Dysfunctional striatal dopamine signaling in Huntington's disease

Dopaminergic system and neurons: Role in multiple neurological diseases

The dopaminergic system is a complex and powerful neurotransmitter system in the brain. It plays an important regulatory role in motivation, reward, cognition, and motor control. In recent decades, research in the field of the dopaminergic system and neurons has increased exponentially and is gradually becoming a point of intervention in the study and understanding of a wide range of neurological diseases related to human health. Studies have shown that the dopaminergic system and neurons are involved in the development of many neurological diseases (including, but not limited to Parkinson's disease, schizophrenia, depression, attention deficit hyperactivity disorder, etc.) and that dopaminergic neurons either have too much stress or too weak function in the dopaminergic system can lead to disease. Therefore, targeting dopaminergic neurons is considered key to treating these diseases. This article provides a comprehensive review of the dopaminergic system and neurons in terms of brain region distribution, physiological function and subtypes of dopaminergic neurons, as well as the role of the dopaminergic system and neurons in a variety of diseases.

Advances in Brain Stimulation, Nanomedicine and the Use of Magnetoelectric Nanoparticles: Dopaminergic Alterations and Their Role in Neurodegeneration and Drug Addiction

Recent advancements in brain stimulation and nanomedicine have ushered in a new era of therapeutic interventions for psychiatric and neurodegenerative disorders. This review explores the cutting-edge innovations in brain stimulation techniques, including their applications in alleviating symptoms of main neurodegenerative disorders and addiction. Deep Brain Stimulation (DBS) is an FDA-approved treatment for specific neurodegenerative disorders, including Parkinson's Disease (PD), and is currently under evaluation for other conditions, such as Alzheimer's Disease. This technique has facilitated significant advancements in understanding brain electrical circuitry by enabling targeted brain stimulation and providing insights into neural network function and dysfunction. In reviewing DBS studies, this review places particular emphasis on the underlying main neurotransmitter modifications and their specific brain area location, particularly focusing on the dopaminergic system, which plays a critical role in these conditions. Furthermore, this review delves into the groundbreaking developments in nanomedicine, highlighting how nanotechnology can be utilized to target aberrant signaling in neurodegenerative diseases, with a specific focus on the dopaminergic system. The discussion extends to emerging technologies such as magnetoelectric nanoparticles (MENPs), which represent a novel intersection between nanoformulation and brain stimulation approaches. These innovative technologies offer promising avenues for enhancing the precision and effectiveness of treatments by enabling the non-invasive, targeted delivery of therapeutic agents as well as on-site, on-demand stimulation. By integrating insights from recent research and technological advances, this review aims to provide a comprehensive understanding of how brain stimulation and nanomedicine can be synergistically applied to address complex neuropsychiatric and neurodegenerative disorders, paving the way for future therapeutic strategies.

A simple and accurate ratiometric sensor for determination of dopamine based on dual-emission carbon dots

Dopamine (DA) is a neuromodulatory molecule that plays critical roles in many biological processes. The dysfunctions of the DA system are closely associated with several nervous system diseases. Therefore, it is urgent to establish a simple and accurate method for DA analysis. In this study, an economic and accurate DA ratiometric sensor was established using dual-emission carbon dots (DE-CDs). DE-CDs were first synthesized by the one-step solvothermal method and two separate fluorescence emission peaks at 340 and 500 nm were observed under the excitation of 310 nm. In the presence of Hg2+, the fluorescence signal at 340 nm was significantly quenched, while the signal at 500 nm keeps stable. Upon adding DA, the quenched signal at 340 nm was significantly recovered, whereas the signal at 500 nm remains stable. Therefore, a novel ratiometric sensor for DA analysis was established. This method shows a good linear range from 500 nM to 100 μM, and the detection limit was calculated to be 80 nM. Moreover, this established method shows excellent specificity and could be applied in real sample analysis, showing great potential for application in clinical research.

