Mammalian Metallothionein-3: New Functional and Structural Insights

Abstract

Metallothionein-3 (MT-3), a member of the mammalian metallothionein (MT) family, is mainly expressed in the central nervous system (CNS). MT-3 possesses a unique neuronal growth inhibitory activity, and the levels of this intra- and extracellularly occurring metalloprotein are markedly diminished in the brain of patients affected by a number of metal-linked neurodegenerative disorders, including Alzheimer's disease (AD). In these pathologies, the redox cycling of copper, accompanied by the production of reactive oxygen species (ROS), plays a key role in the neuronal toxicity. Although MT-3 shares the metal-thiolate clusters with the well-characterized MT-1 and MT-2, it shows distinct biological, structural and chemical properties. Owing to its anti-oxidant properties and modulator function not only for Zn, but also for Cu in the extra- and intracellular space, MT-3, but not MT-1/MT-2, protects neuronal cells from the toxicity of various Cu(II)-bound amyloids. In recent years, the roles of zinc dynamics and MT-3 function in neurodegeneration are slowly emerging. This short review focuses on the recent developments regarding the chemistry and biology of MT-3.

Keywords: amyloid; copper; metal homeostasis; metal-thiolate clusters; metallothionein-3; neurodegeneration; reactive oxygen species; zinc.

Figure 1

Metal-thiolate clusters in metallothionein-3. (A) The NMR solution structure of the Cd₄-α-domain of human Cd₇MT-3. The Cd(II) ions are shown as light-orange spheres connected to the protein backbone by cysteine thiolate ligands. The model was generated with PyMOL using the Protein Data Bank coordinates of 2FJ5 [28]; (B) model of the Cu(I)₄CysS₅ cluster, derived from the spectroscopic characterization of Cu(I)₄,Zn₄MT-3 [29,30]. Metal ions are shown as dark gray spheres connected to cysteine sulfur ligands (yellow).

Figure 2

Metal-swap reactions and Cu(II) redox-silencing by Zn₇MT-3 in neurodegenerative disorders. The protective effect of human Zn₇MT-3 (with the structure of β-domain modeled by that of rat Zn₂Cd₅MT-2 [77]) from the copper-mediated toxicity in AD, PD and prion diseases is summarized. The metal swap between Zn₇MT-3 and the disease specific amyloidogenic Cu(II)–Aβ_1–40 peptide [46], the Cu(II)–α-Syn [75] and Cu(II)–PrP proteins [76] abolishes the ROS production and the related cellular toxicity. In this process, Cu(II) is reduced by the protein thiolates concomitant with its binding into the N-terminal β-domain, forming the Cu(I)₄Zn₄MT-3 species and the non-redox-active Zn(II)–Aβ_1–40, Zn(II)–α-Syn and Zn(II)–PrP. In Cu(I)₄Zn₄MT-3 an air-stable Cu(I)₄-thiolate cluster and two disulfide bonds are present the N-terminal β-domain.