Huntington disease: new insights into the relationship between CAG expansion and disease

Profile of pridopidine and its potential in the treatment of Huntington disease: the evidence to date

Huntington disease (HD) is a chronic, genetic, neurodegenerative disease for which there is no cure. The main symptoms of HD are abnormal involuntary movements (chorea and dystonia), impaired voluntary movements (ie, incoordination and gait balance), progressive cognitive decline, and psychiatric disturbances. HD is caused by a CAG-repeat expanded mutation in the HTT gene, which encodes the huntingtin protein. The inherited mutation results in the production of an elongated polyQ mutant huntingtin protein (mHtt). The cellular functions of the Htt protein are not yet fully understood, but the functions of its mutant variant are thought to include alteration of gene transcription and energy production, and dysregulation of neurotransmitter metabolism, receptors, and growth factors. The phenylpiperidines pridopidine (4-[3-methanesulfonyl-phenyl]-1-propyl-piperidine; formerly known as ACR16) and OSU6162 ([S]-[-]-3-[3-methane [sulfonyl-phenyl]-1-propyl-piperidine) are members of a new class of pharmacologic agents known as “dopamine stabilizers”. Recent clinical trials have highlighted the potential of pridopidine for symptomatic treatment of patients with HD. More recently, the analysis of HD models (ie, in vitro and in mice) highlighted previously unknown effects of pridopidine (increase in brain-derived neurotrophic factor, reduction in mHtt levels, and σ-1 receptor binding and modulation). These additional functions of pridopidine suggest it might be a neuroprotective and disease-modifying drug. Data from ongoing clinical trials of pridopidine will help define its place in the treatment of HD. This commentary examines the available preclinical and clinical evidence regarding the use of pridopidine in HD.

Role of p53 in neurodegenerative diseases

BACKGROUND

p53 plays an important role in many areas of cellular physiology and biology, ranging from cellular development and differentiation to cell cycle arrest and apoptosis. Many of its functions are attributed to its role in assuring proper cellular division. However, since the establishment of its role in cell cycle arrest, damage repair, and apoptosis (thus also establishing its importance in cancer development), numerous reports have demonstrated additional functions of p53 in various cells. In particular, p53 appears to have important functions as it relates to neurodegeneration and synaptic plasticity.

OBJECTIVE

In this review, we will address p53 functions as it relates to various neurodegenerative diseases, mainly its implications in the development of HIV-associated neurocognitive disorders.

CONCLUSION

p53 plays a pivotal role in the development of neurodegenerative diseases through its interaction with cellular factors, viral factors, and/or small RNAs that have the ability to promote the development of these diseases. Hence, inhibition of p53 may present an ideal target to restore neuronal functions.

Huntington's disease and dentatorubral-pallidoluysian atrophy: proteins, pathogenesis and pathology

Each of the glutamine repeat neurodegenerative diseases has a particular pattern of pathology largely restricted to the CNS. However, there is considerable overlap among the regions affected, suggesting that the diseases share pathogenic mechanisms, presumably involving the glutamine repeats. We focus on Huntington's disease (HD) and Dentatorubral-pallidoluysian atrophy (DRPLA) as models for this family of diseases, since they have striking similarities and also notable differences in their clinical features and pathology. We review the pattern of pathology in adult and juvenile onset cases. Despite selective pathology, the disease genes and their protein products (huntingtin and atrophin-1) are widely expressed. This presents a central problem for all the glutamine repeat diseases-how do widely expressed gene products give rise to restricted pathology? The pathogenic effects are believed to occur via a "gain of function" mechanism at the protein level. Mechanisms of cell death may include excitotoxicity, metabolic toxicity, apoptosis, and free radical stress. Emerging data indicate that huntingtin and atrophin-1 may have distinct protein interactions. The specific interaction partners may help explain the selective pathology of these diseases.

