Role of the Mitochondrial Protein Import Machinery and Protein Processing in Heart Disease

Mitochondrial quality control pathways sense mitochondrial protein import

Mitochondrial quality control (MQC) mechanisms are required to maintain a functional proteome, which enables mitochondria to perform a myriad of important cellular functions from oxidative phosphorylation to numerous other metabolic pathways. Mitochondrial protein homeostasis begins with the import of over 1000 nuclear-encoded mitochondrial proteins and the synthesis of 13 mitochondrial DNA-encoded proteins. A network of chaperones and proteases helps to fold new proteins and degrade unnecessary, damaged, or misfolded proteins, whereas more extensive damage can be removed by mitochondrial-derived vesicles (MDVs) or mitochondrial autophagy (mitophagy). Here, focusing on mechanisms in mammalian cells, we review the importance of mitochondrial protein import as a sentinel of mitochondrial function that activates multiple MQC mechanisms when impaired.

Uncovering the proximal proteome of CTR1 through TurboID-mediated proximity labeling

Protein-protein interactions play a crucial role in driving cellular processes and enabling appropriate physiological responses in organisms. The plant hormone ethylene signaling pathway is complex and regulated by the spatiotemporal regulation of its signaling molecules. Constitutive Triple Response 1 (CTR1), a key negative regulator of the pathway, regulates the function of Ethylene-Insensitive 2 (EIN2), a positive regulator of ethylene signaling, at the endoplasmic reticulum (ER) through phosphorylation. Our recent study revealed that CTR1 can also translocate from the ER to the nucleus in response to ethylene and positively regulate ethylene responses by stabilizing EIN3. To gain further insights into the role of CTR1 in plants, we used TurboID-based proximity labeling and mass spectrometry to identify the proximal proteomes of CTR1 in Nicotiana benthamiana. The identified proximal proteins include known ethylene signaling components, as well as proteins involved in diverse cellular processes such as mitochondrial respiration, mRNA metabolism, and organelle biogenesis. Our study demonstrates the feasibility of proximity labeling using the N. benthamiana transient expression system and identifies the potential interactors of CTR1 in vivo, uncovering the potential roles of CTR1 in a wide range of cellular processes.

Rescue of mitochondrial import failure by intercellular organellar transfer

Mitochondria are the powerhouses of eukaryotic cells, composed mostly of nuclear-encoded proteins imported from the cytosol. Thus, problems with the import machinery will disrupt their regenerative capacity and the cell's energy supplies - particularly troublesome for energy-demanding cells of nervous tissue and muscle. Unsurprisingly then, import breakdown is implicated in disease. Here, we explore the consequences of import failure in mammalian cells; wherein, blocking the import machinery impacts mitochondrial ultra-structure and dynamics, but, surprisingly, does not affect import. Our data are consistent with a response involving intercellular mitochondrial transport via tunnelling nanotubes to import healthy mitochondria and jettison those with blocked import sites. These observations support the existence of a widespread mechanism for the rescue of mitochondrial dysfunction.

GTPase-activating protein ARAP1 regulates circular dorsal ruffles as a nutrient uptake mechanism in the Hep3B hepatocellular carcinoma cell line

Circular dorsal ruffles (CDRs), large-scale rounded membrane ruffles, function as precursors of macropinocytosis. We recently reported that CDRs are exposed in the Hep3B hepatocellular carcinoma cell line, while not in other hepatocellular carcinoma cell lines, indicating that the CDRs in Hep3B are associated with malignant potential. In this study, we investigated the cellular function of CDRs in Hep3B cells by focusing on the molecular mechanisms of the GTPase-activating protein ARAP1. ARAP1 was localized to the CDRs, the sizes of which were reduced by deletion of this protein. High-resolution scanning electron micrographs revealed that CDRs comprise small vertical lamellipodia, the expression pattern of which was disrupted in ARAP1 KO cells. Extracellular solute uptake, rate of cell growth, and malignant potential were attenuated in the KO cells. ARAP1 is also localized in Hep3B cell mitochondria, although not in those of the Huh7 hepatocellular carcinoma cell line. On the basis of these findings, we propose that the aberrant expression of ARAP1 in Hep3B cells modulates CDRs, thereby resulting in an excess uptake of nutrients as an initial event in cancer development.

