Abstract
Hazardous contaminants, such as nitrite and microcystin-leucine arginine (MC-LR), are released into water bodies during cyanobacterial blooms and may adversely influence the normal physiological function of hydrobiontes. The combined effects of nitrite and MC-LR on the antioxidant defense and innate immunity were evaluated through an orthogonal experimental design (nitrite: 0, 29, 290 μM; MC-LR: 0, 3, 30 nM). Remarkable increases in malondialdehyde (MDA) levels have suggested that nitrite and/or MC-LR exposures induce oxidative stress in fish spleen, which were indirectly confirmed by significant downregulations of total antioxidant capacity (T-AOC), glutathione (GSH) contents, as well as transcriptional levels of antioxidant enzyme genes cat1, sod1 and gpx1a. Simultaneously, nitrite and MC-LR significantly decreased serum complement C3 levels as well as the transcriptional levels of splenic c3b, lyz, il1β, ifnγ and tnfα, and indicated that they could jointly impact the innate immunity of fish. The severity and extent of splenic lesions were aggravated by increased concentration of nitrite or MC-LR and became more serious in combined groups. The damages of mitochondria and pseudopodia in splenic macrophages suggest that oxidative stress exerted by nitrite and MC-LR aimed at the membrane structure of immune cells and ultimately disrupted immune function. Our results clearly demonstrate that nitrite and MC-LR exert synergistic suppressive effects on fish innate immunity via interfering antioxidant responses, and their joint toxicity should not be underestimated in eutrophic lakes.
Highlights
During the last decade, the issue of harmful cyanobacteria blooms has become a worldwide concern since such blooms may cause oxygen depletion of water body and release cyanotoxins [1,2,3].Microcystins are a family of cyclic heptapeptide toxins released by cyanobacteria [4]
The maximum deviation between measured concentrations of nitrite or microcystin-leucine arginine (MC-LR) and the corresponding nominal concentrations in each treatment group was less than 20%, which proved that the concentrations of these two contaminants within the experimental tanks were relatively stable during the whole exposure period
A dose-dependent increase of MDA in the spleen was found in the groups exposed to single factor nitrite or MC-LR, indicating that both nitrite and MC-LR promoted the production of reactive oxygen species (ROS) and induced oxidative stress
Summary
The issue of harmful cyanobacteria blooms has become a worldwide concern since such blooms may cause oxygen depletion of water body and release cyanotoxins [1,2,3]. Microcystins are a family of cyclic heptapeptide toxins released by cyanobacteria [4]. Among approximately 100 microcystin isoforms, microcystin-leucine arginine (MC-LR) is considered to be the Toxins 2018, 10, 512; doi:10.3390/toxins10120512 www.mdpi.com/journal/toxins. Toxins 2018, 10, 512 most toxic and widely distributed in freshwater bodies [5,6]. The toxins are environmentally stable and can persist in aqueous for days due to the cyclic structure [7,8]. The inhibition of serine/threonine protein phosphatase 1 and 2A is considered to be a classic toxic mechanism of MCs [13]
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