Striatal synaptic dysfunction and hippocampal plasticity deficits in the Hu97/18 mouse model of Huntington 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.

Mild cognitive impairment in Huntington's disease: challenges and outlooks

Although Huntington's disease (HD) has classically been viewed as an autosomal-dominant inherited neurodegenerative motor disorder, cognitive and/or behavioral changes are predominant and often an early manifestation of disease. About 40% of individuals in the presymptomatic period of HD meet the criteria for mild cognitive impairment, later progressing to dementia. The heterogenous spectrum of cognitive decline is characterized by deficits across multiple domains, particularly executive dysfunctions, but the underlying pathogenic mechanisms are still poorly understood. Investigating the pathophysiology of cognitive changes may give insight into important and early neurodegenerative events. Multimodal imaging revealed circuit-wide gray and white matter degenerative processes in several key brain regions, affecting prefronto-striatal/cortico-basal ganglia circuits and many other functional brain networks. Studies in transgenic animal models indicated early synaptic dysfunction, deficient neurotrophic transport and other molecular changes contributing to neuronal death. Synaptopathy within the cerebral cortex, striatum and hippocampus may be particularly important in mediating cognitive and neuropsychiatric manifestations of HD, although many other neuronal systems are involved. The interaction of mutant huntingtin protein (mHTT) with tau and its implication for cognitive impairment in HD is a matter of discussion. Further neuroimaging and neuropathological studies are warranted to better elucidate early pathophysiological mechanisms and to develop validated biomarkers to detect patients' cognitive status during the early stages of the condition significantly to implement effective preventing or management strategies.

Reduced lysosomal density in neuronal dendrites mediates deficits in synaptic plasticity in Huntington's disease

Huntington's disease (HD) usually causes cognitive disorders, including learning difficulties, that emerge before motor symptoms. Mutations related to lysosomal trafficking are linked to the pathogenesis of neurological diseases, whereas the cellular mechanisms remain elusive. Here, we discover a reduction in the dendritic density of lysosomes in the hippocampus that correlates with deficits in synaptic plasticity and spatial learning in early CAG-140 HD model mice. We directly manipulate intraneuronal lysosomal positioning with light-induced CRY2:CIB1 dimerization and demonstrate that lysosomal abundance in dendrites positively modulates long-term potentiation of glutamatergic synapses onto the neuron. This modulation depends on lysosomal Ca2+ release, which further promotes endoplasmic reticulum (ER) entry into spines. Importantly, optogenetically restoring lysosomal density in dendrites rescues the synaptic plasticity deficit in hippocampal slices of CAG-140 mice. Our data reveal dendritic lysosomal density as a modulator of synaptic plasticity and suggest a role of lysosomal mispositioning in cognitive decline in HD.

Structural Plasticity of the Hippocampus in Neurodegenerative Diseases

Neuroplasticity is the capacity of neural networks in the brain to alter through development and rearrangement. It can be classified as structural and functional plasticity. The hippocampus is more susceptible to neuroplasticity as compared to other brain regions. Structural modifications in the hippocampus underpin several neurodegenerative diseases that exhibit cognitive and emotional dysregulation. This article reviews the findings of several preclinical and clinical studies about the role of structural plasticity in the hippocampus in neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and multiple sclerosis. In this study, literature was surveyed using Google Scholar, PubMed, Web of Science, and Scopus, to review the mechanisms that underlie the alterations in the structural plasticity of the hippocampus in neurodegenerative diseases. This review summarizes the role of structural plasticity in the hippocampus for the etiopathogenesis of neurodegenerative diseases and identifies the current focus and gaps in knowledge about hippocampal dysfunctions. Ultimately, this information will be useful to propel future mechanistic and therapeutic research in neurodegenerative diseases.

Amyloid Prefibrillar Oligomers: The Surprising Commonalities in Their Structure and Activity

