The parkinsonism producing neurotoxin MPP+affects microtubule dynamics by acting as a destabilising factor

Evaluating the Role of Lin-11, Isl-1, and Mec-3 Kinases in Dopaminergic Neurodegeneration in a Subacute 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine Model of Parkinson's Disease

The malfunctioning of microtubules is highly correlated with neurodegenerative disorders such as Parkinson's disease (PD), although whether it is a cause or an effect of neurodegeneration is yet unknown. Lin-11, Isl-1, and Mec-3 kinases (LIMKs), being one of the important kinases, regulate the neuronal cytoskeleton by controlling the phosphorylation of the cofilin/actin-depolymerizing factor. Recently, we showed that upregulation of phosphorylated LIMK1 (p-LIMK1) affects the microtubule dynamics in a central nervous system traumatic injury. The goal of this study is to correlate the expression of LIMK1 with dopaminergic neuron death in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of PD, one of the well-established subacute models of PD, where the neurotoxin acts via inhibition of mitochondrial complex I of the electron transport chain. Herein, we found that LIMK1 expression was increased and correlated to dopaminergic neuronal death. Finally, we demonstrated that the treatment with LIMK inhibitor BMS-5 significantly reversed the neurodegeneration, along with an upregulation of the dynamic tubulins, indicating the relevance of LIMKs and microtubule dynamics in neurodegeneration. Therefore, targeting the microtubules, an integral part of the neuronal cytoskeleton and neurite formation, can be a promising strategy to combat degeneration of dopaminergic neurons.

Doxycycline inhibits dopaminergic neurodegeneration through upregulation of axonal and synaptic proteins

Doxycycline (DOX) is a widely used antibiotic that is able to cross the blood-brain barrier. Several studies have shown its neuroprotective effect against neurodegeneration and have associated it with antioxidant, anti-apoptotic, and anti-inflammatory mechanisms. We have recently demonstrated that DOX mimics nerve growth factor (NGF) signaling in PC12 cells. However, the involvement of this mechanism in the neuroprotective effect of DOX is unknown. Axonal degeneration and synaptic loss are key events at the early stages of neurodegeneration, and precede the neuronal death in neurodegenerative diseases, including Parkinson's disease (PD). Therefore, the regeneration of the axonal and synaptic network might be beneficial in PD. The effect of DOX in PC12 cells treated with the Parkinsonian neurotoxin 1-methyl-4-phenylpyridinium (MPP+) was addressed. Doxycycline reduced the inhibition of neuritogenesis induced by MPP+, even in cells deprived of NGF. The mechanism involved the upregulation of GAP-43, synapsin I, β-III-tubulin, F-actin, and neurofilament-200, proteins that are associated with axonal and synaptic plasticity. Considering the role of axonal degeneration and synaptic loss at the initial stages of PD, the recent advances in early diagnosis of neurodegeneration, and the advantages of drug repurposing, doxycycline is a promising candidate to treat PD.

The Parkinson's Disease-Associated Protein DJ-1 Protects Dictyostelium Cells from AMPK-Dependent Outcomes of Oxidative Stress

Mitochondrial dysfunction has been implicated in the pathology of Parkinson’s disease (PD). In Dictyostelium discoideum, strains with mitochondrial dysfunction present consistent, AMPK-dependent phenotypes. This provides an opportunity to investigate if the loss of function of specific PD-associated genes produces cellular pathology by causing mitochondrial dysfunction with AMPK-mediated consequences. DJ-1 is a PD-associated, cytosolic protein with a conserved oxidizable cysteine residue that is important for the protein’s ability to protect cells from the pathological consequences of oxidative stress. Dictyostelium DJ-1 (encoded by the gene deeJ) is located in the cytosol from where it indirectly inhibits mitochondrial respiration and also exerts a positive, nonmitochondrial role in endocytosis (particularly phagocytosis). Its loss in unstressed cells impairs endocytosis and causes correspondingly slower growth, while also stimulating mitochondrial respiration. We report here that oxidative stress in Dictyostelium cells inhibits mitochondrial respiration and impairs phagocytosis in an AMPK-dependent manner. This adds to the separate impairment of phagocytosis caused by DJ-1 knockdown. Oxidative stress also combines with DJ-1 loss in an AMPK-dependent manner to impair or exacerbate defects in phototaxis, morphogenesis and growth. It thereby phenocopies mitochondrial dysfunction. These results support a model in which the oxidized but not the reduced form of DJ-1 inhibits AMPK in the cytosol, thereby protecting cells from the adverse consequences of oxidative stress, mitochondrial dysfunction and the resulting AMPK hyperactivity.

Defective Mitochondrial Dynamics Underlie Manganese-Induced Neurotoxicity

Perturbations in mitochondrial dynamics have been observed in most neurodegenerative diseases. Here, we focus on manganese (Mn)-induced Parkinsonism-like neurodegeneration, a disorder associated with the preferential of Mn in the basal ganglia where the mitochondria are considered an early target. Despite the extensive characterization of the clinical presentation of manganism, the mechanism by which Mn mediated mitochondrial toxicity is unclear. In this study we hypothesized whether Mn exposure alters mitochondrial activity, including axonal transport of mitochondria and mitochondrial dynamics, morphology, and network. Using primary neuron cultures exposed to 100 μM Mn (which is considered the threshold of Mn toxicity in vitro) and intraperitoneal injections of MnCl2 (25mg/kg) in rat, we observed that Mn increased mitochondrial fission mediated by phosphorylation of dynamin-related protein-1 at serine 616 (p-s616-DRP1) and decreased mitochondrial fusion proteins (MFN1 and MFN2) leading to mitochondrial fragmentation, defects in mitochondrial respiratory capacity, and mitochondrial ultrastructural damage in vivo and in vitro. Furthermore, Mn exposure impaired mitochondrial trafficking by decreasing dynactin (DCTN1) and kinesin-1 (KIF5B) motor proteins and increasing destabilization of the cytoskeleton at protein and gene levels. In addition, mitochondrial communication may also be altered by Mn exposure, increasing the length of nanotunnels to reach out distal mitochondria. These findings revealed an unrecognized role of Mn in dysregulation of mitochondrial dynamics providing a potential explanation of early hallmarks of the disorder, as well as a possible common pathway with neurological disorders arising upon chronic Mn exposure.

Patterning Microtubule Network Organization Reshapes Cell-Like Compartments

Eukaryotic cells contain a cytoskeletal network comprised of dynamic microtubule filaments whose spatial organization is highly plastic. Specialized microtubule architectures are optimized for different cell types and remodel with the oscillatory cell cycle. These spatially distinct microtubule networks are thought to arise from the activity and localization of microtubule regulators and motors and are further shaped by physical forces from the cell boundary. Given complexities and redundancies of a living cell, it is challenging to disentangle the separate biochemical and physical contributions to microtubule network organization. Therefore, we sought to develop a minimal cell-like system to manipulate and spatially pattern the organization of cytoskeletal components in real-time, providing an opportunity to build distinct spatial structures and to determine how they are shaped by or reshape cell boundaries. We constructed a system for induced spatial patterning of protein components within cell-sized emulsion compartments and used it to drive microtubule network organization in real-time. We controlled dynamic protein relocalization using small molecules and light and slowed lateral diffusion within the lipid monolayer to create stable micropatterns with focused illumination. By fusing microtubule interacting proteins to optochemical dimerization domains, we directed the spatial organization of microtubule networks. Cortical patterning of polymerizing microtubules leads to symmetry breaking and forces that dramatically reshape the compartment. Our system has applications in cell biology to characterize the contributions of biochemical components and physical boundary conditions to microtubule network organization. Additionally, active shape control has uses in protocell engineering and for augmenting the functionalities of synthetic cells.

