Repopulation of a microglia-depleted central nervous system : molecular characterization during homeostasis and disease

<p>Microglia are predominant tissue resident macrophages within the central nervous system (CNS), and contribute to both CNS development and homeostasis. During disease conditions microglia undergo transcriptional re-programming and their dysfunction is implicated in a multitude of disorders, such as multiple sclerosis (MS). How microglia could be therapeutically targeted is a current research focus. Recent experimental microglial depletion methods using conditional genetic targeting and pharmacological therapies have broadened our perspective of these multi-tasking microglia. Newly repopulated microglia following experimental microglial ablation hold great promise for reducing neuroinflammation and treating a variety of neurological disorders.</p><p>In Study 1 our results indicated that microglia could be ablated (approximately 95%) by systemic use of tamoxifen in Cx3cr1CreER/+Rosa26DTA/+ mice. Microglial repopulation ensued through both the proliferation of surviving microglia in the CNS, and from the infiltration of Ly6Chi monocytes. Under this condition infiltrating monocytes could be shaped into microglia-like cells by the CNS microenvironment. Furthermore, isolated newly repopulated resident microglia and infiltrating microglia-like cells following experimental depletion exhibited differential functionality in vitro, such as phagocytic capacity and cytokine production.</p><p>In Study 2 we used the microglial depletion and repopulation model mentioned above and demonstrated that the presence of infiltrating microglia-like cells following ablation could exacerbate experimental autoimmune encephalomyelitis (EAE) symptoms in Cx3cr1CreER/+Rosa26DTA/+ female mice. This was not evident in male mice, indicating a potential sex effect. Under this condition there was a higher expression of major histocompatibility complex class II and a greater secretion of proinflammatory cytokines during the acute period in the female mice.</p><p>In Study 3 we discovered a novel subpopulation of microglia that escape the genetic modification of Cx3cr1 in Cx3cr1CreER-EYFP/+Rosa26DTA/+ mice. Following microglial depletion using tamoxifen, newly repopulated Cx3cr1highEYFP– microglia had an advantage over Cx3cr1CreER-EYFP/+ and Cx3cr1lowEYFP+ microglia. We also found that microglial repopulation was tightly regulated by the CX3CL1-CX3CR1 signaling. The numbers of repopulated CNS-resident microglia were significantly decreased, while the numbers of infiltrating microglia-like cells were increased during repopulation in mice devoid of Cx3cr1.</p><p>In Study 4 we demonstrated that experimentally removing microglia using both Cx3cr1CreER/+Rosa26DTA/+ mice and PLX3397 treatment had crucial effects on circulating monocytes and splenic macrophages, a finding that had previously received little attention. We therefore proposed that clinical translation of preclinical studies using microglial depletion should take peripheral effects into consideration.</p><h3>List of scientific papers</h3><p>I. Competitive repopulation of an empty microglial niche yields functionally distinct subsets of microglia-like cells. Harald Lund, Melanie Pieber, Roham Parsa, Jinming Han, David Grommisch, Ewoud Ewing, Lara Kular, Maria Needhamsen, Alexander Espinosa, Emma Nilsson, Anna K. Överby, Oleg Butovsky, Maja Jagodic, Xing-Mei Zhang, Robert A. Harris. Nature Communications. 2018 Nov 19;9(1):4845. <br><a href="https://doi.org/10.1038/s41467-018-07295-7">https://doi.org/10.1038/s41467-018-07295-7</a><br><br> </p><p>II. Sex-specific effects of microglia-like cell engraftment during experimental autoimmune encephalomyelitis. Jinming Han, Keying Zhu, Kai Zhou, Ramil Hakim, Sreenivasa Raghavan Sankavaram, Klas Blomgren, Harald Lund, Xing-Mei Zhang, Robert A. Harris. International Journal of Molecular Sciences. 2020 Sep 17;21(18):6824. <br><a href="https://doi.org/10.3390/ijms21186824">https://doi.org/10.3390/ijms21186824</a><br><br> </p><p>III. Microglial niche repopulation competition following genetic depletion is regulated by CX3CL1-CX3CR1 signaling. Kai Zhou, Jinming Han, Harald Lund, Nageswara Rao Boggavarapu, Volker Lauschke, Shinobu Goto, Ahmed M Osman, Yuyu Wang, Asuka Tachi, Cuicui Xie, Ying Sun, Dong Liang, Wei Han, Keying Zhu, Kristina Gemzell-Danielsson, Christer Betsholtz, Xing-Mei Zhang, Changlian Zhu, Bertrand Joseph, Robert A. Harris, Klas Blomgren. [Manuscript]</p><p>IV. Underestimated peripheral effects following pharmacological and conditional genetic microglial depletion. Jinming Han, Yueshan, Fan, Kai Zhou, Keying Zhu, Klas Blomgren, Harald Lund, Xing-Mei Zhang, Robert A. Harris. International Journal of Molecular Sciences. 2020 Nov 15;21(22):8603. <br><a href="https://doi.org/10.3390/ijms21228603">https://doi.org/10.3390/ijms21228603</a><br><br> </p>