Individual Final Project

Dual-peptide Gene-Activated Scaffolds for Combined Osteogenesis and Angiogenesis

Section 1 — Abstract

Significance. Critical size bone defects, which are too large to heal on their own, remain one of the hardest problems in orthopaedic surgery. The current treatment, a recombinant protein called BMP-2, is expensive, requires very high doses, and causes side effects including ectopic bone growth, swelling, and inflammation. BMP-2 also only triggers bone formation; it does nothing for blood vessel growth. Without a blood supply, new bone cannot survive, so this gap is one of the main reasons large defects fail to heal.

Broad objective. The goal of this project is to design a calcium phosphate bone scaffold that delivers DNA instead of recombinant protein. Once implanted, cells migrating into the scaffold take up the DNA and produce the healing peptides themselves, locally at the defect site over the healing window. A single piece of DNA produces two peptides at once: one for bone formation and one for blood vessel growth.

Hypothesis. A 438 bp DNA construct encoding two scaffold anchored peptides, BMP2-MP for osteogenesis and SVVYGLR for angiogenesis, separated by a T2A self-cleaving sequence, can produce both peptides from a single mRNA. Each peptide carries a polyaspartate (polyD₈) tail that sticks to the calcium on the scaffold surface, holding the peptide in place where new tissue is forming.

Specific aims. Aim 1 (this project) covers the DNA design, AlphaFold2 structural validation of the peptides, plasmid amplification and sequence verification, fabricating a porous β-TCP/HA scaffold and loading it with DNA nanoparticles. Aim 2 will test whether the construct works in mammalian cells, using HEK293T transfection followed by osteogenic and angiogenic assays. In parallel, Aim 2 will upgrade the scaffold to a 3D printed ceramic format. Aim 3 envisions the system as a cheaper, more accessible alternative to BMP-2 for clinical bone defect repair.

Methods. The project combines molecular biology (gene design and codon optimisation, Twist Bioscience synthesis, bacterial transformation, miniprep, whole plasmid nanopore sequencing), computational structural biology (AlphaFold2 for predicting peptide geometry), and biomaterials engineering (porogen leached β-TCP/HA scaffold fabrication and calcium phosphate co-precipitation nanoparticle synthesis). Together these establish the foundation for a new class of gene activated bone scaffolds that deliver bone growth and blood vessel signals together.

Section 2 — Project Aims

Aim 1 — Experimental Aim

The first aim of my final project is to design and prepare a dual-peptide gene-activated calcium phosphate scaffold for coupled osteogenesis and angiogenesis by:

• Designing a 438 bp DNA construct co-expressing the osteogenic peptide BMP2-MP and the angiogenic peptide SVVYGLR, each anchored to hydroxyapatite via a polyD₈ tail, separated by a T2A self-cleaving sequence so a single mRNA produces both peptides;
• Codon-optimising the construct for mammalian expression and submitting it as a clonal gene order to Twist Bioscience;
• Validating the predicted structural topology of the two peptides using AlphaFold2 to confirm that the polyD₈ anchor sits far enough from the bioactive site to avoid interfering with receptor binding;
• Preparing a verified plasmid stock through DH5α transformation, miniprep, and whole-plasmid sequence verification;
• Preparing porogen-leached β-TCP/HA scaffold fabrication and surface adsorption loading of DNA-calcium-phosphate nanoparticles.

The detailed step-by-step experimental plan for Aim 1 is provided in Section 4 (Experimental Design).

Aim 2 — Development Aim

The second aim is to validate construct biological function and upgrade the scaffold fabrication to a 3D-printed format, building directly on the verified plasmid produced in Aim 1.

Biological validation will be performed in mammalian cell culture: HEK293T cells, and conditioned media will be analysed by anti-FLAG and anti-His Western blots to confirm that both peptides are produced. Osteogenic activity will be measured by alkaline phosphatase (ALP) assay on pre-osteoblast cells, and angiogenic activity by HUVEC tube-formation assay.