Is there a special relationship between complex I activity and nigral neuronal loss in Parkinson's disease? A critical reappraisal

Parkinson’s disease (PD) is a progressive neurodegenerative disease manifesting both motor and non-motor symptoms. The motor features are generally ascribed to the selective loss of dopamine neurons within the substantia nigra pars compacta. While the precise etiology of PD remains elusive, multiple genetic and environmental elements have emerged as contributing factors. The discovery of MPTP-induced parkinsonism directed intense inquiry towards mitochondrial pathways, with a specific focus on mitochondrial complex I. Consisting of more than 40 subunits, complex I is the first enzyme of the electron transport chain that is required for mitochondrial ATP production. In this review, we present a critical analysis of studies assessing the prevalence and specificity of mitochondrial complex I deficiency in PD. In addition, we take the novel view of incorporating the features of genetically-defined bona fide complex I disorders and the prevalence of nigral involvement in such cases. Through this innovative bi-directional view, we consider both complex I changes in a disease of the substantia nigra and nigral changes in diseases of complex I. We assess the strength of association between nigral cell loss and complex I deficits, as well as the oft under-appreciated heterogeneity of complex I deficiency disorders and the variability of the PD data.

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.

Iron-oxide minerals in the human tissues

Iron is critically important and highly regulated trace metal in the human body. However, in its free ion form, it is known to be cytotoxic; therefore, it is bound to iron storing protein, ferritin. Ferritin is a key regulator of body iron homeostasis able to form various types of minerals depending on the tissue environment. Each mineral, e.g. magnetite, maghemite, goethite, akaganeite or hematite, present in the ferritin core carry different characteristics possibly affecting cells in the tissue. In specific cases, it can lead to disease development. Widely studied connection with neurodegenerative conditions is widely studied, including Alzheimer disease. Although the exact ferritin structure and its distribution throughout a human body are still not fully known, many studies have attempted to elucidate the mechanisms involved in its regulation and pathogenesis. In this review, we try to summarize the iron uptake into the body. Next, we discuss the known occurrence of ferritin in human tissues. Lastly, we also examine the formation of iron oxides and their involvement in brain functions.

Emerging Role of Sirtuin 2 in Parkinson's Disease

Parkinson’s disease (PD), the main risk factor of which is age, is one of the most common neurodegenerative diseases, thus presenting a substantial burden on the health of affected individuals as well as an economic burden. Sirtuin 2 (SIRT2), a subtype in the family of sirtuins, belongs to class III histone deacetylases (HDACs). It is known that SIRT2 levels increase with aging, and a growing body of evidence has been accumulating, showing that the activity of SIRT2 mediates various processes involved in PD pathogenesis, including aggregation of α-synuclein (α-syn), microtubule function, oxidative stress, inflammation, and autophagy. There have been conflicting reports about the role of SIRT2 in PD, in that some studies indicate its potential to induce the death of dopaminergic (DA) neurons, and that inhibition of SIRT2 may, therefore, have protective effects in PD. Other studies suggest a protective role of SIRT2 in the context of neuronal damage. As current treatments for PD are directed at alleviating symptoms and are very limited, a comprehensive understanding of the enzymology of SIRT2 in PD may be essential for developing novel therapeutic agents for the treatment of this disease. This review article will provide an update on our knowledge of the structure, distribution, and biological characteristics of SIRT2, and highlight its role in the pathogenesis of PD.

Effect of Methionine Diet on Metabolic and Histopathological Changes of Rat Hippocampus

Hyperhomocysteinemia (hHcy) is regarded as an independent and strong risk factor for cerebrovascular diseases, stroke, and dementias. The hippocampus has a crucial role in spatial navigation and memory processes and is being constantly studied for neurodegenerative disorders. We used a moderate methionine (Met) diet at a dose of 2 g/kg of animal weight/day in duration of four weeks to induce mild hHcy in adult male Wistar rats. A novel approach has been used to explore the hippocampal metabolic changes using proton magnetic resonance spectroscopy (1H MRS), involving a 7T MR scanner in combination with histochemical and immunofluorescence analysis. We found alterations in the metabolic profile, as well as remarkable histo-morphological changes such as an increase of hippocampal volume, alterations in number and morphology of astrocytes, neurons, and their processes in the selective vulnerable brain area of animals treated with a Met-enriched diet. Results of both methodologies suggest that the mild hHcy induced by Met-enriched diet alters volume, histo-morphological pattern, and metabolic profile of hippocampal brain area, which might eventually endorse the neurodegenerative processes.

Redox Mechanisms in Neurodegeneration: From Disease Outcomes to Therapeutic Opportunities

SIGNIFICANCE

Once considered to be mere by-products of metabolism, reactive oxygen, nitrogen and sulfur species are now recognized to play important roles in diverse cellular processes such as response to pathogens and regulation of cellular differentiation. It is becoming increasingly evident that redox imbalance can impact several signaling pathways. For instance, disturbances of redox regulation in the brain mediate neurodegeneration and alter normal cytoprotective responses to stress. Very often small disturbances in redox signaling processes, which are reversible, precede damage in neurodegeneration. Recent Advances: The identification of redox-regulated processes, such as regulation of biochemical pathways involved in the maintenance of redox homeostasis in the brain has provided deeper insights into mechanisms of neuroprotection and neurodegeneration. Recent studies have also identified several post-translational modifications involving reactive cysteine residues, such as nitrosylation and sulfhydration, which fine-tune redox regulation. Thus, the study of mechanisms via which cell death occurs in several neurodegenerative disorders, reveal several similarities and dissimilarities. Here, we review redox regulated events that are disrupted in neurodegenerative disorders and whose modulation affords therapeutic opportunities.

CRITICAL ISSUES

Although accumulating evidence suggests that redox imbalance plays a significant role in progression of several neurodegenerative diseases, precise understanding of redox regulated events is lacking. Probes and methodologies that can precisely detect and quantify in vivo levels of reactive oxygen, nitrogen and sulfur species are not available.

FUTURE DIRECTIONS

Due to the importance of redox control in physiologic processes, organisms have evolved multiple pathways to counteract redox imbalance and maintain homeostasis. Cells and tissues address stress by harnessing an array of both endogenous and exogenous redox active substances. Targeting these pathways can help mitigate symptoms associated with neurodegeneration and may provide avenues for novel therapeutics. Antioxid. Redox Signal. 30, 1450-1499.

Dexmedetomidine Ameliorates the Neurotoxicity of Sevoflurane on the Immature Brain Through the BMP/SMAD Signaling Pathway

Numerous studies have demonstrated that general anesthetics might damage the nervous system, thus, the effect of general anesthetics on the developing brain has attracted much attention. Dexmedetomidine (Dex) exhibits a certain neuroprotective effect, but the mechanism is obscure. In our study, pregnant rats on gestational day 20 (G20) were exposed to 3% sevoflurane for 2 h or 4 h, and the neuronal apoptosis in hippocampal CA1 region of the offspring rats was detected by quantification of TUNEL positive cells and cleaved-caspase3 (cl-caspase3). Different doses of Dex were intraperitoneally injected before sevoflurane anesthesia; then, the expression of apoptotic-related proteins including BCL-2, BAX and cl-caspase3 as well as amyloid precursor protein (APP, a marker of axonal injury), p-CRMP-2 and CRMP-2 were measured at postnatal days 0, 1and 3 (P0, P1, and P3, respectively). As an antagonist of the bone morphgenetic proteins (BMP) receptor, DMH1 was co-administered with sevoflurane plus Dex to investigate whether BMP/SMAD is associated with the neuroprotective effects of Dex. The results showed that prenatal sevoflurane anesthesia for 4 h activated apoptosis transiently, as manifested by the caspase3 activity peaked on P1 and disappeared on P3. In addition, the expressions of APP and p-CRMP-2/CRMP-2 in postnatal rat hippocampus were significantly increased, which revealed that prenatal sevoflurane anesthesia caused axonal injury of offspring. The long-term learning and memory ability of offspring rats was also impaired after prenatal sevoflurane anesthesia. These damaging effects of sevoflurane could be mitigated by Dex and DMH1 reversed the neuroprotective effect of Dex. Our results indicated that prenatal exposure to 3% sevoflurane for 4 h increased apoptosis and axonal injury, even caused long-term learning and memory dysfunction in the offspring rats. Dex dose-dependently reduced sevoflurane- anesthesia-induced the neurotoxicity by activating the BMP/SMAD signaling pathway.