SUMMARY STATEMENT

ARAP1 regulates circular dorsal ruffles (CDRs) in the Hep3B HCC cell line and deletion of this protein attenuates malignant potential, thereby indicating the involvement of CDRs in cancer development.

Uncovering the Localization and Function of a Novel Read-Through Transcript 'TOMM40-APOE'

Recent advancements in genome analysis technology have revealed the presence of read-through transcripts in which transcription continues by skipping the polyA signal. We here identified and characterized a new read-through transcript, TOMM40-APOE. With cDNA amplification from THP-1 cells, the TOMM40-APOE3 product was successfully generated. We also generated TOMM40-APOE4, another isoform, by introducing point mutations. Notably, while APOE3 and APOE4 exhibited extracellular secretion, both TOMM40-APOE3 and TOMM40-APOE4 were localized exclusively to the mitochondria. But functionally, they did not affect mitochondrial membrane potential. Cell death induction studies illustrated increased cell death with TOMM40-APOE3 and TOMM40-APOE4, and we did not find any difference in cellular function between the two isoforms. These findings indicated that the new mitochondrial protein TOMM40-APOE has cell toxic ability.

RSV Induces Activation of Intracellular EGFR on the Mitochondrial Membrane for Virus Propagation

Respiratory syncytial virus (RSV) infects people of all ages and is one of the most common causative agents of lower respiratory tract infections, such as pneumonia, especially in infants under one year of age. However, no direct treatment has been developed for RSV infections. Maintenance of mitochondrial homeostasis and epidermal growth factor receptor (EGFR) activity is important for human cell growth. This study reported that RSV infection maintained the total cellular ATP levels and promoted the intracellular activity of EGFR to replicate RSV. RSV activates the intracellular EGFR-mediated cell survival signaling cascade and maintains mitochondrial EGFR expression for viral production during early events after infection. The approved EGFR inhibitor, vandetanib, markedly reduces RSV propagation, suggesting that EGFR is an attractive host target for RSV therapeutics. Our results suggest that RSV infection maintains cellular ATP levels and promotes the activation of intracellular EGFR in the mitochondrial membrane, significantly contributing to robust RSV propagation.

Mitochondrial microproteins: critical regulators of protein import, energy production, stress response pathways, and programmed cell death

Mitochondria rely upon the coordination of protein import, protein translation, and proper functioning of oxidative phosphorylation (OXPHOS) complexes I-V to sustain the activities of life for an organism. Each process is dependent upon the function of profoundly large protein complexes found in the mitochondria [translocase of the outer mitochondrial membrane (TOMM) complex, translocase of the inner mitochondrial membrane (TIMM) complex, OXPHOS complexes, mitoribosomes]. These massive protein complexes, in some instances more than one megadalton, are built up from numerous protein subunits of varying sizes, including many proteins that are ≤100-150 amino acids. However, these small proteins, termed microproteins, not only act as cogs in large molecular machines but also have important steps in inhibiting or promoting the intrinsic pathway of apoptosis, coordinate responses to cellular stress, and even act as hormones. This review focuses on microproteins that occupy the mitochondria and are critical for its function. Although the microprotein field is relatively new, researchers have long recognized the existence of these mitochondrial proteins as critical components of virtually all aspects of mitochondrial biology. Thus, recent studies estimating that hundreds of new microproteins of unknown function exist and are missing from current genome annotations suggests that the mitochondrial "microproteome" is a rich area for future biological investigation.

