Reduced SNAP-25 increases PSD-95 mobility and impairs spine morphogenesis.

Impairment of synaptic function can lead to neuropsychiatric disorders collectively referred to as synaptopathies. The SNARE protein SNAP-25 is implicated in several brain pathologies and, indeed, brain areas of psychiatric patients often display reduced SNAP-25 expression. It has been recently found that acute downregulation of SNAP-25 in brain slices impairs long-term potentiation; however, the processes through which this occurs are still poorly defined. We show that in vivo acute downregulation of SNAP-25 in CA1 hippocampal region affects spine number. Consistently, hippocampal neurons from SNAP-25 heterozygous mice show reduced densities of dendritic spines and defective PSD-95 dynamics. Finally, we show that, in brain, SNAP-25 is part of a molecular complex including PSD-95 and p140Cap, with p140Cap being capable to bind to both SNAP-25 and PSD-95. These data demonstrate an unexpected role of SNAP-25 in controlling PSD-95 clustering and open the possibility that genetic reductions of the protein levels - as occurring in schizophrenia - may contribute to the pathology through an effect on postsynaptic function and plasticity.

In utero gene therapy rescues microcephaly caused by Pqbp1-hypofunction in neural stem progenitor cells.

Human mutations in PQBP1, a molecule involved in transcription and splicing, result in a reduced but architecturally normal brain. Examination of a conditional Pqbp1-knockout (cKO) mouse with microcephaly failed to reveal either abnormal centrosomes or mitotic spindles, increased neurogenesis from the neural stem progenitor cell (NSPC) pool or increased cell death in vivo. Instead, we observed an increase in the length of the cell cycle, particularly for the M phase in NSPCs. Corresponding to the developmental expression of Pqbp1, the stem cell pool in vivo was decreased at E10 and remained at a low level during neurogenesis (E15) in Pqbp1-cKO mice. The expression profiles of NSPCs derived from the cKO mouse revealed significant changes in gene groups that control the M phase, including anaphase-promoting complex genes, via aberrant transcription and RNA splicing. Exogenous Apc4, a hub protein in the network of affected genes, recovered the cell cycle, proliferation, and cell phenotypes of NSPCs caused by Pqbp1-cKO. These data reveal a mechanism of brain size control based on the simple reduction of the NSPC pool by cell cycle time elongation. Finally, we demonstrated that in utero gene therapy for Pqbp1-cKO mice by intraperitoneal injection of the PQBP1-AAV vector at E10 successfully rescued microcephaly with preserved cortical structures and improved behavioral abnormalities in Pqbp1-cKO mice, opening a new strategy for treating this intractable developmental disorder.

The X-chromosome-linked intellectual disability protein PQBP1 is a component of neuronal RNA granules and regulates the appearance of stress granules.

The polyglutamine-binding protein 1 (PQBP1) has been linked to several X-linked intellectual disability disorders and progressive neurodegenerative diseases. While it is currently known that PQBP1 localizes in nuclear speckles and is engaged in transcription and splicing, we have now identified a cytoplasmic pool of PQBP1. Analysis of PQBP1 complexes revealed six novel interacting proteins, namely the RNA-binding proteins KSRP, SFPQ/PSF, DDX1 and Caprin-1, and two subunits of the intracellular transport-related dynactin complex, p150(Glued) and p27. PQBP1 protein complex formation is dependent on the presence of RNA. Immunofluorescence studies revealed that in primary neurons, PQBP1 co-localizes with its interaction partners in specific cytoplasmic granules, which stained positive for RNA. Our results suggest that PQBP1 plays a role in cytoplasmic mRNA metabolism. This is further supported by the partial co-localization and interaction of PQBP1 with the fragile X mental retardation protein (FMRP), which is one of the best-studied proteins found in RNA granules. In further studies, we show that arsenite-induced oxidative stress caused relocalization of PQBP1 to stress granules (SGs), where PQBP1 co-localizes with the new binding partners as well as with FMRP. Additional results indicated that the cellular distribution of PQBP1 plays a role in SG assembly. Together these data demonstrate a role for PQBP1 in the modulation of SGs and suggest its involvement in the transport of neuronal RNA granules, which are of critical importance for the development and maintenance of neuronal networks, thus illuminating a route by which PQBP1 aberrations might influence cognitive function.

