Effect of osteoporosis treatment agents on the cortical bone osteocyte microenvironment in adult estrogen-deficient, osteopenic rats

Abstract

Though osteoporosis is a significant cause of disability worldwide, treatment with pharmacologic agents decreases risk of fragility fracture. Though these treatments act through the bone remodeling system to improve bone mass, it is unclear if they alter the response of bone to mechanical loading at the level of the osteocyte. This pre-clinical study determined the relationship between microstructural bone tissue properties and osteocyte lacunar size and density to strain around osteocytes with standard osteoporosis treatment or sequential therapies. Six-month-old female ovariectomized (OVX) Sprague-Dawley rats were cycled through various sequences of pharmacological treatments [alendronate (Aln), raloxifene (Ral) and human parathyroid hormone-1,34 (PTH)] for three month intervals, over nine months. Linear nanoindentation mapping was used to determine Young's modulus in perilacunar and bone matrix regions around cortical bone osteocyte lacunae. Measurements of lacunar diameter and density were completed. Treatment-related differences in Young's modulus in the perilacunar and bone matrix regions were not observed. We confirmed previous data that showed that the bone matrix region was stiffer than the perilacunar matrix region. Whole bone material properties were correlated to perilacunar matrix stiffness. Finite element models predicted a range of mechanical strain amplification factors estimated at the osteocyte across treatment groups. In summary, though the perilacunar matrix near cortical osteocyte lacuna is not as stiff as bone matrix further away, osteoporosis treatment agents do not affect the stiffness of bone tissue near osteocyte lacunae.

Sequential Treatment of Estrogen Deficient, Osteopenic Rats with Alendronate, Parathyroid Hormone (1–34), or Raloxifene Alters Cortical Bone Mineral and Matrix Composition

“…The composition of the perilacunar region differed from that of the non-perilacunar bone matrix region. Based on these results and the a priori knowledge that osteocytes can remove and replace perilacunar mineral [14][15][16][18][19][20], we speculate that sequential treatments with osteoporosis treatment agents cause osteocytes to alter bone tissue composition in their surrounding microenvironment to reduce collagen maturity without changing mineral:matrix ratio, carbonate:phosphate ratio, and crystallinity.…”

“…Care was taken to be sure that no portion of any linear path passed within 20 µm of a neighboring lacuna. Raman spectra were separated into two bone matrix regions as previously reported: (1) the perilacunar region (1-5 µm from the lacunar wall) [14,16,22]; and (2) the non-perilacunar region (16-20 µm from the lacunar wall) [14]. Thus, 40-50 Raman spectra were collected from each rat in each tissue region.…”

“…These regions may be functionally distinct in that under certain conditions, such as during lactation [47], exercise [22], or PTH treatment [15,16], the perilacunar region undergoes osteocyte-mediated remodeling, a localized process that does not affect the non-perilacunar bone matrix tissue region [18][19][20]. The elastic modulus of the perilacunar region differs from that in the non-perilacunar bone matrix region [14,51], which modulates the strain fields experienced by osteocytes. Though we did not directly examine osteocytes for morphologic signs of treatment-related differences in activity, we speculate that the different bone composition outcomes following treatment may be an indicator of such activity.…”

“…In addition to changing bone composition through their influence on bone turnover, boneactive agents may also alter tissue hardness and modulus by changing the microscopic mechanical environment in bone tissue near the osteocyte [9]. Young's modulus of bone tissue near osteocyte lacunae is lower than in more distant bone tissue [14]. Previous studies report changes in osteocytes of rats given continuous PTH [15], and in bone tissue composition surrounding osteocytes with intermittent PTH [16], along with diffusion of BPs into the canalicular network [17].…”

The influence of estrogen deficiency on the structural and mechanical properties of rat cortical bone

“…Sharma and colleagues ( Sharma et al, 2018 ) reported that estrogen deficiency causes an increase in cortical bone vascular porosity in the proximal tibial metaphysis of rats, with enlarged vascular pores, but little change in tissue mineral density. However, studies reporting the effect of estrogen deficiency on the cortical bone of the rat ( Sharma et al, 2012 ; Stern et al, 2018 ) have not investigated the effects separately in the two distinct zones of the bone (lamellar vs. disorganized) and the duration of these studies was short (weeks after ovariectomy), while the results reported here were found 6 months after OVH was performed.…”