The inositol 1,4,5-trisphosphate receptor (IP3R) and its regulators: sometimes good and sometimes bad teamwork

In both nonexcitable and excitable cells, the inositol 1,4,5-trisphosphate receptor (IP(3)R) is the primary cytosolic target responsible for the initiation of intracellular calcium (Ca(2+)) signaling. To fulfill this function, the IP(3)R depends on interaction with accessory subunits and regulatory proteins. These include proteins that reside in the lumen of the endoplasmic reticulum (ER), such as chromogranin A and B and ERp44, and cytosolic proteins, such as neuronal Ca(2+) sensor 1, huntingtin, cytochrome c, IP(3)R-binding protein released with inositol 1,4,5-trisphosphate, Homer, and 4.1N. Specific interactions between these modulatory proteins and the IP(3)R have been described, making it clear that the controlled modulation of the IP(3)R by its binding partners is necessary for physiological cell regulation. The functional coupling of these modulators with the IP(3)R can control apoptosis, intracellular pH, the initiation and regulation of neuronal Ca(2+) signaling, exocytosis, and gene expression. The pathophysiological relevance of IP(3)R modulation is apparent when the functional interaction of these proteins is enhanced or abolished by mutation or overexpression. The subsequent deregulation of the IP(3)R leads to pathological changes in Ca(2+) signaling, signal initiation, the amplitude and frequency of Ca(2+) signals, and the duration of the Ca(2+) elevation. Consequences of this deregulation include abnormal growth and apoptosis. Complex regulation of Ca(2+) signaling is required for the cell to live and function, and this difficult task can only be managed when the IP(3)R teams up and acts properly with its numerous binding partners.

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.

Prevention of cytosolic IAPs degradation: a potential pharmacological target in Huntington's Disease

Huntington's Disease (HD) is a neurodegenerative disorder caused by an abnormally expanded polyglutamine trait in the amino-terminal region of huntingtin. Pathogenic mechanisms involve a gained toxicity of mutant huntingtin and a potentially reduced neuroprotective function of the wild-type allele. Among the molecular abnormalities reported, HD cells are characterized by the presence of aggregates, transcriptional dysregulation, altered mitochondrial membrane potential and aberrant Ca++ handling. In addition, upon exposure to toxic stimuli, increased mitochondrial release of cytochrome C and activation of caspase-9 and caspase-3 are found in HD cells and tissue. Here we report that HTRA2 and Smac/DIABLO, two additional mitochondrial pro-apoptotic factors, are aberrantly released from brain-derived cells expressing mutant huntingtin. This event causes a reduction in levels of the cytosolic IAP1 (Inhibitor of Apoptosis Protein-1) and XIAP (X-linked inhibitor apoptosis) antiapoptotic IAP family members. Reduced IAP levels are also found in post-mortem HD brain tissue. Treatment with ucf101, a serine protease HTRA2 specific inhibitor, counteracts IAPs degradation in HD cells and increases their survival. These results point to the IAPs as potential pharmacological targets in Huntington's Disease.

HAP1 facilitates effects of mutant huntingtin on inositol 1,4,5-trisphosphate-induced Ca2+ release in primary culture of striatal medium spiny neurons

Huntington's disease is caused by polyglutamine expansion (exp) in huntingtin (Htt). Htt-associated protein-1 (HAP1) was the first identified Htt-binding partner. The type 1 inositol (1,4,5)-trisphosphate receptor (InsP3R1) is an intracellular Ca2+ release channel that plays an important role in neuronal function. Recently, we identified a InsP3R1-HAP1A-Htt ternary complex in the brain and demonstrated that Httexp, but not normal Htt, activates InsP3R1 in bilayers and facilitates InsP3R1-mediated intracellular Ca2+ release in medium spiny striatal neurons [MSN; T.-S. Tang et al. (2003) Neuron, 39, 227-239]. Here we took advantage of mice with targeted disruption of both HAP1 alleles (HAP1 -/-) to investigate the role of HAP1 in functional interactions between Htt and InsP3R1. We determined that: (i) HAP1 is expressed in the MSN; (ii) HAP1A facilitates functional effects of Htt and Htt(exp) on InsP3R1 in planar lipid bilayers; (iii) HAP1 is required for changes in MSN basal Ca2+ levels resulting from Htt or Htt(exp) overexpression; (iv) HAP1 facilitates potentiation of InsP3R1-mediated Ca2+ release by Htt(exp) in mouse MSN. Our present results indicate that HAP1 plays an important role in functional interactions between Htt and InsP3R1.

