Huntington's disease: a synaptopathy?

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.

Toxic effects of mutant huntingtin in axons are mediated by its proline-rich domain

Huntington's disease results from expansion of a polyglutamine tract (polyQ) in mutant huntingtin (mHTT) protein, but mechanisms underlying polyQ expansion-mediated toxic gain-of-mHTT function remain elusive. Here, deletion and antibody-based experiments revealed that a proline-rich domain (PRD) adjacent to the polyQ tract is necessary for mHTT to inhibit fast axonal transport and promote axonal pathology in cultured mammalian neurons. Further, polypeptides corresponding to subregions of the PRD sufficed to elicit the toxic effect on fast axonal transport, which was mediated by c-Jun N-terminal kinases (JNKs) and involved PRD binding to one or more SH3-domain containing proteins. Collectively, these data suggested a mechanism whereby polyQ tract expansion in mHTT promotes aberrant PRD exposure and interactions of this domain with SH3 domain-containing proteins including some involved in activation of JNKs. In support, biochemical and immunohistochemical experiments linked aberrant PRD exposure to increased JNK activation in striatal tissues of the zQ175 mouse model and from post-mortem Huntington's disease patients. Together, these findings support a critical role of PRD on mHTT toxicity, suggesting a novel framework for the potential development of therapies aimed to halt or reduce axonal pathology in Huntington's disease.

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.

Estimating the synaptic density deficit in Alzheimer's disease using multi-contrast CEST imaging

In vivo noninvasive imaging of neurometabolites is crucial to improve our understanding of the underlying pathophysiological mechanism in neurodegenerative diseases. Abnormal changes in synaptic organization leading to synaptic degradation and neuronal loss is considered as one of the primary factors driving Alzheimer's disease pathology. Magnetic resonance based molecular imaging techniques such as chemical exchange saturation transfer (CEST) and magnetic resonance spectroscopy (MRS) can provide neurometabolite specific information which may relate to underlying pathological and compensatory mechanisms. In this study, CEST and short echo time single voxel MRS was performed to evaluate the sensitivity of cerebral metabolites to beta-amyloid (Aβ) induced synaptic deficit in the hippocampus of a mouse model of Alzheimer's disease. The CEST based spectra (Z-spectra) were acquired on a 9.4 Tesla small animal MR imaging system with two radiofrequency (RF) saturation amplitudes (1.47 μT and 5.9 μT) to obtain creatine-weighted and glutamate-weighted CEST contrasts, respectively. Multi-pool Lorentzian fitting and quantitative T1 longitudinal relaxation maps were used to obtain metabolic specific apparent exchange-dependent relaxation (AREX) maps. Short echo time (TE = 12 ms) single voxel MRS was acquired to quantify multiple neurometabolites from the right hippocampus region. AREX contrasts and MRS based metabolite concentration levels were examined in the ARTE10 animal model for Alzheimer's disease and their wild type (WT) littermate counterparts (age = 10 months). Using MRS voxel as a region of interest, group-wise analysis showed significant reduction in Glu-AREX and Cr-AREX in ARTE10, compared to WT animals. The MRS based results in the ARTE10 mice showed significant decrease in glutamate (Glu) and glutamate-total creatine (Glu/tCr) ratio, compared to WT animals. The MRS results also showed significant increase in total creatine (tCr), phosphocreatine (PCr) and glutathione (GSH) concentration levels in ARTE10, compared to WT animals. In the same ROI, Glu-AREX and Cr-AREX demonstrated positive associations with Glu/tCr ratio. These results indicate the involvement of neurotransmitter metabolites and energy metabolism in Aβ-mediated synaptic degradation in the hippocampus region. The study also highlights the feasibility of CEST and MRS to identify and track multiple competing and compensatory mechanisms involved in heterogeneous pathophysiology of Alzheimer's disease in vivo.

A clinical study and future prospects for bioactive compounds and semi-synthetic molecules in the therapies for Huntington's disease

A neurodegenerative disorder (ND) refers to Huntington's disease (HD) which affects memory loss, weight loss, and movement dysfunctions such as chorea and dystonia. In the striatum and brain, HD most typically impacts medium-spiny neurons. Molecular genetics, excitotoxicity, oxidative stress (OS), mitochondrial, and metabolic dysfunction are a few of the theories advanced to explicit the pathophysiology of neuronal damage and cell death. Numerous in-depth studies of the literature have supported the therapeutic advantages of natural products in HD experimental models and other treatment approaches. This article briefly discusses the neuroprotective impacts of natural compounds against HD models. The ability of the discovered natural compounds to suppress HD was tested using either in vitro or in vivo models. Many bioactive compounds considerably lessened the memory loss and motor coordination brought on by 3-nitropropionic acid (3-NP). Reduced lipid peroxidation, increased endogenous enzymatic antioxidants, reduced acetylcholinesterase activity, and enhanced mitochondrial energy generation have profoundly decreased the biochemical change. It is significant since histology showed that therapy with particular natural compounds lessened damage to the striatum caused by 3-NP. Moreover, natural products displayed varying degrees of neuroprotection in preclinical HD studies because of their antioxidant and anti-inflammatory properties, maintenance of mitochondrial function, activation of autophagy, and inhibition of apoptosis. This study highlighted about the importance of bioactive compounds and their semi-synthetic molecules in the treatment and prevention of HD.

Lifestyle strategies to promote proteostasis and reduce the risk of Alzheimer's disease and other proteinopathies

Unhealthy lifestyle choices, poor diet, and aging can have negative influences on cognition, gradually increasing the risk for mild cognitive impairment (MCI) and the continuum comprising early dementia. Aging is the greatest risk factor for age-related dementias such as Alzheimer's disease, and the aging process is known to be influenced by life events that can positively or negatively affect age-related diseases. Remarkably, life experiences that make the brain vulnerable to dementia, such as seizure episodes, neurotoxin exposures, metabolic disorders, and trauma-inducing events (e.g. traumatic injuries or mild neurotrauma from a fall or blast exposure), have been associated with negative effects on proteostasis and synaptic integrity. Functional compromise of the autophagy-lysosomal pathway, a major contributor to proteostasis, has been implicated in Alzheimer's disease, Parkinson's disease, obesity-related pathology, Huntington's disease, as well as in synaptic degeneration which is the best correlate of cognitive decline. Correspondingly, pharmacological and non-pharmacological strategies that positively modulate lysosomal proteases are recognized as synaptoprotective through degradative clearance of pathogenic proteins. Here, we discuss life-associated vulnerabilities that influence key hallmarks of brain aging and the increased burden of age-related dementias. Additionally, we discuss exercise and diet among the lifestyle strategies that regulate proteostasis as well as synaptic integrity, leading to evident prevention of cognitive deficits during brain aging in pre-clinical models.

α-Synuclein induces deficiency in clathrin-mediated endocytosis through inhibiting synaptojanin1 expression

Parkinson's disease (PD) is an age-related chronic neurological disorder, mainly characterized by the pathological feature of α-synuclein (α-syn) aggregation, with the exact disease pathogenesis unclear. During the onset and progression of PD, synaptic dysfunction, including dysregulation of axonal transport, impaired exocytosis, and endocytosis are identified as crucial events of PD pathogenesis. It has been reported that over-expression of α-syn impairs clathrin-mediated endocytosis (CME) in the synapses. However, the underlying mechanisms still needs to be explored. In this study, we investigated the molecular events underlying the synaptic dysfunction caused by over-expression of wild-type human α-syn and its mutant form, involving series of proteins participating in CME. We found that excessive human α-syn causes impaired fission and uncoating of clathrin-coated vesicles during synaptic vesicle recycling, leading to reduced clustering of synaptic vesicles near the active zone and increased size of plasma membrane and number of endocytic intermediates. Furthermore, over-expressed human α-syn induced changes of CME-associated proteins, among which synaptojanin1 (SYNJ1) showed significant reduction in various brain regions. Over-expression of SYNJ1 in primary hippocampal neurons from α-syn transgenic mice recovered the synaptic vesicle density, clustering and endocytosis. Using fluorescence-conjugated transferrin, we demonstrated that SYNJ1 re-boosted the CME activity by restoring the phosphatidylinositol-4,5-bisphosphate homeostasis. Our data suggested that over-expression of α-syn disrupts synaptic function through interfering with vesicle recycling, which could be alleviated by re-availing of SYNJ1. Our study unrevealed a molecular mechanism of the synaptic dysfunction in PD pathogenesis and provided a potential therapeutic target for treating PD.

