Impaired TrkB receptor signaling underlies corticostriatal dysfunction in Huntington's disease

Vitamin C Transport Deficiency Alters Striatal Dopamine Gene Expression and Metabolism in YAC128 Huntington Disease Mice

Neurodegeneration in Huntington disease (HD) contributes to dopaminergic system dysfunction via the loss of striatal medium spiny neurons expressing dopamine receptors. Given the key role for ascorbic acid (vitamin C) in dopamine synthesis and neurotransmission, we investigated whether mild cellular ascorbate deficiency accelerates dopaminergic dysfunction in the development of HD pathology and behavioral deficits. YAC128 mice expressing mutant human huntingtin were crossed with SVCT2+/- mice, which carry a heterozygous knockout of the sodium-dependent vitamin C transporter, to generate mice with approximately 30% decreased neuronal vitamin C as well as progressive changes in dopamine signaling. Behavioral and neurochemical outcomes were assessed at early disease stages. At 14 and 20 weeks, YAC128 and SVCT2+/- YAC128 mice showed similar deficits in grip strength, locomotor activity, and rotarod performance compared to controls, suggesting modest ascorbate deficiency did not accelerate motor phenotypes. Gene expression analysis revealed six significantly upregulated genes in the striatum of SVCT2+/- YAC128 mice, including those involved in dopamine synthesis, packaging, and transport. Notably, striatal dopamine and serotonin and their metabolites were decreased in both single mutant mouse lines (YAC128 and SVCT2+/-) but without a compounding effect of the double mutation (SVCT2+/- YAC128). These results indicate that while moderate ascorbate deficiency may not worsen early behavioral phenotypes in the YAC128 model, it does impact dopamine system regulation at the molecular level. These findings highlight the potential importance of ascorbate in modifying disease progression and suggest that humans with HD, who cannot synthesize ascorbate, may be particularly vulnerable to vitamin C deficiency effects on dopamine dynamics.

Upregulation of endocannabinoid signaling in vivo restores striatal synaptic plasticity and motor performance in Huntington's disease mice

BackgroundSynaptic dysfunction underlies early sensorimotor and cognitive deficits in Huntington's disease (HD) and precedes the degeneration of striatal spiny projection neurons and cortical pyramidal neurons. Movement selection and motor learning, which are impaired early in HD, are regulated by connections between the motor cortex, basal ganglia and thalamus. In particular, plasticity at corticostriatal synapses, including endocannabinoid-mediated long-term depression (LTD), is critical for motor learning. Previously, we found impaired endocannabinoid-mediated LTD, induced by high frequency stimulation (HFS) at corticostriatal synapses in brain slice recordings from pre-manifest HD mouse models, which was corrected by JZL184, an inhibitor of endocannabinoid 2-arachidonoyl glycerol (2-AG) degradation.ObjectiveDetermine the effects of in vivo JZL184 administration on YAC128 HD model and wild-type (WT) littermate mice.MethodsJZL184 was administered to mice orally over a 3-week period and their motor function was assessed using several behavioral tasks. In addition, brain tissue was collected from mice in order to quantify changes in endocannabinoid levels and measure HFS-induced plasticity at corticostriatal synapses.ResultsOral administration of JZL184 significantly increased levels of 2-AG in striatal tissue. While JZL184 treatment had no impact on open field behavior, the treatment eliminated the difference in motor learning on the rotarod task between YAC128 and WT mice. Moreover, HFS-induced striatal plasticity in YAC128 mice was normalized to WT levels after JZL184 treatment.ConclusionsThese results suggest a novel target for mitigating early symptoms of HD and support the need for clinical trials of therapies that modulate the endocannabinoid system.

Targeting TrkB-PSD-95 coupling to mitigate neurological disorders

Tropomyosin receptor kinase B (TrkB) signaling plays a pivotal role in dendritic growth and dendritic spine formation to promote learning and memory. The activity-dependent release of brain-derived neurotrophic factor at synapses binds to pre- or postsynaptic TrkB resulting in the strengthening of synapses, reflected by long-term potentiation. Postsynaptically, the association of postsynaptic density protein-95 with TrkB enhances phospholipase Cγ-Ca2+/calmodulin-dependent protein kinase II and phosphatidylinositol 3-kinase-mechanistic target of rapamycin signaling required for long-term potentiation. In this review, we discuss TrkB-postsynaptic density protein-95 coupling as a promising strategy to magnify brain-derived neurotrophic factor signaling towards the development of novel therapeutics for specific neurological disorders. A reduction of TrkB signaling has been observed in neurodegenerative disorders, such as Alzheimer's disease and Huntington's disease, and enhancement of postsynaptic density protein-95 association with TrkB signaling could mitigate the observed deficiency of neuronal connectivity in schizophrenia and depression. Treatment with brain-derived neurotrophic factor is problematic, due to poor pharmacokinetics, low brain penetration, and side effects resulting from activation of the p75 neurotrophin receptor or the truncated TrkB.T1 isoform. Although TrkB agonists and antibodies that activate TrkB are being intensively investigated, they cannot distinguish the multiple human TrkB splicing isoforms or cell type-specific functions. Targeting TrkB-postsynaptic density protein-95 coupling provides an alternative approach to specifically boost TrkB signaling at localized synaptic sites versus global stimulation that risks many adverse side effects.

Neurexin1 level in Huntington's Disease and decreased Neurexin1 in disease progression

Huntington's disease (HD) is a neurodegenerative disorder characterized by the presence of abnormally expanded polyglutamine tracts in huntingtin protein (HTT). Mutant HTT disrupts synaptic transmission and plasticity, particularly in the striatum and cortex, leading to early dysfunctions, such as altered neurotransmitter release, impaired synaptic vesicle recycling, and disrupted postsynaptic receptor function. Synaptic loss precedes neuronal degeneration and contributes to disease progression. Neurexin1 (NRXN1), a synaptic cell adhesion molecule primarily located in the presynaptic membrane, plays a crucial role in maintaining synaptic integrity. The present study investigated the role of NRXN1 in HD. This study researched whether the changed level has been related to expanded polyQ stretch and disease progression. Here, we report a reduction in NRXN1 levels in post-symptomatic HD mice and in neuronal cells expressing abnormally expanded polyQ tracts. Mutant HTT was found to decrease NRXN1 levels while increasing LAMP2A levels, which promotes lysosomal degradation of NRXN1. In HD cells expressing Q111, downregulated LAMP2A restored NRXN1 levels and maintained cell proliferation compared with cells expressing Q7. These findings suggest that NRXN1 is regulated by LAMP2A-mediated way and that decreased NRXN1 levels are associated with symptomatic progression and neuronal cell loss in HD.

AMPA receptor diffusional trapping machinery as an early therapeutic target in neurodegenerative and neuropsychiatric disorders

Over the past two decades, there has been a growing recognition of the physiological importance and pathological implications surrounding the surface diffusion of AMPA receptors (AMPARs) and their diffusional trapping at synapses. AMPAR surface diffusion entails the thermally powered random Brownian lateral movement of these receptors within the plasma membrane, facilitating dynamic exchanges between synaptic and extrasynaptic compartments. This process also enables the activity-dependent diffusional trapping and accumulation of AMPARs at synapses through transient binding to synaptic anchoring slots. Recent research highlights the critical role of synaptic recruitment of AMPARs via diffusional trapping in fundamental neural processes such as the development of the early phases of long-term potentiation (LTP), contextual fear memory, memory consolidation, and sensory input-induced cortical remapping. Furthermore, studies underscore that regulation of AMPAR diffusional trapping is altered across various neurological disease models, including Huntington's disease (HD), Alzheimer's disease (AD), and stress-related disorders like depression. Notably, pharmacological interventions aimed at correcting deficits in AMPAR diffusional trapping have demonstrated efficacy in restoring synapse numbers, LTP, and memory functions in these diverse disease models, despite their distinct pathogenic mechanisms. This review provides current insights into the molecular mechanisms underlying the dysregulation of AMPAR diffusional trapping, emphasizing its role as a converging point for multiple pathological signaling pathways. We propose that targeting AMPAR diffusional trapping represents a promising early therapeutic strategy to mitigate synaptic plasticity and memory deficits in a spectrum of brain disorders, encompassing but not limited to HD, AD, and stress-related conditions. This approach underscores an integrated therapeutic target amidst the complexity of these neurodegenerative and neuropsychiatric diseases.

