Cognitive scores in carriers of Huntington's disease gene compared to noncarriers

In Search of a Function for the N6-Methyladenosine in Epitranscriptome, Autophagy and Neurodegenerative Diseases

Changes in epitranscriptome with N6-methyladenine (m6A) modification could be involved in the development of multiple diseases, which might be a prevalent modification of messenger RNAs (mRNAs) in eukaryotes. The m6A modification might be performed through the action of methyltransferases, demethylases, and methylation-binding proteins. Importantly, the m6A methylation may be associated with various neurological disorders including Alzheimer's disease (AD), Parkinson's disease (PD), depression, aging-related diseases, and/or aging itself. In addition, the m6A methylation might functionally regulate the eukaryotic transcriptome by influencing the splicing, export, subcellular localization, translation, stability, and decay of mRNAs. Neurodegenerative diseases may possess a wide variety of phenotypes, depending on the neurons that degenerate on occasion. Interestingly, an increasing amount of evidence has indicated that m6A modification could modulate the expression of autophagy-related genes and promote autophagy in neuronal cells. Oxidative stresses such as reactive oxygen species (ROS) could stimulate the m6A RNA methylation, which may also be related to the regulation of autophagy and/or the development of neurodegenerative diseases. Both m6A modification and autophagy could also play critical roles in regulating the health condition of neurons. Therefore, a comprehensive understanding of the m6A and autophagy relationship in human diseases may benefit in developing therapeutic strategies in the future. This paper reviews advances in the understanding of the regulatory mechanisms of m6A modification in the occurrence and development of neurodegenerative diseases and/or aging, discussing the possible therapeutic procedures related to mechanisms of m6A RNA methylation and autophagy.

Altered m6A RNA methylation contributes to hippocampal memory deficits in Huntington's disease mice

N6-methyladenosine (m6A) regulates many aspects of RNA metabolism and is involved in learning and memory processes. Yet, the impact of a dysregulation of post-transcriptional m6A editing on synaptic impairments in neurodegenerative disorders remains unknown. Here we investigated the m6A methylation pattern in the hippocampus of Huntington’s disease (HD) mice and the potential role of the m6A RNA modification in HD cognitive symptomatology. m6A modifications were evaluated in HD mice subjected to a hippocampal cognitive training task through m6A immunoprecipitation sequencing (MeRIP-seq) and the relative levels of m6A-modifying proteins (FTO and METTL14) by subcellular fractionation and Western blot analysis. Stereotaxic CA1 hippocampal delivery of AAV-shFTO was performed to investigate the effect of RNA m6A dysregulation in HD memory deficits. Our results reveal a m6A hypermethylation in relevant HD and synaptic related genes in the hippocampal transcriptome of Hdh^+/Q111 mice. Conversely, m6A is aberrantly regulated in an experience-dependent manner in the HD hippocampus leading to demethylation of important components of synapse organization. Notably, the levels of RNA demethylase (FTO) and methyltransferase (METTL14) were modulated after training in the hippocampus of WT mice but not in Hdh^+/Q111 mice. Finally, inhibition of FTO expression in the hippocampal CA1 region restored memory disturbances in symptomatic Hdh^+/Q111 mice. Altogether, our results suggest that a differential RNA methylation landscape contributes to HD cognitive symptoms and uncover a role of m6A as a novel hallmark of HD.

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

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

Language disintegration in spontaneous speech in Huntington's disease: a more fine-grained analysis

Huntington's disease (HD) is a neurodegenerative disease causing motor symptoms along with cognitive and affective problems. Recent evidence suggests that HD also affects language across core levels of linguistic organization, including at stages of the disease when standardized neuropsychological test profiles are still normal and motor symptoms do not yet reach clinical thresholds ('pre-manifest HD'). The present study aimed to subject spontaneous speech to a more fine-grained linguistic analysis in a sample of 20 identified HD gene-carriers, 10 with pre-manifest and 10 with early manifest HD. We further explored how language performance related to non-linguistic cognitive impairment, using standardized neuropsychological measures. A distinctive pattern of linguistic impairments marked off participants with both pre-manifest and manifest HD from healthy controls and each other. Fluency patterns in premanifest HD were marked by prolongations, filled pauses, and repetitions, which shifted to a pattern marked by empty (unfilled) pauses, re-phrasings, and truncations in manifest HD. Both HD groups also significantly differed from controls and each other in how they grammatically connected clauses and used noun phrases referentially. Functional deficits in language occurred in pre-manifest HD in the absence of any non-linguistic neuropsychological impairment and did largely not correlate with standardized neuropsychological measures in manifest HD. These results further corroborate that language can act as a fine-grained clinical marker in HD, which can track disease progression from the pre-manifest stage, define critical remediation targets, and inform the role of the basal ganglia in language processing.

The Neuropsychology of Huntington's Disease

Huntington's disease is an inherited, degenerative brain disease, characterized by involuntary movements, cognitive disorder and neuropsychiatric change. Men and women are affected equally. Symptoms emerge at around 40 years, although there is wide variation. A rare juvenile form has onset in childhood or adolescence. The evolution of disease is insidious and structural and functional brain changes may be present more than a decade before symptoms and signs become manifest. The earliest site of pathology is the striatum and neuroimaging measures of striatal change correlate with neurological and cognitive markers of disease. Chorea and other aspects of the movement disorder are the most visible aspect of the disease. However, non-motor features have greatest affect on functional independence and quality of life, so require recognition and management. The evidence-base for non-pharmacological treatments in Huntington's disease is currently limited, but recent intervention studies are encouraging.

A systematic linguistic profile of spontaneous narrative speech in pre-symptomatic and early stage Huntington's disease

Cognitive decline accompanying the clinically more salient motor symptoms of Huntington's disease (HD) has been widely noted and can precede motor symptoms onset. Less clear is how such decline bears on language functions in everyday life, though a small number of experimental studies have revealed difficulties with the application of rule-based aspects of language in early stages of the disease. Here we aimed to determine whether there is a systematic linguistic profile that characterizes spontaneous narrative speech in both pre-manifest and/or early manifest HD, and how it is related to striatal degeneration and neuropsychological profiles. Twenty-eight early-stage patients (19 manifest and 9 gene-carriers in the pre-manifest stage), matched with 28 controls, participated in a story-telling task. Speech was blindly scored by independent raters according to fine-grained linguistic variables distributed over 5 domains for which composite scores were computed (Quantitative, Fluency, Reference, Connectivity, and Concordance). Voxel-based morphometry (VBM) was used to link specific brain degeneration patterns to loci of linguistic decline. In all of these domains, significant differences were observed between groups. Deficits in Reference and Connectivity were seen in the pre-manifest stage, where no other neuropsychological impairment was detected. Among HD patients, there was a significant positive correlation only between the values in the Quantitative domain and gray matter volume bilaterally in the putamen and pallidum. These results fill the gap of qualitative data of spontaneous narrative speech in HD and reveal that HD is characterized by systematic linguistic impairments leading to dysfluencies and disorganization in core domains of grammatical organization. This includes the referential use of noun phrases and the embedding of clauses, which mediate crucial dimensions of meaning in language in its normal social use. Moreover, such impairment is seen prior to motor symptoms onset and when standardized neuropsychological test profiles are otherwise normal.

