Differences in transductional tropism of adenoviral and lentiviral vectors in the rat brainstem

Adenoviral vectors (AVVs) and lentiviral vectors (LVVs) are highly useful research tools which can be used to investigate the function of specific cell phenotypes in the brain. The transductional tropism of viral vectors has a critical impact upon the transgene expression in different brain areas. This largely depends on the properties of the viral particles, which for AVVs are most commonly analogous to the serotype 5 adenovirus and, in the case of LVVs, are determined by the envelope used for pseudotyping, for example the vesicular stomatitis virus coat (VSVG). We have created a matching set of shuttle plasmids that allow a one-step transfer of an entire expression cassette between the backbones of AVVs and LVVs. This has permitted a fair assessment of the impact of the vector type on tropism for both AVVs and LVVs. Thus, the aims of this study were twofold: (i) to develop and demonstrate the validity of a transgene 'swap' system between AVVs and LVVs; and (ii) using this system, to assess the tropism of AVVs and LVVs for neuronal versus glial cell types. We have constructed AVVs and VSVG-coated LVVs to express monomeric red fluorescent protein (mRFP) driven by the human cytomegalovirus promoter (hCMV). Transgene expression in neurones and glia in the hypoglossal and dorsal vagal motor nuclei of the rat brainstem was compared by determining the colocalization with immunostaining for the neuronal marker NeuN (neuronal nuclear antigen) and the glial marker GFAP (glial fibrillatory acidic protein). We found that 55% of mRFP-expressing cells transduced with AVVs were immunopositive for GFAP, while only 38% were NeuN-immunoreactive. In contrast, when the same expression cassette was delivered by VSVG-coated LVVs, the neurone/glia ratio of mRFP expression was reversed with 56% of mRFP-positive cells identified as neurones and 26% as glia. Thus, the present study provides compelling evidence that VSVG-coated LVVs significantly shift transgene expression towards neurones while transduction with AVVs favours glia.