Methylene Blue Ameliorates Olfactory Dysfunction and Motor Deficits in a Chronic MPTP/Probenecid Mouse Model of Parkinson's Disease

Mitochondrial dysfunction and oxidative stress are very prominent and early features in Parkinson's disease (PD) and in animal models of PD. Thus, antioxidant therapy for PD has been proposed, but in clinical trials such strategies have met with very limited success. Methylene blue (MB), a small-molecule synthetic heterocyclic organic compound that acts as a renewable electron cycler in the mitochondrial electron transport chain, manifesting robust antioxidant and cell energetics-enhancing properties, has recently been shown to have significant beneficial effects in reducing nigrostriatal dopaminergic loss and motor impairment in acute toxin models of PD. However, no studies have investigated the impact of this promising agent in chronic models or for olfactory dysfunction, an early non-motor feature of PD. To test the efficacy of low-dose MB for olfactory dysfunction, motor symptoms, and dopaminergic neurodegeneration, mice were injected with ten subcutaneous doses of 25 mg/kg MPTP, plus 250 mg/kg intraperitoneal probenecid or saline/probenecid at 3.5-day intervals. Following the onset of olfactory dysfunction, MPTP/probenecid (MPTP/p) and saline/probenecid mice were provided drinking water with or without 1 mg/kg/day MB. Oral delivery of low-dose MB significantly ameliorated MPTP/p-induced deficits in motor coordination, as well as degeneration of tyrosine hydroxylase (TH)-positive neurons of the substantia nigra and TH-positive terminals in the striatum. Importantly, olfactory dysfunction was ameliorated by MB treatment, whereas this benefit is not observed with currently available anti-Parkinsonian medications. These results indicate that low-dose MB is a promising neuroprotective intervention for both motor and non-motor features of PD.

Metabolic Dysfunction in Parkinson's Disease: Bioenergetics, Redox Homeostasis and Central Carbon Metabolism

The loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and the accumulation of protein inclusions (Lewy bodies) are the pathological hallmarks of Parkinson's disease (PD). PD is triggered by genetic alterations, environmental/occupational exposures and aging. However, the exact molecular mechanisms linking these PD risk factors to neuronal dysfunction are still unclear. Alterations in redox homeostasis and bioenergetics (energy failure) are thought to be central components of neurodegeneration that contribute to the impairment of important homeostatic processes in dopaminergic cells such as protein quality control mechanisms, neurotransmitter release/metabolism, axonal transport of vesicles and cell survival. Importantly, both bioenergetics and redox homeostasis are coupled to neuro-glial central carbon metabolism. We and others have recently established a link between the alterations in central carbon metabolism induced by PD risk factors, redox homeostasis and bioenergetics and their contribution to the survival/death of dopaminergic cells. In this review, we focus on the link between metabolic dysfunction, energy failure and redox imbalance in PD, making an emphasis in the contribution of central carbon (glucose) metabolism. The evidence summarized here strongly supports the consideration of PD as a disorder of cell metabolism.

Altered microtubule dynamics in neurodegenerative disease: Therapeutic potential of microtubule-stabilizing drugs

Many neurodegenerative diseases are characterized by deficiencies in neuronal axonal transport, a process in which cellular cargo is shuttled with the aid of molecular motors from the cell body to axonal termini and back along microtubules (MTs). Proper axonal transport is critical to the normal functioning of neurons, and impairments in this process could contribute to the neuronal damage and death that is characteristic of neurodegenerative disease. Although the causes of axonal transport abnormalities may vary among the various neurodegenerative conditions, in many cases it appears that the transport deficiencies result from a diminution of axonal MT stability. Here we review the evidence of MT abnormalities in a number of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and traumatic brain injury, and highlight the potential benefit of MT-stabilizing agents in improving axonal transport and nerve function in these diseases. Moreover, we discuss the challenges associated with the utilization of MT-stabilizing drugs as therapeutic candidates for neurodegenerative conditions.

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.

Biallelic Mutations in TBCD, Encoding the Tubulin Folding Cofactor D, Perturb Microtubule Dynamics and Cause Early-Onset Encephalopathy

Microtubules are dynamic cytoskeletal elements coordinating and supporting a variety of neuronal processes, including cell division, migration, polarity, intracellular trafficking, and signal transduction. Mutations in genes encoding tubulins and microtubule-associated proteins are known to cause neurodevelopmental and neurodegenerative disorders. Growing evidence suggests that altered microtubule dynamics may also underlie or contribute to neurodevelopmental disorders and neurodegeneration. We report that biallelic mutations in TBCD, encoding one of the five co-chaperones required for assembly and disassembly of the αβ-tubulin heterodimer, the structural unit of microtubules, cause a disease with neurodevelopmental and neurodegenerative features characterized by early-onset cortical atrophy, secondary hypomyelination, microcephaly, thin corpus callosum, developmental delay, intellectual disability, seizures, optic atrophy, and spastic quadriplegia. Molecular dynamics simulations predicted long-range and/or local structural perturbations associated with the disease-causing mutations. Biochemical analyses documented variably reduced levels of TBCD, indicating relative instability of mutant proteins, and defective β-tubulin binding in a subset of the tested mutants. Reduced or defective TBCD function resulted in decreased soluble α/β-tubulin levels and accelerated microtubule polymerization in fibroblasts from affected subjects, demonstrating an overall shift toward a more rapidly growing and stable microtubule population. These cells displayed an aberrant mitotic spindle with disorganized, tangle-shaped microtubules and reduced aster formation, which however did not alter appreciably the rate of cell proliferation. Our findings establish that defective TBCD function underlies a recognizable encephalopathy and drives accelerated microtubule polymerization and enhanced microtubule stability, underscoring an additional cause of altered microtubule dynamics with impact on neuronal function and survival in the developing brain.

A Case for Microtubule Vulnerability in Amyotrophic Lateral Sclerosis: Altered Dynamics During Disease

Amyotrophic lateral sclerosis (ALS) is an aggressive multifactorial disease converging on a common pathology: the degeneration of motor neurons (MNs), their axons and neuromuscular synapses. This vulnerability and dysfunction of MNs highlights the dependency of these large cells on their intracellular machinery. Neuronal microtubules (MTs) are intracellular structures that facilitate a myriad of vital neuronal functions, including activity dependent axonal transport. In ALS, it is becoming increasingly apparent that MTs are likely to be a critical component of this disease. Not only are disruptions in this intracellular machinery present in the vast majority of seemingly sporadic cases, recent research has revealed that mutation to a microtubule protein, the tubulin isoform TUBA4A, is sufficient to cause a familial, albeit rare, form of disease. In both sporadic and familial disease, studies have provided evidence that microtubule mediated deficits in axonal transport are the tipping point for MN survivability. Axonal transport deficits would lead to abnormal mitochondrial recycling, decreased vesicle and mRNA transport and limited signaling of key survival factors from the neurons peripheral synapses, causing the characteristic peripheral "die back". This disruption to microtubule dependant transport in ALS has been shown to result from alterations in the phenomenon of microtubule dynamic instability: the rapid growth and shrinkage of microtubule polymers. This is accomplished primarily due to aberrant alterations to microtubule associated proteins (MAPs) that regulate microtubule stability. Indeed, the current literature would argue that microtubule stability, particularly alterations in their dynamics, may be the initial driving force behind many familial and sporadic insults in ALS. Pharmacological stabilization of the microtubule network offers an attractive therapeutic strategy in ALS; indeed it has shown promise in many neurological disorders, ALS included. However, the pathophysiological involvement of MTs and their functions is still poorly understood in ALS. Future investigations will hopefully uncover further therapeutic targets that may aid in combating this awful disease.

Reactive oxygen species induce neurite degeneration before induction of cell death

Reactive oxygen species (ROS) induce neuronal cell death in a time- and concentration-dependent manner. Treatment of cultured cells with a low concentration of hydrogen peroxide induces neurite degeneration, but not cell death. Neurites (axons and dendrites) are vulnerable to ROS. Neurite degeneration (shrinkage, accumulation, and fragmentation) has been found in neurodegenerative disorders, such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. However, the mechanism of ROS-related neurite degeneration is not fully understood. Many studies have demonstrated the relationship between mitochondrial dysfunction and microtubule destabilization. These dysfunctions are deeply related to changes in calcium homeostasis and ROS production in neurites. Treatment with antioxidant substances, such as vitamin E, prevents neurite degeneration in cultured cells. This review describes the possibility that ROS induces neurite degeneration before the induction of cell death.

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.

