Familial-associated mutations differentially disrupt the solubility, localization, binding and ubiquitination properties of parkin

Inflammatory mediators leading to protein misfolding and uncompetitive/fast off-rate drug therapy for neurodegenerative disorders

Inflammatory mediators, including free radicals such as nitric oxide (NO) and reactive oxygen species (ROS), can contribute to neurodegenerative diseases in part by triggering protein misfolding. In this chapter, we will discuss a newly discovered pathway for this phenomenon and possible novel treatments. Excitotoxicity, defined as overstimulation of glutamate receptors, has been implicated in a final common pathway contributing to neuronal injury and death in a wide range of acute and chronic neurological disorders, ranging from Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis, and Alzheimer's disease (AD) to stroke and trauma. Excitotoxic cell death is due, at least in part, to excessive activation of N-methyl-d-aspartate (NMDA)-type glutamate receptors, leading to excessive Ca(2+) influx through the receptor's associated ion channel and subsequent free radical production, including NO and ROS. These free radicals can trigger a variety of injurious pathways, but newly discovered evidence suggests that some proteins are S-nitrosylated (transfer of NO to a critical thiol group), and this reaction can mimic the effect of rare genetic mutations. This posttranslational modification can contribute to protein misfolding, triggering neurodegenerative diseases. One such molecule affected is protein disulfide isomerase (PDI), an enzyme responsible for normal protein folding in the endoplasmic reticulum (ER). We found that when PDI is S-nitrosylation (forming SNO-PDI), the function of the enzyme is compromised, leading to misfolded proteins and contributing to neuronal cell injury and loss. Moreover, SNO-PDI occurs at pathological levels in several human diseases, including AD and PD. This discovery thus links protein misfolding to excitotoxicity and free radical formation in a number of neurodegenerative disorders. Another molecule whose S-nitrosylation can lead to abnormal protein accumulation is the E3 ubiquitin ligase, parkin, which contributes to the pathogenesis of PD. One way to ameliorate excessive NO production and hence abnormal S-nitrosylations would be to inhibit NMDA receptors. In fact, blockade of excessive NMDA receptor activity can in large measure protect neurons from this type of injury and death. However, inhibition of the NMDA receptor by high-affinity antagonists also blocks the receptor's normal function in synaptic transmission and leads to unacceptable side effects. For this reason, many NMDA receptor antagonists have disappointingly failed in advanced clinical trials. Our group was the first to demonstrate that gentle blockade of NMDA receptors by memantine, via a mechanism of uncompetitive open-channel block with a rapid "off-rate," can prevent this type of damage in a clinically efficacious manner without substantial side effects. For these Uncompetitive/Fast Off-rate therapeutics, we use the term "UFO drugs" because like Unidentified Flying Objects, they leave very quickly as soon as their job is finished. As a result, memantine blocks excessive NMDA receptor activity without disrupting normal activity. Memantine does this by preferentially entering the receptor-associated ion channel when it is excessively open, and, most importantly, when its off-rate from the channel is relatively fast so that it does not accumulate to interfere with normal synaptic transmission. Hence, memantine is clinically well tolerated, has been used in Europe for PD for many years, and recently passed multiple phase III trials for dementia, leading to its approval by the FDA and European Union for moderate-to-severe AD. Clinical studies of memantine for additional neurological disorders, including other dementias, neuropathic pain, and glaucoma, are underway. We have also developed a series of second-generation drugs that display greater neuroprotective properties than memantine. These second-generation drugs take advantage of the fact that the NMDA receptor has other modulatory sites, including critical thiol groups that are S-nitrosylated. In this case, in contrast to PDI or parkin, S-nitrosylation proves to be neuroprotective by decreasing excessive NMDA receptor activity. Targeted S-nitrosylation of the NMDA receptor can be achieved by coupling NO to memantine, yielding second-generation "UFO drugs" known as NitroMemantines.

