Oral Presentation 30th Australian and New Zealand Bone and Mineral Society Annual Scientific Meeting 2020

Loss of Tsc2 in Cathepsin k expressing cells results in a high bone mass phenotype: not the usual suspects (#51)

Renee T Ormsby 1 , Nicola Alesi 2 , Molly Zgoda 1 , Elizabeth Henske 2 , Julia F Charles 1
  1. Orthopedic Research, Brigham and Womens Hospital, Boston, MA, United States
  2. Department of Medicine, Brigham and Womens Hospital, Boston, MA, United States

Tuberous sclerosis is a congenital disorder that occurs due to loss of function of the tuberous sclerosis complex, most commonly due to mutation of TSC2. Patients with this disorder develop tumours and various pathologies including sclerotic bone lesions. Loss of Tsc2 in mice similarly results in sclerotic bone lesions. In order to test whether loss of Tsc2 in osteoclasts was sufficient to cause sclerotic bone lesions, we crossed Tsc2fl/fl mice with cathepsin K Cre promoter (CtskCre) mice to specifically target osteoclasts. Micro-CT analysis of the femurs of both male and female Tsc2fl/fl;CtskCre mice showed increased cortical BV/TV, increased periosteal diameter and increased trabecular bone volume 1mm below the growth plate. Unexpectedly, histological analysis using TRAP staining showed no decrease in osteoclast number. In contrast, serum analysis showed increased expression of P1NP, a marker of bone formation, and dynamic histomorphometry revealed increased periosteal and trabecular bone formation rates. It has been shown that osteoblast lineage cells can express cathepsin K under some conditions. Cathepsin K is expressed by osteocytes during lactation and periprosthetic osteolysis, and by a population of periosteal stem cells during development, suggesting that the Tsc2fl/fl;CtskCre bone phenotype could reflect the function of TSC2 in one of these cell types. To determine the cellular location of Cre-mediated deletion, Tsc2fl/fl;CtskCre was crossed to the ROSA26mTmG reporter line. Immunofluorescence using GFP showed green fluorescence which indicated Tsc2 deletion in both osteocytes and periosteal lining cells. Furthermore, bone marrow chimera experiments demonstrated that the cortical phenotype is independent of Tsc2 genotype in hematopoietic stem cells. The Tsc2fl/fl;CtskCre bone phenotype depends on intact mTORC activity as it is abrogated by deletion of Raptor. Taken together, our data suggests a crucial role for TSC2 in regulating the periosteal bone formation function in osteoblast lineage cells via regulation of mTORC, linking energy sensing to bone growth.