Ever since the invention of SELEX (systematic evolution of ligands by exponential enrichment), there has been rapid development for aptamers over the last two decades, making them a promising approach in therapeutic applications as either drug candidates or diagnostic tools. For therapeutic purposes, a durable performance of aptamers in biofluids is required, which is, however, hampered by the lack of stability of most aptamers. Not only are the nucleic acid aptamers susceptible to nucleases, the peptide aptamers are also subjective to degradation by proteases. With the advancement of chemical biology, numerous attempts have been made to overcome this obstacle, many resulting in significant improvements in stability. In this review, chemical modifications to increase the stability of three main types of aptamers, DNA, RNA and peptide are comprehensively summarized. For nucleic acid aptamers, development of modified SELEX coupled with mutated polymerase is discussed, which is adaptive to a number of modifications in aptamers and in a large extent facilitates the research of aptamer-modifications. For peptide aptamers, approaches in molecular biology with introduction of stabilizing protein as well as the switch of scaffold protein are included, which may represent a future direction of chemical conjugations to aptamers.

Significance: Functional and pathological amyloid fibrils bind metal ions, but no metal-bound amyloid structures have been determined. Using solid-state NMR and structural bioinformatics, we have determined the oligomeric structure and coordination geometry of a Zn ²⁺ -mediated amyloid fibril that catalyzes ester hydrolysis. The peptide assembles into parallel β-sheets in which histidines bridge zinc ions to promote β-strand association in a geometry that mediates water activation for catalysis. The study demonstrates an approach for determining the structures of metalloamyloids. The resulting structure defines how metal ions can stabilize amyloids, lends support to the hypothesis that amyloids can serve as well-structured intermediates between amino acids and proteins during the evolution of life, and provides a framework for potential applications in material science.

Misfolding of the microtubule-binding protein tau into filamentous aggregates is characteristic of many neurodegenerative diseases such as Alzheimer’s disease and progressive supranuclear palsy. Determining the structures and dynamics of these tau fibrils is important for designing inhibitors against tau aggregation. Tau fibrils obtained from patient brains have been found by cryo-electron microscopy to adopt disease-specific molecular conformations. However, in vitro heparin-fibrillized 2N4R tau, which contains all four microtubule-binding repeats (4R), was recently found to adopt polymorphic structures. Here we use solid-state NMR spectroscopy to investigate the global fold and dynamics of heparin-fibrillized 0N4R tau. A single set of ¹³C and ¹⁵N chemical shifts was observed for residues in the four repeats, indicating a single β-sheet conformation for the fibril core. This rigid core spans the R2 and R3 repeats and adopts a hairpin-like fold that has similarities to but also clear differences from any of the polymorphic 2N4R folds. Obtaining a homogeneous fibril sample required careful purification of the protein and removal of any proteolytic fragments. A variety of experiments and polarization transfer from water and mobile side chains indicate that 0N4R tau fibrils exhibit heterogeneous dynamics: Outside the rigid R2–R3 core, the R1 and R4 repeats are semirigid even though they exhibit β-strand character and the proline-rich domains undergo large-amplitude anisotropic motions, whereas the two termini are nearly isotropically flexible. These results have significant implications for the structure and dynamics of 4R tau fibrils in vivo. Significance The microtubule-binding protein tau misfolds into filamentous aggregates in many neurodegenerative diseases. Understanding the structure and dynamics of tau fibrils is important for designing anti-tau inhibitors. Patient-brain-derived tau fibrils have been shown to adopt disease-specific molecular conformations, but in vitro heparin-fibrillized tau was recently proposed to exhibit significant structural polymorphism. Using solid-state NMR, we show that the β-sheet core of heparin-fibrillized 0N4R tau adopts a single molecular conformation that encompasses the second and third microtubule-binding repeats, and both cysteine residues in the protein are involved in side-chain packing with other β-sheet residues. Extensive characterization of the protein dynamics indicates that these tau fibrils are heterogeneously dynamic, and the first and fourth microtubule-binding repeats are semirigid while retaining β-sheet character.