Treadmill exercise delays the onset of non-motor behaviors and striatal pathology in the CAG140 knock-in mouse model of Huntington's disease

Depression, cognitive impairments, and other neuropsychiatric disturbances are common during the prodromal phase of Huntington's disease (HD) well before the onset of classical motor symptoms of this degenerative disorder. The purpose of this study was to examine the potential impact of physical activity in the form of exercise on a motorized treadmill on non-motor behavioral features including depression-like behavior and cognition in the CAG₁₄₀ knock-in (KI) mouse model of HD. The CAG₁₄₀ KI mouse model has a long lifespan compared to other HD rodent models with HD motor deficits emerging after 12months of age and thus provides the opportunity to investigate early life interventions such as exercise on disease progression. Motorized treadmill running was initiated at 4weeks of age (1h per session, 3 times per week) and continued for 6months. Non-motor behaviors were assessed up to 6months of age and included analysis of depression-like behavior (using the tail-suspension and forced-swim tests) and cognition (using the T-maze and object recognition tests). At both 4 and 6months of age, CAG₁₄₀ KI mice displayed significant depression-like behavior in the forced swim and tail suspension tests and cognitive impairment by deficits in reversal relearning in the T-maze test. These deficits were not evident in mice engaged in treadmill running. In addition, exercise restored striatal dopamine D2 receptor expression and dopamine neurotransmitter levels both reduced in sedentary HD mice. Finally, we examined the pattern of striatal expression of mutant huntingtin (mHTT) protein and showed that the number and intensity of immunohistochemical staining patterns of intranuclear aggregates were significantly reduced with exercise. Altogether these findings begin to address the potential impact of lifestyle and early intervention such as exercise on modifying HD progression.

Phosphodiesterase Inhibitors as a Therapeutic Approach to Neuroprotection and Repair

A wide diversity of perturbations of the central nervous system (CNS) result in structural damage to the neuroarchitecture and cellular defects, which in turn are accompanied by neurological dysfunction and abortive endogenous neurorepair. Altering intracellular signaling pathways involved in inflammation and immune regulation, neural cell death, axon plasticity and remyelination has shown therapeutic benefit in experimental models of neurological disease and trauma. The second messengers, cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP), are two such intracellular signaling targets, the elevation of which has produced beneficial cellular effects within a range of CNS pathologies. The only known negative regulators of cyclic nucleotides are a family of enzymes called phosphodiesterases (PDEs) that hydrolyze cyclic nucleotides into adenosine monophosphate (AMP) or guanylate monophosphate (GMP). Herein, we discuss the structure and physiological function as well as the roles PDEs play in pathological processes of the diseased or injured CNS. Further we review the approaches that have been employed therapeutically in experimental paradigms to block PDE expression or activity and in turn elevate cyclic nucleotide levels to mediate neuroprotection or neurorepair as well as discuss both the translational pathway and current limitations in moving new PDE-targeted therapies to the clinic.

Cognitive assessment strategies in Huntington's disease research

The number of studies examining cognition in Huntington's disease (HD) has increased dramatically in recent decades, and cognitive research methods in HD have become much more sophisticated. In this review, we provide a summary of the advances in cognitive research in HD to date, and outline the key considerations for researchers planning to include cognitive assessment in their studies of HD. In particular, we discuss consideration of structure-function relationships, selection of tests appropriate to the population, choice of materials and issues of intellectual property, consideration of variables which can confound studies of cognition in HD, practice effects, and specific issues for multi-site research. Finally, we discuss future directions for cognitive assessment in HD research.

A role for Kalirin-7 in corticostriatal synaptic dysfunction in Huntington's disease

Cognitive dysfunction is an early clinical hallmark of Huntington's disease (HD) preceding the appearance of motor symptoms by several years. Neuronal dysfunction and altered corticostriatal connectivity have been postulated to be fundamental to explain these early disturbances. However, no treatments to attenuate cognitive changes have been successful: the reason may rely on the idea that the temporal sequence of pathological changes is as critical as the changes per se when new therapies are in development. To this aim, it becomes critical to use HD mouse models in which cognitive impairments appear prior to motor symptoms. In this study, we demonstrate procedural memory and motor learning deficits in two different HD mice and at ages preceding motor disturbances. These impairments are associated with altered corticostriatal long-term potentiation (LTP) and specific reduction of dendritic spine density and postsynaptic density (PSD)-95 and spinophilin-positive clusters in the cortex of HD mice. As a potential mechanism, we described an early decrease of Kalirin-7 (Kal7), a guanine-nucleotide exchange factor for Rho-like small GTPases critical to maintain excitatory synapse, in the cortex of HD mice. Supporting a role for Kal7 in HD synaptic deficits, exogenous expression of Kal7 restores the reduction of excitatory synapses in HD cortical cultures. Altogether, our results suggest that cortical dysfunction precedes striatal disturbances in HD and underlie early corticostriatal LTP and cognitive defects. Moreover, we identified diminished Kal7 as a key contributor to HD cortical alterations, placing Kal7 as a molecular target for future therapies aimed to restore corticostriatal function in HD.

Neurotrophin receptor p75(NTR) mediates Huntington's disease-associated synaptic and memory dysfunction

Learning and memory deficits are early clinical manifestations of Huntington's disease (HD). These cognitive impairments have been mainly associated with frontostriatal HD pathology; however, compelling evidence provided by several HD murine models suggests that the hippocampus may contribute to synaptic deficits and memory dysfunction in HD. The neurotrophin receptor p75(NTR) negatively regulates spine density, which is associated with learning and memory; therefore, we explored whether disturbed p75(NTR) function in the hippocampus could contribute to synaptic dysfunction and memory deficits in HD. Here, we determined that levels of p75(NTR) are markedly increased in the hippocampus of 2 distinct mouse models of HD and in HD patients. Normalization of p75(NTR) levels in HD mutant mice heterozygous for p75(NTR) prevented memory and synaptic plasticity deficits and ameliorated dendritic spine abnormalities, likely through normalization of the activity of the GTPase RhoA. Moreover, viral-mediated overexpression of p75(NTR) in the hippocampus of WT mice reproduced HD learning and memory deficits, while knockdown of p75(NTR) in the hippocampus of HD mice prevented cognitive decline. Together, these findings provide evidence of hippocampus-associated memory deficits in HD and demonstrate that p75(NTR) mediates synaptic, learning, and memory dysfunction in HD.

The effect of interference on temporal order memory in premanifest and manifest Huntington's disease

BACKGROUND

Frontal-striatal dysfunction has been linked to cognitive impairment in Huntington's disease (HD). The frontal lobes play a role in memory for the temporal order in which items occur in a sequence. However, little is known about temporal order memory in HD or how it may be affected by interference.

