Hypoxanthine metabolism in salmon: roles in osmoregulation and the innate immune response.

鲑鱼中的次黄嘌呤代谢：在渗透压调节和先天免疫反应中的作用。

• 批准号: BB/M026019/1
• 负责人: Gordon Cramb
• 金额: $ 31.24万
• 依托单位: University of St Andrews
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2015
• 资助国家: 英国
• 起止时间: 2015 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=BB%2FM026019%2F1
• 关键词: Hypoxanthine; metabolism; salmon; roles; osmoregulation

Atlantic salmon accumulate various metabolites in the skin and fin when fish are in seawater (SW) environments. One metabolite, called guanine, is known to be responsible for the skin "silvering" that accompanies developmental changes associated with the migration of young salmon (called smolts) to SW. The elevated concentrations in the skin result in the formation of guanine crystals that give the skin its classic iridescent/pearlescent metallic lustre, characteristic of all salmonids. Recently we have identified that a structurally related metabolite called hypoxanthine, is also found at high levels within the skin and fin of Atlantic salmon. The functional role of this metabolite however, appears to be completely different from guanine. Although guanine is essentially insoluble in the aqueous environment of the skin cells (and hence explains its precipitation as translucent crystals), hypoxanthine is eminently soluble and in SW salmon, accumulates to very high concentrations suggesting that it may function as an organic osmolyte. Organic osmolytes increase the concentration inside cells to near-equivalent concentrations to that of the sodium chloride and other ions in the surrounding SW. This equilibration of solute concentrations inside and outside the cell limits the osmotic loss of cell water to the extracellular SW environment and therefore prevents cell dehydration, shrinkage and inevitable cell death. Another potential function of hypoxanthine, is that of an anti-microbial compound. In addition to being a precursor of guanine, hypoxanthine can be enzymatically converted to uric acid, but at the same time this processes releases hydrogen peroxide. Essentially three times as much hydrogen peroxide can be generated as hypoxanthine present within the cell. It is proposed that this highly reactive metabolite acts as a cellular disinfectant and the first line of the host's defense against microorganisms or parasites that invade the skin or fin. In higher vertebrates, hydrogen peroxide and uric acid are known to function as so-called chemo-attractant molecules, stimulating the recruitment of blood lymphocytes and macrophages to sites of injury and initiating an immune response against any invading organism. The same functions are proposed to take place in the salmon. Preliminary results suggest that this hypothesis may indeed be the case as skin regions chronically infected with sea lice exhibit higher levels of expression of a key enzyme involved in this hydrogen peroxide generating pathway. Salmon are an important species in the aquaculture industry, especially in Scotland, and a critical period in the farming of this fish is during and just after SW transfer. This is a very stressful period of the salmon's life cycle and if fish are not transferred at the optimal time the physiological adaptations required for life in SW can be compromised, resulting in increased fish mortalities from osmotic disturbances or increased infection rates. Virtually nothing is known about the roles of hypoxanthine during this critical period of the salmon's life cycle. This project will characterize the role of hypoxanthine as an osmolyte in cell volume regulation and its potential function as a hydrogen peroxide generator in skin and fin cells during sea lice infestation. The project will also determine whether various nutritional supplements added to the diet of pre-SW transfer smolts will i) affect the expression of enzymes and transporters associated with the general metabolism of hypoxanthine, ii) influence hypoxanthine levels in skin and fin, iii) alter the osmoregulatory capacity of fish following acute SW transfer and determine if iv) selective nutritional supplements can reduce the incidence of stress-induced skin lesions/infections and mortalities that often arise within 2-3 months of SW transfer.

大西洋鲑鱼在海水环境中会在皮肤和鳍中积累各种代谢物。一种叫做鸟嘌呤的代谢物被认为是导致皮肤“变银”的原因，这种皮肤“变银”伴随着幼鲑鱼（称为小鲑鱼）向SW迁移的发育变化。皮肤中鸟嘌呤浓度的升高导致鸟嘌呤晶体的形成，使皮肤具有典型的彩虹/珠光金属光泽，这是所有鲑科鱼的特征。最近，我们发现了一种结构相关的代谢物，叫做次黄嘌呤，在大西洋鲑鱼的皮肤和鳍中也有很高的含量。然而，这种代谢物的功能作用似乎与鸟嘌呤完全不同。虽然鸟嘌呤在皮肤细胞的水环境中基本上是不溶的（因此解释了它以半透明晶体的形式沉淀），但次黄嘌呤是显著可溶的，并且在SW鲑鱼中积累到非常高的浓度，表明它可能是一种有机渗透剂。有机渗透剂将细胞内的浓度增加到与周围SW中氯化钠和其他离子的浓度几乎相等的浓度。这种细胞内外溶质浓度的平衡限制了细胞水分向细胞外SW环境的渗透损失，从而防止细胞脱水、收缩和不可避免的细胞死亡。次黄嘌呤的另一个潜在功能是抗微生物化合物。除了作为鸟嘌呤的前体外，次黄嘌呤还可以被酶转化为尿酸，但与此同时，这个过程会释放过氧化氢。过氧化氢在细胞内产生的量是次黄嘌呤的三倍。有人提出，这种高活性代谢物作为细胞消毒剂和宿主防御入侵皮肤或鳍的微生物或寄生虫的第一道防线。在高等脊椎动物中，过氧化氢和尿酸被认为是所谓的化学引诱分子，刺激血液淋巴细胞和巨噬细胞聚集到损伤部位，并启动针对任何入侵生物的免疫反应。同样的功能也被认为发生在鲑鱼身上。初步结果表明，这一假设可能确实成立，因为长期感染海虱的皮肤区域表现出更高水平的过氧化氢生成途径中涉及的一种关键酶的表达。鲑鱼是水产养殖业的重要物种，特别是在苏格兰，在SW转移期间和之后是这种鱼养殖的关键时期。这是鲑鱼生命周期中一个非常紧张的时期，如果鱼没有在最佳时间转移，在SW生活所需的生理适应可能会受到损害，导致渗透干扰或感染率增加的鱼死亡率增加。事实上，我们对次黄嘌呤在鲑鱼生命周期的关键时期所扮演的角色一无所知。本项目将描述次黄嘌呤作为渗透剂在细胞体积调节中的作用，以及它在海虱侵袭期间作为皮肤和鳍细胞过氧化氢发生器的潜在功能。该项目还将确定在sw转移前幼鱼的日粮中添加各种营养补充剂是否会i)影响与次黄嘌呤一般代谢相关的酶和转运体的表达，ii)影响皮肤和鳍中的次黄嘌呤水平。iii)在急性SW转移后改变鱼类的渗透调节能力，并确定iv)选择性营养补充剂是否可以减少SW转移后2-3个月内经常出现的应激性皮肤病变/感染和死亡率。

项目成果

Myo-inositol phosphate synthase expression in the European eel (Anguilla anguilla) and Nile tilapia (Oreochromis niloticus): effect of seawater acclimation.

• DOI: 10.1152/ajpregu.00056.2016
• 发表时间: 2016-08-01
• 期刊: American journal of physiology. Regulatory, integrative and comparative physiology
• 作者: Kalujnaia S;Hazon N;Cramb G
• 通讯作者: Cramb G