Neurodegeneration and microtubule dynamics: death by a thousand cuts

Microtubules form important cytoskeletal structures that play a role in establishing and maintaining neuronal polarity, regulating neuronal morphology, transporting cargo, and scaffolding signaling molecules to form signaling hubs. Within a neuronal cell, microtubules are found to have variable lengths and can be both stable and dynamic. Microtubule associated proteins, post-translational modifications of tubulin subunits, microtubule severing enzymes, and signaling molecules are all known to influence both stable and dynamic pools of microtubules. Microtubule dynamics, the process of interconversion between stable and dynamic pools, and the proportions of these two pools have the potential to influence a wide variety of cellular processes. Reduced microtubule stability has been observed in several neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), Amyotrophic Lateral Sclerosis (ALS), and tauopathies like Progressive Supranuclear Palsy. Hyperstable microtubules, as seen in Hereditary Spastic Paraplegia (HSP), also lead to neurodegeneration. Therefore, the ratio of stable and dynamic microtubules is likely to be important for neuronal function and perturbation in microtubule dynamics might contribute to disease progression.

The Docosanoid Neuroprotectin D1 Induces TH-Positive Neuronal Survival in a Cellular Model of Parkinson's Disease

Parkinson's disease (PD) does not manifest clinically until 80 % of striatal dopamine is reduced, thus most neuronal damage takes place before the patient presents clinical symptoms. Therefore, it is important to develop preventive strategies for this disease. In the experimental models of PD, 1-methyl-4-phenylpyridinium ion (MPP+) and rotenone induce toxicity in dopaminergic neurons. Neuroprotectin D1 (NPD1) displays neuroprotection in cells undergoing proteotoxic and oxidative stress. In the present report, we established an in vitro model using a primary neuronal culture from mesencephalic embryonic mouse tissue in which 17-20 % of neurons were TH-positive when differentiated in vitro. NPD1 (100 nM) rescued cells from apoptosis induced by MPP+ and rotenone, and the dendritic arbor of surviving neurons was examined using Sholl analysis. Rotenone, as well as MPP+ and its precursor 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), severely promoted retraction of dendritic arbor distal segments, thus decreasing the maximum branch order reached. On average, NPD1 counteracted the effects of MPP+ on the dendritic arborization, but failed to do so in the rotenone-treated neurons. However, rotenone did decrease the Sholl intersection number from radii 25 to 125 µm, and NPD1 did restore the pattern to control levels. Similarly, NPD1 partially reverted the dendrite retraction caused by MPP+ and MPTP. These results suggest that the apoptosis occurring in mesencephalic TH-positive neurons is not a direct consequence of mitochondrial dysfunction alone and that NPD1 signaling may be counteracting this damage. These findings lay the groundwork for the use of the in vitro model developed for future studies and for the search of specific molecular events that NPD1 targets to prevent early neurodegeneration in PD.

Diquat causes caspase-independent cell death in SH-SY5Y cells by production of ROS independently of mitochondria

Evidence indicates that Parkinson's disease (PD), in addition to having a genetic aetiology, has an environmental component that contributes to disease onset and progression. The exact nature of any environmental agent contributing to PD is unknown in most cases. Given its similarity to paraquat, an agrochemical removed from registration in the EU for its suspected potential to cause PD, we have investigated the in vitro capacity of the related herbicide Diquat to cause PD-like cell death. Diquat showed greater toxicity towards SH-SY5Y neuroblastoma cells and human midbrain neural cells than paraquat and also MPTP, which was independent of dopamine transporter-mediated uptake. Diquat caused cell death independently of caspase activation, potentially via RIP1 kinase, with only a minor contribution from apoptosis, which was accompanied by enhanced reactive oxygen species production in the absence of major inhibition of complex I of the mitochondrial respiratory chain. No changes in α-synuclein expression were observed following 24-h or 4-week exposure. Diquat may, therefore, kill neural tissue by programmed necrosis rather than apoptosis, reflecting the pathological changes seen following high-level exposure, although its ability to promote PD is unclear.

Microtubule-stabilizing agents as potential therapeutics for neurodegenerative disease

Microtubules (MTs), cytoskeletal elements found in all mammalian cells, play a significant role in cell structure and in cell division. They are especially critical in the proper functioning of post-mitotic central nervous system neurons, where MTs serve as the structures on which key cellular constituents are trafficked in axonal projections. MTs are stabilized in axons by the MT-associated protein tau, and in several neurodegenerative diseases, including Alzheimer's disease, frontotemporal lobar degeneration, and Parkinson's disease, tau function appears to be compromised due to the protein dissociating from MTs and depositing into insoluble inclusions referred to as neurofibrillary tangles. This loss of tau function is believed to result in alterations of MT structure and function, resulting in aberrant axonal transport that likely contributes to the neurodegenerative process. There is also evidence of axonal transport deficiencies in other neurodegenerative diseases, including amyotrophic lateral sclerosis and Huntington's disease, which may result, at least in part, from MT alterations. Accordingly, a possible therapeutic strategy for such neurodegenerative conditions is to treat with MT-stabilizing agents, such as those that have been used in the treatment of cancer. Here, we review evidence of axonal transport and MT deficiencies in a number of neurodegenerative diseases, and summarize the various classes of known MT-stabilizing agents. Finally, we highlight the growing evidence that small molecule MT-stabilizing agents provide benefit in animal models of neurodegenerative disease and discuss the desired features of such molecules for the treatment of these central nervous system disorders.

Role of the Akt/GSK-3β/CRMP-2 pathway in axon degeneration of dopaminergic neurons resulting from MPP+ toxicity

Parkinson׳s disease (PD) is the most common neurodegenerative disease of the basal ganglia. Earlier reports suggest that the main pathological change in PD is due to apoptosis of dopaminergic neuronal soma in the substantia nigra (SN). The therapies for PD are also largely focused on the prevention of degeneration of the neuronal soma. However, these treatments can only provide temporary relief by delaying the progression of the disease and are therefore unable to prevent the long term neurodegeneration process. This limitation of the existing therapeutic treatment indicates that there may be other causes that either occur earlier or are independent of apoptosis of neuronal soma. Previous studies have shown that axon degeneration may play an important role in PD, and that this may occur at an early stage of the disease. Thus, preventing axon degeneration may be a potential new approach for therapeutic treatment for PD and future therapies can be useful if emphasis is given on the mechanisms of axon degeneration. It has been recognized that microtubule disassembly leads to axon degeneration because the depolymerized microtubules are more likely to be degraded. Previous studies have shown that glycogen synthase kinase-3β (GSK-3β)/collapsin response mediator protein 2 (CRMP-2) signaling pathway could be regulated by Akt for axonal-dendritic polarity. CRMP-2 is critical for specifying axon/dendrite fate possibly by promoting neurite elongation via microtubule assembly. However, whether Akt could regulate GSK-3β/CRMP-2 pathway and the possible effects of this regulation is unclear in dopaminergic axon degeneration induced by 1-methyl-4-phenylpyridiniumion (MPP+). In this study, we observe the degeneration of axon and neuronal soma by scanning electron microscope and tyrosine hydroxylase staining (TH) using a PD model in dopaminergic neurons in vitro. In addition to this, we detect the expression of total and phosphorylated form of Akt, GSK-3β and CRMP-2, as well as the axonal injury marker amyloid precursor protein (APP). From our studies, we observe that axon degeneration is a characteristic feature in the cascade of events that follow when neurons are induced by MPP+. This degeneration process occurs earlier in case of PD and is more severe than the degeneration of the neuronal soma and Akt/ GSK-3β/CRMP-2 pathway is involved in this process.

Caenorhabditis elegans: a model to investigate oxidative stress and metal dyshomeostasis in Parkinson's disease

Parkinson's disease (PD) is characterized by progressive motor impairment attributed to progressive loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta. Additional clinical manifestations include non-motor symptoms such as insomnia, depression, psychosis, and cognitive impairment. PD patients with mild cognitive impairment have an increased risk of developing dementia. The affected brain regions also show perturbed metal ion levels, primarily iron. These observations have led to speculation that metal ion dyshomeostasis plays a key role in the neuronal death of this disease. However, the mechanisms underlying this metal-associated neurodegeneration have yet to be completely elucidated. Mammalian models have traditionally been used to investigate PD pathogenesis. However, alternate animal models are also being adopted, bringing to bear their respective experimental advantage. The nematode, Caenorhabditis elegans, is one such system that has well-developed genetics, is amenable to transgenesis and has relatively low associated experimental costs. C. elegans has a well characterized neuronal network that includes a simple DAergic system. In this review we will discuss mechanisms thought to underlie PD and the use of C. elegans to investigate these processes.