It has been proposed that a “common core” of pathologic pathways exists for the large family of amyloid-associated neurodegenerations, including Alzheimer’s, Parkinson’s, type II diabetes and Creutzfeldt–Jacob’s Disease. Aggregates of the involved proteins, independently from their primary sequence, induced neuron membrane permeabilization able to trigger an abnormal Ca²⁺ influx leading to synaptotoxicity, resulting in reduced expression of synaptic proteins and impaired synaptic transmission. Emerging evidence is now focusing on low-molecular-weight prefibrillar oligomers (PFOs), which mimic bacterial pore-forming toxins that form well-ordered oligomeric membrane-spanning pores. At the same time, the neuron membrane composition and its chemical microenvironment seem to play a pivotal role. In fact, the brain of AD patients contains increased fractions of anionic lipids able to favor cationic influx. However, up to now the existence of a specific “common structure” of the toxic aggregate, and a “common mechanism” by which it induces neuronal damage, synaptotoxicity and impaired synaptic transmission, is still an open hypothesis. In this review, we gathered information concerning this hypothesis, focusing on the proteins linked to several amyloid diseases. We noted commonalities in their structure and membrane activity, and their ability to induce Ca²⁺ influx, neurotoxicity, synaptotoxicity and impaired synaptic transmission.

Do Changes in Synaptic Autophagy Underlie the Cognitive Impairments in Huntington's Disease?

Although Huntington's disease (HD) is classically considered from the perspective of the motor syndrome, the cognitive changes in HD are prominent and often an early manifestation of disease. As such, investigating the underlying pathophysiology of cognitive changes may give insight into important and early neurodegenerative events. In this review, we first discuss evidence from both HD patients and animal models that cognitive changes correlate with early pathological changes at the synapse, an observation that is similarly made in other neurodegenerative conditions that primarily affect cognition. We then describe how autophagy plays a critical role supporting synaptic maintenance in the healthy brain, and how autophagy dysfunction in HD may thereby lead to impaired synaptic maintenance and thus early manifestations of disease.

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.

What is the Pathogenic CAG Expansion Length in Huntington's Disease?

Huntington's disease (HD) (OMIM 143100) is caused by an expanded CAG repeat tract in the HTT gene. The inherited CAG length is known to expand further in somatic and germline cells in HD subjects. Age at onset of the disease is inversely correlated with the inherited CAG length, but is further modulated by a series of genetic modifiers which are most likely to act on the CAG repeat in HTT that permit it to further expand. Longer repeats are more prone to expansions, and this expansion is age dependent and tissue-specific. Given that the inherited tract expands through life and most subjects develop disease in mid-life, this implies that in cells that degenerate, the CAG length is likely to be longer than the inherited length. These findings suggest two thresholds- the inherited CAG length which permits further expansion, and the intracellular pathogenic threshold, above which cells become dysfunctional and die. This two-step mechanism has been previously proposed and modelled mathematically to give an intracellular pathogenic threshold at a tract length of 115 CAG (95% confidence intervals 70- 165 CAG). Empirically, the intracellular pathogenic threshold is difficult to determine. Clues from studies of people and models of HD, and from other diseases caused by expanded repeat tracts, place this threshold between 60- 100 CAG, most likely towards the upper part of that range. We assess this evidence and discuss how the intracellular pathogenic threshold in manifest disease might be better determined. Knowing the cellular pathogenic threshold would be informative for both understanding the mechanism in HD and deploying treatments.

Neuronal Plasticity: Neuronal Organization is Associated with Neurological Disorders

Stimuli from stressful events, attention in the classroom, and many other experiences affect the functionality of the brain by changing the structure or reorganizing the connections between neurons and their communication. Modification of the synaptic transmission is a vital mechanism for generating neural activity via internal or external stimuli. Neuronal plasticity is an important driving force in neuroscience research, as it is the basic process underlying learning and memory and is involved in many other functions including brain development and homeostasis, sensorial training, and recovery from brain injury. Indeed, neuronal plasticity has been explored in numerous studies, but it is still not clear how neuronal plasticity affects the physiology and morphology of the brain. Thus, unraveling the molecular mechanisms of neuronal plasticity is essential for understanding the operation of brain functions. In this timeline review, we discuss the molecular mechanisms underlying different forms of synaptic plasticity and their association with neurodegenerative/neurological disorders as a consequence of alterations in neuronal plasticity.

The Role of the SLC Transporters Protein in the Neurodegenerative Disorders

The solute carrier (SLC) superfamily is one of the major sub-groups of membrane proteins in mammalian cells. The solute carrier proteins include more than 400 different membrane-spanning solute carriers organized with 65 families in the human. In solute carrier family neurons, neurotransmitter is considered to be a pharmacological target of neuropsychiatric drugs because of their important role in the recovery of neurotransmitters such as GABA, glutamate, serotonin, dopamine and noradrenaline and regulation of their concentration in synaptic regions. Therefore, solute carrier transporters play vital and different roles in neurodegenerative disorders. In this article, the role of solute carrier transporters in neurodegenerative disorders such as Alzheimer disease, amyotrophic lateral sclerosis, Huntington disease, Parkinson’s diseases, depression, post-traumatic stress disorder, dementia, schizophrenia, and Epilepsy reviewed and discussed to see how defects or absences in SLC transporter cause neurodegenerative disorders. In this review, we try to summarize what is known about solute carriers with respect to brain distribution and expression. The review summarizes current knowledge on the roles of solute carrier transporters in neurodegenerative disorders.

