Growing evidence points to mitochondrial quality control as a key feature of familial Parkinson’s disease (PD) and related disorders. An August 11 Nature Neuroscience paper bolsters that notion. Researchers led by Helene Plun-Favreau at University College London, UK, connect the parkinsonism-linked gene F-box only protein 7 (Fbxo7) to mitophagy—a system cells use to dispose of damaged mitochondria. The mitochondrial kinase Pink1 and parkin, a ubiquitin ligase, drive mitophagy, and genetic mutations in each cause early onset PD. The new research shows that Fbxo7, another ubiquitin ligase, binds parkin and helps direct it to mitochondria. This triggers mitophagy to remove dysfunctional mitochondria in neurons, Plun-Favreau said. Co-corresponding authors Heike Laman at the University of Cambridge, and Alexander Whitworth at the University of Sheffield collaborated on the study. While the findings strengthen the case for mitophagy regulators as potential therapeutic targets for familial PD, it remains unclear to what extent this process relates to the more common sporadic forms of PD.

In 2008, scientists found mutations in Fbxo7, a.k.a. PARK 15, in people with a rare form of recessive parkinsonism called parkinsonian-pyramidal disease (see Shojaee et al., 2008; Di Fonzo et al., 2009; Paisán-Ruiz et al., 2010). Their symptoms resemble those of PD patients with parkin and Pink1 mutations, but strike earlier and include spasticity due to white matter lesions (see Deng et al., 2013).

Noticing a putative mitochondrial targeting sequence at Fbxo7’s N-terminal end, co-lead author Victoria Burchell and colleagues, including co-first authors David Nelson and Alvaro Sanchez-Martinez, began exploring a mitochondrial role for the protein by testing if it interacts with parkin (a.k.a. PARK 2). The researchers overexpressed tagged versions of parkin and Fbxo7 in several human cell lines and detected each in immunoprecipitates of the other, concluding that the two proteins interact. They found that Fbxo7’s N-terminal ubiquitin-like region is necessary and sufficient for parkin binding. Both proteins reside primarily in the cytosol but appeared to associate with each other more in mitochondria, as judged by Western blotting of immunoprecipitates from cellular subfractions.

Fbxo7 also binds Pink1 (PARK 6), the scientists found. However, that association required a small stretch of Fbxo7 just downstream of the N-terminal ubiquitin-like domain needed for parkin binding, “raising the possibility that the interactions of Fbxo7, parkin, and Pink1 may be mutually exclusive or form ternary complexes,” the authors wrote.

How might mutations in these three proteins lead to PD? It is well established that parkin and Pink1 drive mitophagy, with the latter functioning upstream. Pink1 accumulates quickly on mitochondria damaged by CCCP, a compound that disrupts the organelle’s electrochemical gradient (see Narendra et al., 2010). The damage somehow nudges Pink1 onto the impaired mitochondria where it recruits parkin (see Narendra et al., 2008; Vives-Bauza et al., 2010). The current study proposes that Fbxo7 facilitates this translocation.

Compared to normal cells, Fbxo7 glommed onto parkin in mitochondria of SH-SY5Y human neuroblastoma cells that were treated with CCCP, suggesting Fbxo7 and parkin interact more readily in mitochondria on the wane. Conversely, silencing Fbxo7 kept parkin out of the damaged mitochondria. Fbxo7 knockdown also reduced ubiquitination of mitofusin 1, a GTPase on the outer mitochondrial membrane that scientists identified as a parkin substrate earlier this year (see ARF news story on Kim et al., 2013). Fbxo7 overexpression restored mitofusin ubiquitination. Patient fibroblasts homozygous for a pathogenic Fbxo7 mutation had similar abnormalities as Fbxo7-deficient neuroblastoma cells.

Work on Drosophila indicates these interactions are functionally important. Overexpression of Fbxo7 corrected motor deficiencies in Parkin-deficient flies, whereas Fbxo7 with pathogenic mutations (T22M, R378G or R498X), or lacking the mitochondrial targeting sequence, failed. It is curious that Fbxo7 rescued flies with no parkin, noted Mark Cookson of the National Institute on Aging in Bethesda, Maryland. This suggests the rescue could not result from physical interaction between Fbxo7 and parkin, wrote Cookson (see full comment below). The authors propose Fbxo7 may interact with another ubiquitin ligase.

