Parkin protects dopaminergic neurons against microtubule-depolymerizing toxins by attenuating microtubule-associated protein kinase activation

Linking acetylated α-Tubulin redistribution to α-Synuclein pathology in brain of Parkinson's disease patients

Highly specialized microtubules in neurons are crucial to both health and disease of the nervous system, and their properties are strictly regulated by different post-translational modifications, including α-Tubulin acetylation. An imbalance in the levels of acetylated α-Tubulin has been reported in experimental models of Parkinson's disease (PD) whereas pharmacological or genetic modulation that leads to increased acetylated α-Tubulin successfully rescues axonal transport defects and inhibits α-Synuclein aggregation. However, the role of acetylation of α-Tubulin in the human nervous system is largely unknown as most studies are based on in vitro evidence. To capture the complexity of the pathological processes in vivo, we analysed post-mortem human brain of PD patients and control subjects. In the brain of PD patients at Braak stage 6, we found a redistribution of acetylated α-Tubulin, which accumulates in the neuronal cell bodies in subcortical structures but not in the cerebral cortex, and decreases in the axonal compartment, both in putamen bundles of fibres and in sudomotor fibres. High-resolution and 3D reconstruction analysis linked acetylated α-Tubulin redistribution to α-Synuclein oligomerization and to phosphorylated Ser 129 α-Synuclein, leading us to propose a model for Lewy body (LB) formation. Finally, in post-mortem human brain, we observed threadlike structures, resembling tunnelling nanotubes that contain α-Synuclein oligomers and are associated with acetylated α-Tubulin enriched neurons. In conclusion, we support the role of acetylated α-Tubulin in PD pathogenesis and LB formation.

Kynurenine attenuates mitochondrial depolarization and neuronal cell death induced by rotenone exposure independently of AhR-mediated parkin induction in SH-SY5Y differentiated cells

Rotenone is a pesticide commonly used in agriculture that is associated with the risk of developing Parkinson's disease (PD) by inducing mitochondrial damage. As a protective cell response to different challenges, they activate mitophagy, which involves parkin activity. Parkin is an E3 ubiquitin ligase necessary in the initial steps of mitophagy, and its overexpression protects against parkinsonian effects in different models. Recent studies have reported that the aryl hydrocarbon receptor (AHR), a ligand-dependent transcription factor, induces parkin expression. Kynurenine, an endogenous AHR ligand, promotes neuroprotection in chronic neurodegenerative disorders, such as PD, although its neuroprotective mechanism needs to be fully understood. Therefore, we evaluated whether the overexpression of parkin by AHR activation with kynurenine promotes autophagy and reduces the neurotoxicity induced by rotenone in SH-SY5Y cells differentiated to dopaminergic neurons. SH-SY5Y neurons were treated with rotenone or pretreated with kynurenine or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and parkin levels, apoptosis, mitochondrial potential membrane, and autophagy were determined. The results showed that kynurenine and TCDD treatments induced parkin expression in an AHR-dependent manner. Kynurenine pretreatment inhibited rotenone-induced neuronal apoptosis in 17%, and the loss of mitochondrial membrane potential in 30% when compare to rotenone alone, together with a decrease in autophagy. By contrast, although TCDD treatment increased parkin levels, non-neuroprotective effects were observed. The kynurenine protective activity was AHR independent, suggesting that parkin induction might not be related to this effect. On the other hand, kynurenine treatment inhibited alpha amine-3-hydroxy-5-methyl-4-isoxazol propionic acid and N-methyl-D-aspartate receptors, which are well-known excitotoxicity mediators activated by rotenone exposure.

Microtubule acetylation dyshomeostasis in Parkinson's disease

The inter-neuronal communication occurring in extensively branched neuronal cells is achieved primarily through the microtubule (MT)-mediated axonal transport system. This mechanistically regulated system delivers cargos (proteins, mRNAs and organelles such as mitochondria) back and forth from the soma to the synapse. Motor proteins like kinesins and dynein mechanistically regulate polarized anterograde (from the soma to the synapse) and retrograde (from the synapse to the soma) commute of the cargos, respectively. Proficient axonal transport of such cargos is achieved by altering the microtubule stability via post-translational modifications (PTMs) of α- and β-tubulin heterodimers, core components constructing the MTs. Occurring within the lumen of MTs, K40 acetylation of α-tubulin via α-tubulin acetyl transferase and its subsequent deacetylation by HDAC6 and SIRT2 are widely scrutinized PTMs that make the MTs highly flexible, which in turn promotes their lifespan. The movement of various motor proteins, including kinesin-1 (responsible for axonal mitochondrial commute), is enhanced by this PTM, and dyshomeostasis of neuronal MT acetylation has been observed in a variety of neurodegenerative conditions, including Alzheimer's disease and Parkinson's disease (PD). PD is the second most common neurodegenerative condition and is closely associated with impaired MT dynamics and deregulated tubulin acetylation levels. Although the relationship between status of MT acetylation and progression of PD pathogenesis has become a chicken-and-egg question, our review aims to provide insights into the MT-mediated axonal commute of mitochondria and dyshomeostasis of MT acetylation in PD. The enzymatic regulators of MT acetylation along with their synthetic modulators have also been briefly explored. Moving towards a tubulin-based therapy that enhances MT acetylation could serve as a disease-modifying treatment in neurological conditions that lack it.

GSK-3β: An exuberating neuroinflammatory mediator in Parkinson's disease

Neuroinflammation is a critical degradative condition affecting neurons in the brain. Progressive neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease (PD) have been strongly linked to neuroinflammation. The trigger point for inflammatory conditions in the cells and body is the physiological immune system. The immune response mediated by glial cells and astrocytes can rectify the physiological alterations occurring in the cell for the time being but prolonged activation leads to pathological progression. The proteins mediating such an inflammatory response, as per the available literature, are undoubtedly GSK-3β, NLRP3, TNF, PPARγ, and NF-κB, along with a few other mediatory proteins. NLRP3 inflammasome is undeniably a principal instigator of the neuroinflammatory response, but the regulatory pathways controlling its activation are still unclear, besides less clarity for the interplay between different inflammatory proteins. Recent reports have suggested the involvement of GSK-3β in regulating NLRP3 activation, but the exact mechanistic pathway remains vague. In the current review, we attempt to provide an elaborate description of crosstalk between inflammatory markers and GSK-3β mediated neuroinflammation progression, linking it to regulatory transcription factors and posttranslational modification of proteins. The recent clinical therapeutic advances targeting these proteins are also discussed in parallel to provide a comprehensive view of the progress made in PD management and lacunas still existing in the field.