Hippocampal Synaptic Dysfunction in a Mouse Model of Huntington Disease Is Not Alleviated by Ceftriaxone Treatment

Glutamate transporters, particularly glutamate transporter 1 (GLT-1), help to prevent the adverse effects associated with glutamate toxicity by rapidly clearing glutamate from the extracellular space. Since GLT-1 expression and/or function are reduced in many neurodegenerative diseases, upregulation of GLT-1 is a favorable approach to treat the symptoms of these diseases. Ceftriaxone, a β-lactam antibiotic reported to increase GLT-1 expression, can exert neuroprotective effects in a variety of neurodegenerative diseases; however, many of these diseases do not exhibit uniform brain pathology. In contrast, as a drug that readily crosses the blood–brain barrier, ceftriaxone administration is likely to increase GLT-1 levels globally throughout the neuroaxis. In Huntington disease (HD), low GLT-1 expression is observed in the striatum in postmortem tissue and animal models. While ceftriaxone was reported to increase striatal GLT-1 and ameliorate the motor symptoms in a mouse model of HD, the extrastriatal effects of ceftriaxone in HD are unknown. Using electrophysiology and high-speed imaging of the glutamate biosensor iGluSnFR, we quantified real-time glutamate dynamics and synaptic plasticity in the hippocampus of the Q175FDN mouse model of HD, following intraperitoneal injections of either saline or ceftriaxone. We observed an activity-dependent increase in extracellular glutamate accumulation within the HD hippocampus, which was not the result of reduced GLT-1 expression. Surprisingly, ceftriaxone had little effect on glutamate clearance rates and negatively impacted synaptic plasticity. These data provide evidence for glutamate dysregulation in the HD hippocampus but also caution the use of ceftriaxone as a treatment for HD.

Lack of mutant huntingtin in cortical efferents improves behavioral inflexibility and corticostriatal dynamics in Huntington's disease mice

Abnormal communication between cerebral cortex and striatum plays a major role in the motor symptoms of Huntington's disease (HD), a neurodegenerative disorder caused by a mutation of the huntingtin gene (mHTT). Because cortex is the main driver of striatal processing, we recorded local field potential (LFP) activity simultaneously in primary motor cortex (M1) and dorsal striatum (DS) in BACHD mice, a full-length HD gene model, and in a conditional BACHD/Emx-1 Cre (BE) model in which mHTT is suppressed in cortical efferents, while mice freely explored a plus-shaped maze beginning at 20 wk of age. Relative to wild-type (WT) controls, BACHD mice were just as active across >40 wk of testing but became progressively less likely to turn into a perpendicular arm as they approached the choice point of the maze, a sign of HD motor inflexibility. BE mice, in contrast, turned as freely as WT throughout testing. Although BE mice did not exactly match WT in LFP activity, the reduction in alpha (8-13 Hz), beta (13-30 Hz), and low-gamma (30-50 Hz) power that occurred in M1 of turning-impaired BACHD mice was reversed. No reversal occurred in DS. In fact, BE mice showed further reductions in DS theta (4-8 Hz), beta, and low-gamma power relative to the BACHD model. Coherence analysis indicated a dysregulation of corticostriatal information flow in both BACHD and BE mice. Collectively, our results suggest that mHTT in cortical outputs drives the dysregulation of select cortical frequencies that accompany the loss of behavioral flexibility in HD.NEW & NOTEWORTHY BACHD mice, a full-length genetic model of Huntington's disease (HD), express aberrant local field potential (LFP) activity in primary motor cortex (M1) along with decreased probability of turning into a perpendicular arm of a plus-shaped maze, a motor inflexibility phenotype. Suppression of the mutant huntingtin gene in cortical output neurons prevents decline in turning and improves alpha, beta, and low-gamma activity in M1. Our results implicate cortical networks in the search for therapeutic strategies to alleviate HD motor signs.