Insulin-Degrading Enzyme Interacts with Mitochondrial Ribosomes and Respiratory Chain Proteins

Insulin-degrading enzyme (IDE) is a highly conserved metalloprotease that is mainly localized in the cytosol. Although IDE can degrade insulin and some other low molecular weight substrates efficiently, its ubiquitous expression suggests additional functions supported by experimental findings, such as a role in stress responses and cellular protein homeostasis. The translation of a long full-length IDE transcript has been reported to result in targeting to mitochondria, but the role of IDE in this compartment is unknown. To obtain initial leads on the function of IDE in mitochondria, we used a proximity biotinylation approach to identify proteins interacting with wild-type and protease-dead IDE targeted to the mitochondrial matrix. We find that IDE interacts with multiple mitochondrial ribosomal proteins as well as with proteins involved in the synthesis and assembly of mitochondrial complex I and IV. The mitochondrial interactomes of wild type and mutant IDE are highly similar and do not reveal any likely proteolytic IDE substrates. We speculate that IDE could adopt similar additional non-proteolytic functions in mitochondria as in the cytosol, acting as a chaperone and contributing to protein homeostasis and stress responses.

Grpel2 maintains cardiomyocyte survival in diabetic cardiomyopathy through DLST-mediated mitochondrial dysfunction: a proof-of-concept study

BACKGROUND

Diabetic cardiomyopathy (DCM) has been considered as a major threat to health in individuals with diabetes. GrpE-like 2 (Grpel2), a nucleotide exchange factor, has been shown to regulate mitochondrial import process to maintain mitochondrial homeostasis. However, the effect and mechanism of Grpel2 in DCM remain unknown.

METHODS

The streptozotocin (STZ)-induced DCM mice model and high glucose (HG)-treated cardiomyocytes were established. Overexpression of cardiac-specific Grpel2 was performed by intramyocardial injection of adeno-associated virus serotype 9 (AAV9). Bioinformatics analysis, co-immunoprecipitation (co-IP), transcriptomics profiling and functional experiments were used to explore molecular mechanism of Grpel2 in DCM.

RESULTS

Here, we found that Grpel2 was decreased in DCM induced by STZ. Overexpression of cardiac-specific Grpel2 alleviated cardiac dysfunction and structural remodeling in DCM. In both diabetic hearts and HG-treated cardiomyocytes, Grpel2 overexpression attenuated apoptosis and mitochondrial dysfunction, including decreased mitochondrial ROS production, increased mitochondrial respiratory capacities and increased mitochondrial membrane potential. Mechanistically, Grpel2 interacted with dihydrolipoyl succinyltransferase (DLST), which positively mediated the import process of DLST into mitochondria under HG conditions. Furthermore, the protective effects of Grpel2 overexpression on mitochondrial function and cell survival were blocked by siRNA knockdown of DLST. Moreover, Nr2f6 bond to the Grpel2 promoter region and positively regulated its transcription.

CONCLUSION

Our study provides for the first time evidence that Grpel2 overexpression exerts a protective effect against mitochondrial dysfunction and apoptosis in DCM by maintaining the import of DLST into mitochondria. These findings suggest that targeting Grpel2 might be a promising therapeutic strategy for the treatment of patients with DCM.

The Journey of Mitochondrial Protein Import and the Roadmap to Follow

Mitochondria are double membrane-bound organelles that play critical functions in cells including metabolism, energy production, regulation of intrinsic apoptosis, and maintenance of calcium homeostasis. Mitochondria are fascinatingly equipped with their own genome and machinery for transcribing and translating 13 essential proteins of the oxidative phosphorylation system (OXPHOS). The rest of the proteins (99%) that function in mitochondria in the various pathways described above are nuclear-transcribed and synthesized as precursors in the cytosol. These proteins are imported into the mitochondria by the unique mitochondrial protein import system that consists of seven machineries. Proper functioning of the mitochondrial protein import system is crucial for optimal mitochondrial deliverables, as well as mitochondrial and cellular homeostasis. Impaired mitochondrial protein import leads to proteotoxic stress in both mitochondria and cytosol, inducing mitochondrial unfolded protein response (UPR^mt). Altered UPR^mt is associated with the development of various disease conditions including neurodegenerative and cardiovascular diseases, as well as cancer. This review sheds light on the molecular mechanisms underlying the import of nuclear-encoded mitochondrial proteins, the consequences of defective mitochondrial protein import, and the pathological conditions that arise due to altered UPR^mt.