The present findings add Fbxo7 to a growing list of parkinsonism-associated genes involved in mitochondrial dynamics and confirms the importance of mitophagy in early onset PD-like disorders. However, Taylor noted, “the big question is whether patients with these rare familial syndromes reflect what’s happening in the more common sporadic forms of disease.” Juvenile forms make up less than a tenth of all PD and parkinsonism cases, and parkin mutations account for about 40 percent of the early onset patients. Mutations in Pink1 cause 10 percent of early onset cases, and Fbxo7 mutations are rarer still.

Plun-Favreau said her team plans to analyze Fbxo7 and the other mitophagy players in neurons differentiated from fibroblasts of patients with PD/parkinsonism mutations. She also hopes to collaborate with industry to find compounds that enhance parkin relocation to unhealthy mitochondria or otherwise drive mitophagy.—Esther Landhuis

Comments

Make a Comment

To make a comment you must login or register.

Comments on News and Primary Papers

  1. The key thing these authors show in this paper, and this is why it is an important study, is that Fbxo7 mutations interact with an established pathway for recessive parkinsonism. The authors clearly show that the mutations are loss of function, which is probably predicted based on the genetics, but is very helpful to demonstrate. This is shown in patient samples, which is helpful, and I think it confirms that mitophagy is likely important in recessive forms of parkinsonism.

    There are some things that still need to be resolved. As the authors themselves state, the rescue of parkin null phenotypes by over expression of Fbxo7 cannot be due to physical interaction between Fbxo7 and parkin, and they suggest another (unidentified) E3 ligase must be involved. I think it's also intriguing that Fbxo7 doesn't rescue loss of PINK1. These are fine grained mechanistic details, and don’t detract from the current paper, but I am intrigued to know the answer. This is of particular interest in relation to Drosophila, which to my best knowledge, has no endogenous Fbxo7 homologue. So how Fbxo7 rescues those animals is very interesting.

    View all comments by Mark Cookson

References

News Citations

  1. Protein Destroying Muscle, Bone, Nerves Parks on Mitochondria

Paper Citations

  1. . Genome-wide linkage analysis of a Parkinsonian-pyramidal syndrome pedigree by 500 K SNP arrays. Am J Hum Genet. 2008 Jun;82(6):1375-84. PubMed.
  2. . FBXO7 mutations cause autosomal recessive, early-onset parkinsonian-pyramidal syndrome. Neurology. 2009 Jan 20;72(3):240-5. PubMed.
  3. . Early-onset L-dopa-responsive parkinsonism with pyramidal signs due to ATP13A2, PLA2G6, FBXO7 and spatacsin mutations. Mov Disord. 2010 Sep 15;25(12):1791-800. PubMed.
  4. . VCP is essential for mitochondrial quality control by PINK1/Parkin and this function is impaired by VCP mutations. Neuron. 2013 Apr 10;78(1):65-80. PubMed.
  5. . PINK1 is selectively stabilized on impaired mitochondria to activate Parkin. PLoS Biol. 2010 Jan;8(1):e1000298. PubMed.
  6. . Parkin is recruited selectively to impaired mitochondria and promotes their autophagy. J Cell Biol. 2008 Dec 1;183(5):795-803. PubMed.
  7. . PINK1-dependent recruitment of Parkin to mitochondria in mitophagy. Proc Natl Acad Sci U S A. 2010 Jan 5;107(1):378-83. PubMed.

Further Reading

Papers

  1. . Drosophila pink1 is required for mitochondrial function and interacts genetically with parkin. Nature. 2006 Jun 29;441(7097):1162-6. PubMed.
  2. . F-box only protein 7 gene in parkinsonian-pyramidal disease. JAMA Neurol. 2013 Jan;70(1):20-4. PubMed.
  3. . Mitochondrial dysfunction in Drosophila PINK1 mutants is complemented by parkin. Nature. 2006 Jun 29;441(7097):1157-61. PubMed.
  4. . VCP is essential for mitochondrial quality control by PINK1/Parkin and this function is impaired by VCP mutations. Neuron. 2013 Apr 10;78(1):65-80. PubMed.

Primary Papers

  1. . The Parkinson's disease-linked proteins Fbxo7 and Parkin interact to mediate mitophagy. Nat Neurosci. 2013 Sep;16(9):1257-65. PubMed.