Parkin is not regulated by the unfolded protein response in human neuroblastoma cells

Small, N-terminal tags activate Parkin E3 ubiquitin ligase activity by disrupting its autoinhibited conformation

Parkin is an E3 ubiquitin ligase, mutations in which cause Autosomal Recessive Parkinson's Disease. Many studies aimed at understanding Parkin function, regulation and dysfunction are performed using N-terminal epitope tags. We report here that the use of small tags such as FLAG, cMyc and HA, influence the physical stability and activity of Parkin in and out of cells, perturbing the autoinhibited native state of Parkin, resulting in an active-for-autoubiquitination species.

Autoregulation of Parkin activity through its ubiquitin-like domain

Parkin is an E3-ubiquitin ligase belonging to the RBR (RING-InBetweenRING-RING family), and is involved in the neurodegenerative disorder Parkinson's disease. Autosomal recessive juvenile Parkinsonism, which is one of the most common familial forms of the disease, is directly linked to mutations in the parkin gene. However, the molecular mechanisms of Parkin dysfunction in the disease state remain to be established. We now demonstrate that the ubiquitin-like domain of Parkin functions to inhibit its autoubiquitination. Moreover pathogenic Parkin mutations disrupt this autoinhibition, resulting in a constitutively active molecule. In addition, we show that the mechanism of autoregulation involves ubiquitin binding by a C-terminal region of Parkin. Our observations provide important molecular insights into the underlying basis of Parkinson's disease, and in the regulation of RBR E3-ligase activity.

Parkin, A Top Level Manager in the Cell's Sanitation Department

Parkin belongs to a class of multiple RING domain proteins designated as RBR (RING, in between RING, RING) proteins. In this review we examine what is known regarding the structure/function relationship of the Parkin protein. Parkin contains three RING domains plus a ubiquitin-like domain and an in-between-RING (IBR) domain. RING domains are rich in cysteine amino acids that act as ligands to bind zinc ions. RING domains may interact with DNA or with other proteins and perform a wide range of functions. Some function as E3 ubiquitin ligases, participating in attachment of ubiquitin chains to signal proteasome degradation; however, ubiquitin may be attached for purposes other than proteasome degradation.

It was determined that the C-terminal most RING, RING2, is essential for Parkin to function as an E3 ubiquitin ligase and a number of substrates have been identified. However, Parkin also participates in a number of other fiunctions, such as DNA repair, microtubule stabilization, and formation of aggresomes. Some functions, such as participation in a multi-protein complex implicated in NMDA activity at the post synaptic density, do not require ubiquitination of substrate molecules. Recent observations of RING proteins suggest their function may be regulated by zinc ion binding. We have modeled the three RING domains of Parkin and have identified a new set of RING2 ligands. This set allows for binding of two rather than just one zinc ion, opening the possibility that the number of zinc ions bound acts as a molecular switch to modulate Parkin function.

Parkin reverses intracellular beta-amyloid accumulation and its negative effects on proteasome function

The significance of intracellular beta-amyloid (Abeta(42)) accumulation is increasingly recognized in Alzheimer's disease (AD) pathogenesis. Abeta removal mechanisms that have attracted attention include IDE/neprilysin degradation and antibody-mediated uptake by immune cells. However, the role of the ubiquitin-proteasome system (UPS) in the disposal of cellular Abeta has not been fully explored. The E3 ubiquitin ligase Parkin targets several proteins for UPS degradation, and Parkin mutations are the major cause of autosomal recessive Parkinson's disease. We tested whether Parkin has cross-function to target misfolded proteins in AD for proteasome-dependent clearance in SH-SY5Y and primary neuronal cells. Wild-type Parkin greatly decreased steady-state levels of intracellular Abeta(42), an action abrogated by proteasome inhibitors. Intracellular Abeta(42) accumulation decreased cell viability and proteasome activity. Accordingly, Parkin reversed both effects. Changes in mitochondrial ATP production from Abeta or Parkin did not account for their effects on the proteasome. Parkin knock-down led to accumulation of Abeta. In AD brain, Parkin was found to interact with Abeta and its levels were reduced. Thus, Parkin is cytoprotective, partially by increasing the removal of cellular Abeta through a proteasome-dependent pathway.

