Why do humans have short toe bones and long leg bones, and how does skeletal proportion vary in species that dig, fly, run, and swim? Most mutations in genes that are required for bone elongation produce a proportionately dwarfed skeleton. Scientists know very little about how a bone's growth rate is locally determined to be faster or slower than other bones in the skeleton. By comparing gene expression differences in two rodents with very different skeletons, the mouse and bipedal jerboa with extremely long feet, the Cooper laboratory identified a number of genes that are excellent candidates to control growth rate differences within and between species. The project detailed in this NSF CAREER award will take advantage of the ability to quickly and easily over-express genes in the developing chicken skeleton to test the prediction that 30 of these candidate genes, which are not known to control skeletal development, are each sufficient to increase or decrease bone growth rate. The methods to visualize and analyze these manipulated skeletons are ideally suited to engage high school students in hands-on research experience. This project is therefore designed to provide a potentially transformative learning opportunity for hundreds of "at risk" students in an under-served community, in addition to expanding knowledge of the genetic control of skeletal growth and proportion.
The Cooper laboratory has capitalized on the striking difference in hindlimb proportions of the mouse and bipedal jerboa, similarity of their forelimbs, and the relatively close phylogenetic relationship (last common ancestor ~55 million years ago) to identify genes by RNA-Seq with expression differences that robustly associate with relative growth rates. These genes form a predicted interaction network that is highly enriched for pathways that are known to influence skeletal growth (e.g. WNT, Notch, and proteoglycan signaling), though many of the individual genes that were identified have no known growth plate function. They will focus on a set of 30 genes that show the strongest evidence for modular expression in the limb skeleton; gene expression changed during the evolution of accelerated elongation of the jerboa foot and remained unchanged in the forelimb that grows similar to mouse. In order to identify causative mechanisms that modulate skeletal growth rate, the lab will implement a powerful retroviral mis-expression system in the developing chicken wing skeleton to test the hypothesis that each gene is sufficient to accelerate or inhibit skeletal elongation. The objectives will rapidly expand what is known of the molecular toolkit that controls skeletal proportion and will contribute to mapping modular enhancers that control growth plate-specific gene expression and thus the development and evolution of skeletal proportion.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