Proteotoxic stresses stimulate dissociation of UBL4A from the tail-anchored protein recognition complex

Inclusion body formation is associated with cytotoxicity in a number of neurodegenerative diseases. However, the molecular basis of the toxicity caused by the accumulation of aggregation-prone proteins remains controversial. In this study, we found that disease-associated inclusions induced by elongated polyglutamine chains disrupt the complex formation of BAG6 with UBL4A, a mammalian homologue of yeast Get5. UBL4A also dissociated from BAG6 in response to proteotoxic stresses such as proteasomal inhibition and mitochondrial depolarization. These findings imply that the cytotoxicity of pathological protein aggregates might be attributed in part to disruption of the BAG6-UBL4A complex that is required for the biogenesis of tail-anchored proteins.

Comprehensive Insights into Biological Roles of Rosmarinic Acid: Implications in Diabetes, Cancer and Neurodegenerative Diseases

Phytochemicals are abundantly occurring natural compounds extracted from plant sources. Rosmarinic acid (RA) is an abundant phytochemical of Lamiaceae species with various therapeutic implications for human health. In recent years, natural compounds have gained significant attention as adjuvant and complementary therapies to existing medications for various diseases. RA has gained popularity due to its anti-inflammatory and antioxidant properties and its roles in various life-threatening conditions, such as cancer, neurodegeneration, diabetes, etc. The present review aims to offer a comprehensive insight into the multifaceted therapeutic properties of RA, including its potential as an anticancer agent, neuroprotective effects, and antidiabetic potential. Based on the available evidences, RA could be considered a potential dietary component for treating various diseases, including cancer, diabetes and neurodegenerative disorders.

Hyperexcitability and translational phenotypes in a preclinical model of SYNGAP1 mutations

SYNGAP1 is a critical gene for neuronal development, synaptic structure, and function. Although rare, the disruption of SYNGAP1 directly causes a genetically identifiable neurodevelopmental disorder (NDD) called SYNGAP1 -related intellectual disability. Without functional SynGAP1 protein, patients present with intellectual disability, motor impairments, and epilepsy. Previous work using mouse models with a variety of germline and conditional mutations has helped delineate SynGAP1's critical roles in neuronal structure and function, as well as key biochemical signaling pathways essential to synapse integrity. Homozygous loss of SYNGAP1 is embryonically lethal. Heterozygous mutations of SynGAP1 result in a broad range of phenotypes including increased locomotor activity, impaired working spatial memory, impaired cued fear memory, and increased stereotypic behavior. Our in vivo functional data, using the original germline mutation mouse line from the Huganir laboratory, corroborated robust hyperactivity and learning and memory deficits. Here, we describe impairments in the translational biomarker domain of sleep, characterized using neurophysiological data collected with wireless telemetric electroencephalography (EEG). We discovered Syngap1+/- mice exhibited elevated spike trains in both number and duration, in addition to elevated power, most notably in the delta power band. Primary neurons from Syngap1+/- mice displayed increased network firing activity, greater spikes per burst, and shorter inter-burst intervals between peaks using high density micro-electrode arrays (HD-MEA). This work is translational, innovative, and highly significant as it outlines functional impairments in Syngap1 mutant mice. Simultaneously, the work utilized untethered, wireless neurophysiology that can discover potential biomarkers of Syngap1 RI-D, for clinical trials, as it has done with other NDDs. Our work is substantial forward progress toward translational work for SynGAP1R-ID as it bridges in-vitro electrophysiological neuronal activity and function with in vivo neurophysiological brain activity and function. These data elucidate multiple quantitative, translational biomarkers in vivo and in vitro for the development of treatments for SYNGAP1-related intellectual disability.

Altered exocytosis of inhibitory synaptic vesicles at single presynaptic terminals of cultured striatal neurons in a knock-in mouse model of Huntington's disease

Huntington's disease (HD) is a progressive dominantly inherited neurodegenerative disease caused by the expansion of a cytosine-adenine-guanine (CAG) trinucleotide repeat in the huntingtin gene, which encodes the mutant huntingtin protein containing an expanded polyglutamine tract. One of neuropathologic hallmarks of HD is selective degeneration in the striatum. Mechanisms underlying selective neurodegeneration in the striatum of HD remain elusive. Neurodegeneration is suggested to be preceded by abnormal synaptic transmission at the early stage of HD. However, how mutant huntingtin protein affects synaptic vesicle exocytosis at single presynaptic terminals of HD striatal neurons is poorly understood. Here, we measured synaptic vesicle exocytosis at single presynaptic terminals of cultured striatal neurons (mainly inhibitory neurons) in a knock-in mouse model of HD (zQ175) during electrical field stimulation using real-time imaging of FM 1-43 (a lipophilic dye). We found a significant decrease in bouton density and exocytosis of synaptic vesicles at single presynaptic terminals in cultured striatal neurons. Real-time imaging of VGAT-CypHer5E (a pH sensitive dye conjugated to an antibody against vesicular GABA transporter (VGAT)) for inhibitory synaptic vesicles revealed a reduction in bouton density and exocytosis of inhibitory synaptic vesicles at single presynaptic terminals of HD striatal neurons. Thus, our results suggest that the mutant huntingtin protein decreases bouton density and exocytosis of inhibitory synaptic vesicles at single presynaptic terminals of striatal neurons, causing impaired inhibitory synaptic transmission, eventually leading to the neurodegeneration in the striatum of HD.

Hyperexcitability and translational phenotypes in a preclinical model of SYNGAP1 mutations

SYNGAP1 is a critical gene for neuronal development, synaptic structure, and function. Although rare, the disruption of SYNGAP1 directly causes a genetically identifiable neurodevelopmental disorder (NDD) called SYNGAP1-related intellectual disability. Without functional SynGAP1 protein, patients present with intellectual disability, motor impairments, and epilepsy. Previous work using mouse models with a variety of germline and conditional mutations has helped delineate SynGAP1's critical roles in neuronal structure and function, as well as key biochemical signaling pathways essential to synapse integrity. Homozygous loss of SYNGAP1 is embryonically lethal. Heterozygous mutations of SynGAP1 result in a broad range of phenotypes including increased locomotor activity, impaired working spatial memory, impaired cued fear memory, and increased stereotypic behavior. Our in vivo functional data, using the original germline mutation mouse line from the Huganir laboratory, corroborated robust hyperactivity and learning and memory deficits. Here, we describe impairments in the translational biomarker domain of sleep, characterized using neurophysiological data collected with wireless telemetric electroencephalography (EEG). We discovered Syngap1 +/- mice exhibited elevated spike trains in both number and duration, in addition to elevated power, most notably in the delta power band. Primary neurons from Syngap1 +/- mice displayed increased network firing activity, greater spikes per burst, and shorter inter-burst intervals between peaks using high density micro-electrode arrays (HD-MEA). This work is translational, innovative, and highly significant as it outlines functional impairments in Syngap1 mutant mice. Simultaneously, the work utilized untethered, wireless neurophysiology that can discover potential biomarkers of Syngap1R-ID, for clinical trials, as it has done with other NDDs. Our work is substantial forward progress toward translational work for SynGAP1R-ID as it bridges in-vitro electrophysiological neuronal activity and function with in vivo neurophysiological brain activity and function. These data elucidate multiple quantitative, translational biomarkers in vivo and in vitro for the development of treatments for SYNGAP1-related intellectual disability.

Age-dependent increase in tau phosphorylation at serine 396 in Huntington's disease pre-frontal cortex

Background

To date, it is still controversial whether tau phosphorylation plays a role in Huntington's disease (HD), as previous studies demonstrated either no alterations or increases in phosphorylated tau (pTau) in HD post-mortem brain and mouse models.

