Distinct Connectivity and Functionality of Aldehyde Dehydrogenase 1a1-Positive Nigrostriatal Dopaminergic Neurons in Motor Learning

Patch and matrix striatonigral neurons differentially regulate locomotion

Striatonigral neurons, known to promote locomotion, reside in both the patch and matrix compartments of the dorsal striatum. However, their compartment-specific contributions to locomotion remain largely unexplored. Using molecular identifier Kremen1 and Calb1 , we showed in mouse models that patch and matrix striatonigral neurons exert opposite influences on locomotion. Matrix striatonigral neurons reduced their activity before the cessation of self-paced locomotion, while patch striatonigral neuronal activity increased, suggesting an inhibitory function. Indeed, optogenetic activation of patch striatonigral neurons suppressed ongoing locomotion with reduced striatal dopamine release, contrasting with the locomotion-promoting effect of matrix striatonigral neurons, which showed an initial increase in dopamine release. Furthermore, genetic deletion of the GABA-B receptor in Aldehyde dehydrogenase 1A1-positive (ALDH1A1 + ) nigrostriatal dopaminergic neurons completely abolished the locomotion-suppressing effect of patch striatonigral neurons. Our findings unravel a compartment-specific mechanism governing locomotion in the dorsal striatum, where patch striatonigral neurons suppress locomotion by inhibiting ALDH1A1 + nigrostriatal dopaminergic neurons.

Molecular and spatial transcriptomic classification of midbrain dopamine neurons and their alterations in a LRRK2G2019S model of Parkinson's disease

Several studies have revealed that midbrain dopamine (DA) neurons, even within a single neuroanatomical area, display heterogeneous properties. In parallel, studies using single cell profiling techniques have begun to cluster DA neurons into subtypes based on their molecular signatures. Recent work has shown that molecularly defined DA subtypes within the substantia nigra (SNc) display distinctive anatomic and functional properties, and differential vulnerability in Parkinson's disease (PD). Based on these provocative results, a granular understanding of these putative subtypes and their alterations in PD models, is imperative. We developed an optimized pipeline for single-nuclear RNA sequencing (snRNA-seq) and generated a high-resolution hierarchically organized map revealing 20 molecularly distinct DA neuron subtypes belonging to three main families. We integrated this data with spatial MERFISH technology to map, with high definition, the location of these subtypes in the mouse midbrain, revealing heterogeneity even within neuroanatomical sub-structures. Finally, we demonstrate that in the preclinical LRRK2G2019S knock-in mouse model of PD, subtype organization and proportions are preserved. Transcriptional alterations occur in many subtypes including those localized to the ventral tier SNc, where differential expression is observed in synaptic pathways, which might account for previously described DA release deficits in this model. Our work provides an advancement of current taxonomic schemes of the mouse midbrain DA neuron subtypes, a high-resolution view of their spatial locations, and their alterations in a prodromal mouse model of PD.

Autism patient-derived SHANK2BY29X mutation affects the development of ALDH1A1 negative dopamine neuron

Autism spectrum disorder (ASD) encompasses a range of neurodevelopmental conditions. Different mutations on a single ASD gene contribute to heterogeneity of disease phenotypes, possibly due to functional diversity of generated isoforms. SHANK2, a causative gene in ASD, demonstrates this phenomenon, but there is a scarcity of tools for studying endogenous SHANK2 proteins in an isoform-specific manner. Here, we report a point mutation on SHANK2, which is found in a patient with autism, located on exon of the SHANK2B transcript variant (NM_133266.5), hereby SHANK2BY29X. This mutation results in an early stop codon and an aberrant splicing event that impacts SHANK2 transcript variants distinctly. Induced pluripotent stem cells (iPSCs) carrying this mutation, from the patient or isogenic editing, fail to differentiate into functional dopamine (DA) neurons, which can be rescued by genetic correction. Available SMART-Seq single-cell data from human midbrain reveals the abundance of SHANK2B transcript in the ALDH1A1 negative DA neurons. We then show that SHANK2BY29X mutation primarily affects SHANK2B expression and ALDH1A1 negative DA neurons in vitro during early neuronal developmental stage. Mice knocked in with the identical mutation exhibit autistic-like behavior, decreased occupancy of ALDH1A1 negative DA neurons and decreased dopamine release in ventral tegmental area (VTA). Our study provides novel insights on a SHANK2 mutation derived from autism patient and highlights SHANK2B significance in ALDH1A1 negative DA neuron.

Distributional coding of associative learning in discrete populations of midbrain dopamine neurons

Midbrain dopamine neurons are thought to play key roles in learning by conveying the difference between expected and actual outcomes. Recent evidence suggests diversity in dopamine signaling, yet it remains poorly understood how heterogeneous signals might be organized to facilitate the role of downstream circuits mediating distinct aspects of behavior. Here, we investigated the organizational logic of dopaminergic signaling by recording and labeling individual midbrain dopamine neurons during associative behavior. Our findings show that reward information and behavioral parameters are not only heterogeneously encoded but also differentially distributed across populations of dopamine neurons. Retrograde tracing and fiber photometry suggest that populations of dopamine neurons projecting to different striatal regions convey distinct signals. These data, supported by computational modeling, indicate that such distributional coding can maximize dynamic range and tailor dopamine signals to facilitate specialized roles of different striatal regions.

Dopamine across timescales and cell types: Relevance for phenotypes in Parkinson's disease progression

Dopamine neurons in the substantia nigra pars compacta (SNc) synthesize and release dopamine, a critical neurotransmitter for movement and learning. SNc dopamine neurons degenerate in Parkinson's Disease (PD), causing a host of motor and non-motor symptoms. Here, we review recent conceptual advances in our basic understanding of the dopamine system - including our rapidly advancing knowledge of dopamine neuron heterogeneity - with special attention to their importance for understanding PD. In PD patients, dopamine neuron degeneration progresses from lateral SNc to medial SNc, suggesting clinically relevant heterogeneity in dopamine neurons. With technical advances in dopamine system interrogation, we can understand the relevance of this heterogeneity for PD progression and harness it to develop new treatments.

