Centrosome disorganization in fibroblast cultures derived from R6/2 Huntington's disease (HD) transgenic mice and HD patients

Hdac6 knock-out increases tubulin acetylation but does not modify disease progression in the R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disorder for which there is no effective disease modifying treatment. Following-on from studies in HD animal models, histone deacetylase (HDAC) inhibition has emerged as an attractive therapeutic option. In parallel, several reports have demonstrated a role for histone deacetylase 6 (HDAC6) in the modulation of the toxicity caused by the accumulation of misfolded proteins, including that of expanded polyglutamine in an N-terminal huntingtin fragment. An important role for HDAC6 in kinesin-1 dependent transport of brain-derived neurotrophic factor (BDNF) from the cortex to the striatum has also been demonstrated. To elucidate the role that HDAC6 plays in HD progression, we evaluated the effects of the genetic depletion of HDAC6 in the R6/2 mouse model of HD. Loss of HDAC6 resulted in a marked increase in tubulin acetylation throughout the brain. Despite this, there was no effect on the onset and progression of a wide range of behavioural, physiological, molecular and pathological HD-related phenotypes. We observed no change in the aggregate load or in the levels of soluble mutant exon 1 transprotein. HDAC6 genetic depletion did not affect the efficiency of BDNF transport from the cortex to the striatum. Therefore, we conclude that HDAC6 inhibition does not modify disease progression in R6/2 mice and HDAC6 should not be prioritized as a therapeutic target for HD.

Association of micronucleus frequency with neurodegenerative diseases

Micronuclei (MNi) can originate either from chromosome breakage or chromosome malsegregation events and are therefore ideal biomarkers to investigate genomic instability. Studies in peripheral lymphocytes of patients with neurodegenerative diseases, mainly Alzheimer's disease (AD) and Parkinson's disease (PD), revealed an increased micronucleus (MN) frequency in both disorders but originating mainly from chromosome malsegregation events in AD and from chromosome breakage events in PD. Studies in other neurodegenerative diseases are largely missing, and some data in premature ageing disorders characterised by neurodegeneration and/or neurological complications, such as Ataxia telangiectasia, Werner's syndrome, Down's syndrome (DS) and Cockayne's syndrome, indicate that MNi increase with ageing in cultured cells. An increased frequency of aneuploidy characterises several tissues of AD patients, as well as of individuals at increased risk to develop AD, such as mothers of DS individuals and DS subjects themselves. The use of the buccal MN cytome assay in AD and DS subjects allowed finding significant changes in the MN frequency as well as other cellular modifications reflecting reduced regenerative capacity compared to age- and gender-matched controls. These changes in buccal cytome ratios may prove useful as potential future diagnostics to identify individuals of increased risk for these disorders.

Identical oligomeric and fibrillar structures captured from the brains of R6/2 and knock-in mouse models of Huntington's disease

Huntington's disease (HD) is a late-onset neurodegenerative disorder that is characterized neuropathologically by the presence of neuropil aggregates and nuclear inclusions. However, the profile of aggregate structures that are present in the brains of HD patients or of HD mouse models and the relative contribution of specific aggregate structures to disease pathogenesis is unknown. We have used the Seprion ligand to develop a highly sensitive enzyme-linked immunosorbent assay (ELISA)-based method for quantifying aggregated polyglutamine in tissues from HD mouse models. We used a combination of electron microscopy, atomic force microscopy (AFM) and sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) to investigate the aggregate structures isolated by the ligand. We found that the oligomeric, proto-fibrillar and fibrillar aggregates extracted from the brains of R6/2 and HdhQ150 knock-in mice were remarkably similar. Using AFM, we determined that the nanometre globular oligomers isolated from the brains of both mouse models have dimensions identical to those generated from recombinant huntingtin exon 1 proteins. Finally, antibodies that detect exon 1 Htt epitopes differentially recognize the ligand-captured material on SDS–PAGE gels. The Seprion-ligand ELISA provides an assay with good statistical power for use in preclinical pharmacodynamic therapeutic trials or to assess the effects of the genetic manipulation of potential therapeutic targets on aggregate load. This, together with the ability to identify a spectrum of aggregate species in HD mouse tissues, will contribute to our understanding of how these structures relate to the pathogenesis of HD and whether their formation can be manipulated for therapeutic benefit.

Formation of polyglutamine inclusions in a wide range of non-CNS tissues in the HdhQ150 knock-in mouse model of Huntington's disease

Background

Huntington's disease (HD) is an inherited progressive neurodegenerative disorder caused by a CAG repeat expansion in the ubiquitously expressed HD gene resulting in an abnormally long polyglutamine repeat in the huntingtin protein. Polyglutamine inclusions are a hallmark of the neuropathology of HD. We have previously shown that inclusion pathology is also present in the peripheral tissues of the R6/2 mouse model of HD which expresses a small N-terminal fragment of mutant huntingtin. To determine whether this peripheral pathology is a consequence of the aberrant expression of this N-terminal fragment, we extend this analysis to the genetically precise knock-in mouse model of HD, HdhQ150, which expresses mutant mouse huntingtin.

