A missense mutation (L166P) in DJ-1, linked to familial Parkinson's disease, confers reduced protein stability and impairs homo-oligomerization

The identification of genetic mutations responsible for rare familial forms of Parkinson's disease (PD) have provided tremendous insight into the molecular pathogenesis of this disorder. Mutations in the DJ-1 gene cause autosomal recessive early onset PD in two European families. A Dutch kindred displays a large homozygous genomic deletion encompassing exons 1-5 of the DJ-1 gene, whereas an Italian kindred harbors a single homozygous L166P missense mutation. A homozygous M26I missense mutation was also recently reported in an Ashkenazi Jewish patient with early onset PD. Mutations in DJ-1 are predicted to be loss of function. The recent determination of the crystal structure of human DJ-1 demonstrates that it exists in a homo-dimeric form in vitro, whereas the L166P mutant exists only as a monomer. Here, we examine the in vivo effects of the pathogenic L166P and M26I mutations on the properties of DJ-1 in cell culture. We report that the L166P mutation confers markedly reduced protein stability to DJ-1, which results from enhanced degradation by the 20S/26S proteasome but not from a loss of mRNA expression. Furthermore, the L166P mutant protein exhibits an impaired ability to self-interact to form homo-oligomers. In contrast, the M26I mutation does not appear to adversely affect either protein stability, turnover by the proteasome, or the capacity of DJ-1 to form homo-oligomers. These properties of the L166P mutation may contribute to the loss of normal DJ-1 function and are likely to be the underlying cause of early onset PD in affected members of the Italian kindred.     

Familial Parkinson's disease-associated L166P mutation disrupts DJ-1 protein folding and function

Mutations in DJ-1, a protein of unknown function, were recently identified as the cause for an autosomal recessive, early onset form of familial Parkinson's disease. Here we report that DJ-1 is a dimeric protein that exhibits protease activity but no chaperone activity. The protease activity was abolished by mutation of Cys-106 to Ala, suggesting that DJ-1 functions as a cysteine protease. Our studies revealed that the Parkinson's disease-linked L166P mutation impaired the intrinsic folding propensity of DJ-1 protein, resulting in a spontaneously unfolded structure that was incapable of forming a homodimer with itself or a heterodimer with wild-type DJ-1. Correlating with the disruption of DJ-1 structure, the L166P mutation abolished the catalytic function of DJ-1. Furthermore, as a result of protein misfolding, the L166P mutant DJ-1 was selectively polyubiquitinated and rapidly degraded by the proteasome. Together these findings provide insights into the molecular mechanism by which loss-of-function mutations in DJ-1 lead to Parkinson's disease.     

BAG1 restores formation of functional DJ-1 L166P dimers and DJ-1 chaperone activity

Mutations in the gene coding for DJ-1 protein lead to early-onset recessive forms of Parkinson’s disease. It is believed that loss of DJ-1 function is causative for disease, although the function of DJ-1 still remains a matter of controversy. We show that DJ-1 is localized in the cytosol and is associated with membranes and organelles in the form of homodimers. The disease-related mutation L166P shifts its subcellular distribution to the nucleus and decreases its ability to dimerize, impairing cell survival. Using an intracellular foldase biosensor, we found that wild-type DJ-1 possesses chaperone activity, which is abolished by the L166P mutation. We observed that this aberrant phenotype can be reversed by the expression of the cochaperone BAG1 (Bcl-2–associated athanogene 1), restoring DJ-1 subcellular distribution, dimer formation, and chaperone activity and ameliorating cell survival.     

Molecular basis for the structural instability of human DJ-1 induced by the L166P mutation associated with Parkinson's disease

DJ-1 is a dimeric protein of unknown function in vivo. A mutation in the human DJ-1 gene causing substitution of proline for leucine at residue 166 (L166P) has been linked to early onset Parkinson's disease. Lack of structural stability has precluded experimental determination of atomic-resolution structures of the L166P DJ-1 polymorph. We have performed multiple molecular dynamics (MD) simulations ( approximately 1/3 mus) of the wild-type and L166P DJ-1 polymorph at physiological temperature to predict specific structural effects of the L166P substitution. L166P disrupted helices alpha1, alpha5, alpha6 and alpha8 with alpha8 undergoing particularly severe disruption. Secondary structural elements critical for protein stability and dimerization were significantly disrupted across the entire dimer interface, as were extended hydrophobic surfaces involved in dimer formation. Relative to wild-type DJ-1, L166P DJ-1 populated a broader ensemble of structures, many of which corresponded to distorted conformations. In a L166P dimer model the substitution significantly destabilized the dimer interface, interrupting >100 intermolecular contacts that are important for dimer formation. The L166P substitution also led to major perturbations in the region of a highly conserved cysteine residue (Cys-106) that participates in dimerization and that is critical for a proposed chaperone function of DJ-1. Cys-106 is located approximately 16 A from the substitution site, demonstrating that structural disruptions propagate throughout the whole protein. Furthermore, L166P DJ-1 showed a significant increase in hydrophobic surface area relative to wild-type protein, possibly explaining the tendency of the mutant protein to aggregate. These simulations provide details about specific structural disturbances throughout L166P DJ-1 that previous studies have not revealed.     