Notch signalling in adult neurons: a potential target for microtubule stabilization

Cytoskeletal dysfunction has been proposed during the last decade as one of the main mechanisms involved in the aetiology of several neurodegenerative diseases. Microtubules are basic elements of the cytoskeleton and the dysregulation of microtubule stability has been demonstrated to be causative for axonal transport impairment, synaptic contact degeneration, impaired neuronal function leading finally to neuronal loss. Several pathways are implicated in the microtubule assembly/disassembly process. Emerging evidence is focusing on Notch as a microtubule dynamics regulator. We demonstrated that activation of Notch signalling results in increased microtubule stability and changes in axonal morphology and branching. By contrast, Notch inhibition leads to an increase in cytoskeleton plasticity with intense neurite remodelling. Until now, several microtubule-binding compounds have been tested and the results have provided proof of concept that microtubule-binding agents or compounds with the ability to stabilize microtubules may have therapeutic potential for the treatment of Alzheimer's disease and other neurodegenerative diseases. In this review, based on its key role in cytoskeletal dynamics modulation, we propose Notch as a new potential target for microtubule stabilization.

Microtubule alterations occur early in experimental parkinsonism and the microtubule stabilizer epothilone D is neuroprotective

The role of microtubule (MT) dysfunction in Parkinson's disease is emerging. It is still unknown whether it is a cause or a consequence of neurodegeneration. Our objective was to assess whether alterations of MT stability precede or follow axonal transport impairment and neurite degeneration in experimental parkinsonism induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in C57Bl mice. MPTP induced a time- and dose-dependent increase in fibres with altered mitochondria distribution, and early changes in cytoskeletal proteins and MT stability. Indeed, we observed significant increases in neuron-specific βIII tubulin and enrichment of deTyr tubulin in dopaminergic neurons. Finally, we showed that repeated daily administrations of the MT stabilizer Epothilone D rescued MT defects and attenuated nigrostriatal degeneration induced by MPTP. These data suggest that alteration of ΜΤs is an early event specifically associated with dopaminergic neuron degeneration. Pharmacological stabilization of MTs may be a viable strategy for the management of parkinsonism.

The rescue of microtubule-dependent traffic recovers mitochondrial function in Parkinson's disease

In Parkinson's disease mitochondrial dysfunction can lead to a deficient ATP supply to microtubule protein motors leading to mitochondrial axonal transport disruption. Compromised axonal transport will then lead to a disorganized distribution of mitochondria and other organelles in the cell, as well as, the accumulation of aggregated proteins like alpha-synuclein. Moreover, axonal transport disruption can trigger synaptic accumulation of autophagosomes packed with damaged mitochondria and protein aggregates promoting synaptic failure. We previously observed that neuronal-like cells with an inherent mitochondrial impairment derived from PD patients contain a disorganized microtubule network, as well as, alpha-synuclein oligomer accumulation. In this work we provide new evidence that an agent that promotes microtubule network assembly, NAP (davunetide), improves microtubule-dependent traffic, restores the autophagic flux and potentiates autophagosome-lysosome fusion leading to autophagic vacuole clearance in Parkinson's disease cells. Moreover, NAP is capable of efficiently reducing alpha-synuclein oligomer content and its sequestration by the mitochondria. Most interestingly, NAP decreases mitochondrial ubiquitination levels, as well as, increases mitochondrial membrane potential indicating a rescue in mitochondrial function. Overall, we demonstrate that by improving microtubule-mediated traffic, we can avoid mitochondrial-induced damage and thus recover cell homeostasis. These results prove that NAP may be a promising therapeutic lead candidate for neurodegenerative diseases that involve axonal transport failure and mitochondrial impairment as hallmarks, like Parkinson's disease and related disorders.

Compartmentalized oxidative stress in dopaminergic cell death induced by pesticides and complex I inhibitors: distinct roles of superoxide anion and superoxide dismutases

The loss of dopaminergic neurons induced by the parkinsonian toxins paraquat, rotenone, and 1-methyl-4-phenylpyridinium (MPP(+)) is associated with oxidative stress. However, controversial reports exist regarding the source/compartmentalization of reactive oxygen species (ROS) generation and its exact role in cell death. We aimed to determine in detail the role of superoxide anion (O2(•-)), oxidative stress, and their subcellular compartmentalization in dopaminergic cell death induced by parkinsonian toxins. Oxidative stress and ROS formation were determined in the cytosol, intermembrane (IMS), and mitochondrial matrix compartments, using dihydroethidine derivatives and the redox sensor roGFP, as well as electron paramagnetic resonance spectroscopy. Paraquat induced an increase in ROS and oxidative stress in both the cytosol and the mitochondrial matrix prior to cell death. MPP(+) and rotenone primarily induced an increase in ROS and oxidative stress in the mitochondrial matrix. No oxidative stress was detected at the level of the IMS. In contrast to previous studies, overexpression of manganese superoxide dismutase (MnSOD) or copper/zinc SOD (CuZnSOD) had no effect on alterations in ROS steady-state levels, lipid peroxidation, loss of mitochondrial membrane potential (ΔΨm), and dopaminergic cell death induced by MPP(+) or rotenone. In contrast, paraquat-induced oxidative stress and cell death were selectively reduced by MnSOD overexpression, but not by CuZnSOD or manganese-porphyrins. However, MnSOD also failed to prevent ΔΨm loss. Finally, paraquat, but not MPP(+) or rotenone, induced the transcriptional activation of the redox-sensitive antioxidant response elements (ARE) and nuclear factor kappa-B (NF-κB). These results demonstrate a selective role of mitochondrial O2(•-) in dopaminergic cell death induced by paraquat, and show that toxicity induced by the complex I inhibitors rotenone and MPP(+) does not depend directly on mitochondrial O2(•-) formation.

Neuropathology of partial PGC-1α deficiency recapitulates features of mitochondrial encephalopathies but not of neurodegenerative diseases

BACKGROUND

Deficient peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) function is one component of mitochondrial dysfunction in neurodegenerative diseases. Current molecular classification of such diseases is based on the predominant protein accumulating as intra- or extracellular aggregates. Experimental evidence suggests that mitochondrial dysfunction and impaired protein processing are closely interrelated. In vitro findings further indicate that PGC-1α dysfunction may contribute to protein misfolding in neurodegeneration.

OBJECTIVE

To systematically evaluate the neuropathological alterations of mice lacking the expression of the full-length PGC-1α protein (FL-PGC-1α) but expressing an N-truncated fragment.

METHODS

To assess the pattern of neurodegeneration-related proteins, we performed immunostaining for Tau, pTau, α-synuclein, amyloid-β, amyloid precursor protein, prion protein, FUS, TDP-43 and ubiquitin. Using hematoxylin and eosin, Klüver-Barrera and Bielschowsky silver stainings and anti-GFAP immunohistochemistry, we performed an anatomical mapping to provide a lesion profile.

RESULTS

The immunohistochemical pattern of neurodegeneration-related proteins did not differ between FL-PGC-1α knockout and wild-type animals, and there was a complete lack of protein deposits or ubiquitin-positive inclusions. The analysis of neuropathological alterations revealed widespread vacuolation predominating in the cerebral white matter, caudate-putamen, thalamus and brainstem, and reactive astrogliosis in the brainstem and cerebellar nuclei. This morphological phenotype was thus reminiscent of human mitochondrial encephalopathies, especially the Kearns-Sayre syndrome.

CONCLUSION

We conclude that the lack of FL-PGC-1α per se is insufficient to recapitulate major features of neurodegenerative diseases, but evokes a pathology seen in mitochondrial encephalopathies, which makes PGC-1α-deficient mice a valuable model for this yet incurable group of diseases.

