Zinc(II) Interactions with Brain-Derived Neurotrophic Factor N-Terminal Peptide Fragments: Inorganic Features and Biological Perspectives

Brain-derived neurotrophic factor (BDNF) is a neurotrophin essential for neuronal differentiation, growth, and survival; it is involved in memory formation and higher cognitive functions. The N-terminal domain of BDNF is crucial for the binding selectivity and activation of its specific TrkB receptor. Zn2+ ion binding may influence BDNF activity. Zn2+ complexes with the peptide fragment BDNF(1-12) encompassing the sequence 1-12 of the N-terminal domain of BDNF were studied by means of potentiometry, electrospray mass spectrometry, NMR, and density functional theory (DFT) approaches. The predominant Zn2+ complex species, at physiological pH, is [ZnL] in which the metal ion is bound to an amino, an imidazole, and two water molecules (NH2, N-lm, and 2O(water)) in a tetrahedral environment. DFT-based geometry optimization of the zinc coordination environment showed a hydrogen bond between the carboxylate and a water molecule bound to zinc in [ZnL]. The coordination features of the acetylated form [AcBDNF(1-12)] and of a single mutated peptide [BDNF(1-12)D3N] were also characterized, highlighting the role of the imidazole side chain as the first anchoring site and ruling out the direct involvement of the aspartate residue in the metal binding. Zn2+ addition to the cell culture medium induces an increase in the proliferative activity of the BDNF(1-12) peptide and of the whole protein on the SHSYSY neuroblastoma cell line. The effect of Zn2+ is opposite to that previously observed for Cu2+ addition, which determines a decrease in the proliferative activity for both peptide and protein, suggesting that these metals might discriminate and modulate differently the activity of BDNF.