PINK1 and Parkin flag Miro to direct mitochondrial traffic.

The Parkinson's disease proteins PINK1 and Parkin are proposed guardians of mitochondrial fidelity, targeting damaged mitochondria for degradation by mitophagy. In this issue of Cell, Wang et al. (2011) now show that PINK1 and Parkin also regulate mitochondrial trafficking and quarantine damaged mitochondria by severing their connection to the microtubule network.

SARM1 is responsible for calpain-dependent dendrite degeneration in mouse hippocampal neurons.

Sterile alpha and toll/interleukin receptor motif-containing 1 (SARM1) is a critical regulator of axon degeneration that acts through hydrolysis of NAD+ following injury. Recent work has defined the mechanisms underlying SARM1's catalytic activity and advanced our understanding of SARM1 function in axons, yet the role of SARM1 signaling in other compartments of neurons is still not well understood. Here, we show in cultured hippocampal neurons that endogenous SARM1 is present in axons, dendrites, and cell bodies and that direct activation of SARM1 by the neurotoxin Vacor causes not just axon degeneration, but degeneration of all neuronal compartments. In contrast to the axon degeneration pathway defined in dorsal root ganglia, SARM1-dependent hippocampal axon degeneration in vitro is not sensitive to inhibition of calpain proteases. Dendrite degeneration downstream of SARM1 in hippocampal neurons is dependent on calpain 2, a calpain protease isotype enriched in dendrites in this cell type. In summary, these data indicate SARM1 plays a critical role in neurodegeneration outside of axons and elucidates divergent pathways leading to degeneration in hippocampal axons and dendrites.

The ubiquitin kinase PINK1 recruits autophagy receptors to induce mitophagy.

Protein aggregates and damaged organelles are tagged with ubiquitin chains to trigger selective autophagy. To initiate mitophagy, the ubiquitin kinase PINK1 phosphorylates ubiquitin to activate the ubiquitin ligase parkin, which builds ubiquitin chains on mitochondrial outer membrane proteins, where they act to recruit autophagy receptors. Using genome editing to knockout five autophagy receptors in HeLa cells, here we show that two receptors previously linked to xenophagy, NDP52 and optineurin, are the primary receptors for PINK1- and parkin-mediated mitophagy. PINK1 recruits NDP52 and optineurin, but not p62, to mitochondria to activate mitophagy directly, independently of parkin. Once recruited to mitochondria, NDP52 and optineurin recruit the autophagy factors ULK1, DFCP1 and WIPI1 to focal spots proximal to mitochondria, revealing a function for these autophagy receptors upstream of LC3. This supports a new model in which PINK1-generated phospho-ubiquitin serves as the autophagy signal on mitochondria, and parkin then acts to amplify this signal. This work also suggests direct and broader roles for ubiquitin phosphorylation in other autophagy pathways.

High-content genome-wide RNAi screens identify regulators of parkin upstream of mitophagy.

An increasing body of evidence points to mitochondrial dysfunction as a contributor to the molecular pathogenesis of neurodegenerative diseases such as Parkinson's disease. Recent studies of the Parkinson's disease associated genes PINK1 (ref. 2) and parkin (PARK2, ref. 3) indicate that they may act in a quality control pathway preventing the accumulation of dysfunctional mitochondria. Here we elucidate regulators that have an impact on parkin translocation to damaged mitochondria with genome-wide small interfering RNA (siRNA) screens coupled to high-content microscopy. Screening yielded gene candidates involved in diverse cellular processes that were subsequently validated in low-throughput assays. This led to characterization of TOMM7 as essential for stabilizing PINK1 on the outer mitochondrial membrane following mitochondrial damage. We also discovered that HSPA1L (HSP70 family member) and BAG4 have mutually opposing roles in the regulation of parkin translocation. The screens revealed that SIAH3, found to localize to mitochondria, inhibits PINK1 accumulation after mitochondrial insult, reducing parkin translocation. Overall, our screens provide a rich resource to understand mitochondrial quality control.

Polyubiquitin-sensor proteins reveal localization and linkage-type dependence of cellular ubiquitin signaling.

