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Sarieva K, Kagermeier T, Khakipoor S, Atay E, Yentür Z, Becker K, Mayer S. Human brain organoid model of maternal immune activation identifies radial glia cells as selectively vulnerable. Mol Psychiatry 2023; 28:5077-5089. [PMID: 36878967 PMCID: PMC9986664 DOI: 10.1038/s41380-023-01997-1] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/09/2022] [Revised: 02/07/2023] [Accepted: 02/10/2023] [Indexed: 03/08/2023]
Abstract
Maternal immune activation (MIA) during critical windows of gestation is correlated with long-term neurodevelopmental deficits in the offspring, including increased risk for autism spectrum disorder (ASD) in humans. Interleukin 6 (IL-6) derived from the gestational parent is one of the major molecular mediators by which MIA alters the developing brain. In this study, we establish a human three-dimensional (3D) in vitro model of MIA by treating induced pluripotent stem cell-derived dorsal forebrain organoids with a constitutively active form of IL-6, Hyper-IL-6. We validate our model by showing that dorsal forebrain organoids express the molecular machinery necessary for responding to Hyper-IL-6 and activate STAT signaling upon Hyper-IL-6 treatment. RNA sequencing analysis reveals the upregulation of major histocompatibility complex class I (MHCI) genes in response to Hyper-IL-6 exposure, which have been implicated with ASD. We find a small increase in the proportion of radial glia cells after Hyper-IL-6 treatment through immunohistochemistry and single-cell RNA-sequencing. We further show that radial glia cells are the cell type with the highest number of differentially expressed genes, and Hyper-IL-6 treatment leads to the downregulation of genes related to protein translation in line with a mouse model of MIA. Additionally, we identify differentially expressed genes not found in mouse models of MIA, which might drive species-specific responses to MIA. Finally, we show abnormal cortical layering as a long-term consequence of Hyper-IL-6 treatment. In summary, we establish a human 3D model of MIA, which can be used to study the cellular and molecular mechanisms underlying the increased risk for developing disorders such as ASD.
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Affiliation(s)
- Kseniia Sarieva
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany
- International Max Planck Research School, Graduate Training Centre of Neuroscience, University of Tübingen, Tübingen, Germany
| | - Theresa Kagermeier
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany
- International Max Planck Research School, Graduate Training Centre of Neuroscience, University of Tübingen, Tübingen, Germany
| | - Shokoufeh Khakipoor
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany
| | - Ezgi Atay
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany
| | - Zeynep Yentür
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany
- International Max Planck Research School, Graduate Training Centre of Neuroscience, University of Tübingen, Tübingen, Germany
- Heidelberger Akademie der Wissenschaften, Heidelberg, Germany
| | - Katharina Becker
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany
| | - Simone Mayer
- Hertie Institute for Clinical Brain Research, University of Tübingen, Tübingen, Germany.
- International Max Planck Research School, Graduate Training Centre of Neuroscience, University of Tübingen, Tübingen, Germany.
- Heidelberger Akademie der Wissenschaften, Heidelberg, Germany.
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Giannaki M, Schrödl-Häußel M, Khakipoor S, Kirsch M, Roussa E. STAT3-dependent regulation of the electrogenic Na +/ HCO 3- cotransporter 1 (NBCe1) functional expression in cortical astrocytes. J Cell Physiol 2021; 236:2036-2050. [PMID: 32761631 DOI: 10.1002/jcp.29990] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2020] [Revised: 07/03/2020] [Accepted: 07/21/2020] [Indexed: 01/10/2023]
Abstract
The electrogenic Na+ /HCO3- cotransporter (NBCe1) in astrocytes is crucial in regulation of acid-base homeostasis in the brain. Since many pathophysiological conditions in the brain have been associated with pH shifts we exposed primary mouse cortical and hippocampal astrocytes to prolonged low or high extracellular pH (pHo ) at constant extracellular bicarbonate concentration and investigated activation of astrocytes and regulation of NBCe1 by immunoblotting, biotinylation of surface proteins, and intracellular H+ recordings. High pHo at constant extracellular bicarbonate caused upregulation of NBCe1 protein, surface expression and activity via upregulation of the astrocytic activation markers signal transducer and activator of transcription 3 (STAT3) signaling and glial fibrillary acidic protein expression. High pHo -induced increased NBCe1 protein expression was prevented in astrocytes from Stat3flox/flox ::GfapCre/+ mice. In vitro, basal and high pHo -induced increased NBCe1 functional expression was impaired following inhibition of STAT3 phosphorylation. These results provide a novel regulation mode of NBCe1 protein and activity, highlight the importance of astrocyte reactivity on regulation of NBCe1 and implicate roles for NBCe1 in altering/modulating extracellular pH during development as well as of the microenvironment at sites of brain injuries and other pathophysiological conditions.