FGF2 and dual agonist of NCAM and FGF receptor 1, Enreptin, rescue neurite outgrowth loss in hippocampal neurons expressing mutated huntingtin proteins

In the present study, we developed an in vitro model of Huntington disease (HD) by transfecting primary rat hippocampal neurons with plasmids coding for m-htt exon 1 with different number of CAG repeats (18, 50 and 115) and demonstrated the influence of the length of polyQ sequence on neurite elongation. We found that exogenously applied FGF2 significantly rescued the m-htt-induced loss of neurite outgrowth. Moreover, the Enreptin peptide, an FGFR1 and NCAM dual agonist, had a similar neuritogenic effect to FGF2 in clinically relevant m-htt 50Q-expressing neurons. This study has developed an in vitro model of primary hippocampal neurons transfected with m-htt-coding vectors that is a powerful tool to study m-htt-related effects on neuronal placticity.

SAP97-mediated rescue of NMDA receptor surface distribution in a neuronal model of Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an expansion of the CAG repeat tract in the HTT gene, leading to motor, cognitive, and psychiatric symptoms. At the cellular level, NMDA-type glutamate receptors are upregulated at glutamatergic extrasynaptic sites in HD, triggering cell death signaling pathways and driving HD neurodegeneration. Extrasynaptic and synaptic glutamate receptor trafficking and surface distribution are regulated by the α and β N-terminal isoforms of SAP97, a postsynaptic density protein localized at glutamatergic synapses. Here we examined the surface distribution of NMDARs and AMPARs in a cellular model of HD, and whether the manipulation of individual SAP97 isoforms can regulate receptor distribution in diseased neurons. Using dSTORM super-resolution imaging, we reveal that mutant HTT drives the elevation of extrasynaptic NMDAR clusters located 100-500 nm from the postsynaptic density. This was accompanied by a decline in synaptic NMDAR-mediated currents while surface NMDAR-mediated currents remained unchanged. These effects were induced within 3 days of mutant HTT expression in rat hippocampal neurons in vitro, and were specific for NMDARs and not observed with AMPARs. Intriguingly, upregulation of either α- or βSAP97 expression increased synaptic and/or perisynaptic NMDAR localization and prevented the shift of NMDARs to extrasynaptic sites in mutant HTT neurons. This was accompanied by the rescue of normal synaptic NMDAR-mediated currents. Taken together, our high-resolution data reveals plasticity in surface NMDAR localization driven by mutant HTT and identifies the similar but independent roles of SAP97 N-terminal isoforms in maintaining normal synaptic function in pathological states.

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.

BDNF Controls Bidirectional Endocannabinoid Plasticity at Corticostriatal Synapses

The dorsal striatum exhibits bidirectional corticostriatal synaptic plasticity, NMDAR and endocannabinoids (eCB) mediated, necessary for the encoding of procedural learning. Therefore, characterizing factors controlling corticostriatal plasticity is of crucial importance. Brain-derived neurotrophic factor (BDNF) and its receptor, the tropomyosine receptor kinase-B (TrkB), shape striatal functions, and their dysfunction deeply affects basal ganglia. BDNF/TrkB signaling controls NMDAR plasticity in various brain structures including the striatum. However, despite cross-talk between BDNF and eCBs, the role of BDNF in eCB plasticity remains unknown. Here, we show that BDNF/TrkB signaling promotes eCB-plasticity (LTD and LTP) induced by rate-based (low-frequency stimulation) or spike-timing–based (spike-timing–dependent plasticity, STDP) paradigm in striatum. We show that TrkB activation is required for the expression and the scaling of both eCB-LTD and eCB-LTP. Using 2-photon imaging of dendritic spines combined with patch-clamp recordings, we show that TrkB activation prolongs intracellular calcium transients, thus increasing eCB synthesis and release. We provide a mathematical model for the dynamics of the signaling pathways involved in corticostriatal plasticity. Finally, we show that TrkB activation enlarges the domain of expression of eCB-STDP. Our results reveal a novel role for BDNF/TrkB signaling in governing eCB-plasticity expression in striatum and thus the engram of procedural learning.