OBJECTIVE

The study assessed temporal order memory in patients with manifest HD (n = 20), premanifest gene carriers for HD (Pre-HD; n = 18), and controls (n = 25) using a computerized radial 8-arm maze.

METHODS

On the sample phase of each trial, participants viewed a random sequence of circles appearing one at a time at the end of each arm. On the choice phase, participants viewed two sample phase circles and chose the circle occurring earliest in the sequence. Manipulations of the temporal lag (defined as the number of circles occurring in the sample phase sequence between the two choice phase circles) were conducted to systematically vary interference. Temporally proximal lags were hypothesized to generate more interference relative to temporally distal lags.

RESULTS

The Pre-HD group was significantly impaired (p < 0.05) compared to controls on proximal temporal lags (high interference) but matched controls on distal lags (low interference). HD patients improved as a function of increased lag but demonstrated significant impairments (p < 0.05) across lags relative to controls.

CONCLUSIONS

Temporal order memory is differentially affected by interference during the premanifest and manifest stages of HD. The study identifies a fundamental, yet relatively unexamined, deficit associated with HD.

Risk-taking and pathological gambling behavior in Huntington's disease

Huntington's disease (HD) is a genetic, neurodegenerative disorder, which specifically affects striatal neurons of the indirect pathway, resulting in a progressive decline in muscle coordination and loss of emotional and cognitive control. Interestingly, predisposition to pathological gambling and other addictions involves disturbances in the same cortico-striatal circuits that are affected in HD, and display similar disinhibition-related symptoms, including changed sensitivity to punishments and rewards, impulsivity, and inability to consider long-term advantages over short-term rewards. Both HD patients and pathological gamblers also show similar performance deficits on risky decision-making tasks, such as the Iowa Gambling Task (IGT). These similarities suggest that HD patients are a likely risk group for gambling problems. However, such problems have only incidentally been observed in HD patients. In this review, we aim to characterize the risk of pathological gambling in HD, as well as the underlying neurobiological mechanisms. Especially with the current rise of easily accessible Internet gambling opportunities, it is important to understand these risks and provide appropriate patient support accordingly. Based on neuropathological and behavioral findings, we propose that HD patients may not have an increased tendency to seek risks and start gambling, but that they do have an increased chance of developing an addiction once they engage in gambling activities. Therefore, current and future developments of Internet gambling possibilities and related addictions should be regarded with care, especially for vulnerable groups like HD patients.

The role of dopamine in Huntington's disease

Alterations in dopamine (DA) neurotransmission in Parkinson's disease are well known and widely studied. Much less is known about DA changes that accompany and underlie some of the symptoms of Huntington's disease (HD), a dominant inherited neurodegenerative disorder characterized by chorea, cognitive deficits, and psychiatric disturbances. The cause is an expansion in CAG (glutamine) repeats in the HTT gene. The principal histopathology of HD is the loss of medium-sized spiny neurons (MSNs) and, to a lesser degree, neuronal loss in cerebral cortex, thalamus, hippocampus, and hypothalamus. Neurochemical, electrophysiological, and behavioral studies in HD patients and genetic mouse models suggest biphasic changes in DA neurotransmission. In the early stages, DA neurotransmission is increased leading to hyperkinetic movements that can be alleviated by depleting DA stores. In contrast, in the late stages, DA deficits produce hypokinesia that can be treated by increasing DA function. Alterations in DA neurotransmission affect glutamate receptor modulation and could contribute to excitotoxicity. The mechanisms of DA dysfunction, in particular the increased DA tone in the early stages of the disease, are presently unknown but may include initial upregulation of DA neuron activity caused by the genetic mutation, reduced inhibition resulting from striatal MSN loss, increased excitation from cortical inputs, and DA autoreceptor dysfunction. Targeting both DA and glutamate receptor dysfunction could be the best strategy to treat HD symptoms.

The importance of considering all attributes of memory in behavioral endophenotyping of mouse models of genetic disease

In order to overcome difficulties in evaluating cognitive function in mouse models of genetic disorders, it is critical to take into account the background strain of the mouse and reported phenotypes in the clinical population being studied. Recent studies have evaluated cognitive function across a number of background strains and found that spatial memory assayed by the water maze and contextual fear conditioning often does not provide optimal results. The logical extension to these results is to emphasize not only spatial, but all attributes or domains of memory function in behavioral phenotyping experiments. A careful evaluation of spatial, temporal, sensory/perceptual, affective, response, executive, proto-linguistic, and social behaviors designed to specifically evaluate the cognitive function each mouse model can be performed in a rapid, relatively high throughput manner. Such results would not only provide a more comprehensive snapshot of brain function in mouse disease models than the more common approach that approaches nonspecific spatial memory tasks to evaluate cognition, but also would better model the disorders being studied.

Phosphodiesterases in neurodegenerative disorders

Cyclic nucleotide phosphodiesterases (PDEs) are responsible for the breakdown of cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). As such, they are crucial regulators of levels of cyclic nucleotide-mediated signaling. cAMP signaling and cGMP signaling have been associated with neuroplasticity and protection, and influencing their levels in the cell by inhibition of PDEs has become a much studied target for treatment in a wide array of disorders, including neurodegenerative disorders. In this review, we will focus on the involvement of PDEs in neurodegenerative disorders. In comparison with preclinical work, data on human patients are scarce. Alzheimer's disease is associated with changes in PDE4, PDE7, and PDE8 expression in the brain. Altered functioning of PDE4 as well as PDE11 is associated with major depressive disorder. In multiple sclerosis, there are indications of alterations in expression of several PDE subtypes in the central nervous system; however, evidence is indirect. In Huntington's disease and Parkinson's disease, most research has focused on PDE1B and PDE10, because of their abundant presence in striatal neurons. In another rare, neurodegenerative striatal motor disorder, that is, autosomal-dominant striatal degeneration, genetic defects in PDE8B gene are thought to underlie the neurodegenerative processes. Although the latter disorder has showed a causative dysfunction of PDEs, this does not hold for the neurodegenerative disorders discussed above, in which changes in PDE levels seemingly rather represent secondary changes and compensation to prior existing dysfunction. However, normalizing cyclic nucleotide signaling via PDE inhibition remains interesting for the treatment of neurodegenerative disorders.

Dysfunction of the ubiquitin ligase Ube3a may be associated with synaptic pathophysiology in a mouse model of Huntington disease

Huntington disease (HD) is a hereditary neurodegenerative disorder characterized by progressive cognitive, psychiatric, and motor symptoms. The disease is caused by abnormal expansion of CAG repeats in the gene encoding huntingtin, but how mutant huntingtin leads to early cognitive deficits in HD is poorly understood. Here, we demonstrate that the ubiquitin ligase Ube3a, which is implicated in synaptic plasticity and involved in the clearance of misfolded polyglutamine protein, is strongly recruited to the mutant huntingtin nuclear aggregates, resulting in significant loss of its functional pool in different regions of HD mouse brain. Interestingly, Arc, one of the substrates of Ube3a linked with synaptic plasticity, is also associated with nuclear aggregates, although its synaptic level is increased in the hippocampus and cortex of HD mouse brain. Different regions of HD mouse brain also exhibit decreased levels of AMPA receptors and various pre- and postsynaptic proteins, which could be due to the partial loss of function of Ube3a. Transient expression of mutant huntingtin in mouse primary cortical neurons further demonstrates recruitment of Ube3a into mutant huntingtin aggregates, increased accumulation of Arc, and decreased numbers of GluR1 puncta in the neuronal processes. Altogether, our results suggest that the loss of function of Ube3a might be associated with the synaptic abnormalities observed in HD.

