Dose–Response Study of Microcystin Congeners MCLA, MCLR, MCLY, MCRR, and MCYR Administered Orally to Mice

Abstract

Microcystins are common freshwater cyanobacterial toxins that affect liver function. The toxicities of five microcystin congeners (microcystin-LA (MCLA), MCLR, MCLY, MCRR, and MCYR) commonly observed in harmful algal blooms (HABs) were evaluated in BALB/c mice after a single oral administration of doses ranging from those that were no observed adverse effect levels (NOAELs) to lowest observed adverse effect levels (LOAELs). Animals were monitored for changes in behavior and appearance, and euthanized 24 h after dosing. Test endpoints included clinical changes, necropsy observations, and serum indicators of hepatic toxicity and general homeostasis. Doses were 0.5–7 mg/kg MCLA, 0.5–11 mg/kg MCLR, 1–7 mg/kg MCLY, 7–22 mg/kg MCRR, and 3–11 mg/kg MCYR. MCLA at 3 mg/kg elevated liver/body weight ratio and liver score, ALT, AST, and GLDH, indicating hepatic toxicity, reduced serum glucose and highly elevated total serum bilirubin. MCLR and MCLY induced similar effects with LOAELs of 5 mg/kg, although a greater extent and severity of effects were observed in MCLR animals. MCRR exposure at 22 mg/kg was associated with reduced serum glucose. MCYR induced scattered liver effects at 7 mg/kg and reduced serum glucose levels at 5 mg/kg. The results indicate significant differences in congener-induced toxicity after microcystin exposure.

Improving the Quantification of Cyanotoxins Using a Mass Balance-Based Effective Concentration-Equivalent Concentration Approach

“…Toxicological studies suggest that MCs with hydrophilic amino acid groups (e.g., MCRR) appear to be less toxic, while MCs with hydrophobic l -amino acids appear to be more toxic (e.g., MCLR) (Table S7). − The trend appears to be similar to the CR values of MCLR and MCRR from the ADDA-ELISA tests (Table S4). However, toxicity studies on most MC variants are still very limited; therefore, it is unclear whether a correlation can be developed between cross-reactivity and toxicity.…”

Variability of Microcystin-LR Standards Available from Seven Commercial Vendors

“…Because of this, the differences in hepatotoxicity endpoints in vivo generally mirror the variations in MCLR standard concentrations (results in Table 2 ). The liver score (defined in Section 4.2.2 ), which assesses the gross appearance of the liver at the time of necropsy [ 10 , 11 ], consistently increased with MCLR dose concentrations and reached significance with males in vendor groups B-G (MCLR dose ≥ 25.6 µg/kg) and females in vendor groups A-G (MCLR dose ≥ 13.2 µg/kg) compared to the controls ( Figure 2 A). The liver/body weight is used as opposed to liver weight alone to normalize for varying body weights.…”

“…All necropsies were done experimenter blind. Necropsies were performed immediately after blood collection, and a gross assessment of liver appearance was recorded and then converted to a liver score [ 10 , 11 ]. The liver score was based on presence/severity of lesions and extent of liver surface area affected.…”

“…Microcystins (MCs) are a class of cyclic heptapeptides with more than 279 derivatives reported [ 1 , 2 ], produced by cyanobacteria (blue-green algae) in freshwater bodies throughout the world [ 3 , 4 , 5 ]. MCs have demonstrated hepatotoxicity in rodents [ 6 , 7 , 8 , 9 , 10 , 11 ] and an association with deaths of wild animals [ 12 , 13 ] as well as domestic pets [ 14 , 15 , 16 ] and livestock [ 17 , 18 ]. Deaths associated with incidental exposure to MCs are usually due to acute, high-dose exposures that inhibit protein phosphatases 1A and 2A in the liver which result in loss of cellular structure, cell death, and ultimately intrahepatic hemorrhage and liver dysfunction [ 19 , 20 ].…”

“…MCs are found worldwide and are the most common freshwater cyanotoxins in U.S. waters [ 34 , 35 , 36 ]. Quantitative monitoring of MCs in impacted waterways [ 37 , 38 ], as well as toxicological studies of microcystin-LR (MCLR) [ 6 , 7 , 8 , 9 , 10 , 11 , 39 ] and related compounds are dependent on access to high-purity and mass-accurate standards. Standards are typically extracted from cyanobacteria biomass as opposed to being synthetically prepared [ 40 ] and require validation to confirm the purity of extracted material and the accuracy of measured standard concentrations.…”