Brain-Derived Neurotrophic Factor (BDNF) in Huntington's Disease: Neurobiology and Therapeutic Potential

Huntington's disease is a hereditary neurodegenerative disorder marked by severe neurodegeneration in the striatum and cortex. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of growth factors. It plays a crucial role in maintaining the survival and proper function of striatal neurons. Depletion of BDNF has been linked to impairment and death of striatal neurons, leading to the manifestation of motor, cognitive, and behavioral dysfunctions characteristic of Huntington's disease. This review highlights the current update on the neurobiology of BDNF in the pathogenesis of Huntington's disease. The molecular evidence and the affected signaling pathways are also discussed. In addition, the impact of experimental manipulation of BDNF levels and its pharmaceutical potential for Huntington's disease treatment are explicitly reviewed.

Restoring Compromised Cl- in D2 Neurons of a Huntington's Disease Mouse Model Rescues Motor Disability

Huntington's disease (HD) is a progressive neurodegenerative disorder with no cure, characterized by significant neurodegeneration of striatal GABAergic medium spiny neurons (MSNs). Early stages of the disease are characterized by the loss of dopamine 2 receptor-expressing MSNs (D2 MSNs) followed by degeneration of dopamine 1 receptor-expressing MSNs (D1 MSNs), leading to aberrant basal ganglia signaling. While the early degeneration of D2 MSNs and impaired GABAergic transmission are well-documented, potassium chloride cotransporter 2 (KCC2), a key regulator of intracellular chloride (Cl-), and therefore GABAergic signaling, has not been characterized in D1 and D2 MSNs in HD. We aimed to investigate whether Cl- regulation was differentially altered in D1 and D2 MSNs and may contribute to the early degeneration of D2 MSNs in male and female symptomatic R6/2 mice. We used electrophysiology to record the reversal potential for GABAA receptors (E GABA), a read-out for the efficacy of Cl- regulation, in striatal D1 and D2 MSNs and their corresponding output structures. During the early symptomatic phase (P55-P65), Cl- impairments were observed in D2 MSNs in R6/2 mice, with no change in D1 MSNs. Cl- regulation was also dysfunctional in the globus pallidus externa, resulting in GABA-mediated excitation. When we overexpressed KCC2 in D2 MSNs using AAV-mediated delivery, we delayed the onset of motor impairments in R6/2 mice. We demonstrate that Cl- homeostasis is differentially altered in D1 and D2 MSNs and may contribute to the enhanced susceptibility of D2 MSNs during HD progression.

Therapeutic approaches targeting aging and cellular senescence in Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disease that is manifested by a gradual loss of physical, cognitive, and mental abilities. As the disease advances, age has a major impact on the pathogenic signature of mutant huntingtin (mHTT) protein aggregation. This review aims to explore the intricate relationship between aging, mHTT toxicity, and cellular senescence in HD. Scientific data on the interplay between aging, mHTT, and cellular senescence in HD were collected from several academic databases, including PubMed, Google Scholar, Google, and ScienceDirect. The search terms employed were "AGING," "HUNTINGTON'S DISEASE," "MUTANT HUNTINGTIN," and "CELLULAR SENESCENCE." Additionally, to gather information on the molecular mechanisms and potential therapeutic targets, the search was extended to include relevant terms such as "DNA DAMAGE," "OXIDATIVE STRESS," and "AUTOPHAGY." According to research, aging leads to worsening HD pathophysiology through some processes. As a result of the mHTT accumulation, cellular senescence is promoted, which causes DNA damage, oxidative stress, decreased autophagy, and increased inflammatory responses. Pro-inflammatory cytokines and other substances are released by senescent cells, which may worsen the neuronal damage and the course of the disease. It has been shown that treatments directed at these pathways reduce some of the HD symptoms and enhance longevity in experimental animals, pointing to a new possibility of treating the condition. Through their amplification of the harmful effects of mHTT, aging and cellular senescence play crucial roles in the development of HD. Comprehending these interplays creates novel opportunities for therapeutic measures targeted at alleviating cellular aging and enhancing HD patients' quality of life.

Reduced inhibitory synaptic transmission onto striatopallidal neurons may underlie aging-related motor skill deficits

Human beings are living longer than ever before and aging is accompanied by an increased incidence of motor deficits, including those associated with the neurodegenerative conditions, Parkinson's disease (PD) and Huntington's disease (HD). However, the biological correlates underlying this epidemiological finding, especially the functional basis at the synapse level, have been elusive. This study reveals that motor skill performance examined via rotarod, beam walking and pole tests is impaired in aged mice. This study, via electrophysiology recordings, further identifies an aging-related reduction in the efficacy of inhibitory synaptic transmission onto dorsolateral striatum (DLS) indirect-pathway medium spiny neurons (iMSNs), i.e., a disinhibition effect on DLS iMSNs. In addition, pharmacologically enhancing the activity of DLS iMSNs by infusing an adenosine A2A receptor (A2AR) agonist, which presumably mimics the disinhibition effect, impairs motor skill performance in young mice, simulating the behavior in aged naïve mice. Conversely, pharmacologically suppressing the activity of DLS iMSNs by infusing an A2AR antagonist, in order to offset the disinhibition effect, restores motor skill performance in aged mice, mimicking the behavior in young naïve mice. In conclusion, this study identifies a functional inhibitory synaptic plasticity in DLS iMSNs that likely contributes to the aging-related motor skill deficits, which would potentially serve as a striatal synaptic basis underlying age being a prominent risk factor for neurodegenerative motor deficits.

BDNF and TRiC-inspired reagent rescue cortical synaptic deficits in a mouse model of Huntington's disease

Synaptic changes are early manifestations of neuronal dysfunction in Huntington's disease (HD). However, the mechanisms by which mutant HTT protein impacts synaptogenesis and function are not well understood. Herein we explored HD pathogenesis in the BACHD mouse model by examining synaptogenesis and function in long term primary cortical cultures. At DIV14 (days in vitro), BACHD cortical neurons showed no difference from WT neurons in synaptogenesis as revealed by colocalization of a pre-synaptic (Synapsin I) and a post-synaptic (PSD95) marker. From DIV21 to DIV35, BACHD neurons showed progressively reduced colocalization of Synapsin I and PSD95 relative to WT neurons. The deficits were effectively rescued by treatment of BACHD neurons with BDNF. The recombinant apical domain of CCT1 (ApiCCT1) yielded a partial rescuing effect. BACHD neurons also showed culture age-related significant functional deficits as revealed by multielectrode arrays (MEAs). These deficits were prevented by BDNF, whereas ApiCCT1 showed a less potent effect. These findings are evidence that deficits in BACHD synapse and function can be replicated in vitro and that BDNF or a TRiC-inspired reagent can potentially be protective against these changes in BACHD neurons. Our findings support the use of cellular models to further explicate HD pathogenesis and potential treatments.

Age- and region-dependent cortical excitability in the zQ175 Huntington disease mouse model

The neurodegenerative disorder, Huntington disease (HD), manifests as disorders of movement, cognition and mood. Although studies report abnormal corticostriatal synaptic function early in HD mouse models, less is known about cortical-cortical activity across brain regions and disease stages. Recently, we reported enhanced mesoscale spread of cortical responses to sensory stimulation in vivo at early-manifest stages of two HD mouse models. Here, we investigated cortical excitability of zQ175 HD-model mice compared to their wild-type littermates across different cell types, ages and/or cortical regions using ex vivo electrophysiology. Cortical pyramidal neurons (CPNs) in somatosensory cortex of zQ175 mice showed intrinsic hyper-excitability at 3-4 months, but hypo-excitability at early-manifest stage (8-9 months); reduced frequency of spontaneous excitatory postsynaptic currents (sEPSCs) was seen at both ages. In contrast, motor cortex CPNs in early-manifest zQ175 mice showed increased intrinsic excitability and sEPSC frequency. Large-amplitude excitatory discharges recorded from CPNs in early-manifest zQ175 mice showed increased frequency only in somatosensory cortex, suggesting the intrinsic hypo-excitability of these CPNs may be compensatory against cortical network hyper-excitability. Similarly, in early-manifest zQ175 mice, region-dependent differences were seen in fast-spiking interneurons (FSIs): somatosensory but not motor FSIs from early-manifest zQ175 mice had reduced intrinsic excitability. Moreover, CPNs showed decreased frequency of spontaneous inhibitory postsynaptic currents and increased excitatory-inhibitory (E-I) balance of evoked synaptic currents in somatosensory cortex. Aberrant large-amplitude discharges and reduced inhibitory drive may therefore underlie E-I imbalances that result in circuit changes and synaptic dysfunction in early-manifest HD.

Rapid and high-purity differentiation of human medium spiny neurons reveals LMNB1 hypofunction and subtype necessity in modeling Huntington's disease

BACKGROUND

Different neural subtypes are selectively lost in diverse neurodegenerative diseases. Huntington's disease (HD) is an inherited neurodegenerative disease characterized by motor abnormalities that primarily affect the striatum. The Huntingtin (HTT) mutation involves an expanded CAG repeat, leading to insoluble polyQ, which renders GABA+ medium spiny neurons (MSN) more venerable to cell death. Human pluripotent stem cells (hPSCs) technology allows for the construction of disease-specific models, providing valuable cellular models for studying pathogenesis, drug screening, and high-throughput analysis.

