Changes in mRNA gene expression during growth in the femoral head of the young rat

Abstract

The rate of physeal growth slows as an animal matures with changes in mRNA gene expression due to the altered cellular activity. To measure the change in gene expression during the juvenile growth period, the femoral head, enclosing the proximal femoral physis, primary spongiosa, and articular cartilage, was collected from both femora of 16 female Sprague–Dawley rats between 4 and 10 weeks of age. One femur of each rat had had a mid-diaphyseal femoral fracture at 4 weeks of age. RNA was extracted and hybridized to 16 Affymetrix Rat Genomic 230 2.0 GeneChip microarrays with probe sets for 31,000 genes of which 18,200 were expressed. Of these, 8002 genes had a significant change in gene expression during growth, about half increasing and half decreasing. These changes included up-regulation with time of genes related to cartilage, blood vessels, osteoprotegerin, osteomodulin, and most ribosomal proteins. There was down-regulation with maturity of genes related to bone, growth-promoting cytokines, G proteins, GTPase-mediated signal transduction factors, cytokine receptors, mitosis, integrin-linked kinase, and the cytoskeleton. In summary, the slowing of growth with maturity was associated with changes in mRNA gene expression in the femoral head for a large number of genes. These changes in gene expression between young and mature rats suggest factors which are important for the support of the rapid linear growth during early life.

Introduction

Young rats grow rapidly to attain their adult size. A young rat is weaned at 3 weeks of age, and at 4 weeks of age physeal growth is near its maximum rate [1]. By 10 weeks of age sexual maturity has been reached, and the linear growth of the long bones has slowed, particularly in females [2], [1]. This linear growth of the skeleton is due to mitosis and hypertrophy in the chondrocytes of the physis.

These changes in the chondrocytes are well described on a morphological and histological basis. Insights have been gained as to the regulation of the growth within the physis [3], [4], [5], [6], [7], [8]. DNA microarray technology has been used to study the differences in gene expression between different zones of the physis [9], [10]. However, less well known are the molecular events which support the change from rapid growth in weanling rats to maintenance of the skeleton without linear growth in the adult. This has been explored by histology [11], [12], by study of the expression of matrix metalloproteinase [13] and collagen genes [13], as well as by generation of EST libraries [14]. Changes in gene expression between young and mature rats would suggest factors important for the support of the rapid linear growth during early life.

The femoral head of the rat femur contains the proximal physis, metaphyseal bone, and articular cartilage [15]. We had conducted an experiment to study the change in mRNA gene expression in the femoral head following unilateral mid-shaft femoral fracture of juvenile rats [15]. We also analyzed the heads of the contralateral unfractured femora as a control. These control samples were striking in the profound changes in gene expression with growth in both the magnitude of the changes as well as the large number of affected genes. This led to the present report of the alterations in mRNA gene expression in the femoral head of juvenile rats as the animals grow and mature.