Date

December 2011

Document Type

Dissertation

Degree Name

Ph.D.

Institution

Oregon Health & Science University

Abstract

Preconditioning induces ischemic tolerance, which confers robust protection against ischemic damage. Ischemic tolerance is a biologic process that can be utilized to enhance the brain’s own endogenous protection mechanisms and as such, holds true promise for patients at risk of ischemic injury. Experimentally, preconditioning with various Toll-like receptor agonists successfully attenuates ischemic damage, in part through genomic reprogramming of the brain’s response to stroke. This treatment diminishes certain damaging inflammatory responses to stroke and at the same time, promotes the production of neuroprotective mediators. Many of the currently identified preconditioning stimuli are not appropriate for regular use in human patients. Therefore, our laboratory sought to identify novel pharmacological preconditioning stimuli that would be useful in a clinical setting. Polyinosinic polycytidylic-acid (poly-IC) treatment leads to robust production of interferon and interferon-related genes, both neuroprotective mediators. Here, we show marked protection with poly-IC preconditioning in three models of murine ischemia-reperfusion injury. Poly-IC preconditioning induced protection against ischemia modeled in vitro in brain cortical cells and in vivo in models of brain ischemia and renal ischemia. Further, unlike other Toll-like receptor ligands, which generally induce significant inflammatory responses, a preconditioning dose of poly-IC elicits only modest systemic inflammation. These results demonstrate that poly-IC is a new, powerful, prophylactic treatment that offers promise as a clinical therapeutic strategy to minimize damage in patient populations at risk of ischemic injury. To investigate endogenous mechanisms of neuroprotection we sought to compare the mechanisms of action of multiple preconditioning stimuli to poly-ICLC. Poly-ICLC preconditioning induces interferon related genes following ischemic challenge that are common to other preconditioning stimuli: LPS, CpG and ischemic preconditioning. This feature suggests that poly-ICLC preconditioning also reprograms the response to stroke. We also identified downstream effectors interferon regulatory factor 7 and type-1 interferon signaling as critical mediators of poly-ICLC neuroprotection. Though we have previously identified key mechanistic components of neuroprotection mediated by preconditioning, little is known about the sites of action of preconditioning stimuli that initiate reprogramming to generate an ischemic tolerant state. Preconditioning with the TLR9 ligand CpG also reduces damage following ischemic injury. TLR9 expression is widespread; therefore, a broad range of potential target cell populations exists. To address this question we focused on the contribution of TLR9-expressing hematopoietic cells and created TLR9KO reciprocal bone marrow chimeric mice lacking TLR9 on either hematopoietic or parenchymal cells. CpG preconditioning did not protect either form of TLR9KO chimeric mice, demonstrating that the expression of TLR9 on hematopoietic and parenchymal cells are both necessary for the protective effects of CpG. Consistent with the critical role of the cytokine TNFα in CpGinduced neuroprotection, we found that both forms of TLR9KO chimeric mice lacked the TNFα mRNA response centrally. These results indicate that TLR9 expression on hematopoietic cells is required but not sufficient to induce preconditioning neuroprotection and that this loss of protection correlates with loss of TNFα mRNA response to CpG in the brain.

Identifier

doi:10.6083/M4QV3JJW

Division

Neuroscience Graduate Program

School

School of Medicine

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