Does the cycad genotoxin MAM implicated in Guam ALS-PDC induce disease-relevant changes in mouse brain that includes olfaction?

Abstract

Western Pacific amyotrophic lateral sclerosis (ALS) and parkinsonism-dementia complex (PDC), a prototypical neurodegenerative disease (tauopathy) affecting distinct genetic groups with common exposure to neurotoxic chemicals in cycad seed, has many features of Parkinson's and Alzheimer's diseases (AD), including early olfactory dysfunction. Guam ALS-PDC incidence correlates with cycad flour content of cycasin and its aglycone methylazoxymethanol (MAM), which produces persistent DNA damage (O(6)-methylguanine) in the brains of mice lacking O(6)-methylguanine methyltransferase (Mgmt(-/-)). We described in Mgmt(-/-)mice up to 7 days post-MAM treatment that brain DNA damage was linked to brain gene expression changes found in human neurological disease, cancer, and skin and hair development. This addendum reports 6 months post-MAM treatment- related brain transcriptional changes as well as elevated mitogen activated protein kinases and increased caspase-3 activity, both of which are involved in tau aggregation and neurofibrillary tangle formation typical of ALS-PDC and AD, plus transcriptional changes in olfactory receptors. Does cycasin act as a "slow (geno)toxin" in ALS-PDC?

Figure 1.

Effect of MAM_late on components of the MAPK signaling pathway and caspase activity. The left half brains of vehicle- (n = 3) and MAM- (n = 4) treated Mgmt^−/− mice were flash frozen in liquid N₂, the frozen tissue subjected to ultrasonication in gel electrophoresis buffer to avoid loss of protein modifications or lysis, and the homogenate heat denatured at 95°C for 5 min. An aliquot of the brain tissue homogenate (50 μg) was resolved on a 10% polyacrylamide gel, transferred to PVDF membranes, the blocked membranes probed with monoclonal antibodies to ERK (ERK1, ERK2), phosphorylated ERK (pERK1, pERK2), PI3K (p110γ) (Santa Cruz Biotechnology, Inc.) and α-fodrin (Chemicon) and the bands visualized by chemiluminescence detection. α-Fodrin cleavage was determined using the 120 kDA band. Membranes were scanned on a Microtek flatbed scanner and each band quantified using Molecular Analyst software (BioRad, Inc) with background subtraction as described previously. Values are the mean ± standard error. Significantly different from vehicle (*p < 0.03 or ** p < 0.01 by ANOVA).