Week 9 HW: Cell-Free Systems & Synthetic Cells

Week 9 Homework: Cell-Free Systems, Synthetic Cells & Space Biology

Cell-free protein synthesis, synthetic minimal cells, freeze-dried materials, and a mock Genes in Space proposal — with a consistent theme: radiation mitigation via SOD3 (extracellular superoxide dismutase) and/or CXCR4 (chemokine receptor–mediated homing to stressed or marrow-associated niches).

     ╔═══════════════════════════════════════════════════════════════╗
     ║  WEEK 9 — CELL-FREE · SMC · MATERIALS · SPACE                 ║
     ║                                                               ║
     ║     ROS  ──►  SOD3  (scavenge O₂⁻)                            ║
     ║      │                                                        ║
     ║      └──►  CXCR4  (homing / niche targeting — radiation axis) ║
     ║                                                               ║
     ║   Parts: General CF · Kate SMC · Peter pitch · Ally Space     ║
     ╚═══════════════════════════════════════════════════════════════╝

Part A — General homework questions (cell-free fundamentals)

1. Advantages of cell-free protein synthesis vs traditional in vivo methods (flexibility & control)

Why cell-free wins on flexibility and control

Advantage	What you control
Open reaction	Add or omit cofactors, chaperones, lipids, detergents, redox buffers, and radiomimetic chemicals without worrying about cytotoxicity or transport into live cells.
No growth phase	Start “expression” immediately; no coupling to doubling time, medium composition for viability, or overflow metabolism.
Template choice	Linear PCR DNA, plasmids, or IVT RNA — fast design–test cycles without cloning into a chassis for every iteration.
Sampling	Aliquot the same batch over time; pair with analytics (gel, activity, mass spec) without lysing a culture.

Two cases where cell-free beats cell production

Rapid prototyping of toxic or burden-heavy proteins (e.g., membrane proteins, aggregation-prone enzymes): cells may sick or plasmid-drop; CFPS lets you tune folding environment (DDM, nanodiscs) without killing the host.
On-demand or deployable synthesis (field, clinic, space): freeze-dried lysates rehydrated with water + template match “use when needed” workflows poorly suited to maintaining sterile cultures.

  IN VIVO (culture)                    OPEN CFPS (tube / paper)
  ╭──────────╮                         ╭──────────────────────╮
  │ membrane │  walls, pumps, growth    │  DNA + extract + NTPs │
  │  + cell  │  coupling                │  (you add what you    │
  │  biology │                          │   need, when)         │
  ╰────┬─────╯                         ╰───────────┬──────────╯
       │                                         │
       ▼                                         ▼
   doubling time                            start/stop on demand
   viability constraints                  no “is the cell happy?”

2. Main components of a cell-free expression system and their roles

   [ DNA or mRNA template ]
            │
            ▼
   ┌────────────────────────────────────────┐
   │  Ribosomes · tRNAs · aaRS · factors    │  translation
   │  NTPs · amino acids                    │  building blocks
   │  Energy system (ATP/GTP + regeneration)│  drives Tx/Tl
   │  Buffer · salts · Mg²⁺ · optional      │  chemistry & folding
   │    chaperones · DTT · PEG · crowding   │
   └────────────────────────────────────────┘

Component	Role
Ribosomes	Peptide bond formation; core of translation.
tRNAs + aminoacyl-tRNA synthetases	Deliver correct amino acids to the ribosome.
Transcription/translation factors	Initiation, elongation, termination (system-specific).
NTPs (ATP, GTP, CTP, UTP)	Energy and RNA synthesis; GTP for translation steps.
Amino acids	Protein polymer building blocks.
Template (DNA or mRNA)	Program for target protein (e.g., SOD3, CXCR4).
Buffer + ions (e.g., Mg²⁺, K⁺)	Optimal pH/ionic strength for enzymes and ribosomes.
Energy regeneration	Recycles ADP/AMP → ATP so Tx/Tl does not stall (see below).
Optional: chaperones, lipids, detergents	Folding helpers; membrane protein expression.