Huntingtin and huntingtin-associated protein 1 influence neuronal calcium signaling mediated by inositol-(1,4,5) triphosphate receptor type 1

Huntington's disease (HD) is caused by polyglutamine expansion (exp) in huntingtin (Htt). The type 1 inositol (1,4,5)-triphosphate receptor (InsP3R1) is an intracellular calcium (Ca2+) release channel that plays an important role in neuronal function. In a yeast two-hybrid screen with the InsP3R1 carboxy terminus, we isolated Htt-associated protein-1A (HAP1A). We show that an InsP3R1-HAP1A-Htt ternary complex is formed in vitro and in vivo. In planar lipid bilayer reconstitution experiments, InsP3R1 activation by InsP3 is sensitized by Httexp, but not by normal Htt. Transfection of full-length Httexp or caspase-resistant Httexp, but not normal Htt, into medium spiny striatal neurons faciliates Ca2+ release in response to threshold concentrations of the selective mGluR1/5 agonist 3,5-DHPG. Our findings identify a novel molecular link between Htt and InsP3R1-mediated neuronal Ca2+ signaling and provide an explanation for the derangement of cytosolic Ca2+ signaling in HD patients and mouse models.

Neurodegenerative disorders of protein aggregation

In recent years, it has become increasingly clear that many neurodegenerative diseases involve aggregation and deposition of misfolded proteins such as amyloid beta, tau, alpha-synuclein and polyglutamine containing proteins. This abnormal deposition of misfolded proteins produce malfunctioning of a distinctive set of neurons. It may also induce oxidative and endoplasmic reticulum stress and proteosomal and mitochondrial dysfunction that ultimately leads to neuronal death. While hereditary forms of disorders are caused by genetic mutations, many sporadic cases are likely to be due to genetic and environmental factors. These disorders are progressive in nature. Therefore, treatment is difficult. However, for some diseases, a growing number of treatment options such as drugs, antioxidants, cell transplantation, surgery, rehabilitation procedures and preimplantation diagnosis is available. It should be noted that many of these treatments produce unacceptable risks or adverse effects and they are of only minimal benefit for patients. In future, an understanding of the causes of protein aggregation and genetic and environmental susceptibility factors of a specific individual (or specific individual determinants) may provide a better opportunity for an effective therapeutic intervention.

An upstream open reading frame impedes translation of the huntingtin gene

Expansion of a CAG tract within the huntingtin gene, leading to the production of a protein with an expanded polyglutamine tract, is responsible for Huntington's disease. We show here that the 5' untranslated region (UTR) of the huntingtin gene plays an important role in controlling the synthesis of huntingtin. In particular, the 5' UTR contains an upstream open reading frame (uORF) encoding a 21 amino acid peptide. We demonstrate that the presence of this uORF negatively influences expression from the huntingtin mRNA. Our results suggest a role for the uORF in limiting ribosomal access to downstream initiation sites. Mechanisms involving the post-transcriptional regulation of huntingtin are not well understood, and this may be an important way of regulating huntingtin protein levels.

Alteration of intracellular structure and function of glyceraldehyde-3-phosphate dehydrogenase: a common phenotype of neurodegenerative disorders?

Recent evidence reveals that glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is not simply a classical glycolytic protein of little interest. Instead, it is a multifunctional protein with diverse cytoplasmic, membrane and nuclear activities. Significantly, each activity is separate and distinctfrom its role in energy production. Its nuclear activities include its emerging role in apoptosis especially in neuronal cells. GAPDH translocates into the nucleus during programmed cell death. Introduction of antisense GAPDH sequences reduces apoptosis and prevents its nuclear translocation. Independent analyses demonstrate that GAPDH may be involved in the cellular phenotype of age-related neurodegenerative disorders. GAPDH binds uniquely in vitro to the beta-amyloid precursor protein (betaAPP), to huntingtin as well as to other triplet repeat neuronal disorder proteins. In Parkinson's disease (PD) cells, immunofluorescent data suggests the co-l localization of GAPDH and alpha-synuclein in Lewy bodies. Drugs used to treat PD bind specifically to GAPDH. Our recent findings (Mazzola and Sirover, 2001) demonstrate a subcellular reduction in GAPDH glycolytic activity in Alzheimer's disease (AD) and in Huntington's disease (HD) cells. The latter may be due to intracellular alteration of GAPDH structure (Mazzola and Sirover 2002). We discuss the hypothesis that the intracellularformation of GAPDH: neuronal protein complexes may represent an emerging cellular phenotype of neurodegenerative disorders. The cytoplasmic binding of neuronal proteins to GAPDH could affect energy production. Nuclear interactions could affect its apoptotic activity. Other functions of this multidimensional protein may also be inhibited. Experimental paradigms to test this hypothesis are considered.

