Inactivation of Pten in osteo-chondroprogenitor cells leads to epiphyseal growth plate abnormalities and skeletal overgrowth.

UNLABELLED: To study the role of the Pten tumor suppressor in skeletogenesis, we generated mice lacking this key phosphatidylinositol 3'-kinase pathway regulator in their osteo-chondroprogenitors. A phenotype of growth plate dysfunction and skeletal overgrowth was observed.
INTRODUCTION: Skeletogenesis is a complex process relying on a variety of ligands that activate a range of intracellular signal transduction pathways. Although many of these stimuli are known to activate phosphatidylinositol 3'-kinase (PI3K), the function of this pathway during cartilage development remains nebulous. To study the role of PI3K during skeletogenesis, we used mice deficient in a negative regulator of PI3K signaling, the tumor suppressor, Pten.
MATERIALS AND METHODS: Pten gene deletion in osteo-chondrodroprogenitors was obtained by interbreeding mice with loxP-flanked Pten exons with mice expressing the Cre recombinase under the control of the type II collagen gene promoter (Pten(flox/flox):Col2a1Cre mice). Phenotypic analyses included microcomputed tomography and immunohistochemistry techniques.
RESULTS: MicroCT revealed that Pten(flox/flox):Col2a1Cre mice exhibited both increased skeletal size, particularly of vertebrae, and massive trabeculation accompanied by increased cortical thickness. Primary spongiosa development and perichondrial bone collar formation were prominent in Pten(flox/flox):Col2a1Cre mice, and long bone growth plates were disorganized and showed both matrix overproduction and evidence of accelerated hypertrophic differentiation (indicated by an altered pattern of type X collagen and alkaline phosphatase expression). Consistent with increased PI3K signaling, Pten-deficient chondrocytes showed increased phospho-PKB/Akt and phospho-S6 immunostaining, reflective of increased mTOR and PDK1 activity. Interestingly, no significant change in growth plate proliferation was seen in Pten-deficient mice, and growth plate fusion was found at 6 months.
CONCLUSIONS: By virtue of its ability to modulate a key signal transduction pathway responsible for integrating multiple stimuli, Pten represents an important regulator of both skeletal size and bone architecture.