Quality Over Quantity: Advantages of Using Alpha-Synuclein Preformed Fibril Triggered Synucleinopathy to Model Idiopathic Parkinson’s Disease

Animal models have significantly advanced our understanding of Parkinson’s disease (PD). Alpha-synuclein (α-syn) has taken center stage due to its genetic connection to familial PD and localization to Lewy bodies, one pathological hallmark of PD. Animal models developed on the premise of elevated alpha-synuclein via germline manipulation or viral vector-mediated overexpression are used to investigate PD pathophysiology and vet novel therapeutics. While these models represented a step forward compared to their neurotoxicant model predecessors, they rely on overexpression of supraphysiological levels of α-syn to trigger toxicity. However, whereas SNCA-linked familial PD is associated with elevated α-syn, elevated α-syn is not associated with idiopathic PD. Therefore, the defining feature of the α-syn overexpression models may fail to appropriately model idiopathic PD. In the last several years a new model has been developed in which α-syn preformed fibrils are injected intrastriatally and trigger normal endogenous levels of α-syn to misfold and accumulate into Lewy body-like inclusions. Following a defined period of inclusion accumulation, distinct phases of neuroinflammation and progressive degeneration can be detected in the nigrostriatal system. In this perspective, we highlight the fact that levels of α-syn achieved in overexpression models generally exceed those observed in idiopathic and even SNCA multiplication-linked PD. This raises the possibility that supraphysiological α-syn expression may drive pathophysiological mechanisms not relevant to idiopathic PD. We argue in this perspective that synucleinopathy triggered to form within the context of normal α-syn expression represents a more faithful animal model of idiopathic PD when examining the role of neuroinflammation or the relationship between a-syn aggregation and toxicity

Loss of VGLUT3 Produces Circadian-Dependent Hyperdopaminergia and Ameliorates Motor Dysfunction and l-Dopa-Mediated Dyskinesias in a Model of Parkinson\u27s Disease.

UNLABELLED: The striatum is essential for many aspects of mammalian behavior, including motivation and movement, and is dysfunctional in motor disorders such as Parkinson\u27s disease. The vesicular glutamate transporter 3 (VGLUT3) is expressed by striatal cholinergic interneurons (CINs) and is thus well positioned to regulate dopamine (DA) signaling and locomotor activity, a canonical measure of basal ganglia output. We now report that VGLUT3 knock-out (KO) mice show circadian-dependent hyperlocomotor activity that is restricted to the waking cycle and is due to an increase in striatal DA synthesis, packaging, and release. Using a conditional VGLUT3 KO mouse, we show that deletion of the transporter from CINs, surprisingly, does not alter evoked DA release in the dorsal striatum or baseline locomotor activity. The mice do, however, display changes in rearing behavior and sensorimotor gating. Elevation of DA release in the global KO raised the possibility that motor deficits in a Parkinson\u27s disease model would be reduced. Remarkably, after a partial 6-hydroxydopamine (6-OHDA)-mediated DA depletion (∼70% in dorsal striatum), KO mice, in contrast to WT mice, showed normal motor behavior across the entire circadian cycle. l-3,4-dihydroxyphenylalanine-mediated dyskinesias were also significantly attenuated. These findings thus point to new mechanisms to regulate basal ganglia function and potentially treat Parkinson\u27s disease and related disorders. SIGNIFICANCE STATEMENT: Dopaminergic signaling is critical for both motor and cognitive functions in the mammalian nervous system. Impairments, such as those found in Parkinson\u27s disease patients, can lead to severe motor deficits. Vesicular glutamate transporter 3 (VGLUT3) loads glutamate into secretory vesicles for neurotransmission and is expressed by discrete neuron populations throughout the nervous system. Here, we report that the absence of VGLUT3 in mice leads to an upregulation of the midbrain dopamine system. Remarkably, in a Parkinson\u27s disease model, the mice show normal motor behavior. They also show fewer abnormal motor behaviors (dyskinesias) in response to l-3,4-dihydroxyphenylalanine, the principal treatment for Parkinson\u27s disease. The work thus suggests new avenues for the development of novel treatment strategies for Parkinson\u27s disease and potentially other basal-ganglia-related disorders

