Microcystin-LR acute exposure increases AChE activity via transcriptional ache activation in zebrafish (Danio rerio) brain

Abstract

Microcystins (MCs) constitute a family of cyanobacterial toxins, with more than 80 variants. These toxins are able to induce hepatotoxicity in several organisms mainly through the inhibition of protein phosphatases PP1 and PP2A and oxidative stress generation. Since recent evidence shows that MCs can either accumulate in brain or alter behavior patterns of fish species, in this study we tested the in vitro and in vivo effects of MC-LR at different concentrations on acetylcholinesterase (AChE) activity in zebrafish brain. In vivo studies showed that 100 μg/L MC-LR led to a significant increase in the AChE activity (27%) when zebrafish were exposed to the toxin dissolved in water, but did not cause any significant changes when injected intraperitoneally. In addition, semiquantitative RT-PCR analysis demonstrated that 100 μg/L MC-LR exposure also increased ache mRNA levels in zebrafish brain. The in vitro assays did not reveal any significant changes in AChE activity. These findings provide the first evidence that brain AChE is another potential target for MCs and suggest that the observed increases in AChE enzymatic activity and in ache transcript levels after MC-LR exposure depend, at least partially, on branchial uptake or ingestion.

Transcriptional and Behavioral Responses of Zebrafish Larvae to Microcystin-LR Exposure

“…Several studies have shown that MCLR accumulates in the brain [9] and exerts toxicity in the nervous system and impacts functions, including behavioral changes [23,38,39]. Furthermore, our data have shown that MCLR affects the transcriptional levels of all visual cycle genes.…”

“…The glutathione pathway is an important biochemical mechanism for the formation of glutathione (GSH) conjugates that increases the water-solubility of MCLR, mediating both its metabolism and elimination [21,22]. Besides the enzymatic activity of PP2A and glutathione transferase, MCLR also affects acetylocholinesterase (AChE), an enzyme that plays key roles in neurotransmitter systems and is widely used as a bioindicator of environmental toxicity [23]. Recently, Zeng et al have shown that MCLR exposure is accompanied by reactive oxygen species (ROS) production that consequently triggers apoptosis in developing zebrafish embryos in a caspase-dependent manner [24].…”

A review of neurotoxicity of microcystins

“…AChE (acetylcholonesterase), with classical functions in terminating neurotransmission at cholinergic synapses and neuromuscular junctions, was increased in MC-LRexposed zebrafish brain (Kist et al 2012). Since MCs can induce toxicity through inhibiting PP1 and PP2A (Campos and Vasconcelos 2010), MCs may influence brain AChE indirectly via the inhibition of serine/threonine phosphatase.…”

Cyanobacterial Toxins of the Laurentian Great Lakes, their Toxicological Effects, and Numerical Limits in Drinking Water

“…Indeed, direct injection of MC to rat hippocampus at femtogram levels has been shown to affect learning and memory [153,154,158]. MCs have also been shown to alter fish behavior and learning as well as increase acetylcholinesterase activity in fish brain [152]. In the well-described human MC intoxication event of dialysis patients in Caruaru, Brazil, patients experienced symptoms of neurotoxicity including deafness, tinnitus, and intermittent blindness [187].…”

“…While the liver is clearly one of the most affected organs, MCs also affect brain and reproductive tissues [146][147][148][149][150][151][152][153][154][155].…”