Fanconi anemia complementation group A cells are hypersensitive to chromium(VI)-induced toxicity

Fanconi anemia (FA) is an autosomal recessive disorder characterized by diverse developmental abnormalities, progressive bone marrow failure, and a markedly increased incidence of malignancy. FA cells are hypersensitive to DNA cross-linking agents, suggesting a general defect in the repair of DNA cross-links. Some forms of hexavalent chromium [Cr(VI)] are implicated as respiratory carcinogens and induce several types of DNA lesions, including ternary DNA-Cr-DNA interstrand cross-links (Cr-DDC). We hypothesized that human FA complementation group A (FA-A) cells would be hypersensitive to Cr(VI) and Cr(VI)-induced apoptosis. Using phosphatidylserine translocation and caspase-3 activation, human FA-A fibroblasts were found to be markedly hypersensitive to chromium-induced apoptosis compared with CRL-1634 cells, which are normal human foreskin fibroblasts (CRL). The clonogenicity of FA-A cells was also significantly decreased compared with CRL cells after Cr(VI) treatment. There was no significant difference in either Cr(VI) uptake or Cr-DNA adduct formation between FA-A and CRL cells. These results show that FA-A cells are hypersensitive to Cr(VI) and Cr-induced apoptosis and that this hypersensitivity is not due to increased Cr(VI) uptake or increased Cr-DNA adduct formation. The results also suggest that Cr-DDC may be proapoptotic lesions. These results are the first to show that FA cells are hypersensitive to an environmentally relevant DNA cross-linking agent.

Fragile X and other trinucleotide repeat diseases

Hereditary unstable DNA is composed of strings of trinucleotide repeats, in which three nucleotides are repeated over and over (ie CAGCAGCAGCAG). These repeats are found in several sites within genes; depending on their location, the number of triplet repeats in a string can change as it is passed on to offspring. When the number of repeats increases to a critical size, it can have a variety of affects on gene function. The repeats may cause a loss in gene function (as in Fragile X) or may result in the gain of a new, abnormal protein and thus a new function (as in myotonic dystrophy and Huntington disease). Although a variety of trinucleotide repeat diseases have been reported and merit consideration, this discussion will focus primarily on Fragile X syndrome, myotonic dystrophy, and Huntington disease.

Psychiatric symptoms in neurologically asymptomatic Huntington's disease gene carriers: a comparison with gene negative at risk subjects

OBJECTIVE

Psychiatric profiles of two at-risk groups [Huntington's disease (HD) gene carriers and non-carriers] were compared by means of a computerized battery and a structured interview.

METHOD

To avoid confounding, only subjects who were free from neurological and cognitive deficits (neurologically asymptomatic) were included in the study. To avoid evaluation biases, all subjects were seen before the genetic testing was undertaken.

RESULTS

Gene carriers had significantly worse recognition memory and scored higher in measures of irritability than controls. The groups also differed in terms of the factor structure of their psychiatric symptoms. None of the subjects qualified for a psychiatric diagnosis at the time of assessment.

CONCLUSION

The groups differed with respect to their profile of psychiatric symptoms. It is hypothesized that these differences are the expression of different mechanisms, i.e. that cognitive deficits relate more to genetic factors and neurotic complaints more to being brought up in a disturbed family background. Issues concerning instrument sensitivity, selection bias and the advantage of seriatim assessments are discussed.

Expression of full-length polyglutamine-expanded Huntingtin disrupts growth factor receptor signaling in rat pheochromocytoma (PC12) cells

