Split-inteins and their bioapplications

Split intein-mediated protein trans-splicing to express large dystrophins

Gene replacement using adeno-associated virus (AAV) vectors is a promising therapeutic approach for many diseases1,2. However, this therapeutic modality is challenged by the packaging capacity of AAVs (approximately 4.7 kilobases)3, limiting its application for disorders involving large coding sequences, such as Duchenne muscular dystrophy, with a 14 kilobase messenger RNA. Here we developed a new method for expressing large dystrophins by utilizing the protein trans-splicing mechanism mediated by split inteins. We identified several split intein pairs that efficiently join two or three fragments to generate a large midi-dystrophin or the full-length protein. We show that delivery of two or three AAVs into dystrophic mice results in robust expression of large dystrophins and significant physiological improvements compared with micro-dystrophins. Moreover, using the potent myotropic AAVMYO4, we demonstrate that low total doses (2 × 1013 viral genomes per kg) are sufficient to express large dystrophins in striated muscles body-wide with significant physiological corrections in dystrophic mice. Our data show a clear functional superiority of large dystrophins over micro-dystrophins that are being tested in clinical trials. This method could benefit many patients with Duchenne or Becker muscular dystrophy, regardless of genotype, and could be adapted to numerous other disorders caused by mutations in large genes that exceed the AAV capacity.

Application of CRISPR-Cas System to Mitigate Superbug Infections

Multidrug resistance in bacterial strains known as superbugs is estimated to cause fatal infections worldwide. Migration and urbanization have resulted in overcrowding and inadequate sanitation, contributing to a high risk of superbug infections within and between different communities. The CRISPR-Cas system, mainly type II, has been projected as a robust tool to precisely edit drug-resistant bacterial genomes to combat antibiotic-resistant bacterial strains effectively. To entirely opt for its potential, advanced development in the CRISPR-Cas system is needed to reduce toxicity and promote efficacy in gene-editing applications. This might involve base-editing techniques used to produce point mutations. These methods employ designed Cas9 variations, such as the adenine base editor (ABE) and the cytidine base editor (CBE), to directly edit single base pairs without causing DSBs. The CBE and ABE could change a target base pair into a different one (for example, G-C to A-T or C-G to A-T). In this review, we addressed the limitations of the CRISPR/Cas system and explored strategies for circumventing these limitations by applying diverse base-editing techniques. Furthermore, we also discussed recent research showcasing the ability of base editors to eliminate drug-resistant microbes.

Construction of IgG-Fab2 bispecific antibody via intein-mediated protein trans-splicing reaction

A bispecific antibody (bsAb) is a class of engineered antibody molecules that simultaneously binds to two different antigens by having two kinds of antigen-binding domains. One of the major obstacles for the bsAb production is the incorrect chain-pairing problem, wherein each heavy and light chain should form pairings with the correct counterpart's chains, but the structural similarity of the incorrect partners also forms the incorrect pairings. This study aimed to demonstrate a bsAb construction method using intein-mediated protein trans-splicing to create IgG-Fab2-type bsAbs, which is a modified antibody with a structure in which two additional Fabs are linked to the N-terminus of the heavy chain of an IgG molecule. The chain-paring problem between a heavy chain and a light chain is circumvented by separate expression and purification of the IgG part and the Fab part. We found that the deletion of a possible glycosylation residue improved the reaction yield and side-reaction cleavage in the protein ligation step. The resulting bsAb, IgG-Fab2 (Her2/CD3), demonstrated target binding activity and cytotoxicity mediated by activated T cells. These results indicate that the use of the protein ligation to produce the IgG-Fab2 type bsAb will expand the bsAb production method.

Efficient splicing of the CPE intein derived from directed evolution of the Cryptococcus neoformans PRP8 intein

Intein-mediated protein splicing has been widely used in protein engineering; however, the splicing efficiency and extein specificity usually limit its further application. Thus, there is a demand for more general inteins that can overcome these limitations. Here, we study the trans-splicing of CPE intein obtained from the directed evolution of Cne PRP8, which shows that its splicing rate is ~29- fold higher than that of the wild-type. When the +1 residue of C-extein is changed to cysteine, CPE also shows high splicing activity. Faster association and higher affinity may contribute to the high splicing rate compared with wild-type intein. These findings have important implications for the future engineering of inteins and provide clues for fundamental studies of protein structure and folding.

