lable at ScienceDirect

Current Anaesthesia & Critical Care 20 (2009) 142–147

Contents lists avai

Current Anaesthesia & Critical Care

journal homepage: www.elsevier .com/locate/cacc

BASIC SCIENCE

Neuroprotective function in brain microglia

Yoshihisa Kitamura*, Daijiro Yanagisawa, Kazuyuki Takata,Takashi TaniguchiDepartment of Neurobiology, 21st Century COE Program, Kyoto Pharmaceutical University, 5 Nakauchi-cho,Misasagi, Yamashina-ku, Kyoto 607-8414, Japan

Keywords:MicrogliaBone marrowNeuroprotectionNeurodegenerationTransplantation

* Corresponding author. Tel.: þ81 75 595 4706; faxE-mail address: yo-kita@mb.kyoto-phu.ac.jp (Y. Ki

0953-7112/$ – see front matter � 2008 Elsevier Ltd.doi:10.1016/j.cacc.2008.12.007

s u m m a r y

Microglia are uniformly distributed throughout the central nervous system. The number of microglia isthought to make up 5–20% of the entire glial cell population. Origin of microglia has been discussed forseveral decades. Recently, microglia are widely considered to originate from mesodermal monocyte/macrophage cell lineage, because of similarities in cell surface molecular phenotype. In normal adultbrain, microglia have finely branched and ramified cell processes that extend in all directions, and surveythe brain microenvironment. When the brain is injured by trauma, stroke and other neurodegenerativedisorders, microglia transform their morphology into an activated phenotype, and accumulate in theaffected sites. Activation of microglia has been believed to lead to neurotoxic effect, because of results ofthe in vitro culture studies, which reflects only one possible phenotype of microglia. However, recentstudies in the in vivo brain revealed that microglia play a crucial role in neuroprotection against neuralcell injury such as cerebral ischemia. Thus, microglial function under pathological conditions in the CNSremains a controversial subject, because of their complexity. In the present review, we summarized thecurrent findings of microglial function, and discussed the possibility of the application to a novel ther-apeutic strategy.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Microglia are traditionally considered to be brain macrophages,and play an important role in immune function in the centralnervous system (CNS).1 In normal adult brain, microglia areuniformly distributed with finely branched and ramified cellprocesses that extend in all directions, described as ramifiedmicroglia. This type of microglia had been considered to be restingphenotype, however, recent studies showed that ramified microgliaare far from inactive. Ramified microglia generally work as sensorsfor detecting abnormalities and disturbances in the CNS.1–5 Whenthe brain is injured by trauma, stroke and other neurodegenerativedisorders, microglia transform their morphology with hypertro-phied, thin and shorter processes, described as reactive microglia.Reactive microglia are characterized not only by a morphologicaltransformation but also by a number of features, such as expressionof a wide range of myeloid markers, production of cytokines, freeradicals and nitric oxide, and the acquisition of a phagocyticphenotype. In addition, reactive microglia express major

: þ81 75 595 4796.tamura).

All rights reserved.

histocompatibility complex (MHC) antigens, suggesting thatmicroglia serve as antigen presenting cells. In neurodegenerativedisorders, such as Alzheimer’s disease, Parkinson’s disease andcerebral ischemia, microglial accumulation with the activatedphenotype is observed in the affected sites. In addition, it isreported that hyperactive microglia in the spinal cord play a crucialrole in neuropathic pain through P2X4 receptors, a subtype of theionotropic ATP receptor.6 These observations suggest that reactivemicroglia play an important role in pathological events in the CNS.

Activation of microglia has been considered as a detrimentalevent. Because lipopolysaccharide (LPS)-stimulated microglia,which has been considered as the gold standard for microglial acti-vation, acquire a phenotype that is both inflammatory and cytotoxicin cell-culture studies, and such a phenotype of microglia wascommonly accepted as typical of all activated microglia.2,7 However,recent in vivo studies suggest that microglia under pathologicalconditions exert neuroprotective functions, such as the productionof trophic factors and the elimination of cellular debris, rather thanneurodegenerative effects. Furthermore, novel therapeutic strate-gies based on the microglial neuroprotective function againstneurodegenerative disorders are arising and being developed.

