Pathological Cell-Cell Interactions Elicited by a Neuropathogenic Form of Mutant Huntingtin Contribute to Cortical Pathogenesis in HD Mice

Full-length human mutant huntingtin with a stable polyglutamine repeat can elicit progressive and selective neuropathogenesis in BACHD mice

To elucidate the pathogenic mechanisms in Huntington's disease (HD) elicited by expression of full-length human mutant huntingtin (fl-mhtt), a bacterial artificial chromosome (BAC)-mediated transgenic mouse model (BACHD) was developed expressing fl-mhtt with 97 glutamine repeats under the control of endogenous htt regulatory machinery on the BAC. BACHD mice exhibit progressive motor deficits, neuronal synaptic dysfunction, and late-onset selective neuropathology, which includes significant cortical and striatal atrophy and striatal dark neuron degeneration. Power analyses reveal the robustness of the behavioral and neuropathological phenotypes, suggesting BACHD as a suitable fl-mhtt mouse model for preclinical studies. Additional analyses of BACHD mice provide novel insights into how mhtt may elicit neuropathogenesis. First, unlike previous fl-mhtt mouse models, BACHD mice reveal that the slowly progressive and selective pathogenic process in HD mouse brains can occur without early and diffuse nuclear accumulation of aggregated mhtt (i.e., as detected by immunostaining with the EM48 antibody). Instead, a relatively steady-state level of predominantly full-length mhtt and a small amount of mhtt N-terminal fragments are sufficient to elicit the disease process. Second, the polyglutamine repeat within fl-mhtt in BACHD mice is encoded by a mixed CAA-CAG repeat, which is stable in both the germline and somatic tissues including the cortex and striatum at the onset of neuropathology. Therefore, our results suggest that somatic repeat instability does not play a necessary role in selective neuropathogenesis in BACHD mice. In summary, the BACHD model constitutes a novel and robust in vivo paradigm for the investigation of HD pathogenesis and treatment.

Huntington's disease: from pathology and genetics to potential therapies

Huntington's disease (HD) is a devastating autosomal dominant neurodegenerative disease caused by a CAG trinucleotide repeat expansion encoding an abnormally long polyglutamine tract in the huntingtin protein. Much has been learnt since the mutation was identified in 1993. We review the functions of wild-type huntingtin. Mutant huntingtin may cause toxicity via a range of different mechanisms. The primary consequence of the mutation is to confer a toxic gain of function on the mutant protein and this may be modified by certain normal activities that are impaired by the mutation. It is likely that the toxicity of mutant huntingtin is revealed after a series of cleavage events leading to the production of N-terminal huntingtin fragment(s) containing the expanded polyglutamine tract. Although aggregation of the mutant protein is a hallmark of the disease, the role of aggregation is complex and the arguments for protective roles of inclusions are discussed. Mutant huntingtin may mediate some of its toxicity in the nucleus by perturbing specific transcriptional pathways. HD may also inhibit mitochondrial function and proteasome activity. Importantly, not all of the effects of mutant huntingtin may be cell-autonomous, and it is possible that abnormalities in neighbouring neurons and glia may also have an impact on connected cells. It is likely that there is still much to learn about mutant huntingtin toxicity, and important insights have already come and may still come from chemical and genetic screens. Importantly, basic biological studies in HD have led to numerous potential therapeutic strategies.

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.

Glial cells as intrinsic components of non-cell-autonomous neurodegenerative disease

A lesson from dominantly inherited forms of diverse neurodegenerative diseases, including amyotrophic lateral sclerosis, spinocerebellar ataxia and Huntington's disease, is that the selective dysfunction or death of the neuronal population most at risk in each disease is not mediated solely by damage from the mutant protein within the target neurons. The disease-causing toxic process, which in each case is caused by mutation in a gene that is widely or ubiquitously expressed, involves damage done by mutant proteins within the non-neuronal glial cells of the central nervous system, especially astrocytes and microglia. The disease mechanism is non-cell-autonomous, with toxicity derived from glia as a prominent contributor driving disease progression and in some instances even disease initiation.