Methodology/Principal Findings

We have previously standardized the CAG repeat size and strain background of the R6/2 and HdhQ150 knock-in mouse models and found that they develop a comparable and widespread neuropathology. To determine whether HdhQ150 knock-in mice also develop peripheral inclusion pathology, homozygous Hdh^Q150/Q150 mice were perfusion fixed at 22 months of age, and tissues were processed for histology and immunohistochemistry with the anti-huntingtin antibody S830. The peripheral inclusion pathology was almost identical to that found in R6/2 mice at 12 weeks of age with minor differences in inclusion abundance.

Conclusions/Significance

The highly comparable peripheral inclusion pathology that is present in both the R6/2 and HdhQ150 knock-in models of HD indicates that the presence of peripheral inclusions in R6/2 mice is not a consequence of the aberrant expression of an N-terminal huntingtin protein. It remains to be determined whether peripheral inclusions are a pathological feature of the human disease. Both mouse models carry CAG repeats that cause childhood disease in humans, and therefore, inclusion pathology may be a feature of the childhood rather than the adult forms of HD. It is important to establish the extent to which peripheral pathology causes the peripheral symptoms of HD from the perspective of a mechanistic understanding and future treatment options.

Huntington's disease: the current state of research with peripheral tissues

Huntington's disease (HD) is a genetically dominant condition caused by expanded CAG repeats. These repeats code for a glutamine tract in the HD gene product huntingtin (htt), which is a protein expressed in almost all tissues. Although most HD symptoms reflect preferential neuronal death in specific brain regions, even before the HD gene was identified numerous reports had described additional abnormalities in the peripheral tissues of HD patients, including weight loss, altered glucose homeostasis, and sub-cellular abnormalities in fibroblasts, lymphocytes and erythrocytes. Several years have elapsed since the HD mutation was discovered, and analyses of peripheral tissues from HD patients have helped to understand the molecular pathogenesis of the disease and revealed that the molecular mechanisms through which mutated htt leads to cell dysfunction are widely shared between central nervous system (CNS) and peripheral tissues. These studies show that in peripheral tissues, mutated htt causes accumulation of intracellular protein aggregates, impairment of energetic metabolism, transcriptional deregulation and hyperactivation of programmed cell-death mechanisms. Here, we review the current knowledge of peripheral tissue alterations in HD patients and in animal models of HD and focus on how this information can be used to identify potential therapeutic possibilities and biomarkers for disease progression.

The ubiquitin-proteasome reporter GFPu does not accumulate in neurons of the R6/2 transgenic mouse model of Huntington's disease

Impairment of the ubiquitin-proteasome system (UPS) has long been considered an attractive hypothesis to explain the selective dysfunction and death of neurons in polyglutamine disorders such as Huntington's disease (HD). The fact that inclusion bodies in HD mouse models and patient brains are rich in ubiquitin and proteasome components suggests that the UPS may be hindered directly or indirectly by inclusion bodies or their misfolded monomeric or oligomeric precursors. However, studies into UPS function in various polyglutamine disease models have yielded conflicting results, suggesting mutant polyglutamine tracts may exert different effects on the UPS depending on protein context, expression level, subcellular localisation and cell-type. To investigate UPS function in a well-characterised mouse model of HD, we have crossed R6/2 HD mice with transgenic UPS reporter mice expressing the GFPu construct. The GFPu construct comprises GFP fused to a constitutive degradation signal (CL-1) that promotes its rapid degradation under conditions of a healthy UPS. Using a combination of immunoblot analysis, fluorescence and immunofluorescence microscopy studies, we found that steady-state GFPu levels were not detectably different between R6/2 and non-R6/2 brain. We observed no correlation between inclusion body formation and GFPu accumulation, suggesting no direct relationship between protein aggregation and global UPS inhibition in R6/2 mice. These findings suggest that while certain branches of the UPS can be impaired by mutant polyglutamine proteins, such proteins do not necessarily cause total blockade of UPS-dependent degradation. It is therefore likely that the relationship between mutant polyglutamine proteins and the UPS is more complex than originally anticipated.

Allele-specific silencing of mutant Huntington's disease gene

Huntington's disease (HD) is an autosomal-dominant neurodegenerative disorder caused by a poly-glutamine expansion in huntingtin, the protein encoded by the HD gene. PolyQ-expanded huntingtin is toxic to neurons, especially the medium spiny neurons of the striatum. At the same time, wild-type huntingtin has important - indeed essential - protective functions. Any effective molecular therapy must preserve the expression of wild-type huntingtin, while silencing the mutant allele. We hypothesized that an appropriate siRNA molecule would display the requisite specificity and efficacy. As RNA interference is incapable of distinguishing among alleles with varying numbers of CAG (glutamine) codons, another strategy is needed. We used HD fibroblasts in which the pathogenic mutation is linked to a polymorphic site: the Delta2642 deletion of one of four tandem GAG triplets. We silenced expression of the harmful Delta2642-marked polyQ-expanded huntingtin without compromising synthesis of its wild-type counterpart. Following this success in HD fibroblasts, we obtained similar results with neuroblastoma cells expressing both wild-type and mutant HD genes. As opposed to the effect of depleting wild-type huntingtin, specifically silencing the mutant species actually lowered caspase-3 activation and protected HD cells under stress conditions. These findings have therapeutic implications not only for HD, but also for other autosomal dominant diseases. This approach has great promise: it may lead to personalized genetic therapy, a holy grail in contemporary medicine.