Here, we reviewed current findings of microglia, especially theirorigin and functions under physiological and pathological

Y. Kitamura et al. / Current Anaesthesia & Critical Care 20 (2009) 142–147 143

conditions. We also speculated the possibility of novel strategies forneuroprotection based on microglial functions.

2. Origin of microglia

Microglia are uniformly distributed with ramified shapethroughout the adult CNS. The number of microglia is thought tomake up 5–20% of the entire glial cell population.1,8 On the otherhand, the origin of microglia is uncertain and has been at the centerof debate for several decades between two schools of thought: (1)mesodermal and (2) neuroectodermal. However, most of the manymarkers that recognize cell surface antigen of microglia cross-reactwith monocytes and macrophages. On the other hand, there are noneuroectodermal markers that recognize microglia, monocytes, ormacrophages. These observations suggest existence of some ofsame surface molecules in both microglia and monocytes/macro-phages, but not neuroectodermal cell lineage. Therefore, microgliaare considered to originate from mesodermal monocyte/macro-phage cell lineage because of the similarities in cell surfacemolecular phenotype, and this school is now widely accepted.9–11

In the developing mouse brain, primitive microglia (fetalmacrophages) were first observed in the yolk sac, which is one ofextramedullary sources of hematopoiesis, at embryonic day (E) 8and these cells were distributed in the mesenchyme associated withthe neuroepithelial basement membrane at E8.5.12 Another reportshowed that Iba1-positive cells, which is a marker for microglia,were apparent and accumulated in the mesenchyme surroundingneural tube at E10.5.9 In perinatal brain, clusters of roundedmicroglial progenitor cells (amoeboid microglia) are foundprimarily in the regions of developing white matter tract, particu-larly corpus callosum.11,13 Microglial progenitor cells in thedeveloping brain act as scavengers of dead cell fragments byphagocytosis, and play a role in the induction of apoptosis and bloodvessel formation.1,10,11 They also participate in the growth andguidance of neurites through the production of trophic factors.

Microglial progenitor cells are recruited from some routes in thedeveloping CNS.10,13,14 Microglial progenitor cells derived frommonocyte-like nature migrate into the brain through the paren-chymal blood vessels of developing white matter since blood–brainbarrier (BBB) forms relatively late. On the other hand, microglialprogenitor cells from fetal macrophage origin enter the developinggray matter from the surrounding meninges. Some of microglialprogenitor cells may reach the brain parenchyma from the ventricle.Microglial progenitor cells, which entered the brain parenchyma,then proliferate and migrate throughout the developing brain.Thereafter, these cells transform into ramified microglia, and thiscell type is the only population of microglial cell type in the whitematter and gray matter from post-natal day (P) 18.13

It has been considered that microglial turnover occurs both bythe proliferation of resident microglia and by the replenishmentby progenitor cells from circulating blood or bone marrow.15

Indeed, recent studies using lethally irradiated green fluorescentprotein (GFP) transgenic bone-marrow chimeric mouse, in whichparenchymal microglia retain the characteristics of the host,whereas peripheral blood cells are reconstituted by GFP-positivebone-marrow cells, showed that bone-marrow-derived GFP-posi-tive cells entered the brain parenchyma, and differentiated intothe microglia.16–20 These observations suggest that microglialprogenitor cells may exist in circulating blood or bone marrow,and that these cells are able to migrate to the brain parenchymathrough the BBB, and replenish or replace resident microglia inthe adult brain. However, studies using two types of non-irradi-ation procedure system (1) parabiosis, a procedure by whichcirculatory system of two animals are joined, and (2) the GFP

transgenic bone-marrow chimeric mouse in which the brain wasprotected from irradiation, showed that no circulating progenitorcells or bone-marrow-derived cells were observed in the brainparenchyma. These observations suggest that the BBB alterationand/or the inflammatory response caused by irradiation may benecessary for migration of microglial progenitor cells into thebrain parenchyma, and therefore, it does not take place underphysiological conditions.21,22 Thus, turnover of microglia is likelyto occur by proliferation and self-renewal of resident microglia,rather than by migration of microglial progenitor cells via thevasculature and meninges in the normal adult CNS.