Comparing tonic and phasic dendritic calcium in cholinergic pedunculopontine neurons and dopaminergic substantia nigra neurons

Several brainstem nuclei degenerate in Parkinson's disease (PD). In addition to the well-characterized dopaminergic neurons of the substantia nigra pars compacta (SNc), the cholinergic neurons of the pedunculopontine nucleus (PPN) also degenerate in PD. One leading hypothesis of selective vulnerability is that pacemaking activity and the activation of low-threshold L-type calcium current are major contributors to tonic calcium load and cellular stress in SNc dopaminergic neurons. However, it is not yet clear whether the vulnerable PPN cholinergic neurons share this property. Therefore, we used two-photon dendritic calcium imaging and whole-cell electrophysiology to evaluate the role of L-type calcium channels in tonic and phasic dendritic calcium signals in PPN and SNc neurons. In addition, we investigated N- and P/Q-type calcium channel regulation of firing properties and dendritic calcium in PPN neurons. We found that blocking L-type channels reduces tonic firing rate and dendritic calcium levels in SNc neurons. By contrast, the tonic calcium load in PPN neurons did not depend on L-, N- or P/Q-type channels. However, we found that blocking either L-type (with nifedipine) or N- and P/Q-type (with omega-conotoxin MVIIC) channels reduces phasic calcium influx in PPN dendrites. Together, these findings show that L-type calcium channels play different roles in the activity of SNc and PPN neurons, and suggest that low-threshold L-type channels are not responsible for tonic calcium levels in PPN cholinergic neurons and are therefore not likely to be a source of selective vulnerability in these cells.

GABA co-released from striatal dopamine axons dampens phasic dopamine release through autoregulatory GABAA receptors

Striatal dopamine axons co-release dopamine and gamma-aminobutyric acid (GABA), using GABA provided by uptake via GABA transporter-1 (GAT1). Functions of GABA co-release are poorly understood. We asked whether co-released GABA autoinhibits dopamine release via axonal GABA type A receptors (GABAARs), complementing established inhibition by dopamine acting at axonal D2 autoreceptors. We show that dopamine axons express α3-GABAAR subunits in mouse striatum. Enhanced dopamine release evoked by single-pulse optical stimulation in striatal slices with GABAAR antagonism confirms that an endogenous GABA tone limits dopamine release. Strikingly, an additional inhibitory component is seen when multiple pulses are used to mimic phasic axonal activity, revealing the role of GABAAR-mediated autoinhibition of dopamine release. This autoregulation is lost in conditional GAT1-knockout mice lacking GABA co-release. Given the faster kinetics of ionotropic GABAARs than G-protein-coupled D2 autoreceptors, our data reveal a mechanism whereby co-released GABA acts as a first responder to dampen phasic-to-tonic dopamine signaling.

Heterogeneity of mesencephalic dopaminergic neurons: From molecular classifications, electrophysiological properties to functional connectivity

The mesencephalic dopamine (DA) system is composed of neuronal subtypes that are molecularly and functionally distinct, are responsible for specific behaviors, and are closely associated with numerous brain disorders. Existing research has made significant advances in identifying the heterogeneity of mesencephalic DA neurons, which is necessary for understanding their diverse physiological functions and disease susceptibility. Moreover, there is a conflict regarding the electrophysiological properties of the distinct subsets of midbrain DA neurons. This review aimed to elucidate recent developments in the heterogeneity of midbrain DA neurons, including subpopulation categorization, electrophysiological characteristics, and functional connectivity to provide new strategies for accurately identifying distinct subtypes of midbrain DA neurons and investigating the underlying mechanisms of these neurons in various diseases.

Distribution, connection and function of ALDH1A1+/TH+ neurons in substantia nigra pars reticulata of mouse

The massive cell death of dopaminergic neurons (DANs) in substantia nigra pars compacta (SNC) is associated with motor diseases, such as Parkinson's disease. Moreover, as a subtype of DANs in SNC, ALDH1A1+ neurons show better resistance to PD related neurotoxin. DANs can also be found in the substantia nigra pars reticulata (SNR), however, whether they are ALDH1A1+ neurons are rarely reported, as well as their projection, function, and reaction in the PD pathology. We studied the distribution of ALDH1A1+ neurons and track their projection by injecting pAAV. We figured out that, in SNR, 87 % neurons are ALDH1A1+/TH+ in ALDH1A1+ cluster averagely, while ALDH1A1+/TH+: TH+ is 52 % averagely. There are two enrichment regions of ALDH1A1+/TH+ neurons at brgma -3.40 mm and brgma -3.70 mm in the SNR of the nTg mice. Nevertheless, in one type of PD-liked mice model, the proportion of ALDH1A1+/TH+: ALDH1A1+ neurons are 98 % averagely, while ALHD1A1+/TH+: TH+ is 57 %. Intriguingly, neuro-tracing discovered that there may be a previously unreported connection between SNR and anterior dorsal thalamus (ADT). The mouse received MPTP stereotactic injection to destroy TH+ neurons in SNR showed depression behavior, indicated the DANs death in SNR may contribute to depression behavior.

Cell type specificity for circuit output in the midbrain dopaminergic system

Midbrain dopaminergic neurons are a relatively small group of neurons in the mammalian brain controlling a wide range of behaviors. In recent years, increasingly sophisticated tracing, imaging, transcriptomic, and machine learning approaches have provided substantial insights into the anatomical, molecular, and functional heterogeneity of dopaminergic neurons. Despite this wealth of new knowledge, it remains unclear whether and how the diverse features defining dopaminergic subclasses converge to delineate functional ensembles within the dopaminergic system. Here, we review recent studies investigating various aspects of dopaminergic heterogeneity and discuss how development, behavior, and disease influence subtype characteristics. We then outline what further approaches could be pursued to gain a more inclusive picture of dopaminergic diversity, which could be crucial to understanding the functional architecture of this system.