Dopamine induces supernumerary centrosomes and subsequent cell death through Cdk2 up-regulation in dopaminergic neuronal cells

Aggregation of proteins in the centrosome is implicated in the pathophysiology of Parkinson's disease. However, the relevance of the centrosome in neurodegeneration is still obscure. Centrosome duplication is initiated by the cyclin E/cyclin-dependent kinase 2 (Cdk2) complex. The present study determined changes in cyclin E or Cdk2 expression and in the centrosomal structure in dopaminergic neuronal CATH.a cells exposed to 50, 100 and 150 micromolar dopamine (DA) for 24 h. DA induced significant increase in Cdk2 protein and cyclin E protein, but not cyclin e mRNA. In DA-treated cells, the intense cyclin E- and Cdk2-immunofluorescence signals were co-localized around large and supernumerary centrosomes, and these two parameters of centrosome amplification were significantly increased compared with the control. Simultaneous co-treatment with DA and a Cdk2 inhibitor blocked centrosome amplification and enhanced cell viability. Our results demonstrated that DA could lead to cyclin E accumulation and Cdk2 up-regulation triggering supernumerary centrosomes and apoptotic cell death.

Live axonal transport disruption by mutant huntingtin fragments in Drosophila motor neuron axons

Huntington's Disease is a neurodegenerative condition caused by a polyglutamine expansion in the huntingtin (Htt) protein, which aggregates and also causes neuronal dysfunction. Pathogenic N-terminal htt fragments perturb axonal transport in vitro. To determine whether this occurs in vivo and to elucidate how transport is affected, we expressed htt exon 1 with either pathogenic (HttEx1Q93) or non-pathogenic (HttEx1Q20) polyglutamine tracts in Drosophila. We found that HttEx1Q93 expression causes axonal accumulation of GFP-tagged fast axonal transport vesicles in vivo and leads to aggregates within larval motor neuron axons. Time-lapse video microscopy, shows that vesicle velocity is unchanged in HttEx1Q93-axons compared to HttEx1Q20-axons, but vesicle stalling occurs to a greater extent. Whilst HttEx1Q93 expression did not affect locomotor behaviour, external heat stress unveiled a locomotion deficit in HttEx1Q93 larvae. Therefore vesicle transport abnormalities amidst axonal htt aggregation places a cumulative burden upon normal neuronal function under stressful conditions.

CAG repeat lengths > or =335 attenuate the phenotype in the R6/2 Huntington's disease transgenic mouse

With spontaneous elongation of the CAG repeat in the R6/2 transgene to > or =335, resulting in a transgene protein too large for passive entry into nuclei via the nuclear pore, we observed an abrupt increase in lifespan to >20 weeks, compared to the 12 weeks common in R6/2 mice with 150 repeats. In the > or =335 CAG mice, large ubiquitinated aggregates of mutant protein were common in neuronal dendrites and perikaryal cytoplasm, but intranuclear aggregates were small and infrequent. Message and protein for the > or =335 CAG transgene were reduced to one-third that in 150 CAG R6/2 mice. Neurological and neurochemical abnormalities were delayed in onset and less severe than in 150 CAG R6/2 mice. These findings suggest that polyQ length and pathogenicity in Huntington's disease may not be linearly related, and pathogenicity may be less severe with extreme repeats. Both diminished mutant protein and reduced nuclear entry may contribute to phenotype attenuation.

The Hdh(Q150/Q150) knock-in mouse model of HD and the R6/2 exon 1 model develop comparable and widespread molecular phenotypes

The identification of the Huntington's disease (HD) mutation as a CAG/polyglutamine repeat expansion enabled the generation of transgenic rodent models and gene-targeted mouse models of HD. Of these, mice that are transgenic for an N-terminal huntingtin fragment have been used most extensively because they develop phenotypes with relatively early ages of onset and rapid disease progression. Although the fragment models have led to novel insights into the pathophysiology of HD, it is important that models expressing a mutant version of the full-length protein are analysed in parallel. We have generated congenic C57BL/6 and CBA strains for the HdhQ150 knock-in mouse model of HD so that homozygotes can be analysed on an F1 hybrid background. Although a significant impairment in grip strength could be detected from a very early age, the performance of these mice in the quantitative behavioural tests most frequently used in preclinical efficacy trials indicates that they are unlikely to be useful for preclinical screening using a battery of conventional tests. However, at 22 months of age, the Hdh(Q150/Q150) homozygotes showed unexpected widespread aggregate deposition throughout the brain, transcriptional dysregulation in the striatum and cerebellum and decreased levels of specific chaperones, all well-characterised molecular phenotypes present in R6/2 mice aged 12 weeks. Therefore, when strain background and CAG repeat length are controlled for, the knock-in and fragment models develop comparable phenotypes. This supports the continued use of the more high-throughput fragment models to identify mechanisms of pathogenesis and for preclinical screening.