Objectives

The goal of this study was to determine whether total tau and pTau levels are altered in HD.

Methods

Immunohistochemistry, cellular fractionations, and western blots were used to measure tau and pTau levels in a large cohort of HD and control post-mortem prefrontal cortex (PFC). Furthermore, western blots were performed to assess tau, and pTau levels in HD and control isogenic embryonic stem cell (ESC)-derived cortical neurons and neuronal stem cells (NSCs). Similarly, western blots were used to assess tau and pTau in Htt Q111 and transgenic R6/2 mice. Lastly, total tau levels were assessed in HD and healthy control plasma using Quanterix Simoa assay.

Results

Our results revealed that, while there was no difference in tau or pTau levels in HD PFC compared to controls, tau phosphorylated at S396 levels were increased in PFC samples from HD patients 60 years or older at time of death. Additionally, tau and pTau levels were not changed in HD ESC-derived cortical neurons and NSCs. Similarly, tau or pTau levels were not altered in Htt Q111 and transgenic R6/2 mice compared to wild-type littermates. Lastly, tau levels were not changed in plasma from a small cohort of HD patients compared to controls.

Conclusion

Together these findings demonstrate that pTau-S396 levels increase significantly with age in HD PFC.

Mitochondrial dysfunction in chromaffin cells from the R6/1 mouse model of Huntington's disease: Impact on exocytosis and calcium current regulation

From a pathogenic perspective, Huntington's disease (HD) is being considered as a synaptopathy. As such, alterations in brain neurotransmitter release occur. As the activity of the sympathoadrenal axis is centrally controlled, deficits in the exocytotic release of catecholamine release may also occur. In fact, in chromaffin cells (CCs) of the adrenal medulla of the R6/1 model of HD, decrease of secretion and altered kinetics of the exocytotic fusion pore have been reported. Those alterations could be linked to mitochondrial deficits occurring in peripheral CCs, similar to those described in brain mitochondria. Here we have inquired about alterations in mitochondrial structure and function and their impact on exocytosis and calcium channel currents (I_Ca). We have monitored various parameters linked to those events, in wild type (WT) and the R6/1 mouse model of HD at a pre-disease stage (2 months age, 2 m), and when motor deficits are present (7 months age, 7 m). In isolated CCs from 7 m and in the adrenal medulla of R6/1 mice, we found the following alterations (with respect 7 m WT mice): (i) augmented fragmented mitochondria and oxidative stress with increased oxidized glutathione; (ii) decreased basal and maximal respiration; (iii) diminution of ATP cell levels; (iv) mitochondrial depolarization; (v) drastic decrease of catecholamine release with poorer potentiation by protonophore FCCP; (vi) decreased I_Ca inhibition by FCCP; and (vii) lesser potentiation by BayK8644 of I_Ca and smaller prolongation of current deactivation. Of note was the fact several of these alterations were already manifested in CCs from 2 m R6/1 mice at pre-disease stages. Based on those results, a plausible hypothesis can be raised in the sense that altered mitochondrial function seems to be an early primary event in HD pathogenesis. This is in line with an increasing number of mitochondrial, metabolic, and inflammatory alterations being recently reported in various HD peripheral tissues.

Alzheimer’s disease as a synaptopathy: Evidence for dysfunction of synapses during disease progression

The synapse has consistently been considered a vulnerable and critical target within Alzheimer’s disease, and synapse loss is, to date, one of the main biological correlates of cognitive decline within Alzheimer’s disease. This occurs prior to neuronal loss with ample evidence that synaptic dysfunction precedes this, in support of the idea that synaptic failure is a crucial stage within disease pathogenesis. The two main pathological hallmarks of Alzheimer’s disease, abnormal aggregates of amyloid or tau proteins, have had demonstrable effects on synaptic physiology in animal and cellular models of Alzheimer’s disease. There is also growing evidence that these two proteins may have a synergistic effect on neurophysiological dysfunction. Here, we review some of the main findings of synaptic alterations in Alzheimer’s disease, and what we know from Alzheimer’s disease animal and cellular models. First, we briefly summarize some of the human evidence to suggest that synapses are altered, including how this relates to network activity. Subsequently, animal and cellular models of Alzheimer’s disease are considered, highlighting mouse models of amyloid and tau pathology and the role these proteins may play in synaptic dysfunction, either in isolation or examining how the two pathologies may interact in dysfunction. This specifically focuses on neurophysiological function and dysfunction observed within these animal models, typically measured using electrophysiology or calcium imaging. Following synaptic dysfunction and loss, it would be impossible to imagine that this would not alter oscillatory activity within the brain. Therefore, this review also discusses how this may underpin some of the aberrant oscillatory patterns seen in animal models of Alzheimer’s disease and human patients. Finally, an overview of some key directions and considerations in the field of synaptic dysfunction in Alzheimer’s disease is covered. This includes current therapeutics that are targeted specifically at synaptic dysfunction, but also methods that modulate activity to rescue aberrant oscillatory patterns. Other important future avenues of note in this field include the role of non-neuronal cell types such as astrocytes and microglia, and mechanisms of dysfunction independent of amyloid and tau in Alzheimer’s disease. The synapse will certainly continue to be an important target within Alzheimer’s disease for the foreseeable future.

Increased Synaptic ATP Release and CD73-Mediated Formation of Extracellular Adenosine in the Control of Behavioral and Electrophysiological Modifications Caused by Chronic Stress

Increased ATP release and its extracellular catabolism through CD73 (ecto-5'-nucleotidase) lead to the overactivation of adenosine A_2A receptors (A_2AR), which occurs in different brain disorders. A_2AR blockade blunts mood and memory dysfunction caused by repeated stress, but it is unknown if increased ATP release coupled to CD73-mediated formation of extracellular adenosine is responsible for A_2AR overactivation upon repeated stress. This was now investigated in adult rats subject to repeated stress for 14 consecutive days. Frontocortical and hippocampal synaptosomes from stressed rats displayed an increased release of ATP upon depolarization, coupled to an increased density of vesicular nucleotide transporters and of CD73. The continuous intracerebroventricular delivery of the CD73 inhibitor α,β-methylene ADP (AOPCP, 100 μM) during restraint stress attenuated mood and memory dysfunction. Slice electrophysiological recordings showed that restraint stress decreased long-term potentiation both in prefrontocortical layer II/III-layer V synapses and in hippocampal Schaffer fibers-CA1 pyramid synapses, which was prevented by AOPCP, an effect occluded by adenosine deaminase and by the A_2AR antagonist SCH58261. These results indicate that increased synaptic ATP release coupled to CD73-mediated formation of extracellular adenosine contributes to mood and memory dysfunction triggered by repeated restraint stress. This prompts considering interventions decreasing ATP release and CD73 activity as novel strategies to mitigate the burden of repeated stress.

Hallmarks of neurodegenerative diseases

Decades of research have identified genetic factors and biochemical pathways involved in neurodegenerative diseases (NDDs). We present evidence for the following eight hallmarks of NDD: pathological protein aggregation, synaptic and neuronal network dysfunction, aberrant proteostasis, cytoskeletal abnormalities, altered energy homeostasis, DNA and RNA defects, inflammation, and neuronal cell death. We describe the hallmarks, their biomarkers, and their interactions as a framework to study NDDs using a holistic approach. The framework can serve as a basis for defining pathogenic mechanisms, categorizing different NDDs based on their primary hallmarks, stratifying patients within a specific NDD, and designing multi-targeted, personalized therapies to effectively halt NDDs.

Accumulation of pTau231 at the Postsynaptic Density in Early Alzheimer's Disease

BACKGROUND

Phosphorylated cytoplasmic tau inclusions correlate with and precede cognitive deficits in Alzheimer's disease (AD). However, pathological tau accumulation and relationships to synaptic changes remain unclear.