Polyubiquitin chain topology is thought to direct modified substrates to specific fates, but this function-topology relationship is poorly understood, as are the dynamics and subcellular locations of specific polyubiquitin signals. Experimental access to these questions has been limited because linkage-specific inhibitors and in vivo sensors have been unavailable. Here we present a general strategy to track linkage-specific polyubiquitin signals in yeast and mammalian cells, and to probe their functions. We designed several high-affinity Lys63 polyubiquitin-binding proteins and demonstrate their specificity in vitro and in cells. We apply these tools as competitive inhibitors to dissect the polyubiquitin-linkage dependence of NF-κB activation in several cell types, inferring the essential role of Lys63 polyubiquitin for signaling via the IL-1ß and TNF-related weak inducer of apoptosis (TWEAK) but not TNF-α receptors. We anticipate live-cell imaging, proteomic and biochemical applications for these tools and extension of the design strategy to other polymeric ubiquitin-like protein modifications.

Discovery of DNL343: A Potent, Selective, and Brain-Penetrant eIF2B Activator Designed for the Treatment of Neurodegenerative Diseases.

Eukaryotic translation initiation factor 2B (eIF2B) is a key component of the integrated stress response (ISR), which regulates protein synthesis and stress granule formation in response to cellular insult. Modulation of the ISR has been proposed as a therapeutic strategy for treatment of neurodegenerative diseases such as vanishing white matter (VWM) disease and amyotrophic lateral sclerosis (ALS) based on its ability to improve cellular homeostasis and prevent neuronal degeneration. Herein, we report the small-molecule discovery campaign that identified potent, selective, and CNS-penetrant eIF2B activators using both structure- and ligand-based drug design. These discovery efforts culminated in the identification of DNL343, which demonstrated a desirable preclinical drug profile, including a long half-life and high oral bioavailability across preclinical species. DNL343 was progressed into clinical studies and is currently undergoing evaluation in late-stage clinical trials for ALS.

Highly divergent mitochondrial ATP synthase complexes in Tetrahymena thermophila.

The F-type ATP synthase complex is a rotary nano-motor driven by proton motive force to synthesize ATP. Its F(1) sector catalyzes ATP synthesis, whereas the F(o) sector conducts the protons and provides a stator for the rotary action of the complex. Components of both F(1) and F(o) sectors are highly conserved across prokaryotes and eukaryotes. Therefore, it was a surprise that genes encoding the a and b subunits as well as other components of the F(o) sector were undetectable in the sequenced genomes of a variety of apicomplexan parasites. While the parasitic existence of these organisms could explain the apparent incomplete nature of ATP synthase in Apicomplexa, genes for these essential components were absent even in Tetrahymena thermophila, a free-living ciliate belonging to a sister clade of Apicomplexa, which demonstrates robust oxidative phosphorylation. This observation raises the possibility that the entire clade of Alveolata may have invented novel means to operate ATP synthase complexes. To assess this remarkable possibility, we have carried out an investigation of the ATP synthase from T. thermophila. Blue native polyacrylamide gel electrophoresis (BN-PAGE) revealed the ATP synthase to be present as a large complex. Structural study based on single particle electron microscopy analysis suggested the complex to be a dimer with several unique structures including an unusually large domain on the intermembrane side of the ATP synthase and novel domains flanking the c subunit rings. The two monomers were in a parallel configuration rather than the angled configuration previously observed in other organisms. Proteomic analyses of well-resolved ATP synthase complexes from 2-D BN/BN-PAGE identified orthologs of seven canonical ATP synthase subunits, and at least 13 novel proteins that constitute subunits apparently limited to the ciliate lineage. A mitochondrially encoded protein, Ymf66, with predicted eight transmembrane domains could be a substitute for the subunit a of the F(o) sector. The absence of genes encoding orthologs of the novel subunits even in apicomplexans suggests that the Tetrahymena ATP synthase, despite core similarities, is a unique enzyme exhibiting dramatic differences compared to the conventional complexes found in metazoan, fungal, and plant mitochondria, as well as in prokaryotes. These findings have significant implications for the origins and evolution of a central player in bioenergetics.

Site-mapping of in vitro S-nitrosation in cardiac mitochondria: implications for cardioprotection.

