Ostrowski Andria, Zou Enmin
Department of Biological Sciences, Nicholls State University, Thibodaux, LA 70310, USA.
Gen Comp Endocrinol. 2018 Nov 1;268:57-63. doi: 10.1016/j.ygcen.2018.07.018. Epub 2018 Jul 26.
The crustacean exoskeleton is composed primarily of chitin, proteins and various inorganic compounds. It is the inorganic compounds, such as calcium and magnesium, that underlie the exoskeletal mineralization process following ecdysis. Little is known about the hormonal mechanism for this process in crustaceans. Carbonic anhydrase (CA) in the epidermis has been suggested to aid in deposition of calcium and magnesium carbonates to the exoskeleton in crustaceans by generating bicarbonate ions, resulting in mineralization. Given a similar pattern of fluctuation in prevalence between epidermal CA and ecdysteroids during the crab molting cycle, it has been proposed in a previous study that post-ecdysial epidermal CA and subsequent metal deposition to the exoskeleton are controlled by the ecdysteroid molting hormones in the blue crab, Callinectes sapidus. This study sought to acquire evidence to support such a proposition. Early postmolt and early intermolt blue crabs were used to quantify epidermal CA mRNA expression when exposed in vitro to three physiologically relevant concentrations of 10 nM, 100 nM and 1 μM 20-hydroxyecdysone (20-HE), using the epidermis-with-exoskeleton (EWE) tissue assay method. It was found that 100 nM 20-HE significantly induced the mRNA of CasCAg, a CA isoenzyme, in epidermal tissues of early intermolt crabs and that injection of 20-HE at a dose of 15 ng/g significantly elevated epidermal CA activity in vivo in early intermolt crabs. These two lines of evidence clearly show that post-ecdysial epidermal CA is influenced by the molting hormone. Interestingly, exoskeletal calcium content was significantly lower in the 20-HE treated crabs than in the control, whereas magnesium content was unchanged. The inhibition of calcification in the post-ecdysial exoskeleton by the exogenous molting hormone may implicate that sclerotization and mineralization of the new shell must be coordinated and that enhanced deposition of carbonate salts as a result of increased epidermal CA activity following the administration of exogenous molting hormone would be inhibited to avoid the formation of a structurally disrupted exoskeleton.
甲壳类动物的外骨骼主要由几丁质、蛋白质和各种无机化合物组成。正是钙和镁等无机化合物构成了蜕皮后外骨骼矿化过程的基础。关于甲壳类动物这一过程的激素机制，人们了解甚少。表皮中的碳酸酐酶（CA）被认为通过产生碳酸氢根离子，帮助甲壳类动物将碳酸钙和碳酸镁沉积到外骨骼中，从而导致矿化。鉴于在螃蟹蜕皮周期中，表皮CA和蜕皮类固醇的流行率波动模式相似，之前的一项研究提出，蜕皮后表皮CA和随后外骨骼的金属沉积受蓝蟹（Callinectes sapidus）蜕皮类固醇蜕皮激素的控制。本研究旨在获取证据支持这一观点。使用带外骨骼的表皮（EWE）组织检测方法，将蜕皮后早期和蜕皮间期早期的蓝蟹在体外暴露于三种生理相关浓度（10 nM、100 nM和1 μM）的20-羟基蜕皮酮（20-HE）下，以量化表皮CA mRNA的表达。研究发现，100 nM的20-HE显著诱导了蜕皮间期早期螃蟹表皮组织中CA同工酶CasCAg的mRNA表达，并且以15 ng/g的剂量注射20-HE可显著提高蜕皮间期早期螃蟹体内的表皮CA活性。这两条证据清楚地表明，蜕皮后表皮CA受蜕皮激素的影响。有趣的是，20-HE处理的螃蟹外骨骼中的钙含量显著低于对照组，而镁含量没有变化。外源性蜕皮激素对蜕皮后外骨骼钙化的抑制作用可能意味着新壳的硬化和矿化必须协调进行，并且在施用外源性蜕皮激素后，由于表皮CA活性增加而导致的碳酸盐盐沉积增强将受到抑制，以避免形成结构破坏的外骨骼。
