Expression of the Primary Coxsackie and Adenovirus Receptor Is Downregulated during Skeletal Muscle Maturation and Limits the Efficacy of Adenovirus-Mediated Gene Delivery to Muscle Cells

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Abstract
Skeletal muscle fibers are infected efficiently by adenoviral vectors only in neonatal animals. This lack of tropism for mature skeletal muscle may be partly due to inefficient binding of adenoviral particles to the cell surface. We evaluated in developing mouse muscle the expression levels of two high-affinity receptors for adenovirus, MHC class I and the coxsackie and adenovirus receptor (CAR). The moderate levels of MHC class I transcripts that were detected in quadriceps, gastrocnemius, and heart muscle did not vary between postnatal day 3 and day 60 adult tissue. A low level of CAR expression was detected on postnatal day 3 in quadriceps and gastrocnemius muscles, but CAR expression was barely detectable in adult skeletal muscle even by reverse transcriptase-polymerase chain reaction. In contrast, CAR transcripts were moderately abundant at all stages of heart muscle development. Ectopic expression of CAR in C2C12 mouse myoblast cells increased their transducibility by adenovirus at all multiplicities of infection (MOIs) tested as measured by lacZ reporter gene activity following AVCMVlacZ infection, with an 80-fold difference between CAR-expressing cells and control C2C12 cells at an MOI of 50. Primary myoblasts ectopically expressing CAR were injected into muscles of syngeneic hosts; following incorporation of the exogenous myoblasts into host myofibers, an increased transducibility of adult muscle fibers by AVCMVlacZ was observed in the host. Expression of the lacZ reporter gene in host myofibers coincided with CAR immunoreactivity. Furthermore, sarcolemmal CAR expression was markedly increased in regenerating muscle fibers of the dystrophic mdx mouse, fibers that are susceptible to adenovirus transduction. These analyses show that CAR expression by skeletal muscle correlates with its susceptibility to adenovirus transduction, and that forced CAR expression in mature myofibers dramatically increases their susceptibility to adenovirus transduction.