NLS-Gal4-VP64

This part of our simulated enhancer hijacking assay system binds to the upstream activation sites (UAS) next to the Oct1 sites (BBa_K5237023), resulting in transactivation of a gene on another plasmid, e.g. a firefly luciferase gene (BBa_K5237024). Furthermore allowing for swift testing of DNA brought into proximity, which can be adapted by other iGEMers for other assay systems.

Figure 1: How Our Part Collection Can Be Used to Engineer New Staples

While synthetic biology has in the past focused on engineering the genomic sequence of organisms, the 3D spatial organization of DNA is well-known to be an important layer of information encoding in particular in eukaryotes, playing a crucial role in gene regulation and hence cell fate, disease development, evolution, and more. However, tools to precisely manipulate and control the genomic spatial architecture are limited, hampering the exploration of 3D genome engineering in synthetic biology. We - the iGEM Team Heidelberg 2024 - have developed PICasSO, a powerful molecular toolbox for rationally engineering genome 3D architectures in living cells, based on various DNA-binding proteins.

The PICasSO part collection offers a comprehensive, modular platform for precise manipulation and re-programming of DNA-DNA interactions using engineered "protein staples" in living cells. This enables researchers to recreate naturally occurring alterations of 3D genomic interactions, such as enhancer hijacking in cancer, or to design entirely new spatial architectures for artificial gene regulation and cell function control. Specifically, the fusion of two DNA binding proteins enables to artificially bring otherwise distant genomic loci into spatial proximity. To unlock the system's full potential, we introduce versatile chimeric CRISPR/Cas complexes, connected either at the protein or - in the case of CRISPR/Cas-based DNA binding moieties - the guide RNA level. These complexes are referred to as protein- or Cas staples, respectively. Beyond its versatility with regard to the staple constructs themselves, PICasSO includes robust assay systems to support the engineering, optimization, and testing of new staples in vitro and in vivo. Notably, the PICasSO toolbox was developed in a design-build-test-learn engineering cycle closely intertwining wet lab experiments and computational modeling and iterated several times, yielding a collection of well-functioning and -characterized parts.

At its heart, the PICasSO part collection consists of three categories.
(i) Our DNA-binding proteins include our finalized Cas staple experimentally validated using an artificial "enhancer hijacking" system as well as "half staples" that can be combined by scientists to compose entirely new Cas staples in the future. We also include our Simple staples comprised of particularly small, simple and robust DNA binding domains well-known to the synthetic biology community, which serve as controls for successful stapling and can be further engineered to create alternative, simpler, and more compact staples.
(ii) As functional elements, we list additional parts that enhance and expand the functionality of our Cas and Basic staples. These consist of staples dependent on cleavable peptide linkers targeted by cancer-specific proteases or inteins that allow condition-specific, dynamic stapling in vivo. We also include several engineered parts that enable the efficient delivery of PICasSO's constructs into target cells, including mammalian cells, with our new interkingdom conjugation system.
(iii) As the final category of our collection, we provide parts that underlie our custom readout systems. These include components of our established FRET-based proximity assay system, enabling users to confirm accurate stapling. Additionally, we offer a complementary, application-oriented testing system based on a luciferase reporter, which allows for straightforward experimental assessment of functional enhancer hijacking events in mammalian cells.

The following table gives a comprehensive overview of all parts in our PICasSO toolbox. The highlighted parts showed exceptional performance as described on our iGEM wiki and can serve as a reference. The other parts in the collection are versatile building blocks designed to provide future iGEMers with the flexibility to engineer their own custom Cas staples, enabling further optimization and innovation in the new field of 3D genome engineering.

Our part collection includes:

1. Sequence overview

2. Usage and Biology

Gal4 is a well-known transcription factor from Saccharomyces cerevisiae that binds specifically to UAS regions on DNA, activating transcription of downstream genes. Its DNA-binding domain has been widely utilized in synthetic biology and gene regulation studies due to its specificity and ability to recruit transcriptional machinery (Kakidani and Ptashne (1988)).
VP64 is a synthetic transcriptional activator composed of four tandem repeats of the Herpes Simplex Virus VP16 transcriptional activation domain. VP64 is commonly used in CRISPR-based gene activation strategies, where it recruits transcriptional machinery to target genes, enhancing transcription (Wang et al., 2016).
Fusions of Gal4 and VP64 create a potent transactivation system. When Gal4 is fused with VP64, the chimeric protein retains Gal4's DNA-binding specificity and gains the strong transactivation capability of VP64, enabling robust gene expression (Lowder et al., 2017).