The insulin-like growth factor pathway is altered in spinocerebellar ataxia type 1 and type 7

Polyglutamine diseases are inherited neurodegenerative disorders caused by expansion of CAG repeats encoding a glutamine tract in the disease-causing proteins. There are nine disorders, each having distinct features but also clinical and pathological similarities. In particular, spinocerebellar ataxia type 1 and 7 (SCA1 and SCA7) patients manifest cerebellar ataxia with degeneration of Purkinje cells. To determine whether the disorders share molecular pathogenic events, we studied two mouse models of SCA1 and SCA7 that express the glutamine-expanded protein from the respective endogenous loci. We found common transcriptional changes, with down-regulation of insulin-like growth factor binding protein 5 (Igfbp5) representing one of the most robust changes. Igfbp5 down-regulation occurred in granule neurons through a non-cell-autonomous mechanism and was concomitant with activation of the insulin-like growth factor (IGF) pathway and the type I IGF receptor on Purkinje cells. These data define one common pathogenic response in SCA1 and SCA7 and reveal the importance of intercellular mechanisms in their pathogenesis.

Expression profiling of Huntington's disease models suggests that brain-derived neurotrophic factor depletion plays a major role in striatal degeneration

Many pathways have been proposed as contributing to Huntington's disease (HD) pathogenesis, but generally the in vivo effects of their perturbation have not been compared with reference data from human patients. Here we examine how accurately mechanistically motivated and genetic HD models recapitulate the striatal gene expression phenotype of human HD. The representative genetic model was the R6/2 transgenic mouse, which expresses a fragment of the huntingtin protein containing a long CAG repeat. Pathogenic mechanisms examined include mitochondrial dysfunction; profiled in 3-nitropropionic acid-treated rats, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mice, and PGC-1alpha knock-out mice; and depletion of brain-derived neurotrophic factor (BDNF) using heterozygous and forebrain-specific BDNF-knock-out mice (BDNF(HET), Emx-BDNF(KO)). Based on striatal gene expression, we find the BDNF models, both heterozygous and homozygous knock-outs, to be more like human HD than the other HD models. This implicates reduced trophic support as a major pathway contributing to striatal degeneration in HD. Because the majority of striatal BDNF is synthesized by cortical neurons, the data also imply that cortical dysfunction contributes to HD's hallmark effects on the basal ganglia. Finally, the results suggest that striatal lesions caused by mitochondrial toxins may arise via pathways different from those that drive neurodegeneration in HD. Based on these findings, we present a testable model of HD pathogenesis that, unlike most models, begins to account for regional specificity in human HD and the absence of such specificity in genetic mouse models of HD.

One-hit stochastic decline in a mechanochemical model of cytoskeleton-induced neuron death I: Cell-fate arrival times

Much experimental evidence shows that the cytoskeleton is a downstream target and effector during cell death in numerous neurodegenerative diseases, including Parkinson's, Huntington's, and Alzheimer's diseases. However, recent evidence indicates that cytoskeletal dysfunction can also trigger neuronal death, by mechanisms as yet poorly understood. This is the first of two papers in which we study a mathematical model of cytoskeleton-induced neuron death. In our model, assembly control of the neuronal cytoskeleton interacts with both cellular stress levels and cytosolic free radical concentrations to trigger neurodegeneration. This trigger mechanism is further modulated by the presence of cell interactions in the form of a diffusible toxic factor released by dying neurons. We find that, consistent with empirical observations, our model produces one-hit exponential and sigmoid patterns of cell dropout. In all cases, cell dropout is exponential-tailed and described accurately by a gamma distribution. The transition between exponential and sigmoidal is gradual, and determined by a synergetic interaction between the magnitude of fluctuations in cytoskeleton assembly control and by the degree of cell coupling. We conclude that a single mechanism involving neuron interactions and fluctuations in cytoskeleton assembly control is compatible with the experimentally observed range of neuronal attrition kinetics.

