Mouse models of senile osteoporosis
According to previous studies, the bone mass of the laboratory mice with a life-span of 2-3years, reaches its maximum between 4 to 8 months, after which the bone mass decreases with time [19, 20]. We purchased 2, 5, 7 and 10 months old C57BL/6 female mice (n = 6) and measured their bone mineral density (BMD) by micro computed tomography (µCT). Three-dimensional reconstruction of these mice using µCT showed that the lumbar and the tibia BMD reached its highest value at 5 months and its lowest at 7 months. Within a five-month period, trabecular thickness (Tb.Th) and cancellous bone volume fraction (BV/TV) showed an increasing trend but then began to decline. The trabecular number (Tb.N) significantly decreased from 5 months, but trabecular separation (Tb.Sp) increased from 5 months (Fig. 1A -C).
It has been pointed out that the bone loss associated with age is based on decreased osteogenic potential. We also found that from 5 or 7 months in BMSCs, the mice showed decreased mRNA levels of osteogenesis marker genes, including osteocalcin (OC), alkaline phosphatase (ALP), and Runx2 (Fig. 2).
To assess the roles of canonical Wnt signaling in senile osteoporosis, we first assayed the mRNA levels of Wnt signaling markers in BMSCs. Based on Fig. 2, Wnt signaling markers including lymphoid enhancer-binding factor-1 (Lef1), CyclinD1 and Dickkopf-1 (Dkk1) were all increased at 5 months. After that, they declined with age.
At menopause, estrogen deficiency increases bone turnover. Since during bone turn over, the bone formation is less than bone resorption, bone loss and finally osteoporosis may result. We examined the levels of thyroxine (T4), parathyroid hormone (Pth), and estrogen in serum from above mice, and found that the levels of T4 and estrogen but not PTH declined with age (Fig. 3A).
Based on these results, we concluded that aged mice may be proper models for senile osteoporosis research.
Increases in bone mass after systemic treated with LiCl in vivo
We tested the effect of LiCl, a known GSK3β inhibitor on senile osteoporosis. We intragastrically administered LiCl into 7-months old mice for 3 months at 100 or 200 mg/kg daily. Because of the death in the process of intragastrically administered LiCl, we got data from six animals per group. As shown by the morphological analysis, LiCl at 200 mg/kg robustly increased the mass of the cancellous bone (Fig. 4A and B). This result was further confirmed by quantitative analysis of bone mass parameters. Compared to saline treatment, LiCl treatment at 100 mg/kg slightly increased BV/TV and Tb.N. There was no significant difference of Tb.Th between LiCl treatment at 100 mg/kg and vehicle treatment mice. LiCl significantly increased BV/TV and Tb.N at 200 mg/kg, and markedly reduced the Tb.Sp, without affecting Tb.Th (Fig. 4C). Moreover, there were more trabecular changes in the histology of lumbar sections from the LiCl group, whereas fewer trabecular number in the vehicle mice (Fig. 5A). Our data indicated that systemic treatment with LiCl resists the bone loss with advanced age.
The effect of LiCl on hormones
Lithium can cause the development of hypothyroidism by inhibiting the release of thyroid hormones [21]. In order to explore the effect of LiCl on the levels of serum hormones in our senile osteoporosis model, we examined the levels of T4, PTH and estrogen. As shown in Fig. 3B, LiCl treatment at 200 mg/kg daily for 3 months, increased the level of T4 but not PTH and estrogen. Thus, these data suggest that the increases in bone mass by treatment with LiCl is not due to hormones.
LiCl stimulates osteoblastic differentiation
To determine whether LiCl enhances bone mass via changes in bone formation or bone resorption, we conducted a study. We first examined the role of LiCl in osteoclastogenesis, by trap staining the lumbar vertebra sections. There was no apparent difference in the number of trap-positive osteoclasts between LiCl treated and vehicle mice (Fig. 5A). We then measured the bone nodules by alizarin-red S staining in BMSCs for bone formation activity. We found that LiCl treatment at both 100 mg/kg and 200 mg/kg, lead to significantly increased number of bone nodules (Fig. 5B and C).
Moreover, LiCl treatment at 100 and 200 mg/kg induced mRNA levels of Lef1 (1.4-fold and 2.8-fold), CyclinD1 (1.2-fold and 2.2-fold), and Dkk1 (2.1-fold and 5.7-fold, respectively) in BMSCs (Fig. 6A). In line with the induction of canonical signaling, LiCl treatment lead to a significant dose-dependent increase in the mRNA levels of osteogenic markers, including ALP (1.5-fold and 2.3-fold), Runx2 (3.1-fold and 12.7-fold), and OC (1.4-fold and 4.5-fold).
Finally, immunohistochemical staining of lumbar slices showed that LiCl robustly increased the numbers of Lef-, ALP- and OC-positive cells (Fig. 6B). In summary, these results demonstrate that LiCl may promote osteogenesis and result in bone mass increase possibly by the activation of Wnt/β-catenin signaling but not hormones in aged mice.