The nigrostriatal system in the presymptomatic and symptomatic stages in the MPTP monkey model: a PET, histological and biochemical study

Nigrostriatal iron accumulation in the progression of Parkinson's disease

Iron deposition in the nigrostriatal system plays a pivotal role in Parkinson's disease (PD) onset and progression. This study explored the time course of nigrostriatal iron accumulation in 54 PD patients at early to moderately advanced stages and 20 age-matched healthy controls. Using multi-echo T2*-MRI and R2* relaxometry, iron content was assessed in the substantia nigra pars compacta (SNpc) and striatum. In vivo findings were contrasted with histological analyses in a progressive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced parkinsonism model involving six non-human primates (NHPs) and two controls using Perls' Prussian blue staining. Complementarily, dopaminergic degeneration was quantified by 6-[18F]-fluoro-L-dopa PET in humans and TH immunohistochemistry in NHPs. Results showed progressive iron accumulation in the SNpc correlating with striatal dopaminergic denervation and neuronal loss. Striatal iron followed a V-shaped progression, decreasing initially and increasing later. Iron in the SNpc may serve as a marker of neurodegeneration in PD, while decreased striatal iron may indicate pathological susceptibility to dopaminergic loss.

α7-Nicotinic Acetylcholine Receptor and Mutated α-Synuclein Interact in Motor Behaviour and Nigrostriatal Dopamine-Findings With Potential Relevance for a Protective Effect of Cigarette Smoking and Parkinson's Disease

Parkinson's disease (PD) occurs less frequently in cigarette smokers than in nonsmokers. Assuming that nicotinic acetylcholine receptors are periodically active by activation through endogenous acetylcholine, we tested whether they act against the effect of α-synuclein, a protein relevant in PD. Transgenic mice with a human α-synuclein containing two mutations that cause familial PD were crossed with mice lacking the nicotinic α7-acetylcholine receptor. Vertical movements determined at 7 and 16 months and nonambulatory movements at 16 months of age were significantly lower in mice with α7-acetylcholine receptor knockout if they express the mutated α-synuclein but not in mice with α-synuclein wild type. Striatal noradrenaline, serotonin and dopamine levels did not differ between the four groups of mice at 21 months; however, striatal dopamine turnover was significantly higher in mice without than with α7-acetylcholine receptor. Stereological counts of nigral cells positive for tyrosine hydroxylase in the left and right hemisphere at 21 months revealed that asymmetry was also significantly higher in mice without than with α7-acetylcholine receptor. In conclusion, up to the age of 16 months, there was no obvious PD behaviour; however, absence of the α7-acetylcholine receptor generally reduced several features of motor behaviour and showed a statistically significant interaction between α7-acetylcholine receptor and mutated α-synuclein. The asymmetry of nigral cell counts and the increased striatal dopamine turnover suggest a stressed nigrostriatal system in mice without α7-acetylcholine receptor and that the neuroprotective effect of smoking might at least partly be mediated by the nicotine in the cigarettes acting via α7-acetylcholine receptors.

Temporal dynamics of neurovascular unit changes following blood-brain barrier opening in the putamen of non-human primates

Low-intensity focused ultrasound (LIFU) combined with intravenously circulating microbubbles has recently emerged as a novel approach for increasing delivery through the blood-brain barrier (BBB). This technique safely and transiently enables therapeutic agents to overcome the BBB, which typically poses a significant obstacle for treatment of brain disorders. However, the full impact of LIFU on the entire neurovascular unit (NVU), as well as the mechanisms and factors involved in restoring BBB integrity still require further elucidation. We conducted immunohistochemical analyses of the putamen in non-human primates to monitor changes over time [immediately post-treatment (3 h) and at 7- and 30-days post-BBB opening] in vascular, glial, and immune cells. Additionally, we examined the dynamic interactions among these elements and their role in the restorative process at the BBB level. A mild inflammatory response primarily involving microglia, astrocytes, and T- and B-lymphocytes was observed in the treated putamen acutely after BBB opening. These cells, recruited in response to the vascular changes, stimulate upregulation of PDGFRβ, a pericyte-specific marker, and VEGF-A, a pro-angiogenic factor. This was associated with vascular sprouting by 7 days post-BBB opening. Importantly, no notable long-term alterations were observed in the NVU 30 days post-BBB opening. These results offer further evidence regarding the efficacy and safety of LIFU in achieving BBB opening in the primate brain, indicating that nearly all changes in the NVU revert to baseline within 30 days post-treatment. This also suggests that angiogenesis may play an important role in restoring vascular integrity after BBB opening.

Calbindin and Girk2/Aldh1a1 define resilient vs vulnerable dopaminergic neurons in a primate Parkinson's disease model

The differential vulnerability of dopaminergic neurons of the substantia nigra pars compacta (SNc) is a critical and unresolved question in Parkinson´s disease. Studies in mice show diverse susceptibility of subpopulations of nigral dopaminergic neurons to various toxic agents. In the primate midbrain, the molecular phenotypes of dopaminergic neurons and their differential vulnerability are poorly characterized. We performed a detailed histological study to determine the anatomical distribution of different molecular phenotypes within identified midbrain neurons and their selective vulnerability in control and MPTP-treated monkeys. In the ventral tier of the SNc (nigrosome), neurons rich in Aldh1a1 and Girk2 are intermingled, whereas calbindin is the marker that best identifies the most resilient neurons located in the dorsal tier and ventral tegmental area, recapitulating the well-defined dorsoventral axis of susceptibility to degeneration of dopaminergic neurons. In particular, a loss of Aldh1a1+ neurons in the ventral SNc was observed in parallel to the progressive development of parkinsonism. Aldh1a1+ neurons were the main population of vulnerable dopaminergic nigrostriatal-projecting neurons, while Aldh1a1- neurons giving rise to nigropallidal projections remained relatively preserved. Moreover, bundles of entwined Aldh1a1+ dendrites with long trajectories extending towards the substantia nigra pars reticulata emerged from clusters of Aldh1a1+ neurons and colocalized with dense cannabinoid receptor 1 afferent fibers likely representing part of the striatonigral projection that is affected in human disorders, including Parkinson´s disease. In conclusion, vulnerable nigrostriatal-projecting neurons can be identified by using Aldh1a1 and Girk2. Further studies are needed to define the afferent/efferent projection patterns of these most vulnerable neurons.

Moderate intensity aerobic exercise alleviates motor deficits in 6-OHDA lesioned rats and reduces serum levels of biomarkers of Parkinson's disease severity without recovery of striatal dopamine or tyrosine hydroxylase

Alleviation of motor impairment by aerobic exercise (AE) in Parkinson's disease (PD) patients points to activation of neurobiological mechanisms that may be targetable by therapeutic approaches. However, evidence for AE-related recovery of striatal dopamine (DA) signaling or tyrosine hydroxylase (TH) loss has been inconsistent in rodent studies. This ambiguity may be related to the timing of AE intervention in relation to the status of nigrostriatal neuron loss. Here, we replicated human PD at diagnosis by establishing motor impairment with >80% striatal DA and TH loss prior to initiating AE, and assessed its potential to alleviate motor decline and restore DA and TH loss. We also evaluated if serum levels of neurofilament light (NfL) and glial fibrillary acidic protein (GFAP), biomarkers of human PD severity, changed in response to AE. 6-hydroxydopamine (6-OHDA) was infused unilaterally into rat medial forebrain bundle to induce progressive nigrostriatal neuron loss over 28 days. Moderate intensity AE (3× per week, 40 min/session), began 8-10 days post-lesion following establishment of impaired forelimb use. Striatal tissue DA, TH protein and mRNA, and serum levels of NfL/GFAP were determined 3-wks after AE began. Despite severe striatal DA depletion at AE initiation, forelimb use deficits and hypokinesia onset were alleviated by AE, without recovery of striatal DA or TH protein loss, but reduced NfL and GFAP serum levels. This proof-of-concept study shows AE alleviates motor impairment when initiated with >80% striatal DA loss without obligate recovery of striatal DA or TH protein. Moreover, the AE-related reduction of NfL and GFAP serum levels may serve as objective blood-based biomarkers of AE efficacy.