3. Role of microglia in the adult brain

Ramified microglia in the adult CNS are believed to be a restingphenotype. However, recent studies suggest that ramified microgliawork as a sensor and housekeeper to maintain the CNS homeo-stasis.4,5,23 Microglia ubiquitously localize in the normal adult CNS,and continuously survey the brain microenvironment with theirmotile ramified processes.5 While detecting slight disturbance ofthe CNS microenvironment, microglia rapidly extend theirprocesses to the injured site, and then, microglia eliminate cellulardebris with their processes by phagocytic activity, and shield theirmicroenvironment.5,23 On the other hand, when marked injuryoccurs in the CNS, microglia transform into the activated phenotypewith thick and shorter processes in response to signals such asneurotransmitters, cytokines, chemokines, growth factors, andplasma components from damaged tissues.2–4 Reactive microgliaproduce proinflammatory cytokines, such as tumor necrosis factor-a (TNF-a), interleukine-1b (IL-1b) and IL-6, as well as free radicals,which are believed to provoke neuronal damage in neurodegen-erative disorders. On the other hand, reactive microglia alsoproduce neurotrophic and growth factors, such as glial cellline-derived neurotrophic factor (GDNF), brain-derived neuro-trophic factor (BDNF), insulin-like growth factor (IGF), hepatocytegrowth factor (HGF), neurotrophins (NTs) and platelet-derivedgrowth factor (PDGF), and eliminate dying or dead cells byphagocytosis, which may contribute to the neuroprotective events.Thus, reactive microglia display both neuroinflammatory andneuroprotective effects, and thereby, the role of reactive microgliaremains a controversial subject.

Recent studies suggest that microglia display dystrophicchanges like senescence in the aging human brain.24 These cells arecharacterized by abnormalities in their cytoplasmic structure, suchas deramified, atrophic, fragmented or unusually tortuousprocesses, frequently beading spheroidal or bulbous swelling.24

Such age-related morphological changes in microglia may beinvolved in the development of age-related neurodegenerativedisorders.25

4. Microglial neuroprotective function

As mentioned above, microglia accumulate in the region whereneurons are injured with morphological changes into activatedphenotypes. Studies to reveal the role of microglia in pathologicalconditions have been mainly performed using in vitro culturedmicroglia. However, there are limitations of in vitro studies usingcultured microglia, because it reflects only one possible phenotypeof microglia in physiological and/or pathological conditions in thein vivo brain, and lacks the interaction with the microenvironmentcomponents such as neurons and astrocytes. Therefore, to examinemicroglial function under the neurodegenerative condition in thein vivo brain, we directly injected a fluorescent dye PKH26-labeledprimary-cultured microglial cells from a newborn rat into the

Fig. 1. Neuroprotective effect of exogenous microglia on rat cerebral ischemia. Intracerebroventricular injection of primary-cultured rat microglia inhibited behavioral dysfunctionin rota-rod test (A) and loss of microtubule-associated protein-2 (MAP2, a neural marker) immunoreactive area (B) in ischemia rats induced by middle cerebral artery occlusion.(C) In confocal laser scanning analysis, microglia (Iba1 immunopositive cells, green) accumulated in and around ischemic regions. At that time, some PKH26-labeled (red) exogenousmicroglia also accumulated. *P< 0.05 vs. vehicle. Scale bar: 50 mm.