Unique functional responses differentially map onto genetic subtypes of dopamine neurons

Dopamine neurons are characterized by their response to unexpected rewards, but they also fire during movement and aversive stimuli. Dopamine neuron diversity has been observed based on molecular expression profiles; however, whether different functions map onto such genetic subtypes remains unclear. In this study, we established that three genetic dopamine neuron subtypes within the substantia nigra pars compacta, characterized by the expression of Slc17a6 (Vglut2), Calb1 and Anxa1, each have a unique set of responses to rewards, aversive stimuli and accelerations and decelerations, and these signaling patterns are highly correlated between somas and axons within subtypes. Remarkably, reward responses were almost entirely absent in the Anxa1+ subtype, which instead displayed acceleration-correlated signaling. Our findings establish a connection between functional and genetic dopamine neuron subtypes and demonstrate that molecular expression patterns can serve as a common framework to dissect dopaminergic functions.

Impact of dietary vitamin A on striatal function in adult rats

The striatum is a brain structure involved in the control of voluntary movement. Striatum contains high amounts of retinoic acid, the active metabolite of vitamin A, as well as retinoid receptors, RARβ and RXRγ. Previous studies revealed that disruption of retinoid signaling initiated during development is deleterious for striatal physiology and related motor functions. However, the alteration of retinoid signaling, and the importance of vitamin A supply during adulthood on striatal physiology and function has never been established. In the present study, we investigated the impact of vitamin A supply on striatal function. Adult Sprague-Dawley rats were fed with three specific diets, either sub-deficient, sufficient, or enriched in vitamin A (0.4, 5, and 20 international units [IU] of retinol per g of diet, respectively) for 6 months. We first validated that vitamin A sub-deficient diet in adult rats constitutes a physiological model of retinoid signaling reduction in the striatum. We then revealed subtle alterations of fine motor skills in sub-deficient rats using a new behavioral apparatus specifically designed to test forepaw reach-and-grasp skills relying on striatal function. Finally, we showed using qPCR analysis and immunofluorescence that the striatal dopaminergic system per se was not affected by vitamin A sub-deficiency at adult age. Rather, cholinergic synthesis in the striatum and μ-opioid receptor expression in striosomes sub-territories were the most affected by vitamin A sub-deficiency starting at adulthood. Taken together these results revealed that retinoid signaling alteration at adulthood is associated with motor learning deficits together with discrete neurobiological alterations in the striatum.

Effects of Voluntary Wheel Running Exercise on Chemotherapy-Impaired Cognitive and Motor Performance in Mice

Chemotherapy-induced cognitive impairment (chemobrain) and muscle wasting (cachexia) are persisting side effects which adversely affect the quality of life of cancer survivors. We therefore investigated the efficacy of physical exercise as a non-pharmacological intervention to reverse the adverse effects of chemotherapy. We examined whether physical exercise in terms of voluntary wheel running could prevent chemotherapy-induced cognitive and motor impairments in mice treated with the multi-kinase inhibitor sorafenib. Adult male BALB/c mice were subdivided into runner and non-runner groups and orally administered with sorafenib (60 mg/kg) or vehicle continuously for four weeks. Mice could freely access the running wheel anytime during sorafenib or vehicle treatment. We found that sorafenib treatment reduced body weight gain (% of change, vehicle: 3.28 ± 3.29, sorafenib: -9.24 ± 1.52, p = 0.0004), impaired hippocampal-dependent spatial memory in the Y maze (exploration index, vehicle: 35.57 ± 11.38%, sorafenib: -29.62 ± 7.90%, p < 0.0001), increased anhedonia-like behaviour in the sucrose preference test (sucrose preference, vehicle: 66.57 ± 3.52%, sorafenib: 44.54 ± 4.25%, p = 0.0005) and impaired motor skill acquisition in rotarod test (latency to fall on day 1: 37.87 ± 8.05 and day 2: 37.22 ± 12.26 s, p > 0.05) but did not induce muscle wasting or reduce grip strength. Concomitant voluntary running reduced anhedonia-like behaviour (sucrose preference, sedentary: 44.54 ± 4.25%, runners: 59.33 ± 4.02%, p = 0.0357), restored impairment in motor skill acquisition (latency to fall on day 1: 50.85 ± 15.45 and day 2: 168.50 ± 37.08 s, p = 0.0004), but failed to rescue spatial memory deficit. Immunostaining results revealed that sorafenib treatment did not affect the number of proliferating cells and immature neurons in the hippocampal dentate gyrus (DG), whereas running significantly increased cell proliferation in both vehicle- (total Ki-67⁺ cells, sedentary: 16,687.34 ± 72.63, exercise: 3320.03 ± 182.57, p < 0.0001) and sorafenib-treated mice (Ki-67⁺ cells in the ventral DG, sedentary: 688.82.34 ± 38.16, exercise: 979.53 ± 73.88, p < 0.0400). Our results suggest that spatial memory impairment and anhedonia-like behaviour precede the presence of muscle wasting, and these behavioural deficits are independent of the changes in adult hippocampal neurogenesis. Running effectively prevents body weight loss, improves motor skill acquisition and reduces anhedonia-like behaviour associated with increased proliferating cells and immature neurons in DG. Taken together, they support physical exercise rehabilitation as an effective strategy to prevent chemotherapy side effects in terms of mood dysregulation and motor deficit.

The dopamine neuron synaptic map in the striatum

Dopamine neurons project to the striatum to control movement, cognition, and motivation via slower volume transmission as well as faster dopamine, glutamate, and GABA synaptic actions capable of conveying the temporal information in dopamine neuron firing. To define the scope of these synaptic actions, recordings of dopamine-neuron-evoked synaptic currents were made in four major striatal neuron types, spanning the entire striatum. This revealed that inhibitory postsynaptic currents are widespread, while excitatory postsynaptic currents are localized to the medial nucleus accumbens and the anterolateral-dorsal striatum, and that all synaptic actions are weak in the posterior striatum. Synaptic actions in cholinergic interneurons are the strongest, variably mediating inhibition throughout the striatum and excitation in the medial accumbens, capable of controlling their activity. This mapping shows that dopamine neuron synaptic actions extend throughout the striatum, preferentially target cholinergic interneurons, and define distinct striatal subregions.