In parallel, the scaffold itself will be advanced from the porogen-leached prototype of Aim 1 to a 3D-printed β-TCP/HA scaffold using a ceramic 3D printing platform; either direct ink writing or lithography-based ceramic manufacturing. The 3D-printed scaffolds will then be loaded with the same DNA-nHA nanoparticles developed in Aim 1, and scaffold-mediated transfection efficiency will be validated using seeded HEK293T cells.

Aim 3 — Visionary Aim

The long-term vision is to develop gene-activated dual-function bone scaffolds as a clinical alternative to recombinant BMP-2 for repairing critical-size bone defects.

Current rhBMP-2 protein is expensive, requires supraphysiological dosing that causes ectopic bone formation, swelling, and inflammation, and addresses only osteogenesis; leaving the parallel problem of vascularisation unsolved. My approach replaces protein delivery with continuously replenished, locally secreted, cell-derived peptide signalling, co-expressing osteogenic and angiogenic factors from a single transcript so bone formation and blood vessel growth occur at the same time and the same place. This project directly addressing the critical-size defect bottleneck.

If fully realised, this technology could:

• Reduce per-treatment cost, enabling accessible bone regeneration in low-resource settings where Infuse is unaffordable;
• Eliminate supraphysiological dosing by relying on physiological peptide concentrations produced by the patient’s own cells;
• Serve as a modular platform for other dual-function gene-activated scaffolds (e.g., osteo-immunomodulatory or osteo-antibacterial constructs) by simply swapping the bioactive cassettes;
• Enable point-of-care manufacturing, where a clinician prints a patient-specific scaffold geometry with pre-loaded gene cassettes on demand, using direct-ink-write ceramic 3D printers in a hospital setting.

Section 3 — Background and Literature Context

3.1 Summary of two peer-reviewed citations

Liu et al. (2021), Journal of Biological Engineering 15:21. Liu and colleagues developed a 3D-printed hydroxyapatite scaffold loaded with the BMP-2 mimetic peptide BMP2-MP, anchored to the scaffold by fusing the peptide to a hydroxyapatite-binding sequence. The tethering strategy worked in vitro: peptide retention increased about 10-fold compared with unmodified peptides, and the loaded scaffolds drove significantly stronger osteoblast differentiation. However, the study was entirely cell-based, contained no angiogenic component, and used pre-deposited synthetic peptides rather than gene-encoded production. The result is a demonstration that scaffold-anchored osteogenic peptides outperform soluble peptides in vitro, but with no in vivo testing, no vascular outcome, and a finite peptide dose once consumed.

Raftery et al. (2018), Journal of Controlled Release 283:20-31. Raftery and colleagues built the first cell-free gene-activated scaffold for bone repair, using chitosan-DNA nanoparticles carrying an optimised BMP-2 plasmid loaded onto a collagen-nanohydroxyapatite matrix. When implanted into a 7 mm rat skull defect, the scaffold accelerated early osteogenesis at 4 weeks compared with control scaffolds, establishing that local gene delivery from a bone-mimicking matrix is a viable strategy. However, the study was osteogenesis-only: no angiogenic gene was co-delivered, no vascular endpoint was measured, and the secreted BMP-2 protein had no anchoring mechanism, so it was free to diffuse away from the defect. The rat calvarial model used is also small and highly vascularised by surrounding tissue, meaning the scaffold did not have to supply its own vasculature; in larger, deeper, or less vascular defects, the coupling between osteogenesis and angiogenesis becomes the rate-limiting problem (Grosso et al. 2017), and gene-activated osteogenesis alone is not sufficient.

3.2 Novelty

The headline novelty of this project is the co-expression of an osteogenic peptide (BMP2-MP, Saito et al. 2003) and an angiogenic peptide (SVVYGLR, derived from osteopontin, Hamada et al. 2003) from a single 438 bp mRNA, separated by a T2A self-cleaving sequence that releases both peptides from one transcript at a defined ratio (Liu et al. 2017). To my knowledge, no published gene-activated scaffold has used a single polycistronic transgene to deliver both an osteogenic and an angiogenic peptide mimetic in this way. The dual cassette directly addresses the osteogenesis-angiogenesis coupling problem identified in Grosso et al. (2017), which the gene-activated osteogenesis-only approach in Raftery et al. (2018) leaves unsolved.