Exploring molecular mechanisms, therapeutic strategies, and clinical manifestations of Huntington's disease

Huntington's disease (HD) is a paradigm of a genetic neurodegenerative disorder characterized by the expansion of CAG repeats in the HTT gene. This extensive review investigates the molecular complexities of HD by highlighting the pathogenic mechanisms initiated by the mutant huntingtin protein. Adverse outcomes of HD include mitochondrial dysfunction, compromised protein clearance, and disruption of intracellular signaling, consequently contributing to the gradual deterioration of neurons. Numerous therapeutic strategies, particularly precision medicine, are currently used for HD management. Antisense oligonucleotides, such as Tominersen, play a leading role in targeting and modulating the expression of mutant huntingtin. Despite the promise of these therapies, challenges persist, particularly in improving delivery systems and the necessity for long-term safety assessments. Considering the future landscape, the review delineates promising directions for HD research and treatment. Innovations such as Clustered regularly interspaced short palindromic repeats associated system therapies (CRISPR)-based genome editing and emerging neuroprotective approaches present unprecedented opportunities for intervention. Collaborative interdisciplinary endeavors and a more insightful understanding of HD pathogenesis are on the verge of reshaping the therapeutic landscape. As we navigate the intricate landscape of HD, this review serves as a guide for unraveling the intricacies of this disease and progressing toward transformative treatments.

Regulatory mechanism of circular RNAs in neurodegenerative diseases

BACKGROUND

Neurodegenerative disease is a collective term for a category of diseases that are caused by neuronal dysfunction, such as Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS). Circular RNAs (circRNAs) are a class of non-coding RNAs without the 3' cap and 5' poly(A) and are linked by covalent bonds. CircRNAs are highly expressed in brain neurons and can regulate the pathological process of neurodegenerative diseases by affecting the levels of various deposition proteins.

AIMS

This review is aiming to suggest that the majority of circRNAs influence neurodegenerative pathologies mainly by affecting the abnormal deposition of proteins in neurodegenerative diseases.

METHODS

We systematically summarized the pathological features of neurodegenerative diseases and the regulatory mechanisms of circRNAs in various types of neurodegenerative diseases.

RESULTS

Neurodegenerative disease main features include intercellular ubiquitin-proteasome system abnormalities, changes in cytoskeletal proteins, and the continuous deposition of insoluble protein fragments and inclusion bodies in the cytoplasm or nucleus, resulting in impairment of the normal physiological processes of the neuronal system. CircRNAs have multiple mechanisms, such as acting as microRNA sponges, binding to proteins, and regulating transcription. CircRNAs, which are highly stable molecules, are expected to be potential biomarkers for the pathological detection of neurodegenerative diseases such as AD and PD.

CONCLUSIONS

In this review, we describe the regulatory roles and mechanisms of circRNAs in neurodegenerative diseases and aim to employ circRNAs as biomarkers for the diagnosis and treatment of neurodegenerative diseases.

Tyrosinase-Based Proximity Labeling in Living Cells and In Vivo

Characterizing the protein constituents of a specific organelle and protein neighbors of a protein of interest (POI) is essential for understanding the function and state of the organelle and protein networks associated with the POI. Proximity labeling (PL) has emerged as a promising technology for specific and efficient spatial proteomics. Nevertheless, most enzymes adopted for PL still have limitations: APEX requires cytotoxic H2O2 for activation and thus is poor in biocompatibility for in vivo application, BioID shows insufficient labeling kinetics, and TurboID suffers from high background biotinylation. Here, we introduce a bacterial tyrosinase (BmTyr) as a new PL enzyme suitable for H2O2-free, fast (≤10 min in living cells), and low-background protein tagging. BmTyr is genetically encodable and enables subcellular-resolved PL and proteomics in living cells. We further designed a strategy of ligand-tethered BmTyr for in vivo PL, which unveiled the surrounding proteome of a neurotransmitter receptor (Grm1 and Drd2) in its resident synapse in a live mouse brain. Overall, BmTyr is one promising enzyme that can improve and expand PL-based applications and discoveries.

Levothyroxine attenuates behavioral impairment and improves oxidative stress and histological alteration 3-nitropropionic acid induced experimental Huntington's disease in rats