Y. Kitamura et al. / Current Anaesthesia & Critical Care 20 (2009) 142–147144

lateral ventricle in a rat model of focal cerebral ischemia. Themicroglial injection significantly inhibited both behavioraldysfunction and neurodegeneration induced by focal cerebralischemia (Fig. 1A,B), and PKH26-labeled exogenous microgliaaccumulated in the ischemic region (Fig. 1C).26 The production ofGDNF and early clearance of neurotoxic debris by exogenousmicroglia seem to be involved in this neuroprotective event. Othergroups also revealed that systemically injected exogenous micro-glia migrated into the injured site in the brain parenchyma, andcontribute to neuroprotection through the production of neuro-trophic factors such as GDNF and BDNF.27,28 Furthermore, Neumannet al. suggested that the cell–cell interaction of exogenous microgliawith injured and/or dead neurons in first neuronal layer in orga-notypic hippocampal slice cultures, which is described as ‘capping’,is a trigger of the trophic support mechanism, including phagocyticremoval of dying/dead neurons and the release of neurotrophic/growth factors, to improve the survival of the neurons in the deeperlayers in the slice.29 These observations suggest that the exogenousmicroglia serve as neuroprotection against cerebral ischemiathrough the production of neurotrophic factors and the elimination

of dead cell debris. On the other hand, Lalancette-Hebert et al.demonstrated that resident microglia play a key role in endogenousneuroprotection against cerebral ischemia.30 They showed thatselective ablation of proliferating microglia, using CD11b-thymi-dine kinase mutant-30 (TKmt-30) transgenic mouse and gansiclo-vir (GCV) treatment, exacerbated ischemia-inducedneurodegeneration, because of decrease in the level of IGF that wasexpressed by resident microglia in the peri-infarct area. Thus,taking into account these in vivo recent studies, microglia are likelyto contribute to neuroprotection in the acute neuronal injury suchas cerebral ischemia. In contrast, microglial accumulation isobserved in the chronic neurodegenerative disease such as Alz-heimer’s disease. In Alzheimer’s disease, microglia accumulatearound the senile plaque, which is the extracellular deposition offibrillar amyloid-b (Ab) and one of the pathological features ofAlzheimer’s disease. These finding indicates that there may bea relationship between microglial function and the pathology ofAlzheimer’s disease, especially Ab plaques. Indeed, it has beenreported that microglia contribute to the reduction in the level ofAb by phagocytosis.31,32 In addition, we showed that even

Fig. 2. Microglial function and their therapeutic applications. Microglia play a crucial role in neuroprotection, rather than neuroinflammation, under pathological conditions. Recentfindings suggest that microglia can be obtained from various cell sources by the in vitro differentiation procedures, and may have great potentials to provide direct neuroprotection,to serve as vehicle for gene delivery to the CNS, and further, to reconstitute the damaged brain. Abbreviations: ES cells, embryonic stem cells; iPS cells, induced pluripotent stemcells; BM cells, bone-marrow cells; ROS, reactive oxygen species; NO, nitric oxide.

Y. Kitamura et al. / Current Anaesthesia & Critical Care 20 (2009) 142–147 145

exogenous microglia, which were injected into lateral ventricle inrats previously injected with Ab into the hippocampus, migratedand accumulated around Ab deposit. Therein, exogenous microgliadisplayed phagocytic activity, and these events resulted in signifi-cant reduction of the level of Ab.33 Taken together, we speculate thepossibility that microglial function is a useful target for develop-ment of new therapeutic strategy in neurodegenerative disorders.On the other hand, microglia, but not macrophages specificallymigrated into the brain after peripheral injection,34 and exhibitspecific affinity for damaged sites.35 Accordingly, we also speculatethat peripherally introduction of microglia into the brain paren-chyma is one possible therapeutic strategy for neurodegenerativedisorders.