The second component of the novelty is the gene-activated production format. Following Raftery et al. (2018), infiltrating cells transcribe and translate the plasmid locally, producing the bioactive peptides continuously over the healing window rather than relying on a one-time peptide deposit. This contrasts with the synthetic peptide-soaking approach of Liu et al. (2021), where the dose is finite and consumed once. Continuous local production lowers the peptide mass required and sustains the signal for as long as the plasmid is expressed (D’Mello et al. 2017).

The third component is scaffold anchoring. Each peptide carries a polyaspartate (polyD8) tail that immobilises it on the calcium phosphate scaffold after secretion, exploiting the well-characterised affinity of acidic amino acid stretches for hydroxyapatite (Murphy et al. 2007; Hunter and Goldberg 1994). This addresses the diffusion problem that limits both current recombinant BMP-2 protein therapy and Raftery’s gene-activated approach, where the secreted protein has no mechanism to stay at the defect site.

The novelty therefore lies in the integration of these three features — dual osteogenic and angiogenic co-expression, continuous gene-activated production, and electrostatic scaffold anchoring.

3.3 Why this matters

The problem addressed. Critical-sized bone defects do not heal on their own. They are caused by trauma, tumour resection, congenital conditions, and severe infection, and they leave patients with lasting disability if untreated.

Importance of the problem. The current biologic standard, recombinant human BMP-2 protein therapy, is expensive and is associated with side effects including unwanted bone growth at non-target sites, postoperative swelling, and inflammation (Tannoury and An 2014). Beyond cost and safety, the underlying biology has been clear for over a decade: bone regeneration cannot be uncoupled from vascular ingrowth, because new bone requires perfusion for mineral, oxygen, and progenitor cell delivery (Grosso et al. 2017). Most current scaffolds still target osteogenesis alone, which is why even successful osteogenesis can fail to produce stable, integrated bone tissue in larger or less vascular defects.

Broader societal contribution. A substantially cheaper alternative to current protein therapy could make bone defect treatment viable in healthcare systems where it is not currently affordable, particularly in low-resource settings.

Advancement of knowledge or capability. The same single-mRNA, scaffold-anchored cassette is a generalisable platform that could be retargeted to other dual-function applications, such as combining osteogenesis with antibacterial or immunomodulatory peptides, simply by swapping the second cassette.

Field-level change. This would shift bone tissue engineering away from expensive recombinant protein delivery toward locally synthesised peptide signalling produced by the patient’s own infiltrating cells. The scaffold becomes both a structural support and a local source of the biological signals a healing defect needs, rather than a passive carrier for an externally manufactured protein.

3.4 Ethics

This project carries low immediate ethical risk because it is a paper-stage protocol with no human or animal work performed, but the technology it sets up does have real ethical weight if it advances. The strongest principles in play are justice and beneficence: recombinant BMP-2 protein therapy is effective but expensive, leaving many patients with critical-sized bone defects without access to current biologic treatment. A gene-encoded alternative that lowers per-treatment cost could meaningfully improve access in low-resource settings. Working against this is non-maleficence: the construct uses a non-integrating plasmid, so expression is transient and lower-risk than viral gene therapy, but transient does not mean zero risk. Immune responses to the plasmid or to the secreted peptides, off-target signalling if peptides diffuse before binding, and ectopic mineralisation all remain credible concerns once the system is tested in vivo. Responsibility at the communication level is also important: this is a paper protocol, not a validated therapy, and writing about it should not exceed what has actually been demonstrated.

The construct must be sequence-verified before any biological use, and any future in vivo testing would require ethical and animal welfare approval. Several things could still go wrong. The most important is the assumption that short peptide mimetics fully recapitulate the activity of their parent proteins; if they do not, the in vivo effect may be smaller than the literature suggests. Polyaspartate anchoring may also be less complete than predicted, allowing some secreted peptide to act on neighbouring tissue. If the dual-peptide design fails, established alternatives remain available, including full-length BMP-2 protein delivery and separate VEGF protein delivery for vascularisation, which are the technologies this project is trying to improve on rather than replace overnight.

