Molecular Perspectives of Mitochondrial Adaptations and Their Role in Cardiac Proteostasis

Abstract

Mitochondria are the key to properly functioning energy generation in the metabolically demanding cardiomyocytes and thus essential to healthy heart contractility on a beat-to-beat basis. Mitochondria being the central organelle for cellular metabolism and signaling in the heart, its dysfunction leads to cardiovascular disease. The healthy mitochondrial functioning critical to maintaining cardiomyocyte viability and contractility is accomplished by adaptive changes in the dynamics, biogenesis, and degradation of the mitochondria to ensure cellular proteostasis. Recent compelling evidence suggests that the classical protein quality control system in cardiomyocytes is also under constant mitochondrial control, either directly or indirectly. Impairment of cytosolic protein quality control may affect the position of the mitochondria in relation to other organelles, as well as mitochondrial morphology and function, and could also activate mitochondrial proteostasis. Despite a growing interest in the mitochondrial quality control system, very little information is available about the molecular function of mitochondria in cardiac proteostasis. In this review, we bring together current understanding of the adaptations and role of the mitochondria in cardiac proteostasis and describe the adaptive/maladaptive changes observed in the mitochondrial network required to maintain proteomic integrity. We also highlight the key mitochondrial signaling pathways activated in response to proteotoxic stress as a cellular mechanism to protect the heart from proteotoxicity. A deeper understanding of the molecular mechanisms of mitochondrial adaptations and their role in cardiac proteostasis will help to develop future therapeutics to protect the heart from cardiovascular diseases.

Keywords: cardiac proteostasis; mitochondria; mitochondrial dysfunction; mitochondrial proteostasis; mitochondrial unfolded protein response; proteotoxicity.

FIGURE 1

Mitochondrial oxidative stress contributes to both cytosolic and mitochondrial protein aggregation. The electron transport chain (ETC) of the mitochondria, located in the inner mitochondrial membrane is composed of five multi-subunit enzyme complexes (denoted by I, II, III, IV, and V). Electron (e-) flow through the ETC follows the order of electrons donated by coenzymes (NADH and FADH₂) as they are accepted and transferred to complex I (NADH ubiquinone reductase) or complex II (succinate dehydrogenase) and then consecutively to complex III (ubiquinol-cytochrome C reductase), complex IV (cytochrome c oxidase), and finally to oxygen (O₂) to produce water (H₂O). The electrochemical gradient established through the electron transfer along with the ETC couple protons transport across the inner membrane, resulting in ATP generation through complex V (F0F1 ATP synthase). Electron leakage in the ETC, mainly from complex I and III, generates reactive oxygen species such as superoxide anion (O₂^–). The transfer of electrons to O₂ generates O₂^–, which is then converted to hydrogen peroxide (H₂O₂) by the enzymes superoxide dismutase (MnSOD) and catalase in the mitochondria. O₂^– can oxidize either the mitochondrial protein inside the mitochondria, forming the misfolded protein aggregate that affects the mitochondrial function, or the cytosolic protein after being released into the cytosol, leading to the formation of protein aggregate in the cytosol.

FIGURE 2

Protein aggregate interacts with VDAC leading to an increase in Ca²⁺ susceptibility. Aggregate proteins may interact with the VDAC, affecting the entry of Ca²⁺ into the mitochondria. Ca²⁺ enters the matrix via the mitochondrial calcium uniporter (MCU). After entering the matrix, Ca²⁺ binds to the mitochondrial permeability transition pore (MPTP) and triggers it to open. Excessive mitochondrial Ca²⁺ causes the MPTP to remain open for a long time. As a result, the mitochondria undergo swelling and rupture, releasing apoptosis-inducing factors into the cytosol.

FIGURE 3

Mitochondrial quality control by chaperones and proteases. Mitochondrial proteins enter into the mitochondria through the TOM and TIM complexes with the help of HSP70, which disaggregates the protein. Following a cascade of the chaperone proteins TRAP1, HSP10, and HSP60, the protein develops its native conformation. ROS generated from the leakage of electrons from the electron transport chain can affect mitochondrial protein conformation. The inner membrane-embedded protease i-AAA degrades the misfolded proteins in the intermembrane space, while m-AAA acts on misfolded proteins in the matrix. Misfolded proteins can be refolded to their native forms by the chaperone proteins, including TRAP1, HSP60, and HSP10. The soluble proteases LonP1 and ClpP clear the aggregates in the matrix. If excessive ROS generation compromises the function of LonP1 and ClpP, the protein cannot be refolded, resulting in aggregate formation in the mitochondria leading to mitochondrial dysfunction.

FIGURE 4

Mitochondrial and nuclear communication during UPR^mt. Mitochondrial and nuclear communication is mediated through the transcription factor ATFS1 in C. elegans and ATF5 in mammals. ATFS1/ATF5 contains both a mitochondrial target sequence (MTS) and a nuclear localization signal (NLS). Under basal conditions, the mitochondrial transport system functions optimally. ATFS1/ATF5 preferentially enters the mitochondria and is degraded by mitochondrial proteases, including LonP1 and ClpP. Under stress conditions, mitochondrial entry systems are compromised due to excessive ROS generation by the mitochondria, making conditions favorable for ATFS1/ATF5 to enter the nucleus and trigger the array of gene transcription for UPR^mt.

FIGURE 5

Cytosolic protein clearance by mitochondrial guidance. In addition to classical mitochondrial proteostasis, the cytosolic aggregate can be cleared by the MAGIC (mitochondria as guidance in the cytosol) pathway. Presumably, the cytosolic aggregates are disaggregated by HSC70 or FUNDC1, followed by the entry of disaggregated protein into the mitochondria. Finally, cytosolic protein transported to the mitochondria undergoes degradation by mitochondrial protease LonP1 and ClpP.

FIGURE 6

Mitochondrial dynamics in cellular proteostasis. Proteotoxicity associated with DRC leads to the accumulation of cytosolic toxic protein aggregates that activate sequential events leading to cardiomyocyte death. These cytosolic aggregate proteins activate Drp1-dependent aberrant mitochondrial fission, activate Bax-dependent MPTP opening, result in excessive ROS generation, and ultimately lead to a reduction in ATP production. The mitochondrial membrane potential is decreased as a consequence of exacerbated ROS production, which, coupled with other signaling molecules, ultimately increases the mitochondrial membrane permeability, and causes the release of apoptotic contents into the cytosol, activating apoptotic cell death. This results in decreased levels of cellular ATP, aberrant mitochondrial fragmentation, and a release of excessive ROS, which contributes to dysfunctional mitochondrial proteostasis and exacerbates the accumulation of protein aggregates. Inhibition of aberrant mitochondrial fragmentation induced by DRC in cardiomyocytes by treatment with mitochondrial fission inhibitor (mdivi-1) restored mitochondrial function and reduced the cytosolic aggregate load.