Dentatorubral and pallidoluysian atrophy is associated with expansion of an unstable CAG repeat on chromosome 12p. The nucleotide sequences of overlapping cDNA clones has been determined and the gene structure has been deduced. The gene is ubiquitously expressed to form a single 4.5 kb transcript and encoded by an open reading frame of 1184 amino acids (aa), in which a polyglutamine track with variable length starts at aa 484. The sequence contains several interesting motifs consisting of a simple repeated amino acid sequence, a homo-proline track, two stretches of arginine-glutamic acid dipeptides and a stretch of alternative histidine residues. These results provide clues toward understanding neurodegenerative diseases associated with triplet repeat expansion (Nagafuchi, 1994).

Hereditary dentatorubral-pallidoluysian atrophy (DRPLA) is an autosomal dominant neurologic disorder characterized by variable combinations of myoclonus, epilepsy, cerebellar ataxia, choreoathetosis and dementia. By specifically searching published brain cDNA sequences for the presence of CAG repeats, unstable expansion of a CAG was identified in a gene on chromosome 12 in all the 22 DRPLA patients examined. A good correlation between the size of the CAG repeat expansion and the ages of disease onset is found in this group. Patients with earlier onset tended to have a phenotype of progressive myoclonus epilepsy and larger expansions. It is proposed that the wide variety of clinical manifestations of DRPLA can now be explained by the variable unstable expansion of the CAG repeat (Nagafuchi, 1994).

DRPLA is associated with the expansion of an unstable CAG repeat. Using antibodies against a synthetic peptide corresponding to the sequence of the DRPLA gene product C terminus, the DRPLA gene product was identified in normal human brains as an approximately 190 kD protein. A larger approximately 205 kD protein is also found specifically in DRPLA brains. Immunohistochemically, the DRPLA gene product is observed mainly in the neuronal cytoplasm. These results demonstrate the existence of the expanded CAG repeat gene product and support the possibility that the expanded CAG-encoded polyglutamine stretch may participate in the pathological process of the similar trinucleotide repeat diseases (Yazawa, 1995).

DRPLA is one of the family of neurodegenerative diseases caused by expansion of a polyglutamine tract. The drpla gene product, atrophin-1, is widely expressed, has no known function or activity, and is found in both the nuclear and cytoplasmic compartments of neurons. Truncated fragments of atrophin-1 accumulate in neuronal nuclei in a transgenic mouse model of DRPLA, and may underlie the disease phenotype. Using the yeast two-hybrid system, studies have identified ETO/MTG8, a component of nuclear receptor corepressor complexes, as an atrophin-1-interacting protein. When cotransfected into Neuro-2a cells, atrophin-1 and ETO/MTG8 colocalize in discrete nuclear structures that contain endogenous mSin3A and histone deacetylases. These structures are sodium dodecyl sulfate-soluble and associated with the nuclear matrix. Cotransfection of ETO/MTG8 with atrophin-1 recruits atrophin-1 to the nuclear matrix, while atrophin-1 and ETO/MTG8 cofractionate in nuclear matrix preparations from brains of DRPLA transgenic mice. Furthermore, in a cell transfection-based assay, atrophin-1 represses transcription. Together, these results suggest that atrophin-1 associates with nuclear receptor corepressor complexes and is involved in transcriptional regulation. Emerging links between disease-associated polyglutamine proteins, nuclear receptors, translocation-leukemia proteins, and the nuclear matrix may have important repercussions for the pathobiology of this family of neurodegenerative disorders (Wood, 2000).

Utilizing an affinity-purified antiserum directed against the carboxyl terminal region of atrophin-1/drplap (residues 1170-1185), the expression and distribution of the protein was examined in a variety of neuronal and non-neuronal tissues. The protein is widely distributed throughout the cerebrum and cerebellum. Labeling is primarily cytoplasmic within neuronal cell bodies and dendrites. Prominently staining regions include layers II, III, V, and VI of cerebral cortex, CA1-4 of the hippocampus, caudate nucleus, putamen, globus pallidus, amygdala, thalamus, red nucleus, pons, Purkinje cells, and deep cerebellar nuclei. A 190 kDa band is detected in extract from numerous mammals, but no cross-reactive material was detected in an extract of avian central nervous system (CNS) tissue. In the rat, expression of the protein is predominantly neuronal. These investigations suggest a ubiquitous expression of atrophin-1/drplap in mammalian CNS tissue (Knight, 1997).

