Mortalin, apoptosis, and neurodegeneration

Abstract

Mortalin is a highly conserved heat-shock chaperone usually found in multiple subcellular locations. It has several binding partners and has been implicated in various functions ranging from stress response, control of cell proliferation, and inhibition/prevention of apoptosis. The activity of this protein involves different structural and functional mechanisms, and minor alterations in its expression level may lead to serious biological consequences, including neurodegeneration. In this article we review the most current data associated with mortalin's binding partners and how these protein-protein interactions may be implicated in apoptosis and neurodegeneration. A complete understanding of the molecular pathways in which mortalin is involved is important for the development of therapeutic strategies for cancer and neurodegenerative diseases.

Figure 1

Molecular modeling of Mortalin. Representation of the 3D structure of mortalincreated by homology modeling with the program PyMOL (The PyMOL Molecular Graphics System [22]) and energy-minimized with Hyperchem 8.0 (Hypercube, Inc. Gainsville, FL. USA). The (N-terminal binding domain, NBD; amino acid residues 1–443) includes the N-terminal region (blue) and includes the ATP binding motif (amino acid residues 61–443; indicated in green); the substrate binding domain (SBD; amino acid residues 444–679) is shown in yellow and includes the peptide binding domain (PBD; amino acid residues 444–581, indicated in red) [2,5,12]. p53 binds mortalin somewhere in the peptide-binding domain of mortalin (black arrow) [23].

Figure 2

Multiple functions and multiple localizations of mortalin. Mortalin is involved in mitochondrial, nuclear, plasma membrane and endoplasmic reticulum processes. The distribution of mortalin is highly dependent on cellular conditions. Mortalin interacts with the following proteins: 1. mitochondrial pre-proteins interact with mortalin and Hsp60 upon entering the mitochondrial matrix compartment; following these interactions, the mortalin/Hsp60 complex acts as a mitochondrial import motor. This coupling process allows proteins to refold, assemble, sort, and perform their corresponding functions; 2. mortalin interacts with VDAC1 and modulates its channel properties; 3. p66Shc localizes into the mitochondria and forms a complex with mortalin that modulates the mitochondrial pathway of apoptosis; 4. binding of mortalin to MVD1 (that inhibits p21(ras)-induced growth arrest) may represent another pathway to immortalization and may be a part of mechanisms of cell proliferation; 5. mortalin associates with the IL-1R (interleukin-1 receptor) protein leading to receptor internalization and downstream signaling cascades; 6. mortalin binds p53 thereby inactivating p53 translocation to the nucleus and inhibiting its activity as an apoptosis inducer; and 7. mortalin promotes intracellular trafficking of FGF-1.

Figure 3

Role of mortalin in oxidative stress-induced apoptosis. Mortalin has different functions under cellular stress or under non-stressed conditions. 1. Exposure of cells to stress induces the phosphorylation of p53 and its interaction with mortalin. Mortalin tries to protect the cells against oxidative damage; however, if the cells cannot recover, p53 induces the transcriptional activation of Bax, and BH3-only proteins including Noxa and PUMA, resulting in apoptosis; 2. increased levels of cellular oxidative stress can alter mortalin’s function; 3. In non-stressed (normal) or mildly-stressed conditions the phosphorylation levels of p53 are low, and mortalin does not interact with p53.

Figure 4

Potential interaction between mortalin and ApoE in Alzheimer’s disease. (a) Astrocytes from hApoE ε2/2-TR, hApoE ε3/3-TR, and hApoE ε4/4-TR mice were solubilized and immunoprecipitated with mortalin (Mot-Ab) or ApoE (ApoE-Ab) antibodies (Left panel). The ApoE-Ab immunoprecipitants were challenged with the Mot-Ab and only the hApoE ε4/4-TR astrocytes displayed interaction between ApoE and mortalin ((a) Right panel, indicated with a white arrow on the right panel). (b) Human brain tissues from hippocampus, were solubilized as described [20], followed by immunoprecipitation with an ApoE antibody. The immunoprecipitated proteins were separated in a 10% SDS-PAGE gel, transferred to a PVDF membrane, and immunoblotted with a Mot-Ab. Quantitation of the mortalin-apoE bands indicates that the binding is genotype- and disease-dependent (c). The complementary experiment, in which mortalin is immunoprecipitated with the Mot-Ab, and the IP is immunoblotted against ApoE-Ab (d,e) shows almost identical results. Proteins were identified by MALDI-TOF/TOF mass spectrometry (white arrow). “M” indicates proteins that were immunoprecipitated with the mortalin antibody and identified by mass spectrometry; correspondingly, “E” represents proteins that were immunoprecipitated with the apoE antibody.

Figure 5

A schematic model showing the mechanism of action of mortalin. (a) The normal function of mortalin. Under normal conditions, mortalin associates with certain chaperones, including Dj-1 (1 in the figure); following this interaction, the complex travels to the mitochondria, where mortalin is detached and enters the mitochondria (2). A magnified image (3) shows that mortalin crosses the outer membrane (TOM) (4) and the inner membrane (TIM) where it performs multiple functions, including chaperoning of the precursor proteins into the mitochondrial matrix (5). During, this process, mortalin binds simultaneously to TIM 44 (a peripheral membrane protein) and to the Tim23 complex (5). Next, the mature protein is transferred by mortalin to Hsp60 (6). Under normal conditions, mitochondrial mortalin forms stable complexes with p66Shc and Apaf-1 (7), which are released under cellular stress. (b) Under OS, mortalin levels are increased in the mitochondria, inhibiting ROS accumulation, and acting as a cytoprotective protein while maintaining cell viability (8). The magnified portion of the image shows that under stress conditions however, thelevels of mortalin needed to control other activities are reduced and the cells can suffer an imbalance; for example, the interaction between mortalin and p66Shc can be disrupted, and p66Shc oxidizes cyt-c and promotes the opening of the mitochondrial permeability transition pore triggering apoptosis (9); alternatively, Apaf-1 can induce apoptotic pathways (10). As a consequence, apoptosis increases in the absence of free mortalin, as a result of a rich OS environment.