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

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 surf ace. We evaluated in developing mouse muscle the expression levels of two h igh-affinity receptors for adenovirus, MHC class I and the coxsackie and ad enovirus receptor (CAR). The moderate levels of MHC class I transcripts tha t 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 expres sion was detected on postnatal day 3 in quadriceps and gastrocnemius muscle s, but CAR expression was barely detectable in adult skeletal muscle even b y reverse transcriptase-polymerase chain reaction. In contrast, CAR transcr ipts were moderately abundant at all stages of heart muscle development. Ec topic expression of CAR in C2C12 mouse myoblast cells increased their trans ducibility by adenovirus at all multiplicities of infection (MOIs) tested a s measured by lacZ reporter gene activity following AVCMVlacZ infection, wi th an 80-fold difference between CAR-expressing cells and control C2C12 cel ls at an MOI of 50. Primary myoblasts ectopically expressing CAR were injec ted into muscles of syngeneic hosts; following incorporation of the exogeno us myoblasts into host myofibers, an increased transducibility of adult mus cle fibers by AVCMVlacZ was observed in the host. Expression of the lacZ re porter gene in host myofibers coincided with CAR immunoreactivity. Furtherm ore, sarcolemmal CAR expression was markedly increased in regenerating musc le fibers of the dystrophic mdx mouse, fibers that are susceptible to adeno virus transduction. These analyses show that CAR expression by skeletal mus cle correlates with its susceptibility to adenovirus transduction, and that forced CAR expression in mature myofibers dramatically increases their sus ceptibility to adenovirus transduction.