Parkinson's disease DJ-1 L166P alters rRNA biogenesis by exclusion of TTRAP from the nucleolus and sequestration into cytoplasmic aggregates via TRAF6

Mutations in PARK7/DJ-1 gene are associated to autosomal recessive early onset forms of Parkinson's disease (PD). Although large gene deletions have been linked to a loss-of-function phenotype, the pathogenic mechanism of missense mutations is less clear. The L166P mutation causes misfolding of DJ-1 protein and its degradation. L166P protein may also accumulate into insoluble cytoplasmic aggregates with a mechanism facilitated by the E3 ligase TNF receptor associated factor 6 (TRAF6). Upon proteasome impairment L166P activates the JNK/p38 MAPK apoptotic pathway by its interaction with TRAF and TNF Receptor Associated Protein (TTRAP). When proteasome activity is blocked in the presence of wild-type DJ-1, TTRAP forms aggregates that are localized to the cytoplasm or associated to nucleolar cavities, where it is required for a correct rRNA biogenesis. In this study we show that in post-mortem brains of sporadic PD patients TTRAP is associated to the nucleolus and to Lewy Bodies, cytoplasmic aggregates considered the hallmark of the disease. In SH-SY5Y neuroblastoma cells, misfolded mutant DJ-1 L166P alters rRNA biogenesis inhibiting TTRAP localization to the nucleolus and enhancing its recruitment into cytoplasmic aggregates with a mechanism that depends in part on TRAF6 activity. This work suggests that TTRAP plays a role in the molecular mechanisms of both sporadic and familial PD. Furthermore, it unveils the existence of an interplay between cytoplasmic and nucleolar aggregates that impacts rRNA biogenesis and involves TRAF6.     

Down regulation of DJ-1 enhances cell death by oxidative stress, ER stress, and proteasome inhibition

Mutations in DJ-1 gene have been linked to autosomal recessive early onset parkinsonism (AR-EOP). Although the mechanism of neuronal cell death due to DJ-1 mutation has not been fully elucidated, loss of DJ-1 function was considered to cause the phenotype. Here, we demonstrated that the down regulation of endogenous DJ-1 of the neuronal cell line by siRNA enhanced the cell death which was induced by oxidative stress, ER stress, and proteasome inhibition, but not by pro-apoptotic stimulus. The cell death with hydrogen peroxide was dramatically rescued by over-expression of wild-type DJ-1, but not by that of L166P mutant DJ-1. Furthermore, DJ-1 rescued the cell death caused by over-expression of Pael receptor, which was a substrate of Parkin, another gene product for autosomal recessive juvenile parkinsonism. These results suggest that loss of protective activity of DJ-1 from neuro-toxicity induced by these stresses contributes to neuronal cell death in AR-EOP with mutant DJ-1.     

Differential effects of Parkinson's disease-associated mutations on stability and folding of DJ-1

Mutations in the PARK7/DJ-1 gene cause autosomal-recessive Parkinson's disease. In some patients the gene is deleted. The molecular basis of disease in patients with point mutations is less obvious. We have investigated the molecular properties of [L166P]DJ-1 and the novel variant [E64D]DJ-1. When transfected into non-neuronal and neuronal cell lines, steady-state expression levels of [L166P]DJ-1 were dramatically lower than wild-type [WT]DJ-1 and [E64D]DJ-1. Cycloheximide and pulse-chase experiments revealed that the decreased expression levels of [L166P]DJ-1 were because of accelerated protein turnover. Proteasomal degradation was not the major pathway of DJ-1 breakdown because treatment with the proteasome inhibitor MG-132 caused only minimal accumulation of DJ-1, even of the very unstable [L166P]DJ-1 mutant. Because of the structural resemblance of DJ-1 with bacterial cysteine proteases, we considered an autoproteolytic mechanism. However, neither pharmacological inhibition nor site-directed mutagenesis of the putative active site residue Cys-106 stabilized DJ-1. To gain further insight into the structural defects of DJ-1 mutants, human [WT]DJ-1 and both mutants were expressed in Escherichia coli. As in eukaryotic cells, expression levels of [L166P]DJ-1 were dramatically reduced compared with [WT]DJ-1 and [E64D]DJ-1. Circular dichroism spectrometry revealed that the solution structures of [WT]DJ-1 and [E64D]DJ-1 are rich in beta-strand and alpha-helix conformation. Alpha-helices were more susceptible to thermal denaturation than the beta-sheet, and [WT]DJ-1 was more flexible in this regard than [E64D]DJ-1. Thus, structural defects of [E64D]DJ-1 only become apparent upon denaturing conditions, whereas the L166P mutation causes a drastic defect that leads to excessive degradation.     

