Metallothionein-3 as a multifunctional player in the control of cellular processes and diseases

Abstract

Transition metals, such as iron, copper, and zinc, play a very important role in life as the regulators of various physiochemical reactions in cells. Abnormal distribution and concentration of these metals in the body are closely associated with various diseases including ischemic seizure, Alzheimer's disease, diabetes, and cancer. Iron and copper are known to be mainly involved in in vivo redox reaction. Zinc controls a variety of intracellular metabolism via binding to lots of proteins in cells and altering their structure and function. Metallothionein-3 (MT3) is a representative zinc binding protein predominant in the brain. Although the role of MT3 in other organs still needs to be elucidated, many reports have suggested critical roles for the protein in the control of a variety of cellular homeostasis. Here, we review various biological functions of MT3, focusing on different cellular molecules and diseases involving MT3 in the body.

Keywords: Alzheimer’s disease; Autophagy; Lysosome; Metallothionein-3; Neurodegenerative disease; Oxidative stress; Zinc.

Fig. 1

Schematic diagram of MT3 amino acid sequences and structure and its response to oxidative stress. a Amino acid sequences alignments of human MT1, MT2, MT3, and MT4. Conserved cysteine residues are highlighted in dark green. The distinctive sequence differences of MT3 from other MTs include an insertion of Thr5, a conservative CPCP (6–9) sequences, and insertion of the charged hexapeptide EAAEAE (55–60). b Simplified structure of MT3. The β-domain of the N-terminal contains nine cysteine residues, but α-domain of the C-terminal contains 11 cysteine residues, providing three and four zinc binding sites, respectively. c Metal swap in MT3 under oxidative injury. MT3 without any metals (apo-MT3) can bind up to seven zinc. Under ROS or NO stress, three Zn in β-domain can be released by swapping with radicals and the metal exchange ability may protect cells from the oxidative injury

Fig. 2

Representative images of various intracellular responses to the absence of MT3. a Neuronal outgrowth induced in MT3 knockout (KO) mice. Cortical astrocytes from MT3 wild type (WT, ^+/+) and KO (^−/−) mice were used as the feeder cells. Cortical neuronal cells from ICR mice were cultured onto the astrocytes from the indicated sources. b-d Confocal images of altered localization and expression of intracellular components, such as endosomes b, Golgi c, F-actin d, in cortical astrocytes from MT3 WT and KO mice. e Fluorescence images of lysosensor-stained MT3 WT and KO cortical astrocytes. f Confocal images of altered localization and expression of vATPase Voa1 in cortical astrocytes from MT3 WT and KO mice. g-h Live-cell confocal microscopic images of WT and MT3-null astrocytes. Cortical astrocytes were exposed to 100 μm of H₂O₂ and the intracellular labile zinc was traced using a FluoZin-3 g, free zinc staining fluorescence dye, and Lysotracker h, a fluorescence dye staining of lysosomes. i-m Confocal images of altered localization and expression of intracellular proteins; Cortical astrocytes from WT and MT3-null astrocytes positive for GFP-LC3 i, fillipin and Lamp2 j, lipofuscin k, presenilin l, and GFP-mHttQ74 m. Cells were stained with filipin (green) j and lipofuscin (cyan) k against cellular lipids. Lamp2 was used to determine the cellular localization of intracellular fillipin-positive signals. GFP-mHttQ74 (green dot) was overexpressed in cortical astrocytes and its expression was compared between WT and KO of MT3. n Live-cell confocal microscopic images of WT and MT3-null astrocytes. The plasma membrane of astrocytes was stained with phalloidin, a membrane stain, and FITC-Aβ_1–42 was loaded and the intracellular uptake into astrocytes was traced. FITC-Aβ_1–42 was uptaken into cells 15 min after loading

Fig. 3

Summary diagrams depicting MT3 effects on diseases. MT3 contributes to the normal progression of redox reaction, lysosomal biogenesis, autophagy, apoptosis, endocytosis, and cytoskeleton, which are all beneficial for cell survival. However, altered expression of MT3 in cortical astrocytes of null mice leads to various pathological conditions, such as Acute and chronic neuronal diseases, cancer, retinopathy, and diabetes. AD, Alzheimer disease; HD, Huntington disease; ALS, amyotrophic lateral sclerosis; AMD, age-related macular degeneration