The ubiquitin ligase F-box/G-domain protein 1 promotes the degradation of the disease-linked protein torsinA through the ubiquitin-proteasome pathway and macroautophagy.

DYT1 dystonia is a dominantly inherited, disabling neurological disorder with low penetrance that is caused by the deletion of a glutamic acid (ΔE) in the protein torsinA. We previously showed that torsinA(wt) is degraded through macroautophagy while torsinA(ΔE) is targeted to the ubiquitin-proteasome pathway (UPP). The different catabolism of torsinA(wt) and (ΔE) potentially modulates torsinA(wt):torsinA(ΔE) stoichiometry. Therefore, gaining a mechanistic understanding on how the protein quality control machinery clears torsinA(ΔE) in neurons may uncover important regulatory steps in disease pathogenesis. Here, we asked whether F-box/G-domain protein 1 (FBG1), a ubiquitin ligase known to degrade neuronal glycoproteins, is implicated in the degradation of torsinA(ΔE) by the UPP. In a first set of studies completed in cultured cells, we show that FBG1 interacts with and influences the steady-state levels of torsinA(wt) and (ΔE). Interestingly, FBG1 achieves this effect promoting the degradation of torsinA not only through the UPP, but also by macroautophagy. To determine the potential clinical significance of these findings, we asked if eliminating expression of Fbg1 triggers a motor phenotype in torsinA(ΔE) knock in (KI) mice, a model of non-manifesting DYT1 mutation carriers. We detected differences in spontaneous locomotion between aged torsinA(ΔE) KI-Fbg1 knock out and control mice. Furthermore, neuronal levels of torsinA were unaltered in Fbg1 null mice, indicating that redundant systems likely compensate in vivo for the absence of this ubiquitin ligase. In summary, our studies support a non-essential role for FBG1 on the degradation of torsinA and uncover a novel link of FBG1 to the autophagy pathway.

Degeneration of ingestion-related brainstem nuclei in spinocerebellar ataxia type 2, 3, 6 and 7.

Dysphagia, which can lead to nutritional deficiencies, weight loss and dehydration, represents a risk factor for aspiration pneumonia. Although clinical studies have reported the occurrence of dysphagia in patients with spinocerebellar ataxia type 2 (SCA2), type 3 (SCA3), type 6 (SCA6) and type 7 (SCA7), there are neither detailed clinical records concerning the kind of ingestive malfunctions which contribute to dysphagia nor systematic pathoanatomical studies of brainstem regions involved in the ingestive process. In the present study we performed a systematic post mortem study on thick serial tissue sections through the ingestion-related brainstem nuclei of 12 dysphagic patients who suffered from clinically diagnosed and genetically confirmed spinocerebellar ataxias assigned to the CAG-repeat or polyglutamine diseases (two SCA2, seven SCA3, one SCA6 and two SCA7 patients) and evaluated their medical records. Upon pathoanatomical examination in all of the SCA2, SCA3, SCA6 and SCA7 patients, a widespread neurodegeneration of the brainstem nuclei involved in the ingestive process was found. The clinical records revealed that all of the SCA patients were diagnosed with progressive dysphagia and showed dysfunctions detrimental to the preparatory phase of the ingestive process, as well as the lingual, pharyngeal and oesophageal phases of swallowing. The vast majority of the SCA patients suffered from aspiration pneumonia, which was the most frequent cause of death in our sample. The findings of the present study suggest (i) that dysphagia in SCA2, SCA3, SCA6 and SCA7 patients may be associated with widespread neurodegeneration of ingestion-related brainstem nuclei; (ii) that dysphagic SCA2, SCA3, SCA6 and SCA7 patients may suffer from dysfunctions detrimental to all phases of the ingestive process; and (iii) that rehabilitative swallow therapy which takes specific functional consequences of the underlying brainstem lesions into account might be helpful in preventing aspiration pneumonia, weight loss and dehydration in SCA2, SCA3, SCA6 and SCA7 patients.