Abnormal error-related antisaccade activation in premanifest and early manifest Huntington disease

OBJECTIVE

Individuals with the trinucleotide CAG expansion (CAG+) that causes Huntington's disease (HD) have impaired performance on antisaccade (AS) tasks that require directing gaze in the mirror opposite direction of visual targets. This study aimed to identify the neural substrates underlying altered antisaccadic performance.

METHOD

Three groups of participants were recruited: (1) Imminent and early manifest HD (early HD, n = 8); (2) premanifest (presymptomatic) CAG+ (preHD, n = 10); and (3) CAG unexpanded (CAG-) controls (n = 12). All participants completed a uniform study visit that included a neurological evaluation, neuropsychological battery, molecular testing, and functional MRI during an AS task. The blood oxygenation level dependent (BOLD) response was obtained during saccade preparation and saccade execution for both correct and incorrect responses using regression analysis.

RESULTS

Significant group differences in BOLD response were observed when comparing incorrect AS to correct AS execution. Specifically, as the percentage of incorrect AS increased, BOLD responses in the CAG- group decreased progressively in a well-documented reward detection network that includes the presupplementary motor area and dorsal anterior cingulate cortex. In contrast, AS errors in the preHD and early HD groups lacked this relationship with BOLD signal in the error detection network, and BOLD responses to AS errors were smaller in the two CAG+ groups as compared with the CAG- group.

CONCLUSIONS

These results are the first to suggest that abnormalities in an error-related response network may underlie early changes in AS eye movements in premanifest and early manifest HD. (PsycINFO Database Record (c) 2011 APA, all rights reserved).

Biological markers of cognition in prodromal Huntington's disease: a review

Huntington's disease (HD), an autosomal-dominant genetic disorder, has historically been viewed as a degenerative movement disorder but it also includes psychiatric symptoms and progressive cognitive decline. There has been a lack of consensus in the literature about whether or not cognitive signs can be detected in carriers before clinical (motor) onset of the disease, i.e., prodromal HD. However, recently validated mathematical formulas to estimate age of clinical onset, refined over the past 5-7 years, have allowed researchers to overcome the methodological limitation of treating all prodromal carriers as a homogenous high-risk group (i.e., whether they may be 2 or 15 years from diagnosis). Here we review 23 articles on the HD prodrome, all of which related cognition to a biological marker of disease burden (i.e., genetic load, neuroimaging). All studies found at least one cognitive domain was associated with disease burden in prodromal HD participants. There was greater variability in both the detection and cognitive domain affected in those farther from onset (or those with less pathology) while most studies reliably found declines in visuomotor performance and working memory in those closer to onset. These findings indicate that cognitive signs can be reliably detected in the HD prodrome when comparing cognition to additional disease markers, however, there continues to be significant variability on cognitive findings among large and methodologically rigorous studies. This may reflect true heterogeneity in the prodromal HD phenotype which must be further explored by analyzing intra-individual variance, determining demographic risk factors associated with decline/protection, and examining if particular HD families exhibit distinct cognitive profiles. These and additional future directions are discussed.

Behavioral and in vivo electrophysiological evidence for presymptomatic alteration of prefrontostriatal processing in the transgenic rat model for huntington disease

Cognitive decline precedes motor symptoms in Huntington disease (HD). A transgenic rat model for HD carrying only 51 CAG repeats recapitulates the late-onset HD phenotype. Here, we assessed prefrontostriatal function in this model through both behavioral and electrophysiological assays. Behavioral examination consisted in a temporal bisection task within a supra-second range (2 vs.8 s), which is thought to involve prefrontostriatal networks. In two independent experiments, the behavioral analysis revealed poorer temporal sensitivity as early as 4 months of age, well before detection of overt motor deficits. At a later symptomatic age, animals were impaired in their temporal discriminative behavior. In vivo recording of field potentials in the dorsomedial striatum evoked by stimulation of the prelimbic cortex were studied in 4- to 5-month-old rats. Input/output curves, paired-pulse function, and plasticity induced by theta-burst stimulation (TBS) were assessed. Results showed an altered plasticity, with higher paired-pulse facilitation, enhanced short-term depression, as well as stronger long-term potentiation after TBS in homozygous transgenic rats. Results from the heterozygous animals mostly fell between wild-type and homozygous transgenic rats. Our results suggest that normal plasticity in prefrontostriatal circuits may be necessary for reliable and precise timing behavior. Furthermore, the present study provides the first behavioral and electrophysiological evidence of a presymptomatic alteration of prefrontostriatal processing in an animal model for Huntington disease and suggests that supra-second timing may be the earliest cognitive dysfunction in HD.

Increased PKA signaling disrupts recognition memory and spatial memory: role in Huntington's disease

Huntington's disease (HD) patients and mouse models show learning and memory impairment even before the onset of motor symptoms. However, the molecular events involved in this cognitive decline are still poorly understood. Here, using three different paradigms, the novel object recognition test, the T-maze spontaneous alternation task and the Morris water maze, we detected severe cognitive deficits in the R6/1 mouse model of HD before the onset of motor symptoms. When we examined the putative molecular pathways involved in these alterations, we observed hippocampal cAMP-dependent protein kinase (PKA) hyper-activation in naïve R6/1 mice compared with wild-type (WT) mice, whereas extracellular signal-regulated kinase 1/2 and calcineurin activities were not modified. Increased PKA activity resulted in hyper-phosphorylation of its substrates N-methyl-D-aspartate receptor subunit 1, Ras-guanine nucleotide releasing factor-1 and striatal-enriched protein tyrosine phosphatase, but not cAMP-responsive element binding protein or the microtubule-associated protein tau. In correlation with the over-activation of the PKA pathway, we found a down-regulation of the protein levels of some phosphodiesterase (PDE) 4 family members. Similar molecular changes were found in the hippocampus of R6/2 mice and HD patients. Furthermore, chronic treatment of WT mice with the PDE4 inhibitor rolipram up-regulated PKA activity, and induced learning and memory deficits similar to those seen in R6 mice, but had no effect on R6/1 mice cognitive impairment. Importantly, hippocampal PKA inhibition by infusion of Rp-cAMPS restored long-term memory in R6/2 mice. Thus, our results suggest that occlusion of PKA-dependent processes is one of the molecular mechanisms underlying cognitive decline in R6 animals.

