect on docked granules or the distribution and clustering of syntaxin-1. Cell permeabilization by -toxin or formaldehyde-fixation caused PtdIns marker to slowly cluster, in part near docked granules. In summary, our data indicate that PtdIns(4,5)P-2 accelerates granule priming, but challenge a role of PtdIns in secretory granule docking or clustering of syntaxin-1 at the release site.

Keywords
exocytosis, insulin, live cell imaging, phosphoinositides, PtdIns(4, 5)P-2, syntaxin clustering, vesicle docking
National Category
Cell and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-356858 (URN)10.1111/tra.12562 (DOI)000432037000005 ()29542271 (PubMedID)
Funder
Swedish Research CouncilSwedish Child Diabetes FoundationSwedish Society for Medical Research (SSMF)Novo NordiskThe Swedish Brain FoundationErnfors Foundation
Available from: 2018-08-15 Created: 2018-08-15 Last updated: 2019-08-02Bibliographically approved
Yin, P., Gandasi, N. R., Arora, S., Omar-Hmeadi, M., Saras, J. & Barg, S. (2018). Syntaxin clusters at secretory granules in a munc18-bound conformation. Molecular Biology of the Cell, 29(22), 2700-2708
Open this publication in new window or tab >>Syntaxin clusters at secretory granules in a munc18-bound conformation
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2018 (English)In: Molecular Biology of the Cell, ISSN 1059-1524, E-ISSN 1939-4586, Vol. 29, no 22, p. 2700-2708Article in journal (Refereed) Published
Abstract [en]

Syntaxin (stx)-1 is an integral plasma membrane protein that is crucial for two distinct steps of regulated exocytosis, docking of secretory granules at the plasma membrane and membrane fusion. During docking, stx1 clusters at the granule docking site, together with the S/M protein munc18. Here we determined features of stx1 that contribute to its clustering at granules. In live insulin-secreting cells, stx1 and stx3 (but not stx4 or stx11) accumulated at docked granules, and stx1 (but not stx4) rescued docking in cells expressing botulinum neurotoxin-C. Using a series of stx1 deletion mutants and stx1/4 chimeras, we found that all four helical domains (Ha, Hb, Hc, SNARE) and the short N-terminal peptide contribute to recruitment to granules. However, only the Hc domain confers specificity, and it must be derived from stx1 for recruitment to occur. Point mutations in the Hc or the N-terminal peptide designed to interfere with binding to munc18-1 prevent stx1 from clustering at granules, and a mutant munc18 deficient in binding to stx1 does not cluster at granules. We conclude that stx1 is recruited to the docking site in a munc18-1-bound conformation, providing a rationale for the requirement for both proteins for granule docking.

National Category
Cell and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-375859 (URN)10.1091/mbc.E17-09-0541 (DOI)000455641000013 ()30156474 (PubMedID)
Funder
Swedish Research CouncilSwedish Child Diabetes FoundationSwedish Diabetes AssociationThe Swedish Brain FoundationNovo NordiskGöran Gustafsson Foundation for Research in Natural Sciences and MedicineErnfors FoundationSwedish Society of Medicine
Available from: 2019-02-01 Created: 2019-02-01 Last updated: 2019-02-01Bibliographically approved
Omar-Hmeadi, M., Gandasi, N. R. & Barg, S. (2017). Plasma membrane PI(4,5)P-2 is critical for secretory granule exocytosis. Paper presented at ASCB/EMBO Meeting, DEC 02-06, 2017, Philadelphia, PA.. Molecular Biology of the Cell, 28(26), 3727-3727
Open this publication in new window or tab >>Plasma membrane PI(4,5)P-2 is critical for secretory granule exocytosis
2017 (English)In: Molecular Biology of the Cell, ISSN 1059-1524, E-ISSN 1939-4586, Vol. 28, no 26, p. 3727-3727Article in journal, Meeting abstract (Other academic) Published
Place, publisher, year, edition, pages
Philadelphia: American Society for Cell Biology (ASCB), 2017
National Category
Cell and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-352940 (URN)10.1091/mbc.E17-10-0618 (DOI)000426664300402 ()29237772 (PubMedID)
Conference
ASCB/EMBO Meeting, DEC 02-06, 2017, Philadelphia, PA.
Note

See: Supplementary Material, 2017 ASCB-EMBO Meeting-Poster Abstracts, p. Sunday-258-Sunday-259, Meeting Abstract: P1403.

Available from: 2018-06-12 Created: 2018-06-12 Last updated: 2019-02-01Bibliographically approved
Omar-Hmeadi, M., Gucek, A. & Barg, S.Fusion pore regulation by transient local generation of PI(4,5)P2 in pancreatic in β-cells.
Open this publication in new window or tab >>Fusion pore regulation by transient local generation of PI(4,5)P2 in pancreatic in β-cells
(English)Manuscript (preprint) (Other academic)
National Category
Cell and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-389794 (URN)
Available from: 2019-07-27 Created: 2019-07-27 Last updated: 2019-08-02
Omar-Hmeadi, M., Lund, P.-E., Gandasi, N., Tengholm, A. & Barg, S.Paracrine control of α-cell glucagon exocytosis is compromised in human type-2 diabetes.
Open this publication in new window or tab >>Paracrine control of α-cell glucagon exocytosis is compromised in human type-2 diabetes
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(English)Manuscript (preprint) (Other academic)
National Category
Cell and Molecular Biology
Identifiers
urn:nbn:se:uu:diva-389793 (URN)
Available from: 2019-07-27 Created: 2019-07-27 Last updated: 2019-08-02
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-8893-7348

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