Introduction of a foreign gene (Escherichia coli lacZ) into rat neostriatal neurons using herpes simplex virus mutants: a light and electron microscopic study

Introducing genes into adult neurons in vivo may be a useful experimental tool for studying and modifying neuronal function. In this study two herpes simplex virus type 1 (HSV-1) mutants were used to examine the capability of different types of neostriatal neurons to express a foreign gene introduced through viral infection. In these HSV-1 mutants (7134 and RH105) the Escherichia coli gene, lacZ, under the control of viral promoters active during the early phase of infection, was substituted for viral genes (ICPO and TK, respectively) needed for efficient replication in the nervous system. Adult male rats received unilateral injections of HSV-1 mutant 7134 or RH105 into the neostriatum. Animals survived for 1 to 70 days with no apparent adverse physiological or behavioral effects. At the injection site, both mutant viruses produced focal tissue necrosis and reactive gliosis. Histochemical detection of the lacZ gene product, beta-galactosidase (beta Gal), revealed extensive labeling of neurons with mutant 7134 and relatively limited neuronal labeling with the mutant RH105. Mutant 7134, which is capable of some replication in cells, conferred beta Gal expression in cells over an area that was twofold greater than the necrotic area. In contrast, mutant RH105, which cannot replicate in cells, produced a zone of beta Gal-labeled cells only two-thirds the area of the necrotic core. Both medium- and large-sized neostriatal neurons were positive for beta Gal, and a higher proportion of large cells were labeled as compared to other neuronal populations in the normal striatum. A few glial cells were also beta Gal-positive. Retrograde transport of virus to the substantia nigra pars compacta and to the cortex was minimal and occurred only with mutant 7134. No evidence was seen for anterograde transport. Immunohistochemical localization of beta Gal at the ultrastructural level after inoculation with mutant 7134 revealed that both types of medium-sized neurons (spiny and aspiny types), as well as large neurons, were infected 3 days following inoculation. Immunoreactive neurons ranged from severely pathologic to remarkably healthy. Some of the axon terminals that contacted beta Gal-immunoreactive dendrites and spines were degenerated. These results demonstrate that in the adult rat replication-deficient HSV-1 vectors injected intrastriatally can be used to express a foreign gene in at least three types of neostriatal neurons, while maintaining the long-term survival and general health of the injected animals. The neurotoxicity induced by HSV-1 mutants may still be considerable, however, and ways of minimizing neuropathological effects need to be addressed.