Role of apoptosis in acute neurodegenerative disorders.
Many toxic factors are generated during stroke that contribute directly to the death of neurons. Several recent studies suggest that a suicide-like phenomena similar to apoptosis or programmed cell death also contributes to the loss of neurons in stroke. The evidence implicating apoptosis in stroke can be divided into three categories; biochemical, molecular and pharmacological. Biochemical evidence: One hallmark of apoptosis is the early activation of destructive enzymes, including endonucleases and proteases. Endonuclease-mediated DNA fragmentation can be observed within 4 h after focal cerebral ischemia and precedes morphological evidence of cell death. Cells with damaged DNA appear to concentrate in the salvageable tissue of the penumbra while necrosis predominates in areas where the sustained lack of blood flow may make tissue salvage impossible. Molecular evidence: Bcl-2 is an anti-apoptotic gene that confers the ability to block apoptosis from a wide variety of stimuli. The levels of bcl-2 can be enhanced by viral gene delivery or transgenic methodology. In cortical tissue where bcl-2 was elevated, neurons were protected from a subsequent ischemic attack. In contrast to bcl-2, p53 is a pro-apoptotic protein. Levels of p53 are elevated after cerebral ischemia and transgenic p53 knockouts exhibit smaller infarcts than wild type control mice. Pharmacological evidence: The process of apoptosis typically involves the activation of enzymes and genes, leading to an irreversible committment to die. Inhibition of new protein synthesis by cycloheximide reduces brain damage after a stroke, suggesting that newly synthesized proteins are contributing to the death of neurons. In addition, inhibition of calpain (an enzyme implicated in certain forms of apoptosis) protects neurons in models of global ischemia, focal ischemia, and hypoxia. In conclusion, the observation that an apoptotic-like process contributes to stroke may have important therapeutic implications since therapies that inhibit apoptosis improve outcome in experimental stroke.