Umbelliferone protects against cerebral ischemic injury through selective autophagy of mitochondria

Effective therapeutic treatments for ischemic stroke are limited. Previous studies suggest selective activation of mitophagy alleviates cerebral ischemic injury while excessive autophagy is detrimental. However, few compounds are available to selectively activate mitophagy without affecting autophagy flux. Here, we found that acute administration of Umbelliferone (UMB) upon reperfusion exerted neuroprotective effects against ischemic injury in mice subjected to transient middle cerebral artery occlusion (tMCAO) and suppressed oxygen-glucose deprivation reperfusion (OGD-R)-induced apoptosis in SH-SY5Y cells. Interestingly, UMB promoted the translocation of mitophagy adaptor SQSTM1 to mitochondria and further reduced the mitochondrial content as well as the expression of SQSTM1 in SHSY5Y cells after OGD-R. Importantly, both the mitochondrial loss and reduction of SQSTM1 expression after UMB incubation can be reversed by autophagy inhibitor chloroquine and wortmannin, proving the mitophagy activation by UMB. Nevertheless, UMB failed to further affect neither LC3 lipidation nor the number of autophagosomes after cerebral ischemia in vivo and in vitro. Furthermore, UMB facilitated OGD-R-induced mitophagy in a Parkin-dependent manner. Inhibition of autophagy/mitophagy either pharmaceutically or genetically abolished the neuroprotective effects of UMB. Taken all, these results suggest that UMB protects against cerebral ischemic injury, both in vivo and in vitro, via promoting mitophagy without increasing the autophagic flux. UMB might serve as a potential leading compound for selectively activating mitophagy and the treatment of ischemic stroke.

Microtubule stabilization promotes the synthesis of type 2 collagen in nucleus pulposus cell by activating hippo-yap pathway

Intervertebral disc degeneration (IDD) is the cardinal pathological mechanism that underlies low back pain. Mechanical stress of the intervertebral disc may result in a change in nucleus pulposus cells state, matrix degradation, and degeneration of the disc. Microtubules, which are components of the cytoskeleton, are involved in driving or regulating signal pathways, which sense and transmit mechano-transduction. Microtubule and the related proteins play an important role in the development of many diseases, while little is known about the role of microtubules in nucleus pulposus cells. Researchers have found that type II collagen (COL2) expression is promoted by microtubule stabilization in synovial mesenchymal stem cells. In this study, we demonstrated that microtubule stabilization promotes the expression of COL2 in nucleus pulposus cells. Stabilized microtubules stimulating Hippo signaling pathway, inhibiting YAP protein expression and activity. In addition, microtubules stabilization promotes the expression of COL2 and alleviates disc degeneration in rats. In summary, our study for the first time, identifies microtubule as a promising therapeutic target for IDD, up-regulating the synthesis of COL2 via Hippo-Yap pathway. Our findings may provide new insights into the etiologies and pathology for IDD, further, targeting of microtubule acetylation may be an effective strategy for the treatment of IDD.

Involvement of heat shock proteins and parkin/α-synuclein axis in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurological diseases, next only to Alzheimer's disease (AD) in terms of prevalence. It afflicts about 2-3% of individuals over 65 years old. The etiology of PD is unknown and several environmental and genetic factors are involved. From a pathological point of view, PD is characterized by the loss of dopaminergic neurons in the substantia nigra, which causes the abnormal accumulation of α-synuclein (α-syn) (a component of Lewy bodies), which subsequently interact with heat shock proteins (HSPs), leading to apoptosis. Apoptosis is a vital pathway for establishing homeostasis in body tissues, which is regulated by pro-apoptotic and anti-apoptotic factors. Recent findings have shown that HSPs, especially HSP27 and HSP70, play a pivotal role in regulating apoptosis by influencing the factors involved in the apoptosis pathway. Moreover, it has been reported that the expression of these HSPs in the nervous system is high. Apart from this finding, investigations have suggested that HSP27 and HSP70 (related to parkin) show a potent protective and anti-apoptotic impact against the damaging outcomes of mutant α-syn toxicity to nerve cells. Therefore, in this study, we aimed to investigate the relationship between these HSPs and apoptosis in patients with PD.

The Role of Membrane-Associated E3 Ubiquitin Ligases in Cancer

The cell membrane system comprises the plasma membrane, endoplasmic reticulum, Golgi apparatus, lysosome, mitochondria, and nuclear membrane, which are essential for maintaining normal physiological functions of cells. The proteins associated with these membrane-organelles are frequently modified to regulate their functions, the most common of which is ubiquitin modification. So far, many ubiquitin E3 ligases anchored in the membrane system have been identified as critical players facilitating intracellular biofunctions whose dysfunction is highly related to cancer. In this review, we summarized membrane-associated E3 ligases and revealed their relationship with cancer, which is of great significance for discovering novel drug targets of cancer and may open up new avenues for inducing ubiquitination-mediated degradation of cancer-associated membrane proteins via small chemicals such as PROTAC and molecular glue.

Microtubules as Regulators of Neural Network Shape and Function: Focus on Excitability, Plasticity and Memory

Neuronal microtubules (MTs) are complex cytoskeletal protein arrays that undergo activity-dependent changes in their structure and function as a response to physiological demands throughout the lifespan of neurons. Many factors shape the allostatic dynamics of MTs and tubulin dimers in the cytosolic microenvironment, such as protein–protein interactions and activity-dependent shifts in these interactions that are responsible for their plastic capabilities. Recently, several findings have reinforced the role of MTs in behavioral and cognitive processes in normal and pathological conditions. In this review, we summarize the bidirectional relationships between MTs dynamics, neuronal processes, and brain and behavioral states. The outcomes of manipulating the dynamicity of MTs by genetic or pharmacological approaches on neuronal morphology, intrinsic and synaptic excitability, the state of the network, and behaviors are heterogeneous. We discuss the critical position of MTs as responders and adaptative elements of basic neuronal function whose impact on brain function is not fully understood, and we highlight the dilemma of artificially modulating MT dynamics for therapeutic purposes.