REVIEW ARTILCE: Cell therapy and stem cells in animal models of motor neuron disorders

Amyotrophic lateral sclerosis (ALS), spinal bulbar muscular atrophy (or Kennedy's disease), spinal muscular atrophy and spinal muscular atrophy with respiratory distress 1 are neurodegenerative disorders mainly affecting motor neurons and which currently lack effective therapies. Recent studies in animal models as well as primary and embryonic stem cell models of ALS, utilizing over-expression of mutated forms of Cu/Zn superoxide dismutase 1, have shown that motor neuron degeneration in these models is in part a non cell-autonomous event and that by providing genetically non-compromised supporting cells such as microglia or growth factor-excreting cells, onset can be delayed and survival increased. Using models of acute motor neuron injury it has been shown that embryonic stem cell-derived motor neurons implanted into the spinal cord can innervate muscle targets and improve functional recovery. Thus, a rationale exists for the development of cell therapies in motor neuron diseases aimed at either protecting and/or replacing lost motor neurons, interneurons as well as non-neuronal cells. This review evaluates approaches used in animal models of motor neuron disorders and their therapeutic relevance.

fMRI detection of early neural dysfunction in preclinical Huntington's disease

Neuropsychological and neuroimaging changes have been observed in individuals with the Huntington's disease (HD) gene expansion prior to the onset of manifest HD. This cross-sectional fMRI study of preclinical HD (pre-HD) individuals was conducted to determine if functional brain changes precede deficits in behavioral performance and striatal atrophy. Twenty-six pre-HD and 13 demographically matched healthy participants performed a time reproduction task while undergoing fMRI scanning. Pre-HD participants were divided into two groups (n=13 each): FAR (>12 years to estimated onset [YEO] of manifest HD) and CLOSE (<12 YEO). The CLOSE group demonstrated behavioral deficits, striatal atrophy, and reduced neural activation in the left putamen, SMA, left anterior insula and right inferior frontal gyrus. The FAR group showed reduced neural activation in the right anterior cingulate and right anterior insula. The FAR group also demonstrated increased neural activation in the left sensorimotor, left medial frontal gyrus, left precentral gyrus, bilateral superior temporal gyri and right cerebellum. The fMRI changes in the FAR group occurred in the relative absence of striatal atrophy and behavioral performance deficits. These results suggest that fMRI is sensitive to neural dysfunction occurring more than 12 years prior to the estimated onset of manifest HD.

Glutamate receptor abnormalities in the YAC128 transgenic mouse model of Huntington's disease

A yeast artificial chromosome (YAC) mouse model of Huntington's disease (YAC128) develops motor abnormalities, age-dependent striatal atrophy and neuronal loss. Alteration of neurotransmitter receptors, particularly glutamate and dopamine receptors, is a pathological hallmark of Huntington's disease. We therefore analyzed neurotransmitter receptors in symptomatic YAC128 Huntington's disease mice. We found significant increases in N-methyl-d-aspartate, AMPA and metabotropic glutamate receptor binding, which were not due to increases in receptor subunit mRNA expression levels. Subcellular fractionation analysis revealed increased levels of glutamate receptor subunits in synaptic membrane fractions from YAC128 mice. We found no changes in dopamine, GABA or adenosine receptor binding, nor did we see alterations in dopamine D1, D2 or adenosine A2a receptor mRNA levels. The receptor abnormalities in YAC128 transgenic mice thus appear limited to glutamate receptors. We also found a significant decrease in preproenkephalin mRNA in the striatum of YAC128 mice, which contrasts with the lack of change in levels of mRNA encoding neurotransmitter receptors. Taken together, the abnormal and selective increases in glutamate receptor subunit expression and binding are not due to increases in receptor subunit expression and may exert detrimental effects. The decrease in preproenkephalin mRNA suggests a selective transcriptional deficit, as opposed to neuronal loss, and could additionally contribute to the abnormal motor symptoms in YAC128 mice.

Pathological cell-cell interactions are necessary for striatal pathogenesis in a conditional mouse model of Huntington's disease

A critical issue in understanding Huntington's disease (HD) pathogenesis is how the ubiquitously expressed mutant huntingtin (mhtt) with an expanded polyglutamine repeat can cause selective toxicity of striatal and cortical neurons. Two potential cellular models may contribute to such specificity: expression of mhtt in these vulnerable neurons alone may be sufficient to result in their dysfunction and/or degeneration (cell-autonomous model); or mhtt in other cell types can elicit pathological cell-cell interactions to cause the vulnerable neurons to become dysfunctional and be at risk for degeneration (cell-cell interaction model). To distinguish between these two models, we have selectively expressed a neuropathogenic fragment of mhtt-exon1 in striatal medium spiny neurons (MSNs) by crossing a conditional mouse model of HD with a striatal-specific Cre mouse line. In this striatal model of HD, we observed progressive and cell-autonomous nuclear accumulation of mhtt aggregates in MSNs. Surprisingly, unlike the mouse model expressing mhtt-exon1 in all the neurons in the brain, the striatal model lacks significant locomotor deficits and striatal neuropathology including gliosis and dark degenerating neurons. Electrophysiological findings from acutely dissociated MSNs revealed a cell-autonomous deficit in N-methyl-d-aspartate (NMDA) receptor sensitivity to Mg2+, a deficit also present in other mouse models of HD. In conclusion, this study provides the first in vivo genetic evidence that pathological cell-cell interactions are necessary for striatal pathogenesis in a conditional mouse model of HD, and suggests a ''two-hit'' hypothesis in which both cell-autonomous toxicity and pathological cell-cell interactions are critical to HD pathogenesis.