OBJECTIVE

To address this, we examined postmortem brain from 50 individuals with the full spectrum of AD (clinically and neuropathologically). Total tau, pTau231, and AMPA GluR1 were compared across two brain regions (entorhinal and middle frontal cortices), as well as clinically stratified groups (control, amnestic mild cognitive impairment, AD dementia), NIA-AA Alzheimer's Disease Neuropathologic Change designations (Not, Low, Intermediate, High), and Braak tangle stages (1-6). Significant co-existing pathology was excluded to isolate changes attributed to pathologic AD.

METHODS

Synaptosomal fractionation and staining were performed to measure changes in total Tau, pTau231, and AMPA GluR1. Total Tau and pTau231 were quantified in synaptosomal fractions using Quanterix Simoa HD-X.

RESULTS

Increasing pTau231 in frontal postsynaptic fractions correlated positively with increasing clinical and neuropathological AD severity. Frontal cortex is representative of early AD, as it does not become involved by tau tangles until late in AD. Entorhinal total tau was significantly higher in the amnestic mild cognitive impairment group when compared to AD, but only after accounting for AD associated synaptic changes. Alterations in AMPA GluR1 observed in the entorhinal cortex, but not middle frontal cortex, suggest that pTau231 mislocalization and aggregation in postsynaptic structures may impair glutamatergic signaling by promoting AMPA receptor dephosphorylation and internalization.

CONCLUSION

Results highlight the potential effectiveness of early pharmacological interventions targeting pTau231 accumulation at the postsynaptic density.

Age-Dependent Increase in Tau Phosphorylation at Serine 396 in Huntington's Disease Prefrontal Cortex

BACKGROUND

To date, it is still controversial whether tau phosphorylation plays a role in Huntington's disease (HD), as previous studies demonstrated either no alterations or increases in phosphorylated tau (pTau) in HD postmortem brain and mouse models.

OBJECTIVE

The goal of this study was to determine whether total tau and pTau levels are altered in HD.

METHODS

Immunohistochemistry, cellular fractionations, and western blots were used to measure total tau and pTau levels in a large cohort of HD and control postmortem prefrontal cortex (PFC). Furthermore, western blots were performed to assess tau, and pTau levels in HD and control isogenic embryonic stem cell (ESC)-derived cortical neurons and neuronal stem cells (NSCs). Similarly, western blots were used to assess tau and pTau levels in HttQ111 and transgenic R6/2 mice. Lastly, total tau levels were assessed in HD and healthy control plasma using Quanterix Simoa assay.

RESULTS

Our results revealed that, while there was no difference in total tau or pTau levels in HD PFC compared to controls, the levels of tau phosphorylated at S396 were increased in PFC samples from HD patients 60 years or older at time of death. Additionally, tau and pTau levels were not changed in HD ESC-derived cortical neurons and NSCs. Similarly, total tau or pTau levels were not altered in HttQ111 and transgenic R6/2 mice compared to wild-type littermates. Lastly, tau levels were not changed in plasma from a small cohort of HD patients compared to controls.

CONCLUSIONS

Together these findings demonstrate that pTau-S396 levels increase significantly with age in HD PFC.

An Optimized Comparative Proteomic Approach as a Tool in Neurodegenerative Disease Research

Recent advances in proteomic technologies now allow unparalleled assessment of the molecular composition of a wide range of sample types. However, the application of such technologies and techniques should not be undertaken lightly. Here, we describe why the design of a proteomics experiment itself is only the first step in yielding high-quality, translatable results. Indeed, the effectiveness and/or impact of the majority of contemporary proteomics screens are hindered not by commonly considered technical limitations such as low proteome coverage but rather by insufficient analyses. Proteomic experimentation requires a careful methodological selection to account for variables from sample collection, through to database searches for peptide identification to standardised post-mass spectrometry options directed analysis workflow, which should be adjusted for each study, from determining when and how to filter proteomic data to choosing holistic versus trend-wise analyses for biologically relevant patterns. Finally, we highlight and discuss the difficulties inherent in the modelling and study of the majority of progressive neurodegenerative conditions. We provide evidence (in the context of neurodegenerative research) for the benefit of undertaking a comparative approach through the application of the above considerations in the alignment of publicly available pre-existing data sets to identify potential novel regulators of neuronal stability.

Glutamatergic Synapse Dysfunction in Drosophila Neuromuscular Junctions Can Be Rescued by Proteostasis Modulation

Glutamate is the major excitatory neurotransmitter in the nervous system, and the Drosophila glutamatergic neuromuscular junctions (NMJs) offer a tractable platform to understand excitatory synapse biology both in health and disease. Synaptopathies are neurodegenerative diseases that are associated with synaptic dysfunction and often display compromised proteostasis. One such rare, progressive neurodegenerative condition, Spinocerebellar Ataxia Type 3 (SCA3) or Machado-Joseph Disease (MJD), is characterized by cerebellar ataxia, Parkinsonism, and degeneration of motor neuron synapses. While the polyQ repeat mutant protein ataxin-3 is implicated in MJD, it is unclear how it leads to impaired synaptic function. In this study, we indicated that a Drosophila model of MJD recapitulates characteristics of neurodegenerative disorders marked by motor neuron dysfunction. Expression of 78 polyQ repeats of mutant ataxin-3 protein in Drosophila motor neurons resulted in behavioral defects, such as impaired locomotion in both larval and adult stages. Furthermore, defects in eclosion and lifespan were observed in adult flies. Detailed characterization of larval glutamatergic neuromuscular junctions (NMJs) revealed defects in morphological features along with compromised NMJ functioning. Autophagy, one of the key proteostasis pathways, is known to be impaired in the case of several synaptopathies. Our study reveals that overexpression of the autophagy-related protein Atg8a rescued behavioral defects. Thus, we present a model for glutamatergic synapse dysfunction that recapitulates synaptic and behavioral deficits and show that it is an amenable system for carrying out genetic and chemical biology screens to identify potential therapeutic targets for synaptopathies.

Imaging Synaptic Density: The Next Holy Grail of Neuroscience?

The brain is the central and most complex organ in the nervous system, comprising billions of neurons that constantly communicate through trillions of connections called synapses. Despite being formed mainly during prenatal and early postnatal development, synapses are continually refined and eliminated throughout life via complicated and hitherto incompletely understood mechanisms. Failure to correctly regulate the numbers and distribution of synapses has been associated with many neurological and psychiatric disorders, including autism, epilepsy, Alzheimer’s disease, and schizophrenia. Therefore, measurements of brain synaptic density, as well as early detection of synaptic dysfunction, are essential for understanding normal and abnormal brain development. To date, multiple synaptic density markers have been proposed and investigated in experimental models of brain disorders. The majority of the gold standard methodologies (e.g., electron microscopy or immunohistochemistry) visualize synapses or measure changes in pre- and postsynaptic proteins ex vivo. However, the invasive nature of these classic methodologies precludes their use in living organisms. The recent development of positron emission tomography (PET) tracers [such as (¹⁸F)UCB-H or (¹¹C)UCB-J] that bind to a putative synaptic density marker, the synaptic vesicle 2A (SV2A) protein, is heralding a likely paradigm shift in detecting synaptic alterations in patients. Despite their limited specificity, novel, non-invasive magnetic resonance (MR)-based methods also show promise in inferring synaptic information by linking to glutamate neurotransmission. Although promising, all these methods entail various advantages and limitations that must be addressed before becoming part of routine clinical practice. In this review, we summarize and discuss current ex vivo and in vivo methods of quantifying synaptic density, including an evaluation of their reliability and experimental utility. We conclude with a critical assessment of challenges that need to be overcome before successfully employing synaptic density biomarkers as diagnostic and/or prognostic tools in the study of neurological and neuropsychiatric disorders.