METHODS

In this study, we established a method that allows for rapid and efficient generation of MSNs (> 90%) within 21 days from hPSC-derived neural progenitor cells, by introducing a specific combination of transcription factors.

RESULTS

We efficiently induced several neural subtypes, in parallel, based on the same cell source, and revealed that, compared to other neural subtypes, MSNs exhibited higher polyQ aggregation propensity and overexpression toxicity, more severe dysfunction in BDNF/TrkB signaling, greater susceptibility to BDNF withdrawal, and more severe disturbances in nucleocytoplasmic transport (NCT). We further found that the nuclear lamina protein LMNB1 was greatly reduced in HD neurons and mislocalized to the cytoplasm and axons. Knockdown of HTT or treatment with KPT335, an orally selective inhibitor of nuclear export (SINE), effectively attenuated the pathological phenotypes and alleviated neuronal death caused by BDNF withdrawal.

CONCLUSIONS

This study thus establishes an effective method for obtaining MSNs and underscores the necessity of using high-purity MSNs to study HD pathogenesis, especially the MSN-selective vulnerability.

Transplanted human neural stem cells rescue phenotypes in zQ175 Huntington's disease mice and innervate the striatum

Huntington's disease (HD), a genetic neurodegenerative disorder, primarily affects the striatum and cortex with progressive loss of medium-sized spiny neurons (MSNs) and pyramidal neurons, disrupting cortico-striatal circuitry. A promising regenerative therapeutic strategy of transplanting human neural stem cells (hNSCs) is challenged by the need for long-term functional integration. We previously described that, with short-term hNSC transplantation into the striatum of HD R6/2 mice, human cells differentiated into electrophysiologically active immature neurons, improving behavior and biochemical deficits. Here, we show that long-term (8 months) implantation of hNSCs into the striatum of HD zQ175 mice ameliorates behavioral deficits, increases brain-derived neurotrophic factor (BDNF) levels, and reduces mutant huntingtin (mHTT) accumulation. Patch clamp recordings, immunohistochemistry, single-nucleus RNA sequencing (RNA-seq), and electron microscopy demonstrate that hNSCs differentiate into diverse neuronal populations, including MSN- and interneuron-like cells, and form connections. Single-nucleus RNA-seq analysis also shows restoration of several mHTT-mediated transcriptional changes of endogenous striatal HD mouse cells. Remarkably, engrafted cells receive synaptic inputs, innervate host neurons, and improve membrane and synaptic properties. Overall, the findings support hNSC transplantation for further evaluation and clinical development for HD.

Activin A targets extrasynaptic NMDA receptors to ameliorate neuronal and behavioral deficits in a mouse model of Huntington disease

Cortical-striatal synaptic dysfunction, including enhanced toxic signaling by extrasynaptic N-methyl-d-aspartate receptors (eNMDARs), precedes neurodegeneration in Huntington disease (HD). A previous study showed Activin A, whose transcription is upregulated by calcium influx via synaptic NMDARs, suppresses eNMDAR signaling. Therefore, we examined the role of Activin A in the YAC128 HD mouse model, comparing it to wild-type controls. We found decreased Activin A secretion in YAC128 cortical-striatal co-cultures, while Activin A overexpression in this model rescued altered eNMDAR expression. Striatal overexpression of Activin A in vivo improved motor learning on the rotarod task, and normalized striatal neuronal eNMDAR-mediated currents, membrane capacitance and spontaneous excitatory postsynaptic current frequency in the YAC128 mice. These results support the therapeutic potential of Activin A signaling and targeting eNMDARs to restore striatal neuronal health and ameliorate behavioral deficits in HD.

Distributed dopaminergic signaling in the basal ganglia and its relationship to motor disability in Parkinson's disease

The degeneration of mesencephalic dopaminergic neurons that innervate the basal ganglia is responsible for the cardinal motor symptoms of Parkinson's disease (PD). It has been thought that loss of dopaminergic signaling in one basal ganglia region - the striatum - was solely responsible for the network pathophysiology causing PD motor symptoms. While our understanding of dopamine (DA)'s role in modulating striatal circuitry has deepened in recent years, it also has become clear that it acts in other regions of the basal ganglia to influence movement. Underscoring this point, examination of a new progressive mouse model of PD shows that striatal dopamine DA depletion alone is not sufficient to induce parkinsonism and that restoration of extra-striatal DA signaling attenuates parkinsonian motor deficits once they appear. This review summarizes recent advances in the effort to understand basal ganglia circuitry, its modulation by DA, and how its dysfunction drives PD motor symptoms.

Neuroprotective effects of hepatoma-derived growth factor in models of Huntington's disease

Huntington's disease (HD) is a movement disorder caused by a mutation in the Huntingtin gene that leads to severe neurodegeneration. Molecular mechanisms of HD are not sufficiently understood, and no cure is currently available. Here, we demonstrate neuroprotective effects of hepatoma-derived growth factor (HDGF) in cellular and mouse HD models. We show that HD-vulnerable neurons in the striatum and cortex express lower levels of HDGF than resistant ones. Moreover, lack of endogenous HDGF exacerbated motor impairments and reduced the life span of R6/2 Huntington's disease mice. AAV-mediated delivery of HDGF into the brain reduced mutant Huntingtin inclusion load, but had no significant effect on motor behavior or life span. Interestingly, both nuclear and cytoplasmic versions of HDGF were efficient in rescuing mutant Huntingtin toxicity in cellular HD models. Moreover, extracellular application of recombinant HDGF improved viability of mutant Huntingtin-expressing primary neurons and reduced mutant Huntingtin aggregation in neural progenitor cells differentiated from human patient-derived induced pluripotent stem cells. Our findings provide new insights into the pathomechanisms of HD and demonstrate neuroprotective potential of HDGF in neurodegeneration.

Microglia and complement mediate early corticostriatal synapse loss and cognitive dysfunction in Huntington's disease

Huntington's disease (HD) is a devastating monogenic neurodegenerative disease characterized by early, selective pathology in the basal ganglia despite the ubiquitous expression of mutant huntingtin. The molecular mechanisms underlying this region-specific neuronal degeneration and how these relate to the development of early cognitive phenotypes are poorly understood. Here we show that there is selective loss of synaptic connections between the cortex and striatum in postmortem tissue from patients with HD that is associated with the increased activation and localization of complement proteins, innate immune molecules, to these synaptic elements. We also found that levels of these secreted innate immune molecules are elevated in the cerebrospinal fluid of premanifest HD patients and correlate with established measures of disease burden.In preclinical genetic models of HD, we show that complement proteins mediate the selective elimination of corticostriatal synapses at an early stage in disease pathogenesis, marking them for removal by microglia, the brain's resident macrophage population. This process requires mutant huntingtin to be expressed in both cortical and striatal neurons. Inhibition of this complement-dependent elimination mechanism through administration of a therapeutically relevant C1q function-blocking antibody or genetic ablation of a complement receptor on microglia prevented synapse loss, increased excitatory input to the striatum and rescued the early development of visual discrimination learning and cognitive flexibility deficits in these models. Together, our findings implicate microglia and the complement cascade in the selective, early degeneration of corticostriatal synapses and the development of cognitive deficits in presymptomatic HD; they also provide new preclinical data to support complement as a therapeutic target for early intervention.

Dopaminergic Input Regulates the Sensitivity of Indirect Pathway Striatal Spiny Neurons to Brain-Derived Neurotrophic Factor

Motor dysfunction in Parkinson's disease (PD) is closely linked to the dopaminergic depletion of striatal neurons and altered synaptic plasticity at corticostriatal synapses. Dopamine receptor D1 (DRD1) stimulation is a crucial step in the formation of long-term potentiation (LTP), whereas dopamine receptor D2 (DRD2) stimulation is needed for the formation of long-term depression (LTD) in striatal spiny projection neurons (SPNs). Tropomyosin receptor kinase B (TrkB) and its ligand brain-derived neurotrophic factor (BDNF) are centrally involved in plasticity regulation at the corticostriatal synapses. DRD1 activation enhances TrkB's sensitivity for BDNF in direct pathway spiny projection neurons (dSPNs). In this study, we showed that the activation of DRD2 in cultured striatal indirect pathway spiny projection neurons (iSPNs) and cholinergic interneurons causes the retraction of TrkB from the plasma membrane. This provides an explanation for the opposing synaptic plasticity changes observed upon DRD1 or DRD2 stimulation. In addition, TrkB was found within intracellular structures in dSPNs and iSPNs from Pitx3-/- mice, a genetic model of PD with early onset dopaminergic depletion in the dorsolateral striatum (DLS). This dysregulated BDNF/TrkB signaling might contribute to the pathophysiology of direct and indirect pathway striatal projection neurons in PD.