FKBP51 modulates hippocampal size and function in post-translational regulation of Parkin

FK506-binding protein 51 (encoded by Fkpb51, also known as Fkbp5) has been associated with stress-related mental illness. To investigate its function, we studied the morphological consequences of Fkbp51 deletion. Artificial Intelligence-assisted morphological analysis revealed that male Fkbp51 knock-out (KO) mice possess more elongated dentate gyrus (DG) but shorter hippocampal height in coronal sections when compared to WT. Primary cultured Fkbp51 KO hippocampal neurons were shown to exhibit larger dendritic outgrowth than wild-type (WT) controls and pharmacological manipulation experiments suggest that this may occur through the regulation of microtubule-associated protein. Both in vitro primary culture and in vivo labeling support a role for FKBP51 in the regulation of microtubule-associated protein expression. Furthermore, Fkbp51 KO hippocampi exhibited decreases in βIII-tubulin, MAP2, and Tau protein levels, but a greater than 2.5-fold increase in Parkin protein. Overexpression and knock-down FKBP51 demonstrated that FKBP51 negatively regulates Parkin in a dose-dependent and ubiquitin-mediated manner. These results indicate a potential novel post-translational regulatory mechanism of Parkin by FKBP51 and the significance of their interaction on disease onset. KO has more flattened hippocampus using AI-assisted measurement Both pyramidal cell layer (PCL) of CA and granular cell layer (GCL) of DG distinguishable as two layers: deep cell layer and superficial layer. Distinct MAP2 expression between deep and superficial layer between KO and WT, Higher Parkin expression in KO brain Mechanism of FKBP51 inhibition resulting in Parkin, MAP2, Tau, and Tubulin expression differences between KO and WT mice, and resulting neurite outgrowth differences.

βIII-Tubulin Structural Domains Regulate Mitochondrial Network Architecture in an Isotype-Specific Manner

βIII-tubulin is a neuronal microtubule protein that is aberrantly expressed in epithelial cancers. The microtubule network is implicated in regulating the architecture and dynamics of the mitochondrial network, although the isotype-specific role for β-tubulin proteins that constitute this microtubule network remains unclear. High-resolution electron microscopy revealed that manipulation of βIII-tubulin expression levels impacts the volume and shape of mitochondria. Analysis of the structural domains of the protein identifies that the C-terminal tail of βIII-tubulin, which distinguishes this protein from other β-tubulin isotypes, significantly contributes to the isotype-specific effects of βIII-tubulin on mitochondrial architecture. Mass spectrometry analysis of protein–protein interactions with β-tubulin isotypes identifies that βIII-tubulin specifically interacts with regulators of mitochondrial dynamics that may mediate these functional effects. Advanced quantitative dynamic lattice light sheet imaging of the mitochondrial network reveals that βIII-tubulin promotes a more dynamic and extended reticular mitochondrial network, and regulates mitochondrial volume. A regulatory role for the βIII-tubulin C-terminal tail in mitochondrial network dynamics and architecture has widespread implications for the maintenance of mitochondrial homeostasis in health and disease.

Mitophagy in aging and longevity

The clearance of damaged or unwanted mitochondria by autophagy (also known as mitophagy) is a mitochondrial quality control mechanism postulated to play an essential role in cellular homeostasis, metabolism, and development and confers protection against a wide range of diseases. Proper removal of damaged or unwanted mitochondria is essential for organismal health. Defects in mitophagy are associated with Parkinson's, Alzheimer's disease, cancer, and other degenerative disorders. Mitochondria regulate organismal fitness and longevity via multiple pathways, including cellular senescence, stem cell function, inflammation, mitochondrial unfolded protein response (mtUPR), and bioenergetics. Thus, mitophagy is postulated to be pivotal for maintaining organismal healthspan and lifespan and the protection against aged-related degeneration. In this review, we will summarize recent understanding of the mechanism of mitophagy and aspects of mitochondrial functions. We will focus on mitochondria-related cellular processes that are linked to aging and examine current genetic evidence that supports the hypothesis that mitophagy is a pro-longevity mechanism.

The Tyrosine Phosphatase hPTPRβ Controls the Early Signals and Dopaminergic Cells Viability via the P2X7 Receptor

ATP, one of the signaling molecules most commonly secreted in the nervous system and capable of stimulating multiple pathways, binds to the ionotropic purinergic receptors, in particular, the P2X₇ receptor (P2X₇R) and stimulates neuronal cell death. Given this effect of purinergic receptors on the viability of dopaminergic neurons model cells and that Ras GTPases control Erk1/2-regulated mitogen-activated cell proliferation and survival, we have investigated the role of the small GTPases of the Ras superfamily, together with their regulatory and effector molecules as the potential molecular intermediates in the P2X₇R-regulated cell death of SN4741 dopaminergic neurons model cells. Here, we demonstrate that the neuronal response to purinergic stimulation involves the Calmodulin/RasGRF1 activation of the small GTPase Ras and Erk1/2. We also demonstrate that tyrosine phosphatase PTPRβ and other tyrosine phosphatases regulate the small GTPase activation pathway and neuronal viability. Our work expands the knowledge on the intracellular responses of dopaminergic cells by identifying new participating molecules and signaling pathways. In this sense, the study of the molecular circuitry of these neurons is key to understanding the functional effects of ATP, as well as considering the importance of these cells in Parkinson’s Disease.

Macroautophagy and Mitophagy in Neurodegenerative Disorders: Focus on Therapeutic Interventions

Macroautophagy, a quality control mechanism, is an evolutionarily conserved pathway of lysosomal degradation of protein aggregates, pathogens, and damaged organelles. As part of its vital homeostatic role, macroautophagy deregulation is associated with various human disorders, including neurodegenerative diseases. There are several lines of evidence that associate protein misfolding and mitochondrial dysfunction in the etiology of Alzheimer’s, Parkinson’s, and Huntington’s diseases. Macroautophagy has been implicated in the degradation of different protein aggregates such as Aβ, tau, alpha-synuclein (α-syn), and mutant huntingtin (mHtt) and in the clearance of dysfunctional mitochondria. Taking these into consideration, targeting autophagy might represent an effective therapeutic strategy to eliminate protein aggregates and to improve mitochondrial function in these disorders. The present review describes our current understanding on the role of macroautophagy in neurodegenerative disorders and focuses on possible strategies for its therapeutic modulation.

Regulation Of Microtubule: Current Concepts And Relevance To Neurodegenerative Diseases

Neurodevelopmental disorders (NDDs) are abnormalities linked to neuronal structure and irregularities associated with the proliferation of cells, transportation, and differentiation. NDD also involves synaptic circuitry and neural network alterations known as synaptopathies. Microtubules (MTs) and MTs-associated proteins help to maintain neuronal health as well as their development. The microtubular dynamic structure plays a crucial role in the division of cells and forms mitotic spindles, thus take part in initiating stages of differentiation and polarization for various types of cells. The MTs also take part in the cellular death but MT-based cellular degenerations are not yet well excavated. In the last few years, studies have provided the protagonist activity of MTs in neuronal degeneration. In this review, we largely engrossed our discussion on the change of MT cytoskeleton structure, describing their organization, dynamics, transportation, and their failure causing NDDs. At end of this review, we are targeting the therapeutic neuroprotective strategies on clinical priority and also try to discuss the clues for the development of new MT-based therapy as a new pharmacological intervention. This will be a new potential site to block not only neurodegeneration but also promotes the regeneration of neurons.