Evaluation of damage discrimination in dopaminergic neurons using dopamine transporter PET tracer [18F]FECNT-d4

BACKGROUND

Parkinson's disease (PD) is a prevalent neurodegenerative disorder worldwide, diagnosed based on classic symptoms like motor dysfunction and cognitive impairments. With the development of various radioactive ligands, positron emission tomography (PET) imaging combined with specific radiolabelling probes has proven to be effective in aiding clinical PD diagnosis. Among these probes, 2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-[18F]-fluoroethyl) nortropane ([18F]FECNT) has been utilized as a PET tracer to image dopamine transporter (DAT) integrity in striatal presynaptic dopaminergic terminals. However, the presence of brain-penetrant radioactive metabolites produced by [18F]FECNT may impact the accuracy of PET imaging. In previous research, we developed 2β-Carbomethoxy-3β-(4-chlorophenyl)-8-(2-[18F]-fluoroethyl-1,1,2,2-d4) nortropane ([18F]FECNT-d4), a deuterated derivative with enhanced stability in plasma and the striatum, along with a slower washout rate. In this study, we further investigated the potential of [18F]FECNT-d4 to detect dopaminergic neuron degeneration in Parkinson's disease. This involved PET imaging in unilaterally-lesioned PD model rats and in vitro autoradiography conducted on postmortem brain sections.

RESULTS

PET images revealed reduced specific uptake in the ipsilateral striatum of rats stereotactically injected with 6-hydroxydopamine hydrochloride (6-OHDA). Compared to the sham group, the ratio of standardized uptake value (SUV) in the ipsilateral to contralateral striatum decreased by 13%, 23%, and 63% in the mild, moderate, and severe lesioned groups, respectively. Dopaminergic denervation observed in PET imaging was further supported by behavioral assessments, immunostaining, and monoamine concentration tests. Moreover, the microPET results exhibited positive correlations with these measurements, except for the apomorphine-induced rotational behavior test, which showed a negative correlation. Additionally, [18F]FECNT-d4 uptake was approximately 40% lower in the postmortem striatal sections of a PD patient compared to a healthy subject. Furthermore, estimated human dosimetry (effective dose equivalent: 5.06 E-03 mSv/MBq), extrapolated from rat biodistribution data, remained below the current Food and Drug Administration limit for radiation exposure.

CONCLUSION

Our findings demonstrate that [18F]FECNT-d4 accurately estimates levels of dopaminergic neuron degeneration in the 6-OHDA-induced PD rat model and effectively distinguishes between PD patients and healthy individuals. This highly sensitive and safe PET probe holds promising potential for clinical application in the diagnosis and monitoring of Parkinson's disease.

Neurovascular and immune factors of vulnerability of substantia nigra dopaminergic neurons in non-human primates

Dopaminergic neurons in the ventral tier of the substantia nigra pars compacta (SNc) degenerate prominently in Parkinson's disease (PD), while those in the dorsal tier and ventral tegmental area are relatively spared. The factors determining why these neurons are more vulnerable than others are still unrevealed. Neuroinflammation and immune cell infiltration have been demonstrated to be a key feature of neurodegeneration in PD. However, the link between selective dopaminergic neuron vulnerability, glial and immune cell response, and vascularization and their interactions has not been deciphered. We aimed to investigate the contribution of glial cell activation and immune cell infiltration in the selective vulnerability of ventral dopaminergic neurons within the midbrain in a non-human primate model of PD. Structural characteristics of the vasculature within specific regions of the midbrain were also evaluated. Parkinsonian monkeys exhibited significant microglial and astroglial activation in the whole midbrain, but no major sub-regional differences were observed. Remarkably, the ventral substantia nigra was found to be typically more vascularized compared to other regions. This feature might play some role in making this region more susceptible to immune cell infiltration under pathological conditions, as greater infiltration of both T- and B- lymphocytes was observed in parkinsonian monkeys. Higher vascular density within the ventral region of the SNc may be a relevant factor for differential vulnerability of dopaminergic neurons in the midbrain. The increased infiltration of T- and B- cells in this region, alongside other molecules or toxins, may also contribute to the susceptibility of dopaminergic neurons in PD.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

A primate nigrostriatal atlas of neuronal vulnerability and resilience in a model of Parkinson's disease

The degenerative process in Parkinson's disease (PD) causes a progressive loss of dopaminergic neurons (DaNs) in the nigrostriatal system. Resolving the differences in neuronal susceptibility warrants an amenable PD model that, in comparison to post-mortem human specimens, controls for environmental and genetic differences in PD pathogenesis. Here we generated high-quality profiles for 250,173 cells from the substantia nigra (SN) and putamen (PT) of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian macaques and matched controls. Our primate model of parkinsonism recapitulates important pathologic features in nature PD and provides an unbiased view of the axis of neuronal vulnerability and resistance. We identified seven molecularly defined subtypes of nigral DaNs which manifested a gradient of vulnerability and were confirmed by fluorescence-activated nuclei sorting. Neuronal resilience was associated with a FOXP2-centered regulatory pathway shared between PD-resistant DaNs and glutamatergic excitatory neurons, as well as between humans and nonhuman primates. We also discovered activation of immune response common to glial cells of SN and PT, indicating concurrently activated pathways in the nigrostriatal system. Our study provides a unique resource to understand the mechanistic connections between neuronal susceptibility and PD pathophysiology, and to facilitate future biomarker discovery and targeted cell therapy.

The importance of social behavior in nonhuman primate studies of aging: A mini-review

Social behavior plays an important role in supporting both psychological and physical health across the lifespan. People's social lives change as they age, and the nature of these changes differ based on whether people are on healthy aging trajectories or are experiencing neurodegenerative diseases that cause dementia, such as Alzheimer's disease and Parkinson's disease. Nonhuman primate models of aging have provided a base of knowledge comparing aging trajectories in health and disease, but these studies rarely emphasize social behavior changes as a consequence of the aging process. What data exist hold particular value, as negative effects of disease and aging on social behavior are likely to have disproportionate impacts on quality of life. In this mini review, we examine the literature on nonhuman primate models of aging with a focus on social behavior, in the context of both health and disease. We propose that adopting a greater focus on social behavior outcomes in nonhuman primates will improve our understanding of the intersection of health, aging and sociality in humans.

Nigral-specific increase in ser31 phosphorylation compensates for tyrosine hydroxylase protein and nigrostriatal neuron loss: Implications for delaying parkinsonian signs

Compensatory mechanisms that augment dopamine (DA) signaling are thought to mitigate onset of hypokinesia prior to major loss of tyrosine hydroxylase (TH) in striatum that occurs in Parkinson's disease. However, the identity of such mechanisms remains elusive. In the present study, the rat nigrostriatal pathway was unilaterally-lesioned with 6-hydroxydopamine (6-OHDA) to determine whether differences in DA content, TH protein, TH phosphorylation, or D1 receptor expression in striatum or substantia nigra (SN) aligned with hypokinesia onset and severity at two time points. In striatum, DA and TH loss reached its maximum (>90%) 7 days after lesion induction. However, in SN, no DA loss occurred, despite ∼60% TH loss. Hypokinesia was established at 21 days post-lesion and maintained at 28 days. At this time, DA loss was ∼60% in the SN, but still of lesser magnitude than TH loss. At day 7 and 28, ser31 TH phosphorylation increased only in SN, corresponding to less DA versus TH protein loss. In contrast, ser40 TH phosphorylation was unaffected in either region. Despite DA loss in both regions at day 28, D1 receptor expression increased only in lesioned SN. These results support the concept that augmented components of DA signaling in the SN, through increased ser31 TH phosphorylation and D1 receptor expression, contribute as compensatory mechanisms against progressive nigrostriatal neuron and TH protein loss, and may mitigate hypokinesia severity.