3. Why energy regeneration is critical; continuous ATP supply

Why it matters: Transcription and translation consume ATP and GTP continuously. Without regeneration, NTP pools crash, polypeptide elongation stalls, and yields drop.

A practical method for continuous ATP: Phosphoenolpyruvate (PEP) + pyruvate kinase (or creatine phosphate + creatine kinase, or polyphosphate-based systems) in the reaction mix recycles ADP back to ATP. Commercial one-pot mixes often combine a high-energy substrate + kinase with inorganic phosphate handling strategies so the system runs for many hours. For your experiment: use a validated regeneration module at manufacturer-recommended ratios, titrate Mg²⁺ (ATP chelates Mg), and consider substrate feeding or semi-continuous addition in long reactions.

        ENERGY LOOP (why the reaction does not die in 5 minutes)
        ═══════════════════════════════════════════════════════

              ┌─────────────┐
         ┌───►│ Tx / Tl     │───┐
         │    │ (burns NTPs)│   │
         │    └─────────────┘   │
         │                      ▼
         │               ADP + Pi  (pool would empty)
         │                      │
         │    ┌─────────────────┴─────────────────┐
         │    │  PEP + PK  or  CrP + CK  or  polyP  │
         └─── │       “regeneration module”         │
              └─────────────────┬─────────────────┘
                                ▼
                              ATP  ──► back to ribosome / polymerases

4. Prokaryotic vs eukaryotic cell-free systems + one protein each

Aspect	Prokaryotic (e.g., E. coli extract)	Eukaryotic (e.g., wheat germ, insect, HEK lysate)
Strengths	High yield, inexpensive, fast, well-characterized	Better for disulfides, glycosylation, some GPCRs
PTMs	Limited	Closer to mammalian N-glycans (still platform-dependent)
Promoters / regulation	Strong bacterial promoters	May need eukaryotic elements if you use certain mammalian switches

Example proteins

System	Protein	Why this system
Prokaryotic CFPS	Truncated or tag-fused SOD3 variant for activity assays	Fast iteration of soluble antioxidant enzyme domains; bacterial CF is cheap for screening fusion partners and solubility tags before mammalian polish.
Eukaryotic CFPS	Full-length CXCR4 (or a stable nanobody against CXCR4)	GPCR folding and ligand binding benefit from eukaryotic membranes/chaperones; use for radiation-homing logic in a nanodisc or proteoliposome readout.

Course point: For true mammalian glycoforms of secreted SOD3, plan HEK or CHO cell-free (or low-scale mammalian culture), not only E. coli lysate.

   PROKARYOTIC EXTRACT              EUKARYOTIC EXTRACT
   (E. coli lysate)                (wheat germ / HEK lysate)
        │                                  │
        │  fast · cheap                    │  PTMs · some GPCRs
        │  good for screens                │  slower / pricier
        ▼                                  ▼
   SOD3 domain fusions                 full-length CXCR4
   solubility tags                    + nanodisc / CHS

5. Designing a cell-free experiment for a membrane protein (e.g., CXCR4) — challenges & fixes

Goal: Express CXCR4 in a defined lipid environment to study SDF-1α/CXCL12 binding in a radiation-relevant context (e.g., niche homing).

Setup sketch

   MSP + lipid  ──►  nanodiscs
         +
   CFPS reaction  ──►  CXCR4 co-translationally inserted
         │
         ▼
   [ ligand binding / FRET / radioligand displacement ]

        NANODISC + GPCR (idea)
        ══════════════════════

           MSP (scaffold protein)
        ~~~╱‾‾‾‾‾‾‾‾‾‾‾‾‾╲~~~
       ╱    lipid bilayer    ╲
      │   ◄── CXCR4 7TM ──►   │
       ╲_______╱‾‾╲________╱
            │      │
            └── SDF-1α / CXCL12 (ligand) binds here

Challenges and mitigations

Challenge	Mitigation
Aggregation	Lower temperature, titrate Mg²⁺, add chaperones (e.g., DnaK system in bacterial extract where applicable), use C-terminal fusion (e.g., BRIL) for stability.
Incorrect topology	Supply lipid nanodiscs or detergent below critical micelle concentration; consider eukaryotic extract for eukaryotic GPCRs.
Low functional fraction	Add fluorescent ligand binding or structural readout (e.g., stable-isotope labeling where available); compare total protein (gel) vs specific activity.