Connexin 43 in Mitochondria: What Do We Really Know About Its Function?

Connexins are known for their ability to mediate cell-cell communication via gap junctions and also form hemichannels that pass ions and molecules over the plasma membrane when open. Connexins have also been detected within mitochondria, with mitochondrial connexin 43 (Cx43) being the best studied to date. In this review, we discuss evidence for Cx43 presence in mitochondria of cell lines, primary cells and organs and summarize data on its localization, import and phosphorylation status. We further highlight the influence of Cx43 on mitochondrial function in terms of respiration, opening of the mitochondrial permeability transition pore and formation of reactive oxygen species, and also address the presence of a truncated form of Cx43 termed Gja1-20k. Finally, the role of mitochondrial Cx43 in pathological conditions, particularly in the heart, is discussed.

Mitochondrial Protein Homeostasis and Cardiomyopathy

Human mitochondrial disorders impact tissues with high energetic demands and can be associated with cardiac muscle disease (cardiomyopathy) and early mortality. However, the mechanistic link between mitochondrial disease and the development of cardiomyopathy is frequently unclear. In addition, there is often marked phenotypic heterogeneity between patients, even between those with the same genetic variant, which is also not well understood. Several of the mitochondrial cardiomyopathies are related to defects in the maintenance of mitochondrial protein homeostasis, or proteostasis. This essential process involves the importing, sorting, folding and degradation of preproteins into fully functional mature structures inside mitochondria. Disrupted mitochondrial proteostasis interferes with mitochondrial energetics and ATP production, which can directly impact cardiac function. An inability to maintain proteostasis can result in mitochondrial dysfunction and subsequent mitophagy or even apoptosis. We review the known mitochondrial diseases that have been associated with cardiomyopathy and which arise from mutations in genes that are important for mitochondrial proteostasis. Genes discussed include DnaJ heat shock protein family member C19 (DNAJC19), mitochondrial import inner membrane translocase subunit TIM16 (MAGMAS), translocase of the inner mitochondrial membrane 50 (TIMM50), mitochondrial intermediate peptidase (MIPEP), X-prolyl-aminopeptidase 3 (XPNPEP3), HtraA serine peptidase 2 (HTRA2), caseinolytic mitochondrial peptidase chaperone subunit B (CLPB) and heat shock 60-kD protein 1 (HSPD1). The identification and description of disorders with a shared mechanism of disease may provide further insights into the disease process and assist with the identification of potential therapeutics.

Targeting mitochondria in dermatological therapy: Beyond oxidative damage and skin aging

INTRODUCTION

The analysis of the role of the mitochondria in oxidative damage and skin aging is a significant aspect of dermatological research. Mitochondria generate most reactive oxygen species (ROS); however, excessive ROS are cytotoxic and DNA-damaging and promote (photo-)aging. ROS also possesses key physiological and regulatory functions and mitochondrial dysfunction is prominent in several skin diseases including skin cancers. Although many standard dermatotherapeutics modulate mitochondrial function, dermatological therapy rarely targets the mitochondria. Accordingly, there is a rationale for "mitochondrial dermatology"-based approaches to be applied to therapeutic research.