Transcriptional signatures in Huntington's disease

While selective neuronal death has been an influential theme in Huntington's disease (HD), there is now a preponderance of evidence that significant neuronal dysfunction precedes frank neuronal death. The best evidence for neuronal dysfunction is the observation that gene expression is altered in HD brain, suggesting that transcriptional dysregulation is a central mechanism. Studies of altered gene expression began with careful observations of postmortem human HD brain and subsequently were accelerated by the development of transgenic mouse models. The application of DNA microarray technology has spurred tremendous progress with respect to the altered transcriptional processes that occur in HD, through gene expression studies of both transgenic mouse models as well as cellular models of HD. Gene expression profiles are remarkably comparable across these models, bolstering the idea that transcriptional signatures reflect an essential feature of disease pathogenesis. Finally, gene expression studies have been applied to human HD, thus not only validating the approach of using model systems, but also solidifying the idea that altered transcription is a key mechanism in HD pathogenesis. In the future, gene expression profiling will be used as a readout in clinical trials aimed at correcting transcriptional dysregulation in Huntington's disease.

NMDA inhibitors cause apoptosis of pyramidal neurons in mature piriform cortex: evidence for a nitric oxide-mediated effect involving inhibitory interneurons

Pyramidal relay neurons in limbic cortex are vulnerable to denervation lesions, i.e. pyramidal neurons in layer IIalpha of piriform cortex undergo transsynaptic apoptosis after lesions that interrupt their inputs from the olfactory bulb. We have previously established the role of inhibitory interneurons in elaborating signals that lead to the apoptosis of projection neurons in these lesion models, i.e. the upregulation of neuronal NOS and release of nitric oxide. Thus, we have proposed that cortical interneurons play an essential role in transducing injury to degenerative effects for nearby pyramidal neurons. In the present study, we extend the previous findings to a toxic model of degeneration of pyramidal neurons in the adult paralimbic cortex, i.e. after exposure to the NMDA channel blocker MK801. Our findings indicate that treatment of adult rats with MK801 in doses previously found to cause alterations in pyramidal neurons of the retrosplenial cortex (5mg/kg) results in an active caspase 3 (+), ultrastructurally apoptotic type of cell death involving the same projection neurons of layer IIalpha that are also vulnerable to bulbotomy lesions. Interneurons of layer I are induced by MK801 treatment to higher levels of nNOS expression and the selective nNOS inhibitor BRNI ameliorates pyramidal cell apoptosis caused by MK801. Our results indicate that certain pyramidal neurons in piriform cortex are very sensitive to NMDA blockade as they are to disconnection from modality-specific afferents and that inhibitory interneurons play significant roles in mediating various types of pro-apoptotic insults to cortical projection neurons via nNOS/NO signaling.

Molecular mechanisms mediating pathological plasticity in Huntington's disease and Alzheimer's disease

Neurodegenerative diseases such as Huntington's disease and Alzheimer's disease, although very different in etiology, share common degenerative processes. These include neuronal dysfunction, decreased neural connectivity, and disruption of cellular plasticity. Understanding the molecular mechanisms underlying the neural plasticity deficits in these devastating conditions may lead the way toward new therapeutic targets, both disease-specific and more generalized, which can ameliorate degenerative cognitive deficits. Furthermore, investigations of 'pathological plasticity' in these diseases lend insight into normal brain function. This review will present evidence for altered plasticity in Huntington's and Alzheimer's diseases, relate these findings to symptomatology, and review possible causes and commonalities.