Mutant Huntingtin Causes a Selective Decrease in the Expression of Synaptic Vesicle Protein 2C

Huntington's disease (HD) is a neurodegenerative disease caused by a polyglutamine expansion in the huntingtin (Htt) protein. Mutant Htt causes synaptic transmission dysfunctions by interfering in the expression of synaptic proteins, leading to early HD symptoms. Synaptic vesicle proteins 2 (SV2s), a family of synaptic vesicle proteins including 3 members, SV2A, SV2B, and SV2C, plays important roles in synaptic physiology. Here, we investigated whether the expression of SV2s is affected by mutant Htt in the brains of HD transgenic (TG) mice and Neuro2a mouse neuroblastoma cells (N2a cells) expressing mutant Htt. Western blot analysis showed that the protein levels of SV2A and SV2B were not significantly changed in the brains of HD TG mice expressing mutant Htt with 82 glutamine repeats. However, in the TG mouse brain there was a dramatic decrease in the protein level of SV2C, which has a restricted distribution pattern in regions particularly vulnerable in HD. Immunostaining revealed that the immunoreactivity of SV2C was progressively weakened in the basal ganglia and hippocampus of TG mice. RT-PCR demonstrated that the mRNA level of SV2C progressively declined in the TG mouse brain without detectable changes in the mRNA levels of SV2A and SV2B, indicating that mutant Htt selectively inhibits the transcriptional expression of SV2C. Furthermore, we found that only SV2C expression was progressively inhibited in N2a cells expressing a mutant Htt containing 120 glutamine repeats. These findings suggest that the synaptic dysfunction in HD results from the mutant Htt-mediated inhibition of SV2C transcriptional expression. These data also imply that the restricted distribution and decreased expression of SV2C contribute to the brain region-selective pathology of HD.

Restoring GABAergic inhibition rescues memory deficits in a Huntington's disease mouse model

Huntington's disease (HD) is classically characterized as a movement disorder, however cognitive impairments precede the motor symptoms by ∼15 y. Based on proteomic and bioinformatic data linking the Huntingtin protein (Htt) and KCC2, which is required for hyperpolarizing GABAergic inhibition, and the important role of inhibition in learning and memory, we hypothesized that aberrant KCC2 function contributes to the hippocampal-associated learning and memory deficits in HD. We discovered that Htt and KCC2 interact in the hippocampi of wild-type and R6/2-HD mice, with a decrease in KCC2 expression in the hippocampus of R6/2 and YAC128 mice. The reduced expression of the Cl--extruding cotransporter KCC2 is accompanied by an increase in the Cl--importing cotransporter NKCC1, which together result in excitatory GABA in the hippocampi of HD mice. NKCC1 inhibition by the FDA-approved NKCC1 inhibitor bumetanide abolished the excitatory action of GABA and rescued the performance of R6/2 mice on hippocampal-associated behavioral tests.

Cognitive Phenotypes for Biomarker Identification in Mental Illness: Forward and Reverse Translation

Psychiatric illness has been acknowledged for as long as people were able to describe behavioral abnormalities in the general population. In modern times, these descriptions have been codified and continuously updated into manuals by which clinicians can diagnose patients. None of these diagnostic manuals have attempted to tie abnormalities to neural dysfunction however, nor do they necessitate the quantification of cognitive function despite common knowledge of its ties to functional outcome. In fact, in recent years the National Institute of Mental Health released a novel transdiagnostic classification, the Research Domain Criteria (RDoC), which utilizes quantifiable behavioral abnormalities linked to neurophysiological processes. This reclassification highlights the utility of RDoC constructs as potential cognitive biomarkers of disease state. In addition, with RDoC and cognitive biomarkers, the onus of researchers utilizing animal models no longer necessitates the recreation of an entire disease state, but distinct processes. Here, we describe the utilization of constructs from the RDoC initiative to forward animal research on these cognitive and behavioral processes, agnostic of disease. By linking neural processes to these constructs, identifying putative abnormalities in diseased patients, more targeted therapeutics can be developed.

Murine Models of Huntington's Disease for Evaluating Therapeutics

Huntington's disease (HD) is an autosomal dominant progressive neurological disorder characterized by motor, cognitive, and psychiatric symptoms that typically present later on in life, although juvenile cases do exist. The identification of the disease-causing mutation, a CAG triplet repeat expansion in the HTT gene, in 1993 generated numerous investigations into the cellular and molecular pathways underlying the disorder. HD mouse models have played a prominent role in these studies, and the use of these mouse models of HD in the development and evaluation of novel therapeutic strategies is reviewed in this chapter. As new interventions and therapeutic approaches are evaluated and implemented, genetic mouse models will continue to be used with the hope of developing effective treatments for HD.

