Mitochondrial Quality Control as a Therapeutic Target

Abstract

In addition to oxidative phosphorylation (OXPHOS), mitochondria perform other functions such as heme biosynthesis and oxygen sensing and mediate calcium homeostasis, cell growth, and cell death. They participate in cell communication and regulation of inflammation and are important considerations in aging, drug toxicity, and pathogenesis. The cell's capacity to maintain its mitochondria involves intramitochondrial processes, such as heme and protein turnover, and those involving entire organelles, such as fusion, fission, selective mitochondrial macroautophagy (mitophagy), and mitochondrial biogenesis. The integration of these processes exemplifies mitochondrial quality control (QC), which is also important in cellular disorders ranging from primary mitochondrial genetic diseases to those that involve mitochondria secondarily, such as neurodegenerative, cardiovascular, inflammatory, and metabolic syndromes. Consequently, mitochondrial biology represents a potentially useful, but relatively unexploited area of therapeutic innovation. In patients with genetic OXPHOS disorders, the largest group of inborn errors of metabolism, effective therapies, apart from symptomatic and nutritional measures, are largely lacking. Moreover, the genetic and biochemical heterogeneity of these states is remarkably similar to those of certain acquired diseases characterized by metabolic and oxidative stress and displaying wide variability. This biologic variability reflects cell-specific and repair processes that complicate rational pharmacological approaches to both primary and secondary mitochondrial disorders. However, emerging concepts of mitochondrial turnover and dynamics along with new mitochondrial disease models are providing opportunities to develop and evaluate mitochondrial QC-based therapies. The goals of such therapies extend beyond amelioration of energy insufficiency and tissue loss and entail cell repair, cell replacement, and the prevention of fibrosis. This review summarizes current concepts of mitochondria as disease elements and outlines novel strategies to address mitochondrial dysfunction through the stimulation of mitochondrial biogenesis and quality control.

Fig. 1.

Normal mitochondrial structure in the heart. (Left) Representative micrographs of section of the mouse left ventricle showing immunofluorescence staining for TCA cycle enzyme citrate synthase. The mitochondria appear as tiny green, bead-like structures (Alexa 488, green fluorescence). Nuclei are stained blue (DAPI; 4′,6-diamidino-2-phenylindole). Scale bar = 10 µm. (Middle) Isolated rat cardiomyocytes showing the intracellular mitochondrial network by confocal fluorescence microscopy stained with MitoTracker red. The cytoskeleton protein F-actin is green and the cell nucleus is blue (DAPI; 4′,6-diamidino-2-phenylindole). 600×. Scale bar = 0.4 µm. (Right) Electron micrograph of normal mouse heart mitochondria. Cardiomyocyte exhibits abundant electron-dense mitochondria (m) surrounded by smooth endoplasmic reticulum (SER). N = cell nucleus. Scale bar = 0.2 μm.

Fig. 2.

Schematic of the major steps of mtDNA replication and transcription. The double-stranded mtDNA molecule is replicated in a strand a synchronous manner and L-strand replication starts at its origin of replication (OL) after the nascent H-strand has completed about two-thirds of the circle proceeding in the opposite direction. H-strand replication is initiated using a short L-strand mRNA transcript synthesized by mitochondrial RNA polymerase (POLRMT). POLRMT accesses its DNA template by binding of mitochondrial transcription factor A, B1, and B2 (Tfam, TFB1M, and TFB2M) at the mtDNA duplex. Precursor RNA primer is cleaved by processing endonuclease (RNase) at the H-strand origin of replication (OH). Bases are added to free 3′-termini by mitochondrial DNA polymerase gamma (Polγ). Twinkle is required for preparation of single-stranded templates for Polγ activity. H-strand transcription is initiated from two promoter sites, HSP1 and HSP2. HSP1 transcripts are terminated at the termination sequence (TERM) within the tRNA-Leu (UUR) gene where MTERF1 binds. HSP2 generate near full-length polycistronic transcripts that are processed into individual RNAs. L-strand transcription is initiated from a single promoter site LSP, which generates near full-length polycistronic messages. The replication origins of both the H- and L-strands are indicated (OH and OL, respectively). CSB, conserved sequence block; HSP, heavy-strand promoter; LS, light-strand promoter; MTERF1, mitochondrial termination factor 1; MtSSB, mitochondrial single-stranded binding protein.

Fig. 3.