6. Low yield — three causes and troubleshooting

Possible cause	What to check	Strategy
Degraded or poor template	Agarose gel of DNA; A260/280	Fresh PCR, codon optimization, clean-up beads, stronger T7 promoter layout.
Energy exhaustion	Time course of luciferase control	Increase regeneration components, shorten reaction, or fed-batch addition.
Toxic misfolding / aggregation	Pellet vs supernatant, smear on gel	Lower temperature, fusion tags, chaperones, redox (for disulfides), or switch to eukaryotic extract for SOD3/CXCR4.

  LOW YIELD?  ──►  check template  ──►  still bad?  ──►  check energy (ATP)
        │                    │                              │
        │                    │                              │
        ▼                    ▼                              ▼
   new DNA / codons     gel + A260/280              add PEP / shorten run
   stronger promoter     PCR cleanup                 luciferase control

Part B — Homework question from Kate Adamala: synthetic minimal cell

Theme: A synthetic minimal compartment that supports radiation-stress mitigation by producing SOD3 and presenting CXCR4 for homing to SDF-1–rich niches (e.g., marrow/stromal signals relevant after damage).

Pick a function

Function: “Radiation-response micro-factory + homing beacon” — sense a proxy of oxidative stress or an external trigger, synthesize SOD3 locally, and display CXCR4 to engage CXCL12 gradients near repair niches.

Input / output


Input	H₂O₂ (ROS proxy) or gamma/UV pulse to the compartment environment (conceptual stand-in for radiation-induced ROS); optionally theophylline (small molecule) if using a riboswitch for tight Tx control.
Output	Secreted/active SOD3 (reduce local O₂⁻); surface-exposed CXCR4 for adhesion/homing assays toward CXCL12.

Could this work with cell-free Tx/Tl alone, no encapsulation?

Partially, but the full “compartmentalized + spatially localized homing particle” does not. Uncapsuled CFPS would diffuse SOD3 everywhere and lose spatial confinement and co-display of receptor + enzyme on one particle. Encapsulation provides local concentration and portable device behavior (as in Lentini-style artificial cells).

Could a genetically modified natural cell do it?

Yes — an engineered HEK or MSC could co-express SOD3 and CXCR4. Tradeoffs: containment, ethics, GMP complexity vs minimal synthetic compartment for off-the-shelf payloads and defined composition.

Desired outcome

Outcome: After stress, elevated local antioxidant capacity (SOD3) plus CXCR4-mediated binding to CXCL12-presenting surfaces — a testable in vitro model for radiation mitigation and stem-cell niche targeting.

Membrane composition

Synthetic lipids: e.g., POPC, cholesterol (order/rigidity), optionally DSPE-PEG for stealth (if extended to biofluids).

What to encapsulate

Mammalian or hybrid cell-free Tx/Tl (for SOD3 secretion competence and CXCR4 folding).
DNA: SOD3 transgene; CXCR4 with export/folding helpers if co-expression.
Energy mix, crowding agents (e.g., PEG), glutathione for redox.
Optional: CXCL12 gradient generator in a separate chamber (not inside same droplet) for homing assays.

Tx/Tl source: bacterial OK or mammalian?

Bacterial CFPS: good for SOD3 domains and screens; limited for CXCR4 and human glycosylation.
Mammalian (e.g., CHO/HEK lysate) or wheat germ for CXCR4 + full-length SOD3 quality.
Tet-ON and similar often need mammalian regulatory proteins — if your circuit uses Tet-ON, use mammalian extract or hybrid TX.