Huntington's disease (HD) is a neurodegenerative disorder characterized by degeneration of the striatum; it results in oxidative stress and motor deficits. Thyroid hormones regulate oxidative metabolism. In the present study, we evaluated the effect of administration of levothyroxine (LT-4) on neurobehavioral, oxidative stress, and histological changes in a rat model of HD. Forty-eight Wistar male rats were divided into the following six groups (n = 8): Group 1 (control) received physiological saline intraperitoneally (ip). Groups 2 and 3 received L-T4,30 and L-T4100 (μg/kg, ip, respectively) daily for 7 days. Group 4 (HD) received 3-nitropropionic acid (3-NP) (25 mg/kg, ip) daily for 7 days. Groups 5 and 6 received L-T4,30 and L-T4100 (μg/kg, ip, respectively) 30 min after 3-NP (25 mg/kg, ip) injection for the same duration. On the 8th day, behavioral parameters were evaluated with the Rotarod, Narrow beam walk, and Limb withdrawal tests. Oxidative markers such as Malondialdehyde (MDA) and Glutathione (GSH) levels and Superoxide dismutase (SOD) activity, in striatum tissue were measured. Moreover, striatum tissues were analyzed by Hematoxylin-eosin staining for histological alterations. We found that 3-NP administration caused motor incoordination and induced oxidative stress increased but reduced free radical scavenging. Also, increased amounts of lipid peroxides caused striatal damage as shown by histopathological evaluation. Administration of L-T4 led to increased falling time in the Rotarod, but reduced the time taken in Narrow beam walking and Limb withdrawal test. Furthermore, L-T4 increased antioxidant activity, decreased lipid peroxidation and ameliorated 3-NP-induced degeneration in neurons.

The improvement of motor symptoms in Huntington's disease during cariprazine treatment

BACKGROUND

Huntington's disease (HD) is a progressive neurodegenerative disease, characterised by motor disturbances and non-motor (i.e., psychiatric) symptoms. Motor symptoms are the hallmark features of HD and take many forms. Their emergence is related to alterations in striatal dopaminergic neurotransmission: dopamine levels increase in the early stages of the disease, while more advanced stages are characterised by reduced dopamine levels. Such a biphasic change potentially explains the alterations in motor symptoms: increased dopamine-production induces hyperkinetic movements early in the disease course, while depleted dopamine storage leads to hypokinetic symptoms in the advanced phase. Dopamine D2-D3 partial agonists could be a promising treatment option in HD, as they have the potential to either elevate or lower the surrounding dopamine levels if the levels are too low or too high, respectively, potentially offering symptom-relief across the illness-course. Therefore, the present study aimed at exploring the effects of cariprazine, a dopamine D2-D3 partial agonist with high affinity to D3 receptors, on motor symptoms associated with HD.

METHODS

This was a single-centre, retrospective study where sixteen patients received off-label cariprazine treatment for 12 weeks (1.5-3 mg/day). Motor symptoms were evaluated using the Motor Assessment of the Unified Huntington's Disease Rating Scale. Least Square (LS) Mean Changes from Baseline (BL) to Week 8 and Week 12 in the Total Motor Score (TMS) were analysed using the Mixed Model for Repeated Measures method. In addition, improvement from BL to Week 8 and 12 was calculated for all motor items.

RESULTS

Data of 16 patients were collected, but data of only 15 patients were analysed as one patient dropped out due to non-compliance. Significant changes were observed from BL to Week 8 (LS Mean Change: -9.4, p < 0.0001) and to Week 12 (LS Mean Change: -12.8, p < 0.0001) in the TMS. The improvement was captured in the majority of motor functions, excluding bradykinesia and gait. Mild akathisia was the most commonly reported side-effect, affecting 3 patients.

CONCLUSION

This is the first study investigating the effectiveness of a D2-D3 partial agonist, cariprazine, in the treatment of HD. The findings of this study revealed that cariprazine was effective in the treatment of a wide range of motor symptoms associated with HD.

The role of glycogen synthase kinase 3 beta in neurodegenerative diseases

Neurodegenerative diseases (NDDs) pose an increasingly prevalent threat to the well-being and survival of elderly individuals worldwide. NDDs include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), and so on. They are characterized by progressive loss or dysfunction of neurons in the central or peripheral nervous system and share several cellular and molecular mechanisms, including protein aggregation, mitochondrial dysfunction, gene mutations, and chronic neuroinflammation. Glycogen synthase kinase-3 beta (GSK-3β) is a serine/threonine kinase that is believed to play a pivotal role in the pathogenesis of NDDs. Here we summarize the structure and physiological functions of GSK3β and explore its involvement in NDDs. We also discussed its potential as a therapeutic target.

Huntington disease exacerbates action impulses

Background

Impulsivity is a common clinical feature of Huntington disease (HD), but the underlying cognitive dynamics of impulse control in this population have not been well-studied.

Objective

To investigate the temporal dynamics of action impulse control in HD patients using an inhibitory action control task.