Dopamine and vesicular monoamine transport loss supports incidental Lewy body disease as preclinical idiopathic Parkinson

Incidental Lewy body disease (ILBD) is a neuropathological diagnosis of brains with Lewy bodies without clinical neuropsychiatric symptoms. Dopaminergic deficits suggest a relationship to preclinical Parkinson's disease (PD). We now report a subregional pattern of striatal dopamine loss in ILBD cases, with dopamine found significantly decreased in the putamen (-52%) and only to a lower extent in the caudate (-38%, not statistically significant); this is similar to the pattern in idiopathic PD in various neurochemical and in vivo imaging studies. We aimed to find out if our recently reported impaired storage of dopamine in striatal synaptic vesicles prepared from striatal tissue of cases with idiopathic PD might be an early or even causative event. We undertook parallel measurements of [3H]dopamine uptake and vesicular monoamine transporter (VMAT)2 binding sites by the specific label [3H]dihydrotetrabenazine on vesicular preparation from caudate and putamen in ILBD. Neither specific uptake of dopamine and binding of [3H]dihydrotetrabenazine, nor mean values of the calculated ratios of dopamine uptake and VMAT2 binding, a measure of uptake rate per transport site, were significantly different between ILBD and controls. ATP-dependence of [3H]dopamine uptake revealed significantly higher rates in putamen than in caudate at saturating concentrations of ATP in controls, a subregional difference lost in ILBD. Our findings support a loss of the normally higher VMAT2 activity in putamen as a contributing factor to the higher susceptibility of the putamen to dopamine depletion in idiopathic PD. Moreover, we suggest ILBD postmortem tissue as a valuable source for testing hypotheses on processes in idiopathic PD.

In vivo18F-DOPA PET imaging identifies a dopaminergic deficit in a rat model with a G51D α-synuclein mutation

Parkinson's disease (PD) is a neurodegenerative condition with several major hallmarks, including loss of substantia nigra neurons, reduction in striatal dopaminergic function, and formation of α-synuclein-rich Lewy bodies. Mutations in SNCA, encoding for α-synuclein, are a known cause of familial PD, and the G51D mutation causes a particularly aggressive form of the condition. CRISPR/Cas9 technology was used to introduce the G51D mutation into the endogenous rat SNCA gene. SNCA^G51D/+ and SNCA^G51D/G51D rats were born in Mendelian ratios and did not exhibit any severe behavourial defects. L-3,4-dihydroxy-6-¹⁸F-fluorophenylalanine (¹⁸F-DOPA) positron emission tomography (PET) imaging was used to investigate this novel rat model. Wild-type (WT), SNCA^G51D/+ and SNCA^G51D/G51D rats were characterized over the course of ageing (5, 11, and 16 months old) using ¹⁸F-DOPA PET imaging and kinetic modelling. We measured the influx rate constant (K_i) and effective distribution volume ratio (EDVR) of ¹⁸F-DOPA in the striatum relative to the cerebellum in WT, SNCA^G51D/+ and SNCA^G51D/G51D rats. A significant reduction in EDVR was observed in SNCA^G51D/G51D rats at 16 months of age indicative of increased dopamine turnover. Furthermore, we observed a significant asymmetry in EDVR between the left and right striatum in aged SNCA^G51D/G51D rats. The increased and asymmetric dopamine turnover observed in the striatum of aged SNCA^G51D/G51D rats reflects one aspect of prodromal PD, and suggests the presence of compensatory mechanisms. SNCA^G51D rats represent a novel genetic model of PD, and kinetic modelling of ¹⁸F-DOPA PET data has identified a highly relevant early disease phenotype.

BBB opening with focused ultrasound in nonhuman primates and Parkinson's disease patients: Targeted AAV vector delivery and PET imaging

Intracerebral vector delivery in nonhuman primates has been a major challenge. We report successful blood-brain barrier opening and focal delivery of adeno-associated virus serotype 9 vectors into brain regions involved in Parkinson's disease using low-intensity focus ultrasound in adult macaque monkeys. Openings were well tolerated with generally no associated abnormal magnetic resonance imaging signals. Neuronal green fluorescent protein expression was observed specifically in regions with confirmed blood-brain barrier opening. Similar blood-brain barrier openings were safely demonstrated in three patients with Parkinson's disease. In these patients and in one monkey, blood-brain barrier opening was followed by 18F-Choline uptake in the putamen and midbrain regions based on positron emission tomography. This indicates focal and cellular binding of molecules that otherwise would not enter the brain parenchyma. The less-invasive nature of this methodology could facilitate focal viral vector delivery for gene therapy and might allow early and repeated interventions to treat neurodegenerative disorders.

Mapping the primate thalamus: systematic approach to analyze the distribution of subcortical neuromodulatory afferents

Neuromodulatory afferents to thalamic nuclei are key for information transmission and thus play critical roles in sensory, motor, and limbic processes. Over the course of the last decades, diverse attempts have been made to map and describe subcortical neuromodulatory afferents to the primate thalamus, including axons using acetylcholine, serotonin, dopamine, noradrenaline, adrenaline, and histamine. Our group has been actively involved in this endeavor. The published descriptions on neuromodulatory afferents to the primate thalamus have been made in different laboratories and are not fully comparable due to methodological divergences (for example, fixation procedures, planes of cutting, techniques used to detect the afferents, different criteria for identification of thalamic nuclei…). Such variation affects the results obtained. Therefore, systematic methodological and analytical approaches are much needed. The present article proposes reproducible methodological and terminological frameworks for primate thalamic mapping. We suggest the use of standard stereotaxic planes to produce and present maps of the primate thalamus, as well as the use of the Anglo-American school terminology (vs. the German school terminology) for identification of thalamic nuclei. Finally, a public repository of the data collected under agreed-on frameworks would be a useful tool for looking up and comparing data on the structure and connections of primate thalamic nuclei. Important and agreed-on efforts are required to create, manage, and fund a unified and homogeneous resource of data on the primate thalamus. Likewise, a firm commitment of the institutions to preserve experimental brain material is much needed because neuroscience work with non-human primates is becoming increasingly rare, making earlier material still more valuable.

Cytology, architecture, development, and connections of the primate striatum: Hints for human pathology

Degeneration of neurons and circuits across the striatum shows stereotyped time-course and spatial topography patterns that are distinct for Huntington's disease, Parkinson's disease, or the Tauopathies. These patterns of neurodegeneration in humans have not yet been systematically related to developmental, connectional, cellular, and chemical factors studied in human and non-human primates, that may underlie potential differences in selective vulnerability across striatal sectors. Relating primate anatomy to human pathology could provide new venues for identifying molecular, cellular, and connectional factors linked to the degeneration of striatal neurons and circuits. This review describes and summarizes several developmental, cellular, structural, and connectional features of the primate striatum in relation to patterns of neurodegeneration in the striatum of humans and of non-human primate models. We review (1) the types of neurons in the primate striatum, (2) the cyto-, myelo-, and chemoarchitecture of the primate striatum, (3) the developmental origin of the striatum in light of modern patterning studies, (4) the organization of corticostriatal projections in relation to cortical types, and (5) the topography and time-course of neuron loss, glial reaction, and protein aggregation induced by neurodegenerative diseases in humans and in non-human primate models across striatal sectors and their corresponding cortical areas. We summarize current knowledge about key aspects of primate striatal anatomy and human pathology and indicate knowledge gaps that should be addressed in future studies. We aim to identify factors for selective vulnerability to neurodegeneration of striatal neurons and circuits and obtain hints that could help elucidate striatal pathology in humans.

