Evidence for more widespread cerebral pathology in early HD: an MRI-based morphometric analysis

Irritability in pre-clinical Huntington's disease

Irritability, together with depression and anxiety, form three salient clinical features of pre-symptomatic Huntington's disease (HD). To date, the understanding of irritability in HD suffers from a paucity of experimental data and is largely based on questionnaires or clinical anecdotes. Factor analysis suggests that irritability is related to impulsivity and aggression and is likely to engage the same neuronal circuits as these behaviours, including areas such as medial orbitofrontal cortex (OFC) and amygdala. 16 pre-symptomatic gene carriers (PSCs) and 15 of their companions were asked to indicate the larger of two squares consecutively shown on a screen while undergoing functional magnetic resonance imaging (fMRI). Despite correct identification of the larger square, participants were often told that they or their partner had given the wrong answer. Size differences were subtle to make negative feedback credible but detectable. Although task performance, baseline irritability, and reported task-induced irritation were the same for both groups, fMRI revealed distinct neuronal processing in those who will later develop HD. In controls but not PSCs, task-induced irritation correlated positively with amygdala activation and negatively with OFC activation. Repetitive negative feedback induced greater amygdala activations in controls than PSCs. In addition, the inverse functional coupling between amygdala and OFC was significantly weaker in PSCs compared to controls. Our results argue that normal emotion processing circuits are disrupted in PSCs via attenuated modulation of emotional status by external or internal indicators. At later stages, this dysfunction may increase the risk for developing recognised, HD-associated, psychiatric symptoms such as irritability.

Cortical dopamine dysfunction in symptomatic and premanifest Huntington's disease gene carriers

We used (11)C-raclopride PET, a marker of D(2) dopamine receptor binding, and statistical parametric mapping (SPM) to localise cortical D(2) receptor dysfunction in individual Huntington's disease (HD) gene carriers (16 symptomatic and 11 premanifest subjects) and assess its clinical significance. 62.5% of symptomatic HD patients and 54.5% of premanifest carriers showed cortical reductions in D(2) binding. The most frequent decreases in cortical binding in individual HD subjects were seen in temporal and frontal areas. Symptomatic HD subjects with decreased cortical D(2) binding had worse scores on neuropsychological tests assessing attention and executive functions than subjects without cortical dopamine dysfunction, notwithstanding comparable reduction in striatal D(2) binding and motor disability. Our results indicate that cortical dopaminergic dysfunction is common in both symptomatic and premanifest HD gene carriers. It is an early event in HD pathophysiology and could contribute to the impairment in neuropsychological performance in these patients.

Parallel explicit and implicit control of reaching

Background

Human movement can be guided automatically (implicit control) or attentively (explicit control). Explicit control may be engaged when learning a new movement, while implicit control enables simultaneous execution of multiple actions. Explicit and implicit control can often be assigned arbitrarily: we can simultaneously drive a car and tune the radio, seamlessly allocating implicit or explicit control to either action. This flexibility suggests that sensorimotor signals, including those that encode spatially overlapping perception and behavior, can be accurately segregated to explicit and implicit control processes.

Methodology/Principal Findings

We tested human subjects' ability to segregate sensorimotor signals to parallel control processes by requiring dual (explicit and implicit) control of the same reaching movement and testing for interference between these processes. Healthy control subjects were able to engage dual explicit and implicit motor control without degradation of performance compared to explicit or implicit control alone. We then asked whether segregation of explicit and implicit motor control can be selectively disrupted by studying dual-control performance in subjects with no clinically manifest neurologic deficits in the presymptomatic stage of Huntington's disease (HD). These subjects performed successfully under either explicit or implicit control alone, but were impaired in the dual-control condition.

Conclusion/Significance

The human nervous system can exert dual control on a single action, and is therefore able to accurately segregate sensorimotor signals to explicit and implicit control. The impairment observed in the presymptomatic stage of HD points to a possible crucial contribution of the striatum to the segregation of sensorimotor signals to multiple control processes.

Magnetization transfer imaging in 'premanifest' Huntington's disease

To investigate whether magnetization transfer imaging (MTI) is a useful detector of diffuse brain abnormalities in ‘premanifest’ carriers of the Huntington’s disease (HD) gene mutation. Furthermore we examined the relations between MTI, clinical measures and CAG repeat length. Sixteen premanifest carriers of the HD gene without motor manifestation and 14 non-carriers underwent a clinical evaluation and a MRI scan. MTI analysis of whole brain, grey matter and white matter was performed producing magnetization transfer ratio (MTR) histograms. A lower peak height of the grey matter MTR histogram in carriers was significantly associated with more UHDRS motor abnormalities. Furthermore, a lower peak height of the whole brain, grey and white matter was strongly associated with a longer CAG repeat length. MTI measures themselves did not differ significantly between carriers and non-carriers. In premanifest HD mutation carriers, a lower MTR peak height, reflecting worse histological brain composition, was related to subtle motor abnormalities and higher CAG repeat length. Although we could not detect altered MTI characteristics in carriers of the HD gene mutation without clinical manifestations, we did provide evidence that the MTR peak height might reflect genetic and subclinical disease burden and may be of value in monitoring further disease progression and provide insight in clinical heterogeneity.

Murine models of Huntington disease

Huntington disease is an inherited neurodegenerative disorder associated with inexorable progression. To date, no therapy has proved effective in modifying the disease course or improving survival. We present here an evaluation of our most valuable asset in the development of a cure – the mouse model. In particular, we will reflect on the relative strengths and weaknesses of current models in reprising Huntington disease pathology, evaluate their role in the development of novel therapeutic agents through preclinical trials and consider their future impact on Huntington disease research.

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.

Abnormal motor cortex excitability in preclinical and very early Huntington's disease

BACKGROUND

In Huntington's disease (HD), the cerebral cortex is involved early in the disease process. The study of cortical excitability can therefore contribute to understanding HD pathophysiology.

METHODS

With transcranial magnetic stimulation (TMS) we examined motor cortex excitability in 8 premanifest HD gene carriers, 8 very early symptomatic HD patients, and 22 healthy control subjects. Electrophysiological measures were correlated with the clinical stage of HD to identify motor cortical dysfunction before overt clinical disease onset.

