Designed Regular Tetragon-Shaped RNA-Protein Complexes with Ribosomal Protein L1 for Bionanotechnology and Synthetic Biology

Nucleic acid nanoassembly-enhanced RNA therapeutics and diagnosis

RNAs are involved in the crucial processes of disease progression and have emerged as powerful therapeutic targets and diagnostic biomarkers. However, efficient delivery of therapeutic RNA to the targeted location and precise detection of RNA markers remains challenging. Recently, more and more attention has been paid to applying nucleic acid nanoassemblies in diagnosing and treating. Due to the flexibility and deformability of nucleic acids, the nanoassemblies could be fabricated with different shapes and structures. With hybridization, nucleic acid nanoassemblies, including DNA and RNA nanostructures, can be applied to enhance RNA therapeutics and diagnosis. This review briefly introduces the construction and properties of different nucleic acid nanoassemblies and their applications for RNA therapy and diagnosis and makes further prospects for their development.

Self-Assembling Nucleic Acid Nanostructures Functionalized with Aptamers

Researchers have worked for many decades to master the rules of biomolecular design that would allow artificial biopolymer complexes to self-assemble and function similarly to the diverse biochemical constructs displayed in natural biological systems. The rules of nucleic acid assembly (dominated by Watson-Crick base-pairing) have been less difficult to understand and manipulate than the more complicated rules of protein folding. Therefore, nucleic acid nanotechnology has advanced more quickly than de novo protein design, and recent years have seen amazing progress in DNA and RNA design. By combining structural motifs with aptamers that act as affinity handles and add powerful molecular recognition capabilities, nucleic acid-based self-assemblies represent a diverse toolbox for use by bioengineers to create molecules with potentially revolutionary biological activities. In this review, we focus on the development of self-assembling nucleic acid nanostructures that are functionalized with nucleic acid aptamers and their great potential in wide ranging application areas.

The kink-turn in the structural biology of RNA

The kink-turn (k-turn) is a widespread structural motif found in functional RNA species. It typically comprises a three-nucleotide bulge followed by tandem trans sugar edge-Hoogsteen G:A base pairs. It introduces a sharp kink into the axis of duplex RNA, juxtaposing the minor grooves. Cross-strand H-bonds form at the interface, accepted by the conserved adenine nucleobases of the G:A basepairs. Alternative acceptors for one of these divides the k-turns into two conformational classes N3 and N1. The base pair that follows the G:A pairs (3b:3n) determines which conformation is adopted by a given k-turn. k-turns often mediate tertiary contacts in folded RNA species and frequently bind proteins. Common k-turn binding proteins include members of the L7Ae family, such as the human 15·5k protein. A recognition helix within these proteins binds in the widened major groove on the outside of the k-turn, that makes specific H-bonds with the conserved guanine nucleobases of the G:A pairs. L7Ae binds with extremely high affinity, and single-molecule data are consistent with folding by conformational selection. The standard, simple k-turn can be elaborated in a variety of ways, that include the complex k-turns and the k-junctions. In free solution in the absence of added metal ions or protein k-turns do not adopt the tightly-kinked conformation. They undergo folding by the binding of proteins, by the formation of tertiary contacts, and some (but not all) will fold on the addition of metal ions. Whether or not folding occurs in the presence of metal ions depends on local sequence, including the 3b:3n position, and the -1b:-1n position (5' to the bulge). In most cases -1b:-1n = C:G, so that the 3b:3n position is critical since it determines both folding properties and conformation. In general, the selection of these sequence matches a given k-turn to its biological requirements. The k-turn structure is now very well understood, to the point at which they can be used as a building block for the formation of RNA nano-objects, including triangles and squares.

Self-Assembling RNA Nanoparticle for Gene Expression Regulation in a Model System

In the search for enzymatically processed RNA fragments, we found the novel three-way junction motif. The structure prediction suggested the arrangement of helices at acute angle approx. 60°. This allows the design of a trimeric RNA nanoparticle that can be functionalized with multiple regulatory fragments. Such RNA nano-object of equilateral triangular shape was applied for gene expression regulation studies in two independent cellular systems. Biochemical and functional studies confirmed the predicted shape and structure of the nanoparticle. The regulatory siRNA fragments incorporated into the nanoparticle were effectively released and triggered gene silencing. The regulatory effect was prolonged when induced with structuralized RNA compared to unstructured siRNAs. In these studies, the enzymatic processing of the motif was utilized for function release from the nanoparticle, enabling simultaneous delivery of different regulatory functions. This methodology of sequence search, RNA structural prediction, and application for rational design opens a new way for creating enzymatically processed RNA nanoparticles.