An integrase toolbox to record gene-expression during plant development

There are many open questions about the mechanisms that coordinate the dynamic, multicellular behaviors required for organogenesis. Synthetic circuits that can record in vivo signaling networks have been critical in elucidating animal development. Here, we report on the transfer of this technology to plants using orthogonal serine integrases to mediate site-specific and irreversible DNA recombination visualized by switching between fluorescent reporters. When combined with promoters expressed during lateral root initiation, integrases amplify reporter signal and permanently mark all descendants. In addition, we present a suite of methods to tune the threshold for integrase switching, including: RNA/protein degradation tags, a nuclear localization signal, and a split-intein system. These tools improve the robustness of integrase-mediated switching with different promoters and the stability of switching behavior over multiple generations. Although each promoter requires tuning for optimal performance, this integrase toolbox can be used to build history-dependent circuits to decode the order of expression during organogenesis in many contexts.

Universal Design of Luciferase Fusion Proteins for Epigenetic Modifications Detection Based on Bioluminescence Resonance Energy Transfer

Global hypomethylation and promoter hypermethylation of tumor-suppressor genes are the hallmarks of cancer. We previously reported a global DNA methylation level sensing system based on dual-color bioluminescence resonance energy transfer (BRET) using methyl-CpG binding domain (MBD)-fused firefly luciferase (Fluc) and unmethyl-CpG binding domain (CXXC)-fused Oplophorus luciferase (Oluc). Moreover, BRET-based hydroxymethylation and hemi-methylation level sensing systems have been developed using hydroxymethyl-CpG and hemi-methyl-CpG binding domain-fused Fluc. These studies suggest that target epigenetic modifications can be simultaneously quantified using target-modification-binding protein-fused luciferases. In this study, we focused on the SnoopTag (SnT)/SnoopCatcher (SnC) protein ligation system to establish a universal design for fusion protein construction for any combination. SnT spontaneously forms an isopeptide bond with SnC; therefore, any kind of fusion protein would be constructed by the SnT/SnC system. To establish the proof of concept, MBD-SnT, CXXC-SnT, and SnC-Oluc were prepared and ligated MBD-SnT or CXXC-SnT to SnC-Oluc. The ligation products of MBD-SnT-SnC-Oluc and CXXC-SnT-SnC-Oluc showed luciferase activity and specific binding activity to methyl-CpG and unmethyl-CpG, respectively. The BRET signal using MBD-SnT-SnC-Oluc and CXXC-SnT-SnC-Oluc increased the amount of methyl-CpG and unmethyl-CpG in genomic DNA, respectively. There was a significant negative correlation between the BRET signals; therefore, the global DNA methylation level was quantified using the BRET signals (R² = 0.99, and R.S.D. <3.5%). These results indicate that the SnT/SnC protein ligation system can be utilized to construct target modification-binding protein-fused luciferases in any combination that detects target modifications in genomic DNA based on BRET.

Full-length ATP7B reconstituted through protein trans-splicing corrects Wilson disease in mice

Wilson disease (WD) is a genetic disorder of copper homeostasis, caused by deficiency of the copper transporter ATP7B. Gene therapy with recombinant adeno-associated vectors (AAV) holds promises for WD treatment. However, the full-length human ATP7B gene exceeds the limited AAV cargo capacity, hampering the applicability of AAV in this disease context. To overcome this limitation, we designed a dual AAV vector approach using split intein technology. Split inteins catalyze seamless ligation of two separate polypeptides in a highly specific manner. We selected a DnaE intein from Nostoc punctiforme (Npu) that recognizes a specific tripeptide in the human ATP7B coding sequence. We generated two AAVs expressing either the 5′-half of a codon-optimized human ATP7B cDNA followed by the N-terminal Npu DnaE intein or the C-terminal Npu DnaE intein followed by the 3′-half of ATP7B cDNA, under the control of a liver-specific promoter. Intravenous co-injection of the two vectors in wild-type and Atp7b^−/− mice resulted in efficient reconstitution of full-length ATP7B protein in the liver. Moreover, Atp7b^−/− mice treated with intein-ATP7B vectors were protected from liver damage and showed improvements in copper homeostasis. Taken together, these data demonstrate the efficacy of split intein technology to drive the reconstitution of full-length human ATP7B and to rescue copper-mediated liver damage in Atp7b^−/− mice, paving the way to the development of a new gene therapy approach for WD.

Circumventing the packaging limit of AAV-mediated gene replacement therapy for neurological disorders

INTRODUCTION

Gene therapy provides the exciting opportunity of a curative single treatment for devastating diseases, eradicating the need for chronic medication. Adeno-associated viruses (AAVs) are among the most attractive vector carriers for gene replacement in vivo. Yet, despite the success of recent AAV-based clinical trials, the clinical use of these vectors has been limited. For instance, the AAV packaging capacity is restricted to ~4.7 kb, making it a substantial challenge to deliver large gene products.

AREAS COVERED

In this review, we explore established and emerging strategies that circumvent the packaging limit of AAVs to make them effective vehicles for gene replacement therapy of monogenic disorders, with a particular focus on diseases affecting the nervous system. We report historical references, design remarks, as well as strengths and weaknesses of these approaches. We additionally discuss examples of neurological disorders for which such strategies have been attempted.

