Possible roles of microglial cells for neurotoxicity in clinical neurodegenerative diseases and experimental animal models.

Microglia has been demonstrated to play critical roles in various neurodegenerative disorders, such as Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD) as well as neuroinflammatory disorders including AIDS encephalitis, multiple sclerosis. In this manuscript, we review the possible roles of microglial cells in animal models of these clinical disorders and human clinical cases. Activated microglia has been demonstrated in various brain regions, such as the hippocampus, substantia nigra and cortex in PD, AD and HD. The contribution of microglial cells to these neurodegenerative disorders is supported by findings in animal experiments: (1) microglial activation precedes the neurodegenerative changes; (2) activated microglia surround the region that undergo neurodegeneration and phagocytose the degenerating cells; (3) activated microglia release neurotoxic molecules such as interleukin(IL)-1beta, IL-6, TNF-alpha, nitric oxide, reactive oxygen species; (4) inhibition of microglial activation leads to the amelioration of neurodegeneration, (5) microglia derived from aged animal exert more toxicity to neurons in an age-dependent fashion, in the same way neurodegenerative disorders occur. Although roles of activated microglia in those clinical disorders needs to be further investigated, these findings suggest that microglial cells may contribute to the progression of neurodegenerative changes as well as inflammation in the brain. Thus, the treatment to target microglial inhibition may help to develop the pharmaceutical approaches for those clinical disorders.

Axotomy-induced dopaminergic neurodegeneration is accompanied with c-Jun phosphorylation and activation transcription factor 3 expression.

Accumulating evidence has shown that both phosphorylated c-Jun (pc-Jun) and activating transcription factor 3 (ATF3) were upregulated in a variety of tissue injuries and proposed to play an important role in cell death/survival. To elucidate the significance and functional role of these immediate-early genes during neuronal damage in the central nervous system, we examined temporal and spatial profiles of pc-Jun and ATF3 in dopaminergic neurons of the substantia nigra (SN) following transection of the medial forebrain bundle (MFB) in adult rats. Morphological characteristics of pc-Jun-positive dopaminergic neurons as well as microglial reaction in response to the axotomy-induced neurodegeneration were also investigated. Following MFB transection, both c-Jun phosphorylation and ATF3 were found in the nuclei of tyrosine hydroxylase-immunoreactive (TH-ir) neurons of the ipsilateral SN, but not in those of the contralateral SN. In the ipsilateral SN, the number of pc-Jun- and ATF3-positive nuclei was increased by 5-7 days post-lesion, and then progressively decreased probably due to the loss of neurons. Retrograde tracing with FluoroGold (FG) in hemi-axotomized rat brain demonstrated that none of the intact, unaxotomized (FG-ir) neurons was pc-Jun-positive, indicating phosphorylation of c-Jun occurs only in axotomized neurons. Concomitant co-localization of pc-Jun and ATF3 in the same TH-ir neuron was also demonstrated by triple immunofluorescence labeling. Many TH-ir neurons that underwent various steps of consecutive neurodegenerative changes retained pc-Jun in the condensed or fragmented nuclei. Moreover, numerous activated microglia, identified by both phagocytic (ED1) and MHC II (OX6) markers, closely apposed to these neurons throughout the entire neurodegenerative process, suggesting that they are actively phagocytosing dying neurons. Taken together, these results support the idea that pc-Jun and its putative dimeric partner ATF3 may be closely participating in axotomy-induced neurodegeneration.

Pathological dynamics of activated microglia following medial forebrain bundle transection.

To elucidate the role and pathological dynamics of activated microglia, this study assessed the phagocytic, immunophenotypic, morphological, and migratory properties of activated microglia in the medial forebrain bundle (MFB) axotomized rat brain. Activated microglia were identified using two different monoclonal antibodies: ED1 for phagocytic activity and OX6 for major histocompatibility complex (MHC) class II. Phagocytic microglia, characterized by ED1-immunoreactivity or ED1- and OX6-immunoreactivity, appeared in the MFB and substantia nigra (SN) as early as 1-3 days post-lesion (dpl), when there was no apparent loss of SN dopamine (DA) neurons. Thereafter, a great number of activated microglia selectively adhered to degenerating axons, dendrites and DA neuronal somas of the SN. This was followed by significant loss of these fibers and nigral DA neurons. Activation of microglia into phagocytic stage was most pronounced between 14 approximately 28 dpl and gradually subsided, but phagocytic microglia persisted until 70 dpl, the last time point examined. ED1 expression preceded MHC II expression in phagocytic microglia. All phagocytic microglia sticking to DA neurons showed activated but ramified form with enlarged somas and thickened processes. They were recruited to the SNc from cranial, dorsal and ventral aspects along various structures and finally stuck to DA neurons of the SNc. Characteristic rod-shaped microglia in the white matter were thought to migrate a long distance. The present study strongly suggests that neurons undergoing delayed neurodegeneration may be phagocytosed by numerous phagocytic, ramified microglia at various sites where specific surface signals are exposed or diffusible molecules are released.