Interaction between neuronal intranuclear inclusions and promyelocytic leukemia protein nuclear and coiled bodies in CAG repeat diseases

Nuclear localization of a non-caspase truncation product of atrophin-1, with an expanded polyglutamine repeat, increases cellular toxicity

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. We now show that 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. We have tested the hypothesis 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.

p14ARF induces the relocation of HDM2 and p53 to extranucleolar sites that are targeted by PML bodies and proteasomes

BACKGROUND

p14ARF is a protein product of the alternative reading frame of the human INK4a locus. It functions as a tumor suppressor protein. p14ARF suppresses growth through p53-dependent and p53-independent pathways.

RESULTS

p14ARF protein localizes primarily to the nucleoli. Here we show that in transfected cells p14ARF also appears in Hsp70 positive extranucleolar inclusions. The formation of p14ARF inclusions induces the parallel re-localization p53 and HDM2 to these sites that are also targeted by PML bodies and proteasomes.

CONCLUSION

Our data show that co-localization between p53, HDM2 and p14ARF occurs at extranucleolar sites. Accumulation of PML and proteasomes at these sites suggest that the components of the nuclear inclusions are targeted for proteasome-mediated degradation.

Genotype-phenotype correlation in CAG-repeat diseases

The expansion of a CAG repeat is a common causative gene mutation in several hereditary neurodegenerative disorders, including dentatorubral-pallidoluysian atrophy (DRPLA). Although, in DRPLA, it is revealed that the variety of clinical manifestations is related to the variable expansion of the CAG repeat, there are still many problems in the correlation between the symptoms and neuropathologic findings. Recent immunohistochemical studies have revealed that diffuse intranuclear accumulation of mutant proteins with expanded polyglutamine stretches is a significant pathology in DRPLA, and involves a wide range of the CNS regions far beyond the lesion distribution previously established by neuronal loss. This novel pathology may become a clue for elucidating molecular mechanisms of neuronal dysfunction and establishing clinicopathological correlations in CAG-repeat diseases.

Promyelocytic leukemia protein is redistributed during the formation of intranuclear inclusions independent of polyglutamine expansion: an immunohistochemical study on Marinesco bodies

Marinesco bodies (MBs) are ubiquitinated intranuclear inclusions observed in nigral pigmented neurons. They increase in number during aging, and their formation is considered to represent a cellular reaction to stress, but is not always associated with neuronal degeneration. We conducted immunohistochemical studies on MBs abundant in myotonic dystrophy brains and compared their nature with that of neuronal intranuclear inclusions (NIIs) in polyglutamine diseases. First, we examined the relationship between MBs and polyglutamine proteins and demonstrated that one of the polyglutamine proteins, ataxin-3, as well as a 19S proteasomal protein, was preferentially recruited into MBs even in the absence of expanded polyglutamine. This indicates that an alternative mechanism during the formation of MBs that is not related to polyglutamine expansion or neuronal degeneration may recruit ataxin-3 into nuclear inclusions in a protein-specific manner. Secondly, we investigated the relationship between MBs and promyelocytic leukemia protein (PML), a nuclear matrix-associated protein that is normally localized to intranuclear punctate structures (PML nuclear bodies) and is known to reorganize itself in association with polyglutamine aggregation. In nigral pigmented neurons in myotonic dystrophy, spherical, hemispherical or rod-like PML-immunoreactive structures, in addition to punctate structures, were observed in their nuclei. Similar PML redistribution was also observed in nigral pigmented neurons in aged controls and cases of hepatic encephalopathy, 2 other conditions in which abundant MBs are formed. Double immunofluorescence study revealed that these PML-positive structures undergo morphological changes in association with ubiquitin accumulation during MB formation. It is therefore indicated that PML reorganization does not represent a specific nuclear event involved in the pathogenesis of polyglutamine diseases, but may commonly occur during the formation of intranuclear inclusions as a reaction against various stresses that involve the ubiquitin-proteasome pathway.

Cajal bodies and coilin--moving towards function

Many nuclear factors are concentrated within nonmembrane-bound subnuclear bodies. The Cajal body is an example of a conserved nuclear compartment that has been linked to molecular disease. Recent studies have shown Cajal bodies to be surprisingly mobile and offer clues about their function in the cell.

Involvement of lysosomes in the pathogenesis of CAG repeat diseases

In CAG repeat diseases, affected neurons possess many cytoplasmic granules immunopositive for expanded polyglutamine stretches. Electron microscopic immunohistochemistry showed that the granules corresponded to lysosomes of primitive type. The results suggest that, in addition to the ubiquitin/proteasome pathway, mutant proteins with expanded polyglutamine stretches are involved in the lysosomal pathway for protein degradation and that this processing mechanism may serve as a target for a new therapeutic approach to CAG repeat diseases.