5. Therapeutic application in neurodegenerative disorders

Microglia are believed to originate from mesodermal monocyte/macrophage cell lineage, and have neuroprotective potential in theCNS under pathological conditions. In GFP transgenic bone-marrowchimeric mouse, microglial progenitor cells from bone marrowentered the CNS parenchyma and differentiated into microglia.Then bone-marrow-derived microglia accumulated in and aroundthe neurodegenerative sites induced by cerebral ischemia.36–38

Furthermore, Simard et al. suggest that bone-marrow-derivedmicroglia, rather than resident microglia, are substantial for theremoval of Ab in mouse model of Alzheimer’s disease.32 Theseobservations suggest that introduction of exogenous microglia,which are derived from bone-marrow cells, into the affected brainparenchyma may be a powerful tool for the improvement ofpathological events. In addition, recruitment of exogenous micro-glia may also be a novel therapeutic strategy for gene delivery to theCNS.17,36

To achieve the therapeutic strategy using the introduction ofexogenous microglia into the brain, it is necessary to resolve whatkind of cell population is able to differentiate into the microglia.Recent studies revealed that CCR2 is critical to the recruitment ofbone-marrow-derived microglia, and that monocytes expressinghigh level of Ly-6C antigen (Ly-6Chi) and equipped with CCR2, arepreferentially recruited into the CNS.22,39 CCR2 is a receptor formonocyte chemoattractantprotein-1 (MCP-1), also named CCL2,a member of the CC family chemokine, and this receptor isexpressed selectively on cells of monocyte/macrophage lineage. Onthe other hand, Ly-6Chi mouse monocytes preferentially populatesites of experimentally induced inflammation, and therefore thesecells are termed inflammatory monocytes. These observations

suggest that a particular cell population of microglial progenitorcells, such as Ly-6ChiCCR2þ mouse monocytes, exists in circulatingblood and enters the CNS, where they subsequently differentiateinto the microglia. It is also suggested that Ly-6ChiCCR2þ mousemonocytes are suitable for cell source of microglia.

However, specific conditions such as BBB alteration induced byirradiation are required for the migration of microglial progenitorcells in circulating blood or bone marrow, such as Ly-6ChiCCR2þ

mouse monocytes, into the brain parenchyma,21,22 unlike nativemicroglia.34,35 Therefore, the problem is how to induce and obtainmicroglia, but not progenitor cells, from cell sources, such ascirculating blood cells or bone-marrow cells. It was reported thatmouse CD34þ myeloid progenitor cells that were isolated frommacrophage-colony stimulating factor (M-CSF)-expanded bone-marrow cells displayed differentiation into microglia-like cells,when cultured in the presence of mixed glial cell-conditionedmedium.40 These bone-marrow-derived microglia-like cells con-sisted of MHC II�/B220�/CD86low/CD11bþ cells with a subpopula-tion of cells expressing CD34, and displayed a ramified morphologyand pseudopodia and filament-like structures, which is a pheno-type comparable with native microglia in the adult CNS. In addition,a recent study showed that circulating monocytes from humanperipheral blood differentiated into microglia-like cells in thepresence of astrocyte-conditioned medium.41 Monocyte-derivedmicroglia-like cells harbored several features of parenchymalmicroglia, such as ramified morphology, electrophysiological andpharmacological response, and a cell membrane phenotype. Thesefindings suggest that bone-marrow cells and circulating blood cellshave a great potential as a cell source of microglia, and the devel-opment of the in vitro induction methods of microglia from bone-marrow cells or circulating blood cells offers autotransplantation ofmicroglia as a viable therapeutic approach in neurodegenerativedisorders. Furthermore, it was reported that microglia wereinduced from mouse embryonic stem (ES) cells using a modifiedfive-step method, and these cells displayed the capability of specificmigration into the CNS after intravenously transplantation,42

similarly to native microglia.34,35 This result supports that it may bepossible that other pluripotent stem cells, such as hematopoieticstem cells, mesenchymal stem cells,43 and induced pluripotentstem (iPS) cells,44 can differentiate into microglia in the in vitroculture system, and that pluripotent stem cells are another cellsource of microglia. Taken together, although further studies areneeded, autotransplantation of microglia, which are derived fromvarious cell sources, to provide direct neuroprotective effect and/orto serve as vehicle for gene delivery to the CNS may be achieved inthe near future (Fig. 2). In addition, Yokoyama et al. suggested that

Y. Kitamura et al. / Current Anaesthesia & Critical Care 20 (2009) 142–147146

in vitro cultured microglia were dedifferentiated under low glucoseconditions with a high concentration of fetal bovine serum, and,surprisingly, a small population among these dedifferentiated cellsturned into neurons under subsequent serum-free conditions,while others became astrocytes, oligodendrocytes or micro-glia.45,46 Thus, microglia may have the ability to dedifferentiateand then turn into neurons, and these findings may lead to thedevelopment of brain regenerative therapy for neurodegenerativedisorders based on microglial transdifferentiation (Fig. 2).

