Week 9 HW: Cell-Free Systems

General homework questions

1- Explain the main advantages of cell-free protein synthesis over traditional in vivo methods, specifically in terms of flexibility and control over experimental variables. Name at least two cases where cell-free expression is more beneficial than cell production.

In traditional in vivo (inside the cell) methods, the cell’s own survival is the priority. In CFPS, your protein is the priority. You can add non-natural amino acids, chaperones, or labeling molecules directly to the mix without worrying about transport across a cell membrane. You can monitor and tweak variables like pH, temperature, and redox potential in real-time. If a protein is lethal to a living cell (e.g., a pore-forming toxin), CFPS is the only way to produce it because there is no “cell” to kill. You can go from a linear DNA template (PCR product) to a protein in hours, whereas cell production requires days for cloning and transformation.

2- Describe the main components of a cell-free expression system and explain the role of each component.

Main Components and Roles Crude Extract: This is the “machinery.” It contains ribosomes, aminoacyl-tRNA synthetases, and translation factors from a lysed cell (like E. coli or yeast).

Energy Solution: Contains ATP and GTP to fuel the reaction, plus an energy regeneration source (like Phosphoenolpyruvate).

Amino Acids: The building blocks for the protein chain.

Cofactors and Salts: Magnesium (Mg2+) and Potassium (K+) are critical for ribosome stability and function.

DNA Template: The “blueprint” (usually a plasmid or PCR fragment) encoding your target protein.

3- Why is energy provision regeneration critical in cell-free systems? Describe a method you could use to ensure continuous ATP supply in your cell-free experiment.

Translation is energy-expensive. For every amino acid added to a chain, multiple high-energy phosphate bonds (ATP/GTP) are consumed. Without regeneration, ATP levels drop rapidly, and inhibitory byproducts (like inorganic phosphate) build up, stopping the reaction in minutes. You can add Creatine Phosphate along with the enzyme Creatine Kinase. This enzyme “recharges” spent ADP back into ATP by transferring a phosphate group from the creatine phosphate, keeping the “battery” full for several hours.

4- Compare prokaryotic versus eukaryotic cell-free expression systems. Choose a protein to produce in each system and explain why.

Prokaryotic vs. Eukaryotic Systems Prokaryotic (e.g., E. coli): Fast, high yield, and cheap. It lacks complex “post-translational modifications” (PTM) like glycosylation.

Protein to produce: GFP (Green Fluorescent Protein). It’s a simple, robust protein that doesn’t need complex folding or sugar tags to function.

Eukaryotic (e.g., CHO or Wheat Germ): Slower and more expensive, but capable of complex folding and adding sugar groups (PTMs).

Protein to produce: Human Erythropoietin (EPO). This protein requires specific glycosylation patterns to be biologically active in humans, which only eukaryotic machinery can provide.

5- How would you design a cell-free experiment to optimize the expression of a membrane protein? Discuss the challenges and how you would address them in your setup.

Membrane proteins are “greasy” (hydrophobic). If produced in a watery cell-free mix, they will clump together (aggregate) and become useless because they have no “home” (lipid bilayer) to sit in. We can maybe add Nanodiscs or Liposomes. Include synthetic lipid bilayers in the reaction. As the protein is synthesized, it can insert directly into these membranes, maintaining its native shape and/or use mild detergents to shield the hydrophobic parts of the protein until it can be purified.

6- Imagine you observe a low yield of your target protein in a cell-free system. Describe three possible reasons for this and suggest a troubleshooting strategy for each.

Homework question from Kate Adamala

Filamentous bulking is a nightmare for wastewater treatment plant operators; it occurs when long, thread-like bacteria (like Microthrix parvicella) overgrow, preventing the “floc” (sludge) from settling properly. This results in poor water quality and sludge carryover. Using the Synthetic Minimal Cell (SMC) framework, we can design a targeted “Seek and Destroy” sentinel to combat this issue without the risks of broad-spectrum biocides like chlorine.