RNA nanostructures and scaffolds for biotechnology applications

RNA plays important roles in the regulation of gene expressions and other cellular functions. It functions as both as an informational carrier and a nanomachine due to its complementary base-pairing ability and complexed three-dimensional structure. Several nanostructures have been designed and constructed by exploiting these natural RNA properties. In this review, we will introduce the design principles of RNA nanostructures and their biotechnology applications as molecular scaffolds. RNA-based molecular scaffolds can control the accumulation and interaction of target proteins at nanometer-scale to regulate the function of bacterial and mammalian cells. Combining useful property of RNA as a nano-material and a molecular scaffold may provide us powerful tools in biological research, bioengineering, and future medicine.

Synthetic switch-based baculovirus for transgene expression control and selective killing of hepatocellular carcinoma cells

Baculovirus (BV) holds promise as a vector for anticancer gene delivery to combat the most common liver cancer-hepatocellular carcinoma (HCC). However, in vivo BV administration inevitably results in BV entry into non-HCC normal cells, leaky anticancer gene expression and possible toxicity. To improve the safety, we employed synthetic biology to engineer BV for transgene expression regulation. We first uncovered that miR-196a and miR-126 are exclusively expressed in HCC and normal cells, respectively, which allowed us to engineer a sensor based on distinct miRNA expression signature. We next assembled a synthetic switch by coupling the miRNA sensor and RNA binding protein L7Ae for translational repression, and incorporated the entire device into a single BV. The recombinant BV efficiently entered HCC and normal cells and enabled cis-acting transgene expression control, by turning OFF transgene expression in normal cells while switching ON transgene expression in HCC cells. Using pro-apoptotic hBax as the transgene, the switch-based BV selectively killed HCC cells in separate culture and mixed culture of HCC and normal cells. These data demonstrate the potential of synthetic switch-based BV to distinguish HCC and non-HCC normal cells for selective transgene expression control and killing of HCC cells.

An RNA Origami Octahedron with Intrinsic siRNAs for Potent Gene Knockdown

The fields of DNA and RNA nanotechnology have established nucleic acids as valuable building blocks for functional nanodevices with applications in nanomedicine. Here, a simple method for designing and assembling a 3D scaffolded RNA origami wireframe structure with intrinsic functioning small interfering RNAs (siRNAs) embedded is introduced. Uniquely, the method uses an mRNA fragment as scaffold strand, which is folded by sequence-complementarity of nine shorter synthetic strands. High-yield production of the intended 3D structure is verified by transmission electron microscopy (TEM). Production of functional siRNAs is facilitated by incorporating recognition sites for Dicer at selected locations in the structure, and efficient silencing of a target reporter gene is demonstrated.

Protein-driven RNA nanostructured devices that function in vitro and control mammalian cell fate

Nucleic acid nanotechnology has great potential for future therapeutic applications. However, the construction of nanostructured devices that control cell fate by detecting and amplifying protein signals has remained a challenge. Here we design and build protein-driven RNA-nanostructured devices that actuate in vitro by RNA-binding-protein-inducible conformational change and regulate mammalian cell fate by RNA–protein interaction-mediated protein assembly. The conformation and function of the RNA nanostructures are dynamically controlled by RNA-binding protein signals. The protein-responsive RNA nanodevices are constructed inside cells using RNA-only delivery, which may provide a safe tool for building functional RNA–protein nanostructures. Moreover, the designed RNA scaffolds that control the assembly and oligomerization of apoptosis-regulatory proteins on a nanometre scale selectively kill target cells via specific RNA–protein interactions. These findings suggest that synthetic RNA nanodevices could function as molecular robots that detect signals and localize target proteins, induce RNA conformational changes, and programme mammalian cellular behaviour.

Nucleic acid nanotechnology has great potential for future therapeutic applications. Here the authors build protein-driven RNA nanostructures that can function within mammalian cells and regulate the cell fate.