DNA Methylation of Cannabinoid Receptor Interacting Protein 1 Promotes Pathogenesis of Intrahepatic Cholangiocarcinoma Through Suppressing Parkin-Dependent Pyruvate Kinase M2 Ubiquitination

BACKGROUND AND AIMS

Methylation landscape is important for maintaining the silence of cannabinoid receptor-interacting protein 1 (CNRIP1) in some tumors. However, the role of CNRIP1 in intrahepatic cholangiocarcinoma (ICC) remains poorly defined.

APPROACH AND RESULTS

In our study, we showed that CNRIP1 was down-regulated in ICC tissues, and low expression of CNRIP1 was significantly associated with poor prognosis of patients with ICC in 3-year overall survival and tumor-free survival. Investigating the genomic DNA methylation profile, we disclosed a CpG island site named CNRIP1 MS-2 (CNRIP1 methylation site-2) that contributes to the down-regulation of CNRIP1. In addition, the methylation level of CNRIP1 MS-2 was correlated to the pathological grade, metastasis, and tumor-node-metastasis classification in ICC. Notably, we observed that CNRIP1 suppressed tumor cell migration, invasion, and proliferation by inhibiting the activity of pyruvate kinase M2 (PKM2). Sustained overexpression of CNRIP1 suppressed the in vivo tumor growth in a mouse xenograft model. It was also found that CNRIP1 overexpression activated Parkin (an E3 ubiquitin ligase), which resulted in the protein degradation of PKM2 in ICC cells.

CONCLUSIONS

We identified that CNRIP1 acted as a putative tumor suppressor in ICC, which suggested that CNRIP1 could be a candidate biomarker for predicting tumor recurrence in patients with ICC. Furthermore, these findings highlight a potential therapeutic approach in targeting the CNRIP1/Parkin/PKM2 pathway for the treatment of ICC.

Microtubule Dysfunction: A Common Feature of Neurodegenerative Diseases

Neurons are particularly susceptible to microtubule (MT) defects and deregulation of the MT cytoskeleton is considered to be a common insult during the pathogenesis of neurodegenerative disorders. Evidence that dysfunctions in the MT system have a direct role in neurodegeneration comes from findings that several forms of neurodegenerative diseases are associated with changes in genes encoding tubulins, the structural units of MTs, MT-associated proteins (MAPs), or additional factors such as MT modifying enzymes which modulating tubulin post-translational modifications (PTMs) regulate MT functions and dynamics. Efforts to use MT-targeting therapeutic agents for the treatment of neurodegenerative diseases are underway. Many of these agents have provided several benefits when tested on both in vitro and in vivo neurodegenerative model systems. Currently, the most frequently addressed therapeutic interventions include drugs that modulate MT stability or that target tubulin PTMs, such as tubulin acetylation. The purpose of this review is to provide an update on the relevance of MT dysfunctions to the process of neurodegeneration and briefly discuss advances in the use of MT-targeting drugs for the treatment of neurodegenerative disorders.

Modeling the pathophysiology of Parkinson's disease in patient-specific neurons

The 30 trillion cells that self-assemble into a human being originate from the pluripotent stem cells in the inner cell mass of a human blastocyst. The discovery of induced pluripotent stem cells (iPSCs) makes it possible to approximate various aspects of this natural developmental process artificially by generating materials that can be used in invasive mechanistic studies of virtually all human conditions. In Parkinson's disease, instructions computed by the basal ganglia to control voluntary motor functions break down, leading to widespread rhythmic bursting activities in the basal ganglia and beyond. It is thought that these oscillatory neuronal activities, which disrupt aperiodic neurotransmission in a normal brain, may reduce information content in the instructions for motor control. Using midbrain neuronal cultures differentiated from iPSCs of Parkinson's disease patients with parkin mutations, we find that parkin mutations cause oscillatory neuronal activities when dopamine D1-class receptors are activated. This system makes it possible to study the molecular basis of rhythmic bursting activities in Parkinson's disease. Further development of stem cell models of Parkinson's disease will enable better approximation of the situation in the brain of Parkinson's disease patients. In this review, I will discuss what has been found in the past about the pathophysiology of motor dysfunction in Parkinson's disease, especially oscillatory neuronal activities and how stem cell technologies may transform our abilities to understand the pathophysiology of Parkinson's disease.

Tau and Alpha Synuclein Synergistic Effect in Neurodegenerative Diseases: When the Periphery Is the Core

In neuronal cells, tau is a microtubule-associated protein placed in axons and alpha synuclein is enriched at presynaptic terminals. They display a propensity to form pathologic aggregates, which are considered the underlying cause of Alzheimer's and Parkinson's diseases. Their functional impairment induces loss of axonal transport, synaptic and mitochondrial disarray, leading to a "dying back" pattern of degeneration, which starts at the periphery of cells. In addition, pathologic spreading of alpha-synuclein from the peripheral nervous system to the brain through anatomical connectivity has been demonstrated for Parkinson's disease. Thus, examination of the extent and types of tau and alpha-synuclein in peripheral tissues and their relation to brain neurodegenerative diseases is of relevance since it may provide insights into patterns of protein aggregation and neurodegeneration. Moreover, peripheral nervous tissues are easily accessible in-vivo and can play a relevant role in the early diagnosis of these conditions. Up-to-date investigations of tau species in peripheral tissues are scant and have mainly been restricted to rodents, whereas, more evidence is available on alpha synuclein in peripheral tissues. Here we aim to review the literature on the functional role of tau and alpha synuclein in physiological conditions and disease at the axonal level, their distribution in peripheral tissues, and discuss possible commonalities/diversities as well as their interaction in proteinopathies.

Modeling Parkinson's Disease Using Induced Pluripotent Stem Cells

Parkinson's disease (PD) is the second most common neurodegenerative disease. The molecular mechanisms of PD at the cellular level involve oxidative stress, mitochondrial dysfunction, autophagy, axonal transport, and neuroinflammation. Induced pluripotent stem cells (iPSCs) with patient-specific genetic background are capable of directed differentiation into dopaminergic neurons. Cell models based on iPSCs are powerful tools for studying the molecular mechanisms of PD. The iPSCs used for PD studies were mainly from patients carrying mutations in synuclein alpha (SNCA), leucine-rich repeat kinase 2 (LRRK2), PTEN-induced putative kinase 1 (PINK1), parkin RBR E3 ubiquitin protein ligase (PARK2), cytoplasmic protein sorting 35 (VPS35), and variants in glucosidase beta acid (GBA). In this review, we summarized the advances in molecular mechanisms of Parkinson's disease using iPSC models.

Neuronal microtubules and proteins linked to Parkinson's disease: a relevant interaction?