The NONRATT023402.2/rno-miR-3065-5p/NGFR axis affects levodopa-induced dyskinesia in a rat model of Parkinson's disease

Levodopa-induced dyskinesia (LID) is a common motor complication in Parkinson's disease. However, few studies have focused on the pathogenesis of LID at the transcriptional level. NONRATT023402.2, a long non-coding RNA (lncRNA) that may be related to LID was discovered in our previous study and characterized in rat models of LID. In the present study, NONRATT023402.2 was overexpressed by injection of adeno-associated virus (AAV) in striatum of LID rats, and 48 potential target genes, including nerve growth factor receptor (NGFR) were screened using next-generation sequencing and target gene predictions. The NONRATT023402.2/rno-miR-3065-5p/NGFR axis was verified using a dual luciferase reporter gene. Overexpression of NONRATT023402.2 significantly increased the abnormal involuntary movements (AIM) score of LID rats, activated the PI3K/Akt signaling pathway, and up-regulated c-Fos in the striatum. NGFR knockdown by injection of ShNGFR-AAV into the striatum of LID rats resulted in a significant decrease in the PI3K/Akt signaling pathway and c-Fos expression. The AIM score of LID rats was positively correlated with the expressions of NONRATT023402.2 and NGFR. A dual luciferase reporter assay showed that c-Fos, as a transcription factor, bound to the NONRATT023402.2 promoter and activated its expression. Together, the results showed that NONRATT023402.2 regulated NGFR expression via a competing endogenous RNA mechanism, which then activated the PI3K/Akt pathway and promoted c-Fos expression. This suggested that c-Fos acted as a transcription factor to activate NONRATT023402.2 expression, and form a positive feedback regulation loop in LID rats, thus, aggravating LID symptoms. NONRATT023402.2 is therefore a possible novel therapeutic target for LID.

Brain-Derived Neurotrophic Factor Dysregulation as an Essential Pathological Feature in Huntington's Disease: Mechanisms and Potential Therapeutics

Brain-derived neurotrophic factor (BDNF) is a major neurotrophin whose loss or interruption is well established to have numerous intersections with the pathogenesis of progressive neurological disorders. There is perhaps no greater example of disease pathogenesis resulting from the dysregulation of BDNF signaling than Huntington's disease (HD)-an inherited neurodegenerative disorder characterized by motor, psychiatric, and cognitive impairments associated with basal ganglia dysfunction and the ultimate death of striatal projection neurons. Investigation of the collection of mechanisms leading to BDNF loss in HD highlights this neurotrophin's importance to neuronal viability and calls attention to opportunities for therapeutic interventions. Using electronic database searches of existing and forthcoming research, we constructed a literature review with the overarching goal of exploring the diverse set of molecular events that trigger BDNF dysregulation within HD. We highlighted research that investigated these major mechanisms in preclinical models of HD and connected these studies to those evaluating similar endpoints in human HD subjects. We also included a special focus on the growing body of literature detailing key transcriptomic and epigenetic alterations that affect BDNF abundance in HD. Finally, we offer critical evaluation of proposed neurotrophin-directed therapies and assessed clinical trials seeking to correct BDNF expression in HD individuals.

Cell and circuit complexity of the external globus pallidus

The external globus pallidus (GPe) of the basal ganglia has been underappreciated owing to poor understanding of its cells and circuits. It was assumed that the GPe consisted of a homogeneous neuron population primarily serving as a 'relay station' for information flowing through the indirect basal ganglia pathway. However, the advent of advanced tools in rodent models has sparked a resurgence in interest in the GPe. Here, we review recent data that have unveiled the cell and circuit complexity of the GPe. These discoveries have revealed that the GPe does not conform to traditional views of the basal ganglia. In particular, recent evidence confirms that the afferent and efferent connections of the GPe span both the direct and the indirect pathways. Furthermore, the GPe displays broad interconnectivity beyond the basal ganglia, consistent with its emerging multifaceted roles in both motor and non-motor functions. In summary, recent data prompt new proposals for computational rules of the basal ganglia.

DRD1 signaling modulates TrkB turnover and BDNF sensitivity in direct pathway striatal medium spiny neurons

Disturbed motor control is a hallmark of Parkinson's disease (PD). Cortico-striatal synapses play a central role in motor learning and adaption, and brain-derived neurotrophic factor (BDNF) from cortico-striatal afferents modulates their plasticity via TrkB in striatal medium spiny projection neurons (SPNs). We studied the role of dopamine in modulating the sensitivity of direct pathway SPNs (dSPNs) to BDNF in cultures of fluorescence-activated cell sorting (FACS)-enriched D1-expressing SPNs and 6-hydroxydopamine (6-OHDA)-treated rats. DRD1 activation causes enhanced TrkB translocation to the cell surface and increased sensitivity for BDNF. In contrast, dopamine depletion in cultured dSPN neurons, 6-OHDA-treated rats, and postmortem brain of patients with PD reduces BDNF responsiveness and causes formation of intracellular TrkB clusters. These clusters associate with sortilin related VPS10 domain containing receptor 2 (SORCS-2) in multivesicular-like structures, which apparently protects them from lysosomal degradation. Thus, impaired TrkB processing might contribute to disturbed motor function in PD.

Axonal ER Ca2+ Release Selectively Enhances Activity-Independent Glutamate Release in a Huntington Disease Model

Action potential (AP)-independent (miniature) neurotransmission occurs at all chemical synapses but remains poorly understood, particularly in pathologic contexts. Axonal endoplasmic reticulum (ER) Ca2+ stores are thought to influence miniature neurotransmission, and aberrant ER Ca2+ handling is implicated in progression of Huntington disease (HD). Here, we report elevated mEPSC frequencies in recordings from YAC128 mouse (HD-model) neurons (from cortical cultures and striatum-containing brain slices, both from male and female animals). Pharmacological experiments suggest that this is mediated indirectly by enhanced tonic ER Ca2+ release. Calcium imaging, using an axon-localized sensor, revealed slow AP-independent ER Ca2+ release waves in both YAC128 and WT cultures. These Ca2+ waves occurred at similar frequencies in both genotypes but spread less extensively and were of lower amplitude in YAC128 axons, consistent with axonal ER Ca2+ store depletion. Surprisingly, basal cytosolic Ca2+ levels were lower in YAC128 boutons and YAC128 mEPSCs were less sensitive to intracellular Ca2+ chelation. Together, these data suggest that elevated miniature glutamate release in YAC128 cultures is associated with axonal ER Ca2+ depletion but not directly mediated by ER Ca2+ release into the cytoplasm. In contrast to increased mEPSC frequencies, cultured YAC128 cortical neurons showed less frequent AP-dependent (spontaneous) Ca2+ events in soma and axons, although evoked glutamate release detected by an intensity-based glutamate-sensing fluorescence reporter in brain slices was similar between genotypes. Our results indicate that axonal ER dysfunction selectively elevates miniature glutamate release from cortical terminals in HD. This, together with reduced spontaneous cortical neuron firing, may cause a shift from activity-dependent to -independent glutamate release in HD, with potential implications for fidelity and plasticity of cortical excitatory signaling.SIGNIFICANCE STATEMENT Miniature neurotransmitter release persists at all chemical neuronal synapses in the absence of action potential firing but remains poorly understood, particularly in disease states. We show enhanced miniature glutamate release from cortical neurons in the YAC128 mouse Huntington disease model. This effect is mediated by axonal ER Ca2+ store depletion, but is not obviously due to elevated ER-to-cytosol Ca2+ release. Conversely, YAC128 cortical pyramidal neurons fired fewer action potentials and evoked cortical glutamate release was similar between WT an YAC128 preparations, indicating axonal ER depletion selectively enhances miniature glutamate release in YAC128 mice. These results extend our understanding of action potential independent neurotransmission and highlight a potential involvement of elevated miniature glutamate release in Huntington disease pathology.

DNA double-strand break-free CRISPR interference delays Huntington's disease progression in mice

Huntington's disease (HD) is caused by a CAG repeat expansion in the huntingtin (HTT) gene. CRISPR-Cas9 nuclease causes double-strand breaks (DSBs) in the targeted DNA that induces toxicity, whereas CRISPR interference (CRISPRi) using dead Cas9 (dCas9) suppresses the target gene expression without DSBs. Delivery of dCas9-sgRNA targeting CAG repeat region does not damage the targeted DNA in HEK293T cells containing CAG repeats. When this study investigates whether CRISPRi can suppress mutant HTT (mHTT), CRISPRi results in reduced expression of mHTT with relative preservation of the wild-type HTT in human HD fibroblasts. Although both dCas9 and Cas9 treatments reduce mHTT by sgRNA targeting the CAG repeat region, CRISPRi delays behavioral deterioration and protects striatal neurons against cell death in HD mice. Collectively, CRISPRi can delay disease progression by suppressing mHtt, suggesting DNA DSB-free CRISPRi is a potential therapy for HD that can compensate for the shortcoming of CRISPR-Cas9 nuclease.