Allele-specific RNA interference for neurological disease.

Suppressing the expression of toxic genes through RNAi holds great promise for the treatment of human disease. Allele-specific approaches have now been used to silence dominant toxic genes implicated in several neurological disorders. Here, we review strategies used to achieve allele-specific silencing in light of recent developments in the field of RNAi biology. In particular, new insights into siRNA and miRNA processing may be used to improve efficiency and specificity of RNAi therapy. We further discuss steps that can be taken to maximize the therapeutic benefits of this powerful technology.

The nucleus raphe interpositus in spinocerebellar ataxia type 3 (Machado-Joseph disease).

The nucleus raphe interpositus (RIP) plays an important role in the premotor network for saccades. Its omnipause neurons gate the activity of the burst neurons for vertical saccades lying within the rostral interstitial nucleus of the medial longitudinal fascicle and that for horizontal saccades residing in the caudal subnucleus of the pontine reticular formation. In the present study we investigated the RIP in five patients with clinically diagnosed and genetically confirmed spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease. Polyethylene glycol-embedded 100 microm serial sections stained for lipofuscin pigment and Nissl material as well as paraffin-embedded Nissl stained thin sections revealed the hitherto overlooked involvement of this pontine nucleus in the degenerative process underlying SCA3, whereby in four of our SCA3 patients the RIP underwent a conspicuous loss of presumed omnipause neurons. As observed in other affected brain structures, the RIP of all our SCA3 patients displayed reactive astrocytes and activated microglial cells, while some of the few of its surviving neurons harbored an ataxin-3-immunopositive intranuclear inclusion body. The findings of the present pathoanatomical study suggest that (1) neurodegeneration in the brain stem of terminal SCA3 patients is more widespread than previously thought and is not confined to cranial nerve nuclei involved in the generation of saccades but likewise involves the premotor network for saccades and (2) damage to the RIP may contribute to slowing of horizontal saccades in SCA3 patients but is not associated with saccadic oscillations as occasionally speculated.

Guidelines for the pathoanatomical examination of the lower brain stem in ingestive and swallowing disorders and its application to a dysphagic spinocerebellar ataxia type 3 patient.

Despite the fact that considerable progress has been made in the last 20 years regarding the three-phase process of ingestion and the lower brain stem nuclei involved in it, no comprehensive descriptions of the ingestion-related lower brain stem nuclei are available for neuropathologists confronted with ingestive malfunctions. Here, we propose guidelines for the pathoanatomical investigation of these nuclei based on current knowledge with respect to ingestion and the nuclei responsible for this process. The application of these guidelines is described by drawing upon the example of the lower brain stem of a male patient with spinocerebellar ataxia type 3, also known as Machado-Joseph disease, who displayed malfunctions during the preparatory phase of ingestion, as well as lingual and pharyngeal phases of swallowing. By way of the representative application of the recommended investigation procedure to 100 microm serial sections through the patient's brain stem stained for lipofuscin pigment and Nissl material, we observed neuronal loss together with astrogliosis in nearly all of the ingestion-related lower brain stem nuclei (motor, principal and spinal trigeminal nuclei; facial nucleus; parvocellular reticular nucleus; ambiguous nucleus, motor nucleus of the dorsal glossopharyngeal and vagal area; gelatinous, medial, parvocellular and pigmented solitary nuclei; hypoglossal nucleus). In view of their known functional role in the three-phase process of ingestion, damage to these nuclei not only offers an explanation of the patient's malfunctions related to the preparatory phase of ingestion and lingual and pharyngeal phases of swallowing, but also suggests that the patient may have suffered from additional esophageal phase swallowing malfunctions not mentioned in his medical records.

Degeneration of the external cuneate nucleus in spinocerebellar ataxia type 3 (Machado-Joseph disease).

