A Manganese-Handling Deficit in Huntington’s Disease Selectively Impairs ATM-p53 Signaling
Tidball, Andrew Martin
The essential micronutrient manganese is enriched in brain, especially the basal ganglia. We sought to identify neuronal signaling pathways responsive to neurologically relevant manganese levels, as previous data suggested manganese alterations occur in Huntington’s disease (HD). We found that p53 phosphorylation is highly responsive to manganese levels in human and mouse striatal-like neuroprogenitors. The Ataxia Telangiectasia Mutated (ATM) kinase is responsible for this manganese-dependent phosphorylation of p53. Activation of ATM-p53 by manganese was severely blunted by pathogenic alleles of Huntingtin. HD neuroprogenitors exhibited a highly manganese selective deficit in ATM kinase activation, since DNA damage and oxidative injury, canonical activators of ATM, did not show similar deficits. Manganese was previously shown to activate ATM kinase in cell-free assays. We found that human HD neuroprogenitors have reduced intracellular manganese with neurologically relevant manganese exposures. Pharmacological manipulation to equalize manganese between HD and control neuroprogenitors rescued the ATM-p53 signaling deficit. The compound that normalized these levels was the small molecule, KB-R7943, a known inhibitor sodium/calcium exchanger (NCX) inhibitor. However, the mechanism by which KB-R7943 corrects manganese accumulation does not seem to be via direct inhibition of the NCX transporters. We also demonstrated a severe deficit in NCX1 expression in HD cells that may also play a key role in the HD manganese deficiency. Huntington’s disease cells also show increased genomic instability and DNA damage signaling under basal conditions. Manganese is known to be an important cofactor for several enzymes involved in DNA repair and replication, and we found that the manganese deficiency was most severe in the nucleus compared with other compartments. Manganese supplementation reduced the elevated DNA damage signaling to those found in non-HD cells suggesting that manganese deficiency underlies this phenotype In short, the ATM-p53 signaling pathway is a manganese responsive signaling pathway. Manganese is an important cofactor with diminished accumulation in HD cell models. These reduced levels may be the reason for observed increases in DNA damage and genomic instability. Further experimentation is needed to elucidate the mechanism of manganese accumulation deficiency mechanism in HD and the KB-R7943 rescue.