Mitochondrial Morphology as Hallmark in PINK1/Parkin-induced Pathology
This excellent paper by Leo Pallanck and colleagues genetically manifests the interplay between PINK1 or parkin deficiency and changes in mitochondrial morphology. The authors were able to rescue PINK1 or parkin loss-of-function defects by genetic modifications that promote mitochondrial fission. Supporting these data, in Drosophila S2 cells the authors observed a fused mitochondrial phenotype after reduction of PINK1 or parkin expression by RNAi.

Recent data from human HeLa cells showed that after RNAi-mediated knockdown of PINK1, a reduction in cristae density and in mitochondrial membrane potential was accompanied by changes in morphology that similarly occur in primary cells from patients with PINK1-associated PD after shift to low glucose conditions (1). These observations, as well as the fact that the phenotype could be rescued by increased expression of parkin, are very consistent with previous studies from Drosophila that reported fragmented cristae, reduced ATP content,...  Read more

Mitochondrial Morphology as Hallmark in PINK1/Parkin-induced Pathology
This excellent paper by Leo Pallanck and colleagues genetically manifests the interplay between PINK1 or parkin deficiency and changes in mitochondrial morphology. The authors were able to rescue PINK1 or parkin loss-of-function defects by genetic modifications that promote mitochondrial fission. Supporting these data, in Drosophila S2 cells the authors observed a fused mitochondrial phenotype after reduction of PINK1 or parkin expression by RNAi.

Recent data from human HeLa cells showed that after RNAi-mediated knockdown of PINK1, a reduction in cristae density and in mitochondrial membrane potential was accompanied by changes in morphology that similarly occur in primary cells from patients with PINK1-associated PD after shift to low glucose conditions (1). These observations, as well as the fact that the phenotype could be rescued by increased expression of parkin, are very consistent with previous studies from Drosophila that reported fragmented cristae, reduced ATP content, and altered mitochondrial morphology in PINK1-deficient flies (2-4). These parallels point out the enormous potential of genetic studies in Drosophila to discover fundamental mechanisms that are relevant for human diseases.

The human data diverge in one detail from what was here elucidated in the flies. In Drosophila, everything points toward a fission-promoting role of PINK1 and parkin, while in human cells truncation and fragmentation of mitochondria was described after downregulation of PINK1. In our view, this difference allows the hypothesis that the components involved in regulation of mitochondrial dynamics might not be direct targets of a PINK1/parkin signaling pathway, as this should lead to exact congruence of the PINK1-deficient mitochondrial morphology phenotypes. We rather suggest a stress-response mechanism that is induced by consequences of PINK1 deficiency. This may be mimicked in Drosophila by genetic modifications that favor mitochondrial fission.

Fission of defective mitochondria is a protective mechanism that allows sequestration of damaged mitochondria and their elimination by mitophagy (5). The mechanism by which parkin exerts its rescuing activity during PINK1 loss-of-function was already suggested to be based on its neuroprotective potential. Recently, it was demonstrated that parkin can act through activation of NF-κB signaling to protect the cell against excitotoxicity and complex I inhibition (6). The antiapoptotic protein Bcl-2 as well was able to rescue PINK1 deficiency in Drosophila (3), which might also be mediated by a stress response mechanism. Bcl-2 is not restricted to mitochondria but also localizes to the ER, where it is involved in Ca2+ dynamics or in regulation of autophagy (7). Although by now several proteins are known to revert PINK1 phenotypes, the mechanisms by which they act could be based on general stress-protective attributes. Future studies, preferably in mammalian systems, are required to reveal the connection between PINK1 and its co-players.

References:
1. Exner N, Treske B, Paquet D, Holmström K, Schiesling C, Gispert S, Carballo-Carbajal I, Berg D, Hoepken HH, Gasser T, Krüger R, Winklhofer KF, Vogel F, Reichert AS, Auburger G, Kahle PJ, Schmid B, Haass C. Loss-of-function of human PINK1 results in mitochondrial pathology and can be rescued by parkin. J Neurosci. 2007 Nov 7;27(45):12413-8. Abstract

2. Clark IE, Dodson MW, Jiang C, Cao JH, Huh JR, Seol JH, Yoo SJ, Hay BA, Guo M. Drosophila pink1 is required for mitochondrial function and interacts genetically with parkin. Nature. 2006 Jun 29;441(7097):1162-6. Abstract

3. Park J, Lee SB, Lee S, Kim Y, Song S, Kim S, Bae E, Kim J, Shong M, Kim JM, Chung J. Mitochondrial dysfunction in Drosophila PINK1 mutants is complemented by parkin. Nature. 2006 Jun 29;441(7097):1157-61. Abstract

4. Yang Y, Gehrke S, Imai Y, Huang Z, Ouyang Y, Wang JW, Yang L, Beal MF, Vogel H, Lu B. Mitochondrial pathology and muscle and dopaminergic neuron degeneration caused by inactivation of Drosophila Pink1 is rescued by Parkin. Proc Natl Acad Sci U S A. 2006 Jul 11;103(28):10793-8. Abstract