Methods

Sixteen motor manifest HD patients and seventeen age-matched healthy controls (HC) completed the action control task. We applied the activation-suppression theoretical model and distributional analytic techniques to differentiate the strength of fast impulses from their top-down suppression.

Results

Overall, HD patients produced slower and less accurate reactions than HCs. HD patients also exhibited an exacerbated interference effect, as evidenced by a greater slowing of RT on non-corresponding compared to corresponding trials. HD patients made more fast, impulsive errors than HC, evidenced by significantly lower accuracy on their fastest reaction time trials. The slope reduction of interference effects as reactions slowed was similar between HD and controls, indicating preserved impulse suppression.

Conclusion

Our results indicate that patients with HD show a greater susceptibility to act rapidly on incorrect motor impulses but preserved proficiency of top-down suppression. Further research is needed to determine how these findings relate to clinical behavioral symptoms.

Neurodegenerative Diseases: Can Caffeine Be a Powerful Ally to Weaken Neuroinflammation?

In recent years, there has been considerable research showing that coffee consumption seems to be beneficial to human health, as it contains a mixture of different bioactive compounds such as chlorogenic acids, caffeic acid, alkaloids, diterpenes and polyphenols. Neurodegenerative diseases (NDs) are debilitating, and non-curable diseases associated with impaired central, peripheral and muscle nervous systems. Several studies demonstrate that neuroinflammation mediated by glial cells—such as microglia and astrocytes—is a critical factor contributing to neurodegeneration that causes the dysfunction of brain homeostasis, resulting in a progressive loss of structure, function, and number of neuronal cells. This happens over time and leads to brain damage and physical impairment. The most known chronic NDs are represented by Alzheimer’s disease (AD), Parkinson’s disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS) and Huntington’s disease (HD). According to epidemiological studies, regular coffee consumption is associated with a lower risk of neurodegenerative diseases. In this review, we summarize the latest research about the potential effects of caffeine in neurodegenerative disorders prevention and discuss the role of controlled caffeine delivery systems in maintaining high plasma caffeine concentrations for an extended time.

Easily automated radiosynthesis of [18F]P10A-1910 and its clinical translation to quantify phosphodiesterase 10A in human brain

Our previous work showed that [18F]P10A-1910 was a potential radioligand for use in imaging phosphodiesterase 10A (PDE10A). Specifically, it had high brain penetration and specific binding that was demonstrated in both rodents and non-human primates. Here, we present the first automatic cGMP-level production of [18F]P10A-1910 and translational PET/MRI study in living human brains. Successful one-step radiolabeling of [18F]P10A-1910 on a GE TRACERlab FX2N synthesis module was realized via two different methods. First, formulated [18F]P10A-1910 was derived from heating spirocyclic iodonium ylide in a tetra-n-butyl ammonium methanesulfonate solution. At the end of synthesis, it was obtained in non-decay corrected radiochemical yields (n.d.c. RCYs) of 12.4 ± 1.3%, with molar activities (MAs) of 90.3 ± 12.6 μmol (n = 7) (Method I). The boronic pinacol ester combined with copper and oxygen also delivered the radioligand with 16.8 ± 1.0% n. d.c. RCYs and 77.3 ± 20.7 GBq/μmol (n = 7) MAs after formulation (Method II). The radiochemical purity, radionuclidic purity, solvent residue, sterility, endotoxin content and other parameters were all validated for human use. Consistent with the distribution of PDE10A in the brain, escalating uptake of [18F]P10A-1910 was observed in the order of cerebellum (reference region), substantial nigra, caudate and putamen. The non-displaceable binding potential (BPND) was estimated by simplified reference-tissue model (SRTM); linear regressions demonstrated that BPND was well correlated with the most widely used semiquantitative parameter SUV. The strongest correlation was observed with SUV(50–60 min) (R2 = 0.966, p &lt; 0.01). Collectively, these results indicated that a static scan protocol could be easily performed for PET imaging of PDE10A. Most importantly, that [18F]P10A-1910 is a promising radioligand to clinically quantify PDE10A.

TRAX Provides Neuroprotection for Huntington's Disease Via Modulating a Novel Subset of MicroRNAs

BACKGROUND

Huntington's disease (HD) is a neurodegenerative disease caused by CAG-repeat expansions (>36) in exon 1 of HTT, which dysregulates multiple cellular machineries. Translin-associated protein X (TRAX) is a scaffold protein with diverse functions, including suppressing the microRNA (miRNA)-mediated silencing by degrading pre-miRNA. To date, the role of TRAX in neurodegenerative diseases remains unknown.