Striatal specificity of gene expression dysregulation in Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder caused by an expanded CAG repeat region in exon 1 of the HD gene. This mutation results in the presence of an abnormally long polyglutamine tract in the encoded protein, huntingtin (htt). A major question in this field is how the mutant htt protein, which is expressed ubiquitously throughout the brain and body, causes severe neuropathologic changes predominantly in the striatum. The mechanisms accounting for this specificity are unknown. The role of transcriptional dysregulation in the pathophysiology of HD has gained much attention in recent years, however, this theory has been unable to explain the specificity of dysfunction and degeneration in HD. Microarray studies have showed hundreds of gene expression changes in mouse models of HD and in post-mortem brain samples from HD subjects. Among the genes whose expression levels are preferentially altered are those that exhibit enriched expression in the striatum, which we have argued are the most relevant to disease pathology. These "striatal-enriched" genes are associated with several systems previously implicated in HD pathology, especially disturbances in transcriptional processes and calcium homeostasis. Large-scale changes in striatal gene expression in this manner would likely have particularly devastating effects to normal striatal function and could explain the specificity of striatal dysfunction and ultimate neurodegeneration observed in HD.

Multiple pathways contribute to the pathogenesis of Huntington disease

Huntington disease (HD) is caused by expansion of a polyglutamine (polyQ) domain in the protein known as huntingtin (htt), and the disease is characterized by selective neurodegeneration. Expansion of the polyQ domain is not exclusive to HD, but occurs in eight other inherited neurodegenerative disorders that show distinct neuropathology. Yet in spite of the clear genetic defects and associated neurodegeneration seen with all the polyQ diseases, their pathogenesis remains elusive. The present review focuses on HD, outlining the effects of mutant htt in the nucleus and neuronal processes as well as the role of cell-cell interactions in HD pathology. The widespread expression and localization of mutant htt and its interactions with a variety of proteins suggest that mutant htt engages multiple pathogenic pathways. Understanding these pathways will help us to elucidate the pathogenesis of HD and to target therapies effectively.

The corticostriatal pathway in Huntington's disease

The corticostriatal pathway provides most of the excitatory glutamatergic input into the striatum and it plays an important role in the development of the phenotype of Huntington's disease (HD). This review summarizes results obtained from genetic HD mouse models concerning various alterations in this pathway. Evidence indicates that dysfunctions of striatal circuits and cortical neurons that make up the corticostriatal pathway occur during the development of the HD phenotype, well before there is significant neuronal cell loss. Morphological changes in the striatum are probably primed initially by alterations in the intrinsic functional properties of striatal medium-sized spiny neurons. Some of these alterations, including increased sensitivity of N-methyl-D-aspartate receptors in subpopulations of neurons, might be constitutively present but ultimately require abnormalities in the corticostriatal inputs for the phenotype to be expressed. Dysfunctions of the corticostriatal pathway are complex and there are multiple changes as demonstrated by significant age-related transient and more chronic interactions with the disease state. There also is growing evidence for changes in cortical microcircuits that interact to induce dysfunctions of the corticostriatal pathway. The conclusions of this review emphasize, first, the general role of neuronal circuits in the expression of the HD phenotype and, second, that both cortical and striatal circuits must be included in attempts to establish a framework for more rational therapeutic strategies in HD. Finally, as changes in cortical and striatal circuitry are complex and in some cases biphasic, therapeutic interventions should be regionally specific and take into account the temporal progression of the phenotype.

Androgen-dependent pathology demonstrates myopathic contribution to the Kennedy disease phenotype in a mouse knock-in model

Kennedy disease, a degenerative disorder characterized by androgen-dependent neuromuscular weakness, is caused by a CAG/glutamine tract expansion in the androgen receptor (Ar) gene. We developed a mouse model of Kennedy disease, using gene targeting to convert mouse androgen receptor (AR) to human sequence while introducing 113 glutamines. AR113Q mice developed hormone and glutamine length-dependent neuromuscular weakness characterized by the early occurrence of myopathic and neurogenic skeletal muscle pathology and by the late development of neuronal intranuclear inclusions in spinal neurons. AR113Q males unexpectedly died at 2-4 months. We show that this androgen-dependent death reflects decreased expression of skeletal muscle chloride channel 1 (CLCN1) and the skeletal muscle sodium channel alpha-subunit, resulting in myotonic discharges in skeletal muscle of the lower urinary tract. AR113Q limb muscles show similar myopathic features and express decreased levels of mRNAs encoding neurotrophin-4 and glial cell line-derived neurotrophic factor. These data define an important myopathic contribution to the Kennedy disease phenotype and suggest a role for muscle in non-cell autonomous toxicity of lower motor neurons.

