Synergistic effects on dopamine cell death in a Drosophila model of chronic toxin exposure

Understanding role of pesticides in development of Parkinson's disease: Insights from Drosophila and rodent models

Parkinson's disease is a neurodegenerative illness linked to ageing, marked by the gradual decline of dopaminergic neurons in the midbrain. The exact aetiology of Parkinson's disease (PD) remains uncertain, with genetic predisposition and environmental variables playing significant roles in the disease's frequency. Epidemiological data indicates a possible connection between pesticide exposure and brain degeneration. Specific pesticides have been associated with important characteristics of Parkinson's disease, such as mitochondrial dysfunction, oxidative stress, and α-synuclein aggregation, which are crucial for the advancement of the disease. Recently, many animal models have been developed for Parkinson's disease study. Although these models do not perfectly replicate the disease's pathology, they provide valuable insights that improve our understanding of the condition and the limitations of current treatment methods. Drosophila, in particular, has been useful in studying Parkinson's disease induced by toxins or genetic factors. The review thoroughly analyses many animal models utilised in Parkinson's research, with an emphasis on issues including pesticides, genetic and epigenetic changes, proteasome failure, oxidative damage, α-synuclein inoculation, and mitochondrial dysfunction. The text highlights the important impact of pesticides on the onset of Parkinson's disease (PD) and stresses the need for more research on genetic and mechanistic alterations linked to the condition.

Sexually dimorphic mechanisms of VGLUT-mediated protection from dopaminergic neurodegeneration

Parkinson's disease (PD) targets some dopamine (DA) neurons more than others. Sex differences offer insights, with females more protected from DA neurodegeneration. The mammalian vesicular glutamate transporter VGLUT2 and Drosophila ortholog dVGLUT have been implicated as modulators of DA neuron resilience. However, the mechanisms by which VGLUT2/dVGLUT protects DA neurons remain unknown. We discovered DA neuron dVGLUT knockdown increased mitochondrial reactive oxygen species in a sexually dimorphic manner in response to depolarization or paraquat-induced stress, males being especially affected. DA neuron dVGLUT also reduced ATP biosynthetic burden during depolarization. RNA sequencing of VGLUT+ DA neurons in mice and flies identified candidate genes that we functionally screened to further dissect VGLUT-mediated DA neuron resilience across PD models. We discovered transcription factors modulating dVGLUT-dependent DA neuroprotection and identified dj-1β as a regulator of sex-specific DA neuron dVGLUT expression. Overall, VGLUT protects DA neurons from PD-associated degeneration by maintaining mitochondrial health.

Identification of UBA1 as the causative gene of an X-linked non-Kennedy SBMA

BACKGROUND

Spinal-bulbar muscular atrophy (SBMA; Kennedy's Disease) is a motor neuron disease (MND). Kennedy's Disease is nearly exclusively caused by mutations in the androgen receptor encoding gene (AR). We report results of studies aimed at identification of the genetic cause of a disease that best approximates SBMA in a pedigree (four patients) without mutations in AR.

METHODS

Clinical investigations included thorough neurologic and non-neurologic examinations and testings. Genetic analysis was performed by exome sequencing using standard protocols. UBA1 mutations were modeled on the crystal structure of UBA1.

RESULTS

The clinical features of the patients are described in detail. A missense mutation in UBA1 (c.T1499C; p.Ile500Thr) was identified as the probable cause of the non-Kennedy SBMA in the pedigree. Like AR, UBA1 is positioned on Chromosome X. UBA1 is a highly conserved gene. It encodes ubiquitin like modifier activating enzyme 1 (UBA1) which is the major E1 enzyme of the ubiquitin-proteasome system. Interestingly, UBA1 mutations can also cause infantile-onset X-linked spinal muscular atrophy (XL-SMA). The mutation identified here and the XL-SMA causative mutations were shown to affect amino acids positioned in the vicinity of UBA1's ATP binding site and to cause structural changes.

CONCLUSION

UBA1 was identified as a novel SBMA causative gene. The gene affects protein homeostasis which is one of most important components of the pathology of neurodegeneration. The contribution of this same gene to the etiology of XL-SMA is discussed.

Transcriptional responses are oriented towards different components of the rearing environment in two Drosophila sibling species

BACKGROUND

The chance to compare patterns of differential gene expression in related ecologically distinct species can be particularly fruitful to investigate the genetics of adaptation and phenotypic plasticity. In this regard, a powerful technique such as RNA-Seq applied to ecologically amenable taxa allows to address issues that are not possible in classic model species. Here, we study gene expression profiles and larval performance of the cactophilic siblings Drosophila buzzatii and D. koepferae reared in media that approximate natural conditions and evaluate both chemical and nutritional components of the diet. These closely related species are complementary in terms of host-plant use since the primary host of one is the secondary of the other. D. koepferae is mainly a columnar cactus dweller while D. buzzatii prefers Opuntia hosts.

RESULTS

Our comparative study shows that D. buzzatii and D. koepferae have different transcriptional strategies to face the challenges posed by their natural resources. The former has greater transcriptional plasticity, and its response is mainly modulated by alkaloids of its secondary host, while the latter has a more canalized genetic response, and its transcriptional plasticity is associated with the cactus species.