Development, wiring and function of dopamine neuron subtypes

The midbrain dopamine (mDA) system is composed of molecularly and functionally distinct neuron subtypes that mediate specific behaviours and are linked to various brain diseases. Considerable progress has been made in identifying mDA neuron subtypes, and recent work has begun to unveil how these neuronal subtypes develop and organize into functional brain structures. This progress is important for further understanding the disparate physiological functions of mDA neurons and their selective vulnerability in disease, and will ultimately accelerate therapy development. This Review discusses recent advances in our understanding of molecularly defined mDA neuron subtypes and their circuits, ranging from early developmental events, such as neuron migration and axon guidance, to their wiring and function, and future implications for therapeutic strategies.

Dendritic involvement in inhibition and disinhibition of vulnerable dopaminergic neurons in healthy and pathological conditions

Dopaminergic neurons in the substantia nigra pars compacta (SNc) differentially degenerate in Parkinson's Disease, with the ventral region degenerating more severely than the dorsal region. Compared with the dorsal neurons, the ventral neurons in the SNc have distinct dendritic morphology, electrophysiological characteristics, and circuit connections with the basal ganglia. These characteristics shape information processing in the ventral SNc and structure the balance of inhibition and disinhibition in the striatonigral circuitry. In this paper, I review foundational studies and recent work comparing the circuitry of the ventral and dorsal SNc neurons and discuss how loss of the ventral neurons early in Parkinson's Disease could affect the overall balance of inhibition and disinhibition of dopamine signals.

Pitx3 deficiency promotes age-dependent alterations in striatal medium spiny neurons

Background

The classical motor symptoms of Parkinson's disease (PD) are tightly linked to the gradual loss of dopamine within the striatum. Concomitantly, medium spiny neurons (MSNs) also experience morphological changes, such as reduced dendritic complexity and spine density, which may be potentially associated with motor dysfunction as well. Thus, MSNs may serve as the emerging targets for PD therapy besides the midbrain dopaminergic neurons.

Results

To comprehensively examine pathological alterations of MSNs longitudinally, we established a TH Cre/ Pitx3 fl/fl (Pitx3cKO ) mouse model that developed canonical PD features, including a significant loss of SNc DAergic neurons and motor deficits. During aging, the targeted neurotransmitter, MSNs morphology and DNA methylation profile were significantly altered upon Pitx3 deficiency. Specifically, dopamine, GABA and glutamate decreased in the model at the early stage. While nuclear, soma and dendritic atrophy, as well as nuclear invaginations increased in the aged MSNs of Pitx3cko mice. Furthermore, more nuclear DNA damages were characterized in MSNs during aging, and Pitx3 deficiency aggravated this phenomenon, together with alterations of DNA methylation profiling associated with lipoprotein and nucleus pathway at the late stage.

Conclusion

The early perturbations of the neurotransmitters within MSNs may potentially contribute to the alterations of metabolism, morphology and epigenetics within the striatum at the late stage, which may provide new perspectives on the diagnosis and pathogenesis of PD.

Estrogen-related receptor gamma regulates mitochondrial and synaptic genes and modulates vulnerability to synucleinopathy

Many studies implicate mitochondrial dysfunction as a key contributor to cell loss in Parkinson disease (PD). Previous analyses of dopaminergic (DAergic) neurons from patients with Lewy-body pathology revealed a deficiency in nuclear-encoded genes for mitochondrial respiration, many of which are targets for the transcription factor estrogen-related receptor gamma (Esrrg/ERRγ). We demonstrate that deletion of ERRγ from DAergic neurons in adult mice was sufficient to cause a levodopa-responsive PD-like phenotype with reductions in mitochondrial gene expression and number, that partial deficiency of ERRγ hastens synuclein-mediated toxicity, and that ERRγ overexpression reduces inclusion load and delays synuclein-mediated cell loss. While ERRγ deletion did not fully recapitulate the transcriptional alterations observed in postmortem tissue, it caused reductions in genes involved in synaptic and mitochondrial function and autophagy. Altogether, these experiments suggest that ERRγ-deficient mice could provide a model for understanding the regulation of transcription in DAergic neurons and that amplifying ERRγ-mediated transcriptional programs should be considered as a strategy to promote DAergic maintenance in PD.

Deficiency in endocannabinoid synthase DAGLB contributes to early onset Parkinsonism and murine nigral dopaminergic neuron dysfunction

Endocannabinoid (eCB), 2-arachidonoyl-glycerol (2-AG), the most abundant eCB in the brain, regulates diverse neural functions. Here we linked multiple homozygous loss-of-function mutations in 2-AG synthase diacylglycerol lipase β (DAGLB) to an early onset autosomal recessive Parkinsonism. DAGLB is the main 2-AG synthase in human and mouse substantia nigra (SN) dopaminergic neurons (DANs). In mice, the SN 2-AG levels were markedly correlated with motor performance during locomotor skill acquisition. Genetic knockdown of Daglb in nigral DANs substantially reduced SN 2-AG levels and impaired locomotor skill learning, particularly the across-session learning. Conversely, pharmacological inhibition of 2-AG degradation increased nigral 2-AG levels, DAN activity and dopamine release and rescued the locomotor skill learning deficits. Together, we demonstrate that DAGLB-deficiency contributes to the pathogenesis of Parkinsonism, reveal the importance of DAGLB-mediated 2-AG biosynthesis in nigral DANs in regulating neuronal activity and dopamine release, and suggest potential benefits of 2-AG augmentation in alleviating Parkinsonism.