Dentatorubral and pallidoluysian atrophy (DRPLA) is a member of a family of progressive neurodegenerative diseases caused by polyglutamine repeat expansion. Transgenic mice expressing full-length human atrophin-1 with 65 consecutive glutamines exhibit ataxia, tremors, abnormal movements, seizures, and premature death. These mice accumulate atrophin-1 immunoreactivity and inclusion bodies in the nuclei of multiple populations of neurons. Subcellular fractionation reveal 120 kDa nuclear fragments of mutant atrophin-1, whose abundance increased with age and phenotypic severity. Brains of DRPLA patients contain apparently identical 120 kDa nuclear fragments. By contrast, mice overexpressing atrophin-1 with 26 glutamines are phenotypically normal and do not accumulate the 120 kDa fragments. It is concluded that the evolution of neuropathology in DRPLA involves proteolytic processing of mutant atrophin-1 and nuclear accumulation of truncated fragments (Schilling, 1999).

Dentatorubro-pallidoluysian atrophy (DRPLA) is one of eight autosomal dominant neurodegenerative disorders characterized by an abnormal CAG repeat expansion which results in the expression of a protein with a polyglutamine stretch of excessive length. Four of the gene products (huntingtin, atrophin-1 (DRPLA), ataxin-3, and androgen receptor) associated with these open reading frame triplet repeat expansions are substrates for the cysteine protease cell death executioners, the caspases. This led to a hypothesis that caspase cleavage of these proteins may represent a common step in the pathogenesis of each of these four neurodegenerative diseases. Evidence is provided that caspase cleavage of atrophin-1 modulates cytotoxicity and aggregate formation. Cleavage of atrophin-1 at Asp109 by caspases is critical for cytotoxicity because a mutant atrophin-1 that is resistant to caspase cleavage is associated with significantly decreased toxicity. Further, the altered cellular localization within the nucleus and aggregate formation associated with the expanded form of atrophin-1 are completely suppressed by mutation of the caspase cleavage site at Asp109. These results provide support for the toxic fragment hypothesis whereby cleavage of atrophin-1 by caspases may be an important step in the pathogenesis of DRPLA. Therefore, inhibiting caspase cleavage of the polyglutamine-containing proteins may be a feasible therapeutic strategy to prevent cell death (Ellerby. 1999).

Dentatorubral-pallidoluysian atrophy (DRPLA) is one of the hereditary neurodegenerative disorders caused by expansion of CAG/glutamine repeats. To investigate the normal function of the DRPLA gene and the pathogenic mechanism of neuron death in specific areas of the brain, a gene that shares a notable motif with DRPLA, arginine-glutamic acid (RE) dipeptide repeats, was isolated and analyzed. The gene isolated, designated RERE, has an open reading frame of 1566 amino acids, of which the C-terminal portion has 67% homology to DRPLA, whereas the N-terminal portion is distinctive. RERE also contains arginine-aspartic acid (RD) dipeptide repeats and putative nuclear localization signal sequences, but no polyglutamine tracts. RERE is expressed at a low level in most tissues examined. Immunoprecipitation and in vitro binding assays demonstrate that the DRPLA and RERE proteins bind each other, for which one of the RE repeats has a primary role, and extended polyglutamine enhances the binding. With engineered constructs fused with a tag, the RERE protein localizes predominantly in the nucleus. Moreover, when RERE is overexpressed, the distribution of endogenous DRPLA protein alters from the diffused to the speckled pattern in the nucleus so as to co-localize with RERE. More RERE protein is recruited into nuclear aggregates of the DRPLA protein with extended polyglutamine than into those of pure polyglutamine. These results reveal a function for the DRPLA protein in the nucleus and the RE repeat in the protein-protein interaction (Tanagisawa, 2000).