Bergmann glia expression of polyglutamine-expanded ataxin-7 produces neurodegeneration by impairing glutamate transport

Non-neuronal cells may be pivotal in neurodegenerative disease, but the mechanistic basis of this effect remains ill-defined. In the polyglutamine disease spinocerebellar ataxia type 7 (SCA7), Purkinje cells undergo non-cell-autonomous degeneration in transgenic mice. We considered the possibility that glial dysfunction leads to Purkinje cell degeneration, and generated mice that express ataxin-7 in Bergmann glia of the cerebellum with the Gfa2 promoter. Bergmann glia-specific expression of mutant ataxin-7 was sufficient to produce ataxia and neurodegeneration. Expression of the Bergmann glia-specific glutamate transporter GLAST was reduced in Gfa2-SCA7 mice and was associated with impaired glutamate transport in cultured Bergmann glia, cerebellar slices and cerebellar synaptosomes. Ultrastructural analysis of Purkinje cells revealed findings of dark cell degeneration consistent with excitotoxic injury. Our studies indicate that impairment of glutamate transport secondary to glial dysfunction contributes to SCA7 neurodegeneration, and suggest a similar role for glial dysfunction in other polyglutamine diseases and SCAs.

CA150 expression delays striatal cell death in overexpression and knock-in conditions for mutant huntingtin neurotoxicity

Transcriptional dysregulation caused by expanded polyglutamines (polyGlns) in huntingtin (htt) may be central to cell-autonomous mechanisms for neuronal cell death in Huntington's disease (HD) pathogenesis. We hypothesized that these mechanisms may involve the dysfunction of the transcriptional regulator CA150, a putative modifier of onset age in HD, because it binds to htt and accumulates in an HD grade-dependent manner in striatal and cortical neurons. Consistently, we report herein that CA150 expression rescues striatal cell death in lentiviral overexpression (rats) and knock-in (mouse cells) conditions for mutant htt neurotoxicity. In both systems, rescue was dependent on the (Gln-Ala)38 repeat normally found in CA150. We excluded the possibility that rescue may be caused by the (Gln-Ala)38 repeat interacting with polyGlns and, by doing so, blocking mutant htt toxicity. In contrast, we found the (Gln-Ala)38 repeat is required for the nuclear restriction of exogenous CA150, suggesting that rescue requires nuclear CA150. Additionally, we found the (Gln-Ala)38 repeat was dispensable for CA150 transcriptional repression ability, suggesting further that CA150 localization is critical to rescue. Finally, rescue was associated with increased neuritic aggregation, with no reduction of nuclear inclusions, suggesting the solubilization and nuclear export of mutant htt. Together, our data indicate that mutant htt may induce CA150 dysfunction in striatal neurons and suggest that the restoration of nuclear protein cooperativity may be neuroprotective.

Stochastic kinetics of intracellular huntingtin aggregate formation

Neurodegeneration in Huntington disease is described by neuronal loss in which the probability of cell death remains constant with time. However, the quantitative connection between the kinetics of cell death and the molecular mechanism initiating neurodegeneration remains unclear. One hypothesis is that nucleation of protein aggregates containing exon I fragments of the mutant huntingtin protein (mhttex1), which contains an expanded polyglutamine region in patients with the disease, is the explanation for the infrequent but steady occurrence of neuronal death, resulting in adult onset of the disease. Recent in vitro evidence suggests that sufficiently long polyglutamine peptides undergo a unimolecular conformational change to form a nucleus that seeds aggregation. Here we use this nucleation mechanism as the basis to derive a stochastic mathematical model describing the probability of aggregate formation in cells as a function of time and mhttex1 protein concentration, and validate the model experimentally. These findings suggest that therapeutic strategies for Huntington disease predicated on reducing the rate of mhttex1 aggregation need only make modest reductions in huntingtin expression level to substantially increase the delay time until aggregate formation.

