Maternal protein intake as a determinant of the total number of motor units and muscle fibres in mice

Abstract

Age‐related sarcopenia is characterized by reduction in the number of myofibres and an additional weakening of the remaining fibres, resulting in a reduction in muscle mass and function. Studies associate these changes with poor maternal malnutrition. However, very little is known about the effect of the maternal protein intake on the number of motor units in offspring. The aim of this study was to examine the effect of maintaining females on a low‐protein diet on the number of muscle fibres in offspring and whether any changes can be modulated by optimization of the nutrition in early post‐natal stages. Studies were undertaken in Thy1‐YFP16 mice that express yellow fluorescent protein only in neuronal cells. Female mice were fed either a normal (20%) or a low‐protein (5%) diet. Newborn pups were cross‐fostered to different lactating dams (maintained on 20% or 5% diet) within 24 h after birth, generating 4 groups: Normal‐to‐Normal (N‐N), Normal‐to‐Low (N‐L), Low‐to‐Normal (L‐N) and Low‐to‐Low (L‐L). Pups were culled at weaning (21 days) by cervical dislocation and the body weight recorded. One EDL was removed and snap frozen, sectioned and the total number of fibres counted. To visualize the neuromuscular junctions (NMJs), the second EDL muscle was removed and fixed in 4% p‐formaldehyde. Muscles were incubated for 30‐min with 5 μg/mL Alexa 594‐conjugated a‐Bungarotoxin (BTX).Data showed that mice born from dams on 20% diet but fed post‐natally by a foster dam on 5% diet (N‐L) show significant reductions in body size and body weight whereas mice born from dams on 5% diet but fed post‐natally by a dam on 20% diet (L‐N) did not show such changes when compared with pups from dams on 20% diet throughout the in utero and post‐natal periods [(N‐N); N‐N: 6.6±0.3g vs. N‐L: 3.56±0.15g)]. Muscle fibre cross sectional areas (CSAs) and innervation and the total number of muscle fibres were reduced in the N‐L compared with the N‐N group whereas in the L‐N only the total number of muscle fibres where affected (N‐N: 1450±51 vs. N‐L: 1231±67 vs. L‐N: 1270±51). Mice born from dams on 5% diet and fed post‐natally by a dam on a 5% diet (L‐L) show a reduced body size, body weight, muscle size and muscle fibre CSAs compared with the N‐N but these reductions were not as profound as in the N‐L group. Thus, data suggests that, in mice, poor maternal protein intake, either only in utero or only post‐natally, results in a significant loss of muscle fibres and that interventions post‐natally are able to reverse the detrimental effects in utero.Support or Funding InformationSupported by the British Physiological Society