Comparison of 18 F-DOPA and 18 F-DTBZ for PET/CT Imaging of Idiopathic Parkinson Disease

OBJECTIVE

The aim of this study was to compare 2 imaging tracers, 18 F-DOPA and 18 F-DTBZ, for PET/CT imaging in idiopathic Parkinson disease (PD).

METHODS

We recruited 32 PD patients and 12 healthy controls in this study. All subjects underwent both 18 F-DOPA and 18 F-DTBZ PET/CT, and the results were interpreted by visual analysis and semiquantitative analysis (specific uptake ratios [SURs]). A 1-way analysis of variance was used to compare the clinical data and the SURs among the patients at different stages. Regression analysis was performed to analyze the correlation between the SURs and the clinical data.

RESULTS

Among the PD patients, there were 7 patients in Hoehn and Yahr stage I, 14 patients in stage II, and 11 patients in stage III. Linear correlation was found in striatal SURs between the 2 tracers ( P < 0.05). In patients of early stages, the striatal SUR decrease percent of 2 tracers had no statistical difference (paired t test, P > 0.05). By initial visual analysis, all the patients were interpreted as positive with 18 F-DBTZ (6 unilaterally, 26 bilaterally), and 31 cases were regarded as positive with 18 F-DOPA (8 unilaterally, 23 bilaterally). After setting the upper limit of SUR images with the putamen SURs of healthy controls (SUR T ), all patients were interpreted as positive with both tracers ( 18 F-DTBZ: 5 unilaterally, 27 bilaterally; 18 F-DOPA: 4 unilaterally, 28 bilaterally).

CONCLUSION

18 F-DTBZ and 18 F-DOPA could reflect the same level of dopaminergic neuron degeneration for PD in early stages, and they have the consistent visual analysis results.

Unbiased Stereological Estimates of Dopaminergic and GABAergic Neurons in the A10, A9, and A8 Subregions in the Young Male Macaque

The ventral midbrain is the primary source of dopamine- (DA) expressing neurons in most species. GABA-ergic and glutamatergic cell populations are intermixed among DA-expressing cells and purported to regulate both local and long-range dopamine neuron activity. Most work has been conducted in rodent models, however due to evolutionary expansion of the ventral midbrain in primates, the increased size and complexity of DA subpopulations warrants further investigation. Here, we quantified the number of DA neurons, and their GABA-ergic complement in classic DA cell groups A10 (midline ventral tegmental area nuclei [VTA] and parabrachial pigmented nucleus [PBP]), A9 (substantia nigra, pars compacta [SNc]) and A8 (retrorubral field [RRF]) in the macaque. Because the PBP is a disproportionately expanded feature of the A10 group, and has unique connectional features in monkeys, we analyzed A10 data by dividing it into 'classic' midline nuclei and the PBP. Unbiased stereology revealed total putative DA neuron counts to be 210,238 ± 17,127 (A10 = 110,319 ± 9649, A9 = 87,399 ± 7751 and A8 = 12,520 ± 827). Putative GABAergic neurons were fewer overall, and evenly dispersed across the DA subpopulations (GAD67 = 71,215 ± 5663; A10 = 16,836 ± 2743; A9 = 24,855 ± 3144 and A8 = 12,633 ± 3557). Calculating the GAD67/TH ratio for each subregion revealed differential balances of these two cell types across the DA subregions. The A8 subregion had the highest complement of GAD67-positive neurons compared to TH-positive neurons (1:1), suggesting a potentially high capacity for GABAergic inhibition of DA output in this region.

Onset pattern of nigrostriatal denervation in early Parkinson's disease

The striatal dopaminergic deficit in Parkinson's disease exhibits a typical pattern, extending from the caudal and dorsal putamen at onset to its more rostral region as the disease progresses. Clinically, upper-limb onset of cardinal motor features is the rule. Thus, according to current understanding of striatal somatotopy (i.e. the lower limb is dorsal to the upper limb) the assumed pattern of early dorsal striatal dopaminergic denervation in Parkinson's disease does not fit with an upper-limb onset. We have examined the topography of putaminal denervation in a cohort of 23 recently diagnosed de novo Parkinson's disease patients and 19 age-/gender-matched healthy subjects assessed clinically and by 18F-DOPA PET; 15 patients were re-assessed after 2 years. There was a net upper-limb predominance of motor features at onset. Caudal denervation of the putamen was confirmed in both the more- and less-affected hemispheres and corresponding hemibodies. Spatial covariance analysis of the most affected hemisphere revealed a pattern of 18F-DOPA uptake rate deficit that suggested focal dopamine loss starting in the posterolateral and intermediate putamen. Functional MRI group-activation maps during a self-paced motor task were used to represent the somatotopy of the putamen and were then used to characterize the decline in 18F-DOPA uptake rate in the upper- and lower-limb territories. This showed a predominant decrement in both hemispheres, which correlated significantly with severity of bradykinesia. A more detailed spatial analysis revealed a dorsoventral linear gradient of 18F-DOPA uptake rate in Parkinson's disease patients, with the highest putamen denervation in the caudal intermediate subregion (dorsoventral plane) compared to healthy subjects. The latter area coincides with the functional representation of the upper limb. Clinical motor assessment at 2-year follow-up showed modest worsening of parkinsonism in the primarily affected side and more noticeable increases in the upper limb in the less-affected side. Concomitantly, 18F-DOPA uptake rate in the less-affected putamen mimicked that recognized on the most-affected side. Our findings suggest that early dopaminergic denervation in Parkinson's disease follows a somatotopically related pattern, starting with the upper-limb representation in the putamen and progressing over a 2-year period in the less-affected hemisphere. These changes correlate well with the clinical presentation and evolution of motor features. Recognition of a precise somatotopic onset of nigrostriatal denervation may help to better understand the onset and progression of dopaminergic neurodegeneration in Parkinson's disease and eventually monitor the impact of putative therapies.

Neuron types in the primate striatum: stereological analysis of projection neurons and interneurons in control and parkinsonian monkeys

AIMS

The striatum is mainly composed of projection neurons. It also contains interneurons, which modulate and control striatal output. The aim of the present study was to assess the percentages of projection neurons and interneuron populations in the striatum of control monkeys and of parkinsonian monkeys.

METHODS

Unbiased stereology was used to estimate the volume density of every neuron population in the caudate, putamen and ventral striatum of control monkeys and of monkeys treated with MPTP, which results in striatal dopamine depletion. The various neuron population phenotypes were identified by immunohistochemistry. All analyses were performed within the same subjects using similar processing and analysis parameters, thus allowing for reliable data comparisons.

RESULTS

In control monkeys, the projection neurons, which express the Dopamine-and-cAMP-Regulated-Phosphoprotein, 32-KDa (DARPP-32), were the most abundant: ~86% of the total neurons counted. The interneurons accounted for the remaining 14%. Among the interneurons, those expressing Calretinin were the most abundant (Cr+: ~57%; ~8% of the total striatal neurons counted), followed those expressing Parvalbumin (Pv+: ~18 %; 2.6%), Dinucleotide Phosphate-Diaphorase (NADPH+: ~13 %; 1.8%), Choline Acetyltransferase (ChAT+: ~11%; 1.5%) and Tyrosine Hydroxylase (TH+: ~0.5%; 0.1%). No significant changes in volume densities occurred in any population following dopamine depletion, except for the TH+ interneurons, which increased in parkinsonian non-symptomatic monkeys and even more in symptomatic monkeys.

CONCLUSIONS

These data are relevant for translational studies targeting specific neuron populations of the striatum. The fact that dopaminergic denervation does not cause neuron loss in any population has potential pathophysiological implications.

Motor and non-motor circuit disturbances in early Parkinson disease: which happens first?

