Self-Assembly of Ultrasmall 3D Architectures of (l)-Acyclic Threoninol Nucleic Acids with High Thermal and Serum Stability.

The primary challenge of implementing DNA nanostructures in biomedical applications lies in their vulnerability to nuclease degradation and variations in ionic strength. Furthermore, the size minimization of DNA and RNA nanostructures is limited by the stability of the DNA and RNA duplexes. This study presents a solution to these problems through the use of acyclic (l)-threoninol nucleic acid (aTNA), an artificial acyclic nucleic acid, which offers enhanced resilience under physiological conditions. The high stability of homo aTNA duplexes enables the design of durable nanostructures with dimensions below 5 nm, previously unattainable due to the inherent instability of DNA structures. The assembly of a stable aTNA-based 3D cube and pyramid that involves an i-motif formation is demonstrated. In particular, the cube outperforms its DNA-based counterparts in terms of stability. We furthermore demonstrate the successful attachment of a nanobody to the aTNA cube using the favorable triplex formation of aTNA with ssDNA. The selective in vitro binding capability to human epidermal growth factor receptor 2 is demonstrated. The presented research presents the use of aTNA for the creation of smaller durable nanostructures for future medical applications. It also introduces a new method for attaching payloads to these structures, enhancing their utility in targeted therapies.

Site-specific nanobody-oligonucleotide conjugation for super-resolution imaging.

Camelid single-domain antibody fragments, also called nanobodies, constitute a class of binders that are small in size (~15 kDa) and possess antigen-binding properties similar to their antibody counterparts. Facile production of recombinant nanobodies in several microorganisms has made this class of binders attractive within the field of molecular imaging. Particularly, their use in super-resolution microscopy has improved the spatial resolution of molecular targets due to a smaller linkage error. In single-molecule localization microscopy techniques, the effective spatial resolution can be further enhanced by site-specific fluorescent labeling of nanobodies owing to a more homogeneous protein-to-fluorophore stoichiometry, reduced background staining and a known distance between dye and epitope. Here, we present a protocol for site-specific bioconjugation of DNA oligonucleotides to three distinct nanobodies expressed with an N- or C-terminal unnatural amino acid, 4-azido-L-phenylalanine (pAzF). Using copper-free click chemistry, the nanobody-oligonucleotide conjugation reactions were efficient and yielded highly pure bioconjugates. Target binding was retained in the bioconjugates, as demonstrated by bio-layer interferometry binding assays and the super-resolution microscopy technique, DNA points accumulation for imaging in nanoscale topography (PAINT). This method for site-specific protein-oligonucleotide conjugation can be further extended for applications within drug delivery and molecular targeting where site-specificity and stoichiometric control are required.

Functionalized Acyclic (l)-Threoninol Nucleic Acid Four-Way Junction with High Stability In Vitro and In Vivo.

Oligonucleotides are increasingly being used as a programmable connection material to assemble molecules and proteins in well-defined structures. For the application of such assemblies for in vivo diagnostics or therapeutics it is crucial that the oligonucleotides form highly stable, non-toxic, and non-immunogenic structures. Only few oligonucleotide derivatives fulfil all of these requirements. Here we report on the application of acyclic l-threoninol nucleic acid (aTNA) to form a four-way junction (4WJ) that is highly stable and enables facile assembly of components for in vivo treatment and imaging. The aTNA 4WJ is serum-stable, shows no non-targeted uptake or cytotoxicity, and invokes no innate immune response. As a proof of concept, we modify the 4WJ with a cancer-targeting and a serum half-life extension moiety and show the effect of these functionalized 4WJs in vitro and in vivo, respectively.