Cholinergic deficits selectively boost cortical intratelencephalic control of striatum in male Huntington's disease model mice

Huntington's disease (HD) is a progressive, neurodegenerative disease caused by a CAG triplet expansion in huntingtin. Although corticostriatal dysfunction has long been implicated in HD, the determinants and pathway specificity of this pathophysiology are not fully understood. Here, using a male zQ175^+/- knock-in mouse model of HD we carry out optogenetic interrogation of intratelencephalic and pyramidal tract synapses with principal striatal spiny projection neurons (SPNs). These studies reveal that the connectivity of intratelencephalic, but not pyramidal tract, neurons with direct and indirect pathway SPNs increased in early symptomatic zQ175^+/- HD mice. This enhancement was attributable to reduced pre-synaptic inhibitory control of intratelencephalic terminals by striatal cholinergic interneurons. Lowering mutant huntingtin selectively in striatal cholinergic interneurons with a virally-delivered zinc finger repressor protein normalized striatal acetylcholine release and intratelencephalic functional connectivity, revealing a node in the network underlying corticostriatal pathophysiology in a HD mouse model.

KIF4 regulates neuronal morphology and seizure susceptibility via the PARP1 signaling pathway

Epilepsy is a common neurological disease worldwide, and one of its causes is genetic abnormalities. Here, we identified a point mutation in KIF4A, a member of kinesin superfamily molecular motors, in patients with neurological disorders such as epilepsy, developmental delay, and intellectual disability. KIF4 is involved in the poly (ADP-ribose) polymerase (PARP) signaling pathway, and the mutation (R728Q) strengthened its affinity with PARP1 through elongation of the KIF4 coiled-coil domain. Behavioral tests showed that KIF4-mutant mice exhibited mild developmental delay with lower seizure threshold. Further experiments revealed that the KIF4 mutation caused aberrant morphology in dendrites and spines of hippocampal pyramidal neurons through PARP1-TrkB-KCC2 pathway. Furthermore, supplementing NAD, which activates PARP1, could modulate the TrkB-KCC2 pathway and rescue the seizure susceptibility phenotype of the mutant mice. Therefore, these findings indicate that KIF4 is engaged in a fundamental mechanism regulating seizure susceptibility and could be a potential target for epilepsy treatment.

Failure to Thrive: Impaired BDNF Transport along the Cortical-Striatal Axis in Mouse Q140 Neurons of Huntington's Disease

Boosting trophic support to striatal neurons by increasing levels of brain-derived neurotrophic factor (BDNF) has been considered as a target for therapeutic intervention for several neurodegenerative diseases, including Huntington's disease (HD). To aid in the implementation of such a strategy, a thorough understanding of BDNF cortical-striatal transport is critical to help guide its strategic delivery. In this manuscript, we investigate the dynamic behavior of BDNF transport along the cortical-striatal axis in Q140 primary neurons, a mouse model for HD. We examine this by using single-molecule labeling of BDNF conjugated with quantum dots (QD-BDNF) to follow the transport along the cortical-striatal axis in a microfluidic chamber system specifically designed for the co-culture of cortical and striatal primary neurons. Using this approach, we observe a defect of QD-BDNF transport in Q140 neurons. Our study demonstrates that QD-BDNF transport along the cortical-striatal axis involves the impairment of anterograde transport within axons of cortical neurons, and of retrograde transport within dendrites of striatal neurons. One prominent feature we observe is the extended pause time of QD-BDNF retrograde transport within Q140 striatal dendrites. Taken together, these finding support the hypothesis that delinquent spatiotemporal trophic support of BDNF to striatal neurons, driven by impaired transport, may contribute to the pathogenesis of HD, providing us with insight into how a BDNF supplementation therapeutic strategy may best be applied for HD.

The Emerging Landscape of Natural Small-molecule Therapeutics for Huntington's Disease

Huntington's disease (HD) is a rare and fatal neurodegenerative disorder with no diseasemodifying therapeutics. HD is characterized by extensive neuronal loss and is caused by the inherited expansion of the huntingtin (HTT) gene that encodes a toxic mutant HTT (mHTT) protein having expanded polyglutamine (polyQ) residues. Current HD therapeutics only offer symptomatic relief. In fact, Food and Drug Administration (FDA) approved two synthetic small-molecule VMAT2 inhibitors, tetrabenazine (1) and deutetrabenazine (2), for managing HD chorea and various other diseases in clinical trials. Therefore, the landscape of drug discovery programs for HD is evolving to discover disease- modifying HD therapeutics. Likewise, numerous natural products are being evaluated at different stages of clinical development and have shown the potential to ameliorate HD pathology. The inherent anti-inflammatory and antioxidant properties of natural products mitigate the mHTT-induced oxidative stress and neuroinflammation, improve mitochondrial functions, and augment the anti-apoptotic and pro-autophagic mechanisms for increased survival of neurons in HD. In this review, we have discussed HD pathogenesis and summarized the anti-HD clinical and pre-clinical natural products, focusing on their therapeutic effects and neuroprotective mechanism/s.

Pharmacological Co-Activation of TrkB and TrkC Receptor Signaling Ameliorates Striatal Neuropathology and Motor Deficits in Mouse Models of Huntington's Disease

BACKGROUND

Loss of neurotrophic support in the striatum, particularly reduced brain-derived neurotrophic factor (BDNF) levels, contributes importantly to Huntington's disease (HD) pathogenesis. Another neurotrophin (NT), NT-3, is reduced in the cortex of HD patients; however, its role in HD is unknown. BDNF and NT-3 bind with high affinity to the tropomyosin receptor-kinases (Trk) B and TrkC, respectively. Targeting TrkB/TrkC may be an effective HD therapeutic strategy, as multiple links exist between their signaling pathways and HD degenerative mechanisms. We developed a small molecule ligand, LM22B-10, that activates TrkB and TrkC to promote cell survival.

OBJECTIVE

This study aimed to determine if upregulating TrkB/TrkC signaling with LM22B-10 would alleviate the HD phenotype in R6/2 and Q140 mice.

METHODS

LM22B-10 was delivered by concomitant intranasal-intraperitoneal routes to R6/2 and Q140 mice and then motor performance and striatal pathology were evaluated.

RESULTS

NT-3 levels, TrkB/TrkC phosphorylation, and AKT signaling were reduced in the R6/2 striatum; LM22B-10 counteracted these deficits. LM22B-10 also reduced intranuclear huntingtin aggregates, dendritic spine loss, microglial activation, and degeneration of dopamine- and cyclic AMP-regulated phosphoprotein with a molecular weight of 32 kDa (DARPP-32) and parvalbumin-containing neurons in the R6/2 and/or Q140 striatum. Moreover, both HD mouse models showed improved motor performance after LM22B-10 treatment.

CONCLUSIONS

These results reveal an NT-3/TrkC signaling deficiency in the striatum of R6/2 mice, support the idea that targeting TrkB/TrkC alleviates HD-related neurodegeneration and motor dysfunction, and suggest a novel, disease-modifying, multi-target strategy for treating HD.

The endocytosis, trafficking, sorting and signaling of neurotrophic receptors

Neurotrophins are soluble factors secreted by neurons themselves as well as by post-synaptic target tissues. Neurotrophic signaling regulates several processes such as neurite growth, neuronal survival and synaptogenesis. In order to signal, neurotrophins bind to their receptors, the tropomyosin receptor tyrosine kinase (Trk), which causes internalization of the ligand-receptor complex. Subsequently, this complex is routed into the endosomal system from where Trks can start their downstream signaling. Depending on their endosomal localization, co-receptors involved, but also due to the expression patterns of adaptor proteins, Trks regulate a variety of mechanisms. In this chapter, I provide an overview of the endocytosis, trafficking, sorting and signaling of neurotrophic receptors.