For the last two decades, pathogenic concepts in Parkinson disease (PD) have revolved around the toxicity and spread of α-synuclein. Thus, α-synuclein would follow caudo-rostral propagation from the periphery to the central nervous system, first producing non-motor manifestations (such as constipation, sleep disorders and hyposmia), and subsequently impinging upon the mesencephalon to account for the cardinal motor features before reaching the neocortex as the disease evolves towards dementia. This model is the prevailing theory of the principal neurobiological mechanism of disease. Here, we scrutinize the temporal evolution of motor and non-motor manifestations in PD and suggest that, even though the postulated bottom-up mechanisms are likely to be involved, early involvement of the nigrostriatal system is a key and prominent pathophysiological mechanism. Upcoming studies of detailed clinical manifestations with newer neuroimaging techniques will allow us to more closely define, in vivo, the role of α-synuclein aggregates with respect to neuronal loss during the onset and progression of PD.

Glial activation precedes alpha-synuclein pathology in a mouse model of Parkinson's disease

Neuroinflammation is increasingly recognized as an important feature in the pathogenesis of Parkinson's disease (PD). However, it remains unclear whether neuroinflammation contributes to nigral degeneration in PD or is merely a secondary marker of neurodegeneration. We aimed to investigate the temporal relationship between synucleopathy, neuroinflammation and nigrostriatal degeneration in a mouse model of PD. Mice received unilateral intrastriatal injection of alpha-synuclein pre-formed fibrils, alpha-synuclein monomer or vehicle and were sacrificed at 15, 30 and 90 days post-injection. Intrastriatal inoculation of alpha-synuclein fibrils led to significant alpha-synuclein aggregation in the substantia nigra peaking at 30 days after injection while the significant increase in Iba-1 cells, GFAP cells and IL-1β expression peaked earlier at 15 days. At 90 days, the striatal dopaminergic denervation was associated with astroglial activation. Alpha-synuclein monomer did not result in long-term glia activation or increase in inflammatory markers. The spread of alpha-synuclein aggregates into the cortex was not associated with any changes to neuroinflammatory markers. Our results demonstrate that in the substantia nigra glial activation is an early event that precedes alpha-synuclein inclusion formation, suggesting neuroinflammation could play an important early role in the pathogenesis of PD.

Stem Cell-Based Therapies for Parkinson Disease

Parkinson disease (PD) is a neurological movement disorder resulting primarily from damage to and degeneration of the nigrostriatal dopaminergic pathway. The pathway consists of neural populations in the substantia nigra that project to the striatum of the brain where they release dopamine. Diagnosis of PD is based on the presence of impaired motor features such as asymmetric or unilateral resting tremor, bradykinesia, and rigidity. Nonmotor features including cognitive impairment, sleep disorders, and autonomic dysfunction are also present. No cure for PD has been discovered, and treatment strategies focus on symptomatic management through restoration of dopaminergic activity. However, proposed cell replacement therapies are promising because midbrain dopaminergic neurons have been shown to restore dopaminergic neurotransmission and functionally rescue the dopamine-depleted striatum. In this review, we summarize our current understanding of the molecular pathogenesis of neurodegeneration in PD and discuss the development of new therapeutic strategies that have led to the initiation of exploratory clinical trials. We focus on the applications of stem cells for the treatment of PD and discuss how stem cell research has contributed to an understanding of PD, predicted the efficacy of novel neuroprotective therapeutics, and highlighted what we believe to be the critical areas for future research.

Molecular targets for endogenous glial cell line-derived neurotrophic factor modulation in striatal parvalbumin interneurons

Administration of recombinant glial cell line-derived neurotrophic factor into the putamen has been tested in preclinical and clinical studies to evaluate its neuroprotective effects on the progressive dopaminergic neuronal degeneration that characterizes Parkinson’s disease. However, intracerebral glial cell line-derived neurotrophic factor infusion is a challenging therapeutic strategy, with numerous potential technical and medical limitations. Most of these limitations could be avoided if the production of endogenous glial cell line-derived neurotrophic factor could be increased. Glial cell line-derived neurotrophic factor is naturally produced in the striatum from where it exerts a trophic action on the nigrostriatal dopaminergic pathway. Most of striatal glial cell line-derived neurotrophic factor is synthesized by a subset of GABAergic interneurons characterized by the expression of parvalbumin. We sought to identify molecular targets specific to those neurons and which are putatively associated with glial cell line-derived neurotrophic factor synthesis. To this end, the transcriptomic differences between glial cell line-derived neurotrophic factor-positive parvalbumin neurons in the striatum and parvalbumin neurons located in the nearby cortex, which do not express glial cell line-derived neurotrophic factor, were analysed. Using mouse reporter models, we have defined the genomic signature of striatal parvalbumin interneurons obtained by fluorescence-activated cell sorting followed by microarray comparison. Short-listed genes were validated by additional histological and molecular analyses. These genes code for membrane receptors (Kit, Gpr83, Tacr1, Tacr3, Mc3r), cytosolic proteins (Pde3a, Crabp1, Rarres2, Moxd1) and a transcription factor (Lhx8). We also found the proto-oncogene cKit to be highly specific of parvalbumin interneurons in the non-human primate striatum, thus highlighting a conserved expression between species and suggesting that specific genes identified in mouse parvalbumin neurons could be putative targets in the human brain. Pharmacological stimulation of four G-protein-coupled receptors enriched in the striatal parvalbumin interneurons inhibited Gdnf expression presumably by decreasing cyclic adenosine monophosphate formation. Additional experiments with pharmacological modulators of adenylyl cyclase and protein kinase A indicated that this pathway is a relevant intracellular route to induce Gdnf gene activation. This preclinical study is an important step in the ongoing development of a specific pro-endo-glial cell line-derived neurotrophic factor pharmacological strategy to treat Parkinson’s disease.

Reduced Midbrain Dopamine Neuron Number in the Adult Non-human Primate Brain after Fetal Radiation Exposure

Early gestation is a neurodevelopmental period that is especially vulnerable to environmental insult and one in which neurogenesis features prominently. Prenatal perturbation during early gestation has been linked to neuropsychiatric illnesses such as autism and schizophrenia, and severe environmental insult during this period can result in profound mental impairment. Midbrain dopamine neurons are generated during early gestation and play a key role in the motor, cognitive and reward circuitries implicated in neuropsychiatric disease and addiction. This study examined the impact of curtailing neurogenesis in early gestation on neuron number in the midbrain dopamine group, i.e., the substantia nigra and contiguous ventral tegmental area. Rhesus macaque monkeys were exposed in utero on embryonic days 39-41 to x-irradiation (3-4 exposures of 50 cGy over 3-7 days totalling <200 cGy) and allowed to mature to full adulthood. Stereologic cell counts of tyrosine hydroxylase-positive neurons in the midbrain dopamine group were performed in adult monkeys, as were measurements of somal size. Mean total neuron number in the irradiated monkeys was significantly reduced on average by 33% compared to that of the control group. Somal size did not differ between the groups, suggesting that the integrity of survivor populations was not impacted. Reduced midbrain dopamine neuron number in fetally irradiated, adult monkeys indicates that radiation exposure during the critical period of neurogenesis results in an enduring reduction of this population and underscores the susceptibility of early neurodevelopmental processes to irreversible damage from environmental exposures.

A Proposed Roadmap for Parkinson's Disease Proof of Concept Clinical Trials Investigating Compounds Targeting Alpha-Synuclein

The convergence of human molecular genetics and Lewy pathology of Parkinson's disease (PD) have led to a robust, clinical-stage pipeline of alpha-synuclein (α-syn)-targeted therapies that have the potential to slow or stop the progression of PD and other synucleinopathies. To facilitate the development of these and earlier stage investigational molecules, the Michael J. Fox Foundation for Parkinson's Research convened a group of leaders in the field of PD research from academia and industry, the Alpha-Synuclein Clinical Path Working Group. This group set out to develop recommendations on preclinical and clinical research that can de-risk the development of α-syn targeting therapies. This consensus white paper provides a translational framework, from the selection of animal models and associated end-points to decision-driving biomarkers as well as considerations for the design of clinical proof-of-concept studies. It also identifies current gaps in our biomarker toolkit and the status of the discovery and validation of α-syn-associated biomarkers that could help fill these gaps. Further, it highlights the importance of the emerging digital technology to supplement the capture and monitoring of clinical outcomes. Although the development of disease-modifying therapies targeting α-syn face profound challenges, we remain optimistic that meaningful strides will be made soon toward the identification and approval of disease-modifying therapeutics targeting α-syn.