Owing to its anatomical connections, the external cuneate nucleus (ECU) plays a crucial role in processing proprioceptive input from the upper trunk and upper limbs. Here, we studied this dorsal column nucleus post-mortem in five individuals with clinically diagnosed and genetically confirmed spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease, who had manifested upper trunk and upper limb ataxia. Polyethylene glycol-embedded 100-microm sections stained for lipofuscin pigment and Nissl material, as well as paraffin-embedded Nissl-stained thin sections, revealed serious neuronal loss in the ECU of all five SCA3 patients. As observed in other affected central nervous system structures, the ECU of these individuals displayed an astrogliosis, and some of the few surviving neurons harbored one or even two ataxin-3-immunopositive intranuclear inclusion bodies. The findings of the present study suggest that (1) the ECU is among the consistent targets of the degenerative process underlying SCA3 and (2) interruption of the proprioceptive pathway at the level of the ECU contributes significantly to upper limb and trunk ataxia in SCA3 patients.

Spinocerebellar ataxia type 3 (Machado-Joseph disease): severe destruction of the lateral reticular nucleus.

The lateral reticular nucleus (LRT) of the medulla oblongata is a precerebellar nucleus involved in proprioception and somatomotor automatisms. We investigated this nucleus in five individuals with clinically diagnosed and genetically confirmed spinocerebellar ataxia type 3 (SCA3, Machado-Joseph disease). Polyethylene glycol-embedded 100 micro m thick sections stained for lipofuscin granules and Nissl material as well as Nissl-stained paraffin-embedded sections revealed severe destruction of the LRT in all SCA3 brains examined. Some of the few surviving neurones contained ataxin-3-immunopositive intranuclear inclusion bodies, as noted in other affected brain regions in SCA3. Along with the severe neuronal depletion, obvious astrogliosis was seen in the LRT of all SCA3 patients. The findings suggest that the LRT is a consistent target of the pathological process underlying SCA3. In view of its afferent and efferent connections, destruction of the LRT probably contributes to gait ataxia in individuals suffering from SCA3.

The role of protein composition in specifying nuclear inclusion formation in polyglutamine disease.

Intracellular inclusions are a unifying feature of polyglutamine (polyQ) neurodegenerative diseases, yet each polyQ disease displays a unique pattern of neuronal degeneration. This implies that the protein context of expanded polyQ plays an important role in establishing selective neurotoxicity. Here, in studies of the spinocerebellar ataxia type 3 disease protein ataxin-3, we demonstrate that the protein sequence surrounding polyQ specifies the constituents of nuclear inclusions (NI) formed by the disease protein. The nuclear proteins cAMP response element-binding protein-binding protein (CBP) and Mastermind-like-1 strongly colocalize only to NI formed by full-length ataxin-3, whereas the splicing factor SC35 colocalizes only to NI formed by a polyQ-containing, carboxyl-terminal fragment of ataxin-3. These differences in NI formation reflect specific protein interactions normally undertaken by ataxin-3, as both normal and mutant full-length ataxin-3 co-immunoprecipitate with CBP and sediment on density gradients as macromolecular complexes. Moreover, normal ataxin-3 represses cAMP response element-binding protein-mediated transcription, indicating a functional consequence of ataxin-3 interactions with CBP. Finally, we show that mutant ataxin-3 forms insoluble intranuclear complexes, or microaggregates, before NI can be detected, implying a precursor-product relationship. These results suggest that protein context-dependent recruitment of nuclear proteins to intranuclear microaggregates, and subsequently to NI, may contribute to selective neurotoxicity in polyQ diseases.

Ataxia and hereditary disorders.

The last decade has seen great changes in the diagnosis of inherited ataxias. Previously mysterious diseases are now recognized to be caused by specific mutations for which genetic screening is readily available. In many cases, the discovery of the molecular basis has broadened the definition of possible clinical manifestations of particular inherited ataxias. The type of mutation underlying the more common forms of inherited ataxia-unstable trinucleotide repeat expansions-helps to explain some of the unusual features of these diseases. This article reviews recent genetic advances in ataxia. The aim is not to present an exhaustive summary but rather to provide guidance in evaluating ataxia, particularly with respect to recent molecular genetic findings.