Dopamine signaling in the dorsomedial striatum promotes compulsive behavior

Compulsive behavior is a defining feature of disorders such as substance use disorders. Current evidence suggests that corticostriatal circuits control the expression of established compulsions, but little is known about the mechanisms regulating the development of compulsions. We hypothesized that dopamine, a critical modulator of striatal synaptic plasticity, could control alterations in corticostriatal circuits leading to the development of compulsions (defined here as continued reward seeking in the face of punishment). We used dual-site fiber photometry to measure dopamine axon activity in the dorsomedial striatum (DMS) and the dorsolateral striatum (DLS) as compulsions emerged. Individual variability in the speed with which compulsions emerged was predicted by DMS dopamine axon activity. Amplifying this dopamine signal accelerated animals' transitions to compulsion, whereas inhibition delayed it. In contrast, amplifying DLS dopamine signaling had no effect on the emergence of compulsions. These results establish DMS dopamine signaling as a key controller of the development of compulsive reward seeking.

Preventive Vitamin A Supplementation Improves Striatal Function in 6-Hydroxydopamine Hemiparkinsonian Rats

Background

The mechanisms leading to a loss of dopaminergic (DA) neurons from the substantia nigra pars compacta (SNc) in Parkinson's disease (PD) have multifactorial origins. In this context, nutrition is currently investigated as a modifiable environmental factor for the prevention of PD. In particular, initial studies revealed the deleterious consequences of vitamin A signaling failure on dopamine-related motor behaviors. However, the potential of vitamin A supplementation itself to prevent neurodegeneration has not been established yet.

Objective

The hypothesis tested in this study is that preventive vitamin A supplementation can protect DA neurons in a rat model of PD.

Methods

The impact of a 5-week preventive supplementation with vitamin A (20 IU/g of diet) was measured on motor and neurobiological alterations induced by 6-hydroxydopamine (6-OHDA) unilateral injections in the striatum of rats. Rotarod, step test and cylinder tests were performed up to 3 weeks after the lesion. Post-mortem analyses (retinol and monoamines dosages, western blots, immunofluorescence) were performed to investigate neurobiological processes.

Results

Vitamin A supplementation improved voluntary movements in the cylinder test. In 6-OHDA lesioned rats, a marked decrease of dopamine levels in striatum homogenates was measured. Tyrosine hydroxylase labeling in the SNc and in the striatum was significantly decreased by 6-OHDA injection, without effect of vitamin A. By contrast, vitamin A supplementation increased striatal expression of D2 and RXR receptors in the striatum of 6-OHDA lesioned rats.

Conclusions

Vitamin A supplementation partially alleviates motor alterations and improved striatal function, revealing a possible beneficial preventive approach for PD.

Sox6 expression distinguishes dorsally and ventrally biased dopamine neurons in the substantia nigra with distinctive properties and embryonic origins

Dopamine (DA) neurons in the ventral tier of the substantia nigra pars compacta (SNc) degenerate prominently in Parkinson's disease, while those in the dorsal tier are relatively spared. Defining the molecular, functional, and developmental characteristics of each SNc tier is crucial to understand their distinct susceptibility. We demonstrate that Sox6 expression distinguishes ventrally and dorsally biased DA neuron populations in the SNc. The Sox6+ population in the ventral SNc includes an Aldh1a1+ subset and is enriched in gene pathways that underpin vulnerability. Sox6+ neurons project to the dorsal striatum and show activity correlated with acceleration. Sox6- neurons project to the medial, ventral, and caudal striatum and respond to rewards. Moreover, we show that this adult division is encoded early in development. Overall, our work demonstrates a dual origin of the SNc that results in DA neuron cohorts with distinct molecular profiles, projections, and functions.

Synaptic dysfunction of Aldh1a1 neurons in the ventral tegmental area causes impulsive behaviors

Background

Aldh1a1 neurons are a subtype of gamma-aminobutyric acid (GABA) inhibitory neurons that use Aldh1a1 rather than glutamate decarboxylase (GAD) as an enzyme for synthesizing GABA transmitters. However, the behaviors and circuits of this newly identified subtype of inhibitory interneurons remain unknown.

Methods

We generated a mutant mouse line in which cyclization recombination enzyme (CRE) was expressed under the control of the Aldh1a1 promotor (Aldh1a1-CRE mice). Using this mutant strain of mice together with the heterozygous male Alzheimer’s disease (AD) related model mice (APPswe/PSEN1dE9, or AD mice) and a genetically modified retrograde and anterograde synaptic tracing strategy, we have studied a specific synaptic circuit of Aldh1a1 neurons with system-level function and disease progression in AD mice.

Results

We demonstrate that Aldh1a1 neurons encode delay of gratification that measures self-control skills in decision making by projecting inhibitory synapses directly onto excitatory glutamate neurons in the intermediate lateral septum (EGNIS) and receiving synaptic inputs from layer 5b pyramidal neurons in the medial prefrontal cortex (L5PN). L5PN → Aldh1a1 synaptic transmission undergoes long-term potentiation (LTP). Pathway specific inhibition by either genetic silencing presynaptic terminals or antagonizing postsynaptic receptors impairs delay of gratification, resulting in the impulsive behaviors. Further studies show that reconstitution of Aldh1a1-deficient neurons with the expression of exogenous Aldh1a1 (eAldh1a1) restores Aldh1a1 → EGNIS synaptic transmission and rescues the impulsive behaviors in AD mice.

Conclusions

These results not only identify a specific function and circuit of Aldh1a1 neurons but also provide a cellular point of entry to an important but understudied synaptic mechanism for the induction of impulsive behaviors at an early stage of AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00494-9.