The Trail Making Test in prodromal Huntington disease: contributions of disease progression to test performance

We examined the Trail Making Test (TMT) in a sample of 767 participants with prodromal Huntington disease (prodromal HD) and 217 healthy comparisons to determine the contributions of motor, psychiatric, and cognitive changes to TMT scores. Eight traditional and derived TMT scores were also evaluated for their ability to differentiate prodromal participants closer to estimated age of diagnosis from those farther away and prodromal individuals from healthy comparisons. Results indicate that motor signs only mildly affected Part A, and psychiatric symptoms did not affect either part. Tests of perceptual processing, visual scanning, and attention were primarily associated with Part A, and executive functioning (response inhibition, set-shifting), processing speed, and working memory were associated with Part B. Additionally, TMT scores differentiated between healthy comparisons and prodromal HD individuals as far as 9-15 years before estimated diagnosis. In participants manifesting prodromal motor signs and psychiatric symptoms, the TMT primarily measures cognition and is able to discriminate between groups based on health status and estimated time to diagnosis.

Impaired long-term potentiation in the prefrontal cortex of Huntington's disease mouse models: rescue by D1 dopamine receptor activation

BACKGROUND

The introduction of gene testing for Huntington's disease (HD) has enabled the neuropsychiatric and cognitive profiling of human gene carriers prior to the onset of overt motor and cognitive symptoms. Such studies reveal an early decline in working memory and executive function, altered EEG and a loss of striatal dopamine receptors. Working memory is processed in the prefrontal cortex and modulated by extrinsic dopaminergic inputs.

OBJECTIVE

We sought to study excitatory synaptic function and plasticity in the medial prefrontal cortex of mouse models of HD.

METHODS

We have used 2 mouse models of HD, carrying 89 and 116 CAG repeats (corresponding to a preclinical and symptomatic state, respectively) and performed electrophysiological field recording in coronal slices of the medial prefrontal cortex.

RESULTS

We report that short-term synaptic plasticity and long-term potentiation (LTP) are impaired and that the severity of impairment is correlated with the size of the CAG repeat. Remarkably, the deficits in LTP and short-term plasticity are reversed in the presence of a D(1) dopamine receptor agonist (SKF38393).

CONCLUSION

In a previous study, we demonstrated that a deficit in long-term depression (LTD) in the perirhinal cortex could also be reversed by a dopamine agonist. These and our current data indicate that inadequate dopaminergic modulation of cortical synaptic function is an early event in HD and may provide a route for the alleviation of cognitive dysfunction.

Role of ubiquitin-proteasome-mediated proteolysis in nervous system disease

Proteolysis by the ubiquitin-proteasome pathway (UPP) is now widely recognized as a molecular mechanism controlling myriad normal functions in the nervous system. Also, this pathway is intimately linked to many diseases and disorders of the brain. Among the diseases connected to the UPP are neurodegenerative disorders such as Alzheimer's, Parkinson's and Huntington's diseases. Perturbation in the UPP is also believed to play a causative role in mental disorders such as Angelman syndrome. The pathology of neurodegenerative diseases is characterized by abnormal deposition of insoluble protein aggregates or inclusion bodies within neurons. The ubiquitinated protein aggregates are believed to result from dysfunction of the UPP or from structural changes in the protein substrates which prevent their recognition and degradation by the UPP. An early effect of abnormal UPP in diseases of the nervous system is likely to be impairment of synaptic function. Here we discuss the UPP and its physiological roles in the nervous system and how alterations in the UPP relate to development of nervous system diseases. This article is part of a Special Issue entitled The 26S Proteasome: When degradation is just not enough!

Capzb2 PROTEIN EXPRESSION IN THE BRAINS OF PATIENTS DIAGNOSED WITH ALZHEIMER'S DISEASE AND HUNTINGTON'S DISEASE

The silencing of actin capping protein β2, Capzb2, by RNAi in developing cultured neurons results in short, dystrophic neurites reminiscent of cytoskeletal changes seen in diverse neurodegenerative diseases, including Alzheimer's disease (AD) and Huntington's disease (HD). Actin and tubulin are two major cytoskeletal proteins indispensable for normal neurite development and regenerative responses to injury and neurodegenerative stimuli. We have previously shown that Capzb2 binds tubulin and, in the presence of microtubule- associated protein tau, affects microtubule polymerization necessary for neurite outgrowth and normal growth cone morphology. Accordingly, Capzb2 silencing in hippocampal neurons results in short neurites with abnormal growth cones. Decreased neurite length is found in both AD and HD. In the first step towards uncovering the possible role of Capzb2 in these diseases, we studied Capzb2 protein expression in the postmortem brains of AD and HD patients. To determine whether disease-specific changes in Capzb2 protein accompany the progression of neurodegeneration, we performed Western Blot analysis of prefrontal cortices (PFC) and hippocampi (HPC) in AD patients and of PFC and heads of caudate nuclei (HCN) in HD patients. Our results show disease- and area-specific dynamics in the levels of Capzb2 protein expression in the progressive stages of AD and HD.

A double blind evaluation of cognitive decline in a Norwegian cohort of asymptomatic carriers of Huntington's disease

Previous studies investigating subclinical signs of cognitive decline in presymptomatic carriers of Huntington's disease (HD) have shown conflicting results. The current study examines cognition in 105 at-risk individuals, using a broad neuropsychological test battery and adopting strict inclusion criteria for attaining a homogeneous sample. Results obtained by analyses of variance and effect size calculations indicate no clinical evidence of significant cognitive decline in asymptomatic HD carriers very far from onset of illness compared to noncarriers. Closeness to disease onset amongst gene carriers influenced cognition negatively whereas cytosine-adenine-guanine (CAG) repeat size did not. The findings call for longitudinal follow-up studies using a combination of clinical instruments and experimental paradigms to pinpoint when subtle cognitive deficits occur and within which of the cognitive domains.

Temporal order memory deficits prior to clinical diagnosis in Huntington's disease

The current study examined temporal order memory in preclinical Huntington's disease (pre-HD). Participants were separated into less than 5 years (pre-HD near) and more than 5 years (pre-HD far) from estimated age of clinical diagnosis. Participants completed a temporal order memory task on a computerized radial eight-arm maze. On the study phase of each trial, participants viewed a random sequence of circles appearing one at a time at the end of each arm. On the choice phase, participants viewed two circles at the end of the study phase arms and chose the circle occurring earliest in the sequence. The task involved manipulations of the temporal lag, defined as the number of arms occurring in the sample phase sequence between the two choice phase arms. Research suggests that there is more interference for temporally proximal stimuli relative to temporally distal stimuli. There were no significant differences between the pre-HD far group and controls on the temporal order memory task. The pre-HD near group demonstrated significant impairments relative to the other groups on closer temporal lags, but were normal on the furthest temporal lag. Therefore, temporal order memory declines with increased temporal interference in pre-HD close to estimated diagnosis of HD.