Mechanisms of chaperone suppression of polyglutamine disease: selectivity, synergy and modulation of protein solubility in Drosophila.

At least eight dominant human neurodegenerative diseases are due to the expansion of a polyglutamine within the disease proteins. This confers toxicity on the proteins and is associated with nuclear inclusion formation. Recent findings indicate that molecular chaperones can modulate polyglutamine pathogenesis, but the basis of polyglutamine toxicity and the mechanism by which chaperones suppress neurodegeneration remains unknown. In a Drosophila: disease model, we demonstrate that chaperones show substrate specificity for polyglutamine protein, as well as synergy in suppression of neurotoxicity. Our analysis also reveals that chaperones alter the solubility properties of the protein, indicating that chaperone modulation of neurodegeneration in vivo is associated with altered biochemical properties of the mutant polyglutamine protein. These findings have implications for these and other human neurodegenerative diseases associated with abnormal protein aggregation.

CREB-binding protein sequestration by expanded polyglutamine.

Spinal and bulbar muscular atrophy (SBMA) is one of eight inherited neurodegenerative diseases known to be caused by CAG repeat expansion. The expansion results in an expanded polyglutamine tract, which likely confers a novel, toxic function to the affected protein. Cell culture and transgenic mouse studies have implicated the nucleus as a site for pathogenesis, suggesting that a critical nuclear factor or process is disrupted by the polyglutamine expansion. In this report we present evidence that CREB-binding protein (CBP), a transcriptional co-activator that orchestrates nuclear response to a variety of cell signaling cascades, is incorporated into nuclear inclusions formed by polyglutamine-containing proteins in cultured cells, transgenic mice and tissue from patients with SBMA. We also show CBP incorporation into nuclear inclusions formed in a cell culture model of another polyglutamine disease, spinocerebellar ataxia type 3. We present evidence that soluble levels of CBP are reduced in cells expressing expanded polyglutamine despite increased levels of CBP mRNA. Finally, we demonstrate that over-expression of CBP rescues cells from polyglutamine-mediated toxicity in neuronal cell culture. These data support a CBP-sequestration model of polyglutamine expansion disease.

Toward an understanding of polyglutamine neurodegeneration.

Polyglutamine expansion is now recognized to be a major cause of inherited human neurodegenerative disease. The polyglutamine expansion diseases identified so far are slowly progressive disorders in which distinct yet overlapping brain regions are selectively vulnerable to degeneration. Despite their clinical differences these diseases likely share a common pathogenic mechanism, occurring at the protein level and centered on an abnormal conformation of expanded polyglutamine in the respective disease proteins. Recently there has been remarkable progress in our understanding of polyglutamine disease, but still there are many unanswered questions. In this review, I first outline some of the shared features of polyglutamine diseases and then discuss several issues relevant to an understanding of pathogenesis, paying particular attention to possible mechanisms of neurotoxicity.

Suppression of polyglutamine-mediated neurodegeneration in Drosophila by the molecular chaperone HSP70.

At least eight inherited human neurodegenerative diseases are caused by expansion of a polyglutamine domain within the respective proteins. This confers dominant toxicity on the proteins, leading to dysfunction and loss of neurons. Expanded polyglutamine proteins form aggregates, including nuclear inclusions (NI), within neurons, possibly due to misfolding of the proteins. NI are ubiquitinated and sequester molecular chaperone proteins and proteasome components, suggesting that disease pathogenesis includes activation of cellular stress pathways to help refold, disaggregate or degrade the mutant disease proteins. Overexpression of specific chaperone proteins reduces polyglutamine aggregation in transfected cells, but whether this alters toxicity is unknown. Using a Drosophila melanogaster model of polyglutamine disease, we show that directed expression of the molecular chaperone HSP70 suppresses polyglutamine-induced neurodegeneration in vivo. Suppression by HSP70 occurred without a visible effect on NI formation, indicating that polyglutamine toxicity can be dissociated from formation of large aggregates. Our studies indicate that HSP70 or related molecular chaperones may provide a means of treating these and other neurodegenerative diseases associated with abnormal protein conformation and toxicity.