EXPERT OPINION

The field of AAV-gene therapy has experienced enormous advancements in the last decade. However, there is still ample space for improvement aimed at overcoming existing challenges that are slowing down the progressive trajectory of this field.

Peptide/protein-based macrocycles: from biological synthesis to biomedical applications

Living organisms have evolved cyclic or multicyclic peptides and proteins with enhanced stability and high bioactivity superior to their linear counterparts for diverse purposes. Herein, we review recent progress in applying this concept to artificial peptides and proteins to exploit the functional benefits of these macrocycles. Not only have simple cyclic forms been prepared, numerous macrocycle variants, such as knots and links, have also been developed. The chemical tools and synthetic strategies are summarized for the biological synthesis of these macrocycles, demonstrating it as a powerful alternative to chemical synthesis. Its further application to therapeutic peptides/proteins has led to biomedicines with profoundly improved pharmaceutical performances. Finally, we present our perspectives on the field and its future developments.

An intein-split transactivator for intersectional neural imaging and optogenetic manipulation

The cell-type-specific recording and manipulation is instrumental to disentangle causal neural mechanisms in physiology and behavior and increasingly requires intersectional control; however, current approaches are largely limited by the number of intersectional features, incompatibility of common effectors and insufficient gene expression. Here, we utilized the protein-splicing technique mediated by intervening sequences (intein) and devised an intein-based intersectional synthesis of transactivator (IBIST) to selectively control gene expression of common effectors in multiple-feature defined cell types in mice. We validated the specificity and sufficiency of IBIST to control fluorophores, optogenetic opsins and Ca²⁺ indicators in various intersectional conditions. The IBIST-based Ca²⁺ imaging showed that the IBIST can intersect five features and that hippocampal neurons tune differently to distinct emotional stimuli depending on the pattern of projection targets. Collectively, the IBIST multiplexes the capability to intersect cell-type features and controls common effectors to effectively regulate gene expression, monitor and manipulate neural activities.

Cell-type-specific recording and manipulation is important for understanding neural circuits. Here the authors describe molecular tools to access cell types based on genetics and connectivity in the brain, and demonstrated the utility of these tools in neural recording and manipulations.

High Level Expression and Purification of Cecropin-like Antimicrobial Peptides in Escherichia coli

Cecropins are a family of antimicrobial peptides (AMPs) that are widely found in the innate immune system of Cecropia moths. Cecropins exhibit a broad spectrum of antimicrobial and anticancer activities. The structures of Cecropins are composed of 34–39 amino acids with an N-terminal amphipathic α-helix, an AGP hinge and a hydrophobic C-terminal α-helix. KR12AGPWR6 was designed based on the Cecropin-like structural feature. In addition to its antimicrobial activities, KR12AGPWR6 also possesses enhanced salt resistance, antiendotoxin and anticancer properties. Herein, we have developed a strategy to produce recombinant KR12AGPWR6 through a salt-sensitive, pH and temperature dependent intein self-cleavage system. The His6-Intein-KR12AGPWR6 was expressed by E. coli and KR12AGPWR6 was released by the self-cleavage of intein under optimized ionic strength, pH and temperature conditions. The molecular weight and structural feature of the recombinant KR12AGPWR6 was determined by MALDI-TOF mass, CD, and NMR spectroscopy. The recombinant KR12AGPWR6 exhibited similar antimicrobial activities compared to the chemically synthesized KR12AGPWR6. Our results provide a potential strategy to obtain large quantities of AMPs and this method is feasible and easy to scale up for commercial production.

Liver gene therapy with intein-mediated F8 trans-splicing corrects mouse haemophilia A

Liver gene therapy with adeno‐associated viral (AAV) vectors is under clinical investigation for haemophilia A (HemA), the most common inherited X‐linked bleeding disorder. Major limitations are the large size of the F8 transgene, which makes packaging in a single AAV vector a challenge, as well as the development of circulating anti‐F8 antibodies which neutralise F8 activity. Taking advantage of split‐intein‐mediated protein trans‐splicing, we divided the coding sequence of the large and highly secreted F8‐N6 variant in two separate AAV‐intein vectors whose co‐administration to HemA mice results in the expression of therapeutic levels of F8 over time. This occurred without eliciting circulating anti‐F8 antibodies unlike animals treated with the single oversized AAV‐F8 vector under clinical development. Therefore, liver gene therapy with AAV‐F8‐N6 intein should be considered as a potential therapeutic strategy for HemA.