Formation of hippocampal mHTT aggregates leads to impaired spatial memory, hippocampal activation and adult neurogenesis

Huntington's disease (HD) is a genetic neurodegenerative disorder characterized by a triad of motor, psychiatric and cognitive deficits with the latter classically attributed to disruption of fronto-striatal circuits. However, emerging evidence suggests that some of the cognitive deficits in HD may have their origin in other structures including the hippocampus. Hippocampal abnormalities have been reported in HD mouse models particularly in terms of performance on the Morris Water Maze. However, in these animals, it is difficult to be certain whether the spatial memory deficits are due to local pathology within this structure or their poor mobility and motivation. Thus, a better model of hippocampal dysfunction in HD is needed especially given that we have previously shown that patients with HD have hippocampal-related problems from the very earliest stages of disease. In this study, our aim was therefore to understand the cellular and behavioural consequences of local overexpression of mutant huntingtin (mHTT) in the hippocampus of adult mice. We found that a targeted injection of a lentivirus, encoding an N-terminal of mHTT with 82 CAG repeats, into the murine hippocampus led to the focal formation of mHTT aggregates, long-term spatial memory impairments with decreased neurogenesis and expression of the immediate early gene c-fos. This study has therefore shown for the first time that local expression of mHTT in the dentate gyrus has deleterious effects, including its neurogenic capacity, with functional behavioural consequences, which fits well with recent data on hippocampal deficits seen in patients with HD.

Neural plasticity and network remodeling: From concepts to pathology

Neuroplasticity has been subject to a great deal of research in the last century. Recently, significant emphasis has been placed on the global effect of localized plastic changes throughout the central nervous system, and on how these changes integrate in a pathological context. Specifically, alterations of network functionality have been described in various pathological contexts to which corresponding structural alterations have been proposed. However, considering the amount of literature and the different pathological contexts, an integration of this information is still lacking. In this paper we will review the concepts of neural plasticity as well as their repercussions on network remodeling and provide a possible explanation to how these two concepts relate to each other. We will further examine how alterations in different pathological contexts may relate to each other and will discuss the concept of plasticity diseases, its models and implications.

Sudden death due to paralysis and synaptic and behavioral deficits when Hip14/Zdhhc17 is deleted in adult mice

BACKGROUND

Palmitoylation, the addition of palmitate to proteins by palmitoyl acyltransferases (PATs), is an important regulator of synaptic protein localization and function. Many palmitoylated proteins and PATs have been implicated in neuropsychiatric diseases, including Huntington disease, schizophrenia, amyotrophic lateral sclerosis, Alzheimer disease, and X-linked intellectual disability. HIP14/DHHC17 is the most conserved PAT that palmitoylates many synaptic proteins. Hip14 hypomorphic mice have behavioral and synaptic deficits. However, the phenotype is developmental; thus, a model of post-developmental loss of Hip14 was generated to examine the role of HIP14 in synaptic function in the adult.

RESULTS

Ten weeks after Hip14 deletion (iHip14 Δ/Δ ), mice die suddenly from rapidly progressive paralysis. Prior to death the mice exhibit motor deficits, increased escape response during tests of anxiety, anhedonia, a symptom indicative of depressive-like behavior, and striatal synaptic deficits, including reduced probability of transmitter release and increased amplitude but decreased frequency of spontaneous post-synaptic currents. The mice also have increased brain weight due to microgliosis and astrogliosis in the cortex.

CONCLUSIONS

Behavioral changes and electrophysiological measures suggest striatal dysfunction in iHip14 Δ/Δ mice, and increased cortical volume due to astrogliosis and microgliosis suggests a novel role for HIP14 in glia. These data suggest that HIP14 is essential for maintenance of life and neuronal integrity in the adult mouse.

Striatal synaptic dysfunction and altered calcium regulation in Huntington disease

Synaptic dysfunction and altered calcium homeostasis in the brain is common to many neurodegenerative disorders. Among these, Huntington disease (HD), which is inherited in an autosomal dominant fashion, can serve as a model for investigating these mechanisms. HD generally manifests in middle age as a disorder of movement, mood and cognition. An expanded polymorphic CAG repeat in the HTT gene results in progressive neurodegeneration that impacts striatal spiny projection neurons (SPNs) earliest and most severely. Striatal SPNs receive massive glutamatergic input from cortex and thalamus, and these excitatory synapses are a focus for early changes that can trigger aberrant downstream signaling to disrupt synaptic plasticity and lead to later degeneration. Mitochondrial dysfunction and altered intracellular calcium-induced calcium release and sequestration mechanisms add to the impairments in circuit function that may underlie prodromal cognitive and subtle motor deficits. These mechanisms and implications for developing disease-modifying therapy will be reviewed here.