The operant serial implicit learning task reveals early onset motor learning deficits in the HdhQ92knock-in mouse model of Huntington's disease

A range of genetic mouse models of Huntington's disease have been created. However, as knock-in models typically have milder phenotypes, they have frequently been overlooked as therapeutic tools in favour of the transgenic models that display severe behavioural symptoms. More sensitive tests are therefore required to reveal abnormalities and release the potential of knock-in lines. An implicit learning task for mice has been developed in the nine-hole operant box test apparatus, in which mice must respond to a sequence of lights in order to earn a reward. A light stimulus was presented randomly in one of five holes, to which a nose poke response resulted in the light being extinguished and a second light illuminated in a different hole. Response to the second light resulted in a liquid reward. To probe implicit learning, a predictable stimulus sequence was embedded among many unpredictable sequences. In the current study, the Hdh(Q92) mouse model of Huntington's disease was examined. At 4 months of age, Hdh(Q92/Q92) mice demonstrated clear and significant deficits in both accuracy and reaction time on all trials of the implicit learning task, with improved performance on predictable trials. We believe this to be the earliest reported behavioural deficit in the Hdh(Q92) knock-in mouse line. The results of this study validate the serial implicit learning task as a sensitive tool for the examination of implicit and motor learning deficits in mutant mice, and may provide a powerful test for probing potential treatments for Huntington's disease.

Deletion of the triplet repeat encoding polyglutamine within the mouse Huntington's disease gene results in subtle behavioral/motor phenotypes in vivo and elevated levels of ATP with cellular senescence in vitro

Huntingtin (htt), the protein encoded by the Huntington's disease (HD) gene, contains a polymorphic stretch of glutamines (polyQ) near its N-terminus. When the polyQ stretch is expanded beyond 37Q, HD results. However, the role of the normal polyQ stretch in the function of htt is still unknown. To determine the contribution of the polyQ stretch to normal htt function, we have generated mice with a precise deletion of the short CAG triplet repeat encoding 7Q in the mouse HD gene (Hdh(DeltaQ)). Hdh(DeltaQ/DeltaQ) mice are born with normal Mendelian frequency and exhibit no gross phenotypic differences in comparison to control littermates, suggesting that the polyQ stretch is not essential for htt's functions during embryonic development. Adult mice, however, commit more errors initially in the Barnes circular maze learning and memory test and perform slightly better than wild-type controls in the accelerating rotarod test for motor coordination. To determine whether these phenotypes may reflect an altered cellular physiology in the Hdh(DeltaQ) mice, we characterized the growth and energy status of primary embryonic and adult Hdh(DeltaQ/DeltaQ) fibroblasts in culture. The Hdh(DeltaQ) fibroblasts exhibited elevated levels of ATP, but senesced prematurely in comparison with wild-type fibroblasts. Taken altogether, these results suggest that htt's polyQ stretch is required for modulating longevity in culture and support the hypothesis that the polyQ stretch may also modulate a htt function involved in regulating energy homeostasis.

Evaluation of the benzothiazole aggregation inhibitors riluzole and PGL-135 as therapeutics for Huntington's disease

Huntington's disease (HD) is an inherited progressive neurological disorder for which there is no effective therapy. It is caused by a CAG/polyglutamine repeat expansion that leads to abnormal protein aggregation and deposition in the brain. Several compounds have been shown to disrupt the aggregation process in vitro, including a number of benzothiazoles. To further explore the therapeutic potential of the benzothiazole aggregation inhibitors, we assessed PGL-135 and riluzole in hippocampal slice cultures derived from the R6/2 mouse, confirming their ability to inhibit aggregation with an EC50 of 40 microM in this system. Preliminary pharmacological work showed that PGL-135 was metabolically unstable, and therefore, we conducted a preclinical trial in the R6/2 mouse with riluzole. At the maximum tolerated dose, we achieved steady-state riluzole levels of 100 microM in brain. However, this was insufficient to inhibit aggregation in vivo and we found no improvement in the disease phenotype.

Huntingtin fragmentation and increased caspase 3, 8 and 9 activities in lymphoblasts with heterozygous and homozygous Huntington's disease mutation

Huntington's disease (HD) is caused by mutated huntingtin (htt), a toxic protein ubiquitously expressed in nervous and non-nervous system tissues. Fragmentation of htt by caspases and further accumulation in cells of protein aggregates contribute to cell dysfunction and death. In the attempt to elucidate whether this mechanism depends on patients' genotype, we analysed the pattern of htt fragmentation, the caspase 3, 8 and 9 activities and their variation in lymphoblasts with heterozygous and homozygous CAG mutation and in controls. Cells homozygous for expanded mutation showed greater amount of mutated fragments than heterozygotes and controls, caspase 3, 8 and 9 activities greater in mutated than control cell lines, after cyanide treatment, the caspase 3 and 8 particularly increased in homozygotes. This data offers a biological explanation to the clinical in-patients evidence of mutation homozygosity associated with more severe phenotype.

Repeat instability: mechanisms of dynamic mutations

Disease-causing repeat instability is an important and unique form of mutation that is linked to more than 40 neurological, neurodegenerative and neuromuscular disorders. DNA repeat expansion mutations are dynamic and ongoing within tissues and across generations. The patterns of inherited and tissue-specific instability are determined by both gene-specific cis-elements and trans-acting DNA metabolic proteins. Repeat instability probably involves the formation of unusual DNA structures during DNA replication, repair and recombination. Experimental advances towards explaining the mechanisms of repeat instability have broadened our understanding of this mutational process. They have revealed surprising ways in which metabolic pathways can drive or protect from repeat instability.