Section 4 — Experimental Design, Techniques, Tools, and Technology

4.1 Project overview and Aim 1 scope

This section documents the wet-lab workflow for Aim 1: receive the Twist-synthesised dual-peptide construct, amplify it in E. coli, sequence-verify the plasmid, fabricate a porogen-leached β-TCP/HA ceramic scaffold, and load DNA-calcium-phosphate nanoparticles onto the scaffold by surface adsorption. The protocol is written as a 9-day paper protocol that I will not execute before the HTGAA presentation because we have not received our Twist orders yet.

4.2 DNA Construct Design

The full construct is a 438 bp insert encoding two peptide cassettes separated by a T2A “self-cleaving” sequence so that one mRNA produces two separate peptides. The cassette layout, cloned into the pTwist CMV vector, is:

Kozak — Igκ#1 — D — BMP2-MP — (GGGGS)₂ — polyD8 — FLAG — GSG — T2A — Igκ#2 — D — SVVYGLR — (GGGGS)₂ — polyD8 — GSG — 6xHis — Stop

Domain-by-domain function

Design rationale and codon optimisation

T2A was selected over alternative 2A peptides (F2A, E2A, P2A) because Liu et al. 2017 reported it is among the top-performing self-cleaving peptides when paired with an upstream GSG linker; exactly the arrangement I have used before each T2A. The polyD8 anchor was chosen over single-domain hydroxyapatite-binding peptides (e.g., HABP) because Murphy 2007 demonstrates that an ~8-residue aspartate run is the minimum length for high-affinity calcium-mineral binding while remaining short enough not to disturb the upstream peptide fold. Using polyD8 also avoids dependence on secondary structure; it works through electrostatics regardless of how the peptide folds, which is important for a gene-encoded design where folding cannot be guaranteed.

The two cassettes are duplicated (Igκ leader + bioactive peptide + (GGGGS)₂ linker + polyD8 anchor + epitope tag) but use codon-diverged Igκ sequences (Igκ#1 and Igκ#2) to prevent the bacterial host from recombining the two identical regions during plasmid replication; a standard precaution when including repeat sequences in a single construct.

The final 438 bp design was passed through Twist’s V1 codon optimiser to maximise mammalian expression while preserving the NcoI start codon and avoiding internal BseSI, SgrAI, and MspA1I restriction sites.

4.3 AlphaFold2 structural validation

To verify that my construct design would produce peptides with the correct spatial geometry — specifically, that the polyD8 anchor would sit far enough away from the bioactive site to not interfere with receptor binding — I performed structural prediction on three peptides using AlphaFold2 via ColabFold (Mirdita 2022, Jumper 2021).

What I did:

1. For each peptide, I prepared the full mature sequence as it would exist after secretion and processing (the bioactive site + (GGGGS)₂ linker + polyD8 anchor + epitope tag), giving 30–50 residues per peptide.
2. The three peptides submitted were:
   • My osteogenic peptide (BMP2-MP + (GGGGS)₂ + polyD8 + FLAG; 47 residues)
   • My angiogenic peptide (SVVYGLR + (GGGGS)₂ + polyD8 + 6xHis; 35 residues)
   • Liu 2021 benchmark (BMP2-MP + (GGGGS)₂ + HABP; 50 residues) — the published reference design I am improving on.
3. Using a short Python script with the BioPython Bio.PDB module, I extracted the alpha-carbon (Cα) positions of all residues, computed the centroid (average position) of the bioactive site and the polyD8 anchor, and measured the 3D distance between them.

What this tells me: the distance between the bioactive site and the polyD8 anchor confirms whether the design hypothesis holds — that the (GGGGS)₂ linker physically separates the two domains so that the anchor doesn’t sterically block receptor binding. Comparison with Liu 2021’s published HABP-based design shows whether my polyD8 strategy achieves comparable spatial topology to a literature benchmark.

The full quantitative results, pLDDT confidence plots, and three-way comparison figure are in Section 5 (Validation).