Parkin is an E3 ubiquitin-ligase for normal and mutant ataxin-2 and prevents ataxin-2-induced cell death

Expansion of the polyQ repeat in ataxin-2 results in degeneration of Purkinje neurons and other neuronal groups including the substantia nigra in patients with spinocerebellar ataxia type 2 (SCA2). In animal and cell models, overexpression of mutant ataxin-2 induces cell dysfunction and death, but little is known about steady-state levels of normal and mutant ataxin-2 and cellular mechanisms regulating their abundance. Based on preliminary findings that ataxin-2 interacted with parkin, an E3 ubiquitin ligase mutated in an autosomal recessive form of Parkinsonism, we sought to determine whether parkin played a role in regulating the steady-state levels of ataxin-2. Parkin interacted with the N-terminal half of normal and mutant ataxin-2, and ubiquitinated the full-length form of both wild-type and mutant ataxin-2. Parkin also regulated the steady-state levels of endogenous ataxin-2 in PC12 cells with regulatable parkin expression. Parkin reduced abnormalities in Golgi morphology induced by mutant ataxin-2 and decreased ataxin-2 induced cytotoxicity. In brains of SCA2 patients, parkin labeled cytoplasmic ataxin-2 aggregates in Purkinje neurons. These studies suggest a role for parkin in regulating the intracellular levels of both wild-type and mutant ataxin-2, and in rescuing cells from ataxin-2-induced cytotoxicity. The role of parkin variants in modifying the SCA2 phenotype and its use as a therapeutic target should be further investigated.

Parkin Protects against Neurotoxicity in the 6-Hydroxydopamine Rat Model for Parkinson's Disease

Loss-of-function mutations in the PARK2 gene are the major cause of early onset familial Parkinson's disease. The gene product, parkin, is an E3 ligase of the ubiquitin-proteasome pathway involved in protein degradation. Dopaminergic neuron loss may result from the toxic accumulation of parkin substrates, suggesting a key role for parkin in dopaminergic neuron survival. In this study, we have investigated the neuroprotective capacity of parkin in the 6-OHDA rat model for Parkinson's disease. 6-OHDA induces the generation of reactive oxygen species leading to the degeneration of catecholaminergic neurons, but may also impair proteasome activity. Lentiviral vectors encoding human wild-type parkin or green fluorescent protein were stereotactically injected into the substantia nigra 2 weeks prior to a striatal 6-OHDA lesion. Histological analysis 1 and 3 weeks after lesioning showed a significant preservation of dopaminergic cell bodies and nerve terminals. Moreover, lesioned rats overexpressing parkin displayed a corresponding behavioral improvement as measured by the amphetamine-induced rotation test and the cylinder test. The improved performance in the amphetamine-induced rotation test lasted until 20 weeks after lesioning. Our results demonstrate that parkin acts as a potent neuroprotective agent in vivo against 6-OHDA toxic insults. These data support the therapeutic potential of parkin for the treatment of not only familial but also sporadic Parkinson's disease.

Parkin: a multifaceted ubiquitin ligase

Mutations in the parkin gene are a common cause of autosomal recessive early-onset parkinsonism. Parkin functions as an E3 ubiquitin ligase where it can polyubiquitinate a number of its protein substrates, thus targeting them for degradation by the 26 S proteasomal complex. Recent studies have demonstrated that alternative modes of parkin-mediated ubiquitination may serve other non-degradative regulatory roles. In addition, parkin appears to function as a multipurpose neuroprotectant in a number of toxic paradigms. Coupled with these observations, parkin may integrate other gene products associated with parkinsonism, including α-synuclein, LRRK2 (leucine-rich repeat kinase 2), DJ-1 and PINK1 [PTEN (phosphatase and tensin homologue deleted on chromosome 10)-induced putative kinase 1], into a common biochemical pathway of potential relevance to disease pathogenesis. Parkin therefore represents a unique multifaceted ubiquitin ligase consistent with an important housekeeping role in maintaining the integrity or survival of dopaminergic neurons.

Parkin potentiates ATP-induced currents due to activation of P2X receptors in PC12 cells