Behavioral phenotyping of a rat model of the BDNF Val66Met polymorphism reveals selective impairment of fear memory

The common brain-derived neurotrophic factor (BDNF) Val66Met polymorphism is associated with reduced activity-dependent BDNF release and increased risk for anxiety disorders and PTSD. Here we behaviorally phenotyped a novel Val66Met rat model with an equivalent valine to methionine substitution in the rat Bdnf gene (Val68Met). In a three-day fear conditioning protocol of fear learning and extinction, adult rats with the Met/Met genotype demonstrated impaired fear memory compared to Val/Met rats and Val/Val controls, with no genotype differences in fear learning or extinction. This deficit in fear memory occurred irrespective of the sex of the animals and was not seen in adolescence (4 weeks of age). There were no changes in open-field locomotor activity or anxiety measured in the elevated plus maze (EPM) nor in other types of memory measured using the novel-object recognition test or Y-maze. BDNF exon VI expression in the dorsal hippocampus was higher and BDNF protein level in the ventral hippocampus was lower in female Val/Met rats than female Val/Val rats, with no other genotype differences, including in total BDNF, BDNF long, or BDNF IV mRNA. These data suggest a specific role for the BDNF Met/Met genotype in fear memory in rats. Further studies are required to investigate gene–environment interactions in this novel animal model

Next-Generation Diamond Electrodes for Neurochemical Sensing: Challenges and Opportunities

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. Carbon-based electrodes combined with fast-scan cyclic voltammetry (FSCV) enable neurochemical sensing with high spatiotemporal resolution and sensitivity. While their attractive electrochemical and conductive properties have established a long history of use in the detection of neurotransmitters both in vitro and in vivo, carbon fiber microelectrodes (CFMEs) also have limitations in their fabrication, flexibility, and chronic stability. Diamond is a form of carbon with a more rigid bonding structure (sp3-hybridized) which can become conductive when boron-doped. Boron-doped diamond (BDD) is characterized by an extremely wide potential window, low background current, and good biocompatibility. Additionally, methods for processing and patterning diamond allow for high-throughput batch fabrication and customization of electrode arrays with unique architectures. While tradeoffs in sensitivity can undermine the advantages of BDD as a neurochemical sensor, there are numerous untapped opportunities to further improve performance, including anodic pretreatment, or optimization of the FSCV waveform, instrumentation, sp2 /sp3 character, doping, surface characteristics, and signal processing. Here, we review the state-of-the-art in diamond electrodes for neurochemical sensing and discuss potential opportunities for future advancements of the technology. We highlight our team’s progress with the development of an all-diamond fiber ultramicroelectrode as a novel approach to advance the performance and applications of diamond-based neurochemical sensors

Induction of antidepressive activity by monoaminergic transplants in rat neocortex

To assess the ability of monoaminergic transplants to reduce immobility in the forced swimming test (FST), either adrenal medullary tissue, pineal gland tissue, or equal volumes of sciatic nerve were transplanted into the rat frontal neocortex. In the FST the duration of immobility is thought to indicate the level of antidepressant activity, as immobility times are reliably reduced by antidepressant therapies. Immobility times were reduced in rats with adrenal medullary grafts and pineal grafts to the rat frontal neocortex. In contrast, immobility times were not reduced in control sciatic nerve tissue grafts. Biochemical analysis using HPLC revealed that pineal-grafted neocortex contained higher levels of serotonin (5-HT) and adrenal medullary-grafted neocortex contained higher levels of norepinephrine (NE) than sciatic nerve-grafted or nongrafted controls. Immunocytochemical studies showed that the monoaminergic grafts survived well and continue to produce high levels of monoamines. These results support an important role for neocortical 5-HT and NE transmission in antidepressant activity and suggest that transplants of monoaminergic-containing tissue can reduce biochemical deficits in depression

Leveraging the preformed fibril model to distinguish between alpha-synuclein inclusion- and nigrostriatal degeneration-associated immunogenicity