Intein-Mediated Protein Engineering for Cell-Based Biosensors

Cell-based sensors provide a flexible platform for screening biologically active targets and for monitoring their interactions in live cells. Their applicability extends across a vast array of biological research and clinical applications. Particularly, cell-based sensors are becoming a potent tool in drug discovery and cell-signaling studies by allowing function-based screening of targets in biologically relevant environments and enabling the in vivo visualization of cellular signals in real-time with an outstanding spatiotemporal resolution. In this review, we aim to provide a clear view of current cell-based sensor technologies, their limitations, and how the recent improvements were using intein-mediated protein engineering. We first discuss the characteristics of cell-based sensors and present several representative examples with a focus on their design strategies, which differentiate cell-based sensors from in vitro analytical biosensors. We then describe the application of intein-mediated protein engineering technology for cell-based sensor fabrication. Finally, we explain the characteristics of intein-mediated reactions and present examples of how the intein-mediated reactions are used to improve existing methods and develop new approaches in sensor cell fabrication to address the limitations of current technologies.

Protein Splicing of Inteins: A Powerful Tool in Synthetic Biology

Inteins are protein segments that are capable of enabling the ligation of flanking extein into a new protein, a process known as protein splicing. Since its discovery, inteins have become powerful biotechnological tools for applications such as protein engineering. In the last 10 years, the development in synthetic biology has further endowed inteins with enhanced functions and diverse utilizations. Here we review these efforts and discuss the future directions.

Inteins as Drug Targets and Therapeutic Tools

Multidrug-resistant pathogens are of significant concern in recent years. Hence new antifungal and anti-bacterial drug targets are urgently needed before the situation goes beyond control. Inteins are polypeptides that self-splice from exteins without the need for cofactors or external energy, resulting in joining of extein fragments. Inteins are present in many organisms, including human pathogens such as Mycobacterium tuberculosis, Cryptococcus neoformans, C. gattii, and Aspergillus fumigatus. Because intein elements are not present in human genes, they are attractive drug targets to develop antifungals and antibiotics. Thus far, a few inhibitors of intein splicing have been reported. Metal-ions such as Zn²⁺ and Cu²⁺, and platinum-containing compound cisplatin inhibit intein splicing in M. tuberculosis and C. neoformans by binding to the active site cysteines. A small-molecule inhibitor 6G-318S and its derivative 6G-319S are found to inhibit intein splicing in C. neoformans and C. gattii with a MIC in nanomolar concentrations. Inteins have also been used in many other applications. Intein can be used in activating a protein inside a cell using small molecules. Moreover, split intein can be used to deliver large genes in experimental gene therapy and to kill selected species in a mixed population of microbes by taking advantage of the toxin-antitoxin system. Furthermore, split inteins are used in synthesizing cyclic peptides and in developing cell culture model to study infectious viruses including SARS-CoV-2 in the biosafety level (BSL) 2 facility. This mini-review discusses the recent research developments of inteins in drug discovery and therapeutic research.

Dual-AAV delivering split prime editor system for in vivo genome editing

Prime editor (PE), a new genome editing tool, can generate all 12 possible base-to-base conversions, insertion, and deletion of short fragment DNA. PE has the potential to correct the majority of known human genetic disease-related mutations. Adeno-associated viruses (AAVs), the safe vector widely used in clinics, are not capable of delivering PE (∼6.3 kb) in a single vector because of the limited loading capacity (∼4.8 kb). To accommodate the loading capacity of AAVs, we constructed four split-PE (split-PE994, split-PE1005, split-PE1024, and split-PE1032) using Rma intein (Rhodothermus marinus). With the use of a GFP-mutated reporter system, PE reconstituting activities were screened, and two efficient split-PEs (split-PE1005 and split-PE1024) were identified. We then demonstrated that split-PEs delivered by dual-AAV1, especially split-PE1024, could mediate base transversion and insertion at four endogenous sites in human cells. To test the performance of split-PE in vivo, split-PE1024 was then delivered into the adult mouse retina by dual-AAV8. We demonstrated successful editing of Dnmt1 in adult mouse retina. Our study provides a new method to deliver PE to adult tissue, paving the way for in vivo gene-editing therapy using PE.

Artificial protein assemblies with well-defined supramolecular protein nanostructures

Nature uses a wide range of well-defined biomolecular assemblies in diverse cellular processes, where proteins are major building blocks for these supramolecular assemblies. Inspired by their natural counterparts, artificial protein-based assemblies have attracted strong interest as new bio-nanostructures, and strategies to construct ordered protein assemblies have been rapidly expanding. In this review, we provide an overview of very recent studies in the field of artificial protein assemblies, with the particular aim of introducing major assembly methods and unique features of these assemblies. Computational de novo designs were used to build various assemblies with artificial protein building blocks, which are unrelated to natural proteins. Small chemical ligands and metal ions have also been extensively used for strong and bio-orthogonal protein linking. Here, in addition to protein assemblies with well-defined sizes, protein oligomeric and array structures with rather undefined sizes (but with definite repeat protein assembly units) also will be discussed in the context of well-defined protein nanostructures. Lastly, we will introduce multiple examples showing how protein assemblies can be effectively used in various fields such as therapeutics and vaccine development. We believe that structures and functions of artificial protein assemblies will be continuously evolved, particularly according to specific application goals.