Neuronal microtubules are key determinants of cell morphology, differentiation, migration and polarity, and contribute to intracellular trafficking along axons and dendrites. Microtubules are strictly regulated and alterations in their dynamics can lead to catastrophic effects in the neuron. Indeed, the importance of the microtubule cytoskeleton in many human diseases is emerging. Remarkably, a growing body of evidence indicates that microtubule defects could be linked to Parkinson's disease pathogenesis. Only a few of the causes of the progressive neuronal loss underlying this disorder have been identified. They include gene mutations and toxin exposure, but the trigger leading to neurodegeneration is still unknown. In this scenario, the evidence showing that mutated proteins in Parkinson's disease are involved in the regulation of the microtubule cytoskeleton is intriguing. Here, we focus on α-Synuclein, Parkin and Leucine-rich repeat kinase 2 (LRRK2), the three main proteins linked to the familial forms of the disease. The aim is to dissect their interaction with tubulin and microtubules in both physiological and pathological conditions, in which these proteins are overexpressed, mutated or absent. We highlight the relevance of such an interaction and suggest that these proteins could trigger neurodegeneration via defective regulation of the microtubule cytoskeleton.

Much More Than a Scaffold: Cytoskeletal Proteins in Neurological Disorders

Recent observations related to the structure of the cytoskeleton in neurons and novel cytoskeletal abnormalities involved in the pathophysiology of some neurological diseases are changing our view on the function of the cytoskeletal proteins in the nervous system. These efforts allow a better understanding of the molecular mechanisms underlying neurological diseases and allow us to see beyond our current knowledge for the development of new treatments. The neuronal cytoskeleton can be described as an organelle formed by the three-dimensional lattice of the three main families of filaments: actin filaments, microtubules, and neurofilaments. This organelle organizes well-defined structures within neurons (cell bodies and axons), which allow their proper development and function through life. Here, we will provide an overview of both the basic and novel concepts related to those cytoskeletal proteins, which are emerging as potential targets in the study of the pathophysiological mechanisms underlying neurological disorders.

Loss of HIPK2 Protects Neurons from Mitochondrial Toxins by Regulating Parkin Protein Turnover

Mitochondria are important sources of energy, but they are also the target of cellular stress, toxin exposure, and aging-related injury. Persistent accumulation of damaged mitochondria has been implicated in many neurodegenerative diseases. One highly conserved mechanism to clear damaged mitochondria involves the E3 ubiquitin ligase Parkin and PTEN-induced kinase 1 (PINK1), which cooperatively initiate the process called mitophagy that identifies and eliminates damaged mitochondria through the autophagosome and lysosome pathways. Parkin is a mostly cytosolic protein, but is rapidly recruited to damaged mitochondria and target them for mitophagy. Moreover, Parkin interactomes also involve signaling pathways and transcriptional machinery critical for survival and cell death. However, the mechanism that regulates Parkin protein level remains poorly understood. Here, we show that the loss of homeodomain interacting protein kinase 2 (HIPK2) in neurons and mouse embryonic fibroblasts (MEFs) has a broad protective effect from cell death induced by mitochondrial toxins. The mechanism by which Hipk2-/- neurons and MEFs are more resistant to mitochondrial toxins is in part due to the role of HIPK2 and its kinase activity in promoting Parkin degradation via the proteasome-mediated mechanism. The loss of HIPK2 leads to higher cytosolic Parkin protein levels at basal conditions and upon exposure to mitochondrial toxins, which protects mitochondria from toxin-induced damage. In addition, Hipk2-/- neurons and MEFs show increased expression of PGC-1α (peroxisome proliferator-activated receptor-γ coactivator 1), a Parkin downstream target that can provide additional benefits via transcriptional activation of mitochondrial genes. Together, these results reveal a previously unrecognized avenue to target HIPK2 in neuroprotection via the Parkin-mediated pathway.SIGNIFICANCE STATEMENT In this study, we provide evidence that homeodomain interacting protein kinase 2 (HIPK2) and its kinase activity promote Parkin degradation via the proteasome-mediated pathway. The loss of HIPK2 increases cytosolic and mitochondrial Parkin protein levels under basal conditions and upon exposure to mitochondrial toxins, which protect mitochondria from toxin-induced damage. In addition, Hipk2-/- neurons and mouse embryonic fibroblasts also show increased expression of PGC-1α (peroxisome proliferator-activated receptor-γ coactivator 1), a Parkin downstream target that can provide additional benefits via transcriptional activation of mitochondrial genes. These results indicate that targeting HIPK2 and its kinase activity can have neuroprotective effects by elevating Parkin protein levels.

Parkin is transcriptionally regulated by the aryl hydrocarbon receptor: Impact on α-synuclein protein levels

Parkin (PRKN) is a ubiquitin E3 ligase that catalyzes the ubiquitination of several proteins. Mutations in the human Parkin gene, PRKN, leads to degeneration of dopaminergic (DA) neurons, resulting in autosomal recessive early-onset parkinsonism and the loss of PRKN function is linked to sporadic Parkinson's disease (PD). Additionally, several in vitro studies have shown that overexpression of exogenous PRKN protects against the neurotoxic effects induced by a wide range of cellular stressors, emphasizing the need to study the mechanism(s) governing PRKN expression and induction. Here, Prkn was identified as a novel target gene of the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor and member of the bHLH/PAS (basic helix-loop-helix/Per-Arnt-Sim) superfamily. AhR binds and transactivates the Prkn gene promoter. We also demonstrated that AhR is expressed in DA neurons and that its activation upregulates Prkn mRNA and protein levels in the mouse ventral midbrain. Additionally, the AhR-dependent increase in PRKN levels is associated with a decrease in the protein levels of its target substrate, α-synuclein, in an AhR-dependent manner, because this effect is not observed in Ahr-null mice. These results suggest that treatments designed to induce PRKN expression through the use of nontoxic AhR agonist ligands may be novel strategies to prevent and delay PD.

Prionoid Proteins in the Pathogenesis of Neurodegenerative Diseases

There is a growing body of evidence that prionoid protein behaviors are a core element of neurodegenerative diseases (NDs) that afflict humans. Common elements in pathogenesis, pathological effects and protein-level behaviors exist between Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). These extend beyond the affected neurons to glial cells and processes. This results in a complicated system of disease progression, which often takes advantage of protective processes to promote the propagation of pathological protein aggregates. This review article provides a current snapshot of knowledge on these proteins and their intrinsic role in the pathogenesis and disease progression seen across NDs.