1. Function and Logic The Function: A “Filament-Specific Lysis Sentinel.” It detects high concentrations of specific metabolites or signal molecules (like long-chain fatty acids) used by bulking filaments and responds by releasing specialized enzymes to break them down.

Input/Output:

Input: Oleic Acid (a common substrate and signal for M. parvicella).

Output: Chitinase or Lysozyme (enzymes that degrade the cell walls of specific filamentous bacteria).

Why Encapsulate? In the turbulent environment of a wastewater tank, enzymes would be diluted instantly. Encapsulation allows the SMC to protect its “payload” until it is in the heart of a sludge floc where the concentration of filaments is highest.

Natural Cell Alternative? Using a genetically modified bacterium (GMM) in wastewater is legally and ethically difficult because they can “escape” into the environment. An SMC is a non-living, non-replicating machine that “runs out of batteries” and disappears after its task is done.

3. Design of Components Membrane: POPC and Palmitic Acid. Including a fatty acid in the membrane helps the SMC “blend in” and adhere to the lipid-loving filamentous bacteria.

   Internal Contents: * E. coli cell-free Tx/Tl system.

   ATP/GTP and an energy regeneration system (Creatine Phosphate).

   Plasmids containing the sensor-actuator circuit.

   Organism Source: Bacterial (E. coli). It provides the most robust and rapid protein production for enzymatic payloads.

   Communication: Small fatty acids are permeable to the liposome membrane. The output enzyme (Chitinase) requires a pore to exit.

4. Experimental Details

   The “Sensor” Gene: fadR (a fatty acid-responsive regulator). In its default state, FadR represses a promoter. When Oleic Acid (the input) enters the SMC, it binds FadR, releasing the repression.

   The “Pore” Gene: α-hemolysin (αHL). This gene is placed under the control of a FadR-repressed promoter. Presence of filament-related lipids → FadR releases → αHL is expressed → Membrane becomes porous.

   The “Actuator” Gene: ChiA (Chitinase). This enzyme specifically degrades the complex polysaccharides in certain filamentous cell walls. It is constitutively expressed but remains trapped until the αHL pore opens.

   Measurements:

    Sludge Volume Index (SVI): In a lab-scale bioreactor, measure the settling rate of the sludge before and after adding the SMCs. A decrease in SVI indicates a successful reduction in bulking.
   
    Microscopy: Use Gram staining or FISH (Fluorescence In Situ Hybridization) to visually observe the physical degradation of the long filaments after SMC treatment.
   

Desired Outcome: The SMCs are added to the “Return Activated Sludge” (RAS) line. They float into the aeration tank and become trapped within the tangled filaments. Once they sense the high lipid concentration of the filaments, they “fire,” releasing a concentrated burst of Chitinase directly onto the target. This breaks the filaments into smaller pieces, allowing the healthy sludge flocs to settle normally and restoring the plant’s efficiency without harming the beneficial “floc-forming” bacteria.

Homework question from Peter Nguyen

Carbon monoxide (CO) is often called the “silent killer” because it is colorless and odorless. Integrating a cell-free biosensor into heating systems or textiles (like curtains near a furnace) could provide a life-saving, zero-electricity backup to traditional electronic detectors.

1. The Pitch: A bio-synthetic “smart vent” filter that detects dangerous carbon monoxide levels from faulty heaters and undergoes a rapid, irreversible color change to provide a visible emergency alert before toxic levels are reached.

2. The Concept: The core of this system is the CooA protein, a natural carbon monoxide sensor found in bacteria like Rhodospirillum rubrum. In our cell-free system, freeze-dried E. coli extract is engineered with the coo promoter system. When CO gas is present, it binds to the CooA transcription factor, causing a structural shift that “turns on” the expression of a highly concentrated red chromoprotein (like amilCP). This system would be embedded in a porous, breathable mesh placed over heating vents or furnace enclosures. As the heater pushes air through the filter, any CO present rehydrates the encapsulated cell-free “pellets” (utilizing the ambient humidity or a small, integrated moisture-release bead), triggering the rapid production of the red pigment.