We reported previously that normal Huntingtin is associated with epidermal growth factor receptor (EGF) signaling complex (Liu, Y. F., Deth, C. R., and Devys, D. (1997) J. Biol. Chem. 272, 8121-8124). To investigate the potential role of normal and polyglutamine-expanded Huntingtin in the regulation of growth factor receptor-mediated cellular signaling and biological function, we stably transfected full-length Huntingtin containing 16, 48, or 89 polyglutamine repeats into PC12 cells where cellular signaling mechanisms, mediated by nerve growth factor (NGF) or EGF receptors, are well characterized. Expression of polyglutamine-expanded Huntingtin, but not normal Huntingtin, leads to a dramatic morphological change. In clones carrying the mutated Huntingtin, both NGF and EGF receptor-mediated activation of mitogen-activated protein kinase, c-Jun N-terminal kinase, and Akt are significantly attenuated, and NGF receptor-mediated neurite outgrowth is blocked. Co-immunoprecipitation studies show that the associations of NGF or EGF receptors with growth factor receptor-binding protein 2 (Grb2) and phosphoinositide 3-kinase are significantly inhibited. NGF-induced tyrosine phosphorylation of NGF receptors (TrkA) is also consistently suppressed. Our data demonstrate that polyglutamine-expanded Huntingtin disrupts cellular signaling mediated by both EGF and NGF receptors in PC12 cells. It is known that Huntington's disease patients exhibit an extremely low incidence of a variety of cancers and are deficient in glucose metabolism. Thus, our results may reflect an important molecular mechanism for the pathogenesis of the disease.

Psychiatric symptoms and CAG repeats in neurologically asymptomatic Huntington's disease gene carriers

The putative relationship between the psychiatric profile of a sample of neurologically asymptomatic Huntington's disease gene carriers and CAG repeats was investigated. The psychiatric assessments (by consultant psychiatrist and computerised battery) were undertaken before the genetic testing was carried out. In this way, the informational distortions caused by neurological and cognitive deficits were avoided. The hypothesis that there is a relationship between psychiatric and CAG repeats was tested by seeking direct correlations between psychiatric systems and CAG repeats, and also by correcting the correlation by the number of years above or below the estimated age of onset in Huntington's disease. Scores for irritability and cognitive failures were high in the sample. There was no correlation between any psychiatric variable and CAG repeats. Possible explanations for this lack of correlations are discussed.

Identification of the full-length huntingtin- interacting protein p231HBP/HYPB as a DNA-binding factor

Neurodegeneration in Huntington's disease (HD) is associated with an elongated glutamine tract in the widely expressed huntingtin protein. Although the pathogenic mechanisms are still unknown, the distinct physical properties of mutant huntingtin in the brain suggest that other factors including huntingtin-interacting proteins might play a specific role. We have previously identified a DNA-binding motif in the proximal E1A promoter of adenovirus serotype 12 as responsible for E1A autoregulation. Here, we identified the p231HBP protein as a DNA-binding factor, the C-terminal portion of which has recently been characterized as the huntingtin-interacting protein HYPB of unknown function. We have determined the full-length cDNA sequence, identified several domains supporting its gene regulatory functions, and mapped the HBP231 gene to chromosome 3p21.2-p21.3. Our results provide an interesting molecular link between huntingtin and a DNA-binding factor, implicating that this interaction might result in the alteration of cellular gene expression involved in HD pathogenesis.

Ionizing radiation and genetic risks. XII. The concept of "potential recoverability correction factor" (PRCF) and its use for predicting the risk of radiation-inducible genetic disease in human live births

Genetic risks of radiation exposure of humans are generally expressed as expected increases in the frequencies of genetic diseases over those that occur naturally in the population as a result of spontaneous mutations. Since human data on radiation-induced germ cell mutations and genetic diseases remain scanty, the rates derived from the induced frequencies of mutations in mouse genes are used for this purpose. Such an extrapolation from mouse data to the risk of genetic diseases will be valid only if the average rates of inducible mutations in human genes of interest and the average rates of induced mutations in mice are similar. Advances in knowledge of human genetic diseases and in molecular studies of radiation-induced mutations in experimental systems now question the validity of the above extrapolation. In fact, they (i) support the view that only in a limited number of genes in the human genome, induced mutations may be compatible with viability and hence recoverable in live births and (ii) suggest that the average rate of induced mutations in human genes of interest from the disease point of view will be lower than that assumed from mouse results. Since, at present, there is no alternative to the use of mouse data on induced mutation rates, there is a need to bridge the gap between these and the risk of potentially inducible genetic diseases in human live births. In this paper, we advance the concept of what we refer to here as "the potential recoverability correction factor" (PRCF) to bridge the above gap in risk estimation and present a method to estimate PRCF. In developing the concept of PRCF, we first used the available information on radiation-induced mutations recovered in experimental studies to define some criteria for assessing potential recoverability of induced mutations and then applied these to human genes on a gene-by-gene basis. The analysis permitted us to estimate unweighted PRCFs (i.e. the fraction of genes among the total studied that might contribute to recoverable induced mutations) and weighted PRCFs (i.e. PRCFs weighted by the incidences of the respective diseases). The estimates are: 0.15 (weighted) to 0.30 (unweighted) for autosomal dominant and X-linked diseases and 0.02 (weighted) to 0.09 (unweighted) for chronic multifactorial diseases. The PRCF calculations are unnecessary for autosomal recessive diseases since the risks projected for the first few generations even without using PRCFs are already very small. For congenital abnormalities, PRCFs cannot be reliably estimated. With the incorporation of PRCF into the equation used for predicting risk, the risk per unit dose becomes the product of four quantities (risk per unit dose=Px(1/DD)xMCxPRCF) where P is the baseline frequency of the genetic disease, 1/DD is the relative mutation risk per unit dose, MC is the mutation component and PRCF is the disease-class-specific potential recoverability correction factor instead of the first three (as has been the case thus far). Since PRCF is a fraction, it is obvious that the estimate of risk obtained with the revised risk equation will be smaller than previously calculated values.