Initial motor reserve and long-term prognosis in Parkinson's disease

There are individual differences in motor deficits, despite a similar degree of dopamine neuronal loss in Parkinson's disease (PD), called motor reserve (MR). Factors enhancing MR have been documented previously, but the influence of initial MR on the long-term prognosis remains unclear. In this longitudinal study, we enrolled 205 patients with de novo PD to estimate individual MR based on initial motor deficits and striatal dopamine depletion using the residual-based approach. We assessed the risk of developing levodopa-induced dyskinesia (LID) or freezing of gait (FOG) and longitudinal increases in levodopa-equivalent dose (LED) according to MR estimates using the Cox regression model and linear mixed model, respectively. Throughout the follow-up period (≥3 years), greater MR estimates were associated with a lower risk for LID and FOG. In addition, patients with high MR received lower LED than those with low MR. These findings suggest that the initial MR, that is, individual's capacity to cope with PD-related pathologies, can be maintained with disease progression and can modulate the risk for LID or FOG.

Combining the lack of chromogranins with chronic L-DOPA treatment affects motor activity in mice

We have tested whether the lack of chromogranins (Cgs) A and B could provoke CNS disorders when combined with an excess of dopamine. We chronically treated (over 6 months) mice lacking both chromogranins A and B (Cgs-KO) with a low oral dosage of L-DOPA/benserazide (10/2.5 mg/kg). Motor performance in the rota-rod test, open field activity, and metabolic cages indicated a progressive impairment in motor coordination in these mice, and an increase in rearing behavior, which was accompanied by an increase in DA within the substantia nigra. We conclude that mild chronic L-DOPA treatment does not produce nigro-striatal toxicity that could be associated with parkinsonism, neither in control nor Cgs-KO mice. Rather, Cgs-KO mice exhibit behaviors compatible with an amphetamine-like effect, probably caused by the excess of catecholamines in the CNS.

Observation of magnetic susceptibility changes within the thalamus: a comparative study between healthy and Parkinson’s disease afflicted cynomolgus monkeys using 7 T MRI

Background

Although the thalamus is known to modulate basal ganglia function related to motor control activity, the abnormal changes within the thalamus during distinct medical complications have been scarcely investigated. In order to explore the feasibility of assessing iron accumulation in the thalamus as an informative biomarker for Parkinson’s disease (PD), this study was designed to employ quantitative susceptibility mapping using a 7 T magnetic resonance imaging system in cynomolgus monkeys. A 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-injected cynomolgus monkey and a healthy control (HC) were examined by 7 T magnetic resonance imaging. Positron emission tomography with 18F-N-(3-fluoro propyl)-2ß-carboxymethoxy-3ß-(4-iodophenyl) nortropane was also employed to identify the relationship between iron deposits and dopamine depletion. All acquired values were averaged within the volume of interest of the nigrostriatal pathway.

Findings

Compared with the HC, the overall elevation of iron deposition within the thalamus in the Parkinson’s disease model (about 53.81% increase) was similar to that in the substantia nigra (54.81%) region. Substantial susceptibility changes were observed in the intralaminar part of the thalamus (about 70.78% increase). Additionally, we observed that in the Parkinson’s disease model, binding potential values obtained from positron emission tomography were considerably decreased in the thalamus (97.51%) and substantia nigra (92.48%).

Conclusions

The increased iron deposition in the thalamus showed negative correlation with dopaminergic activity in PD, supporting the idea that iron accumulation affects glutaminergic inputs and dopaminergic neurons. This investigation indicates that the remarkable susceptibility changes in the thalamus could be an initial major diagnostic biomarker for Parkinson’s disease-related motor symptoms.

Changes in thalamic dopamine innervation in a progressive Parkinson's disease model in monkeys

BACKGROUND

Dopamine loss beyond the mesostriatal system might be relevant in pathogenic mechanisms and some clinical manifestations in PD. The primate thalamus is densely and heterogeneously innervated with dopaminergic axons, most of which express the dopamine transporter, as does the nigrostriatal system. We hypothesized that dopamine depletion may be present in the thalamus of the parkinsonian brain and set out to ascertain possible regional differences.

METHODS

The toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was administered to adult macaque monkeys using a slow intoxication protocol. The treated macaques were classified into 2 groups according to their motor status: nonsymptomatic and parkinsonian. Dopamine innervation was studied with immunohistochemistry for the dopamine transporter. Topographic maps of the dopamine transporter-immunoreactive axon distribution were generated and the total length and length density of these axons stereologically estimated using a 3-dimensional fractionator.

RESULTS

Parkinsonian macaques exhibited lower dopamine transporter-immunoreactive axon length density than controls in mediodorsal and centromedian-parafascicular nuclei. Dopamine denervation in the mediodorsal nucleus was already noticeable in nonsymptomatic macaques and was even greater in parkinsonian macaques. Reticular nucleus dopamine transporter-immunoreactive axon length density presented an inverse pattern, increasing progressively to the maximum density seen in parkinsonian macaques. No changes were observed in ventral thalamic nuclei. Dopamine transporter-immunoreactive axon maps supported the quantitative findings.

CONCLUSIONS

Changes in the dopamine innervation of various thalamic nuclei are heterogeneous and start in the premotor parkinsonian stage. These changes may be involved in some poorly understood nonmotor manifestations of PD. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Decreased Striatal Vesicular Monoamine Transporter Type 2 Correlates With the Nonmotor Symptoms in Parkinson Disease

OBJECTIVE

Nonmotor symptoms (NMS) are critical players in the patients' quality of life in Parkinson disease (PD). Vesicular monoamine transporter type 2 (VMAT2) has been reported owing to a role in affecting dopamine neurons in the striatum. Therefore, this study set out to characterize the relationship between VMAT2 distribution in the striatum in relation to the NMS in PD.

METHODS

Totally, 21 age-matched normal controls and 37 patients with PD in the moderate stages were included, followed by examination using F-DTBZ (F-AV133) PET/CT. The specific uptake ratio (SUR) of each striatal subregion was then determined with the occipital cortex as the reference background region. The overall NMSs of each individual patient were evaluated. Finally, the role of the striatal SURs in the clinical symptom scores were evaluated through the application of a Spearman correlation analysis as well as a multivariable stepwise regression analysis.

RESULTS

Patients with PD, particularly those at a more advanced stage, exhibited a more pronounced reduction in SURs in the bilateral putamen and caudate nucleus (P < 0.05, vs healthy controls). Meanwhile, patients at more advanced PD stages were found to have significantly worse scores in NMS except cognitive function. The Spearman correlation analysis demonstrated that NMS scores, with the exception of cognition scores, were correlated with striatal SURs (P < 0.05).

CONCLUSION

The key findings of the study identified a correlation between decreased striatal VMAT2 with a broad spectrum of NMS in patients with PD, highlighting the association between diminished dopamine supply and the development of NMS in PD.