RESULTS

Premanifest and early manifest HD patients had higher resting and active motor cortex thresholds than control subjects (p = .024). At rest, recruitment of motor evoked potentials was more gradual in both patient groups than in control subjects (p = .001). When active, recruitment and the duration of the cortical silent period were similar in all groups. There was a tendency for short interval intra-cortical inhibition to have a higher threshold in all patients taken together but not in each group separately. Short latency afferent inhibition (SAI) was reduced in early manifest patients compared with control subjects and premanifest patients (p < .001) and in contrast to all other measures was inversely associated with estimated time to onset (p = .013, adjusted R(2) = .32) and the Unified Huntington's Disease Rating Scale motor score (p = .001, adjusted R(2) = .5). A combination of age, CAG repeat length, and SAI strongly predicted the Unified Huntington's Disease Rating Scale motor score (p = .001, adjusted R(2) = .68).

CONCLUSIONS

Our findings add to the evidence for early cortical involvement in HD possibly before HD signs appear.

MRI-derived measurements of human subcortical, ventricular and intracranial brain volumes: Reliability effects of scan sessions, acquisition sequences, data analyses, scanner upgrade, scanner vendors and field strengths

Automated MRI-derived measurements of in-vivo human brain volumes provide novel insights into normal and abnormal neuroanatomy, but little is known about measurement reliability. Here we assess the impact of image acquisition variables (scan session, MRI sequence, scanner upgrade, vendor and field strengths), FreeSurfer segmentation pre-processing variables (image averaging, B1 field inhomogeneity correction) and segmentation analysis variables (probabilistic atlas) on resultant image segmentation volumes from older (n=15, mean age 69.5) and younger (both n=5, mean ages 34 and 36.5) healthy subjects. The variability between hippocampal, thalamic, caudate, putamen, lateral ventricular and total intracranial volume measures across sessions on the same scanner on different days is less than 4.3% for the older group and less than 2.3% for the younger group. Within-scanner measurements are remarkably reliable across scan sessions, being minimally affected by averaging of multiple acquisitions, B1 correction, acquisition sequence (MPRAGE vs. multi-echo-FLASH), major scanner upgrades (Sonata-Avanto, Trio-TrioTIM), and segmentation atlas (MPRAGE or multi-echo-FLASH). Volume measurements across platforms (Siemens Sonata vs. GE Signa) and field strengths (1.5 T vs. 3 T) result in a volume difference bias but with a comparable variance as that measured within-scanner, implying that multi-site studies may not necessarily require a much larger sample to detect a specific effect. These results suggest that volumes derived from automated segmentation of T1-weighted structural images are reliable measures within the same scanner platform, even after upgrades; however, combining data across platform and across field-strength introduces a bias that should be considered in the design of multi-site studies, such as clinical drug trials. The results derived from the young groups (scanner upgrade effects and B1 inhomogeneity correction effects) should be considered as preliminary and in need for further validation with a larger dataset.

Focal nature of neurological disorders necessitates isotype-selective histone deacetylase (HDAC) inhibitors

Histone deacetylase (HDAC) inhibitors represent a promising new avenue of therapeutic options for a range of neurological disorders. Within any particular neurological disorder, neuronal damage or death is not widespread; rather, particular brain regions are preferentially affected. Different disorders exhibit distinct focal pathologies. Hence, understanding the region-specific effects of HDAC inhibitors is essential for targeting appropriate brain areas and reducing toxicity in unaffected areas. The outcome of HDAC inhibition depends on several factors, including the diversity in the central nervous system expression of HDAC enzymes, selectivity of a given HDAC inhibitor for different HDAC enzymes, and the presence or absence of cofactors necessary for enzyme function. This review will summarize brain regions associated with various neurological disorders and factors affecting the consequences of HDAC inhibition.

MRI in dementia

With cognitive disorders increasingly common, clinicians urgently need faster and more accurate tools to classify such disorders and to noninvasively monitor therapeutic interventions. In this review, we provide information on MRI techniques that enable the study of the morphology, neuronal integrity, and metabolism of dementing illnesses. In addition, we explore the usefulness of such techniques as surrogate markers of these diseases.

The current clinical management of Huntington's disease

Huntington's disease is a neurodegenerative condition, characterized by movement disorders, cognitive decline, and psychiatric disturbance. We review the pharmacological management of the various movement disorders associated with the disease, the cognitive decline and the commonly encountered behavioral disturbances. We discuss the nonclassical features of the disease, important in the management of these patients. Nonpharmacological support including genetic counseling and therapy and the importance of palliative care are also addressed. Finally, experimental approaches that may soon impact upon clinical practice are discussed.

Increased hypothalamic-pituitary-adrenal axis activity in Huntington's disease

CONTEXT

Huntington's disease (HD) is a fatal hereditary neurodegenerative disorder characterized by motor, cognitive, and behavioral disturbances. Hypothalamic-pituitary-adrenal (HPA) axis dysfunction could contribute to a number of HD signs and symptoms; however, no data are available on cortisol diurnal variations and secretory dynamics in HD patients.

OBJECTIVE

The aim of the study was to perform a detailed analysis of HPA axis function in HD patients in relation to clinical signs and symptoms.

DESIGN, SETTING, AND PARTICIPANTS

Twenty-four-hour cortisol secretion was studied in eight early-stage, medication-free HD patients and eight age-, sex-, and body mass index-matched controls in a clinical research laboratory. Cortisol levels were measured every 10 min.

MAIN OUTCOME MEASURES

Multiparameter autodeconvolution and cosinor regression were applied to quantify basal, pulsatile, and total cortisol secretion rates as well as diurnal variations in cortisol levels.

RESULTS

Total cortisol secretion rate and the amplitude of the diurnal cortisol profile were both significantly higher in HD patients compared with controls (3490 +/- 320 vs. 2500 +/- 220 nmol/liter/24 h, P = 0.023; and 111 +/- 14 vs. 64 +/- 8 nmol/liter, P = 0.012, respectively). Cortisol concentrations in patients were particularly increased in the morning and early afternoon period. In HD patients, mean 24-h cortisol levels significantly correlated with total motor score, total functional capacity, as well as body mass index.

CONCLUSIONS

HPA axis hyperactivity is an early feature of HD and is likely to result from a disturbed central glucocorticoid feedback due to hypothalamic pathology. HPA axis dysfunction may contribute to some signs and symptoms in HD patients.