Human huntingtin-associated protein (HAP-1) gene: genomic organisation and an intragenic polymorphism

The huntingtin-associated protein (HAP-1) interacts with the Huntington disease gene product, huntingtin. It is predominantly expressed in the brain and shows an increased affinity for mutant huntingtin. We have sequenced an 18,656bp genomic region encompassing the entire human HAP-1 gene and determined its genomic organisation, with 11 exons spanning 12.1kb. We have also found an intragenic polymorphism within intron 6 of HAP-1. We have recently shown that HAP-1 maps to a region of the genome which has been implicated in a variety of neurological conditions, including progressive supranuclear palsy (PSP), a late-onset atypical parkinsonian disorder. The detailed characterisation of the genomic organisation of HAP-1 and the presence of an intragenic polymorphism will be helpful in evaluating its role in different disorders, using candidate gene approaches.

The neuronal microtubule-associated protein tau is a substrate for caspase-3 and an effector of apoptosis

We have identified a class of tau fragments inducing apoptosis in different cellular contexts, including a human teratocarcinoma-derived cell line (NT2 cells) representing committed human neuronal precursors. We have found a transition point inside the tau molecule beyond which the fragments lose their ability to induce apoptosis. This transition point is located around one of the putative caspase-3 cleavage sites. This is the only site that can be effectively used by caspase-3 in vitro, releasing the C-terminal 19 amino acids of tau. These results establish tau as a substrate for an apoptotic protease that turns tau itself into an effector of apoptosis. Accordingly, tau may be involved in a self-propagating process like what has been predicted for the pathogenesis of different neurodegenerative disorders.

Inhibiting caspase cleavage of huntingtin reduces toxicity and aggregate formation in neuronal and nonneuronal cells

Huntington's disease is a neurodegenerative disorder caused by CAG expansion that results in expansion of a polyglutamine tract at the extreme N terminus of huntingtin (htt). htt with polyglutamine expansion is proapoptotic in different cell types. Here, we show that caspase inhibitors diminish the toxicity of htt. Additionally, we define htt itself as an important caspase substrate by generating a site-directed htt mutant that is resistant to caspase-3 cleavage at positions 513 and 530 and to caspase-6 cleavage at position 586. In contrast to cleavable htt, caspase-resistant htt with an expanded polyglutamine tract has reduced toxicity in apoptotically stressed neuronal and nonneuronal cells and forms aggregates at a much reduced frequency. These results suggest that inhibiting caspase cleavage of htt may therefore be of potential therapeutic benefit in Huntington's disease.

Nuclear proteins and cell death in inherited neuromuscular disease

X-linked Emery-Dreifuss muscular dystrophy is caused by mutations in emerin, a novel nuclear membrane protein. Other major inherited neuromuscular diseases have now also been shown to involve proteins which localize and function at least partly in the cell nucleus. These include lamin A/C in autosomal dominant Emery-Dreifuss muscular dystrophy, SMN in spinal muscular atrophy, SIX5 in myotonic dystrophy, calpain3 in type 2A limb-girdle muscular dystrophy, PABP2 in oculopharyngeal dystrophy, androgen receptor in spinal and bulbar muscular atrophy and the ataxins in hereditary ataxias. This review compares the molecular basis for these various disorders and considers the role of cell death, including apoptosis, in their pathogenesis.