Visual scanning and cognitive performance in prediagnostic and early-stage Huntington's disease

The objective of this study was to evaluate visual scanning strategies in carriers of the Huntington disease (HD) gene expansion and to test whether there is an association between measures of visual scanning and cognitive performance. The study sample included control (NC, n = 23), prediagnostic (PDHD, n = 21), and subjects recently diagnosed with HD (HD, n = 19). All participants completed a uniform clinical evaluation that included examination by neurologist and molecular testing. Eye movements were recorded during completion of the Digit Symbol Subscale (DS) test. Quantitative measures of the subject's visual scanning were evaluated using joint analysis of eye movements and performance on the DS test. All participants employed a simple visual scanning strategy when completing the DS test. There was a significant group effect and a linear trend of decreasing frequency and regularity of visual scanning from NC to PDHD to HD. The performance of all groups improved slightly and in a parallel fashion across the duration of the DS test. There was a strong correlation between visual scanning measures and the DS cognitive scores. While all individuals employed a similar visual scanning strategy, the visual scanning measures grew progressively worse from NC to PDHD to HD. The deficits in visual scanning accounted, at least in part, for the decrease in the DS score.

Ten-year rate of longitudinal change in neurocognitive and motor function in prediagnosis Huntington disease

Longitudinal studies of neurocognitive function in prediagnosis Huntington disease (pre-HD) have been few, and duration of follow-up has been brief. In this study, 155 individuals at-risk for HD completed a battery of cognitive and motor tasks at two study visits approximately 10 years apart. Participants were classified as: (1) at-risk, without the CAG expansion (healthy controls, NC; n = 112), or (2) CAG expanded (CAG+; n = 43). To examine the rate of decline at different stages of the pre-HD period, participants in the CAG+ group were further characterized as converters (i.e., individuals who developed manifest HD over the course of the study; n = 21) or nonconverters (n = 22), and their performances were compared. The CAG+ group exhibited faster rates of neurocognitive decline over the course of the study, relative to the NC group. In addition, more rapid decline was associated with closer proximity to estimated age of disease onset in the CAG+ group. Faster rates of motor and psychomotor decline were observed in the CAG+ converter group, relative to the nonconverters. These findings suggest that neurocognitive decline in pre-HD, particularly in motor and psychomotor domains, begins insidiously and accelerates as individuals approach disease onset.

Brain-derived neurotrophic factor modulates the severity of cognitive alterations induced by mutant huntingtin: involvement of phospholipaseCgamma activity and glutamate receptor expression

The involvement of brain-derived neurotrophic factor (BDNF) in cognitive processes and the decrease in its expression in Huntington's disease suggest that this neurotrophin may play a role in learning impairment during the disease progression. We therefore analyzed the onset and severity of cognitive deficits in two different mouse models with the same mutant huntingtin but with different levels of BDNF (R6/1 and R6/1:BDNF+/- mice). We observed that BDNF modulates cognitive function in different learning tasks, even before the onset of motor symptoms. R6/1:BDNF+/- mice showed earlier and more accentuated cognitive impairment than R6/1 mice at 5 weeks of age in discrimination learning; at 5 weeks of age in procedural learning; and at 9 weeks of age in alternation learning. At the earliest age at which cognitive impairment was detected, electrophysiological analysis was performed in the hippocampus. All mutant genotypes showed reduced hippocampal long term potentiation (LTP) with respect to wild type but did not show differences between them. Thus, we evaluated the involvement of BDNF-trkB signaling and glutamate receptor expression in the hippocampus of these mice. We observed a decrease in phospholipaseCgamma activity, but not ERK, in R61, BDNF+/- and R6/1:BDNF+/- hippocampus at the age when LTP was altered. However, a specific decrease in the expression of glutamate receptors NR1, NR2A and GluR1 was detected only in R6/1:BDNF+/- hippocampus. Therefore, these results show that BDNF modulates the learning and memory alterations induced by mutant huntingtin. This interaction leads to intracellular changes, such as specific changes in glutamate receptors and in BDNF-trkB signaling through phospholipaseCgamma.

Detection of Huntington's disease decades before diagnosis: the Predict-HD study

OBJECTIVE

The objective of the Predict-HD study is to use genetic, neurobiological and refined clinical markers to understand the early progression of Huntington's disease (HD), prior to the point of traditional diagnosis, in persons with a known gene mutation. Here we estimate the approximate onset and initial course of various measurable aspects of HD relative to the time of eventual diagnosis.

METHODS

We studied 438 participants who were positive for the HD gene mutation, but did not yet meet the diagnostic criteria for HD and had no functional decline. Predictability of baseline cognitive, motor, psychiatric and imaging measures was modelled non-linearly using estimated time until diagnosis (based on CAG repeat length and current age) as the predictor.

RESULTS

Estimated time to diagnosis was related to most clinical and neuroimaging markers. The patterns of association suggested the commencement of detectable changes one to two decades prior to the predicted time of clinical diagnosis. The patterns were highly robust and consistent, despite the varied types of markers and diverse measurement methodologies.

CONCLUSIONS

These findings from the Predict-HD study suggest the approximate time scale of measurable disease development, and suggest candidate disease markers for use in preventive HD trials.

Visual perception in prediagnostic and early stage Huntington's disease

Disturbances of visual perception frequently accompany neurodegenerative disorders but have been little studied in Huntington's disease (HD) gene carriers. We used psychophysical tests to assess visual perception among individuals in the prediagnostic and early stages of HD. The sample comprised four groups, which included 201 nongene carriers (NG), 32 prediagnostic gene carriers with minimal neurological abnormalities (PD1); 20 prediagnostic gene carriers with moderate neurological abnormalities (PD2), and 36 gene carriers with diagnosed HD. Contrast sensitivity for stationary and moving sinusoidal gratings, and tests of form and motion discrimination, were used to probe different visual pathways. Patients with HD showed impaired contrast sensitivity for moving gratings. For one of the three contrast sensitivity tests, the prediagnostic gene carriers with greater neurological abnormality (PD2) also had impaired performance as compared with NG. These findings suggest that early stage HD disrupts visual functions associated with the magnocellular pathway. However, these changes are only observed in individuals diagnosed with HD or who are in the more symptomatic stages of prediagnostic HD.

Neuropsychological deficits in Huntington's disease gene carriers and correlates of early "conversion"

The authors examined whether the baseline cognitive functioning of 21 clinically normal huntingtin mutation carriers who developed manifest Huntington's disease on follow-up differed from that of 49 mutation carriers who remain asymptomatic over the same period in a longitudinal study. One hundred thirty-four gene-negative offspring of Huntington's disease patients were studied as well. Overall, there were no differences in cognitive test performance among the three groups. However, "converters" who developed signs of Huntington's disease within 8.6 years demonstrated poorer performance on the Wisconsin Card Sorting Test at baseline. People with the Huntington's disease mutation who are carefully examined neurologically and found to be asymptomatic have, at most, very minimal problem-solving impairment, and only if they are within a few years of clinical onset.

