Assembly of recombinant tau into filaments identical to those of Alzheimer's disease and chronic traumatic encephalopathy.

Abundant filamentous inclusions of tau are characteristic of more than 20 neurodegenerative diseases that are collectively termed tauopathies. Electron cryo-microscopy (cryo-EM) structures of tau amyloid filaments from human brain revealed that distinct tau folds characterise many different diseases. A lack of laboratory-based model systems to generate these structures has hampered efforts to uncover the molecular mechanisms that underlie tauopathies. Here, we report in vitro assembly conditions with recombinant tau that replicate the structures of filaments from both Alzheimer's disease (AD) and chronic traumatic encephalopathy (CTE), as determined by cryo-EM. Our results suggest that post-translational modifications of tau modulate filament assembly, and that previously observed additional densities in AD and CTE filaments may arise from the presence of inorganic salts, like phosphates and sodium chloride. In vitro assembly of tau into disease-relevant filaments will facilitate studies to determine their roles in different diseases, as well as the development of compounds that specifically bind to these structures or prevent their formation.

Many neurodegenerative diseases, including Alzheimer's disease, the most common form of dementia, are characterised by knotted clumps of a protein called tau. In these diseases, tau misfolds, stacks together and forms abnormal filaments, which have a structured core and fuzzy coat. These sticky, misfolded proteins are thought to be toxic to brain cells, the loss of which ultimately causes problems with how people move, think, feel or behave. Reconstructing the shape of tau filaments using an atomic-level imaging technique called electron cryo-microscopy, or cryo-EM, researchers have found distinct types of tau filaments present in certain diseases. In Alzheimer's disease, for example, a mixture of paired helical and straight filaments is found. Different tau filaments are seen again in chronic traumatic encephalopathy (CTE), a condition associated with repetitive brain trauma. It remains unclear, however, how tau folds into these distinct shapes and under what conditions it forms certain types of filaments. The role that distinct tau folds play in different diseases is also poorly understood. This is largely because researchers making tau proteins in the lab have yet to replicate the exact structure of tau filaments found in diseased brain tissue. Lövestam et al. describe the conditions for making tau filaments in the lab identical to those isolated from the brains of people who died from Alzheimer's disease and CTE. Lövestam et al. instructed bacteria to make tau protein, optimised filament assembly conditions, including shaking time and speed, and found that bona fide filaments formed from shortened versions of tau. On cryo-EM imaging, the lab-produced filaments had the same left-handed twist and helical symmetry as filaments characteristic of Alzheimer's disease. Adding salts, however, changed the shape of tau filaments. In the presence of sodium chloride, otherwise known as kitchen salt, tau formed filaments with a filled cavity at the core, identical to tau filaments observed in CTE. Again, this structure was confirmed on cryo-EM imaging. Being able to make tau filaments identical to those found in human tauopathies will allow scientists to study how these filaments form and elucidate what role they play in disease. Ultimately, a better understanding of tau filament formation could lead to improved diagnostics and treatments for neurodegenerative diseases involving tau.

2.4-Å structure of the double-ring Gemmatimonas phototrophica photosystem.

Phototrophic Gemmatimonadetes evolved the ability to use solar energy following horizontal transfer of photosynthesis-related genes from an ancient phototrophic proteobacterium. The electron cryo-microscopy structure of the Gemmatimonas phototrophica photosystem at 2.4 Å reveals a unique, double-ring complex. Two unique membrane-extrinsic polypeptides, RC-S and RC-U, hold the central type 2 reaction center (RC) within an inner 16-subunit light-harvesting 1 (LH1) ring, which is encircled by an outer 24-subunit antenna ring (LHh) that adds light-gathering capacity. Femtosecond kinetics reveal the flow of energy within the RC-dLH complex, from the outer LHh ring to LH1 and then to the RC. This structural and functional study shows that G. phototrophica has independently evolved its own compact, robust, and highly effective architecture for harvesting and trapping solar energy.

Time-lapse microscopic observation of non-dividing cells in cultured human osteosarcoma MG-63 cell line.

Cancer stem cells resemble normal tissue-specific stem cells in many aspects, such as self-renewal and plasticity. Like their non-malignant counterparts, cancer stem cells are suggested to exhibit a relative quiescence. The established cancer cell lines reportedly harbor slow-proliferating cells that are positive for some cancer stem cells markers. However, the fate of these cells and their progeny remains unknown. We used time-lapse microscopy and the contrast-based segmentation algorithm to identify and monitor actively dividing and non-dividing cells in human osteosarcoma MG-63 cell line. Within the monitored field of view the non-dividing cells were represented by three cells that never divided, and one cell that attempted to divide, but failed cytokinesis, and later, after significantly prolonged division, produced the progeny with enlarged segmented nuclei, thus pointing to a possible mitotic catastrophe. Together, these cells initially constituted about 6.2% of the total number of seeded cells, yet only 0.02% of all cells at the end of the observation period when cells became confluent. Non-dividing cells were characterized by rounded shape, dark nuclei, random cytoplasmic streaming and subtle oscillatory movement, however, they did not migrate and rarely formed cell-cell contacts as compared to actively dividing cells. Our data indicate that the observed non-dividing MG-63 cells do not have a growth advantage over other cells and, therefore, they do not contribute to the cancer stem cells pool.