Analysis of the role of heat shock protein (Hsp) molecular chaperones in polyglutamine disease.

Polyglutamine (polygln) diseases are a group of inherited neurodegenerative disorders characterized by protein misfolding and aggregation. Here, we investigate the role in polygln disease of heat shock proteins (Hsps), the major class of molecular chaperones responsible for modulating protein folding in the cell. In transfected COS7 and PC12 neural cells, we show that Hsp40 and Hsp70 chaperones localize to intranuclear aggregates formed by either mutant ataxin-3, the disease protein in spinocerebellar ataxia type 3/Machado-Joseph disease (SCA3/MJD), or an unrelated green fluorescent protein fusion protein containing expanded polygln. We further demonstrate that expression of expanded polygln protein elicits a stress response in cells as manifested by marked induction of Hsp70. Studies of SCA3/MJD disease brain confirm these findings, showing localization of Hsp40 and, less commonly, Hsp70 chaperones to intranuclear ataxin-3 aggregates. In transfected cells, overexpression of either of two Hsp40 chaperones, the DNAJ protein homologs HDJ-1 and HDJ-2, suppresses aggregation of truncated or full-length mutant ataxin-3. Finally, we extend these studies to a PC12 neural model of polygln toxicity in which we demonstrate that overexpression of HDJ-1 suppresses polygln aggregation with a parallel decrease in toxicity. These results suggest that expanded polygln protein induces a stress response and that specific molecular chaperones may aid the handling of misfolded or aggregated polygln protein in neurons. This study has therapeutic implications because it suggests that efforts to increase chaperone activity may prove beneficial in this class of diseases.

Ataxin-3 with an altered conformation that exposes the polyglutamine domain is associated with the nuclear matrix.

Spinocerebellar ataxia type-3 or Machado-Joseph disease (SCA3/MJD) is a member of the CAG/polyglutamine repeat disease family. In this family of disorders, a normally polymorphic CAG repeat becomes expanded, resulting in expression of an expanded polyglutamine domain in the disease gene product. Experimental models of polyglutamine disease implicate the nucleus in pathogenesis; however, the link between intranuclear expression of expanded polyglutamine and neuronal dysfunction remains unclear. Here we demonstrate that ataxin-3, the disease protein in SCA3/MJD, adopts a unique conformation when expressed within the nucleus of transfected cells. The monoclonal antibody 1C2 is known preferentially to bind expanded polyglutamine, but we find that it also binds a fragment of ataxin-3 containing a normal glutamine repeat. In addition, expression of ataxin-3 within the nucleus exposes the glutamine domain of the full-length non-pathological protein, allowing it to bind the monoclonal antibody 1C2. Fractionation and immunochemical experiments indicate that this novel conformation of intranuclear ataxin-3 is not due to proteolysis, suggesting instead that association with nuclear protein(s) alters the structure of full-length ataxin-3 which exposes the polyglutamine domain. This conformationally altered ataxin-3 is bound to the nuclear matrix. The pathological form of ataxin-3 with an expanded polyglutamine domain also associates with the nuclear matrix. These data suggest that an early event in the pathogenesis of SCA3/MJD may be an altered conformation of ataxin-3 within the nucleus that exposes the polyglutamine domain.

Evidence for proteasome involvement in polyglutamine disease: localization to nuclear inclusions in SCA3/MJD and suppression of polyglutamine aggregation in vitro.