Influence of cortical synaptic input on striatal neuronal dendritic arborization and sensitivity to excitotoxicity in corticostriatal coculture

Corticostriatal cocultures are utilized to recapitulate the cortex-striatum connection in vitro as a convenient model to investigate the development, function, and regulation of synapses formed between cortical and striatal neurons. However, optimization of this dissociated neuronal system to more closely reproduce in vivo circuits has not yet been explored. We studied the effect of varying the plating ratio of cortical to striatal neurons on striatal spiny projection neuron (SPN) characteristics in primary neuronal cocultures. Despite the large difference in cortical-striatal neuron ratio (1:1 vs. 1:3) at day of plating, by 18 days in vitro the difference became modest (∼25% lower cortical-striatal neuron ratio in 1:3 cocultures) and the neuronal density was lower in the 1:3 cocultures, indicating enhanced loss of striatal SPNs. Comparing SPNs in cocultures plated at a 1:1 vs. 1:3 ratio, we found that resting membrane potential, input resistance, current injection-induced action potential firing rates, and input-output curves were similar in the two conditions. However, SPNs in the cocultures plated at the lower cortical ratio exhibited reduced membrane capacitance along with significantly shorter total dendritic length, decreased dendritic complexity, and fewer excitatory synapses, consistent with their trend toward reduced miniature excitatory postsynaptic current frequency. Strikingly, the proportion of NMDA receptors found extrasynaptically in recordings from SPNs was significantly higher in the less cortical coculture. Consistently, SPNs in cocultures with reduced cortical input showed decreased basal pro-survival signaling through cAMP response element binding protein and enhanced sensitivity to NMDA-induced apoptosis. Altogether, our study indicates that abundance of cortical input regulates SPN dendritic arborization and survival/death signaling.

Real-time imaging of glutamate clearance reveals normal striatal uptake in Huntington disease mouse models

It has become well accepted that Huntington disease (HD) is associated with impaired glutamate uptake, resulting in a prolonged time-course of extracellular glutamate that contributes to excitotoxicity. However, the data supporting this view come largely from work in synaptosomes, which may overrepresent nerve-terminal uptake over astrocytic uptake. Here, we quantify real-time glutamate dynamics in HD mouse models by high-speed imaging of an intensity-based glutamate-sensing fluorescent reporter (iGluSnFR) and electrophysiological recordings of synaptically activated transporter currents in astrocytes. These techniques reveal a disconnect between the results obtained in synaptosomes and those in situ. Exogenous glutamate uptake is impaired in synaptosomes, whereas real-time measures of glutamate clearance in the HD striatum are normal or even accelerated, particularly in the aggressive R6/2 model. Our results highlight the importance of quantifying glutamate dynamics under endogenous release conditions, and suggest that the widely cited uptake impairment in HD does not contribute to pathogenesis.

Huntington disease (HD) has been linked via biochemical uptake assays to impaired glutamate clearance and resultant excitotoxicity. Here, utilizing a fluorescent reporter, the authors measure real-time glutamate dynamics in mouse model HD brain slices and find normal or even accelerated glutamate clearance.

Quantitative Electroencephalographic Analysis Provides an Early-Stage Indicator of Disease Onset and Progression in the zQ175 Knock-In Mouse Model of Huntington's Disease

STUDY OBJECTIVES

Patients with Huntington's disease (HD) show a high prevalence of sleep disorders that typically occur prior to the onset of motoric symptoms and neurodegeneration. Our understanding of the pathophysiological alterations in premanifest HD is limited, hindering the ability to measure disease modification in response to treatment. We used a full-length knock-in HD model to determine early changes in the electroencephalogram (EEG) and sleep that may predict the onset and progression of the disease.

METHODS

A 10-month longitudinal study was designed to determine the effect of the HD mutation on the EEG and sleep/wake changes in heterozygous (HET) and homozygous (HOM) zQ175 mice and wild-type (WT) littermates from 8 to 48 w of age. Mice were instrumented with tethered headmounts to record EEG/electromyography signals. Telemeters were implanted to continuously measure locomotor activity (LMA) and body temperature (Tb). Sleep deprivation (SDep) was performed at 8, 12, 16, 24, 32, and 48 w of age.