Contribution of nuclear and extranuclear polyQ to neurological phenotypes in mouse models of Huntington's disease

In postmortem Huntington's disease brains, mutant htt is present in both nuclear and cytoplasmic compartments. To dissect the impact of nuclear and extranuclear mutant htt on the initiation and progression of disease, we generated a series of transgenic mouse lines in which nuclear localization or nuclear export signal sequences have been placed N-terminal to the htt exon 1 protein carrying 144 glutamines. Our data indicate that the exon 1 mutant protein is present in the nucleus as part of an oligomeric or aggregation complex. Increasing the concentration of the mutant transprotein in the nucleus is sufficient for and dramatically accelerates the onset and progression of behavioral phenotypes. Furthermore, nuclear exon 1 mutant protein is sufficient to induce cytoplasmic neurodegeneration and transcriptional dysregulation. However, our data suggest that cytoplasmic mutant exon 1 htt, if present, contributes to disease progression.

Euploidy in somatic cells from R6/2 transgenic Huntington's disease mice

BACKGROUND

Huntington's disease (HD) is a hereditary neurodegenerative disorder caused by a CAG repeat expansion in the HD gene. The huntingtin protein expressed from HD has an unknown function but is suggested to interact with proteins involved in the cell division machinery. The R6/2 transgenic mouse is the most widely used model to study HD. In R6/2 fibroblast cultures, a reduced mitotic index and high frequencies of multiple centrosomes and aneuploid cells have recently been reported. Aneuploidy is normally a feature closely connected to neoplastic disease. To further explore this unexpected aspect of HD, we studied cultures derived from 6- and 12-week-old R6/2 fibroblasts, skeletal muscle cells, and liver cells.

RESULTS

Cytogenetic analyses revealed a high frequency of polyploid cells in cultures from both R6/2 and wild-type mice with the greatest proportions of polyploid cells in cultures derived from skeletal muscle cells of both genotypes. The presence of polyploid cells in skeletal muscle in vivo was confirmed by fluorescence in situ hybridisation with centromeric probes. Enlarged and supernumerary centrosomes were found in cultures from both R6/2 and wild-type mice. However, no aneuploid cells could be found in any of the tissues.

CONCLUSION

We conclude that polyploid cells are found in fibroblast and skeletal muscle cultures derived from both R6/2 and wild-type littermate mice and that aneuploidy is unlikely to be a hallmark of HD.

Disease-related prion protein forms aggresomes in neuronal cells leading to caspase activation and apoptosis

The molecular basis for neuronal death in prion disease is not established, but putative pathogenic roles for both disease-related prion protein (PrP(Sc)) and accumulated cytosolic PrP(C) have been proposed. Here we report that only prion-infected neuronal cells become apoptotic after mild inhibition of the proteasome, and this is strictly dependent upon sustained propagation of PrP(Sc). Whereas cells overexpressing PrP(C) developed cytosolic PrP(C) aggregates, this did not cause cell death. In contrast, only in prion-infected cells, mild proteasome impairment resulted in the formation of large cytosolic perinuclear aggresomes that contained PrP(Sc), heat shock chaperone 70, ubiquitin, proteasome subunits, and vimentin. Similar structures were found in the brains of prion-infected mice. PrP(Sc) aggresome formation was directly associated with activation of caspase 3 and 8, resulting in apoptosis. These data suggest that neuronal propagation of prions invokes a neurotoxic mechanism involving intracellular formation of PrP(Sc) aggresomes. This, in turn, triggers caspase-dependent apoptosis and further implicates proteasome dysfunction in the pathogenesis of prion diseases.

Rotenone induces aggregation of gamma-tubulin protein and subsequent disorganization of the centrosome: relevance to formation of inclusion bodies and neurodegeneration

Neurodegenerative disorders are characterized by progressive loss of specific neurons in the central nervous system. Although they have different etiologies and clinical manifestations, most of them share similar histopathologic characteristics such as the presence of inclusion bodies in both neurons and glial cells, which represent intracellular aggregation of misfolded or aberrant proteins. In Parkinson's disease, formation of inclusion bodies has been associated with the aggresome-related process and consequently with the centrosome. However, the significance of the centrosome in the neurodegenerative process remains obscure. In the present study, the morphological and functional changes in the centrosome induced by rotenone, a common insecticide used to produce experimental Parkinsonism, were examined both in vitro and in vivo. Aggregation of gamma-tubulin protein, which is a component of the centrosome matrix and recently identified in Lewy bodies of Parkinson's disease, was observed in primary cultures of mesencephalic cells treated with rotenone. Rotenone-treated neurons and astrocytes showed enlarged and multiple centrosomes. These centrosomes also displayed multiple aggregates of alpha-synuclein protein. Neurons with disorganized centrosomes exhibited neurite retraction and microtubule destabilization, and astrocytes showed disturbances of mitotic spindles. The Golgi apparatus, which is closely related to the centrosome, was dispersed in both rotenone-treated neuronal cells and the substantia nigra of rotenone-treated rats. Our findings suggested that recruitment of abnormal proteins in the centrosome contributed to the formation of inclusion bodies, and that rotenone markedly affected the structure and function of the centrosome with consequent induction of cytoskeleton disturbances, disassembly of the Golgi apparatus and collapse of neuronal cells.