DRPLA is an autosomal dominant neurodegenerative disease caused by a CAG repeat expansion, resulting in ubiquitinated inclusions and diffuse accumulation of mutant atrophin-1 in the neuronal nuclei in many regions of the central nervous system. In the cerebellar cortex, such nuclear abnormalities occur in the granule cells. In the present study, ultrastructural and morphometric analyses were performed on the nuclei of the cerebellar granule cells from eight patients with DRPLA (four with juvenile-onset disease and four with adult-onset disease) in an attempt to obtain further insight into the neuronal nuclear alterations that occur in CAG-repeat diseases. Ultrastructurally, all patients had intranuclear filamentous inclusions (NIIs, neuronal intranuclear inclusions) and nuclear membrane indentations (NMIs) in some of their granule cells, and chromatin tended to be sparse in the nucleoplasm of the affected nuclei. In all patients there was an association between NIIs and NMIs, and nuclei with NIIs and/or NMIs were larger than those without such changes. However, the nuclear enlargement was not due solely to the NIIs -- even nuclei without NIIs or NMIs were larger in the patients than in the controls. In the DRPLA patients, there was a significant inverse correlation between the cross-sectional area of the nuclei and the disease duration. These findings indicate that NIIs and NMIs are features in the disease and occur in association with each other, and that nuclear enlargement -- the degree of which may decrease with time after onset of the illness -- is a more prevalent abnormality than the formation of NIIs or NMIs (Takahashi, 2001).

Dentatorubral-pallidoluysian atrophy (DRPLA) is an autosomal dominant neurodegenerative disorder caused by a CAG repeat expansion. Diffuse accumulation of mutant atrophin-1 has been demonstrated in the neuronal nuclei, rather than the formation of neuronal intranuclear inclusions (NIIs), is the predominant pathologic condition and involves a wide range of central nervous system regions far beyond the systems previously reported to be affected. In the neuronal nuclei harboring NIIs, promyelocytic leukemia protein (PML) nuclear bodies are redistributed into a single NII, and the CREB (cAMP-responsive element-binding protein)-binding protein is also recruited into NIIs. The results suggest that the novel lesion distribution revealed by the diffuse nuclear labeling may be responsible for a variety of clinical features, such as dementia and epilepsy in DRPLA, and that certain transcriptional abnormalities may be induced secondarily in neuronal nuclei with the formation of NIIs (Yamada, 2001).

To investigate the implication of small ubiquitin-related modifier-1 (SUMO-1; see Drosophila SUMO) in the formation of neuronal intranuclear inclusions in polyglutamine diseases, the localization of SUMO-1 was found in dentatorubral-pallidoluysian atrophy (DRPLA) brain tissues and PC12 cells expressing truncated atrophin-1 with expanded poly-glutamine stretches. SUMO-1 co-localizes with neuronal intranuclear inclusions in DRPLA brain and the DRPLA model cells, which showed that the aggregates formed by expanded polyglutamine stretches are highly SUMOlylated. In addition, to examine the role of SUMO-1 in nuclear aggregate formation and cell death, either SUMO-1 or DeltaSUMO-1, which is a SUMOlylation defective mutant lacking the C-terminal motif, was co-transfected with atrophin-1 with expanded polyglutamine stretches. Co-transfection of DeltaSUMO-1 decreased the number of cells with nuclear aggregates and consequent apoptosis of PC12 cells, both of which were markedly enhanced by co-transfection of SUMO-1 with atrophin-1 with expanded polyglutamine stretches. These results suggest that SUMO-1 is implicated in the pathogenesis of DRPLA and accelerates aggregate formation and cell death (Terashima, 2002)