Communication with environment

                 SYNTHETIC MINIMAL CELL (side view)
        ╭──────────────────────────────────────────────────────╮
  out   │  O₂ / H₂O₂ (small)  ──diffuse──►                     │
        │         │              ╭──────────────╮              │
        │         ▼              │  POPC + chol │  lumen     │
        │    ┌─────────┐         │   bilayer    │  ┌────────┐  │
        │    │ CXCL12  │◄──────►│  CXCR4 (7TM) │  │ Tx/Tl  │  │
        │    │ gradient│ binds  │              │  │ + DNA  │  │
        │    └─────────┘         ╰──────────────╯  │ SOD3→  │  │
        │                                          └────────┘  │
        ╰──────────────────────────────────────────────────────╯
              Large proteins need secretion, pore, or lysis

        outside              membrane bilayer              inside
    ┌────────────────┐   ┌────────────────────────┐   ┌────────────────┐
    │ H₂O₂ / ROS     │──►│ small & neutral: slips  │   │ riboswitch /   │
    │ (or inducer)   │   │ through (no pore)       │──►│ sensor logic   │
    └────────────────┘   └────────────────────────┘   └────────────────┘
    ┌────────────────┐   ┌────────────────────────┐   ┌────────────────┐
    │ CXCL12 ligand  │◄─►│ CXCR4 (receptor face)   │   │ SOD3 synthesis │
    │ (gradient)     │   │ on outer leaflet        │   │ + release path │
    └────────────────┘   └────────────────────────┘   └────────────────┘

H₂O₂ is membrane-permeable; large proteins are not — SOD3 must be secreted or released after compartment lysis or fusion.
CXCR4 sits in the membrane (proteoliposome or nanodisc-coated vesicle). CXCL12 binds externally.

Experimental details — lipids and genes (bonus: specific examples)

Class	Examples
Lipids	POPC, cholesterol, DOPC (optional mixing for fluidity)
Genes	SOD3 (human SOD3); CXCR4 (CXCR4); optional BRIL fusion for GPCR stability; T7 or CMV depending on extract
Controls	Empty vector, catalase-only, CXCR4 without SOD3

How to measure function

SOD3: Cytochrome c reduction assay or fluorescent superoxide probe (compartment vs bulk).
CXCR4: Alexa-CXCL12 binding, flow cytometry on giant vesicles, or SPR on reconstituted membranes.
Radiation proxy: Clonogenic partner cells with H₂O₂ challenge ± vesicles.

Part C — Homework question from Peter Nguyen: freeze-dried cell-free in materials

Field: Textiles / protective wear (first-responder / aerospace / radiology-adjacent contexts).

One-sentence pitch

Freeze-dried E. coli or mammalian CFPS in a hydrogel–textile laminate produces antioxidant SOD3 on hydration to buffer acute ROS after exposure to ionizing-radiation–induced oxidative stress.

How it works (3–4+ sentences)

A nonwoven or knit carries alginate–PEG patches spotted with BioBits-style freeze-dried lysate and plasmid DNA encoding SOD3 (or a secretion-competent variant). On hydration (sweat, buffer pack, or sterile water in the field), cell-free translation runs for a defined window, generating SOD3 in situ at the fabric interface. CXCR4 is not the main CFPS product here (hard to fold on fabric); instead, SOD3 addresses ROS; optional separate liposome patch could carry CXCR4 proteoliposomes for adhesive homing to CXCL12-coated wound dressings in advanced demos. Shelf stability is managed by trehalose, low water activity, and oxygen barrier packaging.

Societal / market need

Occupational radiation exposure, cancer therapy skin injury, and spaceflight oxidative stress all need rapid, infrastructure-light countermeasures beyond static materials.

Limitations (water activation, stability, one-shot)

Limitation	Mitigation
Needs water	Pair with single-use ampoule or sweat-activated reservoir in garment seam.
Stability	Freeze-dry, desiccant pouch, cold chain optional variants.
Single use	Market as disposable patch (ethical clarity); or modular replaceable inserts.