A MAPK/c-Jun-mediated switch regulates the initial adaptive and cell death responses to mitochondrial damage in a neuronal cell model

Parkinson's disease (PD) is defined by the progressive loss of dopaminergic neurons. Mitochondrial dysfunction and oxidative stress are associated with PD although it is not fully understood how neurons respond to these stresses. How adaptive and apoptotic neuronal stress response pathways are regulated and the thresholds at which they are activated remains ambiguous. Utilising SH-SY5Y neuroblastoma cells, we show that MAPK/AP-1 pathways are critical in regulating the response to mitochondrial uncoupling. Here we found the AP-1 transcription factor c-Jun can act in either a pro- or anti-apoptotic manner, depending on the level of stress. JNK-mediated cell death in differentiated cells only occurred once a threshold of stress was surpassed. We also identified a novel feedback loop between Parkin activity and the c-Jun response, suggesting defective mitophagy may initiate MAPK/c-Jun-mediated neuronal loss observed in PD. Our data supports the hypothesis that blocking cell death pathways upstream of c-Jun as a therapeutic target in PD may not be appropriate due to crossover of the pro- and anti-apoptotic responses. Boosting adaptive responses or targeting specific aspects of the neuronal death response may therefore represent more viable therapeutic strategies.

Parkin absence accelerates microtubule aging in dopaminergic neurons

Loss-of-function caused by mutations in the parkin gene (PARK2) lead to early-onset familial Parkinson's disease. Recently, mechanistic studies proved the ability of parkin in regulating mitochondria homeostasis and microtubule (MT) stability. Looking at these systems during aging of PARK2 knockout mice, we found that loss of parkin induced an accelerated (over)acetylation of MT system both in dopaminergic neuron cell bodies and fibers, localized in the substantia nigra and corpus striatum, respectively. Interestingly, in PARK2 knockout mice, changes of MT stability preceded the alteration of mitochondria transport. Moreover, in-cell experiments confirmed that loss of parkin affects mitochondria mobility and showed that this defect depends on MT system as it is rescued by paclitaxel, a well-known MT-targeted agent. Furthermore, both in PC12 neuronal cells and in patients' induced pluripotent stem cell-derived midbrain neurons, we observed that parkin deficiencies cause the fragmentation of stable MTs. Therefore, we suggest that parkin acts as a regulator of MT system during neuronal aging, and we endorse the hypothesis that MT dysfunction may be crucial in the pathogenesis of Parkinson's disease.

Microtubule dynamics and the neurodegenerative triad of Alzheimer's disease: The hidden connection

Alzheimer's disease (AD) is the most common neurodegenerative disorder and is, on a histopathological level, characterized by the presence of extracellular amyloid plaques composed of the protein fragment Aβ, and intracellular neurofibrillary tangles, which contain the microtubule-associated protein tau in a hyperphosphorylated state. In AD defects in microtubule (MT) assembly and organization have also been reported; however, it is unclear whether MT abnormalities have a causal and early role in the disease process or represent a common end point downstream of the neurodegenerative cascade. Recent evidence indicates that microtubule-stabilizing drugs prevent axonopathy in animal models of tauopathies and reverse Aβ-induced loss of synaptic connectivity in an ex vivo model of amyloidosis. This could suggest that MT dysfunction connects some of the degenerative events and provides a useful target to simultaneously prevent several neurodegenerative processes in AD. Here, we describe how changes in the structure and dynamics of MTs are involved in the different aspects of the neurodegenerative triad of AD. We discuss evidence that MTs are affected both by tau-dependent and tau-independent mechanisms but appear to be regulated in a distinct way in different neuronal compartments. We argue that modulation of MT dynamics could be of potential benefit but needs to be precisely controlled in a cell and compartment-specific manner to avoid harmful side effects. This article is part of the series "Beyond Amyloid".

Mechanisms underlying neurodegeneration in Huntington disease: applications to novel disease-modifying therapies

The CAG repeat expansion mutation that causes Huntington Disease (HD) was discovered more than 20 years ago, yet no treatment has yet been developed to stop the relentless course of the disease. Nonetheless, substantial progress has been made in understanding HD pathogenesis. We review insights that have been gleaned from HD genetics, metabolism, and pathology; HD mouse and cell models; the structure, function and post-translational modification of normal and mutant huntingtin (htt) protein; gene expression profiles in HD cells and tissue; the neurotoxicy of mutant htt RNA; and the expression of an antisense transcript from the HD locus. We conclude that rationale therapeutics for HD is within sight, though many questions remain to be answered.

Microtubule Destabilization Paves the Way to Parkinson's Disease

Microtubules are dynamic structures normally associated to the cell division, during which they form the mitotic spindle, as well as to the initial phases of specification and polarization of various cell types, including neurons. Although microtubules could have a role in the death of many cells and tissues, the microtubule-based degenerative mechanisms have been poorly investigated; nevertheless, during the last two decades, many clues have been accumulated suggesting the importance of the microtubule system during neurodegeneration. Thus, the aim of this review is to analyse how the changes of the microtubule cytoskeleton, in terms of organization and dynamics, as well as the failure of the microtubule-dependent neuronal processes, as axonal transport, may play a pivotal role in the chain of events leading to Parkinson's disease. Last but not least, since disease-modifying or neuroprotective strategies are a clinical priority in Parkinson's disease, we will also present the hints about the concrete possibility of a microtubule-targeted therapy, which would have the potentiality to block the running degenerative events and to prompt the regeneration of the lost tissues.

Back to the tubule: microtubule dynamics in Parkinson's disease

Cytoskeletal homeostasis is essential for the development, survival and maintenance of an efficient nervous system. Microtubules are highly dynamic polymers important for neuronal growth, morphology, migration and polarity. In cooperation with several classes of binding proteins, microtubules regulate long-distance intracellular cargo trafficking along axons and dendrites. The importance of a delicate interplay between cytoskeletal components is reflected in several human neurodegenerative disorders linked to abnormal microtubule dynamics, including Parkinson’s disease (PD). Mounting evidence now suggests PD pathogenesis might be underlined by early cytoskeletal dysfunction. Advances in genetics have identified PD-associated mutations and variants in genes encoding various proteins affecting microtubule function including the microtubule-associated protein tau. In this review, we highlight the role of microtubules, their major posttranslational modifications and microtubule associated proteins in neuronal function. We then present key evidence on the contribution of microtubule dysfunction to PD. Finally, we discuss how regulation of microtubule dynamics with microtubule-targeting agents and deacetylase inhibitors represents a promising strategy for innovative therapeutic development.

