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 Ca(2+)
associated with activation of mu-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 Ca(2+) from the ER lumen. A kinase that shuts off translation initiation
by phosphorylating the alpha-subunit of eukaryotic initiation factor-2 (eIF2alpha)
is activated either by adenosine degradation products or depletion of ER lumenal Ca(2+).
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 eIF2alpha 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.