Serotonergic innervation of the striatum in a nonhuman primate model of Parkinson's disease

Parkinson's disease (PD) is characterized by dopaminergic neurodegeneration in the substantia nigra and dopamine depletion in the striatum. Non-dopaminergic systems are also affected, including the serotonergic system. Enhanced striatal serotonergic innervation is a proposed compensatory mechanism for the dopaminergic deficit. Meanwhile a serotonergic deficit has been suggested as preceding the nigrostriatal dopaminergic pathology in PD. Our aim was to assess the serotonergic innervation of the striatum in a model of progressive experimental parkinsonism in macaques, from pre-symptomatic to symptomatic stages. The neurotoxin 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine (MPTP) was administered to adult macaque monkeys using a slow intoxication protocol. The intoxicated animals were classified into asymptomatic, recovered, moderate and severe parkinsonian, based on their motor behavior. The serotonergic innervation was studied by immunohistochemistry against serotonin (5-HT). In the striatum, the density of 5-HT-immunoreactive (5-HT+) axons was estimated with stereology. Images of the striatum in the immunostained sections were taken to compare the distribution patterns of the serotonergic innervation between groups. These patterns were apparently similar among the groups. Axonal density estimations showed no differences in striatal 5-HT+ innervation between the intoxicated groups and the control group. Accordingly, this study fails to find significant changes in the striatal serotonergic axonal innervation in MPTP-treated monkeys, coinciding with previous biochemical findings in our model. However, it is possible that alterations in the serotonergic system in PD could be independent of axonal density changes. Consequently, the proposed role for striatal serotonin serving as a compensatory mechanism for dopaminergic denervation merits further study. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Local Gastrointestinal Injury Exacerbates Inflammation and Dopaminergic Cell Death in Parkinsonian Mice

The cause of progressive degeneration in Parkinson's disease is not clear, although, in the last years, different studies have suggested that both brain and peripheral inflammation could play a key role in the progression of this disorder. In our study, we aimed to analyze the effect of an acute inflammation confined to the colon on dopaminergic neuronal death and glial response in mice intoxicated with MPTP. The results obtained show a very significant decrease of dopaminergic neurons in the SNpc as well as a significant decrease of dopaminergic fibers in the striatum of the MPTP+DSS-treated group compared with the control animals. In addition, there was a significant exacerbation of microglial and astrocytes activation in MPTP+DSS animals compared with the control group. This data suggests that a specific gastrointestinal injury, which induces a systemic inflammatory response, is able to exacerbate cell death mechanisms of the remaining dopaminergic neurons and then contributes to the persistent progression of the disease. These results leave open new lines of research on the role of exclusive colonic inflammation and the progression of nigrostriatal dopaminergic degeneration.

Unexpected Exacerbation of Neuroinflammatory Response After a Combined Therapy in Old Parkinsonian Mice

The design of therapeutic strategies that focus on the repositioning of anti-inflammatory and antioxidant drugs are a great bet to slow down the progression of neurodegenerative disorders. Despite the fact that Parkinson’s disease (PD) is an age-related pathology, almost all experimental studies are carried out in young animals. Here, we evaluated the possible neuroprotective effect of the combination of the antioxidant N-acetylcysteine (NAC) and the anti-inflammatory HA-1077 in aged 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice (C57BL/6 mice, 20 months old), whose individual treatment has been shown to have neuroprotective effects in this Parkinsonism model. Interestingly, NAC+HA-1077-based treatment produced a significant increase in dopaminergic neuronal death accompanied by an increase in microglial and astroglial activation in the Substantia Nigra pars compacta (SNpc) and striatum of old-Parkinsonian mice compared to their control group. The astroglial response was also explored by co-immunostaining for GFAP and S100b together with p-JNK and it was found to be particularly exacerbated in the MPTP+NAC+HA-1077 group. The unexpected toxic effects found in the combined use of NAC and HA-1077 in old-Parkinsonian mice highlight the importance of taking into account that in elderly Parkinsonian patients the combination of some drugs (most of them used for other different age-related alterations) can have side effects that may result in the exacerbation of the neurodegenerative process.

Effect of NAC treatment and physical activity on neuroinflammation in subchronic Parkinsonism; is physical activity essential?

BACKGROUND

Neuroprotective strategies are becoming relevant to slow down dopaminergic cell death and inflammatory processes related to the progressive neurodegeneration in Parkinson's disease (PD). Interestingly, among others, physical activity (PA) or anti-oxidant agents (such as N-acetyl-L-cysteine, NAC) are common therapeutic strategies. Therefore, this study aims to analyze if there is a synergistic effect of physical activity along with NAC treatment on dopaminergic degeneration and neuroinflammatory response in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinsonism model after subchronic intoxication.

METHODS

To ascertain this possibility, 48 8-week-old male mice (C57BL/6 strain) were used. Twenty four of them were placed individually in cages where voluntary physical activity was automatically monitored during 30 days and were divided into groups: (i) control; (ii) NAC; (iii) MPTP, and (iv) MPTP+NAC. The other 24 mice were divided into the same four groups but without physical activity.

RESULTS

The data collected during the treatment period showed that there was an overall increase in the total running distance in all groups under physical activity, including Parkinsonian animals. However, the monitoring data per day showed that the activity routine by MPTP and MPTP+NAC groups was disrupted by alterations in the circardian rhythm because of MPTP intoxication. Results from post-mortem studies in the substantia nigra pars compacta (SNpc) showed significant decrease in the number of TH+ cells in all MPTP groups. Moreover, TH+ expression in the striatum was significantly decreased in all MPTP groups. Thus, PA + NAC treatment do not protect dopaminergic neurons against a subchronic intoxication of MPTP. Regarding glial response, the results obtained from microglial analysis do not show significant increase in the number of Iba-1+ cell in MPTP+NAC and MPTP+PA + NAC. In the striatum, a significant decrease is observed only in the MPTP+NAC group compared with that of the MPTP group. The microglial results are reinforced by those obtained from the analysis of astroglial response, in which a decrease in the expression of GFAP+ cells are observed in MPTP+NAC and MPTP+PA + NAC compared with MPTP groups both in the SNpc and in the striatum. Finally, from the study of the astroglial response by the co-localization of GFAP/S100b, we described some expression patterns observed based on the severity of the damage produced by the MPTP intoxication in the different treated groups.

CONCLUSIONS

These results suggest that the combination of physical activity with an anti-oxidant agent does not have a synergistic neuroprotective effect in the nigrostriatal pathway. Our results show a potential positive effect, only due to NAC treatment, on the neuroinflammatory response after subchronic MPTP intoxication. Thus, physical activity is not essential, under these conditions. However, we believe that physical activity, used for therapeutic purposes, has a beneficial long-term effect. In this line, these results open the door to design longer studies to demonstrate its promising effect as neuroprotective strategy.

Upgraded Methodology for the Development of Early Diagnosis of Parkinson's Disease Based on Searching Blood Markers in Patients and Experimental Models

Numerous attempts to develop an early diagnosis of Parkinson's disease (PD) by searching biomarkers in biological fluids were unsuccessful. The drawback of this methodology is searching markers in patients at the clinical stage without guarantee that they are also characteristic of either preclinical stage or prodromal stage (preclinical-prodromal stage). We attempted to upgrade this methodology by selecting only markers that are found both in patients and in PD animal models. HPLC and RT-PCR were used to estimate the concentration of amino acids, catecholamines/metabolites in plasma and gene expression in lymphocytes in 36 untreated early-stage PD patients and 52 controls, and in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice at modeling the clinical ("symptomatic") stage and preclinical-prodromal ("presymptomatic") stage of PD. It was shown that among 13 blood markers found in patients, 7 markers are characteristic of parkinsonian symptomatic mice and 3 markers of both symptomatic and presymptomatic mice. According to our suggestion, the detection of the same marker in patients and symptomatic animals indicates adequate reproduction of pathogenesis along the corresponding metabolic pathway, whereas the detection of the same marker in presymptomatic animals indicates its specificity for preclinical-prodromal stage. This means that the minority of markers found in patients-decreased concentration of L-3,4-dihydroxyphenylalanine (L-DOPA) and dihydroxyphenylacetic acid (DOPAC) and increased dopamine D3 receptor gene expression-are specific for preclinical-prodromal stage and are suitable for early diagnosis of PD. Thus, we upgraded a current methodology for development of early diagnosis of PD by searching blood markers not only in patients but also in parkinsonian animals.