Parkin is expressed in vascular endothelial cells

Mutations in the parkin gene are related with early-onset Parkinson's disease. Parkin is identified as an E3-ligase that combines target proteins with ubiquitin. alpha-Synuclein and synphilin-1 are substrates for E3-ligase activity of parkin and considered to be involved in the pathogenesis of Parkinson's disease. We previously demonstrated both alpha-synuclein and synphilin-1 are expressed in vascular endothelial cells (VEC). In the present study, we addressed possible expression of parkin in VEC. Parkin immunoreactivity was detected in vascular endothelial cells in postmortem human brain. Expressions of parkin mRNA and protein in human umbilical vein endothelial cells (HUVEC) were demonstrated by reverse-transcription polymerase-chain reaction (RT-PCR) and western blotting. Expression of parkin in HUVEC was not altered with tunicamycin treatment, which exerts unfolded protein stress on cells. We conclude that parkin is expressed in VEC, and that unfolded protein stress may not regulate its expression.

Cell type-specific upregulation of Parkin in response to ER stress

Parkin is the gene responsible for a familial form of Parkinson's disease (PD) termed autosomal recessive juvenile parkinsonism (AR-JP)/PARK2. Parkin has been shown to protect cells from endoplasmic reticulum (ER) stress and oxidative stress, presumably due to its ubiquitin ligase (E3) activity that targets proteins for proteasomal degradation. Although the authors showed that parkin is upregulated in response to ER stress, subsequent reports suggest that it does not represent a universal unfolded protein response (UPR). Here the authors report different regulation of parkin in response to ER stress in different cell lines, demonstrating upregulation of parkin as a cell type-specific response to ER stress. 2-Mercaptoethanol (2-ME) and tunicamycin increased the expression of parkin in SH-SY5Y (H) cells, Neuro2a cells, Goto-P3 cells, but not in SH-SY5Y (J) cells and IMR32 cells. In parallel with these studies, similar upregulation of the parkin coregulated gene (PACRG)/gene adjacent to parkin (Glup) was also observed by ER stress. Luciferase assays failed to detect the transcriptional activation of 500 bp parkin/Glup promoter in response to ER stress. These results indicate that induction of parkin by ER stress represents a cell type-specific response.

Parkin-deficient mice are not more sensitive to 6-hydroxydopamine or methamphetamine neurotoxicity

Background

Autosomal recessive juvenile parkinsonism (AR-JP) is caused by mutations in the parkin gene which encodes an E3 ubiquitin-protein ligase. Parkin is thought to be critical for protecting dopaminergic neurons from toxic insults by targeting misfolded or oxidatively damaged proteins for proteasomal degradation. Surprisingly, mice with targeted deletions of parkin do not recapitulate robust behavioral or pathological signs of parkinsonism. Since Parkin is thought to protect against neurotoxic insults, we hypothesized that the reason Parkin-deficient mice do not develop parkinsonism is because they are not exposed to appropriate environmental triggers. To test this possibility, we challenged Parkin-deficient mice with neurotoxic regimens of either methamphetamine (METH) or 6-hydroxydopamine (6-OHDA). Because Parkin function has been linked to many of the pathways involved in METH and 6-OHDA toxicity, we predicted that Parkin-deficient mice would be more sensitive to the neurotoxic effects of these agents.

Results

We found no signs consistent with oxidative stress, ubiquitin dysfunction, or degeneration of striatal dopamine neuron terminals in aged Parkin-deficient mice. Moreover, results from behavioral, neurochemical, and immunoblot analyses indicate that Parkin-deficient mice are not more sensitive to dopaminergic neurotoxicity following treatment with METH or 6-OHDA.

Conclusion

Our results suggest that the absence of a robust parkinsonian phenotype in Parkin-deficient mice is not due to the lack of exposure to environmental triggers with mechanisms of action similar to METH or 6-OHDA. Nevertheless, Parkin-deficient mice could be more sensitive to other neurotoxins, such as rotenone or MPTP, which have different mechanisms of action; therefore, identifying conditions that precipitate parkinsonism specifically in Parkin-deficient mice would increase the utility of this model and could provide insight into the mechanism of AR-JP. Alternatively, it remains possible that the absence of parkinsonism in Parkin-deficient mice could reflect fundamental differences between the function of human and mouse Parkin, or the existence of a redundant E3 ubiquitin-protein ligase in mouse that is not found in humans. Therefore, additional studies are necessary to understand why Parkin-deficient mice do not display robust signs of parkinsonism.