Identification of caspases that cleave presenilin-1 and presenilin-2. Five presenilin-1 (PS1) mutations do not alter the sensitivity of PS1 to caspases

Mutations in the presenilin (PS) genes PSI and PS2 are involved in Alzheimer's disease (AD). Recently, apoptosis-associated cleavage of PS proteins was identified. Here we demonstrate that PS1 as well as PS2 are substrates for different members of the caspase protein family. Remarkably, the caspases acting on PS1 could be subdivided in two groups. One group, containing caspase-8, -6 and -11, cleaved PSI after residues ENDD329 and to a lesser extent after residues AQRD341. A second group consisting of caspase-3, -7 and -1 acted uniquely on AQRD341. Importantly, these two cleavage sites were also recognized by caspases in the C-terminal PS1 fragment produced by constitutive proteolysis. In decreasing order of activity, caspase-8, -3, -1, -6 and -7 proteolysed PS2 at the recognition site D326SYD329. Caspase-8 and -3 exhibited the highest proteolytic activity on both PS1 and PS2. PS1 and PS2 were not hydrolyzed by caspase-2 and PS2 also not by caspase-11. None of five missense mutations affected the sensitivity of PSI to caspase-mediated cleavage. This suggests that AD pathogenesis associated with PS1 missense mutations cannot be explained by a change in caspase-dependent processing.

Using knockout and transgenic mice to study neurophysiology and behavior

Reverse genetics, in which detailed knowledge of a gene of interest permits in vivo modification of its expression or function, provides a powerful method for examining the physiological relevance of any protein. Transgenic and knockout mouse models are particularly useful for studies of complex neurobiological problems. The primary aims of this review are to familiarize the nonspecialist with the techniques and limitations of mouse mutagenesis, to describe new technologies that may overcome these limitations, and to illustrate, using representative examples from the literature, some of the ways in which genetically altered mice have been used to analyze central nervous system function. The goal is to provide the information necessary to evaluate critically studies in which mutant mice have been used to study neurobiological problems.

Ectopically expressed CAG repeats cause intranuclear inclusions and a progressive late onset neurological phenotype in the mouse

The mutations responsible for several human neurodegenerative disorders are expansions of translated CAG repeats beyond a normal size range. To address the role of repeat context, we have introduced a 146-unit CAG repeat into the mouse hypoxanthine phosphoribosyltransferase gene (Hprt). Mutant mice express a form of the HPRT protein that contains a long polyglutamine repeat. These mice develop a phenotype similar to the human translated CAG repeat disorders. Repeat containing mice show a late onset neurological phenotype that progresses to premature death. Neuronal intranuclear inclusions are present in affected mice. Our results show that CAG repeats do not need to be located within one of the classic repeat disorder genes to have a neurotoxic effect.

Transgenic animals as models of neurodegenerative diseases in humans

Neurodegenerative diseases are of major socioeconomic importance and represent an enormous challenge for the scientific and medical communities. Advances in molecular genetics during the past decade have begun to provide approaches for the establishment of animal models for these disorders using transgenic technology. Their analysis will lead to better understanding of disease pathogenesis and will be invaluable for the identification of novel diagnostic and therapeutic agents. With the current pace of genomic research, the generation of transgenic animal models, reproducing in full the pathology and symptoms of even complex disorders such as Alzheimer's disease, must now be considered achievable.

The neuropathology of CAG repeat diseases: review and update of genetic and molecular features

Classification of inherited neurodegenerative diseases is increasingly based on their genetic features, which supplement, clarify, and sometimes replace the older clinical and pathologic schemata. This change has been particularly rapid and impressive for the CAG repeat disorders. In Huntington's disease, X-linked spinobulbar muscular atrophy, dentatorubropallidoluysian atrophy, and a series of autosomal dominant cerebellar atrophies, genetic advances have resolved many nosologic issues, and opened new avenues for exploration of pathogenesis. In this review, we summarize classic and current concepts in neuropathology of these CAG repeat diseases.