Intein Inhibitors as Novel Antimicrobials: Protein Splicing in Human Pathogens, Screening Methods, and Off-Target Considerations

Protein splicing is a post-translational process by which an intervening polypeptide, or intein, catalyzes its own removal from the flanking polypeptides, or exteins, concomitant with extein ligation. Although inteins are highly abundant in the microbial world, including within several human pathogens, they are absent in the genomes of metazoans. As protein splicing is required to permit function of essential proteins within pathogens, inteins represent attractive antimicrobial targets. Here we review key proteins interrupted by inteins in pathogenic mycobacteria and fungi, exciting discoveries that provide proof of concept that intein activity can be inhibited and that this inhibition has an effect on the host organism's fitness, and bioanalytical methods that have been used to screen for intein activity. We also consider potential off-target inhibition of hedgehog signaling, given the similarity in structure and function of inteins and hedgehog autoprocessing domains.

Novel intein-based self-cleaving affinity tag for recombinant protein production in Escherichia coli

We evaluated several intein-based self-cleaving affinity tags for expression and single-step affinity chromatography purification of recombinant proteins produced in Escherichia coli. We used human growth hormone (hGH) as target protein that contains two internal disulfide bridges and an N-terminal phenylalanine. Use of N-terminal thiol-induced Sce VMA1 intein affinity tag resulted in purified hGH deficient in disulfide bonds. Inteins with self-cleavage inducible by pH and/or temperature shift were analyzed. N-terminal Ssp DnaX intein affinity tag resulted in a completely cleaved cytosolic protein, whereas N-terminal Ssp DnaB intein affinity tag resulted in a cytosolic fusion protein incapable of releasing hGH. Periplasmic expression of target protein was analyzed using an N-terminal signal peptide and C-terminal Ssp DnaX pH-inducible self-cleaving affinity tag. The fusion protein was properly expressed in pH 8 buffered culture medium. Fusion of a periplasmic signal peptide to the N-terminus of the POI allowed secretion to the periplasmic region and presence of the natural N-terminal amino acid of the POI following cleavage. Periplasmic expression of hGH fused to this novel C-terminal DnaX intein-based self-cleaving affinity tag made possible expression and purification of hGH protein containing disulfide bonds and free of extra amino acids.

Development of HBc virus-like particles as modular nanocarrier by intein-mediated trans-splicing

Hepatitis B virus core protein (HBc) spontaneously assembles as Virus-like particles (VLPs) in Escherichia coli (E. coli) which is extensively used as a nanocarrier to boost antigen immunogenicity. Genetic fusion of cargo protein with HBc occasionally forms inclusion bodies instead of properly assembled VLPs. To this end, we devised HBc VLPs as a modular nanocarrier for antigen delivery by intein-mediated trans-splicing (TS). We introduced split inteinC (intC) to the C-terminus of split HBc N-core to employ intein-mediated TS technology to HBc VLPs. Split HBc with the insertion of intC at N-core C-terminus (designated as HBc N-intC-C) existed in inclusion bodies. Interestingly, introduction of a soluble tag, gb1, to intC C-terminus remarkably improved the solubility of recombinant protein (named HBc N-intC-gb1-C). Moreover, newly designed recombinant spontaneously assembled as VLPs and endowed efficiently coupling two different model antigens onto HBc N-intC-gb1-C VLPs. Furthermore, model antigens delivered by HBc VLPs induced a dramatically enhanced antigen-specific immune responses. Antigen proteins mainly elicited Th2 IgG responses while antigens delivered by HBc VLPs steered Th1/Th2 balanced IgG responses. Taken together, intein-mediated TS was amenable to decorate HBc VLPs with antigens and showed good potential for antigen delivery.

Therapeutic applications of trans-splicing

BACKGROUND

RNA trans-splicing joins exons from different pre-mRNA transcripts to generate a chimeric product. Trans-splicing can also occur at the protein level, with split inteins mediating the ligation of separate gene products to generate a mature protein.

SOURCES OF DATA

Comprehensive literature search of published research papers and reviews using Pubmed.

AREAS OF AGREEMENT

Trans-splicing techniques have been used to target a wide range of diseases in both in vitro and in vivo models, resulting in RNA, protein and functional correction.

AREAS OF CONTROVERSY

Off-target effects can lead to therapeutically undesirable consequences. In vivo efficacy is typically low, and delivery issues remain a challenge.

GROWING POINTS

Trans-splicing provides a promising avenue for developing novel therapeutic approaches. However, much more research needs to be done before developing towards preclinical studies.

