Parathyroid hormone attenuates radiation-induced increases in collagen crosslink ratio at periosteal surfaces of mouse tibia

Abstract

As part of our ongoing efforts to understand underlying mechanisms contributing to radiation-associated bone fragility and to identify possible treatments, we evaluated the longitudinal effects of parathyroid hormone (PTH) treatment on bone quality in a murine model of limited field irradiation. We hypothesized PTH would mitigate radiation-induced changes in the chemical composition and structure of bone, as measured by microscope-based Raman spectroscopy. We further hypothesized that collagen crosslinking would be especially responsive to PTH treatment. Raman spectroscopy was performed on retrieved tibiae (6-7/group/time point) to quantify metrics associated with bone quality, including: mineral-to-matrix ratio, carbonate-to-phosphate ratio, mineral crystallinity, collagen crosslink (trivalent:divalent) ratio, and the mineral and matrix depolarization ratios. Irradiation disrupted the molecular structure and orientation of bone collagen, as evidenced by a higher collagen crosslink ratio and lower matrix depolarization ratio (vs. non-irradiated control bones), persisting until 12 weeks post-irradiation. Radiation transiently affected the mineral phase, as evidenced by increased mineral crystallinity and mineral-to-matrix ratio at 4 weeks compared to controls. Radiation decreased bone mineral depolarization ratios through 12 weeks, indicating increased mineral alignment. PTH treatment partially attenuated radiation-induced increases in collagen crosslink ratio, but did not restore collagen or mineral alignment. These post-radiation matrix changes are consistent with our previous studies of radiation damage to bone, and suggest that the initial radiation damage to bone matrix has extensive effects on the quality of tissue deposited thereafter. In addition to maintaining bone quality, preventing initial radiation damage to the bone matrix (i.e. crosslink ratio, matrix orientation) may be critical to preventing late-onset fragility fractures.

Longitudinal Effects of Single Hindlimb Radiation Therapy on Bone Strength and Morphology at Local and Contralateral Sites

“…Data from histological and biochemical analyses indicate roles for both altered cellular activity (osteoclasts, osteoblasts, adipocytes) and extracellular matrix modifications (alignment, cross‐linking) in radiotherapy‐induced bone disease (Fig. ) . In this model, the increase in metaphyseal cortical bone quantity may represent a compensatory response to the biomechanical effects of metaphyseal trabecular bone resorption .…”

“…Summary of post‐irradiation changes in bone morphology, density, mechanics, and biochemistry as a function of time after treatment in this murine model of limited field radiotherapy. Biochemical and histomorphologic data derived from ( A ) Oest and colleagues; ( B ) Gong and colleagues; ( C ) Oest and colleagues; and ( D ) Oest and colleagues . Tt.Ar = total area; Ct.Th = cortical thickness; Ct.TMD = cortical tissue mineral density; MAR = mineral apposition rate; Tb.…”

“…Work in mouse models (total body and limited field RTx) consistently demonstrates an early increase in osteoclast numbers at 1 to 2 weeks post‐RTx, followed by long‐term depletion of osteoclasts . This results in early loss of trabecular bone and persistence of highly cross‐linked, highly aligned bone matrix due to loss of remodeling activity . At this point, it is unknown whether loss of local osteoclasts after limited field irradiation is because of 1) the absence of osteoclast progenitor cells, or 2) an inability of osteoclast precursors to multinucleate, differentiate, or adhere.…”

“…A consistent pattern of increased osteoclastic resorption followed by loss of trabecular bone has emerged . Bone density is not consistently decreased in irradiated animals, although a consistent pattern of radiation‐induced alterations to the tissue matrix has emerged, including increased trivalent:divalent collagen cross‐link ratios, formation of pathologic glycation end products, and overly aligned mineral and collagen phases . Biomechanical studies have indicated that changes in bone morphology and mineral content alone do not fully explain post‐radiotherapy bone fragility, suggesting a role for material embrittlement .…”

The lack of neuropeptide Y‐Y 1 receptor signaling modulates the chemical and mechanical properties of bone matrix

“…Another aspect that might compromise bone matrix quality is the occurrence of collagen malformations due to augmented deposition of collagen fibers. 38 Although we have observed an acceleration on bone matrix formation by the increased levels of collagen fibers and maturation in Y 1 R −/− compact bone, Raman spectroscopy analysis demonstrated that there is no deviation from normal collagen crosslink, as well as, no pathological accumulation of immature/mature collagen crosslinks, as determined by collagen crosslink ratio. Moreover, Raman analysis revealed there is excessed number of type-B substitution of carbonate in locations of phosphate ions of crystal lattice, as seen by an increase on carbonate-to-phosphate matrix, upon the abolition of Y 1 R sympathetic signaling.…”