The centrosome in human genetic disease

The centrosome is an indispensable component of the cell-cycle machinery of eukaryotic cells, and the perturbation of core centrosomal or centrosome-associated proteins is linked to cell-cycle misregulation and cancer. Recent work has expanded our understanding of the functional complexity and importance of this organelle. The centrosomal localization of proteins that are involved in human genetic disease, and the identification of novel centrosome-associated proteins, has shown that numerous, seemingly unrelated, cellular processes can be perturbed by centrosomal dysfunction. Here, we review the mechanistic relationship between human disease phenotypes and the function of the centrosome, and describe some of the newly-appreciated functions of this organelle in animal cells.

Mosaicism of the CAG repeat sequence in the Huntington disease gene in a pair of monozygotic twins

We report on a pair of monozygotic twins belonging to a family segregating Huntington disease (HD). In routine DNA analysis of blood cells, they displayed three alleles of the CAG repeat sequence in the HD gene. Two different cell lines, carrying the normal allele together with either an expanded allele with 47 CAGs or an intermediate allele with 37 CAGs, were detected in blood and buccal epithelium from both twins. To our knowledge, this is the first case described of HD gene CAG repeat length mosaicism in blood cells. Haplotype analysis established that the 37 CAG allele most likely arose by contraction of the maternal 47 CAG allele. The contraction must have taken place postzygotically, possibly at a very early stage of development, and probably before separation of the twins. One of the twins has presented symptoms of HD for 4 years; his skin fibroblasts and hair roots carried only the cell line with the 47 CAG repeat allele. The other twin, who is without symptoms at present, displayed mosaicism in skin fibroblasts and hair roots. If the proportion of the two cell lines in the brain of each twin resembles that of their hair roots (another tissue originating from the ectoderm), the mosaicism in the unaffected twin would mean that only a part of his brain cells carried the expanded allele, which could explain why he, in contrast to his brother, has no symptoms at this time.

Expanded polyglutamine peptides disrupt EGF receptor signaling and glutamate transporter expression in Drosophila

Huntington's disease (HD) is a late onset heritable neurodegenerative disorder caused by expansion of a polyglutamine (polyQ) sequence in the protein huntingtin (Htt). Transgenic models in mice have suggested that the motor and cognitive deficits associated to this disease are triggered by extended neuronal and possibly glial dysfunction, whereas neuronal death occurs late and selectively. Here, we provide in vivo evidence that expanded polyQ peptides antagonize epidermal growth factor receptor (EGFR) signaling in Drosophila glia. We targeted the expression of the polyQ-containing domain of Htt or an extended polyQ peptide alone in a subset of Drosophila glial cells, where the only fly glutamate transporter, dEAAT1, is detected. This resulted in formation of nuclear inclusions, progressive decrease in dEAAT1 transcription and shortened adult lifespan, but no significant glial cell death. We observed that brain expression of dEAAT1 is normally sustained by the EGFR-Ras-extracellular signal-regulated kinase (ERK) signaling pathway, suggesting that polyQ could act by antagonizing this pathway. We found that the presence of polyQ peptides indeed abolished dEAAT1 upregulation by constitutively active EGFR and potently inhibited EGFR-mediated ERK activation in fly glial cells. Long polyQ also limited the effect of activated EGFR on Drosophila eye development. Our results further indicate that the polyQ acts at an upstream step in the pathway, situated between EGFR and ERK activation. This suggests that disruption of EGFR signaling and ensuing glial cell dysfunction could play a direct role in the pathogenesis of HD and other polyQ diseases in humans.

The R6/2 transgenic mouse model of Huntington's disease develops diabetes due to deficient beta-cell mass and exocytosis

Diabetes frequently develops in Huntington's disease (HD) patients and in transgenic mouse models of HD such as the R6/2 mouse. The underlying mechanisms have not been clarified. Elucidating the pathogenesis of diabetes in HD would improve our understanding of the molecular mechanisms involved in HD neuropathology. With this aim, we examined our colony of R6/2 mice with respect to glucose homeostasis and islet function. At week 12, corresponding to end-stage HD, R6/2 mice were hyperglycemic and hypoinsulinemic and failed to release insulin in an intravenous glucose tolerance test. In vitro, basal and glucose-stimulated insulin secretion was markedly reduced. Islet nuclear huntingtin inclusions increased dramatically over time, predominantly in beta-cells. beta-cell mass failed to increase normally with age in R6/2 mice. Hence, at week 12, beta-cell mass and pancreatic insulin content in R6/2 mice were 35+/-5 and 16+/-3% of that in wild-type mice, respectively. The normally occurring replicating cells were largely absent in R6/2 islets, while no abnormal cell death could be detected. Single cell patch-clamp experiments revealed unaltered electrical activity in R6/2 beta-cells. However, exocytosis was virtually abolished in beta- but not in alpha-cells. The blunting of exocytosis could be attributed to a 96% reduction in the number of insulin-containing secretory vesicles. Thus, diabetes in R6/2 mice is caused by a combination of deficient beta-cell mass and disrupted exocytosis.

