Newly identified transmembrane protein 106B amyloid fibrils in the human brain: pathogens or by-products?

Neurodegenerative diseases (NDs) such as Alzheimer’s disease (AD) and Parkinson’s disease (PD) constitute a spectrum of diseases characterized by the abnormal aggregation of specific amyloid fibrillar proteins; these include β-amyloid (Aβ) and tau in the form of the extracellular Aβ plaques and neuronal neurofibrillary tangles in AD and fibrillar α-synuclein aggregation in the form of Lewy bodies and Lewy neurites in PD. Transmembrane protein 106B (TMEM106B) is a type II transmembrane lysosomal protein that participates in lysosome morphology, localization, acidification, and trafficking; t is involved in the pathogenesis of several NDs, especially frontotemporal lobular degeneration with TAR DNA-binding protein immunoreactive inclusions (FTLD-TDP). Studies from four independent research groups revealed that the luminal domain of TMEM106B (120-254aa) forms amyloid fibrils in several brain regions in patients with a series of NDs and neurologically normal older adults. Given its potentially critical roles in the pathogenesis of NDs and brain aging, this surprising finding has focused attention on TMEM106B and suggested that it is nearly as fundamental as other pathogenic amyloid proteins (e.g., Aβ, tau, α-syn); nevertheless, new questions surrounding TMEM106B must be asked. In this review,we firstly introduce the physiological function of TMEM106B and its involvement in NDs. Then, we elucidate the identification and cryo-electronic microscopic structure of TMEM106B fibrils and analyze the factors that contribute to the polymorphism of TMEM106B fibrils. Finally, the potential pathogenic role of TMEM106B fibrils is discussed, and the future directions for TMEM106 research in NDs are briefly summarized.