Polyglutamine Expansion in Huntingtin and Mechanism of DNA Damage Repair Defects in Huntington’s Disease

Emerging evidence suggests that DNA repair deficiency and genome instability may be the impending signs of many neurological diseases. Genome-wide association (GWAS) studies have established a strong correlation between genes that play a role in DNA damage repair and many neurodegenerative diseases, including Huntington’s disease (HD), and several other trinucleotides repeat expansion-related hereditary ataxias. Recently, many reports have documented a significant role played by the DNA repair processes in aging and in modifying many neurodegenerative diseases, early during their progression. Studies from our lab and others have now begun to understand the mechanisms that cause defective DNA repair in HD and surprisingly, many proteins that have a strong link to known neurodegenerative diseases seem to be important players in these cellular pathways. Mutations in huntingtin (HTT) gene that lead to polyglutamine repeat expansion at the N-terminal of HTT protein has been shown to disrupt transcription-coupled DNA repair process, a specialized DNA repair process associated with transcription. Due to the recent progress made in understanding the mechanisms of DNA repair in relation to HD, in this review, we will mainly focus on the mechanisms by which the wild-type huntingtin (HTT) protein helps in DNA repair during transcription, and the how polyglutamine expansions in HTT impedes this process in HD. Further studies that identify new players in DNA repair will help in our understanding of this process in neurons. Furthermore, it should help us understand how various DNA repair mechanism(s) coordinate to maintain the normal physiology of neurons, and provide insights for the development of novel drugs at prodromal stages of these neurodegenerative diseases.

Synaptopathies in Developmental and Epileptic Encephalopathies: A Focus on Pre-synaptic Dysfunction

The proper connection between the pre- and post-synaptic nervous cells depends on any element constituting the synapse: the pre- and post-synaptic membranes, the synaptic cleft, and the surrounding glial cells and extracellular matrix. An alteration of the mechanisms regulating the physiological synergy among these synaptic components is defined as “synaptopathy.” Mutations in the genes encoding for proteins involved in neuronal transmission are associated with several neuropsychiatric disorders, but only some of them are associated with Developmental and Epileptic Encephalopathies (DEEs). These conditions include a heterogeneous group of epilepsy syndromes associated with cognitive disturbances/intellectual disability, autistic features, and movement disorders. This review aims to elucidate the pathogenesis of these conditions, focusing on mechanisms affecting the neuronal pre-synaptic terminal and its role in the onset of DEEs, including potential therapeutic approaches.

Recent developments in the management of Huntington's disease

Huntington's disease (HD) is a rare, incurable, inheritedneurodegenerative disorder manifested by chorea, hyperkinetic, and hypokinetic movements. The FDA has approved only two drugs, viz. tetrabenazine, and deutetrabenazine, to manage the chorea associated with HD. However, several other drugs are used as an off-label to manage chorea and other symptoms such as depression, anxiety, muscle tremors, and cognitive dysfunction associated with HD. So far, there is no disease-modifying treatment available. Drug repurposing has been a primary drive to search for new anti-HD drugs. Numerous molecular targets along with a wide range of small molecules and gene therapies are currently under clinical investigation. More than 200 clinical studies are underway for HD, 75% are interventional, and 25% are observational studies. The present review discusses the small molecule clinical pipeline and molecular targets for HD. Furthermore, the biomarkers, diagnostic tests, gene therapies, behavioral and observational studies for HD were also deliberated.

Synaptic pathology in Huntington's disease: Beyond the corticostriatal pathway

Huntington's disease (HD) is a heritable, fatal neurodegenerative disorder caused by a mutation in the Huntingtin gene. It is characterized by chorea, as well as cognitive and psychiatric symptoms. Histopathologically, there is a massive loss of striatal projection neurons and less but significant loss in other areas throughout the cortico-basal ganglia-thalamocortical (CBGTC) loop. The mutant huntingtin protein has been implicated in numerous functions, including an important role in synaptic transmission. Most studies on anatomical and physiological alterations in HD have focused on striatum and cerebral cortex. However, based on recent CBGTC projectome evidence, the need to study other pathways has become increasingly clear. In this review, we examine the current status of our knowledge of morphological and electrophysiological alterations of those pathways in animal models of HD. Based on recent studies, there is accumulating evidence that synaptic disconnection, particularly along excitatory pathways, is pervasive and almost universal in HD, thus supporting a critical role of the huntingtin protein in synaptic transmission.

Real-time three-dimensional tracking of single vesicles reveals abnormal motion and pools of synaptic vesicles in neurons of Huntington's disease mice

Although defective synaptic transmission was suggested to play a role in neurodegenerative diseases, the dynamics and vesicle pools of synaptic vesicles during neurodegeneration remain elusive. Here, we performed real-time three-dimensional tracking of single synaptic vesicles in cortical neurons from a mouse model of Huntington's disease (HD). Vesicles in HD neurons had a larger net displacement and radius of gyration compared with wild-type neurons. Vesicles with high release probability (P_r) were interspersed with low-P_r vesicles in HD neurons, whereas high-P_r vesicles were closer to fusion sites than low-P_r in wild-type neurons. Non-releasing vesicles in HD neurons had an abnormally high prevalence of irregular oscillatory motion. These abnormal dynamics and vesicle pools were rescued by overexpressing Rab11, and the abnormal irregular oscillatory motion was rescued by jasplakinolide. Our studies reveal the abnormal dynamics and pools of synaptic vesicles in the early stages of HD, suggesting a possible pathogenic mechanism of neurodegenerative diseases.

Huntingtin and the Synapse

Huntington disease (HD) is a monogenic disease that results in a combination of motor, psychiatric and cognitive symptoms. HD is caused by a CAG trinucleotide repeat expansion in the huntingtin (HTT) gene, which results in the production of a pathogenic mutant HTT protein (mHTT). Although there is no cure at present for HD, a number of RNA-targeting therapies have recently entered clinical trials which aim to lower mHTT production through the use of antisense oligonucleotides (ASOs) and RNAi. However, many of these treatment strategies are non-selective in that they cannot differentiate between non-pathogenic wild type HTT (wtHTT) and the mHTT variant. As HD patients are already born with decreased levels of wtHTT, these genetic therapies may result in critically low levels of wtHTT. The consequence of wtHTT reduction in the adult brain is currently under debate, and here we argue that wtHTT loss is not well-tolerated at the synaptic level. Synaptic dysfunction is an extremely sensitive measure of subsequent cell death, and is known to precede neurodegeneration in numerous brain diseases including HD. The present review focuses on the prominent role of wtHTT at the synapse and considers the consequences of wtHTT loss on both pre- and postsynaptic function. We discuss how wtHTT is implicated in virtually all major facets of synaptic neurotransmission including anterograde and retrograde transport of proteins to/from terminal buttons and dendrites, neurotransmitter release, endocytic vesicle recycling, and postsynaptic receptor localization and recycling. We conclude that wtHTT presence is essential for proper synaptic function.

Identification of Full-Length Wild-Type and Mutant Huntingtin Interacting Proteins by Crosslinking Immunoprecipitation in Mice Brain Cortex

Background:

Huntington’s disease is a neurodegenerative disorder caused by a CAG expansion in the huntingtin gene, resulting in a polyglutamine expansion in the ubiquitously expressed mutant huntingtin protein.

Objective:

Here we set out to identify proteins interacting with the full-length wild-type and mutant huntingtin protein in the mice cortex brain region to understand affected biological processes in Huntington’s disease pathology.

Methods:

Full-length huntingtin with 20 and 140 polyQ repeats were formaldehyde-crosslinked and isolated via their N-terminal Flag-tag from 2-month-old mice brain cortex. Interacting proteins were identified and quantified by label-free liquid chromatography-mass spectrometry (LC-MS/MS).

Results:

We identified 30 interactors specific for wild-type huntingtin, 14 interactors specific for mutant huntingtin and 14 shared interactors that interacted with both wild-type and mutant huntingtin, including known interactors such as F8a1/Hap40. Syt1, Ykt6, and Snap47, involved in vesicle transport and exocytosis, were among the proteins that interacted specifically with wild-type huntingtin. Various other proteins involved in energy metabolism and mitochondria were also found to associate predominantly with wild-type huntingtin, whereas mutant huntingtin interacted with proteins involved in translation including Mapk3, Eif3h and Eef1a2.

Conclusion:

Here we identified both shared and specific interactors of wild-type and mutant huntingtin, which are involved in different biological processes including exocytosis, vesicle transport, translation and metabolism. These findings contribute to the understanding of the roles that wild-type and mutant huntingtin play in a variety of cellular processes both in healthy conditions and Huntington’s disease pathology.