Diverse midbrain dopaminergic neuron subtypes and implications for complex clinical symptoms of Parkinson's disease

Parkinson’s disease (PD), the most common degenerative movement disorder, is clinically manifested with various motor and non-motor symptoms. Degeneration of midbrain substantia nigra pas compacta (SNc) dopaminergic neurons (DANs) is generally attributed to the motor syndrome. The underlying neuronal mechanisms of non-motor syndrome are largely unexplored. Besides SNc, midbrain ventral tegmental area (VTA) DANs also produce and release dopamine and modulate movement, reward, motivation, and memory. Degeneration of VTA DANs also occurs in postmortem brains of PD patients, implying an involvement of VTA DANs in PD-associated non-motor symptoms. However, it remains to be established that there is a distinct segregation of different SNc and VTA DAN subtypes in regulating different motor and non-motor functions, and that different DAN subpopulations are differentially affected by normal ageing or PD. Traditionally, the distinction among different DAN subtypes was mainly based on the location of cell bodies and axon terminals. With the recent advance of single cell RNA sequencing technology, DANs can be readily classified based on unique gene expression profiles. A combination of specific anatomic and molecular markers shows great promise to facilitate the identification of DAN subpopulations corresponding to different behavior modules under normal and disease conditions. In this review, we first summarize the recent progress in characterizing genetically, anatomically, and functionally diverse midbrain DAN subtypes. Then, we provide perspectives on how the preclinical research on the connectivity and functionality of DAN subpopulations improves our current understanding of cell-type and circuit specific mechanisms of the disease, which could be critically informative for designing new mechanistic treatments.

The transcription factor BCL11A defines distinct subsets of midbrain dopaminergic neurons

Midbrain dopaminergic (mDA) neurons are diverse in their projection targets, effect on behavior, and susceptibility to neurodegeneration. Little is known about the molecular mechanisms establishing this diversity during development. We show that the transcription factor BCL11A is expressed in a subset of mDA neurons in the developing and adult murine brain and in a subpopulation of pluripotent-stem-cell-derived human mDA neurons. By combining intersectional labeling and viral-mediated tracing, we demonstrate that Bcl11a-expressing mDA neurons form a highly specific subcircuit within the murine dopaminergic system. In the substantia nigra, the Bcl11a-expressing mDA subset is particularly vulnerable to neurodegeneration upon α-synuclein overexpression or oxidative stress. Inactivation of Bcl11a in murine mDA neurons increases this susceptibility further, alters the distribution of mDA neurons, and results in deficits in skilled motor behavior. In summary, BCL11A defines mDA subpopulations with highly distinctive characteristics and is required for establishing and maintaining their normal physiology.

ASL expression in ALDH1A1+ neurons in the substantia nigra metabolically contributes to neurodegenerative phenotype

Argininosuccinate lyase (ASL) is essential for the NO-dependent regulation of tyrosine hydroxylase (TH) and thus for catecholamine production. Using a conditional mouse model with loss of ASL in catecholamine neurons, we demonstrate that ASL is expressed in dopaminergic neurons in the substantia nigra pars compacta, including the ALDH1A1⁺ subpopulation that is pivotal for the pathogenesis of Parkinson disease (PD). Neuronal loss of ASL results in catecholamine deficiency, in accumulation and formation of tyrosine aggregates, in elevation of α-synuclein, and phenotypically in motor and cognitive deficits. NO supplementation rescues the formation of aggregates as well as the motor deficiencies. Our data point to a potential metabolic link between accumulations of tyrosine and seeding of pathological aggregates in neurons as initiators for the pathological processes involved in neurodegeneration. Hence, interventions in tyrosine metabolism via regulation of NO levels may be therapeutic beneficial for the treatment of catecholamine-related neurodegenerative disorders.

Motor Sequence Learning Deficits in Idiopathic Parkinson's Disease Are Associated With Increased Substantia Nigra Activity

Previous studies have shown that persons with Parkinson’s disease (pwPD) share specific deficits in learning new sequential movements, but the neural substrates of this impairment remain unclear. In addition, the degree to which striatal dopaminergic denervation in PD affects the cortico-striato-thalamo-cerebellar motor learning network remains unknown. We aimed to answer these questions using fMRI in 16 pwPD and 16 healthy age-matched control subjects while they performed an implicit motor sequence learning task. While learning was absent in both pwPD and controls assessed with reaction time differences between sequential and random trials, larger error-rates during the latter suggest that at least some of the complex sequence was encoded. Moreover, we found that while healthy controls could improve general task performance indexed by decreased reaction times across both sequence and random blocks, pwPD could not, suggesting disease-specific deficits in learning of stimulus-response associations. Using fMRI, we found that this effect in pwPD was correlated with decreased activity in the hippocampus over time. Importantly, activity in the substantia nigra (SN) and adjacent bilateral midbrain was specifically increased during sequence learning in pwPD compared to healthy controls, and significantly correlated with sequence-specific learning deficits. As increased SN activity was also associated (on trend) with higher doses of dopaminergic medication as well as disease duration, the results suggest that learning deficits in PD are associated with disease progression, indexing an increased drive to recruit dopaminergic neurons in the SN, however, unsuccessfully. Finally, there were no differences between pwPD and controls in task modulation of the cortico-striato-thalamo-cerebellar network. However, a restricted nigral-striatal model showed that negative modulation of SN to putamen connection was larger in pwPD compared to controls during random trials, while no differences between the groups were found during sequence learning. We speculate that learning-specific SN recruitment leads to a relative increase in SN- > putamen connectivity, which returns to a pathological reduced state when no learning takes place.

Integrating the Roles of Midbrain Dopamine Circuits in Behavior and Neuropsychiatric Disease

Dopamine (DA) is a behaviorally and clinically diverse neuromodulator that controls CNS function. DA plays major roles in many behaviors including locomotion, learning, habit formation, perception, and memory processing. Reflecting this, DA dysregulation produces a wide variety of cognitive symptoms seen in neuropsychiatric diseases such as Parkinson’s, Schizophrenia, addiction, and Alzheimer’s disease. Here, we review recent advances in the DA systems neuroscience field and explore the advancing hypothesis that DA’s behavioral function is linked to disease deficits in a neural circuit-dependent manner. We survey different brain areas including the basal ganglia’s dorsomedial/dorsolateral striatum, the ventral striatum, the auditory striatum, and the hippocampus in rodent models. Each of these regions have different reported functions and, correspondingly, DA’s reflecting role in each of these regions also has support for being different. We then focus on DA dysregulation states in Parkinson’s disease, addiction, and Alzheimer’s Disease, emphasizing how these afflictions are linked to different DA pathways. We draw upon ideas such as selective vulnerability and region-dependent physiology. These bodies of work suggest that different channels of DA may be dysregulated in different sets of disease. While these are great advances, the fine and definitive segregation of such pathways in behavior and disease remains to be seen. Future studies will be required to define DA’s necessity and contribution to the functional plasticity of different striatal regions.