AREAS TIMELY FOR DEVELOPING RESEARCH

Increasing trans-splicing efficacy and specificity by rational design, screening and competitive inhibition of endogenous cis-splicing.

Engineering spatiotemporal organization and dynamics in synthetic cells

Constructing synthetic cells has recently become an appealing area of research. Decades of research in biochemistry and cell biology have amassed detailed part lists of components involved in various cellular processes. Nevertheless, recreating any cellular process in vitro in cell-sized compartments remains ambitious and challenging. Two broad features or principles are key to the development of synthetic cells-compartmentalization and self-organization/spatiotemporal dynamics. In this review article, we discuss the current state of the art and research trends in the engineering of synthetic cell membranes, development of internal compartmentalization, reconstitution of self-organizing dynamics, and integration of activities across scales of space and time. We also identify some research areas that could play a major role in advancing the impact and utility of engineered synthetic cells. This article is categorized under: Biology-Inspired Nanomaterials > Lipid-Based Structures Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

A gene therapy for inherited blindness using dCas9-VPR-mediated transcriptional activation

Catalytically inactive dCas9 fused to transcriptional activators (dCas9-VPR) enables activation of silent genes. Many disease genes have counterparts, which serve similar functions but are expressed in distinct cell types. One attractive option to compensate for the missing function of a defective gene could be to transcriptionally activate its functionally equivalent counterpart via dCas9-VPR. Key challenges of this approach include the delivery of dCas9-VPR, activation efficiency, long-term expression of the target gene, and adverse effects in vivo. Using dual adeno-associated viral vectors expressing split dCas9-VPR, we show efficient transcriptional activation and long-term expression of cone photoreceptor-specific M-opsin (Opn1mw) in a rhodopsin-deficient mouse model for retinitis pigmentosa. One year after treatment, this approach yields improved retinal function and attenuated retinal degeneration with no apparent adverse effects. Our study demonstrates that dCas9-VPR-mediated transcriptional activation of functionally equivalent genes has great potential for the treatment of genetic disorders.

Intein-mediated protein trans-splicing expands adeno-associated virus transfer capacity in the retina

Retinal gene therapy with adeno-associated viral (AAV) vectors holds promises for treating inherited and noninherited diseases of the eye. Although clinical data suggest that retinal gene therapy is safe and effective, delivery of large genes is hindered by the limited AAV cargo capacity. Protein trans-splicing mediated by split inteins is used by single-cell organisms to reconstitute proteins. Here, we show that delivery of multiple AAV vectors each encoding one of the fragments of target proteins flanked by short split inteins results in protein trans-splicing and full-length protein reconstitution in the retina of mice and pigs and in human retinal organoids. The reconstitution of large therapeutic proteins using this approach improved the phenotype of two mouse models of inherited retinal diseases. Our data support the use of split intein-mediated protein trans-splicing in combination with AAV subretinal delivery for gene therapy of inherited blindness due to mutations in large genes.

Control of ϕC31 integrase-mediated site-specific recombination by protein trans-splicing

Serine integrases are emerging as core tools in synthetic biology and have applications in biotechnology and genome engineering. We have designed a split-intein serine integrase-based system with potential for regulation of site-specific recombination events at the protein level in vivo. The ϕC31 integrase was split into two extein domains, and intein sequences (Npu DnaEN and Ssp DnaEC) were attached to the two termini to be fused. Expression of these two components followed by post-translational protein trans-splicing in Escherichia coli generated a fully functional ϕC31 integrase. We showed that protein splicing is necessary for recombination activity; deletion of intein domains or mutation of key intein residues inactivated recombination. We used an invertible promoter reporter system to demonstrate a potential application of the split intein-regulated site-specific recombination system in building reversible genetic switches. We used the same split inteins to control the reconstitution of a split Integrase-Recombination Directionality Factor fusion (Integrase-RDF) that efficiently catalysed the reverse attR x attL recombination. This demonstrates the potential for split-intein regulation of the forward and reverse reactions using the integrase and the integrase-RDF fusion, respectively. The split-intein integrase is a potentially versatile, regulatable component for building synthetic genetic circuits and devices.

Versatile Functionalization of Ferritin Nanoparticles by Intein-Mediated Trans-Splicing for Antigen/Adjuvant Co-delivery

Self-assembling protein nanoparticles are extensively and increasingly engineered to integrate adjuvants with antigens to elicit potent and long-term immunity due to uniform architecture, inherent biocompatibility, and excellent plasticity. However, functionalization of nanoparticles by surface tailoring has two common problems: (1) disassembly caused by loaded cargoes; and (2) an adjuvant that is inconvenient to co-deliver with an antigen by genetic fusion. Here, we report an intein-mediated trans-splicing approach that overcomes the detrimental effects of loaded proteins on ferritin nanoparticle stability and allows concurrent display of antigen and adjuvant in a facile, efficient, and site-specific manner. An immunization study with an epitope-based model antigen reveals that antigen and adjuvant co-delivery nanoparticles induce a more potent protective immunity than other formulations do. Our results demonstrate that protein engineering represents an intriguing approach for antigen/adjuvant co-delivery to potentiate antigen-associated immune responses.

