Artificial Selection for Increased Voluntary Wheel Running Alters Limb Skeleton Shape and Exercise Plasticity in Mice

Abstract

Individual skeletal morphology is determined by genetic effects and within‐generation phenotypic plasticity in response to mechanical loading. Changes in limb bone morphology associated with locomotor activity through ontogeny and across evolutionary time can provide insight into adaptation in locomotor systems. Here, we investigate the effects of long‐term selection for locomotor activity on the gross morphology and exercise response of the hind limb skeleton using a mouse model. A founding population of Hsd:ICR outbred mouse stock was divided into closed lines of “high runner” (HR) and control (C) animals. HR mice were bred for increased voluntary wheel‐running distance by breeding animals (n=10 pairs per generation) with the highest average wheel revolutions on days 5 and 6 of a 6‐day wheel exposure protocol. Control animals were also wheel‐tested but were randomly paired. Male HR and C mice from experimental generation 82 were randomly assigned at 5 weeks of age to be permitted or denied wheel access for 4 weeks, resulting in 4 experimental groups: C wheel (n=12), C no wheel (n=12), HR wheel (n=10), and HR no wheel (n= 10). Following wheel access, mice were euthanized and right hind limbs were micro‐CT scanned (~20 um voxel size). Three‐dimensional shapes of the os coxa, femur, and tibia were defined by digitizing 18, 23, and 18 landmarks, respectively, on each specimen. To adjust for size, individual bones were scaled to the geometric mean of all possible linear distances among landmarks. Between‐group differences were analyzed using canonical variates analysis (CVA) and localized using the Euclidean distance matrix analysis (EDMA) Form procedure. EDMA Growth procedure was used to compare the trajectories of exercise‐induced skeletal plasticity between HR and C animals. Differences ≥3% were considered substantial for both EDMA methods. Shape analyses revealed differences in morphological lability among the three limb bones examined and in subsections within each bone. More specifically, the mean hind limb skeletal morphology, especially of the pelvis and proximal femur, substantially differed between HR and C mice. Plastic bony responses to exercise also differed, most substantially in the iliac blade, ischial tuberosity, and proximal femur. Finally, throughout the hind limb, the baseline skeletal morphology of HR mice did not resemble the exercise response of C animals, indicating that genetic assimilation of plastic bone responses did not occur in this model. The results suggest regional sensitivity of the hind limb skeleton to plastic and evolved morphological change, provide further evidence of genetic influence on bone functional adaptation, and inform our understanding of the evolutionary lability of the musculoskeletal system over evolutionary timescales.Support or Funding InformationUniversity of Missouri School of Medicine, University of Missouri Dissertation Research Travel AwardThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.