饲料中钙磷水平和比例对花鲈钙磷周转代谢的影响机制

The replacement of fish meal in aquafeeds has received heightened attention from academic institutions to industry. In feed industry, the differences of calcium and phosphorus between fishmeal and alternative ingredient are not focused very much on. Most commercial aquafeeds with high animal protein contain excessive amount of calcium, and available phosphorus content is generally not enough in high plant protein aquafeeds. Our previous studies showed that dietary high calcium supplementation reduced significantly apparent phosphorus digestibility of diet and the phosphorus concentrations of vertebra and scales for Japanese seabass, which implied high dietary calcium has negative effects on digestibility and retention of dietary phosphorus. The present project will focus particularly on nutritional regulation mechanism of dietary calcium and phosphorus supplementation on Japanese seabass. A 12-week feeding trial will be conducted to investigate the effects of dietary calcium and phosphorus level and calcium to phosphorus ratio on the intestine, kidney, gill, vertebra, scales, pituitary gland, liver transcriptomic sequencing in Japanese seabass via transcriptomics technology, and then screen differential expression genes. The ultimate objective of the present study is to find out the key genes, metabolic enzymes and regulatory factors affecting absorption and metabolism of calcium and phosphorus by using known molecular and metabolic pathways, and thereby to construct network describing interaction of calcium and phosphorus and illuminate the regulatory metabolism on calcium and phosphorus turnover (digestibility, absorption, retention and excretion) for Japanese seabass. The results would not only enrich the basic theory of fish calcium and phosphorus nutrition, but also provide research approach for exploring interaction among other mineral elements.

鱼粉替代是水产动物营养与饲料领域的研究热点之一，然而在实际生产中替代原料与鱼粉间钙磷含量的差异常被忽视。商业高动物蛋白鱼类饲料多为高钙饲料，高植物蛋白饲料多为低有效磷饲料。前期研究表明，高钙饲料显著降低花鲈对饲料磷的消化率以及脊椎骨和鳞片中磷的含量，表现出高钙负面影响磷消化和沉积的效应。本项目拟用转录组学技术，比较分析不同钙磷水平和比例下花鲈肠、肾、鳃、脊椎骨、鳞片、脑垂体、肝组织基因表达的变化，筛选钙磷吸收或代谢相关的差异表达基因，构建钙磷营养的交互作用网络，并结合传统鱼类营养学研究方法，探索花鲈机体钙磷周转代谢的调控机制。本项目研究结果将对阐释鱼类钙磷营养学机理有重要意义，为探索矿物元素间更广泛的交互作用机制提供研究思路。

鱼类摄食的饲料中大部分磷排放到水体，这是鱼类养殖对水环境的污染主要来源。影响鱼类磷排放的因素很多，其中饲料中高水平的钙元素或高钙/磷可能影响鱼类对磷的利用。本项目研究了饲料钙水平、磷水平和钙磷比对花鲈生长、体组成、钙磷消化率、钙磷沉积和骨骼矿化的影响，并应用转录组学比较分析了花鲈肠、肾、鳃、脊椎骨、鳞片、脑垂体、肝组织的差异表达基因。结果显示，饲料低磷（0.40%）抑制了花鲈的生长，也不利于花鲈渗透压的维持；饲料高钙（1.50%）降低了花鲈的生长和饲料磷的表观消化率，减少了全体磷的沉积。肝脏转录组分析发现，低磷（0.40%）与高磷（1.50%）组相比，组间平均99个显著差异基因，而低钙（0.40%）与高钙（1.50%）组相比，组间仅有8个基因表达有显著差异。KEGG通路分析发现，低磷会导致氨基酸代谢、脂肪酸代谢、糖代谢相关通路的变化，从而对其物质代谢产生影响。脊椎骨转录组分析发现，低磷与高磷组相比，组间平均200个显著差异基因，而低钙与高钙组相比，组间有1478个基因表达有显著差异，涉及软骨细胞、成骨细胞和破骨细胞发生过程的相关基因。饲料中钙磷水平通过影响肠道NaPiIIa、NaPiIIb、PiT1和PiT2的表达来调控磷的吸收，而在肾脏中通过FGF23负反馈调节NaPiIIa的表达来调控磷的重吸收，高钙降低了鳃、肠道和肾与磷吸收相关基因的表达。综上所述，本研究表明花鲈可以从水体吸收钙以部分满足其需求，对饲料钙不存在绝对需要量，而对饲料可利用磷的需要量为0.72%。低磷饲料会导致肝脏氨基酸分解代谢加强、脂肪酸合成加强，脊椎骨成骨细胞活力降低，从而导致低磷组蛋白质效率降低，肝脏脂肪沉积、骨骼发育异常。饲料中钙若以低溶解度的状态存在，则在生长表现方面不存在与磷的交互作用，但若以可溶钙存在，则会对生长以及磷的吸收和沉积产生负面影响。该研究结果可为花鲈饲料中钙磷元素的合理应用提供理论依据。