Bio-engineering of bacterial microcompartments: a mini review

Bacterial microcompartments (BMCs) are protein-bound prokaryotic organelles, discovered in cyanobacteria more than 60 years ago. Functionally similar to eukaryotic cellular organelles, BMCs compartment metabolic activities in the cytoplasm, foremost to increase local enzyme concentration and prevent toxic intermediates from damaging the cytosolic content. Advanced knowledge of the functional and structural properties of multiple types of BMCs, particularly over the last 10 years, have highlighted design principles of microcompartments. This has prompted new research into their potential to function as programmable synthetic nano-bioreactors and novel bio-materials with biotechnological and medical applications. Moreover, due to the involvement of microcompartments in bacterial pathogenesis and human health, BMCs have begun to gain attention as potential novel drug targets. This mini-review gives an overview of important synthetic biology developments in the bioengineering of BMCs and a perspective on future directions in the field.

High-Affinity Antibody Detection with a Bivalent Circularized Peptide Containing Antibody-Binding Domains

Direct chemical labeling of antibody produces molecules with poorly defined modifications. Use of a small antibody-binding protein as an adapter can simplify antibody functionalization by forming a specific antibody-bound complex and introducing site-specific modifications. To stabilize a noncovalent antibody complex that may be used without chemical crosslinking, a bivalent antibody-binding protein is engineered with an improved affinity of interaction by joining two Z domains with a conformationally flexible linker. The linker is essential for the increase in affinity because it allows simultaneous binding of both domains. The molecule is further circularized using a split intein, creating a novel adapter protein ("lasso"), which binds human immunoglobulin G1 (IgG1) with K _D  = 0.53 n m and a dissociation rate that is 55- to 84-fold slower than Z. The lasso contains a unique cysteine for conjugation with a reporter and may be engineered to introduce other functional groups, including a biotin tag and protease recognition sequences. When used in enzyme-linked immunosorbent assay (ELISA), the lasso generates a stronger reporter signal compared to a secondary antibody and lowers the limit of detection by 12-fold. The small size of the lasso and a long half-life of dissociation make the peptide a useful tool in antibody detection and immobilization.

Adeno-Associated Viral Vectors as a Tool for Large Gene Delivery to the Retina

Gene therapy using adeno-associated viral (AAV) vectors currently represents the most promising approach for the treatment of many inherited retinal diseases (IRDs), given AAV's ability to efficiently deliver therapeutic genes to both photoreceptors and retinal pigment epithelium, and their excellent safety and efficacy profiles in humans. However, one of the main obstacles to widespread AAV application is their limited packaging capacity, which precludes their use from the treatment of IRDs which are caused by mutations in genes whose coding sequence exceeds 5 kb. Therefore, in recent years, considerable effort has been made to identify strategies to increase the transfer capacity of AAV vectors. This review will discuss these new developed strategies, highlighting the advancements as well as the limitations that the field has still to overcome to finally expand the applicability of AAV vectors to IRDs due to mutations in large genes.

Biotechnological Applications of Protein Splicing

Protein splicing domains, also called inteins, have become a powerful biotechnological tool for applications involving molecular biology and protein engineering. Early applications of inteins focused on self-cleaving affinity tags, generation of recombinant polypeptide α-thioesters for the production of semisynthetic proteins and backbone cyclized polypeptides. The discovery of naturallyoccurring split-inteins has allowed the development of novel approaches for the selective modification of proteins both in vitro and in vivo. This review gives a general introduction to protein splicing with a focus on their role in expanding the applications of intein-based technologies in protein engineering and chemical biology.

Segmental isotopic labeling of HIV-1 capsid protein assemblies for solid state NMR

