Cadmium association with DREAM promotes DREAM interactions with intracellular partners in a similar manner to its physiological ligand, calcium

Abstract

Cd²⁺ exposure has been associated with neurodegenerative diseases and other pathologies, but the underlying mechanism through which it exerts toxic effects remain unresolved. Using calorimetric and spectroscopic techniques, we show that Cd²⁺ binds to EF-hands in DREAM (downstream regulatory element antagonist modulator) with an equilibrium dissociation constant of 89 ± 10 nM, which is superior to that determined for Ca²⁺ (K_d = 1000 nM). Analogous to Ca²⁺ binding, Cd²⁺ binding triggers changes in the protein secondary and tertiary structure, including increased exposure of the hydrophobic cavities, as determined using a fluorescent probe, 1-anilinonaphthalene-8-sulfonic acid. In addition, we demonstrate that Cd²⁺ binding modulates DREAM interactions with FITC-labeled peptides that mimic binding sites of DREAM effector proteins; helix-9 of presenilin-1, and site-1 and site 2 of potassium voltage channel 4.3 (residues 2–22 and 70–90, respectively). Cd²⁺ association with DREAM increases its affinity for helix 9 of presenilin roughly 30-times compared to metal-free DREAM. The DREAM affinity for site-1 and site 2 is elevated approximately 7 and 15 times, respectively, in the presence of Cd²⁺. The above results suggest that DREAM and probably other members of the neuronal calcium sensor family bind Cd²⁺ with an affinity that is superior to that for Ca²⁺ and the interactions between toxic Cd²⁺ and DREAM and other neuronal calcium sensors provide novel insight into the molecular mechanism of Cd²⁺ neurotoxicity.