Huntingtin interacts with the receptor sorting family protein GASP2.

Protein interaction networks are useful resources for the functional annotation of proteins. Recently, we have generated a highly connected protein-protein interaction network for Huntington's disease (HD) by automated yeast two-hybrid (Y2H) screening (Goehler et al., 2004). The network included several novel direct interaction partners for the disease protein huntingtin (htt). Some of these interactions, however, have not been validated by independent methods. Here we describe the verification of the interaction between htt and GASP2 (G protein-coupled receptor associated sorting protein 2), a protein involved in membrane receptor degradation. Using membrane-based and classical coimmunoprecipitation assays we demonstrate that htt and GASP2 form a complex in cotransfected mammalian cells. Moreover, we show that the two proteins colocalize in SH-SY5Y cells, raising the possibility that htt and GASP2 interact in neurons. As the GASP protein family plays a role in G protein-coupled receptor sorting, our data suggest that htt might influence receptor trafficking via the interaction with GASP2.

Parkin interacts with the proteasome subunit alpha4.

Mutations in the parkin gene encoding an E3 ligase are responsible for autosomal recessive Parkinson's disease. Putative parkin substrates and interacting partners have been identified, but the molecular mechanism underlying parkin-related neurodegeneration is still unclear. We have identified the 20S proteasomal subunit alpha4 (synonyms: PSMA7, XAPC7, subunit alpha type 7) as a new interacting partner of parkin. The C-terminal IBR-RING domain of parkin and the C-terminal part of alpha4 were essential for the interaction. Biochemical studies revealed that alpha4 was not a substrate for parkin-dependent ubiquitylation. Putative functions of the interaction might therefore be substrate presentation to the proteasome or regulation of proteasomal activity. Full-length parkin and parkin lacking the N-terminal ubiquitin-like domain slightly increased the proteasomal activity in HEK 293T cells, in line with the latter hypothesis.

Aggregation of proteins with expanded glutamine and alanine repeats of the glutamine-rich and asparagine-rich domains of Sup35 and of the amyloid beta-peptide of amyloid plaques.

The exon-1 peptide of huntingtin has 51 Gln repeats and produces the symptoms of Huntington's disease in transgenic mice. Aggregation of the yeast Sup35 protein into prions has been attributed to its glutamine-rich and asparagine-rich domain. Here, we show that poly-L-asparagine forms polar zippers similar to those of poly-L-glutamine. In solution at acid pH, the glutamine-rich and asparagine-rich 18-residue Sup35 peptide, rendered soluble by the addition of two aspartates at the amino end and two lysines at the carboxyl end, gives a beta-sheet CD spectrum; it aggregates at neutral pH. A poly-alanine peptide D(2)A(10)K(2) gives an alpha-helical CD spectrum at all pHs and does not aggregate; a peptide with the sequence of the C-terminal helix of the alpha-chain of human hemoglobin, preceded by two aspartates and followed by two lysines, exhibits a random coil spectrum and does not aggregate either. Alignment of several beta-strands with the sequence of the 42-residue Alzheimer's amyloid beta-peptide shows that they can be linked together by a network of salt bridges. We also asked why single amino acid replacements can so destabilize the native structures of proteins that they unfold and form amyloids. The difference in free energy of a protein molecule between its native, fully ordered structure and an amorphous mixture of randomly coiled chains is only of the order of 10 kcal/mol. Theory shows that destabilization of the native structure by no more than 2 kcal/mol can increase the probability of nucleation of disordered aggregates from which amyloids could grow 130,000-fold.

Centrosome disorganization in fibroblast cultures derived from R6/2 Huntington's disease (HD) transgenic mice and HD patients.

Huntington's disease (HD) is a progressive neurological disorder caused by a CAG/polyglutamine repeat expansion. We have previously generated the R6/2 mouse model that expresses exon 1 of the human HD gene containing CAG repeats in excess of 150. These mice develop a progressive neurological phenotype with a rapid onset and progression. We show here that it is impossible to establish fibroblast lines from these mice at 12 weeks of age, whilst this can be achieved without difficulty at 6 and 9 weeks. Cultures derived from mice at 12 weeks contained a high frequency of dysmorphic cells, including cells with an aberrant nuclear morphology and a high frequency of micronuclei and large vacuoles. All of these features were also present in a line derived from a juvenile HD patient. Fibroblast lines derived from R6/2 mice and from HD patients were found to have a high frequency of multiple centrosomes which could account for all of the observed phenotypes including a reduced mitotic index, high frequency of aneuploidy and persistence of the midbody. We were unable to detect large insoluble polyglutamine aggregates in either the mouse or human lines. We have identified a novel progressive HD pathology that occurs in cells of non-central nervous system origin. An investigation of the pathological consequences of the HD mutation in these cells will provide insight into cellular basis of the disease.

