Herpes Simplex Virus Type 1 Vector-Mediated Expression of Nerve Growth Factor Protects Dorsal Root Ganglion Neurons from Peroxide Toxicity

Nerve growth factor β subunit (β-NGF) transgene delivery and expression by herpes simplex virus type 1 (HSV-1) vectors was examined in a cell culture model of neuroprotection from hydrogen peroxide toxicity. Replication-competent (tk− K mutant background) and replication-defective (ICP4−;tk− S mutant background) vectors were engineered to contain the murine β-NGF cDNA under transcriptional control of either the human cytomegalovirus immediate-early gene promoter (HCMV IEp) (e.g., KHN and SHN) or the latency-active promoter 2 (LAP2) (e.g., KLN and SLN) within the viral thymidine kinase (tk) locus. Infection of rat B103 and mouse N2A neuronal cell lines, 9L rat glioma cells, and Vero cells with the KHN or SHN vectors resulted in the production of β-NGF-specific transcripts and β-NGF protein reaching a maximum at 3 days postinfection (p.i.). NGF protein was released into the culture media in amounts ranging from 10.83 to 352.86 ng/ml, with the highest levels being achieved in B103 cells, and was capable of inducing neurite sprouting of PC-12 cells. The same vectors produced high levels of NGF in primary dorsal root ganglion (DRG) cultures at 3 days. In contrast to HCMV IEp-mediated expression, the LAP2-NGF vectors showed robust expression in primary DRG neurons at 14 days. The neuroprotective effect of vector produced NGF was assessed by its ability to inhibit hydrogen peroxide-induced neuron toxicity in primary DRG cultures. Consistent with the kinetics of vector-mediated NGF expression, HCMV-NGF vectors were effective in abrogating the toxic effects of peroxide at 3 but not 14 days p.i. whereas LAP2-NGF vector transduction inhibited apoptosis in DRG neurons at 14 days p.i. but was ineffective at 3 days p.i. Similar kinetics of NGF expression were observed with the KHN and KLN vectors in latently infected mouse trigeminal ganglia, where high levels of β-NGF protein expression were detected at 4 wks p.i. only from the LAP2; HCMV-NGF-driven expression peaked at 3 days but could not be detected during HSV latency at 4 weeks. Together, these results indicate that (i) NGF vector-infected cells produce and secrete mature, biologically active β-NGF; (ii) vector-synthesized NGF was capable of blocking peroxide-induced apoptosis in primary DRG cultures; and (iii) the HCMV-IEp functioned to produce high levels of NGF for several days; but (iv) only the native LAP2 was capable of long-term expression of a therapeutic gene product in latently infected neurons in vivo.