6. Conclusion

Microglial function is of great interest, because of theircomplexity and the relationship with the pathology in neurode-generative disorders. In this review, we summarized the origin ofmicroglia and roles under not only physiological condition but alsopathological conditions. Microglia, which are considered to origi-nate from bone-marrow-derived mesodermal monocyte/macro-phage cell lineage, are uniformly distributed throughout the adultCNS, and survey the brain microenvironment. In neurodegenerativedisorders, microglia transform their morphology into reactivephenotype, and accumulate in the affected sites. Therefore,microglia play a crucial role in neuroprotection, rather thanneurotoxic events, against neural cell injury through the produc-tion of neurotrophic factors and the elimination of dead cell debris.Furthermore, the introduction of exogenous microglia into thebrain parenchyma can be one of the future effective therapeuticapproaches in neurodegenerative disorders. Thus, we believe thatresearch focused on microglial function provides a novel thera-peutic strategy for neurodegenerative disorders.

Acknowledgement

This study was supported in part by the 21st Century Center ofExcellence (COE) and Open Research Programs, and by grants-in-aid from the Ministry of Education, Culture, Sports, Science andTechnology of Japan.

References

1. Kettenmann H, Ransom BR. Neuroglia. 2nd ed. New York: Oxford UniversityPress; 2005.

2. Hanisch UK, Kettenmann H. Microglia: active sensor and versatile effector cellsin the normal and pathologic brain. Nat Neurosci 2007 Nov;10(11):1387–94.

3. Kitamura Y, Nomura Y. Stress proteins and glial functions: possible therapeutictargets for neurodegenerative disorders. Pharmacol Ther 2003 Jan;97(1):35–53.

4. Kreutzberg GW. Microglia: a sensor for pathological events in the CNS. TrendsNeurosci 1996 Aug;19(8):312–8.

5. Nimmerjahn A, Kirchhoff F, Helmchen F. Resting microglial cells are highlydynamic surveillants of brain parenchyma in vivo. Science 2005 May27;308(5726):1314–8.

6. Tsuda M, Shigemoto-Mogami Y, Koizumi S, Mizokoshi A, Kohsaka S, Salter MW,et al. P2X4 receptors induced in spinal microglia gate tactile allodynia afternerve injury. Nature 2003 Aug 14;424(6950):778–83.

7. Schwartz M, Butovsky O, Bruck W, Hanisch UK. Microglial phenotype: is thecommitment reversible? Trends Neurosci 2006 Feb;29(2):68–74.

8. Lawson LJ, Perry VH, Dri P, Gordon S. Heterogeneity in the distribution andmorphology of microglia in the normal adult mouse brain. Neuroscience1990;39(1):151–70.

9. Chan WY, Kohsaka S, Rezaie P. The origin and cell lineage of microglia: newconcepts. Brain Res Rev 2007 Feb;53(2):344–54.

10. Cuadros MA, Navascues J. The origin and differentiation of microglial cellsduring development. Prog Neurobiol 1998 Oct;56(2):173–89.

11. Streit WJ. Microglia and macrophages in the developing CNS. Neurotoxicology2001 Oct;22(5):619–24.

12. Kaur C, Hao AJ, Wu CH, Ling EA. Origin of microglia. Microsc Res Tech 2001 Jul1;54(1):2–9.

13. Dalmau I, Vela JM, Gonzalez B, Finsen B, Castellano B. Dynamics of microglia inthe developing rat brain. J Comp Neurol 2003 Mar 31;458(2):144–57.