4.4 Day 0 — Receive and resuspend Twist plasmid

1. Spin the tube briefly to bring the dried DNA to the bottom.
2. Add 20 µL of Buffer EB (a mild Tris buffer that protects DNA from degradation during long-term storage).
3. Vortex briefly, then let it sit at room temperature for 5 minutes to dissolve.
4. Measure the concentration on a NanoDrop (a small spectrophotometer that reads DNA concentration by shining UV light through a tiny droplet).
5. Aliquot into small tubes and store at −20 °C.

4.5 Day 1 — Heat-shock transformation into DH5α

I will use NEB DH5α competent cells.

1. Thaw a 50 µL tube of competent cells on ice for 10 minutes.
2. Add 1 µL of my plasmid; flick the tube gently to mix. Don’t vortex.
3. Incubate on ice for 30 minutes.
4. Heat-shock at exactly 42 °C for 45 seconds in a water bath.
5. Return to ice for 2 minutes.
6. Add 950 µL of room-temperature SOC medium.
7. Recover at 37 °C, 250 rpm shaking, for 60 minutes.
8. Plate 100 µL onto an LB-agar plate containing 100 µg/mL ampicillin.
9. Incubate inverted overnight at 37 °C.

The next morning, I should see between 50 and 500 isolated bacterial colonies on the plate.

4.6 Day 2 — Pick colonies and grow overnight cultures

1. Pick 4 well-isolated colonies using sterile pipette tips (picking 4 gives backup options in case some colonies have plasmid problems).
2. Drop each tip into its own 5 mL tube of LB broth with 100 µg/mL ampicillin.
3. Shake at 37 °C, 250 rpm, for 16 hours.

By morning each tube will be cloudy with bacteria, each carrying millions of plasmid copies.

4.7 Day 3 — Miniprep

I will use the Qiagen QIAprep Spin Miniprep Kit, following the standard protocol.

1. Spin down each 5 mL culture to pellet the bacteria.
2. Resuspend the pellet in Buffer P1 (gentle salt buffer that breaks up the pellet without harming cells).
3. Add Buffer P2 to lyse the cells (alkaline detergent that opens bacterial membranes and releases all DNA). Don’t leave longer than 5 minutes — it damages plasmid DNA.
4. Add Buffer N3 to neutralize (acidic salt that selectively re-folds plasmid DNA while genomic DNA stays tangled and falls out).
5. Spin down the precipitated debris. The supernatant contains pure plasmid + RNA + small proteins.
6. Apply supernatant to the silica column. Wash with Buffer PE.
7. Elute the pure plasmid in 50 µL of Buffer EB.
8. Quality check on NanoDrop: aim for A260/A280 between 1.8 and 2.0 (this confirms DNA purity).

Expected yield: 5–30 µg per sample.

4.8 Day 4 — Submit for sequencing (Plasmidsaurus)

Even with high-quality DNA synthesis, occasional errors can happen during synthesis or bacterial replication. The polyD8 region of my construct is repetitive (8 × DDDDDDDD), and bacteria sometimes “slip” during replication of repeats and accidentally delete a few bases. Sequencing catches these errors before I waste downstream experiments on a broken plasmid.

1. Take 10 µL from each miniprep.
2. Submit to Plasmidsaurus for whole-plasmid nanopore sequencing (a service that reads my entire plasmid in one go using nanopore technology, which threads DNA through tiny pores and reads each base electrically).

Results return overnight.

4.9 Day 5 — Receive sequence, verify, glycerol stock

1. Open the Plasmidsaurus result file in Benchling.
2. Align it against my designed sequence. Pass criteria:
   • Zero SNPs (no single-base mismatches anywhere)
   • Zero indels (no insertions or deletions, especially in the polyD8 regions)
   • ≥30× coverage (each base read at least 30 times for confidence)
3. If at least one of my 4 samples passes, I pick that as my verified working clone.
4. Make a glycerol stock for permanent backup: mix 500 µL of overnight culture with 500 µL of 50% glycerol; store at −80 °C (this lets me regrow the plasmid forever from this single tube).

