Kynurenine promotes Calcitonin secretion and reduces cortisol in the Japanese flounder Paralichthys olivaceus.

Deep ocean water (DOW) exerts positive effects on the growth of marine organisms, suggesting the presence of unknown component(s) that facilitate their aquaculture. We observed that DOW suppressed plasma cortisol (i.e., a stress marker) concentration in Japanese flounder (Paralichthys olivaceus) reared under high-density condition. RNA-sequencing analysis of flounder brains showed that when compared to surface seawater (SSW)-reared fish, DOW-reared fish had lower expression of hypothalamic (i.e., corticotropin-releasing hormone) and pituitary (i.e., proopiomelanocortin, including adrenocorticotropic hormone) hormone-encoding genes. Moreover, DOW-mediated regulation of gene expression was linked to decreased blood cortisol concentration in DOW-reared fish. Our results indicate that DOW activated osteoblasts in fish scales and facilitated the production of Calcitonin, a hypocalcemic hormone that acts as an analgesic. We then provide evidence that the Calcitonin produced is involved in the regulatory network of genes controlling cortisol secretion. In addition, the indole component kynurenine was identified as the component responsible for osteoblast activation in DOW. Furthermore, kynurenine increased plasma Calcitonin concentrations in flounders reared under high-density condition, while it decreased plasma cortisol concentration. Taken together, we propose that kynurenine in DOW exerts a cortisol-reducing effect in flounders by facilitating Calcitonin production by osteoblasts in the scales.

Osteoclastic and Osteoblastic Responses to Hypergravity and Microgravity: Analysis Using Goldfish Scales as a Bone Model.

It is known that the bone matrix plays an important role in the response to physical stresses such as hypergravity and microgravity. In order to accurately analyze the response of bone to hypergravity and microgravity, a culture system under the conditions of coexistence of osteoclasts, osteoblasts, and bone matrix was earnestly desired. The teleost scale is a unique calcified organ in which osteoclasts, osteoblasts, and the two layers of bone matrix, i.e., a bony layer and a fibrillary layer, coexist. Therefore, we have developed in vitro organ culture systems of osteoclasts and osteoblasts with the intact bone matrix using goldfish scales. Using the scale culture system, we examined the effects of hypergravity with a centrifuge and simulated ground microgravity (g-µG) with a three-dimensional clinostat on osteoclasts and osteoblasts. Under 3-gravity (3G) loading for 1 day, osteoclastic marker mRNA expression levels decreased, while the mRNA expression of the osteoblastic marker increased. Upon 1 day of exposure, the simulated g-µG induced remarkable enhancement of osteoclastic marker mRNA expression, whereas the osteoblastic marker mRNA expression decreased. In response to these gravitational stimuli, osteoclasts underwent major morphological changes. By simulated g-µG treatments, morphological osteoclastic activation was induced, while osteoclastic deactivation was observed in the 3G-treated scales. In space experiments, the results that had been obtained with simulated g-µG were reproduced. RNA-sequencing analysis showed that osteoclastic activation was induced by the down-regulation of Wnt signaling under flight-microgravity. Thus, goldfish scales can be utilized as a bone model to analyze the responses of osteoclasts and osteoblasts to gravity.