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Affiliation(s)
- Marina Giannaki
- Department of Molecular Embryology, Medical Faculty, Institute of Anatomy and Cell Biology, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
| | - Magdalena Schrödl-Häußel
- Department of Molecular Embryology, Medical Faculty, Institute of Anatomy and Cell Biology, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
| | - Shokoufeh Khakipoor
- Department of Molecular Embryology, Medical Faculty, Institute of Anatomy and Cell Biology, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
| | - Matthias Kirsch
- Department of Neuroanatomy, Medical Faculty, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
| | - Eleni Roussa
- Department of Molecular Embryology, Medical Faculty, Institute of Anatomy and Cell Biology, Albert-Ludwigs-Universität Freiburg, Freiburg, Germany
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Khakipoor S, Crouch EE, Mayer S. Human organoids to model the developing human neocortex in health and disease. Brain Res 2020; 1742:146803. [PMID: 32240655 DOI: 10.1016/j.brainres.2020.146803] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2019] [Revised: 01/28/2020] [Accepted: 03/24/2020] [Indexed: 02/06/2023]
Abstract
Rodent models have catalyzed major discoveries in the neocortex, a brain region unique to mammals. However, since the neocortex has expanded considerably in primates, employing rodent models has limitations. Human fetal brain tissue is a scarce resource with limitations for experimental manipulations. In order to create an experimentally tractable representation of human brain development, a number of labs have recently created in vitro models of the developing human brain. These models, generated using human embryonic stem cells or induced pluripotent stem cells, are called "organoids". Organoids have successfully and rapidly uncovered new mechanisms of human brain development in health and disease. In the future, we envision that this strategy will enable faster and more efficient translation of basic neuroscience findings to therapeutic applications. In this review, we discuss the generation of the first human cerebral organoids, progress since their debut, and challenges to be overcome in the future.
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Affiliation(s)
- Shokoufeh Khakipoor
- Hertie Institute for Clinical Brain Research, University of Tübingen, 72076 Tübingen, Germany
| | - Elizabeth E Crouch
- Department of Pediatrics, University of California, San Francisco, San Francisco, CA 94143, USA
| | - Simone Mayer
- Hertie Institute for Clinical Brain Research, University of Tübingen, 72076 Tübingen, Germany.