A potent small molecule inhibits polyglutamine aggregation in Huntington's disease neurons and suppresses neurodegeneration in vivo

Polyglutamine (polyQ) disorders, including Huntington's disease (HD), are caused by expansion of polyQ-encoding repeats within otherwise unrelated gene products. In polyQ diseases, the pathology and death of affected neurons are associated with the accumulation of mutant proteins in insoluble aggregates. Several studies implicate polyQ-dependent aggregation as a cause of neurodegeneration in HD, suggesting that inhibition of neuronal polyQ aggregation may be therapeutic in HD patients. We have used a yeast-based high-throughput screening assay to identify small-molecule inhibitors of polyQ aggregation. We validated the effects of four hit compounds in mammalian cell-based models of HD, optimized compound structures for potency, and then tested them in vitro in cultured brain slices from HD transgenic mice. These efforts identified a potent compound (IC50=10 nM) with long-term inhibitory effects on polyQ aggregation in HD neurons. Testing of this compound in a Drosophila HD model showed that it suppresses neurodegeneration in vivo, strongly suggesting an essential role for polyQ aggregation in HD pathology. The aggregation inhibitors identified in this screen represent four primary chemical scaffolds and are strong lead compounds for the development of therapeutics for human polyQ diseases.

The Centrosome in Higher Organisms: Structure, Composition, and Duplication

The centrosome found in higher organisms is an organelle with a complex and dynamic architecture and composition. This organelle not only functions as a microtubule-organizing center, but also is integrated with or impacts a number of cellular processes. Defects associated with this organelle have been linked to a variety of human diseases including several forms of cancer. Here we review the emerging picture of how the structure, composition, duplication, and function of the centrosome found in higher organisms are interrelated.

Inclusion formation in Huntington's disease R6/2 mouse muscle cultures

Huntington's disease (HD) is an autosomal dominant disorder caused by an expansion in the number of glutamine repeats in the N-terminal region of the huntingtin protein. Nuclear and cytoplasmic aggregates of the N-terminal portion of huntingtin have been found in the brains of HD patients and the brains and non-neuronal tissues of the R6/2 HD transgenic mouse. We have cultured myoblasts and myotubes from transgenic R6/2 mice and littermate controls to investigate the formation of these inclusions in post mitotic cells. Huntingtin immunoreactivity was intense in differentiating, desmin positive myoblasts and myotubes from both control and R6/2 mice suggesting that it may play a role in myotube differentiation. Following differentiation huntingtin and ubiquitin positive aggregates were observed in R6/2 but not control cultures. After 3 weeks in differentiation medium cytoplasmic huntingtin and ubiquitin immunoreactive aggregates were observed in non-myotube cells, while nuclear huntingtin aggregates were seen in a proportion of myotubes after 6 weeks. Growth in the absence of serum resulted in a marked increase in the number of R6/2 myotubes containing nuclear inclusions after 6 weeks demonstrating that environmental factors influenced huntingtin aggregate formation in these cells. Consequently, cultured myotubes from R6/2 mice may be a useful post mitotic cell culture model to study both the biochemical consequences of huntingtin aggregates and the factors that may influence aggregate formation.

Minocycline and doxycycline are not beneficial in a model of Huntington's disease

Huntington's Disease (HD) is an inherited neurological disorder causing movement impairment, personality changes, dementia, and premature death, for which there is currently no effective therapy. The modified tetracycline antibiotic, minocycline, has been reported to ameliorate the disease phenotype in the R6/2 mouse model of HD. Because the tetracyclines have also been reported to inhibit aggregation in other amyloid disorders, we have investigated their ability to inhibit huntingtin aggregation and further explored their efficacy in preclinical mouse trials. We show that tetracyclines are potent inhibitors of huntingtin aggregation in a hippocampal slice culture model of HD at an effective concentration of 30 microM. However, despite achieving tissue levels approaching this concentration by oral treatment of R6/2 mice with minocycline, we observed no clear difference in their behavioral abnormalities, or in aggregate load postmortem. In the light of these new data, we would advise that caution be exercised in proceeding into human clinical trials of minocycline.

Higher order structure of the PCM adjacent to the centriole

The centrosome is the major microtubule organizing center in most animal cells. This cytoplasmic organelle consists of two components : a mature centriole (or a pair of centrioles) and a mass of pericentriolar material (PCM). The PCM has been described as either a cloud of material that encases the entire centriole or as a cluster of proteins divided into two subsets, one that adheres to the lateral surface of the centriole and another that extends outward from this region as a cloud of material. In contrast to these protein distribution patterns, we demonstrated in a previous study that a subset of proteins present within the PCM is integrated together to form a tube (PCM tube) with an open and closed end that is duplicated in concert with centrosome duplication. The present study was undertaken to determine if this tubular conformation represents proteins that are confined to the surface of the centriole or if it represents a subset of proteins within the cloud of material that extends outward from the centriole. We document that : (1) the PCM tube represents a portion of the PCM directly associated with the centriole; (2) the PCM tube has a specific and reproducible relationship to the polar structure of the centriole; (3) the tube is a site of cytoplasmic microtubule organization, and has a structure that influences the initial pattern of microtubule assembly within the juxta-centriolar region; and (4) the PCM tube has a structural relationship with respect to the centriole, which allows the simultaneous expression of centriole- and PCM-based functions (e.g., ciliogenesis and cytoplasmic microtubule organization). Based on these findings, we propose a new model of the PCM at the centriole. This model highlights the role played by the proximal end of the centriole in the nucleation and organization of centriole-associated PCM, and indicates that the centrosome has an overall polarity in the region of the centriole.