Spinocerebellar ataxia type 3, also known as Machado-Joseph disease (SCA3/MJD), is one of at least eight inherited neurodegenerative diseases caused by expansion of a polyglutamine tract in the disease protein. Here we present two lines of evidence implicating the ubiquitin-proteasome pathway in SCA3/MJD pathogenesis. First, studies of both human disease tissue and in vitro models showed redistribution of the 26S proteasome complex into polyglutamine aggregates. In neurons from SCA3/MJD brain, the proteasome localized to intranuclear inclusions containing the mutant protein, ataxin-3. In transfected cells, the proteasome redistributed into inclusions formed by three expanded polyglutamine proteins: a pathologic ataxin-3 fragment, full-length mutant ataxin-3 and an unrelated GFP-polyglutamine fusion protein. Inclusion formation by the full-length mutant ataxin-3 required nuclear localization of the protein and occurred within specific subnuclear structures recently implicated in the regulation of cell death, promyelocytic leukemia antigen oncogenic domains. In a second set of experiments, inhibitors of the proteasome caused a repeat length-dependent increase in aggregate formation, implying that the proteasome plays a direct role in suppressing polyglutamine aggregation in disease. These results support a central role for protein misfolding in the pathogenesis of SCA3/MJD and suggest that modulating proteasome activity is a potential approach to altering the progression of this and other polyglutamine diseases.

Recruitment and the role of nuclear localization in polyglutamine-mediated aggregation.

The inherited neurodegenerative diseases caused by an expanded glutamine repeat share the pathologic feature of intranuclear aggregates or inclusions (NI). Here in cell-based studies of the spinocerebellar ataxia type-3 disease protein, ataxin-3, we address two issues central to aggregation: the role of polyglutamine in recruiting proteins into NI and the role of nuclear localization in promoting aggregation. We demonstrate that full-length ataxin-3 is readily recruited from the cytoplasm into NI seeded either by a pathologic ataxin-3 fragment or by a second unrelated glutamine-repeat disease protein, ataxin-1. Experiments with green fluorescence protein/polyglutamine fusion proteins show that a glutamine repeat is sufficient to recruit an otherwise irrelevant protein into NI, and studies of human disease tissue and a Drosophila transgenic model provide evidence that specific glutamine-repeat-containing proteins, including TATA-binding protein and Eyes Absent protein, are recruited into NI in vivo. Finally, we show that nuclear localization promotes aggregation: an ataxin-3 fragment containing a nonpathologic repeat of 27 glutamines forms inclusions only when targeted to the nucleus. Our findings establish the importance of the polyglutamine domain in mediating recruitment and suggest that pathogenesis may be linked in part to the sequestering of glutamine-containing cellular proteins. In addition, we demonstrate that the nuclear environment may be critical for seeding polyglutamine aggregates.

A novel F box protein, NFB42, is highly enriched in neurons and induces growth arrest.

NFB42 (neural F Box 42 kDa) is a novel gene product that is highly enriched in the nervous system. Its predicted protein contains an F box, a motif recently shown to couple cell cycle regulation to the proteasome pathway (Bai, C., Sen, P., Hofmann, K., Ma, L., Goebl, M., Harper, J. W., and Elledge, S. (1996) Cell 86, 263-274). NFB42 mRNA and protein are expressed in all major areas of the adult rat brain but are not detected in non-neural tissues. NFB42 protein is localized primarily to the cytoplasm of neurons and does not appear to be present in glia. The presence of an F box in NFB42 suggests that it may be involved in cell cycle regulation; however, its expression in postmitotic neurons indicates that it is not involved in regulating typical cell cycle events. In an initial attempt to characterize the function of this protein, NFB42 was transfected into N1E-115 neuroblastoma and Chinese hamster ovary cells. The expression of full-length NFB42, but not an F box deletion mutant, inhibits proliferation in both cell lines. Additional experiments demonstrate that NFB42 interacts with Skp1p, a component of the proteasome pathway, and deletion of the F box also inhibits this interaction. Overall, the expression pattern of NFB42, along with the presence of an F box domain and the ability to inhibit growth, suggests that it may play a role in maintaining neurons in a postmitotic state.