OBJECTIVES

We delineated the role of TRAX upregulation during HD progression.

METHODS

Expression of TRAX in the brains of humans and three mouse models with HD were analyzed by immunohistochemistry staining, western blot, and quantitative reverse transcription-polymerase chain reaction. Adeno-associated viruses harboring TRAX short hairpin RNA were intrastriatally injected into HD mice to downregulate TRAX. HD-like symptoms were analyzed by behavioral and biochemical assessments. The miRNA-sequencing and RNA-sequencing analyses were used to identify the TRAX- regulated miRNA-messenger RNA (mRNA) axis during HD progression. The identified gene targets were validated biochemically in mouse and human striatal cells.

RESULTS

We discovered that TRAX was upregulated in the brains of HD patients and three HD mouse models. Downregulation of TRAX enhanced 83 miRNAs (including miR-330-3p, miR-496a-3p) and subsequently changed the corresponding mRNA networks critical for HD pathogenesis (eg, DARPP-32 and brain-derived neurotrophic factor). Disruption of the TRAX-mediated miRNA-mRNA axis accelerated the progression of HD-like symptoms, including the degeneration of motor function, accumulation of mHTT aggregates, and shortened neurite outgrowth.

CONCLUSIONS

We demonstrated that TRAX upregulation is authentic and protective in HD. Our study provides a novel layer of regulation for HD pathogenesis and may lead to the development of new therapeutic strategies for HD. © 2022 International Parkinson and Movement Disorder Society.

Presynaptic cGMP sets synaptic strength in the striatum and is important for motor learning

The striatum is a subcortical brain region responsible for the initiation and termination of voluntary movements. Striatal spiny projection neurons receive major excitatory synaptic input from neocortex and thalamus, and cyclic nucleotides have long been known to play important roles in striatal function. Yet, the precise mechanism of action is unclear. Here, we combine optogenetic stimulation, 2‐photon imaging, and genetically encoded scavengers to dissect the regulation of striatal synapses in mice. Our data show that excitatory striatal inputs are tonically depressed by phosphodiesterases (PDEs), in particular PDE1. Blocking PDE activity boosts presynaptic calcium entry and glutamate release, leading to strongly increased synaptic transmission. Although PDE1 degrades both cAMP and cGMP, we uncover that the concentration of cGMP, not cAMP, controls the gain of striatal inputs. Disturbing this gain control mechanism in vivo impairs motor skill learning in mice. The tight dependence of striatal excitatory synapses on PDE1 and cGMP offers a new perspective on the molecular mechanisms regulating striatal activity.

SERS determination of dopamine using metal-organic frameworks decorated with Ag/Au noble metal nanoparticle composite after azo derivatization with p-aminothiophenol

A specific surface-enhanced Raman scattering (SERS) assay for dopamine (DA) based on an azo derivatization reaction is proposed for the first time by preparation of p-aminothiophenol (PATP)-modified composite SERS substrate, composed of metal-organic framework (MIL-101) decorated with Au and Ag nanoparticles. As the result, the SERS method for detection of the azo reaction between PATP and DA exhibits superior sensitivity, selectivity, and stability. A reasonable linearity in the range 10^-6 to 10^-10 mol∙L^-1 is achieved, and the limit of detection is 1.2 × 10^-12 mol∙L^-1. The reactive SERS assay is free from interference in complex physiological fluid. The feasibility of the proposed SERS method for the detection of DA levels in fetal bovine serum (FBS) samples and human serum samples is validated by HPLC-MS methods, displaying promising application potential in early disease diagnosis.