Dentatorubral and pallidoluysian atrophy (DRPLA) is an autosomal dominant neurodegenerative disorder similar to Huntington's disease, with clinical manifestations including chorea, incoordination, ataxia, and dementia. It is caused by an expansion of a CAG trinucleotide repeat encoding polyglutamine in the atrophin-1 gene. Both patients and DRPLA transgenic mice have nuclear accumulation of atrophin-1, especially an approximately 120-kDa fragment, which appears to represent a cleavage product. This is an N-terminal fragment that does not correspond to the previously described caspase-3 fragment, or any other known caspase cleavage product. The atrophin-1 sequence contains a putative nuclear localization signal in the N terminus of the protein and a putative nuclear export signal in the C terminus. The hypothesis was tested that endogenous localization signals are functional in atrophin-1, and that nuclear localization and proteolytic cleavage contribute to atrophin-1 cell toxicity. In transient cell transfection experiments using a neuroblastoma cell line, full-length atrophin-1 with 26 (normal) or 65 (expanded) glutamines localized to both nucleus and cytoplasm, with no significant difference in toxicity between the normal and mutant proteins. A construct with 65 glutamine repeats encoding an N-terminal fragment (which removes an NES) of atrophin-1 similar in size to the truncation product in DRPLA patient tissue, showed increased nuclear labeling, and an increase in cellular toxicity, compared with a similar fragment with 26 glutamines. Full-length atrophin-1 with 65 polyglutamine repeats and mutations inactivating the NES also yielded increased nuclear localization and increased toxicity. These data suggest that truncation enhances cellular toxicity of the mutant protein, and that the NES is a relevant region deleted during truncation. Furthermore, mutating the NLS in the truncated protein shifted atrophin-1 more to the cytoplasm and eliminated the increased toxicity, consistent with the idea that nuclear localization enhances toxicity. In none of the experiments were inclusions visible in the nucleus or cytoplasm, suggesting that inclusion formation is unrelated to cell death. These data indicate that truncation of atrophin-1 may alter its ability to shuttle between the nucleus and cytoplasm, leading to abnormal nuclear interactions and cell toxicity (Nucifora, 2003).

Atrophin 2 recruits histone deacetylase and is required for the function of multiple signaling centers during mouse embryogenesis

Atrophins are evolutionarily conserved proteins that are thought to act as transcriptional co-repressors. Mammalian genomes contain two atrophin genes. Dominant polyglutamine-expanded alleles of atrophin 1 have been identified as the cause of dentatorubralpallidoluysian atrophy, an adult-onset human neurodegenerative disease with similarity to Huntington's. In a screen for recessive mutations that disrupt patterning of the early mouse embryo, a line named openmind was identified carrying a mutation in atrophin 2. openmind homozygous embryos exhibit a variety of patterning defects that first appear at E8.0. Defects include a specific failure in ventralization of the anterior neural plate, loss of heart looping and irregular partitioning of somites. In mutant embryos, Shh expression fails to initiate along the anterior midline at E8.0, and Fgf8 is delocalized from the anterior neural ridge at E8.5, revealing a crucial role for atrophin 2 in the formation and function of these two signaling centers. Atrophin 2 is also required for normal organization of the apical ectodermal ridge, a signaling center that directs limb pattern. Elevated expression of atrophin 2 in neurons suggests it may interact with atrophin 1 in neuronal development or function. Atrophin 2 associates with histone deacetylase 1 in mouse embryos, providing a biochemical link between Atr2 and a chromatin-modifying enzyme. Based on these results it is proposed that atrophin proteins act as transcriptional co-repressors during embryonic development (Zoltewicz, 2004).

Sodium butyrate ameliorates histone hypoacetylation and neurodegenerative phenotypes in a mouse model for DRPLA

Dentatorubral-pallidoluysian atrophy (DRPLA) is a progressive neurodegenerative disease caused by polyglutamine expansion within the Atrophin-1 protein. To study the mechanism of this disease and to test potential therapeutic methods, Atro-118Q transgenic mice were established that express in neurons a mutant human Atrophin-1 protein that contains an expanded stretch of 118 glutamines. Consistent with the results from previous studies on transgenic mice that express mutant Atrophin-1 with 65 glutamines, Atro-118Q mice exhibit several neurodegenerative phenotypes that are commonly seen in DRPLA patients, including ataxia, tremors, and other motor defects. Overexpression of wild-type human Atrophin-1 could not rescue the motor and survival defects in Atro-118Q mice, indicating that the mutant protein with polyglutamine expansion does not simply function in a dominant negative manner. Biochemical analysis of Atro-118Q mice revealed hypoacetylation of histone H3 in brain tissues and thus suggests that global gene repression is an underlying mechanism for neurodegeneration in this mouse model. Intraperitoneal administration of sodium butyrate, a histone deacetylase inhibitor, ameliorates the histone acetylation defects, significantly improves motor performance, and extends the average life span of Atro-118Q mice. These results support the hypothesis that transcription deregulation plays an important role in the pathogenesis of polyglutamine expansion diseases and suggest that reversion of transcription repression with small molecules such as sodium butyrate is a feasible approach to treating DRPLA symptoms (Ying, 2005).

