Global deletion ofBCATmincreases expression of skeletal muscle genes associated with protein turnover

Abstract

Lynch CJ, Kimball SR, Xu Y, Salzberg AC, Kawasawa YI. Global deletion of BCATm increases expression of skeletal muscle genes associated with protein turnover. Physiol Genomics 47: 569-580, 2015. First published September 8, 2015 doi:10.1152/physiolgenomics.00055.2015.-Consumption of a protein-containing meal by a fasted animal promotes protein accretion in skeletal muscle, in part through leucine stimulation of protein synthesis and indirectly through repression of protein degradation mediated by its metabolite, ␣-ketoisocaproate. Mice lacking the mitochondrial branched-chain aminotransferase (BCATm/Bcat2), which interconverts leucine and ␣-ketoisocaproate, exhibit elevated protein turnover. Here, the transcriptomes of gastrocnemius muscle from BCATm knockout (KO) and wildtype mice were compared by next-generation RNA sequencing (RNA-Seq) to identify potential adaptations associated with their persistently altered nutrient signaling. Statistically significant changes in the abundance of 1,486/ϳ39,010 genes were identified. Bioinformatics analysis of the RNA-Seq data indicated that pathways involved in protein synthesis [eukaryotic initiation factor (eIF)-2, mammalian target of rapamycin, eIF4, and p70S6K pathways including 40S and 60S ribosomal proteins], protein breakdown (e.g., ubiquitin mediated), and muscle degeneration (apoptosis, atrophy, myopathy, and cell death) were upregulated. Also in agreement with our previous observations, the abundance of mRNAs associated with reduced body size, glycemia, plasma insulin, and lipid signaling pathways was altered in BCATm KO mice. Consistently, genes encoding anaerobic and/or oxidative metabolism of carbohydrate, fatty acids, and branched chain amino acids were modestly but systematically reduced. Although there was no indication that muscle fiber type was different between KO and wild-type mice, a difference in the abundance of mRNAs associated with a muscular dystrophy phenotype was observed, consistent with the published exercise intolerance of these mice. The results suggest transcriptional adaptations occur in BCATm KO mice that along with altered nutrient signaling may contribute to their previously reported protein turnover, metabolic and exercise phenotypes.

LAT1 Protein Content Increases Following 12 Weeks of Resistance Exercise Training in Human Skeletal Muscle

“…These reactions serve to meet metabolic demands while indirectly stimulating protein synthesis by limiting oxidation of amino acids (e.g., leucine). In spite of the fact that BCAT2 protein levels have been shown to be involved with metabolism and endurance performance ( 50 , 51 ), our results suggest resistance exercise training or different supplementation paradigms do not impact this marker. The BCKDH complex is critical for BCAA metabolism given that it is the rate-limiting enzyme and is the commitment step toward generation of ATP from BCAA catabolism.…”

Suppression of p16 Induces mTORC1-Mediated Nucleotide Metabolic Reprogramming

“…The cDNA libraries were prepared using KAPA Stranded RNA-Seq Kits with RiboErase (Kapa Biosystems). Next generation sequencing was performed in The Penn State College of Medicine Genome Sciences and Bioinformatics Core facility as previously described in Lynch et al (2015) using a HiSeq 2500 sequencer (Illumina). Demultiplexed and quality-filtered mRNA-Seq reads were then aligned to human reference genome (GRCh38) using TopHat (v.2.0.9).…”

Role of branched‐chain amino acid–catabolizing enzymes in intertissue signaling, metabolic remodeling, and energy homeostasis

“…These mice display lower plasma leptin, insulin, and norepinephrine and have a metabolically healthy phenotype with increased glucose disposal, improved glucose tolerance, and enhanced insulin sensitivity (28). Physiologic changes observed in BCAT KO mice were supported by transcriptomics RNA sequencing studies in the skeletal muscle, which showed an up-regulation of the pathways involved in protein synthesis, degradation, and muscle degeneration and a coordinated decrease in the mRNA levels of genes modulating body weight, glycemia, plasma insulin, and lipid signaling (29). Though lack of BCAT presented a favorable metabolic milieu for protection against metabolic syndrome, gastrocnemius muscles of BCATm KO mice had decreased mass and were functionally intolerant of exercise (28).…”

“…Exercise intolerance and reduced muscle mass in BCATm KO mouse are likely outcomes of unavailability of gluconeogenic amino acids derived from glucose during exercise, which exacerbates interruptions in the muscle malate-aspartate cycle and lactate flux into pyruvate during the Cori cycle (28). Decreased muscle mass is also a consequence of reduced levels of a-ketoisocaproic acid (KIC), a leucine metabolite, which under conditions of food withdrawal represses protein degradation (29). BCATc…”