  TEXTILE + FREEZE-DRIED CFPS (concept)
  ═════════════════════════════════════

      woven / knit fabric
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      [≈]  [≈]  [≈]   ← hydrogel dots: lysate + SOD3 DNA
       │    │    │
       └──┬─┴────┘
          ▼
    add H₂O (sweat / ampoule)  ──►  CFPS runs ──►  local SOD3 vs ROS

Part D — Homework question from Ally Huang: mock Genes in Space proposal

Toolkit: BioBits® cell-free protein synthesis, miniPCR®, P51 Molecular Fluorescence Viewer.
Theme: Radiation mitigation — SOD3 expression as a readout of successful DNA repair template function; CXCR4 transcript as a stem-cell / niche marker in a radiation model (conceptual).

Background

Ionizing radiation damages DNA and elevates ROS, risking long-term health on long-duration missions. Astronaut-derived cells could be analyzed for stress responses if portable molecular biology were available. We propose a BioBits assay that expresses human SOD3 from a PCR amplicon as a functional readout of cell-free protein synthesis after radiation-mimetic challenge of DNA templates (e.g., damaged plasmid vs repaired control). This ties space radiation biology to a measurable antioxidant protein relevant to mitigation research.

Molecular / genetic target

Target: Human SOD3 cDNA and CXCR4 amplicon (qPCR-style monitoring optional); GFP reporter cassette for P51 fluorescence.

How target relates to the challenge

SOD3 neutralizes superoxide, a major ROS after radiation. CXCR4 expression marks niche-homing pathways relevant to hematopoietic recovery after radiation — a secondary transcript target. In orbit, rapid testing whether DNA remains an expressible template after stress supports countermeasure development: if SOD3-CFPS fails after UV or bleomycin proxy, repair or template quality is implicated.

Hypothesis / goal

Hypothesis: BioBits reactions programmed with SOD3 DNA produce enzymatic activity proportional to template integrity after radiation-mimetic insult; GFP fluorescence on P51 correlates with yield. Goal: Establish a student-feasible pipeline — miniPCR amplifies SOD3 from synthetic gBlocks, BioBits expresses SOD3–His, and P51 reads GFP internal control. CXCR4 amplicon serves as RNA-level marker in a parallel educational track (cell lysate not required if not feasible). Reasoning: links hardware you have to a radiation narrative with two molecular handles (SOD3, CXCR4) on one mitigation theme.

Experimental plan

Samples: Undamaged plasmid vs UV-treated SOD3 template; no-DNA negative. miniPCR amplifies insert; BioBits 37 °C reaction 2–4 h; P51 measures GFP if co-expressed. Controls: GFP-only, stop codon control. Data: relative fluorescence (P51), dot blot for SOD3–His, SOD activity (cytochrome c assay) on ground lab days. CXCR4: optional gel of PCR product from cDNA if RNA available.

  GENES IN SPACE–STYLE PIPELINE (mock experiment)
  ═══════════════════════════════════════════════

   template DNA          amplify               express (BioBits)
  ┌──────────┐      ┌──────────┐            ┌───────────────────┐
  │ SOD3 +   │ ───► │ miniPCR  │ ─────────► │ 37 °C · 2–4 h     │
  │ GFP ctrl │      │ amplicon │            │ cell-free Tx/Tl   │
  └──────────┘      └──────────┘            └─────────┬─────────┘
                                                     │
         ┌───────────────────────────────────────────┴───────────────┐
         ▼                                                           ▼
   ┌─────────────┐                                           ┌─────────────┐
   │ P51 viewer  │  fluorescence (GFP control)               │ assay bench │
   │ (portable)  │                                           │ SOD3 · dot  │
   └─────────────┘                                           │ blot · etc. │
                                                             └─────────────┘

Quick reference links

Genes in Space: https://www.genesinspace.org/
Lentini-style artificial cells (example class paper): Lentini, R. et al., 2014. Nat. Commun. 5, 4012.
Theophylline aptamer context (example): Martini, L. & Mansy, S.S., 2011. Chem. Commun. 47, 10734.