In vivo evidence for the selective subcortical degeneration in Huntington's disease

Although Huntington's disease is largely considered to be a subcortical disease, there is no clear consensus on whether all deep grey matter loss is a direct downstream consequence of the massive degeneration of the medium-size spiny neurons in the striatum. Our aim was to characterise in vivo such preferential degeneration by analysing various distinct diffusion imaging measures including mean diffusivity, anisotropy, fibre orientation (using the information of the principal diffusion direction) and white matter tractography. All results converged to demonstrate the selective degeneration of connections in subcortical grey and white matter, degeneration which was likely to originate with the death of the striatal medium-size spiny neurons. Indeed, we found a significant increase of MD and FA in all the subcortical grey matter structures involved in the cortico-striato-thalamo-cortical loops. The atypical striatal and pallidal increase of FA was concurrent to a decrease of the dispersion of the fibre orientation, unambiguously characterising a preferential loss of connections along specific radiating directions from these structures while some others are comparatively spared. Analysis of striatal and pallidal white matter tracts revealed that striato-pallidal projections were the most affected. The ability of DTI to uncover the impact of such neurodegenerative disease on some specific neuronal/axonal populations is a further step towards the future definition of a surrogate marker of this disease. Beyond Huntington's disease, we prove here that diffusion imaging technique, associated to adequate methodological analyses, can provide insight into any neurodegenerative disorder for which some neuronal populations or connections are selectively targeted over others.

Inducible mutant huntingtin expression in HN10 cells reproduces Huntington's disease-like neuronal dysfunction

BACKGROUND

Expansion of a polyglutamine repeat at the amino-terminus of huntingtin is the probable cause for Huntington's disease, a lethal progressive autosomal-dominant neurodegenerative disorders characterized by impaired motor performance and severe brain atrophy. The expanded polyglutamine repeat changes the conformation of huntingtin and initiates a range of pathogenic mechanisms in neurons including intracellular huntingtin aggregates, transcriptional dysregulation, energy metabolism deficits, synaptic dystrophy and ultimately neurodegeneration. It is unclear how these events relate to each other or if they can be reversed by pharmacological intervention. Here, we describe neuronal cell lines expressing inducible fragments of normal and mutant huntingtin.

RESULTS

In HN10 cells, the expression of wild type and mutant huntingtin fragments was dependent on the induction time as well as on the concentration of the RheoSwitch(R) inducing ligand. In order to analyze the effect of mutant huntingtin expression on cellular functions we concentrated on the 72Q exon1 huntingtin expressing cell line and found that upon induction, it was possible to carefully dissect mutant huntingtin-induced phenotypes as they developed over time. Dysregulation of transcription as a result of mutant huntingtin expression showed a transcription signature replicating that reported in animal models and Huntington's disease patients. Crucially, triggering of neuronal differentiation in mutant huntingtin expressing cell resulted in the appearance of additional pathological hallmarks of Huntington's disease including cell death.

CONCLUSION

We developed neuronal cell lines with inducible expression of wild type and mutant huntingtin. These new cell lines represent a reliable in vitro system for modeling Huntington's disease and should find wide use for high-throughput screening application and for investigating the biology of mutant huntingtin.

Magnetic resonance imaging of Huntington's disease: preparing for clinical trials

The known genetic mutation causing Huntington's disease (HD) makes this disease an important model to study links between gene and brain function. An autosomal dominant family history and the availability of a sensitive and specific genetic test allow pre-clinical diagnosis many years before the onset of any typical clinical signs. This review summarizes recent magnetic resonance imaging (MRI)–based findings in HD with a focus on the requirements if imaging is to be used in treatment trials. Despite its monogenetic cause, HD presents with a range of clinical manifestations, not explained by variation in the number of CAG repeats in the affected population. Neuroimaging studies have revealed a complex pattern of structural and functional changes affecting widespread cortical and subcortical regions far beyond the confines of the striatal degeneration that characterizes this disorder. Besides striatal dysfunction, functional imaging studies have reported a variable pattern of increased and decreased activation in cortical regions in both pre-clinical and clinically manifest HD-gene mutation carriers. Beyond regional brain activation changes, evidence from functional and diffusion-weighted MRI further suggests disrupted connectivity between corticocortical and corticostriatal areas. However, substantial inconsistencies with respect to structural and functional changes have been reported in a number of studies. Possible explanations include methodological factors and differences in study samples. There may also be biological explanations but these are poorly characterized and understood at present. Additional insights into this phenotypic variability derived from study of mouse models are presented to explore this phenomenon.

Animal models of neurodegenerative diseases

Animal models of neurodegenerative disease are excellent tools for studying pathogenesis and therapies including cellular transplantation. In this chapter, we describe different models of Huntington's disease and Parkinson's disease, stereotactic surgery (used in creation of lesion models and transplantation) and finally transplantation studies in these models.

Voxel-based morphometry in individual patients: a pilot study in early Huntington disease

BACKGROUND AND PURPOSE

Voxel-based morphometry (VBM) has proved a powerful method to detect subtle changes of gray matter (GM) at the group level but the role of VBM for the detection of GM changes in single subjects, especially in those with suspected neurodegenerative disorder, remains uncertain. Here, we performed single subject analyses in 22 patients in early stages of Huntington disease (HD), a neurodegenerative disorder with a well-known and characteristic pattern of GM loss.

MATERIALS AND METHODS

We applied an ANCOVA with age and gender as covariates and corrected for multiple statistical tests by false discovery rate (P < 0.05). Each patient was compared to 133 healthy controls. The same procedure was applied to 22 of the controls matched for age and gender in a pair-wise manner.

RESULTS

Our analyses yielded biologically plausible results in HD patients in which GM decrease within the caudate nucleus could be identified in 15 of the 16 most affected patients while GM decrease was found in only 1 control subject. Lowering the size of the control group yielded comparable results with 99 and 66 control subjects whereas sensitivity decreased with 33 control subjects.

CONCLUSIONS

Our pilot study demonstrates a potential role of VBM for the detection of cerebral GM changes in single subjects with suspected neurodegenerative disorder.

Basal ganglia volume and clinical correlates in 'preclinical' Huntington's disease

OBJECTIVE

To establish differences in basal ganglia and thalamic volume between preclinical carriers and non-carriers of the Huntington's disease (HD) gene and to link the volume to motor, cognitive and behavioural characteristics in carriers.

METHODS

Sixteen HD gene carriers without overt clinical motor signs and 14 non-gene carriers underwent clinical evaluation and a MRI scan. Volumes of the caudate nucleus, putamen, gobus pallidus and thalamus were measured using T1-weighted MR images. Motor, cognitive and behavioural functioning was assessed using the Unified Huntington's Disease Rating Scale (UHDRS), cognitive testing and the Beck Depression Inventory (BDI-II).

RESULTS

Volumes of the caudate nucleus, putamen and globus pallidus were significantly smaller in carriers than in non-carriers while no differences between groups were found on clinical evaluation. In gene carriers smaller globus pallidus volume was associated with more motor abnormalities. A smaller putamen volume correlated significantly with worse psychomotor function on the Symbol Digit Modalities Task and the Trail Making Test B.

