[Background and Purpose] UBQLN2 was recently identified as a causative gene of amyotrophic lateral sclerosis (ALS) / frontotemporal dementia (FTD). UBQLN2 accumulates in the protein inclusions observed in the brains and spinal cords of ALS/FTD patients with or without UBQLN2 mutation as well as disease-specific protein inclusions of various other neurodegenerative diseases. UBQLN2 contains ubiquitin-like (UBL) domain, ubiquitin-associated (UBA) domain and heat-shock-chaperonin binding motifs (STI1), which suggest the involvement of UBQLN2 in multiple processes of protein quality control system. In addition, UBQLN2 contains PXX domain of unknown function. Intriguingly, all the highly-penetrant UBQLN2 mutations identified so far exist in this PXX domain. We took multiple complementary approaches to elucidate the pathomechanism of UBQLN2-related neurodegeneration.
[Methods] We performed a mass spectrometry analysis to identify candidate proteins that interact differently with wild type UBQLN2 and disease-related mutant UBQLN2. Coimmunoprecipitation experiments were performed for several proteins of interest to confirm their difference in the binding property to wild type UBQLN2 and to mutant UBQLN2. Immunocytochemistry experiments were performed to analyze the effect of mutant UBQLN2 in cultured cell lines. We also generated transgenic (Tg) mice lines which express either wild type or mutant UBQLN2 under a brain-specific promoter and analyzed their pathology.
[Results] Several proteins were confirmed to bind differently to wild type UBQLN2 and disease-related mutant UBQLN2. Overexpression of mutant UBQLN2 but not wild type UBQLN2 in cultured cells led to abnormal Golgi morphology, which is a shared pathology with that observed in the spinal cords of ALS/FTD patients. Tg mice expressing mutant UBQLN2 exhibited ubiquitin-positive inclusions in the neuronal cell bodies and processes.
[Conclusion] Mutant UBQLN2 induces abnormal Golgi morphology which could lead to abnormal protein accumulation observed in ALS/FTD.