S-nitrosation (SNO) of mitochondrial protein cysteines can be cardioprotective. Several targets have been implicated, yet the scope and identification of specific residues has not been fully assessed. To address this, a comprehensive assessment of mitochondrial SNO-modifiable cysteines was performed to determine nitric oxide (NO) susceptible pathways and identify novel mechanisms of oxidative cardioprotection. The biotin switch assay and mass spectrometry were used on rat cardiac mitochondrial lysates treated with the nitric oxide donor, S-nitrosoglutathione, and controls (n=3) to map 83 SNO-modified cysteine residues on 60 proteins. Of these, three sites have been reported, 30 sites are new to 21 proteins previously known to be S-nitrosated but which lacked site-specific information and 50 sites were found on 39 proteins not previously implicated in SNO pathways. The SNO-modifications occurred in only a subset of available cysteines, indicating a specific targeted effect. Functional annotation and site-specificity analysis revealed a twofold greater nitric oxide-susceptibility for proteins involved in transport; including regulators of mitochondrial permeability transition suggesting SNO-regulation and a possible protective mechanism. Additionally, we identified many novel SNO-modified proteins with cardioprotective potential involved in the electron transport chain, tricarboxylic acid cycle, oxidative stress defense, fatty acid and amino acid metabolism. These findings suggest that SNO-modification may represent a novel mechanism for the regulation of oxidative phosphorylation and/or cell death. S-nitrosation of mitochondrial permeability transition-associated proteins represents an intriguing potential link to cardioprotection.

Type I keratin 17 protein is phosphorylated on serine 44 by p90 ribosomal protein S6 kinase 1 (RSK1) in a growth- and stress-dependent fashion.

Keratin 17 (K17) is a type I intermediate filament protein that is constitutively expressed in ectoderm-derived epithelial appendages and robustly induced in epidermis following injury, during inflammation, and in chronic diseases such as psoriasis and cancer. Mutations within K17 are responsible for two rare diseases related to ectodermal dysplasias. Studies in K17-null mice uncovered several roles for K17, including structural support, resistance to TNFα-induced apoptosis, regulation of protein synthesis, and modulation of cytokine expression. Yet, little is known about the regulation of K17 protein via post-translational modification. Here, we report that serine 44 in the N-terminal head domain of K17 (K17-Ser(44)) is phosphorylated in response to extracellular stimuli (serum, EGF, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate) that alter skin keratinocyte growth, and to cellular stresses (sorbitol-induced hyperosmotic shock, UV irradiation, and hydrogen peroxide-induced oxidative stress). It also occurs in basaloid skin tumors in situ. Upon its stimulation in skin keratinocytes, K17-Ser(44) phosphorylation is induced rapidly but stays on transiently. The majority of the phosphorylated K17-Ser(44) pool is polymer-bound and is not obviously related to a change in filament organization. The amino acid sequence surrounding K17-Ser(44) matches the consensus for the AGC family of basophilic kinases. We show that p90 RSK1, an AGC kinase involved in the regulation of cell survival and proliferation, phosphorylates K17-Ser(44) in skin keratinocytes. These findings confirm and expand the tight link that has emerged between K17 up-regulation and growth and stress responses in the skin epithelium.

Phosphorylation of the F(1)F(o) ATP synthase beta subunit: functional and structural consequences assessed in a model system.

RATIONALE: We previously discovered several phosphorylations to the beta subunit of the mitochondrial F(1)F(o) ATP synthase complex in isolated rabbit myocytes on adenosine treatment, an agent that induces cardioprotection. The role of these phosphorylations is unknown. OBJECTIVE: The present study focuses on the functional consequences of phosphorylation of the ATP synthase complex beta subunit by generating nonphosphorylatable and phosphomimetic analogs in a model system, Saccharomyces cerevisiae. METHODS AND RESULTS: The 4 amino acid residues with homology in yeast (T58, S213, T262, and T318) were studied with respect to growth, complex and supercomplex formation, and enzymatic activity (ATPase rate). The most striking mutant was the T262 site, for which the phosphomimetic (T262E) abolished activity, whereas the nonphosphorylatable strain (T262A) had an ATPase rate equivalent to wild type. Although T262E, like all of the beta subunit mutants, was able to form the intact complex (F(1)F(o)), this strain lacked a free F(1) component found in wild-type and had a corresponding increase of lower-molecular-weight forms of the protein, indicating an assembly/stability defect. In addition, the ATPase activity was reduced but not abolished with the phosphomimetic mutation at T58, a site that altered the formation/maintenance of dimers of the F(1)F(o) ATP synthase complex. CONCLUSIONS: Taken together, these data show that pseudophosphorylation of specific amino acid residues can have separate and distinctive effects on the F(1)F(o) ATP synthase complex, suggesting the possibility that several of the phosphorylations observed in the rabbit heart can have structural and functional consequences to the F(1)F(o) ATP synthase complex.