Parkin Regulation and Neurodegenerative Disorders

Parkin is a unique, multifunctional ubiquitin ligase whose various roles in the cell, particularly in neurons, are widely thought to be protective. The pivotal role that Parkin plays in maintaining neuronal survival is underscored by our current recognition that Parkin dysfunction represents not only a predominant cause of familial parkinsonism but also a formal risk factor for the more common, sporadic form of Parkinson’s disease (PD). Accordingly, keen research on Parkin over the past decade has led to an explosion of knowledge regarding its physiological roles and its relevance to PD. However, our understanding of Parkin is far from being complete. Indeed, surprises emerge from time to time that compel us to constantly update the paradigm of Parkin function. For example, we now know that Parkin’s function is not confined to mere housekeeping protein quality control (QC) roles but also includes mitochondrial homeostasis and stress-related signaling. Furthermore, emerging evidence also suggest a role for Parkin in several other major neurodegenerative diseases including Alzheimer’s disease (AD) and Amyotrophic Lateral Sclerosis (ALS). Yet, it remains truly amazing to note that a single enzyme could serve such multitude of functions and cellular roles. Clearly, its activity has to be tightly regulated. In this review, we shall discuss this and how dysregulated Parkin function may precipitate neuronal demise in various neurodegenerative disorders.

Linking microtubules to Parkinson's disease: the case of parkin

Microtubules (MTs) are dynamic polymers consisting of α/β tubulin dimers and playing a plethora of roles in eukaryotic cells. Looking at neurons, they are key determinants of neuronal polarity, axonal transport and synaptic plasticity. The concept that MT dysfunction can participate in, and perhaps lead to, Parkinson's disease (PD) progression has been suggested by studies using toxin-based and genetic experimental models of the disease. Here, we first learn lessons from MPTP and rotenone as well as from the PD related genes, including SNCA and LRRK2, and then look at old and new evidence regarding the interplay between parkin and MTs. Data from experimental models and human cells point out that parkin regulates MT stability and strengthen the link between MTs and PD paving the way to a viable strategy for the management of the disease.

Parkin represses 6-hydroxydopamine-induced apoptosis via stabilizing scaffold protein p62 in PC12 cells

AIM

Parkin has been shown to exert protective effects against 6-hydroxydopamine (6-OHDA)-induced neurotoxicity in different models of Parkinson disease. In the present study we investigated the molecular mechanisms underlying the neuroprotective action of parkin in vitro.

METHODS

HEK293, HeLa and PC12 cells were transfected with parkin, parkin mutants, p62 or si-p62. Protein expression and ubiquitination were assessed using immunoblot analysis. Immunoprecipitation assay was performed to identify the interaction between parkin and scaffold protein p62. PC12 and SH-SY5Y cells were treated with 6-OHDA (200 μmol/L), and cell apoptosis was detected using PI and Hoechst staining.

RESULTS

In HEK293 cells co-transfected with parkin and p62, parkin was co-immunoprecipitated with p62, and parkin overexpression increased p62 protein levels. In parkin-deficient HeLa cells, transfection with wild-type pakin, but not with ligase activity-deficient pakin mutants, significantly increased p62 levels, suggesting that parkin stabilized p62 through its E3 ligase activity. Transfection with parkin or p62 significantly repressed ERK1/2 phosphorylation in HeLa cells, but transfection with parkin did not repress ERK1/2 phosphorylation in p62-knockdown HeLa cells, suggesting that p62 was involved in parkin-induced inhibition on ERK1/2 phosphorylation. Overexpression of parkin or p62 significantly repressed 6-OHDA-induced ERK1/2 phosphorylation in PC12 cells, and parkin overexpression inhibited 6-OHDA-induced apoptosis in PC12 and SH-SY5Y cells.

CONCLUSION

Parkin protects PC12 cells against 6-OHDA-induced apoptosis via ubiquitinating and stabilizing scaffold protein p62, and repressing ERK1/2 activation.

Parkin mutations reduce the complexity of neuronal processes in iPSC-derived human neurons

Parkinson's disease (PD) is characterized by the degeneration of nigral dopaminergic (DA) neurons and non-DA neurons in many parts of the brain. Mutations of parkin, an E3 ubiquitin ligase that strongly binds to microtubules, are the most frequent cause of recessively inherited PD. The lack of robust PD phenotype in parkin knockout mice suggests a unique vulnerability of human neurons to parkin mutations. Here, we show that the complexity of neuronal processes as measured by total neurite length, number of terminals, number of branch points, and Sholl analysis was greatly reduced in induced pluripotent stem cell (iPSC)-derived TH(+) or TH(-) neurons from PD patients with parkin mutations. Consistent with these, microtubule stability was significantly decreased by parkin mutations in iPSC-derived neurons. Overexpression of parkin, but not its PD-linked mutant nor green fluorescent protein, restored the complexity of neuronal processes and the stability of microtubules. Consistent with these, the microtubule-depolymerizing agent colchicine mimicked the effect of parkin mutations by decreasing neurite length and complexity in control neurons while the microtubule-stabilizing drug taxol mimicked the effect of parkin overexpression by enhancing the morphology of parkin-deficient neurons. The results suggest that parkin maintains the morphological complexity of human neurons by stabilizing microtubules.

Agmatine Protects Against 6-OHDA-Induced Apoptosis, and ERK and Akt/GSK Disruption in SH-SY5Y Cells

6-Hydroxydopamine (6-OHDA), a metabolite of dopamine is known to induce dopaminergic cell toxicity which makes that a suitable agent inducing an experimental model of Parkinson's disease (PD). Agmatine has been shown to protect against some cellular and animal PD models. This study was aimed to assess whether agmatine prevents 6-OHDA-induced SH-SY5Y cell death and if yes, then how it affects Akt/glycogen synthesis kinase-3β (GSK-3β) and extracellular signal-regulated kinases (ERK) signals. The cells were treated with different drugs, and their viability was examined via MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide) assay and morphological observation. Western blot studies were done to assess cleaved caspase-3, Akt/GSK-3β, and ERK proteins. 6-OHDA-induced cell death and caspase-3 cleavage, while agmatine prevented those changes. 6-OHDA also decreased the amount of phosphorylated Akt (pAkt)/Akt while increased GSK-3β activity which was prevented by agmatine. Additionally, this toxin increased pERK/ERK ratio which was averted again by agmatine. The PI3/Akt inhibitor, LY294002, impeded the changes induced by agmatine, while ERK inhibitor (PD98059) did not disturb the effects of agmatine, and by itself, it preserved the cells against 6-OHDA toxicity. This study revealed that agmatine is protective in 6-OHDA model of PD and affects Akt/GSK-3β and ERK pathways.

iPS models of Parkin and PINK1

Parkinson disease (PD) is a degenerative disorder of the central nervous system resulting from depletion of dopaminergic neurons and currently remains incurable despite enormous international research efforts. The development of induced pluripotent stem cell (iPSC) technology opened up the unique possibility of studying disease mechanisms in human tissue that was otherwise not accessible, such as the brain. Of particular interest are the monogenetic forms of PD as they closely resemble the more common ‘idiopathic’ PD and, through the mutated protein, provide a clear research target in iPSC-derived neurons. Recessively inherited Parkin and PTEN-induced putative kinase 1 (PINK1) mutations have been investigated in this context and the present review describes the first insights gained from studies in iPSC-derived dopaminergic neurons, which comprise abnormalities in mitochondrial and dopamine homoeostasis, microtubular stability and axonal outgrowth. These new models of PD have a high translational potential that includes the identification of druggable targets, testing of known and novel therapeutic agents in the disease-relevant tissue using well-defined read-outs and potential regenerative approaches.