3. Assembly and Part Evolution

The construct was provided by our PI, used for the assembly of our Cathepsin B-Cleavable Trans-Activator (BBa_K5237020) and mainly used in the enhancer hijacking assay for the Cas staples (Fig. 2)

4. Results

To show that the Cas staple can staple two DNA loci together, and thereby induce proximity between two separate functional elements, we employed the NLS-Gal4-VP64 fusion as the transactivator.
For this, an enhancer plasmid (containing BBa_K5237023) and a reporter plasmid (containing BBa_K5237024) were used. The reporter plasmid has firefly luciferase behind several repeats of a Cas9 targeted sequence. The enhancer plasmid has a Gal4 binding site behind several repeats of a Cas12a targeted sequence. By introducing a fgRNA staple and the NLS-Gal4-VP64 fusion, expression of the luciferase is induced (Fig. 2A).

Figure 2: Applying Fusion Guide RNAs for Cas Staples. A, schematic overview of the assay. An enhancer plasmid and a reporter plasmid are brought into proximity by an fgRNA Cas staple complex binding both plasmids. Target sequences were included in multiple repeats prior to the functional elements. Firefly luciferase serves as the reporter gene, the enhancer is constituted by multiple Gal4 repeats that are bound by a Gal4-VP64 fusion. B, results of using a fgRNA Cas staple for trans activation of firefly luciferase. Firefly luciferase activity was measured 48h after transfection. Normalized against ubiquitously expressed Renilla luciferase. Statistical significance was calculated with ordinary One-way ANOVA with Dunn's method for multiple comparisons (*p < 0.05; **p < 0.01; ***p < 0.001; mean +/- SD). The assay included sgRNAs and fgRNAs with linker lengths from 0 nt to 40 nt.

This transactivation has also been shown using our fusion dCas protein (BBa_K5237003) in a Cas staple with fgRNAs of different linker lengths (Fig. 3)

Figure 3: Results of Implementing Fusion Cas Proteins in Trans Activation A, schematic overview of the assay. An enhancer plasmid and a reporter plasmid are brought into proximity by a fgRNA Cas staple complex binding both plasmids. Target sequences were included in multiple repeats prior to the functional elements. Firefly luciferase serves as the reporter gene, the enhancer is constituted by multiple Gal4 repeats that are bound by a Gal4-VP64 fusion. B, results of using a fgRNA Cas staple for trans activation of firefly luciferase. Firefly luciferase activity was measured 48h after transfection. Normalized against ubiquitously expressed Renilla luciferase. Statistical significance was calculated with ordinary One-way ANOVA with Dunn's method for multiple comparisons (*p < 0.05; **p < 0.01; ***p < 0.001; mean +/- SD). The assay included sgRNAs and fgRNAs with linker lengths from 0 nt to 40 nt.

Figure 4: SDS-PAGE and EMSA Analysis of the TetR-Oct1 Fusion Protein. Left: Fractions were loaded on a 4-15 % SDS-PAGE gel and stained with coomassie blue. Lane 1: raw lysate, Lane 2: flow through, Lane 3: purified fraction. Right: Electrophoretic Mobility Shift Assay of tetR-Oct1 in PBS (15 µM, 7.5 µM, 3.75 µM, 1.875 µM, 0.9375 µM protein with 0.5 µM DNA)

5. References

Kakidani, H., & Ptashne, M. (1988). GAL4 activates gene expression in mammalian cells. Cell, 52, 161-167. https://doi.org/10.1016/0092-8674(88)90504-1.

Lowder, L., Zhou, J., Zhang, Y., Malzahn, A., Zhong, Z., Hsieh, T., Voytas, D., Zhang, Y., & Qi, Y. (2017). Robust Transcriptional Activation in Plants Using Multiplexed CRISPR-Act2.0 and mTALE-Act Systems. Molecular Plant, 11(2), 245-256. https://doi.org/10.1016/j.molp.2017.11.010.

Wang, J., Wu, F., Zhu, S., Xu, Y., Cheng, Z., Wang, J., Li, C., Sheng, P., Zhang, H., Cai, M., Guo, X., Zhang, X., Wang, C., & Wan, J. (2016). Overexpression of OsMYB1R1–VP64 fusion protein increases grain yield in rice by delaying flowering time. FEBS Letters, 590. https://doi.org/10.1002/1873-3468.12374.