Mitochondrial Abnormalities and Synaptic Damage in Huntington's Disease: a Focus on Defective Mitophagy and Mitochondria-Targeted Therapeutics

Huntington's disease (HD) is a fatal and pure genetic disease with a progressive loss of medium spiny neurons (MSN). HD is caused by expanded polyglutamine repeats in the exon 1 of HD gene. Clinically, HD is characterized by chorea, seizures, involuntary movements, dystonia, cognitive decline, intellectual impairment, and emotional disturbances. Several years of intense research revealed that multiple cellular changes, including defective axonal transport, protein-protein interactions, defective bioenergetics, calcium dyshomeostasis, NMDAR activation, synaptic damage, mitochondrial abnormalities, and selective loss of medium spiny neurons are implicated in HD. Recent research on mutant huntingtin (mHtt) and mitochondria has found that mHtt interacts with the mitochondrial division protein, dynamin-related protein 1 (DRP1), enhances GTPase DRP1 enzymatic activity, and causes excessive mitochondrial fragmentation and abnormal distribution, leading to defective axonal transport of mitochondria and selective synaptic degeneration. Recent research also revealed that failure to remove dead and/or dying mitochondria is an early event in the disease progression. Currently, efforts are being made to reduce abnormal protein interactions and enhance synaptic mitophagy as therapeutic strategies for HD. The purpose of this article is to discuss recent research in HD progression. This article also discusses recent developments of cell and mouse models, cellular changes, mitochondrial abnormalities, DNA damage, bioenergetics, oxidative stress, mitophagy, and therapeutics strategies in HD.

circRNA Regulates Dopaminergic Synapse, MAPK, and Long-term Depression Pathways in Huntington Disease

Huntington disease (HD) is the most common neurogenetic disorder caused by expansion of the CAG repeat in the HTT gene; nevertheless, the molecular bases of the disease are not fully understood. Non-coding RNAs have demonstrated to be involved in the physiopathology of HD. However, the role of circRNAs has not been investigated. The aim of this study was to identify the circRNAs with differential expression in a murine cell line model of HD and to identify the biological pathways regulated by the differentially expressed circRNAs. CircRNA expression was analyzed through a microarray, which specifically detects circular species of RNA. The expression patterns between a murine cell line expressing mutant Huntingtin and cells expressing wild-type Huntingtin were compared. We predicted the miRNAs with binding sites for the differentially expressed circRNAs and the corresponding target genes for those miRNAs. Using the target genes, we performed a function enrichment analysis. We identified 23 circRNAs differentially expressed, 19 downregulated and four upregulated. Most of the downregulated circRNAs derive from the Rere gene. The dopaminergic synapse, MAPK, and long-term depression pathways were significantly enriched. The three identified pathways have been previously associated with the physiopathology of HD. The understanding of the circRNA-miRNA-mRNA network involved in the molecular mechanisms driving HD can lead us to identify novel biomarkers and potential therapeutic targets. To the best of our knowledge, this is the first study analyzing circRNAs in a model of Huntington disease.

Purinergic Signaling in the Pathophysiology and Treatment of Huntington's Disease

Huntington’s disease (HD) is a devastating, progressive, and fatal neurodegenerative disorder inherited in an autosomal dominant manner. This condition is characterized by motor dysfunction (chorea in the early stage, followed by bradykinesia, dystonia, and motor incoordination in the late stage), psychiatric disturbance, and cognitive decline. The neuropathological hallmark of HD is the pronounced neuronal loss in the striatum (caudate nucleus and putamen). The striatum is related to the movement control, flexibility, motivation, and learning and the purinergic signaling has an important role in the control of these events. Purinergic signaling involves the actions of purine nucleotides and nucleosides through the activation of P2 and P1 receptors, respectively. Extracellular nucleotide and nucleoside-metabolizing enzymes control the levels of these messengers, modulating the purinergic signaling. The striatum has a high expression of adenosine A_2A receptors, which are involved in the neurodegeneration observed in HD. The P2X7 and P2Y2 receptors may also play a role in the pathophysiology of HD. Interestingly, nucleotide and nucleoside levels may be altered in HD animal models and humans with HD. This review presents several studies describing the relationship between purinergic signaling and HD, as well as the use of purinoceptors as pharmacological targets and biomarkers for this neurodegenerative disorder.

D1R- and D2R-Medium-Sized Spiny Neurons Diversity: Insights Into Striatal Vulnerability to Huntington's Disease Mutation

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an aberrant expansion of the CAG tract within the exon 1 of the HD gene, HTT. HD progressively impairs motor and cognitive capabilities, leading to a total loss of autonomy and ultimate death. Currently, no cure or effective treatment is available to halt the disease. Although the HTT gene is ubiquitously expressed, the striatum appears to be the most susceptible district to the HD mutation with Medium-sized Spiny Neurons (MSNs) (D1R and D2R) representing 95% of the striatal neuronal population. Why are striatal MSNs so vulnerable to the HD mutation? Particularly, why do D1R- and D2R-MSNs display different susceptibility to HD? Here, we highlight significant differences between D1R- and D2R-MSNs subpopulations, such as morphology, electrophysiology, transcriptomic, functionality, and localization in the striatum. We discuss possible reasons for their selective degeneration in the context of HD. Our review suggests that a better understanding of cell type-specific gene expression dysregulation within the striatum might reveal new paths to therapeutic intervention or prevention to ameliorate HD patients' life expectancy.