Versatile RNA tetra-U helix linking motif as a toolkit for nucleic acid nanotechnology

RNA nanotechnology employs synthetically modified ribonucleic acid (RNA) to engineer highly stable nanostructures in one, two, and three dimensions for medical applications. Despite the tremendous advantages in RNA nanotechnology, unmodified RNA itself is fragile and prone to enzymatic degradation. In contrast to use traditionally modified RNA strands e.g. 2'-fluorine, 2'-amine, 2'-methyl, we studied the effect of RNA/DNA hybrid approach utilizing a computer-assisted RNA tetra-uracil (tetra-U) motif as a toolkit to address questions related to assembly efficiency, versatility, stability, and the production costs of hybrid RNA/DNA nanoparticles. The tetra-U RNA motif was implemented to construct four functional triangles using RNA, DNA and RNA/DNA mixtures, resulting in fine-tunable enzymatic and thermodynamic stabilities, immunostimulatory activity and RNAi capability. Moreover, the tetra-U toolkit has great potential in the fabrication of rectangular, pentagonal, and hexagonal NPs, representing the power of simplicity of RNA/DNA approach for RNA nanotechnology and nanomedicine community.

A quasi-cyclic RNA nano-scale molecular object constructed using kink turns

k-Turns are widespread RNA architectural elements that mediate tertiary interactions. We describe a double-kink-turn motif comprising two inverted k-turns that forms a tight horse-shoe structure that can assemble into a variety of shapes by coaxial association of helical ends. Using X-ray crystallography we show that these assemble with two (dumbell), three (triangle) and four units (square), with or without bound protein, within the crystal lattice. In addition, exchange of a single basepair can almost double the pore radius or shape of a molecular assembly. On the basis of this analysis we synthesized a 114 nt self-complementary RNA containing six k-turns. The crystal structure of this species shows that it forms a quasi-cyclic triangular object. These are randomly disposed about the three-fold axis in the crystal lattice, generating a circular RNA of quasi D3 symmetry with a shape reminiscent of that of a cyclohexane molecule in its chair conformation. This work demonstrates that the k-turn is a powerful building block in the construction of nano-scale molecular objects, and illustrates why k-turns are widely used in natural RNA molecules to organize long-range architecture and mediate tertiary contacts.

Ring Catalog: A resource for designing self-assembling RNA nanostructures

Designing self-assembling RNA ring structures based on known 3D structural elements connected via linker helices is a challenging task due to the immense number of motif combinations, many of which do not lead to ring-closure. We describe an in silico solution to this design problem by combinatorial assembly of RNA 3-way junctions, bulges, and kissing loops, and tabulating the cases that lead to ring formation. The solutions found are made available in the form of a web-accessible Ring Catalog. As an example of a potential use of this resource, we chose a predicted RNA square structure consisting of five RNA strands and demonstrate experimentally that the self-assembly of those five strands leads to the formation of a square-like complex. This is a demonstration of a novel "design by catalog" approach to RNA nano-structure generation. The URL https://rnajunction.ncifcrf.gov/ringdb can be used to access the resource.

The Use of Minimal RNA Toeholds to Trigger the Activation of Multiple Functionalities

Current work reports the use of single-stranded RNA toeholds of different lengths to promote the reassociation of various RNA-DNA hybrids, which results in activation of multiple split functionalities inside human cells. The process of reassociation is analyzed and followed with a novel computational multistrand secondary structure prediction algorithm and various experiments. All of our previously designed RNA/DNA nanoparticles employed single-stranded DNA toeholds to initiate reassociation. The use of RNA toeholds is advantageous because of the simpler design rules, the shorter toeholds, and the smaller size of the resulting nanoparticles (by up to 120 nucleotides per particle) compared to the same hybrid nanoparticles with single-stranded DNA toeholds. Moreover, the cotranscriptional assemblies result in higher yields for hybrid nanoparticles with ssRNA toeholds.

RNA and RNP as Building Blocks for Nanotechnology and Synthetic Biology

Recent technologies that aimed to elucidate cellular function have revealed essential roles for RNA molecules in living systems. Our knowledge concerning functional and structural information of naturally occurring RNA and RNA-protein (RNP) complexes is increasing rapidly. RNA and RNP interaction motifs are structural units that function as building blocks to constitute variety of complex structures. RNA-central synthetic biology and nanotechnology are constructive approaches that employ the accumulated information and build synthetic RNA (RNP)-based circuits and nanostructures. Here, we describe how to design and construct synthetic RNA (RNP)-based devices and structures at the nanometer-scale for biological and future therapeutic applications. RNA/RNP nanostructures can also be utilized as the molecular scaffold to control the localization or interactions of target molecule(s). Moreover, RNA motifs recognized by RNA-binding proteins can be applied to make protein-responsive translational "switches" that can turn gene expression "on" or "off" depending on the intracellular environment. This "synthetic RNA and RNP world" will expand tools for nanotechnology and synthetic biology. In addition, these reconstructive approaches would lead to a greater understanding of building principle in naturally occurring RNA/RNP molecules and systems.