Altered Cortical Glutamate Receptor Function in the R6/2 Model of Huntington's Disease

Alterations in pyramidal neurons from the sensorimotor cortex may be responsible for some of the cognitive and motor symptoms of Huntington's disease (HD). The present experiments used R6/2 transgenic mice that express exon 1 of the human HD gene with an expanded number of CAG repeats. We characterized α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) currents and their modulation by cyclothiazide (CTZ) as well as N-methyl-d-aspartate (NMDA) currents and their Mg2+sensitivity in acutely dissociated cortical pyramidal neurons in R6/2 transgenic and wild-type (WT) mice at 21 days (before overt symptoms), 40 days (when symptoms begin), and 80 days (fully symptomatic). AMPA currents, alone or in the presence of CTZ, were smaller in 21- and 40-day-old R6/2 groups compared with WT mice. In R6/2 mice, more neurons displayed desensitizing AMPA currents in the presence of CTZ, indicating increased expression of “flop” splice variants, whereas the majority of WT cells expressed the “flip” variants of AMPA receptor subunits. NMDA peak currents also were smaller in R6/2 pyramidal neurons at 21 days. At 40 days, NMDA currents were similar in WT and R6/2 mice but Mg2+sensitivity was greater in R6/2 mice, resulting in smaller NMDA currents in the presence of Mg2+. Differences in AMPA and NMDA currents between WT and R6/2 cells were no longer detected at 80 days. Our findings indicate that currents induced by glutamate receptor agonists are decreased in isolated cortical pyramidal neurons from R6/2 mice and that this decrease occurs early. Altered glutamate receptor function could contribute to changes in cortical output and may underlie some of the cognitive and motor impairments in this animal model of HD.

Expression of mutant huntingtin in glial cells contributes to neuronal excitotoxicity

Huntington disease (HD) is characterized by the preferential loss of striatal medium-sized spiny neurons (MSNs) in the brain. Because MSNs receive abundant glutamatergic input, their vulnerability to excitotoxicity may be largely influenced by the capacity of glial cells to remove extracellular glutamate. However, little is known about the role of glia in HD neuropathology. Here, we report that mutant huntingtin accumulates in glial nuclei in HD brains and decreases the expression of glutamate transporters. As a result, mutant huntingtin (htt) reduces glutamate uptake in cultured astrocytes and HD mouse brains. In a neuron-glia coculture system, wild-type glial cells protected neurons against mutant htt-mediated neurotoxicity, whereas glial cells expressing mutant htt increased neuronal vulnerability. Mutant htt in cultured astrocytes decreased their protection of neurons against glutamate excitotoxicity. These findings suggest that decreased glutamate uptake caused by glial mutant htt may critically contribute to neuronal excitotoxicity in HD.

Selective neuronal degeneration in Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder that generally begins in middle age with abnormalities of movement, cognition, personality, and mood. Neuronal loss is most marked among the medium-sized projection neurons of the dorsal striatum. HD is an autosomal dominant genetic disorder caused by a CAG expansion in exon 1 of the HD gene, encoding an expanded polyglutamine (polyQ) tract near the N-terminus of the protein huntingtin. Despite identification of the gene mutation more than a decade ago, the normal function of this ubiquitously expressed protein is still under investigation and the mechanisms underlying selective neurodegeneration in HD remain poorly understood. Detailed postmortem analyses of brains of HD patients have provided important clues, and HD transgenic and knock-in mouse models have facilitated investigations into potential pathogenic mechanisms. Subcellular fractionation and immunolocalization studies suggest a role for huntingtin in organelle transport, protein trafficking, and regulation of energy metabolism. Consistent with this, evidence from vertebrate and invertebrate models of HD indicates that expression of the polyQ-expanded form of huntingtin results in early impairment of axonal transport and mitochondrial function. As well, alteration in activity of the N-methyl-d-aspartate (NMDA) type glutamate receptor, which has been implicated as a main mediator of excitotoxic neuronal death, especially in the striatum, is an early effect of mutant huntingtin. Proteolysis and nuclear localization of huntingtin also occur relatively early, while formation of ubiquitinated aggregates of huntingtin and transcriptional dysregulation occur as late effects of the gene mutation. Although each of these processes may contribute to neuronal loss in HD, here we review the data to support a strong role for NMDA receptor (NMDAR)-mediated excitotoxicity and mitochondrial dysfunction in conferring selective neuronal vulnerability in HD.