Intracellular localization of proteasomes

Proteasomes are present in the cytoplasm and in the nuclei of all eukaryotic cells, however their relative abundance within those compartments is highly variable. In the cytoplasm, proteasomes associate with the centrosomes, cytoskeletal networks and the outer surface of the endoplasmic reticulum (ER). In the nucleus, proteasomes are present throughout the nucleoplasm but are void from the nucleoli. Sometimes they associate with discrete subnuclear domains called the PML nuclear bodies (POD domains). PML bodies in the nucleus, and the pericentrosomal area of the cytoplasm may function as proteolytic centers of the cell, since they are enriched in components of the proteasome system. Under conditions of impaired proteolysis proteasomes and ubiquitinated proteins further accumulate at these locations, forming organized aggregates. In case of the pericentrosomal area those aggregates have been termed "aggresomes". Once formed, aggresomes can impair the function of the proteasome system, which may promote apoptosis. Under favorable conditions they can be cleared, probably by autophagy.

Suberoylanilide hydroxamic acid, a histone deacetylase inhibitor, ameliorates motor deficits in a mouse model of Huntington's disease

Huntington's disease (HD) is an inherited, progressive neurological disorder that is caused by a CAG/polyglutamine repeat expansion and for which there is no effective therapy. Recent evidence indicates that transcriptional dysregulation may contribute to the molecular pathogenesis of this disease. Supporting this view, administration of histone deacetylase (HDAC) inhibitors has been shown to rescue lethality and photoreceptor neurodegeneration in a Drosophila model of polyglutamine disease. To further explore the therapeutic potential of HDAC inhibitors, we have conducted preclinical trials with suberoylanilide hydroxamic acid (SAHA), a potent HDAC inhibitor, in the R6/2 HD mouse model. We show that SAHA crosses the blood-brain barrier and increases histone acetylation in the brain. We found that SAHA could be administered orally in drinking water when complexed with cyclodextrins. SAHA dramatically improved the motor impairment in R6/2 mice, clearly validating the pursuit of this class of compounds as HD therapeutics.

Chromosomal abnormalities in embryos derived from testicular sperm extraction

OBJECTIVE

To compare the rate of chromosome abnormalities in embryos obtained from karyotypically normal patients with nonobstructive azoospermia undergoing testicular sperm extraction (TESE) to those from patients undergoing intracytoplasmic sperm injection (ICSI) with ejaculated sperm.

DESIGN

Retrospective analysis.

SETTING

IVF centers.

PATIENT(S)

Male partners had either nonobstructive zoospermia or oligospermia.

INTERVENTION(S)

Preimplantation genetic diagnosis. Chromosome enumeration was performed by fluorescence in situ hybridization (FISH). Embryos classified as abnormal were reanalyzed to study mosaicism.

MAIN OUTCOME MEASURE(S)

Chromosome abnormalities in embryos.

RESULT(S)

Embryos from ICSI cycles with ejaculated sperm (group 1) were 41.8% normal, 26.2% aneuploid, and 26.5% mosaic. In contrast, the embryos from ICSI cycles with TESE for nonobstructive azoospermia (group 2) were 22% normal, 17% aneuploid, and 53% mosaic. The difference in mosaicism rate between the two groups of embryos was highly significant.

CONCLUSION(S)

The present study results indicate a high incidence of mosaicism in embryos derived from TESE in men with a severe deficit in spermatogenesis. Sperm derived from TESE for nonobstructive azoospermia may have a higher rate of compromised or immature centrosome structures leading to mosaicism in the embryo.

Centrosome replication, genomic instability and cancer

Karyotypic alterations, including whole chromosome loss or gain, ploidy changes, and a variety of chromosome aberrations are common in cancer cells. If proliferating cells fail to coordinate centrosome duplication with DNA replication, this will inevitably lead to a change in ploidy, and the formation of monopolar or multipolar spindles will generally provoke abnormal segregation of chromosomes. Indeed, it has long been recognized that errors in the centrosome duplication cycle may be an important cause of aneuploidy and thus contribute to cancer formation. This view has recently received fresh impetus with the description of supernumerary centrosomes in almost all solid human tumors. As the primary microtubule organizing center of most eukaryotic cells, the centrosome assures symmetry and bipolarity of the cell division process, a function that is essential for accurate chromosome segregation. In addition, a growing body of evidence indicates that centrosomes might be important for initiating S phase and completing cytokinesis. Centrosomes undergo duplication precisely once before cell division. Recent reports have revealed that this process is linked to the cell division cycle via cyclin-dependent kinase (cdk) 2 activity that couples centriole duplication to the onset of DNA replication at the G(1)/S phase transition. Alterations in G(1)/S phase regulating proteins like the retinoblastoma protein, cyclins D and E, cdk4 and 6, cdk inhibitors p16(INK4A) and p15(INK4B), and p53 are among the most frequent aberrations observed in human malignancies. These alterations might not only lead to unrestrained proliferation, but also cause karyotypic instability by uncontrolled centrosome replication. Since several excellent reports on cell cycle regulation and cancer have been published, this review will focus on the role of centrosomes in cell cycle progression, as well as causes and consequences of aberrant centrosome replication in human neoplasias.