Impaired inhibitory GABAergic synaptic transmission and transcription studied in single neurons by Patch-seq in Huntington's disease

Transcriptional dysregulation in Huntington's disease (HD) causes functional deficits in striatal neurons. Here, we performed Patch-sequencing (Patch-seq) in an in vitro HD model to investigate the effects of mutant Huntingtin (Htt) on synaptic transmission and gene transcription in single striatal neurons. We found that expression of mutant Htt decreased the synaptic output of striatal neurons in a cell autonomous fashion and identified a number of genes whose dysregulation was correlated with physiological deficiencies in mutant Htt neurons. In support of a pivotal role for epigenetic mechanisms in HD pathophysiology, we found that inhibiting histone deacetylase 1/3 activities rectified several functional and morphological deficits and alleviated the aberrant transcriptional profiles in mutant Htt neurons. With this study, we demonstrate that Patch-seq technology can be applied both to better understand molecular mechanisms underlying a complex neurological disease at the single-cell level and to provide a platform for screening for therapeutics for the disease.

Current molecular approaches to investigate pre-synaptic dysfunction

Over the course of the last few decades it has become clear that many neurodevelopmental and neurodegenerative disorders have a synaptic defect, which contributes to pathogenicity. A rise in new techniques, and in particular '-omics'-based methods providing large datasets, has led to an increase in potential proteins and pathways implicated in synaptic function and related disorders. Additionally, advancements in imaging techniques have led to the recent discovery of alternative modes of synaptic vesicle recycling. This has resulted in a lack of clarity over the precise role of different pathways in maintaining synaptic function and whether these new pathways are dysfunctional in neurodevelopmental and neurodegenerative disorders. A greater understanding of the molecular detail of pre-synaptic function in health and disease is key to targeting new proteins and pathways for novel treatments and the variety of new techniques currently available provides an ideal opportunity to investigate these functions. This review focuses on techniques to interrogate pre-synaptic function, concentrating mainly on synaptic vesicle recycling. It further examines techniques to determine the underlying molecular mechanism of pre-synaptic dysfunction and discusses methods to identify molecular targets, along with protein-protein interactions and cellular localization. In combination, these techniques will provide an expanding and more complete picture of pre-synaptic function. With the application of recent technological advances, we are able to resolve events with higher spatial and temporal resolution, leading research towards a greater understanding of dysfunction at the presynapse and the role it plays in pathogenicity.

Super-resolution imaging reveals extrastriatal synaptic dysfunction in presymptomatic Huntington disease mice

Synaptic structure and function are compromised prior to cell death and symptom onset in a variety of neurodegenerative diseases. In Huntington disease (HD), a CAG repeat expansion in the gene encoding the huntingtin protein results in a presymptomatic stage that typically spans multiple decades and is followed by striking degeneration of striatal tissue and the progression of debilitating motor symptoms. Many lines of evidence demonstrate that the HD presymptomatic window is associated with injurious effects to striatal synapses, many of which appear to be prerequisites to subsequent cell death. While the striatum is the most vulnerable region in the HD brain, it is widely recognized that HD is a brain-wide disease, affecting numerous extrastriatal regions that contribute to debilitating non-motor symptoms including cognitive dysfunction. Currently, we have a poor understanding of the synaptic integrity, or lack thereof, in extrastriatal regions in the presymptomatic HD brain. If early therapeutic intervention seeks to maintain healthy synaptic function, it is important to understand early HD-associated synaptopathy at a brain-wide, rather than striatal-exclusive, level. Here, we focused on the hippocampus as this structure is generally thought to be affected only in manifest HD despite the subtle cognitive deficits known to emerge in prodromal HD. We used super-resolution microscopy and multi-electrode array electrophysiology as sensitive measures of excitatory synapse structure and function, respectively, in the hippocampus of presymptomatic heterozygous HD mice (Q175FDN model). We found clear evidence for enhanced AMPA receptor subunit clustering and hyperexcitability well before the onset of a detectable HD-like behavioral phenotype. In addition, activity-dependent re-organization of synaptic protein nanostructure, and functional measures of synaptic plasticity were impaired in presymptomatic HD mice. These data demonstrate that synaptic abnormalities in the presymptomatic HD brain are not exclusive to the striatum, and highlight the need to better understand the region-dependent complexities of early synaptopathy in the HD brain.

Protective Effect of Natural Products against Huntington's Disease: An Overview of Scientific Evidence and Understanding Their Mechanism of Action

Huntington's disease (HD), a neurodegenerative disease, normally starts in the prime of adult life, followed by a gradual occurrence of characteristic psychiatric disturbances and cognitive and motor dysfunction. To the best of our knowledge, there is no treatment available to completely mitigate the progression of HD. Among various therapeutic approaches, exhaustive literature reports have confirmed the medicinal benefits of natural products in HD experimental models. Building on this information, this review presents a brief overview of the neuroprotective mechanism(s) of natural products against in vitro/in vivo models of HD. Relevant studies were identified from several scientific databases, including PubMed, ScienceDirect, Scopus, and Google Scholar. After screening through literature from 2005 to the present, a total of 14 medicinal plant species and 30 naturally isolated compounds investigated against HD based on either in vitro or in vivo models were included in the present review. Behavioral outcomes in the HD in vivo model showed that natural compounds significantly attenuated 3-nitropropionic acid (3-NP) induced memory loss and motor incoordination. The biochemical alteration has been markedly alleviated with reduced lipid peroxidation, increased endogenous enzymatic antioxidants, reduced acetylcholinesterase activity, and increased mitochondrial energy production. Interestingly, following treatment with certain natural products, 3-NP-induced damage in the striatum was ameliorated, as seen histologically. Overall, natural products afforded varying degrees of neuroprotection in preclinical studies of HD via antioxidant and anti-inflammatory properties, preservation of mitochondrial function, inhibition of apoptosis, and induction of autophagy.

Novel PET Biomarkers to Disentangle Molecular Pathways across Age-Related Neurodegenerative Diseases

There is a need to disentangle the etiological puzzle of age-related neurodegenerative diseases, whose clinical phenotypes arise from known, and as yet unknown, pathways that can act distinctly or in concert. Enhanced sub-phenotyping and the identification of in vivo biomarker-driven signature profiles could improve the stratification of patients into clinical trials and, potentially, help to drive the treatment landscape towards the precision medicine paradigm. The rapidly growing field of neuroimaging offers valuable tools to investigate disease pathophysiology and molecular pathways in humans, with the potential to capture the whole disease course starting from preclinical stages. Positron emission tomography (PET) combines the advantages of a versatile imaging technique with the ability to quantify, to nanomolar sensitivity, molecular targets in vivo. This review will discuss current research and available imaging biomarkers evaluating dysregulation of the main molecular pathways across age-related neurodegenerative diseases. The molecular pathways focused on in this review involve mitochondrial dysfunction and energy dysregulation; neuroinflammation; protein misfolding; aggregation and the concepts of pathobiology, synaptic dysfunction, neurotransmitter dysregulation and dysfunction of the glymphatic system. The use of PET imaging to dissect these molecular pathways and the potential to aid sub-phenotyping will be discussed, with a focus on novel PET biomarkers.

The Therapeutic Potential of Neuronal K-Cl Co-Transporter KCC2 in Huntington's Disease and Its Comorbidities

Intracellular chloride levels in the brain are regulated primarily through the opposing effects of two cation-chloride co-transporters (CCCs), namely K+-Cl- co-transporter-2 (KCC2) and Na+-K+-Cl- co-transporter-1 (NKCC1). These CCCs are differentially expressed throughout the course of development, thereby determining the excitatory-to-inhibitory γ-aminobutyric acid (GABA) switch. GABAergic excitation (depolarisation) is important in controlling the healthy development of the nervous system; as the brain matures, GABAergic inhibition (hyperpolarisation) prevails. This developmental switch in excitability is important, as uncontrolled regulation of neuronal excitability can have implications for health. Huntington's disease (HD) is an example of a genetic disorder whereby the expression levels of KCC2 are abnormal due to mutant protein interactions. Although HD is primarily considered a motor disease, many other clinical manifestations exist; these often present in advance of any movement abnormalities. Cognitive change, in addition to sleep disorders, is prevalent in the HD population; the effect of uncontrolled KCC2 function on cognition and sleep has also been explored. Several mechanisms by which KCC2 expression is reduced have been proposed recently, thereby suggesting extensive investigation of KCC2 as a possible therapeutic target for the development of pharmacological compounds that can effectively treat HD co-morbidities. Hence, this review summarizes the role of KCC2 in the healthy and HD brain, and highlights recent advances that attest to KCC2 as a strong research and therapeutic target candidate.