Cognitive changes in asymptomatic carriers of the Huntington disease mutation gene

Huntington disease (HD) is a neurodegenerative disorder due to an excessive number of CAG repeats in the IT15 gene on chromosome 4. Studies of cognitive function in asymptomatic gene carriers have yielded contradictory results. This study compared cognitive performance in 44 subjects with the HD mutation (group of carriers) who had no clinical signs of HD and 39 at-risk individuals without HD mutation (group of non-carriers). Neuropsychological evaluation focused on global cognitive efficiency, psychomotor speed, attentional, executive and memory functions. Significant differences, with lower performances in the group of gene carriers, were detected for some measures of psychomotor speed, attention and executive functioning (all P < 0.01). More differences between groups were observed for memory measures, in particular on the California Verbal Memory Test. Complementing these observations, cognitive scores were correlated with age in the group of gene carriers, but not in the group of non-carriers. This suggests that the cognitive changes precede the appearance of the motor and psychiatric symptoms in HD and that tests proved to be sensitive to early HD deficiencies are better suited than global cognitive efficiency scales to observe them.

Impairments in source memory for olfactory and visual stimuli in preclinical and clinical stages of Huntington's disease

Individuals in preclinical and clinical stages of Huntington's disease (HD) demonstrate impairments in olfactory functioning. In addition, HD patients are impaired in source memory for verbal stimuli. A task combining both source and odor memory may be particularly sensitive to early changes in HD. The present study examined source and item memory for olfactory and visual stimuli in 10 individuals with HD, 10 asymptomatic HD gene carriers, 8 nongene carriers who had a parent with HD, and 20 normal controls. During the study phase, a male and a female experimenter (sources) presented odors and objects to the participant in an alternating sequence. To assess item memory, a stimulus from the study phase (target) and a novel stimulus (distractor) were presented, and the participant was asked to choose the target. To assess source memory, the experimenter presented a stimulus and asked whether the male or female experimenter had previously presented the stimulus. Results indicate that source memory for both visual and olfactory stimuli was impaired in HD patients compared to normal controls. In asymptomatic gene carriers, however, source memory for olfactory stimuli, but not visual stimuli, was more impaired than in nongene carriers and normal controls. Furthermore, gene carriers and HD patients showed a similar degree of impairment in source memory for olfactory stimuli. The only significant impairment found in item memory was for olfactory stimuli in HD patients. These results suggest that source memory for olfactory stimuli may be particularly sensitive to neuropathological changes in preclinical stages of HD.

Abnormal cortical synaptic plasticity in a mouse model of Huntington's disease

Huntington's disease is a fatal neurodegenerative disorder characterised by a progressive motor, psychiatric and cognitive decline and associated with a marked loss of neurons in the cortex and striatum of affected individuals. The disease is inherited in an autosomal dominant fashion and is caused by a trinucleotide (CAG) repeat expansion in the gene encoding the protein huntingtin. Predictive genetic testing has revealed early cognitive deficits in asymptomatic gene carriers such as altered working memory, executive function and recognition memory. The perirhinal cortex is believed to process aspects of recognition memory. Evidence from primate studies suggests that decrements in neuronal firing within this cortical region encode recognition memory and that the underlying mechanism is an activity-dependent long-term depression (LTD) of excitatory neurotransmission, the converse of long-term potentiation (LTP). We have used the R6/1 mouse model of HD to assess synaptic plasticity in the perirhinal cortex. This mouse model provides an ideal tool for investigating early and progressive changes in synaptic function in HD. We report here that LTD at perirhinal synapses is markedly reduced in R6/1 mice. We also provide evidence to suggest that a reduction in dopamine D2 receptor signalling may be implicated.

Effects of SCA1, MJD, and DPRLA triplet repeat polymorphisms on cognitive phenotypes in a normal population of adolescent twins

The expansion of unstable trinucleotide CAG repeat polymorphisms of a number of genes causes several neurodegenerative disorders with decreased cognitive function, the severity of the disorder being related to allele length at the triplet repeat locus. While the effects of repeat length have been well studied in clinical samples, there has been little investigation of the effects of triplet repeat variation in the normal range for these genes. We have, therefore, examined linkage and association for three CAG triplet repeat markers (Spinocerebellar Ataxia Type 1, SCA1; Machado-Joseph Disease, MJD; Dentatorubro-pallidoluysian Atrophy, DRPLA) to assess their contribution to variation in cognitive ability (IQ, reading ability, processing speed) in a normal, unselected sample of adolescent twins (248 dizygotic (DZ) sibling pairs, aged 16 years). Association tests, performed in Mx and QTDT, showed a consistent positive association of SCA1 with Arithmetic (P = 0.04). While association was supported between SCA1 and Cambridge reading scores and between DRPLA and inspection time, results were inconsistent across software packages. Given the number of statistical tests performed, it is unlikely that trinucleotide repeat variation in the normal range for these genes influences variation in normal cognition.

Aberrant cortical synaptic plasticity and dopaminergic dysfunction in a mouse model of huntington's disease

Predictive genetic testing for Huntington's disease (HD) has revealed early cognitive deficits in asymptomatic gene carriers, such as altered working memory, executive function and impaired recognition memory. The perirhinal cortex processes aspects of recognition memory and the underlying mechanism is believed to be long-term depression (LTD) of excitatory neurotransmission, the converse of long-term potentiation (LTP). We have used the R6/1 mouse model of HD to assess synaptic plasticity in the perirhinal cortex. We report here a progressive derailment of both LTD and short-term plasticity at perirhinal synapses. Layer II/III neurones gradually lose their ability to support LTD, show early nuclear localization of mutant huntingtin and display a progressive loss of membrane integrity (depolarization and loss of cell capacitance) accompanied by a reduction in the expression of D1 and D2 dopamine receptors visualized in layer I of the perirhinal cortex. Importantly, abnormalities in both short-term and long-term plasticity can be reversed by the introduction of a D2 dopamine receptor agonist (Quinpirole), suggesting that alterations in dopaminergic signalling may underlie early cognitive dysfunction in HD.

Early development of aberrant synaptic plasticity in a mouse model of Huntington's disease

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by progressive motor, psychiatric and cognitive decline. Marked neuronal loss occurs in the cortex and striatum. HD is inherited in an autosomal dominant fashion and caused by a trinucleotide repeat expansion (CAG) in the gene encoding the protein huntingtin. Predictive genetic testing has revealed early cognitive deficits in asymptomatic gene carriers at a time when there is little evidence for cell death, suggesting that impaired cognition results from a cellular or synaptic deficit, such as aberrant synaptic plasticity. Altered hippocampal long-term potentiation has been reported in mouse models of HD; however, the relationship between synaptic dysfunction and phenotype progression has not previously been characterized. We examined the age-dependency of aberrant hippocampal synaptic plasticity in the R6/1 mouse model of HD. Long-term depression (LTD) is a developmentally regulated form of plasticity, which normally declines by early adulthood. Young R6/1 mice follow the same pattern of LTD expression as controls, in that they express LTD in the first weeks of life, and then lose the ability with age. Unlike controls, R6/1 synapses later regain the ability to support LTD. This is associated with nuclear localization of mutant huntingtin, but occurs months prior to the formation of nuclear aggregates. We present the first detailed description of a progressive derailment of a functional neural correlate of cognitive processing in HD.