3. Societal Challenge and Market Need: Carbon monoxide poisoning causes thousands of hospitalizations annually, often due to faulty space heaters or furnaces during power outages (e.g., during winter storms). Electronic detectors are effective but rely on batteries or grid power, which can fail. This biological sensor acts as a fail-safe, passive indicator. It requires no electricity and can be integrated directly into low-cost household materials like vent filters, curtains, or even “stickers” placed on the side of a water heater, making high-level safety accessible to low-income households or in off-grid disaster scenarios.

4. Addressing Limitations: Activation: To solve the rehydration problem, the cell-free pellets can be co-packaged with deliquescent salts (which pull moisture from the air) or a specialized hydrogel that maintains a specific “ready-state” water activity without allowing the reaction to start prematurely.

   Stability: Carbon monoxide is a highly stable gas, and the CooA protein is remarkably robust. By using trehalose-based freeze-drying, the biological machinery can remain shelf-stable for 1–2 years inside the filter packaging.

   One-time Use: In this context, one-time use is a safety advantage. Once the filter turns red, it serves as a permanent, “latched” record that a CO event occurred. Even if the gas clears, the red stain remains, forcing the user to acknowledge the danger and call a technician to repair the faulty heater before replacing the sens

Homework question from Ally Huang

1. Background: The Challenge Deep-space missions expose astronauts to high-LET (Linear Energy Transfer) radiation, which triggers chronic oxidative stress and neuroinflammation. This damage threatens long-term cognitive function and mission safety. Currently, we lack real-time, lightweight tools to monitor neurological health in orbit. Understanding how the blood-brain barrier (BBB) integrity shifts and how specific neuro-protective proteins respond is vital for humanity to become a multi-planetary species. This research is scientifically significant because it explores the limits of biological resilience in extreme environments, offering insights into neurodegenerative diseases on Earth.

2. The Target Our target is the Glial Fibrillary Acidic Protein (GFAP) and its encoding mRNA. GFAP is a hallmark clinical biomarker for astrocyte activation and brain injury.

3. Relation to Space Biology In microgravity, “fluid shifts” increase intracranial pressure, potentially stressing astrocytes—the brain’s support cells. Combined with cosmic radiation, this causes astrocytes to overexpress GFAP as they become reactive. Monitoring GFAP mRNA levels provides a “real-time” look at neural stress before physical symptoms manifest. By using cell-free systems to detect these specific transcripts from a blood or saliva sample, we can quantify neuro-environmental stress without the need for complex, heavy lab equipment like traditional immunoassays, making it an ideal diagnostic approach for resource-constrained spacecraft.

4. Hypothesis and Research Goal Hypothesis: We hypothesize that BioBits® cell-free pellets can be engineered into a rapid, “just-add-water” diagnostic tool to quantify GFAP mRNA levels using a fluorescence-based riboswitch, and that these levels will be significantly higher in samples exposed to simulated cosmic radiation.

Goal: Our goal is to validate a modular detection system using the BioBits® kit. By creating a specialized RNA sensor (a “toehold switch”) that only triggers the production of a fluorescent protein when it binds to GFAP mRNA, we can create a visual “red-light/green-light” safety test for astronaut neural health. This reasoning is based on the high sensitivity of cell-free systems to small RNA concentrations and the portability of fluorescence viewers, providing a low-mass alternative to terrestrial diagnostic labs. 5. Experimental Plan

We will test three samples:

Synthetic GFAP mRNA (positive control).

Scrambled RNA (negative control).

Astronaut-derived RNA (simulated via irradiated human cell culture extracts).

Samples will be added to BioBits® pellets containing the GFAP toehold-switch DNA. If GFAP mRNA is present, the BioBits® machinery will produce a fluorescent protein. We will use the miniPCR® for precise incubation temperatures (37°C) and the P51 Molecular Fluorescence Viewer to record light intensity. Data will consist of fluorescence brightness, quantified against a standard curve to determine mRNA concentration.

Homework Part B: Individual Final Project