Ribosome abundance regulates the recovery of skeletal muscle protein mass upon recuperation from postnatal undernutrition in mice.

Nutritionally-induced growth faltering in the perinatal period has been associated with reduced adult skeletal muscle mass; however, the mechanisms responsible for this are unclear. To identify the factors that determine the recuperative capacity of muscle mass, we studied offspring of FVB mouse dams fed a protein-restricted diet during gestation (GLP) or pups suckled from postnatal day 1 (PN1) to PN11 (E-UN), or PN11 to PN22 (L-UN) on protein-restricted or control dams. All pups were refed under control conditions following the episode of undernutrition. Before refeeding, and 2, 7 and 21 days later, muscle protein synthesis was measured in vivo. There were no long-term deficits in protein mass in GLP and E-UN offspring, but in L-UN offspring muscle protein mass remained significantly smaller even after 18 months (P < 0.001). E-UN differed from L-UN offspring by their capacity to upregulate postprandial muscle protein synthesis when refed (P < 0.001), a difference that was attributable to a transient increase in ribosomal abundance, i.e. translational capacity, in E-UN offspring (P < 0.05); translational efficiency was similar across dietary treatments. The postprandial phosphorylation of Akt and extracellular signal-regulated protein kinases were similar among treatments. However, activation of the ribosomal S6 kinase 1 via mTOR (P < 0.02), and total upstream binding factor abundance were significantly greater in E-UN than L-UN offspring (P < 0.02). The results indicate that the capacity of muscles to recover following perinatal undernutrition depends on developmental age as this establishes whether ribosome abundance can be enhanced sufficiently to promote the protein synthesis rates required to accelerate protein deposition for catch-up growth.

Dystropathology increases energy expenditure and protein turnover in the mdx mouse model of duchenne muscular dystrophy.

The skeletal muscles in Duchenne muscular dystrophy and the mdx mouse model lack functional dystrophin and undergo repeated bouts of necrosis, regeneration, and growth. These processes have a high metabolic cost. However, the consequences for whole body energy and protein metabolism, and on the dietary requirements for these macronutrients at different stages of the disease, are not well-understood. This study used juvenile (4- to 5- wk-old) and adult (12- to 14-wk-old) male dystrophic C57BL/10ScSn-mdx/J and age-matched C57BL/10ScSn/J control male mice to measure total and resting energy expenditure, food intake, spontaneous activity, body composition, whole body protein turnover, and muscle protein synthesis rates. In juvenile mdx mice that have extensive muscle damage, energy expenditure, muscle protein synthesis, and whole body protein turnover rates were higher than in age-matched controls. Adaptations in food intake and decreased activity were insufficient to meet the increased energy and protein needs of juvenile mdx mice and resulted in stunted growth. In (non-growing) adult mdx mice with less severe dystropathology, energy expenditure, muscle protein synthesis, and whole body protein turnover rates were also higher than in age-matched controls. Food intake was sufficient to meet their protein and energy needs, but insufficient to result in fat deposition. These data show that dystropathology impacts the protein and energy needs of mdx mice and that tailored dietary interventions are necessary to redress this imbalance. If not met, the resultant imbalance blunts growth, and may limit the benefits of therapies designed to protect and repair dystrophic muscles.

In utero glucocorticoid exposure reduces fetal skeletal muscle mass in rats independent of effects on maternal nutrition.

Maternal stress and undernutrition can occur together and expose the fetus to high glucocorticoid (GLC) levels during this vulnerable period. To determine the consequences of GLC exposure on fetal skeletal muscle independently of maternal food intake, groups of timed-pregnant Sprague-Dawley rats (n = 7/group) were studied: ad libitum food intake (control, CON); ad libitum food intake with 1 mg dexamethasone/l drinking water from embryonic day (ED)13 to ED21 (DEX); pair-fed (PF) to DEX from ED13 to ED21. On ED22, dams were injected with [(3)H]phenylalanine for measurements of fetal leg muscle and diaphragm fractional protein synthesis rates (FSR). Fetal muscles were analyzed for protein and RNA contents, [(3)H]phenylalanine incorporation, and MuRF1 and atrogin-1 (MAFbx) mRNA expression. Fetal liver tyrosine aminotransferase (TAT) expression was quantified to assess fetal exposure to GLCs. DEX treatment reduced maternal food intake by 13% (P < 0.001) and significantly reduced placental mass relative to CON and PF dams. Liver TAT expression was elevated only in DEX fetuses (P < 0.01). DEX muscle protein masses were 56% and 70% than those of CON (P < 0.01) and PF (P < 0.05) fetuses, respectively; PF muscles were 80% of CON (P < 0.01). Muscle FSR decreased by 35% in DEX fetuses (P < 0.001) but were not different between PF and CON. Only atrogin-1 expression was increased in DEX fetus muscles. We conclude that high maternal GLC levels and inadequate maternal food intake impair fetal skeletal muscle growth, most likely through different mechanisms. When combined, the effects of decreased maternal intake and maternal GLC intake on fetal muscle growth are additive.

In vivo measurement of muscle protein synthesis rate using the flooding dose technique.

Skeletal muscle mass is determined by the balance between rates of protein synthesis and degradation. Protein synthesis rates can be measured in vivo by administering an amino acid as a tracer that is labeled with an isotope (radioactive or stable) of C, H, or N. The rate at which the labeled amino acid is incorporated into muscle protein, as a function of the amount of labeled amino acid in the precursor pool at the site of translation, reflects the rate of protein synthesis. There are a number of approaches for performing this measurement depending on the question being addressed and the experimental system being studied. In this chapter, we describe the "flooding dose" approach using L-[(3)H]-phenylalanine as the tracer and that is suitable for determining the rate of skeletal muscle protein synthesis (total and myofibrillar proteins) over an acute period (ideally less than 30 min) in any size animal; details for working with mice are presented. The method describes how to administer the tracer without anesthesia, the tissue collection, and the preparation of muscle and blood samples for analysis of the tracer and tracee amino acids in the precursor pool and in muscle proteins.