Role of microglia in ischemic focal stroke and recovery: focus on Toll-like receptors

Abstract

Stroke is the leading cause of disability in adults. Drug treatments that target stroke-induced pathological mechanisms and promote recovery are desperately needed. In the brain, an ischemic event triggers major inflammatory responses that are mediated by the resident microglial cells. In this review, we focus on the microglia activation after ischemic brain injury as a target of immunomodulatory therapeutics. We divide the microglia-mediated events following ischemic stroke into three categories: acute, subacute, and long-term events. This division encompasses the spatial and temporal dynamics of microglia as they participate in the pathophysiological changes that contribute to the symptoms and sequela of a stroke. The importance of Toll-like receptor (TLR) signaling in the outcomes of these pathophysiological changes is highlighted. Increasing evidence shows that microglia have a complex role in stroke pathophysiology, and they mediate both detrimental and beneficial effects on stroke outcome. So far, most of the pharmacological studies in experimental models of stroke have focused on neuroprotective strategies which are impractical for clinical applications. Post-ischemic inflammation is long lasting and thus, could provide a therapeutic target for novel delayed drug treatment. However, more studies are needed to elucidate the role of microglia in the recovery process from an ischemic stroke and to evaluate the therapeutic potential of modulating post-ischemic inflammation to promote functional recovery.

Keywords: NF-κB; TLR; inflammation; nuclear factor kappa B.

Figure 1. Microglia can be classified into four different phenotypes based on the morphology of the cell: A) ramified, B) intermediate, C) amoeboid and D) round type

90 minutes transient MCAo was induced in adult male Sprague Dawley rats as described earlier (Airavaara et al., 2009). Rat brains were perfusion-fixed 7 days post-stroke, embedded in paraffin, sectioned, and immunostained for Iba1. Ramified microglial cells are found in the contralateral side of the brain. Intermediate and amoeboid type cells are found in the peri-infarct region. Round type cells are found in the ischemic core region (dark brown). Categorization of microglia morphology is adopted from Lehrmann et al., 1997; Thored et al., 2009. Scale bar is 10 µm.

Figure 2. Time-scale of microglia-mediated events after ischemic stroke

The acute ischemic damage is induced within minutes to hours in the ischemic core region. The microglial responses include release of pro-inflammatory cytokines, chemokines and neurotransmitters. Using protective molecules targeted to microglia, the lesion size can be decreased. In the subacute phase the lesion continues to develop. There is edema and degeneration of microglial cells in the lesion area. In the peri-infarct region microglia polarize and get activated. There are no morphological changes in areas distal to the ischemic core region. The subacute phase also includes infiltration of inflammatory cells. Long-term changes include accumulation of phagocytic microglia to the core area and clearance of dead tissue. Microglial activation can be observed also in the distal areas and there is formation of secondary damage. Schematic time-line is modified from studies of Anttila and Airavaara, 2016 unpublished results; Ito et al., 2001; Kato et al., 1996; Lehrmann et al., 1997; Perego et al., 2011; Ritzel et al., 2015; Rupalla et al., 1998; Thored et al., 2009.

Figure 3. Toll-like receptor (TLR) signaling in microglial cell

Activation of TLR leads to signaling via two principal pathways: MyD88-dependent pathway and TRIF-dependent pathway. TLRs 1, 2, 4, 5, 6, 7 and 9 (Akira and Takeda, 2004) signal via MyD88-dependent pathway where myeloid differentiation primary response gene 88 (MyD88) and MyD88 adaptor-like protein (Mal) recruitment leads to activation of IL-1 receptor-associated kinases (IRAKs) and tumor necrosis factor receptor-associated factor 6 (TRAF6). Phosphorylation of the inhibitory unit of the transcription factor NF-κB complex, κKα/β, leads to nuclear translocation and activation of NF-κB and production of proinflammatory cytokines. TLR4 signals also via TRIF-dependent pathway. TIR-domain-containing adapter-inducing interferon-β (TRIF) and TRIF-related adaptor molecule (TRAM) recruitment leads to activation of the transcription factor interferon regulatory factor 3 (IRF3) and production of interferons. TRIF interacts also with receptor interacting protein 1 (RIP1) and TRAF6 leading to activation of NF-κB as in MyD88-dependent pathway.