Herpes Simplex Viral Vector Gene-Based Therapy to Prevent Loss of Auditory Sensory Cells from Stress-Induced Apoptosis

Abstract

Apoptosis (programmed cell death) plays an important role in both the normal development of the inner ear as well as the response of mature inner sensory cells to damage by toxins, radiation and/or loss of trophic factor support. In the present studies, two Herpes simplex viral vectors (i.e. HSVbdnflac and HSVbcl-2) were used to prevent stress-induced apoptosis of auditory neurons and hair cells, respectively. The first vector, HSVbdnflac, expresses genes for both a growth factor (brain derived growth factor) and a cell expression marker (bacterial galactosidase), In vitro studies have shown that this BDNF gene therapy vector can elicit a biological response (i,e. neuritogenesis) from postnatal rat spiral ganglion explants, In an in vivo study, transduction with the HSVbdnflac vector prevented the loss of spiral ganglion neurons from adult mice in which all of the auditory hair cells were destroyed by a scala tympani injection of neomycin. Double label for neurons and viral vector marker protein showed a high level of expression of HSVbndflac in the auditory neurons of the spiral ganglion at 1 month post-transduction, At 1 month post-transduction, we also observed stable, functional production of BDNF that supported the survival of nearly all (i.e. 94.7%) of the spiral ganglion neurons and prevented their loss from trophic factor deprivation-induced apoptosis (i.e. 64.3% loss). Experiments on neuronal cultures and with in vivo models of CNS anoxia-reperfusion injury have demonstrated that overexpression of bcl-2 (human homologue of ced9, an anti-apoptotic protein) can prevent neuronal degeneration. Cisplatin treatment of organ of Corti explants is a well-defined model of oxidative stress/free radical damage that involves the initiation of the apoptotic cell death pathway and subsequent loss of auditory hair cells. Using a Herpes simplex gene therapy vector that expresses a copy of the human bcl-2 gene (i.e. HSVbcl-2 in a pilot series of in vitro studies), we tested a bcI-2 gene therapy protection strategy in organ of Corti explants. Transfection of organ of Corti explants 24 hrs prior to exposure to CDDP provided protection of the auditory hair cells from cisplatin-damage induced apoptosis. The hair cells are damaged by exposure to CDDP but their loss is prevented much in the same manner as treatment with a caspace inhibitor which also acts by interrupting the apoptotic pathway of the damaged hair cells. Herpes simplex viral vector based gene therapy is an effective strategy to prevent apoptosis of both hair cells and the auditory neurons of the cochlea.