4.10 Day 6 — Fabricate β-TCP/HA porogen-leached scaffold

The locked formulation is a gelatin-bonded ceramic scaffold with NaCl as a sacrificial porogen. β-TCP is the resorbable osteoconductive ceramic, nano-hydroxyapatite (nHA) provides the polyD8-binding surface, and the 200–500 µm NaCl porogen produces macropores in the size range needed for cell migration and capillary formation (Karageorgiou & Kaplan 2005).

Formulation per ~1 g scaffold:

• β-TCP powder: 60 wt% = 600 mg
• Nano-hydroxyapatite (nHA): 10 wt% = 100 mg
• NaCl crystals, sieved 200–500 µm: 30 wt% = 300 mg
• 10 wt% gelatin solution: 0.5 mL (binder, kept at 50 °C)

Steps:

1. Sieve NaCl through stacked 200 µm and 500 µm sieves; collect the 200–500 µm fraction. Discard fines.
2. Make 10% gelatin: dissolve 1.0 g gelatin in 9 mL warm dH₂O at 50 °C with magnetic stirring, ~15 min until clear.
3. Dry-mix powders (β-TCP + nHA + sieved NaCl) in a clean agate mortar for 60 sec.
4. Add 0.5 mL warm gelatin solution to the mortar; mix to a homogeneous paste, ~60 sec (keep gelatin warm or it gels mid-mix).
5. Press paste into mould. Preferred: 8 mm × 3 mm silicone cylinder mould. Acceptable alternatives: 35 mm petri dish at 3 mm thickness for hand-shaped disc, or a 24-well plate as an improvised mould.
6. Smooth the top surface flat with a spatula.
7. Cool at 4 °C for 30 min (gelatin sets, immobilising the powders).
8. Dry at 37 °C overnight (16–24 h) in a standard incubator.
9. Demould carefully with a flat spatula.

4.11 Day 7 — Salt leaching (porogenesis)

1. Transfer demoulded scaffolds to 50 mL Falcon tubes containing 30 mL sterile dH₂O at 37 °C.
2. Place on a tube roller at 10 rpm for 48 h (gentle agitation, no mechanical erosion).
3. Exchange water at 24 h to maintain dissolution.

After 48 h, the spaces once occupied by NaCl crystals are open 200–500 µm macropores connected by smaller channels; this is the porous scaffold structure that cells will infiltrate.

4.12 Day 8 — Final scaffold drying

1. Transfer scaffolds to fresh sterile dH₂O for 1 h to rinse residual NaCl.
2. Air-dry in a desiccator over silica gel at 4 °C for 48 h.
3. Store dry in a sealed glass vial with desiccant at room temperature.
4. Sterilise by UV exposure (15 min in a biosafety cabinet) immediately before use.

4.13 Day 9 — Prepare DNA-nHA nanoparticles

The plasmid DNA needs to be packaged into calcium phosphate nanoparticles so that cells can take it up via endocytosis. Naked DNA cannot enter cells efficiently because both DNA and cell membranes are negatively charged and repel each other. The nanoparticle wrapping solves this problem (Raftery 2018).

This method uses calcium phosphate co-precipitation: when calcium ions and phosphate ions meet in solution at the right pH, they form tiny calcium phosphate particles. If DNA is present at the same time, it gets trapped inside the particles as they form.

Reagents — 2× HBS (HEPES Buffered Saline) at pH 7.05:

• NaCl (salt for physiological ionic strength)
• HEPES (buffer that keeps pH stable at 7.05)
• Na₂HPO₄ (provides the phosphate that reacts with calcium)
• KCl (mimics intracellular potassium levels)
• Glucose (mimics blood sugar)

pH adjusted to exactly 7.05. This is critical because pH changes the size of the particles formed.

Steps per scaffold:

1. Tube A: mix 20 µg of my verified plasmid with calcium chloride (CaCl₂) in 250 µL of sterile dH₂O.
2. Tube B: 250 µL of 2× HBS at pH 7.05.
3. Set Tube B on a vortex at medium speed.
4. While vortexing, add Tube A dropwise into Tube B over ~25 seconds (adding slowly while vortexing ensures DNA gets trapped inside the particles as they form, rather than just sticking to the outside).
5. Let sit at room temperature for 25 minutes. A faint cloudy appearance after 5–10 minutes confirms nanoparticles are forming.
6. Use within 45 minutes; particles grow too large after that.

