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

Neuroprotection by ADAM10 inhibition requires TrkB signaling in the Huntington's disease hippocampus

Synaptic dysfunction is an early pathogenic event leading to cognitive decline in Huntington's disease (HD). We previously reported that the active ADAM10 level is increased in the HD cortex and striatum, causing excessive proteolysis of the synaptic cell adhesion protein N-Cadherin. Conversely, ADAM10 inhibition is neuroprotective and prevents cognitive decline in HD mice. Although the breakdown of cortico-striatal connection has been historically linked to cognitive deterioration in HD, dendritic spine loss and long-term potentiation (LTP) defects identified in the HD hippocampus are also thought to contribute to the cognitive symptoms of the disease. The aim of this study is to investigate the contribution of ADAM10 to spine pathology and LTP defects of the HD hippocampus. We provide evidence that active ADAM10 is increased in the hippocampus of two mouse models of HD, leading to extensive proteolysis of N-Cadherin, which has a widely recognized role in spine morphology and synaptic plasticity. Importantly, the conditional heterozygous deletion of ADAM10 in the forebrain of HD mice resulted in the recovery of spine loss and ultrastructural synaptic defects in CA1 pyramidal neurons. Meanwhile, normalization of the active ADAM10 level increased the pool of synaptic BDNF protein and activated ERK neuroprotective signaling in the HD hippocampus. We also show that the ADAM10 inhibitor GI254023X restored LTP defects and increased the density of mushroom spines enriched with GluA1-AMPA receptors in HD hippocampal neurons. Notably, we report that administration of the TrkB antagonist ANA12 to HD hippocampal neurons reduced the beneficial effect of GI254023X, indicating that the BDNF receptor TrkB contributes to mediate the neuroprotective activity exerted by ADAM10 inhibition in HD. Collectively, these findings indicate that ADAM10 inhibition coupled with TrkB signaling represents an efficacious strategy to prevent hippocampal synaptic plasticity defects and cognitive dysfunction in HD.

Pre-symptomatic synaptic dysfunction and longitudinal decay of hippocampal synaptic function in APPPS1 mouse model of Alzheimer's disease is sex-independent

Alzheimer's disease (AD) is an incurable, age-related and progressive neurodegenerative disease characterized by cognitive impairments. Deficits in synaptic plasticity were reported in various models of AD-like pathology and are considered as an early contributing factor of cognitive impairment. However, the majority of previous studies were focused on overt, symptomatic stages of pathology and assessed long-term potentiation (LTP), whereas long-term depression (LTD) was much less investigated and the precise nature of its involvement remains poorly defined. To better understand the earliest synaptic dysfunctions along the pre-symptomatic stage of AD-like pathology, we performed a detailed analysis of underlying mechanisms and quantified basal synaptic activity, presynaptic release probability, and synaptic plasticity such as post-tetanic potentiation (PTP), as well as LTP and LTD. These parameters were studied in APPPS1 mouse model at two time points (early- and mid-) along the pre-symptomatic stage, which were compared with alterations monitored at two later time-points, i.e. the onset of cognitive deficits and the overt stage of full-blown pathology. Because sex is known to be an instrumental biological parameter in AD pathophysiology, all alterations were assessed in both males and females. Our data show that, as compared to wild-type (WT) littermates, initial neuronal hyperexcitability, seen at early pre-symptomatic stage shifts subsequently towards hypoexcitability at mid-pre-symptomatic stage and remains impaired at advanced stages. The pre-symptomatic changes also involve increased synaptic plasticity as assessed by paired-pulse facilitation (PPF), which returns to basal level at the onset of pathology and remains stable afterwards. Synaptic plasticity is impaired by mid-pre-symptomatic stage and manifests as lowered LTP and absence of LTD induction, the latter being reported here for the first time. Observed LTP and LTD impairments both persist in older APPPS1 mice. Remarkably, none of the observed differences was gender-dependent. Altogether, our data evidence that major impairments in basal synaptic efficacy and plasticity are detectable already during mid-pre-symptomatic stage of AD-like pathogenesis and likely involve hyperexcitability as the underlying mechanism. Our study also uncovers synaptic alterations that may become critical read-outs for testing the efficiency of novel, pre-symptomatic stage-targeted therapies for AD.

Longitudinal spatial mapping of lipid metabolites reveals pre-symptomatic changes in the hippocampi of Huntington's disease transgenic mice

In Huntington's disease (HD), a key pathological feature includes the development of inclusion-bodies of fragments of the mutant huntingtin protein in the neurons of the striatum and hippocampus. To examine the molecular changes associated with inclusion-body formation, we applied MALDI-mass spectrometry imaging and deuterium pulse labelling to determine lipid levels and synthesis rates in the hippocampus of a transgenic mouse model of HD (R6/1 line). The R6/1 HD mice lacked inclusions in the hippocampus at 6 weeks of age (pre-symptomatic), whereas inclusions were pervasive by 16 weeks of age (symptomatic). Hippocampal subfields (CA1, CA3 and DG), which formed the highest density of inclusion formation in the mouse brain showed a reduction in the relative abundance of neuron-enriched lipids that have roles in neurotransmission, synaptic plasticity, neurogenesis, and ER-stress protection. Lipids involved in the adaptive response to ER stress (phosphatidylinositol, phosphatidic acid, and ganglioside classes) displayed increased rates of synthesis in HD mice relative to WT mice at all the ages examined, including prior to the formation of the inclusion bodies. Our findings, therefore, support a role for ER stress occurring pre-symptomatically and potentially contributing to pathological mechanisms underlying 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.

