Dramatic progress has been made over recent years toward the elucidation of the mechanisms regulating lineage determination and cell survival in the developing peripheral nervous system. However, our understanding of Schwann cell development is limited. This is partly due to the difficulties in culturing primary Schwann cell precursor cells, the earliest developmental stage of the Schwann cell lineage defined to date. Both the inability to maintain cultured Schwann cell precursor cells in an undifferentiated state and the technical difficulties involved in their isolation have hampered progress. We have conditionally immortalized rat Schwann cell precursor cells using a retrovirally encoded EGFR/neu fusion protein to circumvent these problems and to generate a source of homogeneous cells. The resulting SpL201 cell line expresses p75 and nestin, two proteins expressed by neural crest-derived cells, as well as peripheral myelin protein 22, protein zero, and Oct-6 as markers of the Schwann cell lineage. When cultured in EGF-containing medium, the SpL201 cells proliferate and maintain an undifferentiated, Schwann cell precursor cell-like state. The cell line is dependent on EGF for survival but can differentiate into early Schwann cell-like cells in response to exogenous factors. Like primary rat Schwann cells, SpL201 cells upregulate Oct-6 and myelin gene expression in response to forskolin treatment. Furthermore, the SpL201 cell line can form myelin in the presence of axons in vitro and is capable of extensively remyelinating a CNS white matter lesion in vivo. Thus, this cell line provides a valuable and unique tool to study the Schwann cell lineage, including differentiation from the Schwann cell precursor cell stage through to myelination.

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Mutations in the gene encoding for the myelinating Schwann cell protein P0 have been linked to inherited peripheral neuropathies, including the Charcot-Marie-Tooth type 1B disease (CMT1B) and Dejerine-Sottas syndrome (DSS). Recently generated mice deficient in the P0 gene (P0-/- mice) resemble cases of CMT1B and DSS with impaired myelin dosage (Martini et al., 1995a). Potential approaches to treat such diseases include the introduction of the normal gene in the nerves of strongly affected patients. In the present study we used P0-/- mice to evaluate the efficiency of a replication-defective, E1-deleted adenovirus vector carrying the lacZ (Ad-RSV-lacZ) or P0 (Ad-RSV-P0) gene to infect abnormally myelinating Schwann cells. The Ad-RSV-lacZ vector suspension was injected into the left sciatic nerve ofPO-/- mice and the nerves examined for beta-galactosidase activity by X-gal histochemistry. Contralateral nerves injected with vehicle solution or non-injected served as controls. Beta-galactosidase activity was detected in nerves injected with the Ad-RSV-lacZ vector up to 2 weeks post-injection. Immunosuppressing the mice with FK506 to decrease the infiltration of activated T-cells in infected nerves lengthened beta-galactosidase activity to 8 weeks, the longest time point examined. Ultrastructural analysis indicated that X-gal crystals were present mostly in abnormally myelinating Schwann cells. These findings demonstrate that an adenovirus vector can successfully infect Schwann cells in P0-/- mice and expression can be maintained for several weeks. The Ad-RSV-P0 suspension was then injected in the sciatic nerve of immunosuppressed P0-/- mice. Two and four weeks post-injection both P0 mRNA and protein could be detected by in situ hybridization and Western blotting in some of the nerves. Furthermore, P0 protein expression was observed in myelin-like structures and onion bulb-like cells by immunohistochemistry. These results indicate that Schwann cells in P0-/- mice can be induced to produce P0 protein after gene transfer. Genetic repair of abnormal Schwann cells by using adenovirus vectors might be a possible technique to treat animal models of inherited peripheral neuropathies.