Co-expression of MyHC-15 with other known isoforms in rat muscle spindles.

Muscle spindles are skeletal muscle sensory receptors composed of intrafusal fibres, partially encapsulated by connective tissue capsule. This capsule encloses the central A and B regions while leaving the distal C region extracapsular. Several past studies in rat have shown that muscle spindles typically contain a single bag1 fibre, a single bag2 fibre, and two smaller chain fibres. Intrafusal fibres co-express multiple myosin heavy chain (MyHC) isoforms: -slow or -1, -slow-tonic, -α, -2a, -2b, -embryonic, and -neonatal. While MyHC-2x was previously thought absent, the recently discovered MyHC-15 isoform has been identified in the C region of rat bag fibres. Using antibodies specific for nine MyHC isoforms and analyzing four different rat skeletal muscles-soleus, extensor digitorum longus, and the lateral and medial heads of gastrocnemius-we aimed to further characterize the co-expression pattern of MyHC-15 with other known isoforms and to determine whether MyHC-2x is expressed in rat intrafusal fibres. While rodents are widely used as animal models in skeletal muscle research, notable species-specific differences in MyHC isoform expression exist. Our findings revealed that MyHC-15 expression in rat intrafusal fibres is less abundant than in human fibres. MyHC-15 was primarily observed in bag fibres but was not detected in the C region, contrary to previous reports in both rat and human. We confirmed the absence of MyHC-2x in rat intrafusal fibres. Similarly, MyHC-embryonic and -neonatal were not detected in the analyzed spindles, suggesting that previously used antibodies may have cross-reacted with MyHC-2a and -2b. While our results partially corroborate previous extensive studies, discrepancies suggest that MyHC expression in intrafusal fibres varies not only along the fibre length but also across muscles.