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Annexin VII: an astroglial protein exhibiting a Ca2+-dependent subcellular distribution

Institute of Biochemistry, Medical Faculty, University of Cologne, Joseph-Stelzmann-Str. 52, 50931 Cologne, Germany.
Neuroreport (Impact Factor: 1.64). 06/2001; 12(6):1139-44. DOI: 10.1097/00001756-200105080-00018
Source: PubMed

ABSTRACT A fundamental issue in neuronal and glial cells is how intracellular rises in Ca2+ are coupled to signaling cascades and changes in subcellular morphology. We studied the expression and localization of annexin VII (synexin), a Ca(2+)-/GTP-dependent membrane fusion protein, in the human CNS. Here, we demonstrate the presence of two annexin VII isoforms (47 and 51 kDa) in human brain tissue as well as its exclusive expression in astroglial cells. An in vitro study of astrocyte-derived C6 rat glioblastoma cells expressing a GFP tagged annexin VII fusion protein demonstrates a sequential redistribution of the fusion protein in response to rising intracellular Ca2+ concentrations. Our findings indicate a role of annexin VII in the regulation of intracellular Ca(2+)-dependent processes in astroglial cells.

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    • "Immuno-staining for A7 revealed its presence in the cytoplasmic, nuclear and membrane compartments. Nuclear localization of A7 has been previously reported in astroglial cells by immunostaining and in astrocyte derived C6 rat glioblastoma cells by localization of green fluorescence protein (GFP)-A7 [28]. As observed in our study, this report also demonstrated that calcium ionophore A23187 increased membrane association of GFP-A7. "
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    ABSTRACT: Annexin A7 ist ein Ca2+- und Phospholipid-bindendes Protein, für das verschiedene Funktionen vorgeschlagen wurden wie die Fusion von Vesikeln, Aufbau von Ca2+-Kanälen und die Beteiligung an exozytotischen /sekretorischen Vorgängen. Die physiologische Funktion des Proteins in vivo ist jedoch noch nicht geklärt. In der vorliegenden Arbeit wurde die Verteilung von Annexin A7 in der Embryonalentwicklung der Maus untersucht. Eine Expression des Proteins konnte in allen Geweben ab dem 5. Tag p.c. mittels immunhistologischen und Northern Blot Analysen dargestellt werden. Es zeigte sich eine subzelluläre Verteilung im Zytosol. Weitere Versuche konzentrierten sich auf die Detektion des Proteins im sich entwickelnden murinen Gehirn. Die zelluläre und subzelluläre Lokalisation von Annexin A7 änderte sich während der Differenzierung des Gehirns. Am 13.-15. Tag p.c. wiesen nur die undifferenzierten Zellen des Stratum germinativum im Zytosol eine Annexin A7 Immunoreaktivität auf. Mit fortschreitender Entwicklung (ab E16) zeigte sich eine Präsenz des Proteins im Kern der sich differenzierenden Zellen in der Intermediärschicht und im Neocortexbereich. Diese Umlokalisation spiegelt die Wanderung der Neuronen und Astrogliazellen während ihres Differenzierungsprozesses wider und ist als Hinweis zu deuten, dass die subzelluläre Verteilung von Annexin A7 abhängig vom Entwicklungsgrad einer Zelle bzw. vom Zelltyp ist. Die Expression von Annexin A7 wurde auch im Gehirn von adulten Mäusen untersucht. Auffällig war seine Kernlokalisation in Neuronen der Hippocampusformation, des Cerebellums und des Cortex temporalis. Darüber hinaus kommt Annexin A7 auch in glialen Zellen vor, die mit einer GFAP-Markierung als Astrozyten charakterisiert wurden. Frühere Studien an der AnxA7-/- Maus lieferten Hinweise auf eine gestörte Ca2+ Homöostase in den Kardiomyozyten nach Verlust des Proteins (Herr et al., 2001). Aus diesem Grund konzentrierte sich die Arbeit auf die Charakterisierung des kardiovaskulären Phänotyps und auf die Analyse der Herzfunktion der AnxA7-/- Maus. Das Herz der AnxA7-/- Tiere war normal entwickelt und zeigte keine pathologische Veränderung. In EKGMessungen ließ sich bei den Annexin A7 defizienten Mäuse eine verkürzte PQ- und QRSKomplex- Zeit sowie eine Bradykardie beobachten. Dies ist ein Anhaltspunkt für die Rolle von Annexin A7 in der elektromechanischen Koppelung von Herzmuskelzellen. Die Induktion einer linksventrikulären Myokardhypertrophie bei den WT- und AnxA7-/- Mäusen schlug aufgrund der Sensibilität des Mausstamms SV129 gegenüber den verabreichten Medikamenten fehl. Annexin A7 is a Ca2+- and phospholipid binding protein. Many functions have been proposed including the fusion of vesicles, acting as a Ca2+ channel and involvement in exocytosis/secretion. As with other family members, the physiological function in vivo remains to be determined. In the present study we explored the appearence of annexin A7 in mouse development. Immunohistochemistry and northern blot analysis showed expression of annexin A7 starting with day E5 in all tissues examined. At the subcellular level, annexin A7 was present in punctate structures, mainly in the cytosol. By contrast, the subcellular distribution in the developing brain changed. In the day 13-15 mouse embryo, the undifferentiated cells in the ventricular germinative zone were labeled mainly in the cytosol. One day later the first annexin A7 positive signals could be observed in the cells of the intermediate zone and the neopallium cortex. The cells showed a predominantly nuclear staining. The migratory pattern of annexin A7 positive cells from the ventricular germinative zone to the neocortex observed in this study was very similar to the developmental pattern of neuronal and astroglial precursors following the guidance of the radial glial cells. All these findings suggested that the subcellular localization of annexin A7 is regulated depending on the developmental stages and the cell types. Previous studies demonstrated the exclusive expression of annexin A7 in astroglial cells in human brain tissue (Clemen et al., 2001). We studied its expression in the adult mouse brain and found that annexin A7 is present in astrocytes and in neurons with distinct subcellular localization in these cells. Neurons showed an intense immunoreactivity in the nucleus and a weaker signal in the cytosol, whereas in astrocytes the protein was localized to punctate structures mainly in the cytosol. To understand the function of annexin A7 an Anxa7-/- mouse was generated (Herr, 2000). Ca2+ regulation studied in isolated cardiomyocytes showed that while cells from early embryos displayed intact Ca2+ homeostasis, cardiomyocytes from adult Anxa7-/-mice exhibited an altered cell shortening-frequency relationship when stimulated with high frequency. This defect does not interfere with the viability of animals maintained under normal conditions. Here we examined the cardiovascular phenotype of the knock-out mouse and the heart function under stress. Heart weight and weight of the ventricles is normal in the annexin A7 mouse. Analysing electrocardiograms from annexin A7 deficient mice we observed a reduced PQ-time and QRS-complex-time and a bradycardia. This suggested a role for annexin A7 in electromechanical coupling, probably through Ca2+ homeostasis. At last, it should be noted that the induction of a cardial hypertrophy failed in our study. This appears to be due to the high sensitivity of the mouse strain SV129 to the applicated drugs.
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