Expected result: nanoparticles of 100–300 nm containing my plasmid DNA.

4.14 Day 9 — Surface adsorption loading

The nanoparticles now need to be coated onto the scaffold so that cells migrating into the scaffold encounter them.

1. Mix the 500 µL of fresh nanoparticle suspension with 50 µL of sucrose solution. Why sucrose: sucrose acts as a protective shield around the DNA during drying — it prevents ice damage during the drying process.
2. UV-sterilize the scaffold in a biosafety cabinet for 15 minutes.
3. Pipette the nanoparticle-sucrose mixture onto the scaffold in 10 drops of 50 µL each, distributed evenly across the surface.
4. Let each drop wick into the scaffold (~5 seconds, the porous structure pulls the liquid in by capillary action) before placing the next drop.
5. Dry the loaded scaffold: air-dry in a desiccator (a sealed box containing silica gel beads) at 4 °C for 48 hours. This makes the scaffold shelf-stable for storage.
6. Store finished scaffolds in sealed glass vials with desiccant at −20 °C.

Section 4.2 — Synthetic Biology Techniques Checklist

The following techniques from the HTGAA list are relevant to my project:

• Pipetting category: Pipetting, Lab Safety, and Bioethical Considerations.
• DNA Gel Art category: DNA Sequencing, DNA Construct Design, and Databases (NCBI, UniProt, GenBank, Benchling annotation).
• Lab Automation category: Designing a Twist Order.
• Protein Design category: Protein Design, Use of Benchling, Models and Notebooks (AlphaFold2 via ColabFold), and Databases.
• Bioproduction category: Bioproduction, Chassis Selection (DH5α), Plasmid Preparation, Bacterial Culturing, Quality Control/Analysis, and Bacterial Processing (centrifugation, alkaline lysis, silica DNA purification).

Section 4.17 — Two Techniques Expanded

4.17.1 DNA Construct Design

The 17-domain dual-cassette was designed in Benchling by laying out each functional element as an annotated block. T2A was selected over alternative self-cleaving peptides (F2A, E2A, P2A) because Liu et al. 2017 reported it is among the top-performing options when paired with an upstream Gly-Ser-Gly linker, which is the context I have used before each T2A. The polyD8 anchor was chosen over single-domain hydroxyapatite-binding peptides because Murphy 2007 demonstrates an 8-residue aspartate run is the minimum length for high-affinity calcium-mineral binding while remaining short enough not to disturb the upstream peptide fold. The final 438 bp design was codon-optimised in Twist’s V1 optimiser to maximise mammalian expression while preserving the NcoI start codon and avoiding internal restriction sites, so the synthesised insert drops cleanly into the pTwist-CMV backbone.

4.17.2 Use of Benchling and Models/Notebooks (AlphaFold2 / ColabFold)

Three peptides were submitted to ColabFold running AlphaFold2: my osteogenic cassette, my angiogenic cassette, and a published reference benchmark. For each peptide I submitted the full expressed sequence (Igκ leader, bioactive site, linker, polyD8 anchor) so I could measure the spatial distance between the bioactive site and the polyD8 anchor in the predicted 3D structure. Distances were extracted from the predicted PDB files using BioPython’s Bio.PDB module in a Jupyter notebook. The results were consistent with the design hypothesis that the polyD8 anchor sits far enough from the bioactive site to avoid steric interference with receptor binding; full structural results are in Section 5.

Section 5 — Validation Results

5.1 Sequence design validation

The 438 bp construct passed Twist Bioscience’s automated design review on the first submission (V1). Twist’s design screen checks for synthesis-blocking features (repetitive sequences, extreme GC content, problematic secondary structure) and for the restriction sites that must be preserved or avoided for downstream cloning. The V1 design preserved the NcoI start codon and avoided internal BseSI, SgrAI, and MspA1I sites, so the synthesised insert drops cleanly into the pTwist-CMV backbone. This validates the sequence at the design level before any biological work begins.

