ANALYSIS OF THE SKELETAL TRAITS OF THE ESTROGEN RECEPTOR (ALPHA) DEFICIENT MOUSE

Participants: K.M. Kozloff, M. Spurchise, E.P. Frankenburg, S.A. Goldstein, L. McCauley

Keywords: bone mechanics, architecture, estrogen receptor, animal model, osteoporosis

Introduction

Estrogen is a hormone that protects bone by inhibiting osteoclast activation. In post-menopausal women, osteoporosis is correlated with a loss of estrogen, however estrogen deficiency may also be a factor in bone loss in men. The precise roles of the two known estrogen receptors, ERa and ERb , in these mechanisms is currently unknown. The goal of the present study is to characterize the skeletal traits of the ERa knockout mouse.

Materials and Methods

All investigations were performed on male and female mice of wild type, heterzygote, and knockout genotypes. Femurs from 8-9 month mice are imaged by micro-ct to obtain architectural properties of cortical bone (M +/+, n=5; M -/-, n=4; M +/-, n=5; F +/+, n=6; F -/-, n=9; F +/-, n=4). 10 week old femurs are subjected to 4 point bending to obtain mechanical properties of early-aged cortical bone (M +/+, n=20; M -/-, n=20; M +/-, n=12; F +/+, n=17; F -/-, n=16; F +/-, n=16). 10 week old vertebral bodies are imaged via micro-ct to investigate the architectural properties of trabecular bone, and these same vertebral bodies are subjected to compression testing for mechanical properties (M +/+, n=9; M -/-, n=10; M +/-, n=6; F +/+, n=7; F -/-, n=7; F +/-, n=7). Parallel DEXA and ash weight measures were performed in Dr. McCauley's laboratory on 10 week, 16 week, and 8-9 month mice.

Results and Discussion

At 10 weeks, the male knockout mice showed decreased femoral stiffness, and decreased bone mineral content (BMC), bone mineral density (BMD) and area of the whole and distal femur as measured by DEXA, when compared to both the wild type and heterozygote. Conversely, the female knockout mice showed no change in femoral stiffness, and decreased BMD of the distal femur when compared only to the heterozygote. The female knockouts also showed greater bone volume fraction and trabecular number, and decreased trabecular spacing in the vertebral bodies, a trend not as evident in the male knockouts. Thus, at 10 weeks, it appears that the male knockouts are more affected than the females.

At 16 weeks, the male knockout mice still showed decreased BMC and BMD in the whole and distal femur by DEXA, decreased area in the distal femur, however ash results show that size of the bones may dominate this phenomena. The female knockouts displayed decreased BMC, BMD, and area in the distal femur, and decreased BMC and area in the whole femur. No ash weight data was available for the females at this point in time.

At 8-9 months, the male knockout mice show decreased BMC, BMD and area of the distal femur, and decreased BMC and area of the whole femur by DEXA. Micro-ct results suggest no difference in cortical geometry in the male knockouts when compared to the other male genotypes. The female knockouts showed no difference in DEXA parameters of BMD, BMC, or area compared to the wild type or heterozygote, and micro-ct showed no cortical geometry changes either. Thus, at 8-9 months, the female knockouts actually appear to be no different than the other female genotypes, whereas the male knockouts may be adapting their phenotype geometrically.

Histologic investigations of the growth plate are underway to help determine the cause for the increases in trabecular bone in the female 10 week knockout. Mechanical tests on these vertebrae are also in progress.