Parkin structure and function

Mutations in the parkin or PINK1 genes are the leading cause of the autosomal recessive form of Parkinson’s disease. The gene products, the E3 ubiquitin ligase parkin and the serine/threonine kinase PINK1, are neuroprotective proteins, which act together in a mitochondrial quality control pathway. Here, we review the structure of parkin and mechanisms of its autoinhibition and function as a ubiquitin ligase. We present a model for the recruitment and activation of parkin as a key regulatory step in the clearance of depolarized or damaged mitochondria by autophagy (mitophagy). We conclude with a brief overview of other functions of parkin and considerations for drug discovery in the mitochondrial quality control pathway.

Cytoskeletal disruption activates the DLK/JNK pathway, which promotes axonal regeneration and mimics a preconditioning injury

Nerve injury can lead to axonal regeneration, axonal degeneration, and/or neuronal cell death. Remarkably, the MAP3K dual leucine zipper kinase, DLK, promotes each of these responses, suggesting that DLK is a sensor of axon injury. In Drosophila, mutations in proteins that stabilize the actin and microtubule cytoskeletons activate the DLK pathway, suggesting that DLK may be activated by cytoskeletal disruption. Here we test this model in mammalian sensory neurons. We find that pharmacological agents designed to disrupt either the actin or microtubule cytoskeleton activate the DLK pathway, and that activation is independent of calcium influx or induction of the axon degeneration program. Moreover, activation of the DLK pathway by targeting the cytoskeleton induces a pro-regenerative state, enhancing axon regeneration in response to a subsequent injury in a process akin to preconditioning. This highlights the potential utility of activating the DLK pathway as a method to improve axon regeneration. Moreover, DLK is required for these responses to cytoskeletal perturbations, suggesting that DLK functions as a key neuronal sensor of cytoskeletal damage.

JNK inhibition of VMAT2 contributes to rotenone-induced oxidative stress and dopamine neuron death

Treatment with rotenone, both in vitro and in vivo, is widely used to model dopamine neuron death in Parkinson's disease upon exposure to environmental neurotoxicants and pesticides. Mechanisms underlying rotenone neurotoxicity are still being defined. Our recent studies suggest that rotenone-induced dopamine neuron death involves microtubule destabilization, which leads to accumulation of cytosolic dopamine and consequently reactive oxygen species (ROS). Furthermore, the c-Jun N-terminal protein kinase (JNK) is required for rotenone-induced dopamine neuron death. Here we report that the neural specific JNK3 isoform of the JNKs, but not JNK1 or JNK2, is responsible for this neuron death in primary cultured dopamine neurons. Treatment with taxol, a microtubule stabilizing agent, attenuates rotenone-induced phosphorylation and presumably activation of JNK. This suggests that JNK is activated by microtubule destabilization upon rotenone exposure. Moreover, rotenone inhibits VMAT2 activity but not VMAT2 protein levels. Significantly, treatment with SP600125, a pharmacological inhibitor of JNKs, attenuates rotenone inhibition of VMAT2. Furthermore, decreased VMAT2 activity following in vitro incubation of recombinant JNK3 protein with purified mesencephalic synaptic vesicles suggests that JNK3 can inhibit VMAT2 activity. Together with our previous findings, these results suggest that rotenone induces dopamine neuron death through a series of sequential events including microtubule destabilization, JNK3 activation, VMAT2 inhibition, accumulation of cytosolic dopamine, and generation of ROS. Our data identify JNK3 as a novel regulator of VMAT2 activity.

ERK/MAPK regulates ERRγ expression, transcriptional activity and receptor-mediated tamoxifen resistance in ER+ breast cancer

Selective estrogen receptor modulators such as tamoxifen (TAM) significantly improve breast cancer-specific survival for women with estrogen receptor-positive (ER+) disease. However, resistance to TAM remains a major clinical problem. The resistant phenotype is usually not driven by loss or mutation of the estrogen receptor; instead, changes in multiple proliferative and/or survival pathways over-ride the inhibitory effects of TAM. Estrogen-related receptor γ (ERRγ) is an orphan member of the nuclear receptor superfamily that promotes TAM resistance in ER+ breast cancer cells. This study sought to clarify the mechanism(s) by which this orphan nuclear receptor is regulated, and hence affects TAM resistance. mRNA and protein expression/phosphorylation were monitored by RT-PCR and western blotting, respectively. Site-directed mutagenesis was used to disrupt consensus extracellular signal-regulated kinase (ERK) target sites. Cell proliferation and cell-cycle progression were measured by flow cytometric methods. ERRγ transcriptional activity was assessed by dual-luciferase promoter-reporter assays. We show that ERRγ protein levels are affected by the activation state of ERK/mitogen-activated protein kinase, and mutation of consensus ERK target sites impairs ERRγ-driven transcriptional activity and TAM resistance. These findings shed new light on the functional significance of ERRγ in ER+ breast cancer, and are the first to demonstrate a role for kinase regulation of this orphan nuclear receptor.

Parkin and mitochondrial quality control: toward assembling the puzzle

Parkin is an E3 ubiquitin ligase associated with autosomal-recessive Parkinsonism. Moreover, parkin inactivation has been found in sporadic Parkinson's disease (PD), suggesting a wider pathogenic impact than initially predicted. Beyond its role in PD, parkin has also been implicated in innate immune responses. Since its discovery, mounting evidence indicates that parkin can mediate degradative as well as nondegradative ubiquitination. Here we review recent insights into the structure of parkin, the mechanism of its E3 ligase activity, and its functional versatility in an attempt to merge controversial aspects into a more comprehensive picture of this multifaceted E3 ubiquitin ligase.

An emerging role of PARK2 in cancer

PARK2 (PARKIN) is an E3 ubiquitin ligase involved in multiple signaling pathways and cellular processes. Activity of PARK2 is tightly regulated through inter- and intra-molecular interactions. Dysfunction of PARK2 is associated with the progression of parkinsonism. Notably, frequent PARK2 inactivation has been identified in various human cancers. Park2-deficient mice are more susceptible to tumorigenesis, indicating its crucial role as a tumor suppressor. However, biological studies also show that PARK2 possesses both pro-survival and growth suppressive functions. Here, we summarize the genetic lesions of PARK2 in human cancers and discuss the current knowledge of PARK2 in cancer progression. We further highlight future efforts for the study of PARK2 in cancer.