L166P mutant DJ-1 promotes cell death by dissociating Bax from mitochondrial Bcl-XL

ABSTRACT: BACKGROUND: Mutations or deletions in DJ-1/PARK7 gene are causative for recessive forms of early onset Parkinson's disease (PD). Wild-type DJ-1 has cytoprotective roles against cell death through multiple pathways. The most commonly studied mutant DJ-1(L166P) shifts its subcellular distribution to mitochondria and renders cells more susceptible to cell death under stress stimuli. We previously reported that wild-type DJ-1 binds to Bcl-XL and stabilizes it against ultraviolet B (UVB) irradiation-induced rapid degradation. However, the mechanisms by which mitochondrial DJ-1(L166P) promotes cell death under death stimuli are largely unknown. RESULTS: We show that DJ-1(L166P) is more prone to localize in mitochondria and it binds to Bcl-XL more strongly than wild-type DJ-1. In addition, UVB irradiation significantly promotes DJ-1(L166P) translocation to mitochondria and binding to Bcl-XL. DJ-1(L166P) but not wild-type DJ-1 dissociates Bax from Bcl-XL, thereby leading to Bax enrichment at outer mitochondrial membrane and promoting mitochondrial apoptosis pathway in response to UVB irradiation. CONCLUSION: Our findings suggest that wild-type DJ-1 protects cells and DJ-1(L166P) impairs cells by differentially regulating mitochondrial Bax/Bcl-XL functions.     

MicroRNA-452 promotes tumorigenesis in hepatocellular carcinoma by targeting cyclin-dependent kinase inhibitor 1B

Rare genetic mutations in the DJ-1 and Parkin genes cause recessive Parkinsonism, however, the relationship between these two genes is not fully elucidated. Current emerging evidence suggests that these genes are involved in mitochondrial homeostasis, and that a deficiency in either of these two genes is associated with damages in mitochondrial function and morphology. In this study, we demonstrated that knockdown of DJ-1 expression or the overexpression of the DJ-1 L166P mutation results in a damaged phenotype in mitochondria and a hypersensitivity to H₂O₂-induced cell apoptosis. These phenotypes result from increased levels of endogenous oxidative stress. However, overexpression of wild-type Parkin rescued the phenotypes observed in the mitochondria of DJ-1 knockdown and DJ-1 L166P mutant cells. We also determined that there were differences between the two cell models. Furthermore, both H₂O₂ treatment and the DJ-1 L166P mutation weakened the interaction between DJ-1 and Parkin. Taken together, these findings suggested that DJ-1 and Parkin were linked through oxidative stress, and that overexpression of Parkin protects DJ-1 protein-deficient and DJ-1 L166P mutant-expressing cells via inhibition of oxidative stress.     

A Two-step Protein Quality Control Pathway for a Misfolded DJ-1 Variant in Fission Yeast

A mutation, L166P, in the cytosolic protein, PARK7/DJ-1, causes protein misfolding and is linked to Parkinson disease. Here, we identify the fission yeast protein Sdj1 as the orthologue of DJ-1 and calculate by in silico saturation mutagenesis the effects of point mutants on its structural stability. We also map the degradation pathways for Sdj1-L169P, the fission yeast orthologue of the disease-causing DJ-1 L166P protein. Sdj1-L169P forms inclusions, which are enriched for the Hsp104 disaggregase. Hsp104 and Hsp70-type chaperones are required for efficient degradation of Sdj1-L169P. This also depends on the ribosome-associated E3 ligase Ltn1 and its co-factor Rqc1. Although Hsp104 is absolutely required for proteasomal degradation of Sdj1-L169P aggregates, the degradation of already aggregated Sdj1-L169P occurs independently of Ltn1 and Rqc1. Thus, our data point to soluble Sdj1-L169P being targeted early by Ltn1 and Rqc1. The fraction of Sdj1-L169P that escapes this first inspection then forms aggregates that are subsequently cleared via an Hsp104- and proteasome-dependent pathway.