Expanded polyglutamine (polyQ) repeats cause neurodegenerative disorders, but their cytotoxic structures remain to be elucidated. Although soluble polyQ oligomers have been proposed as a cytotoxic structure, the cytotoxicity of soluble polyQ oligomers, not inclusion bodies (IBs), has not been proven in living cells. To clarify the cytotoxicity of soluble polyQ oligomers, we carried our fluorescence resonance energy transfer (FRET) confocal microscopy and distinguished oligomers from monomers and IBs in a single living cell. FRET signals were detected when donor and acceptor fluorescent proteins were attached to the same side, not the opposite side, of polyQ repeats, which agrees with a parallel beta-sheet or a head-to-tail cylindrical beta-sheet model. These FRET signals disappeared in semi-intact cells, indicating that these polyQ oligomers are soluble. PolyQ monomers assembled into soluble oligomers in a length-dependent manner, which was followed by the formation of IBs. Notably, survival assay of neuronally differentiated cells revealed that cells with soluble oligomers died faster than those with IBs or monomers. These results indicate that a length-dependent formation of oligomers is an essential mechanism underlying neurodegeneration in polyQ-mediated disorders.

Although the genetic basis of polyglutamine diseases has been recognized for 20 years, their molecular basis is still unclear. We have no therapeutic strategies for these intractable neurodegenerative disorders. To adequately treat patients, we must clarify the molecular basis of polyglutamine diseases. Three main issues address their molecular pathogenesis: whether the specific structure of expanded polyglutamine diseases results in cellular toxicity; what type of dysfunction causes them; and how the toxic structure causes dysfunction, that is, the link between structure and dysfunction. For structures, expanded polyglutamine proteins undergo transformation from monomers to oligomers and inclusions. One can hypothesize that one of these structures might cause the polyglutamine disease. Although the expanded polyglutamine protein is toxic, it does not explain the selective vulnerability of specific neurons in each polyglutamine disease. The normal function of each protein, including protein-protein interaction and modification, might also be crucial for pathogenesis. For dysfunction, various molecular mechanisms have been proposed, including dysregulation of transcription, impairment of the ubiquitin-proteasome system, mitochondrial dysfunction, dysregulation of intracellular Ca(2+) homeostasis, impairment of axonal transport and genotoxic stress. These hypotheses might correlate with each other. In addition, the disease pathogenesis of might not be exclusive to one particular structure or dysfunction. To develop a therapeutic strategy for patients with polyglutamine disease, identifying the most toxic structure and the earliest event in the pathogenesis is important. We review the current understanding of the toxic structure and dysfunction by expanded polyglutamine proteins and suggest directions for future studies of polyglutamine diseases.

Cerebral small-vessel disease is a common disorder in elderly populations; however, its molecular basis is not well understood. We recently demonstrated that mutations in the high-temperature requirement A (HTRA) serine peptidase 1 (HTRA1) gene cause a hereditary cerebral small-vessel disease, cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL). HTRA1 belongs to the HTRA protein family, whose members have dual activities as chaperones and serine proteases and also repress transforming growth factor-β (TGF-β) family signaling. We demonstrated that CARASIL-associated mutant HTRA1s decrease protease activity and fail to decrease TGF-β family signaling. However, the precise molecular mechanism for decreasing the signaling remains unknown. Here we show that increased expression of ED-A fibronectin is limited to cerebral small arteries and is not observed in coronary, renal arterial or aortic walls in patients with CARASIL. Using a cell-mixing assay, we found that HTRA1 decreases TGF-β1 signaling triggered by proTGF-β1 in the intracellular space. HTRA1 binds and cleaves the pro-domain of proTGF-β1 in the endoplasmic reticulum (ER), and cleaved proTGF-β1 is degraded by ER-associated degradation. Consequently, the amount of mature TGF-β1 is reduced. These results establish a novel mechanism for regulating the amount of TGF-β1, specifically, the intracellular cleavage of proTGF-β1 in the ER.

Objective: To report a novel autoimmune encephalitis in which the antibodies target neurexin-3a, a cell adhesion molecule involved in the development and function of synapses.Methods: Five patients with encephalitis and antibodies with a similar pattern of brain reactivity were selected. Antigen precipitation and determination of antibody effects on cultured rat embryonic neurons were performed with reported techniques.Results: Immunoprecipitation and cell-based assays identified neurexin-3a as the autoantigen of patients' antibodies. All 5 patients (median age 44 years, range 23-50; 4 female) presented with prodromal fever, headache, or gastrointestinal symptoms, followed by confusion, seizures, and decreased level of consciousness. Two developed mild orofacial dyskinesias, 3 needed respiratory support, and 4 had findings suggesting propensity to autoimmunity. CSF was abnormal in all patients (4 pleocytosis, 1 elevated immunoglobulin G [IgG] index), and brain MRI was abnormal in 1 (increased fluid-attenuated inversion recovery/T2 in temporal lobes). All received steroids, 1 IV immunoglobulin, and 1 cyclophosphamide; 3 partially recovered, 1 died of sepsis while recovering, and 1 had a rapid progression to death. At autopsy, edema but no inflammatory cells were identified. Cultures of neurons exposed during days in vitro (div) 7-17 to patients' IgG showed a decrease of neurexin-3a clusters as well as the total number of synapses. No reduction of synapses occurred in mature neurons (div 18) exposed for 48 hours to patients' IgG. Neuronal survival, dendritic morphology, and spine density were unaffected. Conclusion:Neurexin-3a autoantibodies associate with a severe but potentially treatable encephalitis in which the antibodies cause a decrease of neurexin-3a and alter synapse development. Encephalitis is a severe inflammatory disorder of the brain with many possible causes and a complex differential diagnosis. Studies from different countries and a recent meta-analysis showed that in about 40% of patients with encephalitis the cause is never identified.1,2 Without reliable biomarkers, a response to empiric immunotherapy is frequently used to support that the disorder is immune-mediated, but a lack of response does not rule out an immune-mediated pathogenesis. For example, approximately 40% of patients with anti-NMDA receptor (NMDAR) encephalitis fail first-line immunotherapy (steroids, plasma exchange, or IV immunoglobulin [IVIg]) and require second-line therapies (rituximab or cyclophosphamide).3,4 However, second-line therapies are rarely used in encephalitis of unclear cause unless evidence of autoimmunity is provided. In this setting, the demonstration of autoantibodies to neuronal cell