Loss-of-function mutations of the parkin gene causes an autosomal recessive juvenile-onset form of Parkinson's disease (AR-JP). Parkin was shown to function as a RING-type E3 ubiquitin protein ligase. However, the function of parkin in neuronal cells remains elusive. Here, we show that expression of parkin-potentiated adenosine triphosphate (ATP)-induced currents that result from activation of the P2X receptors which are widely distributed in the brain and involved in neurotransmission. ATP-induced inward currents were measured in mock-, wild-type or mutant (T415N)-parkin-transfected PC12 cells under the conventional whole-cell patch clamp configuration. The amplitude of ATP-induced currents was significantly greater in wild-type parkin-transfected cells. However, the immunocytochemical study showed no apparent increase in the number of P2X receptors or in ubiquitin levels. The increased currents were attenuated by inhibition of cAMP-dependent protein kinase (PKA) but not protein kinase C (PKC) or Ca2+ and calmodulin-dependent protein kinase (CaMKII). ATP-induced currents were also regulated by phosphatases and cyclin-dependent protein kinase 5 (CDK5) via dopamine and cyclic AMP-regulated phosphoprotein (DARPP-32), though the phosphorylation at Thr-34 and Thr-75 were unchanged or rather attenuated. We also tried to investigate the effect of alpha-synuclein, a substrate of parkin and also forming Lysine 63-linked multiubiquitin chains. Expression of alpha-synuclein did not affect the amplitude of ATP-induced currents. Our finding provides the evidence for a relationship between parkin and a neurotransmitter receptor, suggesting that parkin may play an important role in synaptic activity.

Understanding the molecular causes of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease that is both common and incurable. The majority of cases are sporadic and of unknown origin but several genes have been identified that, when mutated, give rise to rare, familial forms of the disease. The principal genes that have been shown to cause PD are alpha-synuclein (SNCA), parkin, leucine-rich repeat kinase 2 (LRRK2), PTEN-induced putative kinase 1 (PINK1) and DJ-1. Here, we discuss what has been learnt from the study of these genes and what has been elucidated of the molecular pathways that lead to cell degeneration. Of importance is what these molecular events and pathways tell scientists of the common sporadic form of PD. Although complete knowledge of these genes' functions remains elusive, recent work implicates abnormal protein accumulation, protein phosphorylation, mitochondrial dysfunction and oxidative stress as common pathways to PD pathogenesis.

The genetics of Parkinson disease: Implications for neurological care

The identification of single genes linked to heritable forms of Parkinson disease (PD) has challenged the previously held view of a nongenetic etiology for this progressive movement disorder. Detailed analyses of individuals with mutations in SNCA, Parkin, PINK1, DJ1 or LRRK2 have greatly advanced our knowledge of preclinical and clinical, morphological, and pathological changes in PD. These genetic breakthroughs have had profound implications for scientists, neurologists and patients alike. Such advances have provided unique opportunities to pursue the mechanisms of neuronal degeneration in models of PD pathogenesis, thereby reinforcing the significance of oxidative stress and mitochondrial dysfunction. With emerging clues from familial variants, researchers have begun to explore factors that lead to the expression of the more common, sporadic disease phenotype (idiopathic PD), including interactions between various genes, modifying effects of susceptibility alleles and epigenetic factors, and the influence of environmental agents and aging on the expression of PD-linked genes. These genetic leads have added to the urgency of developing translational drug treatments, and neurologists and their patients are confronting considerations relating to DNA testing. In this article, we summarize recent progress in establishing a neurogenetic component of PD, emphasize the need for developing PD biomarkers to improve diagnostic accuracy (in both clinical practice and therapeutic trials), and discuss scenarios in which specific DNA tests might be considered for diagnostic purposes. In the absence of consensus guidelines for DNA testing in PD and of any neuroprotective treatment for this nonfatal disorder, we remind ourselves of the omnipresent mandate, 'Primum nil nocere!' ('First, do no harm!').

Cellular parkin mutants are soluble under non-stress conditions

The parkin gene encodes an E3 ubiquitin ligase and loss of function mutations herein are the most frequent cause of early-onset Parkinson's disease. Reports have suggested that aggregation of mutant protein is the cause of the loss of function. We established stably transfected SH-SY5Y dopaminergic cell lines expressing wild-type and mutant parkin proteins. All the mutant proteins were soluble but could be rendered insoluble by subjecting the cellsto stress by proteasomal inhibition, treatment with oxidants and upon transient expression of the mutant proteins. A functional assay demonstrated that the R42P mutant retained functional activity in contrast to the W453stop mutant. Accordingly, the functional impairment by the mutations is not simply caused by turning the proteins insoluble.