5.2 Structural validation (AlphaFold2 / ColabFold)

To confirm that the polyD8 anchor in each cassette would sit far enough from the active peptide site to avoid steric interference with receptor binding, I performed structural prediction on three peptides using AlphaFold2 via ColabFold:

1. My osteogenic cassette (BMP2-MP, (GGGGS)₂ linker, polyD8 anchor, FLAG tag)
2. My angiogenic cassette (SVVYGLR, (GGGGS)₂ linker, polyD8 anchor, 6xHis tag)
3. Liu et al. 2021 benchmark design (BMP2-MP fused to HABP), used as a published reference

For each peptide, I submitted the full expressed sequence and extracted the predicted 3D structure. Using BioPython’s Bio.PDB module in a Jupyter notebook, I measured the 3D distance between the centroid of the active peptide site and the centroid of the anchor segment.

Results:

Interpretation. All three distances fall in the same range, with my designs producing equal or greater separation than the published reference. This supports the design hypothesis that the polyD8 anchor sits far enough from the active site to avoid blocking receptor binding. The result also confirms that the (GGGGS)₂ flexible linker provides sufficient physical separation between the two functional regions, consistent with the role this linker plays in the literature.

5.3 Confidence and limitations of the structural validation

AlphaFold2 predictions are computational, not experimental. Each prediction comes with per-residue confidence scores (pLDDT) and pairwise error estimates (PAE), and the predictions used here had confidence patterns typical for short peptides with flexible linkers, namely high confidence in the active site and anchor segments and lower confidence in the linker region itself. This is expected and does not affect the distance measurement, because the active site and the anchor are the high-confidence regions being measured. The relevant limitation is that structure prediction cannot verify peptide activity, receptor binding, or anchoring strength; it only confirms that the spatial geometry of the design is plausible. Real biological validation requires the wet-lab assays planned in Aim 2.

5.4 What has not yet been validated

The following experimental readouts are planned but have not been performed in this project (Aim 1) and constitute the immediate next steps under Aim 2:

• Construct expression and T2A self-cleavage in mammalian cells (HEK293T transient transfection followed by anti-FLAG and anti-His Western blots).
• Osteogenic activity of the secreted BMP2-MP peptide (alkaline phosphatase assay on osteoblast precursor cells).
• Angiogenic activity of the secreted SVVYGLR peptide (endothelial cell tube formation assay on Matrigel).
• Scaffold-mediated DNA delivery (transfection efficiency when cells are seeded onto the loaded scaffold).
• In vivo bone defect repair (Aim 3 future work).

The Aim 1 deliverables are therefore the verified construct design, the AlphaFold2 structural confirmation, and plasmid amplification, scaffold fabrication, and nanoparticle loading documented in Section 4.

Section 6 — References

1. Liu Z, Wu S, Li J, Zhang L, Wang Y, Gao H, Cao J. Three-dimensional printed hydroxyapatite bone tissue engineering scaffold with antibacterial and osteogenic ability. J Biol Eng. 2021;15(1):21. doi:10.1186/s13036-021-00273-6
2. Raftery RM, Mencía-Castaño I, Sperger S, et al. Delivery of the improved BMP-2-Advanced plasmid DNA within a gene-activated scaffold accelerates mesenchymal stem cell osteogenesis and critical size defect repair. J Control Release. 2018;283:20-31. doi:10.1016/j.jconrel.2018.05.022
3. Saito A, Suzuki Y, Ogata S, et al. Activation of osteo-progenitor cells by a novel synthetic peptide derived from the bone morphogenetic protein-2 knuckle epitope. Biochim Biophys Acta. 2003;1651(1-2):60-67. doi:10.1016/S1570-9639(03)00235-8
4. Hamada Y, Nokihara K, Okazaki M, et al. Angiogenic activity of osteopontin-derived peptide SVVYGLR. Biochem Biophys Res Commun. 2003;310(1):153-157. doi:10.1016/j.bbrc.2003.09.001
5. Murphy MB, Hartgerink JD, Goepferich A, Mikos AG. Synthesis and in vitro hydroxyapatite binding of peptides conjugated to calcium-binding moieties. Biomacromolecules. 2007;8(7):2237-2243. doi:10.1021/bm070121s
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