The huntingtin interacting protein HIP1 is a clathrin and alpha-adaptin-binding protein involved in receptor-mediated endocytosis.

The huntingtin interacting protein (HIP1) is enriched in membrane-containing cell fractions and has been implicated in vesicle trafficking. It is a multidomain protein containing an N-terminal ENTH domain, a central coiled-coil forming region and a C-terminal actin-binding domain. In the present study we have identified three HIP1 associated proteins, clathrin heavy chain and alpha-adaptin A and C. In vitro binding studies revealed that the central coiled-coil domain is required for the interaction of HIP1 with clathrin, whereas DPF-like motifs located upstream to this domain are important for the binding of HIP1 to the C-terminal 'appendage' domain of alpha-adaptin A and C. Expression of full length HIP1 in mammalian cells resulted in a punctate cytoplasmic immunostaining characteristic of clathrin-coated vesicles. In contrast, when a truncated HIP1 protein containing both the DPF-like motifs and the coiled-coil domain was overexpressed, large perinuclear vesicle-like structures containing HIP1, huntingtin, clathrin and endocytosed transferrin were observed, indicating that HIP1 is an endocytic protein, the structural integrity of which is crucial for maintenance of normal vesicle size in vivo.

Geldanamycin activates a heat shock response and inhibits huntingtin aggregation in a cell culture model of Huntington's disease.

Huntington's disease (HD) is a progressive neurodegenerative disorder with no effective treatment. Geldanamycin is a benzoquinone ansamycin that binds to the heat shock protein Hsp90 and activates a heat shock response in mammalian cells. In this study, we show by using a filter retardation assay and immunofluorescence microscopy that treatment of mammalian cells with geldanamycin at nanomolar concentrations induces the expression of Hsp40, Hsp70 and Hsp90 and inhibits HD exon 1 protein aggregation in a dose-dependent manner. Similar results were obtained by overexpression of Hsp70 and Hsp40 in a separate cell culture model of HD. This is the first demonstration that huntingtin protein aggregation in cells can be suppressed by chemical compounds activating a specific heat shock response. These findings may provide the basis for the development of a novel pharmacotherapy for HD and related glutamine repeat disorders.

Accumulation of mutant huntingtin fragments in aggresome-like inclusion bodies as a result of insufficient protein degradation.

The huntingtin exon 1 proteins with a polyglutamine repeat in the pathological range (51 or 83 glutamines), but not with a polyglutamine tract in the normal range (20 glutamines), form aggresome-like perinuclear inclusions in human 293 Tet-Off cells. These structures contain aggregated, ubiquitinated huntingtin exon 1 protein with a characteristic fibrillar morphology. Inclusion bodies with truncated huntingtin protein are formed at centrosomes and are surrounded by vimentin filaments. Inhibition of proteasome activity resulted in a twofold increase in the amount of ubiquitinated, SDS-resistant aggregates, indicating that inclusion bodies accumulate when the capacity of the ubiquitin-proteasome system to degrade aggregation-prone huntingtin protein is exhausted. Immunofluorescence and electron microscopy with immunogold labeling revealed that the 20S, 19S, and 11S subunits of the 26S proteasome, the molecular chaperones BiP/GRP78, Hsp70, and Hsp40, as well as the RNA-binding protein TIA-1, the potential chaperone 14-3-3, and alpha-synuclein colocalize with the perinuclear inclusions. In 293 Tet-Off cells, inclusion body formation also resulted in cell toxicity and dramatic ultrastructural changes such as indentations and disruption of the nuclear envelope. Concentration of mitochondria around the inclusions and cytoplasmic vacuolation were also observed. Together these findings support the hypothesis that the ATP-dependent ubiquitin-proteasome system is a potential target for therapeutic interventions in glutamine repeat disorders.

