Glucocorticoids Induce Osteoporosis through Decreased Sensory Innervation
Parrott, Kendall   (Johns Hopkins University)
Mentor: Crane, Janet L. (Johns Hopkins University School of Medicine)
Glucocorticoid therapy is the most common cause of secondary osteoporosis in children and the leading iatrogenic cause of the disease. The risk of fracture escalates by as much as 75% within the first 3 months after the initiation of glucocorticoid therapy and fracture risk increases before changes in bone density are observed, suggesting other factors are involved. Sensory innervation of bone has recently been associated with neuronal regulation of bone mass. We hypothesized that glucocorticoids impair bone innervation by decreasing paracrine factors regulating sensory innervation. Using osteoclast cultured media in a microfluidic culture platform with dorsal root ganglion neurons, we found that osteoclasts secrete Netrin to stimulate sensory innervation. To test if glucocorticoids impair sensory innervation by decreasing osteoclast netrin secretion, we first examined wild type (WT) mice treated intraperitoneally with either Vehicle (1xPBS) or Prednisolone 10mg/m- daily from 2 to 6 weeks of age and compared the effects of glucocorticoids to Cathepsin K knockout (Ctsk-/-) mice, as Ctsk-/- have an osteoclast-rich form of osteopetrosis. Glucocorticoids decrease both osteoclast activity and sensory innervation in WT mice, as assessed by Tartrate-resistant acid phosphatase (Trap) and Calcitonin gene-related peptide (CGRP) staining, respectively. Trap/Netrin costaining was also reduced in glucocorticoid (6.083 ± 1.433; n=12) relative to vehicle treated WT mice (30.33 ± 2.273; n=9). Ctsk-/- mice treated with glucocorticoids had lower numbers of TRAP/Netrin 1 positive cells (12 ± 6.715; n=7) relative to the vehicle Ctsk-/- group (37.29 ± 11.75; n=12). Ctsk-/- mice treated with glucocorticoids also had a significantly lower percentage of CGRP (3.832 ± 0.7026; n=24) than the vehicle Ctsk-/- group (11.2 ± 2.558; n=18), but similar to WT groups (vehicle = 5.799 ± 0.8963; n=24; glucocorticoid = 2.571 ± 0.5528; n=24). Genetic loss of Cathepsin K therefore partially abrogated glucocorticoids reduction in sensory innervation. We also examined Trap-Cre::Netrinf/f (NetrinTrap-/-) and Netrinf/f (WT) mice using Microcomputer tomography to assess the role of Netrin in regulation of bone mass. Using MicroCT, we found that specific deletion of Netrin-1 in Trap expressing cells results in decreased bone mass. These data suggest that glucocorticoids cause a decrease in Netrin 1 to impair bone sensory innervation, which may contribute to decreased bone density and osteoporosis. These observations have implications for a possible treatment target to reduce glucocorticoid induced bone disease.