Neuroinflammation has become a well-accepted pathologic hallmark of Parkinson's disease (PD). However, it remains unclear whether inflammation, triggered by α-syn aggregation and/or degeneration, contributes to the progression of the disease. Studies examining neuroinflammation in PD are unable to distinguish between Lewy body-associated inflammation and degeneration-associated inflammation, as both pathologies are present simultaneously. Intrastriatal and intranigral injections of alpha-synuclein (α-syn) preformed fibrils (PFFs) results in two distinct pathologic phases: Phase 1: The accumulation and peak formation of α-syn inclusions in nigrostriatal system and, Phase 2: Protracted dopaminergic neuron degeneration. In this review we summarize the current understanding of neuroinflammation in the α-syn PFF model, leveraging the distinct Phase 1 aggregation phase and Phase 2 degeneration phase to guide our interpretations. Studies consistently demonstrate an association between pathologic α-syn aggregation in the substantia nigra (SN) and activation of the innate immune system. Further, major histocompatibility complex-II (MHC-II) antigen presentation is proportionate to inclusion load. The α-syn aggregation phase is also associated with peripheral and adaptive immune cell infiltration to the SN. These findings suggest that α-syn like aggregates are immunogenic and thus have the potential to contribute to the degenerative process. Studies examining neuroinflammation during the neurodegenerative phase reveal elevated innate, adaptive, and peripheral immune cell markers, however limitations of single time point experimental design hinder interpretations as to whether this neuroinflammation preceded, or was triggered by, nigral degeneration. Longitudinal studies across both the aggregation and degeneration phases of the model suggest that microglial activation (MHC-II) is greater in magnitude during the aggregation phase that precedes degeneration. Overall, the consistency between neuroinflammatory markers in the parkinsonian brain and in the α-syn PFF model, combined with the distinct aggregation and degenerative phases, establishes the utility of this model platform to yield insights into pathologic events that contribute to neuroinflammation and disease progression in PD

Monoaminergic neural transplants prevent learned helplessness in a rat depression model

Current theories of the etiology of depression implicate disturbances and imbalances in the junction of monoaminergic systems, particularly involving serotonin and norepinephrine. Neural transplantation is a potential approach towards restoring balanced functioning in the central nervous system. The purpose of the present study was to determine the utility of transplanting monoamine-producing cells into the brain to alleviate behavioral depression. Serotonin-containing pineal gland tissue, catecholamine-containing adrenal medullary tissue, a combination of both, and a control of striated muscle tissue were implanted into the frontal neocortex of adult rats. The ability of these grafts to prevent the development of learned helplessness, a widely accepted model for depression, was assessed 6–8 weeks following transplantation. The monoamine-containing transplants, but not the control transplants, were able to prevent the development of learned helplessness. Immunocytochemical and ultrastructural studies revealed that the grafted monoaminergic tissues survived and continued to produce high levels of monoamines. These results suggest that neural transplants may provide a long-term local source of monoamines as a potentially new approach for alleviating some forms of depression

In Vivo Release of Catecholamines from Xenogeneic Chromaffin Cell Grafts with Antidepressive Activity

Previous studies in our laboratory have demonstrated that allografts of adrenal medullary tissue and xenografts of isolated bovine chromaffin cells to the rat frontal cortex can increase antidepressive activity in two separate animal models. Biochemical and pharmacological evidence suggest that the most likely mechanism of these antidepressive effects is via local release of catecholamines into the surrounding cortical parenchyma. The aim of the present study was to directly characterize the antidepressive mechanism of chromaffin cell xenografts by utilizing in vivo microdialysis to measure extracellular catecholamine levels from bovine chromaffin cell and control implanted rat frontal cortex. Following transplantation, only bovine chromaffin cell grafted rats displayed significant increases in antidepressive activity, as assessed by the forced swimming test, compared to rats with grafts of bovine adrenal medullary fibroblasts or nontransplanted rats. In vivo microdialysis results revealed remarkably elevated levels of epinephrine (EPI) and norepinephrine (NE), but not dopamine, in dialysates from bovine chromaffin cell-transplanted frontal cortex. The most likely source of these enhanced EPI and NE levels is the grafted xenogeneic chromaffin cells. The results of this study directly demonstrate that xenografts of bovine chromaffin cells to the rat frontal cortex provide a releasable pool of catecholamines for antidepressive activity