Identification and localization of huntingtin in brain and human lymphoblastoid cell lines with anti-fusion protein antibodies

Expression of dentatorubral-pallidoluysian atrophy (DRPLA) proteins in patients

The genetic defect dentatorubral-pallidoluysian atrophy (DRPLA) is caused by expansion of a CAG trinucleotide repeat. The mutant gene is translated into protein whose electrophoretic mobility correlates to the number of expanded CAG trinucleotide repeats, indicating that the protein carries an expanded glutamine repeat. Using two polyclonal antibodies raised against the DRPLA gene product in immunoblotting, we determined the untruncated DRPLA proteins, and showed that the amounts of mutant and wild-type DRPLA proteins were similar in DRPLA brain tissues and lymphoblastoid cells, suggesting that regulation of the level of translation of the DRPLA gene is not central to the development of the disease.

HIP-I: a huntingtin interacting protein isolated by the yeast two-hybrid system

We report the discovery of the huntingtin interacting protein I (HIP-I) which binds specifically to the N-terminus of human huntingtin, both in the two-hybrid screen and in in vitro binding experiments. For the interaction in vivo, a protein region downstream of the polyglutamine stretch in huntingtin is essential. The HIP1 cDNA isolated by the two-hybrid screen encodes a 55 kDa fragment of a novel protein. Using an affinity-purified polyclonal antibody raised against recombinant HIP-I, a protein of 116 kDa was detected in brain extracts by Western blot analysis. The predicted amino acid sequence of the HIP-I fragment exhibits significant similarity to cytoskeleton proteins, suggesting that HIP-I and huntingtin play a functional role in the cell filament networks. The HIP1 gene is ubiquitously expressed in different brain regions at low level. HIP-I is enriched in human brain but can also be detected in other human tissues as well as in mouse brain. HIP-I and huntingtin behave almost identically during subcellular fractionation and both proteins are enriched in the membrane containing fractions.

Expression of normal and mutant huntingtin in the developing brain

Huntington's disease (HD) is caused by a genetic mutation that results in a polyglutamine expansion in huntingtin. The time course of neuronal loss in the HD striatum and other affected brain regions before the onset of symptoms is unknown. To determine the potential influence of huntingtin on brain development, we examined its expression in the developing mouse and in human control and HD brain. By Western blot, huntingtin was detected throughout the adult mouse brain and at all stages of embryonic and postnatal brain development. The protein increased significantly between postnatal day 7 (P7) and P15, which marks a period of active neuronal differentiation and enhanced sensitivity to excitotoxic injury in the rodent striatum. Immunoreactivity was found in neurons throughout the brain and localized mostly to the somatodendritic cytoplasm and to axons in fiber bundles. Staining was variable in different groups of neurons and within the same cell population. In developing brain, huntingtin was limited primarily to neuronal perikarya. Increased immunoreactivity in large neurons followed the gradient of neurogenesis and appeared in the basal forebrain and brainstem by embryonic days 15-17, in regions of cortex by P0-P1, and in the striatum by P7. In human brain at midgestation (19-21 weeks), huntingtin was detected in all regions. The brain of a 10-week-old infant with the expanded HD allele expressed a higher molecular weight mutant form of huntingtin at levels comparable to those of the wild-type protein. Thus, mutant huntingtin is expressed before neuronal maturation is complete. Results suggest that huntingtin has an important constitutive role in neurons during brain development, that heterogeneity in neuronal expression of the protein is developmentally regulated, and that the intraneuronal distribution of huntingtin increases in parallel with neuronal maturation. The presence of mutant huntingtin in the immature HD brain raises the possibility that neurons may be affected during brain development and possibly in the postnatal period when vulnerability to excitotoxic injury is at its peak.

Huntington's disease: translating a CAG repeat into a pathogenic mechanism

The specific pattern of neuronal cell death in Huntington's disease (HD) is triggered by an abnormal version of the huntingtin protein, which is produced by translation of the HD gene defect, an expanded CAG repeat in a novel 4p16.3 gene. The extended amino-terminal polyglutamine segment may act via the protein's inherent activity, increasing it or decreasing it in a graded fashion, or, alternatively, it may confer the ability to interact with a completely different set of cellular pathways, focusing attention on the HD protein's normal and abnormal physiological functions.