5. Barsoum MJ, Yuan H, Gerencser AA, Liot G, Kushnareva Y, Gräber S, Kovacs I, Lee WD, Waggoner J, Cui J, White AD, Bossy B, Martinou JC, Youle RJ, Lipton SA, Ellisman MH, Perkins GA, Bossy-Wetzel E. Nitric oxide-induced mitochondrial fission is regulated by dynamin-related GTPases in neurons. EMBO J. 2006 Aug 23;25(16):3900-11. Abstract

6. Henn IH, Bouman L, Schlehe JS, Schlierf A, Schramm JE, Wegener E, Nakaso K, Culmsee C, Berninger B, Krappmann D, Tatzelt J, Winklhofer KF. Parkin mediates neuroprotection through activation of IkappaB kinase/nuclear factor-kappaB signaling. J Neurosci. 2007 Feb 21;27(8):1868-78. Abstract

7. Hetz C, Glimcher L. The daily job of night killers: alternative roles of the BCL-2 family in organelle physiology. Trends Cell Biol. 2008 Jan 1;18(1):38-44. Abstract

View all comments by Nicole Exner
View all comments by Christian Haass

This paper represents an important extension of our previously published work demonstrating that PINK1 and Parkin interact genetically with components of the mitochondrial morphogenesis machinery. The main findings of Yang et al. are essentially what we reported previously (1), namely that PINK1 acts genetically upstream of the mitochondrial fission promoting component Drp1. But unlike our previous work, which was confined primarily to indirect flight muscle, the current paper demonstrates that this pathway is also conserved in a vertebrate cell line and is relevant to dopaminergic neurons, the cell type that degenerates in Parkinson disease. Thus, two of the three major phenotypes of Drosophila PINK1 and parkin mutants, flight muscle degeneration and dopamine neuron dysfunction, appear to derive from defective mitochondrial fission. The third major phenotype of PINK1 and parkin mutants, a failure to form mature sperm cells, has not yet been shown to derive from an alteration in mitochondrial dynamics, but previous work strongly suggests that a defect in mitochondrial...  Read more

This paper represents an important extension of our previously published work demonstrating that PINK1 and Parkin interact genetically with components of the mitochondrial morphogenesis machinery. The main findings of Yang et al. are essentially what we reported previously (1), namely that PINK1 acts genetically upstream of the mitochondrial fission promoting component Drp1. But unlike our previous work, which was confined primarily to indirect flight muscle, the current paper demonstrates that this pathway is also conserved in a vertebrate cell line and is relevant to dopaminergic neurons, the cell type that degenerates in Parkinson disease. Thus, two of the three major phenotypes of Drosophila PINK1 and parkin mutants, flight muscle degeneration and dopamine neuron dysfunction, appear to derive from defective mitochondrial fission. The third major phenotype of PINK1 and parkin mutants, a failure to form mature sperm cells, has not yet been shown to derive from an alteration in mitochondrial dynamics, but previous work strongly suggests that a defect in mitochondrial dynamics underlies this phenotype (2). Indeed, unpublished work in the laboratory of Dr. Ming Guo (personal communication) indicates that the germline phenotype of PINK1 mutants is also influenced by alterations in mitochondrial dynamics factors in a fashion that is consistent with what has been reported by our group and by Yang et al.

In addition to advancing our previous finding that Parkin and PINK1 promote mitochondrial fission, the studies of Yang et al. fill in several gaps that were not thoroughly examined in our work, and also raise several new questions. For example, we found that overexpression of either Drosophila or human PINK1 resulted in a mild rough eye phenotype. Although we showed that this phenotype could be modified by altering the dosage of mitochondrial fission and fusion-promoting factors, we did not explore the underlying cell biology responsible for the PINK1 overexpression phenotype. Yang et al. show that overexpression of PINK1 in dopaminergic neurons results in mitochondrial clustering, suggesting that the eye phenotypes we reported also derive from an underlying alteration in mitochondrial morphology. However, it is unclear whether these mitochondrial clusters represent aggregates of small mitochondria, or a single mitochondrial entity. Moreover, in contrast to our previous work, Yang et al. were unable to influence the PINK1 overexpression phenotype by altering the dosage of Drp1 or Opa1. Perhaps more surprisingly, Yang et al. show that while Parkin overexpression also leads to the formation of mitochondrial clusters, this phenotype requires PINK1 activity—a genetic argument that Parkin acts upstream of PINK1, in contrast to previous work in Drosophila demonstrating that PINK1 acts upstream of Parkin. There are several plausible explanations for these discordant findings and further work should resolve these matters.

