Microglial depletion abolishes ischemic preconditioning in white matter

Abstract

Ischemic preconditioning (IPC) is a phenomenon whereby a brief, non-injurious ischemic exposure enhances tolerance to a subsequent ischemic challenge. The mechanism of IPC has mainly been studied in rodent stroke models where gray matter (GM) constitutes about 85% of the cerebrum. In humans, white matter (WM) is 50% of cerebral volume and is a critical component of stroke damage. We developed a novel CNS WM IPC model using the mouse optic nerve (MON) and identified the involved immune signaling pathways. Here we tested the hypothesis that microglia are necessary for WM IPC. Microglia were depleted by treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. MONs were exposed to transient ischemia in vivo, acutely isolated 72 h later, and subjected to oxygenglucose deprivation (OGD) to simulate a severe ischemic injury (i.e., stroke).Functional and structural axonal recovery was assessed by recording compound action potentials (CAPs) and by microscopy using quantitative stereology. Microglia depletion eliminated IPC-mediated protection. In control mice, CAP recovery was improved in preconditioned MONs compared with non-preconditioned MONs, however, in PLX5622-treated mice, we observed no difference in CAP recovery between preconditioned and non-preconditioned MONs. Microgliadepletion also abolished IPC protective effects on axonal integrity and survival of mature (APC + ) oligodendrocytes after OGD. IPC-mediated protection was independent of retinal injury suggesting it results from mechanistic processes intrinsic to ischemia-exposed WM. We conclude that preconditioned microglia are critical for IPC in WM. The "preconditioned microglia" phenotype might protect against other CNS pathologies and is a neurotherapeutic horizon worth exploring.

Impact of inflammatory preconditioning on murine microglial proteome response induced by focal ischemic brain injury

“…Hence, microglia are viewed as key target cell type for improved insights into the complex molecular reprogramming due to brain I/R and endogenous neuroprotection by PC ( 71 ). Brain I/R and PC-related changes in microglial gene expression signatures have been widely described ( 28 , 72 – 74 ), but less is known about the extent and functional impact of proteomic alterations in microglia. For the first time, we comprehensively report the effects of LPS-induced PC on microglial proteomic changes induced by transient MCAO in adult mice.…”

Inhibiting CSF1R alleviates cerebrovascular white matter disease and cognitive impairment

“…Depletion of microglia prior to the onset of hypoperfusion using the CSF1R inhibitor PLX3397 reduces white matter damage and associated cognitive impairment (Kakae et al, 2019 ). However, depletion of microglia is not always beneficial as shown by a study in which mice were treated with the CSF1R inhibitor PLX5622 which abolished microglia and the protective effects of ischaemic preconditioning in white matter (Hamner et al, 2022 ). In this study, we specifically selected the dose of GW2580 as it does not ablate microglia (Neal et al, 2020 ; Olmos‐Alonso et al, 2016 ).…”

Role of microglia in stroke

“…Ischemic preconditioning after a short episode of 15‐min ischemia induces microglia cell proliferation 3 days later and protects against a second severe episode of ischemia of 60‐min duration (McDonough et al, 2020). Moreover, microglia depletion by treatment with CSF1R inhibitors abolished the preconditioning effect (Hamner et al, 2022). Remarkably, the phenomenon of preconditioning is induced by stimuli priming microglia such as mild/moderate activation of TLR‐4 and other TLRs (Hamner et al, 2015; Hua et al, 2008; Karikó et al, 2004; Pradillo et al, 2009; Rosenzweig et al, 2004; Stevens et al, 2008; Tasaki et al, 1997; Wang et al, 2016), and induction of type‐I IFN signaling (Gesuete et al, 2012; Hamner et al, 2015; Kundu et al, 2022; Leung et al, 2012; Stevens et al, 2011) several days prior to ischemia (Figure 2c).…”