Function and Regulation of ALDH1A1-Positive Nigrostriatal Dopaminergic Neurons in Motor Control and Parkinson's Disease

Dopamine is an important chemical messenger in the brain, which modulates movement, reward, motivation, and memory. Different populations of neurons can produce and release dopamine in the brain and regulate different behaviors. Here we focus our discussion on a small but distinct group of dopamine-producing neurons, which display the most profound loss in the ventral substantia nigra pas compacta of patients with Parkinson's disease. This group of dopaminergic neurons can be readily identified by a selective expression of aldehyde dehydrogenase 1A1 (ALDH1A1) and accounts for 70% of total nigrostriatal dopaminergic neurons in both human and mouse brains. Recently, we presented the first whole-brain circuit map of these ALDH1A1-positive dopaminergic neurons and reveal an essential physiological function of these neurons in regulating the vigor of movement during the acquisition of motor skills. In this review, we first summarize previous findings of ALDH1A1-positive nigrostriatal dopaminergic neurons and their connectivity and functionality, and then provide perspectives on how the activity of ALDH1A1-positive nigrostriatal dopaminergic neurons is regulated through integrating diverse presynaptic inputs and its implications for potential Parkinson's disease treatment.

Dopamine, Updated: Reward Prediction Error and Beyond

Dopamine neurons have been intensely studied for their roles in reinforcement learning. A dominant theory of how these neurons contribute to learning is through the encoding of a reward prediction error (RPE) signal. Recent advances in dopamine research have added nuance to RPE theory by incorporating the ideas of sensory prediction error, distributional encoding, and belief states. Further nuance is likely to be added shortly by convergent lines of research on dopamine neuron diversity. Finally, a major challenge is to reconcile RPE theory with other current theories of dopamine function to account for dopamine's role in movement, motivation, and goal-directed planning.

Connectivity and Functionality of the Globus Pallidus Externa Under Normal Conditions and Parkinson's Disease

The globus pallidus externa (GPe) functions as a central hub in the basal ganglia for processing motor and non-motor information through the creation of complex connections with the other basal ganglia nuclei and brain regions. Recently, with the adoption of sophisticated genetic tools, substantial advances have been made in understanding the distinct molecular, anatomical, electrophysiological, and functional properties of GPe neurons and non-neuronal cells. Impairments in dopamine transmission in the basal ganglia contribute to Parkinson's disease (PD), the most common movement disorder that severely affects the patients' life quality. Altered GPe neuron activity and synaptic connections have also been found in both PD patients and pre-clinical models. In this review, we will summarize the main findings on the composition, connectivity and functionality of different GPe cell populations and the potential GPe-related mechanisms of PD symptoms to better understand the cell type and circuit-specific roles of GPe in both normal and PD conditions.

Role and Mechanism of Vitamin A Metabolism in the Pathophysiology of Parkinson's Disease

Evidence shows that altered retinoic acid signaling may contribute to the pathogenesis and pathophysiology of Parkinson's disease (PD). Retinoic acid is the bioactive derivative of the lipophilic vitamin A. Vitamin A is involved in several important homeostatic processes, such as cell differentiation, antioxidant activity, inflammation and neuronal plasticity. The role of vitamin A and its derivatives in the pathogenesis and pathophysiology of neurodegenerative diseases, and their potential as therapeutics, has drawn attention for more than 10 years. However, the literature sits in disparate fields. Vitamin A could act at the crossroad of multiple environmental and genetic factors of PD. The purpose of this review is to outline what is known about the role of vitamin A metabolism in the pathogenesis and pathophysiology of PD. We examine key biological systems and mechanisms that are under the control of vitamin A and its derivatives, which are (or could be) exploited for therapeutic potential in PD: the survival of dopaminergic neurons, oxidative stress, neuroinflammation, circadian rhythms, homeostasis of the enteric nervous system, and hormonal systems. We focus on the pivotal role of ALDH1A1, an enzyme expressed by dopaminergic neurons for the detoxification of these neurons, which is under the control of retinoic acid. By providing an integrated summary, this review will guide future studies on the potential role of vitamin A in the management of symptoms, health and wellbeing for PD patients.

Functional Dissection of Basal Ganglia Inhibitory Inputs onto Substantia Nigra Dopaminergic Neurons

Substantia nigra (SNc) dopaminergic neurons respond to aversive stimuli with inhibitory pauses in firing followed by transient rebound activation. We tested integration of inhibitory synaptic inputs onto SNc neurons from genetically defined populations in dorsal striatum (striosome and matrix) and external globus pallidus (GPe; parvalbumin- and Lhx6-positive), and examined their contribution to pause-rebound firing. Activation of striosome projections, which target "dendron bouquets" in the pars reticulata (SNr), consistently quiets firing and relief from striosome inhibition triggers rebound activity. Striosomal inhibitory postsynaptic currents (IPSCs) display a prominent GABA-B receptor-mediated component that strengthens the impact of SNr dendrite synapses on somatic excitability and enables rebounding. By contrast, GPe projections activate GABA-A receptors on the soma and proximal dendrites but do not result in rebounding. Lastly, optical mapping shows that dorsal striatum selectively inhibits the ventral population of SNc neurons, which are intrinsically capable of rebounding. Therefore, we define a distinct striatonigral circuit for generating dopamine rebound.

