Selective breeding as a tool to probe skeletal response to high voluntary locomotor activity in mice

Abstract

We present a novel mouse-model for the study of skeletal structure and evolution, based on selective breeding for high levels of voluntary wheel running. Whereas traditional models (originally inbred strains, more recently knockouts and transgenics) rely on the study of mutant or laboratory-manipulated phenotypes, we have studied changes in skeletal morphometrics resulting from many generations of artificial selection for high activity in the form of wheel running, in which mice engage voluntarily. Mice from the four replicate High Runner (HR) lines run nearly three times as many revolutions during days 5 and 6 of a 6-day exposure to wheels (1.12 m circumference). We have found significant changes in skeletal dimensions of the hind limbs, including decreased directional asymmetry, larger femoral heads, and wider distal femora. The latter two have been hypothesized as evolutionary adaptations for long-distance locomotion in hominids. Exercise-training studies involving experimental groups with and without access to wheels have shown increased diameters of both femora and tibiafibulae, and suggest genetic effects on trainability (genotype-by-environment interactions). Reanalysis of previously published data on bone masses of hind limbs revealed novel patterns of change in bone mass associated with access to wheels for 2 months. Without access to wheels, HR mice have significantly heavier tibiafibulae and foot bones, whereas with chronic access to wheels, a significant increase in foot bone mass that was linearly related to increases in daily wheel running was observed. Mice exhibiting a recently discovered small-muscle phenotype ("mini-muscle," [MM] caused by a Mendelian recessive gene), in which the mass of the triceps surae muscle complex is ∼50% lower than in normal individuals, have significantly longer and thinner bones in the hind limb. We present new data for the ontogenetic development of muscle mass in Control, HR, and MM phenotypes in mice of 1-7 weeks postnatal age. Statistical comparisons reveal highly significant differences both in triceps surae mass and mass-corrected triceps surae mass between normal and MM mice at all but the postnatal age of 1 week. Based on previously observed differences in distributions of myosin isoforms in adult MM mice, we hypothesize that a reduction of myosin heavy-chain type-IIb isoforms with accounts for our observed ontogenetic changes in muscle mass.

Fig. 1

Voluntary wheel running (mean revolutions per day) on days 5 and 6 of a 6-day exposure to wheels (1.12 m circumference). Solid line with black squares is average value of the four replicate HR lines, solid line with open circles is average of four nonselected C lines, and dashed line is the differences between those averages. For both the HR and C lines, about one-third of the total revolutions (and hence distance) per day are attributable to “coasting” (Koteja et al. 1999). After generation 31, mice were moved from Wisconsin to California, and wheel running was not recorded for four generations. Data for subsequent generations are not shown.

Fig. 2

Bone performance during loading is primarily governed by bone structure (from microstructure to macrostructure). Genetics and history of loading (exercise) are the two main determinants of bone structure (Eisman ; Ferrari et al. 1999). Genetic background can mediate the effect of exercise in two ways. First, the propensity to exercise has a genetic basis, and this desire for activity is the target of selection (gray line). Second, the physiological response to exercise can be modulated by genetic background (dashed line). For example, Kodama et al. (2000) showed differential sensitivity to exercise in C57BL/6J mice compared with C3H/HeJ mice, with a greater response to loading in the former.

Fig. 3

Bone mass in relation to amount of running in a wheel (revolutions/day) during the last 6 days of an 8-week access to wheels (1.12 m circumference) as described by Kelly et al. (2006). Individuals plotted near the left axis, above the value of zero revolutions, were housed in ordinary cages without wheels (points are offset from zero for clarity). Data based on Kelly et al. (2006).

Fig. 4

Comparison of the size of the triceps surae muscle in the MM phenotype (left; Mouse ID 53283; body mass = 20.4 g, age = 66 days) and in normal (right; Mouse ID 53277; body mass = 26.1 g, age = 66 days) mice. The muscles of the MM phenotype are ∼50% the mass of normal muscles (0.055 versus 0.129 g). Scale bar = 1 mm. Supplementary online figure is in color.

Fig. 5

Ontogenetic change in body mass (top row) and in mass of the triceps surae muscle normalized to body mass (bottom row) during postnatal weeks 1–7 in C, normal HR, and MM-phenotype mice for females (left) and males (right). Values are simple means ± 1 SD (some of which are so small as to be obscured by the mean point). Points are slightly offset in age (weeks) for clarity.