Intellectual enrichment and genetic modifiers of cognition and brain volume in Huntington's disease

An important step towards the development of treatments for cognitive impairment in ageing and neurodegenerative diseases is to identify genetic and environmental modifiers of cognitive function and understand the mechanism by which they exert an effect. In Huntington's disease, the most common autosomal dominant dementia, a small number of studies have identified intellectual enrichment, i.e. a cognitively stimulating lifestyle and genetic polymorphisms as potential modifiers of cognitive function. The aim of our study was to further investigate the relationship and interaction between genetic factors and intellectual enrichment on cognitive function and brain atrophy in Huntington's disease. For this purpose, we analysed data from Track-HD, a multi-centre longitudinal study in Huntington's disease gene carriers and focused on the role of intellectual enrichment (estimated at baseline) and the genes FAN1, MSH3, BDNF, COMT and MAPT in predicting cognitive decline and brain atrophy. We found that carrying the 3a allele in the MSH3 gene had a positive effect on global cognitive function and brain atrophy in multiple cortical regions, such that 3a allele carriers had a slower rate of cognitive decline and atrophy compared with non-carriers, in agreement with its role in somatic instability. No other genetic predictor had a significant effect on cognitive function and the effect of MSH3 was independent of intellectual enrichment. Intellectual enrichment also had a positive effect on cognitive function; participants with higher intellectual enrichment, i.e. those who were better educated, had higher verbal intelligence and performed an occupation that was intellectually engaging, had better cognitive function overall, in agreement with previous studies in Huntington's disease and other dementias. We also found that intellectual enrichment interacted with the BDNF gene, such that the positive effect of intellectual enrichment was greater in Met66 allele carriers than non-carriers. A similar relationship was also identified for changes in whole brain and caudate volume; the positive effect of intellectual enrichment was greater for Met66 allele carriers, rather than for non-carriers. In summary, our study provides additional evidence for the beneficial role of intellectual enrichment and carrying the 3a allele in MSH3 in cognitive function in Huntington's disease and their effect on brain structure.

Pridopidine rescues BDNF/TrkB trafficking dynamics and synapse homeostasis in a Huntington disease brain-on-a-chip model

Huntington disease (HD) is a neurodegenerative disorder caused by polyglutamine-encoding CAG repeat expansion in the huntingtin (HTT) gene. HTT is involved in the axonal transport of vesicles containing brain-derived neurotrophic factor (BDNF). In HD, diminished BDNF transport leads to reduced BDNF delivery to the striatum, contributing to striatal and cortical neuronal death. Pridopidine is a selective and potent sigma-1 receptor (S1R) agonist currently in clinical development for HD. The S1R is located at the endoplasmic reticulum (ER)-mitochondria interface, where it regulates key cellular pathways commonly impaired in neurodegenerative diseases. We used a microfluidic device that reconstitutes the corticostriatal network, allowing the investigation of presynaptic dynamics, synaptic morphology and transmission, and postsynaptic signaling. Culturing primary neurons from the HD mouse model Hdh^CAG140/+ provides a "disease-on-a-chip" platform ideal for investigating pathogenic mechanisms and drug activity. Pridopidine rescued the trafficking of BDNF and TrkB resulting in an increased neurotrophin signaling at the synapse. This increased the capacity of HD neurons to release glutamate and restored homeostasis at the corticostriatal synapse. These data suggest that pridopidine enhances the availability of corticostriatal BDNF via S1R activation, leading to neuroprotective effects.

Pathogenesis of Huntington's Disease: An Emphasis on Molecular Pathways and Prevention by Natural Remedies

BACKGROUND

Huntington's disease is an inherited autosomal dominant trait neuro-degenerative disorder caused by changes (mutations) of a gene called huntingtin (htt) that is located on the short arm (p) of chromosome 4, CAG expansion mutation. It is characterized by unusual movements, cognitive and psychiatric disorders.

OBJECTIVE

This review was undertaken to apprehend biological pathways of Huntington's disease (HD) pathogenesis and its management by nature-derived products. Natural products can be lucrative for the management of HD as it shows protection against HD in pre-clinical trials. Advanced research is still required to assess the therapeutic effectiveness of the known organic products and their isolated compounds in HD experimental models.

SUMMARY

Degeneration of neurons in Huntington's disease is distinguished by progressive loss of motor coordination and muscle function. This is due to the expansion of CAG trinucleotide in the first exon of the htt gene responsible for neuronal death and neuronal network degeneration in the brain. It is believed that the factors such as molecular genetics, oxidative stress, excitotoxicity, mitochondrial dysfunction, neuroglia dysfunction, protein aggregation, and altered UPS leads to HD. The defensive effect of the natural product provides therapeutic efficacy against HD. Recent reports on natural drugs have enlightened the protective role against HD via antioxidant, anti-inflammatory, antiapoptotic, and neurofunctional regulation.

Early Changes in Striatal Activity and Motor Kinematics in a Huntington's Disease Mouse Model

BACKGROUND

Huntington's disease is a progressive neurodegenerative disorder with no disease-modifying treatments. Patients experience motor, cognitive, and psychiatric disturbances, and the dorsal striatum is the main target of neurodegeneration. Mouse models of Huntington's disease show altered striatal synaptic signaling in vitro, but how these changes relate to behavioral deficits in vivo is unclear.

OBJECTIVES

We aimed to investigate how striatal activity correlates with behavior in vivo during motor learning and spontaneous behavior in a Huntington's disease mouse model at two disease stages.

METHODS

We used fiber photometry to record jGCaMP7f fluorescence, a read-out of neuronal activity, in the dorsal striatum of YAC128 (yeast artificial chromosome-128CAG) mice during accelerating rotarod and open-field behavior.

RESULTS

Mice showed increased striatal activity on the rotarod, which diminished by late stages of learning, leading to an inverse correlation between latency to fall and striatal activity. The 2- to 3-month-old YAC128 mice did not show a deficit in latency to fall, but displayed significant differences in paw kinematics, including increased paw slip frequency and variability in paw height. These mice exhibited a weaker correlation between latency to fall and striatal activity and aberrant striatal activity during paw slips. At 6 to 7 months, the YAC128 mice showed significantly reduced latency to fall, impaired paw kinematics, and increased striatal activity while on the rotarod. In the open field, the YAC128 mice showed elevated neuronal activity at rest.

CONCLUSIONS

We uncovered impaired motor coordination at a stage thought to be premotor manifest in YAC128 mice and aberrant striatal activity during the accelerating rotarod and open-field exploration. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

New Frontiers in Neurodegeneration and Regeneration Associated with Brain-Derived Neurotrophic Factor and the rs6265 Single Nucleotide Polymorphism

Brain-derived neurotrophic factor is an extensively studied neurotrophin implicated in the pathology of multiple neurodegenerative and psychiatric disorders including, but not limited to, Parkinson's disease, Alzheimer's disease, Huntington's disease, traumatic brain injury, major de-pressive disorder, and schizophrenia. Here we provide a brief summary of current knowledge on the role of BDNF and the common human single nucleotide polymorphism, rs6265, in driving the pathogenesis and rehabilitation in these disorders, as well as the status of BDNF-targeted therapies. A common trend has emerged correlating low BDNF levels, either detected within the central nervous system or peripherally, to disease states, suggesting that BDNF replacement therapies may hold clinical promise. In addition, we introduce evidence for a distinct role of the BDNF pro-peptide as a biologically active ligand and the need for continuing studies on its neurological function outside of that as a molecular chaperone. Finally, we highlight the latest research describing the role of rs6265 expression in mechanisms of neurodegeneration as well as paradoxical advances in the understanding of this genetic variant in neuroregeneration. All of this is discussed in the context of personalized medicine, acknowledging there is no "one size fits all" therapy for neurodegenerative or psychiatric disorders and that continued study of the multiple BDNF isoforms and genetic variants represents an avenue for discovery ripe with therapeutic potential.

Signal Transduction Regulators in Axonal Regeneration

Intracellular signal transduction in response to growth factor receptor activation is a fundamental process during the regeneration of the nervous system. In this context, intracellular inhibitors of neuronal growth factor signaling have become of great interest in the recent years. Among them are the prominent signal transduction regulators Sprouty (SPRY) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN), which interfere with major signaling pathways such as extracellular signal-regulated kinase (ERK) or phosphoinositide 3-kinase (PI3K)/Akt in neurons and glial cells. Furthermore, SPRY and PTEN are themselves tightly regulated by ubiquitin ligases such as c-casitas b-lineage lymphoma (c-CBL) or neural precursor cell expressed developmentally down-regulated protein 4 (NEDD4) and by different microRNAs (miRs) including miR-21 and miR-222. SPRY, PTEN and their intracellular regulators play an important role in the developing and the lesioned adult central and peripheral nervous system. This review will focus on the effects of SPRY and PTEN as well as their regulators in various experimental models of axonal regeneration in vitro and in vivo. Targeting these signal transduction regulators in the nervous system holds great promise for the treatment of neurological injuries in the future.