Mutation of the atrophin2 gene in the zebrafish disrupts signaling by fibroblast growth factor during development of the inner ear

The development of the vertebrate inner ear depends on the precise expression of fibroblast growth factors. In a mutagenesis screen for zebrafish with abnormalities of inner-ear development and behavior, a mutant line, ru622, was isolated whose phenotypic characteristics resembled those of null mutants for the gene encoding fibroblast growth factor 8 (Fgf8): an inconsistent startle response, circular swimming, fused otoliths, and abnormal semicircular canals. Positional cloning disclosed that the mutant gene encodes the transcriptional corepressor Atrophin2. Both the Fgf8 protein and zebrafish 'similar expression to fgf genes' protein (Sef), an antagonist of fibroblast growth factors induced by Fgf8 itself, were found to be overexpressed in ru622 mutants. It was therefore hypothesized that an excess of Sef eliminates Fgf8 signals and produces an fgf8 null phenotype in ru622 mutants. In support of this idea, larvae whose atrophin2 expression had been diminished with morpholinos could be rescued by reducing the expression of Sef as well. It is proposed that Atrophin2 plays a role in the feedback regulation of Fgf8 signaling. When mutation of the atrophin2 gene results in the overexpression of both Fgf8 and Sef, the excessive Sef inhibits Fgf8 signaling. The resultant imbalance of Fgf8 and Sef signals then underlies the abnormal aural development observed in ru622 (Asai, 2006).

Functional architecture of atrophins

Vertebrate genomes harbor two Atrophin genes, Atrophin-1 (Atn1) and Atrophin-2 (Atn2). The Atn1 locus produces a single polypeptide, whereas two different protein products are expressed from the Atn2 (also known as Rere) locus. A long, or full-length, form contains an amino-terminal MTA-2-homologous domain followed by an Atrophin-1-related domain. A short form, expressed via an internal promoter, consists solely of the Atrophin domain. Atrophin-1 can be co-immunoprecipitated along with Atrophin-2, suggesting that the Atrophins ordinarily function together. Mutations that disrupt the expression of the long form of Atrophin-2 disrupt early embryonic development. To determine the requirement for Atrophin-1 during development a null allele was generated. Somewhat surprisingly it was found that Atrophin-1 function is dispensable. To gain a better understanding of the requirement for Atrophin function during development, an analysis of the functional domains of the three different gene products was carried out. Taken together, these data suggest that Atrophins function as bifunctional transcriptional regulators. The long form of Atrophin-2 has a transcriptional repression activity that is not found in the other Atrophin polypeptides and that is required for normal embryogenesis. Atrophin-1 and the short form of Atrophin-2, in contrast, can act as potent and evolutionarily conserved transcriptional activators (Shen, 2007).

Nuclear receptor TLX prevents retinal dystrophy and recruits the corepressor atrophin1

During mammalian embryogenesis, precise coordination of progenitor cell proliferation and differentiation is essential for proper organ size and function. The involvement of TLX (NR2E1), an orphan nuclear receptor, has been implicated in ocular development; Tlx–/– mice exhibit visual impairment. Using genetic and biochemical approaches, this study shows that TLX modulates retinal progenitor cell proliferation and cell cycle re-entry by directly regulating the expression of Pten and its target cyclin D1. Additionally, TLX finely tunes the progenitor differentiation program by modulating the phospholipase C and mitogen-activated protein kinase (MAPK) pathways and the expression of an array of cell type-specific transcriptional regulators. Consequently, Tlx–/– mice have a dramatic reduction in retina thickness and enhanced generation of S-cones, and develop severe early onset retinal dystrophy. Furthermore, TLX interacts with atrophin1 (Atn1), a corepressor that is involved in human neurodegenerative dentatorubral-pallidoluysian atrophy (DRPLA) and that is essential for development of multiple tissues. Together, these results reveal a molecular strategy by which an orphan nuclear receptor can precisely orchestrate tissue-specific proliferation and differentiation programs to prevent retinal malformation and degeneration (Zhang, 2006: full text of article).

Grunge/Atrophin: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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