AIF, reactive oxygen species, and neurodegeneration: a "complex" problem

Apoptosis-inducing factor (AIF) is a flavin-binding mitochondrial intermembrane space protein that is implicated in diverse but intertwined processes that include maintenance of electron transport chain function, reactive oxygen species regulation, cell death, and neurodegeneration. In acute brain injury, AIF acquires a pro-death role upon translocation from the mitochondria to the nucleus, where it initiates chromatin condensation and large-scale DNA fragmentation. Although harlequin mice exhibiting an 80-90% global reduction in AIF protein are resistant to numerous forms of acute brain injury, they paradoxically undergo slow, progressive neurodegeneration beginning at three months of age. Brain deterioration, accompanied by markers of oxidative stress, is most pronounced in the cerebellum and retina, although it also occurs in the cortex, striatum, and thalamus. Loss of an AIF pro-survival function linked to assembly or stabilization of electron transport chain complex I underlies chronic neurodegeneration. To date, most studies of neurodegeneration have failed to adequately separate the relative importance of the mitochondrial and nuclear functions of AIF in determining the extent of injury, or whether oxidative stress plays a causative role. This review explores the complicated relationship among AIF, complex I, and the regulation of mitochondrial reactive oxygen species levels. It also discusses the controversial role of complex I deficiency in Parkinson's disease, and what can be learned from the AIF- and complex I-depleted harlequin mouse.

N-terminal cleavage of the mitochondrial fusion GTPase OPA1 occurs via a caspase-independent mechanism in cerebellar granule neurons exposed to oxidative or nitrosative stress

Neuronal cell death via apoptosis or necrosis underlies several devastating neurodegenerative diseases associated with aging. Mitochondrial dysfunction resulting from oxidative or nitrosative stress often acts as an initiating stimulus for intrinsic apoptosis or necrosis. These events frequently occur in conjunction with imbalances in the mitochondrial fission and fusion equilibrium, although the cause and effect relationships remain elusive. Here, we demonstrate in primary rat cerebellar granule neurons (CGNs) that oxidative or nitrosative stress induces an N-terminal cleavage of optic atrophy-1 (OPA1), a dynamin-like GTPase that regulates mitochondrial fusion and maintenance of cristae architecture. This cleavage event is indistinguishable from the N-terminal cleavage of OPA1 observed in CGNs undergoing caspase-mediated apoptosis (Loucks et al., 2009) and results in removal of a key lysine residue (K301) within the GTPase domain. OPA1 cleavage in CGNs occurs coincident with extensive mitochondrial fragmentation, disruption of the microtubule network, and cell death. In contrast to OPA1 cleavage induced in CGNs by removing depolarizing extracellular potassium (5K apoptotic conditions), oxidative or nitrosative stress-induced OPA1 cleavage caused by complex I inhibition or nitric oxide, respectively, is caspase-independent. N-terminal cleavage of OPA1 is also observed in vivo in aged rat and mouse midbrain and hippocampal tissues. We conclude that N-terminal cleavage and subsequent inactivation of OPA1 may be a contributing factor in the neuronal cell death processes underlying neurodegenerative diseases, particularly those associated with aging. Furthermore, these data suggest that OPA1 cleavage is a likely convergence point for mitochondrial dysfunction and imbalances in mitochondrial fission and fusion induced by oxidative or nitrosative stress.

Microtubule destabilization is shared by genetic and idiopathic Parkinson's disease patient fibroblasts

Data from both toxin-based and gene-based models suggest that dysfunction of the microtubule system contributes to the pathogenesis of Parkinson's disease, even if, at present, no evidence of alterations of microtubules in vivo or in patients is available. Here we analyze cytoskeleton organization in primary fibroblasts deriving from patients with idiopathic or genetic Parkinson's disease, focusing on mutations in parkin and leucine-rich repeat kinase 2. Our analyses reveal that genetic and likely idiopathic pathology affects cytoskeletal organization and stability, without any activation of autophagy or apoptosis. All parkinsonian fibroblasts have a reduced microtubule mass, represented by a higher fraction of unpolymerized tubulin in respect to control cells, and display significant changes in microtubule stability-related signaling pathways. Furthermore, we show that the reduction of microtubule mass is so closely related to the alteration of cell morphology and behavior that both pharmacological treatment with microtubule-targeted drugs, and genetic approaches, by transfecting the wild type parkin or leucine-rich repeat kinase 2, restore the proper microtubule stability and are able to rescue cell architecture. Taken together, our results suggest that microtubule destabilization is a point of convergence of genetic and idiopathic forms of parkinsonism and highlight, for the first time, that microtubule dysfunction occurs in patients and not only in experimental models of Parkinson's disease. Therefore, these data contribute to the knowledge on molecular and cellular events underlying Parkinson's disease and, revealing that correction of microtubule defects restores control phenotype, may offer a new therapeutic target for the management of the disease.

WldS but not Nmnat1 protects dopaminergic neurites from MPP+ neurotoxicity

BACKGROUND

The WldS mouse mutant ("Wallerian degeneration-slow") delays axonal degeneration in a variety of disorders including in vivo models of Parkinson's disease. The mechanisms underlying WldS -mediated axonal protection are unclear, although many studies have attributed WldS neuroprotection to the NAD+-synthesizing Nmnat1 portion of the fusion protein. Here, we used dissociated dopaminergic cultures to test the hypothesis that catalytically active Nmnat1 protects dopaminergic neurons from toxin-mediated axonal injury.

RESULTS

Using mutant mice and lentiviral transduction of dopaminergic neurons, the present findings demonstrate that WldS but not Nmnat1, Nmnat3, or cytoplasmically-targeted Nmnat1 protects dopamine axons from the parkinsonian mimetic N-methyl-4-phenylpyridinium (MPP+). Moreover, NAD+ synthesis is not required since enzymatically-inactive WldS still protects. In addition, NAD+ by itself is axonally protective and together with WldS is additive in the MPP+ model.

CONCLUSIONS

Our data suggest that NAD+ and WldS act through separate and possibly parallel mechanisms to protect dopamine axons. As MPP+ is thought to impair mitochondrial function, these results suggest that WldS might be involved in preserving mitochondrial health or maintaining cellular metabolism.

Axon degeneration in Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative disease of the basal ganglia. Like other adult-onset neurodegenerative disorders, it is without a treatment that forestalls its chronic progression. Efforts to develop disease-modifying therapies to date have largely focused on the prevention of degeneration of the neuron soma, with the tacit assumption that such approaches will forestall axon degeneration as well. We herein propose that future efforts to develop neuroprotection for PD may benefit from a shift in focus to the distinct mechanisms that underlie axon degeneration. We review evidence from human post-mortem studies, functional neuroimaging, genetic causes of the disease and neurotoxin models that axon degeneration may be the earliest feature of the disease, and it may therefore be the most appropriate target for early intervention. In addition, we present evidence that the molecular mechanisms of degeneration of axons are separate and distinct from those of neuron soma. Progress is being made in understanding these mechanisms, and they provide possible new targets for therapeutic intervention. We also suggest that the potential for axon re-growth in the adult central nervous system has perhaps been underestimated, and it offers new avenues for neurorestoration. In conclusion, we propose that a new focus on the neurobiology of axons, their molecular pathways of degeneration and growth, will offer novel opportunities for neuroprotection and restoration in the treatment of PD and other neurodegenerative diseases.

Increase in endoplasmic reticulum-associated tissue transglutaminase and enzymatic activation in a cellular model of Parkinson's disease

Parkinson's disease (PD) is characterized by accumulation of α-synuclein aggregates and degeneration of melanized, catecholaminergic neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD, tTG levels are increased and cross-linking has been identified as an important factor in α-synuclein aggregation. In our quest to link tTGs distribution in the human brain to the hallmarks of PD pathology, we recently reported that catecholaminergic neurons in PD disease-affected brain areas display typical endoplasmic reticulum (ER) granules showing tTG immunoreactivity. In the present study, we set out to elucidate the nature of the interaction between tTG and the ER in PD pathogenesis, using retinoic-acid differentiated SH-SY5Y cells exposed to the PD-mimetic 1-methyl-4-phenylpyridinium (MPP(+)). Alike our observations in PD brain, MPP(+)-treated cells displayed typical TG-positive granules, that were also induced by other PD mimetics and by ER-stress inducing toxins. Additional immunocytochemical and biochemical investigation revealed that tTG is indeed associated to the ER, in particular at the cytoplasmic face of the ER. Upon MPP(+) exposure, additional recruitment of tTG toward the ER was found. In addition, we observed that MPP(+)-induced tTG activity results in transamidation of ER membrane proteins, like calnexin. Our data provide strong evidence for a, so far unrecognized, localization of tTG at the ER, at least in catecholaminergic neurons, and suggests that in PD activation of tTG may have a direct impact on ER function, in particular via post-translational modification of ER membrane proteins.

