Effect of botulinum toxin A-induced paralysis and exercise training on mechanosensing and signalling gene expression in juvenile rat gastrocnemius muscle

Intramuscular injections of the paralytic botulinum neurotoxin A (Btx) and physical exercise are used in the treatment of chronic spasticity in children with cerebral palsy. We tested whether Btx-induced paralysis and/or exercise training would have differential effects on the expression of mechanosensing and signalling genes implicated in the adaptive remodelling of skeletal muscle. Juvenile (29-day-old) male rats were injected with Btx or saline (NoBtx) into the right gastrocnemius and housed in standard cages (NoEx) or with running wheels (Ex), for 3 weeks (n = 6 per group). The mRNA expression of nine sarcomere-associated genes in the medial gastrocnemius was then determined by quantitative reverse transcriptase-polymerase chain reaction. The Btx-injected muscles weighed 50% less than NoBtx muscles, but Ex had no effect on the wet mass of Btx or NoBtx muscles. Atrogenic MuRF1, sarcomeric Titin and myogenic MyoD were upregulated (2-fold) with the elimination of contractile activity in Btx muscle. Expression of CARP, Ankrd2 and MLP was increased with mechanical stimuli associated with Btx (5- to 10-fold) or Ex (2- to 4-fold). Expression of CARP and Ankrd2 increased synergistically in Btx-Ex muscle (> or = 20-fold), indicating that these genes may be sensitive to passive stretch of the sarcomeric I-band region of titin to which their proteins bind. Tcap, Myopalladin and Atrogin1 were not, or were no longer responsive to the altered mechanical stimuli after 3 weeks of Btx or Ex. The expression of Ankrd2, CARP and MLP may thus be enhanced by passive stretch within the Btx-paralysed and/or exercising gastrocnemius and contribute to adaptations, other than muscle mass, in juvenile rats.