CONCLUSIONS

In line with previous research we demonstrated that basal ganglia abnormalities precede overt disease manifestation of HD. Besides we showed that smaller basal ganglia volumes are related to subtle motor abnormalities and worse psychomotor performance in gene carriers without clinical diagnosis. Motor and psychomotor measures may be suitable clinical markers in future neuroprotective trials when combined with volumetric imaging.

Diffusion Tensor Imaging in Preclinical Huntington's Disease

Diffusion tensor imaging was used to study brain related changes in white matter that may be associated with Huntington's Disease progression. Thirty-one preclinical gene-mutation carriers were imaged cross-sectionally using diffusion tensor and anatomical brain imaging. Subjects were individuals who had a known gene mutation for HD but did not manifest motor diagnostic criteria for HD. Fractional anisotropy scalar maps showed a positive correlation with five year probability of diagnosis (based upon gene repeat length and current age) in the putamen and a negative correlation in the external capsule. This study shows that scalar maps generated from diffusion tensor imaging may be directly related to the earliest stages of disease progression within HD, even before a diagnosis is given. Findings suggest that DTI measures, therefore, may have the ability to act as a biomarker for disease progression in clinical trials of pre-manifest subjects.

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.

Striatal degeneration impairs language learning: evidence from Huntington's disease

Although the role of the striatum in language processing is still largely unclear, a number of recent proposals have outlined its specific contribution. Different studies report evidence converging to a picture where the striatum may be involved in those aspects of rule-application requiring non-automatized behaviour. This is the main characteristic of the earliest phases of language acquisition that require the online detection of distant dependencies and the creation of syntactic categories by means of rule learning. Learning of sequences and categorization processes in non-language domains has been known to require striatal recruitment. Thus, we hypothesized that the striatum should play a prominent role in the extraction of rules in learning a language. We studied 13 pre-symptomatic gene-carriers and 22 early stage patients of Huntington's disease (pre-HD), both characterized by a progressive degeneration of the striatum and 21 late stage patients Huntington's disease (18 stage II, two stage III and one stage IV) where cortical degeneration accompanies striatal degeneration. When presented with a simplified artificial language where words and rules could be extracted, early stage Huntington's disease patients (stage I) were impaired in the learning test, demonstrating a greater impairment in rule than word learning compared to the 20 age- and education-matched controls. Huntington's disease patients at later stages were impaired both on word and rule learning. While spared in their overall performance, gene-carriers having learned a set of abstract artificial language rules were then impaired in the transfer of those rules to similar artificial language structures. The correlation analyses among several neuropsychological tests assessing executive function showed that rule learning correlated with tests requiring working memory and attentional control, while word learning correlated with a test involving episodic memory. These learning impairments significantly correlated with the bicaudate ratio. The overall results support striatal involvement in rule extraction from speech and suggest that language acquisition requires several aspects of memory and executive functions for word and rule learning.

Voxel-based morphometry in the R6/2 transgenic mouse reveals differences between genotypes not seen with manual 2D morphometry

The R6/2 mouse is the most common mouse model used for Huntington's disease (HD), a fatal, inherited neurodegenerative CAG disorder characterized by marked brain atrophy. We scanned 47 R6/2 transgenic and 42 wildtype (WT) ex vivo mouse brains at 18 weeks of age using high resolution, three-dimensional magnetic resonance imaging (MRI) for automated voxel-based morphometry (VBM) analysis. We found differences between genotypes in specific brain structures. Many of these changes were bilateral and were found in regions known to be involved in the behavioral deficits present in both R6/2 mice and HD patients. In particular, changes were evident in the basal ganglia, hippocampus, cortex and hypothalamus. In the striatum, changes were heterogenous and reminiscent of striosomal distribution. Changes were also seen in the cerebellum, as might be expected in a mouse carrying a repeat length typical of juvenile onset HD. Many of these changes were not detected by manual 2D morphometry from the same MR images. These data indicate that VBM will be a valuable technique for in vivo measurement of developing pathology in HD transgenic mice, and may be particularly useful for correlating histologically undetectable changes with behavioral deficits.

Plasma 24S-hydroxycholesterol and caudate MRI in pre-manifest and early Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disorder for which biological indicators of disease progression, or disease stage, would be especially important for therapeutic trials. 24S-hydroxycholesterol (24OHC) is a brain-generated cholesterol metabolite which has been associated with neurodegeneration, and alterations of cholesterol metabolism in murine HD models and patients' tissues have been recently identified. On these grounds, and with the aim of identifying putative biomarkers in HD, we studied cholesterol metabolism through the analysis in vivo of plasma 24OHC and cholesterol in two independent cohorts of controls and patients of Italian and British origin. We analysed a total of 62 controls, 96 HD symptomatic patients at different disease stages (stage 1-3), and 33 HD gene-positive pre-manifest subjects [pre-manifest HD (pre-HD)]. Cholesterol and 24OHC plasma levels were comparable in both the British and Italian subjects, and were not influenced by fasting or post-meal status. Cholesterol levels did not show differences between controls, pre-HD subjects and HD patients. In contrast, the plasma levels of 24OHC were significantly higher in controls than in HD patients at all disease stages (P < 0.001). Interestingly, in pre-HD subjects plasma 24OHC concentrations were similar to those of controls, and thus significantly greater than those of HD patients at any disease stage (P < 0.001). As expected, significant differences in caudate volumes between stage 1-2 HD patients and pre-HD subjects, and pre-HD subjects and controls were found. The pre-HD cohort of subjects was heterogeneous as to 24OHC levels, since subjects closer to predicted development of motor signs of disease had lower 24OHC levels than those far from onset. Our data indicate that the brain-generated cholesterol metabolite 24OHC measured in plasma was significantly depleted in HD patients at any disease stage, and it could discriminate pre-manifest subjects from patients with overt motor disease. However, 24OHC levels failed to mark further disease progression in patients with manifest HD. Overall, we demonstrate that 24OHC levels parallel the large decrease in caudate volumes observed in gene-positive subjects from pre-manifest to HD stage 1, thus reflecting a critical phase characterized by neuronal loss. We conclude that that 24OHC levels complement MRI morphometry as a valuable tool to follow neurodegenerative changes in the early stages of Huntington disease.

