BackgroundSarcopenia, the progressive decline in skeletal muscle mass and function with age, is a debilitating condition. It leads to inactivity, falls, and loss of independence. Despite this, its cause(s) and the underlying mechanism(s) are still poorly understood.MethodsIn this study, small skeletal muscle fibre bundles isolated from the extensor digitorum longus (a fast‐twitch muscle) and the soleus (a slow‐twitch muscle) of adult mice of different ages (range 100–900 days old) were used to investigate the effects of ageing and dihydrotestosterone (DHT) treatment on protein synthesis as well as the expression and function of two amino acid transporters; the sodium‐coupled neutral amino acid transporter (SNAT) 2, and the sodium‐independent L‐type amino‐acid transporter (LAT) 2.ResultsAt all ages investigated, protein synthesis was always higher in the slow‐twitch than in the fast‐twitch muscle fibres and decreased with age in both fibre types. However, the decline was greater in the fast‐twitch than in the slow‐twitch fibres and was accompanied by a reduction in the expression of SNAT2 and LAT2 at the protein level. Again, the decrease in the expression of the amino acid transporters was greater in the fast‐twitch than in the slow‐twitch fibres. In contrast, ageing had no effect on SNAT2 and LAT2 expressions at the mRNA level. Treating the muscle fibre bundles with physiological concentrations (~2 nM) of DHT for 1 h completely reversed the effects of ageing on protein synthesis and the expression of SNAT2 and LAT2 protein in both fibre types.ConclusionFrom the observations that ageing is accompanied by a reduction in protein synthesis and transporter expression and that these effects are reversed by DHT treatment, we conclude that sarcopenia arises from an age‐dependent reduction in protein synthesis caused, in part, by the lack of or by the low bioavailability of the male sex steroid, DHT.

Background Skeletal muscle mass and function are partly maintained by the supply of amino acids, altered amino acid transport is an important cause of frailty that can lead to decreased independence with increasing age and slow trauma recovery. The system‐A sodium coupled neutral amino acid transporter (SNAT)‐2 coded by gene family SLC38A2 generates a 506 amino acid 56 kDa protein that is an important transporter of amino acids in skeletal muscle. Ageing is associated with a decrease in expression of SNAT2 transporters. Methods In this study, we used the C2C12 cell line, using myoblast cells and cells differentiated into myotubes. We investigated if the expression of SNAT2 DNA would enhance intracellular amino acid levels and increase their availability for protein synthesis. Results In control myoblasts and myotubes, we found significantly decreased expression of SNAT2 (6.5× decrease, n = 4 per group, P < 0.05) in myotubes than found in myoblasts. After transfection with a SNAT2‐eGFP cDNA plasmid, C2C12 myoblasts significantly increased perinuclear punctate SNAT2‐eGFP expression that persisted and was more cytoplasmic after differentiation into myotubes. Interestingly, transfected cells were significantly more responsive to the hormone 5α‐dihydrotestosterone (DHT, 4.5 nM, by 1.6×, n = 3 per group, P < 0.04). Starvation significantly enhanced the amino acid C14‐MeAIB transport (1.7×, n = 3 per group, P < 0.05) indicating increased function of SNAT2. Inhibiting SNAT2 with high concentrations of MeAIB (3.3 or 5 mM) significantly reduced C14‐Isoleucine transport by L‐type amino acid transporter (LAT2, 52.8% and 77%, respectively, n = 3 per group, P < 0.05). However, there was no increase in the LAT2 transport of C14‐isoleucine detectable in SNAT2‐eGFP transfected cells after DHT (4.5 nM) exposure. This indicated that small amino acid availability was not rate limiting to LAT2 function in myoblasts. Conclusions Overall, these data show that transfection of SNAT2‐eGFP expression enhanced its function following starvation and treatment with physiological levels of DHT. Enhanced SNAT2 expression in muscle cells offers a viable epigenetic target in pathological conditions associated with altered amino acid transport.

BackgroundSarcopenia, the progressive decline in skeletal muscle mass and function with age, is a debilitating condition. It leads to inactivity, increased susceptibility to falls and eventually to loss of independent living. Despite this, the causes and mechanism(s) underlying sarcopenia are still poorly understood.MethodsSmall muscle fibre bundles isolated from the extensor digitorum longus (edl; a fast‐twitch muscle) and the soleus (a slow‐twitch muscle) of young, old and elderly mice were used to investigated the effects of age and dihydrotestosterone (DHT) treatment on the expression and function of the sodium‐coupled neutral amino‐ amino acid transporter (SNAT) 2 and the sodium‐independent L‐type amino‐acid transporter (LAT) 2.ResultsAt all ages investigated, protein synthesis was ~2 times higher in slow‐twitch fibres than in the fast‐twitch fibres. Ageing led to a decrease in protein synthesis in both fibre types. However, the decline was greater in the fast‐twitch than in the slow‐twitch fibres. Moreover, it was accompanied by a reduction in the expression of both SNAT 2 and LAT2 proteins. The decrease in SNAT 2 protein was greater in the fast‐twitch fibres than in the slow‐twitch fibres. In contrast, ageing had no effect on the expression of SNAT2 and LAT2 mRNA. Neither did it affect the uptake of the non‐metabolisable amino‐acid, α‐methylamino‐isobutyric acid. Treating the muscle fibre bundles with physiological concentrations of DHT significantly increased protein synthesis in the fast‐twitch fibres of both young and elderly mice and that of the slow‐twitch fibres in elderly mice only. At both protein and mRNA levels, 5α‐reductase (SRD) 1 was more abundant in skeletal muscle than 5α‐reductase (SRD) 2. It was expressed mostly in skin, tibialis anterior and soleus muscle and its expression increased significantly with age. Chloroquine treatment abolished the increased protein synthesis caused by DHT treatment.ConclusionFrom these results we suggest that sarcopenia arises from an age‐dependent reduction in protein synthesis caused by lack of anabolic stimulus. We also suggest that muscles such as soleus and the tibialis anterior are resistance to sarcopenia because they are able to synthesis more DHT locally. The mechanism of action involves the recruitment of SNAT2 from an intracellular vesicle pool into functioning membrane transport proteins.