CONCLUSIONS

Our study unveils a complex pleiotropic genetic landscape in both species, with functional links that relate detox responses and redox mechanisms with developmental and neurobiological processes. These results contribute to deepen our understanding of the role of host plant shifts and natural stress driving ecological specialization.

Chronic Exposure to Paraquat Induces Alpha-Synuclein Pathogenic Modifications in Drosophila

Parkinson's disease (PD) is characterized by the progressive accumulation of neuronal intracellular aggregates largely composed of alpha-Synuclein (αSyn) protein. The process of αSyn aggregation is induced during aging and enhanced by environmental stresses, such as the exposure to pesticides. Paraquat (PQ) is an herbicide which has been widely used in agriculture and associated with PD. PQ is known to cause an increased oxidative stress in exposed individuals but the consequences of such stress on αSyn conformation remains poorly understood. To study αSyn pathogenic modifications in response to PQ, we exposed Drosophila expressing human αSyn to a chronic PQ protocol. We first showed that PQ exposure and αSyn expression synergistically induced fly mortality. The exposure to PQ was also associated with increased levels of total and phosphorylated forms of αSyn in the Drosophila brain. Interestingly, PQ increased the detection of soluble αSyn in highly denaturating buffer but did not increase αSyn resistance to proteinase K digestion. These results suggest that PQ induces the accumulation of toxic soluble and misfolded forms of αSyn but that these toxic forms do not form fibrils or aggregates that are detected by the proteinase K assay. Collectively, our results demonstrate that Drosophila can be used to study the effect of PQ or other environmental neurotoxins on αSyn driven pathology.

Exposure to paraquat associated with periodontal disease causes motor damage and neurochemical changes in rats

Exposure to paraquat is possibly involved with the development of several conditions, including neurodegenerative diseases, such as Parkinson's disease (PD). This condition is mainly characterized by the loss of dopaminergic neurons in the nigrostriatal pathway and the development of classical motor symptoms. Etiology includes exposure to environmental factors, such as the paraquat exposure, and inflammatory diseases may exacerbate paraquat neurotoxicity. The aim of the study was to investigate whether the exposure to paraquat associated with the presence of periodontal disease is able to induce motor and biochemical changes in rats similar to that observed in PD. Adult male Wistar rats were sent to ligature. After 48 h, they were sent to daily treatment paraquat (1 mg/kg/day; 2 mL/kg; intragastric) or vehicle for 4 weeks. Twenty-four hours after the last administration, the open field test was performed. The rats were euthanized and the left hemimandibles and striatum were dissected for the analysis of dopaminergic and inflammatory markers. Only the combination of periodontal disease model plus paraquat exposure induced motor impairments. Remarkably, the paraquat exposure increased the ligature-induced alveolar bone loss in hemimandibles. Moreover, only the combination of periodontal disease and paraquat exposure induced the loss of dopaminergic neurons and astrocyte activation in the striatum.

The pivotal role of ubiquitin-activating enzyme E1 (UBA1) in neuronal health and neurodegeneration

Ubiquitin-activating enzyme E1, UBA1, functions at the apex of the enzymatic ubiquitylation cascade, catalysing ubiquitin activation. UBA1 is thus of fundamental importance to the modulation of ubiquitin homeostasis and to all downstream ubiquitylation-dependent cellular processes, including proteolysis through the ubiquitin-proteasome system and selective autophagy. The proteasome-dependent and -independent functions of UBA1 contribute significantly to a range of processes crucial to neuronal health. The significance of UBA1 activity to neuronal health is clear in light of accumulating evidence implicating impaired UBA1 activity in a range of neurodegenerative conditions, including Parkinson's disease, Alzheimer's disease, Huntington's disease and spinal muscular atrophy. Moreover, ubiquitylation-independent functions of UBA1 of importance to neuronal functioning have been proposed. Here, we summarise findings supporting the significant role of UBA1 in regulating neuronal functioning, and discuss the detrimental consequences of UBA1 impairment that contribute to neuronal dysfunction and degeneration.

Elucidating Conserved Transcriptional Networks Underlying Pesticide Exposure and Parkinson's Disease: A Focus on Chemicals of Epidemiological Relevance