Huntington's disease: from gene to potential therapy.

Huntington's disease (HD) is a progressive, late-onset neurodegenerative illness with autosomal dominant inheritance that affects one in 10 000 individuals in Western Europe. The disease is caused by a polyglutamine repeat expansion located in the N-terminal region of the huntingtin protein. The mutation is likely to act by a gain of function, but the molecular mechanisms by which it leads to neuronal dysfunction and cell death are not yet known. The normal function of huntingtin in cell metabolism is also unclear. There is no therapy for HD. Research on HD should help elucidate the pathogenetic mechanism of this illness in order to develop successful treatments to prevent or slow down symptoms. This article presents new results in HD research focusing on in vivo and in vitro model systems, potential molecular mechanisms of HD, and the development of therapeutic strategies.

Protein aggregation and pathogenesis of Huntington's disease: mechanisms and correlations.

The formation of insoluble protein aggregates is a hallmark of Huntington's disease (HD) and related neurodegenerative disorders, such as dentatorubral pallidoluysian atrophy (DRPLA), spinal bulbar muscular atrophy (SBMA) and the spinocerebellar ataxia (SCA) type 1, 2, 3, 6 and 7. These disorders are caused by an expanded polyglutamine (polyQ) tract in otherwise unrelated proteins. They are characterized by late-onset, selective neuropathology, a pathogenic polyQ threshold and a relationship between polyQ length and disease progression. Thus, molecular models of HD and related glutamine-repeat disorders must account for these characteristic features. During the last three years, considerable effort has been invested in the development of in vitro and in vivo model systems to study the mechanisms of protein aggregation in glutamine-repeat disorders and its potential effects on disease progression and neurodegeneration. A selection of these studies is reviewed here. Furthermore, the correlation between aggregate formation and development of HD is discussed.

Protein aggregation in Huntington's and Parkinson's disease: implications for therapy.

The accumulation of highly insoluble intracellular protein aggregates in neuronal inclusions is a hallmark of Huntington's disease (HD) and Parkinson's disease (PD) as well as several other late-onset neurodegenerative disorders. The aggregates formed in vitro and in vivo generally have a fibrillar morphology, consist of individual beta-strands and are resistant to proteolytic degradation. Although the causal relationship between aggregate formation and disease remains to be proven, the gradual deposition of mutant protein in neurons is consistent with the late-onset and progressive nature of symptoms. Recently, circumstantial evidence from mouse and Drosophila model systems suggests that abnormal protein folding and aggregation play a key role in the pathogenesis of both HD and PD. Therefore, a detailed understanding of the molecular mechanisms of protein aggregation and its effects on neuronal cell death could open new opportunities for therapy.

Inhibition of huntingtin fibrillogenesis by specific antibodies and small molecules: implications for Huntington's disease therapy.

The accumulation of insoluble protein aggregates in intra and perinuclear inclusions is a hallmark of Huntington's disease (HD) and related glutamine-repeat disorders. A central question is whether protein aggregation plays a direct role in the pathogenesis of these neurodegenerative diseases. Here we show by using a filter retardation assay that the mAb 1C2, which specifically recognizes the elongated polyglutamine (polyQ) stretch in huntingtin, and the chemical compounds Congo red, thioflavine S, chrysamine G, and Direct fast yellow inhibit HD exon 1 protein aggregation in a dose-dependent manner. On the other hand, potential inhibitors of amyloid-beta formation such as thioflavine T, gossypol, melatonin, and rifampicin had little or no inhibitory effect on huntingtin aggregation in vitro. The results obtained by the filtration assay were confirmed by electron microscopy, SDS/PAGE, and MS. Furthermore, cell culture studies revealed that the Congo red dye at micromolar concentrations reduced the extent of HD exon 1 aggregation in transiently transfected COS cells. Together, these findings contribute to a better understanding of the mechanism of huntingtin fibrillogenesis in vitro and provide the basis for the development of new huntingtin aggregation inhibitors that may be effective in treating HD.

Hsp70 and hsp40 chaperones can inhibit self-assembly of polyglutamine proteins into amyloid-like fibrils.