Recent studies of noncrystalline HIV-1 capsid protein (CA) assemblies by our laboratory and by Polenova and coworkers (Protein Sci 19:716-730, 2010; J Mol Biol 426:1109-1127, 2014; J Biol Chem 291:13098-13112, 2016; J Am Chem Soc 138:8538-8546, 2016; J Am Chem Soc 138:12029-12032, 2016; J Am Chem Soc 134:6455-6466, 2012; J Am Chem Soc 132:1976-1987, 2010; J Am Chem Soc 135:17793-17803, 2013; Proc Natl Acad Sci USA 112:14617-14622, 2015; J Am Chem Soc 138:14066-14075, 2016) have established the capability of solid state nuclear magnetic resonance (NMR) measurements to provide site-specific structural and dynamical information that is not available from other types of measurements. Nonetheless, the relatively high molecular weight of HIV-1 CA leads to congestion of solid state NMR spectra of fully isotopically labeled assemblies that has been an impediment to further progress. Here we describe an efficient protocol for production of segmentally labeled HIV-1 CA samples in which either the N-terminal domain (NTD) or the C-terminal domain (CTD) is uniformly 15N,13C-labeled. Segmental labeling is achieved by trans-splicing, using the DnaE split intein. Comparisons of two-dimensional solid state NMR spectra of fully labeled and segmentally labeled tubular CA assemblies show substantial improvements in spectral resolution. The molecular structure of HIV-1 assemblies is not significantly perturbed by the single Ser-to-Cys substitution that we introduce between NTD and CTD segments, as required for trans-splicing.

Invention of stimulus-responsive peptide-bond-cleaving residue (Spr) and its application to chemical biology tools

Elucidation of biological functions of peptides and proteins is essential for understanding peptide/protein-related biological events and developing drugs. Caged peptides and proteins that release a parent active peptide/protein by photo-irradiation have successfully been employed to elucidate the functions. Whereas the usual caged peptide/protein enables conversion of an inactive form to an active form (OFF-to-ON conversion) by photo-induced deprotection, photo-triggered main chain cleavage is reported to be applicable to ON-to-OFF conversion. These peptides and proteins are photo-responsive; however, if peptides and proteins could respond to other stimuli such as disease-related environment or enzymes, their range of application should be widened. To convert the photo-responsive peptide/protein into other stimulus-responsive peptide/protein, quite laborious de novo design and synthesis of the stimulus-responsive unit are required. In this context, we designed a stimulus-responsive peptide-bond-cleaving residue (Spr) in which the stimuli available for the main chain cleavage vary according to the choice of protecting groups on the residue. In this review, design and synthesis of Spr are introduced, and challenges to apply Spr to other fields to enable, for example, functional control, localization control, delivery of cargos, labeling of a protein of interest in living cells, and identification of target proteins of bioactive ligands are discussed. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Utilizing intein-mediated protein cleaving for purification of uricase, a multimeric enzyme

Uric acid, a side product of nucleotide metabolism, should be cleared from blood stream since its accumulation can cause cardiovascular diseases and gout. Uricase (urate oxidase) converts uric acid to 5-hydroxyisourate, but it is absent in human and other higher apes. Yet, the recombinant form of uricase, Rasburicase, is now commercially available to cure tumor lysis syndrome by lowering serum uric acid level. Developing new methods to efficiently purify pharmaceutical proteins like uricase has attracted researchers' attention. Self-cleaving intein mediated single column purification is one of these novel approaches. Self-cleaving inteins are modified forms of natural inteins that can excise and join only at one junction site. In this study, the synthetic gene of Aspergillus flavus uricase, a homotetrameric protein, was cloned into pTXB1 vector as a fusion with the N-terminal of Mxe GyrA intein and chitin-binding domain (CBD) for simple purification. Expression was confirmed by western blot analysis. The fusion protein containing uricase-intein-CBD was purified on a chitin column. The cleavage was induced by adding DTT,¹ as a reducing agent to release uricase. The purity of uricase and complete excision of the intein and CBD were confirmed by SDS-PAGE² while its proper folding was proved by circular dichroism and fluorescent emission studies. Isoelectric focusing further confirmed its homogeneity when a single protein band was observed at the predicted pI value. This is the first report of successful purification of a multimeric therapeutic enzyme by intein-mediated protein cleaving using a well-established and facile system.

Locked and proteolysis-based transcription activator-like effector (TALE) regulation

Development of orthogonal, designable and adjustable transcriptional regulators is an important goal of synthetic biology. Their activity has been typically modulated through stimulus-induced oligomerization or interaction between the DNA-binding and activation/repression domain. We exploited a feature of the designable Transcription activator-like effector (TALE) DNA-binding domain that it winds around the DNA which allows to topologically prevent it from binding by intramolecular cyclization. This new approach was investigated through noncovalent ligand-induced cyclization or through a covalent split intein cyclization strategy, where the topological inhibition of DNA binding by cyclization and its restoration by a proteolytic release of the topologic constraint was expected. We show that locked TALEs indeed have diminished DNA binding and regain full transcriptional activity by stimulation with the rapamycin ligand or site-specific proteolysis of the peptide linker, with much higher level of activation than rapamycin-induced heterodimerization. Additionally, we demonstrated reversibility, activation of genomic targets and implemented logic gates based on combinations of protein cyclization, proteolytic cleavage and ligand-induced dimerization, where the strongest fold induction was achieved by the proteolytic cleavage of a repression domain from a linear TALE.