Gene therapy for ischemic neuronal injury (2005)

Advances in the area of stroke and other related disorders have identified a variety of molecular targets for potential therapeutic intervention. The use of modified viral vectors has now made it possible to introduce foreign DNA into central nervous system cells, permitting overexpression of a protein of interest. A particular advantage of the herpes simplex system is that the virus is neurotropic, and is therefore suited for relatively selective gene therapy to neurons. The vectors used by our group to date utilize an amplicon based bipromoter system which permits expression of both the gene of interest as well as a reporter gene. Using this strategy, we have shown that potentially neuroprotective genes can be transferred to individual central nervous system cells, and can confer a relative resistance to cerebral ischemic and excitotoxic insults. We previously reported that gene therapy using a neurotropic herpes simplex viral (HSV) vector system containing bipromoter vectors to transfer various protective genes to neurons. Using this system in experimental models of stroke, cardiac arrest and excitotoxicity, we have found that it is possible to enhance neuron survival against such cerebral insults by overexpressing genes that target various facets of injury. Of the genes studied by our labs, we have shown that glucose transporter (GLUT-1), the calcium binding protein calbindin D28 K (CaBP), anti-oxidant genes glutathione peroxidase (GPx) and catalase (CAT), the anti-apoptotic protein Bcl-2 and the 70 kDa heat shock protein (HSP70) improve neuron survival after ischemia and excitotoxicity. Bcl-2, GPx and HSP70 also appear to protect when administered post insult. Because the extent of vector uptake is limited, it is not generally possible to affect overall infarct size or behavior. Regardless, we have demonstrated that for some cases, gene transfer may also improve cell function. Gene transfer can also be used in combination with other potential neuroprotective strategies with synergistic effects. For instance, mild hypothermia is well known to protect the brain from experimental brain ischemia provided brain cooling begins within hours of ischemia onset. This hypothermic neuroprotection is also associated with Bcl-2 upregulation in some instances, and hypothermia suppresses many aspects of apoptotic death. Our recent work has shown that two different kinds of protective therapies, Bcl-2 overexpression and hypothermia, both inhibit aspects of apoptotic cell death cascades, and combination of hypothermia with Bcl-2 gene transfer will prolong the temporal therapeutic window for Bcl-2 gene therapy. Some limitations of this technique exist, the main ones being that of delivery and extent and duration of transfection. Part of the limited duration of gene expression may be due to a localized tissue inflammatory response to the vectors themselves. This problem may be partially surmounted by the coadministration of anti-inflammatory treatments to prolong survival of transfected cells. While neuronal gene therapy for cerebral ischemia is still limited by the numbers of cells which vectors can transfect, it provides a powerful tool to understand mechanisms of cell death and identify potential molecular targets for therapy.