Effects of inorganic mercury and methylmercury on osteoclasts and osteoblasts in the scales of the marine teleost as a model system of bone

To evaluate the effects of inorganic mercury (InHg) and methylmercury (MeHg) on bone metabolism in a marine teleost, the activity of tartrate-resistant acid phosphatase (TRAP) and alkaline phosphatase (ALP) as indicators of such activity in osteoclasts and osteoblasts, respectively, were examined in scales of nibbler fish (Girella punctata). We found several lines of scales with nearly the same TRAP and ALP activity levels. Using these scales, we evaluated the influence of InHg and MeHg. TRAP activity in the scales treated with InHg (10-5 and 10-4 M) and MeHg (10-6 to 10-4 M) during 6 hrs of incubation decreased significantly. In contrast, ALP activity decreased after exposure to InHg (10-5 and 10-4 M) and MeHg (10-6 to 10-4 M) for 18 and 36 hrs, although its activity did not change after 6 hrs of incubation. As in enzyme activity 6 hrs after incubation, mRNA expression of TRAP (osteoclastic marker) decreased significantly with InHg and MeHg treatment, while that of collagen (osteoblastic marker) did not change significantly. At 6 hrs after incubation, the mRNA expression of metallothionein, which is a metal-binding protein in osteoblasts, was significantly increased following treatment with InHg or MeHg, suggesting that it may be involved in the protection of osteoblasts against mercury exposure up to 6 hrs after incubation. To our knowledge, this is the first report of the effects of mercury on osteoclasts and osteoblasts using marine teleost scale as a model system of bone. © 2014 Zoological Society of Japan

Effects of Inorganic Mercury on Osteoclasts and Osteoblasts of the Goldfish Scales In Vitro

The fish scales are the major source of internal calcium requirement due to having a higher internal calcium reservoir than the body skeleton during the periods of drastic calcium demand, such as sexual maturation. Therefore, we developed original in vitro assay system using goldfish scales that contain osteoclasts and osteoblasts, and examined the direct effect of inorganic mercury (HgCl2) on osteoclasts and osteoblasts. In this assay system, we measured the activities of tartrate-resistant acid phosphatase (TRAP) and alkaline phosphatase (ALP) as respective indicators of each activity in osteoclasts and osteoblasts. TRAP activity in the scales significantly decreased by the treatment of HgCl2 (10-5 to 10-3M) during 6 hrs of incubation. In addition, mRNA expressions of osteoclastic markers: TRAP and cathepsin K significantly decreased compared with control. In our knowledge, this is the first report of a direct effect of inorganic mercury on osteoclasts. On the other hand, ALP activity decreased after exposures of HgCl2 at a concentration of 10-6, 10-5 or 10-4M for 36 and 64 hrs, although its activity did not change after 6 and 18 hrs. The mRNA expression of metallothionein (MT) which is a metal-binding-protein that protects the organism from heavy metal, significantly increased by HgCl2 (10-4M) although insulin-like growth factor-I (osteoblastic marker) was less than those of control scales by treatment with HgCl2 (10-4M). These results suggests that osteoblasts may synthesize MT and protect from mercury until 18 hrs incubation. Thus, the scale in vitro assay system would be a useful means for analysis of heavy metal on bone metabolism