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Kluge M, Namkoong E, Khakipoor S, Park K, Roussa E. Differential regulation of vacuolar H + -ATPase subunits by transforming growth factor-β1 in salivary ducts. J Cell Physiol 2019; 234:15061-15079. [PMID: 30648263 DOI: 10.1002/jcp.28147] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2018] [Accepted: 12/20/2018] [Indexed: 01/24/2023]
Abstract
Bicarbonate concentration in saliva is controlled by the action of acid-base transporters in salivary duct cells. We show for the first time expression of ATP6V1B1 in submandibular gland and introduce transforming growth factor-beta (TGF-β) as a novel regulator of V-ATPase subunits. Using QRT-PCR, immunoblotting, biotinylation of surface proteins, immunofluorescence, chromatin immunoprecipitation, and intracellular H(+ ) recording with H(+ )-sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein we show that in the human submandibular gland (HSG) cell line, activation of TGF-β signaling upregulates ATP6V1E1 and ATP6V1B2, downregulates ATP6V1B1, and has no effect on ATP6V1A. TGF-β1 effects on ATP6V1B1 are mediated through the canonical, the soluble adenylate cyclase, and ERK signaling. A CREB binding sequence was identified in the ATP6V1B1 promoter and CREB binding decreased after TGF-β1 treatment. Following acidosis, a bafilomycin-sensitive and Na+ -independent cell pH recovery was observed in HSG cells, an effect that was not influenced after disruption of acidic lysosomes. Moreover, neutralization of TGF-βs, inhibition of TGF-β receptor, or inhibition of the canonical pathway decreased membrane expression of ATP6V1A and prevented the acidosis-induced increased V-ATPase activity. The results suggest multiple modes of action of TGF-β1 on V-ATPase subunits in HSG cells: TGF-β1 may regulate transcription or protein synthesis of certain subunits and trafficking of other subunits in a context-dependent manner. Moreover, surface V-ATPase is active in salivary duct cells and involved in intracellular pH regulation following acidosis.
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Affiliation(s)
- Milena Kluge
- Department of Molecular Embryology, Faculty of Medicine, Institute of Anatomy and Cell Biology, University of Freiburg, Freiburg, Germany
| | - Eun Namkoong
- Department of Physiology, School of Dentistry, Seoul National University, Seoul, South Korea
| | - Shokoufeh Khakipoor
- Department of Molecular Embryology, Faculty of Medicine, Institute of Anatomy and Cell Biology, University of Freiburg, Freiburg, Germany
| | - Kyungpyo Park
- Department of Physiology, School of Dentistry, Seoul National University, Seoul, South Korea
| | - Eleni Roussa
- Department of Molecular Embryology, Faculty of Medicine, Institute of Anatomy and Cell Biology, University of Freiburg, Freiburg, Germany
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Khakipoor S, Giannaki M, Theparambil SM, Zecha J, Küster B, Heermann S, Deitmer JW, Roussa E. Functional expression of electrogenic sodium bicarbonate cotransporter 1 (NBCe1) in mouse cortical astrocytes is dependent on S255‐257 and regulated by mTOR. Glia 2019; 67:2264-2278. [DOI: 10.1002/glia.23682] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2018] [Revised: 06/23/2019] [Accepted: 07/03/2019] [Indexed: 01/14/2023]
Affiliation(s)
- Shokoufeh Khakipoor
- Department of Molecular Embryology, Faculty of Medicine Institute of Anatomy and Cell Biology, Albert‐Ludwigs‐Universität Freiburg Freiburg Germany
| | - Marina Giannaki
- Department of Molecular Embryology, Faculty of Medicine Institute of Anatomy and Cell Biology, Albert‐Ludwigs‐Universität Freiburg Freiburg Germany
| | - Shefeeq M. Theparambil
- Department of General Zoology, FB Biology University of Kaiserslautern Kaiserslautern Germany
| | - Jana Zecha
- Chair of Proteomics and Bioanalytics Technical University of Munich Freising Germany
| | - Bernhard Küster
- Chair of Proteomics and Bioanalytics Technical University of Munich Freising Germany
- Bavarian Biomolecular Mass Spectrometry Center (BayBioMS) Technical University of Munich Freising Germany
| | - Stephan Heermann
- Department of Molecular Embryology, Faculty of Medicine Institute of Anatomy and Cell Biology, Albert‐Ludwigs‐Universität Freiburg Freiburg Germany
| | - Joachim W. Deitmer
- Department of General Zoology, FB Biology University of Kaiserslautern Kaiserslautern Germany
| | - Eleni Roussa
- Department of Molecular Embryology, Faculty of Medicine Institute of Anatomy and Cell Biology, Albert‐Ludwigs‐Universität Freiburg Freiburg Germany