RhoA Signaling in Neurodegenerative Diseases

Ras homolog gene family member A (RhoA) is a small GTPase of the Rho family involved in regulating multiple signal transduction pathways that influence a diverse range of cellular functions. RhoA and many of its downstream effector proteins are highly expressed in the nervous system, implying an important role for RhoA signaling in neurons and glial cells. Indeed, emerging evidence points toward a role of aberrant RhoA signaling in neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. In this review, we summarize the current knowledge of RhoA regulation and downstream cellular functions with an emphasis on the role of RhoA signaling in neurodegenerative diseases and the therapeutic potential of RhoA inhibition in neurodegeneration.

Altered cortical processing of sensory input in Huntington disease mouse models

Huntington disease (HD), a hereditary neurodegenerative disorder, manifests as progressively impaired movement and cognition. Although early abnormalities of neuronal activity in striatum are well established in HD models, there are fewer in vivo studies of the cortex. Here, we record local field potentials (LFPs) in YAC128 HD model mice versus wild-type mice. In multiple cortical areas, limb sensory stimulation evokes a greater change in LFP power in YAC128 mice. Mesoscopic imaging using voltage-sensitive dyes reveals more extensive spread of evoked sensory signals across the cortical surface in YAC128 mice. YAC128 layer 2/3 sensory cortical neurons ex vivo show increased excitatory events, which could contribute to enhanced sensory responses in vivo. Cortical LFP responses to limb stimulation, visual and auditory input are also significantly increased in zQ175 HD mice. Results presented here extend knowledge of HD beyond ex vivo studies of individual neurons to the intact cortical network.

Uninterrupted CAG repeat drives striatum-selective transcriptionopathy and nuclear pathogenesis in human Huntingtin BAC mice

In Huntington's disease (HD), the uninterrupted CAG repeat length, but not the polyglutamine length, predicts disease onset. However, the underlying pathobiology remains unclear. Here, we developed bacterial artificial chromosome (BAC) transgenic mice expressing human mutant huntingtin (mHTT) with uninterrupted, and somatically unstable, CAG repeats that exhibit progressive disease-related phenotypes. Unlike prior mHTT transgenic models with stable, CAA-interrupted, polyglutamine-encoding repeats, BAC-CAG mice show robust striatum-selective nuclear inclusions and transcriptional dysregulation resembling those in murine huntingtin knockin models and HD patients. Importantly, the striatal transcriptionopathy in HD models is significantly correlated with their uninterrupted CAG repeat length but not polyglutamine length. Finally, among the pathogenic entities originating from mHTT genomic transgenes and only present or enriched in the uninterrupted CAG repeat model, somatic CAG repeat instability and nuclear mHTT aggregation are best correlated with early-onset striatum-selective molecular pathogenesis and locomotor and sleep deficits, while repeat RNA-associated pathologies and repeat-associated non-AUG (RAN) translation may play less selective or late pathogenic roles, respectively.

Neuroinflammation as the Underlying Mechanism of Postoperative Cognitive Dysfunction and Therapeutic Strategies

Postoperative cognitive dysfunction (POCD) is a common neurological complication following surgery and general anesthesia, especially in elderly patients. Severe cases delay patient discharge, affect the patient’s quality of life after surgery, and are heavy burdens to society. In addition, as the population ages, surgery is increasingly used for older patients and those with higher prevalences of complications. This trend presents a huge challenge to the current healthcare system. Although studies on POCD are ongoing, the underlying pathogenesis is still unclear due to conflicting results and lack of evidence. According to existing studies, the occurrence and development of POCD are related to multiple factors. Among them, the pathogenesis of neuroinflammation in POCD has become a focus of research in recent years, and many clinical and preclinical studies have confirmed the correlation between neuroinflammation and POCD. In this article, we reviewed how central nervous system inflammation occurred, and how it could lead to POCD with changes in peripheral circulation and the pathological pathways between peripheral circulation and the central nervous system (CNS). Furthermore, we proposed some potential therapeutic targets, diagnosis and treatment strategies at the cellular and molecular levels, and clinical applications. The goal of this article was to provide a better perspective for understanding the occurrence of POCD, its development, and preventive strategies to help manage these vulnerable geriatric patients.

Dysregulation of the Basal Ganglia Indirect Pathway in Early Symptomatic Q175 Huntington's Disease Mice

The debilitating psychomotor symptoms of Huntington's disease (HD) are linked partly to degeneration of the basal ganglia indirect pathway. At early symptomatic stages, before major cell loss, indirect pathway neurons exhibit numerous cellular and synaptic changes in HD and its models. However, the impact of these alterations on circuit activity remains poorly understood. To address this gap, optogenetic- and reporter-guided electrophysiological interrogation was used in early symptomatic male and female Q175 HD mice. D2 dopamine receptor-expressing striatal projection neurons (D2-SPNs) were hypoactive during synchronous cortical slow-wave activity, consistent with known reductions in dendritic excitability and cortical input strength. Downstream prototypic parvalbumin-expressing external globus pallidus (PV+ GPe) neurons discharged at 2-3 times their normal rate, even during periods of D2-SPN inactivity, arguing that defective striatopallidal inhibition was not the only cause of their hyperactivity. Indeed, PV+ GPe neurons also exhibited abnormally elevated autonomous firing ex vivo Optogenetic inhibition of PV+ GPe neurons in vivo partially and fully ameliorated the abnormal hypoactivity of postsynaptic subthalamic nucleus (STN) and putative PV- GPe neurons, respectively. In contrast to STN neurons whose autonomous firing is impaired in HD mice, putative PV- GPe neuron activity was unaffected ex vivo, implying that excessive inhibition was responsible for their hypoactivity in vivo Together with previous studies, these data demonstrate that (1) indirect pathway nuclei are dysregulated in Q175 mice through changes in presynaptic activity and/or intrinsic cellular and synaptic properties; and (2) prototypic PV+ GPe neuron hyperactivity and excessive target inhibition are prominent features of early HD pathophysiology.SIGNIFICANCE STATEMENT The early symptoms of Huntington's disease (HD) are linked to degenerative changes in the action-suppressing indirect pathway of the basal ganglia. Consistent with this linkage, the intrinsic properties of cells in this pathway exhibit complex alterations in HD and its models. However, the impact of these changes on activity is poorly understood. Using electrophysiological and optogenetic approaches, we demonstrate that the indirect pathway is highly dysregulated in early symptomatic HD mice through changes in upstream activity and/or intrinsic properties. Furthermore, we reveal that hyperactivity of external globus pallidus neurons and excessive inhibition of their targets are key features of early HD pathophysiology. Together, these findings could help to inform the development and targeting of viral-based, gene therapeutic approaches for HD.

Striatal synaptic adaptations in Parkinson's disease

The striatum is densely innervated by mesencephalic dopaminergic neurons that modulate acquisition and vigor of goal-directed actions and habits. This innervation is progressively lost in Parkinson's disease (PD), contributing to the defining movement deficits of the disease. Although boosting dopaminergic signaling with levodopa early in the course of the disease alleviates these deficits, later this strategy leads to the emergence of debilitating dyskinesia. Here, recent advances in our understanding of how striatal cells and circuits adapt to this progressive de-innervation and to levodopa therapy are discussed. First, we discuss how dopamine (DA) depletion triggers cell type-specific, homeostatic changes in spiny projection neurons (SPNs) that tend to normalize striatal activity but also lead to disruption of the synaptic architecture sculpted by experience. Second, we discuss the roles played by cholinergic and nitric oxide-releasing interneurons in these adaptations. Third, we examine recent work in freely moving mice suggesting that alterations in the spatiotemporal dynamics of striatal ensembles contributes to PD movement deficits. Lastly, we discuss recently published evidence from a progressive model of PD suggesting that contrary to the classical model, striatal pathway imbalance is necessary but not sufficient to produce frank parkinsonism.

Fading memories in aging and neurodegeneration: Is p75 neurotrophin receptor a culprit?

Aging and age-related neurodegenerative diseases have become one of the major concerns in modern times as cognitive abilities tend to decline when we get older. It is well known that the main cause of this age-related cognitive deficit is due to aberrant changes in cellular, molecular circuitry and signaling pathways underlying synaptic plasticity and neuronal connections. The p75 neurotrophin receptor (p75^NTR) is one of the important mediators regulating the fate of the neurons in the nervous system. Its importance in neuronal apoptosis is well documented. However, the mechanisms involving the regulation of p75^NTR in synaptic plasticity and cognitive function remain obscure, although cognitive impairment has been associated with a higher expression of p75^NTR in neurons. In this review, we discuss the current understanding of how neurons are influenced by p75^NTR function to maintain normal neuronal synaptic strength and connectivity, particularly to support learning and memory in the hippocampus. We then discuss the age-associated alterations in neurophysiological mechanisms of synaptic plasticity and cognitive function. Furthermore, we also describe current evidence that has begun to elucidate how p75^NTR regulates synaptic changes in aging and age-related neurodegenerative diseases, focusing on the hippocampus. Elucidating the role that p75^NTR signaling plays in regulating synaptic plasticity will contribute to a better understanding of cognitive processes and pathological conditions. This will in turn provide novel approaches to improve therapies for the treatment of neurological diseases in which p75^NTR dysfunction has been demonstrated.