An adverse outcome pathway for parkinsonian motor deficits associated with mitochondrial complex I inhibition

Epidemiological studies have observed an association between pesticide exposure and the development of Parkinson's disease, but have not established causality. The concept of an adverse outcome pathway (AOP) has been developed as a framework for the organization of available information linking the modulation of a molecular target [molecular initiating event (MIE)], via a sequence of essential biological key events (KEs), with an adverse outcome (AO). Here, we present an AOP covering the toxicological pathways that link the binding of an inhibitor to mitochondrial complex I (i.e., the MIE) with the onset of parkinsonian motor deficits (i.e., the AO). This AOP was developed according to the Organisation for Economic Co-operation and Development guidelines and uploaded to the AOP database. The KEs linking complex I inhibition to parkinsonian motor deficits are mitochondrial dysfunction, impaired proteostasis, neuroinflammation, and the degeneration of dopaminergic neurons of the substantia nigra. These KEs, by convention, were linearly organized. However, there was also evidence of additional feed-forward connections and shortcuts between the KEs, possibly depending on the intensity of the insult and the model system applied. The present AOP demonstrates mechanistic plausibility for epidemiological observations on a relationship between pesticide exposure and an elevated risk for Parkinson's disease development.

Application of the Physical Disector Principle for Quantification of Dopaminergic Neuronal Loss in a Rat 6-Hydroxydopamine Nigral Lesion Model of Parkinson's Disease

Stereological analysis is the optimal tool for quantitative assessment of brain morphological and cellular changes induced by neurotoxic lesions or treatment interventions. Stereological methods based on random sampling techniques yield unbiased estimates of particle counts within a defined volume, thereby providing a true quantitative estimate of the target cell population. Neurodegenerative diseases involve loss of specific neuron types, such as the midbrain tyrosine hydroxylase-positive dopamine neurons in Parkinson's disease and in animal models of nigrostriatal degeneration. Therefore, we applied an established automated physical disector principle in a fractionator design for efficient stereological quantitative analysis of tyrosine hydroxylase (TH)-positive dopamine neurons in the substantia nigra pars compacta of hemiparkinsonian rats with unilateral 6-hydroxydopamine (6-OHDA) lesions. We obtained reliable estimates of dopamine neuron numbers, and established the relationship between behavioral asymmetry and dopamine neuron loss on the lesioned side. In conclusion, the automated physical disector principle provided a useful and efficient tool for unbiased estimation of TH-positive neurons in rat midbrain, and should prove valuable for investigating neuroprotective strategies in 6-OHDA model of parkinsonism, while generalizing to other immunohistochemically-defined cell populations.

1-Methyl-4-Phenylpyridinium-Induced Death of Differentiated SH-SY5Y Neurons Is Potentiated by Cholesterol

Background/Aims

Hypercholesterolemia is recently considered a risk factor for Parkinson's disease (PD), the most consistent neurodegenerative movement disorder. The study aimed to investigate the effect of exogenous cholesterol on 1-methyl-4-phenylpyridinium (MPP⁺) parkinsonian neurotoxin-induced cell death, loss of mitochondrial membrane potential, and dopaminergic deficiency in a cellular model of PD.

Methods

Cholesterol (50 μM) when added in the culture media alone or in combination with MPP⁺ was studied in SH-SY5Y neuroblastoma cells. There were 4 groups that were studied; SH-SY5Y cells treated with vehicle (control), cells that were treated with 1 mM MPP⁺ (MPP⁺), or cholesterol (chol) or both (M + chol). The loss of cell survival was measured by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay. Dopamine depletion, microtubule-associated protein 2 (MAP-2), and tyrosine hydroxylase (TH)-positive neuronal loss were determined by HPLC-electrochemical detection and TH immunocytochemistry respectively. Mitochondrial membrane potential in cells stained by tetramethylrhodamine methyl ester dye was analysed by flow cytometry.

Results

Cholesterol treatment potentiated a reduction of neuronal viability with loss of TH-positive neurons in cultures. MPP⁺-induced depletion of dopamine level in the post-mitotic MAP-2 immunoreactive neurons and loss of mitochondrial membrane potential were also heightened by cholesterol.

Conclusion

Apparently, changes in neuronal cholesterol content significantly influenced the neurotoxicity and the direct mitochondrial mechanisms involved in MPP⁺-induced cell death. Our observations demonstrate that high cholesterol incorporated into the differentiated human neuroblastoma cells worsened dopaminergic neuronal survivability through increased depolarization of mitochondrial membrane potential, which is a known mechanism of dopaminergic cell death by MPP⁺. The present findings support the hypothesis that hypercholesterolemia could be a risk factor for PD.

Transplantation in the nonhuman primate MPTP model of Parkinson's disease: update and perspectives

In order to calibrate stem cell exploitation for cellular therapy in neurodegenerative diseases, fundamental and preclinical research in NHP (nonhuman primate) models is crucial. Indeed, it is consensually recognized that it is not possible to directly extrapolate results obtained in rodent models to human patients. A large diversity of neurological pathologies should benefit from cellular therapy based on neural differentiation of stem cells. In the context of this special issue of Primate Biology on NHP stem cells, we describe past and recent advances on cell replacement in the NHP model of Parkinson's disease (PD). From the different grafting procedures to the various cell types transplanted, we review here diverse approaches for cell-replacement therapy and their related therapeutic potential on behavior and function in the NHP model of PD.

The End Is the Beginning: Parkinson's Disease in the Light of Brain Imaging

Parkinson's disease (PD), the most common neurodegenerative disorder, is characterized by abnormal accumulation of α-synuclein aggregates known as Lewy bodies (LB) and loss of nigrostriatal dopaminergic neurons. Recent neuroimaging studies suggest that in the early phases of PD, synaptic and axonal damage anticipate the onset of a frank neuronal death. Paralleling, even post mortem studies on the brain of affected patients and on animal models support that synapses might represent the primary sites of functional and pathological changes. Indeed, α-synuclein microaggregation and spreading at terminals, by dysregulating the synaptic junction, would block neurotransmitter release, thus triggering a retrograde neurodegenerative process ending with neuronal cell loss by proceeding through the axons. Rather than neurodegeneration, loss of dopaminergic neuronal endings and axons could thus underlie the onset of connectome dysfunction and symptoms in PD and parkinsonisms. However, the manifold biases deriving from the interpretation of human brain imaging data hinder the validation of this hypothesis. Here, we present pivotal evidence supporting that novel comparative brain imaging studies, in patients and experimental models of PD in preliminary stages of disease, could be instrumental for proving whether synaptic endings are the sites where degeneration begins and initiating the factual achievement of disease modifying approaches. The need for such investigations is timely to define an early therapeutic window of intervention to attempt disease halting by terminal and/or axonal healing.

Compensatory mechanisms in Parkinson's disease: Circuits adaptations and role in disease modification

The motor features of Parkinson's disease (PD) are well known to manifest only when striatal dopaminergic deficit reaches 60-70%. Thus, PD has a long pre-symptomatic and pre-motor evolution during which compensatory mechanisms take place to delay the clinical onset of disabling manifestations. Classic compensatory mechanisms have been attributed to changes and adjustments in the nigro-striatal system, such as increased neuronal activity in the substantia nigra pars compacta and enhanced dopamine synthesis and release in the striatum. However, it is not so clear currently that such changes occur early enough to account for the pre-symptomatic period. Other possible mechanisms relate to changes in basal ganglia and motor cortical circuits including the cerebellum. However, data from early PD patients are difficult to obtain as most studies have been carried out once the diagnosis and treatments have been established. Likewise, putative compensatory mechanisms taking place throughout disease evolution are nearly impossible to distinguish by themselves. Here, we review the evidence for the role of the best known and other possible compensatory mechanisms in PD. We also discuss the possibility that, although beneficial in practical terms, compensation could also play a deleterious role in disease progression.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.