Synaptic Dysfunction in Huntington's Disease: Lessons from Genetic Animal Models

The understanding of the functional and structural changes occurring in the cerebral cortex and basal ganglia in Huntington's disease (HD) has benefited considerably from the generation of genetic animal models. Most studies of synaptic alterations in HD models have focused on the striatum, but a more complete picture of synaptic dysfunction in the cortico-basal ganglia-cortical loop is emerging. Here, we provide a review and analysis of current developments in the study of synaptic alterations in these areas using HD rodent models. Recent evidence indicates that cortical maldevelopment plays a role in synaptic dysfunction along the corticostriatal pathway that may have its roots in the way mutant huntingtin interacts with synaptic proteins. Furthermore, a progressive disconnection in the corticostriatal pathway leads to abnormal function engaging extrasynaptic N-methyl-D-aspartate glutamate receptors that contribute to eventual cell degeneration. In addition, biphasic increases followed by decreases in glutamate and dopamine release in the striatum could explain contrasting symptomatology in early and late stages of the disease. Changes in striatal output regions also are beginning to be examined. Finally, we highlight some therapeutic avenues aimed at rescuing synaptic dysfunction.

Effects of rosmarinic acid on nervous system disorders: an updated review

Nowadays, the worldwide interest is growing to use medicinal plants and their active constituents to develop new potent medicines with fewer side effects. Precise dietary compounds have prospective beneficial applications for various neurodegenerative ailments. Rosmarinic acid is a polyphenol and is detectable most primarily in many Lamiaceae families, for instance, Rosmarinus officinalis also called rosemary. This review prepared a broad and updated literature review on rosmarinic acid elucidating its biological activities on some nervous system disorders. Rosmarinic acid has significant antinociceptive, neuroprotective, and neuroregenerative effects. In this regard, we classified and discussed our findings in different nervous system disorders including Alzheimer's disease, epilepsy, depression, Huntington's disease, familial amyotrophic lateral sclerosis, Parkinson's disease, cerebral ischemia/reperfusion injury, spinal cord injury, stress, anxiety, and pain.

Glycation in Huntington's Disease: A Possible Modifier and Target for Intervention

Glycation is the non-enzymatic reaction between reactive dicarbonyls and amino groups, and gives rise to a variety of different reaction products known as advanced glycation end products (AGEs). Accumulation of AGEs on proteins is inevitable, and is associated with the aging process. Importantly, glycation is highly relevant in diabetic patients that experience periods of hyperglycemia. AGEs also play an important role in neurodegenerative diseases including Alzheimer’s (AD) and Parkinson’s disease (PD). Huntington’s disease (HD) is a hereditary neurodegenerative disease caused by an expansion of a CAG repeat in the huntingtin gene. The resulting expanded polyglutamine stretch in the huntingtin (HTT) protein induces its misfolding and aggregation, leading to neuronal dysfunction and death. HD patients exhibit chorea and psychiatric disturbances, along with abnormalities in glucose and energy homeostasis. Interestingly, an increased prevalence of diabetes mellitus has been reported in HD and in other CAG triplet repeat disorders. However, the mechanisms underlying the connection between glycation and HD progression remain unclear. In this review, we explore the possible connection between glycation and proteostasis imbalances in HD, and posit that it may contribute to disease progression, possibly by accelerating protein aggregation and deposition. Finally, we review therapeutic interventions that might be able to alleviate the negative impact of glycation in HD.

No symphony without bassoon and piccolo: changes in synaptic active zone proteins in Huntington's disease

Prominent features of HD neuropathology are the intranuclear and cytoplasmic inclusions of huntingtin and striatal and cortical neuronal cell death. Recently, synaptic defects have been reported on HD-related studies, including impairment of neurotransmitter release and alterations of synaptic components. However, the definite characteristics of synapse dysfunction and the underlying mechanisms remain largely unknown. We studied the gene expression levels and patterns of a number of proteins forming the cytoskeletal matrix of the presynaptic active zones in HD transgenic mice (R6/1), in hippocampal neuronal cultures overexpressing mutant huntingtin and in postmortem brain tissues of HD patients. To investigate the interactions between huntingtin and active proteins, we performed confocal microscopic imaging and immunoprecipitation in mouse and HEK 293 cell line models. The mRNA and protein levels of Bassoon were reduced in mouse and cell culture models of HD and in brain tissues of patients with HD. Moreover, a striking re-distribution of a complex of proteins including Bassoon, Piccolo and Munc 13-1 from the cytoplasm and synapses into intranuclear huntingtin aggregates with loss of active zone proteins and dendritic spines. This re-localization was age-dependent and coincided with the formation of huntingtin aggregates. Using co-immunoprecipitation, we demonstrated that huntingtin interacts with Bassoon, and that this interaction is likely mediated by a third linking protein. Three structural proteins involved in neurotransmitter release in the presynaptic active zones of neurons are altered in expression and that the proteins are redistributed from their normal functional site into mutant huntingtin aggregates.

Striatal network modeling in Huntington's Disease

Medium spiny neurons (MSNs) comprise over 90% of cells in the striatum. In vivo MSNs display coherent burst firing cell assembly activity patterns, even though isolated MSNs do not burst fire intrinsically. This activity is important for the learning and execution of action sequences and is characteristically dysregulated in Huntington's Disease (HD). However, how dysregulation is caused by the various neural pathologies affecting MSNs in HD is unknown. Previous modeling work using simple cell models has shown that cell assembly activity patterns can emerge as a result of MSN inhibitory network interactions. Here, by directly estimating MSN network model parameters from single unit spiking data, we show that a network composed of much more physiologically detailed MSNs provides an excellent quantitative fit to wild type (WT) mouse spiking data, but only when network parameters are appropriate for the striatum. We find the WT MSN network is situated in a regime close to a transition from stable to strongly fluctuating network dynamics. This regime facilitates the generation of low-dimensional slowly varying coherent activity patterns and confers high sensitivity to variations in cortical driving. By re-estimating the model on HD spiking data we discover network parameter modifications are consistent across three very different types of HD mutant mouse models (YAC128, Q175, R6/2). In striking agreement with the known pathophysiology we find feedforward excitatory drive is reduced in HD compared to WT mice, while recurrent inhibition also shows phenotype dependency. We show that these modifications shift the HD MSN network to a sub-optimal regime where higher dimensional incoherent rapidly fluctuating activity predominates. Our results provide insight into a diverse range of experimental findings in HD, including cognitive and motor symptoms, and may suggest new avenues for treatment.

Loss of egli-1, the Caenorhabditis elegans Orthologue of a Downstream Target of SMN, Leads to Abnormalities in Sensorimotor Integration

The connectome of Caenorhabditis elegans has been extensively studied and fully mapped, allowing researchers to more confidently conclude on the impact of any change in neuronal circuits based on behavioral data. One of the more complex sensorimotor circuits in nematodes is the one that regulates the integration of feeding status with the subsequent behavioral responses that allow animals to adapt to environmental conditions. Here, we have characterized a Caenorhabditis elegans knockout model of the egli-1 gene (previously known as tag-175). This is an orthologue of the stasimon/tmem41b gene, a downstream target of SMN, the depleted protein in spinal muscular atrophy (SMA), which partially recapitulates the SMA phenotype in fly and zebrafish models when mutated. Surprisingly, egli-1 mutants reveal no deficits in motor function. Instead, they show functional impairment of a specific neuronal circuit, leading to defects in the integration of sensorial information related to food abundance, with consequences at the level of locomotion adaptation, egg laying, and the response to aversive chemical stimuli. This work has demonstrated for the first time the relevance of egli-1 in the nervous system, as well as revealed a function for this gene, which had remained elusive so far.

