Gene expression of proteolytic systems and growth regulators of skeletal muscle in horses with myopathy associated with pituitary pars intermedia dysfunction.

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-1beta, interleukin-6, and tumor necrosis factor-alpha) 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-1beta, interleukin-6, and tumor necrosis factor-alpha 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.

Profiles of pro-opiomelanocortin and encoded peptides, and their processing enzymes in equine pituitary pars intermedia dysfunction.

Equine pituitary pars intermedia dysfunction (PPID) is characterized by hyperplasia of the pars intermedia (PI) melanotrophs of the pituitary gland (PG), and increased production of proopiomelanocortin (POMC). POMC is cleaved by prohormone convertase 1 (PC1) to produce adrenocorticotropic hormone (ACTH), and further processing of ACTH by PC2 to produce alpha-melanocyte stimulating hormone (α-MSH) and corticotropin-like intermediate peptide (CLIP). High plasma ACTH concentrations in horses with PPID might be related to reduced conversion of ACTH to α-MSH by PCs. The hypothesis of this study was that PC1 and PC2 expression in the pituitary gland are altered in PPID, resulting in an abnormal relative abundance of POMC derived proteins. The objectives of this study were to identify the partial sequences of equine POMC, PC1, and PC2 mRNAs; and to determine whether the expression of POMC, PC1, and PC2 mRNAs in whole pituitary extracts, and POMC-protein in the cavernous sinus blood of horses are altered in PPID. We confirmed (RT-PCR and sequencing) that the partial sequences obtained match the corresponding regions of predicted equine POMC, PC1 and PC2 sequences. The expression (quantification by RT-qPCR) of POMC, PC1 and PC2 mRNAs were found upregulated in the pituitary of horses with PPID. Plasma (measured using RIA/ELISA) ACTH and α-MSH were elevated in PPID horses. These results indicate distinct differences in gene and protein expression of POMC and its intermediates, and processing enzymes in PPID. It provides evidence to support the notion that local, pituitary-specific inadequacies in prohormone processing likely contribute to equine PPID.