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

The mutation underlying Huntington disease (HD) is CAG expansion beyond 35 repeats within a novel gene. Recently, new insights into the role of the HD protein (huntingtin) in the pathogenesis of HD have emerged. The CAG is translated and expression of mutant huntingtin is essential for neuronal death. Huntingtin is crucial for normal development and may be regarded as a cell survival gene. Huntingtin is specifically cleaved during apoptosis by a key cysteine protease, apopain, known to play a pivotal role in apoptotic cell death. The rate of cleavage is enhanced by longer polyglutamine tracts, suggesting that inappropriate apoptosis underlies HD. Recently, three proteins have been identified and have been shown specifically to interact with huntingtin, two of these interactions being influenced by CAG length. Several different approaches to develop an animal model for HD include cDNA and YAC transgenics, as well as 'knock-in' strategies. Such a model will be critical for the understanding of the natural history of HD and for the testing of new therapeutic modalities.

Expression of normal and mutant huntingtin in the developing brain

Huntington's disease (HD) is caused by a genetic mutation that results in a polyglutamine expansion in huntingtin. The time course of neuronal loss in the HD striatum and other affected brain regions before the onset of symptoms is unknown. To determine the potential influence of huntingtin on brain development, we examined its expression in the developing mouse and in human control and HD brain. By Western blot, huntingtin was detected throughout the adult mouse brain and at all stages of embryonic and postnatal brain development. The protein increased significantly between postnatal day 7 (P7) and P15, which marks a period of active neuronal differentiation and enhanced sensitivity to excitotoxic injury in the rodent striatum. Immunoreactivity was found in neurons throughout the brain and localized mostly to the somatodendritic cytoplasm and to axons in fiber bundles. Staining was variable in different groups of neurons and within the same cell population. In developing brain, huntingtin was limited primarily to neuronal perikarya. Increased immunoreactivity in large neurons followed the gradient of neurogenesis and appeared in the basal forebrain and brainstem by embryonic days 15-17, in regions of cortex by P0-P1, and in the striatum by P7. In human brain at midgestation (19-21 weeks), huntingtin was detected in all regions. The brain of a 10-week-old infant with the expanded HD allele expressed a higher molecular weight mutant form of huntingtin at levels comparable to those of the wild-type protein. Thus, mutant huntingtin is expressed before neuronal maturation is complete. Results suggest that huntingtin has an important constitutive role in neurons during brain development, that heterogeneity in neuronal expression of the protein is developmentally regulated, and that the intraneuronal distribution of huntingtin increases in parallel with neuronal maturation. The presence of mutant huntingtin in the immature HD brain raises the possibility that neurons may be affected during brain development and possibly in the postnatal period when vulnerability to excitotoxic injury is at its peak.

Huntingtin is ubiquitinated and interacts with a specific ubiquitin-conjugating enzyme

Using the yeast two-hybrid system, we have identified a human ubiquitin-conjugating enzyme (hE2-25K) as a protein that interacts with the gene product for Huntington disease (HD) (Huntingtin). This protein has complete amino acid identity with the bovine E2-25K protein and has striking similarity to the UBC-1, -4 and -5 enzymes of Saccharomyces cerevisiae. This protein is highly expressed in brain and a slightly larger protein recognized by an anti-E2-25K polyclonal antibody is selectively expressed in brain regions affected in HD. The huntingtin-E2-25K interaction is not obviously modulated by CAG length. We also demonstrate that huntingtin is ubiquitinated. These findings have implications for the regulated catabolism of the gene product for HD.

Cleavage of huntingtin by apopain, a proapoptotic cysteine protease, is modulated by the polyglutamine tract

Apoptosis has recently been recognized as a mode of cell death in Huntington disease (HD). Apopain, a human counterpart of the nematode cysteine protease death-gene product, CED-3, has a key role in proteolytic events leading to apoptosis. Here we show that apoptotic extracts and apopain itself specifically cleave the HD gene product, huntingtin. The rate of cleavage increases with the length of the huntingtin polyglutamine tract, providing an explanation for the gain-of-function associated with CAG expansion. Our results show that huntingtin is cleaved by cysteine proteases and suggest that HD might be a disorder of inappropriate apoptosis.

