Abnormal Mitochondrial Dynamics-Early Event in Neurodegenerative Disease?

5 February 2010. Mitochondria take the stage in three recent papers—two that flesh out how several Parkinson disease-related proteins regulate degradation of these organelles, and one that finds loss of synaptic mitochondria as an early phenotype in a new Drosophila model for Alzheimer disease. Reporting in the January issue of PLoS Biology, researchers led by Richard Youle, National Institutes of Health, Bethesda, Maryland, describe how Pink1 and parkin work together to help cells deal with malfunctioning mitochondria. Another study—led by Wolfdieter Springer, University of Tübingen, Germany, and published online January 24 in Nature Cell Biology—unravels downstream events of the same pathway, showing how Pink1 and parkin link mitochondrial dysfunction with ubiquitination and eventual autophagy. And last but not least, a research team led by Fu-De Huang, Chinese Academy of Sciences, Shanghai, reports in the January 27 Journal of Neuroscience that Aβ-overexpressing flies lose presynaptic mitochondria before developing obvious problems with synaptic transmission. Using different model systems, the new studies strengthen the case that mitochondrial dysfunction contributes to neurodegenerative disease pathogenesis.

Previous studies have fingered mitochondria as possibly contributing to PD (see ARF related news story), and mitochondrial dynamics are thought to be disrupted by mutations in Pink1 (a mitochondrial kinase) or parkin (an E3 ubiquitin ligase). Loss-of-function mutations in either can cause PD, and the two proteins interact genetically (Clark et al., 2006), with Pink1 acting upstream of parkin (Exner et al., 2007; Park et al., 2006). Furthermore, parkin translocates to damaged mitochondria, triggering their autophagy (aka mitophagy) (Narendra et al., 2008).

As reported in PLoS Biology, Youle, first author Derek Narendra, and colleagues did a series of biochemistry experiments that show how Pink1 helps parkin sense flagging mitochondria in the first place. In their model—derived from experiments using a host of cultured cell types, both neuronal and non-neuronal—cells keep Pink1 at low levels in healthy mitochondria but selectively stabilize the protein on distressed mitochondria. The extra Pink1 signals to parkin, which tags the mitochondria for destruction by autophagy. The researchers went on to show that pathogenic mutations in Pink1 or parkin disrupt this pathway, suggesting that PD may be caused by a failure to clear damaged mitochondria, leading to neuronal death.

In the Nature Cell Biology study, Springer, first author Sven Giesler, and colleagues examined the same Pink1/parkin pathway but focused further downstream—namely, on how parkin’s recruitment to stressed mitochondria promotes mitophagy. The researchers treated neuronal and non-neuronal cultured cells with a compound (CCCP) that induces mitochondrial depolarization, and used immunostaining to follow Pink1 and parkin in the context of ensuing mitochondrial demise. They found that mitophagy requires functional parkin, as cells transfected with pathogenic parkin mutants failed to clear damaged mitochondria. Blocking Pink1 with short interfering RNA and then counteracting the knockdown by transfecting in wild-type protein, the researchers showed that Pink1 is required to recruit parkin to dysfunctional mitochondria. In studies with various Pink1 mutants, they further determined that Pink1 needs proper kinase activity and mitochondrial targeting sequences to direct itself and parkin to damaged mitochondria. Once there, parkin promotes formation of two poly-ubiquitin chains. The researchers identified voltage-dependent anion channel 1 (VDAC1) as a potential target for this ubiquitination, and showed that downstream steps leading from ubiquitination to mitophagy require the adaptor protein p62/SQSTM1. Like Youle and colleagues, Springer’s team shows that PD-associated parkin mutations stymie specific steps within the cascade linking mitochondrial damage, ubiquitination, and autophagy.

"The authors did a marvelous job in providing biochemical and cell biological evidence linking the Pink1/parkin pathway to mitochondrial autophagy," wrote Bingwei Lu of Stanford University, Palo Alto, California, in an e-mail to ARF. "What remains to be determined, though, is the in vivo relevance of the findings to dopaminergic neuron maintenance and survival." (See full comment below.) Toward this end, the German researchers have begun studying neurons from parkin knockout mice and neuronal cells derived from reprogrammed fibroblasts of PD patients, Springer told ARF.