Biochemical analysis of Parkinson's disease-causing variants of Parkin, an E3 ubiquitin–protein ligase with monoubiquitylation capacity

Mutations in the parkin gene, encoding an E3 ubiquitin-protein ligase, are a frequent cause of autosomal recessive parkinsonism and are also involved in sporadic Parkinson's disease. Loss of Parkin function is thought to compromise the polyubiquitylation and proteasomal degradation of specific substrates, leading to their deleterious accumulation. Several studies have analyzed the effects of parkin gene mutations on the biochemical properties of the protein. However, the absence of a cell-free system for studying intrinsic Parkin activity has limited the interpretation of these studies. Here we describe the biochemical characterization of Parkin and 10 pathogenic variants carrying amino-acid substitutions throughout the sequence. Mutations in the RING fingers or the ubiquitin-like domain decreased the solubility of the protein in detergent and increased its tendency to form visible aggregates. None of the mutations studied compromised the binding of Parkin to a series of known protein partners/substrates. Moreover, only two variants with substitutions of conserved cysteine residues of the second RING finger were inactive in a purely in vitro ubiquitylation assay, demonstrating that loss of ligase activity is a minor pathogenic mechanism. Interestingly, in this in vitro assay, Parkin catalyzed the linkage of single ubiquitin molecules only, whereas the ubiquitin-protein ligases CHIP and Mdm2 promoted the formation of polyubiquitin chains. Similarly, in mammalian cells Parkin promoted the multimonoubiquitylation of its substrate p38, rather than its polyubiquitylation. Thus, Parkin may mediate polyubiquitylation or proteasome-independent monoubiquitylation depending on the protein context. The discovery of monoubiquitylated Parkin species in cells hints at a novel post-translational modification potentially involved in the regulation of Parkin function.

Genetics of Parkinson disease: paradigm shifts and future prospects

Parkinson disease is a complex, multifactorial neurodegenerative disease. Although a heritable basis was originally thought unlikely, recent studies have implicated several genes in its pathogenesis, and molecular findings now allow accurate diagnosis and challenge past criteria for defining Parkinson disease. Most importantly, genetic insights provide the rationale for new strategies for prevention or therapy, and have led to animal models of disease in which these strategies can be tested. Neuroprotective therapies can now be designed to slow or halt disease progression in affected subjects and asymptomatic carriers.

Diverse Effects of Pathogenic Mutations of Parkin That Catalyze Multiple Monoubiquitylation in Vitro

Mutational dysfunction of PARKIN gene, which encodes a double RING finger protein and has ubiquitin ligase E3 activity, is the major cause of autosomal recessive juvenile Parkinsonism. Although many studies explored the functions of Parkin, its biochemical character is poorly understood. To address this issue, we established an E3 assay system using maltose-binding protein-fused Parkin purified from Escherichia coli. Using this recombinant Parkin, we found that not the front but the rear RING finger motif is responsible for the E3 activity of Parkin, and it catalyzes multiple monoubiquitylation. Intriguingly, for autosomal recessive juvenile Parkinsonism-causing mutations of Parkin, whereas there was loss of E3 activity in the rear RING domain, other pathogenic mutants still exhibited E3 activity equivalent to that of the wild-type Parkin. The evidence presented allows us to reconsider the function of Parkin-catalyzed ubiquitylation and to conclude that autosomal recessive juvenile Parkinsonism is not solely attributable to catalytic impairment of the E3 activity of Parkin.

LRRK2: a common pathway for parkinsonism, pathogenesis and prevention?

The presence of alpha-synuclein Lewy body pathology is used to distinguish Parkinson's disease from parkinsonism, for which a broader spectrum of neuropathologies, including tau-immunopositive neurofibrillary tangles and ubiquitin inclusions, might accompany nigral neuronal loss. These neuropathologies define the endpoint of many neurodegenerative disorders but might be symptomatic rather than causative. Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) were recently discovered in late-onset parkinsonism, the phenotype of which can be clinically and pathologically indistinguishable from Parkinson's disease. However, in some kindreds with LRRK2- associated disease, pathologically distinct forms of parkinsonism, including nigral neuronal loss with Lewy body disease or tau-immunopositive neurofibrillary tangles, were discovered. Understanding the molecular function of the LRRK2 protein and its associated pathways might elucidate the switch between Lewy body pathology and neurofibrillary tangles, and holds promise for prospective therapeutics that might slow or halt progression of many forms of parkinsonism.

