Dihydrotestosterone treatment rescues the decline in protein synthesis as a result of sarcopenia in isolated mouse skeletal muscle fibres

Abstract

Background: Sarcopenia, 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.

Methods: In 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.

Results: At 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.

Conclusion: From 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.

Keywords: Ageing; Amino acid transporters; Protein synthesis; Sarcopenia; Skeletal muscle.

Figure 1

Ageing decreases skeletal muscle mass and protein synthesis. Bar graphs showing the effects of ageing on muscle weight (A and B) and protein synthesis (C) in the extensor digitorium longus (F; clear bars) and soleus (S; filled bars) of young (Y), middle‐aged (M) and elderly (E) mice. (D) A typical Western blot showing the phosphorylation of eEF2 in the extensor digitorium longus (F) and soleus (S) of mice ~100 days old. Note that ageing decreases skeletal muscle mass and protein synthesis in both fibre types. Additionally, the extensor digitorium longus expresses more phosphor‐eEF2 than the soleus. The data in (A) and (B) are from whole muscles, whereas those in (C) and (D) are from small muscle fibre bundles. *†P < 0.05 when the data from the elderly and the middle‐aged mice were compared with that from the young mice. € ± P < 0.05 when the data from the middle‐aged mice were compared with those from the elderly mice.

Figure 2

Ageing decreases the expression of sodium‐coupled neutral amino acid transporter 2 and LAT2 proteins. Bar graphs and typical immunoblots showing the effects of ageing on the expression of sodium‐coupled neutral amino acid transporter 2 and L‐type amino‐acid transporter proteins in small muscle fibre bundles isolated from the extensor digitorium longus (F; clear bars) and soleus (S; filled bars) of young (Y), middle‐aged (M), and elderly (E) mice. Note that ageing decreases the expression of both proteins in the fast‐twitch and slow‐twitch fibre bundles. *†P < 0.05 when the data from the elderly and the middle‐aged mice were compared with those from the young mice.

Figure 3

Ageing has no effect on sodium‐coupled neutral amino acid transporter 2 and L‐type amino‐acid transporter mRNA. Bar graphs and immunoblots showing the effects of ageing on the expression of sodium‐coupled neutral amino acid transporter 2 and L‐type amino‐acid transporter mRNA in small muscle fibre bundles isolated from the extensor digitorium longus (F; clear bars) and soleus (S; filled bars) of young (Y), middle‐aged (M), and elderly (E) mice. Note that ageing has no significant effect on the mRNA for both proteins in the fast‐twitch and slow‐twitch fibre bundles. L, liver samples.

Figure 4

Dihydrotestosterone treatment reverses the effects of age on protein synthesis and the expression of sodium‐coupled neutral amino acid transporter 2 and L‐type amino‐acid transporter proteins. Bar graphs and typical western blots showing the effects of treating small muscle fibre bundles isolated from the extensor digitorium longus (F; clear bars) and soleus (S; filled bars) of young (Y), middle‐aged (M), and elderly (E) mice with physiological concentrations of dihydrotestosterone for 1 h on protein synthesis (A), sodium‐coupled neutral amino acid transporter 2 protein expression (B), and L‐type amino‐acid transporter protein expression (C). Note that treating the fibre bundles with dihydrotestosterone reverses the age‐dependent decline in protein synthesis as well as the age‐dependent decrease in sodium‐coupled neutral amino acid transporter 2 and L‐type amino‐acid transporter protein expression. *†P < 0.05 when the data from the elderly and the middle‐aged mice were compared with those from the young mice.