The Caenorhabditis elegans cysteine-string protein homologue DNJ-14 is dispensable for neuromuscular junction maintenance across ageing

Maintenance of synaptic function across ageing is vital in sustaining cognitive function. Synaptic dysfunction is a key part of the pathophysiology of a number of neurodegenerative diseases. The synaptic co-chaperone, cysteine-string protein (CSP), is important for synaptic maintenance and function in Drosophila, mice and humans, and disruption of CSP results in synaptic degeneration. We sought to characterise synaptic ageing in Caenorhabditis elegans upon genetic disruption of CSP. To do this, we focused on the worms' neuromuscular junctions, which are the best characterised synapse. CSP mutant worms did not display reduced lifespan or any neuromuscular-dependent behavioural deficits across ageing. Pharmacological interrogation of the neuromuscular synapse of CSP mutant animals showed no sign of synaptic dysfunction even at advanced age. Lastly, patch clamp analysis of neuromuscular transmission across ageing in wild-type and CSP mutant animals revealed no obvious CSP-dependent deficits. Electrophysiological spontaneous postsynaptic current analysis reinforced pharmacological observations that the C. elegans neuromuscular synapse increases in strength during early ageing and remains relatively intact in old, immotile worms. Taken together, this study shows that surprisingly, despite disruption of CSP in other animals having severe synaptic phenotypes, CSP does not seem to be important for maintenance of the neuromuscular junction across ageing in C. elegans.

Epigenetics of the Synapse in Neurodegeneration

PURPOSE OF REVIEW

In the quest for understanding the pathophysiological processes underlying degeneration of nervous systems, synapses are emerging as sites of great interest as synaptic dysfunction is thought to play a role in the initiation and progression of neuronal loss. In particular, the synapse is an interesting target for the effects of epigenetic mechanisms in neurodegeneration. Here, we review the recent advances on epigenetic mechanisms driving synaptic compromise in major neurodegenerative disorders.

RECENT FINDINGS

Major developments in sequencing technologies enabled the mapping of transcriptomic patterns in human postmortem brain tissues in various neurodegenerative diseases, and also in cell and animal models. These studies helped identify changes in classical neurodegeneration pathways and discover novel targets related to synaptic degeneration. Identifying epigenetic patterns indicative of synaptic defects prior to neuronal degeneration may provide the basis for future breakthroughs in the field of neurodegeneration.

The Onset and Progression of Hippocampal Synaptic Plasticity Deficits in the Q175FDN Mouse Model of Huntington Disease

Huntington disease (HD) is an inherited neurodegenerative disease characterized by a clinical triad of motor, psychiatric and cognitive symptoms. HD is caused by a CAG repeat expansion in the gene encoding the huntingtin protein. Homozygosity for the HD-causing mutation is extremely rare; thus, the majority of HD patients express the mutant huntingtin protein in addition to reduced levels of the non-pathogenic huntingtin protein. Deficits in synaptic plasticity, including hippocampal long-term potentiation (LTP), have been identified in various mouse models of HD and are thought to contribute to the debilitating cognitive symptoms associated with the disease. However, the bulk of these studies used N-terminal fragment or homozygous knock-in mouse models of HD at symptomatic ages, and our understanding of the onset and progression of synaptic plasticity deficits in the HD brain is lacking. To better understand the time-course of synaptic plasticity deficits in HD, as well as the impact of heterozygous and homozygous huntingtin mutations, we quantified basal synaptic connectivity, presynaptic release probability, presynaptically mediated post-tetanic potentiation (PTP) and postsynaptically mediated LTP at presymptomatic, early symptomatic and late symptomatic ages in heterozygous and homozygous Q175FDN knock-in HD mice. Our results demonstrate clear age-dependent effects of the HD-causing mutation on both short and long-term plasticity that generally emerge earlier in homozygous mice. Interestingly, deficits in presynaptic short-term plasticity were more closely linked to disease progression than deficits in postsynaptic LTP, and heterozygous mice were more susceptible to an LTP deficit when induced by high frequency stimulation compared to theta burst stimulation. To the best of our knowledge, the present study represents the most thorough characterization to date of the onset and progression of hippocampal synaptic plasticity deficits in a mouse model of HD, and should prove valuable to future studies exploring cellular mechanisms underlying the debilitating cognitive decline in HD.

PET imaging of synaptic density: A new tool for investigation of neuropsychiatric diseases

Synaptic vesicle glycoprotein 2A (SV2A) is expressed ubiquitously in neurons of the central nervous system, and is the binding target of the anti-epileptic drug levetiracetam. Because of the availability of positron emission tomography (PET) ligands targeting SV2A, there is increasing enthusiasm on the use of SV2A PET to study a variety of neuropsychiatric diseases. This review discusses the recent development of radioligands for PET imaging of SV2A and their potential use in the research and diagnosis of CNS diseases.

Huntington Mice Demonstrate Diminished Pain Response in Inflammatory Pain Model

BACKGROUND

Huntington disease (HD) affects the nervous system and leads to mental and motor dysfunction. Previous studies have shown that HD is caused by the exon 1 region of the huntingtin (HTT) gene having expanded CAG trinucleotide repeats. However, few studies have focused on the relationship between HD and pain. The purpose of this study is to investigate the relationship between HD and pain response.

METHODS

We used clinical similar transgenic HD mice carrying a mutant HTT exon 1 containing 84 CAG trinucleotide repeats to evaluate the relationship between HD and pain. Inflammatory pain models were induced by either formalin or complete Freund adjuvant injection over the hind paw. Spinal cord, dorsal root ganglion, and paw skin tissues were harvested at the end of the behavioral inflammatory pain studies. Immunofluorescence assay, Western blotting, and enzyme-linked immunosorbent assay were used to identify changes in cells and cytokines.

RESULTS

Our data demonstrate that preonset HD mice exhibited less pain behavior than wild-type (WT) mice in both young (n = 11 [WT], 13 [HD]) and aged (n = 8 [WT], 9 [HD]) mice. Western blotting and immunohistological examination of lumbar spinal cord tissue and dorsal root ganglion indicate less activation of glial cells and astrocytes in young HD mice (n = 6-7) compared to that in WT mice (n = 6-7). The production levels of tumor necrosis factor-α, interleukin-1β, and substance P were also lower in young HD mice (n = 6-7).

CONCLUSIONS

Our data demonstrate less pain behavior and pain-related cytokine response at the spinal cord level for HD mice compared to those for WT mice. Further studies are needed for determining the mechanism as to how mutant HTT leads to altered pain behavior and pain-related cytokine response.

Altered Synaptic Vesicle Release and Ca2+ Influx at Single Presynaptic Terminals of Cortical Neurons in a Knock-in Mouse Model of Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by the abnormal expansion of CAG repeats in the huntingtin (HTT) gene, which leads to progressive loss of neurons starting in the striatum and cortex. One possible mechanism for this selective loss of neurons in the early stage of HD is altered neurotransmission at synapses. Despite the recent finding that presynaptic terminals play an important role in HD, neurotransmitter release at synapses in HD remains poorly understood. Here, we measured synaptic vesicle release in real time at single presynaptic terminals during electrical field stimulation. We found the increase in synaptic vesicle release at presynaptic terminals in primary cortical neurons in a knock-in mouse model of HD (zQ175). We also found the increase in Ca²⁺ influx at presynaptic terminals in HD neurons during the electrical stimulation. Consistent with increased Ca²⁺-dependent neurotransmission in HD neurons, the increase in vesicle release and Ca²⁺ influx was rescued with Ca²⁺ chelators or by blocking N-type voltage-gated Ca²⁺ channels, suggesting N-type voltage-gated Ca²⁺ channels play an important role in HD. Taken together, our results suggest that the increased synaptic vesicles release due to increased Ca²⁺ influx at presynaptic terminals in cortical neurons contributes to the selective neurodegeneration of these neurons in early HD and provide a possible therapeutic target.