Neocortical expression of mutant huntingtin is not required for alterations in striatal gene expression or motor dysfunction in a transgenic mouse

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disease caused by an expanded polyglutamine tract in the ubiquitously expressed huntingtin protein. Clinically, HD is characterized by motor, cognitive and psychiatric deficits. Striking degeneration of the striatum is observed in HD with the medium spiny neurons (MSNs) being the most severely affected neuronal subtype. Dysfunction of MSNs is marked by characteristic changes in gene expression which precede neuronal death. The ubiquitous expression of the huntingtin protein raises the question as to whether the selective vulnerability of the MSN is cell-autonomous, non-cell-autonomous, or a combination thereof. In particular, growing evidence suggests that abnormalities of the cortex and corticostriatal projections may be primary causes of striatal vulnerability. To examine this issue, we developed transgenic mice that, within the forebrain, selectively express a pathogenic huntingtin species in the MSNs, specifically excluding the neocortex. These mice develop a number of abnormalities characteristic of pan-cellular HD mouse models, including intranuclear inclusion bodies, motor impairment, and changes in striatal gene expression. As this phenotype develops in the presence of normal levels of brain-derived neurotrophic factor and its major striatal receptor, tropomyosin-related kinase B, these data represent the first demonstration of in vivo cell-autonomous transcriptional dysregulation in an HD mouse model. Furthermore, our findings suggest that therapies targeted directly to the striatum may be efficacious at reversing some of the molecular abnormalities present in HD.

Blood level of brain-derived neurotrophic factor mRNA is progressively reduced in rodent models of Huntington's disease: restoration by the neuroprotective compound CEP-1347

Huntington's disease (HD) is an age-related neurodegenerative disorder that is currently untreatable. A prominent feature of HD pathology is the reduction of the pro-survival neurotrophin Brain-Derived Neurotrophic Factor (BDNF). Both mRNA and protein levels of BDNF are decreased in the brains of several HD rodent models and in human HD patients. We now report for the first time that this molecular event is mirrored in blood from HD rodent models. While protein levels of BDNF are undetectable in mouse blood, mRNA levels are measurable and diminish during HD progression in transgenic mouse (R6/2) and rat models of HD. Among the eight different BDNF transcripts, only BDNF exon III is transcribed in mouse blood and its expression is progressively compromised in R6/2 mice with respect to age-matched wild-types. Assessment of BDNF mRNA in HD rat blood shows a similar result, which is reinforced by evidence that protein levels of the neurotrophin are also significantly reduced at a symptomatic stage. Finally, we demonstrate that acute and chronic treatment of R6/2 mice with CEP-1347, a mixed lineage kinase (MLK) inhibitor with neuroprotective and neurotrophic effects, leads to increased total BDNF mRNA in blood when compared to untreated R6/2 mice. Our results indicate that alterations in BDNF mRNA levels in peripheral blood are a readily accessible measurement of disease progression and drug efficacy in HD rodent models.

Cell therapy in Huntington disease

Huntington disease (HD), caused by polyglutamate expansions in the huntingtin protein, is a progressive neurodegenerative disease resulting in cognitive and motor impairments and death. Neuronal dysfunction and degeneration contribute to progressive physiological, motor, cognitive, and emotional disturbances characteristic of HD. A major impetus for research into the treatment of HD has centered on cell therapy strategies to protect vulnerable neuronal cell populations or to replace dysfunctional or dying cells. The work underlying 3 approaches to HD cell therapy includes the potential for self-repair through the manipulation of endogenous stem cells and/or neurogenesis, the use of fetal or stem cell transplantation as a cell replacement strategy, and the administration of neurotrophic factors to protect susceptible neuronal populations. These approaches have shown some promising results in animal models of HD. Although striatal transplantation of fetal-derived cells has undergone clinical assessment since the 1990s, many cell therapy strategies have yet to be applied in the clinic environment. A more thorough understanding of the pathophysiologies underlying HD as well as the response of both endogenous and exogenous cells to the degenerating brain will inform their merit as potential therapeutic agents and enhance the framework by which the success of such strategies are determined.

L-kynurenine: metabolism and mechanism of neuroprotection

L-kynurenine is an intermediate in the pathway of the metabolism of L-tryptophan to nicotinic acid. L-kynurenine is formed in the mammalian brain (40%) and is taken up from the periphery (60%), indicating that it can be transported across the BBB. It was discovered some 30 years ago that compounds in the kynurenine family have neuroactive properties. L-kynurenine, the central agent of this pathway, can be converted into two other important compounds: the neuroprotective kynurenic acid and the neurotoxic quinolinic acid. Kynurenines have been shown to be involved in many diverse physiological and pathological processes. There are a number of neurodegenerative disorders whose pathogenesis has been demonstrated to involve multiple imbalances of the kynurenine pathway metabolism. This review summarizes the main steps of the kynurenine pathway under normal conditions, discusses the metabolic disturbances and changes in this pathway in certain neurodegenerative disorders, and finally introduces the therapeutic possibilities with kynurenines.

Cortical thickness measured from MRI in the YAC128 mouse model of Huntington's disease

A recent study found differences in localised regions of the cortex between the YAC128 mouse model of Huntington's Disease (HD) and wild-type mice. There are, however, few tools to automatically examine shape differences in the cortices of mice. This paper describes an algorithm for automatically measuring cortical thickness across the entire cortex from MRI of fixed mouse brain specimens. An analysis of the variance of the method showed that, on average, a 50 microm (0.05 mm) localised difference in cortical thickness can be measured using MR scans. Applying these methods to 8-month-old YAC128 mouse model mice representing an early stage of HD, we found an increase in cortical thickness in the sensorimotor cortex, and also revealed regions wherein decreasing striatal volume correlated with increasing cortical thickness, indicating a potential compensatory response.

Are cognitive changes progressive in prediagnostic HD?

OBJECTIVE

To characterize neurocognitive signs of disease progression in prediagnosis and early Huntington disease (HD) and compare the sensitivity of 2 disease staging classification schemes for detecting these signs.

METHODS

Three hundred and six individuals at-risk for or recently diagnosed with HD completed the Unified Huntington's Disease Rating Scale, genetic testing, and a neurocognitive battery. Two schemes were used to estimate latency to onset of disease. One was based on genetic information (CAG repeat length) and the other was based on the extent of motor signs. Effect sizes were compared to assess the relative sensitivity of the 2 schemes for detecting signs of disease progression.

RESULTS

CAG-expanded participants far from estimated diagnosis performed similarly to controls, whereas those near to estimated diagnosis were impaired relative to controls. Overall, the method employing genetic information yielded larger effect sizes than the motor scheme, particularly for strategic and executive function measures; the motor scheme resulted in a larger effect size for a measure of motor/psychomotor function.