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

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

Impaired Refinement of Kinematic Variability in Huntington Disease Mice on an Automated Home Cage Forelimb Motor Task

The effective development of novel therapies in mouse models of neurologic disorders relies on behavioral assessments that provide accurate read-outs of neuronal dysfunction and/or degeneration. We designed an automated behavioral testing system (PiPaw), which integrates an operant lever-pulling task directly into the mouse home cage. This task is accessible to group-housed mice 24 h per day, enabling high-throughput longitudinal analysis of forelimb motor learning. Moreover, this design eliminates the need for exposure to novel environments and minimizes experimenter interaction, significantly reducing two of the largest stressors associated with animal behavior. Male mice improved their performance of this task over 1 week of testing by reducing intertrial variability of reward-related kinematic parameters (pull amplitude or peak velocity). In addition, mice displayed short-term improvements in reward rate, and a concomitant decrease in movement variability, over the course of brief bouts of task engagement. We used this system to assess motor learning in mouse models of the inherited neurodegenerative disorder, Huntington disease (HD). Despite having no baseline differences in task performance, male Q175-FDN HD mice were unable to modulate the variability of their movements to increase reward on either short or long timescales. Task training was associated with a decrease in the amplitude of spontaneous excitatory activity recorded from striatal medium spiny neurons in the hemisphere contralateral to the trained forelimb in WT mice; however, no such changes were observed in Q175-FDN mice. This behavioral screening platform should prove useful for preclinical drug trials toward improved treatments in HD and other neurologic disorders.SIGNIFICANCE STATEMENT In order to develop effective therapies for neurologic disorders, such as Huntington disease (HD), it is important to be able to accurately and reliably assess the behavior of mouse models of these conditions. Moreover, these behavioral assessments should provide an accurate readout of underlying neuronal dysfunction and/or degeneration. In this paper, we used an automated behavioral testing system to assess motor learning in mice within their home cage. Using this system, we were able to study motor abnormalities in HD mice with an unprecedented level of detail, and identified a specific behavioral deficit associated with an underlying impairment in striatal neuronal plasticity. These results validate the usefulness of this system for assessing behavior in mouse models of HD and other neurologic disorders.

Isoform-Specific Reduction of the Basic Helix-Loop-Helix Transcription Factor TCF4 Levels in Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder with onset of characteristic motor symptoms at midlife, preceded by subtle cognitive and behavioral disturbances. Transcriptional dysregulation emerges early in the disease course and is considered central to HD pathogenesis. Using wild-type (wt) and HD knock-in mouse striatal cell lines we observed a HD genotype-dependent reduction in the protein levels of transcription factor 4 (TCF4), a member of the basic helix-loop-helix (bHLH) family with critical roles in brain development and function. We characterized mouse Tcf4 gene structure and expression of alternative mRNAs and protein isoforms in cell-based models of HD, and in four different brain regions of male transgenic HD mice (R6/1) from young to mature adulthood. The largest decrease in the levels of TCF4 at mRNA and specific protein isoforms were detected in the R6/1 mouse hippocampus. Translating this finding to human disease, we found reduced expression of long TCF4 isoforms in the postmortem hippocampal CA1 area and in the cerebral cortex of HD patients. Additionally, TCF4 protein isoforms showed differential synergism with the proneural transcription factor ASCL1 in activating reporter gene transcription in hippocampal and cortical cultured neurons. Induction of neuronal activity increased these synergistic effects in hippocampal but not in cortical neurons, suggesting brain region-dependent differences in TCF4 functions. Collectively, this study demonstrates isoform-specific changes in TCF4 expression in HD that could contribute to the progressive impairment of transcriptional regulation and neuronal function in this disease.

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.

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.

Evaluation of early microstructural changes in the R6/1 mouse model of Huntington's disease by ultra-high field diffusion MR imaging

Diffusion MRI (dMRI) has been able to detect early structural changes related to neurological symptoms present in Huntington's disease (HD). However, there is still a knowledge gap to interpret the biological significance at early neuropathological stages. The purpose of this study is two-fold: (i) establish if the combination of Ultra-High Field Diffusion MRI (UHFD-MRI) techniques can add a more comprehensive analysis of the early microstructural changes observed in HD, and (ii) evaluate if early changes in dMRI microstructural parameters can be linked to cellular biomarkers of neuroinflammation. Ultra-high field magnet (16.7T), diffusion tensor imaging (DTI), and neurite orientation dispersion and density imaging (NODDI) techniques were applied to fixed ex-vivo brains of a preclinical model of HD (R6/1 mice). Fractional anisotropy (FA) was decreased in deep and superficial grey matter (GM) as well as white matter (WM) brain regions with well-known early HD microstructure and connectivity pathology. NODDI parameters associated with the intracellular and extracellular compartment, such as intracellular ventricular fraction (ICVF), orientation dispersion index (ODI), and isotropic volume fractions (IsoVF) were altered in R6/1 mice GM. Further, histological studies in these areas showed that glia cell markers associated with neuroinflammation (GFAP & Iba1) were consistent with the dMRI findings. dMRI can be used to extract non-invasive information of neuropathological events present in the early stages of HD. The combination of multiple imaging techniques represents a better approach to understand the neuropathological process allowing the early diagnosis and neuromonitoring of patients affected by HD.

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.

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.