Mitochondrial biogenesis by the endogenous gases. Nitric oxide (NO) and carbon monoxide (CO) are generated enzymatically by the NOS (1-3) and HO (1-2) isoforms, respectively. H₂S is generated enzymatically from l-cysteine. The mitochondrion is a common organelle target of all three gases. Pharmaceutical development has leveraged these systems to design exogenous molecules to simulate the endogenous gases. Illustrated are ROS pathways for CO (and H₂S), activation of cGMP by NO (or CO), and phosphorylation of peroxisome proliferator-activated receptor gamma coactivator (PGC-1α) via PKA. ROS, mainly H₂O₂, can activate Akt, leading to NRF-1 phosphorylation and its nuclear translocation (CO, H₂S and their releasing molecules). NRF-1, NRF-2, and other nuclear transcription factors (NTFs) are coactivated by PGC-1α, leading to transcription of nuclear-encoded mitochondrial proteins (NEMPs), such as mitochondrial transcription factor A (Tfam), needed to activate mtDNA transcription and replication. NEMPs are imported into mitochondria through multiple outer (Tom)- and inner (Tim)-membrane transport machinery, and the proteins are folded and assembled. For the outer membrane, fission through the dynamin-related protein 1 (DRP1) and for the inner membrane OPA1 allow mitochondrial division, whereas mitofusins (Mfn) regulate fusion. Dynamic control of fusion/fission allows for mitochondrial network organization. Damaged mitochondria are tagged by Park and Pink for ubiquitination (Ub) and elimination by mitophagy; CORM, CO-releasing molecule; HO-1, heme oxygenase-1; NOS, NO synthase; OXPHOS: oxidative phosphorylation; PKA protein kinase A; ROS, reactive oxygen species.

Fig. 4.

Mechanisms of estrogen action on mitochondria. Binding and dimerization of estrogen receptors (ERs) by estrogen E2 trigger nuclear and mitochondrial genomic effects. These are mediated by nuclear translocation of E2-ER complex and either 1) direct binding with estrogen response elements (ERE) along with co-activators to form a transcription complex or 2) binding to transcriptional coactivators to induce gene transcription indirectly. ERs may also localize to mitochondria to induce potentially genomic and nongenomic actions, the mechanisms of which are not clearly understood.

Fig. 5.

Pharmacological induction of mitochondrial biogenesis. Schematic representation of pathways for exercise, calorie restriction, stress, or small molecules. During caloric excess, PGC-1α, a key coactivator of mitochondrial biogenesis is in an inactive acetylated (ac) state. In times of caloric restriction, or after exercise, energy becomes scarce, resulting in increases in the AMP/ATP and NAD+/NADH ratios. The former has a direct impact on the activation of AMPK as binding of AMP (or ADP) facilitates phosphorylation (p) of AMPK by upstream kinases. Activated AMPK phosphorylates PGC-1α directly, which may either activate this coactivator or prime PGC-1α for activation via deacetylation by SIRT1/SIRT3. SIRT1 requires NAD+ as the nucleotide is a cosubstrate. The deacetylated PGC-1α is able to coactivate nuclear mitochondrial gene transcription via transcription factors including NRF1/2, estrogen-related receptor (ERR) α,β,γ (ERRs), and PPARα,β,γ. This activates mtDNA transcription, translation, and replication (represented by mtDNA); production of OXPHOS subunits, tricarboxylic acid (TCA) cycle enzymes, enzymes of fatty acid oxidation; and leads to increased mitochondrial mass. Resveratrol is a SIRT1 activator by a mechanism that is not well-unresolved but ultimately leads to PGC-1α pathway activation via coactivator deacetylation. AICAR is phosphorylated to an AMP analog, ZMP, that activates AMPK by promoting phosphorylation by upstream kinases. AMPK causes PGC-1α phosphorylation and SIRT1 activation via increases in the NAD+/NADH ratio. Bezafibrate is a pan-PPAR agonist and induces PGC-1α expression via the PPAR-responsive element in its promoter. See section on Pharmacological Agents for details.

Fig. 6.

Regulation of mitochondrial quality control (QC) via mitochondrial biogenesis and mitophagy. Mitophagy, in conjunction with mitochondrial biogenesis, regulates changes in mitochondrial number required to meet metabolic demand. Activated AMPK triggers ULK1-dependent mitophagy and simultaneously triggers the biogenesis of new mitochondria via effects on PGC-1α-dependent transcription. Conversely, mTOR represses ULK1-dependent mitophagy when nutrients are plentiful. In addition, AMPK can be activated by ROS-mediated opening of calcium release-activated calcium (CRAC) channels, leading to cytosolic calcium flux that activate the AMPK upstream kinase CaMKKβ. These processes controlled by AMPK and mTOR determine the net effect of replacing defective mitochondria with new functional mitochondria. AMPK: AMP-activated protein kinase; mTOR: mammalian target of rapamycin; PGC-1α: PPARγ co-activator 1-alpha; ULK1: the mammalian Atg1 homologs, uncoordinated family member (unc)-51: like kinase 1; ERK2: the extracellular signal-regulated protein kinase 2.