Studies On The Reproductive Toxicity Of Acute And Subchronic Exposure Of Microcystin-LR To Female Zebrafish

The cyanobacterial bloom caused by eutrophication of water damages the ecological environment seriously.Algae cells can also release microcystins(MCs),which cause serious damage to aquatic organisms and human health.The most toxic of MCs is MC-LR(microcystin-LR).Studies have shown that the gonads are the second target organ of MCs other than the liver.Scholars have demonstrated the reproductive toxicity of MC-LR on fish from the aspects of egg production,fertilization rate,sex hormone levels and parental transmission.However,further research is needed on its mechanism of action.In this study,zebrafish(Danio rerio)was used as a research object to study the damages of MC-LR on zebrafish ovaries by subchronic exposure and acute exposure,and explore the molecular mechanism of interfering oocyte development.Zebrafish were i.p.injected with MC-LR to study the damage of zebrafish ovaries and its mechanism.Sexually mature female zebrafish were injected with MC-LR of 50 and 200?g/kg body weight(BW)into the abdominal cavity.Their ovaries were taken out after 3,9,24 and 48 h of injection.Pathological phenomena such as vacuolization of oocytes,and the loss of connective tissue between the oocyte membrane and the follicular cell layer can be found.The results showed that the gonad somatic index(GSI)of the zebrafish in the exposed group decreased after 24 h of MC-LR injection of 200 ?g/kg BW.After acute exposure,MC-LR inhibited zebrafish ovarian protein phosphatase 2A(PP2A)activity,and promoted maturation promoting factor(MPF)activity.The transcription levels of p38 MAPK and extracellular signal-regulated protein kinase 1/2(ERK1/2)in the mitogen-activated protein kinase(MAPK)family in zebrafish ovaries were significantly up-regulated after MC-LR treatment,while JNK did not change significantly.The study believes that MC-LR exposure inhibits zebrafish ovarian PP2 A activity and MC-LR can activate MPF activity and the transcriptional levels of ERK1/2 and p38 MAPK in the MAPK signaling pathway,thereby interfering with the development of oocytes to play reproductive toxicity.In the study of subchronic exposure,sexual mature female zebrafish were exposed to 1,5,20 ?g/L MC-LR for 30 days.Study have shown that MC-LR accumulate in zebrafish ovaries after subchronic exposure.The oocytes in different stages of the ovaries were counted,and the percentage of oocytes that had a germinal vesicle breakdown(GVBD)increased after 5,20 ?g/L of MC-LR exposure.After MC-LR exposure,zebrafish PP2 A activity and oocyte maturation inhibitor(OMI)activity decreased,MPF activity and17?,20?-dihydroxy-4-pregnen-3-one activity increased.After subchronic exposure to MC-LR,the transcription levels of ERK1,p38 MAPK and c-junamino-terminal kinase(JNK)increased.After exposure to MC-LR at 20 ?g/L,the transcription levels of the reproductive-related genes,such as vasa,dazl and nanosl increased.Studies have shown that MC-LR subchronic exposure can inhibit the PP2 A activity of zebrafish ovaries,change the level of sex hormone secretion,and affect the transcription level of MAPK family genes and reproductive related genes,so as to induce GVBD in oocytes and interfere with the development process of oocytes.