David Park of the University of Ottawa, Canada, agrees that the relevance of the current findings to PD pathogenesis remains uncertain. Nevertheless, "It’s an exciting paper," he said. "I think there will be more stories coming out in support of this theme." Park suspects that parkin-mediated mitophagy could turn out to be a general mechanism of mitochondrial quality control with potential implications for normal aging and for other neurodegenerative diseases besides PD.

Abnormal mitochondrial dynamics have also been linked to AD (Wang et al., 2009 and ARF related news story), and the Journal of Neuroscience paper by Huang’s team supports this. First author Xiao-Liang Zhao and colleagues created a new AD fly model by expressing mutant human Aβ in the giant fiber neurons that control flight, and tracking axonal and synaptic changes in the flies using electrophysiology and microscopy studies.

Besides decreased mobility and faster demise, the Aβ-expressing flies developed a host of abnormalities, most notably loss of presynaptic mitochondria, which preceded appearance of synaptic transmission problems. "It’s striking that mitochondrial depletion from axons and presynaptic terminals was the earliest detected phenotype. That’s what I tried to find in in vivo models," said Xiongwei Zhu of Case Western Reserve University in Cleveland, Ohio, who was not involved in this work. In a recent paper (Wang et al., 2008), Zhu and colleagues overexpressed amyloid precursor protein (APP) in human neuroblastoma cells and found changes in mitochondrial morphology and distribution that contributed to neuronal dysfunction.

Interestingly, the Chinese researchers found that while the Aβ-overexpressing flies had lower numbers of axonal mitochondria, those fewer mitochondria were larger than normal. The authors took this as a hint that Aβ may have induced mitochondrial fission defects in the flies. However, other scientists caution that this is pure speculation. "It could simply be caused by damage of mitochondria by Aβ and subsequent removal of abnormal mitochondria," Lu wrote. Zhu agrees, noting that "without detailed characterization of the changes in mitochondrial fission/fusion machinery, it is probably premature at this stage to suggest impaired mitochondrial fission in this model because swelled mitochondria have been found in cells with mitochondrial fusion defects and during cell death (Chen et al., 2007). Zhu pointed out that Pink1 and parkin mutations have had conflicting effects on mitochondrial fission—impairing it in fly models, yet enhancing it in mammalian cells (see full comment below). In any case, Zhu notes, the current findings confirm a previous study of Aβ42-overexpressing flies (Iijima-Ando et al., 2009) and support the idea that Aβ-induced mitochondrial distribution abnormalities can lead to synaptic deficits in vivo.—Esther Landhuis.

References:
Narendra DP, Jin SM, Tanaka A, Suen DF, Gautier CA, Shen J, Cookson MR, Youle RJ. PINK1 is Selectively Stabilized on Impaired Mitochondria to Activate Parkin. PLoS Biol. 2010 Jan 26;8(1):e1000298. Abstract

Geisler S, Holmstroem KM, Skujat D, Fiesel FC, Rothfuss OC, Kahle PJ, Springer W. PINK1/Parkin-mediated mitophagy is dependent on VDAC1 and p62/SQSTM1. Nature Cell Biology. 2010 Jan 24. Abstract

Zhao XL, Wang WA, Tan JX, Huang JK, Zhang X, Zhang BZ, Wang YH, YangCheng HY, Zhu HL, Sun XJ, Huang FD. Expression of beta-amyloid induced age-dependent presynaptic and axonal changes in Drosophila. J Neurosci. 2010 Jan 27;30(4):1512-1522. Abstract


To view commentaries, primary articles and linked stories, go to the original posting on Alzforum.org here.

ALS Forum Note: For an ALS twist on this story, we refer you to a Feb 2010 publication in the Journal of Neurological Science entitled: Induction of parkinsonism-related proteins in the spinal motor neurons of transgenic mice carrying a mutant SOD1 gene

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