Reduced Bone Mass and Increased Osteocyte Tartrate-Resistant Acid Phosphatase (TRAP) Activity, But Not Low Mineralized Matrix Around Osteocyte Lacunae, Are Restored After Recovery From Exogenous Hyperthyroidism in Male Mice

Abstract

Hyperthyroidism causes secondary osteoporosis through favoring bone resorption over bone formation, leading to bone loss with elevated bone fragility. Osteocytes that reside within lacunae inside the mineralized bone matrix orchestrate the process of bone remodeling and can themselves actively resorb bone upon certain stimuli. Nevertheless, the interaction between thyroid hormones and osteocytes and the impact of hyperthyroidism on osteocyte cell function are still unknown. In a preliminary study, we analyzed bones from male C57BL/6 mice with drug‐induced hyperthyroidism, which led to mild osteocytic osteolysis with 1.14‐fold larger osteocyte lacunae and by 108.33% higher tartrate‐resistant acid phosphatase (TRAP) activity in osteocytes of hyperthyroid mice compared to euthyroid mice. To test whether hyperthyroidism‐induced bone changes are reversible, we rendered male mice hyperthyroid by adding levothyroxine into their drinking water for 4 weeks, followed by a weaning period of 4 weeks with access to normal drinking water. Hyperthyroid mice displayed cortical and trabecular bone loss due to high bone turnover, which recovered with weaning. Although canalicular number and osteocyte lacunar area were similar in euthyroid, hyperthyroid and weaned mice, the number of terminal deoxynucleotidyl transferase–mediated deoxyuridine triphosphate nick end labeling (TUNEL)‐positive osteocytes was 100% lower in the weaning group compared to euthyroid mice and the osteocytic TRAP activity was eightfold higher in hyperthyroid animals. The latter, along with a 3.75% lower average mineralization around the osteocyte lacunae in trabecular bone, suggests osteocytic osteolysis activity that, however, did not result in significantly enlarged osteocyte lacunae. In conclusion, we show a recovery of bone microarchitecture and turnover after reversal of hyperthyroidism to a euthyroid state. In contrast, osteocytic osteolysis was initiated in hyperthyroidism, but its effects were not reversed after 4 weeks of weaning. Due to the vast number of osteocytes in bone, we speculate that even minor individual cell functions might contribute to altered bone quality and mineral homeostasis in the setting of hyperthyroidism‐induced bone disease. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Hyperthyroidism-driven bone loss depends on BMP receptor Bmpr1a expression in osteoblasts

“…As such, LysM-Cre can target not only monocytes and macrophages, but also neutrophils 62 and Osx-Cre can target also additional cell populations such as osteocytes, hypertrophic chondrocytes, stromal cells, adipocytes, and perivascular cells of bone marrow 63 . Beside osteoblasts, also osteocytes are crucial for orchestrating bone remodeling by secreting RANKL 64 , can import thyroid hormones 16 and have been shown to exhibit resorptive TRAP activity themselves in response to hyperthyroidism in vivo 65 . Osteocytes are not the main target cells for the Osx-Cre-driven recombination 66 , but are the dominant cells type (90−95%) within bone tissue 67 .…”

“…Then, bones were embedded in methyl methacrylate (Technovit 9100, Heraeus Kulzer, Hanau, Germany) and cut into 7 μm sagittal sections for calcein label quantification of the trabecular bone. The mineralized surface/bone surface (MS/BS), the mineral apposition rate (MAR) and the bone formation rate/bone surface (BFR/BS) were quantified in an area of 1.44 mm² in the center of vertebrae according to established protocols 16,17,26,65,74 using fluorescence microscopy (Microscope Axio Imager M1) and the Osteomeasure software. Representative photos were taken using Axio Vision 3.1 Software (Carl Zeiss Jena, Germany).…”

Efficacy assessment of methylcellulose-based thermoresponsive hydrogels loaded with gallium acetylacetonate in osteoclastic bone resorption

“…To further characterize the effect of GaAcAc and GaMH, we performed western blotting analyses of key markers involved in OC differentiation and function in bone resorption. Previous studies have reported that complementary binding of RANKL to RANK receptor triggered a myriad signaling cascade involving the following markers: cFos, TRAF6, TRAP, and NFAT2 [ 27 , 57 ]. Increasing the concentration of GaAcAc from 10 to 50 µg/mL resulted in marked downregulation of all the expression of all the identified OC differentiation markers (Fig.…”

Thyroid hormone receptor Thra and Thrb knockout differentially affects osteoblast biology and thyroid hormone responsiveness in vitro

“…3,[16][17][18][19][20][21] Recently, osteocytes in bones of hyperthyroid mice have been reported to acquire osteoclast-like features and actively shape their lacunae, a process so-termed osteocytic osteolysis. 22 To exert their physiological actions, TH must bind to thyroid hormone receptors (TR) of the nuclear receptor superfamily that are encoded by the two genes, Thra and Thrb. 3,23 In addition to the functional receptors TRα1, TRβ1, and TRβ2, further isoforms are expressed that fail to bind T 3 and may act as antagonists.…”

“…At the cellular level, TH promote osteoblast differentiation and activity, 9–15 while osteoclasts and their resorptive activity might be mainly indirectly affected by TH‐stimulated cells of the osteoblast lineage within the proximity 3,16–21 . Recently, osteocytes in bones of hyperthyroid mice have been reported to acquire osteoclast‐like features and actively shape their lacunae, a process so‐termed osteocytic osteolysis 22 …”