Viral Gene Delivery to Skeletal Muscle: Insights on Maturation-Dependent Loss of Fiber Infectivity for Adenovirus and Herpes Simplex Type 1 Viral Vectors

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Abstract
The mechanisms causing age-dependent loss of muscle fiber infectivity observed in vivo for both adenoviral (Ad) and herpes simplex virus type 1 (HSV-1) gene delivery vectors remain poorly understood. Here we investigate the possible bases for this phenomenon using the novel application of enzymatically isolated, viable, single muscle fibers. We show that maturation-dependent loss of fiber infectivity is recapitulated in single fibers, and, thus, is not solely due to host immune response. Using localized irradiation of muscle in vivo, we show data suggesting that Ad infectivity of differentiated myofibers depends, at least in part, on myoblasts to mediate fiber transduction. On the other hand, infection of single fibers by HSV-1 is not affected by irradiation. Using confocal microscopy, we show that the basal lamina of myogenic cells efficiently infected by HSV-1 is structurally less organized than that of fibers resistant to infection by HSV-1. As well, we show that single myofibers isolated from adult, basal lamina-defective mice (merosin-deficient, dy/dy) are at least 10-fold more susceptible to infection by HSV-1 than are myofibers isolated from control mice. Together, these observations support the hypothesis that the basal lamina acts as a physical barrier to HSV-1 infection of mature muscle. Maturation-dependent loss of muscle fiber infectivity with adenovirus (Ad) and herpes simplex virus type 1 (HSV-1) limits the usefulness of these vectors for gene transfer to adult muscle. We investigated this phenomenon using myotubes and isolated, single, viable myofibers from locally irradiated and nonirradiated normal and nonirradiated basal lamina defective (dy/dy) mice of specified ages. We show that the myoblast content of muscle may be an important determinant of efficient myofiber infection by Ad. In contrast, infection of myofibers by HSV-1 does not require myoblasts, but instead is affected by the myofiber basal lamina. Our data suggest that augmentation of the number or activity of myoblasts in mature muscle could enhance Ad-mediated gene transfer to muscle, whereas permeabilization of the basal lamina could increase HSV-1 gene transfer.

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