Somatic mosaicism of expanded CAG repeats in brains of patients with dentatorubral-pallidoluysian atrophy: cellular population-dependent dynamics of mitotic instability

Dentatorubral-pallidoluysian atrophy (DRPLA) is an autosomal dominant neurodegenerative disease caused by unstable expansion of a CAG repeat in the DRPLA gene. We performed detailed quantitative analysis of the size and the size distribution (range) of the expanded CAG repeats in various regions of the CNS of eight autopsied patients with DRPLA. Expanded alleles (AE) showed considerable variations in size, as well as in range, depending on the region of the CNS, whereas normal alleles did not show such variations, which indicates the occurrence of somatic mosaicism of AE in the CNS. The AE in the cerebellar cortex were consistently smaller by two to five repeat units than those in the cerebellar white matter. Moreover, the AE in the cerebral cortex were smaller by one to four repeat units than those in the cerebral white matter. These results suggest that the smaller AE in the cerebellar and cerebral cortices represent those of neuronal cells. The ranges of the AE in the cerebral cortex, cerebral white matter, and cerebellar white matter showed considerable variation ranging from 9 to 23 repeat units, whereas those in the cerebellar cortex showed little variance and were approximately 7 repeat units. The ranges of the AE in the cerebral cortex, cerebral white matter, and cerebellar white matter were much broader in patients with higher ages at death than they were in patients with lower ages at death, raising the possibility that the range of AE increases with time, as the result of mitotic instability of AE.

Expansion of polyglutamine repeat in huntingtin leads to abnormal protein interactions involving calmodulin

Huntington's disease (HD) is an inherited neurodegenerative disorder associated with expansion of a CAG repeat in the IT15 gene. The IT15 gene is translated to a protein product termed huntingtin that contains a polyglutamine (polyGln) tract. Recent investigations indicate that the cause of HD is expansion of the polyGln tract. However, the function of huntingtin and how the expanded polyGln tract causes HD is not known. We investigate potential protein-protein interactions of huntingtin using affinity resins. Huntingtin from brain extracts is retained on calmodulin(CAM)-Sepharose in a calcium-dependent fashion. We purify rat huntingtin to apparent homogeneity using a combination of DEAE-cellulose column chromatography, ammonium sulfate precipitation, and preparative SDS/PAGE. Purified rat huntingtin does not interact with CAM directly as revealed by 125I-CAM overlay. Huntingtin forms a large CAM-containing complex of over 1,000 kDa in the presence of calcium, which partially disassociates in the absence of calcium. Furthermore, an increased amount of mutant huntingtin from HD patient brains is retained on CAM-Sepharose compared to normal huntingtin from control patient brains, and the mutant allele is preferentially retained on CAM-Sepharose in the absence of calcium. These results suggest that huntingtin interacts with other proteins including CAM and that the expansion of polyGln alters this interaction.

Huntingtin-associated protein (HAP1): discrete neuronal localizations in the brain resemble those of neuronal nitric oxide synthase

Huntington disease stems from a mutation of the protein huntingtin and is characterized by selective loss of discrete neuronal populations in the brain. Despite a massive loss of neurons in the corpus striatum, NO-generating neurons are intact. We recently identified a brain-specific protein that associates with huntingtin and is designated huntingtin-associated protein (HAP1). We now describe selective neuronal localizations of HAP1. In situ hybridization studies reveal a resemblance of HAP1 and neuronal nitric oxide synthase (nNOS) mRNA localizations with dramatic enrichment of both in the pedunculopontine nuclei, the accessory olfactory bulb, and the supraoptic nucleus of the hypothalamus. Both nNOS and HAP1 are enriched in subcellular fractions containing synaptic vesicles. Immunocytochemical studies indicate colocalizations of HAP1 and nNOS in some neurons. The possible relationship of HAP1 and nNOS in the brain is reminiscent of the relationship of dystrophin and nNOS in skeletal muscle and suggests a role of NO in Huntington disease, analogous to its postulated role in Duchenne muscular dystrophy.

A huntingtin-associated protein enriched in brain with implications for pathology

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expanding polyglutamine repeat in the IT15 or huntingtin gene. Although this gene is widely expressed and is required for normal development, the pathology of HD is restricted to the brain, for reasons that remain poorly understood. The huntingtin gene product is expressed at similar levels in patients and controls, and the genetics of the disorder suggest that the expansion of the polyglutamine repeat induces a toxic gain of function, perhaps through interactions with other cellular proteins. Here we report the identification of a protein (huntingtin-associated protein (HAP)-1) that binds to huntingtin. This binding is enhanced by an expanded polyglutamine repeat, the length of which is also known to correlate with the age of disease onset. The HAP-1 protein is enriched in the brain, suggesting a possible basis for the selective brain pathology of HD.