Discovery of Potent and Selective Dual Leucine Zipper Kinase/Leucine Zipper-Bearing Kinase Inhibitors with Neuroprotective Properties in In Vitro and In Vivo Models of Amyotrophic Lateral Sclerosis.

Dual leucine zipper kinase (DLK) and leucine zipper-bearing kinase (LZK) are regulators of neuronal degeneration and axon growth. Therefore, there is a considerable interest in developing DLK/LZK inhibitors for neurodegenerative diseases. Herein, we use ligand- and structure-based drug design approaches for identifying novel amino-pyrazine inhibitors of DLK/LZK. DN-1289 (14), a potent and selective dual DLK/LZK inhibitor, demonstrated excellent in vivo plasma half-life across species and is anticipated to freely penetrate the central nervous system with no brain impairment based on in vivo rodent pharmacokinetic studies and human in vitro transporter data. Proximal target engagement and disease relevant pathway biomarkers were also favorably regulated in an in vivo model of amyotrophic lateral sclerosis.

Post-translational modifications of ATP synthase in the heart: biology and function.

The ATP synthase complex is a critical enzyme in the energetic pathways of cells because it is the enzyme complex that produces the majority of cellular ATP. It has been shown to be involved in several cardiac phenotypes including heart failure and preconditioning, a cellular protective mechanism. Understanding the regulation of this enzyme is important in understanding the mechanisms behind these important phenomena. Recently there have been several post-translational modifications (PTM) reported for various subunits of this enzyme complex, opening up the possibility of differential regulation by these PTMs. Here we discuss the known PTMs in the heart and other mammalian tissues and their implication to function and regulation of the ATP synthase.

Proteomic analysis of pharmacological preconditioning: novel protein targets converge to mitochondrial metabolism pathways.

Ischemic preconditioning is characterized by resistance to ischemia reperfusion injury in response to previous short ischemic episodes, a protective effect that can be mimicked pharmacologically. The underlying mechanism of protection remains controversial and requires greater understanding before it can be fully exploited therapeutically. To investigate the overall effect of preconditioning on the myocardial proteome, isolated rabbit ventricular myocytes were treated with drugs known to induce preconditioning, adenosine or diazoxide (each at 100 micromol/L for 60 minutes). Their protein profiles were then compared with vehicle-treated controls (n=4 animals per treatment) using a multitiered 2D gel electrophoresis approach. Of 28 significantly altered protein spots, 19 nonredundant proteins were identified (5 spots remained unidentified). The majority of these proteins are involved in mitochondrial energetics, including subunits of tricarboxylic acid cycle enzymes and oxidative phosphorylation complexes. These changes were not indiscriminate, with only a small number of enzymes or complex subunits altered, indicating a very specific and targeted affect of these 2 preconditioning mimetics. Among the changes were shifts in the extent of posttranslational modification of 4 proteins. One of these, the adenosine-induced phosphorylation of the ATP synthase beta subunit, was fully characterized with the identification of 5 novel phosphorylation sites. This proteomics approach provides an overall assessment of the cellular response to pharmacological treatment with adenosine and diazoxide and identifies a distinct subset of enzymes and protein complex subunit that may underlie the preconditioned phenotype.

Subfractionation of heart tissue: the "in sequence" myofilament protein extraction of myocardial tissue.