14. Ling EA, Wong WC. The origin and nature of ramified and amoeboid microglia:a historical review and current concepts. Glia 1993 Jan;7(1):9–18.

15. Lawson LJ, Perry VH, Gordon S. Turnover of resident microglia in the normaladult mouse brain. Neuroscience 1992;48(2):405–15.

16. Asheuer M, Pflumio F, Benhamida S, Dubart-Kupperschmitt A, Fouquet F,Imai Y, et al. Human CD34þ cells differentiate into microglia and expressrecombinant therapeutic protein. Proc Natl Acad Sci U S A 2004 Mar9;101(10):3557–62.

17. Hess DC, Abe T, Hill WD, Studdard AM, Carothers J, Masuya M, et al. Hemato-poietic origin of microglial and perivascular cells in brain. Exp Neurol 2004Apr;186(2):134–44.

18. Massengale M, Wagers AJ, Vogel H, Weissman IL. Hematopoietic cells maintainhematopoietic fates upon entering the brain. J Exp Med 2005 May16;201(10):1579–89.

19. Simard AR, Rivest S. Bone marrow stem cells have the ability to populate theentire central nervous system into fully differentiated parenchymal microglia.FASEB J 2004 Jun;18(9):998–1000.

20. Vallieres L, Sawchenko PE. Bone marrow-derived cells that populate the adultmouse brain preserve their hematopoietic identity. J Neurosci 2003 Jun15;23(12):5197–207.

21. Ajami B, Bennett JL, Krieger C, Tetzlaff W, Rossi FM. Local self-renewal cansustain CNS microglia maintenance and function throughout adult life. NatNeurosci 2007 Dec;10(12):1538–43.

22. Mildner A, Schmidt H, Nitsche M, Merkler D, Hanisch UK, Mack M, et al.Microglia in the adult brain arise from Ly-6ChiCCR2þ monocytes only underdefined host conditions. Nat Neurosci 2007 Dec;10(12):1544–53.

23. Davalos D, Grutzendler J, Yang G, Kim JV, Zuo Y, Jung S, et al. ATP mediates rapidmicroglial response to local brain injury in vivo. Nat Neurosci 2005Jun;8(6):752–8.

24. Streit WJ, Sammons NW, Kuhns AJ, Sparks DL. Dystrophic microglia in the aginghuman brain. Glia 2004 Jan 15;45(2):208–12.

25. Streit WJ. Microglial senescence: does the brain’s immune system have anexpiration date? Trends Neurosci 2006 Sep;29(9):506–10.

26. Kitamura Y, Yanagisawa D, Inden M, Takata K, Tsuchiya D, Kawasaki T, et al.Recovery of focal brain ischemia-induced behavioral dysfunction by intra-cerebroventricular injection of microglia. J Pharmacol Sci 2005Feb;97(2):289–93.

27. Imai F, Suzuki H, Oda J, Ninomiya T, Ono K, Sano H, et al. Neuroprotective effectof exogenous microglia in global brain ischemia. J Cereb Blood Flow Metab 2007Mar;27(3):488–500.

28. Hayashi Y, Tomimatsu Y, Suzuki H, Yamada J, Wu Z, Yao H, et al. The intra-arterial injection of microglia protects hippocampal CA1 neurons against globalischemia-induced functional deficits in rats. Neuroscience 2006 Sep29;142(1):87–96.

29. Neumann J, Gunzer M, Gutzeit HO, Ullrich O, Reymann KG, Dinkel K.Microglia provide neuroprotection after ischemia. FASEB J 2006Apr;20(6):714–6.

30. Lalancette-Hebert M, Gowing G, Simard A, Weng YC, Kriz J. Selective ablation ofproliferating microglial cells exacerbates ischemic injury in the brain. J Neurosci2007 Mar 7;27(10):2596–605.

31. Kakimura J, Kitamura Y, Takata K, Umeki M, Suzuki S, Shibagaki K, et al.Microglial activation and amyloid-beta clearance induced by exogenous heat-shock proteins. FASEB J 2002 Apr;16(6):601–3.

