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Microglial repopulation model reveals a robust homeostatic process for replacing CNS myeloid cells


Varvel, Nicholas H; Grathwohl, Stefan A; Baumann, Frank; Liebig, Christian; Bosch, Andrea; Brawek, Bianca; Thal, Dietmar R; Charo, Israel F; Heppner, Frank L; Aguzzi, Adriano; Garaschuk, Olga; Ransohoff, Richard M; Jucker, Mathias (2012). Microglial repopulation model reveals a robust homeostatic process for replacing CNS myeloid cells. Proceedings of the National Academy of Sciences of the United States of America, 109(44):18150-18155.

Abstract

Under most physiological circumstances, monocytes are excluded from parenchymal CNS tissues. When widespread monocyte entry occurs, their numbers decrease shortly after engraftment in the presence of microglia. However, some disease processes lead to focal and selective loss, or dysfunction, of microglia, and microglial senescence typifies the aged brain. In this regard, the long-term engraftment of monocytes in the microglia-depleted brain remains unknown. Here, we report a model in which a niche for myeloid cells was created through microglia depletion. We show that microglia-depleted brain regions of CD11b-HSVTK transgenic mice are repopulated with new Iba-1-positive cells within 2 wk. The engrafted cells expressed high levels of CD45 and CCR2 and appeared in a wave-like pattern frequently associated with blood vessels, suggesting the engrafted cells were peripheral monocytes. Although two times more numerous and morphologically distinct from resident microglia up to 27 wk after initial engraftment, the overall distribution of the engrafted cells was remarkably similar to that of microglia. Two-photon in vivo imaging revealed that the engrafted myeloid cells extended their processes toward an ATP source and displayed intracellular calcium transients. Moreover, the engrafted cells migrated toward areas of kainic acid-induced neuronal death. These data provide evidence that circulating monocytes have the potential to occupy the adult CNS myeloid niche normally inhabited by microglia and identify a strong homeostatic drive to maintain the myeloid component in the mature brain.

Abstract

Under most physiological circumstances, monocytes are excluded from parenchymal CNS tissues. When widespread monocyte entry occurs, their numbers decrease shortly after engraftment in the presence of microglia. However, some disease processes lead to focal and selective loss, or dysfunction, of microglia, and microglial senescence typifies the aged brain. In this regard, the long-term engraftment of monocytes in the microglia-depleted brain remains unknown. Here, we report a model in which a niche for myeloid cells was created through microglia depletion. We show that microglia-depleted brain regions of CD11b-HSVTK transgenic mice are repopulated with new Iba-1-positive cells within 2 wk. The engrafted cells expressed high levels of CD45 and CCR2 and appeared in a wave-like pattern frequently associated with blood vessels, suggesting the engrafted cells were peripheral monocytes. Although two times more numerous and morphologically distinct from resident microglia up to 27 wk after initial engraftment, the overall distribution of the engrafted cells was remarkably similar to that of microglia. Two-photon in vivo imaging revealed that the engrafted myeloid cells extended their processes toward an ATP source and displayed intracellular calcium transients. Moreover, the engrafted cells migrated toward areas of kainic acid-induced neuronal death. These data provide evidence that circulating monocytes have the potential to occupy the adult CNS myeloid niche normally inhabited by microglia and identify a strong homeostatic drive to maintain the myeloid component in the mature brain.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > University Hospital Zurich > Institute of Neuropathology
Dewey Decimal Classification:570 Life sciences; biology
610 Medicine & health
Language:English
Date:2012
Deposited On:30 Nov 2012 13:55
Last Modified:05 Apr 2016 16:08
Publisher:National Academy of Sciences
Series Name:Proceedings of the National Academy of Sciences USA
ISSN:0027-8424
Publisher DOI:https://doi.org/10.1073/pnas.1210150109
PubMed ID:23071306

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