CONCLUSIONS

Neurocognitive function is not uniformly affected in prediagnosis and early HD; individuals near to their estimated age of diagnosis have cognitive signs similar to HD, whereas individuals far from estimated diagnosis appear cognitively normal. Classification schemes that incorporate both genetic and phenotypic information may be more sensitive for tracking neurocognitive signs of disease progression.

R6/2 neurons with intranuclear inclusions survive for prolonged periods in the brains of chimeric mice

The R6/2 mouse possesses mutant exon 1 of human Hdh, and R6/2 mice with 150 CAG repeats show neurological abnormalities by 10 weeks and die by 15 weeks. Few brain abnormalities, however, are evident at death, other than widespread ubiquitinated neuronal intranuclear inclusions (NIIs). We constructed R6/2t+/t- <--> wildtype (WT) chimeric mice to prolong survival of R6/2 cells and determine if neuronal death and/or neuronal injury become evident with longer survival. ROSA26 mice (which bear a lacZ transgene) were used as WT to distinguish between R6/2 and WT neurons. Chimeric mice consisting partly of R6/2 cells lived longer than pure R6/2 mice (up to 10 months), with the survival proportional to the R6/2 contribution. Genotypically R6/2 cells formed NIIs in the chimeras, and these NIIs grew only slightly larger than in 12-week pure R6/2 mice, even after 10 months. Additionally, neuropil aggregates formed near R6/2 neurons in chimeric mice older than 15 weeks. Thus, R6/2 neurons could survive well beyond 15 weeks in chimeras. Moreover, little neuronal degeneration was evident in either cortex or striatum by routine histological stains. Nonetheless, striatal shrinkage and ventricular enlargement occurred, and striatal projection neuron markers characteristically reduced in Huntington's disease were diminished. Consistent with such abnormalities, cortex and striatum in chimeras showed increased astrocytic glial fibrillary acidic protein. These results suggest that while cortical and striatal neurons can survive nearly a year with nuclear and extranuclear aggregates of mutant huntingtin, such lengthy survival does reveal cortical and striatal abnormality brought on by the truncated mutant protein.

Variation of selective gray and white matter atrophy in Huntington's disease

The relationship between the extent of local gray/white matter atrophy, genetic load, and clinical impairment was studied in Huntington's disease (HD) by means of voxel-based morphometry. T1-weighted brain images from 33 patients (mean age 49.5, range 25-73 years) with HD duration of 1 to 15 years were analyzed by correlation of each voxel intensity with the number of CAG triplets and the UHDRS-motor score (P < 0.001). The CAG number correlated inversely with gray matter intensity in the caudate nuclei and with white matter intensity in the both postcentral gyri and the right cerebellum. The UHDRS-motor score correlated inversely with the atrophy of both caudates, right hippocampus, calcarine fissure, and with the white matter along the fourth and lateral ventricles. While atrophy of the caudate nucleus was related to a higher number of CAG triplets and higher UHDRS-motor score, atrophy in other parts of the brain covaried with the two parameters differently: higher genetic load was associated with greater loss of cortical somatosensory projections and the worse UHDRS-motor score was accompanied by increased atrophy of the internal capsule, lower brainstem, hippocampus, and visual cortex. According to our results, the genetic load in HD predicts partially the extent of selective gray/white brain matter atrophy, which is then reflected in the severity of motor impairment.

Systematic assessment of BDNF and its receptor levels in human cortices affected by Huntington's disease

One cardinal feature of Huntington's disease (HD) is the degeneration of striatal neurons, whose survival greatly depends on the binding of cortical brain-derived neurotrophic factor (BDNF) with high-affinity (TrkB) and low-affinity neurotrophin receptors [p75 pan-neurotrophin receptor (p75(NTR))]. With a few exceptions, results obtained in HD mouse models demonstrate a reduction in cortical BDNF mRNA and protein, although autopsy data from a limited number of human HD cortices are conflicting. These studies indicate the presence of defects in cortical BDNF gene transcription and transport to striatum. We provide new evidence indicating a significant reduction in BDNF mRNA and protein in the cortex of 20 HD subjects in comparison with 17 controls, which supports the hypothesis of impaired BDNF production in human HD cortex. Analyses of the BDNF isoforms show that transcription from BDNF promoter II and IV is down-regulated in human HD cortex from an early symptomatic stage. We also found that TrkB mRNA levels are reduced in caudate tissue but not in the cortex, whereas the mRNA levels of T-Shc (a truncated TrkB isoform) and p75(NTR) are increased in the caudate. This indicates that, in addition to the reduction in BDNF mRNA, there is also unbalanced neurotrophic receptor signaling in HD.

Cortical expression of brain derived neurotrophic factor and type-1 cannabinoid receptor after striatal excitotoxic lesions

An involvement of one particular neurotrophin, namely, the brain-derived neurotrophic factor (BDNF), has been demonstrated in the pathophysiology Huntington's disease. Type-1 cannabinoid (CB1) receptor has been postulated to upregulate BDNF gene transcription. To better understand the relationship between CB1 and BDNF levels in a situation where the striatum is degenerating, we studied, by dual label immunofluorescence, the distribution of CB1 and BDNF in cortical neurons projecting to the striatum in our rat quinolinic acid model of striatal excitotoxicity. We completed our study with quantitative analyses of BDNF protein levels and CB1 binding activity in the cortex. We show that, 2 weeks post lesion, cortical neurons contain more BDNF compared with controls and to earlier time points. Such BDNF up-regulation coincides with a higher binding activity and an increased protein expression of CB1. We suggest that after excitotoxic lesions, CB1 might, at least transiently, upregulate BDNF in the attempt to rescue striatal neurons from degeneration.

MR relaxometry in Huntington's disease: Correlation between imaging, genetic and clinical parameters

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disease. It has been hypothesized that changes of iron content in the brain may be involved in the pathogenesis of HD. To ascertain the hypothesis, we investigated the relationship between T2 relaxation time (T2), the number of cytosine-adenine-guanine triplet repeats (CAG) and clinical status in patients suffering from HD. 34 HD patients (mean age 50.1+/-11.8 standard deviation (SD) years) and 34 control subjects (49.6+/-13.3) were scanned using a 1.5 tesla magnetic resonance (MR) scanner and the patients underwent clinical and genetic testing. A multiple echo sequence was employed for T2 measurements. T2 from healthy volunteers matched previous studies. A T2 shortening was found in the pallidum of HD patients compared to controls (65.4+/-6.4 ms vs. 71.8+/-3.6 ms, P<0.00001). A correlation between the number of CAG and T2 was found for the left pallidum (decrease in T2, P<0.05) and an inverse correlation for the left caudate (increase in T2, P<0.05). In HD patients, alterations in iron levels may be caused by an alteration in its axonal transport. The observed T2/CAG covariations may reflect changes in levels and forms of iron: this suggests that HD patients with a higher genetic load have more ferritin-bound ("safe form") iron in the pallidum and/or more low-molecular ("toxic") iron in the caudate. An increase in "toxic" iron in the caudate may enable oxidative stress and thus underlie progression of the disease.