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Feuerstein M, Chleilat E, Khakipoor S, Michailidis K, Ophoven C, Roussa E. Expression patterns of key Sonic Hedgehog signaling pathway components in the developing and adult mouse midbrain and in the MN9D cell line. Cell Tissue Res 2017; 370:211-225. [PMID: 28799057 PMCID: PMC5640734 DOI: 10.1007/s00441-017-2664-2] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2016] [Revised: 06/22/2017] [Accepted: 06/23/2017] [Indexed: 12/29/2022]
Abstract
The temporal dynamic expression of Sonic Hedgehog (SHH) and signaling during early midbrain dopaminergic (mDA) neuron development is one of the key players in establishing mDA progenitor diversity. However, whether SHH signaling is also required during later developmental stages and in mature mDA neurons is less understood. We study the expression of SHH receptors Ptch1 and Gas1 (growth arrest-specific 1) and of the transcription factors Gli1, Gli2 and Gli3 in mouse midbrain during embryonic development [embryonic day (E) 12.5 onwards)], in newborn and adult mice using in situ hybridization and immunohistochemistry. Moreover, we examine the expression and regulation of dopaminergic neuronal progenitor markers, midbrain dopaminergic neuronal markers and markers of the SHH signaling pathway in undifferentiated and butyric acid-treated (differentiated) MN9D cells in the presence or absence of exogenous SHH in vitro by RT-PCR, immunoblotting and immunocytochemistry. Gli1 was expressed in the lateral mesencephalic domains, whereas Gli2 and Gli3 were expressed dorsolaterally and complemented by ventrolateral expression of Ptch1. Co-localization with tyrosine hydroxylase could not be observed. GAS1 was exclusively expressed in the dorsal mesencephalon at E11.5 and co-localized with Ki67. In contrast, MN9D cells expressed all the genes investigated and treatment of the cells with butyric acid significantly upregulated their expression. The results suggest that SHH is only indirectly involved in the differentiation and survival of mDA neurons and that the MN9D cell line is a valuable model for investigating early development but not the differentiation and survival of mDA neurons.
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Affiliation(s)
- Melanie Feuerstein
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany
| | - Enaam Chleilat
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany
| | - Shokoufeh Khakipoor
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany
| | - Konstantinos Michailidis
- Neuroanatomy, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany
| | - Christian Ophoven
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany
| | - Eleni Roussa
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany. .,Neuroanatomy, Faculty of Medicine, University of Freiburg, Albertstrasse 17, D-79104, Freiburg, Germany.
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Khakipoor S, Ophoven C, Schrödl‐Häußel M, Feuerstein M, Heimrich B, Deitmer JW, Roussa E. TGF-β signaling directly regulates transcription and functional expression of the electrogenic sodium bicarbonate cotransporter 1, NBCe1 (SLC4A4), via Smad4 in mouse astrocytes. Glia 2017; 65:1361-1375. [PMID: 28568893 PMCID: PMC5518200 DOI: 10.1002/glia.23168] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2017] [Revised: 04/27/2017] [Accepted: 05/04/2017] [Indexed: 02/06/2023]
Abstract
The electrogenic sodium bicarbonate cotransporter NBCe1 (SLC4A4) expressed in astrocytes regulates intracellular and extracellular pH. Here, we introduce transforming growth factor beta (TGF-β) as a novel regulator of NBCe1 transcription and functional expression. Using hippocampal slices and primary hippocampal and cortical astrocyte cultures, we investigated regulation of NBCe1 and elucidated the underlying signaling pathways by RT-PCR, immunoblotting, immunofluorescence, intracellular H(+ ) recording using the H(+ ) -sensitive dye 2',7'-bis-(carboxyethyl)-5-(and-6)-carboxyfluorescein, mink lung epithelial cell (MLEC) assay, and chromatin immunoprecipitation. Activation of TGF-β signaling significantly upregulated transcript, protein, and surface expression of NBCe1. These effects were TGF-β receptor-mediated and suppressed following inhibition of JNK and Smad signaling. Moreover, 4-aminopyridine (4AP)-dependent NBCe1 regulation requires TGF-β. TGF-β increased the rate and amplitude of intracellular H+ changes upon challenging NBCe1 in wild-type astrocytes but not in cortical astrocytes from Slc4a4-deficient mice. A Smad4 binding sequence was identified in the NBCe1 promoter and Smad4 binding increased after activation of TGF-β signaling. The data show for the first time that NBCe1 is a direct target of TGF-β/Smad4 signaling. Through activation of the canonical pathway TGF-β acts directly on NBCe1 by binding of Smad4 to the NBCe1 promoter and regulating its transcription, followed by increased protein expression and transport activity.
