19 March 2013. A failure to dispose of underperforming mitochondria may underlie the complex degenerative disorder known as multisystem proteinopathy. Mutations in valosin-containing protein (VCP) cause this syndrome, which affects bone, muscle, and nerves. Two papers in the March 14 Neuron online point to defects in mitochondria—at the level of ATP production and mitophagy of damaged organelles—as problems occurring in cells with faulty VCP. “We think that because cells produce less ATP, they cannot cope with energy-demanding processes,” concluded Helen Plun-Favreau of the University College London Institute of Neurology in the U.K., one of the senior authors. That metabolic vulnerability, plus a second hit from the environment or genes, could predispose energy-hungry cells like neurons to the degeneration that occurs in MSP-related conditions, including frontotemporal dementia, amyotrophic lateral sclerosis, and parkinsonism.
In one paper, Plun-Favreau and co-senior author Andrey Abramov, also at the Institute of Neurology, report that loss of VCP function interferes with ATP synthesis. First author Fernando Bartolome and colleagues used RNA silencing to knock down VCP expression in a variety of cells in culture, including the SH-SY5Y human neuroblastoma line, mouse primary cortical neurons, and primary astrocytes. In addition, they examined fibroblasts taken from three people with MSP-linked VCP mutations, and from three control donors.
Bartolome and colleagues found that the mitochondrial membrane potential was reduced in all VCP-deficient or mutant lines. That signifies weakening of the protein gradient that drives the synthesis of ATP, and the cells produced much less of this energy-rich molecule than they should. Despite this ATP deficiency, the cells gobbled up more oxygen than usual, indicating a hyperactive respiratory chain. Together, these results indicated that VCP dysfunction uncouples ATP production from mitochondrial respiration. “All the oxygen is consumed for nothing, and the ATP is not produced properly,” Plun-Favreau said.
The researchers do not know how VCP mutations uncouple the respiratory chain, or how this defect results in the myriad symptoms of MSP. However, Plun-Favreau noted that malfunctioning mitochondria like these ought to be on track for mitophagy, or cellular digestion of mitochondria. “You do not want these damaged mitochondria to accumulate in the cells, ” she said. The second Neuron paper reports that they do build up, and explains why. VCP participates in the mitophagy pathway, report the authors at St. Jude Children’s Research Hospital in Memphis, Tennessee. “Presumably, the defect we identified in VCP function accounts for the mitochondrial defect [Plun-Favreau and colleagues] document,” wrote senior author Paul Taylor in an e-mail to Alzforum.
Joint first authors Nam Chul Kim and Emilie Tresse and colleagues expressed MSP-linked VCP mutations in the muscles of Drosophila, which caused degeneration and drooping wings. This phenotype reminded Taylor of flies missing parkin and PINK1, regulators of mitochondrial quality control that cause parkinsonism when mutated (see ARF related news story on Greene et al., 2003; ARF news story on Poole et al., 2008). Given the VCP fly phenotype and the fact that MSP sometimes presents as parkinsonism or PD (Kimonis et al., 2008; Spina et al., 2012), he hypothesized that VCP, PINK1, and parkin might work in the same biochemical pathway.
Parkin and PINK1 promote mitophagy (see ARF related news story on Geisler et al., 2010). Because parkin ubiquitinates mitochondrial proteins, and VCP typically binds ubiquitinated protein complexes, Taylor wondered if the parkin might help recruit VCP to damaged mitochondria. To test this, the researchers treated HeLa human cervical cancer cultures with a mitochondrial toxin. Parkin attached itself to the organelles within about 20 minutes. VCP joined 15 minutes later, but not in cells expressing a ubiquitin ligase-defective parkin mutant. Kim and colleagues concluded that VCP relies on ubiquitination of mitochondrial proteins, by parkin, to reach damaged mitochondria.
What might those ubiquitinated proteins be? There are probably many, Taylor said, but they identified mitofusin, an outer mitochondrial membrane GTPase, as one. As the name suggests, it joins mitochondria together. Parkin ubiquitinates mitofusin, and the researchers discovered the GTPase bound VCP as well. Wild-type VCP was required for proper degradation of ubiquitinated mitofusin, leading to mitochondrial fragmentation and mitophagy.
How do these defects add up to MSP? Cells constantly manage their mitochondria, clearing away rotten ones, Taylor said. Faulty VCP function, he suggested, would lead to the accumulation of damaged mitochondria—just as Plun-Favreau and colleagues observed. Taylor plans to examine mitochondrial function in mouse and human tissues, while Plun-Favreau hopes to investigate mitophagy in fibroblasts from VCP carriers.
“VCP-mediated disease would thus be, at least partially, the result of a traffic jam of dysfunctional mitochondria,” Luc Dupuis of the University of Strasbourg in France, who was not part of either study team, wrote to Alzforum (see full comment below). “The two papers complement each other and provide convincing evidence that VCP mutations alter mitochondrial functions through dysfunctional recycling of abnormal mitochondria.” Many other papers have linked neurodegeneration and mitochondrial abnormalities (e.g., see ARF related news story on Duboff et al., 2012; ARF related news story on Lim et al., 2012, and Han et al., 2012; and ARF related news story on Yao et al., 2011). However, both Dupuis and Taylor cautioned that mitochondrial problems need not be central to VCP-mediated pathology.
VCP mutations alone may not cause degeneration. Even in a person with MSP, many tissues remain unaffected. Instead, researchers suggested, the failure to make sufficient ATP would make VCP-mutant cells vulnerable to an additional insult that could create the disease state. Cell types that binge on energy—as do nerves and muscle—might be most susceptible. Plun-Favreau suggested that a second hit, such as ischemia, could push the mutant cells beyond their capacity to maintain adequate ATP supplies. That second hit might be environmental, such as pesticide exposure, suggested Flint Beal, of Weill Medical College of Cornell University in New York City, in an e-mail to Alzforum (see full comment below). Or it could be genetic, said Taylor.
Taylor speculated that second-hit genetic variants might explain why MSP affects different organ systems in different people. One mutation might sensitize muscle, for example, another—nerves. “We suspect that the conspiracy between two different gene variants adds up to cause the phenotype,” he said. “VCP touches so many different systems that it could be a second partner to a lot of different diseases.” A multiple-gene theory has already been proposed for ALS (see ARF related news story). Taylor speculated that antioxidant treatment might set the mitochondria right and alleviate symptoms.—Amber Dance.
Bartolome F, Wu HC, Burchell VS, Preza E, Wray S, Mahoney CJ, Fox NC, Calvo A, Canosa A, Moglia C, Mandrioli J, Chiò A, Orrell RW, Houlden H, Hardy J, Abramov AY, Plun-Favreau H. Pathogenic VCP mutations induce mitochondrial uncoupling and reduced ATP levels. Neuron. 14 Mar 2013. Abstract
Kim NC, Tresse E, Kolaitis RM, Molliex A, Thomas RE, Alami NH, Wang B, Joshi A, Smith RB, Ritson GP, Winborn BJ, Moore J, Lee JY, Yao TP, Pallanck L, Kundu M, Taylor JP. VCP is essential for mitochondrial quality control by PINK1/Parkin and this function is impaired by VCP mutations. Neuron. 14 Mar 2013. Abstract