Disorders characterized by ischemia/reperfusion (I/R), such as myocardial infarction, stroke, and peripheral vascular disease, continue to be among the most frequent causes of debilitating disease and death. Tissue injury and/or death occur as a result of the initial ischemic insult, which is determined primarily by the magnitude and duration of the interruption in the blood supply, and then subsequent damage induced by reperfusion. During prolonged ischemia, ATP levels and intracellular pH decrease as a result of anaerobic metabolism and lactate accumulation. As a consequence, ATPase-dependent ion transport mechanisms become dysfunctional, contributing to increased intracellular and mitochondrial calcium levels (calcium overload), cell swelling and rupture, and cell death by necrotic, necroptotic, apoptotic, and autophagic mechanisms. Although oxygen levels are restored upon reperfusion, a surge in the generation of reactive oxygen species occurs and proinflammatory neutrophils infiltrate ischemic tissues to exacerbate ischemic injury. The pathologic events induced by I/R orchestrate the opening of the mitochondrial permeability transition pore, which appears to represent a common end-effector of the pathologic events initiated by I/R. The aim of this treatise is to provide a comprehensive review of the mechanisms underlying the development of I/R injury, from which it should be apparent that a combination of molecular and cellular approaches targeting multiple pathologic processes to limit the extent of I/R injury must be adopted to enhance resistance to cell death and increase regenerative capacity in order to effect long-lasting repair of ischemic tissues.

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Cell Biology of Ischemia/Reperfusion Injury

Inflammation and glial responses in ischemic brain lesions

Lesions in the nervous system induce rapid activation of glial cells and under certain conditions additional recruitment of granulocytes, T-cells and monocytes/macrophages from the blood stream triggered by upregulation of cell adhesion molecules, chemokines and cytokines. Hematogenous cell infiltration is not restricted to infectious or autoimmune disorders of the nervous system, but also occurs in response to cerebral ischemia and traumatic lesions. Neuroinflammation can cause neuronal damage, but also confers neuroprotection. Granulocytes occlude vessels during reperfusion after transient focal ischemia, while the functional role of T-cells and macrophages in stroke development awaits further clarification. After focal cerebral ischemia neurotoxic mediators released by microglia such as the inducible nitric oxide synthase (leading to NO synthesis) and the cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) are upregulated prior to cellular inflammation in the evolving lesion and functionally contribute to secondary infarct growth as revealed by numerous pharmacological experiments and by use of transgenic animals. On the other hand, cytokine induction remote from ischemic lesions involves NMDA-mediated signalling pathways and confers neuroprotection. After nerve injury T cells can rescue CNS neurons. In the peripheral nervous system neuroinflammation is a prerequisite for successful regeneration that is impeded in the CNS. In conclusion, there is increasing evidence that neuroinflammation represents a double edged sword. The opposing neurotoxic and neuroprotective properties of neuroinflammation during CNS injury provide arich and currently unexplored set of research problems.

Detrimental and Beneficial Effects of Injury-Induced Inflammation and Cytokine Expression in the Nervous System

In mammalian brain the expression of peripheral benzodiazepine receptors (PBRs) can be markedly induced following different types of neuronal injury. PBRs are believed to be expressed on non-neuronal cells in the brain, yet the specific cell type that expresses these receptors following CNS insult has not been defined. In the present study, we investigated the effects of transient global forebrain ischemia on PBRs by autoradiographic localization of 3H-PK11195 binding. The distribution of PBRs was compared to glial fibrillary acidic protein (GFAP) as a marker for astrocytes and OX42 as a marker for microglia. Five to 6 d following four-vessel occlusion (4-VO), an increase in PBRs was seen in the CA1 region of all 15 brains examined. In brains from rats subjected to 4-VO, microglia were selectively activated in stratum pyramidale of the CA1 layer. In contrast, astrocytes appeared to be activated in multiple hippocampal cell layers including stratum radiatum and stratum oriens. Activated astrocytes were also found in regions that did not exhibit increased 3H-PK11195 binding. In some brains, selected regions of secondary lesion, specifically necrotic thalamic nuclei and the isocortex were found to be strongly immunoreactive for OX42 but lacked GFAP immunoreactive cells. In adjacent sections, these same regions displayed high densities of 3H-PK1195 binding. These observations lend further support to the application of 3H-PK11195 binding as a marker of neuronal injury in the brain. Furthermore, the data strongly suggest that activated microglia rather than astrocytes express PBRs following ischemic insults.

https://www.jneurosci.org/content/jneuro/15/7/5263.full.pdf

Peripheral benzodiazepine receptors are colocalized with activated microglia following transient global forebrain ischemia in the rat

Microglial cell activation is a rapidly occurring cellular response to cerebral ischaemia. Microglia proliferate, are recruited to the site of lesion, upregulate the expression of several surface molecules including major histocompatibility complex class I and II antigens, complement receptor and the amyloid precursor protein (APP) as well as newly expressed cytokines, e.g. interleukin-1 and transforming growth factor beta 1. The ischaemia-induced production of APP may contribute to amyloid deposition in the aged brain under conditions of hypofusion. Ultrastructurally, microglia transform into phagocytes removing necrotic neurons but still respecting the integrity of eventually surviving neurons even in the close vicinity of necrotic neurons. Microglial activation starts within a few minutes after ischaemia and thus precedes the morphologically detectable neuronal damage. It additionally involves a transient generalized response within the first 24 hours post-ischaemia even at sites without eventual neuronal cell death. In functional terms, the microglial reaction appears to be a double-edged sword in ischaemia. Activated microglia may exert a cytotoxic effector function by releasing reactive oxygen species, nitric oxide, proteinases or inflammatory cytokines. All of these cytotoxic compounds may cause bystander damage following ischaemia. Pharmacological suppression of microglial activation after ischaemia has accordingly attenuated the extent of cell death and tissue damage. However, activated microglia support tissue repair by secreting factors such as transforming growth factor beta 1 which may limit tissue damage as well as suppress astroglial scar formation. In line with ultrastructural observations microglial activation in ischaemia is a strictly controlled event. By secreting cytokines and growth factors activated microglia most likely serve seemingly opposed functions in ischaemia, i.e. maintenance as well as removal of injured neurons. Post-ischaemic pharmacological modulation of microglial intervention in the cascade of events that lead to neuronal necrosis may help to improve the structural and functional outcome following CNS ischaemia.

Reactive microglia in cerebral ischaemia: an early mediator of tissue damage?

Selective vulnerability and early progression of hippocampal CA1 pyramidal cell degeneration and GFAP-positive astrocyte reactivity in the rat four-vessel occlusion model of transient global ischemia

P. Bialobok

Papers

Selective vulnerability and early progression of hippocampal CA1 pyramidal cell degeneration and GFAP-positive astrocyte reactivity in the rat four-vessel occlusion model of transient global ischemia