斑马鱼脑细胞凋亡基因对束丝藻毒素致毒的响应

RESPONSE OF APOPTOTIC GENES TO APHANTOXIN-PARALYTIC SHELLFISH POISON IN FRESHWATER EXTRACTED FROM THE APHANIZOMENON FLOS-AQUAE DC-1 IN CELLS OF BRAIN ON ZEBRAFISH (DANIO RERIO)

水华束丝藻是淡水湖泊中常见的水华蓝藻,是我国滇池冬春季节常见的优势种群,因其产生麻痹性贝类毒素,损伤人和动物的神经系统而倍受关注。但有关该毒素对动物神经系统损伤的研究较少,特别是对水生脊椎动物中枢神经系统损伤的研究尚无报道,为此本研究通过腹腔注射5.3μg STXeq/kg bw束丝藻毒素,研究了24h内该藻毒素对斑马鱼脑组织超微结构损伤及脑细胞凋亡基因表达的影响,以揭示该毒素对脑组织的损伤及其脑细胞在基因水平对该毒素的响应机理。研究表明,束丝藻毒素引起斑马鱼脑组织超微结构损伤,出现细胞膜发泡和形成凋亡小体等典型的细胞凋亡结构;从分子水平进一步分析显示,该毒素引起脑细胞p53、bax、caspase-3和c-jun等凋亡相关基因的表达上调,其上调量分别是对照组表达上调量的1.92、1.55、1.63和1.55倍,且具有时间-效应关系。这说明该毒素能通过引起脑细胞凋亡基因的表达异常,使脑细胞出现凋亡性形态损伤而导致脑细胞死亡;斑马鱼脑细胞可通过启动p53→bax→caspase-3线粒体径路实现其对该毒素的响应机制;束丝藻毒素具有损伤鱼类脑的神经毒性;这是束丝藻毒素引起脑细胞凋亡基因表达异常及超微结构损伤的直接证据,也是脑细胞在基因水平对束丝藻毒素积极响应分子机理的首次报道。

The Aphanizomenon flos-aquae DC-1,frequently appeares in freshwater,is a dominant species in Dianchi Lake of Yunnan Province,China,in winter and spring.During the past 20 years,blooms of Aphanizomenon flos-aquae DC-1 have occurred in the Dianchi Lake per year because of heavy pollution and the dominant species overgrowth.The blooms always sustained for six months in a year,even throughout a year in some water of Dianchi Lake.And the blooms could also give off distasteful smell into the air.As a result,visitors are more likely to breathe the odorous air than fresh air today,and see a bad scenery of green-mud water than clear blue waters.Especially the dominant species can produce aphantoxins,which is attributed to the paralytic shellfish poisons（PSPs） and influences aquatic ecosystem and damages the nerve system of animals and human.However,little research has been carried out on the toxin,in particular the effects on central nervous system（CNS） of animals and human.Thus the present research is to investigate abnormity of apoptotic gene expression,ultrastructural damage of brain induced by aphantoxin,to reveal mechanism of response of brain on zebrafish to aphantoxins.Virus-free male zebrafish（Danio rerio）,110 days old or so,average bodyweight of（0.4±0.01） g,after 7—10 days of acclimatization,were randomly assigned into control and treated groups（45 fish per group）.The two groups were received 30μL 0.01 mol/L acetic acid solution（control） and 30 μL 0.01 mol/L acetic acid solution contained aphantoxins（5.3 μg STXeq/kg bodyweight,treated） through intraperitoneal injections（i.p.）.At each time point（1,3,6,9,12 and 24h）,five zebrafish in each group were sacrificed by cold shock（embedding into crushed ice at -8℃）,and cerebra were removed from whole brain and washed at once in 0.86% ice-cold physiological saline,prefixed in 2.5% glutaraldehyde solution for electron microscopy.In another 5 fish at each timepoint,whole brains were removed as described above,frozen in liquid nitrogen,and stored at either -70℃ for subsequent RNA analysis.The prefixed cerebra（at 9h） were dehydrated in graded ethanol and propylene oxide,embedded in Epon 812,and cut into the ultra-thin sections using glass knives on an ultramicrotome（Leica,Germany）.Sections were stained with uranyl acetate and lead citrate before electron microscopy（JEM-1230,Japan）.The 45mg frozen（-70 ℃） whole brain（five fish each timepoint） of zebrafish were homogenized（IKA Werke,Germany） in ice-cold TRIzoL reagent（w：v,1：20,30s） to extract total RNA.Gene-specific RNA levels were measured using an RT-PCR kit（Promega,USA） according to the protocol of manufacturer.The system of reactions was carried out into 50μL as follows：Reactions contained 25μL of 2× AccessQuick TM Master Mix,5 μL oligo-dT,10 μmol/L upstream and downstream primers,1 μg template RNA,and RNase-free MilliQ water to 50μL.AMV reverse transcriptase（1μL） was added and incubated at 45℃ for 45 min for reverse transcription.Samples were then heated at 95℃ for 5min before PCR;amplification conditions were 40 cycles of 95℃ for 30s,58℃ for 30s,72℃ for 45s,followed by a final extension step（72℃ for 10min）.RT-PCR amplicons were analyzed by elec-trophoresis on 1.5%—2.0% agarose gels（60—100 V,40 A,30min）,ethidium bromide staining,visualization on a Bio-imaging System（M-20X,UVP,USA）,and densitometry using analytical software（Quantity One v4.62,Bio-Rad,USA）.All statistical analyses employed commercial software（SPSS 13.0,Chicago,IL,USA） and assessed by one-way analy-sis of variance（ANOVA） combined with least significant difference（LSD） post-hoc tests to express significances of differences between groups（P0.05 and P0.01）.The results showed that aphantoxins could induce the apoptosis of brain cells in ultrastructure,such as the membrane blebbing and apoptotic-like body.And it was further analyzed on molecular level to reveal that the expressions of p53,bax,caspase-3 and c-jun genes were increased and followed with time-dependent manner whose level of expression upreguation were 1.92,1.55,1.6 and 1.55 times that of control respectively.These results displayed that aphantoxins could lead to apoptotic ultrastructural damage of brain tissues through increasing expression of genes.And the brain cells could activate the mechnism of response to aphantoxins through the mitochondrial route which was involved in the cascade of p53→bax→caspase-3 genes.Therefore the toxin could result in death of brain cells by apoptotic way and exhibited the neurotoxicity of damage brain.These findings provide direct evidence that freshwater cyanobacterial aphantoxins can exert toxic effects on brain tissue.This is also strong evidences that find the mechanism of response on molecular level from brain cells to cyanobacterial neurotoxins of freshwater.