Review article
Brain ischemia and reperfusion: molecular mechanisms of neuronal injury

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Abstract

Brain ischemia and reperfusion engage multiple independently-fatal terminal pathways involving loss of membrane integrity in partitioning ions, progressive proteolysis, and inability to check these processes because of loss of general translation competence and reduced survival signal-transduction. Ischemia results in rapid loss of high-energy phosphate compounds and generalized depolarization, which induces release of glutamate and, in selectively vulnerable neurons (SVNs), opening of both voltage-dependent and glutamate-regulated calcium channels. This allows a large increase in cytosolic Ca2+ associated with activation of μ-calpain, calcineurin, and phospholipases with consequent proteolysis of calpain substrates (including spectrin and eIF4G), activation of NOS and potentially of Bad, and accumulation of free arachidonic acid, which can induce depletion of Ca2+ from the ER lumen. A kinase that shuts off translation initiation by phosphorylating the α-subunit of eukaryotic initiation factor-2 (eIF2α) is activated either by adenosine degradation products or depletion of ER lumenal Ca2+. Early during reperfusion, oxidative metabolism of arachidonate causes a burst of excess oxygen radicals, iron is released from storage proteins by superoxide-mediated reduction, and NO is generated. These events result in peroxynitrite generation, inappropriate protein nitrosylation, and lipid peroxidation, which ultrastructurally appears to principally damage the plasmalemma of SVNs. The initial recovery of ATP supports very rapid eIF2α phosphorylation that in SVNs is prolonged and associated with a major reduction in protein synthesis. High catecholamine levels induced by the ischemic episode itself and/or drug administration down-regulate insulin secretion and induce inhibition of growth-factor receptor tyrosine kinase activity, effects associated with down-regulation of survival signal-transduction through the Ras pathway. Caspase activation occurs during the early hours of reperfusion following mitochondrial release of caspase 9 and cytochrome c. The SVNs find themselves with substantial membrane damage, calpain-mediated proteolytic degradation of eIF4G and cytoskeletal proteins, altered translation initiation mechanisms that substantially reduce total protein synthesis and impose major alterations in message selection, down-regulated survival signal-transduction, and caspase activation. This picture argues powerfully that, for therapy of brain ischemia and reperfusion, the concept of single drug intervention (which has characterized the approaches of basic research, the pharmaceutical industry, and clinical trials) cannot be effective. Although rigorous study of multi-drug protocols is very demanding, effective therapy is likely to require (1) peptide growth factors for early activation of survival-signaling pathways and recovery of translation competence, (2) inhibition of lipid peroxidation, (3) inhibition of calpain, and (4) caspase inhibition. Examination of such protocols will require not only characterization of functional and histopathologic outcome, but also study of biochemical markers of the injury processes to establish the role of each drug.

Introduction

Stroke is the leading cause of serious, long-term disability in the United States [1], with about 600,000 people suffering a new or recurrent stroke each year. Three million Americans are currently permanently disabled because of ischemic stroke, and 31% of stroke survivors need help caring for themselves, 20% need help walking, 71% have an impaired vocational capacity when examined an average of 7 years later, and 16% have to be institutionalized. The direct and indirect cost of stroke in 1998 is estimated at $43.3 billion [1]. In addition to this, cardiopulmonary resuscitation for victims of cardiac arrest, both within and outside of the hospital, succeeds in restoring spontaneous circulation in about 70,000 patients a year in the United States. At least 60% of these patients subsequently die in the hospital as a result of extensive brain damage; only 3–10% of resuscitated patients are finally able to resume their former lifestyles [2]. Thus, brain injury by transient complete global brain ischemia (cardiac arrest) and regional incomplete brain ischemia (ischemic stroke) afflicts a very large number of patients with death or permanent disability.

In order to reduce this neurologic morbidity, we must sufficiently understand the mechanisms involved in neuronal injury and repair to design clinically effective therapy. There are as yet no clinically effective therapeutic protocols for amelioration of brain damage by ischemia and reperfusion. In the 1980s clinical trials of barbiturate-induced coma [3] or calcium antagonists [4] failed to reduce neurologic damage caused by cardiac arrest and resuscitation. More recently, both the radical scavenger tirilazad [5] and the glutamate receptor antagonist selfotel [6] have been found ineffective in clinical treatment of stroke. Thus the theoretical syntheses that led to these trials were inadequate. Here we will examine new information regarding neuronal injury (Fig. 1) and repair, indicating that:

  • 1.

    Many phenomena observed during brain ischemia and reperfusion can be accounted for by damage to membrane lipids, specifically by lipolysis during ischemia and by radical-mediated peroxidation of polyunsaturated fatty acids (PUFAs) during reperfusion.

  • 2.

    Protein synthesis in vulnerable brain neurons is rapidly and persistently inhibited at the level of translation initiation during post-ischemic reperfusion.

  • 3.