32. Simard AR, Soulet D, Gowing G, Julien JP, Rivest S. Bone marrow-derivedmicroglia play a critical role in restricting senile plaque formation in Alz-heimer’s disease. Neuron 2006 Feb 16;49(4):489–502.

33. Takata K, Kitamura Y, Yanagisawa D, Morikawa S, Morita M, Inubushi T, et al.Microglial transplantation increases amyloid-beta clearance in Alzheimermodel rats. FEBS Lett 2007 Feb 6;581(3):475–8.

34. Sawada M, Imai F, Suzuki H, Hayakawa M, Kanno T, Nagatsu T. Brain-specificgene expression by immortalized microglial cell-mediated gene transfer in themammalian brain. FEBS Lett 1998 Aug 14;433(1–2):37–40.

35. Imai F, Sawada M, Suzuki H, Zlokovic BV, Kojima J, Kuno S, et al. Exogenousmicroglia enter the brain and migrate into ischaemic hippocampal lesions.Neurosci Lett 1999 Sep 10;272(2):127–30.

36. Priller J, Flugel A, Wehner T, Boentert M, Haas CA, Prinz M, et al. Targeting gene-modified hematopoietic cells to the central nervous system: use of greenfluorescent protein uncovers microglial engraftment. Nat Med 2001Dec;7(12):1356–61.

37. Schilling M, Besselmann M, Leonhard C, Mueller M, Ringelstein EB, Kiefer R.Microglial activation precedes and predominates over macrophage infiltra-tion in transient focal cerebral ischemia: a study in green fluorescentprotein transgenic bone marrow chimeric mice. Exp Neurol 2003Sep;183(1):25–33.

38. Tanaka R, Komine-Kobayashi M, Mochizuki H, Yamada M, Furuya T, Migita M,et al. Migration of enhanced green fluorescent protein expressing bonemarrow-derived microglia/macrophage into the mouse brain followingpermanent focal ischemia. Neuroscience 2003;117(3):531–9.

39. Zhang J, Shi XQ, Echeverry S, Mogil JS, De Koninck Y, Rivest S. Expression ofCCR2 in both resident and bone marrow-derived microglia plays a critical rolein neuropathic pain. J Neurosci 2007 Nov 7;27(45):12396–406.

40. Davoust N, Vuaillat C, Cavillon G, Domenget C, Hatterer E, Bernard A, et al. Bonemarrow CD34þ/B220þ progenitors target the inflamed brain and displayin vitro differentiation potential toward microglia. FASEB J 2006Oct;20(12):2081–92.

Y. Kitamura et al. / Current Anaesthesia & Critical Care 20 (2009) 142–147 147

41. Leone C, Le Pavec G, Meme W, Porcheray F, Samah B, Dormont D, et al. Char-acterization of human monocyte-derived microglia-like cells. Glia 2006 Aug15;54(3):183–92.

42. Tsuchiya T, Park KC, Toyonaga S, Yamada SM, Nakabayashi H, Nakai E, et al.Characterization of microglia induced from mouse embryonic stem cells and theirmigration into the brain parenchyma. J Neuroimmunol 2005 Mar;160(1–2):210–8.

43. Colter DC, Sekiya I, Prockop DJ. Identification of a subpopulation of rapidlyself-renewing and multipotential adult stem cells in colonies of humanmarrow stromal cells. Proc Natl Acad Sci U S A 2001 Jul 3;98(14):7841–5.

44. Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K, et al.Induction of pluripotent stem cells from adult human fibroblasts by definedfactors. Cell 2007 Nov 30;131(5):861–72.

45. Yokoyama A, Sakamoto A, Kameda K, Imai Y, Tanaka J. NG2 proteoglycan-expressing microglia as multipotent neural progenitors in normal and patho-logic brains. Glia 2006 May;53(7):754–68.

46. Yokoyama A, Yang L, Itoh S, Mori K, Tanaka J. Microglia, a potentialsource of neurons, astrocytes, and oligodendrocytes. Glia 2004 Jan 1;45(1):96–104.