Gene-environment interactions modulating cognitive function and molecular correlates of synaptic plasticity in Huntington's disease transgenic mice

Huntington's disease (HD) is a fatal neurodegenerative disorder characterized by motor, cognitive and psychiatric symptoms. Here, we show that R6/1 (HD) mice have deficits in short-term hippocampal-dependent memory prior to onset of motor symptoms. HD mice also exhibit impaired performance on a test of long-term spatial memory, however, environmental enrichment enhanced spatial learning and significantly ameliorated this memory deficit in HD mice. Analysis of the presynaptic vesicle protein synaptophysin showed no differences between standard-housed wild-type and HD littermates, however, enrichment increased synaptophysin levels in the frontal cortex and hippocampus in both groups. In comparison, analysis of postsynaptic proteins revealed that HD animals show decreased levels of PSD-95 and GluR1, but no change in levels of gephyrin. Furthermore, at 12 weeks of age when we observe a beneficial effect of enrichment on spatial learning in HD mice, enrichment also delays the onset of a deficit in hippocampal PSD-95 levels. Our results show that cognitive deficits in HD mice can be ameliorated by environmental enrichment and suggest that changes in synaptic composition may contribute to the cognitive alterations observed.

Ubiquitin-proteasome system alterations in a striatal cell model of Huntington's disease

Huntington's disease (HD) is a progressive, autosomal dominant neurodegenerative disease caused by an abnormally expanded CAG repeat in the HD gene. Ubiquitylated aggregates containing mutant huntingtin protein in neurons are hallmarks of HD. Misfolded mutant huntingtin monomers, oligomers, or aggregates may be a result of, and cause, ubiquitin- proteasome dysfunction. To investigate the ubiquitin-proteasome system we designed a series of firefly luciferase reporters targeted selectively to different points along this pathway. These reporters were used to monitor ubiquitin-proteasome system function in a striatal cell culture model of HD. Ubiquitylation processes were not reduced in mutant huntingtin cells but recognition or degradation of ubiquitylated substrates was decreased. We also found mutant huntingtin expressing cells had slight but significant decreases in chymotrypsin-like and caspase-like activities, and an unexpected increase in trypsin-like activity of the proteasome core. General proteasome core inhibitors, as well as selective caspase-like activity inhibitors, were less effective in mutant cells. Finally, treatment with 3-nitropropionic acid, a succinate dehydrogenase inhibitor, had opposite effects on the ubiquitin-proteasome system with activation in wild-type and decreased activity in mutant huntingtin expressing cells. The results of these experiments show clearly selective disruption of the ubiquitin-proteasome system in this cell culture model of HD. The high throughput tools that we have designed and optimized will also be useful in identifying compounds that alter ubiquitin-proteasome system function and to investigate other neurodegenerative diseases such Alzheimer's disease and Parkinson's disease.

Evidence of thalamic dysfunction in Huntington disease by proton magnetic resonance spectroscopy

Our objective was to investigate thalamic neuronal dysfunction in patients with Huntington disease (HD). We performed localized single-voxel proton magnetic resonance spectroscopy (MRS) of the thalamus in 22 HD patients and 25 healthy individuals. The mean age of patients was 48.5 years (ranging from 32 to 71 years). Age at onset varied between 20 and 66 years (mean 38.9 years). The expanded CAG repeat ranged from 40 to 52 (mean 45.2) CAGs. The mean age of control group was 35.4 years, ranging from 19 to 67 years. N-acetylaspartate (NAA) relative to creatine (NAA/Cr) values in the thalamus of HD patients were decreased when compared with controls (P = 0.0001). The spectroscopic findings were not correlated with motor impairment. However, there was a positive correlation between duration of disease and motor impairment (P = 0.02, r = 0.48), and a tendency for positive correlation between duration of disease and NAA/Cr (P = 0.059, r = 0.4). We found decreased NAA/Cr values in the thalamus of patients with HD, indicating neuronal loss or dysfunction. This is in agreement with previous studies that indicated the involvement of mitochondrial dysfunction in the neurodegenerative process of HD.

Longitudinal evaluation of the Hdh(CAG)150 knock-in murine model of Huntington's disease

Several murine genetic models of Huntington's disease (HD) have been developed. Murine genetic models are crucial for identifying mechanisms of neurodegeneration in HD and for preclinical evaluation of possible therapies for HD. Longitudinal analysis of mutant phenotypes is necessary to validate models and to identify appropriate periods for analysis of early events in the pathogenesis of neurodegeneration. Here we report longitudinal characterization of the murine Hdh(CAG)150 knock-in model of HD. A series of behavioral tests at five different time points (20, 40, 50, 70, and 100 weeks) demonstrates an age-dependent, late-onset behavioral phenotype with significant motor abnormalities at 70 and 100 weeks of age. Pathological analysis demonstrated loss of striatal dopamine D1 and D2 receptor binding sites at 70 and 100 weeks of age, and stereological analysis showed significant loss of striatal neuron number at 100 weeks. Late-onset behavioral abnormalities, decrease in striatal dopamine receptors, and diminished striatal neuron number observed in this mouse model recapitulate key features of HD. The Hdh(CAG)150 knock-in mouse is a valid model to evaluate early events in the pathogenesis of neurodegeneration in HD.

Haloperidol protects striatal neurons from dysfunction induced by mutated huntingtin in vivo

Huntington's disease (HD) results from an abnormal polyglutamine extension in the N-terminal region of the huntingtin protein. This mutation causes preferential degeneration of striatal projection neurons. We previously demonstrated, in vitro, that dopaminergic D2 receptor stimulation acted synergistically with mutated huntingtin (expHtt) to increase aggregate formation and striatal death. In the present work, we extend these observations to an in vivo system based on lentiviral-mediated expression of expHtt in the rat striatum. The early and chronic treatment with the D2 antagonist haloperidol decanoate protects striatal neurons from expHtt-induced dysfunction, as analyzed by DARPP-32 and NeuN stainings. Haloperidol treatment also reduces aggregates formation, an effect that is maintained over time. These findings indicate that D2 receptors activation contributes to the deleterious effects of expHtt on striatal function and may represent an interesting early target to alter the subsequent course of neuropathology in HD.