Toward understanding the molecular pathology of Huntington's disease

Huntington's Disease (HD) is caused by expansion of a CAG trinucleotide beyond 35 repeats within the coding region of a novel gene. Recently, new insights into the relationship between CAG expansion in the HD gene and pathological mechanisms have emerged. Survival analysis of a large cohort of affected and at-risk individuals with CAG sizes between 39 and 50 repeats have yielded probability curves of developing HD symptoms and dying of HD by a certain age. Animals transgenic for the first exon of huntingtin with large CAG repeats lengths have been reported to have a complex neurological phenotype that bears interesting similarities and differences to HD. The repertoire of huntingtin-interacting proteins continues to expand with the identification of HIP1, a protein whose yeast homologues have known functions in regulating events associated with the cytoskeleton. The ability of huntingtin to interact with two of its four known protein partners appears to be influenced by CAG length. Caspase 3 (apopain), a key cysteine protease known to play a seminal role in neural apoptosis, has also been demonstrated to specifically cleave huntingtin in a CAG length-dependent manner. Many of these features are combined in a model suggesting mechanisms by which the pathogenesis of HD may be initiated. The development of appropriate in vitro and animal models for HD will allow the validity of these models to be tested.

HIP1, a human homologue of S. cerevisiae Sla2p, interacts with membrane-associated huntingtin in the brain

Huntington disease (HD) is associated with the expansion of a polyglutamine tract, greater than 35 repeats, in the HD gene product, huntingtin. Here we describe a novel huntingtin interacting protein, HIP1, which co-localizes with huntingtin and shares sequence homology and biochemical characteristics with Sla2p, a protein essential for function of the cytoskeleton in Saccharomyces cerevisiae. The huntingtin-HIP1 interaction is restricted to the brain and is inversely correlated to the polyglutamine length in huntingtin. This provides the first molecular link between huntingtin and the neuronal cytoskeleton and suggests that, in HD, loss of normal huntingtin-HIP1 interaction may contribute to a defect in membrane-cytoskeletal integrity in the brain.

Analysis of the 5' upstream sequence of the Huntington's disease (HD) gene shows six new rare alleles which are unrelated to the age at onset of HD

The CAG repeat number in the Huntington's disease (HD) gene accounts for about 50% of the variation seen in age at onset of HD. In order to determine whether promoter sequence variation can contribute to the residual variation in age at onset, we studied the conserved 303 bp region upstream of the +1 translation start site in the HD gene in a population of 56 control East Anglians, 30 Africans, 34 Japanese, and 208 English Huntington's disease patients. A surprisingly high degree of variation was found. Seven alleles were identified, comprising four polymorphisms: two single base pair substitutions, a 6 bp VNTR present as one or two copies, and a 20 bp VNTR with one to three copies of the tandem repeat. No correlation between polymorphisms and age at onset of symptoms was found in HD patients. The 6 bp and 20 bp stretches are present only in single copies in the chimpanzees and gorilla, suggesting that these VNTRs have evolved by duplication of the core sequences in the human lineage.

Glutamine repeats and inherited neurodegenerative diseases: molecular aspects

Several dominantly inherited, late onset, neurodegenerative diseases are due to expansion of CAG repeats, leading to expansion of glutamine repeats in the affected proteins. These proteins are of very different sizes and, with one exception, show no sequence homology to known proteins or to each other; their functions are unknown. In some, the glutamine repeat starts near the N-terminus, in another near the middle and in another near the C-terminus, but regardless of these differences, no disease has been observed in individuals with fewer than 37 repeats, and absence of disease has never been found in those with more than 41 repeats. Protein constructs with more than 41 repeats are toxic to E. coli and to CHO cells in culture, and they elicit ataxia in transgenic mice. These observations argue in favour of a distinct change of structure associated with elongation beyond 37-41 glutamine repeats. The review describes experiments designed to find out what these structures might be and how they could influence the properties of the proteins of which they form part. Poly-L-glutamines form pleated sheets of beta-strands held together by hydrogen bonds between their amides. Incorporation of glutamine repeats into a small protein of known structure made it associate irreversibly into oligomers. That association took place during the folding of the protein molecules and led to their becoming firmly interlocked by either strand- or domain-swapping. Thermodynamic considerations suggest that elongation of glutamine repeats beyond a certain length may lead to a phase change from random coils to hydrogen-bonded hairpins. Possible mechanisms of expansion of CAG repeats are discussed in the light of looped DNA model structures.