Proteomic analysis of heart tissue is complicated by the large dynamic range of its proteins. The most abundant proteins are the myofilament proteins, which comprise the contractile apparatus. This chapter describes a protocol for fractionation of heart tissue that extracts the myofilament proteins into a separate sample fraction, allowing analysis of lower-abundance proteins. Importantly, this is performed in a manner that is compatible with two-dimensional electrophoresis and high-performance liquid chromatography, two of main technologies of proteomics. The method produces three fractions based on solubility at different pHs: (1) cytoplasmic-enriched extract (neutral pH), (2) myofilament-enriched extract (acidic pH), and (3) membrane protein-enriched pellet. Fractionation of heart tissue in this manner provides the basis for in-depth proteomic analysis.

PINK1 phosphorylates ubiquitin to activate Parkin E3 ubiquitin ligase activity.

PINK1 kinase activates the E3 ubiquitin ligase Parkin to induce selective autophagy of damaged mitochondria. However, it has been unclear how PINK1 activates and recruits Parkin to mitochondria. Although PINK1 phosphorylates Parkin, other PINK1 substrates appear to activate Parkin, as the mutation of all serine and threonine residues conserved between Drosophila and human, including Parkin S65, did not wholly impair Parkin translocation to mitochondria. Using mass spectrometry, we discovered that endogenous PINK1 phosphorylated ubiquitin at serine 65, homologous to the site phosphorylated by PINK1 in Parkin's ubiquitin-like domain. Recombinant TcPINK1 directly phosphorylated ubiquitin and phospho-ubiquitin activated Parkin E3 ubiquitin ligase activity in cell-free assays. In cells, the phosphomimetic ubiquitin mutant S65D bound and activated Parkin. Furthermore, expression of ubiquitin S65A, a mutant that cannot be phosphorylated by PINK1, inhibited Parkin translocation to damaged mitochondria. These results explain a feed-forward mechanism of PINK1-mediated initiation of Parkin E3 ligase activity.

Role of PINK1 binding to the TOM complex and alternate intracellular membranes in recruitment and activation of the E3 ligase Parkin.

Mutations in the mitochondrial kinase PINK1 and the cytosolic E3 ligase Parkin can cause Parkinson's disease. Damaged mitochondria accumulate PINK1 on the outer membrane where, dependent on kinase activity, it recruits and activates Parkin to induce mitophagy, potentially maintaining organelle fidelity. How PINK1 recruits Parkin is unknown. We show that endogenous PINK1 forms a 700 kDa complex with the translocase of the outer membrane (TOM) selectively on depolarized mitochondria whereas PINK1 ectopically targeted to the outer membrane retains association with TOM on polarized mitochondria. Inducibly targeting PINK1 to peroxisomes or lysosomes, which lack a TOM complex, recruits Parkin and activates ubiquitin ligase activity on the respective organelles. Once there, Parkin induces organelle selective autophagy of peroxisomes but not lysosomes. We propose that the association of PINK1 with the TOM complex allows rapid reimport of PINK1 to rescue repolarized mitochondria from mitophagy, and discount mitochondrial-specific factors for Parkin translocation and activation.

Mitochondrial fission and fusion and their roles in the heart.

Mitochondria are dynamic organelles that usually exist in extensive and interconnected networks that undergo constant remodeling through fission and fusion. These processes are governed by distinct sets of proteins whose mechanism and regulation we are only beginning to fully understand. Early studies on mitochondrial dynamics were performed in yeast and simple mammalian cell culture models that allowed easy visualization of these intricate networks. Equipped with this core understanding, the field is now expanding into more complex systems. Cardiac cells are a particularly interesting example because they have unique energetic and spatial demands that make the study of their mitochondria both challenging and potentially very fruitful. This review will provide an overview of mitochondrial fission and fusion as well as recent developments in the understanding of these processes in the heart.

Mitochondrial membrane potential regulates PINK1 import and proteolytic destabilization by PARL.