The parkinsonian mimetic, MPP+, specifically impairs mitochondrial transport in dopamine axons

Impaired axonal transport may play a key role in Parkinson's disease. To test this notion, a microchamber system was adapted to segregate axons from cell bodies using green fluorescent protein-labeled mouse dopamine (DA) neurons. Transport was examined in axons challenged with the DA neurotoxin, 1-methyl-4-phenylpyridinium ion (MPP+). MPP+ rapidly reduced overall mitochondrial motility in DA axons; among motile mitochondria, anterograde transport was slower yet retrograde transport was increased. Transport effects were specific for DA mitochondria, which were smaller and transported more slowly than their non-DA counterparts. MPP+ did not affect synaptophysin-tagged vesicles or any other measureable moving particle. Toxin effects on DA mitochondria were not dependent upon ATP, calcium, free radical species, JNK, or caspase3/PKC pathways but were completely blocked by the thiol-anti-oxidant N-acetyl-cysteine or membrane-permeable glutathione. Since these drugs also rescued processes from degeneration, these findings emphasize the need to develop therapeutics aimed at axons as well as cell bodies to preserve "normal" circuitry and function as long as possible.

Following activation of the amyloid cascade, apolipoprotein E4 drives the in vivo oligomerization of amyloid-β resulting in neurodegeneration

According to the amyloid hypothesis, the accumulation of oligomerized amyloid-β (Aβ) is a primary event in the pathogenesis of Alzheimer's disease (AD). The trigger of the amyloid cascade and of Aβ oligomerization in sporadic AD, the most prevalent form of the disease, remains elusive. Here, we examined the hypothesis that apolipoprotein E4 (ApoE4), the most prevalent genetic risk factor for AD, triggers the accumulation of intraneuronal oligomerized Aβ following activation of the amyloid cascade. We investigated the intracellular organelles that are targeted by these processes and govern their pathological consequences. This revealed that activation of the amyloid cascade in vivo by inhibition of the Aβ degrading enzyme neprilysin specifically results in accumulation of Aβ and oligomerized Aβ and of ApoE4 in the CA1 neurons of ApoE4 mice. This was accompanied by lysosomal and mitochondrial pathology and the co-localization of Aβ, oligomerized Aβ, and ApoE4 with enlarged lysosomes and of Aβ and oligomerized Aβ with mitochondria. The time course of the lysosomal effects paralleled that of the loss of CA1 neurons, whereas the mitochondrial effects reached an earlier plateau. These findings suggest that ApoE4 potentiates the pathological effects of Aβ and the amyloid cascade by triggering the oligomerization of Aβ, which in turn, impairs intraneuronal mitochondria and lysosomes and drives neurodegeneration.

Tetrahydroxystilbene glucoside protects human neuroblastoma SH-SY5Y cells against MPP+-induced cytotoxicity

1-methyl-4-phenylpyridinium (MPP+), an inhibitor of mitochondrial complex I, has been widely used as a neurotoxin for inducing a cell model of Parkinson's disease. This study aimed to evaluate the effects of 2,3,5,4'-tetrahydroxystilbene-2-O-β-D-glucoside (TSG), an active component extracted from Polygonum multiflorum, on MPP+-induced cytotoxicity in human dopaminergic neuroblastoma SH-SY5Y cells. The results from the MTT and lactate dehydrogenase (LDH) assays showed that incubating cells with 500 μM MPP+ for 24 h decreased cell viability and increased LDH leakage, whereas preincubating cells with 3.125 to 50 μM TSG for 24 h protected the cells against MPP+-induced cell damage. Using 2',7'-dichlorofluorescin diacetate (DCFH-DA) and rhodamine 123, respectively, we found that TSG inhibited both the elevation of intracellular reactive oxygen species and the disruption of mitochondrial membrane potential induced by MPP+. In addition, TSG suppressed both the upregulation of the ratio of Bax to Bcl-2 and the activation of caspase-3 induced by MPP+, and TSG inhibited apoptosis as detected by flow cytometric analysis using Annexin-V and propidium (PI) label. These results suggest that TSG may protect neurons against MPP+-induced cell death through improving mitochondrial function, decreasing oxidative stress and inhibiting apoptosis, and this may provide a potentially new strategy for preventing and treating neurodegenerative disorders such as Parkinson's disease.

Mitochondrial fusion/fission, transport and autophagy in Parkinson's disease: when mitochondria get nasty

Understanding the molecular basis of Parkinson's disease (PD) has proven to be a major challenge in the field of neurodegenerative diseases. Although several hypotheses have been proposed to explain the molecular mechanisms underlying the pathogenesis of PD, a growing body of evidence has highlighted the role of mitochondrial dysfunction and the disruption of the mechanisms of mitochondrial dynamics in PD and other parkinsonian disorders. In this paper, we comment on the recent advances in how changes in the mitochondrial function and mitochondrial dynamics (fusion/fission, transport, and clearance) contribute to neurodegeneration, specifically focusing on PD. We also evaluate the current controversies in those issues and discuss the role of fusion/fission dynamics in the mitochondrial lifecycle and maintenance. We propose that cellular demise and neurodegeneration in PD are due to the interplay between mitochondrial dysfunction, mitochondrial trafficking disruption, and impaired autophagic clearance.

The biochemical and cellular basis for nutraceutical strategies to attenuate neurodegeneration in Parkinson's disease

Future therapeutic intervention that could effectively decelerate the rate of degeneration within the substantia nigra pars compacta (SNc) could add years of mobility and reduce morbidity associated with Parkinson’s disease (PD). Neurodegenerative decline associated with PD is distinguished by extensive damage to SNc dopaminergic (DAergic) neurons and decay of the striatal tract. While genetic mutations or environmental toxins can precipitate pathology, progressive degenerative succession involves a gradual decline in DA neurotransmission/synaptic uptake, impaired oxidative glucose consumption, a rise in striatal lactate and chronic inflammation. Nutraceuticals play a fundamental role in energy metabolism and signaling transduction pathways that control neurotransmission and inflammation. However, the use of nutritional supplements to slow the progression of PD has met with considerable challenge and has thus far proven unsuccessful. This review re-examines precipitating factors and insults involved in PD and how nutraceuticals can affect each of these biological targets. Discussed are disease dynamics (Sections 1 and 2) and natural substances, vitamins and minerals that could impact disease processes (Section 3). Topics include nutritional influences on α-synuclein aggregation, ubiquitin proteasome function, mTOR signaling/lysosomal-autophagy, energy failure, faulty catecholamine trafficking, DA oxidation, synthesis of toxic DA-quinones, o-semiquinones, benzothiazolines, hyperhomocyseinemia, methylation, inflammation and irreversible oxidation of neuromelanin. In summary, it is clear that future research will be required to consider the multi-faceted nature of this disease and re-examine how and why the use of nutritional multi-vitamin-mineral and plant-based combinations could be used to slow the progression of PD, if possible.

Mitochondrial Dysfunction: The Road to Alpha-Synuclein Oligomerization in PD

While the etiology of Parkinson's disease remains largely elusive, there is accumulating evidence suggesting that mitochondrial dysfunction occurs prior to the onset of symptoms in Parkinson's disease. Mitochondria are remarkably primed to play a vital role in neuronal cell survival since they are key regulators of energy metabolism (as ATP producers), of intracellular calcium homeostasis, of NAD(+)/NADH ratio, and of endogenous reactive oxygen species production and programmed cell death. In this paper, we focus on mitochondrial dysfunction-mediated alpha-synuclein aggregation. We highlight some of the findings that provide proof of evidence for a mitochondrial metabolism control in Parkinson's disease, namely, mitochondrial regulation of microtubule-dependent cellular traffic and autophagic lysosomal pathway. The knowledge that microtubule alterations may lead to autophagic deficiency and may compromise the cellular degradation mechanisms that culminate in the progressive accumulation of aberrant protein aggregates shields new insights to the way we address Parkinson's disease. In line with this knowledge, an innovative window for new therapeutic strategies aimed to restore microtubule network may be unlocked.