Ubiquitin, proteasome and parkin

The ubiquitin-proteasome system (UPS) is important for intracellular proteolysis, and is responsible for a diverse array of biologically important cellular processes, such as cell-cycle progression, signaling cascades and developmental programs. This system is also involved in the protein quality control, which maintains the health of the cell. Thus, the UPS provides a clue for understanding of the molecular mechanisms underlying various neurodegenerative diseases. In the last decade, we witnessed a tremendous progress in uncovering the mechanisms of Parkinson's disease (PD). Of the several genes that can cause familial PD, parkin, the causative gene of autosomal recessive juvenile parkinsonism (ARJP), is of a special interest because it encodes an ubiquitin-protein ligase, which covalently attaches ubiquitin to target proteins, designating them for destruction by the proteasome. This review summarizes recent studies on the UPS pathway with a special reference to parkin, focusing on how parkin is linked to the pathogenesis of ARJP.

How do Parkin mutations result in neurodegeneration?

The gene product responsible for autosomal recessive juvenile Parkinsonism, Parkin, has been observed to have ubiquitin ligase activity. This finding has changed the direction of studies on Parkinson's disease by suggesting that abnormal protein turnover might be involved in its pathogenesis. A number of potentially neurotoxic Parkin-specific substrates have been identified. Further investigation of Parkin knockout mice will hopefully provide new evidence in the search for Parkin's substrates and further clarify their role in Parkinson's disease.

N-myc regulates parkin expression

Mutations in the parkin gene are common in early-onset and familial Parkinson's disease (PD), and the parkin protein interacts in the ubiquitin-proteasome system as an E3 ligase. However, the regulatory pathways that govern parkin expression are unknown. In this study, we showed that a phylogenetically conserved N-myc binding site in the bi-directional parkin promoter interacted with myc-family transcription factors in reporter assays, and N-myc bound to the parkin promoter in chromatin immunoprecipitation assays and repressed transcription activity. Parkin expression was inversely correlated with N-myc levels in the developing mouse and human brain, in human neuroblastoma cell lines with various levels of n-myc amplification, and in an inducible N-myc cell line. Although parkin and N-myc expression were dramatically altered upon retinoic acid-induced differentiation of a human neuroblastoma cell line, modulation of parkin expression did not significantly affect either rates of cellular proliferation or levels of cyclin E. Analysis of additional genes associated with familial PD revealed a shared basis of transcription regulation mediated by N-myc and the cell cycle. Our results, in combination with functional knowledge of the proteins encoded by these genes, suggest a common pathway linking together PD, the ubiquitin-proteasome system, and cell cycle control.

Inhibition of proteasomal activity causes inclusion formation in neuronal and non-neuronal cells overexpressing Parkin

Association between protein inclusions and neurodegenerative diseases, including Parkinson's and Alzheimer's diseases, and polyglutamine disorders, has been widely documented. Although ubiquitin is conjugated to many of these aggregated proteins, the 26S proteasome does not efficiently degrade them. Mutations in the ubiquitin-protein ligase Parkin are associated with autosomal recessive juvenile Parkinsonism. Although Parkin-positive inclusions are not detected in brains of autosomal recessive juvenile Parkinsonism patients, Parkin is found in Lewy bodies in sporadic disease. This suggests that loss of Parkin ligase activity via mutation, or sequestration to Lewy bodies, is a contributory factor to sporadic disease onset. We now demonstrate that decreased proteasomal activity causes formation of large, noncytotoxic inclusions within the cytoplasm of both neuronal and nonneuronal cells overexpressing Parkin. This is not a general phenomenon as there is an absence of similar inclusions when HHARI, a structural homolog of Parkin, is overexpressed. The inclusions colocalize with ubiquitin and with proteasomes. Furthermore, Parkin inclusions colocalize with gamma-tubulin, acetylated alpha-tubulin, and cause redistribution of vimentin, suggesting aggresome-like properties. Our data imply that lower proteasomal activity, previously observed in brain tissue of Parkinson's disease patients, leads to Parkin accumulation and a concomitant reduction in ligase activity, thereby promoting Lewy body formation.