Objective - To investigate gene expression of the major proteolytic systems and growth regulators in skeletal muscle of horses with myopathy associated with pituitary pars intermedia dysfunction (PPID). Animals - 14 horses with PPID-associated myopathy and 7 healthy control horses. Procedures - Horses with PPID and controls were age matched (15 to 28 years old). Muscle biopsy specimens were collected from both groups and processed for RNA and cDNA extraction. Validation of the most stable housekeeping genes for skeletal muscle was performed and used to compare gene expression of the following proteolytic systems: cysteine aspartate protease-dependent systems (caspases), lysosomal-dependent systems (cathepsins), non-lysosomal calcium protease-dependent systems (calpains), and ubiquitin-proteasome-dependent systems (ubiquitins). Gene expression of negative regulators of muscle growth (myostatin and inflammatory cytokines interleukin-1β, interleukin-6, and tumor necrosis factor-α) was also determined. Results - No significant difference between groups was detected in expression of the major proteolytic systems except for m-calpain, which was greater in horses with PPID. No differences in gene expression of myostatin and interleukin-1β, interleukin-6, and tumor necrosis factor-α were detected between groups. Conclusions and Clinical Relevance - Greater expression of m-calpain may suggest that calpains play an important role in development of muscle atrophy in horses with PPID. However, because posttranslational events may alter protein activation, inactivation, and functions not studied here, other mechanisms of muscle atrophy cannot be excluded.
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