Brain phylogeny, ontogeny and dysfunction: integrating evolutionary, developmental and clinical perspectives in cognitive neuroscience

OBJECTIVE

One of the most popular approaches in cognitive neuroscience has been to study the normal adult human brain. However, there are likely to be limits to the knowledge that can be obtained from such studies. If we assume that no single approach can ever provide us with knowledge of causative processes whereby the mind emerges from the brain, then we need to consider how to combine more disparate approaches. I aim to illustrate here how the parallel study of brain phylogeny, ontogeny and dysfunction may bring us towards an integrative understanding of fundamental aspects of cognitive neuroscience.

METHODS

A review of published literature in these research areas was carried out and representative articles selected.

RESULTS

Comparative approaches, utilizing the extraordinary behavioural abilities as well as the structural and functional variants that evolution has thrown up across diverse groups of species, can inform the core neural systems that may be necessary and sufficient to support specific cognitive processes. Similarly, detailed studies of human brain development, focusing on structural and functional maturation correlated with temporal mapping of cognitive processes as they come 'on-line', may provide unique mechanistic insights. Finally, the study of brain dysfunction in neurological and psychiatric disorders such as Huntington's disease, Alzheimer's disease, schizophrenia and depression, may have the beneficial side-effect of greatly enhancing our understanding of healthy brain function.

CONCLUSION

Each approach has its own epistemological advantages and disadvantages, but combined they may lead to more sophisticated, and empirically testable, models. In this review, I outline evidence for their utility, illustrate the approaches using specific examples and suggest how new advances in fields such as genomics, neurophysiology and neuroimaging may provide unprecedented opportunities in cognitive neuroscience.

Brain-derived neurotrophic factor restores synaptic plasticity in a knock-in mouse model of Huntington's disease

Asymptomatic Huntington's disease (HD) patients exhibit memory and cognition deficits that generally worsen with age. Similarly, long-term potentiation (LTP), a form of synaptic plasticity involved in memory encoding, is impaired in HD mouse models well before motor disturbances occur. The reasons why LTP deteriorates are unknown. Here we show that LTP is impaired in hippocampal slices from presymptomatic Hdh(Q92) and Hdh(Q111) knock-in mice, describe two factors contributing to this deficit, and establish that potentiation can be rescued with brain-derived neurotrophic factor (BDNF). Baseline physiological measures were unaffected by the HD mutation, but LTP induction and, to a greater degree, consolidation were both defective. The facilitation of burst responses that normally occurs during a theta stimulation train was reduced in HD knock-in mice, as was theta-induced actin polymerization in dendritic spines. The decrease in actin polymerization and deficits in LTP stabilization were reversed by BDNF, concentrations of which were substantially reduced in hippocampus of both Hdh(Q92) and Hdh(Q111) mice. These results suggest that the HD mutation discretely disrupts processes needed to both induce and stabilize LTP, with the latter effect likely arising from reduced BDNF expression. That BDNF rescues LTP in HD knock-in mice suggests the possibility of treating cognitive deficits in asymptomatic HD gene carriers by upregulating production of the neurotrophin.

Myelin breakdown and iron changes in Huntington's disease: pathogenesis and treatment implications

BACKGROUND

Postmortem and in vivo imaging data support the hypothesis that premature myelin breakdown and subsequent homeostatic remyelination attempts with increased oligodendrocyte and iron levels may contribute to Huntington's Disease (HD) pathogenesis and the symmetrical progress of neuronal loss from earlier-myelinating striatum to later-myelinating regions. A unique combination of in vivo tissue integrity and iron level assessments was used to examine the hypothesis.

METHODS

A method that uses two Magnetic resonance imaging (MRI) instruments operating at different field-strengths was used to quantify the iron content of ferritin molecules (ferritin iron) as well as tissue integrity in eight regions in 11 HD and a matched group of 27 healthy control subjects. Three white matter regions were selected based on their myelination pattern (early to later-myelinating) and fiber composition. These were frontal lobe white matter (Fwm) and splenium and genu of the corpus callosum (Swm and Gwm). In addition, gray matter structures were also chosen based on their myelination pattern and fiber composition. Three striatum structures were assessed [caudate, putamen, and globus pallidus (C, P, and G)] as well as two comparison gray matter regions that myelinate later in development and are relatively spared in HD [Hippocampus (Hipp) and Thalamus (Th)].

RESULTS

Compared to healthy controls, HD ferritin iron levels were significantly increased in striatum C, P, and G, decreased in Fwm and Gwm, and were unchanged in Hipp, Th, and Swm. Loss of tissue integrity was observed in C, P, Fwm, and especially Swm but not Hipp, Th, G, or Gwm. This pattern of findings was largely preserved when a small subset of HD subjects early in the disease process was examined.

CONCLUSIONS

The data suggest early in the HD process, myelin breakdown and changes in ferritin iron distribution underlie the pattern of regional toxicity observed in HD. Prospective studies are needed to verify myelin breakdown and increased iron levels are causal factors in HD pathogenesis. Tracking the effects of novel interventions that reduce myelin breakdown and iron accumulation in preclinical stages of HD could hasten the development of preventive treatments.

Striatal and extrastriatal atrophy in Huntington's disease and its relationship with length of the CAG repeat

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder that affects the striatum most severely. However, except for juvenile forms, relative preservation of the cerebellum has been reported. The objective of the present study was to perform MRI measurements of caudate, putamen, cerebral, and cerebellar volumes and correlate these findings with the length of the CAG repeat and clinical parameters. We evaluated 50 consecutive patients with HD using MRI volumetric measurements and compared them to normal controls. Age at onset of the disease ranged from 4 to 73 years (mean: 43.1 years). The length of the CAG repeat ranged from 40 to 69 (mean: 47.2 CAG). HD patients presented marked atrophy of the caudate and putamen, as well as reduced cerebellar and cerebral volumes. There was a significant correlation between age at onset of HD and length of the CAG repeat, as well as clinical disability and age at onset. The degree of basal ganglia atrophy correlated with the length of the CAG repeat. There was no correlation between cerebellar or cerebral volume and length of the CAG repeat. However, there was a tendency to a positive correlation between duration of disease and cerebellar atrophy. While there was a negative correlation of length of the CAG repeat with age at disease onset and with striatal degeneration, its influence on extrastriatal atrophy, including the cerebellum, was not clear. Extrastriatal atrophy occurs later in HD and may be related to disease duration.