Stress-induced alterations in parkin solubility promote parkin aggregation and compromise parkin's protective function

Mutations in parkin are currently recognized as the most common cause of familial Parkinsonism. Emerging evidence also suggests that parkin expression variability may confer a risk for the development of the more common, sporadic form of Parkinson's disease (PD). Supporting this, we have recently demonstrated that parkin solubility in the human brain becomes altered with age. As parkin apparently functions as a broad-spectrum neuroprotectant, the resulting decrease in the availability of soluble parkin with age may underlie the progressive susceptibility of the brain to stress. Interestingly, we also observed that many familial-PD mutations of parkin alter its solubility in a manner that is highly reminiscent of our observations with the aged brain. The converging effects on parkin brought about by aging and PD-causing mutations are probably not trivial and suggest that environmental modulators affecting parkin solubility would increase an individual's risk of developing PD. Using both cell culture and in vivo models, we demonstrate here that several PD-linked stressors, including neurotoxins (MPP+, rotenone, 6-hydroxydopamine), paraquat, NO, dopamine and iron, induce alterations in parkin solubility and result in its intracellular aggregation. Furthermore, the depletion of soluble, functional forms of parkin is associated with reduced proteasomal activities and increased cell death. Our results suggest that exogenously introduced stress as well as endogenous dopamine could affect the native structure of parkin, promote its misfolding, and concomitantly compromise its protective functions. Mechanistically, our results provide a link between the influence of environmental and intrinsic factors and genetic susceptibilities in PD pathogenesis.

Dopamine covalently modifies and functionally inactivates parkin

Inherited mutations in PARK2, the gene encoding parkin, cause selective degeneration of catecholaminergic neurons in the substantia nigra and locus coeruleus of the brainstem, resulting in early-onset parkinsonism. But the role of parkin in common, sporadic forms of Parkinson disease remains unclear. Here we report that the neurotransmitter dopamine covalently modifies parkin in living dopaminergic cells, a process that increases parkin insolubility and inactivates its E3 ubiquitin ligase function. In the brains of individuals with sporadic Parkinson disease, we observed decreases in parkin solubility consistent with its functional inactivation. Using a new biochemical method, we detected catechol-modified parkin in the substantia nigra but not other regions of normal human brain. These findings show a vulnerability of parkin to modification by dopamine, the principal transmitter lost in Parkinson disease, suggesting a mechanism for the progressive loss of parkin function in dopaminergic neurons during aging and sporadic Parkinson disease.

A Caenorhabditis elegans Parkin mutant with altered solubility couples alpha-synuclein aggregation to proteotoxic stress

Mutations in the human parkin gene encoding an E3 ubiquitin ligase have been associated with early-onset recessive forms of Parkinson's disease (PD). However, the molecular mechanisms by which mutations in the parkin gene cause PD are still under debate. Here, we identified and characterized the Caenorhabditis elegans parkin homolog, pdr-1. PDR-1 protein physically associates and cooperates with a conserved degradation machinery to mediate ubiquitin conjugation. Strikingly, in contrast to pdr-1 loss-of-function mutants, an in-frame deletion variant with altered solubility and intracellular localization properties is hypersensitive toward different proteotoxic stress conditions. Both endoplasmic reticulum-derived folding stress and cytosolic stress conferred by expression of mutant human alpha-synuclein resulted in severe developmental defects and lethality in pdr-1(lg103) mutant background. Furthermore, we show that the corresponding truncated protein PDR-1(Deltaaa24-247) aggregates in cell culture, but still interacts with its ubiquitylation co-enzymes. Thus, it might block the cellular degradation/detoxification machinery and therefore renders worms highly vulnerable to protein folding stress. In contrast to other complete gene knockouts or RNAi models of Parkin function, this C. elegans model recapitulates Parkin insolubility and aggregation similar to several autosomal recessive juvenile parkinsonism (AR-JP)-linked Parkin mutations. We suggest that such Parkin variants that either confer a neomorphic function or a partial loss-of-function may help to further elucidate the biological function of Parkin in vivo and the pathogenic mechanisms resulting in AR-JP. Due to high-throughput capacity of C. elegans, this model is particularly well suited to identify genetic and chemical modifiers of toxicity.