Uncoupling the Excitatory Amino Acid Transporter 2 From Its C-Terminal Interactome Restores Synaptic Glutamate Clearance at Corticostriatal Synapses and Alleviates Mutant Huntingtin-Induced Hypokinesia

Rapid removal of glutamate from the sites of glutamate release is an essential step in excitatory synaptic transmission. However, despite many years of research, the molecular mechanisms underlying the intracellular regulation of glutamate transport at tripartite synapses have not been fully uncovered. This limits the options for pharmacological treatment of glutamate-related motor disorders, including Huntington’s disease (HD). We therefore investigated the possible binding partners of transgenic EAAT2 and their alterations under the influence of mutant huntingtin (mHTT). Mass spectrometry analysis after pull-down of striatal YFP-EAAT2 from wild-type (WT) mice and heterozygote (HET) Q175 mHTT-knock-in mice identified a total of 148 significant (FDR < 0.05) binders to full-length EAAT2. Of them 58 proteins exhibited mHTT-related differences. Most important, in 26 of the 58 mHTT-sensitive cases, protein abundance changed back toward WT levels when the mice expressed a C-terminal-truncated instead of full-length variant of EAAT2. These findings motivated new attempts to clarify the role of astrocytic EAAT2 regulation in cortico-basal movement control. Striatal astrocytes of Q175 HET mice were targeted by a PHP.B vector encoding EAAT2 with different degree of C-terminal modification, i.e., EAAT2-S506X (truncation at S506), EAAT2-4KR (4 lysine to arginine substitutions) or EAAT2 (full-length). The results were compared to HET and WT injected with a tag-only vector (CTRL). It was found that the presence of a C-terminal-modified EAAT2 transgene (i) increased the level of native EAAT2 protein in striatal lysates and perisynaptic astrocyte processes, (ii) enhanced the glutamate uptake of transduced astrocytes, (iii) stimulated glutamate clearance at individual corticostriatal synapses, (iv) increased the glutamate uptake of striatal astrocytes and (iv) alleviated the mHTT-related hypokinesia (open field indicators of movement initiation). In contrast, over-expression of full-length EAAT2 neither facilitated glutamate uptake nor locomotion. Together, our results support the new hypothesis that preventing abnormal protein-protein interactions at the C-terminal of EAAT2 could eliminate the mHTT-related deficits in corticostriatal synaptic glutamate clearance and movement initiation.

Calcineurin and huntingtin form a calcium-sensing machinery that directs neurotrophic signals to the nucleus

When a neurotrophin binds at the presynapse, it sends survival signals all the way to the nucleus on signaling endosomes. These endosomes fuel their own journey with on-board glycolysis—but how is that journey initiated and maintained? Using microfluidic devices and mice, we find that the calcium released upon brain-derived neurotrophic factor (BDNF) binding to its receptor, tropomyosin receptor kinase B (TrkB), is sensed by calcineurin on the cytosolic face of the endosome. Calcineurin dephosphorylates huntingtin, the BDNF scaffold, which sets the endosome moving in a retrograde direction. In an in vitro reconstituted microtubule transport system, controlled calcium uncaging prompts purified vesicles to move to the microtubule minus end. We observed similar retrograde waves of TrkA- and epidermal growth factor receptor (EGFR)-bearing endosomes. Signaling endosomes in neurons thus carry not only their own fuel, but their own navigational system.

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.

Nicotinamide Adenine Dinucleotide Phosphate Oxidases Are Everywhere in Brain Disease, but Not in Huntington's Disease?

Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by neuronal loss and tissue atrophy mainly in the striatum and cortex. In the early stages of the disease, impairment of neuronal function, synaptic dysfunction and white matter loss precedes neuronal death itself. Relative to other neurodegenerative diseases such as Alzheimer's and Parkinson's disease and Amyotrophic Lateral Sclerosis, where the effects of either microglia or NADPH oxidases (NOXs) are recognized as important contributors to disease pathogenesis and progression, there is a pronounced lack of information in HD. This information void contrasts with evidence from human HD patients where blood monocytes and microglia are activated well before HD clinical symptoms (PET scans), and the clear signs of oxidative stress and inflammation in post mortem HD brain. Habitually, NOX activity and oxidative stress in the central nervous system (CNS) are equated with microglia, but research of the last two decades has carved out important roles for NOX enzyme function in neurons. Here, we will convey recent information about the function of NOX enzymes in neurons, and contemplate on putative roles of neuronal NOX in HD. We will focus on NOX-produced reactive oxygen species (ROS) as redox signaling molecules in/among neurons, and the specific roles of NOXs in important processes such as neurogenesis and lineage specification, neurite outgrowth and growth cone dynamics, and synaptic plasticity where NMDAR-dependent signaling, and long-term depression/potentiation are redox-regulated phenomena. HD animal models and induced pluripotent stem cell (iPSC) studies have made it clear that the very same physiological processes are also affected in HD, and we will speculate on possible roles for NOX in the pathogenesis and development of disease. Finally, we also take into account the limited information on microglia in HD and relate this to any contribution of NOX enzymes.

Epilepsy in Neurodegenerative Diseases: Related Drugs and Molecular Pathways

Epilepsy is a chronic disease of the central nervous system characterized by an electrical imbalance in neurons. It is the second most prevalent neurological disease, with 50 million people affected around the world, and 30% of all epilepsies do not respond to available treatments. Currently, the main hypothesis about the molecular processes that trigger epileptic seizures and promote the neurotoxic effects that lead to cell death focuses on the exacerbation of the glutamate pathway and the massive influx of Ca2+ into neurons by different factors. However, other mechanisms have been proposed, and most of them have also been described in other neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, or multiple sclerosis. Interestingly, and mainly because of these common molecular links and the lack of effective treatments for these diseases, some antiseizure drugs have been investigated to evaluate their therapeutic potential in these pathologies. Therefore, in this review, we thoroughly investigate the common molecular pathways between epilepsy and the major neurodegenerative diseases, examine the incidence of epilepsy in these populations, and explore the use of current and innovative antiseizure drugs in the treatment of refractory epilepsy and other neurodegenerative diseases.

CalDAG-GEFI mediates striatal cholinergic modulation of dendritic excitability, synaptic plasticity and psychomotor behaviors

CalDAG-GEFI (CDGI) is a protein highly enriched in the striatum, particularly in the principal spiny projection neurons (SPNs). CDGI is strongly down-regulated in two hyperkinetic conditions related to striatal dysfunction: Huntington’s disease and levodopa-induced dyskinesia in Parkinson’s disease. We demonstrate that genetic deletion of CDGI in mice disrupts dendritic, but not somatic, M1 muscarinic receptors (M1Rs) signaling in indirect pathway SPNs. Loss of CDGI reduced temporal integration of excitatory postsynaptic potentials at dendritic glutamatergic synapses and impaired the induction of activity-dependent long-term potentiation. CDGI deletion selectively increased psychostimulant-induced repetitive behaviors, disrupted sequence learning, and eliminated M1R blockade of cocaine self-administration. These findings place CDGI as a major, but previously unrecognized, mediator of cholinergic signaling in the striatum. The effects of CDGI deletion on the self-administration of drugs of abuse and its marked alterations in hyperkinetic extrapyramidal disorders highlight CDGI’s therapeutic potential.

Cell Reprogramming to Model Huntington's Disease: A Comprehensive Review

Huntington’s disease (HD) is a neurodegenerative disorder characterized by the progressive decline of motor, cognitive, and psychiatric functions. HD results from an autosomal dominant mutation that causes a trinucleotide CAG repeat expansion and the production of mutant Huntingtin protein (mHTT). This results in the initial selective and progressive loss of medium spiny neurons (MSNs) in the striatum before progressing to involve the whole brain. There are currently no effective treatments to prevent or delay the progression of HD as knowledge into the mechanisms driving the selective degeneration of MSNs has been hindered by a lack of access to live neurons from individuals with HD. The invention of cell reprogramming provides a revolutionary technique for the study, and potential treatment, of neurological conditions. Cell reprogramming technologies allow for the generation of live disease-affected neurons from patients with neurological conditions, becoming a primary technique for modelling these conditions in vitro. The ability to generate HD-affected neurons has widespread applications for investigating the pathogenesis of HD, the identification of new therapeutic targets, and for high-throughput drug screening. Cell reprogramming also offers a potential autologous source of cells for HD cell replacement therapy. This review provides a comprehensive analysis of the use of cell reprogramming to model HD and a discussion on recent advancements in cell reprogramming technologies that will benefit the HD field.