The deposition of protein aggregates in neurons is a hallmark of neurodegenerative diseases caused by polyglutamine (polyQ) proteins. We analyzed the effects of the heat shock protein (Hsp) 70 chaperone system on the aggregation of fragments of huntingtin (htt) with expanded polyQ tracts. In vitro, Hsp70 and its cochaperone Hsp40 suppressed the assembly of htt into detergent-insoluble amyloid-like fibrils in an ATP-dependent manner and caused the formation of amorphous, detergent-soluble aggregates. The chaperones were most active in preventing fibrillization when added during the lag phase of the polymerization reaction. Similarly, coexpression of Hsp70 or Hsp40 with htt in yeast inhibited the formation of large, detergent-insoluble polyQ aggregates, resulting in the accumulation of detergent-soluble inclusions. Thus, the recently established potency of Hsp70 and Hsp40 to repress polyQ-induced neurodegeneration may be based on the ability of these chaperones to shield toxic forms of polyQ proteins and to direct them into nontoxic aggregates.

The Huntington's disease protein interacts with p53 and CREB-binding protein and represses transcription.

Huntington's Disease (HD) is caused by an expansion of a polyglutamine tract within the huntingtin (htt) protein. Pathogenesis in HD appears to include the cytoplasmic cleavage of htt and release of an amino-terminal fragment capable of nuclear localization. We have investigated potential consequences to nuclear function of a pathogenic amino-terminal region of htt (httex1p) including aggregation, protein-protein interactions, and transcription. httex1p was found to coaggregate with p53 in inclusions generated in cell culture and to interact with p53 in vitro and in cell culture. Expanded httex1p represses transcription of the p53-regulated promoters, p21(WAF1/CIP1) and MDR-1. httex1p was also found to interact in vitro with CREB-binding protein (CBP) and mSin3a, and CBP to localize to neuronal intranuclear inclusions in a transgenic mouse model of HD. These results raise the possibility that expanded repeat htt causes aberrant transcriptional regulation through its interaction with cellular transcription factors which may result in neuronal dysfunction and cell death in HD.

Aggregation of truncated GST-HD exon 1 fusion proteins containing normal range and expanded glutamine repeats.

We have shown previously by electron microscopy that the purified glutathione S-transferase (GST)-Huntington's disease (HD) exon 1 fusion protein with 51 glutamine residues (GST-HD51) is an oligomer, and that site-specific proteolytic cleavage of this fusion protein results in the formation of insoluble more highly ordered protein aggregates with a fibrillar or ribbon-like morphology (E. Scherzinger et al. (1997) Cell 90, 549-558). Here we report that a truncated GST HD exon 1 fusion protein with 51 glutamine residues, which lacks the proline-rich region C-terminal to the polyglutamine (polyQ) tract (GST-HD51 delta P) self-aggregates into high-molecular-mass protein aggregates without prior proteolytic cleavage. Electron micrographs of these protein aggregates revealed thread-like fibrils with a uniform diameter of ca. 25 nm. In contrast, proteolytic cleavage of GST-HD51 delta P resulted in the formation of numerous clusters of high-molecular-mass fibrils with a different, ribbon-like morphology. These structures were reminiscent of prion rods and beta-amyloid fibrils in Alzheimer's disease. In agreement with our previous results with full-length GST-HD exon 1, the truncated fusion proteins GST-HD20 delta P and GST-HD30 delta P did not show any tendency to form more highly ordered structures, either with or without protease treatment.

Self-assembly of polyglutamine-containing huntingtin fragments into amyloid-like fibrils: implications for Huntington's disease pathology.

Huntington's disease is a progressive neurodegenerative disorder caused by a polyglutamine [poly(Q)] repeat expansion in the first exon of the huntingtin protein. Previously, we showed that N-terminal huntingtin peptides with poly(Q) tracts in the pathological range (51-122 glutamines), but not with poly(Q) tracts in the normal range (20 and 30 glutamines), form high molecular weight protein aggregates with a fibrillar or ribbon-like morphology, reminiscent of scrapie prion rods and beta-amyloid fibrils in Alzheimer's disease. Here we report that the formation of amyloid-like huntingtin aggregates in vitro not only depends on poly(Q) repeat length but also critically depends on protein concentration and time. Furthermore, the in vitro aggregation of huntingtin can be seeded by preformed fibrils. Together, these results suggest that amyloid fibrillogenesis in Huntington's disease, like in Alzheimer's disease, is a nucleation-dependent polymerization.