While a number of genetic mutations are associated with Parkinson's disease (PD), it is also widely acknowledged that the environment plays a significant role in the etiology of neurodegenerative diseases. Epidemiological evidence suggests that occupational exposure to pesticides (e.g., dieldrin, paraquat, rotenone, maneb, and ziram) is associated with a higher risk of developing PD in susceptible populations. Within dopaminergic neurons, environmental chemicals can have an array of adverse effects resulting in cell death, such as aberrant redox cycling and oxidative damage, mitochondrial dysfunction, unfolded protein response, ubiquitin-proteome system dysfunction, neuroinflammation, and metabolic disruption. More recently, our understanding of how pesticides affect cells of the central nervous system has been strengthened by computational biology. New insight has been gained about transcriptional and proteomic networks, and the metabolic pathways perturbed by pesticides. These networks and cell signaling pathways constitute potential therapeutic targets for intervention to slow or mitigate neurodegenerative diseases. Here we review the epidemiological evidence that supports a role for specific pesticides in the etiology of PD and identify molecular profiles amongst these pesticides that may contribute to the disease. Using the Comparative Toxicogenomics Database, these transcripts were compared to those regulated by the PD-associated neurotoxicant MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). While many transcripts are already established as those related to PD (alpha-synuclein, caspases, leucine rich repeat kinase 2, and parkin2), lesser studied targets have emerged as “pesticide/PD-associated transcripts” [e.g., phosphatidylinositol glycan anchor biosynthesis class C (Pigc), allograft inflammatory factor 1 (Aif1), TIMP metallopeptidase inhibitor 3, and DNA damage inducible transcript 4]. We also compared pesticide-regulated genes to a recent meta-analysis of genome-wide association studies in PD which revealed new genetic mutant alleles; the pesticides under review regulated the expression of many of these genes (e.g., ELOVL fatty acid elongase 7, ATPase H+ transporting V0 subunit a1, and bridging integrator 3). The significance is that these proteins may contribute to pesticide-related increases in PD risk. This review collates information on transcriptome responses to PD-associated pesticides to develop a mechanistic framework for quantifying PD risk with exposures.

Drosophila Models of Sporadic Parkinson's Disease

Parkinson’s disease (PD) is the most common cause of movement disorders and is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. It is increasingly recognized as a complex group of disorders presenting widely heterogeneous symptoms and pathology. With the exception of the rare monogenic forms, the majority of PD cases result from an interaction between multiple genetic and environmental risk factors. The search for these risk factors and the development of preclinical animal models are in progress, aiming to provide mechanistic insights into the pathogenesis of PD. This review summarizes the studies that capitalize on modeling sporadic (i.e., nonfamilial) PD using Drosophilamelanogaster and discusses their methodologies, new findings, and future perspectives.

UBA1: At the Crossroads of Ubiquitin Homeostasis and Neurodegeneration

Neurodegenerative diseases are a leading cause of disability and early death. A common feature of these conditions is disruption of protein homeostasis. Ubiquitin-like modifier activating enzyme 1 (UBA1), the E1 ubiquitin-activating enzyme, sits at the apex of the ubiquitin cascade and represents an important regulator of cellular protein homeostasis. Critical contributions of UBA1-dependent pathways to the regulation of homeostasis and degeneration in the nervous system are emerging, including specific disruption of UBA1 in spinal muscular atrophy (SMA) and Huntington's disease (HD). In this review we discuss recent findings that put UBA1 at the centre of cellular homeostasis and neurodegeneration, highlighting the potential for UBA1 to act as a promising therapeutic target for a range of neurodegenerative diseases.

Ziram, a pesticide associated with increased risk for Parkinson's disease, differentially affects the presynaptic function of aminergic and glutamatergic nerve terminals at the Drosophila neuromuscular junction

Multiple populations of aminergic neurons are affected in Parkinson's disease (PD), with serotonergic and noradrenergic loci responsible for some non-motor symptoms. Environmental toxins, such as the dithiocarbamate fungicide ziram, significantly increase the risk of developing PD and the attendant spectrum of both motor and non-motor symptoms. The mechanisms by which ziram and other environmental toxins increase the risk of PD, and the potential effects of these toxins on aminergic neurons, remain unclear. To determine the relative effects of ziram on the synaptic function of aminergic versus non-aminergic neurons, we used live-imaging at the Drosophila melanogaster larval neuromuscular junction (NMJ). In contrast to nearly all other studies of this model synapse, we imaged presynaptic function at both glutamatergic Type Ib and aminergic Type II boutons, the latter responsible for storage and release of octopamine, the invertebrate equivalent of noradrenalin. To quantify the kinetics of exo- and endo-cytosis, we employed an acid-sensitive form of GFP fused to the Drosophila vesicular monoamine transporter (DVMAT-pHluorin). Additional genetic probes were used to visualize intracellular calcium flux (GCaMP) and voltage changes (ArcLight). We find that at glutamatergic Type Ib terminals, exposure to ziram increases exocytosis and inhibits endocytosis. By contrast, at octopaminergic Type II terminals, ziram has no detectable effect on exocytosis and dramatically inhibits endocytosis. In contrast to other reports on the neuronal effects of ziram, these effects do not appear to result from perturbation of the Ubiquitin Proteasome System (UPS) or calcium homeostasis. Unexpectedly, ziram also caused spontaneous and synchronized bursts of calcium influx (measured by GCaMP) and electrical activity (measured by ArcLight) at aminergic Type II, but not glutamatergic Type Ib, nerve terminals. These events are sensitive to both tetrodotoxin and cadmium chloride, and thus appear to represent spontaneous depolarizations followed by calcium influx into Type II terminals. We speculate that the differential effects of ziram on Type II versus Type Ib terminals may be relevant to the specific sensitivity of aminergic neurons in PD, and suggest that changes in neuronal excitability could contribute to the increased risk for PD caused by exposure to ziram. We also suggest that the fly NMJ will be useful to explore the synaptic effects of other pesticides associated with an increased risk of PD.