RESULTS

The HD mutation disrupted the EEG field potential from 8-12 w in an age- and mutant huntington dose-dependent manner, prior to changes in sleep/wake states, LMA, and Tb. Prominent effects of the HD mutation on the EEG included a progressive reduction in low frequency power, a slowing of rapid eye movement peak theta frequency, and the emergence of state-dependent beta/gamma oscillations. There was no effect of genotype on the relative increase in nonrapid eye movement delta power or sleep time in response to SDep.

CONCLUSIONS

The expression of the Huntington's disease (HD) mutation results in complex EEG alterations that occur prior to deficits in behavioral measures and are one of the earliest phenotypes uncovered in this mouse model. Despite these EEG changes, homeostatic responses to sleep loss were preserved in HET and HOM zQ175 mice. Greater insight into the localization and response of these EEG alterations to novel therapies may enable early intervention and improve outcomes for patients with HD.

Impaired development of cortico-striatal synaptic connectivity in a cell culture model of Huntington's disease

Huntington's disease (HD) is a genetically inherited neurodegenerative disease caused by a mutation in the gene encoding the huntingtin protein. This mutation results in progressive cell death that is particularly striking in the striatum. Recent evidence indicates that early HD is initially a disease of the synapse, in which subtle alterations in synaptic neurotransmission, particularly at the cortico-striatal (C-S) synapse, can be detected well in advance of cell death. Here, we used a cell culture model in which striatal neurons are co-cultured with cortical neurons, and monitored the development of C-S connectivity up to 21days in vitro (DIV) in cells cultured from either the YAC128 mouse model of HD or the background strain, FVB/N (wild-type; WT) mice. Our data demonstrate that while C-S connectivity in WT co-cultures develops rapidly and continuously from DIV 7 to 21, YAC128 C-S connectivity shows no significant growth from DIV 14 onward. Morphological and electrophysiological data suggest that a combination of pre- and postsynaptic mechanisms contribute to this effect, including a reduction in both the postsynaptic dendritic arborization and the size and replenishment rate of the presynaptic readily releasable pool of excitatory vesicles. Moreover, a chimeric culture strategy confirmed that the most robust impairment in C-S connectivity was only observed when mutant huntingtin was expressed both pre- and postsynaptically. In all, our data demonstrate a progressive HD synaptic phenotype in this co-culture system that may be exploited as a platform for identifying promising therapeutic strategies to prevent early HD-associated synaptopathy.

Differential changes in thalamic and cortical excitatory synapses onto striatal spiny projection neurons in a Huntington disease mouse model

Huntington disease (HD), a neurodegenerative disorder caused by CAG repeat expansion in the gene encoding huntingtin, predominantly affects the striatum, especially the spiny projection neurons (SPN). The striatum receives excitatory input from cortex and thalamus, and the role of the former has been well-studied in HD. Here, we report that mutated huntingtin alters function of thalamostriatal connections. We used a novel thalamostriatal (T-S) coculture and an established corticostriatal (C-S) coculture, generated from YAC128 HD and WT (FVB/NJ background strain) mice, to investigate excitatory neurotransmission onto striatal SPN. SPN in T-S coculture from WT mice showed similar mini-excitatory postsynaptic current (mEPSC) frequency and amplitude as in C-S coculture; however, both the frequency and amplitude were significantly reduced in YAC128 T-S coculture. Further investigation in T-S coculture showed similar excitatory synapse density in WT and YAC128 SPN dendrites by immunostaining, suggesting changes in total dendritic length or probability of release as possible explanations for mEPSC frequency changes. Synaptic N-methyl-D-aspartate receptor (NMDAR) current was similar, but extrasynaptic current, associated with cell death signaling, was enhanced in YAC128 SPN in T-S coculture. Employing optical stimulation of cortical versus thalamic afferents and recording from striatal SPN in brain slice, we found increased glutamate release probability and reduced AMPAR/NMDAR current ratios in thalamostriatal synapses, most prominently in YAC128. Enhanced extrasynaptic NMDAR current in YAC128 SPN was apparent with both cortical and thalamic stimulation. We conclude that thalamic afferents to the striatum are affected early, prior to an overt HD phenotype; however, changes in NMDAR localization in SPN are independent of the source of glutamatergic input.