PINK1 is a mitochondrial kinase mutated in some familial cases of Parkinson's disease. It has been found to work in the same pathway as the E3 ligase Parkin in the maintenance of flight muscles and dopaminergic neurons in Drosophila melanogaster and to recruit cytosolic Parkin to mitochondria to mediate mitophagy in mammalian cells. Although PINK1 has a predicted mitochondrial import sequence, its cellular and submitochondrial localization remains unclear in part because it is rapidly degraded. In this study, we report that the mitochondrial inner membrane rhomboid protease presenilin-associated rhomboid-like protein (PARL) mediates cleavage of PINK1 dependent on mitochondrial membrane potential. In the absence of PARL, the constitutive degradation of PINK1 is inhibited, stabilizing a 60-kD form inside mitochondria. When mitochondrial membrane potential is dissipated, PINK1 accumulates as a 63-kD full-length form on the outer mitochondrial membrane, where it can recruit Parkin to impaired mitochondria. Thus, differential localization to the inner and outer mitochondrial membranes appears to regulate PINK1 stability and function.

p62/SQSTM1 is required for Parkin-induced mitochondrial clustering but not mitophagy; VDAC1 is dispensable for both.

Mitochondria sustain damage with aging, and the resulting mitochondrial dysfunction has been implicated in a number of diseases including Parkinson disease. We recently demonstrated that the E3 ubiquitin ligase Parkin, which is linked to recessive forms of parkinsonism, causes a dramatic increase in mitophagy and a change in mitochondrial distribution, following its translocation from the cytosol to mitochondria. Investigating how Parkin induces these changes may offer insight into the mechanisms that lead to the sequestration and elimination of damaged mitochondria. We report that following Parkin's translocation from the cytosol to mitochondria, Parkin (but not a pathogenic mutant) promotes the K63-linked polyubiquitination of mitochondrial substrate(s) and recruits the ubiquitin- and LC3-binding protein, p62/SQSTM1, to mitochondria. After its recruitment, p62/SQSTM1 mediates the aggregation of dysfunctional mitochondria through polymerization via its PB1 domain, in a manner analogous to its aggregation of polyubiquitinated proteins. Surprisingly and in contrast to what has been recently reported for ubiquitin-induced pexophagy and xenophagy, p62 appears to be dispensable for mitophagy. Similarly, mitochondrial-anchored ubiquitin is sufficient to recruit p62 and promote mitochondrial clustering, but does not promote mitophagy. Although VDAC1 (but not VDAC2) is ubiquitinated following mitochondrial depolarization, we find VDAC1 cannot fully account for the mitochondrial K63-linked ubiquitin immunoreactivity observed following depolarization, as it is also observed in VDAC1/3-/- mouse embryonic fibroblasts. Additionally, we find VDAC1 and VDAC3 are dispensable for the recruitment of p62, mitochondrial clustering and mitophagy. These results demonstrate that mitochondria are aggregated by p62, following its recruitment by Parkin in a VDAC1-independent manner. They also suggest that proteins other than p62 are likely required for mitophagy downstream of Parkin substrates other than VDAC1.

Modulation of mitochondrial proteome and improved mitochondrial function by biventricular pacing of dyssynchronous failing hearts.

BACKGROUND: Cardiac resynchronization therapy (CRT) improves chamber mechanoenergetics and morbidity and mortality of patients manifesting heart failure with ventricular dyssynchrony; however, little is known about the molecular changes underlying CRT benefits. We hypothesized that mitochondria may play an important role because of their involvement in energy production. METHODS AND RESULTS: Mitochondria isolated from the left ventricle in a canine model of dyssynchronous or resynchronized (CRT) heart failure were analyzed by a classical, gel-based, proteomic approach. Two-dimensional gel electrophoresis revealed that 31 mitochondrial proteins where changed when controlling the false discovery rate at 30%. Key enzymes in anaplerotic pathways, such as pyruvate carboxylation and branched-chain amino acid oxidation, were increased. These concerted changes, along with others, suggested that CRT may increase the pool of Krebs cycle intermediates and fuel oxidative phosphorylation. Nearly 50% of observed changes pertained to subunits of the respiratory chain. ATP synthase-beta subunit of complex V was less degraded, and its phosphorylation modulated by CRT was associated with increased formation (2-fold, P=0.004) and specific activity (+20%, P=0.05) of the mature complex. The importance of these modifications was supported by coordinated changes in mitochondrial chaperones and proteases. CRT increased the mitochondrial respiratory control index with tightened coupling when isolated mitochondria were reexposed to substrates for both complex I (glutamate and malate) and complex II (succinate), an effect likely related to ATP synthase subunit modifications and complex quantity and activity. CONCLUSIONS: CRT potently affects both the mitochondrial proteome and the performance associated with improved cardiac function.