Deficits in experience-dependent cortical plasticity and sensory-discrimination learning in presymptomatic Huntington's disease mice

Huntington's disease (HD) is one of a group of neurodegenerative diseases caused by an expanded trinucleotide (CAG) repeat coding for an extended polyglutamine tract. The disease is inherited in an autosomal dominant manner, with onset of motor, cognitive, and psychiatric symptoms typically occurring in midlife, followed by unremitting progression and eventual death. We report here that motor presymptomatic R6/1 HD mice show a severe impairment of somatosensory-discrimination learning ability in a behavioral task that depends heavily on the barrel cortex. In parallel, there are deficits in barrel-cortex plasticity after a somatosensory whisker-deprivation paradigm. The present study demonstrates deficits in neocortical plasticity correlated with a specific learning impairment involving the same neocortical area, a finding that provides new insight into the cellular basis of early cognitive deficits in HD.

Presymptomatic signs in healthy CJD mutation carriers

Creutzfeldt-Jacob disease (CJD) is a rapidly progressing dementia with neurological, psychiatric and cognitive symptoms. We focused our study on the familial CJD form among Libyan Jews (the E200K mutation), trying to identify preclinical neuropsychological signs in mutation carriers to facilitate early diagnosis of the disease. A wide range of neuropsychological tests was administered to 27 healthy volunteers, all first-degree relatives of genetic CJD patients. Thirteen of our participants were gene mutation carriers (E200K) and 14 controls. The healthy mutation carriers reported significantly lower Trait and higher State anxiety scores. Repeated Measure analysis showed statistical significance. The Anxiety Index (State-Trait Anxiety Score) progressed with age in the carriers' group but not in the controls. Since this was more pronounced in the older subjects, we suggest that abnormal stress mechanisms precede the clinical onset of CJD. Cognitive differences have also been found between carriers and controls, especially in visual recognition of pictured objects. Both kinds of differences (anxiety levels and cognitive deficits) were most pronounced in elderly subjects. This study is the first to show any dysfunction in healthy CJD mutation carriers.

Early changes in Huntington's disease patient brains involve alterations in cytoskeletal and synaptic elements

Huntington's disease (HD) is caused by a polyglutamine repeat expansion in the N-terminus of the huntingtin protein. Huntingtin is normally present in the cytoplasm where it may interact with structural and synaptic elements. The mechanism of HD pathogenesis remains unknown but studies indicate a toxic gain-of-function possibly through aberrant protein interactions. To investigate whether early degenerative changes in HD involve alterations of cytoskeletal and vesicular components, we examined early cellular changes in the frontal cortex of HD presymptomatic (PS), early pathological grade (grade 1) and late-stage (grade 3 and 4) patients as compared to age-matched controls. Morphologic analysis using silver impregnation revealed a progressive decrease in neuronal fiber density and organization in pyramidal cell layers beginning in presymptomatic HD cases. Immunocytochemical analyses for the cytoskeletal markers alpha -tubulin, microtubule-associated protein 2, and phosphorylated neurofilament demonstrated a concomitant loss of staining in early grade cases. Immunoblotting for synaptic proteins revealed a reduction in complexin 2, which was marked in some grade 1 HD cases and significantly reduced in all late stage cases. Interestingly, we demonstrate that two synaptic proteins, dynamin and PACSIN 1, which were unchanged by immunoblotting, showed a striking loss by immunocytochemistry beginning in early stage HD tissue suggesting abnormal distribution of these proteins. We propose that mutant huntingtin affects proteins involved in synaptic function and cytoskeletal integrity before symptoms develop which may influence early disease onset and/or progression.

Cognitive changes in patients with Huntington's disease (HD) and asymptomatic carriers of the HD mutation--a longitudinal follow-up study

OBJECTIVE

Objective information about the onset and progression of cognitive impairment in Huntington's disease (HD) is very important in the light of appropriate outcome measures when conducting clinical trials. Therefore, we evaluated the progression of cognitive functions in HD patients and asymptomatic carriers of the HD mutation (AC) over a 2.5-year period. We also sought to detect the earliest markers of cognitive impairment in AC.

METHODS

A prospective study comparing HD patients, clinically asymptomatic HD mutation-carriers (AC) and non-carriers (NC). These groups were examined three times during a period of 2.5 years. At baseline the study sample consisted of 49 subjects. Forty-two subjects (19 HD patients, 12 AC and 11 NC) completed three assessments. A battery of neuropsychological tests measuring intelligence, attention, memory, language, visuospatial perception, and executive functions was performed.

RESULTS

The performance of HD patients deteriorated on the following cognitive tests: Symbol Digit Modalities Test (SDMT), Stroop Colour and Word, Boston Naming Test (BNT), Object and Space Perception and Trail Making Test-B. Longitudinal comparison of AC and NC revealed that performances on SDMT, Block Span, Digit Span Backwards, Hopkins Verbal Learning Test (learning and delayed recall) and Conditional Associative Learning Test are impaired in AC.

CONCLUSIONS

Tasks measuring mainly attention, object and space perception and executive functions adequately assess the progression of HD disease. Other cognitive functions do not significantly deteriorate. Furthermore, problems in attention, working memory, verbal learning, verbal long-term memory and learning of random associations are the earliest cognitive manifestations in AC.

Ubiquitin-proteasome-mediated local protein degradation and synaptic plasticity

A proteolytic pathway in which attachment of a small protein, ubiquitin, marks the substrates for degradation by a multi-subunit complex called the proteasome has been shown to function in synaptic plasticity and in several other physiological processes of the nervous system. Attachment of ubiquitin to protein substrates occurs through a series of highly specific and regulated steps. Degradation by the proteasome is subject to multiple levels of regulation as well. How does the ubiquitin-proteasome pathway contribute to synaptic plasticity? Long-lasting, protein synthesis-dependent, changes in the synaptic strength occur through activation of molecular cascades in the nucleus in coordination with signaling events in specific synapses. Available evidence indicates that ubiquitin-proteasome-mediated degradation has a role in the molecular mechanisms underlying synaptic plasticity that operate in the nucleus as well as at the synapse. Since the ubiquitin-proteasome pathway has been shown to be versatile in having roles in addition to proteolysis in several other cellular processes relevant to synaptic plasticity, such as endocytosis and transcription, this pathway is highly suited for a localized role in the neuron. Because of its numerous roles, malfunctioning of this pathway leads to several diseases and disorders of the nervous system. In this review, I examine the ubiquitin-proteasome pathway in detail and describe the role of regulated proteolysis in long-term synaptic plasticity. Also, using synaptic tagging theory of synapse-specific plasticity, I provide a model on the possible roles and regulation of local protein degradation by the ubiquitin-proteasome pathway.

Movement Disorders

This chapter covers five movement disorders: Parkinson's disease, dystonia, tic disorders, Huntington's disease, and essential tremor. These disorders demonstrate many of the most common challenges encountered in the epidemiologic investigation of movement disorders. Each section includes a description of the disease, followed by a review of descriptive studies (disease incidence, prevalence, and mortality studies), and discussion of genetic and environmental risk factors for the disorder. At the end of each section, directions for future studies are discussed.