Toward Generating Subtype-Specific Mesencephalic Dopaminergic Neurons in vitro

Mesencephalic dopaminergic (mDA) neurons derived from pluripotent stem cells (PSCs) have proven to be pivotal for disease modeling studies and as a source of transplantable tissue for regenerative therapies in Parkinson's disease (PD). Current differentiation protocols can generate standardized and reproducible cell products of dopaminergic neurons that elicit the characteristic transcriptional profile of ventral midbrain. Nonetheless, dopamine neurons residing in the mesencephalon comprise distinct groups of cells within diffusely defined anatomical boundaries and with distinct functional, electrophysiological, and molecular properties. Here we review recent single cell sequencing studies that are shedding light on the neuronal heterogeneity within the mesencephalon and discuss how resolving the complex molecular profile of distinct sub-populations within this region could help refine patterning and quality control assessment of PSC-derived mDA neurons to subtype-specificity in vitro. In turn, such advances would have important impact in improving cell replacement therapy, disease mechanistic studies and drug screening in PD.

Nurr1Cd11bcre conditional knockout mice display inflammatory injury to nigrostriatal dopaminergic neurons

Nuclear receptor-related 1 protein (NURR1) is essential for the development and maintenance of midbrain dopaminergic (DAergic) neurons. NURR1 also protects DAergic neurons against neuroinflammation. However, it remains to be determined to what extent does NURR1 exerts its protective function through acting autonomously in the microglia. Using Cre/lox gene targeting system, we deleted Nurr1 in the microglia of Nurr1^Cd11bcre conditional knockout (cKO) mice. The Nurr1^Cd11bcre cKO mice displayed age-dependent motor abnormalities and increased microglial activation, but with no obvious DAergic neurodegeneration. To boost the inflammatory injury, we systemically administered endotoxin lipopolysaccharide (LPS) to Nurr1^Cd11bcre mice. As expected, LPS treatment exacerbated the motor phenotypes and inflammatory reactions in Nurr1^Cd11bcre cKO mice. More importantly, LPS administration caused DAergic neuron loss and α-synuclein aggregation, two pathological hallmarks of Parkinson's disease (PD). Therefore, our findings provide in vivo evidence supporting a critical protective role of NURR1 in the microglia against inflammation-induced degeneration of DAergic neurons in PD.

Classification of Midbrain Dopamine Neurons Using Single-Cell Gene Expression Profiling Approaches

Dysfunctional dopamine (DA) signaling has been associated with a broad spectrum of neuropsychiatric disorders, prompting investigations into how midbrain DA neuron heterogeneity may underpin this variety of behavioral symptoms. Emerging literature indeed points to functional heterogeneity even within anatomically defined DA clusters. Recognizing the need for a systematic classification scheme, several groups have used single-cell profiling to catalog DA neurons based on their gene expression profiles. We aim here not only to synthesize points of congruence but also to highlight key differences between the molecular classification schemes derived from these studies. In doing so, we hope to provide a common framework that will facilitate investigations into the functions of DA neuron subtypes in the healthy and diseased brain.

Parkinson's disease-related Leucine-rich repeat kinase 2 modulates nuclear morphology and genomic stability in striatal projection neurons during aging

BACKGROUND

Multiple missense mutations in Leucine-rich repeat kinase 2 (LRRK2) are associated with familial forms of late onset Parkinson's disease (PD), the most common age-related movement disorder. The dysfunction of dopamine transmission contributes to PD-related motor symptoms. Interestingly, LRRK2 is more abundant in the dopaminoceptive striatal spiny projection neurons (SPNs) compared to the dopamine-producing nigrostriatal dopaminergic neurons. Aging is the most important risk factor for PD and other neurodegenerative diseases. However, whether LRRK2 modulates the aging of SPNs remains to be determined.

METHODS

We conducted RNA-sequencing (RNA-seq) analyses of striatal tissues isolated from Lrrk2 knockout (Lrrk2-/-) and control (Lrrk2+/+) mice at 2 and 12 months of age. We examined SPN nuclear DNA damage and epigenetic modifications; SPN nuclear, cell body and dendritic morphology; and the locomotion and motor skill learning of Lrrk2+/+ and Lrrk2-/- mice from 2 to 24 months of age. Considering the strength of cell cultures for future mechanistic studies, we also performed preliminary studies in primary cultured SPNs derived from the Lrrk2+/+ and Lrrk2-/- mice as well as the PD-related Lrrk2 G2019S and R1441C mutant mice.

RESULTS

Lrrk2-deficiency accelerated nuclear hypertrophy and induced dendritic atrophy, soma hypertrophy and nuclear invagination in SPNs during aging. Additionally, increased nuclear DNA damage and abnormal histone methylations were also observed in aged Lrrk2-/- striatal neurons, together with alterations of molecular pathways involved in regulating neuronal excitability, genome stability and protein homeostasis. Furthermore, both the PD-related Lrrk2 G2019S mutant and LRRK2 kinase inhibitors caused nuclear hypertrophy, while the Lrrk2 R1441C mutant and γ-Aminobutyric acid type A receptor (GABA-AR) inhibitors promoted nuclear invagination in the cultured SPNs. On the other hand, inhibition of neuron excitability prevented the formation of nuclear invagination in the cultured Lrrk2-/- and R1441C SPNs.

CONCLUSIONS

Our findings support an important physiological function of LRRK2 in maintaining nuclear structure integrity and genomic stability during the normal aging process, suggesting that PD-related LRRK2 mutations may cause the deterioration of neuronal structures through accelerating the aging process.

A physical therapy decision-making tool for stratifying persons with Parkinson's disease into community exercise classes

Aim: Physical therapy and exercise are considered essential components in the management of Parkinson's disease (PD). Using our retrospective data and years of experience in assigning persons with PD to multilevel group classes we propose a two-part physical therapy decision-making tool consisting of participant and exercise program considerations. Methods: Retrospective medical record review and therapist consensus identified evaluation considerations determined to aide clinical decision making. The ability of these variables (i.e., demographics, clinical characteristics, clinical measures cut-offs) to predict the class assignment decision of PD-specialized physical therapists was evaluated using discriminant function analysis. Results: Therapist-assigned groups differed significantly on all clinical measures (p < 0.001) which provided the categorical data required for discriminant analysis. Using all variables, the discriminant function analysis predicted class assignment of the therapists with 79% agreement. Conclusion: This proposed tool provides a framework that may guide the process for increasing access to multilevel group classes.