Parkin, which tags other proteins with ubiquitin to mark them for destruction, has been linked to sporadic and some familial forms of Parkinson's disease (PD). In the August 25 Nature Neuroscience, scientists led by Ted and Valina Dawson, Johns Hopkins University School of Medicine, Baltimore, Maryland, report that parkin deficiency leads to a toxic buildup of a substrate that goes by the mouthful aminoacyl-tRNA synthetase complex interacting multifunctional protein-2. AIMP2 sets in motion a cell death cascade that specifically targets dopamine cells in transgenic mice, the authors found. Since AIMP2 and some of its downstream partners also appear at higher levels in postmortem tissue from patients with Parkinson's, this pathway could serve as a therapeutic target for the disease, the authors suggested.
AIMP2 stabilizes the aminoacyl-tRNA synthetase complex in the cytosol. It would ordinarily be ubiquitinated by parkin and then degraded in the proteasome. In PD, it instead turns up in Lewy bodies, protein inclusions that mark this disease (see Corti et al., 2003). The Dawson group previously reported that AIMP2 accumulates in animal models of PD and in post-mortem tissue from patients (see Ko et al., 2005). In the current study, they wanted to determine if too much AIMP2 would be toxic to dopamine cells, potentially explaining why parkin deficiency is so deadly.
First author Yunjong Lee and colleagues overexpressed human AIMP2 in mice and found that as the animals aged, they lost dopaminergic neurons and became physically uncoordinated. In 20-month-old mice, overexpression of AIMP2 killed more dopamine neurons in the substantia nigra zona compacta than in the ventral tegmental area, even though expression levels were similar, mirroring the pattern of cell death in PD.
To work out the mechanism of AIMP2 cell death, the researchers overexpressed the protein in SH-SY5Y neuroblastoma cells. According to subcellular fractionation experiments, a small portion of AIMP2 translocated from the cytosol to the nucleus, which happens when a cell is under stress. There, it directly activated poly(ADP-ribose) polymerase-1 (PARP1), a protein that senses damage to DNA and forms a poly(ADP-ribose) (PAR) polymer. PARP1 and PAR activate a type of cell death pathway called parthanatos (see diagram). In this pathway, the PAR polymer makes its way from the nucleus to mitochondria, where it binds apoptosis-inducing factor (AIF), a well-known instigator of that death pathway (see Wang et al., 2011).
To test whether parthanatos accounts for AIMP2-induced dopaminergic loss in vivo, the researchers knocked out or inhibited PARP1 in the AIMP2-overexpressing mice. Doing so protected the neurons, suggesting that AIMP2 does at least some of its damage through the cell death pathway. The authors acknowledged that it might wreak havoc through other mechanisms, as well.
Overexpression experiments are known to have led to artifacts in the past. To get a sense whether these findings might be relevant to human disease, the researchers compared levels of these proteins in diseased and control postmortem brains. AIMP2 was three-fold more abundant in dopaminergic cells from PD patients, while PAR was up 10-fold in the substantia nigra but not the cortex when compared to healthy controls. These results suggest that PARP1 inhibition could be a therapeutic target for PD, wrote the authors.
"The idea that a toxic substrate of Parkin, AIMP2, mediates activation of PARP-1 is novel and intriguing for genetic PD and probably sporadic PD," Imam, University of Texas Health Science Center, San Antonio, wrote to Alzforum in an email. "However, this pathway might be just part of a complex picture of progressive damage, and [inhibitors] might bring a minimal change." He noted that deactivating PARP1 could compromise DNA repair, and possibly increase the risk for cancer. Ted Dawson countered that other DNA repair mechanisms would likely compensate for a lack of PARP1, noting that PARP1 knockout mice live a normal lifespan. PARP1 inhibitors are currently being developed as breast, ovarian, and prostate cancer treatments, and some cross the blood brain barrier (for a review, see Rouleau et al., 2010).
Why does AIMP2 specifically kill dopamine neurons? Because these cells are particularly sensitive to oxidative stress (see ARF related news story), they are more likely to redirect AIMP2 from the cytosol to the nucleus to activate PARP1 and kick off parthanatos, Ted Dawson told Alzforum. Whether this happens in human dopaminergic cells is unclear.
Parkin and the mitochondrial kinase PINK1, whose gene is also linked to parkinsonism, are both known to regulate mitochondrial quality control through the process of mitophagy (see ARF related news story). Which is more important when parkin function is lost to the cell—mitochondrial abnormalities or parthanatos? Dawson could not say, but he pointed out that much of the mitochondrial data comes from flies and tumor cell lines, and research yet needs to show that mitochondrial abnormalities occur in the dopaminergic neurons of mice and humans. Dawson speculated that if they do, then parthanatos could act downstream of mitochondrial problems. Parkin dysfunction might dysregulate mitochondria and the cells’ response to oxidative stress, which would move AIMP2 toward the nucleus and set off the death cascade.—Gwyneth Dickey Zakaib