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Affiliation(s)
- Shokoufeh Khakipoor
- Department of Molecular EmbryologyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
| | - Christian Ophoven
- Department of Molecular EmbryologyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
| | - Magdalena Schrödl‐Häußel
- Department of Molecular EmbryologyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
| | - Melanie Feuerstein
- Department of Molecular EmbryologyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
| | - Bernd Heimrich
- Department of NeuroanatomyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
| | - Joachim W. Deitmer
- Department of General ZoologyFB Biology, University of KaiserslauternP.B. 3049D‐67653KaiserslauternGermany
| | - Eleni Roussa
- Department of Molecular EmbryologyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
- Department of NeuroanatomyInstitute for Anatomy and Cell Biology, Faculty of Medicine, University of FreiburgAlbertstrasse 17D‐79104FreiburgGermany
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Roussa E, Speer JM, Chudotvorova I, Khakipoor S, Smirnov S, Rivera C, Krieglstein K. The membrane trafficking and functionality of the K+-Cl- co-transporter KCC2 is regulated by TGF-β2. J Cell Sci 2016; 129:3485-98. [PMID: 27505893 PMCID: PMC5047681 DOI: 10.1242/jcs.189860] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2016] [Accepted: 08/02/2016] [Indexed: 02/02/2023] Open
Abstract
Functional activation of the neuronal K(+)-Cl(-) co-transporter KCC2 (also known as SLC12A5) is a prerequisite for shifting GABAA responses from depolarizing to hyperpolarizing during development. Here, we introduce transforming growth factor β2 (TGF-β2) as a new regulator of KCC2 membrane trafficking and functional activation. TGF-β2 controls membrane trafficking, surface expression and activity of KCC2 in developing and mature mouse primary hippocampal neurons, as determined by immunoblotting, immunofluorescence, biotinylation of surface proteins and KCC2-mediated Cl(-) extrusion. We also identify the signaling pathway from TGF-β2 to cAMP-response-element-binding protein (CREB) and Ras-associated binding protein 11b (Rab11b) as the underlying mechanism for TGF-β2-mediated KCC2 trafficking and functional activation. TGF-β2 increases colocalization and interaction of KCC2 with Rab11b, as determined by 3D stimulated emission depletion (STED) microscopy and co-immunoprecipitation, respectively, induces CREB phosphorylation, and enhances Rab11b gene expression. Loss of function of either CREB1 or Rab11b suppressed TGF-β2-dependent KCC2 trafficking, surface expression and functionality. Thus, TGF-β2 is a new regulatory factor for KCC2 functional activation and membrane trafficking, and a putative indispensable molecular determinant for the developmental shift of GABAergic transmission.
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Affiliation(s)
- Eleni Roussa
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, Freiburg D-79104, Germany Institute of Anatomy and Cell Biology, Department of Neuroanatomy, Faculty of Medicine, University of Freiburg, Albertstrasse 17, Freiburg D-79104, Germany
| | - Jan Manuel Speer
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, Freiburg D-79104, Germany
| | - Ilona Chudotvorova
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, Freiburg D-79104, Germany
| | - Shokoufeh Khakipoor
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, Freiburg D-79104, Germany
| | - Sergei Smirnov
- Institute of Biotechnology, University of Helsinki, Viikinkaari 9, Helsinki FIN-00014, Finland
| | - Claudio Rivera
- Institute of Biotechnology, University of Helsinki, Viikinkaari 9, Helsinki FIN-00014, Finland
| | - Kerstin Krieglstein
- Institute of Anatomy and Cell Biology, Department of Molecular Embryology, Faculty of Medicine, University of Freiburg, Albertstrasse 17, Freiburg D-79104, Germany
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