AREAS COVERED

This paper examines the functions of mitochondria in cutaneous physiology beyond energy (ATP) and ROS production. Keratinocyte differentiation and epidermal barrier maintenance, appendage morphogenesis and homeostasis, photoaging and skin cancer are considered. Based on related PubMed search results, the paper evaluates thyroid hormones, glucocorticoids, Vitamin D3 derivatives, retinoids, cannabinoid receptor agonists, PPARγ agonists, thyrotropin, and thyrotropin-releasing hormone as instructive lead compounds. Moreover, the mitochondrial protein MPZL3 as a promising new drug target for future "mitochondrial dermatology" is highlighted.

EXPERT OPINION

Future dermatological therapeutic research should have a mitochondrial medicine emphasis. Focusing on selected lead agents, protein targets, in silico drug design, and model diseases will fertilize a mito-centric approach.

SHetA2 Attack on Mortalin and Colleagues in Cancer Therapy and Prevention

Heat Shock Proteins of the 70-kDa family (HSP70s) do not cause cancer by themselves, but instead protect cells as they transform into cancer. These molecular chaperones bind numerous client proteins and utilize ATP hydrolysis to facilitate proper protein folding, formation of functional complexes and cellular localizations, or degradation of irreparably damaged proteins. Their transient upregulation by stressful situations avoids induction of programmed cell death. Continued upregulation of the mortalin, heat shock cognate (hsc70) and glucose regulated protein 78 (Grp78) support cancer development and progression by supporting pro-proliferative and metabolic functions and repressing pro-death functions of oncoproteins and tumor suppressor proteins. This review describes the discovery and development of a lead anti-cancer compound, sulfur heteroarotinoid A2 (SHetA2, NSC726189), which was originally developed to bind retinoic acid receptors, but was subsequently found to work independently of these receptors. The discovery and validation of mortalin, hsc70 and Grp78 as SHetA2 target proteins is summarized. The documented and hypothesized roles of these HSP70 proteins and their clients in the mechanism of SHetA2 inhibition of cancer without toxicity are discussed. Use of this mechanistic data to evaluate drug action in a cancer clinical trial and develop synergistic drug combinations is explained. Knowledge needed to optimize SHetA2 analogs for use in cancer therapy and prevention is proposed as future directions.

Role of Mitochondrial Protein Import in Age-Related Neurodegenerative and Cardiovascular Diseases

Mitochondria play a critical role in providing energy, maintaining cellular metabolism, and regulating cell survival and death. To carry out these crucial functions, mitochondria employ more than 1500 proteins, distributed between two membranes and two aqueous compartments. An extensive network of dedicated proteins is engaged in importing and sorting these nuclear-encoded proteins into their designated mitochondrial compartments. Defects in this fundamental system are related to a variety of pathologies, particularly engaging the most energy-demanding tissues. In this review, we summarize the state-of-the-art knowledge about the mitochondrial protein import machinery and describe the known interrelation of its failure with age-related neurodegenerative and cardiovascular diseases.

With or without You: Co-Chaperones Mediate Health and Disease by Modifying Chaperone Function and Protein Triage

Heat shock proteins (HSPs) are a family of molecular chaperones that regulate essential protein refolding and triage decisions to maintain protein homeostasis. Numerous co-chaperone proteins directly interact and modify the function of HSPs, and these interactions impact the outcome of protein triage, impacting everything from structural proteins to cell signaling mediators. The chaperone/co-chaperone machinery protects against various stressors to ensure cellular function in the face of stress. However, coding mutations, expression changes, and post-translational modifications of the chaperone/co-chaperone machinery can alter the cellular stress response. Importantly, these dysfunctions appear to contribute to numerous human diseases. Therapeutic targeting of chaperones is an attractive but challenging approach due to the vast functions of HSPs, likely contributing to the off-target effects of these therapies. Current efforts focus on targeting co-chaperones to develop precise treatments for numerous diseases caused by defects in protein quality control. This review focuses on the recent developments regarding selected HSP70/HSP90 co-chaperones, with a concentration on cardioprotection, neuroprotection, cancer, and autoimmune diseases. We also discuss therapeutic approaches that highlight both the utility and challenges of targeting co-chaperones.