Midbrain circuits of novelty processing

Novelty triggers an increase in orienting behavior that is critical to evaluate the potential salience of unknown events. As novelty becomes familiar upon repeated encounters, this increase in response rapidly habituates as a form of behavioral adaptation underlying goal-directed behaviors. Many neurodevelopmental, psychiatric and neurodegenerative disorders are associated with abnormal responses to novelty and/or familiarity, although the neuronal circuits and cellular/molecular mechanisms underlying these natural behaviors in the healthy brain are largely unknown, as is the maladaptive processes that occur to induce impairment of novelty signaling in diseased brains. In rodents, the development of cutting-edge tools that allow for measurements of real time activity dynamics in selectively identified neuronal ensembles by gene expression signatures is beginning to provide advances in understanding the neural bases of the novelty response. Accumulating evidence indicate that midbrain circuits, the majority of which linked to dopamine transmission, promote exploratory assessments and guide approach/avoidance behaviors to different types of novelty via specific projection sites. The present review article focuses on midbrain circuit analysis relevant to novelty processing and habituation with familiarity.

Cortical and Striatal Circuits in Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder that typically manifests in midlife with motor, cognitive, and/or psychiatric symptoms. The disease is caused by a CAG triplet expansion in exon 1 of the huntingtin gene and leads to a severe neurodegeneration in the striatum and cortex. Classical electrophysiological studies in genetic HD mouse models provided important insights into the disbalance of excitatory, inhibitory and neuromodulatory inputs, as well as progressive disconnection between the cortex and striatum. However, the involvement of local cortical and striatal microcircuits still remains largely unexplored. Here we review the progress in understanding HD-related impairments in the cortical and basal ganglia circuits, and outline new opportunities that have opened with the development of modern circuit analysis methods. In particular, in vivo imaging studies in mouse HD models have demonstrated early structural and functional disturbances within the cortical network, and optogenetic manipulations of striatal cell types have started uncovering the causal roles of certain neuronal populations in disease pathogenesis. In addition, the important contribution of astrocytes to HD-related circuit defects has recently been recognized. In parallel, unbiased systems biology studies are providing insights into the possible molecular underpinnings of these functional defects at the level of synaptic signaling and neurotransmitter metabolism. With these approaches, we can now reach a deeper understanding of circuit-based HD mechanisms, which will be crucial for the development of effective and targeted therapeutic strategies.

Targeted Depletion of Primary Cilia in Dopaminoceptive Neurons in a Preclinical Mouse Model of Huntington's Disease

Multiple pathomechanisms triggered by mutant Huntingtin (mHTT) underlie progressive degeneration of dopaminoceptive striatal neurons in Huntington’s disease (HD). The primary cilium is a membrane compartment that functions as a hub for various pathways that are dysregulated in HD, for example, dopamine (DA) receptor transmission and the mechanistic target of rapamycin (mTOR) pathway. The roles of primary cilia (PC) for the maintenance of striatal neurons and in HD progression remain unknown. Here, we investigated PC defects in vulnerable striatal neurons in a progressive model of HD, the mHTT-expressing knock-in zQ175 mice. We found that PC length is affected in striatal but not in cortical neurons, in association with the accumulation of mHTT. To explore the role of PC, we generated conditional mutant mice lacking IFT88, a component of the anterograde intraflagellar transport-B complex lacking PC in dopaminoceptive neurons. This mutation preserved the expression of the dopamine 1 receptor (D1R), and the survival of striatal neurons, but resulted in a mild increase of DA metabolites in the striatum, suggesting an imbalance of ciliary DA receptor transmission. Conditional loss of PC in zQ175 mice did not trigger astrogliosis, however, mTOR signaling was more active and resulted in a more pronounced accumulation of nuclear inclusions containing mHTT. Further studies will be required of aged mice to determine the role of aberrant ciliary function in more advanced stages of HD.