Bone is a highly dynamic organ system comprised of various cell types that are constantly working to maintain the health and stability of bone. The main cells involved are the osteoblasts that form bone, the osteoclasts that degrade bone, and the osteocytes that act as sensors of the microenvironment and coordinate a response. An imbalance of the interactions between the cell types can potentially result in pathological states in bone at the microscopic level that can then affect the entire skeleton. Moreover, a number of genetic mutations can also lead to pathogenic changes in bone. An example of such is the development of sclerotic bone lesions in patients with the disease tuberous sclerosis complex.
Tuberous sclerosis complex, or TSC, is an autosomal dominant disorder affecting approximately 1.5 million people worldwide. It is caused by a mutation in one of the genes encoding either member of the TSC1-TSC2 complex. Molecularly, TSC1-TSC2 negatively regulate the mechanistic target of rapamycin (mTOR) kinase in the mutliprotein complex mTORC1. Activation of mTORC1 leads to an upregulation of protein synthesis and cell growth. Tuberous sclerosis patients are heterozygous for TSC1 or TSC2, and post-natal loss of the second allele results in the development of multiple, benign, tumor-like hamartomas in various organ systems, most notably affecting the brain, kidneys, lungs, skin, and heart. Additionally, CT scans of patients reveal multiple loci of dense, compact bone termed sclerotic bone lesions. The bone lesions were most commonly seen in the posterior elements of the vertebrae and while they are asymptomatic, a remarkably high frequency of patients express them.
To further investigate and better understand the mechanisms of tuberous sclerosis complex in bone, we analyzed a mouse model with heterozygous deletion in Tsc2. Initial examination showed the Tsc2+/- mice recapitulated tumors in various organ systems, most notably the kidney, and presented bone lesions in the pelvis and elements of the vertebrae. To further investigate the mechanism driving the disease state, we used a Cre driver thought to be specific for osteoclast (Cathepsin K-Cre, or Ctsk-Cre) to selectively delete Tsc2. Cathepsin K-Cre; Tsc2fl/fl mice exhibit a remarkably high bone mass. This study examined three specific aspects of this high bone mass phenotype. First, we sought to verify that the increased bone mass caused by Ctsk-Cre driven Tsc2 deletion was dependent on mTORC1 upregulation. This was done by generating Ctsk-Cre;Tsc2fl/fl mice lacking Raptor, a mTORC1 component essential for function. Next, we investigated the cell of origin driving the increase bone density by utilizing additional Cre drivers specific for osteoclasts and osteocytes. Additionally, we used radiation chimeras to assess if donated wild type cells could rescue the observed phenotype. We lastly explored the role of a secreted signaling molecule, CTHRC1, that has been proposed as a candidate to mediate osteoclast-osteoblast interaction, in the high bone mass phenotype of Ctsk-Cre;Tsc2fl/fl mice.
Selective deletion of Tsc2 in bone cells provides an excellent model to investigate pathways regulating bone mass and strength and may provide new candidate targets for treating diseases of low bone mass, such as osteoporosis.