While the finding that PINK1 and Parkin interact genetically with the mitochondrial morphogenesis machinery represents an advance, some important questions remain unanswered from this work. Clearly, the two most important questions concern the mechanism by which the PINK1/Parkin pathway promotes mitochondrial fission and the effects of decreased mitochondrial fission on tissue integrity. Yang et al. argue that the localization of PINK1 to the inner mitochondrial membrane constrains the possible range of substrates of this factor, but several recent reports indicate that a substantial fraction of PINK1 also localizes to the cytoplasm (3-6). This raises the possibility that the PINK1 substrates may not be mitochondrial proteins. Moreover, Parkin has been reported to localize to both the cytoplasm and mitochondria (7,8), so the Parkin substrates may also reside in either of these compartments. Yang et al. also suggest that the neurodegeneration accompanying mutations in PINK1 may result from defective synaptic function owing to the previously reported role of mitochondrial fission in the proper distribution of mitochondria in neurons. While this is a reasonable model, it is also important to note that alterations in mitochondrial dynamics can potentially influence many features of mitochondrial biology, including the rates of ATP synthesis and reactive oxygen species production and the turnover of mitochondria through autophagy (9). An alteration in any one of these processes could profoundly influence tissue viability. Resolving these questions will be an important challenge for our understanding of Parkinson disease, as well as our general understanding of the cell biological roles of mitochondrial dynamics.

References:
1. Poole AC, Thomas RE, Andrews LA, McBride HM, Whitworth AJ, Pallanck LJ. The PINK1/Parkin pathway regulates mitochondrial morphology. Proc Natl Acad Sci U S A. 2008 Feb 5;105(5):1638-43. Abstract

2. Riparbelli MG, Callaini G. The Drosophila parkin homologue is required for normal mitochondrial dynamics during spermiogenesis. Dev Biol. 2007 Mar 1;303(1):108-20. Abstract

3. Haque ME, Thomas KJ, D'Souza C, Callaghan S, Kitada T, Slack RS, Fraser P, Cookson MR, Tandon A, Park DS. Cytoplasmic Pink1 activity protects neurons from dopaminergic neurotoxin MPTP. Proc Natl Acad Sci U S A. 2008 Feb 5;105(5):1716-21. Abstract

4. Lin W, Kang UJ. Characterization of PINK1 processing, stability, and subcellular localization. J Neurochem. 2008 Apr 5; Abstract

5. Takatori S, Ito G, Iwatsubo T. Cytoplasmic localization and proteasomal degradation of N-terminally cleaved form of PINK1. Neurosci Lett. 2008 Jan 3;430(1):13-7. Abstract

6. Weihofen A, Ostaszewski B, Minami Y, Selkoe DJ. Pink1 Parkinson mutations, the Cdc37/Hsp90 chaperones and Parkin all influence the maturation or subcellular distribution of Pink1. Hum Mol Genet. 2008 Feb 15;17(4):602-16. Abstract

7. Darios F, Corti O, Lücking CB, Hampe C, Muriel MP, Abbas N, Gu WJ, Hirsch EC, Rooney T, Ruberg M, Brice A. Parkin prevents mitochondrial swelling and cytochrome c release in mitochondria-dependent cell death. Hum Mol Genet. 2003 Mar 1;12(5):517-26. Abstract

8. Kuroda Y, Mitsui T, Kunishige M, Shono M, Akaike M, Azuma H, Matsumoto T. Parkin enhances mitochondrial biogenesis in proliferating cells. Hum Mol Genet. 2006 Mar 15;15(6):883-95. Abstract

9. Berman SB, Pineda FJ, Hardwick JM. Mitochondrial fission and fusion dynamics: the long and short of it. Cell Death Differ. 2008 Apr 25; Abstract

View all comments by Leo Pallanck
View all comments by Alex Whitworth

We thank Dr. Zhu for citing our work (Iijima-Ando et al., 2009) in his comment. In our Aβ42 fly brain neurons, mitochondria were reduced in axons and dendrites, and accumulated in the somata without severe mitochondrial damage or neurodegeneration. At this stage, organization of microtubules and distribution of synaptic vesicle markers were not significantly altered, suggesting that mitochondrial mislocalization occurs without global axonal transport defects.

By knocking down milton, an adaptor protein that links mitochondria and kinesin, we showed that reduction in mitochondria transport exacerbated Aβ42-induced behavioral defects. Furthermore, milton knockdown by itself caused neuronal dysfunction at a later stage. Our results indicate that Aβ42-induced mitochondrial mislocalization contributes to Aβ42-induced neuronal dysfunction in vivo.

References:
Iijima-Ando K, Hearn SA, Shenton C, Gatt A, Zhao L, Iijima K. Mitochondrial mislocalization underlies Abeta42-induced neuronal dysfunction in a Drosophila model of Alzheimer's disease. PLoS One. 2009;4(12):e8310. Abstract

View all comments by Koichi Iijima
View all comments by Kanae Iijima-Ando

For mitochondrial effects in AD please see our Pharmacogenomics Journal article published in 2009. localizing a variable polyT mutation in the translocase of the outer mitochondrial membrane as a diagnostic predictor of risk for AD.

View all comments by Allen Roses

We thank all the commentators on our paper, "Expression of beta amyloid induced age-dependent presynaptic and axonal changes in Drosophila."

Our examination, through genetic manipulation, of the role of critical mitochondria fission and fusion genes in the mitochondrial abnormalities induced by Aβ expression will be finished soon.

View all comments by Fu-De Huang