The role of axonopathy in Parkinson's disease

New genetic and environmental studies of Parkinson's disease have revealed early problems in synaptic function and connectivity indicating that axonal impairment may be an important hallmark in this disorder. Since many studies suggest that axonal dysfunction precedes cell body loss, it is critical to target axons with treatments aimed at preserving "connectivity" as well as to develop and verify "biomarkers" with which to assess disease progression and drug efficacy.

Parkinsonian neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and alpha-synuclein mutations promote Tau protein phosphorylation at Ser262 and destabilize microtubule cytoskeleton in vitro

In Parkinson disease (PD) brain, a progressive loss of dopaminergic neurons leads to dopamine depletion in the striatum and reduced motor function. Lewy bodies, the characteristic neuropathological lesions found in the brain of PD patients, are composed mainly of α-synuclein protein. Three point mutations in the α-synuclein gene are associated with familial PD. In addition, genome-wide association studies indicate that α-synuclein and Tau protein synergistically increase disease susceptibility in the human population. To determine the mechanism by which α-synuclein and Tau act together, we have used PD-causing neurotoxin MPTP and pathogenic α-synuclein mutants A30P, E46K, and A53T as models. We found that exposure of human neuroblastoma M17 cells to MPTP enhances the intracellular α-synuclein protein level, stimulates Tau protein phosphorylation at Ser(262), and induces apoptosis. In mouse brain, ablation of α-synuclein function significantly suppresses Tau phosphorylation at Ser(262). In vitro, α-synuclein binds to phosphorylated Ser(214) of Tau and stimulates PKA-catalyzed Tau phosphorylation at Ser(262). PD-associated α-synuclein mutations increase α-synuclein binding to Tau and stimulate Tau phosphorylation at Ser(262). In HEK-293 cells, α-synuclein and its all PD-associated mutants destabilize the microtubule cytoskeleton in a similar extent. In contrast, when co-expressed with Tau, these PD-associated mutants destabilize microtubules with significantly higher potency than WT. Our results demonstrate that α-synuclein is an in vivo regulator of Tau protein phosphorylation at Ser(262) and suggest that PD-associated risk factors such as environmental toxins and α-synuclein mutations promote Tau phosphorylation at Ser(262), causing microtubule instability, which leads to loss of dopaminergic neurons in PD brain.

Role of gamma-synuclein in microtubule regulation

Gamma-synuclein is a neuronal protein found in peripheral and motor nerve systems. It becomes highly expressed in metastatic but not in primary tumor or normal tissues. The close association between gamma-synuclein expression and cancer spreading has been demonstrated in a broad range of malignancies. Our previous study showed that exogenous expression of gamma-synuclein in ovarian and breast cancer cells significantly enhanced cell migration and resistance to paclitaxel-induced apoptotic death. In our current research, we found that gamma-synuclein can affect microtubule properties and act as a functional microtubule associated protein. In vitro assays revealed that gamma-synuclein can bind and promote tubulin polymerization, induce the microtubule bundling and alter microtubule morphology developed in the presence of microtubule associated protein 2 (MAP2). Using cancer cell lysate, gamma-synuclein protein was found to be localized in both cytosolic compartment and extracted cytoskeleton portion. Immunofluorescence staining demonstrated that gamma-synuclein can colocalize with microtubule in HeLa cells and decrease rigidity of microtubule bundles caused by paclitaxel. In human ovarian cancer epithelial A2780 cells, gamma-synuclein overexpression improved cell adhesion and microtubule structure upon paclitaxel treatment. Importantly, it led to microtubule-dependent mitochondria clustering at perinuclear area. These observations suggest that overexpression of gamma-synuclein may reduce cell chemo-sensitivity of tumor cells through decreasing microtubule rigidity. In summary, our studies suggested that gamma-synuclein can directly participate in microtubule regulation.

Human dental pulp stem cells protect mouse dopaminergic neurons against MPP+ or rotenone

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive death of substantia nigra dopaminergic neurons that results in a regional loss of striatal dopamine (DA) levels. Dental pulp contains ex vivo-expandable cells called dental pulp stem cells (DPSCs), with the capacity to differentiate into multiple cell lineages. More interestingly, due to their embryonic origin, DPSCs express neurotrophic factors such as brain-derived neurotrophic factor, nerve growth factor and glial cell-derived neurotrophic factor. The aim of the present study was to investigate the neuroprotective effects of DPSCs against MPP+ (2.5, 5, and 10 μM) and rotenone (0.25, 0.5 and 1 μM) in an in vitro model of PD, using an indirect co-culture system with mesencephalic cell cultures. When mesencephalic cultures were challenged with MPP+ or rotenone, in the presence of DPSCs a statistically significant protective effect was observed at all the tested doses in terms of DA uptake. DPSCs protective effect on DA neurons was also confirmed by immunocytochemistry: an increased number of spared tyrosine hydroxylase (TH)+ cells was observed in co-culture conditions compared to controls, and neurons showed longer processes in comparison with mesencephalic cells grown without DPSCs. In conclusion, the co-culture with DPSCs significantly attenuated MPP+ or rotenone-induced toxicity in primary cultures of mesencephalic neurons. Considering that the direct contact between the two cell types was prevented, it can be speculated that neuroprotection could be due to soluble factors such as BDNF and NGF, released by DPSCs. Blocking BDNF and NGF with neutralizing antibodies, the neuroprotecting effect of DPSCs was completely abolished. Therefore DPSCs can be viewed as possible candidates for studies on cell-based therapy in neurodegenerative disorders.

Microtubule dysfunction precedes transport impairment and mitochondria damage in MPP+ -induced neurodegeneration

Dysfunction of the microtubule (MT) system is an emerging theme in the pathogenesis of Parkinson's disease. This study was designed to investigate the putative role of MT dysfunction in dopaminergic neuron death induced by the neurotoxin 1-methyl-4-phenylpiridinium (MPP(+)). In nerve growth factor-differentiated PC12 cells, we have analyzed post-translational modifications of tubulin known to be associated with differently dynamic MTs and show that MPP(+) causes a selective loss of dynamic MTs and a concomitant enrichment of stable MTs. Through a direct live cell imaging approach, we show a significant reduction of MT dynamics following exposure to MPP(+) and a reorientation of MTs. Furthermore, these alterations precede the impairment of intracellular transport as revealed by changes in mitochondria movements along neurites and their accumulation into varicosities. We have also analyzed activation of caspase 3 and mitochondrial injury, well-known alterations induced by MPP(+), and found that they are noticeable only when MT dysfunction is already established. These data provide the first evidence that axonal transport impairment and mitochondrial damage might be a consequence of MT dysfunction in MPP(+) -induced neurodegeneration, lending support to the concept that alterations of MT organization and dynamics could play a pivotal role in neuronal death in Parkinson's disease.

Gypenosides protects dopaminergic neurons in primary culture against MPP(+)-induced oxidative injury

Oxidative injury has been implicated in the etiology of Parkinson's disease (PD). Gypenosides (GPs), the saponins extract derived from the Gynostemma pentaphyllum, has various bioactivities. In this study, GPs was investigated for its neuroprotective effects on the 1-methyl-4-phenylpyridinium ion (MPP(+))-induced oxidative injury of dopaminergic neurons in primary nigral culture. It was found that GPs pretreatment, cotreatment or posttreatment significantly and dose-dependently attenuated MPP(+)-induced oxidative damage, reduction of dopamine uptake, loss of tyrosine hydrolase (TH)-immunopositive neurons and degeneration of TH-immunopositive neurites. However, the preventive effect of GPs was more potential than its therapeutical effect. Most importantly, the neuroprotective effect of GPs may be attributed to GPs-induced strengthened antioxidation as manifested by significantly increased glutathione content and enhanced activity of glutathione peroxidase, catalyze and superoxide dismutase in nigral culture. The neuroprotective effects of GPs are specific for dopaminergic neurons and it may have therapeutic potential in the treatment of PD.