    Apoptotic mechanisms are engaged in vulnerable neurons during early reperfusion, and these mechanisms include modifications of translation initiation, activation of proteolysis, and activation of endonucleases.

  • 4.

    These events not only exacerbate damage to critical molecules during reperfusion but also impair the competence of cellular repair processes.

  • 5.

    Neuronal competence for antioxidant defense, translation initiation, inhibition of apoptosis, and repair of damaged cellular organelles is regulated at a fundamental level by signal transduction mechanisms involving growth factors.

Because multiple independently lethal mechanisms (radical damage, loss of translation competence, proteolytic activation, and induction of apoptosis) are involved in post-ischemic neuronal death, single drug intervention will be ineffective. Thus we must undertake a complex experimental effort that will involve both classical characterization of neuronal and functional loss as well as assays of biochemical markers reflecting the success or failure of interventions against the specific injury mechanisms in order to develop and validate an effective multi-drug therapeutic protocol.

Section snippets

Historical observations of major phenomena in brain ischemia and reperfusion

Four major observations have provided the foundation for investigation of brain injury by ischemia and reperfusion [7]: (1) rapid loss of high-energy phosphate compounds during ischemia followed by their recovery within the first 15 min of reperfusion; (2) morphological evidence that most structural damage occurs during reperfusion, especially in selectively vulnerable zones; (3) progressive brain hypoperfusion during post-ischemic reperfusion; and (4) prolonged suppression of protein synthesis

Classical mechanisms implicated in selective vulnerability

Two major hypotheses emerged during the 1980s from efforts to explain the phenomenon of selective vulnerability. One is the excitotoxic neurotransmitter hypothesis directed largely at events during ischemia, and the other is the free radical hypothesis directed largely at events during reperfusion.

Neurons and terminal differentiation

Regulation of the cell cycle may affect resistance to and repair of membrane damage and be important in surviving ischemia and reperfusion. Non-replicating cells, such as those in the glomeruli, the myocardium, and the central nervous system, are quite sensitive to damage by ischemia and reperfusion. The linkage of DNA replication and membrane synthesis follows from the requirement for both for cellular reproduction. In prokaryotes, experimental manipulations that specifically inhibit lipid

Ischemia-induced calpain activation

Calpains (EC 3.4.22.17) are a family of non-lysosomal neutral cysteine proteases [94], [95] whose proteolytic activity is absolutely dependent on calcium but is also regulated by phospholipids, by a specific endogenous inhibitor (calpastatin), and by a specific activator protein [96]. Calpains and calpastatin have been found in all vertebrate tissues studied [97]. The two ubiquitous calpain isoforms are μ-calpain (calpain-I, CANP-I) and m-calpain (calpain-II, CANP-II). Immunohistochemical

General considerations

Post-ischemic suppression of protein synthesis was first reported in 1971 by Kleihues and Hossman [123], who observed a 30% decrease in labeled amino acid incorporation after 4 h reperfusion following 30 min of ischemia. In the SVNs the post-ischemic suppression of brain protein synthesis is prompt, severe, and prolonged. Cooper et al. measured [14C]-phenylalanine incorporation by in vitro translation utilizing the post-mitochondrial supernatant from rat brain homogenates [124]. Incorporation

General considerations

Apoptosis is a process of self-destructive cell death that involves activation of mechanisms encoded in the genomes of all higher eukaryotes [254]. Appreciation of the importance of apoptosis in mammalian systems has only developed during the last 5–7 years [255], and some of the details of the process remain poorly understood. Nevertheless, it has become clear that the process of apoptosis is intimately involved in several acute disease states (including injury mechanisms involved in ischemia

Immediate early genes and ‘heat shock’ proteins

Immediate early genes [357], [358] include proto-oncogenes of the c-fos and c-jun families. These proteins form heterodimers to constitute a variety of AP-1 transcription factor complexes that bind specifically to consensus promoter sequences upstream of target genes [359] and also bind TBP (TATA-binding protein), thus facilitating initiation of transcription [360].

In rats [140] and gerbils [141] c-fos and c-jun transcripts are elevated in brain homogenates within 30 min of post-ischemic

Resuscitation

The data regarding the involvement of insulin in survival signaling for neurons, as well as other cells, raise concern about the use of large doses of epinephrine in resuscitation of patients from cardiac arrest. Catecholamines, such as epinephrine, cause prompt and prolonged inhibition of insulin secretion by the pancreas [453], activate cAMP-induced phosphorylation of the insulin receptor thereby reducing its tyrosine kinase activity [454], and promote iron-mediated lipid peroxidation [455],

Acknowledgements

Supported in part by National Institute of Health Grants NS33196 (Drs. Krause, White, DeGracia, Grossman, and Rafols), NS02008 (Dr. Sullivan), NS01832 (Dr. Neumar), GM48517 (Dr. Grossman), and NS39860 (Dr. Rafols). Dr. DeGracia is also supported in part by a grant from the American Heart Association, and Dr. O’Neil is supported in part by a grant from the Emergency Medicine Foundation.

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